Calculation of Unsteady Hydrodynamic Lubrication and Surface Contact at the Piston-Ring / Cylinder-Liner Interface 981402

Two of the objectives in developing internal-combustion engines are to keep wear and - at the same time - the related friction losses between the piston rings and cylinder liner as low as possible. To this end, optimization of the hydrodynamic conditions and designing for reduction of mixed friction have become the goals of many engine manufacturers and developers.

As part of this project, a two-dimensional computation model is developed to include modelling of four areas: gas dynamics, lubrication-oil hydrodynamics, solid-body contact, and piston-ring dynamics. The gas pressures in the ring pack are calculated either using the one-dimensional Navier-Stokes equation or the unsteady adiabatic-flow model. The hydrodynamic pressure distribution ranging from the piston-ring running surface to the cylinder wall is calculated by solving Reynold's differential equation for rough surfaces /1/. This was accomplished under the use of flow factors. Newton's equation of motion for ring dynamics is solved using the respective forces previously determined. In contrast to preceding models /2/, the integration of all differential equations of the tribological system is effected in one incremental time step /3/.

To describe the highly loaded microcontacts under study, the assumption of purely elastic distortion of the roughness peaks /2/ resulting from contact of the ring and cylinder running surfaces does not lead to adequate contact pressures. As a consequence, the calculation of radial piston-ring motion throughout the full cycle is not possible /3/. However, we overcome this shortcoming by extending the elastic microcontact pressure model to an elastic/plastic model. While calculating the hydrodynamic pressure plot using Reynold's boundary condition, the finite-element area is divided into three pressure ranges and the solution is coupled with the radial piston-ring dynamics. Within the scope of radial piston-ring modelling, the quasi-static, linear-dynamic and nonlinear-dynamic procedures for solution are presented and compared. Additionally, lubrication-oil starvation over the ring running surface as a function of time is presented using a two-zone model (hydrodynamic pressure and cavitation area) /5/.

Calculation examples of differing hone structures and variations of the related parameters demonstrate the possibilities and applications for the model. Above all, possibilites are suggested on how the hydrodynamic behavior of the lubricating film can be decisively improved with respect to the piston ring.