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The straight-eight engine or inline-eight engine is an eight-cylinder internal combustion engine with all eight cylinders mounted in a straight line along the crankcase. The type has been produced in side-valve, overhead-valve, sleeve-valve, and overhead-cam configurations. A straight-8 can be timed for inherent primary and secondary balance, with no unbalanced primary or secondary forces or moments. However, crankshaft torsional vibration, present to some degree in all engines, is sufficient to require the use of a harmonic damper at the accessory end of the crankshaft. Without such damping, fatigue cracking near the rear main bearing journal may occur, leading to engine failure. Although an inline six-cylinder engine can also be timed for inherent primary and secondary balance, a straight-8 develops more power strokes per revolution and, as a result, will run more smoothly under load than an inline six. Also, due to the even number of power strokes per revolution, the straight-8 does not produce unpleasant odd-order harmonic vibration in the vehicle's driveline at low engine speeds. The smooth running characteristics of the straight-8 made it popular in luxury and racing cars of the past. However, the engine's length demanded the use of a long engine compartment, making the basic design unacceptable in modern vehicles. Also, due to the length of the engine, torsional vibration in both crankshaft and camshaft can adversely affect reliability and performance at high speeds. In particular, a phenomenon referred to as "crankshaft whip," caused by the effects of centrifugal force on the crank throws at high engine rpm, could cause physical contact between the connecting rods and crankcase walls, leading to the engine's destruction. As a result, the design has been displaced almost completely by the shorter V8 engine configuration. ==Early period (1903–1918)== The first straight-eight was conceived by Charron, Girardot et Voigt (CGV) in 1903, but never built.〔Georgano, p. ?〕 Great strides were made during World War I, as Mercedes made straight-eight aircraft engines. Advantages of the straight-8 engine for aircraft applications included the aerodynamic efficiency of the long, narrow configuration, and the inherent balance of the engine making counterweights on the crankshaft unnecessary. The disadvantages of crank and camshaft twisting were not considered at this time, since aircraft engines of the time ran at low speeds to keep propeller tip speed below the speed of sound. Unlike the V8 engine configuration, examples of which were used in De Dion-Bouton, Scripps-Booth, and Cadillac automobiles by 1914, no straight-8 engines were used in production cars before 1920. 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「Straight-eight engine」の詳細全文を読む スポンサード リンク
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