In two strokes SI engine a cycle is completed in two stroke of a piston or one complete revolution (360º) of a crankshaft. In this engine the suction stroke and exhaust strokes are eliminated and ports are used instead of valves. Petrol is used in this type of engine.

The major components of a four stroke spark Ignition engine are.

Cylinder: It is a cylindrical vessel in which a piston makes up and down motion.

Piston: It is a cylindrical component making up and down movement in the cylinder.

Combustion Chamber: It is the portion above the cylinder in which the combustion of the Fuel-air mixture takes place.

Inlet and Exhaust ports: The inlet port allows the fresh fuel-air mixture to enter the combustion chamber and the exhaust port discharges the products of combustion.

Crank Shaft: It is a shaft which converts the reciprocating motion of piston into the rotary motion.

Connecting Rod: The connecting rod connects the Piston with the crankshaft.

Cam shaft: The cam shaft controls the opening and closing of inlet and Exhaust valves.

Spark Plug: It is located at the cylinder head. It is used to initiate the combustion process.

Working: When the piston moves from bottom dead centre to top dead centre, the fresh air and fuel mixture enters the crank chamber through the valve. The mixture enters due to the pressure difference between the crank chamber and outer atmosphere. At the same time the fuel-air mixture above the piton is compressed.

            Ignition with the help of spark plug takes place at the end of stroke. Due to the explosion of the gases, the piston moves downward.        When the piston moves downwards the valve closes and the fuel-air mixture inside the crank chamber is compressed. When the piston is at the bottom dead centre, the burnt gases escape from the exhaust port.

At the same time the transfer port is uncovered and the compressed charge from the crank chamber enters into the combustion chamber through transfer port. This fresh charge is deflected upwards by a hump provided on the top of the piston. This fresh charge removes the exhaust gases from the combustion chamber.

            Again the piston moves from bottom dead centre to top dead centre and the fuel-air mixture gets compressed when the both the Exhaust port and Transfer ports are covered. The cycle is repeated.