Piston Ring Conformability in a Distorted Bore 960356
Some different equations to calculate the maximum deformation that a given ring can conform to, are found in the bibliography. These equations do not consider the ring end gap and ovality, gas pressure acting on it, nor the actual bore shape, but only the maximum amplitude for a given term (from a Fourier Series that describes the bore shape). A more exact prediction can be done with Finite Element tools or specific codes for piston ring simulation; those approaches are not usually carried out, except in special cases or in more fundamental studies.
Experimental measurements were carried out to verify the simple conformability criteria. Deformed shapes were produced in a static jig and areas of “non contact”, between ring and the deformed bore shapes, were measured.
Based on these measurements, a semi-empirical equation is proposed to calculate the limit of piston ring conformability. The proposed equation is simple enough to be calculated in the initial engine design phases (where the required inputs to more detailed methods are not available) or on a “day-by-day” basis.
If bore deformation surpasses the ring conformability, the percentage of ring periphery contacting the bore can be estimated, in a first approximation, by the linear regression empirically found in the experiments.