Working of FOUR stroke Petrol engine:
The cycle of events that takes place in 4 stroke petrol engine is shown in figure. Fig. (a) shows the suction of air-fuel mixture in the cylinder during the downward movement of the piston.
The piston moving away from cylinder head creates a pressure reduction or below atmospheric pressure.
This depression is responsible for sucking the air-fuel mixture in the cylinder in naturally aspirated engine. In fig. (b) is shown the compression stroke in which both the inlet and exhaust valves are closed at the end of which the typical cylinder pressure will be from 8 bar to 13 bar with engine running under load.
Towards the end of the compression stroke, combustion of the charge is ignited by the spark plug occurs. This generate the heat and rises pressure.
The burning gases expands as shown in fig (c) pushing the piston downward. This is called the power or expansion stroke.
At the end of power stroke the inlet valve remains closed but exhaust valve opens, the piston moves towards the cylinder head expelling most of the burnt gases to atmosphere (fig. d).
Thus whole cycle is completed in four strokes i.e. two revolution of crankshaft.
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