Thermal Efficiency
The dimensionless ratio of net mechanical work produced by a heat engine to total thermal energy input supplied.
Governing formula
η = W_net / Q_input = 1 - (Q_out / Q_in)
SI unit
Dimensionless (%)
In depth
Always strictly less than 1.0 (100%) due to Second Law constraints ($1 - T_c / T_h$). Modern combined-cycle gas power plants reach ~60% thermal efficiency.
Examples in the real world
A automotive gasoline engine achieving ~30% thermal efficiency, dissipating ~70% of fuel energy as radiator heat and exhaust.