A naturally-aspirated engine (NA - aspiration meaning breathing) refers to an internal combustion engine (normally petrol or diesel powered) that is neither turbocharged nor supercharged. Most automobile engines are naturally-aspirated, though turbochargers and superchargers have enjoyed periods of success, particularly in the late 1980s and the current 2000s era.

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. Most 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.

Natural aspiration gives less power than either turbo or supercharged engines of similar capacity but is cheaper to produce and generally with better fuel efficiency. In drag racing, naturally aspirated vehicles are vehicles that don't run a blower, a turbo, nor 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.

See also

• Carburetor
• Cylinder head
• Forced induction
• Fuel injection
• Turbocharger
• Supercharger
• Helmholtz Resonance
• Variable Length Intake Manifold