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Heat engine

A heat engine takes in heat from a hot reservoir, rejects some to a cold reservoir, and delivers the rest as useful work. The efficiency tells you what fraction of the heat supplied is converted to work, and is always less than 1.

W = Q_H - Q_C

energy balance for a heat engine

\eta = \dfrac{W}{Q_H} = 1 - \dfrac{Q_C}{Q_H}

efficiency

Symbols

$Q_H$	heat from the hot reservoir	J
$Q_C$	heat rejected to the cold reservoir	J
$W$	work delivered	J

Example

An engine takes in $Q_H = 800$ kJ and rejects $Q_C = 500$ kJ. The work is $W = 800 - 500 = 300$ kJ, and the efficiency is $\eta = 300/800 = 0.375$ .

Q_H = Q_C + W

must always hold (energy conservation), regardless of which cycle is involved.

Practise thermodynamic cycles for free →

← Carnot efficiency · Heat pump and COP →

Part of Thermodynamics: Thermodynamic cycles.