Mixed Lubrication and Roughness Surface Effects Application to Piston Rings 2007-01-1246

The mixed lubrication phenomenon is becoming a key issue for reliable and low friction engine design. Three main reasons can explain this fact. The increase of the engine torques at low speed has generated very low oil film thicknesses in lubricated devices. For the last ten years car manufacturers have decreased the viscosity of the oil in order to reduce friction and the oil temperatures on lubricated components have increased along with the specific power increase.

Two kinds of approach, the global and the local approach, must be considered to analyze the mixed lubrication and the lubricated component behavior.

From a global point of view, the various mechanisms to consider to get a realistic modelling of mixed lubrication are considered.

From a local point of view, the modelling of the asperity contacts between antagonist surfaces responsible of the mixed lubrication is detailed. Commonly, a Gaussian type of distribution is used for asperity summit distribution. However, this kind of distribution has a significant influence on the load generated by direct contact, on the friction and heat generated particularly at low contact pressure. In order to show the importance of the asperity summit distribution, at the transition between hydrodynamic and mixed lubrication, one example of modelling is provided. This example refers to piston ring lubrication. This paper leads to the conclusion that in spite of the effort required to introduce contact models in mixed lubrication, it does represent an important contribution for reliability analysis.