Horsepower (hp, HP)

Below is some brief information about horsepower (hp).

Horsepower (hp or HP) is the name of several non-SI (Non International System of Units) units of power. This was originally defined to allow the output of steam engines to be measured, and compared with the power output of draft horses ( This is used in USA, or draught horse (UK), or dray horse).

The horsepower (hp or HP) was widely adopted to measure the output of piston engines, turbines, electric motors, and other machinery.  And different regions adopted different definitions of the unit. Now most countries use the SI unit watt for measurement of power.

The definition of a horsepower (hp) unit is different in different applications; application outside of the context of a particular definition will be inaccurate. Pls refer to the message below:

1. One mechanical horsepower of 550 foot-pounds per second is equivalent to 745.7 watts
2. A metric horsepower of 75 kgf-m per second is equivalent to 735.499 watts
3. 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
4. One horsepower for rating electric motors is equal to 746 watts
5. 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.

Here we only concern the electrical horsepower.

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.

 This unit (German: Pferdestärke = 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)^[7] 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.

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.

Logically, fractional horsepower refers to electrical machines that rate less than 746 Watts.

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.

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".

A few manufacturers such as Honda and Toyota switched to the new ratings immediately, with multi-directional results; the rated output of Cadillac's supercharged Northstar V8 jumped from 440 horsepower (330 kW) to 469 horsepower (350 kW) under the new tests, while the rating for Toyota's Camry 3.0 L 1MZ-FE V6 fell from 210 horsepower (160 kW) to 190 horsepower (140 kW). Much of the drop can be attributed to Toyota now rating engines on 87 octane, compared to Lexus which uses 91 octane. This is why the same 3.3 L 3MZ-FE V6 engine in the Lexus ES330 and Camry SE V6 did not show equal declines. The ES330 and Camry SE V6 were previously rated at 225 hp but the ES330 dropped to 218 hp while the Camry declined to 210 hp. 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 horsepower (370 kW) to 505 horsepower (377 kW).

source: http://www.inverter-china.com/blog/post/34.html

source: http://en.wikipedia.org/wiki/Horsepower