Naturally Aspirated Engine

Naturally aspirated engine are the combustion engine in which air intake depends upon the atmospheric pressure and not depending upon the inducers such as supercharger or turbocharger. Before all of the cars used to have naturally aspirated engines but nowadays are out of use. They are being replaced by turbocharged or supercharged engines to increase performance, reduce fuel consumption and increase combustion efficiency.

In a naturally aspirated engine, air for combustion 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.

They were replaced by smaller and lighter turbocharged engines in order to fulfill stricter emission norms and power requirements that of bigger engines. The only disadvantage of Turbo Charged engines are that they consists of turbo lag. There is no linear power delivery but the power they provide is higher than that of naturally aspirated engine.

Many motor racing series like the big Aussie V8 Super-cars specify that only naturally aspirated engines be used in the cars to limit power and speed. NASCAR, Indy-Car and Formulas One sit also in this category. One of the main reasons for the limiting is also to ensure that the cost of producing the racing engines is not excessive and beyond the reach of some of the manufacturers.