Topic summary
Boxer engine
Extracted from the Wikipedia article Flat engine.
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 vibration, being the only common configuration that has no unbalanced forces, regardless of the number of pairs of cylinders. Boxer engines therefore do not require either 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 the other member of its pair due to the distance between the crankpins along the crankshaft.