In the I.C. Engine, the reciprocating motion of the piston inside the cylinder is converted into rotary motion of the crankshaft. The piston downward motion is caused by the expansion of burnt gases during the combustion of the fuel in the engine cylinder. When the engine is running four events occur in a cyclic manner. They are suction, com pression, power (expansion) and exhaust.


In four stroke engine, all the events, i.e. suction, compression, power and exhaust are completed in two revolutions of crank shaft. They are described below.

Suction stroke

The stroke begins when the piston is at Top Dead Centre (TDC). The piston moves downward. The inlet valve opens. A mixture of petrol and air vapours, mixed proportionately by the carburettor enters the cylinder and fills the space in the cylinder. The piston reaches its lowest position known as Bottom Dead Centre (BDC).

Compression stroke

The piston moves up after the intake stroke. The inlet valve closes when the piston moves upward. The exhaust valve also remains closed. The piston compresses the

fuel-air mixture by pushing it upward into clearance space at the top of the cylinder. Pres sure and temperature of the mixture increases during compression. Power stroke

The fuel-air mixture at high pressure and temperature is ignited by a spark. The spark ignites the fuel-air mixture. This burning increases the pressure and temperature of the gases further. The gases expand producing an explosive force pushing the piston down. Piston moves down at high acceleration. The piston is connected to the connecting rod. The connecting rod is attached to crankshaft. The linear movement of the piston is converted into rotary motion of the crankshaft.

Exhaust stroke

The piston reaches the bottom centre at the end of the power stroke. The exhaust valve opens. The burnt gases escape at greater speed. The piston moves up from bottom centre and pushes out the remaining burnt gases inside the cylinder. At the end of the exhaust stroke, the exhaust valve closes. The cycle is repeated. For every 4-stroke of the piston, the crankshaft rotates 2 revolutions.

Four stroke petrol engine

See More: I.C. Engine

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