23rd International Colloquium Tribology - January 2022 187 Optimizing the Mo concentration in low viscosity fully formulated engine oils Aaron Thornley Insitute of Functional Surfaces, School of Mechanical Engineering, University of Leeds, Leeds, England Corresponding author: mnatho@leeds.ac.uk Yuechang Wang Insitute of Functional Surfaces, School of Mechanical Engineering, University of Leeds, Leeds, England Chun Wang Insitute of Functional Surfaces, School of Mechanical Engineering, University of Leeds, Leeds, England Jiaqi Chen Sinopec Lubricant Company, Beijing, China Haipeng Huang Sinopec Lubricant Company, Beijing, China Hong Liu Sinopec Lubricant Company, Beijing, China Anne Neville Insitute of Functional Surfaces, School of Mechanical Engineering, University of Leeds, Leeds, England Ardian Morina Insitute of Functional Surfaces, School of Mechanical Engineering, University of Leeds, Leeds, England 1. Introduction Original equipment manufacturers (OEMs) are constantly finding new ways to increase the fuel economy for their vehicles. Such drive for better fuel economy has been increased due to strict CO 2 emissions legislation from countries and regions such as China and Europe. Therefore, OEMs have to implement different solutions to meet these requirements set by international governments. One efficient approach to increase the fuel economy that OEMs are taking is to decrease the viscosity of the engine oil. Previous research has shown that a 2.75% fuel efficiency increase can be achieved using an SAE 5W-20 oil compared to an SAE 10W-30 oil. Lowering the viscosity further still produces positive results; a 1.5% increase can be achieved using a 0W-20 compared to a 5W-30. A new oil grade has been introduced recently as SAE 0W-8 to counteract the tighter CO 2 reductions further. Since this is a relatively new oil grade, little research has been conducted on 0W-8 engine oils. However, recent studies have shown that a fuel economy increase of 0.57% and 0.8% can be obtained using a 0W-8 engine oil compared to 0W-16 [1]. Decreasing oil viscosity generates more metal-to-metal contact between the components, especially at high temperatures where the oil is at its thinnest [2]. In addition, components that make up the piston assembly, exhaust, and inlet valve all have increased friction when lowering the oil’s viscosity, as these operate at lower lambda ratios. The increased fuel economy demands are causing countries like Japan to increase the concentration of molybdenum dithiocarbamate (MoDTC) to very high amounts of 1000+ ppm, which supposedly gives prolonged fuel economy boosts. However, when lowering the oil’s viscosity, the concentration of boundary additives has to be increased due to a decrease in lambda ratio at the tribo-contacts and additive depletion due to oxidation [3]. In addition, there is also a need to reduce the amount of sulfur, which is part of the MoDTC molecule within engine oils due to its harmful effects on the engine and its relatively high cost [4,5]. Therefore, understanding the behavior of MoDTC concentration in fresh oils and aged oils is extremely important, which will directly lead to the optimization of MoDTC in low viscosity oils. 188 23rd International Colloquium Tribology - January 2022 Optimizing the Mo concentration in low viscosity fully formulated engine oils 2. Methodology MoDTC needs to perform in the low viscosity conditions but does not need to be present in significant excess such that it is expensive/ inefficient and will increase SAPS level of the oil. It is also not necessarily the highest Mo concentration producing a ≈ 0.04 friction value in the boundary lubrication regime, which is the lowest friction obtained with this additive. A low friction value of ≈ 0.04 must be achieved over a range of lambda ratios to produce an overall performance similar to its higher concentration counterparts, including wear. In total, six different Mo concentrations ranging from 85 to 1000 ppm, including an oil with 0 ppm of Mo, were selected for this research. All friction tests were conducted using a mini traction machine (MTM) with traction and Stribeck phases. A space layer imaging (SLIM) unit attached to the MTM enabled zinc dialkyl dithiophosphate (ZDDP) tribofilm thickness measurements to be obtained at set intervals during the tests. To understand the influence Mo concentration has on tribochemistry, x-ray photoelectron spectroscopy (XPS) and Raman techniques were used. Finally, wear generated during the MTM testing was analyzed using an NPFlex, and tribofilms were removed before wear measurements using EDTA solution. 3. Key results Figure 1 displays the steady-state friction and wear coefficients obtained from MTM and NPFlex analysis for the selected Mo concentrations. Increasing the Mo concentration decreases the coefficient of friction, and there is a threshold value to which the ≈ 0.04 friction is achieved. Wear increases with the addition of MoDTC but does not significantly change when Mo concentration increases past the optimum value. The Stribeck curves taken towards the end of the test, under a varying lambda ratio, are shown in figure 2. Mo concentrations ≥ 350 ppm all produce ≈ 0.04 friction across the range of entrainment speeds/ lambda ratios. The 180 ppm displays increased friction during the Stribeck curve compared to a constant speed due to the Mo concentration influencing the MoS2 layer thickness and its formation and removal rate. Finally, figure 3 displays the total intensity counts of MoS 2 within the tribofilm matrix for each steady-state friction. From the figure, a threshold value of total intensity counts is required to produce a ≈ 0.04 friction tribofilm. As the Mo concentration increases, the total intensity counts increase to a maximum value, to which further increases would not significantly change the intensity counts. Figure 1: Steady-state friction and wear coefficients for selected Mo concentrations. Figure 2: Final Stribeck curves taken for all Mo concentrations. Figure 3: Steady-state friction vs total intensity counts of MoS2 within the tribofilm matrix for all friction-reducing tribofilms. 23rd International Colloquium Tribology - January 2022 189 Optimizing the Mo concentration in low viscosity fully formulated engine oils 4. Conclusion In this research study, the optimum Mo concentration for ultra-low engine oil is between the range of 180 to 350 ppm. The lowest Mo concentration, which produces similar friction and wear results to its higher concentration counterparts. Increasing the Mo concentration decreases the induction time to reach steady-state friction due to increased MoS 2 layer thickness in the ZDDP dominant tribofilm in the beginning stages of film formation. The optical film thickness of the ZDDP tribofilm significantly decreases with the addition of MoDTC due to its decomposition products preventing its formation. Increasing the Mo concentration past the optimum value does not significantly change the steady-state film thickness. The rate of formation and removal of MoS 2 within a ZDDP dominant tribofilm is highly dependent on Mo concentration, and the formation rate becomes more significant with higher Mo concentration. Increasing the Mo concentration increases the total intensity counts of MoS 2 within the ZDDP dominant tribofilm up to a specific concentration, 500 ppm in this study. Any further increase in Mo concentration does not result in higher total intensity counts. 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