The oil control ring is the most critical component for oil consumption and friction from the piston system in internal combustion engines. Three-piece oil control rings are widely used in Spark Ignition (SI) engines. However, the dynamics and lubrication of three piece oil control rings have not been thoroughly studied from the theoretical point of view.
In this work, a model was developed to predict side sealing, bore sealing, friction, and asperity contact between rails and groove as well as between rails and the liner in a Three Piece Oil Control Ring (TPOCR). The model couples the axial and twist dynamics of the two rails of TPOCR and the lubrication between two rails and the cylinder bore. Detailed rail/groove and rail/liner interactions were considered. The pressure distribution from oil squeezing and asperity contact between the flanks of the rails and the groove were both considered for rail/groove interaction. A mixed lubrication model with consideration of shear-thinning effects in multi-grade oils was applied for rail/liner lubrication. The interaction between the rails and expander/spacer was simplified as point forces. Design parameters of rails and expander as well as worn patterns of the running surfaces and flanks of two rails and the groove were taken into account in the model.
Then, the model predictions were illustrated using a standard TPOCR in Sl engines. Dynamics of the rails, side sealing, bore sealing, and friction of TPOCR under different engine speeds and different design parameters were studied. At low engine speed, sudden variation of friction force can lift up the rails at the inside corner and may result in oil leak through rail/groove interface. At high engine speed, it is the sudden change of the inertia force at mid-stroke that may result in oil leak through rail/groove interface. Dynamics of the rails, specifically dynamic twist of the rails, was also found to have significant effects on the lubrication between rails and the liner by changing the widths of the convergent regions on the rail running surface where the hydrodynamic pressure is generated. As a result, oil transport, friction, and asperity contact between rails and the liner are also affected by the dynamic twist of the rails. Simple studies were also made to show the effects of expander ear angle, ring tension and the difference in oil transport between worn and new TPOCR.
Asperity contact between rails and the liner as well as between rails and the groove were also studied and the results show consistency with the worn profiles of the rails.