Flat engine

A flat engine is an internal combustion engine with horizontally-opposed pistons. 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.

Flat-six engine

A flat-6 or horizontally opposed-6 is a flat engine with six cylinders arranged horizontally in two banks of three cylinders on each side of a central crankcase. This should not be confused with the Opposed-piston engine. The pistons are mounted to the crankshaft such that opposing pistons move back and forth in opposite directions at the same time, somewhat like a boxing competitor punching their gloves together before a fight, which has led to it being referred to as a boxer engine.

The configuration results in inherently good balance of the reciprocating parts, a low center of gravity, and a very short engine length. The layout also lends itself to effective air cooling. The shape of the engine suits it better for rear engine and mid-engine designs, where the low center of gravity is an advantage; in front engine designs the width interferes with the ability of the front wheels to steer. However, it is an intrinsically expensive design to manufacture, and somewhat too wide for compact automobile engine compartments, which makes it more suitable for luxury sports cars, cruising motorcycles, and aircraft.

Flat-twelve engine

A flat-12 is a 12-cylinder internal combustion engine in a flat configuration. Rarer, wider, and less tall than a V12, a flat-12 is used exclusively in mid-engined sports cars and rarely on production cars.

The engine has a naturally lower center of gravity than a V12, but must be mounted somewhat higher in the engine bay to provide clearance for the exhaust system (six runners on either side).

Ferrari

Flat-12 engines are generally not true horizontally opposed engines (boxer), but rather 180° V-engines. A true boxer has one crank pin per piston, while in the 180° V-engine, two opposing pistons share the same crank pin. The flat design was used in Formula One and Endurance racing. However, when wing-cars requiring air-flow venturis came along in the late 1970s, the wide, flat layout obstructed the airflow and became obsolete. In 1964-65, at the end of the 1.5 litre F1 era, Ferrari introduced a flat-12 on the Ferrari 1512 F1, but a more classical V12 was chosen for the new 3 litre F1. The Ferrari flat-12 design was successful and influential especially on Italian manufacturers, including Alfa Romeo, who were also successful in endurance racing with a flat-12. The Tecno Formula One flat-12 was a failure, however. A 4.4-5.0 L 180° flat-12 was later introduced by Ferrari in some of their production models, including the Berlinetta Boxer and Testarossa.