Horsepower: understanding mechanical horsepower
A mechanical horsepower is approximately 745.7 watts, while the metric version is approximately 735.5 watts. Horsepower measures power: the rate at which work is done. Engines and motors are familiar settings for the term, but its definitions extend beyond a single engine rating. Imperial horsepower, also abbreviated hp or bhp, is one common standard; metric horsepower can appear as cv or PS. Electric horsepower is exactly 746 watts. Boiler horsepower belongs to another definition, with values of 9809.5 or 9811 watts depending on the exact year. A number beside the word therefore needs a definition alongside it.
The name took shape around steam engines and draft horses. Scottish engineer James Watt adopted the term in the late eighteenth century to compare their outputs. Earlier, Thomas Savery had described engines in terms of the horses whose water-raising work they could replace. Watt used the comparison to market his improved Newcomen steam engine. His earlier arrangement involved royalties equal to one-third of the coal savings over older steam engines. Customers who used horses instead had no existing steam engine against which to measure those savings, creating a different commercial problem for that royalty scheme.
The calculation behind mechanical horsepower connected pulling force, distance, and time. Watt determined that a horse could turn a mill wheel 144 times in an hour, or 2.4 times a minute. The wheel had a radius of 12 feet, and he judged the horse's pulling force to be 180 pounds-force. Those quantities yielded 32,572 foot-pounds-force per minute. Other estimates differed: John Smeaton suggested 22,916 foot-pounds per minute, John Desaguliers 44,000, and Thomas Tredgold 27,500. Watt's brewery-horse experiment in 1782 gave 32,400. Watt and Matthew Boulton standardized the figure at 33,000 foot-pounds per minute the following year.
That standard gives mechanical horsepower its conversion into modern units. One imperial horsepower is defined as 33,000 foot-pounds-force per minute, equivalent to 550 foot-pounds-force per second. The conversion uses the international foot and pound together with standard gravity. One foot is 0.3048 meters, and one pound is 0.45359237 kilograms. Following the conversion through gives 745.69987158227022 watts, commonly rounded to 745.7. The lifting illustration is correspondingly specific: one imperial horsepower lifts 550 pounds through one foot in one second. The unit expresses a rate of work rather than merely a weight or a distance.
Metric horsepower begins with a different lifting task: raising 75 kilograms against Earth's gravitational force through one meter in one second. DIN 66036 defines that standard as Pferdestarke, or PS. Its equivalent is 735.49875 watts, or 98.6% of an imperial horsepower. British manufacturers sometimes intermix metric and mechanical horsepower according to the engine's origin. Names for the metric standard vary by language, including the Italian cavallo vapore and French cheval-vapeur. A 1978 EEC directive required vehicle power figures to be given in kilowatts rather than horsepower, and a 1980 directive harmonized engine-power measurement. Horsepower later remained permitted in the European Union as a supplementary unit.
A horse's output also depends on the duration being measured. In 1993, R. D. Stevenson and R. J. Wassersug summarized measurements and calculations of peak and sustained work rates in correspondence published in Nature. They cited measurements from the 1925 Iowa State Fair showing peak power over a few seconds as high as 14.88 horsepower. For sustained activity, about one horsepower per horse was consistent with agricultural advice from the nineteenth and twentieth centuries. Human power has a similar distinction between brief and sustained output: a healthy person can produce about 1.2 horsepower briefly and sustain about 0.1 horsepower indefinitely.
Engine measurement adds another layer to mechanical horsepower. Indicated horsepower is the theoretical power of a frictionless reciprocating engine; crankshaft horsepower subtracts internal frictional losses. Bearing drag, rod and crankshaft windage, and oil-film drag are among those losses. Shaft horsepower concerns power delivered at an output shaft. Electrical motor nameplates show available shaft output rather than electrical input, ordinarily using watts or kilowatts, with horsepower used in the United States. Drawbar power serves a different task: measuring the power available for a locomotive to haul a train or for an agricultural tractor to pull an implement.
Boiler horsepower concerns the capacity to deliver steam, tying the measurement to a thermal task. The term developed at the Philadelphia Centennial Exhibition in 1876, where steam engines were tested. In 1884, ASME redefined it around evaporating 34.5 pounds of water per hour from and at 212 degrees Fahrenheit. That revision simplified testing and comparisons. Drawbar testing instead uses a dynamometer car behind a locomotive to measure tension and speed continuously. A controllable load, often another locomotive with its brakes applied, allows the test to establish maximum available hauling power.