Flat engine

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Animation of a boxer engine Boxerengineanimation.gif
Animation of a boxer engine

A flat engine, also known as a horizontally opposed engine, is a piston engine where the cylinders are located on either side of a central crankshaft. A flat engine should not be confused with the opposed-piston engine, in which each cylinder has two pistons sharing a central combustion chamber.


The most common configuration of flat engines is the boxer engine, in which the pistons of each opposed pair of cylinders move inwards and outwards at the same time. Boxer engines are a type of flat engine; however, flat engines are not necessarily boxer engines. The other configuration is effectively a V engine with a 180-degree angle between the cylinder banks. Each pair of cylinders shares a single crankpin, so that as one piston moves inward, the other moves outward.

The first flat engine 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 less than six cylinders) and V engines (for engines with six or more cylinders). They are much 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.


Difference between two flat 6 cylinder engines: 180deg V on the left, boxer on the right Moteurs a Plat - V a 180deg vs. Boxer.png
Difference between two flat 6 cylinder engines: 180° V on the left, boxer on the right

The advantages of flat engines are a short length, low centre of mass and suitability for air cooling.

Compared with straight engines— the most common layout for engines with four cylinders or less— 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 have a lower centre of mass (and a better primary balance than V6 engines), however they usually have a larger width. [2]

The most common usages of flat engines are:

Boxer configuration

1954 BMW R68 flat-twin boxer engine R68-opposed-cylinders.jpg
1954 BMW R68 flat-twin boxer engine

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 vibrations, since they are the only common configuration that have no unbalanced forces regardless of the number of cylinders. Boxer engines therefore do not require a balance shaft [4] or counterweights on the crankshaft to balance the weight of the reciprocating parts. However, in the case of boxer engines with fewer than six cylinders, a rocking couple is present, since each cylinder is slightly offset from its opposing pair, due to the distance between the crankpins along the crankshaft. [3]

180-degree V engine

An alternative configuration for flat engines is as a 180-degree V engine, which has been used on most twelve-cylinder flat engines. In this configuration, each pair of pistons shares a crankpin, rather than the boxer configuration where each piston has its own crankpin.

Aviation use

World War II-era Riedel starter motor Riedelanlasser.jpg
World War II-era Riedel starter motor

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 is the 1909 Santos-Dumont Demoiselle range of airplanes, which was powered by boxer-twin engines.[ citation needed ]

Several boxer-four engines have been produced specifically for light aircraft. Several manufacturers produced boxer-six aircraft engines during the 1930s and 1940s.[ citation needed ]

During World War II, boxer-twin engines called the "Riedel starter" were 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 center of mass, low vibration, suitability for shaft drive, and even cooling of the cylinders (for air-cooled engines). [5] [6] The most common design of flat engines for motorcycles is the boxer-twin, beginning with the 1905 Fée manufactured by the Light Motors Company flat-twin, which was the first production motorcycle to use a flat 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.[ citation needed ]

Automotive use

Diesel Engine Type DS140 2C Hino.jpg
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 center 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). [8] 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

Due to the short length of flat engines, locating a flat engine outside of the car's wheelbase results in minimal overhang. [9] 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).


Volkswagen motor cut 1945.JPG
Volkswagen air-cooled boxer-four engine
Subaru BRZ engine (20413082204).jpg
2012-present Subaru FA20 boxer-four engine

The first flat engine was produced in 1897 by German engineer Karl Benz. [5] [10] 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, [11] 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. [12]

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 1960-1969 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. [13] 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" [14] The company also produced boxer-six engines from 1988–1996 and 2001–2019. [15] 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] [16] [17] [18]

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. [19]

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

Related Research Articles

V12 engine Piston engine with 12 cylinders in vee configuration

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.

V-twin engine

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

V6 engine Piston engine with six cylinders in a "V" configuration

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

Straight engine Type of engine

The straight or inline engine is an internal combustion engine with all cylinders aligned in one row and having no offset. Usually found in four, six and eight cylinder configurations, they have been used in automobiles, locomotives and aircraft, although the term in-line has a broader meaning when applied to aircraft engines, see Inline engine (aviation).

V engine Internal combustion engine with two banks of cylinders at an angle resembling a V

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.

Flat-twin engine

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 cylinders move inwards and outwards at the same time.

Flat-four engine

A flat-four engine, also known as a horizontally opposed-four 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 cylinders moves inwards and outwards at the same time.

Straight-twin engine Inline piston engine with two cylinders

A straight-twin engine, also known as an inline-twin, vertical-twin-0=0(2974), or parallel-twin is a two-cylinder piston engine where two cylinders are arranged in a line along a common crankshaft.

Flat-twelve engine

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.

Straight-four engine Inline piston engine with four cylinders

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

Rear-engine, rear-wheel-drive layout

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.

Flat-six engine

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.

Multi-valve Type of car engine

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

Motorcycle engine Engine that powers a motorcycle

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

In a reciprocating piston engine, the stroke ratio, defined by either bore/stroke ratio or stroke/bore ratio, is a term to describe the ratio between cylinder bore diameter and piston stroke length. 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 known as a horizontally opposed-eight, is an eight-cylinder piston engine with four cylinders on each side of a central crankshaft.

Flat-sixteen engine

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.

Twin-turbo refers to an engine in which two turbochargers compress the intake fuel/air mixture. The most common layout features two identical turbochargers in parallel. The two turbochargers can either be identical or different sizes. Twincharger is a combination of supercharger and turbocharger.

V8 engine Piston engine with eight cylinders in vee configuration

A V8 engine is an eight-cylinder piston engine in which the cylinders share a common crankshaft and are arranged in a V configuration.


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