Horsepower is a unit for measuring the power of an engine or motor in the inch-pound system of measurement customarily used in the United States. The term horsepower was first used by the inventor of the steam engine, Scottish engineer James Watt (see Watt, James ). He used it to compare the power of steam engines to the power of horses. Today the term is used to measure the power of devices such as automobile engines, gas turbine engines, electric motors, and nuclear power plants.
Power is a measure of the rate of doing work. The amount of work to move one pound a distance of one foot is one foot-pound. One horsepower equals 550 foot-pounds of work per second, or 33,000 foot-pounds of work per minute. The watt is the unit for measuring power in the metric system. One horsepower equals 745.700 watts. See Foot-pound ; Watt .
If an engine lifts a 550-pound object to a height of 2 feet in 1 second, it is working at a rate of 1,100 foot-pounds per second (550 times 2 divided by 1 equals 1,100). This engine is delivering 2 horsepower (1,100 divided by 550 equals 2). If a person weighing 150 pounds climbs a height of 88 feet, he or she does 13,200 foot-pounds of work (150 times 88 equals 13,200). If the person makes the climb in 60 seconds, he or she is working at the rate of 4/10 horsepower (13,200 divided by 60 equals 220; 220 divided by 550 equals 4/10). A person who is accustomed to hard work can work at a rate of between 1/10 and 1/8 horsepower continuously during an 8-hour day.
The power of an engine can be defined in several ways. Some of the most useful are (1) indicated horsepower, (2) brake horsepower, and (3) SAE horsepower.
Indicated horsepower
measures the power produced inside the cylinders of an engine. First, that power must be calculated by multiplying together the average pressure on the pistons, the area of each piston, the length of the piston’s stroke, the number of power strokes per minute per piston, and the number of pistons in the engine. The indicated horsepower is determined by dividing the power by 33,000 foot-pounds per minute.
Brake horsepower
is sometimes called effective horsepower because it is the amount of power available at the engine’s output shaft. Brake horsepower is determined by the engine’s speed and torque, the amount of twist exerted. A machine called a dynamometer measures brake horsepower. Brake horsepower is the rating most widely used by engineers. It is less than indicated horsepower due to friction and other power losses in the engine.
SAE horsepower
defines the power of an engine under specified conditions and allows valid comparisons between engines. This rating is approved by SAE International, a professional organization for automotive engineers. It is used in determining licensing fees for automobiles in some states.
