Horsepower (hp) is the name of several non-metric units of power. In scientific discourse, the term "horsepower" is rarely used because of its various definitions and the existence of an 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 engines in "horsepower" units of measurement.

There are two important factors to consider when evaluating the measurement of "horsepower":

• The various definitions of the "horsepower" unit itself
• The various standards used in measuring the value 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 metrics have been widely used to define "horsepower":

• Mechanical horsepower — 0.74569987158227022 kW/hp (745.7 watts/hp) (33,000 ft-lbs/min) (550 ft-lbs/sec)
• Metric horsepower — 0.73549875 kW/hp (735.5 watts/hp)
• Electrical horsepower — 0.746 kW/hp (746 watts/hp)
• Boiler horsepower — 9.8095 kW/hp (9810 watts/hp)

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 liter of engine displacement is equal to 10 RAC horsepower)

Mechanical horsepower

See History of the term "horsepower"

The term "horsepower" was coined by the engineer James Watt (1736 to 1819) in 1782 while working on improving the performance of steam engines. This occurred while using a mine pony to lift coal out of a coal mine. He conceived the idea of defining the power exerted by these animals to accomplish this work. He found that, on the average, a mine horse could pull (lift by means of a pulley) 22,000 foot-pounds per minute. Rather than call this "pony" power, he increased these test results by 50 percent, and called it horsepower i.e. 33,000 foot-pounds of work per minute.

Under this system, then, one horsepower is defined as:

1 hp = 33,000 ft-lbs/min (550 ft-lbs/sec) (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)^[1] 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.

In the 19th century the French had their own unit, called the poncelet (p) 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.^{[citation needed]}

pk, hk and hv

The Dutch paardenkracht (pk), the Swedish hästkraft (hk), the Finnish hevosvoima (hv), and the Norwegian and Danish hestekraft (hk) all equal the German Pferdestärke (PS)

CV and cv

Often the French name for the Pferdestärke. In addition, 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.^[2] 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.

In Italian ("Cavalli"), Spanish ("Caballos"), and Portuguese ("Cavalos"), 'CV' is the equivalent to the German 'PS'. In France, this should be written as 'cv'.

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-pounds, 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.lbs./min) / 2Ï€ (radians/revolution),

and 9549 comes from 60 (s/min) x 1000 (W/kW) / 2Ï€ (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 (F / lbf) and speed is measured in miles per hour (v / mph), then the drawbar power in horsepower (P / hp) 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?

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 P = Fv.

RAC horsepower (taxable horsepower)

See Tax "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 10Ï€ then divided again by the stroke in inches.^{[3] [not in citation given]}

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 3.8-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-8s.

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 is derived from the size of the engine and the piston speed and is only accurate at a pressure of 7 psi.^[4]

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.

Nominal horsepower (nhp)

Nominal horsepower 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 lb/sq. in and for the piston speed to be of the order of 180-248 ft/s.^[5]

Indicated horsepower (ihp)

Indicated horsepower is the theoretical power of a reciprocating engine if it is completely efficient in converting the energy contained in the expanding gases in the cylinders. It is calculated from the pressures developed in the cylinders, measured by a device called an engine indicator - hence indicated horsepower. 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.

SAE gross horsepower

Prior to 1972 most American automakers rated their engines in terms of SAE gross horsepower (defined under SAE standards J245 and J1995). Gross hp was measured using a blueprinted test engine running on a stand without accessories, mufflers, or emissions control devices. It therefore reflected a maximum, theoretical value, not the power of an installed engine in a street car. Gross horsepower figures were also subject to considerable adjustment by carmakers: the power ratings of mass-market engines were often exaggerated, while those for the highest-performance muscle car engines were frequently understated.

Brake horsepower (bhp)

Brake horsepower (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. Thus the prefix "brake" refers to where the power is measured: at the engine's output shaft, as on an engine dynamometer. The actual horsepower delivered to the driving wheels is less. An engine would have to be retested to obtain a rating in another system. The term "brake" refers to the original use of a band brake to measure torque during the test (which is multiplied by the engine RPM and a scaling constant to give horsepower).

hp (SAE)

In the United States the term "bhp" fell into disuse after the American Society of Automotive Engineers (SAE) recommended manufacturers use hp (SAE) to indicate the net power of the engine, given that particular car's complete engine installation. It measures engine power at the flywheel, not counting drivetrain losses.

Starting in 1971 automakers began to quote power in terms of SAE net horsepower (as defined by standard J1349). This reflected the rated power of the engine in as-installed trim, with all accessories and standard intake and exhaust systems. By 1972, US carmakers quoted power exclusively in SAE net hp. The change was meant to 'deflate' power ratings to assuage the auto insurance industry and environmental and safety lobbies, as well as to obfuscate the power losses caused by emissions-control equipment.

SAE net ratings, while more accurate than gross ratings, still represent the engine's power at the flywheel. Contrary to some reports, it does not measure power at the drive wheels.

Because SAE gross ratings were applied liberally, at best, there is no precise conversion from gross to net. Comparison of gross and net ratings for unchanged engines shows a variance of anywhere from 40 to 150 horsepower. The Chrysler 426 Hemi, for example, in 1971 carried a 425 hp gross rating (often considered to be understated) and a net rating of 375 hp.

SAE-certified horsepower

In 2005, the Society of Automotive Engineers introduced a new test procedure for engine horsepower and torque.^[6] The procedure eliminates some of the areas of 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, often with surprising results. The rated output of Cadillac's supercharged Northstar V8 jumped from 440 hp (328 kW) to 469 hp (350 kW) under the new tests, while the rating for Toyota's Camry 3.0 L 1MZ-FE V6 fell from 210 hp (157 kW) to 190 hp (142 kW). 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 500 hp (373 kW) to 505 hp (377 kW).

hp (DIN)

DIN horsepower is the power measured according to the German standard DIN 70020 and like the SAE net figure is measured at the flywheel. However, DIN "horsepower" is often expressed in metric (Pferdestärke) rather than mechanical horsepower.

hp (ECE)

ECE R24 is another standard for measuring net horsepower. It is quite similar to the DIN 70020 standard, but the requirement for connecting an engine's fan during testing varies. ECE is seen as slightly more liberal than DIN, and ECE figures tend to be slightly higher than DIN. John Deere is one strong adherent to ECE testing.

9768-EC

9768-EC is a European Union Standard. Generally very similar to ISO-14396 and 9768-EC metrics.

ISO 14396

ISO 14396 is a new standard from the ISO for all engines not intended for on-road use. Generally, ISO-14396 and 9768-EC metrics are very similar.

Note: Brake Horse Power, or indeed any other absolute measurement of power output, is measured where convenient. Whether ancilliary equipment such as fan, alternator water pump etc are connected or not is immaterial to the expression of power measurement.

Obviously, if ancilliary equipment is connected this will consume power and net output will be lower. However, the expression "Brake Horsepower" will still pertain, since it IS an expression and not the measurement, per se.

In the case of an engine dynamometer being used to measure, power is measured at the flywheel. With a chassis dynamometer (or Rolling Road), power output is measured at the driving wheels, thus taking into account power loss through the drive train, which can be significant, particularly with front wheel drive vehicles. As an example, a standard earlier BMC/BLMC/BL Mini 850 c.c produced circa 34 bhp at the flywheel, yet only circa 18.20 bhp at the front - driving wheels!

Manufacturers traditionally quoted net power output in BHP measured at the flywheel. Ancilliary equipment (water pump, cooling fan, aternator, power steering pump, air-conditioning compressor) were removed, thus the power outputs quoted were highly misleading and optimistic!

It matters not which mathematical expression is used to denote power output, since all different expressions can be cross converted, as with any other measurements of energy, such as Kilowatt and BTUs.

Engine designers use other expressions 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.

The term Brake Horsepower comes from the earliest methodology used to measure engine power output, simply a band brake of friction materials set around a steel drum connected to the engine flywheel by (e.g.) a Cardon Shaft, with resistance being applied by a spring balance and as the resistance to rotation was increased by the brake, the spring balance would read higher. The maximum Brake Horsepower is calculated by using the diameter of the brake drum and the weight exerted by the spring balance at the point where the load is greatest (in pounds, kilograms etc)and the engine is at the point just before stall with this load.

Later and traditional dynamometers (always affectionately referred to as a "Brake" or an "Engine Brake"), used a water gate resistance. The leading British manufacturer was Heenan and Froud, who provided most engine and chassis brakes from the 1930s onwards to both manufacturers and engine designers and race engine developers/tuners.

Shaft horsepower (shp)

Shaft horsepower 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, though drivetrain losses can be significant.

Effective horsepower (ehp) / True horsepower (thp) / Wheel Horsepower (whp)

Effective horsepower, True hp or wheel horsepower 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.

"True hp" is generally 10% to 20% less than the engine's "bhp" ratings due to drivetrain losses.

Wheel Horsepower (whp) is the more widely used term for Effective horsepower.

History of the term "horsepower"

The term "horsepower" was coined 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.^[7] 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 × 2π × 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.^[8]

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.^[9]

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 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 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. Wasserzug 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.

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:

• 1 ft = 0.3048 m
• 1 lbf = g_n × 1 lb = 9.80665 m/s² × 1 lb × 0.45359237 kg/lb = 4.44822 kg·m/s² = 4.44822 N
• 60 seconds = 1 minute

And the watt is defined as so the historical figure of 33,000 ft·lbf/min converts exactly to the modern definition.

References

External links

• How Stuff Works - Horsepower
• "Horsepower and Torque" An article showing how power, torque, and gearing affect a vehicle's performance.
• "Horsepower and power losses" Explains Horsepower and the power losses in the engine and powertrain.
• "What is Horsepower?" (picture)
• "What's the difference between horsepower and torque?" at the Straight Dope
• "True hp figures as compared to inflated."
• Horsepower vs. Torque article
• "chassis dyno hp numbers, origin of inflated "
• Bristol Dyno - What is a dynamometer?
• Horsepower listings of all current cars.