1.
Introduction
The spark plug's location exposes it to extreme temperature variations, chemicals, fuels and oils. It is also attacked by cylinder pressures produced by the piston and cam timing, then it is also assaulted by high-output ignition units. As a result of all of this, one can effectively learn what the engine is doing by reading the firing end of the spark plugs. By careful examination of the plug's color, gap, and any deposits that reside on it, you will be shown the efficiencies as well as deficiencies of what is going on in the engine. Spark plugs should be checked at least yearly, and replaced as often as necessary. In most cases you can follow the manufacturers’ recommendations, but in a race car, our replacement intervals are quite frequent.
2.
CONSTRUCTION
Each plug consists of a metallic rod, called an electrode, surrounded by a ceramic insulator. The lower end of the insulator is encased in a threaded metal shell that is screwed into the cylinder head. Another electrode is welded to the shell. it is separated from the center electrode by a small air gap. High-voltage current from the distributor flows down the center electrode and jumps this gap in the form of spark. Every engine has a specified spark plug gap, which may vary from .02 inches(.5mm) to .08 inches (2mm).
3. How a Spark Plug Works. The basics of a spark plug are that it must perform two primary functions. a. b.
To Ignite the Air/Fuel mixture To remove the heat out of the combustion chamber (1)
Spark plugs transmit electrical energy that turns fuel into working energy. A sufficient amount of voltage must be supplied by the ignition system to cause the spark to jump the across the spark plug gap, thus creating what is called Electrical Performance.
(2)
c.
Additionally, the temperature of the spark plug's firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. This is called Thermal Performance and is determined by the heat range of the spark plug.
It is important to understand that spark plugs cannot create heat, only remove it. The spark plug works as a heat exchanger, pulling unwanted thermal energy away from the combustion chamber and transferring the heat
to the engine's cooling system. The heat range is defined as a plug's ability to dissipate heat. The rate of heat is determined by (1) The insulator nose length. (2) Gas volume around the insulator nose. (3) Materials and/or construction of the center electrode and porcelain insulator d. Now to the actual function: As the Ignition is triggered it sends the spark through the rotor, to the cap, down the wire and then it jumps the gap of the spark plug, a spark kernel is created that ignites the air/fuel mixture in the combustion chamber. Proper timing of this spark is not the only concern as described above. You must have the proper heat range (described later) as well as the correct timing.
4.
Spark Plug Heat Range. A spark plug's heat range has no relationship on the actual voltage transferred through the spark plug. Rather, the heat range is a measure of the spark plug's ability to remove heat from the combustion chamber. The heat range measurement is determined by several factors:
a.
The length of the ceramic center insulator nose. (1) The longer the insulator nose gives you a larger surface area exposed (2)
b.
to combustion gasses and heat is dissipated slowly. The insulator nose length is the distance from the firing tip of the insulator to the point where the insulator meets the metal shell.
The insulator nose ability to absorb and transfer combustion heat .
c. d. e.
The material composition of the insulator. The material composition of the center electrode. The spark plug tip temperature. (!) The spark plug tip temperature must remain between 450°C to 850°C. (2) If the tip temperature is lower than 450°C, the insulator area surrounding the center electrode will not be hot enough to deter fouling and carbon deposit build-ups, thus causing misfires. (3)
(4)
If the tip temperature exceeds 850°C, the spark plug will overheat which can cause the ceramic around the the center electrode to blister as well as the electrodes will begin to melt. This may lead to pre-ignition/detonation and expensive engine damage. The firing end appearance also depends on the spark plug tip temperature. There are three basic diagnostic criteria for spark plugs: good, fouled, and overheated.
5. Influences on Spark Plug Temp and Performance Below is a list of possible external influences on a spark plug's operating temperatures. The following symptoms or conditions may have an affect on the actual temperature of the spark plug. The spark plug cannot create these conditions, but it must be able to deal with all the levels of heat, otherwise performance will suffer and engine damage can occur:
a.
Air/Fuel Mixtures. Mixtures seriously affect engine performance
(1) (2)
(3)
b.
Rich air/fuel mixtures cause tip temperature to drop, causing fouling and poor drivability. Lean air/fuel mixtures cause plug tip and cylinder temperatures to increase resulting in pre-ignition, detonation, and possibly serious spark plug and internal engine damage. It is important to read spark plugs many times during the tuning process to achieve optimum air/fuel mixture.
Higher Compression Ratios and Forced Induction
These will elevate spark plug tip and in-cylinder temperatures (1) Compression can be increased by performing any one of the following modifications: (a) reducing combustion chamber volume (i.e.: domed pistons, smaller chamber heads, milling heads, etc.). (b) adding forced induction (Nitrous, Turbo charging, Supercharging). (c) camshaft change.
c.
Advanced Ignition Timing: Advancing timing by 10° causes plug temperature to increase by approximately 70°C to 100°C.
CLASSIFICATION OF PLUGS Plugs are classified on the basis of 3.
a.
HEAT RANGE
Plugs are classified by their heat range ability to transfer heat from insulator tip to the engine block such as
a. HOT PLUG These types of plugs transfer heat slowly and therefore stays hot as they remain too hot may glow and ignite the fuel mixture before it should, and causing pinging. Construction Hot plugs have long insulator tip and a rerelatively continuos heat flow path. Tip remains hot enough to burn off the deposits caused by low speed driving
b.
(1). Cold plug A cold plug transfers heat quickly and remains cool. They may not burn away combustion deposits and will foul, that is carbon deposits will bridge the gap between the electrodes (2)
Construction cold plugs have short insulator tip and and therefore a fairly direct heat flow path used in high speed driving to prevent pinging caused by overheated plugs.
c. UTILIZATION OF HOT AND COLD PLUGS 1). A car used mostly for stop and go driving may need hotter than normal plug to burn off carbon deposits. 2). A car used mainly for high speed or long distance driving may need a colder than a normal plug to avoid over heating.
2. PHYSICAL DIMENSION plugs vary from each other on the basis of the their length, eletrode gap and seat setting such as a. LONG REACH These plugs are used where the cylinder head is very deep. In a thinner head such A plug would project too far into the combustion chamber, possibly striking the piston. Carbon deposits on the exposed plug thresds would make it difficult to remove a plug that is too long
b. SHORT REACH These plugs are used in engines with thin cylinder heads. If used on an engine with a thick cylinder head.This plug electrodes would be recessed too far for efficient ignition.Carbon deposits in the plug hole threads would make it difficult to install the proper plug later.
c. TAPERED SEAT These plugs eliminate the need for a gasket . it is tightened into a machined recess in the cylinder head to make a gas tight seal. Ta pered seat plugs are usually slimmer than gasket plugsallowing room in the cylinder head for bigger valves or larger water jackets
d. WIDE GAP These plugs produce a larger spark. Whioch is needed to ignite the lean air _fuel Mixtures in some modern smog-controlled engines. A very high voltage ignition system is needed to fire such plugs which have gaps that range from .05 to .08 in. (1.25 mm – 2 mm).
Spark Plugs and What They Say Look at the spark plug porcelin for the color and condition of it. Plug colors tell the mechanic how the engine is running and can be very important. Plugs change colors and the different colors can explain such things as if the engine is running too hot or if the
engine is worn. The color should be read by looking at the porcelin insulator and compared to the plugs shown on the next few slides .
Normal Condition:
A grayish-tan to white colored plug means that the engine is running normal and the air/fuel mixture is correct. This is the correct color a spark plug should be and it tells the mechanic everything is fine with the engine. The mechanic would install a new properly gapped plug. When installing a new plug, replace the old one with the same heat range. The plug below the center electrode. A new plug would have square edges thatshows helps normal the plugwear fire in bett
Carbon Fouled:
A black dry fluffy colored plug is caused by Too much rich air/fuel mixture coming from carburetor, or excessive idling on some engines. Faulty timing. Wrong heat range A weak spark Low compression Black smoke coming from the exhaust is a sign of a rich air/fuel mixture. The rich air/fuel mixture must be repaired before installing a new spark plug.
Common causes for a rich air/fuel mixture are: dirty air filter. air mixture screw or carburetor needs adjusting. choke is sticking.
carburetor float height is out of adjustment or float is sticking open. Hotter plugs (within plug maker approved range for your engine) may burn off the deposits but the above mentioned problems must also be repaired.
Oil fouled(wet black)
(1) (2) (3) (4)
Black wet glazed colored plug is caused by the burned oil that is leaking past the piston rings or valve guides and burning in the cylinder. This engine will have blue smoke coming out of the exhaust. It is a sign that the engine will need rebuilding. Two strokes that have this color spar plug are burning the transmission oil from the crankcase. If a new hot spark plug is installed it will foul and quit working. Common causes for this problem are: (a) Worn piston and rings. (b) Worn cylinder bore. (c) worn valve guide.
over heating
A plug white colored insulator and spots on it, is caused by engine overheating. Failure to repair this engine will result in severe engine damage. Common causes for this are: Pinging. Overly advanced timing . * Incorrect spark plug (too hot heat range). * Low octane fuel. * Timing is not set properly. * Cooling problems, (dirty cylinder fins, no or low water if water cooled, low or no engine oil). * Carburetor air/fuel mixture is too lean (too much air). * Leaking crankshaft seals, no oil, base or head gasket leaks, or crankcase leaks on two stroke engines.
Mechanical damage
The plug shows what happen when something from inside the engine has hit the plug and this problem must be repaired before running the engine further. Make sure the spark plug is of correct length for the engine. Pre igintion
Initial preignition
This plug condition was caused by preignition and failure to repair this condition will cause engine damage. Check for the following: * correct heat range plug * over advanced ignition timing * lean air/fuel mixture * cooling system * lack of lubrication*
Sustained Pre-ignition
This will be pretty obvious ... melted and/or missing center and/or ground electrodes as well as a destroyed insulator. Check for incorrect heat range plug, over-advanced timing, lean fuel mixtures look for hot spots or deposit accumulation inside the combustion chamber.
Ash Deposits
These are light-brownish deposits that are encrusted to the ground and/or center electrode(s). This situation is caused by oil and/or fuel additives. This condition can cause misfires.
The cure for this is • To verify worn valve guides or valve seals, Not using fuel additives, • Try changing fuel brands. • A hotter plug is what most people try to fix this problem You need to first • understand that the plug is NOT typically the problem •
Detonation
Excessive detonation has caused the porcelin on this plug to break away. If this engine is allowed to run, engine damage can occur. Make sure the fuel octane is high enough for the engines requirements. Splashed carbon deposits
The plug has fused deposit mass(carbon) bridges gap between electrodes across the plug gap. This causes the permanent misfire, which wastes fuel and causes pollution. Check plugs used in high speed driving every 10,000 miles and 5000 miles for city driving cars.
NORMAL WEAR OR WORN OUT CONDITIONS
1. The plug is worn out from being used for a long period of time. 2. Notice how the center electrode is round and worn from use. 3. A spark plug that is worn takes a lot more voltage to fire and can cause poor engine running.
4. it looks ok color wise, But replace it with same plug or at least compatible heat range.Don’t fix them just change them as much wear has already occurred to the side electrode
