| Poppet Valve |
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Presta and Schrader Valve s used on tires are examples of poppet valves. The Presta valve has no spring and relies on a pressure differential for opening and closing while being inflated. Poppet valves are used in many industrial process from controlling the flow of rocket fuel to controlling the flow of Milk . INTERNAL COMBUSTION ENGINE ''Poppet valves'' are used in most Piston Engine s to open and close the intake and exhaust Ports . 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 Valves 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 . For certain applications the valve stem and disk are made of different steel Alloy s, or the valve stems may be hollow and filled with Sodium to improve Heat transport and transfer. The engine normally operates the valves by pushing on the stems with Cam s and 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 engine. On high performance engines e.g. used in Ferrari cars, the camshaft is moveable 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 . In very early engine designs the valves were 'upside down' in the block, parallel to the Cylinder s - the so called L-head engine because of the shape of the cylinder and combustion space, also called ' Flathead Engine ' as the top of the Cylinder Head is flat. Although this design makes for simplified and cheap construction, it has two major drawbacks; the tortuous path followed by the intake charge limits Air Flow and effectively prevents speeds greater than 2,000-2,500 RPM, and the travels of the exhaust through the block lead to excessive overheating under sustained heavy load. This design therefore evolved into 'Intake Over Exhaust', '''IOE''' or '''F-head''', where the intake valve was in the block and the exhaust valve was in the head; later both valves moved to the head. In most such designs the camshaft remained relatively near the crankshaft, and the valves were operated through Pushrod s and Rocker Arm s. This led to significant energy losses in the engine, but was simpler, especially in a V Engine where one camshaft can actuate the valves for both Cylinder Bank s; for this reason, pushrod engine designs persisted longer in these configurations than others. More modern designs have the camshaft on top of the cylinder head, pushing directly on the valve stem (again through cam followers), a system known as ''overhead camshaft''; if there is just one camshaft, this is a Single Overhead Cam or ''SOHC'' engine. Often there are two camshafts, one for the intake and one for exhaust valves, creating the Dual Overhead Cam , or ''DOHC''. The camshaft is driven by the Crankshaft - through gears, a chain or in modern engines with a Rubber Belt . In the early days of engine building, the poppet valve was a major problem. Metallurgy was not what it is today, 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, hardening the metal. Valve seats made of improved alloys such as Stellite have generally made this problem disappear completely and making leaded fuel unnecessary. ''Poppet valves'' are used in most Piston Engine s to open and close the intake and exhaust Ports . 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 Valves 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 . For certain applications the valve stem and disk are made of different steel Alloy s, or the valve stems may be hollow and filled with Sodium to improve Heat transport and transfer. The engine normally operates the valves by pushing on the stems with Cam s and 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 engine. On high performance engines e.g. used in Ferrari cars, the camshaft is moveable 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 . In very early engine designs the valves were 'upside down' in the block, parallel to the Cylinder s - the so called L-head engine because of the shape of the cylinder and combustion space, also called ' Flathead Engine ' as the top of the Cylinder Head is flat. Although this design makes for simplified and cheap construction, it has two major drawbacks; the tortuous path followed by the intake charge limits Air Flow and effectively prevents speeds greater than 2,000-2,500 RPM, and the travels of the exhaust through the block lead to excessive overheating under sustained heavy load. This design therefore evolved into 'Intake Over Exhaust', '''IOE''' or '''F-head''', where the intake valve was in the block and the exhaust valve was in the head; later both valves moved to the head. In most such designs the camshaft remained relatively near the crankshaft, and the valves were operated through Pushrod s and Rocker Arm s. This led to significant energy losses in the engine, but was simpler, especially in a V Engine where one camshaft can actuate the valves for both Cylinder Bank s; for this reason, pushrod engine designs persisted longer in these configurations than others. More modern designs have the camshaft on top of the cylinder head, pushing directly on the valve stem (again through cam followers), a system known as ''overhead camshaft''; if there is just one camshaft, this is a Single Overhead Cam or ''SOHC'' engine. Often there are two camshafts, one for the intake and one for exhaust valves, creating the Dual Overhead Cam , or ''DOHC''. The camshaft is driven by the Crankshaft - through gears, a chain or in modern engines with a Rubber Belt . In the early days of engine building, the poppet valve was a major problem. Metallurgy was not what it is today, 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, hardening the metal. Valve seats made of improved alloys such as Stellite have generally made this problem disappear completely and making leaded fuel unnecessary. SEE ALSO |
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