Effects of piston elastic deformation on piston secondary motion and skirt lubrication characteristics

Abstract: Piston skirt-liner is one of the primary friction pairs influencing the friction power loss of an internal combustion engine, so it is feasible to improve the fuel economy of engines by studying the lubrication characteristics of the piston skirt-liner to reduce the friction power loss. The piston skirt lubrication is related to the piston secondary motion and the elastic deformation of the piston and liner, and there exists a strong coupled relationship between them. Therefore, it may be more reasonable to couple the elastic deformation equation of piston and liner than ignore the deformation of piston and liner, when the piston skirt lubrication and the piston secondary motion equations are solved. In order to reveal the effects of the piston elastic deformation on the piston secondary motion and skirt lubrication characteristics, the structural dynamics equations of the piston and liner were established based on the finite element modal reduction method, the piston secondary motion equation, the average Reynolds equation and the possible solid-to-solid contact equation in mixed lubrication were solved iteratively based on the parameters of a single-cylinder diesel engine, and the difference between the skirt lubrication performances with or without considering the elastic deformations of piston was analyzed. The results showed that the elastic deformation configurations of the piston skirt thrust and anti-thrust sides experienced different variations with crank angles and the deformation was significant at work stroke. Besides, the maximum deformation region of the piston skirt side was variable at different crank angles, sometimes in the middle of the piston skirt side, and sometimes at the bottom edges of the piston skirt side. It could be found that the piston secondary motion quantities including the piston deformation became bigger, when compared with those excluding the deformation of piston, especially at compress and work strokes. The eccentricity of piston at the bottom of the skirt including the piston deformation was about 1.3 times as big as those excluding the piston deformation in the region from 370 to 500°CA at work stroke. The minimum film thickness was increased at intake, compress and work strokes, with fluctuating at exhaust stroke and significant changes at work stroke. Furthermore, the minimum film thickness considering the deformation of piston were about 2 times as big as those excluding the deformation of piston at work stroke. While the total friction power loss was reduced significantly and was about 0.4 times as big as those excluding the deformation of piston at work stroke, and varied slightly at other strokes. Also, the configurations of the oil film pressure field experienced different variations with crank angles, from parabolic to saddle-shaped ones, when the deformation of piston was not considered and considered. The peak values of oil film pressure field became smaller and were about 0.5 times as big as those excluding the deformation of piston at intake, compress and work strokes. It is necessary to consider the effect of the piston deformation in order to obtain reliable solutions when the piston skirt lubrication is investigated.