Temperature Effect in Minimum Oil Film Thickness Measurements in a Simplified Single-Ring Test Rig Used to Simulate the Piston-Cylinder Assembly

Abstract

As part of the study of the complex lubrication phenomena between the piston-ring and the cylinder liner, it is important to characterize the oil film thickness in view of new designs that can be implemented for the piston-ring assembly. In previous studies parametric results of different lubricants, speed, load, temperature and piston-ring surface were presented to complement on the cavitation rheological phenomena (initiation and development) and to interpret the effect of physical-chemical properties of the lubricants in oil film thickness, friction and oil film pressure. Measurements were conducted in a simplified single-ring test rig, where a steady piston-ring section of overall width 5 mm is placed under a flat surface used as a reciprocating liner, taking advantage of significantly less uncertainties when compared to engine experiments. The advantage of this layout is that it can provide abundance of results that in turn, are being simultaneously interpreted in an easier and safer way prior to engine implementation and testing. In this manner, the effect of different operating conditions is assessed to achieve solid experimental results, useful in engine tribological applications in the piston-cylinder assembly, that comply with the emission regulations of today and the near future. This presentation is focused on the effect of temperature in minimum oil film thickness measurements (MOFT) for different lubricants. An electrical method is used in this set of experiments to measure the oil film thickness (capacitance). The testing is applied for different speeds and loads so that a complete picture of the lubricants behavior can be taken and, in parallel, friction measurements are presented to assess the MOFT results for specific parts of the stroke. The results show the effect of different lubricant properties in MOFT and give an insight of the conditions of cavitation occurrence at the early parts of the stroke as temperature rises. Further processing of these results provides very useful conclusions and the combination of the lubricants behavior under cavitating conditions, can lead to innovative additives design-formulation and new lubricant properties.