Aflat engine is apiston engine where the cylinders are located on either side of a centralcrankshaft. Flat engines are also known ashorizontally opposed engines, however this is distinct from the less commonopposed-piston engine design, whereby each cylinder has two pistons sharing a central combustion chamber.
The most common configuration of flat engines is theboxer 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 aV 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 byKarl Benz. Flat engines have been used in aviation, motorcycle and automobile applications. They are now less common in cars thanstraight engines (for engines with fewer than six cylinders) andV engines (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 replacedradial engines in many smaller installations.
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The advantages of flat engines are a short length, lowcentre of mass and suitability forair cooling (due to the well-exposed, large-surface-area cylinders and cylinder heads, and their short length).
Compared with the more commonstraight engines, flat engines have betterprimary balance (resulting in less vibration);[1] however, the disadvantages are increased width and the need to have two cylinder heads. Compared withV engines – 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.[2]
The most common usages of flat engines are:
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.[3] 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 abalance shaft[4] or counterweights on thecrankshaft to balance the weight of the reciprocating parts. However, arocking couple is present, since each cylinder is slightly offset from the other member of its pair due to the distance between thecrankpins along the crankshaft.[3]
An alternative configuration for flat engines is as a 180-degreeV engine, which has been used on most twelve-cylinder flat engines. In this configuration, each pair of pistons shares acrankpin, 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 1909Santos-Dumont Demoiselle range of aeroplanes, which was powered by boxer-twin engines.[citation needed]
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]
During World War II, a boxer-twin engine called the "Riedel starter" was used as a starter motor/mechanicalAPU for the early German jet engines, such as theJunkers Jumo 004 andBMW 003. Designed byNorbert Riedel, these engines have a veryoversquare stroke ratio of 2:1 so that they could fit within the intake diverter, directly forward of the turbine compressor.[citation needed]
Flat engines offer several advantages for motorcycles including a lowcentre of mass, low vibration, suitability for shaft drive, and equal cooling of the cylinders (for air-cooled engines).[5][6] 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.[7]BMW Motorrad have a long history of boxer-twin motorcycles, beginning in 1923 with theBMW R32[5]
Several motorcycles have been produced with flat-four engines, such as the 1938–1939Zündapp K800 and the 1974–1987Honda Gold Wing. In 1987, the Honda Gold Wing engine was upsized to a flat-six design.[8]
When used in cars, advantages of flat engines are a lowcentre of mass (which improves the handling of the car),[4] short length, low vibration and suitability for air cooling (due to the well exposed, large surface area, cylinder heads and short length).[9] 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]
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]
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Due to the short length of flat engines, locating a flat engine outside of the car'swheelbase results in minimal overhang.[10] Therefore, many cars with flat engines have used arear-engine, rear-wheel-drive layout. Examples include the flat-twinBMW 600 (1957–1959) andBMW 700 (1959–1965); the flat-fourTatra 97 (1936–1939),Volkswagen Beetle (1938–2003) andPorsche 356 (1948–1965); and the flat-sixChevrolet Corvair (1959–1969),Porsche 911 (1963–present), andTucker 48 (1947–1948).
The opposite layout,front-engine front-wheel drive, was also common for cars with flat engines. Examples include theCitroën 2CV (1948–1990),Panhard Dyna X (1948–1954),Lancia Flavia (1961–1970),Citroën GS (1970–1986),Alfa Romeo Alfasud (1971–1989) andSubaru Leone (1971–1994).
Subaru have been producing cars with afront-engine, four-wheel-drive layout powered by flat engines (mostly boxer-four engines) since 1971. Examples include theSubaru Leone (1971–1994),Subaru Legacy (1989-present) andSubaru 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 traditionalfront-engine, rear-wheel-drive layout is relatively uncommon for cars with flat engines, however some examples include theToyota 86 / Subaru BRZ (2012–present),Jowett Javelin (1947–1953),Glas Isar (1958–1965) and theTatra 11 (1923–1927).
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The first flat engine was produced in 1897 by German engineerKarl Benz.[5][11] Called thekontra engine, it was a boxer-twin design. Early uses of flat engines in cars include the 1900Lanchester 8 hp Phaeton boxer-twin, the 1901Wilson-Pilcher boxer-four,[12] the 1904 Wilson-Pilcher18/24 HP boxer-six and the 1903Ford Model A, the 1904Ford Model C and the 1905Ford Model F.[13]
In 1938, theVolkswagen Beetle (then called the "KdF-Wagen") was released with a rear-mounted flat-four engine. ThisVolkswagen air-cooled engine was produced for many years and also used in theVolkswagen Type 2 (Transporter, Kombi or Microbus), theVolkswagen Karmann Ghia sports car and theVolkswagen Type 3 compact car. A water-cooled version, known as theWasserboxer, 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 1948–1965Porsche 356 used an air-cooled boxer-four engine. Also using boxer-fours were the 1969–1976Porsche 914, the 1965–1969Porsche 912, and the 2016–presentPorsche Boxster/Cayman (982). ThePorsche 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 1962Porsche 804 Formula One car and the 1968–1971Porsche 908 sports car. Porsche made a flat-twelve engine for the 1969–1973Porsche 917 sports car.
Chevrolet used a horizontally opposed air-cooled 6 cylinder engine in itsCorvair 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.
TheSubaru EA engine was introduced in 1966 and beganSubaru's line of boxer-four engines that remain in production to this day.[14] Most of Subaru's models are powered by a boxer-four engine in either naturally aspirated or turbocharged form. A print ad for the 1973Subaru GL coupe referred to the engine as "quadrozontal".[15] The company also produced boxer-six engines from 1988–1996 and 2001–2019.[16] In 2008, theSubaru EE engine became the world's first passenger car diesel boxer engine. This engine is a turbocharged boxer-four with common rail fuel injection.[4][17][18][19]
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 1973–1984Ferrari Berlinetta Boxer, 1984–1996Ferrari Testarossa and their derivatives.[20]
Toyota uses the designationToyota 4U-GSE for the boxer-four engine in the Toyota-badged versions of theToyota 86 – Subaru BRZ twins, although the engine is designed and built by Subaru as theSubaru FA20.[3]
Source: 'Subaru' magazine – Subaru 1000 extra edition (issued 1966-05-20)
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