Analysis of circumferential oil film thickness for piston ring in twin-rotor piston engine under differential velocity motion of combined annular cylinder

Abstract

In this paper, a novel type of twin-rotor piston engine (TRPE) is proposed, and the circumferential oil film thickness for the piston ring in TRPE is analysed. Different from the integrated cylinder of a typical engine, the combined annular cylinder (CAC) of TRPE has unique structural composition and differential velocity motion characteristic, making the tribological performance more complicated. The main purpose of this paper is to reveal and explain the special tribological performance of TRPE, which has never been studied. Firstly, the special structural composition of CAC is introduced and the relative velocity between the piston ring and CAC wall at different circumferential positions of the piston ring is analysed. Then, based on the Greenwood-Trip asperity contact theory, radial and axial quasi-static equilibrium equations of the piston ring are both derived. The circumferential oil film thickness is calculated by solving the equilibrium equations and the two-dimensional average Reynolds equation within a cycle. Results indicate that there are significant differences in circumferential oil film thickness of the piston ring due to the special structure and motion of CAC, and the differences become greater as the output shaft speed increases. A long-time engine reverse towing experiment shows an obvious uneven wear phenomenon of the piston ring and CAC wall, which well validates the simulation results of the circumferential oil film thickness. The research work can be used as the basis of equal-wear design for the piston ring in TRPE with the help of surface texture technology, thereby greatly reducing the wear loss.