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A flat-4 is a four cylinder internal combustion engine where the cylinders are arranged in a flat configuration. As of today Flat-4 engines are boxer engine, also known as a horizontally opposed engine, in which the corresponding pistons reach top dead centre simultaneously, thus balancing each other with respect to momentum. Flat engines with four or fewer cylinders are most commonly boxer engines.

This is not a common configuration, but some brands of automobile have favored such engines and it is a common configuration for smaller aircraft engines such as made by Lycoming or Continental. Although they are considered to be superior to "inline-fours" in terms of vibrations, they have largely fallen out of favor because they have two cylinder banks thus requiring twice more camshaft than a straight-4 while the crankshaft is as complex to manufacture. The low center of gravity of the engine is an advantage. The shape of the engine suits it better for mid engine or rear engine designs. With a rear engine body it allows a low-tail body while in front engine designs the width or the engine will interferes with the ability of the front wheels to steer.

The open and exposed design of the engine allows air cooling over water cooling, and in air cooled applications fins are often seen machined into the external cylinder block walls.

Automobile use

Jowetts before the 2nd world war were best known for their flat twin engines, but they made a flat four for the Jason and Jupiter models in the 1930s. Post war the Gerald Palmer designed Javelin saloon and Jupiter sports models usd a totally different design of flat four. Issigonis originally designed the Morris Minor for a flat four, but cost constraints meant it was never used.

Flat-4 engine in a 1955 Porsche 550 Spyder

Volkswagen used air-cooled flat-4s extensively in their early days, in the VW Beetle and most early VW designs. Porsche also used the VW engine in the early Porsche 356. This engine was replaced by a Porsche designed flat-4 in the late 356s and the 912. The 914 that replaced the 912 was built in partnership with VW using a VW engine. VW used a water-cooled flat-4 in the T3 until 1991.

Citroën used an air-cooled flat-4 on the Ami Super, GS, GSA and Axel.

Alfa Romeo introduced a water cooled flat-4 on the Alfa Romeo Alfasud. That engine was later used on the Alfa Romeo Arna, the Alfa Romeo 33, the Alfa Romeo Sprint, the Alfa Romeo 145/146.

Lancia used a water cooled flat-4 on the Flavia and high-end Lancia Gamma.

Subaru produces water-cooled front mounted flat-4 engines marketed as H-4, by which they mean Horizontal rather than the H cross-section normally meant by H engine.

Motorcycle use

Honda introduced water cooled flat-4 on the Honda 1000 Gold Wing.

Aircraft use

Lycoming developed a very successful series of flat-4 aircraft engines, as used in many Cessna aircraft. Similar engines are produced by Continental Motors.

Piston engine configurations
Straight Single, 2, 3, 4, 5, 6, 8, 9, 10, 12, 14
V 2, 4, 5, 6, 8, 10, 12, 16, 20, 24
Flat 2, 4, 6, 8, 10, 12, 16, H
W 8, 9, 12, 16, 18
Other inline H, VR, Opposed, U (Square), X
Other Hemi, Radial, Rotary, Pistonless, Deltic, (Wankel)

Heat engines
Stroke cycles
Engine types
Gas turbinePistonJetRocket engineSteam engineStirling engineTschudiTwingle
Cylinder head portingD slideFour-strokeManifoldMultiPistonPoppetSleeve
Piston layouts
Single cylinderStraightOpposedFlatVWHDelticRadialRocket engine nozzleRotaryStelzerControlled CombustionBourke
Motion mechanisms
CamConnecting rodCoomber rotaryCrankCrank substituteCrankshaftLinkages (EvansPeaucellier-LipkinSector straight-lineWatt) • Double acting/differential cylinder
Thermodynamic cycle