Unit of power; symbol hp; equal to 745·7 W (watt, SI unit); almost obsolete, but still used in engineering to describe the power of machinery; equal to 1·0139 metric horsepower.
The most common definition of horsepower for engines is the one originally proposed by James Watt in 1782. Under this system, one horsepower is defined as:
1 hp = 33,000 ft·pound-force·min−1 = exactly 745.69987158227022 WA common memory aid is based on the fact that Christopher Columbus first sailed to the Americas in 1492.
This measure was instituted by the Royal Automobile Club in Britain and used to denote the power of early 20th century British cars.
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.
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. 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.
In general:
Indicated or gross horsepower (theoretical capability of the engine) minus frictional losses within the engine (bearings, rods, etc), equals Brake or net horsepower (power delivered directly by the engine) minus frictional losses in the transmission (bearings, gears, etc.), equals Shaft horsepower (power delivered to the driveshaft) minus shaft losses (friction, slip, cavitation, etc), equals Effective or wheel horsepowerIndicated horsepower (ihp)
Indicated horsepower is the theoretical power of a reciprocating engine assuming that it is completely efficient in converting the energy contained in the expanding gases in the cylinders. 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 underrated.
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 auxiliaries. Thus the prefix "brake" refers to where the power is measured: at the engine's output shaft, as on an engine dynamometer. The term "brake" refers to the use of a band brake to measure torque during the test (which is multiplied by the engine speed in revs/sec and the circumference of the band to give the power).
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.
SAE net ratings, while more accurate than gross ratings, still represent the engine's power at the flywheel. Comparison of gross and net ratings for unchanged engines show a variance of anywhere from 40 to 150 horsepower.
SAE-certified horsepower
In 2005, the Society of Automotive Engineers introduced a new test procedure (J2723) for engine horsepower and torque. The procedure eliminates some of the areas of flexibility in power measurement, and requires an independent observer present when engines are measured.
hp (DIN)
DIN horsepower is the power measured according to the German standard DIN 70020. However, be aware that 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.
ISO 14396
ISO 14396 is a new method from the International Standards Organization for all engines not intended for on-road use.
Shaft horsepower (shp)
Shaft horsepower is the power delivered to the propellor shaft of a ship or turboprop airplane.
Effective horsepower (ehp)
Effective horsepower is the power converted to useful work.
In automobiles, effective horsepower is often referred to as wheel horsepower. Most automotive dynamometers measure wheel horsepower and then apply a conversion factor to calculate net or brake horsepower at the engine.
History of the term "horsepower"
The term "horsepower" was invented by James Watt to help market his improved steam engine. This royalty scheme did not work with customers who did not have existing steam engines but used horses instead.
Put into perspective, a healthy human can produce about 1.2hp briefly (see Orders of magnitude (power)) and sustain about 0.1hp indefinitely, and trained athletes can manage up to about 0.3 horsepower for a period of several hours.
Horsepower from a horse
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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 = 0.3048m 1 lbf = gn × 1 lb = 9.80665 m/s = 4.44822 N 60 seconds = 1 minuteAnd the watt is defined as so the historical figure of 33,000 ft·lbf/min converts exactly to the modern definition.
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