Normally aspirated

A naturally aspirated engine is an internal combustion engine in which air intake depends solely on atmospheric pressure and which does not rely on forced induction through a turbocharger or a supercharger. Many sports cars specifically use naturally aspirated engines to avoid turbo lag.

In a naturally aspirated engine, air for combustion (diesel cycle in a diesel engine, or specific types of Otto cycle in gasoline engines – namely gasoline direct injection), or an air/fuel mixture (traditional Otto cycle petrol engines) is drawn into the engines cylinders by atmospheric pressure acting against a partial vacuum that occurs as the piston travels downwards toward bottom dead centre during the intake stroke. Owing to innate restriction in the engine's inlet tract which includes the intake manifold, a small pressure drop occurs as air is drawn in, resulting in a volumetric efficiency of less than 100 percent – and a less than complete air charge in the cylinder. 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 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 in increasing the mass of intake air beyond what could be produced by atmospheric pressure alone.

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