Abstract
To reduce mechanical friction, weight and vibration of piston is one of the most important subjects for automobile engines of lower fuel consumption and lower noise. The cost down and the shorter period of engine developments are also crucial subjects. Therefore, an accurate CAE technique to predict the lubrication between piston and cylinder bore, the piston strain and the slap force under running condition is required to optimize multi design criteria of mechanical friction, structural durability and vibration.
The contact surface between piston skirt and cylinder bore is under mixed lubrication where the contact load is shared by oil film and asperity contact. The contact pressure and the friction force are affected both by the microscopic oil film thickness and by the macroscopic motion of the piston. So, a coupling analysis technique of the lubrication and the dynamics problem is necessary to predict the contact behavior precisely.
We have developed a numerical analysis system of kinematic and structural vibration on engine moving components, such as crankshaft based on a flexible multibody dynamics theory. In this system, the kinematic motion and elastic vibration of each moving component were expressed using eigen modes obtained by a commercial finite element solver. The mechanical joints connecting the moving components were modeled by force elements acting as nonlinear springs and dampers.
Extending this analysis system, in this study, we have developed a coupling analysis method between lubrication and dynamics of piston. The dynamics model consisted of piston, connecting rod, crankshaft and cylinder block. The interface between the piston skirt and the cylinder bore was expressed by a force element involving a mixed lubrication model. In the lubrication model Average Reynolds Equation was applied for oil film lubrication The contact and the friction force on the interface between the piston skirt and the cylinder bore were solved numerically by finite-difference method in force element. This lubrication analysis was iteratively coupled with the flexible multibody dynamics analysis. The displacements and the velocities of the structural nodes on the piston surface were mapped to the lubrication nodes by an interpolation method, then the calculated contact force and the friction force were applied to the structural nodes at every time step. The piston ring friction was also considered by a simplified model. This calculation method was applied to real engine models in order to analyze the lubrication behavior of oil film thickness, contact forces, and the friction force on the piston skirt.
Keywords: piston, elastohydrodynamic lubrication, multibody dynamics, slap, friction