Carnot Cycle
An idealised reversible cycle of two isothermal and two adiabatic steps that defines the maximum efficiency of a heat engine.
Formula / equation
η = 1 − T_cold / T_hot
Unit
Dimensionless
In depth
Carnot proved that no engine working between two temperatures can exceed this efficiency, and that the limit depends only on the temperatures, not on the working substance. The result led directly to the definition of the thermodynamic temperature scale and to the concept of entropy.
Examples in the real world
An engine between 600 K and 300 K can be at most 50% efficient, however perfectly it is engineered.