Difference between revisions of "Poppet valve"

''Poppet valves'' are used in most [[piston engine]]s to open and close the intake and exhaust [[cylinder head porting|ports]] in the [[cylinder head]]. The valve is usually a flat disk of metal with a long rod known as the ''valve stem'' out one end. The stem is used to push down on the valve and open it, with a spring generally used to close it when the stem is not being pushed on. [[Desmodromic valve]]s are closed by positive mechanical action instead of by a spring, and are used in some high speed [[motorcycle]] and auto racing engines, eliminating '[[valve float]]' at high [[RPM]].

''Poppet valves'' are used in most [[piston engine]]s to open and close the intake and exhaust [[cylinder head porting|ports]] in the [[cylinder head]]. The valve is usually a flat disk of metal with a long rod known as the ''valve stem'' out one end. The stem is used to push down on the valve and open it, with a spring generally used to close it when the stem is not being pushed on. [[Desmodromic valve]]s are closed by positive mechanical action instead of by a spring, and are used in some high speed [[motorcycle]] and auto racing engines, eliminating '[[valve float]]' at high [[RPM]].

The engine normally operates the valves by pushing on the stems with [[cam]]s and [[Tappet|Cam follower]]s. The shape and position of the cam determines the valve lift and when and how quickly (or slowly) the valve is opened. The cams are normally placed on a fixed [[camshaft]] which is then geared to the [[crankshaft]], running at half crankshaft speed in a [[four stroke cycle|four-stroke]] engine. On high performance engines (e.g., [[Ferrari]] cars), the camshaft is movable and the cams have a varying height, so by axially moving the camshaft in relation with the engine [[RPM]], also the valve lift varies. See [[variable valve timing]].

The engine normally operates the valves by pushing on the stems with [[cam]]s and [[Tappet|Cam follower]]s. The shape and position of the cam determines the valve lift and when and how quickly (or slowly) the valve is opened. The cams are normally placed on a fixed [[camshaft]] which is then geared to the [[crankshaft]], running at half crankshaft speed in a [[four stroke cycle|four-stroke]] engine. On high performance engines (e.g., [[Ferrari]] cars), the camshaft is movable and the cams have a varying height, so by axially moving the camshaft in relation with the engine [[RPM]], also the valve lift varies. See [[variable valve timing]].

===Valve wear===

===Valve wear===

In the early days of engine building, the poppet valve was a major problem. Metallurgy was not what it is today, and the rapid opening and closing of the valves against the cylinder heads led to rapid wear. They would need to be re-ground every two years or so, in an expensive and time consuming process known as a ''[[valve job]]''. Adding [[tetra-ethyl lead]] to the [[petrol]] reduced this problem to some degree as the lead would coat the valve seats, in effect lubricating the metal.  Valve seats made of improved alloys such as stellite have generally made this problem disappear completely and made leaded fuel unnecessary.

In the early days of engine building, the poppet valve was a major problem. Metallurgy was not what it is today, and the rapid opening and closing of the valves against the cylinder heads led to rapid wear. They would need to be re-ground every two years or so, in an expensive and time consuming process known as a ''[[valve job]]''. Adding [[tetra-ethyl lead]] to the [[petrol]] reduced this problem to some degree as the lead would coat the valve seats, in effect lubricating the metal.  Valve seats made of improved alloys such as stellite have generally made this problem disappear completely and made leaded fuel unnecessary.

==References==

==References==

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