A naturally-aspirated engine (N/A) is a reciprocating internal combustion engine that depends solely on atmospheric pressure to draw in combustion air. This is in contrast to a forced induction engine, in which a mechanical or exhaust-driven blower is employed to increase the volume of intake air beyond what could be produced by atmospheric pressure alone.
In a naturally-aspirated engine, air (Diesel cycle) or an air/fuel mixture (Otto cycle) is forced into the cylinder by atmospheric pressure in response to a partial vacuum that occurs as the piston moves toward bottom dead center during the intake stroke. Owing to innate restriction in the engine's induction system, 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.
Most automobile gasoline (petrol) engines, as well as many small engines used for non-automotive purposes, are naturally-aspirated. Most Diesel engines powering highway vehicles are turbocharged to produce a more favorable power-to-weight ratio, as well as better fuel efficiency and lower exhaust emissions. Turbocharging is nearly universal on Diesel engines that are used in railroad, marine, and commercial stationary applications (electrical power generation, for example). Forced induction is also used with reciprocating aircraft engines to negate some of the power loss that occurs as the aircraft climbs to higher altitudes.
A two stroke Diesel engine is incapable of natural aspiration as defined above. Some method of charging the cylinders with scavenging air must be integrated into the engine design. This is usually achieved with a positive displacement blower driven by the crankshaft. The blower does not act as a supercharger in this application, as it is sized to produce a volume of air flow that is in direct proportion to engine displacement and speed. A mechanically-scavenged two-stroke Diesel engine is considered to be naturally aspirated.