Straight-twin engine

A straight-twin engine (also known as an inline-twin, vertical-twin, inline-2, or parallel-twin) is a two-cylinder piston engine whose cylinders are arranged in a line along a common crankshaft. Straight-twin engines are primarily used in motorcycles; other uses include automobiles, marine vessels, snowmobiles, jet skis, all-terrain vehicles, tractors and ultralight aircraft. Various different crankshaft configurations have been used for straight-twin engines, with the most common being 360 degrees, 180 degrees and 270 degrees. The straight-twin layout is also referred to as "parallel-twin", "vertical-twin" and "inline-twin". Some of these terms originally had specific meanings relating to the crankshaft angle or engine orientation; however, they are often also used interchangeably. In the United Kingdom, the term "parallel-twin" is traditionally used for engines with a crankshaft angle of 360 degrees, since the two pistons are in the same direction (i.e. parallel to each other). "Vertical-twin" was used to describe engines with a crankshaft angle of 180 degrees, which causes the pistons to travel in opposite directions. The terms "straight-twin" and "inline-twin" were used more generically for any crankshaft angle. For motorcycles, "inline-twin" has sometimes referred to either a longitudinal engine orientation (i.e. with the crankshaft in line with the chassis) or a U-engine (tandem twin) where the cylinders are arranged longitudinally in the chassis (although the two crankshafts are actually oriented transversely). Compared with V-twin engines and flat-twin engines, straight-twins are more compact, a simpler design and cheaper to produce.