What is the ratio of work output to heat input called for a heat engine?

The ratio of work output to heat input for a heat engine is called thermal efficiency. It is usually written as η = W/Qin, where W is the net mechanical work produced and Qin is the heat absorbed from the hot source.

A heat engine converts some thermal energy into useful work while releasing the remainder to a colder reservoir or the surroundings. Applying the first law gives W = Qin − Qout, so efficiency can also be written as 1 − Qout/Qin. It is normally expressed as a percentage.

The second law explains why no real heat engine reaches 100% efficiency: some heat must be rejected, and practical machines also lose energy through friction, turbulence, electrical resistance, and imperfect combustion. A typical gasoline engine converts only roughly a quarter of its fuel energy into useful shaft work.

Thermal efficiency is not the same as heat capacity, which measures energy needed to change temperature, or latent heat, which concerns phase changes. For an ideal reversible engine, the upper limit is the Carnot efficiency, 1 − TC/TH, using absolute temperatures.