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Flat twin engine

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Title: Flat twin engine  
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Subject: Motorcycle transmission, Flat-ten engine, V3 engine, Watt's linkage, Two-stroke engine
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Flat twin engine

Movement of flat-twin boxer rotating assembly
A 1967 BMW R50/2 with tank removed, showing its flat-twin engine mounted with its crankshaft longitudinal. 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 a flat engine with two cylinders. Used in motorcycles for more than ninety years, flat-twins have also been used in automobiles, light aircraft, stationary powerplants, and household appliances.

Early flat-twin motorcycles had their engines mounted with the cylinders in line with the frame. This caused uneven cooling of the cylinders and required the motorcycle to have a long wheelbase. Later flat-twin motorcycles had their engines mounted with their cylinders across the frame for better air cooling and a shorter wheelbase. Disadvantages of this layout include torque reaction in the motorcycle in turns and the potential to damage cylinder heads.

Crank configurations

Connecting rods at the crank pin of the Brush flat-twin Diesel engine on display at the Snibston Discovery Museum

The most common crank configuration used with flat-twin engine is the boxer twin, with a 180 degree crankshaft with two crankpins, such that the pistons move in and out simultaneously. This, in theory, eliminates unbalanced forces in the reciprocating parts of the drivetrain, but having the pistons offset from each other causes an unbalanced moment on the crankshaft.[1]

Some flat-twin engines, including the Brush Diesel engine, have only one crank throw, with the crank pin shared by both connecting rods.

The flat-twin engine in the 1897 Lanchester 8 hp phaeton had two counter-rotating crankshafts.[2] Each piston was attached to one crankshaft by a thick connecting rod and to the other crankshaft by two thinner connecting rods, one on either side of the other piston's thick connecting rod.[2][3] This allowed both cylinders to have the same axis. It also had the torque reaction of one crankshaft cancel the torque reaction of the other, cancelling torque reaction in the engine.[2] The pistons in the Lanchester engine moved so that one piston was on top while the other was at bottom dead center, creating an uneven firing cycle. Lanchester used this engine design until 1904.[3]

Motorcycle use

The first flat-twin motorcycle engine was the Fée,[4] later renamed "Fairy", which was built by the Light Motors Company from 1905 until the company folded in 1907.[5][6] The Douglas Engineering Company, one of Light Motors' suppliers, took over the manufacturing rights and developed motorcycles based on the Fée system under their own name.[6][7]

BMW Motorrad manufactures a number of flat-twin engine motorcycles,[8][9] as do Ural and Dnepr.

Engine alignment

Cylinders along frame

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

The 1905 Fée bicycle engine system included a flat-twin engine, mounted with the cylinders in line with the frame, with chain drive to a primary shaft with a pulley driving the rear wheel of the bicycle.[7] The 1907 Douglas, developed from the Fée system, had a belt drive to the rear wheel driven directly from the engine.[7] Later developments of the Douglas motorcycle were made with the cylinders in line with the frame until the Second World War.[10]

Other early flat-twin motorcycles used a similar layout, including the Helios in Germany and the Indian Model O and Harley-Davidson Model W in the United States, with their cylinders aligned along the frame, and therefore with the crankshaft running transverse to the frame.[9][11][12] This position allowed the use of a conventional motorcycle drivetrain by belt or chain to the rear wheel.[11][13] Another advantage of this layout is its low centre of gravity.[13] However, in this layout, the front cylinder is more heavily cooled than the rear cylinder,[11][13] and the wheelbase tends to be excessive due to the length of the engine. In 1914 the main supplier of rear-hub gearboxes, Sturmey-Archer, introduced a 3-speed countershaft gearbox with integral kick-starter.[14] The photograph at right shows the gearbox located low down behind the engine where the kick-start is convenient for operation. While this was a relatively easy modification for vertical single-cylinder or V-twin engined motorcycles, the length of the pair of cylinders along the frame made this location difficult without having a long wheelbase. Douglas, and the Fée motorcycle that preceded it, had used a countershaft below the engine;[15] some later models used a gearbox above the engine,[13][16] though where the engine was short enough the preferred option was the conventional layout.[17]

In a review of flat twin engines in 1916, Motor Cycle magazine listed the following (all except the ABC engines were fitted with cylinders in line with the frame):[18]

  • ABC 2.5 hp (bore 60 mm, stroke 44 mm), 249 cc - "the smallest flat-twin at present on the road".[18]
  • ABC 3.5 hp (bore 70 mm, stroke 64 mm), 494 cc - designed by G.W. Bradshaw.
  • Bradbury 3.5 hp (bore 68 mm, stroke 68.7 mm), 499 cc. Introduced c1914.
  • Brough 3.5 hp (bore 70 mm, stroke 64.5 mm) 497 cc, all Brough motorcycles used the flat-twin layout from end of 1914
  • Brough 6 hp (bore 70 mm, stroke 90 mm) 692 cc, developed by Brough during the war and mainly intended for sidecar work after the war
  • Douglas 2.75 hp (bore 60.5 mm, stroke 60 mm) 345 cc, the original Douglas, but with mechanically operated inlet valves from 1912
  • Douglas 4 hp (bore 72 mm, stroke 68 mm) 544 cc, an updated design bearing little in common with the smaller engine
  • Humber 6 hp (bore 78 mm, stroke 78 mm) 746 cc, water-cooled and with Claudel-Hobson automatic carburettor
  • Humber 3.5 hp (bore 68 mm, stroke 68.75 mm) 497 cc, brand new in October 1916 with Longuemare-Hardy carburettor
  • Matchless 6 hp (bore 70 mm, stroke 95 mm) 732 cc, expected for 1917 season
  • Indian 2.5 hp (bore 50.8 mm, stroke 63.5 mm) 257 cc, "The only flat-twin at present on the American market"[18]
  • Montgomery 6 hp (bore 75 mm, stroke 78 mm) 688 cc, produced essentially for sidecar work.
  • Williamson 8 hp (bore 85 mm, stroke 85 mm) 964 cc, made by Douglas intended originally watercooled for cyclecars but made as water and air cooled for Williamson motorcycles

Cylinders across frame

1942 Harley-Davidson XA flat-twin engine

In 1916,[18] 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.[19] The 1923 BMW R32 used a similar engine position with a drive shaft using bevel gears to power the rear axle.[9]

This position allowed both cylinders to protrude into the airflow, providing excellent air cooling for each cylinder.[9][13] 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.[20]

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.[9][13]

A disadvantage of this layout is that it exposes the cylinders and valve covers to the danger of collision damage.[9][13] 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.[21][22]

Another disadvantage of this layout is that the engine has to be high enough in the frame to provide the cylinder heads with banking clearance in turns, which raises the flat-twin engine's otherwise low center of gravity.[23]

Automotive use

Blackjack Avion displaying the cylinders of its Citroën 2CV engine

The Lanchester Engine Company used their twin-crank flat-twin engines from 1897 until 1904 when they were re-incorporated as the Lanchester Motor Company.[2][3]

Benz & Cie showed their first boxer engine, the two-cylinder "contra engine", in 1897.[24]

Flat-twin engines were used in some of Henry Ford's early cars, including the Ford 1903-04 Model A, Model C, and Model F.[25][26]

Flat-twin engines were later used in several economy cars, including Jowett cars from 1906 to 1937[27] and later in the Jowett Bradford van from 1945 to 1954,[28] postwar Citroën and Panhard front wheel drive cars, rear-engined BMW cars, Steyr-Puch 500, DAF Daffodil, and the Toyota Publica and Toyota Sports 800.

Aviation use

Bristol Cherub II installed in aircraft

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.[29] During World War II, the Riedel firm in Germany designed and manufactured a two-stroke flat-twin engine to start Junkers Jumo 004 axial-flow turbojets.[30]

Other uses

Maytag used its Model 72 flat-twin engines to power washing machines, although they were used as proprietary engines for other purposes as well.[31][32] 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.[31][33] Production ended some time between 1952[33] and 1960.[31]

During World War II, motorcycle manufacturer Douglas built generators powered by their flat-twin engines,[34] Norman Engineering Co produced flat twin generators from 1932 to 1968, and Enfield Industrial Engines (part of Enfield Cycle Co) produced 250cc and 350cc flat twin 2-stroke petrol stationary engines during the war which were used for generators and other military uses. After the war they built flat-twin air-cooled diesel engines, with applications ranging from farm use to marine use. Coventry Victor introduced a diesel version of their existing 688cc petrol flat-twin in 1932, and went on to produce flat twin diesel and petrol engines for a variety of industrial and marine uses well into the post-war period.

Two-stroke flat-twins were a popular choice for use in outboard motors, as they were smoother than single-cylinder engines. They lost popularity in the late 1940s to straight-twin two-strokes with 180-degree crankshafts that were easier to start and had an acceptable amount of vibration.[35]

Ignition systems

A Wasted spark ignition system used on a boxer-twin engine

Boxer-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.[36]


  1. ^ Wilson, H. The Encyclopedia of the Motorcycle "Engine blueprint" sidebar on page 27
  2. ^ a b c d Rogliatti, Gianni (1973). Period Cars. Feltham, Middlesex, UK: Hamlyn. p. 140.  
  3. ^ a b c Smith, Sam (October 2010). "The 10 Most Unusual Engines of All Time". car magazine. Hearst. Lanchester Twin-Crank Twin. Archived from the original on 2012-06-03. Retrieved 2013-05-17. One crank lived above the other, and each piston had three connecting rods—two light outer ones and a heavier one in the center. The light rods went to one crank, the heavy rods to the other, and the two shafts counterrotated. 
  4. ^ "The engine of the future".  
  5. ^ Wilson, H. The Encyclopedia of the Motorcycle p. 219
  6. ^ a b Wilson, H. The Encyclopedia of the Motorcycle p. 218
  7. ^ a b c Partridge, Michael (1976), "1905 2 12 hp Joseph Barter motorcycle", Motorcycle Pioneers: The Men, the Machines, the Events 1860-1930, David & Charles (Publishers), p. 42,  
  8. ^ "BMW Motorrad USA - Bikes". Archived from the original on 2008-08-22. Retrieved 2008-12-22. 
  9. ^ a b c d e f Wilson, Hugo (1995). "The A-Z of Motorcycles". The Encyclopedia of the Motorcycle (in UK English). London: Dorling Kindersley. pp. 26–32, 51.  
  10. ^ Wilson, H. The Encyclopedia of the Motorcycle p. 51
  11. ^ 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.  
  12. ^ Indian:
    • "1917 Indian Model O". Motorcycle Hall of Fame Museum.  
  13. ^ a b c d e f g Willoughby, Vic (1977) [1975]. "Douglas". Classic Motorcycles (Third impression ed.). The Hamlyn Publishing Group. p. 23.  
  14. ^ "A Sturmey-Archer Countershaft Gear".  
  15. ^ Drawing of 1911 Douglas
  16. ^ Drawing of 1932 Douglas K32
  17. ^ Drawing of 1926 Douglas
  18. ^ a b c d "Flat Twins".  
  19. ^ Wilson, H. The Encyclopedia of the Motorcycle p. 10
  20. ^ "1942 Harley-Davidson XA". Motorcycle Hall of Fame Museum.  
  21. ^ Friedman, Art; Trevitt, Andrew; Cherney, Andrew; Elvidge, Jamie; Brasfield, Evans (April 2000). "Sport Cruisers Comparison - Seven Sport-Cruiser Motorcycles". Motorcycle Cruiser. Source Interlink Media. "Take a Spin" section, paragraph 4. Retrieved 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. 
  22. ^ Battisson, Stephen (1997). "Developing the V6 - Taming The Beast". The Laverda V6. Stephen Battisson. p. 3. Retrieved 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.  
  23. ^ Cocco, Gaetano (2004). "Chapter 11: The Engine". Motorcycle Design and Technology (English ed.). St. Paul, MN USA: Motorbooks International. p. 118.  
  24. ^ English, Bob (2010-04-29). "The engine that Benz built still survives". The Globe and Mail (Toronto, Canada). Retrieved 2013-12-19. 
  25. ^ Kimes, Beverly (1996). Standard Catalog of American Cars 1805-1942. Krause Publications. p. 572.  
  26. ^ Brooke, Lindsay (2008). "Chapter 1 Before the Model T". Ford Model T: The Car that Put the World on Wheels. Minneapolis, MN USA: Motorbooks. pp. 38–44.  
  27. ^ Specification tables, Culshaw & Horrobin 2013, pp. 188–189
  28. ^ Culshaw, David; Horrobin, Peter (2013) [1974]. "Bradford". The Complete Catalogue of British Cars 1895 - 1975 (e-book ed.). Poundbury, Dorchester, UK: Veloce Publishing. p. 373.  
  29. ^ Chaplin, R. H.; Nixon, F. (1939-04-06). Poulsen, C. M., ed. "Ancillary Power Services". Flight (London) 35 (1580): 357–359. Retrieved 2010-12-29. 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. 
  30. ^  
  31. ^ a b c Shelton, Charles L. (March–April 1999). "Maytag Twins or 'Look-a-Likes'?" (aspx). Gas Engine Magazine (Topeka, Kansas, United States: Ogden Publications). Retrieved 2010-12-28. 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. 
  32. ^ Citations for Maytag 72 engine:
    • Kinney, Keith (2007-02-27). "Maytag Engine-Driven Wringer Washer". Old Iron and Other Americana: The collections of the Kinney family. Retrieved 2009-01-08. 
    • "Maytag Service Instructions" (pdf). pp. 11–16. 
    • Hunn, Peter (Jun 13, 2005). "Short Profiles of Manufacturers". The Small-Engine Handbook. Motorbooks Workshop. MotorBooks International. p. 42.  
  33. ^ a b "Maytag Multi-Motor Engines". Maytag Collector's club. Retrieved 2009-01-08. 
  34. ^ Brown, Roland (November–December 2007). "1955 Douglas Dragonfly". Motorcycle Classics (Ogden Publications). Retrieved 2010-12-28. 
  35. ^ Holcolmb, Hank (October 1964). Juettner, Walter R., ed. "Inside Today's Outboards". MotorBoating (New York, NY USA: Hearst) 114 (4): 34–35.  
  36. ^ 2CV Stuff: A Series Ignition System

External links

  • Detailed BMW Motorcycle Flat Twin Engine Animations
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