Flat four

A flat-four or horizontally opposed-four is a type of four-cylinder engine, a flat engine with four cylinders arranged horizontally in two banks of two cylinders on each side of a central crankcase.

The pistons are usually mounted on the crankshaft so that opposing pistons move back and forth in opposite directions at the same time, somewhat like boxing competitors punching their gloves together before a fight, which has led to it being referred to as a "boxer" engine. The design is rarely seen with shared crank throws (See Coventry Climax FWMW for such a non-boxer flat engine), so "flat-four" and "boxer-four" are usually used synonymously.

The configuration results in inherently good balance of the reciprocating parts, a low centre of gravity, and a very short engine length. The layout also lends itself to efficient air cooling with excellent thermal balance. However, it is an expensive design to manufacture, and somewhat too wide for compact automobile engine compartments, which makes it more suitable for cruising motorcycles and aircraft than ordinary passenger cars.

This is no longer a common configuration, but some brands of automobiles use such engines and it is a common configuration for smaller aircraft engines such as those made by Lycoming, Continental and Rotax. Although they are superior to straight fours in terms of secondary vibration, they have largely fallen out of favour for overhead cam design engines, for which an opposed-four cylinder layout would require twice as many camshafts as a straight-four while the crankshaft is as complex to manufacture. The low centre of gravity of the engine is an advantage. The shape of the engine suits it better for mid engine or rear engine designs. With a rear engine layout, it allows a low-tail body while in front engine designs the width of the engine often interferes with the maximum front wheel steering angle. The latter problem has not stopped Subaru from using it in its all-wheel drive cars, where the difficulty of fitting the wide engine between the front wheels ahead of the front axle is compensated for by the ease of locating the transmission and four-wheel drive mechanisms behind the short length, between the front and rear axles.

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