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Difference between revisions of "Horsepower"
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=== Relationship with torque === | === Relationship with torque === | ||
For a given [[torque]], the equivalent power may be calculated. The standard equation relating torque in | 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: | ||
:<math>P / {\rm hp} = {[\tau / ({\rm ft \cdot lbf})] [\omega / ({\rm r/min})] \over 5252}</math>. | :<math>P / {\rm hp} = {[\tau / ({\rm ft \cdot lbf})] [\omega / ({\rm r/min})] \over 5252}</math>. | ||
This is based on Watt's definition of the mechanical horsepower. The constant 5252 is rounded; the exact value is 16,500/π. See [[torque#Relationship between torque and power|torque]]. | This is based on Watt's definition of the mechanical horsepower. The constant 5252 is rounded; the exact value is 16,500/π. See [[torque#Relationship between torque and power|torque]]. | ||
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This is equal to the displacement in cubic inches divided by 10π then divided again by the stroke in inches. [http://www.designchambers.com/wolfhound/wolfhoundRACHP.htm] | This is equal to the displacement in cubic inches divided by 10π then divided again by the stroke in inches. [http://www.designchambers.com/wolfhound/wolfhoundRACHP.htm] | ||
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 ([[Revolutions per minute|rpm]]), hampering the true power output and efficiency of the engine. | 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 ([[Revolutions per minute|rpm]]), hampering the true 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. | 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== | ==Measurement== | ||
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In general: | In general: | ||
:[[#Indicated horsepower (ihp)|Indicated]] or gross horsepower (theoretical capability of the engine) | :[[#Indicated horsepower (ihp)|Indicated]] or gross horsepower (theoretical capability of the engine) | ||
::minus | ::minus [[friction]]al losses within the engine (bearings, rods, etc), equals | ||
:[[#Brake horsepower (bhp)|Brake]] or net horsepower (power delivered directly by the engine) | :[[#Brake horsepower (bhp)|Brake]] or net horsepower (power delivered directly by the engine) | ||
::minus | ::minus [[friction]]al losses in the transmission (bearings, gears, etc.), equals | ||
:[[#Shaft horsepower (shp)|Shaft]] horsepower (power delivered to the driveshaft) | :[[#Shaft horsepower (shp)|Shaft]] horsepower (power delivered to the driveshaft) | ||
::minus shaft losses (friction, slip, cavitation, etc), equals | ::minus shaft losses ([[friction]], slip, cavitation, etc), equals | ||
:[[#Effective horsepower (ehp)|Effective]] or wheel horsepower | :[[#Effective horsepower (ehp)|Effective]] or wheel horsepower | ||