A naturally-aspirated engine or normally-aspirated engine (or "NA" - 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, because naturally-aspirated diesels generally cannot offer suitable power:weight ratios to be acceptable in the modern car market.
Air or fuel-air mixture is forced into the cylinders by natural atmospheric pressure 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 NA engines today make use of Variable Length Intake Manifolds to harness Helmholtz resonance, which has a mild forced induction effect but is not be 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 NA engines that benefit from higher compression.
Natural aspiration gives less power than either turbo or supercharged engines of same engine displacement and development level but is cheaper to produce and generally operates with better fuel efficiency. In drag racing, naturally-aspirated vehicles are vehicles that do not run a blower, a turbo, nor use nitrous oxide.
Many racing series specify NA 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 the excessive powers being developed by engines with superchargers or turbochargers.