Straight-6 four-stroke cycle engines are in perfect primary and secondary balance and require no balance shaft. They are in primary balance because the two ends of the engines are mirror images of each other and the cylinders move up and down in pairs, one on the compression stroke and the other on the exhaust stroke. Piston #1 balances #6, #2 balances #5, and #3 balances #4, canceling the end-to-end rocking motion that would otherwise result. (This does not apply to two-stroke cycle engines.) Secondary imbalance occurs in straight-4 engines because the two pistons on the upper 180 degrees of the crankshaft rotation move faster than the two pistons on the lower 180 degrees, creating an unequal motion. Straight-6 engines have cranks at 120 degrees to each other, so the differences in speed on different parts of the crankshaft rotation are offset by the changing number of pistons on each portion.
Diesel straight-6 engines
The straight-6 in diesel form with a much larger displacement is commonly used for industrial applications. These include various types of heavy equipment, power generation, as well as transit buses or coaches. Virtually every medium-duty to large over-the-road truck employs an inline six diesel engine. Its virtues are superior low-end torque, very long service life, smooth operation and dependability. On-highway vehicle operators look for straight-6 diesels, which are smooth-operating and quiet. Off-highway applications such as tractors, marine engines, and electric generators need a motor that is rugged and powerful. In these applications, compactness is not as big a factor as in passenger cars, and reliability and maintainability are much more important concerns.
As with everyday passenger vehicles, the smooth running characteristics of the straight-6 engine are what make it desirable for industrial use. In addition, a straight-6 engine is mechanically simpler than a V6 or V8. It has only one cylinder head and half as many camshafts as a V engine. The fact that the the straight-6 is the simplest engine that is in both primary and secondary balance mean it can be scaled up to very large sizes without causing excessive vibration, and the fact that most of the engine components and accessories can be conveniently located along both sides, rather than on top of or underneath the cylinders, means that maintenance is easier than on a V-type engine.
Just some stuff from wiki about the great advantages of I6 engines. A 5 cylinder engine is flawed from the begining since it is basically missing a cylinder.
