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.
The first straight-eight was conceived by Charron, Girardot et Voigt (CGV) in 1903, but never built. 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.
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.
The first straight-eight was conceived by Charron, Girardot et Voigt (CGV) in 1903, but never built. 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.
Mercedes-Benz
would create the last notable straight-8 racing cars in 1955, with the
championship-winning W196 Formula One racing car and the 300SLR sports racing
car. The 300SLR was famous for Stirling Moss and Denis Jenkinson's victory in
the 1955 Mille Miglia, but notorious for Pierre Levegh's notorious accident at
the 1955 24 Hours of Le Mans. The 300SLR was the final development of the Alfa
Romeo design of the early 1930s as not only the camshaft, but now also the
gearbox was driven from the engine's centre. Engineers calculated that
torsional stresses would be too high if they took power from the end of the
long crankshaft, so they put a central gear train in the middle (which also ran
the dual camshafts, dual magnetos, and other accessories) and ran a drive shaft
to the clutch housing at the rear.
https://en.wikipedia.org/wiki/Straight-eight_engine
Straight-8s are currently used in luxury coaches, alongside V12 engines. They are generally preferred by coach drivers for their lower fuel consumption and smoother acceleration.
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Current Use
Straight-8s are currently used in luxury coaches, alongside V12 engines. They are generally preferred by coach drivers for their lower fuel consumption and smoother acceleration.
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