Admission is the first of the four times of an internal combustion engine. At the beginning of the intake phase, the intake valve opens and the piston begins to descend causing a pressure drop in the cylinder, which in turn causes the entry of fresh air from the outside in the case of a diesel engine, or a flammable mixture of air and fuel in the case of an Otto Cycle engine commonly called a gasoline engine.
When the piston has completed its down stroke and the cylinder has been filled with air or flammable mixture, the inlet valve closes and the compression stroke begins.
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