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Power-to-weight ratio

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Power-to-weight ratio (or specific power) is a calculation commonly applied to engines and mobile power sources to enable the comparison of one unit or design to another. Power-to-weight ratio is a measurement of actual performance of any engine or power sources. It is also used a measure of performance of a vehicle as a whole, with the engine's power output being divided by the curb weight of the car, to give an idea of the vehicle's acceleration.

Power to weight (specific power)Edit

The power-to-weight ratio (Specific Power) formula for an engine (power plant) is the power generated by the engine divided by weight of the engine as follows:

Power-to-weight ratio equation


A typical turbocharged V-8 diesel engine might have an engine power of 250 horsepower (190 kW) and a weight of 450 kilograms (1,000 lb), giving it a power to weight ratio of 0.42 kW/kg (0.25 hp/lb).

Examples of high power to weight ratios can often be found in turbines. This is because of their ability to operate at very high speeds. For example, the Space Shuttle's main engines use turbopumps (machines consisting of a pump driven by a turbine engine) to feed the propellants (liquid oxygen and liquid hydrogen) into the engine's combustion chamber. The liquid hydrogen turbopump is slightly larger than an automobile engine (weighing approximately 320 kilograms (700 lb)) and produces nearly 70,000 hp (52.2 MW) for a power to weight ratio of 164 kW/kg (100 hp/lb).

The actual useful power of an entire jet engine or rocket engine can be calculated, but varies with speed (power is force times distance over time or simply force times speed). For jet engines there is often a cruise speed and power can be usefully calculated there, for rockets there is typically no cruise speed, so it is less meaningful.

ExamplesEdit

EnginesEdit

Engine Power to weight ratio Total Power Output
Turbocharged V-8 diesel engine 0.25 hp/lb / 410 W/kg[1] 250 hp / 186  kW
49-PI Type II Wankel engine 1.7 hp/lb / 2.8 kW/kg[2] 1.252 hp / 0.934 kW
BMW P84/5 2005 (Formula 1) 4.6 hp/lb / 7.5 kW/kg[3] 925 hp / 690 kW
Space Shuttle Engine Turbopump 100 hp/lb / 160 kW/kg[4] 70,000 hp / 52,000 kW
Boeing 777 GE90-115B Jet Engine 6.10 hp/lb / 10.0 kW/kg[5] 111,526 hp / 83,164 kW

VehiclesEdit

Power to weight ratios for vehicles are usually calculated using curb weight (for cars) or wet weight (for motorcycles) - in other words, excluding weight of the driver and any cargo. This could be slightly misleading, especially with regard to motorcycles, where the driver might weigh 1/3 to 1/2 as much as the vehicle itself.

Vehicle Power Weight Power to weight ratio
Subaru R2 type S 2003[6] 47 kW / 63 bhp 830 kg / 1830 lb 57 W/kg / 29 lb/hp
Subaru Legacy 2.0R 2005[7] 121 kW / 162 bhp 1370 kg / 3020 lb 88 W/kg / 19 lb/hp
Subaru Outback 2.5i 2008[8] 130.5 kW / 175 bhp 1430 kg / 3153 lb 91 W/kg / 18 lb/hp
Ford Focus 2.0 auto 2007[9] 104.4 kW / 140 bhp 1198 kg / 2641 lb 94 W/kg / 19 lb/hp
Artega GT[10] 220 kW / 300 bhp 1100 kg / 2425 lb 200 W/kg / 8 lb/hp
Lotus Exige GT3 2006[11] 202.1 kW / 271 bhp 980 kg / 2160 lb [12] 206 W/kg / 8 lb/hp
Chevrolet Corvette C6[13] 321 kW / 430 bhp 1441 kg / 3177 lb 223 W/kg / 7 lb/hp
Ultima GTR GTR720 2000[14] 257.3 kW / 345 bhp 1048 kg / 2310 lb 245 W/kg / 7 lb/hp
Chevrolet Corvette C6 Z06[13] 376 kW / 505 bhp 1421 kg / 3133 lb 265 W/kg / 6 lb/hp
McLaren F1 GT 1997[15] 467.6 kW / 627 bhp 1220 kg / 2690 lb 403 W/kg / 4 lb/hp
Honda CBR1000RR 2009[16] 133 kW / 178 bhp 199 kg / 439 lb 668 W/kg / 2 lb/hp
MTT Turbine SUPERBIKE 2008[17] 213.3 kW / 286 bhp 227 kg / 500 lb 940 W/kg / 2 lb/hp
Formula One 2006 582 kW / 780 bhp 605 kg / 1334 lb 962 W/kg / 2 lb/hp

BatteriesEdit

Battery type Power to weight ratio
Nickel hydrogen battery 75 W/kg
Nickel-cadmium battery 150 W/kg
Lead acid battery 180 W/kg
Nickel metal hydride 250[18] (market) –980 W/kg[19] (lab)
Lithium ion battery ~340 W/kg[20] 1700 W/kg (lab)[21]

Electric motorsEdit

Motor type weight power Power to weight ratio
Himax HC6332-230 0.69 kg[22] 2.2 kW[22] 3.19 kW/kg
Hi-Pa Drive[23] 120 kg 235 kW 1.96 kW/kg

Fuel cellsEdit

Fuel cell type Power to weight ratio
PEMFC 967 W/kg (market) - 1,500 W/kg (lab)[24]

Spacecraft solar panelsEdit

Panel type Power to weight ratio
Current ~170 W/kg[25]
Believed possible ~300 W/kg[25]

The inverse of power-to-weight, weight-to-power ratio (power loading) is a calculation commonly applied to aircraft, cars, and vehicles in general, to enable the comparison of one vehicle performance to another. Weight-to-power ratio is a measurement of the acceleration capability (potential) of any land vehicle or climb performance of any aircraft or space vehicle.

See alsoEdit


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