Diesel engines come in two and four cycles; next, the four-cycle diesel engine is described: 

 

1.-Admission
During the admission stroke, the piston moves downwards aspirating the air through the opening of a fan admission valve, and taking it to the combustion chamber. Some motors have more than one induction valve and exhaust valve per cylinder.

 

 

 

2.-Compression  
During the compression stroke, all of the valves are closed, and the piston moves upward in the cylinder reducing the air. As the molecules in the air become compressed, the temperature increases considerably. When the piston reaches the upper part of the stroke, it injects fuel into the combustion chamber on the upper part of the piston. Finally the fuel is mixed with hot compressed air, thus, producing combustion. 

 

 

3.-Power
During the power stroke, the valves are closed as the force of the combustion pushes the piston and the piston rod towards the bottom, which makes the crank rotate. The thermal energy is converted into mechanical energy. 

 

 

4.-Exhaust 
During the exhaust travel, the force of inertia created by the rotation of the steering wheel helps to make the rotation of the crank continue to push the piston upwards within the cylinder, forcing the outlet of the burnt gases through the open exhaust valve. This completes the four strokes of the piston, these are repeated in a cyclical manner while the motor operates.