Fluid Dynamics Analysis of Bubble Generation Process in Engine Oil Based on Model Experiments

Abstract

Bubbles generated in oil pans adversely affect engine lubrication performance. For example, if air bubbles enter the lubrication system to much, they will cause bearing damage and heat exchange performance deterioration. However, the generating process of these bubbles has not been analyzed enough to be used in engine design. Therefore, as a typical case of bubble generation, observation was made with a high-speed camera in a model experiment in which a sprocket on a horizontal axis was rotated near an oil pool surface. These phenomena observed in the experiment were analyzed using fluid dynamics theory. From the high-speed photographs, three phenomena were identified as the bubble generation processes: 1) releasing air trapped in the pockets between the sprocket teeth, 2) separation of bubbles from the bottom of an oil surface valley caused by oil flow around the rotating sprocket, and 3) entrapment of air by oil droplets dropping on the oil surface. The first process mentioned above was found to be explained by a model in which liquid accumulated in a channel flows downstream when a weir is suddenly released. The second process was modeled by the combination of velocity potentials caused by gravity and circulation around the rotating sprocket. The numerical simulation based on this model met well with experiment results. Concerning the third process, the development of oil film ridge on the rotating sprocket, which becomes large oil droplets entrapping air in bulk oil reserved in a pan, was investigated. To predict the influences of design factors, fundamental equations of motion governing the phenomenon were derived. Order estimation of terms in the equations caused by centrifugal force, viscosity and surface tension was conducted. Then, the equations were solved numerically. As a result, effects of such factors as oil viscosity, surface tension, and gravitational potential on the growth of oil ridge were identified. Concerning also the third process, formulation and numerical calculation of air trapping in a cavity caused by oil droplet were conducted using surface tension mechanics. This formulation enabled us to calculate the process in which the gravitational and capillary pressures in the oil pool reduced the cavity inlet diameter of the cavity dug down by the oil drop. This theory can explain the experimental result that two oil droplets falling successively tend to generate bubbles. As a result, we improved the method of predicting bubble generation using dimensionless numbers such as Froude and Weber numbers. These studies will guide our future lubrication system design.