Difference between revisions of "Straight-four"

An even-firing inline-four engine is in primary balance because the pistons are moving in pairs, and one pair of pistons is always moving up at the same time as the other pair is moving down. However, piston acceleration and deceleration are greater in the top half of the crankshaft rotation than in the bottom half, because the connecting rods are not infinitely long, resulting in a non sinusoidal motion. As a result, two pistons are always accelerating faster in one direction, while the other two are accelerating more slowly in the other direction, which leads to a secondary dynamic imbalance that causes an up-and-down vibration at twice crankshaft speed. This imbalance is tolerable in a small, low-displacement, low-power configuration, but the vibrations get worse with increasing size and power.<ref>Nunney, 14-15</ref>

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&nbsp;L in displacement rely on the damping effect of their engine mounts to reduce the vibrations to acceptable levels. Above 2.0&nbsp;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&nbsp;cc engine that was a derivative of the [[Citroën Traction Avant|Traction Avant]] engine, the 1948 [[Austin Motor Company|Austin]] 2,660&nbsp;cc engine used in the [[Austin-Healey 100]] and [[Austin Atlantic]], the 3.3&nbsp;L [[flathead engine]] used in the [[Ford Model A (1927)]], and the 2.5&nbsp;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 [[aluminium]] big-bore inline-four engines (2.5 or later 2.9&nbsp;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>

[[list of automotive superlatives|The smallest]] production motorcycle inline-four engine was the [[four-stroke engine]] powered the 231&nbsp;cc Benelli/Moto Guzzi 254. For racing, Honda built inline-four engines as small as a 125&nbsp;cc for the Honda 125/4. This engine was replaced by a 125&nbsp;cc [[straight-five engine]]. The largest proprietary inline-four engine in a commercially-produced motorcycle is the 1402&nbsp;cc engine in the [[Suzuki GSX1400]].

Modern inline-four motorcycle engines first gained their popularity with [[Honda]]'s [[overhead camshaft#Single overhead camshaft|SOHC]] [[Honda CB750|CB750]] in the 1970s. Since then, the inline-four has become one of the most common engine configurations in street bikes. Outside of the [[cruiser (motorcycle)|cruiser]] category, the inline-four is simply the most common configuration because of its relatively high performance-to-cost ratio. All of the Japanese motorcycle manufacturers offer motorcycles with inline-four engines, as does [[MV Agusta]] and [[BMW motorcycles|BMW]] who employ both [[longitudinal engine|longitudinal]] and [[transverse engine|transverse]]-mounted engines. Even the modern [[Triumph Motorcycles Ltd|Triumph]] company has offered inline-four-powered motorcycles, though they were discontinued in favour of a [[Triumph Triple|triple]].