flattwin

A flattwin, also known as a boxer engine or horizontally opposed engine, is an internal combustion engine configuration in which the pistons move in opposition to each other around a central crankshaft. The cylinders are arranged horizontally on either side of the crankshaft, giving the engine a wide and flat profile. This design leads to a lower center of gravity compared to inline or V-engines, which can improve vehicle handling.

The flattwin engine's opposing piston motion results in inherent primary balance, meaning that the forces generated by the moving pistons are largely self-canceling, leading to smoother operation and reduced vibration. This is often contrasted with inline-four engines, which typically require balance shafts to mitigate vibrations.

The compact dimensions of a flattwin engine, particularly its low height, make it suitable for applications where space is limited, such as in sports cars or motorcycles. The crankshaft is supported by multiple main bearings, contributing to its robustness. Cooling of the horizontally opposed cylinders can be achieved through air or liquid cooling systems.

Historically, flattwin engines have been employed by various automotive manufacturers, including Porsche, Subaru, and Volkswagen. They are also commonly found in aircraft engines and some industrial applications. The design's balance and compact nature have made it a popular choice for specific performance and packaging requirements.