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. In four stroke CI engine the combustion of fuel-air mixture takes place with compression. The engine operates at a high compression ratio of the order of 16 to 20. Due to high compression ratio the mixtures reaches its ignition temperature and the combustion takes place.  

The major components of a four stroke compressed 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.

Fuel Injector: It is located at the top of head to inject the fuel into the combustion chamber.

A four stroke CI engine 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 only air enters 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 air is compressed by the upward movement of the piston. At the end of the compression stroke the fuel is injected into the combustion chamber. An injector is provided to inject the fuel. At the end of compression stroke the temperature is sufficient to ignite the fuel and the combustion of fuel-air mixture takes place.

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.