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A straight-two engine or parallel twin is a two cylinder piston engine that has its cylinders arranged in a single row. It is often improperly referred to as a "straight twin". A true straight twin engine uses a common crank pin for both cylinders; engines with separate crank pins for each cylinder are more properly called inline or straight two cylinder engines, abbreviated as "I-2".

Straight two cylinder designs are mostly used on motorcycles, but in the past they have also been used in very small cars. No current production car uses an I-2 engine; even the smallest displacement cars now use at least a straight-3 because of its superior vibration characteristics.

Most of the British four-stroke cycle straight two cylinder engines had a crank angle of 360°, which means that both pistons have to be in the same position and move in same direction all the time. This leads to a working cycle every 360°. The mechanical balance of this design is no better than that of a similar displacement one-cylinder engine, because the forces of both cylinders add up. The advantage is that the firing is regular, with one cylinder firing each revolution of the crankshaft.

Japanese motorcycles, with the exception of Yamaha, use a crank angle of 180°. This leads to fewer vibrations but uneven firing. The sound of these engines is distinctly different.

With the two-stroke cycle, the crank angle is generally 180°, and a working cycle every 180°. Such an engine will produce fewer vibrations.

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 that all the prototypes built simply exploded during the tests.

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Piston engine configurations
Straight Single, 2, 3, 4, 5, 6, 8, 9, 10, 12, 14
V 2, 4, 5, 6, 8, 10, 12, 16, 20, 24
Flat 2, 4, 6, 8, 10, 12, 16, H
W 8, 9, 12, 16, 18
Other inline H, VR, Opposed, U (Square), X
Other Hemi, Radial, Rotary, Pistonless, Deltic, (Wankel)

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Thermodynamic cycle