Power-to-weight ratio is a measurement of actual performance of any engine or power source.
Power-to-weight is often quoted by manufacturers at the peak value, but the actual value may vary in use and variations will affect performance.
Power-to-weight ratio is equal to thrust per unit mass multiplied by the velocity of any vehicle.
The original liquid hydrogen turbopump is similar in size to an automobile engine (weighing approximately 352 kilograms (775 lb)) and produces 72,000 hp (54 MW)[2] for a power-to-weight ratio of 153 kW/kg (93 hp/lb).
This fact allows one to express the power-to-weight ratio purely by SI base units.
It is typically assumed here that mechanical transmission allows the powerplant to operate at peak output power.
Electric motors do not suffer from this tradeoff, instead trading their high torque for traction at low speed.
Heat engines are able to convert thermal energy in the form of a temperature gradient between a hot source and a cold sink into other desirable mechanical work.
Heat pumps take mechanical work to regenerate thermal energy in a temperature gradient.
Standard definitions should be used when interpreting how the propulsive power of a jet or rocket engine is transferred to its vehicle.
Hydraulic (liquid) and pneumatic (gas) engines convert fluid pressure into other desirable mechanical or electrical work.
A variety of effects can be harnessed to produce thermoelectricity, thermionic emission, pyroelectricity and piezoelectricity.
If the temperature lowers or the power demand increases, the total energy delivered at the point of "discharge" is also reduced.
For example, a battery with a nominal capacity quoted in ampere-hours (Ah) at a C/10 rated discharge current (derived in amperes) may safely provide a higher discharge current – and therefore higher power-to-weight ratio – but only with a lower energy capacity.
Electric double-layer capacitors extend both electrodes with a nanoporous material such as activated carbon to significantly increase the surface area upon which electric charge can accumulate, reducing the dielectric medium to nanopores and a very thin high permittivity separator.
While capacitors tend not to be as temperature sensitive as batteries, they are significantly capacity constrained and without the strength of chemical bonds suffer from self-discharge.
In the sport of competitive cycling athlete's performance is increasingly being expressed in VAMs and thus as a power-to-weight ratio in W/kg.
[103] A locomotive generally must be heavy in order to develop enough adhesion on the rails to start a train.
As the coefficient of friction between steel wheels and rails seldom exceeds 0.25 in most cases, improving a locomotive's power-to-weight ratio is often counterproductive.
Most vehicles are designed to meet passenger comfort and cargo carrying requirements.
Vehicle designs trade off power-to-weight ratio to increase comfort, cargo space, fuel economy, emissions control, energy security and endurance.
Some utility and practical vehicle variants such as hot hatches and sports-utility vehicles reconfigure power (typically increased) and weight to provide the perception of sports car like performance or for other psychological benefit.
Bodies vary from hot hatches, sedans (saloons), coupés, convertibles and roadsters.
[385] Propeller aircraft depend on high power-to-weight ratios to generate sufficient thrust to achieve sustained flight, and then for speed.
Power-to-weight ratio is important in cycling, since it determines acceleration and the speed during hill climbs.
Since a cyclist's power-to-weight output decreases with fatigue, it is normally discussed with relation to the length of time that he or she maintains that power.