Four Stroke Engine / Spark Ignition Engine / Otto Cycle

Four stroke engine cycle was introduced by Otto in 1876. In a four stroke engine, there are 4 strokes or stages which are:

Suction Stroke, Compression Stroke,  Expansion Stroke/Power Stroke and Exhaust Stroke.

1)  Suction Stroke: During this stage, the momentum created in the flywheel by the electric starting motor causes the crank shaft to pull down the piston. This creates a negative pressure inside the cylinder as a result air-fuel mixture from the carburetor enters the cylinder through the open inlet valve. The exhaust valve remains closed during this stroke.

2) Compression Stroke:  The inlet and outlet valve remains closed during this stage. The piston moves upwards compressing the mixture inside the cylinder. The temperature and pressure inside the cylinder rises, which results in a more homogenous mixture and also makes ignition faster. Spark is produced from the spark plug, which ignites the mixture and the combustion is half complete while the piston reaches the top dead center.

3) Expansion/Power Stroke: Both valves remain closed during this stroke also. The heat produced as a result of combustion causes the gases to expand which exerts pressure on the cylinder walls and the piston. This causes the piston to move downwards which rotates the crank shaft and the flywheel. The power for all other strokes is derived from this stroke.

4) Exhaust Stroke: The exhaust valve opens. The piston moves upwards due to the momentum created in the flywheel.   This upwards motion pushes the burnt gases out through the exhaust valve.

After the exhaust stroke, the suction stroke is repeated and the cycle continues.