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The cam-in-block valvetrain layout of piston engines is one where the camshaft is placed within the cylinder block, usually beside and slightly above the crankshaft in a straight engine or directly above the crankshaft in the V of a V engine. This contrasts with an overhead camshaft (OHC) design which places the camshafts within the cylinder head and drives the valves directly or through short rocker arms. Placing the camshaft inside the engine block has a long history in its use in valve-in-block engines, in straight and V configurations, the Ford flathead being exemplary of the type. Pushrod overhead valve engines with the cam in the block were long used in Chevrolet and Buick straight engines from the 1930s through the mid-1950s and in various similar six-cylinder engines until the extensive employment of the V6 configuration in the 1980s. There are three main cam-in-block designs: * L-head, also known as ''L-block'', ''flathead'' or ''sidevalve'' * F-head * I-head, also known as overhead valve (OHV) ==L-head== (詳細はvalves are placed in the engine block beside the pistons. The design was common on early engine designs, but has since fallen from use. Generally L-head engines use a small chamber on one side of the cylinder to carry the valves. This has a number of advantages, primarily in that it makes the cylinder head much simpler. It also means that the valve can be operated by pushing directly up on it, as opposed to needing some sort of mechanical arrangement to push the valves down. It may also lead to slightly easier cooling, as the valves and operating rods are out of the way of the cylinder, making a cooling jacket simpler to construct (but see below). The line of intakes along the side of the engine lead to the name L-head, due to the cylinders having the shape of an upside-down L. This configuration is also known as sidevalve, as the valves are located be''side'' the cylinders. On the downside, the L-head engine also requires the airflow to make at least a 90° turn to enter the cylinder, which makes it less efficient; colloquially it's said that such an engine has poorer "breathing". Breathing was not greatly emphasized in past production cars because engines could not run long and reliably at high speed due to other factors. This was a minor concern given the benefits in simplicity. Although L-head inline 4 and 6-cylinder engines were frequently used for automobiles, tractors, etc., the best known L-head automotive engine is the early 20th century Ford V-8, which has both sets of valves (intake and exhaust) located on the inside of the "Vee," and which are all operated by a single camshaft located above the crankshaft. The exhaust follows a lengthy path to leave the engine. This virtually guarantees that the engine will need an unusually large coolant radiator to avoid overheating under sustained heavy use. A flathead design in a V engine, with the air intake/fuel system and all of the exhaust and intake valves inside of the "V" requires that the exhaust gas be passed between the cylinders to outside of the V to the exhaust system. Exhaust heat is thus passed to the coolant (as it exits the engine between the cylinders). In the Ford V-8 flathead design, manufactured from 1932 through 1952, the center exhaust port on the outside of the block exhausts the gasses from two cylinders, exacerbating the high heat problem. This "very hot in the middle" problem makes this particular engine prone to heat-related stress and cracks in the cylinder block. In line engine exhaust gas exits the block more directly and does not cross between the cylinders and is a more temperature-stable design. Whenever exhaust ports and valves are in the cylinder head, exhausting heat has far less time to heat the coolant, and such engines are more durable under high load conditions and a similar sized engine will require less coolant radiator capacity than a flathead V-8. Due to the heating and efficiency problems, L-head engines fell from high power uses such as aircraft engines fairly quickly, prior to World War I. They lived on for some time in the automotive world and were used in the World War II Jeep, for instance. L-heads are no longer used in automobile engines, although they remain in common use for small-engine applications in lawnmowers and generators. Because of their heat-retaining design, the size of valves and the compression ratio are limited (the valve/combustion chamber is away from the piston top typically creating a larger combustion space--a lower compression ratio), which in turn reduces available power and economy. Not all L-heads are cam-in-block engines; the location of the camshaft varies in this layout. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cam-in-block」の詳細全文を読む スポンサード リンク
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