Transient thermo-rheological mixed-lubrication modelling of a piston ring-cylinder liner conjunction using an accelerated full-state solver

This study examines how prescribed temperature and lubricant shear thinning affect piston ring-cylinder liner lubrication over an engine cycle. A transient mixed-lubrication model combines the averaged Reynolds equation, Patir-Cheng type roughness flow factors, Greenwood-Williamson asperity contact, transient squeeze effects, and Vogel and Carreau–Yasuda viscosity relationships. The new contribution lies in quantifying the combined effects of prescribed temperature and shear thinning on film formation, load sharing, and friction energy, together with the computational gains from full-state warm starting. The model was calibrated and assessed using out-of-calibration load cases validated against experimental minimum film thickness measurements. The independent predictions achieved an RMSE of 0.144 µm, a MAPE of 13.3%, and an R 2 of 0.886. The warm-start strategy reduced the computational time by 53.1% and the number of pressure iterations by 54.3%, while changing the main physical results by less than 0.055%. The numerical simulations show that the most severe lubrication conditions occur near the firing top dead center, where the minimum film thickness decreases to approximately 0.10 µm and asperity contact becomes significant. Compared with the Newtonian assumption, the Carreau-Yasuda model predicts a thinner lubricant film, greater asperity load sharing, and higher cycle-integrated friction. Increasing lubricant temperature further reduces film thickness and shifts the contact toward more severe mixed lubrication. Findings show that thermal and non-Newtonian effects should be considered together to avoid overestimating film formation and underestimating friction and surface interaction in piston ring-cylinder liner contacts.