Sometime, we hear people talk about "NA" engine and "Turbo" engine, or when you need some engine gasket, your supplier may ask you your engine is "NA" type or "Turbo" type?
For "Turbo" type of engine, some may feel very straight forward that is a turbocharger bolt on engine, but for "NA" engine, do you just call it as engine that don't have turbocharger on it? Of course not.
"NA" is actually stand for Naturally Aspirated, so what does it mean? Here again with the help of Google search and Wikipedia, let see how is their definition on it.
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 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 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 altitudeincreases.
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.
As 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's requirement for combustion, at a given power and speed.
By the Society of Automotive Engineer's definition, a mechanically scavenged two-stroke diesel engine is considered to be naturally aspirated.
What is a Naturally Aspirated Engine ?
A naturally-aspirated engine refers to an internal combustion engine that is neither power enhanced with a turbocharger or supercharger. 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. Jaguar enjoys the benefits of its very quick supercharged models. Subaru, Saab, Mitsubishi and Nissan all have turbocharged models in their range of cars. Most road-going diesel-engine vehicles will use turbochargers and intercoolers too. This is because naturally aspirated diesel engines generally cannot offer a suitable level of power for everyday driving conditions.
In a naturally aspirated engine, the air/fuel mixture is forced into the cylinders by a vacuum caused by the cylinder movement, natural atmospheric pressure, and the venturi effect upon the opening of the inlet valves. In a turbocharged or supercharged engine, the fuel mix is forced into the combustion chamber under a greater pressure and this is therefore referred to as a forced induction engine.
When comparing a naturally asperated engine with a turbocharged or supercharged engine of the same displacement, the naturally aspirated engine will generally give less power. The simpler design of the naturally aspirated engines mean that they tend to be cheaper to produce en masse.
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. We hope that helps answer the question ‘What is What is a Naturally Aspirated Engine ?’!
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