Development of a new visualization technique using photochromism for transport process of lubricating oil around the engine piston

Abstract

This study proposes and experimentally validates a new method for the visualization of oil film flow. A photochromic dye is dissolved in the oil and an arbitrary spot of an oil film is illuminated with ultraviolet light, which makes a marker in the oil film via a photochromic reaction. The basic principle of the photochromic reaction and its application to flow visualization are described. The color density of the colored solution is quantified based on the absorbance calculated from images taken before and after coloring. The results confirm that the color density is proportional to the oil film thickness. The color density changes sufficiently slowly at room temperature to make it suitable as a marker for flow visualization. The characteristics of the coloring and fading reactions are examined. The results confirm that increase in the energy density of the ultraviolet light effectively increases the color density and that the optimal energy density of the ultraviolet light can be determined from a model formula for the coloring reaction. The color fading reaction at different temperatures is measured, and the temperature dependence of a solution of spiropyran and ester oil is quantified using an Arrhenius plot. Flow visualization is conducted in a 10- μm-thick flow channel with a complex shape. The test oil flowing through this channel is colored by the focused third harmonic of an Nd:YAG laser, and the flow velocity distribution is visualized using the proposed method. Finally, for experimental validation, the proposed method is used to visualize the movement of an oil film on the piston land of an optical engine. The proposed technique can be applied to investigate the dominant route of oil consumption and the physics involved.