Editing Straight-four


The '''Inline-four engine''' or '''Straight-four engine''' is an [[internal combustion engine]] with all four [[cylinder (engine)|cylinders]] mounted in a straight line, or plane along the [[crankcase]]. The single bank of cylinders may be oriented in either a vertical or an inclined plane with all the [[piston]]s driving a common [[crankshaft]]. Where it is inclined, it is sometimes called a '''slant-four'''. In a specification chart or when an abbreviation is used, an inline-four engine is listed either as '''I4''' or '''L4''' (for ''longitudinal'', to avoid confusion between the digit 1 and the letter I).

The inline-four layout is the simplest design which is in perfect [[engine balance|primary balance]] and confers a degree of mechanical simplicity which makes it popular for economy cars.<ref>Nunney, ''Light and Heavy Vehicle Technology'', page 12</ref>  However, despite its simplicity, it suffers from a secondary imbalance which causes minor vibrations in smaller engines.  These vibrations become worse as engine size and power increase, so the more powerful engines used in larger cars generally are more complex designs with more than four cylinders.

==Displacement==
This [[inline engine]] configuration is the most common in [[automobile|car]]s with a [[engine displacement|displacement]] up to 2.4&nbsp;L. The usual "practical" limit of the displacement of inline-four engines in a car is around 2.7&nbsp;L. However, [[Porsche]] used a 3.0&nbsp;L four in its [[Porsche 944|944 S2]] and [[Porsche 968|968]] sports cars, and Rolls Royce produced several 4-cylinder engines of 2,838&nbsp;cc with basic cylinder dimensions of {{convert|3.5|in|mm|abbr=on}} diameter and {{convert|4.5|in|mm|abbr=on}} stroke (Rolls Royce B40);

Classic and Antique vehicles tended to have larger displacements to develop horsepower and torque.  The Model A Ford was built with a 3.3&nbsp;L inline-four engine.

Four-cylinder diesel engines, which are lower revving than gasoline engines, often exceed 3.0&nbsp;L. Mitsubishi still employs a 3.2&nbsp;L four-cylinder [[turbodiesel]] in its Pajero (called the Shogun or Montero in certain markets), and Tata Motors employs a 3.0&nbsp;L four-cylinder diesel in its Spacio and Sumo Victa.

The [[Toyota B engine|Toyota B engine series]] of diesel engines varies in displacement from 3.0- 4.1&nbsp;L. The largest engine in that series was used in the [[Toyota Mega Cruiser|Mega Cruiser]].

Larger four-cylinder engines are used in industrial applications, such as in small trucks and tractors, are often found with displacements up to about 4.6&nbsp;L. [[Diesel engine]]s for stationary, marine and [[diesel locomotive|locomotive]] use (which run at low speeds) are made in much larger sizes.

Displacement can also be very small, as found in [[kei cars]] sold in Japan, such as the [[Subaru EN engine|Subaru EN series]]; engines that started out at 550&nbsp;cc and are currently at 660&nbsp;cc, with variable valve timing, [[DOHC]] and superchargers resulting in engines that produce {{convert|65|PS|kW bhp|0|abbr=on}}.

==Balance and smoothness==
[[Image:Engine movingparts.jpg|thumb|Computer generated image showing the major internal moving parts of an inline-four engine with belt-driven double overhead camshafts and 4 valves per cylinder.]]

The inline-four engine is much smoother than one, two, and three cylinder engines, and this has resulted in it becoming the engine of choice for most economy cars, although it can be found in some sports cars as well. However, the inline-four is not a fully [[engine balance|balanced]] configuration.

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>

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.



==Motorcycle use==
[[Image:Honda CB750 Engine.jpg|thumb|Honda CB750 engine]]

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]].

The 2009 [[Yamaha R1]] has an interesting inline-four engine that does not fire at even intervals of 180°. Instead, it uses a [[crossplane]] crankshaft that prevents the pistons from simultaneously reaching top dead centre. This results in increased torque at lower engine speeds.

==Notes & references==
{{reflist}}
{{refbegin}}
*{{cite book|last=Ludvigsen|first=Karl|title=Classic Racing Engines|publisher=Haynes Publishing| year=2001|isbn=1-8596-0649-0}}
*{{cite book|last=Nunney|first=M J|title=Light and Heavy Vehicle Technology|edition=4th|publisher= Butterworth-Heinemann|year=2006|isbn=0-7506-8037-7}}
{{refend}}

{{Piston engine configurations}}

{{DEFAULTSORT:Straight-Four Engine}}
[[Category:Engine configurations]]