Define Working Principle of Four Stroke Cycle SI Engine, Diagram, Examples

 working principle of 4 stroke cycle SI engine

As the name suggests, in a four-stroke engine the cycle of operation is completed into four strokes of the piston.

There are 4 stocks in SI engines are: - 

 (i) Suction Stroke - During the suction stroke, the piston moves from Top Dead Center (TDC) to Bottom Dead Center (BDC). The inlet valve is open in this stroke and the fuel-air mixture in correct proportion is sucked inside the cylinder. The exhaust valve remains closed throughout the stroke.

(ii) Compression Stroke - In compression stroke both the valves are closed and the charge is compressed as the piston moves upwards from BDC to TDC. As a result compression pressure and temperature of the charge increases considerably. This completes one revolution of the crankshaft Compression stroke. 

(iii) Expansion or Power Stroke - Just before the piston reaches the TDC during the compression stroke, the air-fuel mixture is ignited with the help of a spark plug. It suddenly increases the pressure and temperature of the products of combustion, but the volume practically remains constant. Due to the rise in pressure, the piston is pushed down with great force. The hot burnt gases expand due to the high speed of the piston. During this expansion, some of the heat energy produced is transformed into mechanical work. In this stroke both the valves are closed.

(iv) Exhaust Stroke - As the piston moves from BDC to TDC the exhaust valve opens and the piston pushes out the products of combustion, from the engine cylinder to the atmosphere. This completes the cycle and the engine cylinder is again ready to suck the charge.