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A flathead engine (aka sidevalve engine (SV), flathead, or flatty)〔''American Rodder'', 6/94, pp.45 & 93.〕 is an internal combustion engine with valves placed in the engine block beside the piston, instead of in the cylinder head, as in an overhead valve engine. As the cylinder cross-section has the shape of an upside-down L, other names such as L-block or L-head are also common. The sidevalve's poppet valves are usually sited on one side of the cylinder(s). A recess in the cylinder head creates a corridor connecting the valves and the combustion chamber. The valve gear comprises a camshaft which operates the valves via simple tappets, without any further valvetrain paraphernalia (such as pushrods, rocker arms, overhead valves or overhead camshafts). The sidevalve arrangement was once the most common across all motor industries (automotive, agricultural, marine, aviation, and others), but it has since fallen from favor in most multicylinder applications, such as automotive and aviation, having been displaced by overhead valve designs. Flathead designs are still commonly built new for many small engine applications of one and two cylinders, such as lawnmowers, rotary tillers, and two-wheel tractors. ==Advantages== The main advantages of a sidevalve engine are simplicity, reliability, cheapness, compactness, responsive low-speed power, low mechanical engine noise and insensitivity to low-octane fuel. The absence of a complicated ohv valvetrain allows a compact engine that is cheaper to manufacture, as the cylinder head may be little more than a simple metal casting. These advantages may explain why ohv designs were first adopted only in high-performance applications such as aircraft, luxury cars, and sports cars, while economy cars, trucks, and agricultural engines continued to use flathead engines for some time. Even today, flatheads are common in lawnmowers and basic farm machinery. An important benefit of the sidevalve design is that, if a valve should seize in its guide (remain partially open), only limited damage to that individual cylinder would occur, and an engine can often operate safely on its other cylinders with a seized valve. If a valve head breaks off, the detached head is much larger in diameter than the combustion chamber, and frequently becomes lodged between the piston and the head causing severe damage. At tdc, the piston gets very close to the flat cylinder head, and the resultant squish turbulence gives excellent fuel/air mixing. The typical flathead engine has its valves adjacent, but the "T-head" variant acts as a "cross-flow" head. 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「Flathead engine」の詳細全文を読む スポンサード リンク
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