Simulated Bearing Durability and Friction Reduction with Ultra-Low Viscosity Oils 2018-01-1802
Legislation aimed at reducing carbon dioxide emissions is forcing significant changes in passenger car engine hardware and lubricants. Reduced viscosity lubricants can reduce friction levels and are therefore helpful to manufacturers seeking legislative compliance. MAHLE and Shell have worked together to determine the crankshaft, bearing and lubricant combination which minimizes friction with an acceptable level of durability. This paper describes the results of our joint simulation studies.
MAHLE Engine Systems have developed in-house simulation packages to predict bearing lubrication performance. SABRE-M is a “routine” simulation tool based on the mobility method  curve fitting from the finite bearing theory to simulate the hydrodynamic lubrication in steady-state conditions. Whereas, SABRE-TEHL is a specialized simulation package used for performing Thermo-Elasto-Hydrodynamic Lubrication (TEHL) analysis of bearing systems. Predictions for bearing severity parameters allow more informed specification of the required bearing material and oil combination without expensive engine tests.
Shell also have an in-house bearing modelling software for conducting bearing simulations for efficient lubricant selection based on journal bearing friction performance and oil film thickness. This model is based on the short bearing approximation and accommodates the consequences of the lubricant’s shear thinning behaviour on hydrodynamic power loss.
Simulation results revealed that Shell calculation trends correlated well with MAHLE SABRE-M results. For a typical mid-size engine, optimal combinations of all the design parameters could create fuel savings in excess of 1% for both the SAE 0W-8 and prototype 0W-4 oils when simulated at conditions typified by the European NEDC and US FTP75. For highly loaded applications, the SAE 0W-8 oil was deemed to be ‘safe’ when tested against the durability criteria. The ultra-thin 0W-4 oil however, led to durability concerns arising from asperity contact. These concerns can be mitigated by following certain restrictions in engine operation and careful attention to hardware design.