Valvetrain Friction and Wear Performance with Fresh and Used Low-Phosphorous Engine Oils

Abstract Continued legislative pressure to reduce automotive exhaust emissions requires an automotive catalyst to operate at its peak efficiency up to 120,000 miles. Catalyst life is shortened by the poisoning of active sites by glazing caused by the deposition of phosphorous. The primary source of phosphorous is zinc dialkyldithiophosphate, an antiwear and antioxidant additive in engine oil. Therefore, the reduction of the phosphorous level in engine oils raises concern for increased wear of engine components. In an engine equipped with a direct acting mechanical bucket-type valvetrain, high contact stress coupled with sliding action at the cam and tappet contact makes it particularly vulnerable to wear. Motored single cam lobe valvetrain experiments were conducted to evaluate the wear protection capability of several 0.05 wt% P containing engine oils while the oil is fresh. The wear protection capability of vehicle drain samples was also evaluated to ensure adequate protection up to the point of oil change. It was observed that used oils provided significantly improved wear protection capability coupled with reduced friction. An analysis of the tappet shim surface showed that the composition of lubricant-derived protective films formed with used oils is very different than that formed with fresh oil, which may very well explain improved wear characteristics and reduced friction with used oils. KEY WORDS: FrictionWearUsed Engine OilValvetrainLow Phosphorous Oil ACKNOWLEDGEMENTS The authors would like to acknowledge Mike Riley for his assistance in providing drain oil samples, Larry P. Haack for the surface analysis of the tappet shims using XPS, and Ron Jensen and Scott McQueen for the discussion and help with the entire test. Presented at the STLE Annual Meeting, Las Vegas, Nevada, May 15-19, 2005 Final manuscript approved April 30, 2007 Review led by Cyril Migdal