2. Operating principles

The fundamental operating principle of a combustion engine lies in the use of the heat released by combustion to increase the temperature, pressure and/or volume of a working fluid, usually a gas. The pressure forces then exert an effect on a moving part (piston, blades, rotor) and thus generate motion, or are used directly to set the fluid in motion in the case of jet engines.

Two effects are at work in the efficiency of such machines: firstly, the maximum temperature that the working fluid can reach from a given initial state, and secondly, the minimum temperature to which this working fluid is returned at the machine outlet while still generating motion. The greater the temperature difference for a given amount of fuel burned, the better the motor's efficiency or energy output. Other performance criteria can also be taken into account, such as the quantity of...