Ideal cyclical sequence of changes of pressure and temperature of a fluid, such as water, used in an engine, such as a steam engine .
Described in 1859 by William Rankine , it is used as a standard for rating the performance of steam power plants. In the Rankine cycle, the working substance of the engine undergoes four successive changes: (1) heating at constant volume (as in a boiler), (2) evaporation and superheating (if any) at constant pressure, (3) isentropic expansion in the engine, and (4) condensation at constant pressure with return of the fluid to the boiler. See also Carnot cycle .