Anaturally aspirated engine, also known as anormally aspirated engine, and abbreviated toN/A orNA, is aninternal combustion engine in which airintake depends solely onatmospheric pressure and does not haveforced induction through aturbocharger or asupercharger.[1]
In a naturally aspirated engine, air for combustion (Diesel cycle in adiesel engine or specific types ofOtto cycle in petrol engines, namelypetrol direct injection) or an air/fuel mixture (traditional Otto cycle petrol engines), is drawn into the engine'scylinders by atmospheric pressure acting against apartial vacuum that occurs as thepiston travels downwards towardbottom dead centre during the intakestroke. Owing to innate restriction in the engine's inlet tract, which includes theintake manifold, a small pressure drop occurs as air is drawn in, resulting in avolumetric 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 theoreticalpower 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 operatingaltitude increases.
This is in contrast to aforced-induction engine, in which a mechanically drivensupercharger or an exhaust-driventurbocharger 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. Nitrous oxide is 36.3% available oxygen by mass after it decomposes as compared with atmospheric air at 20.95%. Nitrous oxide also boils at −127.3 °F (−88.5 °C) at atmospheric pressures and offers significant cooling from the latent heat of vaporization, which also aids in increasing the overall air charge density significantly compared to natural aspiration.
Mostautomobile petrol engines, as well as many small engines used for non-automotive purposes, are naturally aspirated.[2] Most modern diesel engines powering highway vehicles areturbocharged to produce a more favourablepower-to-weight ratio, a highertorque curve, as well as betterfuel efficiency and lowerexhaust emissions. Turbocharging is nearly universal on diesel engines that are used inrailroad,marine engines, and commercialstationary applications (electrical power generation, for example). Forced induction is also used with reciprocatingaircraft engines to negate some of the power loss that occurs as theaircraft climbs to higher altitudes.
The advantages and disadvantages of a naturally aspirated engine in relation to a same-sized engine relying onforced induction include:
Most motor vehicle engines are naturally-aspirated engines; however, turbocharging and supercharging are currently a very popular way of boosting power output for a number car marques.