Editing Flat-four engine (section)

== Design ==
[[File:Vierzylinder Boxermotor.gif|thumb|Boxer-four animation]]
In a flat-four engine, each pair of opposing [[piston]]s successively moves inwards together and outwards together. The advantages of the boxer-four layout are perfect secondary vibration (resulting in minimal vibration), a low [[centre of gravity]], and a short engine length. Flat-four engine have successfully used [[air cooling]], although air-cooled engines are noisier and have a lower power output than an equivalent engine with [[liquid cooling]].  In light aircraft, where lightness is of primary importance, air-cooling has traditionally been common.<ref>Established manufacturers of such air-cooled aero-engines include [[Lycoming Engines|Lycoming]] and [[Continental Aerospace Technologies|Continental]].</ref>

The downsides of boxer-four engines (compared with inline-four engines) are their extra width, the increased costs associated with having two cylinder heads instead of one, and the long exhaust manifold required to achieve evenly spaced exhaust pulses.<ref>{{cite book | last = Nunney | first = M. J. | title = Light and Heavy Vehicle Technology | url = https://archive.org/details/lightheavyvehicl00nunn_220 | url-access = limited | publisher = Butterworth-Heinemann | year = 2007 | page = [https://archive.org/details/lightheavyvehicl00nunn_220/page/n25 13] | isbn = 978-0-7506-8037-0}}</ref> Due to these factors, [[straight-four engine]]s are more common in cars than are flat-four engines, and [[V6 engine]]s are often used where larger displacements are required.<ref> Engine article [https://www.motorauthority.com/news/1111942_whats-the-difference-between-flat-4-and-inline-4-engines]</ref>

=== Engine balance ===
The equal and opposing forces generated in a boxer-four engine result in perfect [[secondary balance]] (unlike the unbalanced vertical forces produced by inline-four engines). Boxer-four engines are therefore better suited to displacements above {{convert|2.0|L|cuin|0|abbr=on}}, since they do not require [[balance shaft]]s to reduce the secondary vibration.

In a boxer engine, each cylinder is slightly offset from its opposing pair due to the distance between the [[crankpin]] journals. This offset gives rise to a slight [[Couple (mechanics)|rocking couple]],<ref name="Wilson">{{cite book |last=Wilson |first=Hugo |title=The Encyclopedia of the Motorcycle |url=https://archive.org/details/encyclopediaofmo00wils_1 |url-access=registration |year=1995 |publisher=Dorling Kindersley |location=London, UK |isbn=0-7513-0206-6 |chapter=The A–Z of Motorcycles }}</ref>{{refpage|page=27}} but any resulting vibration is normally insufficient to require balance shafts.

As with all four-stroke engines of four cylinders or fewer, the lack of overlap in the [[power stroke (engine)|power strokes]] results in a pulsating delivery of torque to the [[flywheel]], causing a torsional vibration along the crankshaft axis.<ref>Thomson. Vibration Theory and Applications. p. 190. Dynamics of reciprocating forces. Prentice Hall Inc., USA, 1965.</ref> Such vibration, if excessive, may be minimised using a [[harmonic damper]].

=== Exhaust manifold ===
The typical firing order for a boxer-four engine is for the left bank of cylinders to ignite one after another, followed by the right bank of cylinders (or vice versa), with the [[Firing order#Even and uneven firing order|firing interval]] evenly spaced at 180 degrees. Traditionally, the exhausts from the two cylinders on each bank were merged, with the resulting uneven exhaust pulses causing a characteristic "flat-four burble" exhaust sound.

The other common exhaust configuration (such as used by Subaru since the mid-2000s) is to pair the cylinders with a firing interval offset of 360 degrees, in order to optimise the [[Exhaust manifold#Exhaust scavenging|exhaust pulses]].<ref>Announcement on Legacy exhaust in Japanese: {{cite web |title=Fuji Heavy Industries, Press Information |url=http://www.fhi.co.jp/news/03_04_06/03_05_23b.htm |date=2003-05-23 |access-date=2013-12-16 |archive-date=2016-05-15 |archive-url=https://web.archive.org/web/20160515173706/http://www.fhi.co.jp/news/03_04_06/03_05_23b.htm |url-status=dead }}</ref><ref>Announcement on Forester exhaust in Japanese: {{cite web |title=Fuji Heavy Industries, News Release |url=http://www.fhi.co.jp/news/05_01_03/05_01_27.html |date=2005-01-27 |access-date=2013-12-16 |archive-date=2016-03-04 |archive-url=https://web.archive.org/web/20160304084234/http://www.fhi.co.jp/news/05_01_03/05_01_27.html |url-status=dead }}</ref><ref>Announcement on Impreza exhaust in Japanese: {{cite web |title=Fuji Heavy Industries, News Release |url=http://www.fhi.co.jp/news/07_04_06/07_06_05_01.html |date=2007-06-05 |access-date=2013-12-16 |archive-date=2016-03-09 |archive-url=https://web.archive.org/web/20160309183635/http://www.fhi.co.jp/news/07_04_06/07_06_05_01.html |url-status=dead }}</ref> This configuration requires long exhaust manifolds, in order to pair the cylinders on opposite banks, and results in a less distinctive exhaust sound.