Construction and Working of 4-Stroke Petrol (SI) Engine

4-Storke SI Engine-

Working cycles of SI engine

• Working Principle: -

        The working cycle of an engine is completed in a 4-stroke of piston i.e. two revolutions of the crankshafts.  Each stroke consists of 180 deg revolution of a crankshaft. Therefore, the cycle consists of 720 deg of crankshaft for 1 cycle. Petrol is used as a fuel for SI engines. These engines work on Otto cycles. Here air & fuel mixture is ignited by producing a spark plug, just before the end of the compression stroke. Therefore, these engines are also called Spark Ignition Engines.

·        Following are the 4-strokes in a petrol engine: -

Cycles of 4-stroke SI Engine 

1. Intake/Suction Stroke: - 

During this stroke, the inlet valve opens and the exhaust valve remains closed. As the piston moves downwards, creates a low-pressure area in the cylinder. Thus, the air-fuel mixture gets sucked into the cylinder. As the piston reaches BDC, the suction valve gets closed completing suction stroke.

2. Compression Stroke: -

During this stroke, the piston starts moving toward TDC from BDC, both valves remain closed. The charge is compressed to clearance volume. Just before the piston reached TDC i.e. before the end of the compression stroke the mixture is ignited with the help of a spark plug.

Combustion is almost completed when the piston is at TDC producing a temperature rise of about 2000^oC and pressure also considerably increased. This completes one revolution of the crankshaft.

3. Expansion/Power Stroke: -

Both valves remain closed during this stroke. The ignited mixture at high pressure and temperature drives the piston downwards. During this stroke, power is obtained. Both pressure and temperature decrease during expansion.

4. Exhaust Stroke: -

In this stroke, the product of combustion is thrown out from the engine cylinder. The exhaust valve remains open during this stroke and the inlet valve remains closed. The piston is moving from BDC to TDC. The exhaust valve closes at the end of the exhaust stroke and some residual gases remain inside the cylinder.

• What do you mean by Valve Overlap? see HERE

References

1. Wikipedia.com