Re: Why are most diesel engines 6 cylinder?

There is no basic difference between gasoline and diesel engines except in 
the way the cylinders are fired - by a spark in gasoline engines and by 
heat of compression in diesels.  Probably the reason you see mostly six 
cylinder diesels is that they are cheaper to make than eight cylinders and 
are suitable for most applications in this horsepower range.

Eight cylinder diesels are manufactured and sold.  Cummins Engine, one of 
the largest diesel engine makers, makes V design diesel engines in 
6,8,12,16,and 18 cylinders, and in-line design diesels in 3,4 and 6 
cylinders.

Source: Telephone conversation with Jim Olham, Cummins Engint.