in heat engines, ideal cyclical sequence of changes of pressures and temperatures of a fluid, such as a gas used in an engine, conceived early in the 19th century by the French engineer Sadi Carnot (Carnot, Sadi). It is used as a standard of performance of all heat engines operating between a high and a low temperature.

In the cycle the working substance of the engine undergoes four successive changes: expansion by heating at a constant high temperature; reversible adiabatic expansion; compression by cooling at a constant low temperature; and reversible adiabatic compression. The engine receives heat (from the heat source) during the expansion at high temperature, delivers work during the reversible adiabatic expansion, rejects heat (to the heat sink) during the compression at low temperature, and receives work during the reversible adiabatic compression. The ratio of the net work output to the heat input is equal to the ratio of the difference between the temperatures of the heat source and the heat sink divided by the temperature of the heat source. It represents Carnot's principle in that it is the largest such ratio of any engine operating between the two temperatures.