Engine Oil Effects on Friction and Wear Using 2.2L Direct Injection Diesel Engine Components for Bench Testing Part 2: Tribology Bench Test Results and Surface Analyses

The effects of lubricating oil on friction and wear were investigated using light-duty 2.2L compression ignition direct injection (CIDI) engine components for bench testing. A matrix of test oils varying in viscosity, friction modifier level and chemistry, and base stock chemistry (mineral and synthetic) was investigated.

Among all engine oils used for bench tests, the engine oil containing MoDTC friction modifier showed the lowest friction compared with the engine oils with organic friction modifier or the other engine oils without any friction modifier. Mineral-based engine oils of the same viscosity grade and oil formulation had slightly lower friction than synthetic-based engine oils.

In the comparison of wear on cylinder bores lubricated with the same viscosity of lubricant, the lubricant containing the MoDTC friction modifier had the lowest wear depth, probably because of a wear-resistant reaction film formed by the reaction of sulfur from ZnDTP (Zinc Dialkyl Dithiophosphate) and MoDTC. The wear depth of the engine oil without any friction modifier was the highest among all lubricants tested. With MoDTC in the engine oil, the wear depths for all tested piston rings were lower than those operating in the absence of MoDTC. This might be caused by a synergistic wear-resistant film formation (both MoS₂ and polyphosphates formed) on both cast iron bores and piston rings as evidenced by EDX and XPS surface analyses. Surface analyses were conducted to help understand the surface mechanisms responsible for friction reduction and the impact of engine materials and additives on wear.