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CATEGORIES (articles) > Engine, Gearbox > Technical > Flat engine explained

Flat engine explained

A flat engine is an internal combustion engine with its pistons parallel to the ground. It can be an inline engine canted 90 degrees from straight up, and also can be boxing engine sometimes referred to as a boxermotor, in which the cylinders are arranged in two banks on either side of a single crankshaft so that the motion of all the pistons is in a single plane.

In 1896, Karl Benz patented his design for the first internal combustion engine with horizontally opposed pistons. There are two main types of these engines:

  • The boxer engine, also known as a horizontally opposed engine, in which the corresponding pistons reach top dead centre simultaneously, thus balancing each other with respect to momentum. Flat engines with four or fewer cylinders are most commonly boxer engines.
  • The 180° V engine, in which corresponding pistons share a crank pin, and thus each will reach top dead centre half a crankshaft revolution after the other. Flat engines with more than eight cylinders are most commonly V engines.
In German, the term for flat engine is boxermotor, which includes both types, not just those known as boxer engines in English. It is often said that Adolf Hitler supposedly coined the term when he was inspecting the engine of the then new Volkswagen in World War II. When the pistons of the motor are observed from the top they resemble the fists of a boxer as he strikes them together before a bout.

Another source of confusion is the English term horizontally opposed engine, which contrasts with an opposed piston engine, a completely different concept.

Flat engines are more compact than in-line engines, and have a lower center of gravity than any other common configuration. Automobiles and motorcycles powered by a flat engine generally have a lower center of gravity, giving better stability and control. However, these engines are also wider than more traditional configurations and are more expensive to build. The extra width may cause problems in fitting the engine into the engine bay of a front-engined car owing to the interference with the steering wheels, and cornering problems for a motorcycle.

The flat configuration also fits very well with air cooling and aircraft engines. Air-cooled designs such as in the VW Beetle used a flat-4, as did the Porsche 356 and 912. The Chevrolet Corvair used an air-cooled flat-6, a rarity in American designs. Both the older and newer models of the 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; the use of air-cooling obviates the need for connecting the engine to a radiator, and also reduces the weight even more.

Front-mounted air-cooled flat-twin engines were used by Citroën in their model 2CV and its derivatives, while the Citroën GS used a flat-4, and a flat-6 was proposed for the Citroën DS but rejected. BMW uses an air-cooled flat-twin in many of their motorbikes.

Boxer engines

Diagram of the opposing pistons in a boxer engine

Boxer engines of up to eight cylinders have proved highly successful in both automobiles and motorcycles, and continue to be popular for light aircraft engines.

One benefit of using a boxer engine versus a V engine is that the design provides good balance because each piston's momentum is counterbalanced by the corresponding piston movement of the opposite side. These engines can run very smoothly and free of vibrations with a four-stroke cycle, regardless of number of cylinders, 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. But boxer engines tend to produce more noise than inline and V-engines because valve clatter is not so well dampened due to lack of covering by air-filters and other components, and produce a larger torsional vibration than a V engine, and so tend to require a larger flywheel.

Notable boxer engines include:

  • In 1896 Karl Benz invented the first internal combustion engine with horizontally opposed pistons.
  • The air-cooled flat-4 engine used in the Volkswagen Beetle and Karmann Ghia, and later developed further for the Volkswagen Type 2(Bus) transporters and Volkswagen Type 3 cars.
  • The air-cooled flat-2 "knee-warmer" motorcycles produced for many years by BMW motorcycles, and as of 2005 now back in their range by popular demand after an attempt to discontinue the design.
  • The Citroën 2CV and Panhard air-cooled flat-2 engines, both influenced by the flat-2s of BMW.
  • The air-cooled Porsche 911 and Chevrolet Corvair flat-6s.
  • The flat-4 engines in Alfa Romeo's Alfasud, Sprint, 33 and early versions of the 145 (last of the line was a 1712 cc flat four, 16 valves, producing up to 137bhp)
  • The flat-12 engine in the 24 Hours of Le Mans-winning Porsche 917.
  • The water-cooled front-mounted flat-4 and flat-6 engines used by Subaru in all of its mid-sized all wheel drive cars. Subaru calls these H engines in defiance of the normal nomenclature.
  • The water-cooled SOHC 1832cc flat-6 as fitted to the Honda Gold Wing.

180° V engines

Flat V engines are used in performance and racing cars, normally a 180 degree V12. Ferrari used a 180° version of the Colombo V12 in the Testarossa and Berlinetta Boxer models.

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