Straight-twin engine

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 more compact, simpler, and usually cheaper to make, but may generate more vibration during operation.

Straight-twin engines have been primarily used in motorcycles, but 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.

Different crankshaft angles are used in four-stroke straight-twins to achieve different characteristics of firing intervals and engine balance, affecting vibrations and power delivery. The traditional British parallel twin (1937 onwards) had 360° crankshafts, while some larger Japanese twins of the 1960s adopted the 180° crankshaft. In the 1990s, new engines appeared with a 270° crankshaft.