This Is AuburnElectronic Theses and Dissertations

Determination Of Lubricant Quality using Maximum Bubble Pressure Method




Banerjee, Ishita

Type of Degree



Materials Engineering


A common lubricant in vehicles is Motor oil. Motor oil deteriorates in quality due to oxidation, thermal breakdown, micro-dieseling, additive depletion, electrostatic spark discharge or contamination. Reaction with oxygen causes spoilage of oil quality. Some effects of oxidation are an increase in viscosity, formation of a varnish, sludge, additive depletion and sediment formation. During the movement of machinery through which fluids flow to keep the parts moving smoothly, heat is generated. This heat causes the oil temperature to increase. Overheating could damage the lubricant. Thermal breakdown can cause breakdown of additives, release of gases, production of insoluble substances. It could also result in a decrease in viscosity. The degraded oil shows a major change in bubble interval. Surface tension may or may not change due to deterioration. Since, the effect of degradation of oil on surface tension is not that obvious. Hence, studying the changes in viscosity of fluid may prove to be more suitable to determine the quality of liquid. Surface tension and viscosity are two properties of fluids that change due to deterioration. Among the many measurement methods, this research uses the Maximum bubble pressure method (MBPM) to study the dynamic surface tension in a liquid-gas interface. Here, a simple setup is proposed to study the two fluid properties. Nitrogen gas ii is injected into different fluids of known surface tension. A new structure, in the form of “J” or inverted tube is suggested to ease the flow of bubbles after release. Water and glycerin were first studied as their surface tension and viscosity values could be determined easily. By using equations found in the literature, the surface tension and correction factors were calculated. Viscosity was calculated in a similar manner. Since the pressure is measured at the top of the capillary tube instead of inside the bubble, and due to aerodynamic and viscosity effects, the pressure is higher than the actual value. The corrected surface tension, however was quite close to the correct values. Similar results were obtained for viscosity. Clean motor oil, Valvoline 10W-30, and used motor oil of the same brand and grade were then studied. The surface tension was lower for the used oil as was the viscosity. The difference in viscosity was much higher than the difference in surface tension. Hence viscosity may be a better determining property for fluid degradation. This thesis highlights some of the existing methods for measurement of surface tension in fluids such as water, glycerin, and oil. The merits and demerits of some of the techniques are listed. A new technique using a “J” tube, was developed during this research to measure surface tension in such fluids. Results presented in this research show the consistency of this technique for such measurements. This technique has the potential for application in measurement of surface tension and viscosity in vehicle engine oil, in extreme operating conditions.