A flat engine is an internal combustion engine with multiple pistons that move in a horizontal plane. Typically, the layout has cylinders arranged in two banks on either side of a single crankshaft and is sometimes known as the boxer, or horizontally opposed engine. The concept was patented in 1896 by engineer Karl Benz, who called it the "contra engine." It should not be confused with the opposed-piston engine, in which each cylinder has a piston at both ends and no cylinder head.
Another widely used form of flat engine consists of a straight engine with two, three, four or more cylinders canted 90 degrees into the horizontal plane, however this is not generally considered significantly different from other straight engines.
Flat engines offer a low centre of gravity improving stability and control for motorcycles, and reducing body roll in automobiles and also enhancing handling precision such as during a sudden lane-change. Flat engines lend themselves well to aircraft engines.
Front-mounted air-cooled flat-twin engines were used in Tatra 11 and Tatra 30, 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. BMW has used air-, air/oil-cooled and air/water-cooled flat-twin engines in its motorcycles from 1923 until the present day. Cars such as the four-cylinder Volkswagen Beetle and the six-cylinder Porsche 911 use a flat-engine at the rear of the car, where its extra width does not interfere with the steering of the front wheels and there is a weight-saving since no prop-shaft is required.
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.
True boxers have each crankpin controlling only one piston/cylinder while 180° engines, which superficially appear very similar, share crankpins. The 180° engine is considered to be a type of V engine. The boxer engine has corresponding pistons reaching top dead centre (TDC) simultaneously.
Boxer engines must not be confused with opposed-piston engines, which are essentially the inverse, with two pistons compressing a single combustion space. These can be used in vehicles such as tanks.
Boxer engines got their name because each pair of pistons moves simultaneously in and out rather than alternately, like boxers clashing their gloved fists together before a fight. Boxer engines have proved to be highly successful with up to twelve cylinders in automobiles and up to six cylinders in motorcycles, and they continue to be popular for light aircraft engines.
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, for example, valve clatter from under the hood is not damped by large air filters and other components. Boxers need no balance weights on the crankshaft, which is lighter and fast-accelerating. They have a smoothness throughout the rev range and offer a low centre of gravity.
Notable flat engines
- In 1896, Karl Benz invented the first internal combustion engine with horizontally opposed pistons.
- In 1901, the Wilson-Pilcher car was launched with a choice of flat-four or flat-six engine mounted at the front of the car with the crankshaft in-line. The water cooled engines had 95mm 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 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.
- 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 min-1 and essentially functioned as a pioneering example of an APU for starting a jet engine. It was considered 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.
- In 1948 Preston Tucker modified a helicopter flat-six to be rear-mounted in his Tucker 48.
- The Volkswagen air-cooled flat-four engine used in the Volkswagen Beetle, SP2 and Karmann Ghia, and later developed further for the Volkswagen Type 2 (Bus) transporters and Volkswagen Type 3 cars. VW was rumoured to have worked on a Diesel version of the aircooled boxer but abandoned this engine due to noise and heat issues. The latest version of the VW boxer was 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.
- 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 rebuild a new big 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-4 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.
- Since its introduction in 1975, the Honda Goldwing has utilized a boxer engine, a four-cylinder until 1987, and six-cylinders since. The water-cooled SOHC 1,832 cc flat-six is fitted to the Honda Goldwing from 2001 on.
- 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
- 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 or Jabiru 2200.
- 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]
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
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Source: 'Subaru' magazine – Subaru 1000 extra edition (issued May 20, 1966)
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- "A True Engineering Revolution". SUBARU BOXER DIESEL. Fuji Heavy Industries Ltd. Retrieved 2013-12-20.
- Bonk, Aaron (2013-02-07). "How Boxer Engines Work". Super Street Magazine. Source Interlink Media. Retrieved 2013-12-21.
- 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.
- "The Wilson-Pilcher Petrol Cars", The Automotor Journal, April 16th, 1904
- "The Buffum Automobile". Farber and Associates.
- Cameron, Kevin (2012-12-21). "BMW’s All-New Water-Cooled Boxer – Tech Preview". Cycle World. Bonnier Corp. Retrieved 2013-12-20.
- "» Vintage Subaru Ad (1973 The Subaru GL Coupe)". Scoobyblog.com. Retrieved 2010-09-18.
- "Greencarcongress". Legacy Diesel Announcement. Retrieved 2008-01-23.
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