Straight-six engine

The inline-six engine (also referred to as an straight-six engine; abbreviated I6 or L6) is a piston engine with six cylinders arranged in a straight line along the crankshaft. A straight-six engine has perfect primary and secondary engine balance, resulting in fewer vibrations than other designs of six or fewer cylinders.

Until the mid-20th century, the straight-six layout was the most common design for engines with six cylinders. However, V6 engines became more common from the 1970s and by the 2000s most straight-six engines had been replaced by V6 engines. An exception to this trend is BMW, who have produced automotive straight-six engines from 1933 to present.