Flat-four Engine

Flat-four Engine

A flat-four or horizontally opposed-4 is 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 such 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 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. 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 automobile use such engines and it is a common configuration for smaller aircraft engines such as made by Lycoming, Continental and Rotax. Although they are somewhat superior to straight-4s in terms of vibrations, they have largely fallen out of favor because they have two cylinder banks thus requiring twice as many camshafts as a straight-4 (if an OHC rather than OHV or F-head configuration is used) 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 interferes with the ability of the front wheels to steer. The latter problem has not stopped Subaru from using it in its all-wheel drive cars, where the difficulty of fitting the short 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 it, between the front and rear axles.

The open and exposed design of the engine allows air cooling as well as water cooling, and in air-cooled applications fins are often cast into the external cylinder block walls to improve the engine cooling.