Thermal efficiency
The fraction of the heat supplied to an engine that emerges as useful work, η = W/Q_h.
Definition
The thermal efficiency of a heat engine is the ratio of the net work it delivers to the heat it absorbs from the hot reservoir: η = W/Q_h = 1 − Q_c/Q_h. It is a dimensionless number between 0 and 1, often quoted as a percentage.
Efficiency measures how much of the energy you pay for is returned as work rather than dumped as waste heat. A perfectly efficient engine (η = 1) would reject no heat at all; a useless one (η = 0) would convert nothing. Real engines sit between: a few percent for early steam engines, around 25% for a car engine, 40% for a coal plant, and up to about 60% for a combined-cycle gas turbine.
No engine's efficiency can exceed the Carnot value 1 − T_c/T_h set by its reservoir temperatures, so improving efficiency in practice usually means raising the combustion temperature or lowering the exhaust temperature rather than refining the mechanism.
History
The pursuit of efficiency drove the entire steam age and motivated Carnot's 1824 inquiry into whether a maximum existed; the modern definition as work over heat input followed the establishment of the first law in the 1840s.