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The '''horsepower''' ('''hp''') is the name of several non-metric units 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 rate of power application of [[internal-combustion engine]]s. | |||
The '''horsepower''' ('''hp''') is the name of several non-metric units 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 | |||
There are two important factors to consider when evaluating a "horsepower" figure: | There are two important factors to consider when evaluating a "horsepower" figure: | ||
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==Definition== | ==Definition== | ||
There have been many definitions for the term over the years since | 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: | ||
* [[#Mechanical horsepower|Mechanical horsepower]] — 0.74569987158227022 [[watt|kW]] (33,000 ft·lbf per minute) | * [[#Mechanical horsepower|Mechanical horsepower]] — 0.74569987158227022 [[watt|kW]] (33,000 ft·lbf per minute) | ||
* [[#Metric horsepower|Metric horsepower]] — 0.73549875 kW | * [[#Metric horsepower|Metric horsepower]] — 0.73549875 kW | ||
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===Mechanical horsepower=== | ===Mechanical horsepower=== | ||
:''See [[#History of the term "horsepower"|History of the term "horsepower"]]'' | :''See [[#History of the term "horsepower"|History of the term "horsepower"]]'' | ||
The most-common definition of horsepower for engines is the one originally proposed by | 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 [[foot (unit of length)|ft]]·[[pound-force|lbf]]·[[minute|min]]<sup>−1</sup> = exactly 0.74569987158227022 kW | : 1 hp = 33,000 [[foot (unit of length)|ft]]·[[pound-force|lbf]]·[[minute|min]]<sup>−1</sup> = exactly 0.74569987158227022 kW | ||
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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. Companies of the United Kingdom often intermix metric horsepower and mechanical horsepower depending on the origin of the engine in question. | 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. Companies of the United Kingdom 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 | 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 ==== | ==== 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)[http://www.ptb.de/] in | 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)[http://www.ptb.de/] in Braunschweig as exactly: | ||
: 1 PS = 75 [[kilopond|kp]]·m/s = 0.73549875 kW = 0.9863201652997627 hp (SAE) | : 1 PS = 75 [[kilopond|kp]]·m/s = 0.73549875 kW = 0.9863201652997627 hp (SAE) | ||
The PS was adopted by the | The PS was adopted by the Deutsches Institut für Normung (DIN) and then by the automotive industry throughout most of Europe, and is always measured at the wheels, as opposed to most factory horsepower figures, which are rated at the crank (which is measured right off the motor, not accounting to any drivetrain loss, which is especially considerable in an All wheel Drive configuration). | ||
In the 19th century, the French did not use this German unit, but had their own, the | In the 19th century, the French did not use this German unit, but had their own, the Poncelet. In 1992, the PS was rendered obsolete by EEC directives, when it was replaced by the kilo[[watt]] as the official power measuring unit, but it continued to be used for commercial and advertising purposes, as customers were not familiar with the use of kilowatts for combustion engines. | ||
==== pk and hk ==== | ==== pk and hk ==== | ||
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==== CV and cv ==== | ==== CV and cv ==== | ||
Often the French name for the Pferdestärke. Also a French unit for | Often the French name for the Pferdestärke. Also a French unit for tax horsepower, short for ''chevaux vapeur'' ("steam horses") or ''cheval-vapeur''.<br> | ||
CV is a nonlinear rating of a motor vehicle for tax purposes[http://home.att.net/~numericana/answer/units.htm]. <br>The CV rating, or fiscal power, is (P/40)<sup>1.6</sup>+ U/45, <br>where P is the maximum power in kW and U is the amount of CO<sub>2</sub> emitted in g/km. The fiscal power has found its way into naming of automobile models, such as the popular [[Citroën 2CV|Citroën deux-chevaux]]. | CV is a nonlinear rating of a motor vehicle for tax purposes[http://home.att.net/~numericana/answer/units.htm]. <br>The CV rating, or fiscal power, is (P/40)<sup>1.6</sup>+ U/45, <br>where P is the maximum power in kW and U is the amount of CO<sub>2</sub> emitted in g/km. The fiscal power has found its way into naming of automobile models, such as the popular [[Citroën 2CV|Citroën deux-chevaux]]. | ||
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=== Drawbar horsepower (dbhp) === | === Drawbar horsepower (dbhp) === | ||
'''Drawbar horsepower''' is the power a [[railroad]] [[locomotive]] has available to haul a | '''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]] (<math>F / {\rm lbf}</math>) and speed is measured in miles per hour (<math>v / ({\rm mi/h})</math>), then the drawbar power in horsepower (<math>P / {\rm hp}</math>) is: | If the drawbar force is measured [[pounds-force]] (<math>F / {\rm lbf}</math>) and speed is measured in miles per hour (<math>v / ({\rm mi/h})</math>), then the drawbar power in horsepower (<math>P / {\rm hp}</math>) is: | ||
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=== RAC horsepower (taxable horsepower) === | === RAC horsepower (taxable horsepower) === | ||
This measure was instituted by the | This measure was instituted by the Royal Automobile Club in Britain and used to denote the power of early 20th century British cars. Many cars took their names from this figure (hence the Austin Seven and Riley Nine), while others had names such as "40/50hp", which indicated the RAC figure followed by the true measured power. | ||
Taxable horsepower does not reflect developed horsepower; rather, it is a calculated figure based on the engine's bore size, number of cylinders, and a (now archaic) presumption of engine efficiency. As new engines were designed with ever-increasing efficiency, it was no longer a useful measure, but was kept in use by UK regulations which used the rating for | Taxable horsepower does not reflect developed horsepower; rather, it is a calculated figure based on the engine's bore size, number of cylinders, and a (now archaic) presumption of engine efficiency. As new engines were designed with ever-increasing efficiency, it was no longer a useful measure, but was kept in use by UK regulations which used the rating for tax purposes. | ||
:<math>RAC h.p. = {D^2 * n}/2.5 \,</math> | :<math>RAC h.p. = {D^2 * n}/2.5 \,</math> | ||
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Since taxable horsepower was computed based on bore and number of cylinders, not based on actual displacement, it gave rise to engines with 'undersquare' dimensions, i.e. relatively narrow bore, but long stroke; this tended to impose an artificially low limit on rotational speed ([[Revolutions per minute|rpm]]), hampering the true power output and efficiency of the engine. | Since taxable horsepower was computed based on bore and number of cylinders, not based on actual displacement, it gave rise to engines with 'undersquare' dimensions, i.e. relatively narrow bore, but long stroke; this tended to impose an artificially low limit on rotational speed ([[Revolutions per minute|rpm]]), hampering the true power output and efficiency of the engine. | ||
The situation persisted for several generations of four- and six-cylinder British engines: for example, | The situation persisted for several generations of four- and six-cylinder British engines: for example, Jaguar's 3.8-litre XK engine had six cylinders with a bore of 87 mm (3.43 inches) and a stroke of 106 mm (4.17 inches), where most American automakers had long since moved to oversquare (wide bore, short stroke) [[V-8]]s. | ||
==Measurement== | ==Measurement== | ||