Energy density is a measure of the amount of energy a battery can store, relative to its weight.

Energy density is often referred to in relation to electric vehicle traction batteries, where the goal is to store as much energy as possible without excessive weight – in other words, to achieve the highest energy density possible.

See also power density.

Efficiency in engineering terms is a comparison between the actual performance of a system and the theoretical maximum performance (which is usually greater).

Common measures of efficiency in vehicles include:

  • thermal efficiency – the ratio of useful work produced by an engine to the fuel energy it consumes
  • volumetric efficiency – the ratio of the intake charge volume (at standard pressure and temperature) in the combustion chamber compared to the actual volume of the chamber
  • transmission efficiency – the ratio of the energy output to the energy input

Efficiency is often expressed as a percentage, or a number between 0 and 1. In engineering calculations efficiency is usually represented by the Greek letter eta (η).

An energy recovery system (ERS) captures kinetic or heat energy which would otherwise be wasted, so that it can be reused to drive the car or to power ancillaries.

In the 2014 F1 technical regulations, the ERS is split into two parts - a kinetic motor generator unit (MGUK) which captures kinetic energy from the transmission, and a heat motor generator unit (MGUH) which captures heat energy from the turbocharger turbine.

Expansion ratio is a measure of the amount that the combustion gases expand in the cylinder after combustion. The greater the expansion ratio, the more work is done and the more thermally efficient the engine is.

Usually the expansion ratio is the same as the compression ratio, but by using sophisticated valve control the compression stroke can be shortened, reducing the compression ratio to avoid knock while still providing a high expansion ratio. This is the basis of the Miller cycle.


End plates are the vertical surfaces added to the end of a wing. End plates improve the wing's efficiency by preventing air flowing over and under the wing from spilling out sideways.

End plates can be simple vertical structures (as above on a GP2 car) but are often much more complex aerodynamic components with several functions, as on a Formula 1 car.