Editing Flat engine (section)

== Design ==
{{More citations needed section|date=February 2021}}
[[File:Moteurs à Plat - V à 180° vs. Boxer.png|thumb|Difference between two flat 6 cylinder engines: 180° V on the left, boxer on the right]]

The advantages of flat engines are a short length, low [[centre of mass]] and suitability for [[Air-cooled engine|air cooling]] (due to the well-exposed, large-surface-area cylinders and cylinder heads, and their short length).

Compared with the more common [[straight engine]]s, flat engines have better [[primary balance]] (resulting in less vibration);<ref>{{cite web |title=The Physics of: Engine Cylinder-Bank Angles |url=https://www.caranddriver.com/features/a15126436/the-physics-of-engine-cylinder-bank-angles-feature/ |website=www.caranddriver.com |access-date=2019-10-18 |date=2019-10-14}}</ref> however, the disadvantages are increased width and the need to have two cylinder heads. Compared with [[V engine]]s – the most common layout for engines with six cylinders or more – flat engines again have a lower centre of mass, and, for six-cylinders, better primary balance; the disadvantage is again their being wider.<ref name="why-subaru">{{cite web |url=https://www.subaru.com.au/why-subaru/performance |title=Subaru Car Performance |publisher=Subaru |location=Australia |access-date=2020-07-01}}</ref>

The most common usages of flat engines are:
* [[Flat-twin engine]]s are mostly used in motorcycles. Occasionally they have been used in light cars, aircraft and industrial applications, mostly up until the 1960s.
* [[Flat-four engine]]s are mostly used in cars (most notably in the earlier [[Volkswagen]] [[Volkswagen Beetle|Types 1]] to [[Volkswagen Type 4|4]], and by [[Subaru]]<ref name="why-subaru"/> in most of their models), and have occasionally been used in motorcycles. Their most common use is in smaller single-engine [[general aviation|general-aviation]] aircraft, for which they are still manufactured and used to this day.
* [[Flat-six engine]]s are mostly used in aircraft and cars (particularly by the [[Porsche]] 911 sports car), and have occasionally been used in motorcycles.
* [[Flat-eight engine]]s have been used in several racing cars, mostly by Porsche in the 1960s.
* [[Flat-ten engine]]s are not known to have reached production. A prototype road car engine was built by [[Chevrolet]] in the 1960s.
* [[Flat-twelve engine]]s have been used in various racing cars, notably the [[Porsche 917K]], during the 1960s and 1970s, and in [[Ferrari]] road cars from 1973 to 1996. The [[Panhard EBR]] armoured car is one of the few military vehicles to have used such an engine.
* [[Flat-sixteen engine]]s are not known to have reached production. Prototype racing car engines were built by [[Coventry Climax|Coventry]] and Porsche in the 1960s and 1970s.

=== Boxer configuration {{anchor|Boxer engine}}===
[[File:R68-opposed-cylinders.jpg|thumb|right|1954 [[BMW R68]] flat-twin boxer engine]]

Most flat engines use a "boxer" configuration, where each pair of opposing pistons move inwards and outwards at the same time, somewhat like boxing competitors punching their gloves together before a fight.<ref name=Bonk>{{cite web |last=Bonk |first=Aaron |title=How Boxer Engines Work |url=http://www.superstreetonline.com/techarticles/sstp_1301_how_boxer_engines_work/ |work=Super Street Magazine |publisher=Source Interlink Media |access-date=2013-12-21 |date=2013-02-07 |archive-date=27 February 2014 |archive-url=https://web.archive.org/web/20140227052947/http://www.superstreetonline.com/techarticles/sstp_1301_how_boxer_engines_work/ |url-status=dead }}</ref> Boxer engines have low vibration, being the only common configuration that has no unbalanced forces, regardless of the number of pairs of cylinders. Boxer engines therefore do not require either a [[balance shaft]]<ref name=Hanlon/> or counterweights on the [[crankshaft]] to balance the weight of the reciprocating parts. However, a [[Couple (mechanics)#Applications|rocking couple]] is present, since each cylinder is slightly offset from the other member of its pair due to the distance between the [[crankpin]]s along the crankshaft.<ref name=Bonk/>

=== 180-degree V engine ===
An alternative configuration for flat engines is as a 180-degree [[V engine]], which has been used on most twelve-cylinder flat engines. In this configuration, each pair of pistons shares a [[crankpin]], this being simpler than the boxer configuration, where each piston has its own separate crankpin.