Beginner Pilot: Power-to-weight ratio

Saturday, July 26, 2008

Power-to-weight ratio

From Wikipedia, the free encyclopedia

Power-to-weight ratio (specific power) is a calculation commonly applied to engines and other 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 kerb 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:

$\begin{matrix} \mbox{P-to-W}&= P/W \\ \end{matrix}$

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 0.56 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 160 kW/kg (100 hp/lb).

Examples

Engines

Engine	Power to weight ratio
Turbocharged V-8 diesel engine	0.25 hp/lb ^[1]
Space shuttle turbopump	100 hp/lb ^[2]

Vehicles

Vehicle	Power to weight ratio
Ford Focus Sedan 2007^[3]	115 hp/ton
Lotus Exige GT3	276 bhp (206 kW)/tonne
McLaren F1	550 hp/ton (403 kW/tonne)
Ultima GTR720	650 bhp (480 kW) per tonne

Batteries

Battery type	Power to weight ratio
Nickel-cadmium battery	150W/kg
Lead acid battery	180 W/kg
Nickel metal hydride	250–1000 W/kg
Lithium ion battery	1800 W/kg

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.