in heat engines, ideal cyclical sequence of changes of pressure and temperature of a fluid, such as water, used in an engine, such as a steam engine. It is used as a thermodynamic standard for rating the performance of steam power plants. The cycle was described in 1859 by the Scottish engineer William J.M. Rankine.

In the Rankine cycle the working substance of the engine undergoes four successive changes: heating at constant pressure, converting the liquid to vapour; reversible adiabatic expansion, performing work (as by driving a turbine); cooling at constant pressure, condensing the vapour to liquid; and reversible adiabatic compression, pumping the liquid back to the boiler.