Numerical investigation of power cylinder lubrication and frictional performance considering piston elastic deformations

This study aims to predict the behavior of an oil-lubricated piston reciprocating inside a cylinder in the context of its lateral motion, and to state its contribution to total piston assembly friction. For this purpose, a detailed model was constructed and a MATLAB code was written solving secondary motion of a piston in mixed lubrication and calculating friction losses arising from skirt-liner interaction. In regard to its effects on the operation of the piston, skirt lubrication was focused on. Effects of surface finish of skirt and liner were taken into account using the modified form of Reynolds equation with pressure and shear flow factors. An asperity contact model was introduced for the effect of solid-to-solid contact of surfaces. In addition, continuity of lubricant film between skirt and liner surfaces was investigated. Reynolds flow separation approach was used to determine film rupture boundary for the two-dimensional flow of lubricant. Thermal expansion of the piston was calculated to obtain the skirt profile under steady-state engine running conditions for a known cold profile. Change in the lubricated surface of skirt during operation was taken into account by calculating the piston's elastic deformation under the effect of lubricant pressure and contact loads at each time step. Simulation results for a number of cases were presented for effects of the skirt profile. Different cases are obtained by changing barrel form, the bulge location and ovality of piston. Setting the radius at the bulge location as the nominal skirt radius, radial reduction towards skirt top and bottom were increased and reduced for the effect of barrel form only. A second set was obtained by moving the bulge location up and down while the radial reduction due to barrel form was kept constant as moved away from the bulge. The third set contains two different oval profiles with increased and reduced radial drops in the circumferential direction while the barrel profile on the plane of symmetry which is perpendicular to pin axis was kept constant. Resulting friction losses were compared and used to suggest an alternative skirt profile for improved lubrication performance. Furthermore, oil viscosity which has transverse effects on hydrodynamic lubrication was selected as another focus design parameter. The model was tested to be used as a tool to predict an optimized viscosity for its effects on viscous shear and solid-to-solid contact behavior. These analyses were carried out with two different inlet oil supply conditions. 20 µm oil thickness available at the leading edge was assumed for partially-flooded inlet whereas 40 µm was taken for fully-flooded condition. Generally it was found that critical period in terms of frictional loss and solid contact started at around 40°CA after FTDC. Boundary friction was more dominant in partially-flooded inlet cases whereas hydrodynamic losses were higher for fully-flooded inlet ones, as expected. In partially-flooded inlet analysis, minimum and maximum frictional loss values were found for the change in oval form of the skirt. It was also seen that baseline oil type can be used since lower viscosities results in increased power loss and higher one offered negligible benefit. In the end, a skirt profile was suggested with the combination of three parameters. However, the friction mean effective pressure was found to be higher than the value obtained by only changing the oval form. Therefore, it was concluded that effects of changes in skirt profile parameters were not linear for the given engine operation condition with the partially-inlet. A reduction in oval form alone could be more beneficial in terms of frictional power loss. Fully-flooded inlet resulted in significantly less frictional power loss due to reduced solid contact. Barrel form had a transverse effect in this case compared to partially-flooded inlet. In addition, it was seen that reduction in power loss with increased oil viscosity was not negligible this time. Combined effect of skirt profile parameters and oil type were observed to give the best frictional performance for the suggested case with a fully-flooded inlet. In conclusion, this study presented a series of recommendations for selected design parameters defining skirt profile and lubricant viscosity in order to achieve an improved frictional performance of a medium-duty diesel engine piston. A comprehensive model for skirt lubrication and piston secondary dynamics were built and used for case studies. Keeping in mind that the design process of an ICE power cylinder requires an integrated analysis of any design parameter with its effects on the performance of other important components such as rings as well, the resulting model can be a base tool to be developed further for use in more advanced analysis.