The engine in which the cycle of operations is completed in two revolutions (720º) of the crank shaft or four strokes of the piston is known as the four stroke engine. One stroke is completed when the piston moves from Top dead centre to Bottom Dead Centre or when the crank rotates through 180º. If the combustion of the fuel-air mixture takes place with the help of spark plug then it is known as four strokes Spark Ignition Engine. The SI engine operates at a compression ratio of 6 to 10.

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 valves: The inlet valves allow the fresh fuel-air mixture to enter the combustion chamber and the exhaust valve 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.

A four stroke engine SI consists of the following four strokes.

1.      Suction or Intake stroke

2.      Compression Stroke

3.      Expansion or power stroke

4.      Exhaust stroke 

1.      Suction Stroke: This stroke starts when the piston is at the top dead centre. When it moves downwards it will create suction and draw the fuel air mixture into the cylinder. The inlet valve is open at this time and exhaust valve is closed. When the piston reaches at the bottom dead centre the inlet valve closes and the suction stroke ends. It all takes place in 180º of the crankshaft rotation.

2.      Compression stroke: In this stroke the piston starts moving upward. During this stroke both the inlet and exhaust valves are closed. The fuel-air mixture is compressed by the upward movement of the piston. At the end of the compression stroke the fuel-air mixture is ignited by the spark plug. Burning of fuel-air mixture takes place instantaneously when the piston is at the top dead centre.

3.      Expansion or Power Stroke: Due to the high pressure of the burnt gases the piston moves towards bottom dead centre. Both the inlet and exhaust valve remains closed during the stroke.

4.      Exhaust stroke: When the piston is at the bottom dead centre the exhaust valve opens. As the pressure falls to atmospheric level. The piston moves from Top dead centre to bottom dead centre and sweeps the products of discharge out at nearly atmospheric pressure. The exhaust valve closes at the end of exhaust stroke. The gases are not fully exhausted. Some of the burnt gases stills remains in the clearance volume.

 These remained gases mixed with the fresh fuel-air mixture entering the chamber.