Naturally aspirated engine

The density of the air charge, and therefore the engine's maximum theoretical power output, in addition to being influenced by induction system restriction, is also affected by engine speed and atmospheric pressure, the latter of which decreases as the operating altitude increases.

This is in contrast to a forced-induction engine, in which a mechanically driven supercharger or an exhaust-driven turbocharger is employed to facilitate increasing the mass of intake air beyond what could be produced by atmospheric pressure alone.

Nitrous oxide can also be used to artificially increase the mass of oxygen present in the intake air.

This is accomplished by injecting liquid nitrous oxide into the intake, which supplies significantly more oxygen in a given volume than is possible with atmospheric air.

[2] Most modern diesel engines powering highway vehicles are turbocharged to produce a more favourable power-to-weight ratio, a higher torque curve, as well as better fuel efficiency and lower exhaust emissions.