By operating cycle of a thermal engine we understand the succession of operations that the active fluid executes in the cylinder and repeats periodically. The duration of the operating cycle is measured by the number of strokes made by the piston to perform it.
In this sense, there are two types of thermal engines:
- 4-stroke engine. In the four-stroke engine the cycle is carried out in 4 strokes of the piston. This engine makes a cycle every two revolutions of the motor shaft.
- 2-stroke engine. The two-stroke cycle is carried out only in 2 strokes of the piston. Two-stroke engines cycle one revolution at a time.
However, engines have been devised in which the operating cycle is verified in 6 piston races. In this particular case, four of the six races are used to execute the same operations of the four-cycle cycle, and the remaining two serve to retain the fuel mixture inside the cylinder in order to allow the fuel, when it is not very volatile, vaporize completely and diffuse in the combustion air, or perform a sweep of the cylinder with pure air. These engines have not had, however, a practical diffusion.