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A flat engine is a 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 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 (for engines with fewer than six cylinders) and (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 in many smaller installations.
Compared with the more common , flat engines have better primary balance (resulting in less vibration); however, the disadvantages are increased width and the need to have two cylinder heads. Compared with – 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.
The most common usages of flat engines are:
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.
During World War II, a boxer-twin engine called the "Riedel starter" was used as a starter motor/mechanical 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 oversquare stroke ratio of 2:1 so that they could fit within the intake diverter, directly forward of the turbine compressor.
Several motorcycles have been produced with flat-four engines, such as the 1938–1939 Zündapp K800 and the 1974–1987 Honda Gold Wing. In 1987, the Honda Gold Wing engine was upsized to a flat-six design.
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.
The opposite 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-present) 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 / Subaru BRZ (2012–present), Jowett Javelin (1947–1953), Glas Isar (1958–1965) and the Tatra 11 (1923–1927).
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 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 Boxster/Cayman (982). The 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 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. 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". The company also produced boxer-six engines from 1988–1996 and 2001–2019. In 2008, the 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.
Ferrari used flat-twelve 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.
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 FA20.
See also
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