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SHP redirects here. For Sosyaldemokrat Halk Partisi please see Social Democratic People's Party (Turkey) The horsepower ('''hp''') is the name of several non-metric Unit s of Power . In scientific discourse the term "horsepower" is rarely used due to the various definitions and the existence of an SI unit for power, the Watt (W). However, the idea of horsepower persists as a legacy term in many languages, particularly in the Automotive industry for listing the maximum power of Internal-combustion Engine s. There are two important factors to consider when evaluating a "horsepower" figure:
These factors can be combined in unexpected ways — the true power output for an engine rated at "100 horsepower" might vary significantly from a reader's expectations. For this reason, various groups have attempted to standardize both the definition and measurement system, often leading to even more confusion. Note also that, although the SI watt is not subject to varying definitions, it can still vary based on the measurement conditions. Definition There have been many definitions for the term over the years since James Watt first coined the term in 1782 . The following metrics have been widely used:
Additionally, the term "horsepower" has been applied to calculated (rather than measured) metrics:
Mechanical horsepower The most-common definition of horsepower for engines is the one originally proposed by James Watt in 1782. Under this system, one horsepower is defined as: : 1 hp = 33,000 Ft · Lbf · Min −1 = exactly 0.74569987158227022 kW A common memory aid is based on the fact that Christopher Columbus first sailed to The Americas in 1492 . The memory aid states that 1 hp = 1/2 Columbus or 746 W. : ''In fourteen hundred and ninety-two'' : ''Columbus sailed the ocean blue''. : ''Divide that Son-of-a-gun by two'' : ''And that's the number of watts in a horsepower too''. Metric horsepower Metric horsepower began in Germany in the nineteenth century and became widely used 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. Companies of the United Kingdom often intermix both 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 due to 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 "horse power", but mathematically different from the British unit. It is defined by the ''Physikalisch-Technische Bundesanstalt'' (PTB) {Link without Title} 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 subsequently, by the automotive industry throughout most of Europe. In the nineteenth century, however, the French did not use this German unit, but had one of their own, the Poncelet . In 1992, the PS was rendered obsolete by EEC directives, where it was replaced by the kilo Watt as the official power measuring unit, but in situations where horsepower was used for commercial and advertising purposes, it continued to be used, as customers are not familiar with the usage of kilowatts for combustion engines. pk A Dutch ''paardekracht'' equals the German ''Pferdestärke'' hence : 1 pk = 0.73549875 kW CV Often the French name for the Pferdestärke. Also a French unit for Tax Horsepower , short for ''chevaux vapeur'' ("steam horses") or ''cheval-vapeur''. In Italian ("Cavalli"), Spanish ("Caballos"), and Portuguese ("Cavalos"), 'CV' is the equivalent to the German 'PS'. ch This is a French 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'. 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 an hour. Electrical horsepower The electrical horsepower is used by the electrical industry for electric motors 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: :. This is based on Watt's definition of the mechanical horsepower. The constant 5252 is rounded; the exact value is 16,500/π. See Torque . Drawbar horsepower (dbhp) Drawbar horsepower is the power a Railroad 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 Railroad 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; this is normally a second locomotive with its brakes applied, in addition to a static load. If the drawbar force is measured 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?
This was rounded to an even 33,000 ft·lbf/min {Link without Title} . Others recount that Watt determined that a pony could lift an average 220 pounds 100 feet (30 m) per minute over a four-hour working shift. Watt then judged a horse was 50% more powerful than a pony and thus arrived at the 33,000 ft·lbf/min figure {Link without Title} . ''Engineering in History'' recounts that John Smeaton initially estimated that a horse could produce 22,916 foot-pounds per minute. John Desaguliers increased that to 27,500 foot-pounds per minute. "Watt found by experiment in 1782 that a 'brewery horse' was able to produce 32,400 foot-pounds per minute". James Watt and Matthew Boulton standardized that figure at 33,000 the next year {Link without Title} . Put into perspective, a healthy human can sustain about 0.1 horsepower, and trained athletes can manage up to about 0.3 horsepower for a period of several hours. Most observers familiar with horses and their capabilities estimate that Watt was either a bit optimistic or intended to underpromise and overdeliver; few horses can maintain that effort for long. Regardless, comparison to a horse proved to be an enduring marketing tool. Conversion of historical definition to watts The historical value of 33,000 ft·lbf/min may be converted to the SI unit of watts by using the following Conversion Of Units factors:
: And the Watt is defined as so the historical figure of 33,000 ft·lbf/min converts exactly to the modern definition. References
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