The boxer engine was first patented by German engineer Karl Benz.

A flat engine is an internal combustion engine with multiple pistons that move in a horizontal plane. Typically, the layout has cylinders arranged in two banks on either side of a single crankshaft and is sometimes known as the boxer, or horizontally-opposed engine. The concept was patented in 1896 by engineer Karl Benz. It should not to be confused with the opposed-piston engine, in which each cylinder has a piston at both ends and no cylinder head.

Another widely used form of flat engine consists of a straight engine with two, three, four or more cylinders canted 90 degrees into the horizontal plane, however this is not generally considered significantly different from other straight engines.

Configuration[edit | edit source]

1954 BMW "Boxer" motorcycle engine. The two cylinders cannot be directly opposite each other.

The Flat-4 Volkswagen air cooled engine

UL260i Flat-4 aircraft engine

Flat engines offer a low centre of gravity and thereby may offer a drive configuration with better stability and control. Flat engines lend themselves well to aircraft engines.

Front-mounted air-cooled flat-twin engines were used in Tatra 11 and Tatra 30, by Citroën in their model 2CV and its derivatives, while the GS and GSA. Oltcit used a flat-four and a flat-six was proposed for the Citroën DS but rejected. BMW has used air- or air/oil-cooled flat-twin engines in its motorcycles from 1923 until the present day. Cars such as the Porsche 911 use a flat-engine (in that particular case a six-cylinder) at the rear of the car, where its extra width does not interfere with the steering of the front wheels and there is a weight-saving since no prop-shaft is required.

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

True boxers have each crankpin controlling only one piston/cylinder while 180° engines, which superficially appear very similar, share crankpins.

  • The boxer engine has corresponding pistons reaching top dead centre (TDC) simultaneously.
  • The 180° V engine has corresponding pistons sharing a crankpin on the crankshaft and reaching TDC 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.[clarification needed]

Boxer engines must not be confused with opposed-piston engines, which are essentially the inverse, with two pistons compressing a single combustion space. These can be used in vehicles such as tanks.

Boxer engines[edit | edit source]

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 are 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 due to the "opposite" cylinders being slightly out of line with each other.

Boxer engines (and flat engines in general) tend to be noisier than other common engines for both intrinsic and other reasons, e.g., in cars, valve clatter from under the hood is not damped by large air filters and other components. Boxers need no balance weights on the crankshaft, which is lighter and fast-accelerating. They have a characteristic smoothness throughout the rev range and offer a low centre 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[edit | edit source]

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. VW was rumoured to have worked on a Diesel version of the aircooled boxer but abandoned this engine due to noise and heat issues. The latest version of the VW boxer was watercooled and thus dubbed the 'Wasserboxer' or waterboxer by enthusiasts. This engine included many developments of the earlier engines. It was offered in capacities of 1.9 and 2.1 litres was used to power T3 buses and transporters.
  • 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 from 1966, when the Subaru 1000 was introduced to current; both closed and semi-closed short blocks have been used. A print add for the 1973 Subaru GL coupe referred to the engine as "quadrozontal"[1] The EJ series of 4 cylinder engines released first in 1990 has been the focus for the development of the Boxer engine in the late 20th century. Ranging from 1.6-2.5 litres, this engine in its 2 litre turbo arrangement has been the power behind World Rally Championship winning cars. Subaru also offers a boxer turbodiesel,[2] 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.
  • Ferrari made use of a flat-12 design in several models, including the Berlinetta Boxer, the Testarossa and its derivatives, such as the 512TR and the F512 M, although this engine design is technically a V12 that has been flattened down to a 180° configuration, and therefore cannot be regarded as a true boxer engine.

Flat engine designs[edit | edit source]

Other engine designs[edit | edit source]

See also[edit | edit source]

References[edit | edit source]

  1. "» Vintage Subaru Ad (1973 The Subaru GL Coupe)". Retrieved on 2010-09-18.
  2. "Greencarcongress". Legacy Diesel Announcement. Retrieved on 2008-01-23.

External links[edit | edit source]

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