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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.

Engine developments have led to higher mechanical and thermal loads on the components, at the same time that lower friction losses are also sought. Therefore, the development of better materials and of surface treatments has received great emphasis. This paper presents the results of dynamometric engine tests with a proposed piston ring pack, composed of a gas nitrided steel top ring, a nitrided gray cast iron second ring and a normal production chrome plated oil ring. The proposed pack showed very low wear when applied to a medium duty diesel engine, besides being a cost-effective alternative to the conventional pack with moly coated and chrome plated (respectively in the top and second) rings. The proposed pack also caused very low wear on the cylinder bore, specially near the TDC, where the bore wear is usually maximum.

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.

This work compares the typical manufacturing process of cast iron piston rings with chromium or molybdenum coating with the more recent nitriding steel process. Environmental impact of the processes is estimated by their material losses, consumption of energy and hazardous waste. Despite all technological development, the nowadays production process of a typical piston ring still implies that the finished part has only 30% of the iron initially cast. A more recent design, nitrided steel piston ring, reduces substantially material losses during the part manufacturing. It also substitutes high polluter processes as chromium plating or metal spray for the lower polluter gas nitriding. Production of Nitrided Steel Rings (NSR) uses 40% less energy, needs 78% less raw material and produces almost 10 times less hazardous waste. NSR has significant lower environmental impact in comparison with the traditional Coated Iron Ring (CIR). NSR also has environmental advantages during use.