The resultant mathematical relation is given by fR=NE p/60 where p represents the
specific order of the engine. In this case, p=0.5 corresponds to half-order excitation even though the fundamental firing order of a four-cylinder engine is 2. This implies
that the occurrence of rumble is either driven by the excitation from a specific
cylinder due to sensitivity of the resonance modes to that one position, or caused by
the non-uniformity clearance problem in only one of the main journal bearings. This
question can only be resolved by performing a series of controlled engine component
dynamometer experiments using crankshafts with known journal clearances, which
is beyond the intended scope of this study and is left for future research.