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.
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.
Cylinders along frame
The earliest flat-twin motorcycles, including Douglas in the United Kingdom, Helios of Germany, and Indian 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. Another advantage of this layout is that it has a low centre of gravity. However, in this layout, the front cylinder is more heavily cooled than the rear cylinder, 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.
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. 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.
A disadvantage of this layout is that it exposes the cylinders and valve covers to the danger of collision damage. 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.
Flat-twin engines were later 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-twins have been used to power light aircraft, although most piston-engined aircraft have used more cylinders for more power. Notable flat-twin aircraft engines include the Aeronca E-107 and E-113, the Praga B2, and the Bristol Cherub.
In larger aircraft, flat-twin engines have been used in auxiliary power units (APUs). A notable example was made by ABC Motors between the World Wars.
Maytag used its Model 72 flat-twin engines to power washing machines, although they were used as proprietary engines for other purposes as well. Maytag began manufacturing the Model 72 engine in 1937 and, after a break in production from May 1942 to June 1945 due to World War II, continued manufacturing them until the 1950s. Production ended some time between 1952 and 1960.
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.
- "BMW Motorrad USA - Bikes". http://www.bmwmotorcycles.com/. Archived from the original on 2008-08-22. Retrieved on 2008-12-22.
- Wilson, Hugo (1995). "The A-Z of Motorcycles", The Encyclopedia of the Motorcycle (in UK English). London: Dorling Kindersley, 26–32, 51. ISBN 0-7513-0206-6.
- 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, 14–17. ISBN 0-517-42464-9.
- "1917 Indian Model O". Motorcycle Hall of Fame Museum. American Motorcyclist Association (2010). Archived from the original on 2012-07-12. Retrieved on 2012-07-12. “Like the early Douglases, the Model O had its engine placed in the frame with the cylinders facing fore and aft, rather than sticking out to each side...”
- (1997) "The Early Years (1903-1928)", Harley-Davidson Chronicle. Lincolnwood, Illinois, USA: Publications International, 44–45. ISBN 0-7853-2514-X.
- "1922 Harley-Davidson Sport Twin". Motorcycle Hall of Fame Museum. American Motorcyclist Association (2010). Archived from the original on 2012-07-12. Retrieved on 2012-07-12. “In building the Sport Twin, Harley took Douglas’ lead in orienting the cylinders in line with the frame.”
- Willoughby, Vic  (1977). "Douglas", Classic Motorcycles, Third impression, The Hamlyn Publishing Group, 23. ISBN 0-600-31870-2.
- Wilson, H. The Encyclopedia of the Motorcycle p. 10 Dorling-Kindersley Limited, 1995 ISBN 0-7513-0206-6
- "1942 Harley-Davidson XA". Motorcycle Hall of Fame Museum. American Motorcyclist Association (2010). Archived from the original on 2012-07-12. Retrieved on 2012-07-12. “Mechanically, the large cooling fins stuck straight out in the breeze, reportedly keeping the XA’s oil temperature 100 degrees cooler than a standard Harley 45.”
- "Sport Cruisers Comparison - Seven Sport-Cruiser Motorcycles". Motorcycle Cruiser. Source Interlink Media (April 2000). Retrieved on 2010-09-10. “"Though the Valkyrie also has a longitudinal crankshaft, this torque reaction has been eliminated by making some of the components, such as the alternator, spin the opposite direction of the engine."”
- Battisson, Stephen (1997). "Developing the V6 - Taming The Beast". The Laverda V6. Stephen Battisson. Retrieved on 2010-09-10. “"By arranging the rest of the engine internals to rotate in the opposite direction to the crankshaft their forces are cancelled out without having to resort to the weight, complexity and friction associated with two crankshafts. "”
- "Kemp G-2 Horizontally-opposed Engine". Smithsonian National Air and Space Museum website. Retrieved on 2012-10-06. “Matthew B. Sellers, builder of the first Kentucky-manufactured aircraft, flew his first quadruplane from 1908 to 1913 powered by various 2-cylinder, horizontally-opposed engines. This Kemp G-2 was one of these engines.”
- Chaplin, R. H.; Nixon, F. (1939-04-06)Poulsen, C. M.ed., "Ancillary Power Services", Flight (London) 35(1580): 357–359, http://www.flightglobal.com/pdfarchive/view/1939/1939%20-%201031.html. Retrieved on "Both lecturers discussed the claims of the auxiliary engine for supplying service power. This is a well known British example, the A.B.C. flat twin.".
- Shelton, Charles L. (March/April 1999), "Maytag Twins or 'Look-a-Likes'?" (aspx), Gas Engine Magazine (Topeka, Kansas, United States: Ogden Publications), http://gasengine.farmcollector.com/Equipment/Maytag-Twins-or-Look-a-Likes.aspx. Retrieved on "The twin, or 72 as it was commonly referred to, was used primarily as a source of power for the Maytag washing machines. Even as late as the early '30s, some brands of washers were hand operated; thus a ready power source such as the twin had a great deal of influence on Americans' work habits."
- Kinney, Keith (2007-02-27). "Maytag Engine-Driven Wringer Washer". Old Iron and Other Americana: The collections of the Kinney family. Retrieved on 2009-01-08.
- "Maytag Service Instructions" (pdf) 11–16.
- Hunn, Peter (Jun 13, 2005). "Short Profiles of Manufacturers", The Small-Engine Handbook, Motorbooks Workshop. MotorBooks International, 42. ISBN 978-0-76032-049-5. Retrieved on 2012-07-05. “Often equipped with a foot pedal kick-starter, Maytag motors were available in both single-cylinder and opposed-twin formats.”
- "Maytag Multi-Motor Engines". Maytag Collector's club. Retrieved on 2009-01-08.
- Brown, Roland (November/December 2007), "1955 Douglas Dragonfly", Motorcycle Classics (Ogden Publications), http://www.motorcycleclassics.com/motorcycle-reviews/1955-douglas-dragonfly.aspx. Retrieved on .
- 2CV Stuff: A Series Ignition System
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