Dr.D wrote at 2014-12-24 04:56:32
Rick left the impression that the residual unbalance is due to firing impulses (which, by the way, in a 4 stroke engine occur every other crank revolution), but that is not actually the source of the balance problem.
The actual difficulty is the variable inertia of the slider-crank mechanism. When the piston is at top dead center, the crank can turn with no piston motion at all, and this his true again when the piston is at bottom dead center. On the other hand, when the piston is about 35 to 40% of the way down from TDC, the piston mass is strongly coupled to the crank. Similarly, at TDC and BDC, the connecting rod inertia is not coupled to the crank, but is later strongly coupled to the crank motion. The upshot of all this is that the effective inertia varies quite significantly through each crank revolution. This gives rise to an effective reaction torque (a d'Alembert force, if you like) that acts alternately in direction and then the other that cannot be fully balanced out.