The Carnot cycle is a thermodynamic transformation cycle for a combustion engine.
This concerns a fluid in a cylinder subjected to a series of adiabatic and isothermal transformations in a cycle such as shown in the diagram below.
The Carnot cycle in a PV diagram.
Such a cycle makes it possible to use reasoning from the second law of thermodynamics and introduce the state function called entropy, as well as the formula for the efficiency of a reversible combustion engine.
Nicholas Sadi Carnot 1796-1832 (Credit: Joachim Reinhardt).