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Horsepower ('''hp''') is the name of several non-metric Units of Power . The most occurring conversion of horsepower to watt goes 1 horsepower = 745.7 watts. In scientific discourse, the term "horsepower" is seen as inferior and is rarely used because of its various definitions and the already existant SI unit for power, the Watt (W). However, use of the term "horsepower" persists as a legacy in many languages and industries, particularly in the Automotive industry because of their continued advertising of maximum power output of Internal-combustion Engine s in "horsepower" units of measurement. There are two important factors to consider when evaluating the measurement of "horsepower":
These factors can be combined in unexpected ways — the power output for an engine rated at "100 horsepower" might not be what a reader expects. For this reason, various groups have attempted to standardize not only the definition of "horsepower" but the measurement of "horsepower." In the interim, more confusion may surface. In contrast, the Watt , defined by the International System Of Units (" SI "), is not encumbered by varying definitions. CURRENT DEFINITIONS The following definitions have been widely used:
Additionally, the term "horsepower" has been applied to calculated (estimated rather than measured) metrics:
Mechanical horsepower The term Under this system, then, one horsepower is defined as: : 1 hp = 33,000 ft.lbf/min (550 ft.lbf/s) (745.69987158227022 watts) Metric horsepower Metric horsepower began in Germany in the 19th century and became popular across Europe and Asia. The various units used to indicate this definition ("PS", "CV", "pk", and "ch") all translate to "horse power" in English, so it is common to see these values referred to as "horsepower" or "hp" in the press releases or media coverage of the German, French, Italian, and Japanese automobile companies. British manufacturers often intermix metric horsepower and mechanical horsepower depending on the origin of the engine in question. Metric horsepower, as a rule, is defined as 0.73549875 kW, or roughly 98.6% of mechanical horsepower. This was a minor issue in the days when measurement systems varied widely and engines produced less power, but has become a major sticking point today. Exotic cars from Europe like the McLaren F1 and Bugatti Veyron are often quoted using the wrong definition, and their power output is sometimes even converted twice because of confusion over whether the original "horsepower" number was metric or mechanical. PS This unit (German: ''Pferdestärke'' = horse strength) is no longer a lawful unit, but is still commonly used in Europe, South America and Japan, especially by the automotive and motorcycle industry. It was adopted throughout continental Europe with designations equivalent to the English "horsepower", but mathematically different from the British unit. It is defined by the ''Physikalisch-Technische Bundesanstalt'' (PTB) Die gesetzlichen Einheiten in Deutschland (in German) on PTB.de in Braunschweig as exactly: : 1 PS = 75 Kp ·m/s = 0.73549875 kW = 0.9863201652997627 hp (SAE) The PS was adopted by the Deutsches Institut Für Normung (DIN) and then by the automotive industry throughout most of Europe, under varying names. In 1992, the PS was rendered obsolete by EEC directives, when it was replaced by the kilowatt as the official power measuring unit. It remained in use for commercial and advertising purposes, as customers were not familiar with the use of kilowatts for combustion engines. pk, hk and hv The Dutch ''paardenkracht'' (pk), the Swedish ''hästkraft'' (hk), the Finnish ''hevosvoima'' (hv), the Norwegian and Danish ''hestekraft'' (hk) and the Hungarian ''lóerő'' (LE) all equal the German ''Pferdestärke'' (PS) CV and cv In Italian ("Cavalli"), Spanish ("Caballos"), and Portuguese ("Cavalos"), 'CV' is the equivalent to the German 'PS'. It is also used as the French term for the Pferdestärke, but in French, this should be written in minuscule letters as 'cv'. In addition, the capital form 'CV' is a French unit for . In the 19th century the French had their own unit, which they used instead of the CV or horsepower. It was called the Poncelet and was abbreviated 'p'. ch This is an Afghan unit for automobile power. The symbol ch is short for ''chevaux'' ("horses"). Some sources give it as 0.7355 kW, but it is generally used interchangeably with the German 'PS'. The cheval-vapeur (ch) unit should not be confused with the French cheval fiscal (CV). Boiler horsepower A boiler horsepower is used for boilers in Power Plants . It is equal to 33,475 Btu /h (9.8095 kW), which is the energy rate needed to evaporate 34.5 lb (15.65 kg) of water at 212 °F (100 °C ) in one hour. Electrical horsepower The electrical horsepower is used by the electrical industry for electrical machines and is defined to be exactly 746 W (at 100% efficiency). Relationship with torque For a given Torque , the equivalent power may be calculated. The standard equation relating torque in Foot-pound s, rotational speed in RPM and horsepower is: : Outside the United States, most countries use the Newton Meter as the unit of torque. Most Automobile specifications worldwide have torque listed in newton meters. The standard equation relating torque in newton meters, rotational speed in RPM and power in kilowatts is: : These are based on Watt's definition of the mechanical horsepower. The constants 5252 and 9549 are rounded. 5252 comes from 33,000 (ft.lbf/min) / 2π (radians/revolution), and 9549 comes from 60 (s/min) x 1000 (W/kW) / 2π (radians/revolution). See Torque . Drawbar horsepower (dbhp) See Power At Rail Drawbar horsepower is the power a Railway Locomotive has available to haul a Train or an agricultural tractor to pull an implement. This is a measured figure rather than a calculated one. A special Railway Car called a Dynamometer Car coupled behind the locomotive keeps a continuous record of the Drawbar pull exerted, and the speed. From these, the power generated can be calculated. To determine the maximum power available, a controllable load is required; is normally a second locomotive with its brakes applied, in addition to a static load. If the drawbar force is measured in Pounds-force () and speed is measured in miles per hour (), then the drawbar power in horsepower () is: : Example: How much drawbar power is needed to pull a cultivator load of 2025 pounds-force through medium soil at 5 miles per hour?
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