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Difference between revisions of "Poppet valve"
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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]]. | ||
Although better heat conductors, aluminum cylinder heads require steel [[valve seat]] inserts while | 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]]. 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 seals are | 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]]. 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 seals are characterized 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. | ||
===Valve position=== | ===Valve position=== | ||