Editing Flat-four engine (section)

{{Short description|Horizontally opposed four-cylinder piston engine}}
{{original research|date=December 2023}}
[[File:UL350iS ULPower aircraft engine.jpg|thumb|ULPower UL360iS aircraft piston engine]]
A '''flat-four engine''', also known as a '''horizontally opposed-four engine''' or '''boxer engine''',<ref>The so-called "boxer" configuration appears somewhat like boxing competitors punching their gloves together before a fight</ref> is a four-cylinder [[piston engine]] with two banks of cylinders lying on opposite sides of a common crankshaft. The most common type of flat-four engine is the '''boxer-four engine''', each pair of opposed pistons moves inwards and outwards at the same time.

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 1897, especially by Volkswagen and Subaru. They have also occasionally been used in motorcycles and frequently in aircraft.  [[Cessna]] and [[Piper Aircraft|Piper]] use flat four engines from [[Lycoming Engines|Lycoming]] and [[Continental Aerospace Technologies|Continental]] in the most [[List of most-produced aircraft|common civil aircraft]] in the world - the [[Cessna 172]], and [[Piper Cherokee]], while many [[Ultralight aviation|ultralight]] and [[Light-sport aircraft|LSA]] planes use versions of the [[Rotax 912]].