Editing Straight-four engine (section)

== Design ==
A four-stroke straight-four engine always has a cylinder on its power stroke, unlike engines with fewer cylinders where there is no power stroke occurring at certain times. Compared with a [[V4 engine]] or a [[flat-four engine]], a straight-four engine only has one [[cylinder head]], which reduces complexity and production cost.

=== Displacement ===
Petrol straight-four engines used in modern production cars typically have a displacement of {{convert|1.3|-|2.5|L|cuin|0|abbr=on}}, but larger engines have been used in the past, for example the 1927–1931 [[Bentley 4½ Litre]].

Diesel engines have been produced in larger displacements, such as a 3.2&nbsp;L turbocharged Mitsubishi engine (used the Pajero/Shogun/Montero SUV) and a 3.0&nbsp;L Toyota engine. European and Asian trucks with a [[gross vehicle weight rating]] between 7.5 and 18 tonnes commonly use inline four-cylinder diesel engines with displacements around 5&nbsp;litres.<ref>{{cite web |url=http://www.man-engines.com/datapool/mediapool/800/D0834_LKW_dt.pdf |title=4-Zylinder Reihenmotor für Nutzfahrzeuge. |access-date=2011-08-01 |url-status=dead |archive-url=https://web.archive.org/web/20110714024640/http://www.man-engines.com/datapool/mediapool/800/D0834_LKW_dt.pdf |archive-date=2011-07-14 }}</ref><ref>{{cite web |url=http://www.mantruckandbus.com/en/Products_and_solutions/MAN_Lkw/TGL/TGL.jsp |title=MAN Truck & Bus - TGL |access-date=2011-05-09 |url-status=dead |archive-url=https://web.archive.org/web/20110523132506/http://www.mantruckandbus.com/en/Products_and_solutions/MAN_Lkw/TGL/TGL.jsp |archive-date=2011-05-23 }}</ref><ref>{{cite web |url=http://www.vario-mobil.com/welcome%20to%20variomobil.html |title=VARIOmobil - Welcome to a lap of luxury coaches - recreation vehicles - motor homes |access-date=2011-05-09 |url-status=dead |archive-url=https://web.archive.org/web/20110827160415/http://www.vario-mobil.com/welcome%20to%20variomobil.html |archive-date=2011-08-27 }}</ref><ref>{{cite web |url=http://www.isuzucv.com/engines/4h_index.html |title=Isuzu Commercial Vehicles - Low Cab Forward Trucks - Commercial Vehicles - 4HK1-TC 5.2L Diesel Engine |access-date=2010-12-14 |url-status=dead |archive-url=https://web.archive.org/web/20101224200033/http://www.isuzucv.com/engines/4h_index.html |archive-date=2010-12-24 }}</ref><ref>{{cite web |url=http://www.isuzutruck.co.uk/downloads/specsheets/fseries/f110.210%20easyshift.pdf |title=Euro 4 'Forward' F11O.21O |access-date=2010-12-14 |url-status=dead |archive-url=https://web.archive.org/web/20101126070600/http://isuzutruck.co.uk/downloads/specsheets/fseries/f110.210%20easyshift.pdf |archive-date=2010-11-26 }}</ref><ref>{{cite web|url=http://www.hino-global.com/products/diesel_engines/index.html |title=Diesel Engines &#124; Products |publisher=Hino Global |access-date=2017-05-23}}</ref><ref>{{cite web |url=http://www.hino-global.com/pdf/catalog/Hino_500S_Catalog_LR.pdf |title=Hino 500 Series |access-date=2010-12-14 |url-status=dead |archive-url=https://web.archive.org/web/20101214123029/http://www.hino-global.com/pdf/catalog/Hino_500S_Catalog_LR.pdf |archive-date=2010-12-14 }}</ref> Larger displacements are found in locomotive, marine and stationary engines.

Displacement can also be very small, as found in [[kei car]]s sold in Japan. Several of these engines had four cylinders at a time when regulations dictated a maximum displacement of 550&nbsp;cc; the maximum size is currently at 660&nbsp;cc.

=== Primary and secondary balance ===
Straight-four engines with the preferred crankshaft configuration have perfect [[primary balance]].<ref name="Nunney">{{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|ref=Nunney}}</ref>{{refpage|p. 12}} This is 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, straight-four engines have a [[Secondary balance|secondary imbalance]]. This is caused by the acceleration/deceleration of the pistons during the top half of the crankshaft rotation being greater than that of the pistons in the bottom half of the crankshaft rotation (because the connecting rods are not infinitely long). 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 common among all piston engines, but the effect is particularly strong on four-stroke inline-four because of the two pistons always moving together.

The strength of this imbalance is determined by the reciprocating mass, the ratio of connecting rod length to stroke, and the peak piston velocity. Therefore, small displacement engines with light pistons show little effect, and racing engines use long connecting rods. However, the effect [[quadratic growth|grows quadratically]] with engine speed (rpm).

=== Pulsations in power delivery ===
[[File:Reihenmotor Vier Zylinder 1-2-4-3.gif|thumb|Animation of an Inline-four engine]]
[[Four-stroke engine]]s with five or more cylinders are able to have at least one cylinder performing its power stroke at any given point in time. However, four-cylinder engines have gaps in the power delivery, since each cylinder completes its power stroke before the next piston starts a new power stroke. This pulsating delivery of power results in more vibrations than engines with more than four cylinders.

=== Usage of balance shafts ===
[[File:MMC Silent shafts.jpg|thumb |Mitsubishi Silent Shaft display]]

A [[balance shaft]] system is sometimes used to reduce the vibrations created by a straight-four engine, most often in engines with larger displacements. The balance shaft system was invented in 1911 and consists of two shafts carrying identical [[Eccentric (mechanism)|eccentric]] weights that rotate in opposite directions at twice the crankshaft's speed.<ref name="Nunney"/>{{refpage|pp. 42–44}} This system was patented by Mitsubishi Motors in the 1970s, introduced in the [[Mitsubishi Astron engine]] with the "Silent Shaft" name, and has since been used under licence by several other companies.<ref name="bbc1">{{cite web |url=http://www.bbc.com/autos/story/20140609-before-they-were-carmakers |title=Before they were carmakers |last=Carney |first=Dan |publisher=BBC |location=UK |date=2014-06-10 |access-date=2018-11-01}}</ref><ref name="PS-Jun89">{{cite magazine |last=Nadel |first=Brian |date=June 1989 |title=Balancing Act |magazine=Popular Science |page =52 }}</ref>

Not all large displacement straight-four engines have used balance shafts, however. Examples of relatively large engines without balance shafts include the 2.4 litre [[Citroën DS]] engine, the 2.6 litre [[Austin-Healey 100]] engine, the 3.3&nbsp;L [[Ford Model A (1927)]] engine and the 2.5&nbsp;L [[GM Iron Duke engine]]. Soviet/Russian [[Volga (automobile)|GAZ Volga]] and [[UAZ]] engines with displacements of up to 2.9 litres were produced without balance shafts from the 1950s to the 1990s, however these were relatively low-revving engines which reduces the need for a balance shaft system.<ref name="Nunney"/>{{refpage|pp. 40–44}}