History Edit

Calculating power Edit

When torque T {\displaystyle T} is in pound-foot units, rotational speed ( N ) {\displaystyle (N)} is in rpm and power is required in horsepower: P / hp = T / ( ft ⋅ lbf ) × N / rpm 5252 {\displaystyle P/{\text{hp}}={\frac {T/({\text{ft}}{\cdot }{\text{lbf}})\times N/{\text{rpm}}}{5252}}} The constant 5252 is the rounded value of (33,000 ft⋅lbf/min)/(2π rad/rev). When torque T {\displaystyle T} is in inch pounds: P / hp = T / ( in ⋅ lbf ) × N / rpm 63,025 {\displaystyle P/{\text{hp}}={\frac {T/({\text{in}}{\cdot }{\text{lbf}})\times N/{\text{rpm}}}{63{,}025}}} The constant 63,025 is the approximation of 33,000 ft ⋅ lbf min × 12 in ft 2 π rad ≈ 63,025 {\displaystyle 33{,}000\,{\frac {{\text{ft}}{\cdot }{\text{lbf}}}{\text{min}}}\times {\frac {12\,{\frac {\text{in}}{\text{ft}}}}{2\pi ~{\text{rad}}}}\approx 63{,}025} . If torque and rotational speed are expressed in coherent SI units, the power is calculated by; P = τ ⋅ ω {\displaystyle P=\tau \cdot \omega } where P {\displaystyle P} is power in watts when τ {\displaystyle \tau } is torque in newton-metres, and ω {\displaystyle \omega } is angular speed in radians per second. When using other units or if the speed is in revolutions per unit time rather than radians, a conversion factor has to be included.

Definitions Edit

Measurement Edit

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.[citation needed] In general: Nominal or rated horsepower is derived from the size of the engine and the piston speed and is only accurate at a steam pressure of 48 kPa (7 psi).[26] Indicated or gross horsepower (theoretical capability of the engine) [ PLAN/ 33000] 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 brake mean effective pressure (BMEP). This is a coefficient of theoretical brake horsepower and cylinder pressures during combustion. Nominal (or rated) horsepower Edit Nominal horsepower (nhp) is an early 19th-century rule of thumb used to estimate the power of steam engines.[26] It assumed a steam pressure of 7 psi (48 kPa).[27] nhp = 7 × area of piston in square inches × equivalent piston speed in feet per minute/33,000 For paddle ships, the Admiralty rule was that the piston speed in feet per minute was taken as 129.7 × (stroke)1/3.38.[26][27] For screw steamers, the intended piston speed was used.[27] The stroke (or length of stroke) was the distance moved by the piston measured in feet. 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 (48 kPa) and for the piston speed to be that generated by the assumed relationship for paddle ships.[26] The French Navy used the same definition of nominal horse power as the Royal Navy.[26] Comparison of nominal and indicated horse power Ship Indicated horse power (ihp) Nominal horse power (nhp) Ratio of ihp to nhp Source Dee 272 200 1.36 [26] Locust 157 100 1.57 [26] Rhadamanthus 400 220 1.82 [26] Albacore 109 60 1.82 [27] Porcupine 285 132 2.16 [26] Harpy 520 200 2.60 [26] Spitfire 380 140 2.70 [26] Spiteful 796 280 2.85 [27] Jackal 455 150 3.03 [26] Supply 265 80 3.31 [27] Simoom 1,576 400 3.94 [27] Hector 3,256 800 4.07 [27] Agincourt 6,867 1,350 5.08 [27] Bellerophon 6,521 1,000 6.52 [27] Monarch 7,842 1,100 7.13 [27] Penelope 4,703 600 7.84 [27] Indicated horsepower Edit Indicated horsepower (ihp) is the theoretical power of a reciprocating engine if it is completely frictionless in converting the expanding gas energy (piston pressure × 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. Indicated horsepower was a better measure of engine power than nominal horsepower (nhp) because it took account of steam pressure. But unlike later measures such as shaft horsepower (shp) and brake horsepower (bhp), it did not take into account power losses due to the machinery internal frictional losses, such as a piston sliding within the cylinder, plus bearing friction, transmission and gear box friction, etc. Brake horsepower Edit Brake horsepower (bhp) is the power measured at the crankshaft just outside the engine, before the losses of power caused by the gearbox efficiency and drive train efficiency.[28] In Europe, the DIN 70020 standard tests the engine fitted with all ancillaries and exhaust system as used in the car. The older American standard (SAE gross horsepower, referred to as bhp) used an engine without alternator, water pump, and other auxiliary components such as power steering pump, muffled exhaust system, etc., so the figures were higher than the European figures for the same engine. The newer American standard (referred to as SAE net horsepower) tests an engine with all the auxiliary components (see "Engine power test standards" below). Brake refers to the device which was used to load an engine and hold it at a desired rotational speed. 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 diagram" (a James Watt invention of the late 18th century), and later by means of a Prony brake connected to the engine's output shaft. More recently, an electrical brake dynamometer is used instead of a Prony brake, in order to measure the engines brake horsepower, the actual output of the engine itself, before losses to the drivetrain. Shaft horsepower Edit Shaft horsepower (shp) is the power delivered to a propeller shaft, a turbine shaft – or to an output shaft of an automotive transmission.[29] Shaft horsepower is a common rating for turboshaft and turboprop engines, industrial turbines, and some marine applications. Equivalent shaft horsepower (eshp) is sometimes used to rate turboprop engines. It includes the equivalent power derived from residual jet thrust from the turbine exhaust.[30]

Engine power test standards Edit

See also Edit