Heat Engine Efficiency - Heat & Thermodynamics Step by Step

A heat engine converts only part of its heat input into useful work.

highlighted = computed this step

Read the engine budget

The engine takes in 30 joules of heat and produces 12 joules of useful work.

Q_{\text{in}} = 30\ \text{J}\qquad W_{\text{out}} = 12\ \text{J}

Efficiency counts the fraction of heat input that becomes useful work.

\eta = \frac{W_{\text{out}}}{Q_{\text{in}}}

Divide 12 joules by 30 joules. The efficiency is two fifths.

\eta = \frac{12\ \text{J}}{30\ \text{J}} = \tfrac{2}{5}