A naturally-aspirated engine or normally-aspirated engine (or "N/A" - aspiration meaning breathing) refers to an internal combustion engine (normally petrol or diesel powered) that is neither turbocharged nor supercharged. Most automobile gasoline (petrol) engines are naturally-aspirated, though turbochargers and superchargers have enjoyed periods of success, particularly in the late 1980s and the current 2000s era. However, most road-going diesel-engined vehicles use turbochargers and intercoolers, because naturally-aspirated diesels generally cannot offer suitable power-to-weight ratios to be acceptable in the modern car market.
Air or air/fuel mixtures are forced into the cylinders by vacuum caused by cylinder movement, natural atmospheric pressure, and venturi effect upon 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 porting design is of premium importance in naturally aspirated engines. Camshafts usually will be more "aggressive", having greater lift and duration. Also, cylinder head gaskets will be thinner, and with the top of the piston rising 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 displacement and development level but tend to be cheaper to produce. In drag racing, naturally-aspirated vehicles are vehicles that do not operate with a blower, a turbo, nor use nitrous oxide.
Many racing series specify N/A engines to limit power and speed. NASCAR, IndyCar, and Formula One are 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 League mandated N/A engines in 1997.
Natural aspiration as defined above, cannot occur in a two-stroke diesel engine. Therefore, 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 supercharger in 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.