The Carnot cycle is an idealization for a heat engine operating reversibly between two reservoirs at temperatures

and

. The working substance is assumed to be one mole of an ideal gas with heat-capacity ratio

. (For a monatomic ideal gas,

has its maximum value at

.) The four steps of the cycle are most commonly plotted on a pressure-volume diagram, shown on the left, with alternate isotherms (red curves of

constant temperature) and adiabatics or isentropics (blue curves of constant entropy). A simple alternative representation is therefore a rectangle on a temperature-entropy diagram, shown as an inset.

A schematic diagram of an idealized heat engine is shown on the right. In each cycle of the engine, a quantity of heat

is withdrawn from the hot reservoir at temperature

. The fraction

is rejected to the cold reservoir at temperature

, with the difference

converted into work. Numerical values of

,

and

in kJ are shown. Since the heat

is essentially wasted, the efficiency of the heat engine is expressed as the ratio

. The efficiency of a Carnot cycle depends only on the temperatures of the two reservoirs:

, where

and

are measured on the absolute temperature scale (in K). The efficiency is always less than 1. To get

, the cold reservoir would have to be at

, absolute zero, which is unattainable. The area enclosed by either the pV or TS curves equals the work produced per cycle.