Editing Flat engine (section)

{{Short description|Combustion engine using pistons facing to the sides on a common crankshaft}}
{{For|similarly-named engines with two pistons per cylinder|opposed-piston engine}}
{{Use dmy dates|date=October 2020}}
{{Use British English|date=October 2024}}

[[File:Boxerengineanimation.gif|thumb|upright=1.35|Animation of a boxer engine. <!-- ERROR in graphic: Right crank throw is backwards, so the right piston moves only half the way of the left. -->]]
[[File:Benz Contramotor IMG 1343 2023-11-25 JM.jpg|thumb|Benz Contramotor, the first commercial flat-engine design, ca. 1899]]

A '''flat engine''' is a [[Internal combustion engine#Reciprocating engines|piston engine]] where the cylinders are located on either side of a central [[crankshaft]]. Flat engines are also known as '''horizontally opposed engines''', however this is distinct from the less common [[opposed-piston engine]] design, whereby each cylinder has two pistons sharing a central combustion chamber.

The most common configuration of flat engines is the [[#Boxer configuration|boxer engine]] configuration, in which the pistons of each opposed pair of cylinders move inwards and outwards at the same time. The other configuration is effectively a [[V engine]] with a 180-degree angle between the cylinder banks: in this configuration each pair of cylinders shares a single crankpin, so that as one piston moves inward, the other moves outward.

The first flat engine (Benz Contramotor) was built in 1897 by [[Karl Benz]]. Flat engines have been used in aviation, motorcycle and automobile applications. They are now less common in cars than [[straight engine]]s (for engines with fewer than six cylinders) and [[V engine]]s (for engines with six or more cylinders). Flat engines are more common in aircraft, where straight engines are a rarity and V engines have almost vanished except in historical aircraft. They have even replaced [[radial engine]]s in many smaller installations.