The Effect of Nonlinear Viscoelasticity of Lubricants on Piston Ring Lubrication

In recent years, engine oils have contained various kinds of additives such as high molecular-weight polymers to improve their performance. Such lubricants exhibit viscoelastic behavior, but the lubricants were assumed to be Newtonian fluids in most of the previous investigations on piston ring lubrication. The objective of this paper is to theoretically clarify the effect of nonlinear viscoelasticity of lubricants on piston ring lubrication by introducing a nonlinear four-element viscoelastic model which consists of two nonlinear dashpots and two nonlinear springs. Numerical solutions show that the effect of viscoelasticity reduces the frictional force and the film thickness. In addition, the phase of the variations of the frictional force and the film thickness of viscoelastic fluids is delayed compared with that of Newtonian fluids.