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 850 cc (51.9 cu in) engine produced about 34 bhp (25.4 kW) at the flywheel, of which only 18.2 bhp (14 kW) reached the driving wheels.[
citation needed]
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.
[8] 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.
[8]
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 blueprinted test engine running on a stand with no belt-driven accessories, air cleaner, mufflers, or emission control devices and sometimes fitted with long tube "test headers" in lieu of the OEM exhaust manifolds.[
citation needed] The atmospheric correction standards for barometric pressure, humidity and temperature were relatively idealistic. The resulting gross power and torque figures therefore reflected a maximum, theoretical value and not the power of an installed engine in a street car. Gross horsepower figures were also subject to considerable adjustment by the manufacturer's advertising and marketing staff under the direction of product managers.[
citation needed] The power ratings of mass-market engines were often exaggerated beyond their actual gross output, while those of the highest-performance
muscle car engines often tended to be closer in actual output to their advertised, gross ratings.[
citation needed]
No pre-1972 engine in its unaltered, production line stock form, as installed in the vehicle, has ever yielded documented, qualified third party validated power figures that equal or exceed its original gross rating.[
citation needed] Claims that such engines were "under-rated" are therefore dubious; for example, the 1969 427 ZL1 Chevrolet, rated at 430 bhp (320.7 kW), is frequently cited[
who?] as an "under-rated" high performance engine, yet it produced only 376 horsepower (280 kW).
[9]
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.
Because SAE gross ratings were applied liberally, 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 and a net rating of 350 hp, while the same company's
225 Slant 6 carried a rating of 145 bhp but 110 net hp.[
citations needed]
SAE certified horsepower
In 2005, the SAE introduced a new test protocol for engine horsepower and
torque.
[10] 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 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). 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).