Flat engine

{{Short description|Combustion engine using pistons facing to the sides on a common crankshaft}} {{For|similarly-named engines with two pistons per cylinder|opposed-piston engine}} {{Use dmy dates|date=October 2020}} {{Use British English|date=October 2024}}

[[File:Boxerengineanimation.gif|thumb|upright=1.35|Animation <small>(with errors)</small> of a boxer engine]] [[File:Benz Contramotor IMG 1343 2023-11-25 JM.jpg|thumb|Benz Contramotor, the first commercial flat-engine design, ca. 1899]]

A '''flat engine''' is a [[Internal combustion engine#Reciprocating engines|piston engine]] where the cylinders are located on either side of a central [[crankshaft]]. Flat engines are also known as '''horizontally opposed engines''', however this is distinct from the less common [[opposed-piston engine]] design, whereby each cylinder has two pistons sharing a central combustion chamber.

The most common configuration of flat engines is the [[#Boxer configuration|boxer engine]] configuration, in which the pistons of each opposed pair of cylinders move inwards and outwards at the same time. The other configuration is effectively a [[V engine]] with a 180-degree angle between the cylinder banks: in this configuration each pair of cylinders shares a single crankpin, so that as one piston moves inward, the other moves outward.

The first flat engine (Benz Contramotor) was built in 1897 by [[Karl Benz]]. Flat engines have been used in aviation, motorcycle and automobile applications. They are now less common in cars than [[straight engine]]s (for engines with fewer than six cylinders) and [[V engine]]s (for engines with six or more cylinders). Flat engines are more common in aircraft, where straight engines are a rarity and V engines have almost vanished except in historical aircraft. They have even replaced [[radial engine]]s in many smaller installations.

== 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]] 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.

In 1902, the [[Pearse monoplane]] (which would later become one of the first aircraft to achieve flight) was powered by a flat-twin engine. Amongst the first commercially produced aircraft to use a flat engine was the 1909 [[Santos-Dumont Demoiselle]] range of aeroplanes, which was powered by boxer-twin engines.{{Citation needed|date=March 2021}}

Several boxer-four engines have been produced specifically for light aircraft. A number of manufacturers produced boxer-six aircraft engines during the 1930s and 1940s.{{Citation needed|date=March 2021}}

During World War II, a boxer-twin engine called the "Riedel starter" was used as a starter motor/mechanical [[Auxiliary power unit#As mechanical "startup" APUs for jet engines|APU]] for the early German jet engines, such as the [[Junkers Jumo 004]] and [[BMW 003]]. Designed by [[Norbert Riedel]], these engines have a very [[Stroke ratio#Oversquare or short-stroke engine|oversquare]] stroke ratio of 2:1 so that they could fit within the intake diverter, directly forward of the turbine compressor.{{Citation needed|date=March 2021}} {{clear right}}

==Motorcycle use== Flat engines offer several advantages for motorcycles including a low [[centre of mass]], low vibration, suitability for shaft drive, and equal cooling of the cylinders (for air-cooled engines).<ref name=English /><ref name=Szymezak1>{{cite web |last=Szymezak |first=Pascal |title=Début de la légende |url=http://fgwcf.org/rever/debut_legende.asp |work=La saga GoldWing |publisher=Fédération des GoldWing Club de France |language=fr |access-date=2013-12-20 |url-status=dead |archive-url=https://web.archive.org/web/20131219232421/http://fgwcf.org/rever/debut_legende.asp |archive-date=2013-12-19 }}</ref> The most common design of flat engine for motorcycles is the boxer-twin, beginning with the 1905 Fée flat-twin engine, manufactured by the Light Motors Company, which was the first production motorcycle engine.<ref name=TMCfuture>{{cite journal |date=1916-10-05 |title=The engine of the future |page=283 |journal=[[The Motor Cycle (magazine)|The Motor Cycle]] |publisher=Iliffe & Sons Ltd.}}</ref> [[BMW Motorrad]] have a long history of boxer-twin motorcycles, beginning in 1923 with the [[BMW R32]]<ref name=English />

Several motorcycles have been produced with flat-four engines, such as the 1938–1939 [[Zündapp|Zündapp K800]] and the 1974–1987 [[Honda Gold Wing]]. In 1987, the Honda Gold Wing engine was upsized to a flat-six design.<ref>{{Cite web |title=Overview – GL1800 Gold Wing – Touring – Range – Motorcycles – Honda |url=https://www.honda.co.uk/motorcycles/range/touring/gl1800-gold-wing/overview.html |access-date=2024-10-09 |website=Honda UK}}</ref>

== Automotive use ==

{{multiple image | align = right | direction = vertical | width = 220 | image1 = Diesel Engine Type DS140 2C Hino.jpg | caption1 = 1969 [[Hino Motors]] DS140 boxer-twelve diesel engine | image2 = ChassisT11.JPG | caption2 = 1923-1927 [[Tatra 11]] chassis and engine— front-mounted flat-twin engine with rear-wheel drive }}

When used in cars, advantages of flat engines are a low [[centre of mass]] (which improves the handling of the car),<ref name=Hanlon>{{cite news |last=Hanlon |first=Mike |title=The world's first horizontally-opposed turbo diesel engine |url=https://www.gizmag.com/go/6800/ |work=Gizmag |date=2001-02-07 |access-date=2013-12-20}}</ref> short length, low vibration and suitability for air cooling (due to the well exposed, large surface area, cylinder heads and short length).<ref>{{Cite book |last= Nunney|first= M. J.|title= Light and Heavy Vehicle Technology|publisher= Heinemann Professional Publishing|location= Oxford, UK|year= 1988|isbn= 0-434-91473-8|url= {{Google books|-vj8BAAAQBAJ|Light and Heavy Vehicle Technology |page=12|plainurl=yes}}|access-date=2015-05-08|pages= 12-13}}</ref> However the larger width of flat engines (compared with the more common inline and V layouts) is a drawback, particularly when the engine is located between the steered wheels.{{Citation needed|date=March 2021}}

Flat engines were used by various automobile manufacturers – mostly with a boxer-four design – up until the late 1990s. Since then, only Porsche and Subaru have remained as significant manufacturers of flat engines.{{Citation needed|date=March 2021}}

=== Drivetrain layout ===

{{More citations needed section|date=February 2021}} Due to the short length of flat engines, locating a flat engine outside of the car's [[wheelbase]] results in minimal overhang.{{Sfn|Nunney|1988|p=12}} Therefore, many cars with flat engines have used a [[rear-engine, rear-wheel-drive layout]]. Examples include the flat-twin [[BMW 600]] (1957–1959) and [[BMW 700]] (1959–1965); the flat-four [[Tatra 97]] (1936–1939), [[Volkswagen Beetle]] (1938–2003) and [[Porsche 356]] (1948–1965); and the flat-six [[Chevrolet Corvair]] (1959–1969), [[Porsche 911]] (1963–present), and [[Tucker 48]] (1947–1948).

The opposite layout, [[front-engine, front-wheel-drive layout|front-engine front-wheel drive]], was also common for cars with flat engines. Examples include the [[Citroën 2CV]] (1948–1990), [[Panhard Dyna X]] (1948–1954), [[Lancia Flavia]] (1961–1970), [[Citroën GS]] (1970–1986), [[Alfa Romeo Alfasud]] (1971–1989) and [[Subaru Leone]] (1971–1994).

Subaru have been producing cars with a [[front-engine, four-wheel-drive layout]] powered by flat engines (mostly boxer-four engines) since 1971. Examples include the [[Subaru Leone]] (1971–1994), [[Subaru Legacy]] (1989-2025) and [[Subaru Impreza]] (1992–present). The front half-shafts come out of a front differential that is part of the gearbox. A rear driveshaft connects the gearbox to the rear half-shafts.

The traditional [[front-engine, rear-wheel-drive layout]] is relatively uncommon for cars with flat engines, however some examples include the [[Toyota 86|Toyota 86 / Subaru BRZ]] (2012–present), [[Jowett Javelin]] (1947–1953), [[Glas Isar]] (1958–1965) and the [[Tatra 11]] (1923–1927).

===History=== {{More citations needed section|date=February 2021}} {{multiple image | align = right | direction = vertical | width = 220 | header = Boxer-four engines | image1 = Volkswagen motor cut 1945.JPG | caption1 = 1936–2006 [[Volkswagen air-cooled engine|Volkswagen air-cooled]] | image2 = Subaru BRZ engine (20413082204).jpg | caption2 = 2012–present [[Subaru FA engine|Subaru FA20]] }}

The first flat engine was produced in 1897 by German engineer [[Karl Benz]].<ref name=English>{{cite news |last=English |first=Bob |title=The engine that Benz built still survives |url=https://www.theglobeandmail.com/globe-drive/car-life/classic-cars/the-engine-that-benz-built-still-survives/article4317376/ |access-date=2013-12-19 |work=The Globe and Mail |location=Canada |date=2010-04-29 |archive-date=20 December 2013 |archive-url=https://web.archive.org/web/20131220085552/http://www.theglobeandmail.com/globe-drive/car-life/classic-cars/the-engine-that-benz-built-still-survives/article4317376/ |url-status=dead}}</ref><ref>{{cite web |last=Hodzic |first=Muamer |title=Mercedes Heritage: Four-cylinder engines from Mercedes-Benz |url=http://www.benzinsider.com/2008/03/mercedes-heritage-four-cylinder-engines-from-mercedes-benz/ |work=Blog |publisher=BenzInsider |access-date=2013-12-20 |date=2008-03-27 |archive-date=22 December 2013 |archive-url=https://web.archive.org/web/20131222171942/http://www.benzinsider.com/2008/03/mercedes-heritage-four-cylinder-engines-from-mercedes-benz/ |url-status=dead }}</ref> Called the ''kontra'' engine, it was a boxer-twin design. Early uses of flat engines in cars include the 1900 [[Lanchester Motor Company|Lanchester 8 hp Phaeton]] boxer-twin, the 1901 [[Wilson-Pilcher]] boxer-four,<ref name="Automotor">{{cite journal |title=The Wilson-Pilcher Petrol Cars |journal=The Automotor Journal |date=1904-04-16}}</ref> the 1904 Wilson-Pilcher ''18/24 HP'' boxer-six and the 1903 [[Ford Model A (1903–1904)|Ford Model A]], the 1904 [[Ford Model C]] and the 1905 [[Ford Model F]].<ref name="Kimes">{{cite book|last=Kimes |first=Beverly |title=Standard Catalog of American Cars 1805–1942 |year=1996 |publisher=Krause Publications |isbn=0-87341-428-4 |page=572}}</ref>

In 1938, the [[Volkswagen Beetle]] (then called the "KdF-Wagen") was released with a rear-mounted flat-four engine. This [[Volkswagen air-cooled engine]] was produced for many years and also used in the [[Volkswagen Type 2]] (Transporter, Kombi or Microbus), the [[Volkswagen Karmann Ghia]] sports car and the [[Volkswagen Type 3]] compact car. A water-cooled version, known as the [[Volkswagen Wasserboxer engine|Wasserboxer]], was introduced in 1982 and eventually replaced the air-cooled versions.

Most Porsche sports cars have been powered by flat engines, starting with its first car: the 1948–1965 [[Porsche 356]] used an air-cooled boxer-four engine. Also using boxer-fours were the 1969–1976 [[Porsche 914]], the 1965–1969 [[Porsche 912]], and the 2016–present [[Porsche 982|Porsche Boxster/Cayman (982)]]. The [[Porsche 911 (classic)|Porsche 911]] has exclusively used boxer-six engines since its introduction, from 1964–present. In 1997, the Porsche 911 changed from being air-cooled to water-cooled.

[[Porsche flat-eight engines]] were used in various racing cars throughout the 1960s, such as the 1962 [[Porsche 804]] Formula One car and the 1968–1971 [[Porsche 908]] sports car. Porsche made a flat-twelve engine for the 1969–1973 [[Porsche 917]] sports car.

[[Chevrolet]] used a horizontally opposed air-cooled 6 cylinder engine in its [[Chevrolet Corvair|Corvair]] line during its entire production run from 1960–1969 in various applications and power ratings, including one of the first uses of a turbocharger in a mass-produced automobile.

The [[Subaru EA engine]] was introduced in 1966 and began [[Subaru]]'s line of boxer-four engines that remain in production to this day.<ref>{{cite web |title=Distinct Engineering Mounted into the Subaru 1000 |url=http://subaru-philosophy.com/post/29112851147/distinct-engineering-mounted-into-the-subaru-1000 |work=Subaru Philosophy |publisher=Fuji Heavy Industries Ltd |access-date=2013-12-20 |date=2012-08-10 |quote=Source: 'Subaru' magazine – Subaru 1000 extra edition (issued 1966-05-20) |archive-date=21 December 2013 |archive-url=https://web.archive.org/web/20131221053109/http://subaru-philosophy.com/post/29112851147/distinct-engineering-mounted-into-the-subaru-1000 |url-status=dead}}</ref> Most of Subaru's models are powered by a boxer-four engine in either naturally aspirated or turbocharged form. A print ad for the 1973 [[Subaru GL]] coupe referred to the engine as "quadrozontal".<ref>{{cite web|url=http://www.scoobyblog.com/2007-01-04/vintage-subaru-ad-1973-the-subaru-gl-coupe |title=Vintage Subaru Ad (1973 The Subaru GL Coupe) |publisher=Scoobyblog.com |access-date=2010-09-18}}</ref> The company also produced boxer-six engines from 1988–1996 and 2001–2019.<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 |date=12 June 2018 |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> In 2008, the [[List of Subaru engines#Subaru EE engine (diesel)|Subaru EE]] engine became the world's first passenger car diesel boxer engine. This engine is a turbocharged boxer-four with common rail fuel injection.<ref name=Hanlon/><ref>{{cite web |title=Greencarcongress |work=Legacy Diesel Announcement |url=http://www.greencarcongress.com/2008/01/subarus-first-b.html |access-date=2008-01-23}}</ref><ref>{{cite web |last=Harwood |first=Allyson |title=2008 Subaru Turbodiesel Boxer - First Drive |url=http://www.motortrend.com/roadtests/sedans/112_0801_subaru_boxer_turbodiesel_first_drive/viewall.html |work=MotorTrend Magazine |publisher=Source Interlink Media |access-date=2013-12-20 |date=January 2008}}</ref><ref>{{cite web |title=A True Engineering Revolution |url=http://www.boxerdiesel.com/engineering/en/01.html |work=Subaru Boxer Diesel |publisher=Fuji Heavy Industries Ltd |access-date=2013-12-20 |archive-url=https://web.archive.org/web/20140301122607/http://boxerdiesel.com/engineering/en/01.html |archive-date=2014-03-01 |url-status=dead}}</ref>

Ferrari used [[Ferrari flat-12 engine|flat-12 engines]] for various Formula One cars in the 1970s. A road car flat-twelve engine (using a 180-degree V12 configuration) was used for the 1973–1984 [[Ferrari Berlinetta Boxer]], 1984–1996 [[Ferrari Testarossa]] and their derivatives.<ref>{{cite web |title=Flat-12 engine |url=http://www.ferrari.com/English/GT_Sport%20Cars/RacingInnovation/Pages/Flat_12_engine.aspx |work=Ferrari.com |publisher=Ferrari S.p.A. |access-date=2013-12-21 |archive-url=https://web.archive.org/web/20131225102235/http://www.ferrari.com/English/GT_Sport%20Cars/RacingInnovation/Pages/Flat_12_engine.aspx |archive-date=2013-12-25}}</ref>

Toyota uses the designation ''Toyota 4U-GSE'' for the boxer-four engine in the Toyota-badged versions of the ''[[Toyota 86]] – Subaru BRZ'' twins, although the engine is designed and built by Subaru as the [[Subaru FA engine#FA20|Subaru FA20]].<ref name=Bonk/>

==See also== * [[H engine]] * [[V engine]] * [[W engine]] * [[X engine]] * [[Radial engine]] * [[History of the internal combustion engine]]

==References== {{Reflist|colwidth=35em}}

[[Category:Flat engines| ]] [[Category:Engines by cylinder layout|Flat]]