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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, who called it the "contra engine."[1][2] It should not 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.
Flat engines offer a low centre of gravity improving stability and control for motorcycles,[1][3] and reducing body roll in automobiles and also enhancing handling precision such as during a sudden lane-change.[4] Flat engines lend themselves well to aircraft engines.[5]
Front-mounted air-cooled flat-twin engines were used in Tatra 11 and Tatra 30, DAF 600, 750, Daffodil, 33 and the 44/46, by Citroën in their model 2CV and its derivatives. The GS and GSA and the Oltcit Club used a flat-four[6] and a flat-six was proposed for the Citroën DS but rejected.[7] BMW has used air-, air/oil-cooled and air/water-cooled flat-twin engines in its motorcycles from 1923 until the present day. Cars such as the four-cylinder Volkswagen Beetle and the six-cylinder Porsche 911 use a flat-engine 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.
Since the Subaru 1000 of 1966,[8] all versions of the Subaru Impreza, Forester, Tribeca, Legacy, Outback, Baja, BRZ and SVX use either a flat-four or flat-six engine, including a unique common rail flat-four turbo-diesel.[9][10]
True boxers have each crankpin controlling only one piston/cylinder while 180° engines, which superficially appear very similar, share crankpins. The 180° engine is considered to be a type of V engine. The boxer engine has corresponding pistons reaching top dead centre (TDC) simultaneously.[11]
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 got their name because each pair of pistons moves simultaneously in and out rather than alternately, like boxers clashing their gloved fists together before a fight.[11] Boxer engines have proved to be highly successful with up to twelve[12] cylinders in automobiles and up to six cylinders in motorcycles, and they continue to be popular for light aircraft engines.[4][5]
The boxer configuration is the only configuration in common use that does not have unbalanced forces with a four-stroke cycle regardless of the number of cylinders, as long as both banks have the same number of cylinders. They do not require a balance shaft or counterweights on the crankshaft to balance the weight of the reciprocating parts, which are required in most other engine configurations.[4] 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.[11] Other engine configurations with natural dynamic balance include the straight-six, the straight-eight, the V12, and the V16.
Boxer engines (and flat engines in general) tend to be noisier than other common engines for both intrinsic and other reasons.[13] In cars, for example, valve clatter from under the hood is not damped by large air filters and other components.[14] Boxers need no balance weights on the crankshaft, which is lighter and fast-accelerating. They have a smoothness throughout the rev range and offer a low centre of gravity.
Source: 'Subaru' magazine – Subaru 1000 extra edition (issued May 20, 1966)
YouTube, Mercedes-Benz, Bertha Benz, Patent, Mannheim
Volvo Cars, Flat engine, World War II, Reciprocating engine, VR6 engine
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