Carnot cycle

The Carnot cycle is a reversible thermodynamic cycle composed of four legs: isothermal expansion at the hot temperature T_h (absorbing heat Q_h), adiabatic expansion (cooling the working substance to T_c), isothermal compression at T_c (rejecting heat Q_c), and adiabatic compression (returning to the start). Only the two isothermal legs exchange heat with the surroundings; the two adiabatic legs are isentropic.

Because every step is reversible — carried out infinitely slowly through a continuous succession of equilibrium states — the Carnot cycle wastes no work to friction, turbulence, or finite-temperature-difference heat flow. It is therefore the most efficient cycle possible between two given temperatures, a result known as Carnot's theorem.

In the pressure–volume plane the cycle is a curved quadrilateral whose enclosed area is the net work; in the temperature–entropy plane it becomes a clean rectangle, since the isotherms are horizontal and the reversible adiabats are vertical lines of constant entropy.

Sadi Carnot described the cycle in his 1824 Réflexions sur la puissance motrice du feu, reasoning about an idealised engine before the first law of thermodynamics or the concept of entropy existed. Clapeyron later drew it on the indicator (P–V) diagram that made it calculable.