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Thermodynamics
Thermodynamics

Heat Engine Efficiency

η=WQH=1−QCQH\eta = \frac{W}{Q_H} = 1 - \frac{Q_C}{Q_H}

Efficiency of any heat engine.

Variables

η
Efficiency
Work output / Heat input
unitless
W
Work output
Useful work
J
Q_H
Heat input
Heat from hot reservoir
J
Q_C
Heat output
Heat to cold reservoir
J

Derivation

η = W/Q_H. Since W = Q_H - Q_C: η = 1 - Q_C/Q_H.

Your notes

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Common questions

What is the Heat Engine Efficiency formula?

Heat Engine Efficiency is written η= W/Qₕ = 1 - Q_C/Qₕ. Efficiency of any heat engine. It belongs to Thermodynamics, under Second Law.

What are the units in η= W/Qₕ = 1 - Q_C/Qₕ?

In SI units, η is efficiency (dimensionless); W is work output, measured in kg·m²/s²; Q_H is heat input, measured in kg·m²/s²; Q_C is heat output, measured in kg·m²/s².

How is the Heat Engine Efficiency formula derived?

η = W/Q_H. Since W = Q_H - Q_C: η = 1 - Q_C/Q_H. The result is η= W/Qₕ = 1 - Q_C/Qₕ.