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:


 * [[Image:Power-to-weight ratio equation.png|left]]

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.

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.

Spacecraft solar panels
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.