ADVANCED IC ENGINE NOTES ME2041 ADVANCED I.C. ENGINES
LTPC 3003
OBJECTIVES: To update the knowledge in engine exhaust emission control and alternate fuels To enable the students to understand the recent developments in IC Engines UNIT I SPARK IGNITION ENGINES 9 Air-fuel ratio requirements, Design of carburetor –fuel jet size and venture size, Stages of combustion-normal and abnormal combustion, Factors affecting knock, Combustion chambers, Introduction to thermodynamic analysis of SI Engine combustion process. UNIT II COMPRESSION IGNITION ENGINES 9 Stages of combustion-normal and abnormal combustion – Factors affecting knock, Direct and Indirect injection systems, Combustion chambers, Turbo charging, Introduction to Thermodynamic Analysis of CI Engine Combustion process. UNIT III ENGINE EXHAUST EMISSION CONTROL 9 Formation of NOX , HC/CO mechanism , Smoke and Particulate emissions, Green House Effect , Methods of controlling emissions , Three way catalytic converter and Particulate Trap, Emission (HC,CO, NO and NOX , ) measuring equipments, Smoke and Particulate measurement, Indian Driving Cycles and emission norms UNIT IV ALTERNATE FUELS 9 Alcohols , Vegetable oils and bio-diesel, Bio-gas, Natural Gas , Liquefied Petroleum Gas ,Hydrogen , Properties , Suitability, Engine Modifications, Performance , Combustion and Emission Characteristics of SI and CI Engines using these alternate fuels. UNIT V RECENT TRENDS 9 Homogeneous Charge Compression Ignition Engine, Lean Burn Engine, Stratified Charge Engine, Surface Ignition Engine, Four Valve and Overhead cam Engines, Electronic Engine Management, Common Rail Direct Injection Diesel Engine, Gasoline Direct Injection Engine, Data Acquisition System –pressure pick up, charge amplifier PC for Combustion and Heat release analysis in Engines. TOTAL: 45 PERIODS TEXT BOOK: 1. Heinz Heisler, ‘Advanced Engine Technology,” SAE International Publications, USA,1998 2. Ganesan V..” Internal Combustion Engines” , Third Edition, Tata Mcgraw-Hill ,2007 REFERENCES: 1. John B Heywood,” Internal Combustion Engine Fundamentals”, Tata McGraw-Hill 1988 2. Patterson D.J. and Henein N.A,“Emissions from combustion engines and their control,” Ann Arbor Science publishers Inc, USA, 1978 3. Gupta H.N, “Fundamentals of Internal Combustion Engines” ,Prentice Hall of India, 2006 4. Ultrich Adler ,” Automotive Electric / Electronic Systems, Published by Robert Bosh GmbH,1995
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UNIT
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SPARK IGNITION ENGINES Air-fuel Requirement in SI Engines The spark-ignition automobile engines run on a mixture of gasoline and air. The amount of mixture the engine can take in depends upon following major factors: (i) Engine displacement. (ii) Maximum revolution per minute (rpm) of engine. (iii) Volumetric efficiency of engine. There is a direct relationship between an engine’s air flow and it’s fuel requirement. This relationship is called the air-fuel ratio. Air-fuel Ratios The air-fuel ratio is the proportions by weight of air and gasoline mixed by the carburetor as required for combustion by the engine. This ratio is extremely important for an engine because there are limits to how rich (with more fuel) or how lean (with less fuel) it can be, and still remain fully combustible for efficient firing. The mixtures with which the engine can operate range from 8:1 to 18.5:1 i.e. from 8 kg of air/kg of fuel to 18.5 kg of air/kg of fuel. Richer or leaner air-fuel ratio limit causes the engine to misfire, or simply refuse to run at all.
Stoichiometric Air-Fuel Ratio The ideal mixture or ratio at which all the fuels blend with all of the oxygen in the air and be completely burned is called the stoichiometric ratio, a chemically perfect combination. In theory, an air fuel ratio of about 14.7:1 i.e. 14.7 kg of air/kg of gasoline produce this ratio, but the exact ratio at which perfect mixture and complete combustion take place depends on the molecular structure of gasoline, which can vary somewhat. Engine Air-fuel Ratios An automobile SI engine, as indicated above, works with the air-fuel mixture ranging from 8:1 to 18.5:1. But the ideal ratio would be one that provides both the maximum power and the best economy, while producing the least emissions. But such a ratio does not exist because the fuel requirements of an engine vary widely depending upon temperature, load, and speed conditions. The best fuel economy is obtained with a 15:1 to 16:1 ratio, while maximum power output is achieved with a 12.5:1 to 13.5:1 ratio. A rich mixture in the order of 11:1 is required for idle heavy load, and high-speed conditions. A lean mixture is required for normal cruising and light load conditions. Figure 9.36 represents the characteristic curves showing the effect of mixture ratio on efficiency and fuel consumption.
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Fig. 9.36. Effect of air-fuel ratio on efficiency and fuel consumption.
Practically for complete combustion, through mixing of the fuel in excess air (to a limited extent above that of the ideal condition) is needed. Lean mixtures are used to obtain best economy through minimum fuel consumption whereas rich mixtures used to suppress combustion knock and to obtain maximum power from the engine. However, improper distribution of mixture to each cylinder and imperfect/incomplete vaporization of fuel in air necessitates the use of rich mixture to obtain maximum power output. A rich mixture is also required to overcome the effect of dilution of incoming mixture due to entrapped exhaust gases in the cylinder and of air leakage because of the high vacuum in the manifold, under idling or no-load condition. Maximum power is desired at full load while best economy is expected at part throttle conditions. Thus required air fuel ratios result from maximum economy to maximum power. The carburetor must be able to vary the air-fuel ratio quickly to provide the best possible mixture for the engine’s requirements at a given moment. The best air-fuel ratio for one engine may not be the best ratio for another, even when the two engines are of the same size and design. To accurately determine the best mixture, the engine should be run on a
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dynamometer to measure speed, load and power requirements for all types of driving conditions. With a slightly rich mixture, the combustion flame travels faster and conversely with a slightly weak mixture, the flame travel becomes slower. If a very rich mixture is used then some “neat” petrol enters cylinder, washes away lubricant from cylinder walls and gets past piston to contaminate engine oil. A very sooty deposit occurs in the combustion chamber. On the other hand, if an engine runs on an excessively weak mixture, then overheating particularly of such parts as valves, pistons and spark plugs occurs. This causes detonation and pre-ignition together or separately. The approximate proportions of air to petrol (by weight) suitable for the different operating conditions are indicated below: Starting 9: 1 Idling 12 : 1 Acceleration 12 : 1 Economy 16: 1 Full power 12 : 1 It makes no difference if an engine is carburetted or fuel injected, the engine still needs the same air-fuel mixture ratios. Carburetion Introduction Spark-ignition engines normally use volatile liquid fuels. Preparation of fuel-air mixture is done outside the engine cylinder and formation of a homogeneous mixture is normally not completed in the inlet manifold. Fuel droplets, which remain in suspension, continue to evaporate and mix with air even during suction and compression processes. The process of mixture preparation is extremely important for sparkignition engines. The purpose of carburetion is to provide a combustible mixture of fuel and air in the required quantity and quality for efficient operation of the engine under all conditions. Definition of Carburetion The process of formation of a combustible fuel-air mixture by mixing the proper amount of fuel with air before admission to engine cylinder is called carburetion and the device which does this job is called a carburetor. Requirements of an automotive carburetor The spark ignition engines fitted to automotive vehicles have to operate under variable speed and load conditions. These engines present the most difficult and stringent requirements to the carburetors. They are as follows:1. Ease of starting the engine, particularly under low ambient conditions. 2. Ability to give full power quickly after starting the engine. 3. Equally good and smooth engine operation at various loads. 4. Good and quick acceleration of the engine. 5. Developing sufficient power at high engine speeds. 6. Simple and compact in construction. 7. Good fuel economy. 8. Absence of racing of the engine under idling conditions. 9. Ensuring full torque at low speeds. Factors Affecting Carburetion Of the various factors, the process of carburetion is influenced by i. The engine speed ii. The vaporization characteristics of the fuel
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iii. The temperature of the incoming air and iv. The design of the carburetor
Principle of Carburetion Both air and gasoline are drawn through the carburetor and into the engine cylinders by the suction created by the downward movement of the piston. This suction is due to an increase in the volume of the cylinder and a consequent decrease in the gas pressure in this chamber. It is the difference in pressure between the atmosphere and cylinder that causes the air to flow into the chamber. In the carburetor, air passing into the combustion chamber picks up discharged from a tube. This tube has a fine orifice called carburetor jet that is exposed to the air path. The rate at which fuel is discharged into the air depends on the pressure difference or pressure head between the float chamber and the throat of the venturi and on the area of the outlet of the tube. In order that the fuel drawn from the nozzle may be thoroughly atomized, the suction effect must be strong and the nozzle outlet comparatively small. In order to produce a strong suction, the pipe in the carburetor carrying air to the engine is made to have a restriction. At this restriction called throat due to increase in velocity of flow, a suction effect is created. The restriction is made in the form of a venturi to minimize throttling losses. The end of the fuel jet is located at the venturi or throat of the carburetor. The geometry of venturi tube is as shown in Fig.16.6. It has a narrower path at the center so that the flow area through which the air must pass is considerably reduced. As the same amount of air must pass through every point in the tube, its velocity will be greatest at the narrowest point. The smaller the area, the greater will be the velocity of the air, and thereby the suction is proportionately increased As mentioned earlier, the opening of the fuel discharge jet is usually loped where the suction is maximum. Normally, this is just below the narrowest section of the venturi tube. The spray of gasoline from the nozzle and the air entering through the venturi tube are mixed together in this region and a combustible mixture is formed which passes through the intake manifold into the cylinders. Most of the fuel gets atomized and simultaneously a small part will be vaporized. Increased air velocity at the throat of the venturi helps he rate of evaporation of fuel. The difficulty of obtaining a mixture of sufficiently high fuel vapour-air ratio for efficient starting of the engine and for uniform fuel-air ratio indifferent cylinders (in case of multi cylinder engine) cannot be fully met by the increased air velocity alone at the venturi throat. The Simple Carburetor Carburetors are highly complex. Let us first understand the working principle bf a simple or elementary carburetor that provides an air fuel mixture for cruising or normal range at a single speed. Later, other mechanisms to provide for the various special requirements like starting, idling, variable load and speed operation and acceleration will be included. Figure 3. shows the details of a simple carburetor.
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Figure: 3 The Simple Carburetor The simple carburetor mainly consists of a float chamber, fuel discharge nozzle and a metering orifice, a venturi, a throttle valve and a choke. The float and a needle valve system maintain a constant level of gasoline in the float chamber. If the amount of fuel in the float chamber falls below the designed level, the float goes down, thereby opening the fuel supply valve and admitting fuel. When the designed level has been reached, the float closes the fuel supply valve thus stopping additional fuel flow from the supply system. Float chamber is vented either to the atmosphere or to the” upstream side of the venturi.During suction stroke air is drawn through the venturi. As already described, venturi is a tube of decreasing cross-section with a minimum area at the throat, Venturi tube is also known as the choke tube and is so shaped that it offers minimum resistance to the air flow. As the air passes through the venturi the velocity increases reaching a maximum at the venturi throat. Correspondingly, the pressure decreases reaching a minimum. From the float chamber, the fuel is fed to a discharge jet, the tip of which is located in the throat of the venturi. Because of the differential pressure between the float chamber and the throat of the venturi, known as carburetor depression, fuel is discharged into the air stream. The fuel discharge is affected by the size of the discharge jet and it is chosen to give the required air-fuel ratio. The pressure at the throat at the fully open throttle condition lies between 4 to 5 cm of Hg, below atmospheric and seldom exceeds8 cm Hg below atmospheric. To avoid overflow of fuel through the jet, the level of the liquid in the float chamber is maintained at a level slightly below the tip of the discharge jet. This is called the tip of the nozzle. The difference in the height between the top of the nozzle and the float chamber level is marked h in Fig.3. The gasoline engine is quantity governed, which means that when power output is to be varied at a particular speed, the amount of charge delivered to the cylinder is varied. This is achieved by means of a throttle valve usually of the butterfly type that is situated after the venturi tube. As the throttle is closed
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less air flows through the venturi tube and less is the quantity of air-fuel mixture delivered to the cylinder and hence power output is reduced. As the” throttle is opened, more air flows through the choke tube resulting in increased quantity of mixture being delivered to the engine. This increases the engine power output. A simple carburetor of the type described above suffers from a fundamental drawback in that it provides the required A/F ratio only at one throttle position. At the other throttle positions the mixture is either leaner or richer depending on whether the throttle is opened less or more. As the throttle opening is varied, the air flow varies and creates a certain pressure differential between the float chamber and the venturi throat. The same pressure differential regulates the flow of fuel through the nozzle. Therefore, the velocity of flow of air II and fuel vary in a similar manner. At the same time, the density I of air decrease as the pressure at the venturi throat decrease with increasing air flow whereas that of the fuel remains unchanged. This results in a simple carburetor producing a progressively rich mixture with increasing throttle opening. The Choke and the Throttle When the vehicle is kept stationary for a long period during cool winter seasons, may be overnight, starting becomes more difficult. As already explained, at low cranking speeds and intake temperatures a very rich mixture is required to initiate combustion. Some times air-fuel ratio as rich as 9:1 is required. The main reason is that very large fraction of the fuel may remain as liquid suspended in air even in the cylinder. For initiating combustion, fuel-vapour and air in the form of mixture at a ratio that can sustain combustion is required. It may be noted that at very low temperature vapour fraction of the fuel is also very small and this forms combustible mixture to initiate combustion. Hence, a very rich mixture must be supplied. The most popular method of providing such mixture is by the use of choke valve. This is simple butterfly valve located between the entrance to the carburetor and the venturi throat as shown in Fig.3. When the choke is partly closed, large pressure drop occurs at the venturi throat that would normally result from the quantity of air passing through the venturi throat. The very large depression at the throat inducts large amount of fuel from the main nozzle and provides a very rich mixture so that the ratio of the evaporated fuel to air in the cylinder is within the combustible limits. Sometimes, the choke valves are spring loaded to ensure that large carburetor depression and excessive choking does not persist after the engine has started, and reached a desired speed. This choke can be made to operate automatically by means of a thermostat so that the choke is closed when engine is cold and goes out of operation when engine warms up after starting. The speed and the output of an engine is controlled by the use of the throttle valve, which is located on the downstream side of the venturi. The more the throttle is closed the greater is the obstruction to the flow of the mixture placed in the passage and the less is the quantity of mixture delivered to .the cylinders. The decreased quantity of mixture gives a less powerful impulse to the pistons and the output of the engine is reduced accordingly. As the throttle is opened, the output of the engine increases. Opening the throttle usually increases the speed of the engine. But this is not always the case as the load on the engine is also a factor. For example, opening the throttle when the motor vehicle is starting to climb a hill may or may not increase the vehicle speed, depending upon the steepness of the hill and the extent of throttle opening. In short, the throttle is simply a means to regulate the output of the engine by varying the quantity of charge going into the cylinder. Stages of Combustion in SI Engine In a spark-ignition engine a sufficiently homogeneous mixture of vaporized fuel, air and residual gases is ignited by a single intense and high temperature spark between the spark plug electrodes (at the moment of discharge the temperature of electrodes exceeds 10,000°C), leaving behind a thin thread of flame. From this thin thread combustion spreads to the envelop of mixture immediately surrounding it at a
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rate which depends primarily upon the temperature of the flame front itself and to a secondary degree, upon both the temperature and the density of the surrounding envelope. In this manner there grows up, gradually at first, a small hollow nucleus of flame, much in the manner of a soap bubble. If the contents of the cylinder were at rest, this flame bubble would expand with steadily increasing speed until extended throughout the whole mass. In the actual engine cylinder, however, the mixture is not at rest. It is, in fact, in a highly turbulent condition the turbulence breaks the filament of flame into a ragged front, thus presenting a far greater surface area from which heat is radiated; hence its advance is speeded up enormously. The rate at which the flame front travels is dependent primarily on the degree of turbulence, but its general direction of/movement, that of radiating outward from the ignition point, is not much affected. According to Ricardo the combustion can be imagined as if developing in two stages, one the growth and development of a semi propagating nucleus of flame called ignition lag or preparation phase, and the other, the spread of the flame throughout the combustion chamber [see Fig. 9].
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Figure: 9. Stages of combustion in SI engine The former is a chemical process depending upon the nature of the fuel, upon temperature and pressure, the proportion of the exhaust gas, and also upon the temperature coefficient of the fuel, that is, the relationship between temperature and rate of acceleration of oxidation or burning. The second stage is a
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mechanical one pure and simple. The two stages are not entirely distinct, since the nature and velocity of combustion change gradually. The starting point of the second stage is where first measurable rise of pressure can be seen on the indicator diagram, i.e., the point where the line of combustion departs from the compression line. In Fig. 14.2(b), A shows the point of passage of spark - (say 28° before TDC), B the point at which the first rise of pressure can be detected (say, 8°before TDC) and C the attainment of peak pressure. Thus AB represents the first stage (about 20° crank angle rotation) and BC the second stage. Although the point C makes the completion of the flame travel, it does not follow that at this point the whole of the heat of the fuel has been liberated, for even after the passage of the flame, some further chemical adjustments due to re-association, etc., and what is generally referred to as after burning, will to a greater or less degree continue throughout the expansion stroke. The first stage AB, by analogy with diesel engines is called ignition lag, which label is wrong in principle. In spark ignition there is practically no ignition lag and a nucleus of combustion arises instantaneously near the spark plug electrodes. But during the initial period flame front spreads very slowly and the fraction of burnt mixture is small so that an increase of pressure cannot be detected on the indicator diagram. The increase of pressure maybe just one per cent of maximum combustion pressure corresponding to burning of about 1.5per cent of the working mixture, and the volume occupied by the combustion products may be about 5 per cent of the combustion chamber space. The stage II is the main stage of combustion. The end of second stage is taken as the moment at which maximum pressure is reached in the indicator diagram (see Fig. 9). However, combustion does not terminate at this point and after burning continues for a rather long time near the walls and behind the turbulent flame front. The combustion rate in the stage III reduces, due to surface of the flame front becoming smaller and reduction in turbulence. About 10 per cent or more of heat is evolved in the afterburning stage and hence the temperature of the gases continues to increase to point D in Fig.9. However, the pressure reduces because the decrease in pressure due to expansion of gases and transfer of heat to walls is more than the increase in pressure due to combustion.
Effect of Engine Variables on Flame Propagation A study of the variables which affect the flame propagation velocity is important because the flame velocity influences the rate of pressure rise in the cylinder, and has bearing or certain types of abnormal combustion . There are several factors which affect the flame speed, the most important being fuel-air ratio and turbulence. 1. Fuel-air ratio: The composition of the working mixture influences the rate of combustion and the amount of heat evolved. With hydrocarbon fuels the maximum flame velocities occur when mixture strength is 110% of stoichiometric (i.e., about 10% richer than stoichiometric). When the mixture is made leaner or is enriched and still more, the velocity of flame diminishes. Lean mixtures release less thermal energy resulting in lower flame temperature and flame speed. Very rich mixtures have incomplete combustion (some carbon only burns to CO and not to CO2) that results in production of less thermal energy and hence flame speed is again low. 2. Compression Ratio: A higher compression ratio increases the pressure and temperature of the working mixture and decreases the concentration of residual gases. These favorable conditions reduce the ignition lag of combustion and hence less ignition advance is needed. High pressures and temperatures of the compressed mixture also speed up the second phase of combustion. Total ignition angle is reduced.
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Maximum pressure and indicated mean effective pressure are increased.. Lastly, use of a higher compression ratio increases the surface to volume ratio of the combustion chamber, thereby increasing the part of the mixture which after-burns in the third phase. The increase in compression ratio results in increase in temperature that increases the tendency of the engine to detonate. 3. Intake temperature and pressure: Increase in intake temperature and pressure increases the flame speed. 4. Engine load: With increase in engine load the cycle pressures increase. Hence the flame speed increases. In SI engines with decrease in load, throttling reduces power of an engine. Due to throttling the initial and final compression pressures decrease and the dilution of the working mixture due to residual gases increases. This makes the smooth development of self propagating nucleus of flame difficult and unsteady and prolongs the ignition lag. The difficulty can be overcome to a certain extent by enriching the mixture at low loads (0.8 to 0.9of stoichiometric) but still it is difficult to avoid after-burning during a substantial part of expansion stroke. In fact, poor combustion at low loads and the necessity of mixture enrichment are among the main disadvantages of spark ignition engines which cause wastage of fuel and discharges of a large amount of products of incomplete combustion like carbon monoxide and other poisonous substances. 5. Turbulence: Turbulence plays a very vital role in combustion phenomenon. The flame speed is very low in non-turbulent mixtures. A turbulent motion of the mixture intensifies the processes of heat transfer and mixing of the burned and unburned portions in the flame front (diffusion). These two factors cause the velocity of turbulent flame to increase practically in proportion to the turbulence velocity. The turbulence of the mixture is due to admission of fuel-air mixture through comparatively narrow sections of the intake pipe, valves, etc. in the suction stroke. The turbulence can be increased at the end of the compression by suitable design of combustion chamber that involves the geometry of cylinder head and piston crown. The degree of turbulence increases directly with the piston speed. If there is no turbulence the time occupied by each explosion would be so great as to make the high speed internal combustion engines impracticable. Insufficient turbulence lowers the efficiency due to incomplete combustion of the fuel. However, excessive turbulence is also undesirable. 6. Engine Speed: The higher the engine speed, the greater the turbulence inside the cylinder. For this reason the flame speed increases almost linearly with engine speed. Thus if the engine speed is doubled the time required, in milliseconds, for the flame to traverse the combustion space would be halved. Double the original speed arid hence half the original time would give the same number of crank degrees for flame propagation. The crank angle required for the flame propagation, which is the main phase of combustion, will remain almost constant at all speeds. This is an important characteristic of spark ignition engines. However, the increase in engine speed would lead to ignition advance due to the first phase of combustion. This can be illustrated with a numerical example. Consider a petrol engine running at 1500rpm. Let us say for the first stage of combustion the ignition lag, the time required in terms of crank angle, is 8° of crank rotation, and for the second stage, the propagation of flame through the combustion space, 12oofcrank rotation is required. Thus the total ignition period is 20°of crank rotation. Now if the engine speed is doubled from 1500 to 3000 rpm, the time required for the second stage will again be 12° of crank rotation (due to doubling of turbulence intensity time in milliseconds is halved and in terms of crank angle remains constant), but for the first stage time in milliseconds is constant and hence in terms of crank angle it will be doubled, i.e., it would be 16°.This would make the total ignition period of 16 + 12 = 28° crank rotation at 3000rpm compared to 8° + 12°= 20° at .1500 rpm. From this it follows that with increase in engine speed ignition must be advanced. This is done in practice by automatic ignition advance mechanism.
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7. Engine size: Engines of similar design generally run at the same piston speed. This is achieved by smaller engines having larger rpm and larger engines having smaller rpm. Due to the same piston speed, the inlet velocity, the degree of turbulence, and flame speed are nearly same in similar engines regardless of the size. However, in small engines the flame travel is small and in large engines large. Therefore, if the engine size is doubled the time required (in milliseconds) for propagation of flame through combustion space will also be doubled. But with lower rpm of larger engines the time for flame propagation in terms of crank angle would be nearly same as in smaller engines. In other words the number of crank degrees required for flame travel will be about the same irrespective of engine size, provided the engines are similar. Rate of Pressure Rise The rate of pressure rise is a very important aspect of flame development from engine design and operation point of view. It considerably influences the maximum cylinder pressure, the power produced and the smooth running of the engine. The rate or pressure rise depends on the mass rate of combustion of the mixture in the cylinder. Fig. 10 shows pressure-crank angle diagrams for three different combustion rates. One is for a high, the second for the usual and the third for a low rate of combustion
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Figure: 10. Relationship b/w pressure and crank angle for different rates of combustion It is clear from the figure that with lower rates of combustion longer time is required for combustion that necessitates the initiation of burning at an earlier point on the compression stroke. With higher rates of
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burning the time required for combustion is smaller and the rate of pressure rise is higher. Also, the peak pressure produced is close to TDC, which is desirable because it produces greater force acting through a large portion of the power stroke. But peak pressure and hence peak temperature too close to TDC gives a long time for rapid heat loss from the cylinder. The higher rate of pressure rise causes rough running of the engine because of vibrations and jerks produced in crankshaft. If the rate of pressure rise is very high it results in abnormal combustion called detonation. In practice the engine is so designed that approximately one-half of the pressure rise takes place as the piston reaches TDC. This results in peak pressure and temperature 10° to 15° after TDC. In this way very small portion of the expansion stroke is-lost and the gain is smooth engine operation and saving an appreciable period of time during which loss of heat is rapid. In the old engines with low compression ratios of 5 to 6 a rate of pressure rise of 2 bar per crank degree used to be thought as optimum. Today with higher compression ratios of the order of 8 to 9, a rate of pressure rise of 3 to 4 bar per crank degree may be employed if the engine mountings are sufficiently stiff and efficient. Gasoline Combustion Vaporization of the hydrocarbons in gasoline and start of decomposition take place at temperatures below 593 K, which exist in the combustion chamber before the initiation of ignition. The products of combustion are mostly gases containing a large quantity of heat. The heat energy increases the gas pressure in the combustion chamber to produce the force on the engine piston, required to operate the engine. The liquid gasoline must be converted to a vapour to burn in an engine. In carburetted engines vaporization of the gasoline must be done in one-third of a second at idle speeds and in one-thirtieth of a second at normal operating speeds. In fuel injected engines this must occur much faster. The carburetor during the process of mixing liquid fuel and air supports the vaporization process by breaking the liquid gasoline into sudsy foam that rapidly mixes with the air. The molecules of fuel and the molecules of oxygen in the air must combine in correct numbers. At sea level the air being dense a relatively small quantity is required for a given amount of gasoline. The air becomes less dense at high altitudes and at high atmospheric temperatures due to which the same volume of air contains a smaller number of oxygen molecules causing the air-fuel mixture to become richer in fuel. This causes problem on some emission controlled engines requiring leaner carburetor settings on automobiles used in the mountains than those used at sea level. Since automobiles are frequently operated in both mountains and at sea level, carburetors are provided with altitude compensation devices to prevent over-rich mixtures at high elevations.
the charge is trapped in the combustion chamber, the molecules of oxygen in the air come into close contact with the hydrocarbon molecules of the gasoline. This causes rapid burning. A litre of gasoline if completely burned produces nearly a litre of water as well as sulphur dioxide in an amount dependent on the sulphur content in the gasoline. As the water is in a vapour form at normal operating temperatures it leaves the cylinder as a part of exhaust gas. When the engine is first started in cold weather condensed water vapour is visible in the exhaust. Condensed moisture with sulphur dioxide produces the acidic water, which is corrosive. During low temperature operating conditions such as suburban driving when the engine is cold, much of the moisture is condensed inside the engine. The combination of corrosion and wear under these conditions is the major reason for excessive wear of the top ring area of the cylinder wall. Normal Combustion In a SI engine a homogeneous air-fuel mixture within the combustible range sustains the progress of a definite flame front across the combustion chamber, and combustion takes place in any location where fuel particle exists. In a CI engine, on the other hand, the air-fuel ratios in the various part of the chamber
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very widely, so no definite flame front is evident, and hence combustion occurs in many locations within the chamber. A spark plug ignites the charge in the combustion chamber near the end of the compression stroke. The spark, produced across the spark plug electrodes at the correct time, must have sufficient energy to raise the gas temperature between the electrodes at a point so that the charge burning becomes self-sustaining. From this point, a flame front moves smoothly across the combustion. The flame front movement during normal combustion is illustrated in Fig. 8.6. Burning of charge takes place during approximately fifty degrees of crankshaft rotation due to which maximum force is exerted on the crankshaft. Actual combustion is much more complex and the combustion gases pass through many phases during the combustion process. For better understanding, the combustion is divided into two phases i.e. pre-flame reactions, and combustion.
As the gases are compressed and the temperature rises, pre-flame chemical reactions take place in the compressed charge thereby changing the character of the charge. These pre-flame reactions prepare the charge for burning. As ignition takes place, depending upon combustion chamber turbulence the flame front moves out in a modified spherical fashion. The heat energy released behind the flame front increases combustion chamber pressure and temperature. Due to higher combustion chamber pressure and temperature the pre-flame reactions are increased in a portion of the charge, called the end gases, which remain ahead of the flame front. Pre-flame reactions increase more rapidly at higher engine compression ratios. If pre-flame reactions become too rapid, abnormal combustion takes place.
Abnormal Combustion Abnormal combustion may be divided into two main types i.e. knock or detonation and surface ignition. Each of these types causes loss of power and excessive temperature. Continued operation under either type of abnormal combustion gives rise to physical damage of the engine. Detonation. Engine knock or detonation is the out come of rapid pre-flame reactions within the highly stressed end gases. Due to the too rapid reactions spontaneous ignition of the end gases takes place as shown in Fig. 8.7. This causes very rapid combustion within the end gases, accompanied by high-frequency pressure waves. These waves hit the combustion chamber walls; as a result vibration noise sets which is called knock or detonation. Detonation is affected by (i) compression ratio, (ii) the temperature and pressure at the end of compression, (Hi) the temperature of combustion chamber wall, (iv) engine speed, (v) fuel mixture strength, (vi) combustion chamber shape, (vii) the type of fuel, (viii) ignition timing, (ix) position of spark plug, and (x) position of exhaust valve.
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Fig. 8.6. Flame front movement during normal combustion.
Fig. 8.7. Flame front movement during detonation.
The tendency of an engine to knock with a given fuel can be suppressed by lowering either combustion pressure or temperature, or both ; or by reducing the time the end gases are subjected to high pressures and temperatures. Also, using a fuel, which is less susceptible to rapid pre-flame reactions, reduces the tendency to knock. Octane rating is a measure of the anti-knock properties of a fuel. A fuel, which has high anti-knock characteristics, has a high octane rating.
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Compression ratio has predominant effects on compression pressure. With the increase of compression pressure the output power of an engine increases. This is due to the higher combustion pressures, which are produced. High combustion pressures, however, increase the knock tendency. Fuels with high antiknock properties are used in higher-compression ratio engines to run engine knock-free while developing increased power. Lower compression ratios are used in low-emission engines so that they can run knock-free on low-octane unleaded gasoline. Combustion chamber design also affects knock tendency. If combustion chambers end gases are in a squash or quench area, the engine has low knocking tendencies. This happens, as the end gases are thin and close to a cool metal surface. Cooling the gases reduces and slows the end gas preflame reactions, thereby decreasing the engine knock tendency. This quenching of end gases is the main reason for a rotating combustion chamber engine to run knock-free on low octane gasoline. Combustion chamber turbulence, as illustrated in Fig. 8.8, also helps to reduce knocking tendency by mixing cool and hot gases, thus preventing a concentration of static hot end gases where rapid pre-flame reactions can take place.
Fig. 8.8. End gases cooled in the quench area.
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Fig. 8.9. Flame front movement during pre-ignition.
The detonation can be reduced by (a) decreasing the combustion pressure and temperature, (b) reducing the time the end gases are subjected to high pressures and temperatures, (c) the use of fuel with a high octane number, (d) proper design of combustion chamber where end gases are in a squash or quench area, and (e) increasing combustion chamber turbulence. Surface Ignition.
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Surface ignition or secondary ignition, an abnormal combustion, starts at any source of ignition other than the spark plug. This is illustrated in Fig. 8.9. As surface ignition produces a secondary ignition source, its effect is to complete the combustion process sooner than normal, thereby developing maximum pressure at a wrong time in the engine cycle producing less power. One potential source of secondary ignition is a hot spot, such as a spark plug electrode, a protruding gasket, a sharp valve edge, etc. These items can become extremely hot during engine operation form ing a second source of ignition. These sources rarely occur in modern engine designs provided the engines are properly maintained. Another source of secondary ignition is combustion chamber deposits, which result from the type of fuel and oil used in the engine as well as from the type of operation of the engine. A deposit ignition source may be a hot loose deposit flake capable of igniting one charge before it is exhausted from the engine with the spent exhaust gases. This is called wild ping. Sometimes, the flake remains attached to the combustion chamber wall. Under this situation, it ignites successive charges until the deposit is consumed or the engine operating conditions are changed. When surface ignition occurs before firing of the spark plug, it is called pre-ignition. It may be audible or inaudible. It may be a wild ping or it may be a continuous runaway surface ignition. If it occurs after the ignition is turned off, it is called run-on or dieseling. Another phenomenon resulting from pre-ignition is engine rumble. Rumble is a low-frequency vibration of the lower part of the engine that occurs when the maximum pressure is reached earlier than normal in the cycle. Rumble has been almost eliminated from modern engines. The knock-resistant fuels and antiknock additives generally tend to increase combustion chamber deposits thereby increasing the tendency to cause surface ignition. Fuel manufacturers therefore, use additional additives in the gasoline to reduce the deposit ignition tendency resulting from the antiknock additives deposits. Abnormal combustion seldom occurs in modern mass-produced automotive engines provided the recommended grade of fuel and motor oil is used and the engine is maintained and adjusted correctly. Some problems may exist in engines that are used exclusively for low-speed, short-trip driving. Abnormal combustion frequently occurs in engines modified for maximum performance and also some in emission controlled engines.
Pre-ignition Ignition of air fuel mixture by some hot spot which exists within the combustion chamber, before the occurrence of spark is called pre-ignition. In a spark ignition engine, the spark that jumps across the terminals of the spark plug initiates combustion. Similarly if there is any other hot source in the combustion chamber it will heat up the air fuel mixture surrounding it. Then preflame reaction will certainly be accelerated by this hot spot. The hot spot may activate. The charge in its immediate vicinity and produce a flame. The flame may then propagate from this point before the occurrence of spark. Pre-ignition combustion can be seen in fig.
As indicated under surface ignition, carbon deposit from fuel or oil, an over heated spark plug center electrode or the edge of the gasket that protrudes into the combustion chamber can act as a hot spot and cause pre-ignition. An overheated exhaust valve head or edge can cause preigniton. Using unsuitable type spark plug (one that runs too hot or has a long reach) or igniton timing too far retarded or mixture too
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weak or rich which gives too slow a burning rate may also cause preigniton. The minimum tendency to preignite exists at fuel air ratios usually richer than the chemically correct. Tetra ethyl lead which is added to a fuel to increase its antiknock characteristics also reduces the tendency to preignite.
The amount of charge that burns instantaneously due to preigniton depends upon the surface area of the hot spot and the temperature of the hot spot. When a considerable amount of charge burns, steep pressure rise and pressure pulsation may occur. A knock, metallic sound will be heard.
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Different abnormal combustion that may take place in a SI engine The definitions that follow the spirit of the CRC report 278, SAE special publication are as follows:Knock – The noise associated with auto-ignition of a portion of the mixture ahead of a flame front advancing at normal velocity (whether or not surface ignition is present). Normal combustion - Combustion initiated by a timed spark, with the flame front moving in a uniform manner at a normal velocity, without auto ignition. Abnormal combustion – Combustion with surface ignition (phosphorous additives to the gasoline are used for control of surface ignition and spark plug fouling). Spark knock – Recurrent knock which can be controlled in intensity (or eliminated) by adjusting the spark advance. Surface ignition – Initiation of a flame front by a hot surface other than the spark. Pre-ignition – Surface ignition occurring before the spark. Post ignition – Surface ignition occurring after the spark. Wild ping – Erratic pings or sharp cracks (probably as the result of early surface ignition from deposit particles) Rumble – A low pitched thud (probably caused by multiple, early, surface ignition raising the pressure greatly with consequent deflection of mechanical parts). Effects of combustion knock The auto ignitions of the charge, steep pressure rise which sets up pressure wave, vibration of the gas and increased heat transfer to the cylinder walls, piston and other engine components during knocking combustion may result in the following: 1. Reduction in power output and efficiency. 2. Burning of piston crown due to increased temperature or due to blow by of very hot gases past the piston rings from the piston top to the crankcase. 3. The impact of the high pressure wave that is set up might even fracture the piston crown. 4. Burning of cylinder head and valve head. 5. Gumming of piston rings in the piston grooves leading to ring sticking. 6. Loosening of valve seat inserts in the cylinder head. 7. Erosion of piston head may occur at the position of the end mixture. The eroded surface has the appearance of being blasted and not melted. Operating conditions causing detonation The following are some of the operating conditions which may cause detonation in an engine. 1. Slow burning lean air fuel mixture supplied by faulty carburetor or fuel injector, fuel pump, blocked fuel filter or fuel line, vacuum leak at higher engine speeds caused by bad positive crankcase ventilation (PCV) valve or exhaust gas recirculation (EGR) valve. 2. Gasoline with low octane or anti clock rating. This is more common with unleaded gasoline. 3. Carbon deposits increasing compression ratio. This is the result of lubricating oil entering the cylinders or poor detergent action of gasoline.
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4. Engine operating at above normal temperature due to low coolant level or circulation, water jacket blockage in the head. 5. Ignition timing very much advanced due to improper setting of initial ignition timing, inaccurate distribution or advance curve etc. 6. Bad rings and / or valve seals allowing oil (low octane hydrocarbon) to be burned in the cylinders. 7. Air cleaner clogged, which allows too much hot exhaust gas to remain in the engine cylinder. 8. Excessive turbocharger boost pressure from a bad pressure limiting valve.
Ways and means of knock reduction Investigations indicate that one or more of the following factors will decrease the possibility of knock in the SI engine. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
Decreasing the compression ratio or reducing the inlet pressure. Decreasing inlet air temperature. Decreasing coolant inlet temperature. Decreasing temperature of the cylinder and combustion chamber walls or part opening of the throttle (decreasing the load). Retarding spark timing. Decreasing the distance of flame travel in order to complete combustion within a shorter period. Increasing the turbulence of the mixture and thus increasing the flame speed. Increasing the engine speed, thus increasing the speed (movement) of the mixture and decreasing the time available for preflame reactions. Increasing octane rating of the fuel. Supplying rich or lean mixtures. Stratifying the mixtures so that the end gas is less reactive. Increasing the humidity of the entering air.
Increase in variable
Major effect on unburned reduce charge
Action to be taken to Knocking
Compression ratio
Increases temperature and pressure
reduce
Mass of charge induced
Increase pressure
Reduce
Inlet temperature
Increases temperature
Reduce
Chamber wall temperature
Increases temperature
Reduce
Spark advance
Increases temperature & pressure
Retard
A / F ratio
Increases temperature & pressure
Make very rich
Turbulence
Decreases time factor
Increases
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Engine speed
Decreases time factor
Increases
Distance of flame travel
Increases time factor
Reduce
Types of combustion chambers Combustion chamber shape depends principally upon the valve arrangement, piston head and combustion chamber contours. Different types of combustion chambers such as T head, L head, F head, L head turbulent, valve in head, valve in head with inclined valves have been tried and used by different engine manufacturers. These can be seen in figure.
The T head design stipulates the use of the lowest compression ratios to prevent knocking with a given fuel. F head design is an improvement over the T head. In this the inlet valves are located in the cylinder head and the exhaust valves are located in the cylinder or vice versa. This improves the volumetric efficiency and also reduces the width of the combustion chambers. T head design stipulates two camshafts one operating the inlet valves and the other operating the exhaust valves. F head and other designs can have a single camshaft operating all the valves. However, F head design presents difficulties in the design of the valve operating mechanism. Overhead valve designs result in higher volumetric efficiency. These may have a single camshaft located by the side of the cylinder operating the valves through tappets, push rods and rocker arms, or a single camshaft located in the cylinder head and operating the valves through rocker arms or a single camshaft located in the cylinder head and operating the valves directly. In the turbulent combustion chamber, very small clearance is provided between the piston crown and the cylinder head over a portion of the piston crown surface. This causes squish turbulence in the mixture, better mixing of fuel and air and improves combustion. Further, this narrow space when made to contain the end mixture, knocking is avoided because of better cooling. Even if knock occurs its severity will be lesser. This feature was incorporated in the General Motors Research combustion chamber and this permitted the use of a 12.5:1 compression ratio with 100 ON fuel. This principle was also incorporated in the Ricardo turbulent combustion chamber.
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QUESTION BANK OF UNIT I Part A 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.
What is steady running What is Transient operation What is back firing Define idling in an engine What is the effect of inlet and exhaust pressure on mixture requirements What are the factors that influence carburetion What are the essential features of a carburetor What is pre ignition What are the effects of pre ignition What are the effects of knock in SI engines Name some types of combustion chambers in SI engines Write a short notes on T- head combustion chamber What are the various additives used to suppress knock in SI engines Why rich mixture is required for idling What is stoichiometric air fuel ratio What are different air fuel mixtures on which an engine can operate How can the location of the spark plug influence knocking tendency What is delay period and what are the factors that affect the delay period? Write any four factors that affect the process of combustion What is a homogeneous and heterogeneous mixture? What is meant by Carburetion? What are the functions of a carburettor? What is ignition lag What is period of afterburning in SI engines What are the variables that affect ignition lag What is the effect of inlet temp and pressure on ignition lag What is the effect of fuel - air ratio on flame propagation What is the effect of compression ratio on flame propagation Write short notes on effect of turbulence on flame propagation What are the engine variables that effect Knock in SI engines. How spark advance effects knock in SI engines Write any four methods of controlling knock What are the methods of detecting knock What are the basic requirements of a good combustion chamber Write short notes on Side Valve(I-Head) combustion chambers Write short notes on Over head valve combustion chamber What are the basic types of carburettor What are the drawbacks of a simple carburettor
Part B 1. Discuss the design criteria for a S.I engine combustion chamber 2. Explain with figures various types of combustion chambers used in SI engines. 3. Explain the effect of various engine variables on SI engine Knock 4. Explain the phenomenon of knock in SI engines.what are the factors which influence the knock.describe the methods used to supress it. 5. Explain the fuel/air mixture requirements for an engine based on various speeds.
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6. With the help of neat sketch explain the working principle of a simple carburettor 7. Explain the following related to SI engines: i)Pre ignition ii)Auto ignition iii)Knock 8. What is Ignition Lag ?Discuss the effect of engine variables on ignition lag 9. Discuss the effect of the following engine variables on flame propagation : a) Fuel - air ratio b) Compression ratio c) engine load d) turbulence e) engine speed 10. Discuss the ill effects of detonation 11. Explain the two theories of detonation 12. Explain the phenomenon of pre-ignition? How pre ignition leads to detonation
UNIT II COMPRESSION IGNITION ENGINES Four stages of combustion in a CI engine Herry Ricardo has investigated the combustion in a compression ignition engine and divided the same into the following four stages: 1. Ignition delay or delay period. 2. Uncontrolled combustion. 3. Controlled combustion. 4. After burning.
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Fig: Pressure time diagram illustrating in a compression ignition engine. 1. 2. 3. 4.
Ignition delay Uncontrolled combustion Controlled combustion After burning.
The details of these stages of combustion are given below: Pressure Vs crank angle of a CI engine in a simplified from is shown in fig. The curved line ABCG represents compression and expansion of the air charge in the engine cylinder when the engine is being motored, without fuel injection. This curve is mirror symmetry with respect to TDC line. The curve ABCDEFH shows the pressure trace of an actual engine. Delay period In an actual engine, fuel injection beings at the point B during the compression stroke. The injected fuel does not ignite immediately. It takes some time to ignite. Ignition sets in at the point C. During the crank travel B to C pressure in the combustion chamber does not rise above the compression curve. The period corresponding to the crank angle B to C is called delay period or ignition delay (about 0.001 seconds). During ignition delay, the following events take place. The injected spray enters the combustion chamber and slowly (at about 55 m/min) bores hole in the air mass, while the fuel particles are stripped away. Some of these particles are vapourized. Thus, the main body of the spray is surrounded by vapour liquid particle air envelope. In small combustion chambers, the spray body may impinge on the walls. Some of the impinged fuel may bounce off the surface, while the rest may glide on the walls. Vapourization of fuel particles tends to lower the compression pressure and temperature slightly. At the same time, the energy released in the pre-flame reactions tends to raise the pressure. Now in the outer
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envelope of the spray, ignition nuclei are formed. Mostly, the nuclei are cool flame reactions, on the verge of auto-ignition. By oxidation or cracking reactions, luminescent carbon particles are formed. Uncontrolled combustion At the end of the delay period i.e. at the point C, fuel starts burning. At this point, a good amount of fuel would have already entered and got accumulated inside the combustion chamber. This fuel charge is surrounded by hot air. The fuel is finely divided and evaporated. Majority of the fuel burns with an explosion like effect. This instantaneous combustion is called uncontrolled combustion. This combustion causes a rapid pressure rise. During uncontrolled combustion the following take place. Flame appears at one or more locations and spreads turbulently, with glowing luminosity. Flame of low luminosity marks regions of vapourized fuel and air (premixed flame. Flames of higher luminosity marks regions of liquid droplets and air (diffusion flame). The initial spreading of non luminous and luminous flame arises from auto-ignition and flame propagation. This is the knock reaction with a high rate of energy release and correspondingly high rate of pressure rise. Combustion during crank travel C to D is called uncontrolled combustion. This is because no control over this combustion is possible by the engine operator. Since this combustion is more or les instantaneous, it is also called rapid combustion. If more fuel is present in the cylinder at the end of delay period, and undergoes rapid combustion when ignition sets in, the rate of pressure rise and the peak pressure attained will be greater. During this combustion the piston is around TDC, and is almost stand still. Too rapid a pressure rise and severe pressure impulse at this position of the piston will result in combustion noise called Diesel Knock. The severity of the knock reactions is in proportion to the mass enflamed. The regions of premixed flame are probably hotter (and older) than the regions where liquid droplets are present. As such, the knock reaction may be propagated mainly in the low luminosity state of the flame. The rate at which the uncontrolled combustion takes place will depend upon the following: 1. The quantity of fuel in the combustion chamber at the point C. This quantity depends upon the rate at which fuel is injected during delay period and the duration of ignition delay. 2. The condition of fuel that has got accumulated in the combustion chamber at the point C. The rate of combustion during the crank travel C to D and the resulting rate of pressure rise determine the quietness and smoothness of operation of the engine. Controlled combustion During controlled combustion, following thing happen. The flame spreads rapidly (but less than 135 m/min), as a turbulent, heterogeneous or diffusion flame with a gradually decreasing rate of energy release. Even in this stage, small auto-ignition regions may be present. The diffusion flame is characterized by its high luminosity. Bright, white carbon flame with a peak temperature of 2500 o C is noticed. In this stage, radiation plays a significant part in engine heat transfer. During the period D to E, combustion is gradual. Further by controlling the rate of fuel injection, complete control is possible over the rate of burning. Therefore, the rate o pressure rise is controllable. Hence, this stage of combustion is called Gradual combustion or Controlled combustion. The period corresponding to the crank travel D to E is called the period of controlled combustion. The rate of burning during the period of controlled combustion depends on the following:
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1. 2. 3. 4.
Rate of fuel injection during the period of controlled combustion. The fineness of atomization of the injected fuel. The uniformity of distribution of the injected fuel in the combustion chamber. Amount and distribution of the oxygen left in the combustion space for reaction of the injected fuel.
At the point E, injection of fuel ends, the period of controlled combustion ends at this point. When the load on the engine is greater, the period of controlled combustion is also greater. During controlled combustion, the pressure in the cylinder may increase or remain constant or decrease. Usually during this period, the combustion is more or less at constant pressure (on a PV diagram) because the downward movement of the piston (i.e. increase in volume) compensates for the effect of heat release and the consequent pressure rise.
After burning At the last stage, i.e. between E and F the fuel that is left in the combustion space when the fuel injection stops is burnt. This stage of combustion is called After burning (burning on the expansion stroke). In the indicator diagram, after burning will not be visible. This is because the downward movement of the piston causes the pressure to drop inspired of the heat that is released by the burning of the last portion of the charge. Increasing excess air, or air motion will shorten after burning i.e. reduce the quantity of fuel that may undergo after burning).
THE PHENOMENON OF KNOCK IN CI ENGINES In CI engines the injection process takes place over a definite interval of time. Consequently, as the first few droplets to be injected are passing through the ignition delay period, additional droplets are being injected into the chamber. If the ignition delay of the fuel being injected is short, the first few droplets will commence the actual burning phase in a relatively short time after injection and a relatively small amount of fuel will be accumulated in the chamber when actual burning commences. Effect of Variables on the Delay Period Increases in variable Cetane number of fuel Injection pressure
Effect on Delay Period Reduces Reduces
Reason Reduces the self-ignition temperature Reduces physical delay due to greater surface
Injection timing advance
Reduces
volume ration Reduced pressures and temperatures when the
Compression ration
Reduces
injection begins Increases air temperature and pressure and
Intake temperature Jacket water temperature Fuel temperature
Reduces Reduces Reduces
reduces auto-ignition temperature Increases air temperature Increases wall and hence air temperature Increases chemical reaction due to better
pressure Reduces
vaporization Increases density and also reduces auto-ignition
Intake (Supercharging) Speed
Increases in terms of
temperature Reduces loss of heat
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crank angle. Reduce in Load (Fuel air ratio) Engine size
Type of combustion chamber
terms of milliseconds Decreases Decrease in terms of crank
angle.
effect
in
Increase the operating temperature Larger engines operator normally at low speeds
Little
terms
of
milliseconds Lower for engines with Due to compactness of the chamber. pre-combustion
CHARACTERISTICS TENDING TO REDUCE DETONATION OR KNOCK S.No 1. 2. 3. 4. 5. 6. 7. 8.
Characteristics Ignition temperature of fuel Ignition delay Compression ratio Inlet temperature Inlet pressure Combustion wall temperature Speed, rpm Cylinder size
SI Engines High Long Low Low Low Low High Small
CI Engines Low Short High High High High Low Large
Factors influencing diesel knock The diesel combustion process which includes ignition delay, premixed burning due to delay period and diffusion burning and injector needle lift and pressure variation with respect to crank angle can be seen in fig. The premixed burning is responsible for diesel knock. The following are the factors which influence ignition delay and thereby contribute to knock: The different engine factors that control diesel knock can be seen in fig.
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Fig: Diesel combustion and injector needle lift Higher inlet air pressure, air temperature and compression ratio reduce knock. Supercharging reduces knock. Increased humidity increases knock. Combustion chamber design and associated air motion influence heat losses from the compressed air. Tendency to knock will be lesser, with less heat losses. A combustion chamber with a minimum surface to volume ratio and with lesser intensity of air motion is desirable. Knocking tendency is lesser in engines where compressed air injects the fuel into the combustion space. In the case of mechanical injection of fuel, finer the atomization of fuel, lesser is the tendency to knock. A fuel with a long preflame reactions (i.e. self ignition possible only at a higher temperature) will result in the injection of a considerable amount of fuel before the initial part ignities. This in turn results in a large amount or number of parts of the mixture to ignite at the same time and produce knock. Thus, a good CI engine fuel should have a short ignition delay and low self ignition temperature, if knock is to be avoided.
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Fig: Factors influencing combustion knock in the CI engine. Ignition delay of fuels is generally measured in terms of cetane number. Fuels of higher cetane number have shorter ignition delay and thus will have a lesser tendency to knock. The ignition delay of CI engine fuels may be decreased by the addition of small amounts of certain compounds (called ignition accelerators or improves). These compounds are ethyl nitrate and amyl thionitrate. These compounds affect the combustion process by speeding the molecular interactions. Direct injection engines – These engines have a single, open combustion chamber into which the entire quantity of fuel is injected directed directly. An open combustion chamber is one in which the combustion space incorporates no restrictions that are sufficiently small to cause large differences in pressure between different parts of the chamber during the combustion process. Indirect injection engines – In these engines the combustion space is divided into two parts and the fuel is injected into the auxiliary chamber which is connected to the main chamber via a nozzle or one or more number of orifices. The main chamber is situated above the piston. The restrictions or throat are so small to cause considerable pressure differences between them during the combustion process. Combustion chambers for CI engines Combustion chamber is the space wherein combustion of fuel with air takes place. In IC engines, combustion chamber is the closed space formed by three engine components, namely, cylinder head, top
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portion of cylinder and piston crown, when the piston is close to TDC, at the end of compression. This space is more or less equal to the clearance volume in an engine. Functions of the combustion chambers are as follows: 1. Efficient preparation of fuel air charge for combustion. This stipulates (i) an even distribution of the injected fuel throughout the compressed air and (ii) a thorough mixing of the fuel with air to ensure complete combustion, with minimum excess air supply. 2. Efficient and smooth combustion. This stipulates (i) a sufficiently high air temperature to cause ignition of fuel, (ii) a small ignition lag or delay period, (iii) a moderate rate of pressure rise during uncontrolled combustion stage, (iv) a controlled, even burning during controlled combustion stage, (v) a minimum of after burning and (vi) minimum heat losses and energy losses to ensure high thermal efficiency.
The open combustion chamber may be located either in the cylinder head or in the piston crown as shown in fig.
Fig: Types of open combustion chambers, changing the shapes of the cavity in the piston crown affects piston height and hence engine size. It may also be partly in the cylinder head and partly in the piston crown. Presence of valves and fuel injector in the cylinder head makes it difficult to locate the combustion chamber in the cylinder head.
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Hence, it is usually located in the piston crown, either centrally or so. Locating the chamber in the piston crown has an advantage i.e. reduced heat losses from the working fluid. Open combustion chamber is invariably circular in plan. This helps organized rotational air movement to prevail. The cross section is of different shapes. The chamber shape usually confirms to the shapes of the fuel spray used. The shape of some of the open combustion chambers used in automotive diesel engines can be seen in fig. How the combustion chamber design affects weight and height of an engine can also be seen n this figure. The fig. shows the performance results of some direct injection engines having different shapes of open combustion chambers. The torroidal shape seems to give better performance over the operating range.
Fig: Performance of D.I. engines having different open combustion chambers In the open combustion chamber, air mass is more or less quiescent in nature. As such atomization (i.e. disintegration of the fuel jet into drops of different sizes), distribution of these drops and
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mixture formation (i.e. mixing of fuel with air) are to be effected by the injection system. Hence, fuel must be injected at high velocity. This means high injection pressure is required. Injector nozzle should also contain more of holes of comparatively small diameter (d or = 0.15 to 0.25 mm) In open combustion chamber even though there may be many sprays, still the air between the sprays may be utilized fully, due to quiescent nature of the air charge. As such, in this chamber the minimum possible excess air coefficient is amin > 1.5 The fuel injector is usually arranged along the chamber axis for effective distribution of fuel spray. With a centrally located multihole injector nozzle, the design goal is to keep the amount of fuel which may impinge on the piston bowl walls to be a minimum. In some engines, injector and combustion chamber are located away from the cylinder axis. This arrangement helps to increase the size of the valves, inlet manifold and exhaust manifold. In the case of open combustion chambers the injection timing, rate of injection, injection pressure, engine speed, size of each fuel orifice and viscosity and ignition quality of the fuel dictate the pressure rise and completeness of combustion. Pre-combustion chamber In some CI engines, combustion space is divided into two parts, namely, pre-combustion chamber and main combustion chamber. Pre-combustion chamber is always located in the cylinder head. Main combustion chamber is enclosed between the piston and cylinder head. The two combustion chambers are interconnected by one or more number of orifices. Pre-combustion chamber is built in various shapes and relative sizes. Pre-combustion chamber volume is about 30 to 40 percent of the total combustion space. The manner in which combustion of fuel in this type of engine is taking place is discussed below: During compression, part of the air in the cylinder enters the pre-combustion chamber. At the end of compression, the whole of the fuel is injected into the pre-combustion chamber. The hot air ignitives the fuel. Combustion starts in this chamber. Pressure rises in it. Rise of pressure in the pre-combustion chamber forces out the products of combustion, partially burned and unburned fuel and remaining air into the main combustion chamber. These constituents flow out at high velocity into the main chamber. As such, these constituents mix thoroughly with the air in the main chamber. The orifices connecting the two chambers are so sized, and shaped and located to effect good mixing. Air motion thus created is called combustion induced swirl or combustion turbulence. In the pre-combustion chamber engine, ignition and combustion starts in the pre-combustion chamber. But the combustion of the entire quantity of the injected fuel will not be completed in this chamber itself. This is because only a smaller quantity of total air sucked in is present in this chamber. About 20 percent of the fuel injected during each cycle, burns in the pre-combustion chamber and the reminder burns in the main combustion chamber.
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Merits and demerits of pre-combustion chamber- the merits and demerits of the ecombustion chamber engines are as follows: 1. There is better mixing of air and fuel due to combustion induced swirl. Air movement is one of turbulence in character. As such, lower fuel injection pressure (60 to 100 kscm) can be used. Lower injection pressure eliminates dripping of fuel from the injector tip. Lower injection pressure necessitates the use of fairly large injection orifices to deliver with carbon particles. Such injectors, therefore, require less frequent maintenance. Because of larger orifices and lower injection pressures, higher viscosity fuels can be used. 2. Brake mean effective pressures are much lower in these engines. 3. Only a fraction of the fuel is burnt in the pre-combustion chamber. Thus combustion process proceeds at a slower rate. As such, rate of pressure rise and peak pressure (seldom exceeds 90 kscm) will be lower. The engine will be very smooth running. The cycle becomes almost a constant pressure cycle. 4. During compression , at any instant, pressure and temperature of air in the pre-combustion chamber will lag behind in magnitude compared to those in the main combustion chamber. Throttling effect of orifices is responsible for this. As such, at the start of fuel injection, pressure and temperature of air in the pre-combustion chamber will be lesser. This factor increases ignition delay. Possibility of knock of knock occurring is greater, especially during cold weather and while starting.
35
5. Heat losses through the orifices are greater during compression. Hence, cold starting is difficult. To effect easy cold starting, electric heater or starting cartridges or higher compression ratios are used. Using higher compression ratio (usually from 16 to 19) results in a relatively heavier engine. 6. Air flows from the main chamber into the pre-combustion chamber during compression. During combustion and expansion, burning gases flow out from the pre-combustion chamber into the main combustion chamber. These fluid flow through the orifices result in higher fluid friction, and energy and heat losses. These aspects reduce power output by about 10 to 15 percent and also reduce thermal efficiency. Specific fuel consumption is more by about 10 to 12percent compared to that of a open combustion chamber. 7. Pre-combustion chamber imprisons the first combustion shock. This prevents high, knocking pressure from being applied on the piston and through the connecting rod to the engine knock on the ening ecomponents, inferior ignition quality fuels can be used. 8. Pre-combustion chambers are suitable and are being used in engines operating at relatively high speed. This becomes possible because of reduced or elimination of the ill effect of knock. 9. Scavenging the pre-combustion chamber is difficult. This causes inefficient combustion. 10. Considerable amount of fuel that is injected burns after the same entering the main combustion chamber. This combustion occurs relatively late in the expansion stroke. This aspect reduces thermal efficiency. 11. Pre-combustion chamber utilizes the energy of initial combustion for creating air movement in the main chamber. Greater will be the air movement if greater amount of fuel is burnt in the precombustion chamber. The amount of fuel that is burnt initially does not depend upon the speed. As such this type of combustion chamber is very much suitable for engines meant for constant speed operation. Swirl combustion chamber The swirl combustion chamber is also a divided type combustion chamber with certain differences and modifications. Swirl chamber is usually located in the cylinder head. In one case, it is located in the cylinder block itself, by the side of the engine cylinder. Swirl chamber is spherical or cylindrical in shape. Volume of the swirl chamber is greater than that of the precombusiton chamber. Volume of the chamber over the piston ranges from a minimum to usually not more than half the total clearance volume. A much larger passage called transfer passage or throat connects the swirl chamber with the chamber in the cylinder. This connection passage is tangential to the swirl chamber. The figure shows the location of the swirl combustion chamber either in the cylinder head or in the cylinder block and the air motion created in them.
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Fig: Different arrangements of swirl combustion chambers
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During compression, air from the cylinder is forced through the throat into the swirl chamber. A tangential velocity of swirl is produced in the swirl chamber. This swirl is called compression swirl. At the end of compression, fuel is injected into the swirl chamber. Vigorous swirl in the chamber helps the injected fuel and air to mix well. Fuel injector is so located and fuel sprays are so aimed to achieve this goal. Ignition and combustion of fuel starts in the swirl chamber. Bulk of the injected fuel burns in the swirl chamber itself. This becomes possible because of the presence of the major portion of air in it. Combustion causes pressure rise. This pressure rise forces combustion products and air fuel mixture into the engine cylinder. Piston is also pushed outward on the working stroke. Further mixing of unburned and partially burnt fuel with air occurs and this results in efficient combustion. Hence, a swirl chamber engine uses both compression induced swirl and combustion induced swirl. ‘M’ Combustion system Dr Meurer of MAN, Germany has developed a simple but a peculiar diesel combustion chamber based upon the following three rules: 1. The fuel must be allowed to oxidize slowly and gradually and must be heated only as vapour in the mixed state. 2. The fuel quantity undergoing auto-ignition must be minimized 3. The mixture of fuel vapour and air must be done faster as combustion proceeds and the mixture must never be richer than the stoichiometric ratio. The M combustions chamber is located in the piston crown as shown in fig. It is open type and is somewhat shallower. It has a recess at the top just below the injector nozzle. The nozzle directs the fuel towards the combustion chamber walls, tangentially. The intake port is inclined. The intake valve has a mask. These create an air swirl about the axis of the cylinder. The direction of the swirl is in the same direction as that of the fuel jet. The fuel particles injected at the first instance meet high resistance due to the dense hot air in the chamber. Hence, these particles get well dispersed into the hot air. The succeeding particles due to lesser air resistance get deposited on the combustion chamber walls, in the form of a thin film. At full load, the thickness of the fuel film be about 0.150 mm. The fuel dispersed into the air mass is only about 5% of the total fuel injected. The bottom surface of the combustion chamber is cooled by the lubricating oil that is splashed continuously from the crankcase. The combustion chamber wall temperature is maintained at about 330 oC. The combustion of the 5% of the fuel which gets injected into air mass starts. It undergoes, usual droplet combustion. But the combustion of the fuel sprayed on the cooled combustion chamber walls does not follow immediately. The wall deposited fuel starts evaporating in the absence of the hot air and moves towards the center. The swirling air removes the fuel vapours from the zone evaporation.
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Fig: M Combustion chamber
The fuel vapours mix with air and after slow oxidation burns. The combustion of the air fuel vapour mixture is initiated by the red hot carbon particles produced by combustion of air deposited fuel (that act like spark produced in a spark ignition engine). As combustion proceeds, the chamber temperature increases. This in turn increase the rate of vapourization and mixture formation. By this controlled evaporation and slow combustion, the fuel has little or no chance to crack resulting in diesel knock and smoky exhause. Hence, combustion is smooth and efficient in this system. A comparison of the indicator diagrams of the conventional diesel engine and the M combustion chamber diesel can also be seen in fig which will reveal this fact. Rate of pressure rise and peak pressure are lesser in the M combustion chamber engine.
Merits and demerits of M combustion system: The advantages of the M combustion system are as follows: In the M combustion system, complete and effective burning of the fuel takes place. This controlled burning eliminates diesel knock and free carbon particles in the exhaust. 1. About 5 – 10 % higher power output is realized. 2. Specific fuel consumption is lesser.
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3. Smooth running of the engine even during idling becomes possible which is very rare in normal diesel engines. 4. Much lower smoke density upto three fourths full load and almost identical with that of a conventional diesel engine at full load. Smoke density is the ratio of carbon present in the exhaust to the amount of carbon in the quantity of fuel injected. 5. Lesser contamination of insoluble in the lubricating oil. In the bohr test, in an ordinary diesel engine, the insolubles were about 0.9% and in the M combustion engine, the insolubles where only about 0.25%. 6. M combustion system is more adaptable for multi fuel operation because of the elimination of diesel knock.
Turbo charging In turbo charging, the supercharger or blower is being driven by a gas turbine which uses the energy in the exhaust gases. In this case, there is no mechanical linkage between the engine and the supercharger. The major parts of a turbocharger are turbine wheel, turbine housing, turbo shaft, compressor wheel, compressor housing and bearing housing.
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During engine operation, hot exhaust gases blow out through the exhaust valve opening into the exhaust manifold. The exhaust manifold and the connecting tubing route these gases into the turbine housing. As the gases pass through the turbine housing, they strike on the fins or blades on the turbine wheel. When the engine load is high enough, there is enough gas flow and this makes the turbine wheel to spin rapidly. The turbine wheel is connected to the compressor wheel by the turboshaft. As such, the compressor wheel rotates with the turbine. Compressor wheel rotation sucks air into the compressor housing. Centrifugal force throws the air outward. This causes the air to flow out of the turbocharger and into the engine cylinder under pressure. In the case of turbocharging, there is a phenomena called turbolag. It refers to the short delay period before the boost or manifold pressure increases. This is due to the time the turbocharger assembly takes the exhaust gases to accelerate the turbine and compressor wheel to speed up.
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If the supercharger is driven directly by the engine, part of the power developed by the engine will be used in running the supercharger.
Fig: Comparative heat balance of naturally aspirated and supercharged diesel engines. If is found that the gain in the power output of an engine due to supercharging will be many time the power required to drive the supercharger. Of course, this is possible only with increased fuel supply to the engine. It is to be noted that at full loads, the compression of the supercharger is not fully utilized. This will result in greater loss. Therefore, the specific fuel consumption of a mechanically driven supercharged engine will be more at part loads when compared to that of a naturally aspirated engine. In the case of the exhaust gas turbine driven supercharger, the engine is not required to supply any power to run the supercharger turbine. This type of supercharging is called turbo charging. The turbo charging gives about 5% higher thermal efficiency at full load. This increase in efficiency results in reduced fuel consumption compared to that of a naturally aspirated engine for the same power output. Effects of turbocharging: The following are the effects of supercharging engines. Some of the points refer to CI engines: 1. Higher power output 2. Mass of charge inducted is greater 3. Better atomization of fuel 4. Better mixing of fuel and air 5. Combustion is more complete and smoother 6. Can use inferior (poor ignition quality) fuels. 7. Scavenging of products is better 8. Improved torque over the whole speed range 9. Quicker acceleration (of vehicle) is possible 10. Reduction in diesel knock tendency and smoother operation 11. Increased detonation tendency in SI engines 12. Improved cold starting 13. Eliminates exhaust smoke
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14. 15. 16. 17. 18. 19. 20. 21.
Lowers specific fuel consumption, in turbocharging Increased mechanical efficiency Extent of supercharging is limited by durability, reliability and fuel economy Increased thermal stresses Increased turbulence may increase heat losses Increased gas loading Valve overlap period has to be increased to about 60 to 160 degrees of crank angle Necessitates better cooling of pistons and valves.
QUESTION BANK OF UNIT II Part A 1. What are the stages of combustion in CI engines. 2. What is Ignition delay period 3. What is period of rapid combustion 4. What is contrilled combustion in CI engines. 5. What is period of after burning. 6. What are the factors that affect delay period. 7. What is knock in CI engines. 8. State different types of combustion chambers in CI engines. 9.Write a short notes on Direct injection combustion chambers. 10. What is Pre - Combustion chamber. 11.What are homogeneous and heterogeneous mixtures. 12. What is turbo charging. 13. What are the advantages of turbo charging. 14. What are the dis-advantages of turbo charging. 15. What is ignition delay period in CI engines 16. What is Uncontrolled combustion in CI engines 17. What is controlled combustion in CI engines 18. What is period of afterburning in CI engines 19. What variables affect delay period 20. What is the affect of size of droplet on delay period 21. What is the affect of compression ratio on delay period 22. List various methods to control delay period in CI engine 23. What are the methods of generating air swirl in CI engine 24. What are the advantages of induction swirl 25. What are various cold starting aids in CI engine. Part B 1. Explain the various stages of Combustion in CI engines. 2. Explain the phenomenon of Knock in CI engines. 3. Explain various types of Combustion chambers used in CI engines with figures. 4. What is meant by delay period. Explain about the types of delay period 5. What are the three methods of generating swirl in CI engine combustion chamber
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UNIT III ENGINE EXHAUST EMISSION CONTROL POLLUTION: The mixing of unwanted and undesirable substances into our surroundings that cause undesirable effects on both living and non living things is known as pollution. AIR POLLUTION: Air pollution is defined as the addition of unwanted and undesirable things to our atmosphere that have harmful effect upon our planned life. Major sources of Air pollution: 1. Automotive Engines 2. Electrical power generating stations 3. Industrial and domestic fuel consumption 4. Refuse burning of industrial processing, wastes etc., Sources of Pollutants from Gasoline Engine: There are four possible sources of atmospheric pollution from a petrol engine powered vehicle. They are 1. Fuel Tank 2. Carburettor 3. Crank case 4. Engine The amount of pollutants contributed by the above mentioned sources are as follows. a.. Fuel tank evaporative loss
5 to 10 % of HC
b. Carburettor evaporative loss 5 % of HC c. Crank case blow by
20 to 35 % of HC
d. Tail Pipe exhaust
50 to 60 % of HC and almost all Co and NOx
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Emittant as a Pollutant: An emittant is said to be a pollutant when it has some harmful effect upon our surroundings. The primary source of energy for our automotive vehicles is crude oil from underground which typically contains varying amounts of sulphur. Much of the sulphur is removed during refining of automotive fuels. Thus the final fuel is hydrocarbon with only a small amount of sulphur. If we neglect sulphur and consider complete combustion, only water and carbon dioxide would appear in the exhaust. Water is not generally considered undesirable and therefore it is not considered as a pollutant. Likewise carbon dioxide is also not considered as pollutant in earlier days. But due to increase in global warming due to CO2 which is a green house gas, now a days CO2 is also considered as unwanted one.
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Then apart from this we get sulphur dioxide a pollutant which is a product of complete combustion. Apart from this all the compounds currently considered as pollutants are the result of imperfect or incomplete combustion. Pollutants
Pollutant Effects
Unburned Hydro Carbons (UBHC)
Photochemical Smog
Nitric Oxide
Toxic , Photochemical Smog
Carbon monoxide
Toxic
Lead compounds
Toxic
Smoke combines with fog and forms a dense invisible layer in the atmosphere which is known as Smog. The effect of Smog is that it reduces visibility. Effect of Pollutants on Environment: a. Unburned Hydro Carbons ( UBHC ): The major sources of UBHC in an automobile are the engine exhaust, evaporative losses from fuel system, blow by loss and scavenging in case of 2-stroke petrol engines. Unburned or partially burned hydrocarbons in gaseous form combine with oxides of nitrogen in the presence of sunlight to form photochemical smog. UBHC + NOx
Photochemical smog
The products of photochemical smog cause watering and burning of the eyes and affect the respiratory system, especially when the respiratory system is marginal for other reasons. Some of the high molecular weight aromatic hydrocarbons have been shown to be carcinogenic in animals. Some of the unburned hydrocarbons also serve as particulate matter in atmosphere. b. Carbon monoxide: Carbon monoxide is formed during combustion in engine only when there is insufficient supply of air. The main source is the engine exhaust. The toxicity of carbon monoxide is well known. The hemoglobin the human blood which carries oxygen to various parts of the body has great affinity towards carbon monoxide than for oxygen. When a human is exposed to an atmosphere containing carbon monoxide, the oxygen carrying capacity of the blood is reduced and results in the formation of carboxy hemoglobin. Due to this the human is subjected to various ill effects and ultimately leads to death. The toxic effects of carbon monoxide are dependent both on time and concentration as shown in the diagram.
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c. Oxides of Nitrogen ( NOx ) : Oxides of nitrogen ( NO, NO 2 , N2O2 etc) are formed at higher combustion temperature present in engines and the engine exhaust is the major source. Like carbon monoxide, oxides of nitrogen also tend to settle on the hemoglobin in blood. Their most undesirable effect is their tendency to join with moisture in the lungs to form dilute nitric acid. Because the amounts formed are minute and dilute, their effect is very small but over a long period of time cam be cumulatively undesirable, especially when the respiratory problems for other reasons are found. Another effect is that, the oxides of nitrogen are also one of the essential component for the formation of photochemical smog. d. Sulphur dioxide: Sulphur dioxide from automotive vehicle is very less when compared to that emitted by burning coal. Sulphur dioxide combines with moisture in atmosphere and forms sulphuric acid at higher temperatures. This comes to the earth as acid rain. Much of the sulphur dioxide combines with other materials in the atmosphere and forms sulphates which ultimately form particulate matter. e. Particulates:
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Particulate matter comes from hydrocarbons, lead additives and sulphur dioxide. If lead is used with the fuel to control combustion almost 70% of the lead is airborne with the exhaust gasses. In that 30% of the particulates rapidly settle to the ground while remaining remains in the atmosphere. Lead is well known toxic compound
.Particulates when inhaled or taken along with food leads to respiratory problems and other infections. Particulates when settle on the ground they spoil the nature of the object on which they are settling. Lead, a particulate is a slow poison and ultimately leads to death. CHEMISTRY OF SI ENGINE COMBUSTION: In a Spark ignition engine a perfectly mixed air fuel mixture enters the engine during suction stroke. The charge is compressed well and at the end of end of compression stroke, the charge is ignited by means of spark from spark plug. The air fuel mixture is delivered to engine by means of carburettor. The quantity and quality of charge entering the engine is controlled according to the engine speed and load conditions. GASOLINE ENGINE EMISSIONS The emissions form gasoline powered automobiles are mainly
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1. Unburned Hydro Carbons 2. Carbon monoxide 3. Oxides of nitrogen 4. Oxides of sulphur and 5. Particulates including smoke Pollutant formation in Gasoline engine: 1. Hydrocarbons: Hydrocarbon exhaust emission may arise from three sources as a. Wall quenching b. Incomplete combustion of charge c. Exhaust scavenging in 2-stroke engines In an automotive type 4-stroke cycle engine, wall quenching is the predominant source of exhaust hydrocarbon under most operating conditions. a. Wall quenching: The quenching of flame near the combustion chamber walls is known as wall quenching. This is a combustion phenomenon which arises when the flame tries to propagate in the vicinity of a wall. Normally the effect of the wall is a slowing down or stopping of the reaction. Because of the cooling, there is a cold zone next to the cooled combustion chamber walls. This region is called the quench zone. Because of the low temperature, the fuel-air mixture fails to burn and remains unburned. Due to this, the exhaust gas shows a marked variation in HC emission. The first gas that exits is from near the valve and is relatively cool. Due to this it is rich in HC. The next part of gas that comes is from the hot combustion chamber and hence a low HC concentration. The last part of the gas that exits is scrapped off the cool cylinder wall and is relatively cool. Therefore it is also rich in HC emission. b. Incomplete combustion: Under operating conditions, where mixtures are extremely rich or lean, or exhaust gas dilution is excessive, incomplete flame propagation occurs during combustion and results in incomplete combustion of the charge. Normally, the carburettor supplies air fuel mixture in the combustible range. Thus incomplete combustion usually results from high exhaust gas dilution arising from high vacuum operation such as idle or deceleration.
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However during transient operation, especially during warm up and deceleration it is possible that some times too rich or too lean mixture enters the combustion chamber resulting in very high HC emission. Factors which promote incomplete flame propagation and misfire include: a. Poor condition of the ignition system, including spark plug b. Low charge temperature c. Poor charge homogeneity d. Too rich or lean mixture in the cylinder e. Large exhaust residual quantity f. Poor distribution of residuals with cylinder Carburetion and mixture preparation, evaporation and mixing in the intake manifold, atomization at the intake valve and swirl and turbulence in the combustion chamber are some factors which influence gaseous mixture ration and degree of charge homogeneity including residual mixing. The engine and intake system temperature resulting from prior operation of the engine affect charge temperature and can also affect fuel distribution.
Valve overlap, engine speed, spark timing, compression ratio, intake and exhaust system back pressure affect the amount and composition of exhaust residual. Fuel volatility of the fuel is also one of the main reasons.
c. Scavenging: In 2-stroke engine a third source of HC emission results from scavenging of the cylinder with fuel air mixture. Due to scavenging part of the air fuel mixture blows through the cylinder directly into exhaust port and escapes combustion process completely. HC emission from a 2-Stroke petrol engine is comparatively higher than 4-Stroke petrol engine.
2. Carbon monoxide: Carbon monoxide remains in the exhaust if the oxidation of CO to CO 2 is not complete. This is because carbon monoxide is an intermediate product in the combustion process. Generally this is due to lack of sufficient oxygen. The emission levels of CO from gasoline engine are highly dependent on A/F ratio. The amount of CO released reduces as the mixture is made leaner. The reason that the CO concentration does not drop to zero when the mixture is chemically correct and leaner arises from a combination of cycle to cycle and cylinder to cylinder mal distribution and slow CO reaction kinetics.
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Better carburetion and fuel distribution are key to low CO emission in addition to operating the engine at increased air-fuel ratio. 3. Oxides of Nitrogen: Nitric oxide is formed within the combustion chamber at the peak combustion temperature and persists during expansion and exhaust in non-equilibrium amount. Upon exposure to additional oxygen in the atmosphere, nitrogen dioxide ( NO2) and other oxides may be formed. It should be noted that although many oxides of nitrogen may be also formed in low concentrations like, Nitrogen trioxide (N 2O3 ), Nitrogen pent oxide (N 2O5 ) etc., they are unstable compounds and may decompose spontaneously at ambient condition to nitrogen dioxide. A study of the equilibrium formation of the different nitrogen oxides showed that No is the only compound having appreciable importance with respect to engine combustion. In engine terminology an unknown mixture or nitrogen oxides usually NO and NO 2 is known as NOx. It is expected that higher temperature and availability of oxygen would promote the formation of oxides of nitrogen. Mechanism of NO formation: The nitric oxide formation during the combustion process is the result of group of elementary reaction involving the nitrogen and oxygen molecules. Different mechanism proposed are discussed below. a. Simple reaction between N2 and O2 N2 + O2 2 NO This mechanism proposed by Eyzat and Guibet predicts NO concentrations much lower that those measured in I.C engines. According to this mechanism, the formation process is too slow for NO to reach equilibrium at peak temperatures and pressures in the cylinders.
b. Zeldovich Chai Reaction mechanism: O2 2 O
------------- ( 1)
O + N2 NO + N ------( 2 ) N + O2 NO + O ------( 3 ) The chain reactions are initiated by the equation ( 2 ) by the atomic oxygen, formed in equation ( 1 ) from the dissociation of oxygen molecules at the high temperatures reached in the combustion process. Oxygen atoms react with nitrogen molecules and produces NO and nitrogen atoms. In the equation ( 3 ) the nitrogen atoms react with oxygen molecule to form nitric oxide and atomic oxygen. According to this mechanism nitrogen atoms do not start the chain reaction because their equilibrium concentration during the combustion process is relatively low compared to that of atomic oxygen. Experiments have shown that equilibrium concentrations of both oxygen atoms and nitric oxide molecules increase with temperature and with leaning of mixtures. It has also been observed that NO
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formed at the maximum cycle temperature does not decompose even during the expansion stroke when the gas temperature decreases. In general it can be expected that higher temperature would promote the formation of NO by speeding the formation reactions. Ample O 2 supplies would also increase the formation of NO. The NO levels would be low in fuel rich operations, i.e. A/F 15, since there is little O 2 left to react with N2 after the hydrocarbons had reacted. The maximum NO levels are formed with AFR about 10 percent above stoichiometric. More air than this reduces the peak temperature, since excess air must be heated from energy released during combustion and the NO concentration fall off even with additional oxygen. Measurements taken on NO concentrations at the exhaust valve indicate that the concentration rises to a peak and then fall as the combustion gases exhaust from the cylinder. This is consistent with the idea that NO is formed in the bulk gases. The first gas exhausted is that near the exhaust valve followed by the bulk gases. The last gases out should be those from near the cylinder wall and should exhibit lower temperatures and lower NO concentration. 4. Particulate matter and Partial Oxidation Products: Organic and inorganic compounds of higher molecular weights and lead compounds resulting from the use of TEL are exhausted in the form of very small size particles of the order of 0.02 to 0.06 microns. About 75% of the lead burned in the engine is exhausted into the atmosphere in this form and rest is deposited on engine parts. Some traces of products of partial oxidation are also present in the exhaust gas of which formaldehyde and acetaldehyde are important. Other constituents are phenolic acids, ketones, ethers etc., These are essentially products of incomplete combustion of the fuel.
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Flame Quenching: The phenomenon of flame quenching at the engine walls and the resulting unburned layer of combustible mixture play a significant role in the overall problem of air pollution. It has long been understood that a flame will not propagate through a narrow passage. It has been found that the walls comprising the narrow passage quench the flame by acting as a sink for energy. The minimum distance between two plates through which a flame will propagate is defined as the quenching distance. The quenching distance is found to be a function of pressure, temperature and reactant composition. When a flame is quenched by a single wall as would be the case in the combustion chamber of a S.I engine, the distance of the closest approach of the flame to the wall is smaller than the quenching distance. This distance is called the dead space. In general, the dead space has been assumed to range from 0.33 to1.0 of the quenching distance. Friedman and Johnson, Green and Agnes, Gottenbery and others have made significant work on this area. The following points are drawn from their experiments. 1. Essentially the expression for quenching distance is of the form 1 qd = ------Pα Tβ Where the values of α and β depends on the stoichiometry of the combustible mixture. 2. Lean mixtures have significantly large quenching distance than stoichiometry or rich mixture at any given pressure. 3. There exist a direct linear relationship between the total exhausted hydrocarbon and surface to volume ratio, a direct linear relationship between the representatives measured quench distance and the quantity of unburned hydrocarbons in the combustion products. 4. The quenching distance of copper, mica, glass and platinum surfaces were the same and hence they concluded that the quenching effect was independent of the surface material. 5. As the temperature of the wall increases, the flame can propagate closer to it. If high temperature materials could be used to make the cylinder walls in an engine capable of withstanding 800 °C to 1200 °C temperature, the quench layer thickness can be reduced to bring down the concentration of hydrocarbons. Danial proposed that the unburned hydrocarbons that are exhausted during the cruise and acceleration modes are due to the quenching of flames by the walls of the combustion chamber piston. He measured the thickness of the dark zone between the flame and the combustion chamber wall in a single cylinder engine that was fitted with a single quartz head. The dark zone or dead space was measured by taking stroboscopic picture of successive cycle through the quartz cylinder head, and he showed that the quantity of fuel trapped in the dead space was sufficient to account for the unburned
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hydrocarbons emitted from the engine. He also reported that the thickness of the dark zone was a function of temperature and pressure as referred by Friedman and Johnson. Tabaczynski proposed that there are four separate quench regions in the cylinder of a S.I engine. As shown in fig 3.1, these four quench layers may be expected to be exhausted from the cylinder at different times during the exhaust stroke. Regions 1 and 2 shown in the figure are the head and side wall quench layers respectively. Region 3 represents the piston face quench layer and region 4 corresponds to the quench volume between the cylinder wall, piston crown and first compression ring. It has been proposed that the head quench layer and part of the side wall quench layer nearest the exhaust valve leave the cylinder when the exhaust valve opens. Due to the low flow velocities near the piston face, the piston face quench layer will probably not leave the cylinder at any time during the stroke. During the expansion stroke, the hydrocarbons from the crevice between the piston crown and the first compression ring are laid along the cylinder wall. As the piston begins its upward stroke, it has been shown that a vortex is formed which scraps up the hydrocarbons along the wall and forces them to be exhausted near the end of the exhaust stroke.
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EFFECT OF DESIGN AND OPERATING VARIABLES ON GASOLINE ENGINE EXHAUST EMISSIONS The exhaust emission of hydrocarbons, carbon monoxide and nitric oxide can be minimized by the control of several inter related engine design and operating parameter. Fuel preparation, distribution and composition are also factors. In this section the effects on emissions of factors which the engineer has under his control when designing and tailoring his engine for minimum exhaust emissions are discussed. The factors include: •
Air fuel ratio
•
Load or power level
•
Speed
•
Spark timing
•
Exhaust back pressure
•
Valve overlap
•
Intake manifold pressure
•
Combustion chamber deposit build up
•
Surface temperature
•
Surface to volume ratio
•
Combustion chamber design
•
Displacement per cylinder
•
Compression ratio
•
Stroke to bore ratio
In the following discussions, the hydrocarbons and CO emissions are treated together; because once they are formed they both can be reduced by chemical oxidation process in either the cylinder or the exhaust system. On the other hand nitric oxide, once formed must be reduced by a chemical reduction process. In the first case for HC and CO reduction, excess O 2 is required where as in the second case for NO reduction a deficiency of O2 is desirable. EFFECTS ON UNBURNED HYDROCARBONS AND CARBON MONOXIDE 1. AIR – FUEL RATIO: a. Hydrocarbon emission:
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Hydrocarbon emissions are high at rich air fuel ratios and decrease as the mixture is leaned up to about 17:1. When operation leaner than 17 or 18:1 is attempted, emissions increases because of incomplete flame propagation and the engine begin to misfire. The basic factor contributing to the shape of the curve for HC emissions are the effect of mixture ratio on quench layer thickness and on fuel concentration within that quench layer, and the effect of mixture ratio on the availability of excess oxygen in the exhaust to complete the combustion and on the exhaust system temperature. When the temperature is over 650 °C and with oxygen available appreciable exhaust after reaction does occur. b. CO emission: CO emissions are high at rich air fuel ratios and decreases as the mixture is leaned. On the richer side, a change of only 1/3 air fuel ratio leads to a change of 1.0% in exhaust CO. The reason that the CO concentration does not drop to zero when the mixture is chemically correct and leaner arises from a combination of cycle to cycle and cylinder to cylinder mal distribution and slow CO kinetics. 2. POWER OUTPUT: a. Hydrocarbon emission: Hydrocarbon concentration does not change as load is increased while speed and mixture ratio are held constant and spark is adjusted to MBT. This result is to be viewed as arising from effects of several factors some of which tend to reduce HC while others tend to increase them, apparently counter balancing one another.
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A factor which increases the HC formation as load increases is the reduced time within the exhaust system. The residence time of the exhaust gas in the very hot section of the exhaust system is very important for increased exhaust after-reaction.
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Factors tending to reduce HC concentration include decreased quench thickness and increased exhaust temperature. Quench layer thickness decreases inversely as pressure increases and the mean cylinder pressure increases linearly with increase in load. Increased temperature with increasing load tends to increase exhaust after-reaction. However, an almost linear increase in HC mass emissions is observed as load is increased. A light car with low power is better than a large car on mass emission basis. b. CO emission: At a fixed air-fuel ratio there is no effect of power output on CO emission concentration. However, as in the case of HC emissions, CO emission on mass basis will increase directly with increasing output, giving advantage for a small light and efficient car. 3. ENGINE SPEED: a. Hydrocarbon emission: HC emission is considerably reduced at higher engine speeds. This is because with increase in engine speed, the combustion process within the cylinder is increased by increasing turbulent mixing and eddy diffusion. In addition, increased exhaust port turbulences at higher speeds promotes exhaust system oxidation reactions through better mixing.
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b. CO emission: Speed has no effect on CO concentration. This is because oxidation of CO in the exhaust is kinetically limited rather than mixing limited at normal exhaust temperatures. 4. SPARK TIMING: a. HC emission: HC emission has huge impact on spark timing. As the timing is retarded, the HC emissions are reduced. This is because, the exhaust gas temperature increases which promotes CO and HC oxidation. This advantage is gained by compromising the fuel economy.
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b. CO emission: Spark timing has very little effect on CO concentration. But at very high retarded timing, the CO emission increases. This is due to lack of time, to complete oxidation of CO. 5. EXHAUST BACK PRESSURE: a. HC emission: Increasing exhaust back pressure increases the amount of residual exhaust gas left in the cylinder at the end of the exhaust. If this increase in dilution does not affect the combustion process adversely, the
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HC emissions would be marginally reduced. The reduction arises from leaving the tail end of the exhaust, which is rich in HC, in the cylinder. This tail will be subsequently burned in the next cycle. If the back pressure is increased more and more, HC emission would rise sharply because of the effect of excessive dilution on combustion. On the other hand, increased dilution at idle increases HC emission concentration. At idle, dilution is already quirt high and combustion is marginal and the engine cannot tolerate much more exhaust dilution. 6. VALVE OVERLAP: a. HC emission: Increasing valve overlap has an effect similar to increasing the back pressure. The charge is further diluted with residual gases. A slight 2 overlap provided minimizes emission due to re burning of exhaust tail gas which is rich in HC. Combustion deteriorates with lean mixture as residual is increased. If the mixture ratio is richened to provide stable idle and off-idle performance, then HC advantage will be lost and CO will be increased. In general, minimum HC emissions are obtained with moderate or low back pressure with minimum overlap. b. CO emission: There is no effect of overlap on CO concentration at a constant mixture ratio. However any increase in richness of the mixture for smooth idle or off idle will increase the CO directly. This is due to lack of insufficient supply of oxygen for complete oxidation of CO.
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7. INTAKE MANIFOLD PRESSURE: a. HC emission: The intake manifold pressure variation reflects the variation in power output of an engine. Between 22cm and 60cm of Hg manifold pressure, the A/F ratio is lean which minimizes HC and CO emissions. Above 60cm of Hg, the engine power increases and the carburettor switch to rich mode. The rich mixture increases HC and CO emissions. This holds good only in case of carbureted engine. At light loads and low manifold pressure, additional HC emissions results from wall quenching accompanying rich mixtures delivered from the carburetor and incomplete combustion at manifold pressures below 15cm of Hg. 8. COMBUSTION CHAMBER DEPOSITS: a. HC emission: It is well known that in a normal engine the major source of combustion chamber deposit is TEL, a fuel additive used to suppress combustion knock. The deposits act to increase the surface area of the chamber because of their irregular porous nature. As a result, the mass of quenched HC increases. Deposits act as a sponge to trap raw fuel which remains unburned and adds to exhaust. All these tend to increase the HC emission. Tests have indicated that removal of deposits, depending on the extent of deposit build up, would reduce about 15% in HC emissions. Addition of fuel additives to reduce deposit build up may be helpful. Ethylene dibromide is commonly added to motor fuel to reduce lead deposits from TEL. Any modification to both fuels and lubricants can indirectly reduce HC emissions through deposit modification. b. CO emission: There is no effect of deposit build up on CO emission. 9. SURFACE TEMPERATURE: a. HC emission: Combustion chamber surface temperature affects the unburned HC emissions by changing the thickness of combustion chamber quench layer and degree of after burning. Higher the combustion chamber surface temperature, the lower are the HC emissions. In addition to changing quench distance and after-reaction, changing engine temperature increases fuel evaporation and distribution, and result in a faster reaction and hence reduced HC emission. b. CO emission: An increase in surface temperature of chamber increases the rate of oxidation of CO and hence reduces CO emission. Further exhaust after reaction also increases resulting in decrease in CO emission. 10. SURFACE TO VOLUME RATIO: a. HC emission:
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Because hydrocarbon emissions arise primarily from quenching at the combustion chamber wall surface, it is desirable to minimize the surface area of the chamber. The ratio of surface area to volume of the combustion chamber (S / V) is useful for interpreting the effects of many designs and operating variables on HC concentration. Lowering the S / V ratio reduces HC emission concentration.
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b. CO emission: CO concentration has no effect on surface to volume ratio.
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11. COMBUSTION CHAMBER DESIGN: One of the most important factors that the emission engineer has under his control is the combustion chamber design. For a given clearance volume, reducing the surface area is an important way of reducing HC emission. Designing a combustion chamber to create better turbulence will reduce both HC and CO emission. 12. STROKE / BORE RATIO: Another design factor is stroke to bore ratio. Engines with small bore and long stroke have lower S / V ratio. Engines with low surface to volume ratio provide a good emission reduction compared to the engine with higher surface to volume ratio. Displacement per cyl. Bore Stroke Stroke/bore s/v
41 4 3.25 .813 8
41 3.62 4 1.105 6.1
The engine with s/v 6.1 should provde good emission result.Unfortunately this requirement is opposed to modern design practice of short stroke for reduced friction, increased power and economy. Long stroke engines tend to be large, heavy and more expensive and they have poor fuel economy and reduced peak power. 13. DISPLACEMENT PER CYLINDER: For a given displacement, engines with larger cylinders have smaller surface to volume ratio. This result suggests that for an engine of given displacement, hydrocarbon emissions can be reduced by decreasing the number of cylinders and increasing the displacement per cylinder. On the other hand, for a given number of cylinders, increasing the engine displacement reduces s/v ratio and reduces HC. Displacement per cyl. Bore Stroke Stroke/bore s/v
41 4 3.25 .813 8
30.2 3.62 2.94 .813 9
14. COMPRESSION RATIO: A decrease in compression ratio decreases surface to volume ratio. Decrease in compression ratio increases the clearance volume greatly with little increase in surface area. Due to this decrease in surface to volume ratio the HC emission is reduced. A decrease in compression ratio decreases the HC emission on a second way also. With reduced compression ratio, thermal efficiency is lowered and as a result exhaust gas temperature is increased. This improves exhaust system after-recirculation and lowers the HC emission even more.
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On the other hand, as engine efficiency is lowered, mass flow is increased for a given horse power level which increases mass emissions. On the other hand with large reduction in compression ratio, the temperature in chamber decreases and it increases both HC and CO emission. EFFECT OF NITRIC OXIDE: The concentration of NO in the exhaust gases depends upon the difference between the rate of its formation at the highest temperature in the cycle and the rate of its decomposition as the temperature decreases during the expansion stroke. A study of the decomposition rate of NO indicates that the amount decomposed is negligible because of the short time available during the expansion stroke. 1. EQUIVALENCE RATIO: The equivalence ratio affects both the gas temperature and the available oxygen during combustion. Theoretically an increase in the equivalence ratio form 1.0 to 1.1 results in an increase of maximum cycle temperature by about 55C while oxygen concentration is reduced by 50%. At equivalence ratio of 1.1, NO in the exhaust is very low. Maximum NO concentration occurs at an equivalence ratio of 0.8. The maximum cycle temperature with this lean mixture is lower than with a rich mixture but available oxygen concentration is much higher.
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With very rich mixtures, low peak combustion temperatures and low oxygen concentration lead to low NO. For mixtures leaner than 15.5:1 there is enough oxygen but the temperature is very less and hence lower the NO formation. Thus NO concentration is very low for very lean as well as very rich mixtures. 2. SPARK TIMING: An advance in spark timing increases the maximum cycle temperature and therefore results in increased NO concentration.
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3. MANIFOLD PRESSURE: An increase in manifold vacuum decreases load and temperature. As a result the ignition delay is increased and the flame speed is reduced. Both these factors increase the time of combustion. This reduces the maximum cycle temperature and thus reducing NO concentration in the exhaust. 4. ENGINE SPEED: An increase in engine speed has little effect on ignition delay. Increase in engine speed results in an increase in flame speed due to turbulence and reduces heat losses per cycle which tends to raise compression and combustion temperature and pressure. If spark timing is held constant, a greater portion of this combustion tends to occur during expansion where temperature and pressure are relatively low.
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This is most pronounced for the slowest burning mixture ratio of 19:1. For richer mixtures which burn faster, the effect of reduced heat losses at higher speeds predominates.
These are two opposing influences – an increase in the rate of NO formation due to reduced heat losses opposed by a reduction in the rate of NO formation due to late burning. For rich mixtures where combustion and NO formation are rapid, the former predominates. For lean mixtures where combustion and NO formation are slow, the later effect predominates. 5. COOLANT TEMPERATURE AND DESPOSIT: An increase in the coolant temperature results in a reduction of heat losses to the cylinder walls and an increase in the maximum gas temperature. This results in an increase in NO concentration. An increase in deposit thickness causes an increase in compression ratio, reduction in heat losses to the coolant and an increase in NO concentration.
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6. HUMIDITY: The reduction in NO formation caused by an increase in mixture humidity is mainly due to the drop in maximum flame temperature. Test on hydrogen-air, and ethylene-air mixture indicates that 1% of water vapour reduces the flame temperature by 20C. This reduces the initial rate of NO production by about 25%.
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7. EXHAUST GAS RECIRCULATION: Recycling of a portion of exhaust gas to inlet charge increases dilution. This reduces peak combustion temperature, since the inert exhaust gas re circulated will act as a heat sink. This also reduces the oxygen availability. About 15% recycle will reduce NOx emission by about 80%. The maximum percentage which can be re circulated is limited by rough engine operation and loss of power.
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8. SURFACE TO VOLUME RATIO: Engine changes which decrease surface to volume ratio reduce heat loss to the coolant. As a result NO concentration may increase. EFFECT OF DESIGN AND OPERATING VARIABLES ON EXHAUST EMISSIONS SL.NO.
VARIABLE
HC
CO
NO
1 2 3
INCREASED Load Speed Spark retard
_ Decrease Decrease
_ -
Increase Increase/Decrease Decrease
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4 5 6
Exhaust back pressure Decrease Valve overlap Decrease Intake manifold -
-
Decrease Decrease Increase
7
pressure Combustion
chamber Increases
-
Increases
8 9
deposit S/V ratio Combustion
Increase chamber Increase
-
-
10 11 12 13 14 15
area Stroke to bore ratio Displacement per cyl. Compression ratio Air Injection Fuel injection Coolant temperature
Decrease Decrease Decrease
Increase Increase Increase Increase
Decrease Decrease Increase Decrease Decrease Decrease
DIESEL ENGIEN EXHAUST EMISSIONS: The pollutants from diesel engines can be categorized into two types: 1. Visible and 2. Invisible. The first one consists of smoke and metallic particulates. Smoke being so conspicuous and odorous is objected to public and also reduces visibility and has smudging character but is not harmful to health. The second type consists of CO, un burnt hydrocarbons including poly nuclear aromatics, oxides of N2, SO2 and partially oxidized organics (aldehydes, ketones etc.,) Among these pollutants smoke, CO, UBHC and oxides of nitrogen are of most immediate concern. FORMATION OF POLLUTANTS IN DIESEL ENGINES: Unlike a gasoline engine, where fuel and air are premixed into a homogenous form before entering the cylinder, in the diesel engine fuel is injected into the compressed air charge inside the cylinder. As the mixing of air and fuel has to take place entirely in the combustion chamber, complete mixing is virtually impossible and infinite variations in air-fuel mixture ratio takes place within the same cylinder. Also as the load requirement is met through variation in the quantity of fuel injected, the overall air fuel ratio varies within wide limits, about 20:1 to 60:1. A normally rated and well maintained engine emits negligible amount of CO and unburnt hydrocarbons, through considerable amount of oxides of nitrogen and smoke are emitted. Carbon monoxide: It is formed during combustion when there is insufficient oxygen to oxidize the fuel fully. Compression ignition engines have long been known to produce low levels of CO because of excess amount of air available for combustion. Theoretically it should not emit any CO as it always operated with large amount of excess air. Nevertheless CO is present in small quantities ( 0.1 to 0.75%) in the exhausts.
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This is possible because of the fact that fuel injected in later part of the injection does not find enough oxygen due to local depletion in certain parts of the combustion chamber. Unburnt Hydrocarbons: The concentrations of hydrocarbons in diesel exhaust varies for a few parts per million to several thousand parts per millions depending on engine speed and load. Hydrocarbons in engine exhaust are composed of many individual hydrocarbons in the fuel supplied to the engine as well as number of hydrocarbons partially unburnt produced during the combustion process. In addition some unburnt hydrocarbons may be from lubricating oils. Tests on engine with single component fuels shows that these engines contained hydrocarbons of higher and lower molecular weights, than original fuel as well as molecules with different structures. Aromatic compounds have been observed in exhaust of engines operated on pure paraffins. Poly nuclear aromatics found in exhaust are products of this synthesis. During the normal operation the relatively cold walls “quench” the fuel air mixture and inhibit combustion leaving a thick skin of unburnt air fuel mixture over the entire envelope of the combustion chamber. The amount of unburnt fuel depends on the thickness of quench zone and the effective combustion chamber area. The thickness of quench zone depends on many variables as combustion temperature, pressure, mixture ratio, turbulence and residual gas dilution. Higher surface to volume ratio of combustion chamber leads to greater fraction of unburnt hydrocarbon from the quench zone. Partially oxidized hydrocarbons (aldehydes) have been associated with diesel exhaust. They produce objectionable odor and are high when engine idles and under cold starting indicating poor combustion. OXIDES OF NITROGEN: This is more significant. The formation of Nitric oxide, the major component of oxides of nitrogen depends on number of operating conditions of diesel engine. The main factors that control this formation are amounts of oxygen available and the peak temperature in the zones with sufficient oxygen and residence times at temperatures above 2000K. Both open and pre-combustion chamber produce small amount of oxides of nitrogen when air fuel ratio is about 0.01 to maximum near air fuel ratio of about 0.035 ratios. Additional fuel tends to lower air fuel ratio; the charge temperature also reduces which consequently reduces oxides of nitrogen. Formation of oxides of nitrogen: Since nitrogen is a high temperature species its formation is influenced by combustion temperature and time available for combustion. Hence NO tends to increase with advanced injection timing. Also NO produced increase with fuel supply. Notable exception is prechamber. In direct injection engines NO reaches maximum value at stoichiometric air fuel ratio, as lean and rich mixtures tend to reduce combustion temperatures. Increase in compression ratio leads to increase in combustion temperature and hence higher NO formation.
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Valve overlap has significant effect on NO formation. Higher valve overlap dilutes the incoming air more and more leading to increasing in fuel/air ratio. This in turn reduces combustion temperatures and hence lowers NO formation. Earlier inlet valve opening before TDC leads to increased dilution of incoming air and hence lower NO. Extended inlet valve opening up to 20 has no effect on NO formation as it does not vary manifold pressure. Extended exhaust valve opening before bottom dead centre results in marginal increase in NO due to better scavenging, conversely later exhaust valve opening leads to delayed scavenging and higher dilution. Exhaust valve closing determine effect of scavenging and pronounced effect on dilution and hence Nitrogen formation. DIESEL ENGINE SMOKE EMISSION: Engine exhaust smoke is a visible indicator of the combustion process in the engine. Smoke is due to incomplete combustion. Smoke in diesel engine can be divided into three categories: blue, white and black. Blue smoke: It results from the burning of engine lubricating oil that reaches combustion chamber due to worn piston rings, cylinder liners and valve guides. White or cold smoke: It is made up of droplets of unburnt or partially burnt fuel droplets and is usually associated with the engine running at less than normal operating temperature after starting, long period of idling, operating under very light load, operating with leaking injectors and water leakage in combustion chamber. This smoke normally fades away as engine is warmed up and brought to normal stage.
Black or hot smoke: It consists of unburnt carbon particles ( 0.5 – 1 microns in diameter) and other solid products of combustion. This smoke appears after engine is warmed up and is accelerating or pulling under load. Formation of smoke in Diesel engines: The main cause of smoke formation is known to be inadequate mixing of fuel and air. Smoke is formed when the local temperature is high enough to decompose fuel in a region where there is insufficient oxygen to burn the carbon that is formed. The formation of over-rich fuel air mixtures either generally or in localized regions will result in smoke. Large amounts of carbons will be formed during the early stage of combustion. This carbon appears as smoke if there is insufficient air, if there is insufficient mixing or if
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local temperatures fall below the carbon reaction temperatures (approximately 1000C) before the mixing occurs. Acceptable performance of diesel engine is critically influenced by exhaust some emissions. Failure of engine to meet smoke legislation requirement prevents sale and particularly for military use, possible visibility by smoke is useful to enemy force. Diesel emissions gives information on effectiveness of combustion, general performance and condition of engine. FACTORS AFFECTING SMOKE FORMATION: The smoke intensity in the diesel exhaust is generally affected by many parameters. By controlling them, smoke intensity may be reduced. 1. Injection timing: Advancing the injection timing in diesel engines with all other parameters kept constant results in longer delay periods, more fuel injected before ignition, higher temperatures in the cycle and earlier ending of the combustion process. The residence time is therefore increased. All these factors have been fond to reduce the smoke intensity in the exhaust. However earlier injection results in more combustion noise, higher mechanical and thermal stresses, and high NO concentration. In a recent study, khan reported that a very late injection reduces the smoke. The timing after which this reduction occurs is that at which the minimum ignition delay occurs. He suggested that one of the factor that contributes to the reduction in smoke at the retarded timing is the reduced rat of formation due to decrease in the temperature of the diffusion flames as most of these flames occur during the expansion stroke. 2. Rate of Injection: Higher initial rates of injection have been found to be effective in reducing the exhaust smoke. 3. Injection nozzle: The size of the nozzle holes and the ratio of the hole length to its diameter have an effect on smoke concentration. A larger hole diameter results in less atomization and increased smoke. An increase in the length/diameter ratio beyond a certain limit also results in increased smoke. 4. Maintenance: The engine condition plays a very important role in deciding the smoke levels. The maintenance affects the injection characteristics and the quantity of lubricating oil which passes across the piston rings and thus a profound effect on smoke generation tendency of the engine. Good maintenance is a must for lower smoke levels.
5. Fuel: Higher cetane number fuels have a tendency to produce more smoke. It is believed to be due to lower stability of these fuels. For a given cetane number less smoke is produced with more volatile fuels.
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6. Load: A rich fuel-air mixture results in higher smoke because the amount of oxygen available is less. Hence any over loading of the engine will result in a very black smoke. The smoke level rises from no load to full load. During the first part, the smoke level is more or less constant as there is always excess air present. However in the higher load range there is an abrupt rise in smoke level due to less available oxygen.
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7. Engine type and speed: Naturally aspirated engines have higher smoke levels at higher loads than turbo charged engines, because the later have sufficient oxygen even at full loads. The smoke is worse at low as well as at high speeds. This follows the volumetric efficiency curve of the engine in some measure as it drops at the extremes of speed. 8. Fuel air ratio: The smoke increases with richening the mixture. The increase in smoke occurs even with as much as 25% excess air in cylinder, cleanly indicating that the diesel engine has a mixing problem.
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CONTROL OF DIESEL ENGINE SMOKE: Smoke can be reduced by some of the following methods: 1. Derating: Derating is nothing but making the engine to run at lower loads. At lower loads more excess air is present in the combustion chamber and hence the smoke developed is less as already discussed. However this means a loss of output. 2. Proper maintenance of the engine: Maintaining the engine properly, especially the injection system, will not only result in reducing smoke but also keep the performance of the engine at its best. 3. Proper choice of combustion chamber design and operating conditions: A proper choice of combustion chamber design results in better mixing of fuel and air in the chamber and hence reduces the smoke level to a considerable level. 4. Use of smoke suppression additives: Some barium compounds if used in fuel reduce the temperature of combustion, thus avoiding the soot formation. Even if formed they break it into fine particles, thus appreciably reducing smoke. However, the use of barium salts increases the deposit formation tendencies of engine and reduces the fuel filter life. 5. Adopting fumigation technique:
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This method consists of introducing a small amount of fuel into the intake manifold. This starts pre-combustion reactions before and during the compression stroke resulting in reduced chemical delay, because the intermediate products such as peroxides and aldehydes react more rapidly with oxygen than original hydrocarbons. The shortening of delay period curbs thermal cracking which is responsible for soot formation.
Fumigation rate of about 15% gives best smoke improvement. However this improvement varies greatly with engine speed. At low engine speeds 50 to 80% smoke reduction is obtained. This decrease as speed increases until a speed at which there is no effect of fumigation. DIESEL ODOUR: Ever since the first diesel engine was developed, the odor from its exhaust has been recognized as undesirable. Determination of the cause of this odor has been difficult because of the complexity of the heterogeneous combustion process and the lack of chemical instruments available. In practice the human nose plays a significant role in odor measurement. The members of the aldehydes family are supposed to be responsible for the pungent odors of diesel exhaust. Though the amount of aldehydes is small being less than 30ppm, the concentration as low as 1ppm are irritating the human eyes and nose. Mechanism of odour production:
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Some experimental results indicate that the products of partial oxidation are the main cause of odor in diesel exhaust. This partial oxidation may be because of either very lean mixture or due to quenching effect.
In diesel combustion there are most probably regions in which the fuel/oxygen/inert mixture is outside the flammability limits. The fuel in these regions which are too lean to burn might only partially oxidize resulting in odors. This is most likely to occur during idling and or part load operation of the engine. Also the fact that chemical reactions take place during the second stage of diesel combustion suggest that if the reactions are quenched during this period, partial oxidation products will result in odour in the diesel exhaust. Barns concluded in his research that diesel odor resulted from partial oxidation reaction in the fuel lean regions which are almost inevitably formed in heterogeneous combustion. Graph shows the relative odor producing capabilities of different air fuel regions. The data shown in graph were obtained using CFR engine and varying air-fuel ratios and the inlet air temperatures. Odor relevant compounds: Until recently very little was known about the compound or compounds that contribute to the odorous qualities of diesel exhaust. Rounds and Pearsall correlate odor with the aldehydes in diesel exhaust gas. However Vogh says that aldehydes are not significant contributors to the overall odor problem. Vogh also says that neither SO2 nor particulate contribute significantly to diesel odor emissions. Research work at the Illinois Institute of Technology, Research Institute ( IITRI ) has contributed significantly to the development of a better understanding of the chemical nature of the odor contributors
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in diesel exhaust. Based on the IITRI work, various high molecular weight cyclic and aromatic hydrocarbons including naphthalene, tetra ling and cyclo paraffins some with olefinic and or paraffinic side changing were reported as major contributors to the burnt odor note of the exhaust. Various non aromatic hydrocarbons with more than one double or triple bond were also reported to contribute to the burnt odor note. Furan aldehydes, aromatic benzene and paraffinic aldehydes from ethanol to n-octanol were found be important odor contributors and have individual odors that varied from pleasant to pungent. Some heterocyclic sulfur compounds, thiophene and benzothiophene derivatives were also reported to be odor contributors. FACTORS AFFECTING ODOR PRODUCTION: 1. Fuel air ratio: The fact that very lean mixtures result in odorous diesel exhaust has already been discussed. 2. Engine operation mode: It has been found that the mode of operation of the engine significantly affects the exhaust odor. Maximum odor occurs while accelerating from idle and minimum odor results when the engine is running at medium sped and or at part loads. Effect of engine operating mode on odor production ( 4-stroke normally aspirated medium speed diesel engine) Engine operation mode Idle Acceleration Part load Full load
Odor intensity ( Turk number) 3.6 4.1 3.0 3.5
3. Engine type: The odor intensity does not vary with the engine type as can be seen from the table. The odor intensity from all the engines is more or less the same. Engine type Odor intensity ( Turk number) Two stroke 3.5 Four stroke normally aspirated 3.3 ( medium speed) Four stroke normally aspirated 3.5 ( high speed) Four stroke – Turbo charged
3.4
4. Fuel composition: It is really surprising to find that the composition of the fuel has no effect on exhaust odor intensity. The changes in fuel composition result in different second stage combustion time in diesel combustion and it is expected that this will affect the degree of oxidation if quenching is taking place. However the results contradict this expectation. 5. Odor suppressant additives:
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It has been claimed from time to time, by different manufactures of odor suppressant additive compounds that they reduce the odor. However small and rather insignificant effects upon the odor has been found in comparison of exhausts from treated and untreated fuels. No predictable and reliable correspondence between the additives and odor is found. Odor Measurement Techniques: Effective chemical or physical methods for the measurement of odor have not been developed, and therefore the human nose plays a significant role in all odor studies. In practice odor is measured by a specially selected, specially trained human panel. When the nose is subjected to an odor, the physiological response to the odor can be classified by either intensity or intensity and quality. The Turk kit contains a number of different standard odors that are classified as a) burnt/smoky b) oily c) pungent/acid and d) aldehydic/aromatic. It has been generally accepted as a standard for rating the intensity and quality of an unknown odorous sample. Odor-detection methods that have been developed todate may be placed in two general categories. The first category includes methods that only rate the over all odor intensity, while the second group is employed to classify odors by quality and intensity. The threshold dilution technique and natural dilution technique that are described below fall into the first category. The threshold dilution technique consists of presenting raw diesel exhaust synthetically diluted with variable quantities of odor free air to a panel of “Sniffers” person who smells. A series of diluted samples, both above and below the threshold dilution ratio, are presented to members of the panel and the individual panel members are asked to determine whether or not any odor is detectable. The odor intensity is assumed to be proportional to the dilution ratio at which the odor is just detectable to the panel. The natural dilution technique was developed in order to determine whether diesel powered vehicles could meet the motor vehicle exhaust odor on standards set by the state of California. During the course of these tests, a panel is seated at varying distance from a vehicle. Both the vehicle and the panel are located inside a large municipal hanger, in order to minimize the effects due to winds and the panelists are asked to determine whether or not they can detect any odor from the vehicle. Their responses are utilized to evaluate threshold response distances. Variations of the direct method have been used to rate the quality and intensity of diesel odor and hence thy fall into the second category of odor detection methods. When applying this method, the exhaust from the diesel engine is usually diluted with odor free air at the engine exhaust pipe and the resulting mixture of gases which consists of raw diesel exhaust mixed with odor free air in ratios ranging from 1 to 200 flows dynamically through a presentation system to the panelists. The panelists who have been previously trained to evaluate both quality and intensity as determined by the Turk kit are asked to record their response to test gases as a function of dilution ratio and experimental parameters. CONTROL OF EMISSIONS FROM SI AND CI ENGINES Design changes:
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The effects of engine design and operating variables on exhaust emission were discussed in a detailed manner already. Based on the discussions made already the engine design modifications approaches to control the pollutants are discussed below. 1. NOx is decreased by A. Decreasing the combustion chamber temperature The combustion chamber temperature can be decreased by 1. Decreasing compression ratio 2. Retarding spark timing 3. Decreasing charge temperature 4. Decreasing engine speed 5. Decreasing inlet charge pressure 6. Exhaust gas recirculation 7. Increasing humidity 8. Water injection 9. Operating the engine with very lean or very rich air fuel ratio 10. Decreasing the coolant temperature 11. Decreasing the deposits 12. Increasing S/V ratio B. By decreasing oxygen available in the flame front The amount of oxygen available in the chamber can be controlled by 1. Rich mixture 2. Stratified charge engine 3. Divided combustion chamber 2. Hydrocarbon emission can be decreased by 1. Decreasing the compression ratio 2. Retarding the spark 3. Increasing charge temperature 4. Increasing coolant temperature 5. Insulating exhaust manifold 6. Increasing engine speed 7. Lean mixture 8. Adding oxygen in the exhaust 9. Decreasing S/V ratio 10. Increasing turbulence 11. Decreasing the deposits 12. Increasing exhaust manifold volume 13. Increasing exhaust back pressure
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3. CO can be decreased by 1. Lean air fuel ratio 2. Adding oxygen in the exhaust 3. Increasing coolant temperature. EXHAUST GAS RECIRCULATION: In exhaust gas recirculation a portion of the exhaust gas is recirculated to the cylinder intake charge. This reduces the peak combustion temperature, since the inert gas serves as a heat sink. This also reduces the quantity of oxygen available for combustion.
The exhaust gas for recirculation is passed through the control valve for regulation of the rate and inducted down to the intake p[ort, The recycle rate control valve is connected to the throttle shaft by means of appropriated linkage and the amount of valve opening is regulated by throttle position. The link is designed so that recycled exhaust is normally shut off during idle to prevent rough engine operation. This is also shut off during full throttle, acceleration to prevent loss of power when maximum performance is needed. The NOx concentration will vary with the amount of recycling of gas at various air fuel ratios. About 15% recycle will reduce NOx emission by about 80%. The maximum percentage which can be circulated is limited by rough engine operation and loss of power. The above figure shows a vacuum controlled EGR valve used to control the recycle rate. A special passage connects the exhaust manifold with the intake manifold. This passage is opened or closed by a vacuum controlled EGR valve. The upper part of the valve is sealed. It is connected by a vacuum line to a vacuum port in the carburettor. When there is no vacuum the port, there is no vacuum applied to the
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diaphragm in the EGR valve. Therefore, the spring holds the valve closed. No exhaust gas recirculates. This is the situation during engine idling when little NOx is formed. As the throttle valve opens it passes the vacuum port in the carburettor. This allows intake manifold vacuum to operate the EGR valve. Then vacuum raises the diaphragm, which lifts the attached valve off its seat. Now exhaust gas flows into the intake manifold. There the exhaust gas mixes with the air fuel mixture and enter the engine cylinders. At wide open throttle, there is little vacuum in the intake manifold. This produces a denser mixture which burns cooler during the combustion process. Therefore at wide open throttle there is less need for exhaust gas recirculation. Due to low vacuum, the EGR valve is nearly closed.
A thermal vacuum switch on many cars prevents exhaust gas recirculation until the engine temperature reaches about 100 F 0r 37.8C. The thermal vacuum switch is also called a coolant temperature override switch (CTO switch). It is mounted in a cooling system water jacket, so it senses coolant temperature. If this temperature is below 100F, the switch remains closed. This prevents the vacuum from reaching the EGR valve, so the exhaust gas does not recirculate. Cold engine performance immediately after starting is improved. After the engine warms up it can tolerate exhaust gas recirculation. Then the CTO valve opens. Now vacuum can get to the EGR valve, so that exhaust gas can recirculate. EGR invariably results in drop in power, increased fuel consumption and rough combustion. In addition excessive intake system deposit buildup and increased oil sludging occur.
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Fumigation technique: This method consists of introducing a small amount of fuel into the intake manifold. This starts pre-combustion reactions before and during the compression stroke resulting in reduced chemical delay, because the intermediate products such as peroxides and aldehydes react more rapidly with oxygen than original hydrocarbons. The shortening of delay period curbs thermal cracking which is responsible for soot formation. Fumigation rate of about 15% gives best smoke improvement. However this improvement varies greatly with engine speed. At low engine speeds 50 to 80% smoke reduction is obtained. This decrease as speed increases until a speed at which there is no effect of fumigation. CRANKCASE EMISSION AND CONTROL During the compression and combustion strokes, highly corrosive blowby gases are forced past the piston rings into the crankcase. The amount of blowby entering the crankcase generally increases with engine speed. The amount of blowby also depends on other conditions including piston, ring and cylinder wear. The actual amount of wear may be small, perhaps only a few thousands of an inch. But almost any wear is enough to weaker the sealing effect of the rings and permit blowby to increase. Blowby gases
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contain burned and unburned fuel, carbon and water vapour from the combustion chamber. When the engine is cold, some of the water vapour of the blowby condenses on the cylinder walls and crankcase. It forms into droplets and runs down into the oil pan. Gasoline vapour also condenses on cold engine parts and drips down into the oil pan. This gasoline dilutes and thins the oil, reducing its lubricating ability. The churning action of the rotating crank shaft can whip the water and engine oil into thick, gummy substance called sludge. The acid compounds from the blowby can get into the sludge and cause corrosion and faster wear of engine parts. Sludge can also clog oil passages and prevent normal engine lubrication, thereby leading to early engine failure. Blowby causes pressure in the crankcase. If this pressure is allowed to build up, engine oil is forced past the oil seals and gaskets and out of the engine. To help to control the effect of blowby, there must be a way to relieve the crankcase pressure caused by blow by gases.
CRANKCASE VENTILATION To avoid the above said problems, the unburned and partly burned gasoline and the combustion gases and water vapour must be cleared out of the crankcase by providing crankcase ventilation systems. In early engines, the crankcase ventilation system was very simple. It provided crankcase breathing by passing fresh oil through the crankcase. On almost all American made automobile engines built prior to 1961, the fresh air entered through an air inlet at the top front of the engine. The fresh air is mixed with the blowby fumes and other vapours in the crankcase. These vapours were routed out of the crankcase through a large hollow tube called the road draft tube, which discharged under the car into the atmosphere. The fresh air inlet was usually the crankcase breather cap. On most engines it also served as the cap for the crankcase oil filler tube. The cap was open, or vented with holes on both sides to let fresh air to pass through. The cap was filled with oil soaked steel wool or similar material to serve as an air filter. The filter prevented dust particle in the air from getting into the crankcase oil and causing engine wear.
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ROAD DRAFT TUBE EMISSIONS:
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The road draft tube system worked well to keep the crankcase free of fumes and pressure build up. However it discharged all the crankcase pollutants into the atmosphere. This discharge through the road draft tube represented about 20% of the total HC emissions from an automobile. Therefore controlling blow by was the first step in eliminating atmospheric pollution from the automobile. OPEN PCV SYSTEM: An early system that partially controlled crankcase emission was installed on cars built for sale in California beginning in 1961. The system was called open positive crankcase ventilation system. In this system a tube is connected between a crank case vent and the intake manifold. While the engine is running, intake manifold vacuum is used to pull vapour from the crankcase through the tube into the intake manifold. Fresh ventilating air is drawn into the crankcase through an open oil filler cap. In the intake manifold, the crankcase vapours are mixed with the incoming air-fuel mixture and sent to the cylinders for burning.
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For the engine to operate properly under all conditions of speed and load, a flow control valve is required. Without a flow control valve, excessive ventilation air passes from the crankcase into the intake
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manifold during idling and low speed. This upsets the engine air fuel ratio and results in poor idling with frequent stalling. The PCV valve is installed in a tube from the crankcase vent to the intake manifold. The PCV valve is a variable orifice valve. A variable orifice is a hole that acts as a valve by changing the size to vary the flow rate through it. This valve is also called a metering valve, a modulator valve and a regulator valve. A typical PCV valve consists of a coil spring, a valve and a two piece outer body which is usually crimped together. At idle or low speed, high intake manifold vacuum tends to pull the valve closed or into its minimum flow condition. As the valve tries to close it compresses the valve spring. The smaller opening now allows a much smaller volume of blow by gas to pass through. At high engine speeds, the compressed spring overcomes the pull of the vacuum on the valve. The spring begins to force the valve open towards the maximum flow condition. As the valve moves open, the flow capacity increases. This is to handle the greater volume of blowby that results from an increase in engine load and speed. CLOSED PCV: The crankcase emission control system described above is not completely effective in controlling crankcase emissions. In open type system, blowby in excess of the PCV valve flow rate escapes to the atmosphere through the open oil filler cap. To overcome this problem, a closed positive crankcase ventilation system was developed. All cars manufactured in California in 1963 and later used a closed type of positive crankcase ventilation system. The blowby gases are turned to the engine cylinder through the intake manifold and under appropriate conditions, through the carburettor air cleaner. The PCV valve described earlier is generally used as the flow control valve. A closed oil filler cap is used. Other possible outlets for blow by gases, such as dipstick tube are sealed.
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102
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All cars are now being equipped with such closed PCV system wherever there are air pollution regulations. These systems have completely eliminated the crankcase as a source of atmospheric contamination and no additional control in future is required in this direction. EVAPORATIVE EMISSIONS AND CONTROL Hydrocarbon evaporative emissions from a vehicle arise from two sources as evaporation of fuel in the carburettor float bowl ( 5-10 percent ) of fuel in the fuel tank ( about 5 percent ). CARBURETTOR EVAPORATIVE LOSSES: Carburettor hydrocarbon vapour losses arise from distillation of fuel from the float bowl. Carburettor fuel temperature often reaches 55°C during warm weather engine operation and may rise up to 80°C during a hot soak. Hot soak is a condition when a running car is stopped and its engine turned off. During the soak a significant fraction of the fuel will boil off and a large portion of the loss finds its way into the atmosphere. There is a considerable rise in fuel system temperature following shut down after a hard run. The basic factors governing the mass of fuel distilled from carburettor during a hot soak period are •
maximum fuel bowl temperature
•
amount of fuel in the bowl
•
amount of after-fill and
•
distillation curve of the fuel
Tests have indicated that a less volatile fuel would reduce the evaporative losses considerably. A fuel with 20% distilled at 72 °C would give 22% less losses as compared to a fuel which distilled 25% at 72 °C . The carburettor bowl volume has a significant effect on evaporative losses. Increase in the volume of the bowl increases the losses linearly. If an insulated spacer is placed between the carburettor and the inlet manifold, almost 50% reduction can be observed. Filling of the carburettor (after-fill) to the original liquid level is similar to an increase in the bowl volume and the distillation losses would increase by about 15%. FUEL TANK EVAPORATIVE LOSSES: Fuel tank losses occur by displacement of vapour during filling of petrol tank, or by vaporization of fuel in the tank, forcing the vapour through a breather vent to the atmosphere. When the temperature is low, the fuel tank breathes in air. When the temperature goes high it breathes out air, loaded with petrol vapours. Fuel tank losses occur because the tank temperature is increased during the vehicle operation which causes an increase in the vapour pressure and thermal expansion of tank vapour. The mechanism of tank loss is as follows: When a partially filled fuel tank is open to atmosphere, the partial pressure of vapour phase hydrocarbons and vapour pressure of the liquid phase are equal and they are in equilibrium. If the temperature of the liquid is increased, say by engine operation, the vapour pressure of the liquid will increase and it will vaporize in an attempt to restore equilibrium. As additional
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liquid vaporize the total pressure of the tank increases and since the tank is open to atmosphere, the vapour will flow out of the tank. This outflow to the vapour will increase if in addition to liquid temperature rise, the vapour temperature is also increased. The evaporation from the tank is affected by a large number of variables of which the ambient and fuel tank temperature, the mode of vehicle operation, the amount of fuel in the tank and the capacity, design and location of the fuel tank with reference to exhaust system and the flow pattern of the heated air underneath the vehicle. Less the tank fill, greater is the evaporative loss. The effect of the tank fill and the temperature are shown in the table. This reflects the difference in the tank vapour space. Also when a car is parked in a hot location, the evaporation of gasoline in the tank accelerates and so the evaporation loss is greater. EFFECT OF FUEL TANK FILL ON EVAPORATIVE LOSS: Tank fill
Ambient temperature Temp.
¼ ½ ¾ Full
C 19 16 18 22
rise
test in °C 7 4 2 3
during Loss
during
operations % 5.7 1.2 0.1 0.0
The operational modes substantially affect the evaporation loss. When the tank temperature rises, the loss increases. The fuel composition also affects the tank losses. About 75% of the HC losses from the tank are C4 and C5 hydrocarbons. Design factors that affect the evaporative losses include the peak tank temperature, the area of the liquid vapour surface, and the amount of agitation. It is obvious that nay design change which reduces the peak tank temperature will reduce the tank loss. Such modifications include tank insulation, lower surface to volume ratio of tank, better tank orientation or location for reduced heat pick up from solar radiation or other heat sources such as the exhaust system. The surface area for evaporation and tank agitation are factor which influence the speed with which equilibrium is achieved. Baffles in the tank can reduce losses by maintaining concentration gradients. EVAPORATIVE EMISSION CONTROL DEVICES: Evaporative emission control devices are designed to virtually eliminate the hydrocarbon vapours emitted by the carburettor and fuel tank during both running and hot soak. During running, fuel tank vapours are inducted and burned in the engine. Carburettor losses are vented to intake system. Vehicles without evaporative controls are estimated to 10se 30 g/day of HC from fuel tank filling and breathing. Another 40 g/day is lost by evaporation from the carburettor ( hot soak loss ) when the vehicle is parked after being operated. ON this basis, evaporative losses are estimated to be 23% of total HC emissions.
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The device as shown in the figure consists of an absorbent chamber, the pressure balance valve and the purge control valve. The absorbent chamber which consists of a charcoal bed or foamed polyurethane holds the hydrocarbon vapour before they can escape to atmosphere. The carburettor bowl and the fuel tank are directly connected to the absorbent chamber.
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During hot soaks, vapours from the fuel tank are routed to a storage device. Carburettor vapours may be vented to the storage system or retained internally in the carburettor or induction system volume. A schematic diagram of this arrangement is shown in the figure. Upon restart, filtered air is drawn through the stored vapours and the mixture is metered into the intake system and burned in the engine. In this manner the storage device is purged (removed off the retained vapour). The operations of the purge control valve are controlled by the exhaust back pressure. The storage system consists of a canister containing activated charcoal located in the engine compartment. Activated charcoal has an affinity for HC and on a recycle basis can store 30-35 grams of fuel per 100 grams of charcoal without breakthrough. Typically 700-800 grams of charcoal are used in a vehicle system.
One problem with any storage system is the possibility of liquid fuel entering the storage device. Ball check valves or vapour liquid separators assure than only fuel vapours reach the storage device. In addition, a dead volume in the tank allow for thermal expansions of a full fuel tank. About 10% of the tank volume is partially walled off from the remainder of the tank. When the tank is filled, this volume remains nearly empty. After a period of time, the fuel fills the additional volume thereby leaving room for expansion in the rest of the tank. Otherwise expansion could force the liquid fuel into the charcoal canister or the crankcase.
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THERMAL REACTORS: Thermal reactor is a chamber in the exhaust system designed to provide sufficient residence time to allow appreciable homogeneous oxidation of HC and CO to occur. In order to improve CO conversion efficiency, the exhaust temperature is increased by retarding spark timing. This however results in fuel economy loss.
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The air is supplied from an engine driven pump through a tube to a place very near to the exhaust valve. To achieve a high degree of exhaust system oxidation of HC and CO, a high exhaust temperature coupled with sufficient oxygen and residence time to complete the combustion is needed. Oxides of nitrogen are not reduced. In fact, they may be increased if sufficiently high exhaust temperature results from the combustion of CO and HC with the added air or if the injected air enters the cylinder during the overlap period, thereby leaning the mixture in the cylinder. Warren has derived the following equation for the concentration of hydrocarbons leaving the exhaust system. Co = Ci * exp Kr O2 P2V K3 T2W Where,
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Co = Concentration of HC leaving the thermal reactor Ci = Concentration of HC leaving cylinders and entering the thermal reactor Kr = Specific reaction rate ft3/lbm – mole/sec K3 = constant O2 = oxygen concentration in exhaust gases. Volume percent P = exhaust pressure ( Psi) V = thermal reactor volume available for reaction Cu.ft T = Absolute temperature C W = Mass flow rate of air ( lb/sec) Note the importance of pressure term. Increasing exhaust system back pressure promote after reaction. However commercially, the possible back pressure increase is small.
The graph shows the effect of temperature on specific reaction rate Kr, calculated from the above equation by warren from his experimental data. The nearness of his curve to a straight line suggests the equation is a good approximation for the overall reactions occurring. Note that a decrease in exhaust temperatures from 1100C to 1000F decrease the reaction rate by a factor of 10.
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The graph shows the effect of temperature and reactor volume on exhaust hydrocarbon concentration at an oxygen input concentration of 3%. Reactor volume may be viewed as the volume of the exhaust system which is insulated and ant the high temperature needed for reaction. Note that if the exhaust temperature were 1400F, only twice the convention system volume is required for virtually complete elimination of the hydrocarbons. On the other hand, if the temperature were only 1200F, eight times the volume would achieve only a 76% reduction. A pair of conventional exhaust manifolds has about 0.09 ft3 of volume.
Increasing the exhaust system volume increases the residence time during which reactions can occur. This is a benefit, providing the added surface area does not result in excessive cooling. Thus when large volume exhaust manifolds are to be used, they must be well insulated. Brownson and stebar have studied thermal reactor performance for a reactor coupled to a single cylinder CFR engine. In their work an insulated exhaust mixing tank of 150 cubic inch was used for some tests. They determined that the basic factors governing the combustion of CO and hydrocarbons in the exhaust system are composition of the reacting mixture, temperature and pressure of the mixture, and residence time of the mixture or time available for reaction.
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The graph shows the hydrocarbon and CO emissions as a function of air-fuel ratio and injected air flow rate. The emission concentration results were corrected for the added air. Injected air flow rate is indicated as a percentage of the engine air volume flow rate. An insulated 150 cubic inch exhaust mixing tank was used.
The minimum HC concentrations occurred at rich mixtures. When too much air was injected, especially at lean mixtures, excessive cooling of the exhaust increased HC concentrations above those with no air. Thus the normal oxidation process was apparently inhibited by this cooling. A small increase in CO occurred slightly richer than stoichiometric. At stoichiometric mixtures and leaner, CO was very low. Best results occurred for rich mixtures with air injection at 20-30% of inlet air flow. The air-fuel ratio for best emission reduction was 13.5:1. Normally engine operation at such a rich mixture would reduce fuel economy by 10%. At each air-fuel ratio there exists one minimum air injection rate that provides maximum emission reduction. Minimum air flow is desired in order to reduce pump power requirement, size and cost. Graph shows the optimum air injection rate for both HC and CO emissions.
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CATALYTIC CONVERTERS: Catalytic converters provide another way to treat the exhaust gas. These devices located in the exhaust system, convert the harmful pollutants into harmless gases.
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In contrast to thermal reactors efficient catalytic oxidation catalysts can control CO and HC emissions almost completely at temperature equivalent to normal exhaust gas temperatures. Thus the fuel economy loss necessary to increase the exhaust temperature is avoided. Inside the catalytic converter the exhaust gases pass over a large surface area coated with a catalyst. A catalyst is a material that causes a chemical reaction without actually becoming a part of the reaction process. Catalytic reaction of NO can be represented as follows: NO + CO CO2 + ½ N2 NO + H2 H2O + ½ N2 10 NO + 4HC 2H2O + 4CO2 + 5 N2 HC / CO oxidation is represented by CO + ½ O2 CO2 4HC + 5 O2 2H2O + 4CO2 The figure shows a single bed catalytic converter. The exhaust gas and air are passed through a bed of platinum coated pellets or honeycomb core. HC and CO react with the oxygen in the air. Harmless ware and carbon dioxide are formed. The catalyst platinum act on the exhaust gas in two ways, converting HC and CO to carbon dioxide and water. So it is called a two way catalyst.
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Figure show a dual bed catalytic converter. The exhaust gas first passes through the upper bed. The upper bed contains a reducing catalyst ( example rhodium). NOx is reduced to nitrogen and oxygen in the upper bed. Then secondary air is mixed with the exhaust gas. The mixture of exhaust gas and secondary air flows to the lower bed. The lower bed contains an oxidizing catalyst ( example platinum). HC and CO are oxidized to water vapour and carbon dioxide in the lower bed. Here the catalyst rhodium is a one way catalyst since it acts o NOx only. Platinum is a two way catalyst since it acts on HC and CO. A three way catalyst is a mixture of platinum and rhodium. It acts on all three of the regulated pollutants ( HC, CO and NOx) but only when the air-fuel ratio is precisely controlled. If the engine is operated with the ideal or stoichiometric air-fuel ratio of 14.7:1. The three way catalyst is very effective. It strips oxygen away from the NOx to form harmless water, carbon dioxide and nitrogen. However the airfuel ratio must be precisely controlled, otherwise the three way catalyst does not work.
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Figure shows a three way catalytic converter. The front section( in the direction of gas flow) handles NOx and partly handles HC and CO. The partly treated exhaust gas is mixed with secondary air. The mixture of partly treated exhaust gas and secondary air flows into the rear section of the chamber. The two way catalyst present in the rear section takes care of HC and CO.
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Generally catalysts are classified as: 1. Supported catalysts based on a. Noble metals b. Transition metals 2. Unsupported metallic alloys NO-Reduction Catalysts: From the literature, it is seen that the following materials have been tried successfully as reduction catalysts in the vehicle emission control
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1. Copper oxide-chromia 2. Copper oxide – Vanadia 3. Iron oxide – Chromia 4. Nickel oxide pelleted on monolithic ceramic and metallic supports 5. Monel metal 6. Rare earth oxides HC/CO oxidation catalysts: 1. Noble metal catalysts such as activated carbon, palladium or platinum 2. Transition metal oxide catalysts such as copper, cobalt, nickel and iron chromate as well as vanadium or manganese promoted versions of these metals. 3. Copper chromite-alumina and platinum oxide –alumina catalysts were developed with sufficient activity, stability and mechanical strength. The catalysts chosen for vehicle emission control should satisfy the following: 1. High conversion efficiency under transient conditions 2. Effective for wide range of temperature ( for ambient to 1600 F) 3. Must withstand the poisoning action of additives in the gasoline that are emitted in the exhaust 4. Must be able to withstand thermal shock 5. Be attrition resistant to highly turbulent flows through the converter 6. Vehicle operation for 50,000 miles 7. Convert into harmless products 8. Cheap and readily available. Converter Design: Converter volume is fixed, based on the space velocity and exhaust flow rate Space velocity = gas flow rate in cm^3 /hr / converter volume in cm^3 The reciprocal of this expression is the residence time. As the exhaust flow rate varies under different modes of vehicle operation, an average gas flow rate of 0.85m^3/min and a space velocity of 15000/hr are normally selected for the preliminary design of the converter. This will give a converter volume of 3,540 cm^3 in each stage. Draw backs of the catalytic converter system: 1. Generally catalysts are active only at relatively high temperature. Emissions during warm-up cannot be catalysed and this period has particularly heavy emissions. 2. Catalysts operate over a wide but not unlimited temperature range. A temperature control is required to avoid burnout temperature at high speeds and loads. 3. Catalysts are poisoned by exhaust constituents in particular lead compounds. Hence conversion efficiency decreases with use. 4. The catalytic bed offers considerable back pressure which increases with use.
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5. Catalytic converters are expensive. Importance of unleaded Petrol: Vehicles equipped with catalytic converters must use only un-leaded gasoline. If the gasoline contains lead, the lead will coat the catalyst and the converter will stop working. OTHER EMISSIN CONTROL DEVICES: 1. Water injection: In this a small amount of water is injected into the combustion chamber. Due to this the peak combustion temperature is reduced and thus NOx emission is reduced. Graph shows nitric oxide reduction as a function of water rate. The spark advance was kept constant and the power loss was balanced by leaning the A/F ratio of the mixture. The specific fuel consumption as clear from the graph, decreases a few percent at medium water injection ratio. So for no attempts have been done to use water as a deice for controlling the NOx, perhaps because of complexity varying the amount of injection rate in relation to engine requirements. 2. Direct air Injection: In this compressed air is introduced into the combustion chamber in addition to air fuel charge from the carburettor. This gives better combustion and hence reduced hydrocarbon and CO emission. This will also give tremendous power boost with some saving in fuel. But extra equipment in the form of air compressor and air valves will raise the cost very much. Also, exhaust gas recirculation will still be needed to curb NOx emissions. 3. Ammonia Injection: In this ammonia is injected into the exhaust gas. Ammonia reacts with NOx in exhaust and forms nitrogen and water. Thus NOx emission is reduced. As a fuel, ammonia does not hold much promise, but if used as an exhaust additive it can give excellent control for NOx emission. Ammonia and nitric oxide interact to form nitrogen and water. Ford motor company has been doing investigations with injecting Ammonia-water in the exhaust manifold, downstream from the port. For an effective utilization of Ammonia injection, the exhaust gas temperature has to be kept within strict limits and the injecting device has to be put sufficiently down to bring the gas temperature to 165C. This also demands a very close tolerance in air-fuel ratio supplied by the carburettor. The present carburettors are incapable of this and it might be necessary to adopt electronic injection system to keep it. 4. Electronic Injection:
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It is possible to develop an electronic injection system with sensors for air temperature, manifold pressure and speed which will precisely regulate the fuel supply giving only such air fuel ratio that will give no hydrocarbon or CO emissions. Since the injection can be affected in individual intake ports, the problem of fuel distribution among various cylinders will automatically be avoided. The emissions on deceleration can be completely removed by shutting off the fuel supply when the throttle is closed. But this system will still not be able to control the HC emission. Combination of electronic injection and ammonia as an exhaust additive has an attractive future. MEASUREMENT TECHNIQUES EMISSION STANDARDS AND TEST PROCEDURE VEHICLE EMISSION STANDARDS: Federal exhaust emission test procedures for light duty vehicles under 6000 lb GVW covering the period 1972 to 1975 assess hydrocarbon, carbon monoxide and nitric oxide emissions in terms of mass of emission emitted over a 7.5 mile chassis dynamometer driving cycle. Results are expressed as grams of pollutant emitted per mile. There are two procedures in using the same test equipment which assess vehicle emissions. One, which is termed as CVS-1 (constant volume sampling), employs a single bag to collect a representative portion of the exhaust for subsequent analysis. This single bag system applied to testing of 1972, 1973 and 1974 vehicles. Based on this test, emission standards for vehicles have been set at Hydrocarbons
3.4 g/mile ( 1972 to 1974 )
Carbon monoxide
3.9 g/mile ( 1972 to 1974 )
Oxide of nitrogen
3.0 g/mile ( 1973 to 1974 )
The second test procedure, termed CVS-3 uses three sampling bags and is designed to give a reduced and more realistic weighing to cold start portion of the test. This three bag system applies to testing of 1975 to 1976 vehicles. Exhaust emission standards based on this test are Hydrocarbons
0.41 g/mile ( 1975 to 1976 )
Carbon monoxide
3.4 g/mile ( 1975 to 1976)
Oxide of nitrogen
3.0 g/mile ( 1975 )
One of the latest U.S standards ( 1982) for passenger cars and equivalents are Hydrocarbons
0.41 g/mile
Carbon monoxide
3.4 g/mile
Oxide of nitrogen
1.5 g/mile
These are measured by following a prescribed test procedure.
Driving Cycle:
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The driving cycle for both CVS-1 and CVS-3 cycles is identical. It involves various accelerations, decelerations and cruise modes of operation. The car is started after soaking for 12 hours in a 60-80 F ambient. A trace of the driving cycle is shown in figure. Miles per hour versus time in seconds are plotted on the scale. Top speed is 56.7 mph. Shown for comparison is the FTP or California test cycle. For many advanced fast warm-up emission control systems, the end of the cold portion on the CVS test is the second idle at 125 seconds. This occurs at 0.68 miles. In the CVS tests, emissions are measured during cranking, start-up and for five seconds after ignition is turned off following the last deceleration. Consequently high emissions from excessive cranking are included. Details of operation for manual transmission vehicles as well as restart procedures and permissible test tolerance are included in the Federal Registers.
CVS-1 system: The CVS-1 system, sometimes termed variable dilution sampling, is designed to measure the true mass of emissions. The system is shown in figure. A large positive displacement pump draws a constant volume flow of gas through the system. The exhaust of the vehicle is mixed with filtered room air and the mixture is then drawn through the pump. Sufficient air is used to dilute the exhaust in order to avoid vapour condensation, which could dissolve some pollutants and reduce measured values. Excessive dilution on the other hand, results in very low concentration with attendant measurement problems. A pump with capacity of 30-350 cfm provides sufficient dilution for most vehicles. Before the exhaust-air mixture enters the pump, its temperature is controlled to within +or – 10F by the heat exchanger. Thus constant density is maintained in the sampling system and pump. A fraction of the diluted exhaust stream is drawn off by a pump P2 and ejected into an initially evacuated plastic bag. Preferably, the bag should be opaque and manufactured of Teflon or Teldar. A single bag is used for the entire test sample in the CVS-1 system. Because of high dilution, ambient traces of HC, CO or NOx
can significantly increase
concentrations in the sample bag. A charcoal filter is employed for leveling ambient HC measurement. To correct for ambient contamination a bag of dilution air is taken simultaneously with the filling of the exhaust bag. HC, CO and NOx measurements are made on a wet basis using FID, NDIR and chemiluminescent detectors respectively. Instruments must be constructed to accurately measure the relatively low concentrations of diluted exhaust.
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Bags should be analyzed as quickly as possible preferably within ten minutes after the test because reactions such as those between NO, NO2 and HC can occur within the bag quite quickly and change the test results. CVS-3 SYSTEM: The CVS-3 system is identical to the CVS-1 system except that three exhaust sample bags are used. The normal test is run from a cold start just like the CVS-1 test. After deceleration ends at 505 seconds, the diluted exhaust flow is switched from the transient bag to the stabilized bag and revolution counter number 1 is switched off and number 2 is activated. The transient bag is analyzed immediately. The rest of the test is completed in the normal fashion and the stabilized bag analyzed. However in the CVS-3 test ten minutes after the test ends the cycle is begun and again run until the end of deceleration at 505 seconds. This second run is termed the hot start run. A fresh bag collects what is termed the hot transient sample. It is assumed that the second half of the hot start run is the same as the second half of the cold start run and is not repeated. In all, three exhaust sample bags are filled. An ambient air sample bag is also filled simultaneously. STANDARDS IN INDIA: The Bureau of Indian Standards ( BIS ) is one of the pioneering organizations to initiate work on air pollution control in India. At present only the standards for the emission of carbon monoxide are being suggested by BIS given in IS:9057-1986. These are based on the size of the vehicle and to be measured under idling conditions. The CO emission values are 5.5 percent for 2 or 3 wheeler vehicles with engine displacement of 75cc or less, 4.5 percent for higher sizes and 3.5 percent for four wheeled vehicles. IS: 8118-1976 Smoke Emission Levels for Diesel vehicles prescribes the smoke limit for diesel engine as 75 Hatridge units or 5.2 Bosch units at full load and 60-70 percent rated speed or 65 Hatridge units under free acceleration conditions. EMISSION MEASURING INSTRUMENTS: Terms of expressions: In emission measurement, volume concentrations of the several components are characteristically expressed in the following terms 1. Carbon dioxide and carbon monoxide are expressed as percent of the sample volume. 2. Nitric oxide and nitrogen dioxide are expressed as volume parts of NO or NO2 per million parts of the sample ( ppm). The total of NO and NO2 is designated as NOx. 3. Hydrocarbon is expressed as i. Parts of hydrocarbon per million parts of the sample or ii. Parts of carbon per million parts of the sample ( ppmc). The latter term is defined as the volume concentration of hydrocarbon in the sample multiplied by the average number of carbon atoms per molecule of that hydrocarbon. Thus 1ppm propane ( C3H8) is the equivalent of 3ppm C hydrocarbon. In the early days of
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emission measurement hydrocarbon emissions were measured in terms of the carbon equivalent of hexane or ppm hexane. Thus in early usage ppm values were often assumed to be ppm hexane even though not
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designated as hexane, this usage is ambiguous and should be avoided.
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Flame Ionisation Detector ( FID ): The unburned hydrocarbons in the exhaust consist of about 200 different compounds, each with different composition and different number of carbon and hydrogen atoms. It is impossible to detect each of these hydrocarbons separately. The over all concentration of the unburned hydrocarbons may be found by measuring the equivalent concentration of n-hexane ( C6H14). An accurate method of measuring the unburned hydrocarbon emissions is to use the Flame Ionisation Detector ( FID ). The working principle of FID is as follows: A hydrogen-air flame contains a negligible amount of ions but if few hydrocarbon molecules are introduced into the flame a larger number of ions are produced. The ion yield is proportional to the amount of hydrocarbon introduced into the flame. The basic elements of a Flame Ionisation Detector are as shown in the figure, a burner and ion collector assembly. In practice, a sample of gas is mixed with hydrogen in the burner assembly and the mixture burned in a diffusion flame. Ions that are produced in the flame move to the negatively polarized collector under the influence of an electrical potential applied between the collector plates. At the negative collector, the ions receive, via a current network, electrons that are collected from the flame zone at the positive collector. Thus a small current proportional to the amount of hydrocarbon entering the flame flows between the collector plates. This small current is amplified using a high impedance direct current amplifier, the output of which becomes an indication of hydrocarbon present. The detector responds to carbon that is linked with hydrogen as in equation 1 and the response is largely independent of the molecular configuration, i.e hydrocarbon species. Thus the detector is essentially a carbon atom counter. The output of the FID depends on the number of carbon atoms passing through the flame in a unit time. Doubling the flow velocity would also double the output. Hexane ( C6H14) would give double the output of propane ( C3H8). Therefore FID output is usually referred to a standard hydrocarbon usually as PPM of normal hexane. Characteristics of the FID are improved with most burned designs if instead of using pure hydrogen fuel, the hydrogen is mixed with inert gas to decrease flame temperature. This mixture of hydrogen and inert gas is referred to as fuel gas or fuel. The FID responds directly to the amount of hydrocarbon entering the flame. Therefore close control of sample flow is required. In general, the sample flow rate is specified at the minimum amount that will give the required sensitivity in any given instrument. Fuel and air flow rates also influence the response characteristics of the detector. Response typically first rises and then fall with increased fuel rate, as shown in the figure. Typical volume rates of instrument gases are sample 3-5 ml/min and fuel gas mixture 75ml/min and air 200ml/min.
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Presence of CO, CO2, NOx, water and nitrogen in the exhaust have no effect on the FID reading. FID analyzer is rapid, continuous and accurate method of measuring HC in the exhaust gas concentrations as low as 1ppb can be measured. CHEMILUMINESCENT NOX ANALYZER Chemiluminescence, a chemical reaction once dismissed as a laboratory curiosity, has become the most widely used NOx emissions monitoring technique in stack emissions and ambient air-monitoring instrumentation. More than 95% of the NOx CEMS used by the electric utility industry, and approximately 99% of the NOx analyzers used for EPA Reference Method 7E and 20 testing employ the chemiluminescence measurement technology. The basic chemiluminescence chemistry was delineated by Clough et al. NO + O3 → NO2+ O2 (Eq. 1) NO + O3 → NO2 + O2 (Eq. 2) NO2 → NO2 + hv (~600 to 3,000nm) (Eq. 3) NO2 + M → NO2 + M (Eq. 4) hv = photons, M = any compound Equations 1 and 3 describe the technique employed in commercial instrumentation to measure nitrogen oxides. When NO reacts with O 3, some electronically-excited NO2 molecules are produced (Equation 1). These molecules may give off energy in the form of light emission with intensity linearly proportional to the concentration of NO (Equation 3). When the emitted radiation is monitored, it becomes a measure of the concentration of the NO in the reacting sample. The light emission occurs between 600 and 3,000 nm, with a peak at about 1,200 nm. Chemiluminescence NO x analyzers measure NO concentrations by using a bandpass filter to select light in the region from about 600 to 900 nm. The reactions described by equations 2 and 4 describe a potential limitation to the measurement technique. Only a fraction of the NO reacts to form NO 2 and emits light. Fortunately, the percentage of NO in the reactor that follows the pathway described by equations 1 and 3 is sufficient to ensure a proportionally linear response in a properly designed instrument. By maintaining the O 3 concentration at a large excess to the NO concentration and with a detection system designed for optimum light-collection efficiency, analyzers have been developed with linear dynamic ranges over 10,000 ppm and with detection limits at the parts per trillion level.
In order to measure NO 2 using the same basic mechanism outlined by Equations 1 through 4, NO 2 is first converted to NO (i.e., NO2 to NO converter), after which the converted molecules react with ozone along with the original NO molecules in the sample. This results in a signal that equals the sum of NO and NO 2. By taking the difference between the converted and non-converted modes, a measure of the NO 2 concentration is obtained. The major advantages of chemiluminescence method over other measurement methods for NO x monitoring include: • Increased sensitivity (detection limit) • Improved specificity (accuracy) • Rapid response time (control) • Linearity over a wide dynamic range (precision) • Continuous monitoring (control and reporting) • Simplicity of design (maintenance)
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Basic NO-NO2-NOx Chemiluminescence Instrumentation The simplified diagram of a NO-NO2-NOx chemiluminescence analyzer as previously described. To measure NO concentrations, the sample mixes with ozone in a flow reactor. The ozone required for the reaction is produced within the instrument from dry air or oxygen. The luminescence that results from the reaction of NO with O3 is monitored through an optical bandpass filter by a high-sensitivity detector positioned at the end of the reactor. The bandpass filter/detector combination responds to the light emission in a narrow-wavelength band. Figure 1 shows the spectral output of the chemiluminescence reaction (Trace 1) along with optical filter (Trace 2) and typical detector (photomultiplier tube [PMT]) response (Trace 3) characteristics. As can be seen, only a small portion of the spectrum is monitored. This aids in gaining specificity for the analyzer. An electronic package takes the detector output signal and processes it to voltage, current level, or digital signal. The sample inlet generally has two flow modes. The first (NO mode) is a direct path of sample to the reaction chamber. Ideally, only the NO in the sample reacts with the ozone to produce light emission (chemiluminescence). In the second mode (NO x mode), the sample is routed through a converter that transforms the NO2 to NO.
Figure 2 – Basic NO-NO2-NOx Chemiluminescence Instrumentation DESCRIPTION OF A TYPICAL SAMPLE GAS FLOW SYSTEM Figure 2 shows a flow diagram of a typical analyzer. The chemiluminescence method relies on the measurement of the number of NO molecules entering the reaction chamber per time. In order to maintain instrument stability, two gas flow systems are used to regulate a constant flow of ozone and sample gas to the reaction chamber. In Figure 3, a single capillary regulates the ozone flow. The function of the capillary is to restrict the flow in the line such that a controllable backpressure is maintained. The pressure upstream of the capillary and the capillary dimensions, and pump characteristics determine the flow. Other flow regulation schemes are possible. Mass flow controller, critical orifices, and the regulation system describe above have been used. All regulation systems are designed for the same purpose: to maintain a constant air or oxygen flow through the ozonator. The prime requirement of the flow system is to maintain a constant flow of the sample gas to the analyzer as the chemiluminescent reaction is extremely flow sensitive. Figure 3 presents one standard industrial approach to fulfill these conditions. The total sample passes into the analyzer through the inlet capillary restrictor. A fraction (about 5%) is bled off through a second smaller restrictor and directed to a mode valve. The portion of the sample bypassing the mode valve (and reaction chamber) exits the instrument through a bypass pump. The sample regulator functions to maintain a constant pressure drop across the smaller capillary. This method of flow control maintains a reaction
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chamber sample flow rate that is insensitive to the sample inlet pressure and total sample flow rate. This type of bypass flow system has the additional advantage of bring the sample to the analyzer. It has the disadvantage of using a fairly large sample flow, which can be a limitation in smog chamber or bag sample studies. For typical process applications, however, there is no sample limitation. Not all chemiluminescent analyzers use such a bypass-flow technique. Most ambient-air analyzers use only one sample restrictor and no bypass flow. Nevertheless, sample flow remains one of the most critical parameters to control. Mass flow controllers, critical orifices, and/or a regulator and capillary system using a pressure sample have been used. Figure 3 – Basic Chemiluminescence Analyzer Flow Diagram
DRIVING CYCLES IN UNIT-3 ENGINE EXHAUST EMISSION CONTROL EMISSION (HC,CO,NO,NOX) MEASURING EQUIPMENTS Vehicles generate potentially harmful and toxic emissions through their exhaust pipes, especially when they are not properly maintained. Many states require that vehicles pass emissions testing before they are deemed road-worthy. Special instrumentation is used to analyze emissions and detect gases like nitrogen oxides (NOx), carbon dioxide (CO2), carbon monoxide (CO) and hydrocarbons(HC) Gas Analyzers Gas pipe exhaust is measured using special equipment such as the range of Dyne System five-gas portable analyzers. These instruments test and measure the amount of engine exhaust for specific gas components, including CO, CO2, NO, HC and oxygen. Equipment is first calibrated using ambient air
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prior to each use to ensure accurate measurements. Analyzers feature a sample line to collect exhaust gases and a two-stage filter system that is capable of detecting and capturing both large and small particles. Systems have touch-screen interfaces that offer control over manual operations. Wall-mounted enclosures are optional equipment to house instruments when not in use. Dynamometer A dynamometer is an electronic roller device used inside vehicle inspection bays to measure tailpipe emissions. The test vehicle is driven onto the dynamometer rollers and then testing begins. This instrument simulates the actual exhaust that the vehicle produces while in driving at low speed (15 mph) and when accelerating, all while still sitting in the bay. It's used in conjunction with a five-gas analyzer for instant data readout. Mostly with the help of engine &chassis dyanamometers analysising of emissions are carried out.
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Chassis & engine dyanamometer used in automobile for measuring emission Engine dynamometer An engine dynamometer measures power and torque directly from the engine's crankshaft (or flywheel), when the engine is removed from the vehicle. These dynos do not account for power losses in the drivetrain, such as the gearbox, transmission or differential etc Chassis dynamometer
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Emissions development and homologation dynamometer test systems often integrate emissions sampling, measurement, engine speed and load control, data acquisition, and safety monitoring into a complete test cell system. These test systems usually include complex emissions sampling equipment (such as constant volume samplers or raw exhaust gas sample preparation systems), and exhaust emissions analyzers. These analyzers are much more sensitive and much faster than a typical portable exhaust gas analyzer. Response times of well under one second are common and required by many transient test cycles Scanning Tools Many vehicles manufactured post-1980, and all models manufactured after 1996 have some form of on-board computerized diagnostic systems (OBDs). These computers can monitor vehicular subsystems like exhaust sensors and oxygen sensors. Scanning tools developed to exploit OBD technology can be readily connected to these on-board computers to test emissions. Hand-held electronic scanning instruments can provide real-time assessments when measuring vehicular emissions. Gas Cap Testing Gas cap testing checks for leaks from around vehicle gas cap seals. Invisible gasoline or diesel fumes are volatile gases and seepage into the atmosphere may go otherwise undetected if inspections are not made. According to the Illinois Environmental Protection Agency, about 40 percent of hydrocarbon emissions in the air is due to evaporation of gasoline from leaky gas caps. The instrument used for testing consists of a short, wide tube with a pressurized gauge attached. One end of the tube connects to the uncovered gas cap on the vehicle, and the gas cap that was removed is screwed onto the other end. Pressure is applied and the reading on the gauge reveals any leaks. INDIAN DRIVING CYCLES & EMISSION NORMS Table 1: Indian Emission Standards (4-Wheel Vehicles) Standard
Reference
Date
Region
India 2000
Euro 1
2000
Nationwide
2001
NCR*, Mumbai, Kolkata, Chennai
2003.04
NCR*, 13 Cities†
2005.04
Nationwide
2005.04
NCR*, 13 Cities†
2010.04
Nationwide
2010.04
NCR*, 13 Cities†
Bharat Stage II
Euro 2
Bharat Stage III
Euro 3
Bharat Stage IV
Euro 4
* National Capital Region (Delhi) † Mumbai, Kolkata, Chennai, Bengaluru, Hyderabad, Ahmedabad, Pune, Surat, Kanpur, Lucknow, Sholapur, Jamshedpur and Agra The above standards apply to all new 4-wheel vehicles sold and registered in the respective regions. In addition, the National Auto Fuel Policy introduces certain emission requirements for interstate buses with routes originating or terminating in Delhi or the other 10 cities. For 2-and 3-wheelers, Bharat Stage II (Euro 2) will be applicable from April 1, 2005 and Stage III (Euro 3) standards would come in force from April 1, 2010. TRUCKS AND BUSES
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Exhaust gases from vehicles form a significant portion of air pollution which is harmful to human health and the environment Emission standards for new heavy-duty diesel engines—applicable to vehicles of GVW > 3,500 kg—are listed in Table 2. Table 2 Emission Standards for Diesel Truck and Bus Engines, g/kWh
Year
Reference
Test
CO
HC
NOx
PM
1992
-
ECE R49
17.3-32.6
2.7-3.7
-
-
1996
-
ECE R49
11.20
2.40
14.4
-
2000
Euro I
ECE R49
4.5
1.1
8.0
0.36*
2005†
Euro II
ECE R49
4.0
1.1
7.0
0.15
2010†
Euro III
ESC
2.1
0.66
5.0
0.10
ETC
5.45
0.78
5.0
0.16
2010‡
Euro IV
ESC
1.5
0.46
3.5
0.02
ETC
4.0
0.55
3.5
0.03
* 0.612 for engines below 85 kW † earlier introduction in selected regions, see Table 1 ‡ only in selected regions, see Table 1 More details on Euro I-III regulations can be found in the EU heavy-duty engine standards page. LIGHT DUTY DIESEL VEHICLES Emission standards for light-duty diesel vehicles (GVW ≤ 3,500 kg) are summarized in Table 3. Ranges of emission limits refer to different classes (by reference mass) of light commercial vehicles; compare the EU light-duty vehicle emission standards page for details on the Euro 1 and later standards. The lowest limit in each range applies to passenger cars (GVW ≤ 2,500 kg; up to 6 seats). Table 3 Emission Standards for Light-Duty Diesel Vehicles, g/km
Year Reference CO
HC
HC+NOx NOx
PM
1992 -
17.3-32.6 2.7-3.7 -
-
-
1996 -
5.0-9.0
-
-
2000 Euro 1
2.72-6.90 -
-
2.0-4.0
0.97-1.70 0.14-0.25 -
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2005† Euro 2
1.0-1.5
2010† Euro 3
0.64 0.80 0.95
2010‡ Euro 4
0.50 0.63 0.74
-
0.7-1.2
0.08-0.17 -
-
0.56 0.72 0.86
0.50 0.65 0.78
0.05 0.07 0.10
-
0.30 0.39 0.46
0.25 0.33 0.39
0.025 0.04 0.06
† earlier introduction in selected regions, see Table 1 ‡ only in selected regions, see Table 1
The test cycle has been the ECE + EUDC for low power vehicles (with maximum speed limited to 90 km/h). Before 2000, emissions were measured over an Indian test cycle. Engines for use in light-duty vehicles can be also emission tested using an engine dynamometer. The respective emission standards are listed in Table 4. Table 4 Emission Standards for Light-Duty Diesel Engines, g/kWh Year
Reference
CO
HC
NOx
PM
1992
-
14.0
3.5
18.0
-
1996
-
11.20
2.40
14.4
-
2000
Euro I
4.5
1.1
8.0
0.36*
2005†
Euro II
4.0
1.1
7.0
0.15
* 0.612 for engines below 85 kW † earlier introduction in selected regions, see Table 1 [edit]Light duty gasoline vehicles [edit]4-wheel vehicles Emissions standards for gasoline vehicles (GVW ≤ 3,500 kg) are summarized in Table 5. Ranges of emission limits refer to different classes of light commercial vehicles (compare the EU light-duty vehicle emission standards page). The lowest limit in each range applies to passenger cars (GVW ≤ 2,500 kg; up to 6 seats). Table 5 Emission Standards for Gasoline Vehicles (GVW ≤ 3,500 kg), g/km Year
Reference
CO
HC
HC+NOx
NOx
1991
-
14.3-27.1
2.0-2.9
-
1996
-
8.68-12.4
-
3.00-4.36
1998*
-
4.34-6.20
-
1.50-2.18
2000
Euro 1
2.72-6.90
-
0.97-1.70
2005†
Euro 2
2.2-5.0
-
0.5-0.7
2010†
Euro 3
2.3 4.17 5.22
0.20 0.25 0.29
-
0.15 0.18 0.21
2010‡
Euro 4
1.0
0.1
-
0.08
137
1.81 2.27
0.13 0.16
0.10 0.11
* for catalytic converter fitted vehicles † earlier introduction in selected regions, see Table 1 ‡ only in selected regions, see Table 1 Gasoline vehicles must also meet an evaporative (SHED) limit of 2 g/test (effective 2000). 3- and 2-wheel vehicles Emission standards for 3- and 2-wheel gasoline vehicles are listed in the following tables. Table 6 Emission Standards for 3-Wheel Gasoline Vehicles, g/km Year
CO
HC
HC+NOx
1991
12-30
8-12
-
1996
6.75
-
5.40
2000
4.00
-
2.00
2005 (BS II)
2.25
-
2.00
2010.04 (BS III)
1.25
-
1.25
Table 7 Emission Standards for 2-Wheel Gasoline Vehicles, g/km Year
CO
HC
HC+NOx
1991
12-30
8-12
-
1996
5.50
-
3.60
2000
2.00
-
2.00
2005 (BS II)
1.5
-
1.5
2010.04 (BS III)
1.0
-
1.0
Table 8 Emission Standards for 2- And 3-Wheel Diesel Vehicles, g/km Year
CO
HC+NOx
PM
2005.04
1.00
0.85
0.10
2010.04 0.50 0.50 Overview of the emission norms in India
0.05
1991 - Idle CO Limits for Gasoline Vehicles and Free Acceleration Smoke for Diesel Vehicles, Mass Emission Norms for Gasoline Vehicles. 1992 - Mass Emission Norms for Diesel Vehicles. 1996 - Revision of Mass Emission Norms for Gasoline and Diesel Vehicles, mandatory fitment of Catalytic Converter for Cars in Metros on Unleaded Gasoline. 1998 - Cold Start Norms Introduced.
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2000 - India 2000 (Equivalent to Euro I) Norms, Modified IDC (Indian Driving Cycle), Bharat Stage II Norms for Delhi. 2001 - Bharat Stage II (Equivalent to Euro II) Norms for All Metros, Emission Norms for CNG & LPG Vehicles. 2003 - Bharat Stage II (Equivalent to Euro II) Norms for 13 major cities. 2005 - From 1 April Bharat Stage III (Equivalent to Euro III) Norms for 13 major cities. 2010 - Bharat Stage III Emission Norms for 4-wheelers for entire country whereas Bharat Stage - IV (Equivalent to Euro IV) for 13 major cities. Bharat Stage IV also has norms on OBD (similar to Euro III but diluted) CO2 emission India’s auto sector accounts for about 18 per cent of the total CO 2 emissions in the country. Relative CO2 emissions from transport have risen rapidly in recent years, but like the EU, currently there are no standards for CO2 emission limits for pollution from vehicles. Obligatory labeling There is also no provision to make the CO 2 emissions labeling mandatory on cars in the country. A system exists in the EU to ensure that information relating to the fuel economy and CO 2 emissions of new passenger cars offered for sale or lease in the Community is made available to consumers in order to enable consumers to make an informed choice.
QUESTION BANK OF UNIT III PART A 1. What are the harmful effects of various pollutants on human beings? 2. How unburnt hydrocarbons are formed in SI engines? 3. What is the effect of inlet air temperature on “NO” formation in SI engines? 4. How aldehyde emissions are formed in CI engines? 5. What is the effect of quantity of fuel injected on black smoke formation in SI engines? 6. What is a thermal reactor? What for it is used? 7. Name few catalysts used for oxidation and reduction of emissions in IC Engine exhaust. 8. What are the sources of pollution in I.C. Engine? 9. What are the factors contributing to NOx emission? 10. What is particulate emission? 11. What is aldehyde emission? 12. What is meant by fumigation? 13. What is gas chromatograph. 14. What are the major emissions that come out of engine exhaust. 15. What are the causes of HC emissions from SI engine. 16. What are the causes of incomplete combustion in an engine. 17. What is crankcase blow-by. 18. Write a short notes on HC emission in CI engine.
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19. What are various Non-Exhaust emissions in CI engine. 20. What is positive crankcase ventilation. 21. What are the major emissions that come out of engine exhaust. 22. How oxides of Nitrogen are formed. 23. What are the causes of HC emissions from SI engine. 24. What are the causes of incomplete combustion in an engine. 25. What is crankcase blow-by. 26. Write a short notes on HC emission in CI engine. 27. What are various Non-Exhaust emissions in CI engine. 28. What is positive crankcase ventilation 29. What are the sources from which pollutants are emitted from SI engine. 30. How compression ratio affects emission control in SI engine. 31. What are the basic requirements of a catalytic converter. 32. What is a three way catalytic converter. 33. What are the various ways of controlling Oxides of nitrogen in exhaust emission 34. Write a short notes on Green house effect. 35. What is Diesel Smoke 36. What are the various types of smoke meters. 37. What is the effect of CO emission on human health. 38. What are the factors that affect the formation of NOx. 39. How are Hydro Carbons formed Part B 1. Explain in detail about the various sources of IC engine responsible for emission and pollutant formation. 2. Illustrate and explain the effects of combustion time and spark timing on “NO” formation in SI engines. 3. How the carbon monoxide is formed in SI engines? Explain the effect of O 2 concentration on “CO” formation in SI engines. 4. Explain in detail about the combined effect of engine speed, cetane number and combustion time on nitric oxide formation in CI engines. 5. Explain in detail about the following: (i) Blake smoke (ii) White smoke (iii) Blue smoke (iv) Particulates. 6. With the aid of a neat cross sectional sketch explain the construction, working and limitations of a 3 way catalytic converter. 7. Explain in detail about the open and closed type PVC system. Also state that what for it is used. 8. With the help of a schematic diagram explain the working of CLA (Chemiluminescent analyzer). What for it is used? 9. Explain the working of an instrument used for the measurement of CO and CO 2 from IC engines. 10. What are the operating variables and how they contribute on emission formation? How can it be controlled? 11.Differentiate smoke and particulate emission. Explain the diffusion funnel method to measure particulate emission in a diesel engine. 12. Explain different methods of smoke measurement. 13. (i)Explain PVC system in a diesel engine.
140
(ii)What is the necessity of exhaust gas re-circulation? Explain in detail. 14. (i) What are thermal reactors? Explain their constructions. (ii) Explain three way catalytic converter.
UNIT IV ALTERNATE FUELS
141
142
143
144
145
146
147
148
149
150
151
152
Problems with alcohol blends :
Advantages of alcohol as Fuel :
153
Fumigation :
Surface ignition of ethanol :
154
155
156
Hydrogen Advantages of Hydrogen as SI engine fuel
Disadvantages of Hydrogen as SI engine fuel
157
Combustion properties of hydrogen as fuel
158
Various ways of using Hydrogen as fuel for SI engine
159
160
161
Natural Gas
Methods of using Natural gas in engines
162
Conversion kits for use of natural gas in CI engine :
163
164
165
166
167
Liquefied Petroleum Gas
168
Bio Gas
Systems required for use of Bio Gas in engines
169
Use of Bio gas in diesel engine :
170
171
172
173
Bio – Diesel
Esterification
174
Transesterification
Advantages of bio diesel
175
Engine modifications:
176
Esterification process
177
Emission characteristics
178
QUESTION BANK OF UNIT IV Part A 1. Why alcohols are considered as good SI engine fuels.
179
2. Mention important property changes that take place by the addition of alcohol to diesel. 3. Define Fumigation 4. What are the advantages of Hydrogen as fuel in SI engine 5. What are the demerits of using Hydrogen as fuel in SI engine 6. What are the methods of using Natural gas in CI engines. 7. What are the advantages of using LPG in engines. 8. What are the limitations of using LPG as fuel in IC engines. 9. Give the general composition of Bio gas. 10. What is Bio diesel. 11. What is meant by esterification. 12. What is meant by Transesterification. 13. What are the advantages of Bio diesel. 14. Name some alternative fuels for IC engine. 15. What is the reason for development of alternative fuels. 16. What are the advantages of using alternative fuels 17. What are the disadvantages of alternative fuels. 18. What is volatility 19. Why volatility is an important quality of SI engine fuel. 20. What are the advantages of alcohol as a fuel. 21. What are the advantages of using Bio-diesel as fuel for IC engines. 22. What are the disadvantages of using Bio-diesel as fuel for IC engines. 23. What are the properties of Biogas. 24. What are the advantages of using Biogas as fuel. 25. What are the disadvantages of using Biogas as fuel for IC engine. 26. What are primary fuels 27. What are secondary fuels. 28. What is the effect of fuel viscosity on diesel engine performance.
Part B 1. What are the methods of using alcohol fuels in diesel engine and explain any one method. 2. What are the methods of using Hydrogen in SI engine. 3. Discuss the emission and performance characteristics using hydrogen in SI engines. 4. Explain the conversion kit required for use of natural gas in CI engines. 5. Explain how Biogas is used in diesel engine. Discus the performance characteristics. 6. What is the engine modification required for using Biodiesel in existing diesel engine. 7. Discuss the suitability of Alcohols in diesl engines 8. With a schematic diagram explain the production of Methanol from coal 9. With a schematic diagram explain the production of Methanol from Municipal solid waste 10. Explain the engine performance of Ethanol over a petrol engine.
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UNIT V RECENT TRENDS HCCI engine Homogeneous charge compression ignition (HCCI) is a form of internal combustion in which well-mixed fuel and oxidizer (typically air) are compressed to the point of auto-ignition. As in other forms of combustion, this exothermic reaction releases chemical energy into a sensible form that can be transformed in an engine into work and heat. Introduction HCCI has characteristics of the two most popular forms of combustion used in SI engines: homogeneous charge spark ignition (gasoline engines) and CI engines: stratified charge compression ignition (diesel engines). As in homogeneous charge spark ignition, the fuel and oxidizer are mixed together. However, rather than using an electric discharge to ignite a portion of the mixture, the density and temperature of the mixture are raised by compression until the entire mixture reacts spontaneously. Stratified charge compression ignition also relies on temperature and density increase resulting from compression, but combustion occurs at the boundary of fuel-air mixing, caused by an injection event, to initiate combustion. The defining characteristic of HCCI is that the ignition occurs at several places at a time which makes the fuel/air mixture burn nearly simultaneously. There is no direct initiator of combustion. This makes the process inherently challenging to control. However, with advances in microprocessors and a physical understanding of the ignition process, HCCI can be controlled to achieve gasoline engine-like emissions along with diesel engine-like efficiency. In fact, HCCI engines have been shown to achieve extremely low levels of Nitrogen oxide emissions (NO x) without an aftertreatment catalytic converter. The unburned hydrocarbon and carbon monoxide emissions are still high (due to lower peak temperatures), as in gasoline engines, and must still be treated to meet automotive emission regulations. Recent research has shown that the use of two fuels with different reactivities (such as gasoline and diesel) can help solve some of the difficulties of controlling HCCI ignition and burn rates. RCCI or Reactivity Controlled Compression Ignition has been demonstrated to provide highly efficient, low emissions operation over wide load and speed ranges *. HCCI engines have a long history, even though HCCI has not been as widely implemented as spark ignition or diesel injection. It is essentially an Otto combustion cycle. In fact, HCCI was popular before electronic spark ignition was used. One example is the hot-bulb engine which used a hot vaporization chamber to help mix fuel with air. The extra heat combined with compression induced the conditions for combustion to occur. Another example is the "diesel" model aircraft engine.
181
Operation Methods A mixture of fuel and air will ignite when the concentration and temperature of reactants is sufficiently high. The concentration and/or temperature can be increased by several different ways:
182
• •
High compression ratio Pre-heating of induction gases
•
Forced induction
•
Retained or re-inducted exhaust gases
•
Once ignited, combustion occurs very quickly. When auto-ignition occurs too early or with too much chemical energy, combustion is too fast and high in-cylinder pressures can destroy an engine. For this reason, HCCI is typically operated at lean overall fuel mixtures.
Advantages
• •
HCCI provides up to a 30-percent fuel savings, while meeting current emissions standards. Since HCCI engines are fuel-lean, they can operate at a Diesel-like compression ratios (>15), thus achieving higher efficiencies than conventional spark-ignited gasoline engines.
•
Homogeneous mixing of fuel and air leads to cleaner combustion and lower emissions. Actually, because peak temperatures are significantly lower than in typical spark ignited engines, NOx levels are almost negligible. Additionally, the premixed lean mixture does not produce soot.
•
HCCI engines can operate on gasoline, diesel fuel, and most alternative fuels.
•
In regards to gasoline engines, the omission of throttle losses improves HCCI efficiency.
Disadvantages • •
High in-cylinder peak pressures may cause damage to the engine. High heat release and pressure rise rates contribute to engine wear.
•
The autoignition event is difficult to control, unlike the ignition event in spark ignition (SI) and diesel engines which are controlled by spark plugs and in-cylinder fuel injectors, respectively.
•
HCCI engines have a small power range, constrained at low loads by lean flammability limits and high loads by in-cylinder pressure restrictions.
•
Carbon monoxide (CO) and hydrocarbon (HC) pre-catalyst emissions are higher than a typical spark ignition engine, caused by incomplete oxidation (due to the rapid combustion event and low in-cylinder temperatures) and trapped crevice gases, respectively.
Control
183
Controlling HCCI is a major hurdle to more widespread commercialization. HCCI is more difficult to control than other popular modern combustion engines, such as Spark Ignition (SI) and Diesel. In a typical gasoline engine, a spark is used to ignite the pre-mixed fuel and air. In Diesel engines, combustion begins when the fuel is injected into compressed air. In both cases, the timing of combustion is explicitly controlled. In an HCCI engine, however, the homogeneous mixture of fuel and air is compressed and combustion begins whenever the appropriate conditions are reached. This means that there is no welldefined combustion initiator that can be directly controlled. Engines can be designed so that the ignition conditions occur at a desirable timing. To achieve dynamic operation in an HCCI engine, the control system must change the conditions that induce combustion. Thus, the engine must control either the compression ratio, inducted gas temperature, inducted gas pressure, fuel-air ratio, or quantity of retained or re-inducted exhaust. Several control approaches are discussed below.
Variable compression ratio There are several methods for modulating both the geometric and effective compression ratio. The geometric compression ratio can be changed with a movable plunger at the top of the cylinder head. This is the system used in "diesel" model aircraft engines. The effective compression ratio can be reduced from the geometric ratio by closing the intake valve either very late or very early with some form of variable valve actuation (i.e. variable valve timing permitting Miller cycle). Both of the approaches mentioned above require some amounts of energy to achieve fast
responses. Additionally, implementation is expensive. Control of an HCCI engine using variable compression ratio strategies has been shown effective. The effect of compression ratio on HCCI combustion has also been studied extensively.
Variable induction temperature In HCCI engines, the autoignition event is highly sensitive to temperature. Various methods have been developed which use temperature to control combustion timing. The simplest method uses resistance heaters to vary the inlet temperature, but this approach is slow (cannot change on a cycle-to-cycle basis). Another technique is known as fast thermal management (FTM). It is accomplished by rapidly varying the cycle to cycle intake charge temperature by rapidly mixing hot and cold air streams. It is also expensive to implement and has limited bandwidth associated with actuator energy.
Variable exhaust gas percentage Exhaust gas can be very hot if retained or re-inducted from the previous combustion cycle or cool if recirculated through the intake as in conventional EGR systems. The exhaust has dual effects on HCCI combustion. It dilutes the fresh charge, delaying ignition and reducing the chemical energy and engine work. Hot combustion products conversely will increase the temperature of the gases in the cylinder and advance ignition. Control of combustion timing HCCI engines using EGR has been shown experimentally. Variable valve actuation
184
Variable valve actuation (VVA) has been proven to extend the HCCI operating region by giving finer control over the temperature-pressure-time history within the combustion chamber. VVA can achieve this via two distinct methods: •
•
Controlling the effective compression ratio: A variable duration VVA system on intake can control the point at which the intake valve closes. If this is retarded past bottom dead center (BDC), then the compression ratio will change, altering the in-cylinder pressure-time history prior to combustion. Controlling the amount of hot exhaust gas retained in the combustion chamber: A VVA system can be used to control the amount of hot internal exhaust gas recirculation (EGR) within the combustion chamber. This can be achieved with several methods, including valve re-opening and changes in valve overlap. By balancing the percentage of cooled external EGR with the hot internal EGR generated by a VVA system, it may be possible to control the in-cylinder temperature.
While electro-hydraulic and camless VVA systems can be used to give a great deal of control over the valve event, the componentry for such systems is currently complicated and expensive. Mechanical variable lift and duration systems, however, although still being more complex than a standard valvetrain, are far cheaper and less complicated. If the desired VVA characteristic is known, then it is relatively simple to configure such systems to achieve the necessary control over the valve lift curve. Also see variable valve timing.
Variable fuel ignition quality Another means to extend the operating range is to control the onset of ignition and the heat release rate is by manipulating fuel itself. This is usually carried out by adopting multiple fuels and blending them "on the fly" for the same engine . Examples could be blending of commercial gasoline and diesel fuels , adopting natural gas or ethanol ". This can be achieved in a number of ways;
•
Blending fuels upstream of the engine: Two fuels are mixed in the liquid phase, one with low resistance to ignition (such as diesel fuel) and a second with a greater resistance (gasoline), the timing of ignition is controlled by varying the compositional ratio of these fuels. Fuel is then delivered using either a port or direct injection event.
•
Having two fuel circuits: Fuel A can be injected in the intake duct (port injection) and Fuel B using a direct injection (in-cylinder) event, the proportion of these fuels can be used to control ignition, heat release rate as well as exhaust gas emissions.
Learn burn engines Principle A lean burn mode is a way to reduce throttling losses. An engine in a typical vehicle is sized for providing the power desired for acceleration, but must operate well below that point in normal steady-speed operation. Ordinarily, the power is cut by partially closing a throttle. However, the extra work done in
185
pumping air through the throttle reduces efficiency. If the fuel/air ratio is reduced, then lower power can be achieved with the throttle closer to fully open, and the efficiency during normal driving (below the maximum torque capability of the engine) can be higher.
The engines designed for lean burning can employ higher compression ratios and thus provide better performance, efficient fuel use and low exhaust hydrocarbon emissions than those found in conventional petrol engines. Ultra lean mixtures with very high air-fuel ratios can only be achieved by direct injection engines.
The main drawback of lean burning is that a complex catalytic converter system is required to reduce NOx emissions. Lean burn engines do not work well with modern 3-way catalytic converter—which require a pollutant balance at the exhaust port so they can carry out oxidation and reduction reactions—so most modern engines run at or near the stoichiometric point. Alternatively, ultra-lean ratios can reduce NOx emissions.
Heavy-duty gas engines Lean burn concepts are often used for the design of heavy-duty natural gas, biogas, and liquefied petroleum gas (LPG) fuelled engines. These engines can either be full-time lean burn, where the engine runs with a weak air-fuel mixture regardless of load and engine speed, or part-time lean burn (also known as "lean mix" or "mixed lean"), where the engine runs lean only during low load and at high engine speeds, reverting to a stoichiometric air-fuel mixture in other cases.
Heavy-duty lean burn gas engines admit as much as 75% more air than theoretically needed for complete combustion into the combustion chambers. The extremely weak air-fuel mixtures lead to lower combustion temperatures and therefore lower NOx formation. While lean-burn gas engines offer higher theoretical thermal efficiencies, transient response and performance may be compromised in certain situations. Lean burn gas engines are almost always turbocharged, resulting high power and torque figures not achieveable with stoichiometric engines due to high combustion temperatures.
Heavy duty gas engines may employ pre-combustion chambers in the cylinder head. A lean gas and air mixture is first highly compressed in the main chamber by the piston. A much richer, though much lesser volume gas/air mixture is introduced to the pre-combustion chamber and ignited by spark plug. The flame front spreads to the lean gas air mixture in the cylinder.
This two stage lean burn combustion produces low NOx and no particulate emissions. Thermal efficiency is better as higher compression ratios are achieved.
Manufacturers of heavy-duty lean burn gas engines include GE Jenbacher, MAN Diesel & Turbo, Wärtsilä, Mitsubishi Heavy Industries and Rolls-Royce plc.
186
Honda lean burn systems One of the newest lean-burn technologies available in automobiles currently in production uses very precise control of fuel injection, a strong air-fuel swirl created in the combustion chamber, a new linear air-fuel sensor (LAF type O2 sensor) and a lean-burn NOx catalyst to further reduce the resulting NOx emissions that increase under "lean-burn" conditions and meet NOx emissions requirements.
This stratified-charge approach to lean-burn combustion means that the air-fuel ratio isn't equal throughout the cylinder. Instead, precise control over fuel injection and intake flow dynamics allows a greater concentration of fuel closer to the spark plug tip (richer), which is required for successful ignition and flame spread for complete combustion. The remainder of the cylinders' intake charge is progressively leaner with an overall average air:fuel ratio falling into the lean-burn category of up to 22:1.
The older Honda engines that used lean burn (not all did) accomplished this by having a parallel fuel and intake system that fed a pre-chamber the "ideal" ratio for initial combustion. This burning mixture was then opened to the main chamber where a much larger and leaner mix then ignited to provide sufficient power. During the time this design was in production this system (CVCC, Compound Vortex Controlled Combustion) primarily allowed lower emissions without the need for a catalytic converter. These were carbureted engines and the relative "imprecise" nature of such limited the MPG abilities of the concept that now under MPI (Multi-Port fuel Injection) allows for higher MPG too.
The newer Honda stratified charge (lean burn engines) operate on air-fuel ratios as high as 22:1. The amount of fuel drawn into the engine is much lower than a typical gasoline engine, which operates at 14.7:1—the chemical stoichiometric ideal for complete combustion when averaging gasoline to the petrochemical industries' accepted standard of C6H8. This lean-burn ability by the necessity of the limits of physics, and the chemistry of combustion as it applies to a current gasoline engine must be limited to light load and lower RPM conditions. A "top" speed cut-off point is required since leaner gasoline fuel mixtures burn slower and for power to be produced combustion must be "complete" by the time the exhaust valve open Stratified charge engines In a stratified charge engine, the fuel is injected into the cylinder just before ignition. This allows for higher compression ratios without "knock," and leaner air/fuel mixtures than in conventional internal combustion engines.
Conventionally, a four-stroke (petrol or gasoline) Otto cycle engine is fuelled by drawing a mixture of air and fuel into the combustion chamber during the intake stroke. This produces a homogeneous charge: a homogeneous mixture of air and fuel, which is ignited by a spark plug at a predetermined moment near the top of the compression stroke.
187
In a homogeneous charge system, the air/fuel ratio is kept very close to stoichiometric. A stoichiometric mixture contains the exact amount of air necessary for a complete combustion of the fuel. This gives stable combustion, but places an upper limit on the engine's efficiency: any attempt to improve fuel economy by running a lean mixture with a homogeneous charge results in unstable combustion; this impacts on power and emissions, notably of nitrogen oxides or NOx.
If the Otto cycle is abandoned, however, and fuel is injected directly into the combustion-chamber during the compression stroke, the petrol engine is liberated from a number of its limitations.
First, a higher mechanical compression ratio (or, with supercharged engines, maximum combustion pressure) may be used for better thermodynamic efficiency. Since fuel is not present in the combustion chamber until virtually the point at which combustion is required to begin, there is no risk of pre-ignition or engine knock.
The engine may also run on a much leaner overall air/fuel ratio, using stratified charge.
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Combustion can be problematic if a lean mixture is present at the spark-plug. However, fueling a petrol engine directly allows more fuel to be directed towards the spark-plug than elsewhere in the combustionchamber. This results in a stratified charge: one in which the air/fuel ratio is not homogeneous throughout the combustion-chamber, but varies in a controlled (and potentially quite complex) way across the volume of the cylinder.
A relatively rich air/fuel mixture is directed to the spark-plug using multi-hole injectors. This mixture is sparked, giving a strong, even and predictable flame-front. This in turn results in a high-quality combustion of the much weaker mixture elsewhere in the cylinder.
Direct fuelling of petrol engines is rapidly becoming the norm, as it offers considerable advantages over port-fuelling (in which the fuel injectors are placed in the intake ports, giving homogeneous charge), with no real drawbacks. Powerful electronic management systems mean that there is not even a significant cost penalty.
With the further impetus of tightening emissions legislation, the motor industry in Europe and north America has now switched completely to direct fuelling for the new petrol engines it is introducing.
It is worth comparing contemporary directly-fuelled petrol engines with direct-injection diesels. Petrol can burn faster than diesel fuel, allowing higher maximum engine speeds and thus greater maximum power for sporting engines. Diesel fuel, on the other hand, has a higher energy density, and in combination with higher combustion pressures can deliver very strong torque and high thermodynamic efficiency for more 'normal' road vehicles. Four-valve engines
‡ A 4-Valve engine is designed for better performance than a regular 2-Valve engine ‡ More power: The 4-valve provides for a greater intake and exhaust area resulting in more power More Mileage: 4-valve not only enhances the performance but also returns a very good fuel economy ‡ More green: Comfortably meets BSIII regulations What is the 4-valve engine? ‡ An engine that has valves that let the air-fuel mixture into the combustion chamber to be burned and then draw out the exhaust gas after the combustion. A conventional engine has one intake valve to let in the air-fuel mixture and one exhaust valve to let out the
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exhaust gases. But the 4-valve engine has two intake and two exhaust valves. A typical 2-valve engine has just 1/3 combustion chamber head area covered by the valves, but a 4-valve head increases that to more than 50%, hence smoother and quicker breathing. ‡ 4-valve design also benefits from a clean and effective combustion, because the spark plug can be placed in the middle. ‡ 4 valves are better to be driven by twin-cam, one for intake valves and one for exhaust valves. 4-valve type and its shape is designed with the minimum area necessary for the two intake and two exhaust valves. ‡ At the same time it is designed with a minimum intake and exhaust valve angles to realize an optimum combustion chamber shape.
M ERITS:‡ The first merit of 4-valve design is that it allows the spark plug to be positioned in the center of the combustion chamber to provide more efficient flame spread and combustion. In other words, it enables highly efficient combustion. ‡ Also, the 4-valve design enables greater overall valve area than a 2-valve system for more efficient (per unit of area) intake and exhaust function. This is the second major merit. COMPARISION OF 4-VALVES OVER 2-VALVES ‡ A conventional engine has one intake valve to let in the air-fuel mixture and one exhaust valve to let out the exhaust gases. But the 4-valve engine has two intake and two exhaust valves ‡ 4-valves is better than two because 4-valves give an engine steadier low-speed performance and a better acceleration feeling ‡ That is why most race engines and high-performance engines have four valves. For example, Yamaha¶s YZR-M1 MotoGP race machine has four valves.
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The name of the game is velocity and turbulence/mixing of the intake charge at differing engine speeds. At low engine speeds, one intake valve gives increased velocity, hence better gas mixing and better cylinder filling. If you open a second intake valve at low engine speed, the velocity drops dramatically, leaving poor intake filling and a lean intake charge. The result is engine knock and less torque. On the other hand, at high RPM, breathing is the name of the game. The valves are open such a short length of time you need the maximum available intake area. Therefore 2 intake valves work better at high RPM. All of this is of course subject to the exhaust system design. A proper extractor exhaust can make a significant difference on a two valve system and a restrictive exhaust can nullify all the gains of a 4 valve system.
‡ When you have only 2 valves, the air/fuel mixture entering the cylinder can be tangential to the circle of the cylinder, giving a high degree of swirl, better air/fuel mixing and hence better performance at lower revs in an SI engine. At higher revs, enough turbulence is available to create good mixing, and so 4 valves are better, as they allow greater airflow.
OHV engine design OHV means OverHead Valve - an engine design where the camshaft is installed inside the engine block and valves are operated through lifters, pushrods and rocker arms (an OHV engine also known as a "Pushrod" engine). Although an OHV design is a bit outdated, it has been successfully used for decades. An OHV engine is very simple, it has more compact size and proven to be durable. On the downside, it's difficult to precisely control the valve timing at high rpm due to higher inertia caused by larger amount of valve train components (lifter-pushrod-rocker arm). Also, it's very difficult to install more than 2 valves per cylinder, or implement some of the latest technologies such as Variable Valve
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Timing - something that could be easily done in a DOHC engine.
OHC or SOHC engine
4-cylinder 8-valve SOHC engine
OHC in general means OverHead Cam while SOHC means Single OverHead Cam. In a SOHC engine the camshaft is installed in the cylinder head and valves are operated either by the rocker arms or directly through the lifters (as in the picture). The advantage is that valves are operated almost directly by the camshaft, which makes it easy to achieve the perfect timing at high rpm. It's also possible to install three or four valves per cylinder The disadvantage is that an OHC engine requires a timing belt or chain with related components, which is more complex and more expensive design.
DOHC or Twin Cam engine DOHC means Double OverHead Cam, or sometimes it could be called "Twin Cam". A DOHC setup is used in most of newer cars. Since it's possible to install multiple valves per cylinder and place intake valves on the opposite side from exhaust vales, a DOHC engine can "breathe" better, meaning that it can produce more horsepower with smaller engine volume. Compare: The 3.5-liter V6 DOHC engine of 2003 Nissan Pathfinder has 240 hp, similar to 245 hp of the 5.9-liter V8 OHV engine of 2003 Dodge Durango. Pros: High efficiency, possible to install multiple valves per cylinder and adopt variable timing. Cons: More complex and more expensive design. 4-cylinder 16-valve DOHC engine
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Electronic Engine Management electronic control systems engines are subject to very high stresses during compression and ignition, and increasingly stringent emission standards have made better control of the diesel combustion process necessary. Electronic controlled diesel systems give very precise control of the fuel injection and combustion process. Electronic controls have delivered other benefits besides a reduction in fuel consumption and emissions, such as an increase in power and torque; improved engine responsiveness; a reduction in engine noise and diesel knock; and improved and expanded diagnostic capabilities through the use of scan tools. electronic control systems monitor and control many variables, including: •
•
Engine speed: o to maintain a smooth functional idle, o
and to limit the maximum safe engine speed, power, and torque;
o
and to keep the engine output to within safe limits.
Fuel injector operation: o
•
Glow plugs and heater elements: o
•
Control of pre-heating of the intake air to support quick cold starting and reduced cold run emissions.
Exhaust emissions: o
•
including the timing, rate and volume of fuel injected.
Analysis of exhaust gas to determine combustion efficiency and pollutants.
And the data bus: o
An electronic communications network that allows exchange of data between computers necessary for efficient operation and fault diagnosis.
Other inputs monitored include: • •
crankshaft position, throttle position,
•
brake and clutch operation,
•
battery voltage,
•
cruise control request,
•
air, oil fuel, exhaust and coolant temperatures,
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•
and intake air, oil and fuel pressures.
The ECU is a micro-computer. It is constructed from printed circuitry, and contains a large number of electrical components, including many semiconductor devices.
Its input devices receive data as electrical signals. They come from sensors and components at various locations around the engine. Its processing unit compares incoming data with data stored in a memory unit. The memory unit contains basic data about how the engine is to operate. And an output device pulses the electrical circuit of the solenoid-type injection valves. It is normally located in a safe place, behind a kick-panel in the foot-well, under the passenger seat, or in the boot, and connected by a multi-plug, or plugs, to the vehicle’s wiring harness.
The core function of a basic ECU in an EFI system is to control the pulse width of the injector. More sophisticated models also control other functions such as idle speed, ignition timing, and the fuel pump. These wider systems are called engine management systems. The more precise control they allow is very effective in reducing fuel consumption and exhaust emissions.
The ECU adjusts quickly to changing conditions by using what are called programmed characteristic maps, stored in the memory unit. They are programmed into the ECU, just as data is programmed into a computer. Characteristics means the engine’s operating conditions. And they are called maps because they map all of the operating conditions for the engine.
They are constructed first from dynamometer tests, then fine-tuned, to optimise the operating conditions and to comply with emission regulations. This data is stored electronically.
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Ignition timing is crucial in this process. Between one spark and the next, the ECU uses data it receives on engine load and speed to determine when the next ignition point will occur. It can also correct the map value, using extra information such as engine coolant temperature, intake air temperature, or throttle position. Putting all of this together, it arrives at the best ignition point for that operating condition.
Common Rail Direct Injection or CRDI CRDI is an intelligent way of controlling a diesel engine with use of modern computer systems. CRDI helps to improve the power, performance and reduce harmful emissions from a diesel engine. Conventional Diesel Engines (non-CRDI engines) are sluggish, noisy and poor in performance compared to a CRDI engine.
CRDI or common rail direct injection system is also sometimes referred to by many similar or different names. Some brands use name CRDe / DICOR / Turbojet / DDIS / TDI etc. All these systems work on same principles with slight variations and enhancements here and there.
CRDI system uses common rail which is like one single rail or fuel channel which contains diesel compresses at high pressure. This is a called a common rail because there is one single pump which compresses the diesel and one single rail which contains that compressed fuel. In conventional diesel engines, there will be as many pumps and fuel rails as there are cylinders.
As an example, for a conventional 4 cylinder diesel engine there will be 4 fuel-pumps, 4 fuel rails each feeding to one cylinder. In CRDI, there will be one fuel rail for all 4 cylinders so that the fuel for all the cylinders is pressurized at same pressure.
The fuel is injected into each engine cylinder at a particular time interval based on the position of moving piston inside the cylinder. In a conventional non-CRDI system, this interval and the fuel quantity was determined by mechanical components, but in a CRDI system this time interval and timing etc are all controlled by a central computer or microprocessor based control system.
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To run a CRDI system, the microprocessor works with input from multiple sensors. Based on the input from these sensors, the microprocessor can calculate the precise amount of the diesel and the timing when the diesel should be injected inside the cylinder. Using these calculations, the CRDI control system delivers the right amount of diesel at the right time to allow best possible output with least emissions and least possible wastage of fuel.
The input sensors include throttle position sensor, crank position sensor, pressure sensor, lambda sensor etc. The use of sensors and microprocessor to control the engine makes most efficient use of the fuel and also improved the power, fuel-economy and performance of the engine by managing it in a much better way.
One more major difference between a CRDI and conventional diesel engine is the way the fuel Injectors are controlled. In case of a conventional Engine, the fuel injectors are controlled by mechanical components to operate the fuel injectors. Use of these mechanical components adds additional noise as there are many moving components in the injector mechanism of a conventional diesel engine. In case of a CRDI engine, the fuel injectors are operated using solenoid valves which operate on electric current and do not require complex and noisy mechanical arrangement to operate the fuel Injection into the cylinder. The solenoid valves are operated by the central microprocessor of the CRDI control system based on the inputs from the sensors used in the system. Gasoline Direct Injection Engine : Port fuel injected (PFI) engines are the most commonly used spark ignition (SI) engine in current vehicles. In certain
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markets, a very small number of direct-injected spark ignition (DISI) engines have been introduced. Both use gasoline fuel. In PFI engines, fuel is injected into the intake port near the closed intake valve, producing a well mixed fuel–air charge in the combustion chamber. This is the most commonly used engine type in current vehicles. These engines are typically operated with a stoichiometric fuel–air ratio, which is the ratio that permits complete conversion of the fuel and oxygen in the intake charge to form CO2 and H2O. As a result of the premixed combustion, it produces very low particulate emissions. The levels of other emissions directly leaving the engine are relatively high, and compliance with regulated emission standards relies on the effectiveness of the three-way catalyst, which reduces emissions by 95–99% as discussed in more detail below. In DISI engines, the fuel is injected directly into the combustion chamber. At higher load, the fuel is injected during the intake stroke to form a nearly homogeneous fuel– air mixture at the time of ignition. At lower load, the injection timing can be delayed until the compression stroke to produce a ‘‘stratified’’ fuel mixture. This mixture is ideally uniform, premixed, and stoichiometric near the center, and devoid of fuel near the cylinder walls. This spatial localization translates into a faster burn and allows the engine to be run more fuellean overall than PFI engines, providing improved fuel economy and better performance during transient acceleration/ deceleration. In practice, however, it is difficult to realize this idealized mixing, and fuel-rich and lean regions result, leading to reduced benefits. Additionally, because this engine injects fuel droplets directly into the combustion chamber, particulate emissions are increased substantially relative to PFI engines. Like PFI engines, DISI engines rely on catalytic devices to significantly reduce engine-out concentrations of regulated emissions.
1. The engines use injectors that can spray fuel directly into the cylinder during the compression stroke, along with an extremely high pressure fuel pump (2,000 PSI). Before GDI, it was far more common to use port fuel injection, where the injector sprayed fuel at low pressure into the intake manifold. 2. The direct injection process allows the fuel to evaporate in the cylinder and cool the air/fuel mixture. That helps avoid premature ignition, so… 3. These engines can increase the compression ratio. The Mazda engine goes to a 14:1 ratio, which has never been seen before in a production gasoline engine. The normal high is 12:1 or so, and that would require premium fuel. 4. Many of the engines are using multiple injector sprays per stroke. One spray occurs as the air starts flowing in on the intake stroke. The second occurs right before the spark plug fires. This creates a stratified charge of fuel for a better burn pattern.
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the advantages of higher compression ratios ,The higher the compression ratio, the more closely packed the molecules of fuel and air are when the mixture is ignited by the sparkplug, this causes a more powerful explosion by making a more violent reaction which produces more power. Higher compression makes the expansion ratio of the exploding hot gas greater which means that more energy is impinged on the piston top, pushing it down harder, making more power. Increasing the compression ratio improves the thermal efficiency of an engine and this is the primary reason why higher compression increases power. Improving thermal efficiency improves fuel economy from getting more power from the same amount of fuel and a reduction of combustion chamber surface area to volume. This means less wasted combustion heat and more expansion being used to drive the piston down.
UNIT V Part A 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
1. 2. 3. 4.
List the features of HCCI. List the features of surface ignition engines. Define charge stratification What are the advantages of Four valve engine. What is a Lambda sensor List out various sensors used on an IC engine. What is a Lean Burn engine. What are the advantages of Gasoline direct engine over conventional gasoline engine What is Surface Ignition. What are the advantages of stratified charge engines. What are the advantages of HCCI engine. What are the disadvantages of HCCI engine. What is the principle of Lean burn engines. What is a Four valve engine. What is a Over Head Valve engine. What is an electronic control unit.
Part B Explain the working of a Homogeneous charge combustion engine. Draw the sketch of an engine and locate various sensors used on it. Explain the working of Common rail direct injection diesel engine with a neat sketch. Explain the working of Gasoline direct injection engine.
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5. 6. 7. 8. 9.
Explain the working of a Four – Valve engine and state the advantages of it. Explain the working of a Lean Burn engine. Explain the working of a Stratified Charge engine. What are the methods of charge stratification What is a stratified charge engine. What are the advantages and disadvantages of stratified charge engine
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