The cylinder is filled with a fresh charge of fuel – air mixture if the intake valve / valves of an engine are opened. This charge (or it could be air for diesel and direct injection engines) rushes into the cylinder because of the surrounding barometric pressure and the area of low pressure made by the descending piston. This kind of power plant is called a naturally aspirated engine. It uses nothing but the atmospheric pressure and a vacuum in the various intakes to fill the cylinders. Naturally aspirated engines vary from force – fed engines which make use of blowers to force more air into the cylinder than it is normally possible. Supercharged and turbocharged engines are not the cases of naturally aspirated engines.
The air for combustion (diesel cycle in a diesel engine or it could be specific types of Otto cycle in gasoline engines – namely gasoline direct injection) or an air / fuel mixture (a traditional Otto cycle petrol engine) is drawn inside 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, in a naturally aspirated cylinders. Owing to instinctive restriction in the engine's inlet tract which includes the various intakes, a small pressure drop happens as air is drawn in, consequently giving a volumetric efficiency of less than 100 percent – and a less than complete air charge inside the cylinder. The mass to the volume ratio of the air charge and thereby the engine's maximum theoretical power output is also affected by engine speed and atmospheric pressure, the latter which decreases when the operating altitude increases, in addition to being affected by induction system restriction.
Most of the automobile petrol engines as well as many small engines made use of non – automotive purposes which are are naturally aspirated. Many now – a – day diesel engines which power highway vehicles are turbocharged and produce a more favourable power – to – weight ratio, as well as give better fuel efficiency and lower exhaust emissions. Also, forced induction is used with reciprocating aircraft engines to negate some of the power loss which occurs when the aircraft climbs to higher altitudes.