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A parallel-twin engine (aka straight twin, straight-two cylinder engine, inline twin, or vertical twin) is a two-cylinder piston engine that has its cylinders arranged side by side. There are three crankshaft configurations for this engine: 360°, 180°, and the new 270°:

• In a 360° engine, both pistons rise and fall together. The dynamic balance is identical to that of a single-cylinder engine, but with twice the number of ignition pulses.
• In a 180° engine, one piston rises as the other falls. This gives good primary balance, albeit with a rocking couple; but results in irregular ignition pulses.
• In a 270° engine, one piston follows a quarter of a turn behind the other. This is a compromise between the first two types, yielding results similar to a V-twin.

Unlike V-twins, straight-twin engines do not use a common crank pin for both connecting rods; each cylinder has its own crank pin. Most British straight-twin motorcycle engines from the 1930s through the 1980s, such as Triumph, BSA, Norton and Royal Enfield, had two main bearings. An exception was the AJS/Machless, which had a third, center main bearing.

Honda straight-twin engines, which began appearing in the late 1950s, had four main bearings. Subsequent straight-twin engines have four, or occasionally three, main bearings.

Automobile use

In the past, straight-twin engines have been used in very small cars (e.g. Microcars, "light" cars, and city cars like the Fiat 500 and Mitsubishi Minica) and in farm equipment, notably by John Deere whose large two-cylinder engines were used in their line of farm tractors up until 1960.

From 1967 to 1972, Honda produced the N360 and its successors N400 and N600 with straight-twin engines in 360 cc, 400 cc, and 600 cc sizes. The Z600 was produced from 1970 to 1972. From 1958 to 1971, Subaru produced the 360 with a rear-mounted, rear-drive 358 cc air-cooled engine.

Current production cars that use an inline twin engine include the Tata Nano, which has a 623 cc engine; and the Fiat 500 TwinAir, which has a turbocharged 875 cc engine.

Ferrari briefly considered creating an inline two-cylinder engine for Formula One use in the 1950s. Aurelio Lampredi worked with Enzo and Dino Ferrari on this design but abandoned the development after some tests of the engine in the F1-racer of that time. They hoped that the good low-end torque would make good for the lesser maximum power on twisting street-courses, but the fast escalating maximum power of the multi-cylinder-engines and the growing number of high-speed-courses killed the project.

Motorcycle use

The motorcycle world generally refers to these engines as "parallel twins" or "vertical twins", and the term "straight-two" is obsolete. "Parallel twin" refers to an engine mounted transversely across the frame; and the term "inline twin" refers to exclusively to an engine mounted inline with the frame, such as the Sunbeam S7. In four-stroke designs, the parallel twin is usually vertical or near vertical. An exception is the racing-only AJS Porcupine, which featured nearly horizontal cylinders, and Suzuki two-stroke, small bore, street-bikes.

Edward Turner's 1937 Triumph Speed Twin started a trend, and up to the mid-1970s four-stroke parallel twins were the most common type of British motorcycles, being produced by Triumph, BSA, Norton, Ariel, Matchless and AJS. Italian, German and USA manufacturers have also made parallel twins, but BMW have made the flat-twin their trademark, while Moto-Guzzi, Ducati and Harley-Davidson have specialised in V-twins. Japanese manufacturers still make parallel twins, particularly for middleweight bikes. USA-made parallel twins include the 1949 440 cc Indian Scout and the 1950 500 cc Indian Warrior.

Although the rise in popularity of the large V-twin motorcycle has seen the parallel twin fall out of favour, the latter retains these advantages over the former:

• A parallel twin is cheaper to make, having only one cylinder block and one cylinder head.
• Both cylinders can have the exhaust pipe exiting at the front, in the cool air stream.
• Siting of ancillaries (air-filter, carburetters, ignition etc.) is simpler.
• This simpler layout can potentially make maintenance access easier.
• The parallel twin is both lighter and shorter, allowing a lighter frame and shorter wheelbase.
• The motorcycle's centre of gravity can be sited optimally, i.e. lower and further forward.
• Provided a 270° crank is used, a parallel twin can simulate the "feel" of a V-twin.

Crank angle (360° and 180°)

All classic British four-stroke parallel twin motorcycles used a crank angle of 360°, which allowed the use of a single carburetor. (180° and 270° twins need twin carburetors). An early Kawasaki was a clone of the 360° British BSA Golden Flash; but in the 1960s, Japanese manufacturers favoured the 180° whose smoothness allowed higher rpm and thus more power. For example, the 1966 Honda 450 cc dohc 180° “Black Bomber" could outperform contemporary British 650 cc 360° twins.

Many small motorcycles of less than 250 cc use a 360° crankshaft as the vibration issue was less significant; examples includes Honda's CB92, CB160, CA72, CA77s, and CM185. Larger twins over 500 cc, such as the Yamaha's XS650 and TX750, have used 360° crankshafts, but such parallel twins tend to feature balance shafts. The Honda CB-series in the 250 to 500 cc range used 180° crankshafts. Both the 1973 Yamaha TX500 and the 1977 Suzuki GS400 featured a 180° crankshaft and a balance shaft, while the 1974 Kawasaki KZ400 used a 360° crankshaft and a balance shaft.

A 180° crankshaft engine suffers fewer pumping losses than a 360° twin, as displacement in the crankcase stays roughly constant. However, a 180° engine requires a separate ignition system, points or otherwise, for each cylinder. The 360° twins can have a single ignition system for both cylinders, with a wasted spark on each cylinder's exhaust stroke. The BMW F800 parallel twin motorcycle is a 360° design. Inherent vibration in the BMW F800 means its engine is limited to 9,000 rpm. BMW reduced the vibration using a third "vestigial" connecting rod to act as a counterbalance.

In two-stroke engines, the crank angle is generally 180° with a working cycle every 180°. Such an engine will produce fewer vibrations. An exception is the Yankee, which featured a 360° crankshaft. The Yankee's configuration, which featured separate combustion chambers for the two cylinders, should not be confused with that of a split-single.

Crank angle (270°)

A modern development is the 270° crank, which imitates the sound and feel of a 90° V-twin. A 270° engine requires a balance shaft to reduce vibration. Effectively, the 270° crank is a compromise that allows a more regular firing pattern than a 180° crank, and less vibration than a 360° crank. Also, just like a 90° V-twin (but unlike the 180° and 360°), the 270° crank never has both pistons stationary, thus aiding crankshaft momentum.

References

1. "Schnelle Motoren" (Author = Helmut Hütten)
2. Fast Bike magazine August 1995 page 21

External links

• A detailed analysis of the effect of different crankshaft offset angles on the engine balance of a straight twin engine.

• Piston engine configurations