The interactions of surface-active engine oil additives in low viscosity engine oils and their effect on tribological performance

One of the main ways of increasing the fuel economy of the internal combustion engine (ICE) is by using lower-viscosity oils, which directly reduces CO2 emissions. The challenges of lower viscosity oils include decreased lambda ratios in tribo-contacts. To address this, the engine oil additive package plays a crucial role in maintaining low friction and ensuring engine durability. High concentrations of molybdenum dithiocarbamate (MoDTC) are added to counteract oil viscosity reduction and oxidation effects. An investigation into the optimization of the MoDTC concentration in low viscosity engine oil is undertaken in this study by conducting friction and wear tests on SAE 0W-8 oil with different MoDTC concentrations using a Mini Traction Machine (MTM) and NPFlex. Tribofilm growth is studied through spacing layering imaging (SLIM) imaging, and chemical composition is analyzed using X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy. The optimum Mo concentration falls between 180 and 350 ppm, significantly influencing friction induction time, MoS2 and ZDDP film thicknesses. Understanding the interactions of boundary active additives becomes critical in low-viscosity engine oil. Detergents, which have received less attention than MoDTC and zinc dithiophosphate (ZDDP), form tribofilms with good wear properties. Investigating different detergents' influence and interactions with ZDDP and MoDTC in ultra-low viscosity oil, three detergent formulations within a 0W-8 oil grade engine oil are examined. Detergents impact friction, wear, and tribofilm composition, showing synergistic effects with ZDDP and MoDTC. The same techniques used in the first study are applied. It has been proven that extending the oil drain interval (ODI) is as essential as reducing the viscosity to reduce CO2 emissions. However, ageing occurs within the oils during operation and is a detrimental process for low viscosity engine oil additives. Little research has been conducted comparing artificial and engine ageing processes. Thus, the same tribological testing equipment is utilised to investigate a comparison study between the two ageing processes as in the previous studies. The artificial aging process cannot fully replicate engine aging effects on oils. Regardless of the engine ageing process, the impact on oils remains relatively constant, while artificial ageing produces inconsistent results.