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Increased engine loads, coupled with low friction rings, renew the attention for predicting the maximum bore deformation which a given ring design can conform to before losing contact. The MIT developed “Software of Ring Design Tools” (RDT) code was used to verify the different published ring conformability criteria by progressively increasing the bore deformation until the model predicted lack of contact. For simple order deformations, the semi-empirical Tomanik criterion was found to correlate with the model predictions. However, as expected, more realistic deformations combining different harmonic orders were able to cause lack of conformability, even with the individual order amplitudes being lower than the criterion limit. In search of a more comprehensive criterion, an automated model ran hundreds of thousands of deformation cases in combination with relative ring angular positions looking at conformability.

Low viscosity combined with appropriated additive technology is one of the main paths to reduce friction on Internal Combustion Engines. Japan is on the cutting edge of low viscosity oils, having already available SAE 0W-8 in the market. On the other hands, in emergent countries like Brazil, SAE 15W-40 is still used in some passenger cars while the Japanese origin car brands use SAE 0W-20. Lubricant friction additives type also differs depending on the original equipment manufacturer (OEM) origin, and the Japanese ones usually containing high amounts of the Molybdenum type. In this paper, some of the advantages and challenges of using low viscosity oils are discussed and emphasis is given in the friction reduction obtained with the synergic effects of the right choice of additives components type and the material/coating used in the engine parts. Ring-liner rig and floating liner engine tests comparing different oils will be presented.

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.