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A flat engine is an internal combustion engine with pistons that are all relatively horizontal. A straight engine canted 90 degrees from straight up is a flat engine, as is one in which the cylinders are arranged in two banks on either side of a single crankshaft. In both configurations, the motion of all the pistons is in the horizontal plane.

German engineer Karl Benz patented his design for the first internal combustion engine with horizontally opposed pistons in 1896.

Usually, each pair of corresponding pistons from each bank of cylinders share one crank pin on the crankshaft, either by master/slave rods or by two ordinary rods side by side. Some authorities divide flat engines into boxer engines which do not share crank pins in this way, and 180° engines which do.

• The boxer engine (also known as a horizontally opposed engine) in which the corresponding pistons reach top dead centre simultaneously. Boxer engines should not be confused with opposed piston engines, which use a completely different concept.

• 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.

Contents 1 Configuration 2 Boxer engines 3 Notable boxer engines 4 180° V-engines 5 See also 5.1 Flat engine designs 5.2 Other engine designs

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. These engines, however, are also wider than more traditional configurations and are more expensive to build. The extra width may cause problems fitting the engine into the engine bay of a front-engined car owing to the interference with the steering wheels, and create 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 motorcycles.

Subaru uses boxer engines.

Boxer engines probably got their name because the pistons of the motor, when observed from the top, resemble the fists of a boxer pummeling his opponent. 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. Boxers are one of only four layouts that have a natural dynamic balance; the others being the straight-6, the V12 and the wankel engine. These engines can run very smoothly and free of vibrations 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 true of all boxer engines regardless of the number of cylinders, but not true for all V and straight, or inline engines.)

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. They have a characteristic smoothness throughout the rev range and, combined with the mounting position immediately ahead of the rear axle, offer a low center of gravity and more neutral handling.

• 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 500cc boxer engine and unit transmission with shaft drive
• 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 air-cooled flat-2 "foot-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 Chevrolet Corvair flat-6
• 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 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 1984 to current, both closed and semi-closed short blocks have been used
• The water-cooled SOHC 1832cc flat-6 as fitted to the Honda Goldwing
• The air-cooled flat-4, flat-6, flat-8 and flat-12 engines were used for many years in early Porsches
• The water-cooled flat-6 engines in the Porsche Boxster, Cayman and later 911 models

• The engines of light private aircraft are often air-cooled flat-4s and flat-6s of the type famously made by the companies Lycoming, and Continental

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.

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

• X4 engine
• H engine
• V engine
• W engine
• Straight engine
• Rotary engine

v • d • e Piston engine configurations
Straight	Single · 2 · 3 · 4 · 5 · 6 · 8 · 9 · 10 · 12 · 14
Flat	2 · 4 · 6 · 8 · 10 · 12 · 16
V	2 · 4 · 5 · 6 · 8 · 10 · 12 · 16 · 20 · 24
W	8 · 12 · 16 · 18
Other inline	H · U · Square four · VR · Opposed · X
Other	Hemi · Radial · Rotary · Pistonless (Wankel)