Thermal efficiency

Thermal efficiency is the ratio of useful work or energy output to heat supplied to an energy-conversion system. It measures how much supplied heat becomes useful output.

Heat engine: A device that takes in heat, produces work, and rejects some heat to a cooler reservoir. Thermal efficiency measures the work produced by a heat engine relative to its heat input.
Internal combustion engine: An engine that burns fuel inside a cylinder or combustion chamber to produce mechanical work. Its thermal efficiency compares useful shaft work with the fuel's chemical energy input.
Coefficient of performance: A heat pump or refrigerator's useful heating or cooling output divided by its electrical work input. Unlike thermal efficiency, it can exceed one because it moves heat rather than converting heat into work.
Carnot's theorem: The theorem that no engine operating between two thermal reservoirs can be more efficient than a reversible engine between them. It sets a universal ceiling on thermal efficiency from the reservoirs' temperatures.
First law of thermodynamics: The law of energy conservation: energy changes form but is neither created nor destroyed. It accounts for how supplied heat is divided between work and energy leaving the system.
Steam turbine: A rotary machine that extracts work from expanding steam through rows of blades. It converts heat supplied in a steam cycle into mechanical power.
Energy efficiency: The ratio of useful energy service or output to energy input for a process or device. Thermal efficiency is a specific form whose input is heat supplied to an energy-conversion system.
Entropy: A thermodynamic state function that quantifies energy dispersal and constrains spontaneous processes. Entropy generation marks irreversible losses that reduce the work obtainable from supplied heat.
Second law of thermodynamics: The law that constrains energy transformations through entropy and forbids complete conversion of heat into work in a cycle. It explains why a cyclic heat engine must reject some heat and cannot reach 100% efficiency.
Combined-cycle power plant: A power plant that uses a gas turbine's exhaust heat to generate steam for a second turbine. Using exhaust heat again raises the plant's overall thermal efficiency.

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