Carnot's theorem

Carnot's theorem states that no heat engine operating between two fixed-temperature reservoirs can be more efficient than a reversible engine operating between those reservoirs. It makes the maximum possible efficiency depend only on reservoir temperatures.

Carnot cycle: An ideal reversible cycle comprising two isothermal and two adiabatic processes. Its efficiency reaches the upper bound Carnot's theorem sets for engines between fixed reservoirs.
Heat engine: A device that converts heat taken from a hot source into work while rejecting some heat to a colder sink. The theorem compares the efficiencies of heat engines sharing the same reservoirs.
Sadi Carnot: A French physicist whose 1824 analysis of heat engines established the importance of reversible cycles. His reasoning supplied the theorem's original comparison of engine performance.
Otto cycle: An idealized cycle used to model spark-ignition internal-combustion engines. Its efficiency can be compared with the Carnot limit for the same temperature extremes.
Thermodynamic temperature: An absolute temperature scale defined through thermodynamic principles rather than a particular material's properties. The theorem expresses maximum efficiency using the reservoirs’ absolute temperatures.
Thermal reservoir: An idealized body that can exchange heat while maintaining constant temperature. Fixed-temperature reservoirs are the boundary conditions in the theorem.
Reflections on the Motive Power of Fire: Sadi Carnot's 1824 treatise analyzing the motive power produced by heat engines. It presented the argument from which Carnot's theorem developed.
Diesel cycle: An idealized cycle used to model compression-ignition internal-combustion engines. It offers a practical engine model whose efficiency remains bounded by Carnot's theorem.
Second law of thermodynamics: The principle that total entropy does not decrease in an isolated system. Its restrictions on entropy changes rule out engines that exceed the reversible limit.
Thermal efficiency: The ratio of useful work output to heat input for a heat engine. Carnot's theorem sets the greatest possible value of this ratio for specified reservoirs.

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