Horsepower (hp) is a
unit of measurement
A unit of measurement, or unit of measure, is a definite magnitude (mathematics), magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other qua ...
of
power, or the rate at which
work
Work may refer to:
* Work (human activity), intentional activity people perform to support themselves, others, or the community
** Manual labour, physical work done by humans
** House work, housework, or homemaking
** Working animal, an ani ...
is done, usually in reference to the output of engines or motors. There are many different standards and types of horsepower. Two common definitions used today are the imperial horsepower as in "hp" or "bhp" which is about , and the metric horsepower as in "cv" or "PS" which is approximately . The electric horsepower "hpE" is exactly , while the boiler horsepower is 9809.5 or 9811 watts, depending on the exact year.
The term was adopted in the late 18th century by
Scottish engineer
James Watt
James Watt (; 30 January 1736 (19 January 1736 OS) – 25 August 1819) was a Scottish inventor, mechanical engineer, and chemist who improved on Thomas Newcomen's 1712 Newcomen steam engine with his Watt steam engine in 1776, which was f ...
to compare the output of
steam engine
A steam engine is a heat engine that performs Work (physics), mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a Cylinder (locomotive), cyl ...
s with the power of
draft horse
A draft horse (US) or draught horse (UK), also known as dray horse, carthorse, work horse or heavy horse, is a large horse bred to be a working animal hauling freight and doing heavy agricultural tasks such as plowing. There are a number o ...
s. It was later expanded to include the output power of other power-generating machinery such as
piston engines
A reciprocating engine, more often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common features of all ...
,
turbine
A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced can be used for generating electrical ...
s, and
electric motor
An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a electromagnetic coil, wire winding to gene ...
s. The definition of the unit varied among geographical regions. Most countries now use the
SI unit
watt
The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
for measurement of power. With the implementation of the EU Directive
80/181/EEC on 1 January 2010, the use of horsepower in the EU is permitted only as a supplementary unit.
History

The development of the
steam engine
A steam engine is a heat engine that performs Work (physics), mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a Cylinder (locomotive), cyl ...
provided a reason to compare the output of horses with that of the engines that could replace them. In 1702,
Thomas Savery
Thomas Savery (; c. 1650 – 15 May 1715) was an English inventor and engineer. He invented the first commercially used steam-powered device, a steam pump which is often referred to as the "Savery engine". Savery's steam pump was a revolutiona ...
wrote in ''The Miner's Friend'':
:So that an engine which will raise as much water as two horses, working together at one time in such a work, can do, and for which there must be constantly kept ten or twelve horses for doing the same. Then I say, such an engine may be made large enough to do the work required in employing eight, ten, fifteen, or twenty horses to be constantly maintained and kept for doing such a work...
The idea was later used by
James Watt
James Watt (; 30 January 1736 (19 January 1736 OS) – 25 August 1819) was a Scottish inventor, mechanical engineer, and chemist who improved on Thomas Newcomen's 1712 Newcomen steam engine with his Watt steam engine in 1776, which was f ...
to help market the
Watt steam engine
The Watt steam engine design was an invention of James Watt that became synonymous with steam engines during the Industrial Revolution, and it was many years before significantly new designs began to replace the basic Watt design.
The Newcomen ...
, an improved
Newcomen steam engine
The atmospheric engine was invented by Thomas Newcomen in 1712, and is sometimes referred to as the Newcomen fire engine (see below) or Newcomen engine. The engine was operated by condensing steam being drawn into the cylinder, thereby creating ...
. He had previously agreed to take royalties of one-third of the savings in coal from the older Newcomen steam engines. This royalty scheme did not work with customers who did not have existing steam engines but used horses instead.
Watt determined that a horse could turn a
mill wheel 144 times in an hour (or 2.4 times a minute).
[Hart-Davis, Adam (2012). ''Engineers''. Dorling Kindersley. p. 121.] The wheel was in radius; therefore, the horse travelled feet in one minute. Watt judged that the horse could pull with a
force
In physics, a force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...
of . So:
:
''Engineering in History'' recounts that
John Smeaton
John Smeaton (8 June 1724 – 28 October 1792) was an English civil engineer responsible for the design of bridges, canals, harbours and lighthouses. He was also a capable mechanical engineer and an eminent scholar, who introduced various ...
initially estimated that a horse could produce per minute.
John Desaguliers had previously suggested per minute, and
Thomas Tredgold
Thomas Tredgold (22 August 1788 – 28 January 1829) was an English engineer and author, known for his early work on railroad construction. His definition of civil engineering formed the basis of the charter of the Institution of Civil Engineers ...
suggested per minute. "Watt found by experiment in 1782 that a '
brewery horse' could produce per minute."
James Watt and
Matthew Boulton
Matthew Boulton ( ; 3 September 172817 August 1809) was an English businessman, inventor, mechanical engineer, and silversmith. He was a business partner of the Scottish engineer James Watt. In the final quarter of the 18th century, the par ...
standardized that figure at per minute the next year.
A common legend states that the unit was created when one of Watt's first customers, a brewer, specifically demanded an engine that would match a horse, and chose the strongest horse he had and driving it to the limit. In that legend, Watt accepted the challenge and built a machine that was actually even stronger than the figure achieved by the brewer, and the output of that machine became the horsepower.
In 1993, R. D. Stevenson and R. J. Wassersug published correspondence in ''
Nature
Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...
'' summarizing measurements and calculations of peak and sustained work rates of a horse.
Citing measurements made at the 1925
Iowa State Fair
The Iowa State Fair is an annual state fair held in Des Moines, Iowa, every August.
It began in 1854 and has been held on the Iowa State Fairgrounds since 1886. It is based in the state capital Des Moines, Iowa over an 11 day period in August ...
, they reported that the peak power over a few seconds has been measured to be as high as and also observed that for sustained activity, a work rate of about per horse is consistent with agricultural advice from both the 19th and 20th centuries and also consistent with a work rate of about four times the
basal rate
Basal metabolic rate (BMR) is the rate of energy expenditure per unit time by endothermic animals at rest.. In other words it is the energy required by body organs to perform normal It is reported in energy units per unit time ranging from watt ( ...
expended by other vertebrates for sustained activity.
When considering
human-powered equipment, a healthy human can produce about briefly (see
orders of magnitude
In a ratio scale based on powers of ten, the order of magnitude is a measure of the nearness of two figures. Two numbers are "within an order of magnitude" of each other if their ratio is between 1/10 and 10. In other words, the two numbers are wi ...
) and sustain about indefinitely; trained athletes can manage up to about briefly
and for a period of several hours. The Jamaican sprinter
Usain Bolt
Usain St. Leo Bolt (; born 21 August 1986) is a Jamaican retired sprinter who is widely regarded as the greatest sprinter of all time. He is an eight-time Olympic gold medalist and the world record holder in the 100 metres, 200 metres, ...
produced a maximum of 0.89 seconds into his 9.58 second sprint world record in 2009.
In 2023 a group of engineers modified a
dynamometer
A dynamometer or "dyno" is a device for simultaneously measuring the torque and rotational speed ( RPM) of an engine, motor or other rotating prime mover so that its instantaneous power may be calculated, and usually displayed by the dyna ...
to be able to measure how much power a horse can produce. This horse was measured to .
Calculating power
When
torque
In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...
is in
pound-foot units,
rotational speed
Rotational frequency, also known as rotational speed or rate of rotation (symbols ''ν'', lowercase Greek nu, and also ''n''), is the frequency of rotation of an object around an axis.
Its SI unit is the reciprocal seconds (s−1); other com ...
is in
rpm
Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or r⋅min−1) is a unit of rotational speed (or rotational frequency) for rotating machines.
One revolution per minute is equivalent to hertz.
Standards
ISO 80000-3:2019 def ...
, the resulting power in horsepower is
:
The constant 5252 is the
rounded value of (33,000 ft⋅lbf/min)/(2π rad/rev).
When torque is in inch-pounds,
:
The constant 63,025 is the approximation of
:
Definitions
Imperial horsepower
Assuming the third
CGPM
The General Conference on Weights and Measures (abbreviated CGPM from the ) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established in 1875 under the terms of the Metre ...
(1901, CR 70) definition of
standard gravity
The standard acceleration of gravity or standard acceleration of free fall, often called simply standard gravity and denoted by or , is the nominal gravitational acceleration of an object in a vacuum near the surface of the Earth. It is a constant ...
, , is used to define the pound-force as well as the kilogram force, and the
international avoirdupois pound (1959), one imperial horsepower is:
:
Or given that 1 hp = 550 ft⋅lbf/s, 1 ft = 0.3048 m, 1 lbf ≈ 4.448 N, 1 J = 1 N⋅m, 1 W = 1 J/s: 1 hp ≈ 745.7 W
Metric horsepower (PS, KM, cv, hk, pk, k, ks, ch)

The various units used to indicate this definition (''PS'', '' KM'', ''cv'', ''hk'', ''pk'', ''k'', ''ks'' and ''ch'') all translate to ''horse power'' in English. British manufacturers often intermix metric horsepower and mechanical horsepower depending on the origin of the engine in question.
DIN
DIN or Din or din may refer to:
People and language
* Din (name), people with the name
* Dīn, an Arabic word with three general senses: judgment, custom, and religion from which the name originates
* Dinka language (ISO 639 code: din), spoken ...
66036 defines one metric horsepower (Pferdestärke, or PS) as the power to raise a mass of 75 kilograms against the Earth's gravitational force over a distance of one metre in one second: = 75
⋅m/s = 1 PS. This is equivalent to 735.49875 W, or 98.6% of an imperial horsepower. In 1972, the PS was replaced by the
kilowatt
The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
as the official power-measuring unit in EEC directives.
Other names for the metric horsepower are the Italian , Dutch , the French , the Spanish and Portuguese , the Russian , the Swedish , the Finnish , the Estonian , the Norwegian and Danish , the Hungarian , the Czech and Slovak or ), the Serbo-Croatian , the Bulgarian , the Macedonian , the Polish (), Slovenian , the Ukrainian , the Romanian , and the German .
In the 19th century,
revolutionary-era France had its own unit used to replace the ''cheval vapeur'' (horsepower); based on a 100
kgf⋅m/s standard, it was called the
poncelet
The poncelet (symbol p) is an obsolete unit of power, once used in France and replaced by (ch, metric horsepower). The unit was named after Jean-Victor Poncelet.François Cardarelli, ''Encyclopaedia of Scientific Units, Weights and Measures: T ...
and was abbreviated ''p''.
Tax horsepower
Tax or fiscal horsepower is a non-linear rating of a motor vehicle for tax purposes. Tax horsepower ratings were originally more or less directly related to the size of the engine; but as of 2000, many countries changed over to systems based on emissions, so are not directly comparable to older ratings. The
Citroën 2CV
The Citroën 2CV (, , lit. "two horses", meaning "two Tax horsepower#France, ''taxable'' horsepower") is an economy car produced by the French company Citroën from 1948 to 1990. Introduced at the 1948 Paris Paris Auto Show, Salon de l'Automobi ...
is named for its French fiscal horsepower rating, "deux chevaux" (2CV).
Electrical horsepower
Nameplates on electrical motors show their available shaft power output, not the electrical power input. This power output is ordinarily stated in watts or kilowatts. In the United States, the power output is stated in horsepower. Wattage is calculated by multiplying voltage by amperage and power factor.
Hydraulic horsepower
Hydraulic horsepower can represent the power available within
hydraulic machinery
Hydraulic machines use liquid fluid power to perform work. Heavy construction vehicles are a common example. In this type of machine, hydraulic fluid is pumped to various hydraulic motors and hydraulic cylinders throughout the machine a ...
, power through the down-hole nozzle of a
drilling rig
A drilling rig is an integrated system that Drilling, drills wells, such as oil or water wells, or holes for piling and other construction purposes, into the earth's subsurface. Drilling rigs can be massive structures housing equipment used to ...
,
or can be used to estimate the mechanical power needed to generate a known hydraulic flow rate.
It may be calculated as
:
where pressure is in psi, and flow rate is in
US gallon
The gallon is a unit of volume in British imperial units and United States customary units.
The imperial gallon (imp gal) is defined as , and is or was used in the United Kingdom and its former colonies, including Ireland, Canada, Australia, ...
s per minute.
Drilling rigs are powered mechanically by rotating the drill pipe from above. Hydraulic power is still needed though, as 1 500 to 5 000 W are required to push
mud
Mud (, or Middle Dutch) is loam, silt or clay mixed with water. Mud is usually formed after rainfall or near water sources. Ancient mud deposits hardened over geological time to form sedimentary rock such as shale or mudstone (generally cal ...
through the drill bit to clear waste rock. Additional hydraulic power may also be used to drive a down-hole mud motor to power
directional drilling.
When using SI units, the equation becomes coherent and there is no dividing constant.
:
where pressure is in pascals (Pa), and flow rate is in
cubic metre
The cubic metre (in Commonwealth English and international spelling as used by the International Bureau of Weights and Measures) or cubic meter (in American English) is the unit of volume in the International System of Units (SI). Its symbol is m ...
s per second (m
3).
Boiler horsepower
Boiler horsepower is a
boiler
A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, centra ...
's capacity to deliver
steam
Steam is water vapor, often mixed with air or an aerosol of liquid water droplets. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization. Saturated or superheated steam is inv ...
to a
steam engine
A steam engine is a heat engine that performs Work (physics), mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a Cylinder (locomotive), cyl ...
and is not the same unit of power as the 550 ft lb/s definition. One boiler horsepower is equal to the thermal energy rate required to evaporate of fresh water at in one hour. In the early days of steam use, the boiler horsepower was roughly comparable to the horsepower of engines fed by the boiler.
The term "boiler horsepower" was originally developed at the
Philadelphia Centennial Exhibition in 1876, where the best steam engines of that period were tested. The average steam consumption of those engines (per output horsepower) was determined to be the evaporation of of water per hour, based on feed water at , and saturated steam generated at . This original definition is equivalent to a boiler heat output of . A few years later in 1884, the
ASME
The American Society of Mechanical Engineers (ASME) is an American professional association that, in its own words, "promotes the art, science, and practice of multidisciplinary engineering and allied sciences around the globe" via "continuing edu ...
re-defined the boiler horsepower as the thermal output equal to the evaporation of 34.5 pounds per hour of water "from and at" . This considerably simplified boiler testing, and provided more accurate comparisons of the boilers at that time. This revised definition is equivalent to a boiler heat output of . Present industrial practice is to define "boiler horsepower" as a boiler thermal output equal to , which is very close to the original and revised definitions.
Boiler horsepower is still used to measure boiler output in industrial boiler engineering in the US. Boiler horsepower is abbreviated BHP, which is also used in many places to symbolize brake horsepower.
Drawbar power
Drawbar power (dbp) is the power a
railway
Rail transport (also known as train transport) is a means of transport using wheeled vehicles running in railway track, tracks, which usually consist of two parallel steel railway track, rails. Rail transport is one of the two primary means of ...
locomotive
A locomotive is a rail transport, rail vehicle that provides the motive power for a train. Traditionally, locomotives pulled trains from the front. However, Push–pull train, push–pull operation has become common, and in the pursuit for ...
has available to haul a
train
A train (from Old French , from Latin">-4; we might wonder whether there's a point at which it's appropriate to talk of the beginnings of French, that is, when it wa ... , from Latin , "to pull, to draw") is a series of connected vehicles th ...
or an agricultural tractor to pull an implement. This is a measured figure rather than a calculated one. A special
railway car
A railroad car, railcar ( American and Canadian English), railway wagon, railway carriage, railway truck, railwagon, railcarriage or railtruck (British English and UIC), also called a train car, train wagon, train carriage or train truc ...
called a
dynamometer car
A dynamometer car is a railroad maintenance of way car used for measuring various aspects of a locomotive's performance. Measurements include tractive effort (pulling force), power, top speed, etc.
History
The first dynamometer car was probably ...
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; it is normally a second locomotive with its brakes applied, in addition to a static load.
If the drawbar force () is measured in pounds-force (lbf) and speed () is measured in miles per hour (mph), then the drawbar power () in horsepower (hp) is
Example: How much power is needed to pull a drawbar load of 2,025 pounds-force at 5 miles per hour?
The constant 375 is because 1 hp = 375 lbf⋅mph. If other units are used, the constant is different. When using coherent
SI units (watts, newtons, and metres per second), no constant is needed, and the formula becomes .
This formula may also be used to calculate the power of a jet engine, using the speed of the jet and the thrust required to maintain that speed.
Example: how much power is generated with a thrust of 4000 pounds at 400 miles per hour?
RAC horsepower (taxable horsepower)
This measure was instituted by the
Royal Automobile Club
The Royal Automobile Club is a British private Club (organization)#Country or sports club, social and athletic club. It has two clubhouses: one in London at 89 Pall Mall, London, Pall Mall, and the other in the countryside at Woodcote Park, ne ...
and was 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 "40/50 hp", 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
tax purposes. The United Kingdom was not the only country that used the RAC rating; many states in Australia used RAC hp to determine taxation. The RAC formula was sometimes applied in British colonies as well, such as
Kenya (British East Africa).
:
where
: ''D'' is the diameter (or
bore) of the cylinder in inches,
: ''n'' is the number of cylinders.
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 (bore smaller than stroke), which tended to impose an artificially low limit on
rotational speed
Rotational frequency, also known as rotational speed or rate of rotation (symbols ''ν'', lowercase Greek nu, and also ''n''), is the frequency of rotation of an object around an axis.
Its SI unit is the reciprocal seconds (s−1); other com ...
, hampering the potential power output and efficiency of the engine.
The situation persisted for several generations of four- and six-cylinder British engines: For example,
Jaguar's 3.4-litre XK engine of the 1950s had six cylinders with a bore of and a stroke of , where most American automakers had long since moved to oversquare (large bore, short stroke)
V8 engine
A V8 engine is an eight- cylinder piston engine in which two banks of four cylinders share a common crankshaft and are arranged in a V configuration.
Origins
The first known V8 was the Antoinette, designed by Léon Levavasseur, a ...
s. See, for example, the early
Chrysler Hemi engine
The Chrysler Hemi engine, known by the trademark Hemi or HEMI, refers to a series of high-performance American overhead valve V8 engines built by Chrysler with hemispherical combustion chambers. Three generations have been produced: the Fire ...
.
Measurement
The power of an engine may be measured or estimated at several points in the transmission of the power from its generation to its application. A number of names are used for the power developed at various stages in this process, but none is a clear indicator of either the measurement system or definition used.
In general:
:
nominal horsepower is derived from the size of the engine and the piston speed and is only accurate at a steam pressure of ;
:
indicated
In medicine, an indication is a valid reason to use a certain test, medication, procedure, or surgery. There can be multiple indications to use a procedure or medication. An indication can commonly be confused with the term diagnosis. A diagnosis ...
or gross horsepower is the theoretical capability of the engine
LAN/ 33000
:
brake
A brake is a machine, mechanical device that inhibits motion by absorbing energy from a moving system. It is used for Acceleration, slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of ...
/net/crankshaft horsepower (power delivered directly to and measured at the engine's crankshaft) equals
::indicated horsepower minus frictional losses within the engine (bearing drag, rod and crankshaft windage losses, oil film drag, etc.);
:
shaft horsepower (power delivered to and measured at the output shaft of the transmission, when present in the system) equals
::crankshaft horsepower minus frictional losses in the transmission (bearings, gears, oil drag, windage, etc.);
:effective or true (thp), commonly referred to as wheel horsepower (whp), equals
::shaft horsepower minus frictional losses in the universal joint/s, differential, wheel bearings, tire and chain, (if present).
All the above assumes that no power inflation factors have been applied to any of the readings.
Engine designers use expressions other than horsepower to denote objective targets or performance, such as brake mean effective pressure (BMEP). This is a coefficient of theoretical brake horsepower and cylinder pressures during combustion.
Nominal horsepower
Nominal horsepower (nhp) is an early 19th-century
rule of thumb
In English language, English, the phrase ''rule of thumb'' refers to an approximate method for doing something, based on practical experience rather than theory. This usage of the phrase can be traced back to the 17th century and has been associat ...
used to estimate the power of steam engines.
It assumed a steam pressure of .
[
Nominal horsepower = 7 × area of piston in square inches × equivalent piston speed in feet per minute/33,000.
For paddle ships, the Admiralty rule was that the piston speed in feet per minute was taken as 129.7 × (stroke)1/3.38.][ For screw steamers, the intended piston speed was used.][
The stroke (or length of stroke) was the distance moved by the piston measured in feet.
For the nominal horsepower to equal the actual power it would be necessary for the mean steam pressure in the cylinder during the stroke to be and for the piston speed to be that generated by the assumed relationship for paddle ships.]
The French Navy used the same definition of nominal horse power as the Royal Navy.
Indicated horsepower
Indicated horsepower (ihp) is the theoretical power of a reciprocating engine if it is completely frictionless in converting the expanding gas energy (piston pressure × displacement) in the cylinders. It is calculated from the pressures developed in the cylinders, measured by a device called an '' engine indicator'' – hence indicated horsepower. As the piston advances throughout its stroke, the pressure against the piston generally decreases, and the indicator device usually generates a graph of pressure vs stroke within the working cylinder. From this graph the amount of work performed during the piston stroke may be calculated.
Indicated horsepower was a better measure of engine power than nominal horsepower (nhp) because it took account of steam pressure. But unlike later measures such as shaft horsepower (shp) and brake horsepower (bhp), it did not take into account power losses due to the machinery internal frictional losses, such as a piston sliding within the cylinder, plus bearing friction, transmission and gear box friction, etc.
Brake horsepower
Brake horsepower (bhp) is the power measured using a brake type (load) dynamometer at a specified location, such as the crankshaft, output shaft of the transmission, rear axle or rear wheels.
In Europe, the DIN 70020 standard tests the engine fitted with all ancillaries and the exhaust system as used in the car. The older American standard ( SAE gross horsepower, referred to as ''bhp'') used an engine without alternator
An alternator (or synchronous generator) is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field wit ...
, water pump, and other auxiliary components such as power steering pump, muffled exhaust system, etc., so the figures were higher than the European figures for the same engine. The newer American standard (referred to as SAE net horsepower) tests an engine with all the auxiliary components (see "Engine power test standards" below).
''Brake'' refers to the device which is used to provide an equal braking force, load to balance, or equal an engine's output force and hold it at a desired rotational speed. During testing, the output torque and rotational speed are measured to determine the brake horsepower. Horsepower was originally measured and calculated by use of the "indicator diagram" (a James Watt invention of the late 18th century), and later by means of a Prony brake connected to the engine's output shaft. Modern dynamometer
A dynamometer or "dyno" is a device for simultaneously measuring the torque and rotational speed ( RPM) of an engine, motor or other rotating prime mover so that its instantaneous power may be calculated, and usually displayed by the dyna ...
s use any of several braking methods to measure the engine's brake horsepower, the actual output of the engine itself, before losses to the drivetrain.
Shaft horsepower
Shaft horsepower (shp) is the power delivered to a propeller or turbine shaft. Shaft horsepower is a common rating for turboshaft and turboprop engines, industrial turbines, and some marine applications.
Equivalent shaft horsepower (eshp) is sometimes used to rate turboprop
A turboprop is a Gas turbine, gas turbine engine that drives an aircraft Propeller (aeronautics), propeller.
A turboprop consists of an intake, reduction drive, reduction gearbox, gas compressor, compressor, combustor, turbine, and a propellin ...
engines. It includes the equivalent power derived from residual jet thrust from the turbine exhaust. of residual jet thrust is estimated to be produced from one unit of horsepower.
Engine power test standards
There exist a number of different standards determining how the power and torque of an automobile engine is measured and corrected. Correction factors are used to adjust power and torque measurements to standard atmospheric conditions, to provide a more accurate comparison between engines as they are affected by the pressure, humidity, and temperature of ambient air. Some standards are described below.
Society of Automotive Engineers/SAE International
Early "SAE horsepower"
In the early twentieth century, a so-called "SAE horsepower" was sometimes quoted for U.S. automobiles. This long predates the Society of Automotive Engineers (SAE) horsepower measurement standards and was another name for the industry standard ALAM
Alam is a masculine name derived from several ancient languages including :
# Arabic: (''ʿĀlam'') meaning "world" or "universe"
# Hebrew: cognate word is transcribed as '' Olam'' or Ulam, also meaning "World"
# Tagalog: ''Alam'' means "K ...
or NACC horsepower figure and the same as the British RAC horsepower also used for tax purposes. Alliance for Automotive Innovation
The Alliance for Automotive Innovation (AAI) is a Washington, D.C.–based trade association and Lobbying, lobby group whose members include international car and Light truck, light duty truck manufacturers that build and sell products in the Uni ...
is the current successor of ALAM and NACC.
SAE gross power
Prior to the 1972 model year, American automakers rated and advertised their engines in brake horsepower, '' bhp'', which was a version of brake horsepower called SAE gross horsepower because it was measured according to Society of Automotive Engineers (SAE) standards (J245 and J1995) that call for a stock test engine without accessories (such as dynamo/alternator, radiator fan, water pump), and sometimes fitted with long tube test headers in lieu of the OEM exhaust manifolds. This contrasts with both SAE net power and DIN 70020 standards, which account for engine accessories (but not transmission losses). The atmospheric correction standards for barometric pressure, humidity and temperature for SAE gross power testing were relatively idealistic.
SAE net power
In the United States, the term '' bhp'' fell into disuse in 1971–1972, as automakers began to quote power in terms of SAE net horsepower in accord with SAE standard J1349. Like SAE gross and other brake horsepower protocols, SAE net hp is measured at the engine's crankshaft, and so does not account for transmission losses. However, similar to the DIN 70020 standard, SAE net power testing protocol calls for standard production-type belt-driven accessories, air cleaner, emission controls, exhaust system, and other power-consuming accessories. This produces ratings in closer alignment with the power produced by the engine as it is actually configured and sold.
SAE certified power
In 2005, the SAE introduced "SAE Certified Power" with SAE J2723. To attain certification the test must follow the SAE standard in question, take place in an ISO 9000
The ISO 9000 family is a set of international standards for Quality (business), quality management systems. It was developed in March 1987 by International Organization for Standardization. The goal of these standards is to help organizations en ...
/9002 certified facility and be witnessed by an SAE approved third party.
A few manufacturers such as Honda and Toyota switched to the new ratings immediately.[Jeff Plungis, ''Asians Oversell Horsepower'', Detroit News] The rating for Toyota's Camry 3.0 L '' 1MZ-FE'' V6 fell from .[ The company's Lexus ES 330 and Camry SE V6 (3.3 L V6) were previously rated at but the ES 330 dropped to while the Camry declined to . The first engine certified under the new program was the 7.0 L LS7 used in the 2006 ]Chevrolet Corvette
The Chevrolet Corvette is a line of American two-door, two-seater sports cars manufactured and marketed by General Motors under the Chevrolet marque since 1953. Throughout eight generations, indicated sequentially as C1 to C8, the Corvette is not ...
Z06. Certified power rose slightly from .
While Toyota and Honda are retesting their entire vehicle lineups, other automakers generally are retesting only those with updated powertrains.[ For example, the 2006 Ford Five Hundred is rated at , the same as that of 2005 model. However, the 2006 rating does not reflect the new SAE testing procedure, as Ford did not opt to incur the extra expense of retesting its existing engines.][ Over time, most automakers are expected to comply with the new guidelines.
SAE tightened its horsepower rules to eliminate the opportunity for engine manufacturers to manipulate factors affecting performance such as how much oil was in the crankcase, engine control system calibration, and whether an engine was tested with high octane fuel. In some cases, such can add up to a change in horsepower ratings.
]
''Deutsches Institut für Normung'' 70020 (DIN 70020)
DIN 70020 is a German DIN
DIN or Din or din may refer to:
People and language
* Din (name), people with the name
* Dīn, an Arabic word with three general senses: judgment, custom, and religion from which the name originates
* Dinka language (ISO 639 code: din), spoken ...
standard for measuring road vehicle horsepower. DIN hp is measured at the engine's output shaft as a form of metric horsepower rather than mechanical horsepower. Similar to SAE net power rating, and unlike SAE gross power, DIN testing measures the engine as installed in the vehicle, with cooling system, charging system and stock exhaust system all connected. DIN hp is often abbreviated as " PS", derived from the German word ''Pferdestärke'' (literally, "horse strength").
CUNA
A test standard by Italian
Italian(s) may refer to:
* Anything of, from, or related to the people of Italy over the centuries
** Italians, a Romance ethnic group related to or simply a citizen of the Italian Republic or Italian Kingdom
** Italian language, a Romance languag ...
CUNA (''Commissione Tecnica per l'Unificazione nell'Automobile'', Technical Commission for Automobile Unification), a federated entity of standards organisation
A standards organization, standards body, standards developing organization (SDO), or standards setting organization (SSO) is an organization whose primary function is developing, coordinating, promulgating, revising, amending, reissuing, interpr ...
UNI, was formerly used in Italy.
CUNA prescribed that the engine be tested with all accessories necessary to its running fitted (such as the water pump), while all others – such as alternator/dynamo, radiator fan, and exhaust manifold – could be omitted. All calibration and accessories had to be as on production engines.
Economic Commission for Europe R24
ECE R24 is a UN standard for the approval of compression ignition engine emissions, installation and measurement of engine power. It is similar to DIN 70020 standard, but with different requirements for connecting an engine's fan during testing causing it to absorb less power from the engine.
Economic Commission for Europe R85
ECE R85 is a UN standard for the approval of internal combustion engines with regard to the measurement of the net power.
80/1269/EEC
80/1269/EEC of 16 December 1980 is a European Union standard for road vehicle engine power.
International Organization for Standardization
The International Organization for Standardization
The International Organization for Standardization (ISO ; ; ) is an independent, non-governmental, international standard development organization composed of representatives from the national standards organizations of member countries.
M ...
(ISO) publishes several standards for measuring engine horsepower.
* ISO 14396 specifies the additional and method requirement for determining the power of reciprocating internal combustion engines when presented for an ISO 8178 exhaust emission test. It applies to reciprocating internal combustion engines for land, rail and marine use excluding engines of motor vehicles primarily designed for road use.
* ISO 1585 is an engine net power test code intended for road vehicles.
* ISO 2534 is an engine gross power test code intended for road vehicles.
* ISO 4164 is an engine net power test code intended for mopeds.
* ISO 4106 is an engine net power test code intended for motorcycles.
* ISO 9249 is an engine net power test code intended for earth moving machines.
Japanese Industrial Standard D 1001
JIS D 1001 is a Japanese net, and gross, engine power test code for automobile
A car, or an automobile, is a motor vehicle with wheels. Most definitions of cars state that they run primarily on roads, Car seat, seat one to eight people, have four wheels, and mainly transport private transport#Personal transport, peopl ...
s or truck
A truck or lorry is a motor vehicle designed to transport freight, carry specialized payloads, or perform other utilitarian work. Trucks vary greatly in size, power, and configuration, but the vast majority feature body-on-frame construct ...
s having a spark ignition, diesel engine
The diesel engine, named after the German engineer Rudolf Diesel, is an internal combustion engine in which Combustion, ignition of diesel fuel is caused by the elevated temperature of the air in the cylinder due to Mechanics, mechanical Compr ...
, or fuel injection engine.
See also
* Brake-specific fuel consumption
Brake-specific fuel consumption (BSFC) is a measure of the fuel efficiency of any prime mover that burns fuel and produces rotational, or shaft power. It is typically used for comparing the efficiency of internal combustion engines with a shaft out ...
– how much fuel an engine consumes per unit energy output
* Dynamometer engine testing
* European units of measurement directives
As of 2009, the European Union had issued two units of measurement directives. In 1971, it issued Directive 71/354/EEC, which required EU member states to standardise on the International System of Units (SI) rather than use a variety of CGS ...
* Horsepower-hour
* Mean effective pressure
The mean effective pressure (MEP) is a quantity relating to the operation of a reciprocating engine and is a measure of an engine's capacity to do work that is independent of engine displacement. Despite having the dimension of pressure, MEP cann ...
* Torque
In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...
References
External links
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{{United States Customary Units
Imperial units
Units of power
Customary units of measurement in the United States
James Watt