Predictive modeling of piston assembly lubrication in reciprocating internal combustion engines

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Xu, Huijie

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The influence of piston assembly lubrication on the reciprocating internal combustion engine performance has received considerable attention for over halfcentury. An in-depth understanding of piston assembly friction and cylinder wear is crucial for achieving a better fuel economy and higher durability engine design. Early studies show hydrodynamic lubrication theory is applicable to the interface of piston assembly and cylinder liner throughout most of the piston middle stroke. However, when the piston motion ceases near top dead center (TDC) or bottom dead center (BDC) of the stroke, the piston velocity is not adequate to establish a hydrodynamic lubrication action. Lubricating films become very thin and contact between the surface asperities on the ring and the liner will support part of the piston ring restoring force. Therefore, wear on the cylinder liner surface may occur in the vicinity of TDC and BDC. Severe surface wear could affect the liner-ring sealing performance and result in excessive gas blow-by and fuel consumption. The objective of this dissertation is to develop a complete mathematical and computational model to predict the piston assembly friction loss in terms of the piston assembly design parameters and cylinder liner surface topography. Piston assembly experiences all three lubrication regimes including hydrodynamic, mixed and boundary lubrication. In order to simplify modeling, early studies usually considered either a full film hydrodynamic lubrication described by Reynolds equation, or a mixed film lubrication described by average Reynolds equation. While our model is based on the real surface interactive between piston assembly and cylinder liner, the latest tribology theory and effective numerical approach have been applied to model piston assembly friction problem. An integrated friction model over three lubrication regimes was developed based on both quasi-static and dynamic equilibrium conditions of the piston assembly. The new model was verified by experimental data with specified pressure and velocity boundaries. Finally, the friction characteristics of a rotating liner engine (RLE) design was investigated as an extension of the conventional piston assembly friction model.