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Cycles of the Thermal Engines

Cycles of the thermal engines

The operating system of the thermal engines is based on a cycle that is constantly repeated. In the case of internal combustion engines this cycle consists of the fuel input (admission), combustion of the fuel, expansion of the cylinder and expulsion of the gases produced. This cycle generates a mechanical movement, if the cycle repeats we will have a constant mechanical movement.

A thermal engine can work through many different cycles. In this section we will explain the different operating cycles that an internal combustion engine can have, the real cycles and the theoretical cycles.

Operating Cycles of a Heat Engine

By operating cycle 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 strokes. 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.

Theoretical Cycles of the Thermal Engines

The theoretical cycle of a thermal engine is a theoretical approximation of its operation to calculate its performance.

The cycle of an internal combustion engine is constituted by the physical and chemical transformations suffered by the fuel during its passage inside the engine (compression, expansion, combustion, transmission of heat with the walls, friction in the mass of the fluid and with the walls, etc.)

The quantitative examination of these phenomena, taking into account all the numerous variables, represents a very complex problem. Therefore, it is usually simplified by resorting to theoretical approaches, each of which is based on different simplified assumptions, which have a gradually increasing approximation.

Actual Cycles of the Thermal Engines

The actual cycle of a thermal engine is obtained experimentally by means of various indicating devices. These devices are capable of recording the pressure diagram according to the volumes, in a running engine cylinder.

The indicated diagram reflects the actual conditions of the cycle and, therefore, also takes into account the heat losses, the duration of combustion, the losses caused by the friction of the fluid, the duration of the opening time of the valves, the time ignition, as well as injection and leakage losses.


Published: November 28, 2017
Last review: November 28, 2017