All Petrol Engine used electric spark ignition.The compressed air-petrol mixture in a petrol engine must be fired at the correct instant so that the resulting rise in pressure acts on the piston when it is at top dead centre.A high voltage is required to jump the gap of a spark plug and give a spark of sufficient energy to ignite the mixture.
As the speed of a spark ignition engine increases,it is desirable that the spark advance increase as the ignition time stays constant but the cycle is completed in less time.Some devices are employed for this purpose.The details about such mechanism is as below,
01) Centrifugal Advance Mechanism:-
In this mechanism,the drive shaft is connected to the ignition cam through base plate,spring loaded weight and an advance cam.The latter integral with the ignition cam.When the speed of the engine increases,the centrifugal force pull the weights out against the spring force thus advancing the angular position of the cam relative to drive shaft.
This action opens the contact points earlier and the ignition timing is therefore advanced.With decrease in the speed the springs pulls the weight inwards and bring back the cam thus reducing the spark in advance.
02) Vacuum Advance Mechanism:-
In this case,flexible diaphragm is kept in the fully retarded position by the force of a spring.One face of diaphragm is exposed to the atmosphere through the air vent,the other face is exposed to vacuum in the sealed vacuum chamber which is connected by a pipe line to the induction manifold.
The part loads due to vacuum the diaphragm is pushed upwards by the air pressure and the angular position of the cam in the distributor is advanced through the actuating rod,so that the spark now occurs earlier.When the engine runs on full load,the suction in the manifold decreases and the diaphragm is pushed downwards by the spring thereby retarding the cam.
This way we can increase the timing of spark ignition in our petrol engine.
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