In heat engines , the ideal cycle of changes of pressures and temperatures of a working fluid, such as steam or ammonia, conceived by Sadi Carnot .
It is the standard of performance of all heat engines operating between a high and a low temperature. In the cycle, the working fluid undergoes four successive changes: (1) the fluid receives heat , expanding at high temperature; (2) it delivers work during the reversible adiabatic expansion (it changes in volume or pressure without losing or gaining heat); (3) it rejects heat (to the heat sink) during compression at low temperature; and (4) it receives work during the reversible adiabatic compression. The efficiency is determined by the difference between the temperatures of the heat source and the heat sink divided by the temperature of the heat source. See also Rankine cycle .