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Horsepower
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Horsepower (hp or HP) is the name of several non-SI units of power. It was originally defined to allow the output of steam engines to be measured and compared with the power output of draft horses. The horsepower was widely adopted to measure the output of piston engines, turbines, electric motors and other machinery. Different regions adopted different definitions of the unit. Most countries now use the SI unit watt for measurement of power.
The definition of a horsepower unit is different in different applications; application outside of the context of a particular definition will be inaccurate.
An ALAM or SAE horsepower is not a unit of power but instead is a measure of internal combustion engine displacement, and an RAC horsepower or tax horsepower is a differently defined unit of displacement.
Where units of horsepower are used for marketing consumer products, often measurement methods are designed by advertisers to maximize the size of the number produced for any product, even if this may not reflect realistic capacity of the product to do work when used in normal conditions.
development of the steam engine provided a reason to equate the output of horses with the engines that could replace them.
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Encyclopedia
Horsepower (hp or HP) is the name of several non-SI units of power. It was originally defined to allow the output of steam engines to be measured and compared with the power output of draft horses. The horsepower was widely adopted to measure the output of piston engines, turbines, electric motors and other machinery. Different regions adopted different definitions of the unit. Most countries now use the SI unit watt for measurement of power.
The definition of a horsepower unit is different in different applications; application outside of the context of a particular definition will be inaccurate.
- One mechanical horsepower of 550 foot-pounds per second is equivalent to 745.7 watts
- A metric horsepower of 75 kgf-m per second is equivalent to 735.499 watts
- A boiler horsepower is used for rating steam boilers and is equivalent to 34.5 pounds of water evaporated per hour at 212 degrees Fahrenheit, or 9809.5 watts
- One horsepower for rating electric motors is equal to 746 watts
- A Pferdestärke is a name for a group of similar power measurements used in Germany around the end of the 19th century, all of about one metric horsepower in size.
An ALAM or SAE horsepower is not a unit of power but instead is a measure of internal combustion engine displacement, and an RAC horsepower or tax horsepower is a differently defined unit of displacement.
Where units of horsepower are used for marketing consumer products, often measurement methods are designed by advertisers to maximize the size of the number produced for any product, even if this may not reflect realistic capacity of the product to do work when used in normal conditions.
History of the term "horsepower"
The development of the steam engine provided a reason to equate the output of horses with the engines that could replace them. In 1702, Thomas Savery 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 term "horsepower" was coined later by James Watt to help market his improved steam engine. 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). The wheel was 12 feet in radius, therefore the horse travelled 2.4 × 2p × 12 feet in one minute. Watt judged that the horse could pull with a force of 180 pounds (assuming that the measurements of mass were equivalent to measurements of force in pounds-force, which were not well-defined units at the time). So:
This was rounded to an even 33,000 ft·lbf/min.
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.
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.
Put into perspective, a healthy human can produce about 1.2 hp briefly (see Orders of magnitude (power)) and sustain about 0.1 hp indefinitely, and trained athletes can manage up to about 2.5 hp briefly and 0.3 hp 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 under promise and over deliver; few horses can maintain that effort for long. Regardless, comparison to a horse proved to be an enduring marketing tool.
Horsepower from a horse
R. D. Stevenson and R. J. Wassersug published an article in Nature 364, 195-195 (15 July 1993) calculating the upper limit to an animal's power output. The peak power over a few seconds has been measured to be as high as 14.9 hp. However, for longer periods, an average horse produces less than one horsepower.
Current definitions
The following definitions have been widely used:
Mechanical horsepower
hp(I) | = 33,000 ft·lbf/min
= 550 ft·lbf/s
= 745.6999 W | Metric horsepower
hp(M) | = 75 kgf·m/s
= 735.49875 W (exactly) | Electrical horsepower
hp(E) | =746 W | Boiler horsepower
hp(S) | = 33,475 Btu/h
=9809.5 W | | Hydraulic horsepower | =Flow Rate(US gal/min)*Pressure(psi)*7/12,000
= 550 ft·lbf/s
= 745.6999 W |
In certain situations it is necessary to distinguish between the various definitions of horsepower and thus a suffix is added: hp(I) for mechanical (or imperial) horsepower, hp(M) for metric horsepower, hp(S) for boiler (or steam) horsepower and hp(E) for electrical horsepower.
Hydraulic horsepower is equivalent to mechanical horsepower. The formula given above is for conversion to mechanical horsepower from the factors acting on a hydraulic system.
Additionally, the term "horsepower" has been applied to calculated (estimated rather than measured) metrics:
- RAC horsepower is based solely on the dimensions of a piston engine (1 litre of engine displacement is equal to 10 RAC horsepower)
Mechanical horsepower
Assuming the third CGPM (1901, CR 70) definition of standard gravity, gn=9.80665 m/s², is used to define the pound-force as well as the kilogram force, and the international avoirdupois pound (1959), one mechanical horsepower is:
| 1 hp | = 33,000 ft·lbf/min | by definition | | | = 550 ft·lbf/s | since | 1 min | = 60 s | | | = 550×0.3048×0.45359237 m·kgf/s | since | 1 ft | = 0.3048 m and | | | = 76.0402249068 kgf·m/s | | 1 lb | = 0.45359237 kg | | | = 76.0402249068×9.80665 kg·m²/s³ | | g | = 9.80665 m/s² | | | = 745.69987158227022 W | since | 1 W | = 1 J/s = 1 N·m/s = 1 (kg·m/s²)·(m/s) |
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 = 746 W
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 ( = horse strength) is no longer a statutory 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) in Braunschweig as exactly:
- 1 PS = 75 kilopond-meters per second
- (75 kp·m/s)×(9.80665 N/kp) = 735.49875 N·m/s ≈ 735.5 N·m/s ≈ 735.5 W ≈ 0.7355 kW ≈ 0.98632 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 is still in use for commercial and advertising purposes, as many customers are not familiar with the use of kilowatts for engines.
pk, hk, hv, LE, k/ks, KM
The Dutch paardenkracht (pk), the Swedish hästkraft (hk), the Finnish hevosvoima (hv), the Norwegian and Danish hestekraft (hk), the Hungarian lóero (LE), the Czech konská síla (k or ks), the Slovak konská sila (k or ks) and the Polish kon mechaniczny (KM) 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 lowercase letters as 'cv'.
In addition, the capital form 'CV' is a French unit for tax horsepower, short for chevaux vapeur ("steam horses") or cheval-vapeur. CV is a non-linear rating of a motor vehicle for tax purposes. The CV rating, or fiscal power, is , where P is the maximum power in kilowatts and U is the amount of CO2 emitted in grams per kilometre. The fiscal power has found its way into naming of automobile models, such as the popular Citroën deux-chevaux. The cheval-vapeur (ch) unit should not be confused with the French cheval fiscal (CV).
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 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 various industrial applications, however it is considered an antiquated term and is not used in modern power plants. It is equal to a boiler thermal output of 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.
The term 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 30 pounds of water per hour, based on feedwater at 100 °F (38 °C), and saturated steam generated at 70 psig (480 kPa gauge pressure). This original definition is equivalent to a boiler heat output of 33,485 Btu/h. Years later in 1884, the ASME re-defined the boiler horsepower as the thermal output equal to the evaporation of 34.5 pounds per hour of water "from and at" 212 °F. 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 33,469 Btu/hr. Present industrial practice is to define "Boiler Horsepower" as a boiler thermal output equal to 33,475 Btu/h, which is very close to the original and revised definitions.
The amount of power that can be obtained by a steam engine or steam turbine based on "boiler horsepower" varies so widely that use of the term is entirely obsolete for these purposes. The term makes no distinction as to the steam pressure or temperature which is produced (both of which can significantly influence engine/turbine output), it merely defines a thermal output of a boiler. Smaller steam engines often require several "boiler horsepower" to make one horsepower, and modern steam turbines can make power with as little as about 0.15 "boiler horsepower" thermal output per actual horsepower developed.
Electrical horsepower
The electrical horsepower is used by the electrical industry for electrical machines and is defined to be exactly 746 W. The nameplates on electrical motors show their power output, not their power input.
Relationship with torque
For a given torque, the equivalent power may be calculated. The standard equation relating torque in foot-pounds, rotational speed in RPM and horsepower is:
Where is power, is torque, and is rotations per minute.
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)/(2p rad/rev),
and 9549 comes from (60 s/min)(1000 W/kW)/(2p rad/rev).
See torque.
Drawbar horsepower
- See also Power at rail
Drawbar horsepower (dbhp) 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 (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 2025 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 a coherent system of units, such as SI (watts, newtons, and metres per second), no constant is needed, and the formula becomes .
RAC horsepower (taxable horsepower)
This measure was instituted by the Royal Automobile Club in Britain 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.
- where
- D is the diameter (or bore) of the cylinder in inches
- n is the number of cylinders
This is equal to the displacement in cubic inches divided by 10p then divided again by the stroke in inches.
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 (rpm), 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 (wide bore, short stroke) V-8s (see, for example, the early Chrysler Hemi).
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 the case of an engine dynamometer, power is measured at the engine's flywheel (i.e., at the crankshaft output). With a chassis dynamometer or "rolling road", power output is measured at the driving wheels. This accounts for the significant power loss through the drive train. As an example, an early-production BL Mini engine produced about at the flywheel, of which only reached the driving wheels.
In general:
- Nominal is derived from the size of the engine and the piston speed and is only accurate at a pressure of 7 lbf/in².
- Indicated or gross horsepower (theoretical capability of the engine)
minus frictional losses within the engine (bearing drag, rod and crankshaft windage losses, oil film drag, etc.), equals
- Brake / net / crankshaft horsepower (power delivered directly to and measured at the engine's crankshaft)
minus frictional losses in the transmission (bearings, gears, oil drag, windage, etc.), equals
- Shaft horsepower (power delivered to and measured at the output shaft of the transmission, when present in the system)
minus frictional losses in the universal joint/s, differential, wheel bearings, tire and chain, (if present), equals
- Effective, True (thp) or commonly referred to as wheel horsepower (whp)
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 BMEP (Brake Mean Effective pressure). This is a coefficient of theoretical brake horsepower and cylinder pressures during combustion.
Nominal horsepower
Nominal horsepower (nhp) is an early Nineteenth Century rule of thumb used to estimate the power of steam engines.
nhp = 7 x area of piston x equivalent piston speed/33,000
For paddle ships the piston speed was estimated as 129.7 x (stroke)1/3.35
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 7 psi and for the piston speed to be of the order of 180-248 ft/min.
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 x 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. It was the figure normally used for steam engines in the 19th century but is misleading because the mechanical efficiency of an engine means that the actual power output may only be 70% to 90% of the indicated horsepower.
Brake horsepower
Brake horsepower (abbreviated bhp) is the measure of an engine's horsepower without the loss in power caused by the gearbox, generator, differential, water pump, and other auxiliary components such as alternator, power steering pump, muffled exhaust system, etc. "Brake" refers to a device which was used to load an engine and hold it at a desired RPM. During testing, the output torque and rotational speed were measured to determine the "brake horsepower". Horsepower was originally measured and calculated by use of the indicator (a James Watt invention of the late 18th century), and later by means of a De Prony brake connected to the engine's output shaft. More recently, an engine dynamometer is used instead of a De Prony brake. The output delivered to the driving wheels is less than that obtainable at the engine's crankshaft.
British horsepower
The acronym bhp may also be used for British horsepower, which has the same definition as the American SAE gross brake horsepower: 33,000 lb·ft/minute. More information on American SAE horsepower measurements is below.
SAE horsepower
SAE gross horsepower
Prior to the 1972 model year, American automakers rated and advertised their engines in brake horsepower (bhp), frequently referred to as SAE gross horsepower, because it was measured in accord with the protocols defined in SAE standards J245 and J1995. As with other brake horsepower test protocols, SAE gross hp was measured using a stock test engine, generally running with few belt-driven accessories and sometimes fitted with long tube (test headers) in lieu of the OEM exhaust manifolds. The atmospheric correction standards for barometric pressure, humidity and temperature for testing were relatively idealistic. It is a misconception that the readings were exaggerated at the time (post-1972), because those accusations do not take into account that emission control devices were not included on factory vehicles of the time. This is because catalytic converters were not used on any vehicle, nor required, and most vehicles had mufflers that didn't take into account noise (a non-noise cancellation muffler), thus resulting in a performance muffler of sorts (generally higher h.p. output). Many vehicles of the time also didn't have extra accessories such as air conditioning and power steering to restrict the drive belt. The resulting gross power and torque figures, therefore, reflected a realistic value. However, performance vehicles of the time were almost always underrated.
SAE net horsepower
In the United States the term "bhp" fell into disuse in 1971-72, 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, the SAE net hp 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. The change to net hp effectively deflated power ratings to assuage the auto insurance industry and environmental and safety lobbies.
SAE certified horsepower
In 2005, the SAE introduced a new test protocol for engine horsepower and torque. The new protocol eliminates some of the flexibility in power measurement, and requires an independent observer present when engines are measured. The test is voluntary, but engines completing it can be advertised as "SAE-certified".
Many manufacturers began switching to the new rating immediately, with multi-directional results; the rated output of Cadillac's supercharged Northstar V8 jumped from to under the new tests, while the rating for Toyota's Camry 3.0 L 1MZ-FE V6 fell from to . The first engine certified under the new program was the 7.0 L LS7 used in the 2006 Chevrolet Corvette Z06. Certified power rose slightly from to .
DIN hp
DIN horsepower is the power measurement protocol in the German DIN standard 70020. It is sometimes abbreviated as "PS", which stands for Pferdestärke, which is the German word for horsepower. DIN hp is measured at the engine's output shaft, and is usually expressed in metric (Pferdestärke) rather than mechanical horsepower.
ECE hp
ECE R24 is the European standard for measuring engine output. It is quite similar to the DIN 70020 standard, but with different requirements for connecting an engine's fan during testing. ECE is seen as slightly more liberal than DIN, and ECE figures tend to be slightly higher than DIN.
9768-EC
9768-EC is a European Union standard, generally very similar to ISO-14396.
ISO 14396
ISO 14396 is a new standard from the ISO for all engines not intended for on-road use.
Shaft horsepower
Shaft horsepower (shp) is the power delivered to the propeller shaft of a ship or an airplane powered by a piston engine or a turbine engine (the combination of turbine engine and propeller commonly called a turboprop). This may be measured, or estimated from the indicated horsepower given a standard figure for the losses in the transmission (typical figures are around 10%). This metric is uncommon in the automobile industry, because drivetrain losses can be significant.
Effective (true, wheel) hp
Effective horsepower (ehp), True horsepower (thp) or wheel horsepower (whp) is the power converted to useful work. In the case of a road vehicle this is the power actually turned into forward motion as measured on a chassis dynamometer. Power available at the road is generally 10% to 20% less than the engine's bhp rating due to "coastdown" losses, most of which are due to the vehicle's rubber tires rather than true transmission losses. Front-wheel drive cars (provided a transverse engine layout is used) suffer slightly lower coastdown losses due to the absence of the bevelled crown and pinion gears used to change the drive direction in the back axle of a RWD car .
For railway locomotives the terms Drawbar Horsepower or Equivalent Drawbar Horsepower (EDHP) refer to the power available to haul a train. This is synonymous with the Effective horsepower. This figure takes into account the horsepower needed to move the locomotive, which is not available for hauling the train. The Rail Horsepower is the power at the wheels of a locomotive, directly comparable to the wheel horsepower of a road vehicle.
See also
External links
- A clear explanation of the relationship between Power and Torque, and how they relate to engine performance.
- Road dynamometer results vs. a chassis dyno.
- An article showing how power, torque, and gearing affect a vehicle's performance.
- Explains Horsepower and the power losses in the engine and powertrain.
- (picture)
- at the Straight Dope
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