- Naturally-aspirated engine
A naturally-aspirated engine or normally-aspirated engine (or "N/A" -
aspirationmeaning breathing) refers to an internal combustion engine(normally petrolor dieselpowered) that is neither turbochargednor supercharged. Most automobilegasoline (petrol) engines are naturally-aspirated, although turbochargers and superchargers have enjoyed periods of success, particularly in the late 1980s and the current 2000s era. Most road-going diesel-engined vehicles use turbochargers and intercoolers because naturally-aspirated diesels generally cannot offer power-to-weight ratios acceptable in the modern car market.
Air or air/fuel mixtures are forced into the cylinders by vacuum caused by piston movement, natural
atmospheric pressure, and venturi effectupon opening of the inlet valve or valves. The pressure within the cylinder is lowered by the action of the piston moving away from the valves (so as to expand the volume available for incoming air). In some cases the lowering of the cylinder pressure is enhanced by a combination of the speed of the exhaust gases leaving the cylinder and the closing of the exhaust valve at the appropriate time. A tuned exhaust can help with this but generally only works at a narrow range of engine speeds and hence is most useful in very high performance cars, aircraft, and helicopters. Many N/A engines today make use of Variable Length Intake Manifolds to harness Helmholtz resonance, which has a mild forced-induction effect but is not considered true forced induction. Cylinder head portingdesign is of premium importance in naturally aspirated engines. Camshaftsusually will be more "aggressive", having greater lift and duration. Cylinder headgaskets will also be thinner, with the top of the pistonrising up into the combustion chamber,= for high-performance N/A engines that benefit from higher compression.
Naturally-aspirated engines generally gives less power than either turbo or supercharged engines of the same
engine displacementand development level but tend to be cheaper to produce. In drag racing naturally-aspirated vehicles are those that do not operate with a supercharger, a turbocharger, nor use nitrous oxide.
Many racing series specify N/A engines to limit power and speed.
NASCAR, IndyCar, and Formula Oneare all in this category. Naturally-aspirated engines have been mandated in Formula One since 1989, in order to curb excessive power output and the high cost of engines with superchargers or turbochargers. The Indy Racing Leaguemandated Naturally-aspirated engines in 1997.
Natural aspiration as defined above cannot occur in a two-stroke diesel engine. Some method of charging the cylinder with scavenging air must be integrated into the engine design, customarily achieved with a positive displacement blower driven by the
crankshaft. The blower does not act as a superchargerin this application as it is sized to produce air flow volume in direct proportion to engine displacement and speed. A mechanically scavenged two-stoke diesel is considered to be naturally aspirated.
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