A flat engine is an internal combustion engine with horizontally-opposed pistons. Typically, the layout has cylinders arranged in two banks on either side of a single crankshaft and is otherwise known as the boxer, or horizontally-opposed engine. The concept was patented in 1896 by engineer Karl Benz, who called it the "contra engine."
A boxer engine should not be confused with the opposed-piston engine, in which each cylinder has two pistons but no cylinder head. Also, if a straight engine is canted 90 degrees into the horizontal plane, it may be thought of as a "flat engine", but this usage is not common.
True boxers have each crankpin controlling only one piston/cylinder while the 180° engines, which superficially appear very similar, share crankpins. The 180° engine, which may be thought of as a type of V engine, is quite uncommon as it has all of the disadvantages of a flat engine, and few of the advantages.
In 1896, Karl Benz invented the first internal combustion engine with horizontally opposed pistons. He called it the kontra engine, as the action of each side opposed the action of the other. This design has since been called the "boxer" engine because each pair of pistons moves in and out together, rather like the gloves of a boxer. The boxer engine has pairs of pistons reaching TDC simultaneously.
The boxer configuration is the only configuration in common use that does not have unbalanced forces with a four-stroke cycle regardless of the number of cylinders, as long as both banks have the same number of cylinders. They do not require a balance shaft or counterweights on the crankshaft to balance the weight of the reciprocating parts, which are required in most other engine configurations. However, in the case of boxer engines with fewer than six cylinders, unbalanced moments (a reciprocating torque also known as a "rocking couple") are unavoidable due to the "opposite" cylinders being slightly out of line with each other. Other engine configurations with natural dynamic balance include the straight-six, the straight-eight, the V12, and the V16.
Boxer engines (and flat engines in general) tend to be noisier than other common engines for both intrinsic and other reasons. In cars, valve clatter from the engine compartment is not damped by air filters or other components.
Multi-cylinder boxer layouts have proved to be well suited as light aircraft engines, as exemplified by Continental, Lycoming, Rotax, Jabiru and Verner. An important factor in aircraft use is the flat engine's absence of vibration, which allows a lighter engine mount.
General aviation aircraft often use air-cooled flat-four and flat-six engines made by companies such as Lycoming and Continental. Ultralight and microlight aircraft often use engines such as the Rotax 912 and Jabiru 2200.
During the Second World War, Boxer engines were used as a starter motor for the first German jet engines to power up the engine at cranking speed. The two-cylinder two-stroke flat engine was developed by Norbert Riedel ("Riedel starter"), had a cylinder capacity of 270 cc and a power of 8 kW (10.5 hp) at 7150 rpm and essentially functioned as a pioneering example of an APU for starting a jet engine. It was an extreme short stroke (bore / stroke: 70 mm / 35 mm = 2:1) design so it could fit in the hub of the turbine compressor and started electrically or with a pull starter. The engine was produced by the Victoria works in Nuremberg and served as a starter for the jet engines Junkers Jumo 004 and BMW 003.
The first motorcycle with a boxer engine was the 1905 Fée flat-twin, which was developed into the 1907 Douglas. Douglas would continue making flat-twin motorcycles until 1957, ending with the Dragonfly. BMW have made motorcycles with flat-twin engines since the BMW R32 of 1923. Unlike contemporary Douglas motorcycles, which had their engines mounted with the cylinders in line with the frame, the R32 had its cylinders mounted across the frame and used a shaft to drive the rear wheel. The drivetrain layout of the R32 has been used, with improvements, in all subsequent BMW flat-twin motorcycles. The Russian Ural and Ukrainian Dnepr flat twins were licensed copies of the pre-WWII military plunger-suspension BMW R71.
In 1923, Max Friz designed the first BMW motorcycles, choosing a 500 cc boxer engine and unit transmission with shaft drive. This engine type is still in production today. The BMW 247 engine, known as an airhead due to its air cooling, was produced until 1995. BMW replaced it with the oilhead engine with partial oil cooling and four valves per cylinder, but still retained the same flat-twin configuration. In 2013 BMW introduced partial ("precision") water-cooled version, first on their BMW GS and planned to replace all oilheads.
Flat-four engines have been used in the 1938–1939 Zündapp K800, the French BFG motorcycle with the Citroën GS engine, and the Honda Gold Wing from 1975 to 1986. Gold Wings since 1987 have used flat-six engines, as have Honda Valkyries.
The low centre of gravity allowed by a flat engine can reduce body roll in automobiles and enhance handling precision. Historically used by Benz, Ford, Tatra, Jowett, Volkswagen, Chevrolet, and Ferrari, they are currently used by Porsche and Subaru.
In 1901, the Wilson-Pilcher car was launched with the choice of a flat-four or flat-six engine mounted at the front of the car with the crankshaft in-line. The water-cooled engines had 95 mm bore and stroke, and the crankshaft was supported with intermediate bearings between each pair of cylinders. The cars were made in London until 1904 when Armstrong Whitworth took over the manufacture, which continued until c. 1907. These cars "created quite a sensation in automobile circles at the time on account of its remarkably silent and smooth running, and of the almost total absence of vibration."
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. 
In the 1920s and 1930s Tatra made a series of automobiles with front-mounted air-cooled flat engines, including the flat-twin 11 and 12 and the flat-four 30, 54 and 75. In the late 1930s Tatra built the rear-engined 97 with a rear-engined flat-four.
Ferdinand Porsche developed the KdF-Wagen partly based on the Tatra 97. Like the 97, the KdF-Wagen had an air-cooled flat-four engine The KdF-Wagen was renamed the Volkswagen and a limited number were made before World War II. The factory was reopened after the war, and the Volkswagen became a commercial success in its own right, and also became the base for other designs, including the Type 2 (bus) transporters, the Karmann Ghia, Volkswagen Type 3 cars and the SP2 sports car. The last versions of the Volkswagen boxer were watercooled and thus dubbed the Wasserboxer (waterboxer) by enthusiasts. This engine included many developments of the earlier engines. It was offered in capacities of 1.9 and 2.1 litres was used to power T3 buses and transporters.
Automobile layouts used with flat engines
Cars such as the four-cylinder Volkswagen Beetle and the six-cylinder Chevrolet Corvair and Porsche 911 use flat-engines mounted at the rear, where the extra width of the engine does not interfere with the steering of the front wheels. These designs also include the reduced weight inherent in rear-engine, rear-wheel-drive layouts with no need for a drive shaft from front to rear.
Notable automotive applications of flat engines
- The Citroën 2CV and Panhard air-cooled flat-twin engines, both influenced by the flat-twins of BMW
- The air-cooled Chevrolet Corvair OHV flat-six
- In 1960 Lancia's flat-four water-cooled engine debuted for the Lancia Flavia model, first Italian front-wheel drive car, like 1500 cc 90 PS (66 kW) coupé version and during the years become a 2000 cc with 142 bhp (106 kW) when was mounted the first electronic injection by Bosch in 1970 in a flat four engine, Lancia also rebuilt a new flat-four engine in 1976, 2484 cc, for his upper size model Lancia Gamma. It was produced until 1984 and was Lancia's last flat-four engine.
- 1970 Citroen GS; work began on the concept of a small family car as early as 1960, with the "C60", to slot between the 1961 Citroën Ami and DS. Engines used were flat-four air-cooled; 1015 cc, 1129 cc, 1222 cc and 1299cc. The Citroen Ami model also adopted the 1015 cc engine.
- The flat-four engines in Alfa Romeo's Alfasud, Sprint, 33 and early versions of the 145. The last of the line was a 1,712 cc flat-four, 16-valves, producing up to 137 PS (101 kW).
- The Toyota Sports 800 was Toyota's first sports car, and contained a two-cylinder boxer engine, the 2U.
- The water-cooled front-mounted flat-four and flat-six engines used by Subaru in all of its mid-sized cars. Subaru refers to these as boxer engines in publicity commentary, and include a variety of naturally aspirated and turbo driven engines from 1966, when the Subaru 1000 was introduced to current; both closed and semi-closed short blocks have been used. A print ad for the 1973 Subaru GL coupe referred to the engine as "quadrozontal" The EJ series of four-cylinder engines released first in 1990 has been the focus for the development of the boxer engine in the late 20th century. Ranging from 1.6–2.5 litres, this engine in its 2-litre turbo arrangement has been the power behind World Rally Championship winning cars. Subaru also offers a boxer turbodiesel, called the Subaru EE series, the world's first to be fitted into a passenger car. In a joint venture with Toyota a 1,998 cc flat-four engine with 200 PS (147 kW) having GDI was developed as the Subaru (FA20) and Toyota (4U-GSE). This was used in the two-door coupe Toyota 86.
- The air-cooled flat-four, flat-six and flat-eight engines were used for many years in early Porsches. The flat-twelve in the 917 model is a 180° V-engine and not a boxer.
- The water-cooled flat-six engines in the Porsche Boxster, Cayman and later 911 models
- Ferrari made use of a flat-twelve design in several models, including the Berlinetta Boxer, the Testarossa and its derivatives, such as the 512TR and the F512 M, although this engine design is technically a V12 that has been flattened down to a 180° configuration, and therefore cannot be regarded as a true boxer engine.[not in citation given]
- The Lykan HyperSport, the third most expensive car in the world as of 2015[update], uses a Porsche derived flat-six engine.
Front-mounted air-cooled flat-twin engines were used in DAF 600, 750, Daffodil, 33 and the 44/46, by Citroën in their model 2CV and its derivatives. The GS and GSA and the Oltcit Club used a flat-four and a flat-six was proposed for the Citroën DS but rejected.
Since the Subaru 1000 of 1966, all versions of the Subaru Impreza, Forester, Tribeca, Legacy, Outback, Baja, BRZ and SVX use either a flat-four or flat-six engine, including a unique common rail flat-four turbo-diesel.
Flat engine designs
- Flat-twin engine
- Flat-four engine
- Flat-six engine
- Flat-eight engine
- Flat-ten engine
- Flat-twelve engine
- Flat-sixteen engine
- H engine
- Controlled combustion engine
- English, Bob (2010-04-29). "The engine that Benz built still survives". The Globe and Mail (Canada). Retrieved 2013-12-19.
- Hodzic, Muamer (2008-03-27). "Mercedes Heritage: Four-cylinder engines from Mercedes-Benz". Blog. BenzInsider. Retrieved 2013-12-20.
- Bonk, Aaron (2013-02-07). "How Boxer Engines Work". Super Street Magazine. Source Interlink Media. Retrieved 2013-12-21.
- Hanlon, Mike (2001-02-07). "The world’s first horizontally-opposed turbo diesel engine". Gizmag. Retrieved 2013-12-20.
- Brown, Stuart F. (2009-07-02). "Handlebars - BMW’s Boxer - A Classic Design Is Updated and Refined". New York Times. Retrieved 2013-12-20.
- Brown, Warren (2013-12-13). "Subaru Impreza is fun, reliable but has darned noisy engine". The Record. Retrieved 2013-12-20.
- Szymezak, Pascal. "Début de la légende". La saga GoldWing (in French). Fédération des GoldWing Club de France. Retrieved 2013-12-20.
- Cameron, Kevin (2012-12-21). "BMW’s All-New Water-Cooled Boxer – Tech Preview". Cycle World. Bonnier Corp. Retrieved 2013-12-20.
- "The Wilson-Pilcher Petrol Cars", The Automotor Journal, April 16th, 1904
- "The Buffum Automobile". Farber and Associates.
- "» Vintage Subaru Ad (1973 The Subaru GL Coupe)". Scoobyblog.com. Retrieved 2010-09-18.
- "Greencarcongress". Legacy Diesel Announcement. Retrieved 2008-01-23.
- "Flat-12 engine". Ferrari.com. Ferrari S.p.A. Retrieved 2013-12-21.
- "Citroën GS + GSA boxer engine". Citroënët. Retrieved 2013-12-20.
- "Citroën DS the birth of the goddess". Citroënët. Retrieved 2013-12-20.
- "Distinct Engineering Mounted into the Subaru 1000". SUBARU Philosophy. Fuji Heavy Industries Ltd. 2012-08-10. Retrieved 2013-12-20.
Source: 'Subaru' magazine – Subaru 1000 extra edition (issued May 20, 1966)
- Harwood, Allyson (January 2008). "2008 Subaru Turbodiesel Boxer - First Drive". MotorTrend Magazine. Source Interlink Media. Retrieved 2013-12-20.
- "A True Engineering Revolution". SUBARU BOXER DIESEL. Fuji Heavy Industries Ltd. Retrieved 2013-12-20.