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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. 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.
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Front-mounted air-cooled flat-twin engines were used in Tatra 11 and Tatra 30, 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.
True boxers have each crankpin controlling only one piston/cylinder while 180° engines, which superficially appear very similar, share crankpins.
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 
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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 eight cylinders in automobiles and up to six cylinders in motorcycles, and they continue to be popular for light aircraft engines.
Boxers are one of only three cylinder layouts that have a natural dynamic balance; the others being the straight-six and the V12. These engines can run very smoothly and free of unbalanced forces with a four-stroke cycle, and they do not require a balance shaft or counterweights on the crankshaft to balance the weight of the reciprocating parts, which are required in other engine configurations. This is generally true of boxer engines regardless of the number of cylinders (assumed to be even), but not true for all V or inline engines. 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.
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 characteristic 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 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.
- In 1948 Preston Tucker modified a helicopter flat-six to be rear-mounted in his Tucker Torpedo
- 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' or 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.
- 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.
- 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, 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.
- A 1,998 cc Flat-four engine with 200 PS (147 kW) having GDI was developed by a joint venture by Subaru (FA20) and Toyota (4U-GSE) for the two-door coupe Toyota 86.
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
See also