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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Motorcycle use
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]
Engine alignment
Cylinders along 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.[2][3] [4] This position allowed the use of a conventional motorcycle drivetrain by belt or chain to the rear wheel.[3] However, in this layout, the front cylinder is heavily cooled and the rear cylinder receives comparatively little cooling.[3]
Cylinders across frame
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.[5] 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] 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.[6]
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]
A disadvantage of this layout is that it exposes the cylinders to the danger of collision damage.[2]
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.[7][8]
See also
References
- ^ "BMW Motorrad USA - Bikes". http://www.bmwmotorcycles.com/. http://www.bmwmotorcycles.com/bikes/index.jsp?bikeSection=. Retrieved 2008-12-22.
- ^ a b c d e f g Wilson, Hugo (1995). "The A-Z of Motorcycles" (in UK English). The Encyclopedia of the Motorcycle. London: Dorling Kindersley. pp. 26-32, 51. ISBN 0 7513 0206 6.
- ^ a b c Norbye, Jan P. (1984). "The Origins of BMW: From Flying Machines to Driving Machines". BMW - Bavaria's Driving Machines. New York, NY, USA: Beekman House. pp. 14-17. ISBN 0-517-42464-9.
- ^ Mitchel, Doug (1997). "The Early Years (1903-1928)". Harley-Davidson Chronicle. Lincolnwood, Illinois, USA: Publications International. pp. 44-45. ISBN 0-7853-2514-X.
- ^ Wilson, H. The Encyclopedia of the Motorcycle p. 10 Dorling-Kindersley Limited, 1995 ISBN 0 7513 0206 6
- ^ AMA Motorcycle Hall of Fame Museum: 1942 Harley-Davidson XA
- ^ "Maytag Multi-Motor Engines". Maytag Collector's club. http://www.maytagclub.com/page-37.htm. Retrieved 2009-01-08.
- ^ Kinney, Keith (2007-02-27). "Maytag Engine-Driven Wringer Washer". Old Iron and Other Americana: The collections of the Kinney family. http://www.herculesengines.com/Maytag/Default.htm. Retrieved 2009-01-08.
External links
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