Flat four

A boxer-four engine has perfect primary and secondary balance, however, the two cylinder heads means the design is more expensive to produce than a straight-four engine. There is a minor, secondary unbalanced rotational torque pulse in the plane of the pistons, when a piston pair at one end of the engine is at TDC and the other pair at BDC. The TDC pair creates a torque greater than the BDC pair, so the net unbalanced torque pulse is the difference. The difference in TDC vs BDC inertial forces is explained in the Engine balance section. Boxer-four engines have been used in cars since 1900, especially by Volkswagen and Subaru. They have also occasionally been used in motorcycles and frequently in aircraft. Cessna and Piper use flat four engines from Lycoming and Continental in the most common civil aircraft in the world - the Cessna 172, and Piper Cherokee, while many ultralight and LSA planes use versions of the Rotax 912.