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.
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.
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.
|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)|
|Stroke cycles |
One • Two • Four • Six •
|Engine types |
Gas turbine • Piston • Jet • Rocket engine • Steam engine • Stirling engine • Tschudi• Twingle
Rotary • Wankel • Free-piston • Britalus • Coomber • Swing-piston • Orbital • Quasiturbine
Cylinder head porting • D slide • Four-stroke • Manifold • Multi • Piston • Poppet • Sleeve
|Piston layouts |
Single cylinder • Straight • Opposed • Flat • V • W • H • Deltic • Radial • Rocket engine nozzle • Rotary • Stelzer • Controlled Combustion • Bourke
|Motion mechanisms |
Cam • Connecting rod • Coomber rotary • Crank • Crank substitute • Crankshaft • Linkages (Evans • Peaucellier-Lipkin • Sector straight-line • Watt) • Double acting/differential cylinder