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

A new visualization technique using photochromism for the movement of oil film was developed and applied to an optical gasoline engine. A photochromic dye was dissolved in the oil and an arbitrary position of the oil film was illuminated by UV laser light, which makes a marker in the oil film via a photochromic reaction. The lifetime of color change by photochromism is relatively long and therefore, by tracking the movement of a marker makes it possible to visualize the movement of oil film directly. The color density was quantified based on the absorbance calculated from images taken before and after coloring in two wavelengths. Through the experimental and theoretical considerations, it was confirmed that the calculated absorbance is effective in reducing a noise originated by the color, the shape of the piston surface, the temporal variation of oil film thickness and the illuminating light intensity distribution. Furthermore, the value of the absorbance is in a very good linear relationship with the oil film thickness. This technique was applied to an optical gasoline engine and confirmed the availability of this technique. In the top land of piston surface, the oil film between the piston and cylinder liner was separated and the majority of the oil is at the piston surface and moved with the piston motion. The oil film thickness on the cylinder liner was very thin. On the contrary, at the piston skirt region, a wider region of the oil film is connected between the linear and the piston skirt. However, there are regions where the oil film between the liner and the skirt is separated by oil film rupture and the majority of the oil film is on the piston skirt. The change of the flow direction by the operating condition, i.e., the throttle condition, was able to clearly visualize, though the movement of oil film on the piston surface was very slow in normal case. The relatively fast complexed flow for opposite direction was also able to visualize by this technique.