In Reply to: A good question merits a good answer... posted by J.D. on October 24, 1999 at 19:23:34:
: Inline/Straight Six:
: The I-6 is one of the "smoothest" engine designs. The layout of the engine naturally counteracts all vibration. For some reason though, the 4.0 L straight six used in the Jeep Grand Cherokee is far from smooth. Leave it to Daimler-Chrysler to screw up an essentially smooth engine.
: In addition to smoothness, I-6s, as a rule of thumb, are often torqueir than an equal sized flat- or V-6. Additionally, they allow for easy packaging in the long, narrow hood of cars like the Z3.
: BMW goes so far as to counter-wieght the camshaft lobes! They want to make sure that the essentially smooth I-6 stays smooth!
Inline-6 engines are so smooth because the primary and secondary forces are fully balanced in this type of engine. Quoting from "The Design and Tuning of Competition Engines":
"The primary forces are fully balanced in the inline 'six'. As two pistons come to rest at each one-sixth of a revolution, the primary vibratory force has a frequency of three times the rpm and the secondaries have twice this frequency. The latter, however, are also balanced except for a minor unbalanced third-order frequency, which can be disregarded."
The same thing goes for all "multiples" of inline-6 engines, e.g. the V-12. Each bank of 6 cylinders in a V-12 is basically an inline-6, so each bank is individualy balanced -- the whole motor is balanced, too.
For anyone who is really interested in why different cams, engine layouts, etc. affect an engine's performance, smoothness, fuel consumption, etc., I highly recommend "The Design and Tuning of Competition Engines" by Philip H. Smith and David N. Wenner. I picked it up at Border's for $27.