In four stroke spark ignition (SI) engine, petrol is used as fuel. It is also called four stroke Otto cycle engines because it is based on the Otto cycle. In Otto cycle the heat is added at constant volume. In this type of engine are there four strokes which are as below.
Suction stroke: During the suction stroke the piston moves from Top dead centre (TDC) to Bottom dead centre (BDC). The inlet valves opens and the fuel-air mixture is sucked into the engine cylinder. The exhaust valve remains closed during the stroke.
Compression stroke: In compression stroke the piston moves from BDC to TDC which compresses the fuel-air mixture. Due to compression the pressure of the mixture rises to 8 bars. At the end of compression stroke the operating plug initiates the spark plug located at the top end which ignites the mixture and combustion takes place at constant volume. Both the inlet valve and exhaust valve remains closed during the stroke.
Expansion or Power stroke: Due to the combustion of mixture the hot gases are produced which drive the piston from TDC to BDC. It is during this stroke when we get work from engine. This energy is stored into the flywheel and supplies it during the idle stroke. Both the inlet and outlet valves remain closed during this stroke.
Exhaust stroke: After the expansion stroke the gases from which work has been collected become useless and made to escape through exhaust valve to the atmosphere. In this stroke the piston moves from BDC to TDC and the exhaust gases are driven out of the engine cylinder through outlet valve. This process is also known as scavenging.
The above processes are repeated again and again for continuous working of engine.