Flat engine

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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. Other forms of flat engines include two stroke diesel employing two crankshafts and pairs of opposing pistons.

Configuration

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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 later 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.

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-4, and a flat-6 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 the most popular.

All versions of the Subaru Impreza, Forester, Tribeca, Legacy 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 con-rods side by side, or use a master/slave 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

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A cutaway of an aircraft boxer engine, view from above.

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 use a lighter crankshaft without need of balance weights, so should be able to accelerate faster - but in practice (eg in cars) they would suffer excessive torsional vibrations at low speeds so are fitted with a larger flywheel that negates the advantage. They have a characteristic smoothness throughout the rev range and, when combined with a mounting position immediately ahead of the rear axle, offer a low center of gravity and largely neutral handling.

180° V-engines

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Non-boxer flat engines have been used in performance and racing cars, notably 180 degree V12s. Ferrari used a 180° version of the Colombo V12 in the Testarossa and Berlinetta Boxer models. The Porsche 917 flat 12 engine is also not a boxer.

This configuration is sometimes termed a 180° V engine because like most other V engine layouts each pair of pistons share a crank pin (also referred to as a crankshaft throw). In a 180° V engine when one piston of a pair is at Top Dead Centre the other will be at BDC. In a boxer engine a pair of pistons will be on crank pins 180° apart so that both pistons in a pair will be at TDC at the same time.

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 bhp (67 kW) din coupè 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, this engine was also tested by Squadra Corse Lancia in 1978 with 16 valves and Turbo supercharged, over 300 bhp (220 kW), for the racing Car Lancia Beta Montecarlo prototype (car after becomes World Champion of Makes in 1979 with a different engine). Last Flat-4 engine by Lancia was produced in 1984 on final production of Lancia Gamma, replaced by new Lancia Thema
• 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 bhp (102 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] 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

• Flat-twin
• Flat-4
• Flat-6
• Flat-8
• Flat-10
• Flat-12
• Flat-16
• H engine
• Controlled Combustion Engine

Other engine designs

• X engine
• V engine
• W engine
• Straight engine
• Rotary engine

See also

• History of the internal combustion engine

References

External links

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• Detailed Flat Twin Engine Animations