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Automotive glossary: What is a Valve or 4-Stroke Cycle Engine Valve?

Definition of Valve: In contrast to two stroke engines, where the piston controls letting in the gasoline air mixture and letting out the incinerated gas, four stroke engines need valves, which open and close at the proper moment, to control the gas flow. The valves are located in the cylinder head and are opened by the camshaft and (usually) closed again by a spring. Thereby the cams (so bulges) of the revolving camshaft (possibly via rocker levers) push on the backside of the valves, so that the sealing part of the valve is being shifted a bit into the combustion chamber and thereby a gap is opened, through which the gas can flow in resp. out. Accordingly, valves are classified as inlet and outlet valves. Valves are exposed to extremely high thermal and mechanic stress.

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