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A straight-twin engine, also known as straight-two, inline-twin, vertical-twin, or parallel-twin is a two-cylinder piston engine which has its cylinders arranged side by side and its pistons connected to a common crankshaft. Compared to V-twins and flat-twins, straight-twins are compact, simple, and inexpensive to make, but generate more vibration during operation.
Straight-twin engines have been widely used in motorcycles. The terms parallel-twin and inline-twin vary in definition in motorcycle applications and may apply to the engine configuration in general, the orientation of the engine in the frame, or the crankshaft angle used in the engine. Different crankshaft angles are used in four-stroke straight-twins to achieve different characteristics regarding vibrations and power delivery.
Straight-twin engines are also used in automobiles and in powersports applications. Automobiles with straight-twin engines are usually very small and include city cars and kei cars. Recent examples of cars with straight-twin engines include the Tata Nano and Fiat Group automobiles using the TwinAir engine. Powersports applications include use in outboard motors, personal water craft, all-terrain vehicles, snowmobiles, and ultralight aircraft.
- 1 Advantages and disadvantages
- 2 Construction
- 3 Motorcycle use
- 4 Automobile use
- 5 Marine engine use
- 6 Aviation use
- 7 Other uses
- 8 References
- 9 External links
Advantages and disadvantages
Straight-twins have the advantage of being compact, relatively simple, and cheap to make in comparison to V- or flat-twins. As with any twin cylinder engine, they have an imbalanced firing action which causes vibration and inline-twins also suffer further from torsional torque reactions and vibration.
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 vintage British straight-twin motorcycle engines, such as Triumph, BSA, Norton and Royal Enfield, had two main bearings, the exception being AJS/Machless, which used a third, center main bearing.
Honda straight-twin engines, which began appearing in the late 1950s, had four main bearings. Subsequent engines had four or occasionally three main bearings, ball bearings being better than shell bearings for this engine configuration.
In motorcycles, as with cars and other vehicles, the terms "parallel-twin", "inline-twin", "vertical-twin" and "straight-two" are used. The term "parallel-twin" is sometimes used with the special meaning that the crankshaft is transverse across the frame, while "inline-twin" would mean that the cylinders are arranged front to rear, in line with the direction of travel. This special meaning for "inline" has been used for motorcycles with a longitudinal crankshaft, such as the Sunbeam S7, and for tandem twins, with a transverse pair of crankshafts, but the cylinders arranged longitudinally, one in front of the other. The term "parallel twin" is also sometimes used to refer specifically to a four-stroke straight-two engine with 360º firing intervals, causing the pistons to travel parallel to each other. Other times, "parallel-twin", "inline-twin" and the other variants are used interchangeably and treated as equivalent.
Edward Turner's 1937 Triumph Speed Twin started a trend, and up to the mid-1970s four-stroke 360º parallel-twins were the most common type of British motorcycles, being produced by Triumph, BSA, Norton, Ariel, Matchless and AJS. Italian and German manufacturers have also made parallel-twins, as had American manufacturer Indian, whose parallel-twins included the 1949 440 cc Indian Scout and the 1950 500 cc Indian Warrior. BMW and Japanese manufacturers still made them as of 2010, particularly for middleweight bikes.
In four-stroke designs, the parallel twin is usually vertical or near vertical. One exception is the only parallel-twin to win a 500cc Grand Prix, the AJS E-90 Porcupine of 1949, which had nearly horizontal cylinders.
Advantages in motorcycle use
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, carburettors, ignition, etc.) is simpler.
- This simpler layout can potentially make maintenance access easier.
- The parallel twin is both lighter and narrower, allowing a lighter frame and shorter wheelbase if placed across the frame, or a narrower frame if placed in line with the frame.
- The motorcycle's centre of gravity can be sited optimally, i.e. lower and further forward.
- Provided a 270° crank is used, a four-stroke parallel twin can simulate the "feel" of a four-stroke V-twin.
There are three main crankshaft configurations for this engine: 360°, 180°, and 270°: There are minor differences in the applications for four stroke and for two stroke engines, largely pertaining to ignition intervals. For example, the 360 twin is the natural configuration for a two cylinder four-stroke engine, since four piston strokes add up to 720°. The below will mostly be concerned with four-stroke engines.
- 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. The firing order is offset, so that cylinder 2 fires 360 degrees after cylinder 1, and 360 degrees later cylinder 1 fires again at 720 total degrees, the beginning of another four-stroke cycle.
- 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. This is because cylinder 2 fires 180 degrees after cylinder 1, and cylinder 1 does not fire again for another 540 degrees - always adding up to the 720 degrees of rotation for a four-stroke cycle.
- In a 270° engine, one piston follows three quarters of a rotation behind the other. This results in a mixture of the imbalances in the first two types and yields firing intervals identical to a 90° V-twin. Firing order here is that cylinder 2 fires 270 degrees (3/4 of a rotation) after cylinder 1, and cylinder 1 fires again 450 degrees (one and a quarter rotations) after cylinder two, again at a total 720 degrees and the beginning of the next cycle.
360° and 180°
From the 1930s, following the work of Val Page, most British four-stroke parallel-twin motorcycles used a crank angle of 360°, which allowed the use of a single carburettor because 180° and 270° twins need twin carburettors, as did an early Meguro was a copy of the 360° British BSA A7. However, 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° parallel-twin “Black Bomber" could challenge 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. The 1978 to 1984 Honda CB 250 N and CB 400 N are 360° designs, too; later Honda straight twins from 1993 onward until today are, again, 180° designs.
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.
A modern development of the straight-two engine is the 270° crank, which imitates the sound and feel of a 90° V-twin, but requires a balance shaft to reduce vibration. Effectively, the 270° crank is a compromise which allows a more regular firing pattern than a 180° crank and less vibration than a 360° crank. As with a 90° V-twin, the pistons in a 270° inline twin engine are never both stationary at the same time, thereby reducing the net momentum exchange between the crank and pistons during a full rotation. The oscillating momentum manifests itself as an oscillating crank rotation speed, which, when paired with a driven-wheel rotating at the more steady road speed, will introduce an oscillating torque in the drivetrain and at the tyre contact patch.
Phil Irving undertook to minimise this oscillating torque, and for one particular connecting rod to stroke ratio, arrived at an optimal separation of 76° (294°), instead of the 90° (270°) described above. The optimum for two pistons is found when one piston is travelling fastest at the same time the other has stopped; maximum piston speed occurs when the connecting rod and crank throw are at right angles, not when the crank throw is at 90° to the cylinder bore. This minimisation of so-called inertial torque was also one of the goals Yamaha achieved with its "cross-plane" R1 engine. Note that in neither case was the oscillation completely eliminated, only reduced significantly.
In two-stroke engines, the crank angle is generally 180°, which gives two power strokes in each revolution. This configuration vibrates at twice the frequency but half the amplitude of a single cylinder engine of the same capacity.
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.
Engine in line with frame
The inline-twin engine design has been used often during the history of motorcycling for both two-stroke and four-stroke engines. Examples include the Dresch 500 cc Monobloc and the Sunbeam S7 and S8.
Although mounting the engine in line with the frame allows for a motorcycle as narrow as a single cylinder engine, they also create a longer engine. A significant disadvantage for air-cooled engines is that the rear cylinder runs hotter than the front cylinder. For motorcycle racing purposes, they minimise the front area of the engine and chassis allowing for a more aerodynamic and narrower front profile equivalent to a single cylinder vehicle.
Inline-twins were used in the early days of motoring in models such as the 1898 Decauville Voiturelle which used a pair of cylinders taken from a de Dion model mounted fore and aft and positioned below the seat.
In 1955, engineer Aurelio Lampredi designed an experimental straight-twin cylinder Formula One engine on the theory that it would provide high levels of torque for tight race circuits. The result was the 2.5-liter Type 116 prototype. Upon testing, it vibrated so much that it broke the test bench. The engine was never used in a racing car. 
Straight-twin engines have been used in very small cars, e.g. microcars, kei cars, and city cars such as the Fiat 500 and 126, NSU Prinz, and Mitsubishi Minica. From 1967 to 1972, Honda produced the N360 and its successors N400 and N600 with straight-two 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.
Straight-twin engines currently used in production cars include the 623 cc engine used in the Tata Nano, and the TwinAir turbocharged 875 cc engine used in the Fiat 500 TwinAir, the Fiat Panda TwinAir, the Fiat Punto TwinAir, the Lancia Ypsilon TwinAir, and the Alfa Romeo MiTo TwinAir.
Marine engine use
From the 1950s, manufacturers of outboard motors had settled on the use of the basic inline engine design, cylinders stacked on top of each other with the crankshaft driving the propellor shaft. An experimental engine used two inline engine blocks joined in order to make a square-four engine. 
Inline-twin engines, such as the Hirth 2704 are commonly used in Ultralight, single seat gyrocopters and small homebuilt aircraft  originally sourced from snowmobiles. Another popular inline-twin two-stroke engine was the Cuyuna 430-D, also a modified snowmobile unit which produced 30 hp (22 kW). More modern ultralights tend to use engines such as the Rotax 503 or Rotax 582 which are designed for the purpose. 
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