Research On Lubrication Of Piston Assembly And Piston Top Fatigue Failure Under Lubrication Conditions

Piston assembly accounts a significant part of all the engine mechanical frictional losses, It is essential to carry out piston assembly lubrication study and reduce friction and wear to improve engine mechanical efficiency and reliability. This dissertation studied the lubrication mechanisms of piston assembly, which includes piston skirt-cylinder liner, piston pinbore-pin, and piston ring-cylinder liner. Piston top fatigue failure analysis was conducted.Piston temperature was measured by using temperature plugs. It was found that the piston top temperature at intake side was lower than that at exhaust side due to the intake air cooling effect. Computational fluid dynamics(CFD) simulation of air intake process was employed to analyze the cooling effect of intake air on the piston top. The heat transfer at the piston top during air intake process was obtained by CFD simulation and was used to correct the thermal boundary conditions for piston temperature analysis in order to improve the accuracy of piston temperature analysis.Piston skirt lubrication analysis was performed by coupling skirt lubrication model and piston secondary motion model, which was implemented by flexible multibody analysis, lubrication characteristic such as oilfilm pressure, asperity pressure,hydrodynamic and asperity friction power were obtained. Piston secondary motion results were also presented. The effect of piston clearance, piston pinbore offset, skirt ovality, skirt profile,skirt surface roughness on piston lubrication and secondary motion was discussed.Piston pinbore lubrication analysis was conducted by coupling pinbore lubrication model and piston assembly flexible multibody analysis, lubrication characteristic such as oilfilm pressure, asperity pressure,hydrodynamic and asperity friction power were obtained. The effect of pinbore-pin deflection, rigidness of the pin, pinbore clearance and pinbore profile on pinbore lubrication was discussed.Piston ring lubrication analysis considering cavitation was carried by coupling ring lubrication analysis and piston ring force balance analysis, lubrication characteristic such as oilfilm pressure, asperity pressure, hydrodynamic and asperity friction power were obtained. The effect of ring normal load, tangential elasticity, axial height on ring lubrication was discussed. Piston ring-cylinder liner tribology investigation was performed with bench experiment, the effect of ring normal load, frequency and stroke of reciprocating motion on friction coefficient was studied.Aiming at the piston top cracking during its development, thermo-mechanical analysis was performed, considering the effect of oilfilm on the distribution of piston side force and the force between pinbore and pin. Based on thermo-mechanical stress results. Scanning electron microscope(SEM)examining, chemical composition and metallographic phase analysis and hardness measurement were employed to study the mechanism of piston top crack origination and propagation.