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ok, continue. now let's consider our three planes.
in the x plane our pistons are moving. we have a flat-6 motor with three pistons on each side. normally with an inline 3, we'd have a rocking couple as a result of the firing order & odd number of cylinders (the forces would attempt to 'rock' the crank fore & aft). this is precisely balanced by the other bank of cylinders, so a flat 6 is a very well balanced engine to begin with. as a result, heavy counterweights are not necessary.
as for the y plane? since there are no counterweights, things are pretty stable.
combine these factors along with the fact that we have seven main bearings, & things are looking nicely balanced & rigid insofar as the engine configuration is concerned.
now we get into torque. every time a piston fires, it pushes down on a rod journal. this force actually deflects (bends) the crankshaft a little bit, & the crank snaps back. this occurs at the natural frequency of the crankshaft. since the crankshaft is only spinning one way, it isn't possible to counterbalance this force. (FYI, some engines use balance shafts, but they only serve to reduce vibration, NOT do un-do this twisting moment). in engines where the natural frequency of the crankshaft is similar to the frequency of the 1st, 2nd or 3rd order harmonic created at any engine speed between idle and redline, a harmonic balancer is used. it is tuned to the natural frequency of the crank, and the elastomer core of the balancer is designed to absorb this frequency. this prevents an 'excited harmonic state' from occuring at the NF of the crank.
part 2 done, more to come.....
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