A straight-twin engine is a two cylinder piston engine that has its cylinders arranged in a single row.

Straight-twin designs are mostly used on motorcycles, but in the past they have also been used in very small cars. No current production car uses a straight-twin; 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-twin 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 lead to a working cycle every 360°. The vibrations are worse than with a similar displacement one-cylinder engine because the forces of both cylinders add up. Counteracting this, 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 engine sound of these engines is distinctively 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 a straight-twin 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's believed that all the prototypes built simply exploded during the tests.

See also: straight engine