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The Boxer engine was first patented by German engineer Karl Benz

A flat engine is an internal combustion engine with multiple pistons that all move in the horizontal plane. The most popular and significant layout has cylinders arranged in two banks on either side of a single crankshaft, generally known as "boxers". There is a widely-used but technologically less significant form consisting of a straight engine with two, three, four or more cylinders canted 90 degrees into the horizontal plane.

The concept of the boxer was patented in 1896 by engineer Karl Benz, eight years after he started producing the world's first successful cars. Flat engines are commonly described as horizontally opposed engines but must not to be confused with opposed-piston engines, which are mechanically quite different.



1954 BMW "Boxer" motorcycle engine. The two cylinders cannot be directly opposite each other.
Famous Flat-4 VW Beetle engine is also air-cooled, this time by fan.
UL260i Flat-4 aircraft engine

Flat engines have a lower center of gravity than any other common configuration, so vehicles using them should benefit from better stability and control. They are, however, also wider than more traditional configurations and the extra width causes problems fitting the engine into the engine bay of a front-engined car. Some well-known forms of flat engines tend to restrict lean and cornering in motorcycles. Flat engines lend themselves well to aircraft engines, where any extra building expense is of minor importance.

The flat configuration lends itself very well to air cooling designs such as the flat-4 in the VW Beetle, Porsche 356 and 912. The Chevrolet Corvair used an air-cooled flat-6, though this is something of a rarity in American designs. Both the older and newer models of the Porsche 911 use a flat-6, at first air-cooled but since the introduction of the Porsche 996 models are water-cooled. These automobiles situate the engine in the rear rather than the front, where its width does not interfere with the steering of the front wheels and to reduce weight by eliminating a front engine that requires a drive train.

Front-mounted air-cooled flat-twin engines were used by Citroën in their model 2CV and its derivatives, while the GS, GSA, Oltcit used a flat-four, and a flat-six was proposed for the DS, but rejected. BMW used an air-cooled flat-twin in almost all of its motorcycles from 1921 until 1980, and still depends heavily on this layout, using it in many models including its most popular bike, the R1200GS

All versions of the Subaru Impreza, Forester, Tribeca, Legacy, Outback and SVX use either a flat-4 or flat-6 engine.

"Boxers" can be subdivided, since true boxers have each crankpin controlling only one piston/cylinder, and 180° engines share crankpins.

  • The boxer engine (the true horizontally opposed engine) has corresponding pistons reaching top dead center (TDC) simultaneously.
  • The 180° V engine has corresponding pistons sharing a crank pin on the crankshaft and reaching top dead center half a crankshaft revolution apart. They may use regular connecting rods side by side, or use a master/slave system, or a fork-and-blade system. Flat engines with more than eight cylinders are most commonly V engines.

Boxer engines must not be confused with opposed piston engines, which are based on a quite different concept using two crankshafts. These can be used in vehicles such as tanks.


Boxer engines

1969 Hino Motors DS140 12 cylinder boxer diesel engine

Boxer engines got their name because each pair of pistons moves simultaneously in and out rather than alternately, like boxers showing they're ready by clashing their gloved fists against each other before a fight. Boxer engines of up to eight cylinders have proved highly successful in automobiles and up to six cylinders in motorcycles, and continue to be popular for light aircraft engines.

Boxers are one of only three cylinder layouts that have a natural dynamic balance; the others being the straight-6 and the V12. These engines can run very smoothly and free of unbalanced forces with a four-stroke cycle and do not require a balance shaft or counterweights on the crankshaft to balance the weight of the reciprocating parts, which are required in other engine configurations. Note that this is generally true of boxer engines regardless of the number of cylinders (assumed to be even), but not true for all V or inline engines. However, in the case of boxer engines with fewer than six cylinders, unbalanced moments (a reciprocating torque also known as a "rocking couple") are unavoidable when the output is a crankshaft due to the "opposite" cylinders being not exactly opposite but offset slightly.

Boxer engines (and flat engines in general) tend to be noisier than other common engines for both intrinsic and other reasons, eg in cars, valve clatter from under the bonnet is not damped by large air-filters and other components. Boxers need no balance weights on the crankshaft, which should be lighter and fast-accelerating - but in practice (e.g. in cars) they need a flywheel to run smoothly at low speeds and this negates the advantage. They have a characteristic smoothness throughout the rev range and offer a low center of gravity. When combined with a mounting position immediately ahead of the rear axle (e.g. Porsche Boxster and Cayman but not Porsche 911 nor Volkswagen Beetle) they have largely neutral handling.

Notable flat engines

Subaru Boxer Turbodiesel engine cutaway display
  • In 1896, Karl Benz invented the first internal combustion engine with horizontally opposed pistons.
  • In 1923 Max Friz designed the first BMW motorcycles, choosing a 500 cc boxer engine and unit transmission with shaft drive. This engine type is still in production today. The BMW 247 engine, known as an airhead due to its air cooling, was produced until 1995. BMW replaced it with the oilhead engine with partial oil cooling and four valves per cylinder, but still retaining the same boxer twin configuration.
  • In 1948 Preston Tucker modified a helicopter flat-6 to be rear mounted in his Tucker Torpedo
  • The Volkswagen air-cooled flat-4 engine used in the Volkswagen Beetle, SP2 and Karmann Ghia, and later developed further for the Volkswagen Type 2 (Bus) transporters and Volkswagen Type 3 cars
  • The Citroën 2CV and Panhard air-cooled flat-2 engines, both influenced by the flat-2s of BMW
  • The air-cooled Chevrolet Corvair OHV flat-6
  • In 1960 Lancia's flat-4 water-cooled engine debuted for the Lancia Flavia model, first Italian front-wheel drive car, like 1500 cc 90 PS (66 kW) coupé version and during the years become a 2000 cc with 142 bhp (106 kW) when was mounted the first electronic Injection by Bosch in 1970 in a flat four engine, Lancia also rebuild a new big flat-4 engine in 1976, 2484 cc, for his upper size model Lancia Gamma. It was produced until 1984 and was Lancia's last flat-4 engine.
  • The flat-4 engines in Alfa Romeo's Alfasud, Sprint, 33 and early versions of the 145. The last of the line was a 1712 cc flat-4, 16 valves, producing up to 137 PS (101 kW).
  • The water-cooled front-mounted flat-4 and flat-6 engines used by Subaru in all of its mid-sized cars; Subaru refers to these as boxer engines in publicity commentary, and include a variety of naturally aspirated and turbo driven engines; in their turbo engines from 1983 to current, both closed and semi-closed short blocks have been used. Subaru also offers a boxer turbodiesel,[1] called the Subaru EE series, the world's first to be fitted into a passenger car.
  • Since its introduction in 1975, the Honda Goldwing has utilized a boxer engine, a 4-cylinder until 1987, and 6-cylinders since. The water-cooled SOHC 1832 cc flat-6 is fitted to the Honda Goldwing from 2001 on.
  • The air-cooled flat-4, flat-6 and flat-8 engines were used for many years in early Porsches. The flat-12 in the 917 model is a 180° V-engine and not a boxer.
  • The water-cooled flat-6 engines in the Porsche Boxster, Cayman and later 911 models
  • General aviation aircraft often use air-cooled flat-4 and flat-6 engines made by companies such as Lycoming, Continental. Ultralight and microlight aircraft often use engines such as the Rotax 912 or Jabiru 2200.

Flat engine designs

Other engine designs

See also


  1. ^ "Greencarcongress". Legacy Diesel Announcement. Retrieved 2008-01-23. 

External links


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