A flat engine is a piston engine where the cylinders are located on either side of a central crankshaft. Flat engines are also known as horizontally opposed engines, however this is distinct from the less common opposed-piston engine design, whereby each cylinder has two pistons sharing a central combustion chamber.
The most common configuration of flat engines is the boxer engine configuration, in which the pistons of each opposed pair of cylinders move inwards and outwards at the same time. The other configuration is effectively a V engine with a 180-degree angle between the cylinder banks; in this configuration each pair of cylinders shares a single crankpin, so that as one piston moves inward, the other moves outward.
The first flat engine was built in 1897 by Karl Benz. Flat engines have been used in aviation, motorcycle and automobile applications. They are now less common in cars than straight engines (for engines with fewer than six cylinders) and V engines (for engines with six or more cylinders). Flat engines are more common in aircraft, where straight engines are a rarity and V engines have almost vanished except in historical aircraft. They have even replaced radial engines in many smaller installations.
Design
The advantages of flat engines are a short length, low centre of mass and suitability for air cooling.
Compared with the more common straight engines, flat engines have better primary balance (resulting in less vibration); however the disadvantages are increased width and the need to have two cylinder heads. Compared with V engines— the most common layout for engines with six cylinders or more— flat engines have a lower centre of mass and better primary balance than V6 engines, however they typically have a larger width.
The most common usages of flat engines are: Flat-twin engines are mostly used in motorcycles. Occasionally, they have been used in light cars, aircraft and industrial applications, mostly up until the 1960s. Flat-four engines are mostly used in cars (particularly in the earlier Volkswagen Type 1 to 4 and by Subaru in most of their models), and have occasionally been used in motorcycles. Their most common use is in smaller single-engine general aviation aircraft in which they are still manufactured and used to this day. Flat-six engines are mostly used in cars (particularly by the Porsche 911 sports car), and have occasionally been used in motorcycles and aircraft. Flat-eight engines have been used in several racing cars, mostly by Porsche in the 1960s. Flat-ten engines are not known to have reached production. A prototype road car engine was built by Chevrolet in the 1960s. Flat-twelve engines have been used in various racing cars, notably the Porsche 917K, during the 1960s and 1970s, and in Ferrari road cars from 1973-1996. Flat-sixteen engines are not known to have reached production. Prototype racing car engines were built by Coventry and Porsche in the 1960s and 1970s.
Boxer configuration
Most flat engines use a "boxer" configuration, where each pair of opposing pistons move inwards and outwards at the same time, somewhat like boxing competitors punching their gloves together before a fight. Boxer engines have low vibrations, since they are the only common configuration that have no unbalanced forces regardless of the number of cylinders. Boxer engines therefore do not require a balance shaft or counterweights on the crankshaft to balance the weight of the reciprocating parts. However, a rocking couple is present, since each cylinder is slightly offset from its opposing pair, due to the distance between the crankpins along the crankshaft.
180-degree V engine
An alternative configuration for flat engines is as a 180-degree V engine, which has been used on most twelve-cylinder flat engines. In this configuration, each pair of pistons shares a crankpin, rather than the boxer configuration where each piston has its own crankpin.
Aviation use
In 1902, the Pearse monoplane (which would later become one of the first aircraft to achieve flight) was powered by a flat-twin engine. Amongst the first commercially produced aircraft to use a flat engine is the 1909 Santos-Dumont Demoiselle range of airplanes, which was powered by boxer-twin engines.
Several boxer-four engines have been produced specifically for light aircraft. Several manufacturers produced boxer-six aircraft engines during the 1930s and 1940s.