Friction and Wear of Low-Phosphorus Engine Oils with Additional Molybdenum and Boron Compounds, Measured on a Reciprocating Lubricant Tester

Abstract

Previous research has already extensively characterized the effects of a variety of engine oils on engine performance. However, low-phosphorus engine oils, which are of growing importance due to the reduction of automotive emissions, have not been studied much. This paper sets forth the results of an initial investigation involving the effects of low-phosphorus engine oils on the friction and wear characteristics of typical engine cylinder kit components from a 3.8-liter, four-cylinder engine. For these tests, sections of molybdenum-coated piston rings were oscillated against segments of production cast iron cylinder bores. The oils were evaluated under two sets of conditions; i.e., the frequency of 20 Hz and a load of 60 N and a frequency of 40 Hz and a load of 120 N. The friction coefficients were continuously recorded, and the wear depths on the cylinder bore segments were measured at the conclusion of each experiment. The oils consisted of low-phosphorus prototype oil (LPPO) containing 0.05 wt% phosphorus, which is half the amount of ZDDP in GF–3 oils, and the LPPO treated with six different supplemental antiwear, anti-friction additives. The remaining additives in the LPPO were present in the amount required for full GF–3 formulation. The supplemental additives were selected to lower friction and replace and/or enhance antioxidant and antiwear properties lost due to lower zinc dithiophosphate (ZDDP) content. The results of low phosphorus concentration oils were compared to the results obtained with formulated ILSAC GF-3 engine oil. The oils that contained organomolybdenum ester showed the greatest benefit in friction reduction in the two different operating conditions. SEM/EDS/XPS surface analyses on the surfaces run with oils containing organomolybdenum ester showed evidence of molybdenum compound formation on the cylinder bore surface. In addition, antiwear film formation mechanisms due to surface interaction among the ZnDTP, organomolybdenum compounds, and metal oxides are discussed.

KEY WORDS:

• Friction
• Wear
• Oil Additives
• Piston Ring
• Cylinder Bore

ACKNOWLEDGMENTS

The authors wish to thank Drs. James A. Spearot and Michael L. McMillan for their technical support. The authors also wish to acknowledge the help of Drs. Zheng Ma and Ben Zhou (GM Powertrain), Mr. Charles Gossel, and Mark Rosetto (North America Materials Testing Group) for wear testing and surface characterization. The authors would also like to thank Maria Militello and Stephen Swarin for the XPS surface analysis. In addition, the authors thank Glenn Clever and Mark Gillman of GM Powertrain for helping to prepare piston ring and cylinder bore materials.

Review led by Elaine Yamaguchi

Notes

*All oils contain 0.2 mass% sulfur.