Straight-6

The straight-6 (also inline-6, I-6, or I6) is an internal combustion engine with six cylinders aligned in a single row. The name slant-6 is sometimes used when the cylinders are at an angle from the vertical.
Straight-6 engines have perfect primary and secondary balance and require no balance shaft. Usually a straight-6 was used for engine displacements between about 2.5 and 4.0 L. It was also sometimes used for smaller engines but these, although very smooth running, tended to be rather expensive to manufacture and they where inevitably physically longer than alternative layouts. The smallest production straight-6 was found in the Benelli 750 Sei motorcycle, displacing 747.7 cc. The largest are used to power ships and have displacements of 1000 L or more.
Straight-6 engines were historically more common than V6s, mainly because the length of such engines was not such a concern in rear wheel drive vehicles but also because V6s (unlike the crossplane V8) were difficult to make smooth-running. The widespread use of front-wheel-drive and transverse ("east-west") engine configurations in smaller cars saw that the shorter engine length of the V6 became highly desirable, and these days most six-cylinder engines are made in the V configuration.