Study on Oil Film Thickness of Piston Skirt Pattern Coating in a Commercial Vehicle Diesel Engine

In recent years, there has been a trend towards lower engine speeds and downsizing of diesel engines to improve fuel efficiency. This has the advantage of reducing frictional losses in the hydrodynamic lubrication condition but causes severe lubrication in the mixed lubrication condition. In order to reduce friction losses without the risk of abnormal wear or seizure, pattern coatings of the piston skirt area have been proposed. In this study, the oil film thickness between piston and cylinder was measured to investigate the effect of pattern coating on the oil film thickness. The oil film thickness between the piston and cylinder were measured by the laser-induced fluorescence method using the optical fibers embedded in the cylinder. The oil film thickness on the piston skirt was successfully measured under the engine operating conditions for the medium duty Direct Injection (DI) diesel engine. The oil film thickness for the pattern coatings was compared with that for the solid coating. It was found that the dimples created by the pattern coating on the piston skirt anti-thrust side caused thicker oil film on the piston skirt upper point in the latter half of compression stroke. Based on the relationship between oil film thickness and the piston speed/side force ratio, it was found that dimples on the piston skirt have the potential to reduce oil starvation. In particular, at measurement points with higher piston speeds, pistons with dimples were confirmed to form a thicker oil film compared to those without dimples. Furthermore, the effect of the depth of the dimple on oil film thickness was also investigated, and it was suggested that an optimal dimple depth might exist.