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Valve (214)

A poppet valve (also called mushroom valve) is a valve typically used to control the timing and quantity of gas or vapour flow into an engine. It consists of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem.

Poppet valves are used in most piston engines to open and close the intake and exhaust ports in the cylinder head.
The poppet valve lifts from the seat with a movement perpendicular to the port. The main advantage of the poppet valve is that it has no movement on the seat, thus requiring no lubrication.

For certain applications the valve stem and disk are made of different steel alloys, or the valve stems may be hollow and filled with sodium to improve heat transport and transfer. Although better heat conductors, aluminum cylinder heads require steel valve seat inserts, while cast-iron cylinder heads often used integral valve seats in the past.

Because the valve stem extends into lubrication in the cam chamber, it must be sealed against blow-by to prevent cylinder gases from escaping into the crankcase, even though the stem to valve clearance is very small, typically 0.04-0.06 mm. A rubber lip-type seal ensures that excessive amounts of oil are not drawn in from the crankcase on the induction stroke and that exhaust gas does not enter the crankcase on the exhaust stroke. Worn valve guides or defective oil seals are characterised by a puff of blue smoke from the exhaust when pressing back down on the accelerator pedal after allowing the engine to over-run, such as when changing gears.

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