Editing Flat-four engine

{{Short description|Horizontally opposed four-cylinder piston engine}}
{{original research|date=December 2023}}
[[File:UL350iS ULPower aircraft engine.jpg|thumb|ULPower UL360iS aircraft piston engine]]
A '''flat-four engine''', also known as a '''horizontally opposed-four engine''' or '''boxer engine''',<ref>The so-called "boxer" configuration appears somewhat like boxing competitors punching their gloves together before a fight</ref> is a four-cylinder [[piston engine]] with two banks of cylinders lying on opposite sides of a common crankshaft. The most common type of flat-four engine is the '''boxer-four engine''', each pair of opposed pistons moves inwards and outwards at the same time.

A boxer-four engine has perfect primary and secondary balance, however, the two cylinder heads means the design is more expensive to produce than a [[straight-four engine]]. There is a minor, secondary unbalanced rotational torque pulse in the plane of the pistons, when a piston pair at one end of the engine is at TDC and the other pair at BDC. The TDC pair creates a torque greater than the BDC pair, so the net unbalanced torque pulse is the difference. The difference in TDC vs BDC inertial forces is explained in the '''Engine balance''' section. Boxer-four engines have been used in cars since 1897, especially by Volkswagen and Subaru. They have also occasionally been used in motorcycles and frequently in aircraft.  [[Cessna]] and [[Piper Aircraft|Piper]] use flat four engines from [[Lycoming Engines|Lycoming]] and [[Continental Aerospace Technologies|Continental]] in the most [[List of most-produced aircraft|common civil aircraft]] in the world - the [[Cessna 172]], and [[Piper Cherokee]], while many [[Ultralight aviation|ultralight]] and [[Light-sport aircraft|LSA]] planes use versions of the [[Rotax 912]].

== 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.

== Use in automobiles ==
=== 1900–1935 ===
[[File:Wilson-Pilcher flat-4 engine.jpg|thumb|1904 [[Wilson-Pilcher]] water-cooled engine]]
In 1900, the first flat-four engine was produced by [[Karl Benz#Benz & Cie. and the Benz Patent Motorwagen|Benz & Cie]], based on Benz's  1897 "contra" flat-twin engine.{{citation needed|date=September 2019}} This engine was used in Benz racing cars, produced {{convert|20|hp|kW|0|abbr=on}}, had a displacement of {{convert|5.4|L|cuin|0|abbr=on}} and was designed by Georg Diehl.

London company [[Wilson-Pilcher]] released its first car in 1901, which was powered by a flat-four engine. This engine was mounted longitudinally in the chassis, water-cooled, produced {{convert|9|hp|kW|0|abbr=on}} and had a displacement of {{convert|2.4|L|cuin|0|abbr=on}}. Unusually for its day, the bore and stroke were equal, with each being {{convert|95|mm|in|1|abbr=on}}.

In 1902 the Buffum automobile was equipped with opposed four cylinder engines that were rated at 16 horsepower. Herbert H. Buffum produced an American Automobile called the Buffum in Abington, Massachusetts from 1903 to 1907.<ref>{{cite web |title=The Buffum Automobile |url=http://www.american-automobiles.com/Buffum.html |publisher=Farber and Associates |access-date=2019-10-27 |archive-date=2019-04-23 |archive-url=https://web.archive.org/web/20190423025054/http://www.american-automobiles.com/Buffum.html |url-status=dead }}</ref>

Having previously produced flat-twin engines, the 1926 [[Tatra 30]] was the Czech company's first model powered by a flat-four engine. Tatra produced various flat-four engined model through the 1920s and 1930s.
{{clear right}}

=== 1936–1999 ===
{{More citations needed section|date=December 2023}}
{{multiple image
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| image1            = Flat Four Jowett Engine from a Jowett Jupiter.jpg
| caption1          = 1952 [[Jowett Jupiter]] water-cooled engine
| image2            = 1955 Porsche 550 Spyder engine.jpg
| caption2          = 1955 [[Porsche 550 Spyder]] air-cooled engine
}}

The 1936 [[Tatra T97]] pioneered the rear-engined, air-cooled flat-four, backbone chassis layout (later used by the [[Volkswagen Beetle]]), and at the same time, though unrelated, came the [[Steyr 50]] from Austria, sporting a front boxer 4 engine with rear wheel drive. Also in 1936, English company [[Jowett Cars|Jowett]] expanded its model range from flat-twin engines to also include flat-four engines. Production of Jowett flat-four engines continued until 1954, when the [[Jowett Javelin]] saloon and [[Jowett Jupiter]] sports models ended production.

The longest production flat-four engine is the [[Volkswagen air-cooled engine]], which was produced from 1938 until 2006 and was most famously used in the rear-engined 1938–2003 [[Volkswagen Beetle]] and 1950–1983 [[Volkswagen Transporter]].<ref>{{cite news |title=The engine that Benz built still survives |url=https://www.theglobeandmail.com/globe-drive/reviews/classics/the-engine-that-benz-built-still-survives/article4317376/ |newspaper=The Globe and Mail |date=29 April 2010 |access-date=1 September 2019}}</ref> This air-cooled engine was designed by Porsche and was also used in the 1948–1965 [[Porsche 356]], 1953–1956 [[Porsche 550]], 1965–1969 [[Porsche 912]] and 1969–1976 [[Porsche 914]]. In 1984, to comply with exhaust emissions regulations a water-cooled version called the [[Volkswagen Wasserboxer engine]] was introduced in the [[Volkswagen Type 2 (T3)|Volkswagen Transporter (T3)]]

During the 1960s and 1970s, several manufacturers produced flat-four engines including the air-cooled [[Citroën GS#Mechanics|Citroën flat-four engine]], the water-cooled [[Alfa Romeo Flat-4 engine|Alfa Romeo flat-four engine]], the water-cooled [[Lancia Flat-4 engine|Lancia flat-four engine]] and the water-cooled [[Subaru EA engine]].

Two important engines designed during this period, but never saw the light of day in series production, were the Morris 800cc side valve engine by Alec Issigonis in 1947 originally destined for the Morris Minor, and the Ferguson 2.2 litre SOHC engine by Claude Hill in 1966 as part of the R5 vehicle research project.

=== 2000–present===
[[File:Subaru BRZ engine (20413082204).jpg|thumb |2012–present [[Subaru FA engine|Subaru FA]] water-cooled engine]]

By 2000, most manufacturers had replaced flat-four engines with inline-four engines. A notable exception is Subaru, with the water-cooled [[Subaru EJ engine]] being available in turbocharged form in the [[Subaru WRX]] sports sedan and its [[World Rally Car]] counterpart. Subaru's adoption of [[all-wheel drive]] was a factor in retaining the flat-four engine, since the shorter length of this engine assists in fitting the all-wheel drive components into the chassis.<ref name=popsci>{{cite journal |last=Stepler |first=Richard |title=Preview Drive: Three Ways to Get a Grip |journal=Popular Science |date=September 1994 |volume=245 |issue=3 |page=44 |url=https://books.google.com/books?id=RudyJP1ByycC&pg=PA44 |access-date=2011-10-23}}</ref>  Although it is more expensive than an inline-four engine, the flat-four engine allows Subaru to build an all-wheel drive vehicle at little extra cost from two-wheel drive.<ref name=popsci/>

In 2012, a naturally aspirated version of the [[Subaru FA engine]] was used in the [[Toyota 86]] (also called the "Subaru BRZ" and "Scion FR-S") rear-wheel drive sports coupe.<ref name=Bonk>{{cite web |last=Bonk |first=Aaron (2013-02-07) |title=How Boxer Engines Work, And Why You Should Care About Subaru's New FA20 |url=http://www.superstreetonline.com/techarticles/sstp_1301_how_boxer_engines_work/ |access-date=8 January 2014 |date=2013-02-07 |archive-date=2014-02-27 |archive-url=https://web.archive.org/web/20140227052947/http://www.superstreetonline.com/techarticles/sstp_1301_how_boxer_engines_work/ |url-status=dead }}</ref> This engine is water-cooled, has [[gasoline direct injection]], produces {{convert|147|kW|hp|0|abbr=on}} and has a displacement of {{convert|2.0|L|cuin|0|abbr=on}}.

The 2016 [[Porsche 982|Porsche Boxster/Cayman (982)]] mid-engined sports cars downsized from a naturally aspirated flat-six engine to a turbocharged flat-four engine, Porsche's first flat-four since the mid-1970s. This engine is produced in displacements of {{convert|2.0|-|2.5|L|cuin|0|abbr=on}} and produces up to {{convert|365|hp|kW|0|abbr=on}}.<ref>{{cite web | url = http://www.porsche.com/canada/en/models/718/718-boxster/drive/performance/ | title = Performance: The new 718 Boxster | date = 2016 | publisher = Porsche | access-date = 2016-11-01}}</ref> Several reviewers criticised the Boxster/Cayman for an uninspiring engine sound.<ref>{{cite web |title=2019 Porsche 718 Cayman |url=https://www.caranddriver.com/porsche/718-cayman |website=www.caranddriver.com |access-date=1 September 2019 |date=14 May 2019}}</ref><ref>{{cite web |title=Porsche 718 Cayman review – The entry-level Porsche punches above its weight |url=https://www.evo.co.uk/porsche/718-cayman |website=www.evo.co.uk |access-date=1 September 2019 |language=en}}</ref><ref>{{cite web |title=2017 Porsche 718 Cayman First Drive Review: Performance Trumps Sound |url=https://www.motortrend.com/cars/porsche/718-cayman/2017/2017-porsche-718-cayman-first-drive-review/ |website=www.motortrend.com |access-date=1 September 2019}}</ref>

In 2025, Chinese automaker BYD introduced its first flat-four engine in the BYD U7 luxury sedan, marking a modern revival of the boxer engine layout. This water-cooled, turbocharged powerplant combined direct injection and hybrid technology, delivering 268 hp (200 kW) while maintaining a compact design ideal for the sedan’s all-wheel-drive system. The engine drew inspiration from Porsche’s turbocharged flat-four units and showcased BYD’s push into high-performance powertrains alongside its electric vehicle lineup.<ref>{{Cite web |last=White |first=Tom |date=28 March 2025 |title=BYD's new engine shock! New Subaru-style turbo boxer engine headlines unique performance hybrid YangWang U7 sedan as the Chinese brand takes aim at the Porsche Panamera, BMW i7, and Audi e-tron GT |url=https://www.carsguide.com.au/car-news/byds-new-engine-shock-new-subaru-style-turbo-boxer-engine-headlines-unique-performance |url-status=live}}</ref><ref>{{Cite web |last=Lewis |first=William Irvin |date=2025-04-06 |title=Chinese EV Maker Creates 268 HP Turbo Boxer Engine |url=https://www.hotcars.com/byd-creates-boxer-engine/ |access-date=2025-05-10 |website=HotCars |language=en}}</ref>

== Use in motorcycles ==
{{See also|Motorcycle engine}}
[[File:Glengine.jpg|thumb|[[Honda Gold Wing|Honda GL1000]] ]]

Most motorcycles with four-cylinder engines use a [[straight-four engine]] layout, however, several flat-four engine engines have been used in [[Drive shaft#Motorcycle drive shafts|shaft drive]] motorcycles:
* 1938–1939 [[Zündapp|Zündapp K800]] (air-cooled)
* 1974–1987 [[Honda Gold Wing]] (liquid-cooled)
* 1955–1956 [[Wooler (motorcycles)#Post WW2 motorcycles|Wooler 500cc]] (air-cooled)
* 1981–1982 {{ill|BFG 1300|fr|BFG (entreprise)}}, using the air-cooled [[Citroën GS#Mechanics|Citroën flat-four]] car engine. Approximately 450 were built, with one quarter of these purchased by the French police.<ref name=CitroenetBFG>{{cite web|author=Julian Marsh |url=http://www.citroenet.org.uk/miscellaneous/bfg/bfg.htm |title=BFG and MF motorcycles |publisher=Citroenet.org.uk |date=2000-06-10 |access-date=2011-12-06}}</ref><ref name=CitroenetBoxer4>{{cite web|author=Julian Marsh |url=http://www.citroenet.org.uk/passenger-cars/michelin/gs/flat-4/engine.html |title=GS & GSA boxer engine |publisher=Citroenet.org.uk |access-date=2011-12-06}}</ref>

== Use in aircraft ==
[[File:ULPower UL260i.jpg|thumb|[[ULPower UL260i]] aircraft engine]]

[[Light aircraft]] commonly use flat-four engines with displacements up to {{convert|390|CID|L|1|abbr=on|order=flip}} from manufacturers such as [[Rotax]], [[Lycoming Engines]], [[Teledyne Continental Motors|Continental Motors]] and [[Franklin Engine Company]].

For [[radio-controlled aircraft]], flat-four engines with displacements of {{convert|40|-|50|cc|cuin|1|abbr=on}} are produced by companies such as [[O.S. Engines]].

A notable recent flat-four aero-engine is the [[Flathead engine|side-valve]] [[Belgium|Belgian]] [[D-Motor LF26]].<ref>A modular variant of the LF26 is the 4-litre [[flat-six]] [[D-Motor LF39]]</ref> Although the side-valve format has long been abandoned for most automotive applications because its combustion chamber is a bar to high engine [[rpm]], the  massively over-square (1.295:1) D-Motor is a very simple, low-revving, compact, reliable lightweight aero-engine (without the heavy complication of [[ohv]] valve-gear)<ref>Note: A low-revving  engine is eminently suitable to drive a propeller, which is typically most effective at about 2,000 rpm. If a valve in a side-valve engine should stick open, it cannot hit the piston and cause engine failure; the engine will continue with just that cylinder inoperative. This is an important safety feature for aero-engines.</ref>

== See also ==
* [[Straight-four engine]]
* [[V4 engine]]

== References ==
{{reflist|30em}}
{{Commons category|Flat-4 engines}}
{{Piston engine configurations}}

{{DEFAULTSORT:Flat-Four Engine}}
[[Category:Flat engines|04]]
[[Category:Piston engine configurations|Flat-04]]