Flat-four engine

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2007 ULPower UL260i aircraft 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.

A boxer-four engine has perfect primary and secondary balance, however the two cylinder heads means the design is more expensive to produce than an inline-four engine. Boxer-four engines have been used in cars since 1900, especially by Volkswagen and Subaru, they have also occasionally been used in motorcycles and airplanes.


Boxer-four animation

Most flat-four engines are designed so that each pair of opposing pistons moves inwards and outwards at the same time, which is known as a "boxer" configuration (somewhat like boxing competitors punching their gloves together before a fight). Therefore, the terms "flat-four" and "boxer-four" are often used synonymously.

The advantages of the boxer-four layout are perfect secondary vibration (resulting in minimal vibration), a low centre of gravity, and a short engine length; the layout also lends itself to efficient air cooling with the airflow being evenly distributed across the four cylinders.

The downsides of boxer-four engines (compared with inline-four engines) are their extra width, the increased costs associated with having two cylinder heads instead of one, and the long exhaust manifold required to achieve evenly spaced exhaust pulses.[1] Due to these factors, inline-four engines are more commonly used than flat-four engines, and V6 engines are often used where larger displacements are required.

Engine balance[edit]

The equal and opposing forces generated in a boxer-four engine results in perfect secondary balance (unlike the unbalanced vertical forces produced by inline-four engines). Boxer-four engines are therefore better suited to displacements above 2.0 L (122 cu in), since they do not require balance shafts to reduce the secondary vibration.

In practice, each cylinder in a boxer engine is slightly offset from its opposing pair due to the distance between the crankpins along the crankshaft; this offset distance means that the equal and opposite forces from each cylinder pair produces a rocking couple.[2](p27) The resulting vibration is not usually high enough to require balance shafts.

As per all four-stroke engines with four cylinder or less, the lack of overlap in the power strokes results in a pulsating delivery of torque to the flywheel, causing a torsional vibration along the crankshaft axis.[citation needed] If necessary, this vibration can be minimised using a harmonic damper.

Exhaust manifold[edit]

The typical firing order for a boxer-four engine is for the left bank of cylinders to ignite one after another, followed by the right bank of cylinders (or vice-versa), with the firing interval evenly spaced at 180 degrees. Traditionally, the exhausts from the two cylinders on each bank were merged together, with the resulting uneven exhaust pulses causing a characteristic "flat-four burble" exhaust sound.

The other common exhaust configuration (such as used by Subaru since the mid-2000s) is to pair the cylinders with a firing interval offset of 360 degrees, in order to optimise the exhaust pulses;[3][4][5] this configuration requires long exhaust manifolds, in order to pair the cylinders on opposite banks, and results in a less distinctive exhaust sound.

Use in automobiles[edit]


1904 Wilson-Pilcher water-cooled engine

In 1900, the first flat-four engine was produced by Benz & Cie, based on Benz's 1897 "contra" flat-twin engine.[citation needed] This engine was used in Benz racing cars, produced 20 hp (15 kW), had a displacement of 5.4 L (330 cu in) and was designed by Georg Diehl.

London company Wilson-Pilcher released its first car in 1901, which was powered by a flat-four engine; this engine was mounted longitudinally in the chassis, water-cooled, produced 9 hp (7 kW) and had a displacement of 2.4 L (146 cu in). Unusually for its day, the bore and stroke were equal, with each being 95 mm (3.7 in).

Having previously produced flat-twin engines, the 1926 Tatra 30 was the Czech company's first model powered by a flat-four engine. Tatra produced various flat-four engined model through the 1920s and 1930s.


1952 Jowett Jupiter water-cooled engine
1955 Porsche 550 Spyder air-cooled engine

The 1936 Tatra T97 pioneered the rear-engined, air-cooled flat-four, backbone chassis layout (later used by the Volkswagen Beetle); also in 1936, English company Jowett expanded its model range from flat-twin engines to also include flat-four engines. Production of Jowett flat-four engines continued until 1954, when the Jowett Javelin saloon and Jowett Jupiter sports models ended production.

The highest production flat-four engine is the Volkswagen air-cooled engine, which was produced from 1938 until 2006 and was most famously used in the rear-engined 1938-2006 Volkswagen Beetle and 1950-1990 Volkswagen Transporter;[6] this air-cooled engine was designed by Porsche and was also used in the 1948-1965 Porsche 356, 1953-1956 Porsche 550, 1965–1969 Porsche 912 and 1969-1976 Porsche 914. In 1982, to comply with exhaust emissions regulations a water-cooled version called the Volkswagen Wasserboxer was introduced in the Volkswagen Transporter (T3)

During the 1960s and 1970s, several manufacturers produced flat-four engines including the air-cooled Citroën flat-four engine, the water-cooled Alfa Romeo flat-four engine, the water-cooled Lancia flat-four engine and the water-cooled Subaru EA engine.


2012-present Subaru FA water-cooled engine

By the year 2000, most manufacturers had replaced flat-four engines with inline-four engines. A notable exceptions is Subaru, with the latest iteration of its flat-four engine, the water-cooled Subaru EJ engine being available in turbocharged form in the iconix Subaru WRX sports sedan and its World Rally Car counterpart. Subaru's adoption of all-wheel drive was a factor in retaining the flat-four engine, since the shorter length of this engine assists in fitting the all-wheel drive components into the chassis.[7] Although it is more expensive than an inline-four engine, the flat-four engine allows Subaru to build an all-wheel drive vehicle at little extra cost from two-wheel drive.[7]

In 2012, a naturally-aspirated version of the Subaru FA engine was used in the Toyota 86 (also called the 'Subaru BRZ' and 'Scion FR-S') rear-wheel drive sports coupe;[8] this engine is water-cooled, has gasoline direct injection, produces 147 kW (197 hp) and has a displacement of 2.0 L (122 cu in)

The 2016 Porsche Boxster/Cayman (982) mid-engined sports cars downsized from a naturally aspirated flat-six engine to a turbocharged flat-four engine, Porsche's first flat-four since the mid-1970s; this engine is produced in displacements of 2.0–2.5 L (122–153 cu in) and produces up to 365 hp (272 kW).[9] Several reviewers criticised the Boxster/Cayman for an uninspiring engine sound.[10][11][12]

Use in motorcycles[edit]

Most motorcycles with four-cylinder engines use an inline-four engine layout, however, several flat-four engine engines have been used in shaft drive motorcycles:

Use in aircraft[edit]

Light aircraft (such as Cessnas) sometimes use flat-four engines with displacements up to 5.9 L (360 cu in) that are produced by manufacturers including Continental Motors and Franklin Engine Company.

For radio-controlled aircraft, flat-four engines with displacements of 40–50 cc (2.4–3.1 cu in) are produced by companies such as O.S. Engines.


  1. ^ Nunney, M J (2007). Light and Heavy Vehicle Technology. Butterworth-Heinemann. p. 13. ISBN 978-0-7506-8037-0.
  2. ^ Wilson, Hugo (1995). "The A-Z of Motorcycles". The Encyclopedia of the Motorcycle. London, UK: Dorling Kindersley. ISBN 0-7513-0206-6.
  3. ^ Announcement on Legacy exhaust in Japanese: "Fuji Heavy Industries, Press Information". 2003-05-23. Retrieved 2013-12-16.
  4. ^ Announcement on Forester exhaust in Japanese: "Fuji Heavy Industries, News Release". 2005-01-27. Retrieved 2013-12-16.
  5. ^ Announcement on Impreza exhaust in Japanese: "Fuji Heavy Industries, News Release". 2007-06-05. Retrieved 2013-12-16.
  6. ^ "The engine that Benz built still survives". www.theglobeandmail.com. Retrieved 1 September 2019.
  7. ^ a b Stepler, Richard (September 1994). "Preview Drive: Three Ways to Get a Grip". Popular Science. 245 (3): 44. Retrieved 2011-10-23.
  8. ^ Bonk, Aaron (2013-02-07) (2013-02-07). "How Boxer Engines Work, And Why You Should Care About Subaru's New FA20". Retrieved 8 January 2014.
  9. ^ "Performance: The new 718 Boxster". Porsche. 2016. Retrieved 2016-11-01.
  10. ^ "2019 Porsche 718 Cayman". www.caranddriver.com. 14 May 2019. Retrieved 1 September 2019.
  11. ^ "Porsche 718 Cayman review – The entry-level Porsche punches above its weight". www.evo.co.uk. Retrieved 1 September 2019.
  12. ^ "2017 Porsche 718 Cayman First Drive Review: Performance Trumps Sound". www.motortrend.com. Retrieved 1 September 2019.
  13. ^ Julian Marsh (2000-06-10). "Citroënët - BFG and MF motorcycles". Citroenet.org.uk. Retrieved 2011-12-06.
  14. ^ Julian Marsh. "Citroënët - GS & GSA boxer engine". Citroenet.org.uk. Retrieved 2011-12-06.