Carnot's theorem

The law that any engine producing work from heat has an efficiency ceiling set only by the temperatures of its hot and cold sides.

In 1824 the French engineer Sadi Carnot argued in "Reflections on the Motive Power of Fire" that no heat engine, however well designed, can exceed a certain efficiency, and that the limit depends not on the fuel or machine but on the temperatures at which heat is taken in and given out^[1]. In modern terms the ceiling is one minus the ratio of the cold to the hot absolute temperature.

It told engineers to raise combustion temperatures to raise efficiency, and it was the starting point of the diesel engine. Fuel cells and electric motors beat engines on efficiency because they are not bound by this ceiling.