(Ignition Advance)-It is of great importance that the ignition should take place at the correct moment at or before the end of the compression stroke so that the fuel may have sufficient time to burn completely for producing maximum pressure on the piston head.
It depends upon the nature of the fuel that has different rates of burning, engine speed, engine temperature and load, compression ratio, air-fuel ratio, and cylinder bore. The pressure will be maximum if the fuel is ignited when confined in the smallest space combustion chamber. At that time the piston is at its top dead center position. But the fuel also takes some time to burn completely for producing maximum pressure.
Hence, the must take place a few degrees earlier of crankshaft rotation when the piston reaches TDC. It depends upon the engine speed also. If the piston is moving slowly, the fuel may be sufficient time of complete combustion by the time the stroke is ended. If the piston is moving at high speed, the ignition must occur much earlier in the stroke to have the same time for complete combustion by the end of the stroke. Therefore, there must be some device to ignite the fuel at the correct moment of compression stroke according to the different operating conditions of the engine.
When the ignition occurs early in the compression stroke, before the top dead center, the ignition or spark is said to be advanced A retarded spark occurs when the compression stroke is more nearly done at the time ignition occurs.
If the spark is advanced too much, the fuel will ignite completely before the piston reaches to TDC and the maximum pressure will be exerted on the piston when it is moving upwards during the compression stroke. It will cause the engine to stop suddenly. Under certain operating conditions, an advanced spark might also cause the fuel to explode or detonate.
If the spark is retarded too much, the fuel will ignite completely too long after the piston has begun to move downward during the stoke. Because of the combustion space increases by the downward movement of the piston, the pressure will also be reduced on the piston. If the spark is still further retarded, the fuel may not burn completely by the time the exhaust valve opens. It will cause loss of power, overheating, carbon deposits and probably burning exhaust valve.