Brake Horsepower

Horsepower as a unit of power output is a historical was an attempt by James Watt, the British engineer and pioneer of the steam engine, to relate the output of steam engines to in terms of the number of horses. One horsepower, hp, was defined as the power needed to lift 33,000 pounds over one foot in one minute on the surface of the Earth, or in SI units,

\[\begin{equation} \begin{aligned} 1 \: hp &= \frac{mg \Delta h}{t} \\ &= \frac{33000 \times 0.4536 \times 9.807 \times 0.3048}{60} \\ &= 746 W \end{aligned} \end{equation}\]

Today brake horsepower (bhp) is used to to measure the power output of an engine at the crankshaft just outside the engine, before the losses of power caused by the gearbox and drive train, and it is common to hear the power of a car given in terms of brake horsepower.