Browse Publications Technical Papers 2018-01-1806
2018-09-10

Prediction of Lubricant Performance in an EHL Valvetrain Simulation Using an Equation of State and Detailed Rheology Characterization Approach 2018-01-1806

With the continued CO2 reduction challenges for automotive engines, it becomes necessary to minimize friction losses.
Many studies in the past have used generic equations of state to account for changes in density, neglecting the compressibility difference between fluids, whereas this study used accurately measured equations of state, for each of the oils. Density change is generally only accounted for in terms of mass conservation, an effort has been made here to develop a link between viscosity characteristics and density, by separating molecular density and molecular interaction contributions rather than using an all-encompassing pressure-viscosity coefficient. Several studies have not accounted for all physical aspects acting within an elasto-hydrodynamic contact, because of the difficulties associated with model convergence. Sub-models were carefully selected between open form, closed form, simplistic and advanced to account for fluid rheology, thermal and surface considerations whilst achieving model convergence.
A detailed quasi-steady EHL model was developed for the counter-formal contact.
In the first instance the model was validated against film thickness measurements obtained from a ball-on-disk EHD rig for the same six oils that will be assessed on a valvetrain. The results were accurate and encouraging.
As expected, the higher grade oils provided the thickest fluid film. However, for a given viscosity grade, the lubricant change in density as a function of pressure did play a role. It was found there were two phenomena counteracting one another. The increase in dynamic viscosity due to increased density would act to increase film thickness and the ‘peak shift’ effect acts to reduce film thickness. The peak shift is a subtle distortion in the pressure profile along a contact which occurs at high entrainment conditions and effectively reduces the integrated pressure, resulting in a thinner film. The peak shift is more pronounced in high entrainment operation with more compressible fluids.

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