In thermodynamics, the ratio of useful work derived from a heat engine to the heat absorbed by it; symbol e, a number between 0 and 1. It represents a theoretical maximum possible efficiency. For car engines, e = 0·5; for coal-fired power stations, e = 0·4 (approximate values).
When transforming thermal energy into mechanical energy, the thermal efficiency of a heat engine is the percentage of energy that is transformed into work. For example, when 1000 joules of thermal energy is transformed in 300 joules of mechanical energy (with the remaining 700 joules dissipated as heat), the thermal efficiency is 30%.
Thermal efficiency is defined as or
where is the thermal efficiency, is the work output, is the rate of heat flow from a high-temperature heat source, and is the heat flow to a low-temperature heat sink.
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