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BMW motorcycle powered by a flat-twin engine
Movement of flat-twin rotating assembly
A 1967 BMW R50/2 longitudinally mounted flat-twin engine, with tank removed. Note that the cylinders are not truly in line but displaced by the width of one crank pin and one crank-shaft web.

A flat-twin is a two cylinder internal combustion engine with the cylinders arranged on opposite sides of the crankshaft. It is part of the class of flat engines, sub-type "boxer", and shares most characteristics of those engines.

Motorcycle use

BMW Motorrad manufactures a number of flat-twin engine motorcycles, as do Ural and Dnepr. The geometry gives good primary balance, but there is an unbalanced moment on the crankshaft caused by the pistons being offset from each other.

Engine alignment

Cylinders along frame

Flat-twin engine in a 1912 Douglas N3, with its cylinders mounted along the frame

The earliest flat-twin motorcycles, including Douglas in the United Kingdom, Helios of Germany, and Harley-Davidson of the United States, had their cylinders aligned along the frame, and therefore with the crankshaft running transverse to the frame. This position allowed the use of a conventional motorcycle drivetrain by belt or chain to the rear wheel. However, in this layout, the front cylinder is heavily cooled and the rear cylinder receives comparatively little cooling.

Citroen 2CV flat twin engine
Rear engine of a Citroën Sahara

Cylinders across frame

1942 Harley-Davidson XA flat-twin engine

In 1919, ABC introduced a motorcycle with a flat-twin engine with the cylinders across the frame, and therefore with the crankshaft running longitudinally when referenced to the frame. To accommodate chain drive, the ABC used a bevel drive at the gearbox to change the direction of the drive through ninety degrees. (Wilson, H. The Encyclopedia of the Motorcycle p. 10 Dorling-Kindersley Limited, 1995 ISBN 0 7513 0206 6) The 1923 BMW R32 used a similar engine position with a drive shaft using bevel gears to power the rear axle.

This position allowed both cylinders to protrude into the airflow, providing excellent air cooling for each cylinder. The Harley-Davidson XA, which used a flat-twin engine with the cylinders across the frame, maintained an oil temperature 100 °F (56 °C) cooler than a Harley-Davidson WLA with a V-twin with the cylinders in line with the frame.

Many motorcyclists appreciate the way the cylinders in this layout provide protection to the rider in the event of a collision or fall, and keeps their feet warm in cold weather.

A disadvantage of this layout is that it exposes the cylinders to the danger of collision damage.

Automotive use

Flat-twin engines were used in several economy cars, including the Citroën 2CV, the Panhard Dyna X and Dyna Z, Steyr-Puch 500, DAF Daffodil, BMW 600, several Jowett cars between World Wars I and II, and the Toyota Publica and Toyota Sport 800.

Other uses

Maytag used its Model 72 flat-twin engines to power washing machines.

External links

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
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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