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Title: Flat-twin  
Author: World Heritage Encyclopedia
Language: English
Subject: V-twin engine, Citroën DS, Citroën LNA, Drive shaft, List of Toyota engines, Panhard Dyna Z, Panhard PL 17, Cyril Pullin, Unit construction, Douglas (motorcycles)
Publisher: World Heritage Encyclopedia


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

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.[1]

Some flat-twin engines[which?], 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 while the cylinders moved in and out at the same time. It also had the torque reaction of one crankshaft cancel the torque reaction of the other, cancelling torque reaction in the engine,[2] although the geared engine output would create torque reactions in the rest of the drivetrain.

Motorcycle use

BMW Motorrad manufactures a number of flat-twin engine motorcycles,[4][5] 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.[5]

Engine alignment

Cylinders along frame

The earliest flat-twin motorcycles, including Douglas motorcycles in the United Kingdom, the Helios in Germany, and the Indian Model O and Harley-Davidson Model W in the United States, had their cylinders aligned along the frame, and therefore with the crankshaft running transverse to the frame.[5][6][7][8] This position allowed the use of a conventional motorcycle drivetrain by belt or chain to the rear wheel.[6][7] Another advantage of this layout is that it has a low centre of gravity.[6] However, in this layout, the front cylinder is more heavily cooled than the rear cylinder,[7][6] 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.[6]

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.[9] The 1923 BMW R32 used a similar engine position with a drive shaft using bevel gears to power the rear axle.[5]

This position allowed both cylinders to protrude into the airflow, providing excellent air cooling for each cylinder.[5][6] 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.[10]

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.[5][6]

A disadvantage of this layout is that it exposes the cylinders and valve covers to the danger of collision damage.[5][6] 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.[11][12]

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.[13]

Automotive use

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.[14][15]

Flat-twin engines were later used in several economy cars, including Jowett cars between World Wars I and II, postwar Citroën and Panhard front wheel drive cars, rear-engined BMW cars, Steyr-Puch 500, DAF Daffodil, and the Toyota Publica and Toyota Sport 800.

Aviation use

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.[16] 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.[17]

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.[18][19] 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.[18][20] Production ended some time between 1952[20] and 1960.[18]

During World War II, motorcycle manufacturer Douglas built generators powered by their flat-twin engines.[21]

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.[22]

Ignition systems

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.[23]



External links

  • Detailed BMW Motorcycle Flat Twin Engine Animations
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