Thermodynamic efficiency

In thermodynamics, the thermal efficiency (${\displaystyle \eta _{th}\,}$) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, a steam turbine or a steam engine, a boiler, a furnace, or a refrigerator for example. For a power cycle, thermal efficiency indicates the extent to which the energy added by heat is converted to net work output. In the case of a refrigeration or heat pump cycle, thermal efficiency indicates the extent to which the energy added by work is converted to net heat output.

In general, energy conversion efficiency is the ratio between the useful output of a device and the input, in energy terms. For thermal efficiency, the input, ${\displaystyle Q_{in}}$, to the device is heat, or the heat-content of a fuel that is consumed. The desired output is mechanical work, ${\displaystyle W_{out}}$, or heat, ${\displaystyle Q_{out}}$, or possibly both. Because the input heat normally has a real financial cost, a memorable, generic definition of thermal efficiency is

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