Horizontally opposed

A flat engine is an internal combustion engine with horizontally-opposed cylinders. Typically, the layout has cylinders arranged in two banks on either side of a single crankshaft and is otherwise known as the boxer, or horizontally-opposed engine. The concept was patented in 1896 by engineer Karl Benz, who called it the "contra engine."

A boxer engine should not be confused with the opposed-piston engine, in which each cylinder has two pistons but no cylinder head. Also, if a straight engine is canted 90 degrees into the horizontal plane, it may be thought of as a "flat engine". Horizontal inline engines are quite common in industrial applications such as underfloor mounting for buses.

True boxers have each crankpin controlling only one piston/cylinder while the 180° engines, which superficially appear very similar, share crankpins. The 180° engine, which may be thought of as a type of V engine, is quite uncommon as it has all of the disadvantages of a flat engine, and few of the advantages.

In 1896, Karl Benz invented the first internal combustion engine with horizontally opposed pistons. He called it the kontra engine, as the action of each side opposed the action of the other. This design has since been called the "boxer" engine because each pair of pistons moves in and out together, rather like the gloves of a boxer. The boxer engine has pairs of pistons reaching TDC simultaneously.

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. 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. Other engine configurations with natural dynamic balance include the straight-six, the straight-eight, the V12, and the V16.

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