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Tribological Investigation of Piston Ring Coatings Operating in an Alternative Fuel and Engine Oil Blend

The tribological performance of piston ring coatings, including Mo, CrN and physical vapor deposited Diamond-Like-Carbon, sliding against cast iron cylinder segments was studied using a reciprocating bench friction and wear machine. The surface interaction of ring coatings with the blend of 85% ethanol I 15% unleaded gasoline (ethanol 85) fuel and energy-conserving engine oil containing molybdenum dialkyldithiocarbamate (MoDTC) was compared and analyzed. The results show that the DLC coating has the lowest coefficient of friction among all the coatings when running in a blend of engine oil and E85 fuel. The CrN coating has the lowest ring wear but produces the highest wear on the mating cylinder segment. With MoDTC in the engine oil, the wear volumes for all ring coatings are higher while the wear volumes of the cylinder segment are lower than those operating without MoDTC. Based on the surface profile and roughness measurements, the surface roughness becomes smoother and the skewness becomes more negative after the running-in process. Presented at the 57th Annual Meeting in Houston, Texas May 19–23, 2002