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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.
The engine normally operates the valves by pushing on the stems with cams 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 cylinderThe word cylinder has several meanings. For the geometric object, see Cylinder (geometry . For the engine component, see Cylinder (engine . In firearms the cylinder is the rotating device that contains the firing chambers of a revolver. The phonograph cyls - the so called L-head engine because of the shape of the cylinder and combustion space, also called 'flat head' engine as the top of the head is flat. This evolved into 'Intake Over Exhaust', IOE or F-head, where the intake valve was in the block and the exhaust valve was in the cylinder headBMW In an internal combustion engine, the cylinder head sits atop the cylinders and consists of a platform containing part of the combustion chamber and the location of the valves and spark plugs. The top half of the cylinder head contains the camshaft in, and later both valves moved to the head.
In most such designs the camshaft remained relatively near the crankshaft, and the valves were operated through pushrodA pushrod or overhead valve (OHV) type piston engine places the camshaft below the pistons and uses pushrods or "rods" to actuate lifters or tappets above the cylinder head to actuate the valves. This contrasts with an overhead cam (OHC) design which placs and rocker arm s. This led to significant energy losses in the engine, but was simpler, especially in a V engineA V engine is a common configuration for an internal combustion engine in which the pistons are aligned so that, if viewed along the line of the crankshaft, they appear to be in a V. The V configuration reduces the overall engine length and weight compare where one camshaft can actuate the valves for both cylinder bankPiston engines are typically arranged with their pistons in rows, moving inside individual cylinders. This allows the engine block to be built from a single piece of metal, which is machined. Engines often have more than one row of pistons, each with theis; 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 camSingle overhead cam (also SOHC refers to the internal combustion engine design where one camshaft is located above the valves. SOHC engines are generally limited to two valves per cylinder, one for intake and one for exhaust. See also Double 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 which is found in almost all modern engines. The camshaft was driven by 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. MetallurgyMetallurgy is a domain of materials science and of materials engineering that studies the physical and chemical behavior of metallic elements and their mixtures, which are called alloys. Extractive metallurgy Extractive metallurgy is the practice of separ 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, but valve seats made of improved alloys such as stellite have generally made this problem disappear completely and making leaded fuel unnecessary.
The poppet valve was also used in a limited fashion in steam engines, particularly steam locomotives. Most steam locomotives used slide valve s or piston valves, but these designs, although mechanically simpler and very rugged, were significantly less efficient than the poppet valve. A number of designs of locomotive poppet valve system were tried, the most popular being the Italian Caprotti valve gear , the British Caprotti valve gear (an improvement of the Italian one), the German Lentz rotary-cam valve gear , and two American versions by Franklin, their oscillating-cam valve gear and rotary-cam valve gear . They were used with some success, but they were less ruggedly reliable than traditional valve gear and did not see widespread adoption.
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