Investigation of the Influence of Different Asperity Contact Models on the Elastohydrodynamic Analysis of a Conrod Small-End/Piston Pin Coupling

Bearings represent one of the main causes of friction losses in internal combustion engines, and their lubrication performance has a crucial influence on the operating condition of the engine. In particular, the conrod small-end bearing is one of the most critical engine parts from a tribological point of view since limited contact surfaces have to support high inertial and combustion forces. In this contribution an analysis is performed of the tribological behavior of the lubricated contact between the piston pin and the conrod small-end of a high performance motorbike engine. A mass-conserving algorithm is employed to solve the Reynolds equation based on a complementarity formulation of the cavitation problem. The analysis of the asperity contact problem is addressed in detail. A comparison between two different approaches is presented, the former based on the standard Greenwood/Tripp theory and the latter based on a complementarity formulation of the asperity contact problem. Differently from the Greenwood/Tripp method, the complementarity formulation needs only one parameter related to the roughness of the mating surfaces to be set and allows a solution to be obtained without the need of an iterative procedure thus avoiding convergence issues. Similar results are obtained comparing both the approaches used to solve the asperity contact problem. However, such an agreement is only reached after an appropriate calibration of the model input data is performed; a remarkable sensitivity of the results obtained using the Greenwood/Tripp model to the adjustment parameters is found and highlighted. Finally, a full model validation is performed by comparing the results obtained adopting the proposed algorithm, using the implementation of the Greenwood/Tripp asperity contact model, with those obtained adopting the commercial software AVL Excite Power Unit.