No engine operating between two heat reservoirs can be more efficient than a Carnot engine operating between the same reservoirs.
The rule was an essential stepping stone towards the formulation of the second law of thermodynamics. When transforming thermal energy into mechanical energy, the thermal efficiency of a heat engine is the percentage of energy that is transformed into work. Thermal efficiency is defined as
Carnot showed that the maximum efficiency possible by any sort of engine has a limit defined by the following efficiency :
Carnot's theorem sets essential limitations on the yield of a cyclic heat engine such as steam engines or internal combustion engines, which operate on the Carnot Cycle. They can extract only a certain proportion of mechanical energy from the heat of the working fluid, and this maximal amount is realized by the ideal Carnot heat engine.