Curved beam based model for piston-ring designs in internal combustion engines

Abstract

Characterizing the piston ring behavior is inherently associated with the oil consumption, friction, wear and blow-by in internal combustion engines. This behavior varies along the ring's circumference and determining these variations is of utmost importance for developing ring-packs achieving desired performances in terms of sealing and conformability. This study based on straight beam model was already developed but does not consider the lubrication sub-models, the tip gap effects and the characterization of the ring free shape based on any final closed shape. In this work, three numerical curved beam based models were developed to study the performance of the piston ring-pack. The conformability model was developed to characterize the behavior of the ring within the engine. In this model, the curved beam model is adopted with considering ring-bore and ring-groove interactions. This interactions include asperity and lubrication forces. Besides, gas forces are included to the model along with the inertia and initial ring tangential load. In this model we also allow for bore, groove upper and lower flanks thermal distortion. We also take into account the thermal expansion effect of the ring and the temperature gradient from inner diameter (ID) to outer diameter (OD) effects. The piston secondary motion and the variation of oil viscosity on the liner with its temperature in addition to the existence of fuel and the different hydrodynamic cases (Partially and fully flooded cases) are considered as well. This model revealed the ring position relative to the groove depending on the friction, inertia and gas pressures. It also characterizes the effect of non-uniform oil distribution on the liner and groove flanks. Finally, the ring gap position within a distorted bore also reveals the sealing performance of the ring. Using the curved beam model we also developed a module determining the twist calculation under fix ID or OD constraint. The static twist is an experimental characterization of the ring during which the user taps on the ring till there is a minimum clearance between the ring lowest point and the lower plate all over the ring's circumference but without any force contact. Our last model includes four sub-models that relate the ring free shape, its final shape when subjected to a constant radial pressure (this final shape is called ovality) and the force distribution in circular bore. Knowing one of these distribution, this model determines the other two. This tool is useful in the sense that the characterization of the ring is carried out by measuring its ovality which is more accurate than measuring its free shape or force distribution in circular bore. Thus, having a model that takes the ovality as an input is more convenient and useful based on the experiments carried out to characterize the ring.