Four cylinder boxer

A flat engine, also known as a horizontally opposed engine, is a piston engine where the cylinders are located on either side of a central crankshaft. The most common configuration of flat engines is the boxer engine, where each pair of opposed cylinders moves inwards and outwards at the same time, which is known as a "boxer" configuration (somewhat like boxing competitors punching their gloves together before a fight). Therefore, the terms "flat-four" and "boxer-four" are often used synonymously. Boxer engines have been used in automobiles, motorcycles and also light aircraft. The advantages of the boxer-four layout are perfect secondary vibration (resulting in minimal vibration), a low centre of gravity, and a short engine length. The layout also lends itself to efficient air cooling as airflow is evenly distributed across the four cylinders. In aircraft, this avoids the need to carry heavy water cooling systems. The downsides of boxer-four engines (compared to inline-four engines) are their extra width, the increased costs associated with having two cylinder heads instead of one, and the long exhaust manifold required to achieve evenly spaced exhaust pulses.