HEAT ENGINES PRESENTATION
INTRODUCTION What is heat engine? An engine may be defined as a device which converts one form of energy into mechanical energy. energy. Mechanical energy can be further further easily converted into electrical energy which is the most required form of energy. energy.
So, Heat Engine is a device which transforms heat energy into mechanical energy. In every heat engine, some form of fuel is used to convert the chemical(heat) chemical(heat) energy energy into useful mechanical mechanical energy energy (or to produce mechanical work).
The Heat Engines are mainly classified into two types:1.
External combustion Engines, and
2.
Internal Combustion Engines
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TYPES OF HEAT ENGINES External Combustion Engines:In this engine, the products of combustion are air and fuel transfer heat to a second fluid which is working fluid of the cycle, as in the case of a steam engine or steam turbine plant. Ex:- The heat of combustion is employed to generate steam which is used in a piston engine or a turbine.
Internal Combustion Engine:In internal combustion engine, the products of combustion are directly the motive fluid. Petrol, gas and diesel engines, open cycle gas turbines are the examples of I. C. Engine. 3/28/11
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CLASSIFICATION OF HEAT ENGINES CLASSIFICATION
NAME OF ENGINE
RECIPROCATING/ROTARY
(A) Internal combustion Engines
Petrol Engine(SI)
Reciprocating
Gas Engine(SI)
Reciprocating
Diesel Engine(CI)
Reciprocating
Wank Wanklle Engi Engine ne(S (SII, CI)
Rotar otary y
Open Open Cycle Cycle Gas Turbin Turbine e
Rotary Rotary
Jet Engine
Rotary
Rocket
No Mechanism
Steam Engine
Reciprocating
Steam Turbine
Rotary
(B)External combustion engines
Sterling Sterling or Hot Hot Air Air Engine Engine Reciprocatin Reciprocating g Closed Cycle Turbi rbine
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ADVANTAGES OF HEAT ENGINES Advantages of Internal Combustion Engines are:1.
Greater mechanica mechanicall simplicity simpl icity..
2.
Lower weight and bulk to o/p ratio.
3.
Lower first cost.
4.
Higher overall ov erall efficiency eff iciency..
5.
Lesser requirements of water for dissipation of energy through cooling system.
Advantages of External Combustion Engines are:1.
Use of cheaper fuels.
2.
High starting torque.
3.
Higher weight and bulk to o/p ratio.
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COMPARISION BETWEEN COMPARISION INTERNAL & EXTERNAL COMBUSTION ENGINES
Parameter
I.C. Engine
External Combustion Engine
Small size upto 1 MW.
Large sizes
2)Size(weight and bulk)
Co Compact
Bulky
3)Fuel
Refined Fuel
Cheaper Fuels
4)Pollution level
Higher
Lower
5)Efficiency
Lower
Higher
6) Complixity of components
Less
More
7)Expansion
Incomplete
Complete
8)Heat Rejection
At higher temperature
At atm. temperature
1)
Sutablity
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ESSENTIAL ELEMENTS OF A HEAT ENGINE 1.
2.
3.
4.
5.
Working Substance:- It is a medium for receiving and rejection rejection heat. Heat Source:- It is reservoir from which the working substance receives heat. Heat Sink:- It is the reservoir where the working substance rejects heat after developing work. It is at a lower temperature than the source temperature. Expander:- It is an enclosure where the working substance does work. It may be either a cylinder or casing. Pump:- it is device used to raise the pressure of the working substance. It utilizes the work give out by the expander. The essential elements of a direct cycle heat engine are shown in fig.
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ESSENTIAL ELEMENTS OF HEA HEAT T CYCLE Qs HEAT SOURCE
PUMP
CONTROL SURFACE EXPANDE R
W
HEAT SINK
Qr
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CARNOT CYCLE Every thermodynamic system exists in a particular thermodynamic state. When a system is taken through a series of different states and finally returned to its initial state, a thermodynamic cycle is said to have occurred. In the process of going through this cycle, the system may perform work on its surroundings, thereby thereby acting as a heat engine. A system undergoing a Carnot cycle is called a Carnot heat engine, although such a 'perfect' engine is only theoretical and cannot be built in practice. The working of carnot cycle is shown in fig. The working substance is enclosed in a cylinder having frictionless piston. The walls of the cylinder and piston are taken as perfect insulators of heat. The cylinder head is assumed diathermic which permits the flow of heat. The heat source is at temperature T1 and heat sink at temperature t2. there is also a lso a work reservoir. reservoir. The carnot consists of the following four reversible processes:1.
Isothermal expansion
2.
Adiabatic expansion
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WORKING OF A CARNOT CYCLE
The carnot consists of the following four reversible processes:1.
Isothermal expansion
2.
Adiabatic expansion
3.
Isothermal compression
4.
Adiabatic compression
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CARNOT CYCLE ON P-V DIAGRAM Click to edit Master text styles Click to edit Master text styles
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CARNOT HEAT ENGINE PROCESS 1-2:- In the first stage, the piston moves downward while the engine absorbs heat from a source (T1)and (T1)an d gas begins to expand. The portion of the graphic from point 1 to point 2 represents this behavior. behavior. Because the temperature of the gas does not change, this kind of expansion is called exothermic. exothermic. (Heat absorption = Q1,T1=C) PROCESS 2-3:- In the second stage, the heat hea t source is removed; the piston continues to move downward and the t he gas is still expanding while lowering in temperature from T2 to T3. It is presented by the graphic from point 2 to point 3. This stage is called a adiabatic expansion. PROCESS 3-4:- The piston begins to move upward and the t he cool gas is recompressed recompressed in the third stage. The heat goes to sink. Point 3 point 4 represents represents the decrease in volume and increase in pressure. The engine gives energy to the environment. This stage is called isothermal compression. (Heat rejection =Q2, T2 = C) PROCESS 4-1:- In the final stage, the piston to move upward and the cool gas is secluded and compressed. Its temperature rises to its original state. Point 3 to point 4 illustrate this behavior; a continuing increase in pressure and
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EFFICIENCY OF CARNOT CYCLE Click to edit Master text styles Second level Third level Fourth level Fifth level ●
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EFFICIENCY OF CARNOT CYCLE Click to edit Master text styles Second level Third level Fourth level Fifth level ●
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RANKINE CYCLE Process 3-4:- The water from the hot well at low pressure p1 pumped isentropically into the boiler at high pressure p2.
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Process 4-5 :- This is the sensible heating of water along the hot water is heated up to the saturation temperature T1. The area a-3-4-5-b-a represents the heat supplied (hf5 – hf4) during this process.
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RANKINE CYCLE Process 5-1 :- The saturated water at pressure p1 and temperature T1 is completely vaporized into steam. The heat added (h1 – hf5) is represented by area b-5-1-c-b, i.e., the latent heat of vaporization. vaporization. The state point 1’ shows wet steam, 1 dry and saturated and 1’’ superheated condition of steam.
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Process 1-2 :- It is the isentropic expansion of steam in the turbine from p1 to p2. Process 2-3 :- The exhaust steam is condensed in the condenser at pressure p2 and temperature 3/28/11 JIGART2 MEVADA
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RANKINE CYCLE ON p-v DIAGRAM
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EFFICIENCY OF RANKINE CYCLE Consider 1 kg of the working fluid, we have (considering pump work).
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EFFICIENCY NEGLECTING PUMP WORK For this process assuming polytropic expansion, work done per kg of steam.
p-v diagram for neglecting pump work P P23
P 14
=
2
Adiab atic Proces s
1
(p1 v1 – p2 v2) V
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This cycle is composed of four internally reversible processes, two adiabatic and two constant volume processes. The p-v and T-s diagrams as shown in fig. Process 1-2:Adiabatic compression(Q=0), Process 2-3:- Heat Hea t addition at v=c, Process 3-4:- Adiabatic expansion (Q=0), Process 4-1:- Heat Rajection at v=c
WORKING OF OTTO CYCLE
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Otto cycle on p-v diagram
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EFFICIENCY OF OTTO CYCLE Consider 1 kg of air flowing through the cycle. Since the air in the cylinder acts as a closed system.So, from first law of thermodynamics for isen tropic compression and expansion, we have q-w = Δu
So, for v=c heat supplied and rejection processes, since ω = 0 then, q=Δu=cv ΔT
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EFFICIENCY OF OTTO CYCLE Click to edit Master text styles Second level Third level Fourth level Fifth level ●
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EFFICIENCY OF OTTO CYCLE Click to edit Master text styles Second level Third level Fourth level Fifth level ●
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The air standard efficiency of otto cycle depends on compression ratio only and increases as compression ratio increases. In actual engine the compression ratio varies from 5 to 8. This is used for S.I. engines working on petrol engine.
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M.E.P. OF OTTO CYCLE M.E.P. M.E.P. = Mean Effective Eff ective Pressure
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DIESEL CYCLE This cycle is used for compresssion ignition internal combustion engines working on diesel oil. The p-v and T-s diagrams are shown in fig. it consists of four internally reversible processes two adiabatic, one p=c and one v=c.
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WORKING OF DIESEL CYCLE Process 1-2:- Adiabatic compression of air, Process 2-3:- Heat addition at p=c Process 3-4:- Adiabatic expansion of air, Process 4-1:- Heat rejection at v=c.
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EFFIECIENCY EFFIECI ENCY OF DIESEL DIESEL CYCLE CYCLE Click to edit Master text styles Second level Third level Fourth level Fifth level ●
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The efficiency of diesel cycle increases as compression ratio increases but decreases as cut-off ratio increases. The thermal efficiency of diesel cycle is less then that of otto cycle. The compression ratio for diesel cycle varies from 14 to 18.
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M.E.P. OF DIESEL CYCLE Click to edit Master text styles Second level Third level Fourth level Fifth level ●
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