Effect of Surface Irregularities of Piston Ring and Sleeve Materials in High-Speed Reciprocating Test

The reciprocating frictional test is a common approach for screening the materials of the piston and sleeve of an automobile engine. The frictional speed of this test is, however, limited mainly by the vibration of test apparatus due to the absence of damping factors in engines. Considering that the frictional velocity between the piston and sleeve reaches around 20 m/s, common test conditions at less than 2 m/s are not sufficient to understand the real phenomena at a frictional interface. We therefore developed a high-speed reciprocating test apparatus that can operate at a much higher speed range and examined two materials used for piston rings and sleeves. For the piston ring material, nitrided SUS440C was used. Plates were made of centrifugal cast iron FC250 or cast aluminum AC2B, which were coated with Nikasil. The experimental results showed that the lubrication regimes of the two plate materials were different even at the same reciprocating speeds. In order to examine the results, we numerically calculated the frictional characteristics based on both mixed and elastohydrodynamic lubrication models. In this analysis, surface roughness and the viscosity characteristic of the lubricant were taken into account as key factors. Whereas the calculation results fit the measured result for FC250, on which surface roughness is dominant, the difference between the calculated and measured values were significant for Nikasil coating, on which waviness due to substrate deformations is dominant. This study not only evaluated the mixed lubrication model for the reciprocating motion, but also validated the importance of developing test apparatus that can operate at a higher speed range, closer to that of real engines.