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Difference between revisions of "Straight-four"
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The reason for the piston's higher speed during the 180° rotation from mid-stroke through top-dead-centre, and back to mid-stroke, is that the minor contribution to the piston's up/down movement from the [[connecting rod]]'s change of angle here has the same direction as the major contribution to the piston's up/down movement from the up/down movement of the crank pin. By contrast, during the 180° rotation from mid-stroke through bottom-dead-centre and back to mid-stroke, the minor contribution to the piston's up/down movement from the [[connecting rod]]'s change of angle has the opposite direction of the major contribution to the piston's up/down movement from the up/down movement of the crank pin. | The reason for the piston's higher speed during the 180° rotation from mid-stroke through top-dead-centre, and back to mid-stroke, is that the minor contribution to the piston's up/down movement from the [[connecting rod]]'s change of angle here has the same direction as the major contribution to the piston's up/down movement from the up/down movement of the crank pin. By contrast, during the 180° rotation from mid-stroke through bottom-dead-centre and back to mid-stroke, the minor contribution to the piston's up/down movement from the [[connecting rod]]'s change of angle has the opposite direction of the major contribution to the piston's up/down movement from the up/down movement of the crank pin. | ||
Most inline-four engines below 2.0 L in displacement rely on the damping effect of their engine mounts to reduce the vibrations to acceptable levels. Above 2.0 L, most modern inline-four engines now use [[balance shaft]]s to eliminate the second-order harmonic vibrations. In a system invented by Dr. [[Frederick W. Lanchester]] in 1911, and popularised by [[Mitsubishi Motors]] in the 1970s, an inline-four engine uses two balance shafts, rotating in opposite directions at twice the crankshaft's speed, to offset the differences in piston speed.<ref>Nunney, 42-44</ref> However, in the past, there were numerous examples of larger inline-fours without balance shafts, such as the [[Citroën DS|Citroën DS 23]] 2,347 cc engine that was a derivative of the [[Citroën Traction Avant|Traction Avant]] engine, the 1948 [[Austin Motor Company|Austin]] 2,660 cc engine used in the [[Austin-Healey 100]] and [[Austin Atlantic]], the 3.3 L [[flathead engine]] used in the [[Ford Model A (1927)]], and the 2.5 L [[GM Iron Duke engine]] used in a number of American cars and trucks. Soviet/Russian [[Volga (automobile)|GAZ Volga]] cars and [[UAZ]] SUVs, vans and light trucks used [[ | Most inline-four engines below 2.0 L in displacement rely on the damping effect of their engine mounts to reduce the vibrations to acceptable levels. Above 2.0 L, most modern inline-four engines now use [[balance shaft]]s to eliminate the second-order harmonic vibrations. In a system invented by Dr. [[Frederick W. Lanchester]] in 1911, and popularised by [[Mitsubishi Motors]] in the 1970s, an inline-four engine uses two balance shafts, rotating in opposite directions at twice the crankshaft's speed, to offset the differences in piston speed.<ref>Nunney, 42-44</ref> However, in the past, there were numerous examples of larger inline-fours without balance shafts, such as the [[Citroën DS|Citroën DS 23]] 2,347 cc engine that was a derivative of the [[Citroën Traction Avant|Traction Avant]] engine, the 1948 [[Austin Motor Company|Austin]] 2,660 cc engine used in the [[Austin-Healey 100]] and [[Austin Atlantic]], the 3.3 L [[flathead engine]] used in the [[Ford Model A (1927)]], and the 2.5 L [[GM Iron Duke engine]] used in a number of American cars and trucks. Soviet/Russian [[Volga (automobile)|GAZ Volga]] cars and [[UAZ]] SUVs, vans and light trucks used [[aluminum]] big-bore inline-four engines (2.5 or later 2.9 L) with no balance shafts from the 1950s-1990s. These engines were generally the result of a long incremental evolution process and their power was kept low compared to their capacity. However, the forces increase with the square of the engine speed — that is, doubling the speed makes the vibration four times worse — so modern high-speed inline-fours have more need to use balance shafts to offset the vibrations.<ref>Nunney, 40-44.</ref> | ||
Four cylinder engines also have a smoothness problem in that the power strokes of the pistons do not overlap. With four cylinders and four cycles to complete, each piston must complete its power stroke and come to a complete stop before the next piston can start a new power stroke, resulting in a pause between each power stroke and a pulsating delivery of power. In engines with more cylinders, the power strokes overlap, which gives them a smoother delivery of power and less vibration than a four can achieve. As a result, six- and eight- cylinder engines are generally used in more luxurious and expensive cars. | Four cylinder engines also have a smoothness problem in that the power strokes of the pistons do not overlap. With four cylinders and four cycles to complete, each piston must complete its power stroke and come to a complete stop before the next piston can start a new power stroke, resulting in a pause between each power stroke and a pulsating delivery of power. In engines with more cylinders, the power strokes overlap, which gives them a smoother delivery of power and less vibration than a four can achieve. As a result, six- and eight- cylinder engines are generally used in more luxurious and expensive cars. | ||