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A straight-two engine, parallel twin or inline twin is a two-cylinder piston engine that has its cylinders arranged in a single row. There are two primary crankshaft configurations for this engine with a new third configuration. One has both pistons moving identically (360° offset) and is commonly called parallel twin. An inline twin or straight-two has one piston moving up as the other moves down (180° offset). The third and new concept offsets the pistons 90° from each other similar to a V-Twin or a crossplane (American) V8.

Straight-two engines regardless of configuration do not use a common crank pin for both cylinders. Even in a parallel twin each cylinder has its own crank pin, they are just set at the same angle.

Automobile use

Straight-two engine in the back of a NSU Prinz 4L

In the past, straight-two engines have been used in very small cars (eg. Microcars and City cars like the Fiat 500) 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-two engines in 360cc, 400cc, and 600cc sizes. The Z600 was produced from 1970 to 1972. From 1958 to 1971, Subaru produced the 360 with a rear mounted, rear drive 358cc air-cooled engine.

The only current production car to use an inline twin engine is the Tata Nano, announced 10 January, 2008, which has a 623 cc engine with a single balance shaft. Most cars now use at least a straight-3 because of its better power characteristics.

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 development due to unsatisfactory balance. It is believed[by whom?] that all the prototypes built simply exploded during tests.

Motorcycle use

An example of a British four stroke "parallel twin" engine, as fitted to a 1974 Norton Commando MKIIA 850 Interstate

The motorcycle world generally refers to these engines as parallel twins or vertical twins. Although the phrase vertical twin is widely used, in many motorcycles the cylinders may be tilted forward, as in the accompanying photo of a Norton. It follows that "parallel twin" is the better description.

Parallel twin engines are very common on motorcycles, eg. Edward Turner's 1937 Triumph Speed Twin and later British twins. Japanese manufacturers have also made many parallel twins, as have Italian and German manufacturers. Examples of American-made parallel twins are the 1949 440cc Indian Scout and the 1950 500cc Indian Warrior.

By far the most common parallel twin configuration is for the engine's crankshaft to sit across the frame. One notable exception to this is the Sunbeam S-series, in which the crankshaft was inline with the frame.

Crank Angle (360 & 180 degrees)

Most British four-stroke cycle parallel twins engines had a crank angle of 360°, which means that both pistons are always in the same position as each other and move in same direction. This leads to a working cycle every 360°. The mechanical balance of this design is no better than that of a similar displacement single-cylinder engine, because the forces of both cylinders are compounded; however, the advantage is that the firing is regular, with one cylinder firing each revolution of the crankshaft rather than every second revolution.

Japanese manufacturers have made four-stroke engines in both the 360° and 180° designs. The 180° crankshaft has superior primary balance at the cost of a rocking couple. Also, the 180° design has uneven firing, resulting in these engines having a distinctive exhaust note similar to 45° v-twin Harley Davidson (differences in pitch due to significantly different capacities notwithstanding). In such an engine, the left cylinder fires, then 180° later the right cylinder fires, then the engine rotates 540° before the left cylinder again fires.

An advantage of 360° engines, given their even firing order with corresponding even pulses in the intake tract, is their suitability to a single carburetor. This fact was exploited by Triumph, BSA, and Honda in the CA series.

Many small motorcycles of less than 250cc use the 360° crankshaft, examples being the Honda CB92, the Honda CB160 and its many derivatives, and the more recent Honda CM185 and its derivatives. Likewise, the Honda CA72 and CA77 models used 360° crankshafts.

Larger parallel twins, over 500cc, also use 360° crankshafts. Yamaha, in particular, has numerous models with this configuration, the XS650 and TX650 of 1970-1983 being well-known examples. Most parallel twins greater than 500cc designed after 1973 featured balance shafts, the Yamaha TX750 being an early example.

Honda models in the 250cc to 500cc range, very common in the 1960s and 1970s, used 180° crankshafts. Examples of such engines are found in the CB72 and CB77, CB250 and CB350, CB360, CB450 and CB500. The Yamaha TX500, introduced in 1973 also used a 180° crankshaft, although it featured a balance shaft. The Suzuki GS400, introduced in 1977, featured a 180° crankshaft and a balance shaft. However, the Kawasaki KZ400, introduced in 1974, featured a 360° crankshaft and a balance shaft.

In addition to reducing vibration (absent a balance shaft), the 180° crankshaft engine suffers fewer pumping losses, 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° parallel twins usually have a single ignition system for both cylinders, with a wasted spark on each cylinder's exhaust stroke.

The parallel twin engine configuration is currently used by BMW Motorrad in its F800 series motorcycles. It uses a 360° firing order, which synchronizes combustion torque and inertial torque from the crankshaft, albeit with relatively long intervals between power strokes as compared to modern inline-four motorcycle engines. This configuration produces exceptional torque, but due to the inherent vibration of a parallel twin piston configuration, is limited to 9,000 rpm. BMW's solution to the vibration problem was to install a third "vestigial" conrod to act as a counterbalance. The sound of this engine is sometimes described as "agricultural" (probably due to the parallel twin configuration of tractor engines), but vibrates much less. Peak horsepower in this design is sacrificed for high torque levels; there are no comparable motorcycles in the marketplace at this time. The choice of the parallel twin engine was likely made to produce an engine note similar to the iconic boxer twin which is the signature BMW Motorrad engine - both engines are twins with 360° firing orders.

In two-stroke cycle 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 Twingle.

In four-stroke designs, the parallel twin is usually vertical or near vertical. An exception is the racing-only AJS Porcupine, which featured horizontal cylinders.

Crank Angle (270 degrees)

A development in recent times is the 270-degree crank, pioneered by the Yamaha TRX850. The TRX used a modified version of the Yamaha TDM850 engine; but the TDMs 360 crank became a 270 crank in the TRX. A 270-degree crank imitates the sound and feel of a 90º V-twin, and there are other advantages. In contrast to the 360 and 180 parallel twins (above) the 270 crank gives a compromise that allows a more regular firing pattern than a 180-crank, and less vibration than a 360-crank. There are two extra benefits as well. Just like a 90-degree V-twin (but unlike the 180 & 360-cranks) the 270-crank never has both pistons stationary, and this gives a dynamic benefit. Also, although the firing interval is not as uneven as the 180-crank or 90-degree Vee (which both fire 1-1-0-0) the 270-crank avoids being perfectly even. This feature is said to allow better power delivery to the rear tyre, giving two fairly close power pulses followed by a "recovery gap". This "big bang" concept has been adopted for the 2009 Yamaha R1 engine, which has a 4-cylinder "cross-plane" crank. The 2009 Triumph Thunderbird 1600, which has a massive 1600cc parallel-twin engine, has adopted a 270-crank to obtain such special benefits; and it is quite probable that 270-cranks may become the norm for parallel twins in future.

See also


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