What engine layout has cylinders arranged in two banks 180 degrees apart?

A flat engine has cylinders arranged in two banks 180 degrees apart on opposite sides of a central crankshaft. It is also commonly called a horizontally opposed engine.

This layout keeps the engine low and wide rather than tall. Its low center of mass can improve vehicle handling, while the opposing cylinders help balance reciprocating forces and reduce vibration. The trade-offs are greater width and the need for two cylinder heads, exhaust systems and related hardware.

Most flat engines use the boxer arrangement, in which opposing pistons move inward and outward together, like boxers throwing punches. However, “flat engine” is the broader term: some 180-degree V engines place opposing pistons on a shared crankpin, so their piston motion differs from a true boxer engine.

Karl Benz built the first flat engine, the Benz Contramotor, in 1897. Flat engines later became important in aircraft, motorcycles and cars. Subaru is strongly associated with flat-four engines, while the Porsche 911 is famous for its flat-six. The layout should not be confused with an opposed-piston engine, which uses two pistons in one cylinder.