Carnot heat engine

An ideal reversible heat engine operating between hot and cold thermal reservoirs. Its efficiency is the maximum possible for any engine working between those same temperatures.

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Carnot cycle: An ideal cycle of two isothermal and two adiabatic processes that returns a working substance to its initial state. The engine produces work by making its working substance traverse this reversible cycle.
Second law of thermodynamics: A principle stating that entropy of an isolated system does not decrease in spontaneous processes. It rules out engines that convert all absorbed heat into work in a cycle.
Sadi Carnot: A French physicist and engineer whose 1824 work analyzed the motive power of heat. Carnot introduced the ideal engine as a way to reason about heat-powered work.
Internal combustion engine: A heat engine that releases energy by burning fuel inside a working chamber. Its efficiency can be compared with the Carnot limit for its operating temperatures.
Otto cycle: An idealized cycle used to model spark-ignition internal combustion engines. Unlike the Carnot cycle, it models heat addition and rejection at constant volume.
Isothermal process: A thermodynamic process that occurs at constant temperature. The cycle absorbs and rejects heat during its two constant-temperature steps.
Absolute temperature: Temperature measured from absolute zero, commonly expressed in kelvins. The efficiency limit uses absolute reservoir temperatures, not Celsius readings.
Reflections on the Motive Power of Fire: Sadi Carnot’s 1824 treatise on heat engines and the production of motive power. This work introduced the reasoning from which the Carnot limit developed.
Steam engine: A heat engine that uses steam to convert thermal energy into mechanical work. Steam engines are practical examples whose performance falls below the reversible limit.
Diesel cycle: An idealized cycle used to model compression-ignition engines, with heat added at constant pressure. It predicts performance for a practical engine model rather than a reversible limit.