Horsepower

Horsepower is a unit of power used to express the rate at which engines and motors perform work. It was introduced by James Watt in the late 18th century to help compare the output of steam engines with that of draft horses. Since then, horsepower has become a standard descriptor of engine performance, particularly in automotive and industrial contexts.

There are several definitions. The most common Imperial or mechanical horsepower is defined as 550 foot-pounds per second, equal to about 745.7 watts. The metric horsepower (often called Pferdestärke, PS) is defined as 75 kilogram-force meters per second, equal to 735.49875 watts. Some ratings refer to electrical horsepower, defined as exactly 746 watts. Consequently, 1 kilowatt equals about 1.341 horsepower (mechanical) and about 1.3596 horsepower (metric).

Power ratings are often given as brake horsepower (bhp) measured at the engine output shaft with a dynamometer, or indicated horsepower (ihp) derived from cylinder pressures. In automotive practice, horsepower is commonly the brake horsepower. The relationship to torque and speed is P = τ·ω; in customary units, horsepower equals torque in lb-ft times rpm divided by 5252. Testing conditions such as gear ratios, altitude, and exhaust restrictions can influence the measured value.

Horsepower remains a useful indicator of performance, though it can be misunderstood if taken in isolation. Peak horsepower reflects the maximum rate of work, but overall acceleration and real-world performance depend on torque, drivetrain, vehicle weight, and gearing.