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Power-to-weight ratio
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)
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:
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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.
Examples
Engines
| Engine | Power to weight ratio | Total Power Output |
|---|---|---|
| Turbocharged V-8 diesel engine | 0.25 hp/lb / 410 W/kg<ref>250 hp (engine power)/1,000 lb (engine weight) / 410 W/kg</ref> | 250 hp / 186 kW |
| 49-PI Type II Wankel engine | 1.7 hp/lb / 2.8 kW/kg<ref>[1]</ref> | 1.252 hp / 0.934 kW |
| BMW P84/5 2005 (Formula 1) | 4.6 hp/lb / 7.5 kW/kg<ref>[2]</ref> | 925 hp / 690 kW |
| Space Shuttle Engine Turbopump | 100 hp/lb / 160 kW/kg<ref>70,000 hp (turbine power)/700 lb (turbine weight)</ref> | 70,000 hp / 52,000 kW |
| Boeing 777 GE90-115B Jet Engine | 6.10 hp/lb / 10.0 kW/kg<ref>111,526 hp (turbine power)/18,260 lb (engine weight) / 10,041 W/Kg</ref> | 111,526 hp / 83,164 kW |
Vehicles
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<ref>[3]</ref> | 47 kW / 63 bhp | 830 kg / 1830 lb | 57 W/kg / 29 lb/hp |
| Subaru Legacy 2.0R 2005<ref>[4]</ref> | 121 kW / 162 bhp | 1370 kg / 3020 lb | 88 W/kg / 19 lb/hp |
| Subaru Outback 2.5i 2008<ref>[5]</ref> | 130.5 kW / 175 bhp | 1430 kg / 3153 lb | 91 W/kg / 18 lb/hp |
| Ford Focus 2.0 auto 2007<ref>[6]</ref> | 104.4 kW / 140 bhp | 1198 kg / 2641 lb | 94 W/kg / 19 lb/hp |
| Artega GT<ref>Motor Authority » Artega GT now on sale</ref> | 220 kW / 300 bhp | 1100 kg / 2425 lb | 200 W/kg / 8 lb/hp |
| Lotus Exige GT3 2006<ref>[7]</ref> | 202.1 kW / 271 bhp | 980 kg / 2160 lb <ref>Lotus Exige</ref> | 206 W/kg / 8 lb/hp |
| Chevrolet Corvette C6<ref name="MSN Autos">Template:Citation/core{{#if:|}}</ref> | 321 kW / 430 bhp | 1441 kg / 3177 lb | 223 W/kg / 7 lb/hp |
| Ultima GTR GTR720 2000<ref>[8]</ref> | 257.3 kW / 345 bhp | 1048 kg / 2310 lb | 245 W/kg / 7 lb/hp |
| Chevrolet Corvette C6 Z06<ref name="MSN Autos"/> | 376 kW / 505 bhp | 1421 kg / 3133 lb | 265 W/kg / 6 lb/hp |
| McLaren F1 GT 1997<ref>[9]</ref> | 467.6 kW / 627 bhp | 1220 kg / 2690 lb | 403 W/kg / 4 lb/hp |
| Honda CBR1000RR 2009<ref>Honda CBR1000RR</ref> | 133 kW / 178 bhp | 199 kg / 439 lb | 668 W/kg / 2 lb/hp |
| MTT Turbine SUPERBIKE 2008<ref>[10]</ref> | 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 |
Batteries
| 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<ref>High Energy Metal Hydride Battery</ref> (market) –980 W/kg<ref>Improvement of Nickel Metal Hydride Battery with Non-foam Nickel Electrode for Hybrid Electric Vehicles Applications</ref> (lab) |
| Lithium ion battery | ~340 W/kg<ref>http://www.panasonic.com/industrial/battery/oem/images/pdf/Panasonic_LiIon_CGA103450A.pdf</ref> 1700 W/kg (lab)<ref>Lithium Ion Battery Research</ref> |
Electric motors
| Motor type | weight | power | Power to weight ratio |
|---|---|---|---|
| Himax HC6332-230 | 0.69 kg<ref name=rctoy>[11]</ref> | 2.2 kW<ref name=rctoy/> | 3.19 kW/kg |
| Hi-Pa Drive<ref>[12]</ref> | 120 kg | 235 kW | 1.96 kW/kg |
Fuel cells
| Fuel cell type | Power to weight ratio |
|---|---|
| PEMFC | 967 W/kg (market) - 1,500 W/kg (lab)<ref>Low-cost light weight high power density PEM fuel cell stack</ref> |
Spacecraft solar panels
| Panel type | Power to weight ratio |
|---|---|
| Current | ~170 W/kg<ref name=rocket>Rocket and spacecraft propulsion By Martin J. L. Turner</ref> |
| Believed possible | ~300 W/kg<ref name=rocket/> |
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.
