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 exhaust valve opens. It will cause loss of power, overheating, carbon deposits and probably burning exhaust valve.
Understand fully the ignition advance process from the video below:
IGNITION ADVANCE MECHANISMS
There are two general methods used in modern engines to advance and retard the ignition timing automatically in relation to engine speed and operating conditions after the initial timing is set manually, most manufacturers call these methods as automatic advance mechanisms :
- Centrifugal advance mechanism.
- Vacuum advance mechanism.
Centrifugal Advance Mechanism
It consists of two flyweights (advance weights), cam, spring and base plate. The flyweights are carried by the distributor drive shaft through the base plate which is fixed to the drive shaft. The flyweights are pivoted on the base plate and attached to the cam through
springs. The cam is connected with the distributor drive shaft through the springs, flywheel and base plate.
|Mechanical or Centrifugal advance mechanism|
When the engine speed increases, the weights move out due to centrifugal force rotating the plate and cam in the anti-clockwise direction. This movement affects the desired advance. This timing of the spark varies from no advance at low speed to full advance at high speed.
Vacuum Advance Mechanism
The figure shows the vacuum advance mechanism. It consists of a diaphragm which automatically advances and retards the ignition timing according to the engine speed and operating condition. The diaphragm forms two chambers, one of which is connected to the induction manifold and the other is open to the atmosphere. A linkage connects the diaphragm to the distributor. When the diaphragm is in its normal position, the contact breaker is held fully retarded. When the engine speed increases the induction manifold depression is high and the diaphragm is pressed up by atmospheric pressure. The movement of the diaphragm moves the contact breaker in the opposite direction to rotate and thereby advancing the ignition. A decrease in vacuum allows the diaphragm to return back to its original setting retarding the ignition.
|Vacuum advance mechanism|
It is seen that the centrifugal advance mechanism takes much care of speed only and not the load conditions, whereas the vacuum advance mechanism takes much care of load conditions. Therefore, a combination of the two mechanisms is applied to the distributor. In this design, part of the ignition advance is due to the centrifugal force and part to the vacuum produced in the intake manifold. The combination of the two mechanisms gives the practically perfect spark timing for all driving conditions.
Explaining all about ignition advance mechanism in the video below:
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