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.
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BMW Motorrad manufactures a number of flat-twin engine motorcycles,[1][2] 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.[2]
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.[2][3] [4] This position allowed the use of a conventional motorcycle drivetrain by belt or chain to the rear wheel.[3][5] Another advantage of this layout is that it has a low centre of gravity.[5] However, in this layout, the front cylinder is more heavily cooled than the rear cylinder,[3][5] and the wheelbase tends to be excessive due to the length of the engine. The wheelbase can be reduced by placing the transmission above the rear cylinder, as done on some Douglas motorcycles.[5]
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.[6] The 1923 BMW R32 used a similar engine position with a drive shaft using bevel gears to power the rear axle.[2]
This position allowed both cylinders to protrude into the airflow, providing excellent air cooling for each cylinder.[2][5] 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.[7]
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.[2][5]
A disadvantage of this layout is that it exposes the cylinders and valve covers to the danger of collision damage.[2][5] Longitudinal crankshaft mounting is also associated with a torque reaction that tends to twist the motorcycle to one side on sharp acceleration or when opening the throttle in neutral and in the opposite direction on sharp deceleration. Many modern motorcycle manufacturers correct for this effect by rotating flywheels or alternators in the opposite direction to that of the crankshaft.[8][9]
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. Flat-twin engines were also used in several early cars, including the Ford 1903-04 Model A, Model C, and Model F.
Maytag used its Model 72 flat-twin engines to power washing machines,[10][11][12] although they were used as proprietary engines for other purposes as well.[10] Maytag began manufacturing the Model 72 engine in 1937[10][13] and, after a break in production from May 1942 to June 1945 due to World War II,[13] continued manufacturing them until the 1950s. Production ended some time between 1952[13] and 1960.[10]
During World War II, motorcycle manufacturer Douglas built generators powered by their flat-twin engines,[14] while ABC Motors used one in an Auxiliary power unit (APU).[15]
Flat-twin engines are well suited to the wasted spark ignition system, a distributor-less ignition system using a double-ended coil firing both spark plugs on each revolution, that is, on both the compression stroke and the exhaust stroke. This system requires only a single contact breaker and single coil to run two cylinders.[16]
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