An efficient piston design methodology including secondary dynamics and elastohydrodynamic lubrication.

摘要

A computationally efficient piston design methodology has been developed which can be used in routine piston design. The methodology is composed of an analytical method to simulate piston secondary dynamics and piston-bore contact for an asymmetric half piston model, including elastohydrodynamic (EHD) lubrication, and surface roughness at the bore-skirt interface. A fast and accurate piston EHD analysis is used based on a finite-difference formulation. The oil film is discretized using a two-dimensional mesh. It is not necessary to generate a fluidity matrix, as is the case with any finite element approach. The Reynolds' equation is solved using a successive over-relaxation algorithm which improves the efficiency of the solution without loss of accuracy. The analysis includes several important physical attributes such as bore distortion effects due to mechanical and thermal deformation, inertia loading and piston barrelity and ovality.;A Newmark-Beta time integration scheme, combined with a Newton-Raphson linearization, calculates the dynamic piston motion. The EHD behavior is coupled with an accurate contact algorithm at the piston-bore interface as well as the piston secondary dynamics. Surface roughness characteristics determine the type of lubrication regime in which the skirt---bore interface operates. It also influences the friction and noise. The Greenwood and Tripp surface roughness model is used to model the roughness of the piston and the liner.;A fast and sufficiently accurate noise prediction algorithm has been also developed to predict the piston slap noise including engine block dynamics. Results of a parametric design study demonstrate the importance of coupling among skirt friction, piston slap noise, lubrication and secondary motion. The effects of some piston design parameters such as piston pin offset, clearance, surface roughness, piston barrel and oval design and location of cylinder peak pressure are investigated at different engine operating conditions. Finally, an optimization study has been performed to design a piston for low friction, noise and scuffing under three different engine running conditions.