Flat engine

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.

Contents

• Design
• Boxer configuration
• 180-degree V engine
• Aviation use
• Motorcycle use
• Automotive use
• Drivetrain layout
• History
• See also
• References

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 straight engines (for engines with fewer than six cylinders) and V 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 replaced radial engines in many smaller installations.

Design

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The advantages of flat engines are a short length, low centre of mass and suitability for air cooling (due to the well-exposed, large-surface-area cylinders and cylinder heads, and their short length).

Compared with the more common straight engines, flat engines have better primary balance (resulting in less vibration); ^[1] however, the disadvantages are increased width and the need to have two cylinder heads. Compared with V 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:

• Flat-twin engines are mostly used in motorcycles. Occasionally they have been used in light cars, aircraft and industrial applications, mostly up until the 1960s.
• Flat-four engines are mostly used in cars (most notably in the earlier Volkswagen Types 1 to 4, and by Subaru ^[2] in most of their models), and have occasionally been used in motorcycles. Their most common use is in smaller single-engine general-aviation aircraft, for which they are still manufactured and used to this day.
• Flat-six engines are mostly used in aircraft and cars (particularly by the Porsche 911 sports car), and have occasionally been used in motorcycles.
• Flat-eight engines have been used in several racing cars, mostly by Porsche in the 1960s.
• Flat-ten engines are not known to have reached production. A prototype road car engine was built by Chevrolet in the 1960s.
• Flat-twelve engines have been used in various racing cars, notably the Porsche 917K, during the 1960s and 1970s, and in Ferrari road cars from 1973 to 1996.
• Flat-sixteen engines are not known to have reached production. Prototype racing car engines were built by Coventry and Porsche in the 1960s and 1970s.

Boxer configuration

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 they are the only common configuration that has no unbalanced forces, regardless of the number of pairs of cylinders. Boxer engines therefore do not require either a balance shaft ^[4] or counterweights on the crankshaft to balance the weight of the reciprocating parts. However, a rocking couple is present, since each cylinder is slightly offset from the other member of its pair due to the distance between the crankpins along the crankshaft. ^[3]

Aviation use

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 1909 Santos-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/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.^{[ citation needed ]}

Motorcycle use

Flat engines offer several advantages for motorcycles including a low centre 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 the BMW R32 ^[5]

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. ^[8]

Automotive use

1969 Hino Motors DS140 boxer-twelve diesel engine

1923-1927 Tatra 11 chassis and engine— front-mounted flat-twin engine with rear-wheel drive

When used in cars, advantages of flat engines are a low centre 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 ]}

Drivetrain layout

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Due to the short length of flat engines, locating a flat engine outside of the car's wheelbase results in minimal overhang. ^[10] Therefore, many cars with flat engines have used a rear-engine, rear-wheel-drive layout. Examples include the flat-twin BMW 600 (1957–1959) and BMW 700 (1959–1965); the flat-four Tatra 97 (1936–1939), Volkswagen Beetle (1938–2003) and Porsche 356 (1948–1965); and the flat-six Chevrolet Corvair (1959–1969), Porsche 911 (1963-present), and Tucker 48 (1947–1948).

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).

History

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Volkswagen air-cooled boxer-four engine

2012-present Subaru FA20 boxer-four engine

The first flat engine was produced in 1897 by German engineer Karl Benz. ^{[5] [11]} Called the kontra engine, it was a boxer-twin design. Early uses of flat engines in cars include the 1900 Lanchester 8 hp Phaeton boxer-twin, the 1901 Wilson-Pilcher boxer-four, ^[12] the 1904 Wilson-Pilcher 18/24 HP boxer-six and the 1903 Ford Model A, the 1904 Ford Model C and the 1905 Ford Model F. ^[13]

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.

The majority of sports cars throughout Porsche's history are powered by flat engines, beginning with its first car; the 1948-1965 Porsche 356 used an air-cooled boxer-four engine. Also using boxer-four engines 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 from its introduction in 1964 until the present. In 1997, the Porsche 911 switched 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. A flat-twelve engine was also produced by Porsche 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 to 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. ^[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 1973 Subaru GL coupe referred to the engine as "quadrozontal". ^[15] The company also produced boxer-six engines from 1988 to 1996 and 2001–2019. ^[16] 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. ^{[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 1973-1984 Ferrari Berlinetta Boxer, 1984-1996 Ferrari Testarossa and their derivatives. ^[20]

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 actually designed and built by Subaru as the Subaru FA20 engine. ^[3]

See also

• H engine
• V engine
• W engine
• X engine
• Radial engine
• History of the internal combustion engine

Related Research Articles

A V12 engine is a twelve-cylinder piston engine where two banks of six cylinders are arranged in a V configuration around a common crankshaft. V12 engines are more common than V10 engines. However, they are less common than V8 engines.

A V-twin engine, also called a V2 engine, is a two-cylinder piston engine where the cylinders are arranged in a V configuration and share a common crankshaft.

A V6 engine is a six-cylinder piston engine where the cylinders and cylinder blocks share a common crankshaft and are arranged in a V configuration.

The straight engine is a configuration of multi-cylinder piston engine where all of the cylinders are arranged in a single row, rather than radially or in two or more cylinder banks.

A V engine, sometimes called a Vee engine, is a common configuration for internal combustion engines. It consists of two cylinder banks—usually with the same number of cylinders in each bank—connected to a common crankshaft. These cylinder banks are arranged at an angle to each other, so that the banks form a "V" shape when viewed from the front of the engine.

The engine configuration describes the fundamental operating principles by which internal combustion engines are categorized.

A flat-twin engine is a two-cylinder internal combustion engine with the cylinders on opposite sides of the crankshaft. The most common type of flat-twin engine is the boxer-twin engine, where both pistons move inwards and outwards at the same time.

A flat-four engine, also known as a horizontally opposed-four engine or boxer engine, is a four-cylinder piston engine with two banks of cylinders lying on opposite sides of a common crankshaft. The most common type of flat-four engine is the boxer-four engine, each pair of opposed pistons moves inwards and outwards at the same time.

A straight-twin engine, also known as an inline-twin, vertical-twin, or parallel-twin, is a two-cylinder piston engine whose cylinders are arranged in a line along a common crankshaft.

A flat-twelve engine, also known as a horizontally opposed-twelve, is a twelve-cylinder piston engine with six cylinders on each side of a central crankshaft.

A straight-four engine is a four-cylinder piston engine where cylinders are arranged in a line along a common crankshaft.

A straight-three engine is a three-cylinder piston engine where cylinders are arranged in a line along a common crankshaft.

In automotive design, an RR, or rear-engine, rear-wheel-drive layout places both the engine and drive wheels at the rear of the vehicle. In contrast to the RMR layout, the center of mass of the engine is between the rear axle and the rear bumper. Although very common in transit buses and coaches due to the elimination of the drive shaft with low-floor buses, this layout has become increasingly rare in passenger cars.

A flat-six engine, also known as a horizontally opposed-six, is a six-cylinder piston engine with three cylinders on each side of a central crankshaft. The most common type of flat-six engine is the boxer-six engine, where each pair of opposed cylinders moves inwards and outwards at the same time. An alternative configuration for flat engines is a 180-degree V engine, where both cylinders move to the right then the left at the same time.

A multi-valve or multivalve engine is one where each cylinder has more than two valves. A multi-valve engine has better breathing, and with more smaller valves may be able to operate at higher revolutions per minute (RPM) than a two-valve engine, delivering more power.

A motorcycle engine is an engine that powers a motorcycle. Motorcycle engines are typically two-stroke or four-stroke internal combustion engines, but other engine types, such as Wankels and electric motors, have been used.

Stroke ratio, today universally defined as bore/stroke ratio, is a term to describe the ratio between cylinder bore diameter and piston stroke length in a reciprocating piston engine. This can be used for either an internal combustion engine, where the fuel is burned within the cylinders of the engine, or external combustion engine, such as a steam engine, where the combustion of the fuel takes place outside the working cylinders of the engine.

A flat-eight engine, also called a horizontally-opposed eight, is an eight-cylinder piston engine with two banks of four inline cylinders, one on each side of a central crankshaft, 180° apart.

A flat-sixteen engine, also known as a horizontally opposed-sixteen, is a sixteen-cylinder piston engine with eight cylinders on each side of a central crankshaft.

The Ferrari flat-12 engine family is a series of flat-12 DOHC petrol engines produced by Ferrari from 1964 to 1996. The first racing Ferrari flat-12, the Mauro Forghieri-designed Tipo 207, was introduced in the Ferrari 1512 F1 car in 1964. Later flat-12 racing engines were used in Ferrari Formula One and sports racing cars from 1968 until 1980, including the 212 E Montagna, 312 B series, 312 PB and 312 T series. The roadgoing flat-12 engines were introduced with the 365 GT4 BB and were produced in various versions until the end of F512M production in 1996.

References