Carnot cycle

The Carnot cycle is an ideal reversible thermodynamic cycle consisting of two isothermal and two adiabatic processes. It sets the maximum possible efficiency for a heat engine operating between two fixed temperatures.

Isothermal process: A thermodynamic process occurring at constant temperature. Heat enters or leaves the working substance during each constant-temperature leg.
Thermodynamic system: A defined quantity of matter or region of space chosen for thermodynamic analysis. The cycle tracks changes in a working substance treated as a thermodynamic system.
Otto cycle: An idealized cycle used to model spark-ignition engines, with heat addition at constant volume. It models practical gasoline engines rather than the Carnot cycle's reversible heat exchange.
Heat engine: A device that converts heat received from a hot source partly into work while rejecting the remainder to a colder sink. The Carnot cycle defines the greatest efficiency such a device can attain between fixed temperatures.
Adiabatic process: A thermodynamic process in which no heat crosses the system boundary. The connecting legs change temperature without transferring heat to the surroundings.
Heat reservoir: A body large enough to exchange heat while remaining at a nearly constant temperature. The ideal engine absorbs and rejects heat at fixed hot- and cold-reservoir temperatures.
Diesel cycle: An idealized cycle used to model compression-ignition engines, with heat addition at constant pressure. Its process sequence reflects diesel-engine operation, unlike the Carnot cycle's isothermal steps.
Refrigerator: A device that uses work to transfer heat from a colder region to a warmer one. Reversed, the Carnot cycle sets the maximum coefficient of performance for refrigeration.
Reversible process: An idealized process that can be reversed without leaving a net change in the system and surroundings. The cycle reaches its limiting efficiency only when every step is reversible.
Thermodynamic temperature: Absolute temperature measured on a scale grounded in thermodynamic principles. The efficiency limit depends on the reservoirs' absolute temperatures, not their Celsius values.

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