Tribological Performance Of Piston Ring Coatings Lubricated By Full-Formulated Engine Oils With Ethanol Fuel Existing

The characteristics of piston ring materials and the formulations of engine oils are main factors influencing the tribological performance of piston ring and cylinder bore systems, which is also affected by the existence of ethanol fuels. The comprehensive and multiplex interactions among them determine the tribological performance of piston ring and cylinder bore system jointly. In this thesis, by the investigation of the tribological performances of four kinds of piston ring materials (Nitrided, CrN, B4C and WS2) lubricated by GF-3 category fully-formulated engine oil without/with molybdenum dithiocarbomate (MoDTC) friction modifier and the tribological impacts of ethanol fuels, the influences of the tribological compatibility among piston ring materials, engine oils and ethanol fuels on the tribological performances of piston ring and cylinder bore systems were studied.The tribological performances of four kinds of piston rings lubricated by GF-3 fully-formulated engine oil were investigated on a SRV high temperature friction and wear tester. Combined with surface analysis results, it can be concluded that the tribofilm which is mainly composed of calcium phosphate mostly exists on the cylinder bore surface. The tribofilm is formed via the reaction between ZDTP and calcium sulfonic detergent, which is based on the tribochemical reaction between ZDTP and ferric oxide on the cylinder bore surface. The piston ring material with higher hardness, lower roughness and better adhesion would accelerate the formation and expansion of the tribofilm, which means broader distribution and better performance.When lubricated by GF-3 fully-formulated engine oil with MoDTC friction modifier, friction coefficients of four kinds of piston rings were greatly decreased, which reached about 0.03 in the end of tribotests. Under the friction, MoO3 and MoS2 are formed by the depolymerization of MoDTC, which disperse in the tribofilm of calcium phosphate and reduce the friction. The decrease speed of friction coefficients depends on the characteristics of piston ring materials. The material with higher hardness, lower roughness and better adhesion would hasten the decreasing.The impacts of ethanol fuels on the tribological performance of CrN ring and cylinder bore were evaluated. Ethanol fuels have limited lubricities. The existence of ethanol fuels do not affect lubricating performances and tribochemical interactions of full-formulated engine oils, but will cause corrosive wears on the rubbing surfaces, which increase with the content or the acidity of ethanol fuels. Esters could be created by ethanol fuels under certain conditions, which provide good lubricating performances.