International Colloquium Tribology
ict
expert verlag Tübingen
125
2022
231
Rheological properties of Lubricants and their correlation with fuel economy performance
125
2022
Maryam Sepher
Sara Zhang
David Morgan
Ramoun Mourhatch
Peter Kleijwegt
Claire Chommeloux
ict2310191
23rd International Colloquium Tribology - January 2022 191 Rheological properties of Lubricants and their correlation with fuel economy performance Maryam Sepehr Chevron Oronite Company LLC, Richmond California, USA Sara Zhang Chevron Oronite Company LLC, Richmond California, USA David Morgan Chevron Oronite Company LLC, Richmond California, USA Ramoun Mourhatch Chevron Oronite Company LLC, Richmond California, USA Peter Kleijwegt Chevron Oronite Technology BV, Rotterdam, The Netherlands Claire Chommeloux Chevron Oronite Company LLC, Richmond California, USA 1. Introduction One of the least expensive pathways to achieving improvements in vehicle fuel economy is through changes to engine lubricant viscosity and composition [1]. Driven by ever more stringent emissions regulations, OEMs are therefore requiring engine oils to continue protecting engines at lower viscosities and reduced friction. Different engine operating conditions represent a range of lubrication conditions, and to better understand the full impact of engine lubrication process, one must understand how oils perform in these conditions. In general, additives in lubricants help to provide the right balance of fuel economy while maintaining durability protection. The focus of the present study is on the hydrodynamic lubrication regime, and rheological properties of oils were investigated and correlated to their fuel economy performance in different engines, Mercedes Benz OM 501 LA and Detroit Diesel DD13, and driving cycles, WHTC (World-Harmonized Transient Cycle) and modal. 2. Testing A series of fully formulated oils were tested in OM 501 LA in WHTC driving cycles, and in DD13 in modal operating conditions. A rotational rheometer MCR 302 (from Anton Paar), and high shear viscometer USV (from PCS Instruments) were used to measure the rheological properties of these oils at different shear rates from 1 to 10 7 s -1 at different temperatures, from 20 to 120°C, with an increment of 20°C. A generalized Newtonian fluid model, Carreau-Yasuda model [2], was used to fit the experimental data, and compared to fuel economy performance of the oils in different engines. 3. Results and discussion An example of rheological profiles of different fully formulated multi-grade lubricating oils is shown at 100°C in Figure 1. Viscosity is lower for lower viscosity grades, and one can also note more distinct shear thinning behaviour for heavier oils, between 10 4 - 10 7 s -1 . These oils were tested in a DD13 engine under modal operating conditions. Results of FEI (Fuel Economy Improvement) compared to a baseline SAE 10W-30 oil are summarized in Figure 2. 192 23rd International Colloquium Tribology - January 2022 Rheological properties of Lubricants and their correlation with fuel economy performance Figure 1: Viscosity profile of different fully formulated multi-grade oils at 100°C. Figure 2: Fuel Economy Improvement of oils tested in DD13 in modal operation. Higher viscosity oils result in lower FEI in the engine. One expects for different viscosity grade oils to see significant differences in their rheological profile and FE performance. A strong correlation can be observed between rheological profile and FEI of these oils in wide range of shear rates. The developed rheological method and profiles can also differentiate oils within same viscosity grade, and correctly rank and predict their FE performance. This is specifically useful in considering weighting of discrete operating points in modal engine operation test procedures. Figures 3 and 4 show the rheological profiles and FEI results of a series of SAE 0W-20 oils tested in OM 501 LA, respectively. For simplicity, the FEI results of run 1 of WHTC and rheological profile of oils at average oil sump temperature of WHTC 1 st run have been shown in these 2 figures. Figure 3: Viscosity profiles of different SAE 0W-20 oils at 82°C, the average oil sump temperature of run 1 of WHTC driving cycle. The blue box represents a proposed engine/ driving operation window of shear rate. Figure 4: Fuel Economy Improvement of SAE 0W-20 oils tested in OM 501 LA. Results correspond to the 1 st run of WHTC. The difference between the rheological profiles (Figure 3) and FEI (Figure 4) of the SAE 0W-20 oils are smaller than the difference seen for different viscosity grade oils (shown in figures 1 and 2). In addition, one can also note that, at constant temperature, the viscosity curve of these oils may cross over at different shear rates, which is caused by different shear-thinning behaviour of these oils. As an example, at a constant temperature, viscosity of Oil E is the lowest at lower shear rate, and the highest at higher shear rates. On the other hand, Oil B shows the highest viscosity at lower shear rate, and the lowest at higher shear rate. Oil B show stronger shear-thinning behaviour compared to Oil E. Depending on the engine/ driving operation window, FEI results may vary for the same oils. In the case of OM 501 LA, applied shear rates in the operation window are not higher than 10 7 s -1 . Therefore, in this engine and driving conditions, Oil E with lower viscosity at low shear rates shows the highest FEI, and Oil B shows one of the lowest 23rd International Colloquium Tribology - January 2022 193 Rheological properties of Lubricants and their correlation with fuel economy performance FEI. It can be noticed that FEI of oils has strong correlation with their rheological profile in proposed the engine/ driving operation window of shear rate. In addition to shear-thinning behaviour of oils, their response to temperature or their VI also plays a role on their response in different engine operating conditions. More examples will be discussed during the presentation, showing how the developed rheological profile can well describe and correlate to FEI of oils, particularly for engines and driving cycles with dominant hydrodynamic lubrication regimes. 4. Summary Rheological properties of lubricating oils are crucial in understanding frictional losses in different parts of an engine, in different operating conditions. In the present study, the rheological properties of a series of fully formulated multi-grade lubricating oils, tested in engines, were investigated using different types of rheometer/ viscometers over a range of temperatures and shear rates, and a generalized Newtonian model was used to fit the experimental data. The results of developed rheological methods can be used to predict relative fuel economy performance of oils in an engine under driving conditions with dominant hydrodynamic regimes. This technique is an effective tool in development of more fuel-efficient motor oils. References [1] Miller, T. “The Road to Improved Heavy Duty Fuel Economy”. Directions in Engine-Efficiency and Emissions Research (DEER) Conference, 2010. [2] Carreau, P; De Kee D; Chhabra, R; “Rheology of Polymeric Systems: Principles and Applications”, Hanser Publishers, 1997
