Editing Flat-six engine

{{Short description|Horizontally opposed 6-cylinder piston engine}}
{{Redirect|Flat-six|the interval|Minor sixth}}
[[File:Continental O-470-13A.jpg|thumb|[[Continental O-470|Continental O-470-13A]] air-cooled aircraft engine]]
A '''flat-six engine''', also known as a '''horizontally opposed-six''', is a [[six-cylinder]] [[Internal combustion engine#Reciprocating engines|piston engine]] with three cylinders on each side of a central [[crankshaft]]. The most common type of flat-six engine is the '''boxer-six''' engine, where each pair of opposed cylinders moves inwards and outwards at the same time. An alternative configuration for flat engines is a 180-degree V engine, where both cylinders move to the right then the left at the same time.

The advantages of the flat-six layout are good engine balance (for reduced vibration), a low center of gravity, short length (compared with an [[inline-six engine]]) and being well suited to air-cooling. The disadvantages are a large width (which can limit the maximum steering angle when used in a front-engined car), a large intake manifold being required when a central [[carburetor]] is used, and duplication of the inlet and outlet connections for water-cooled engines.<ref>{{cite book  | last = Nunney  | first = M J  | title = Light and Heavy Vehicle Technology  | publisher = Butterworth-Heinemann  | year = 2007  | isbn = 978-0-7506-8037-0  | page = 13 |url=https://books.google.com/books?id=xLxySLNAe3YC&pg=PA13 |access-date= 24 September 2019}}</ref>

The first production flat-six engine was in the 1904 ''[[Wilson-Pilcher]] 18/24 HP'' car. The most notable use of flat-six engines is the [[Porsche 911]] sports car, which has used flat-six engines continuously since 1963. Several other car manufacturers, including Subaru, have produced flat-six engines at times. Flat-six engines have also occasionally been used in motorcycles, and commonly in general aviation aircraft, along with the [[flat-four engine]].

== Engine balance ==
{{see|Flat engine}}
A [[boxer engine|boxer-style]] flat-six engine is able to have perfect [[primary balance|primary]] and [[secondary balance]]. As in other six-cylinder engines, the overlapping of the power strokes of the different cylinders (with a firing interval of 120 degrees in a four-stroke engine) reduces the pulsating of the power delivery relative to that of similar engines with fewer cylinders. 

In a [[boxer engine|boxer]] configuration, a flat-six engine does not have a [[Couple (mechanics)|rocking couple]]. The symmetry of the arrangement, in which one bank of three cylinders is mirrored by the other bank, means that there is no net force from any given mirrored cylinder pair along the axis of movement of the pistons. So not only is there no net primary or secondary reciprocating effect, there is no net turning force - moment - that would try to rotate the engine back and forth about its engine mountings. (The moment is not quite zero in practice because each cylinder pair is offset slightly from the other, or else they would clash at the crankshaft.) In this regard, the six cylinder boxer engine is the same as a flat-four boxer engine, which also does not experience any rocking couple.<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=9 September 2019}}</ref>

These characteristics result in low vibration for flat-six engines (as in [[straight-six engine]]s), especially when compared with the imbalances that are present in [[V6 engine]]s with a 90-degree bank angle.

== Use in aircraft ==
[[File:FranklinO335engine1.JPG|thumb|right |1945–1975 [[Franklin O-335]] air-cooled aircraft engine]]

Early flat-six engines include the [[Franklin O-265]] which began production in 1940, and the [[Lycoming O-435]] which began production in 1942. Several manufacturers use the letter ''O'' in their model codes for flat-layout engines as a designation for “opposed” cylinder layouts. The [[Franklin O-335]] was used in the [[Bell 47]] light helicopter. 

Flat engines largely replaced the historically more popular radial engines in small aircraft after World War II because they were less expensive to manufacture. The smaller frontal area compared with a radial engine also results in less drag, although the radial design offers improved cooling.

Some aircraft have used flat-six engines originally designed for cars. The [[Porsche PFM 3200]] engine, produced from 1985 to 1991, was based on the engine used in the Porsche 911 sports car. Several examples of the [[Pietenpol Air Camper]] homebuilt monoplane aircraft have used the air-cooled engine from the [[Chevrolet Corvair]] compact car.<ref>{{cite web |url=http://community.pressenter.net/~apietenp/BHPietenpolAndSonsAirCamperAircraftHistoryofPietenpolAirCamper.html |title=Pietenpol Official Pietenpol Air Camper Family Website |author=<!--Not stated--> |website=community.pressenter.net |publisher=Pietenpol Aircraft Company |access-date=May 6, 2017 |quote=As the years passed, Bernard Pietenpol taught himself how to repair televisions and radios. By 1960, Chevrolet introduced the six cylinder air-cooled Chevrolet Corvair. This would be the last engine Bernard would experiment with in powering his Air Camper. In 1966 Bernard completed his first Corvair powered Air Camper, then another in 1970 which was dubbed "the Last Original". |archive-date=April 23, 2017 |archive-url=https://web.archive.org/web/20170423060658/http://community.pressenter.net/~apietenp/BHPietenpolAndSonsAirCamperAircraftHistoryofPietenpolAirCamper.html |url-status=dead }}</ref> Compared with the engines used in cars, flat-six engines used by helicopters have large displacements and are low revving, producing more torque and less power.<ref>{{cite web |title=The Lycoming O-540 Engine and Why We Use It |url=http://hieldsaviation.co.uk/the-lycoming-o-540-engine-and-why-we-use-it/ |website=www.hieldsaviation.co.uk |access-date=10 September 2019 |date=18 November 2015 |archive-date=19 September 2019 |archive-url=https://web.archive.org/web/20190919085444/http://hieldsaviation.co.uk/the-lycoming-o-540-engine-and-why-we-use-it/ |url-status=dead }}</ref>

A notable recent flat-six aero-engine is the 4-litre [[Flathead engine|side-valve]] [[Belgium|Belgian]] [[D-Motor LF39]], a modular variant of the 2.7-litre [[flat-four engine|flat-four]] [[D-Motor LF26]]. Although the side-valve format has long been abandoned for most automotive applications because its combustion chamber is a bar to high engine [[revolution per minute|rpm]], the  massively over-square (1.295:1) D-Motor is a very simple, low-revving, compact, reliable lightweight aero-engine (without the heavy (and bulky) complication of [[overhead valve engine|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>

== Use in automobiles ==
=== Porsche ===
{{see|Porsche flat-six engine}}
[[File:Porsche 911 2.0 004.JPG|thumb|left|1966 [[Porsche 911 (classic)|Porsche 911]] ]]
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The flat-six engine is often associated with the [[Porsche 911]], a rear-engined sports car which has used flat-six engines exclusively since 1963. The engines were [[Air-cooled engine|air-cooled]] until 1999, when Porsche started using [[Water cooling (engines)|water-cooled engines]].<ref>{{cite web |title=Flat Sixy: The Evolution of Porsche 911 Engine Size, Technology, and Output in the U.S. |url=https://www.caranddriver.com/news/a15366914/flat-sixy-the-evolution-of-porsche-911-engine-size-technology-and-output-in-the-u-s/ |website=www.caranddriver.com |access-date=21 September 2019 |date=13 December 2013}}</ref>

Other Porsche models that use flat-six engines are the 1970–1972 [[Porsche 914|Porsche 914/6]] ([[mid-engine]]), the 1986–1993 [[Porsche 959]] ([[rear engine|rear-engine]]), and the 1996–2021 [[Porsche Boxster/Cayman]] (mid-engine).
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=== Other manufacturers ===
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| image1            = The Wilson-Pilcher Flat-Six engine.jpg
| caption1          = 1904 [[Wilson-Pilcher]] engine 
| image2            = SUBARU_EZ36_001.jpg
| caption2          = 2007–2019 [[Subaru_six-cylinder_engines#EZ36|Subaru EZ36]] engine
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The first car to use a flat-six engine was the 1904–1907 ''[[Wilson-Pilcher]] 18/24 HP'',<ref>{{cite web |title=7 Flat-Six-Powered Vehicles That Aren't Porsches |url=https://www.carthrottle.com/post/7-flat-six-powered-vehicles-that-arent-porsches/ |website=www.carthrottle.com |access-date=21 September 2019 |language=en}}</ref> which used an engine based on a flat-four engine with two cylinders added. The Wilson-Pilcher was a front-engined car with the crankshaft in-line with the chassis and the cylinders between the chassis rails. Reports on this car quote it as being "remarkably silent and smooth running" and "almost total absence of vibration".<ref>"The Wilson-Pilcher Petrol Cars", The Automotor Journal, April 16th, 1904, pp. 463-468</ref>

Two American manufacturers briefly produced cars with flat-six engines—the 1948 [[Tucker 48]] (water-cooled, based on the Franklin O-335) and the 1960–1969 [[Chevrolet Corvair]] (air-cooled).<ref>{{cite web |title=Corvair History |url=http://www.corvair.org/index.php?option=com_k2&view=item&layout=item&id=6&Itemid=648 |website=www.corvair.org |archive-url=https://web.archive.org/web/20140622041530/http://www.corvair.org/index.php?option=com_k2&view=item&layout=item&id=6&Itemid=648 |archive-date=22 June 2014}}</ref> Both are rear-engined cars with rear-wheel drive. Chevrolet produced over 1.8 million Corvairs of various types and utilized a turbocharger on some models; one of the first uses of a turbocharger on a mass-produced automobile.

Japanese manufacturer Subaru produced water-cooled flat-6 engines from 1988–1996 and 2000–2019.<ref>{{cite web |title=Flat-6 Club: 1993 Subaru Impreza w/ Legacy 6-Cyl |url=http://www.dailyturismo.com/2018/06/flat-6-club-1993-subaru-impreza-w.html |website=www.dailyturismo.com |access-date=21 September 2019 |archive-date=21 September 2019 |archive-url=https://web.archive.org/web/20190921022942/http://www.dailyturismo.com/2018/06/flat-6-club-1993-subaru-impreza-w.html |url-status=dead }}</ref> Their first flat-6 engine, the [[Subaru_six-cylinder_engines#ER27|Subaru ER27]], was added to the model range of the [[Subaru XT]] coupe. In 1991, the [[Subaru_six-cylinder_engines#EG33|Subaru EG33]] engine was used in the [[Subaru Alcyone SVX]], the replacement for the Subaru XT.<ref>{{cite web |title=Subaru history in the USA: the models, the changes |url=http://www.cars101.com/subaru/subaru_history.html |website=www.cars101.com |access-date=23 September 2019}}</ref> Following a hiatus of flat-6 engine production for four years, the [[Subaru_six-cylinder_engines#EZ30_and_EZ36|Subaru EZ30]] engine was introduced in the 2000 [[Subaru_Legacy_(third_generation)|Subaru Legacy/Outback]]. The [[Subaru EZ36]] engine was introduced in 2007 and was produced until 2019. All engines were used in front-engine cars with all-wheel drive, plus the XT and Alcyone SVX were also available with front-wheel drive.
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== Use in motorcycles ==
[[Image:flat.six.honda.valkyrie.arp.750pix.jpg|thumb|1997–2004 [[Honda Valkyrie]] engine]]
Most motorcycles use engines with four or fewer cylinders, however the [[Honda Gold Wing]] touring motorcycle has used a water-cooled flat-six engine since 1988.<ref>{{cite web |title=Touring |url=https://powersports.honda.com/street/touring |website=www.honda.com |access-date=23 September 2019 |language=en}}</ref> Initially, the engine had a displacement of {{convert|1.5|L|cuin|0|abbr=on}}, until it was enlarged to {{convert|1.8|L|cuin|0|abbr=on}} in 2001.<ref name="Mitchel">{{cite book |last1=Mitchel |first1=Doug |title=Honda Motorcycles: Everything You Need to Know About Every Honda Motorcycle Ever Built |date=2005 |publisher=Krause Publications |isbn=9780873499668 |language=en}}</ref>{{refpage|page=208}}

The [[Honda Valkyrie]] F6C (1997–2003) was a cruiser based on the GL1500 Gold Wing.<ref name="Mitchel"/>{{refpage|pages=198,218}} The limited edition 2004 Valkyrie Rune was based on the GL1800.<ref name="Mitchel"/>{{refpage|pages=218,220}}
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== See also ==
* [[Inline-six engine]]
* [[V6 engine]]

==References==
{{reflist|30em}}

{{Piston engine configurations}}

[[Category:Piston engine configurations|Flat-06]]
[[Category:Flat engines|06]]