24th International Colloquium Tribology - January 2024 41 Impact of Lubricating Oils on the Performance for Liquid-Cooled Motor and Battery Thermal Control System Applied to Electric Transaxles K. Narita 1* , Y. Nakahara 1 and K. Matsubara 1 1 Lubricants Research Laboratory, Idemitsu Kosan Co., Ltd, Ichihara-shi, Chiba, Japan * Corresponding author: keiichi.narita.0440@idemitsu.com 1. Introduction E-axle (transaxle for electric vehicles) is electric drive unit that integrates a motor, an inverter and reduction gears, which provide an excellent fuel efficiency. E-axle is being developed by various manufactures. For further improvement in motor performance and utilization for EVs, these units are expected to become a downsized transaxle in future. In addition, for street use with frequent starts and stops, motor loss is caused by copper loss, which may be affected by coil temperature 1) . Heat transfer property for between motor and coolant, which is called as motor cooling performance is an important issue for E-axle to improve the efficiency and reliability of driving motors. There are three types for motor cooling: air, water, and oil. Although water itself has cooling ability, it has no insulating capacity and is used for cooling through a jacket, and therefore water-cooled system result in a combination of water coolant and a complex structure. Excellent cooling performance can be obtained for oil-cooled system because oils is highly insulating material when the motor is immersed directly into oil. Automatic transmission fluids (ATFs) are used in some cases as lubricating oil for E-axle 2) . ATFs have complex compositions designed to provide lubrication and friction control for shift devises and are not always optimal as fluids for E-axle. Lubricant additive aspect involves the ability of the oil limit corrosion of copper elements, mainly copper wire, and electric sensors 2) . Lithium-ion batteries are commonly used as energy storage devices in EVs because they have advantages in longer lifetime, low self-discharge rates compared to other batteries 3) . Heat thermal energy is produced inside the battery while charging or discharging the batteries, which lead to a temperature increment 4) . This is caused by the internal resistance and electrochemical reactions occurring inside the battery. Temperature increasing in the batteries give an effect on the life cycle, safety, reliability, and efficiency of the battery. There is an increasing interest in the technical approaches for battery thermal management. Battery cells usually do not have direct contact between the cell and the cooling fluids. Heat capacity of fluids is known to be more effective than air. Lubricating oil is a cooling fluid which does not conduct electricity. Battery pack is often submerged in a cooling system designed so that heat is transferred form batteries to oil directly 3) . Therefore, thermal control for the battery system by oil-cooled coolant may be more attractive. Heat transfer characteristics between oil fluid and motor/ battery pack seems to be related with viscosity, thermal conductivity, specific heat, and density of fluids. At present, the base properties of required for EVs fluids have not been systematically studied. In this study, we report the results aimed mainly at improving cooling performance for motor and battery system by lubricating oils. 2. Impact of lubricating oils on the cooling ability for liquid-cooled motor thermal control system A laboratory test method for the cooling at a forced convection was originally designed for screening the cooling performance of oil-type coolant. It is reported that a threephase induction motor is operated at the maximum temperature of 150-°C 1) . The changes in the copper plate temperature were monitored by thermocouples and then the cooling speed was calculated through the temperature by time. Figure 1 shows the effect of viscosity and type of base oil on the cooling speed. Test oils were prepared so that viscosities at 70-°C was from 1.3 to 16.4 mm 2 / s by using different type base oils. It is obvious that the cooling speed increase with lower viscosity oils comparing between hydrocracked mineral and naphthene mineral base oil similar viscosity 8-9 mm 2 / s, hydrocracked base shows a better cooling ability. In addition, synthetic base oil results in an excellent cooling performance. The difference in cooling speed among base oil type may be caused by their molecular structure. Hydrocracked base oil includes more normal chain saturated hydrocarbons rather than naphthene base oil. Figure 1: Experimental result of cooling performance by oils with different viscosity and base oil type Thermal vibration energy will come down to the main chain, and such energy transferred through collisions with neighboring molecules propagates to the end of the main chain through intermolecular heat transfer, leading to a higher heat conductivity. 42 24th International Colloquium Tribology - January 2024 Impact of Lubricating Oils on the Performance for Liquid-Cooled Motor and Battery Thermal Control System Applied to Electric Transaxles 3. Impact of lubricating oils on the cooling ability for liquid-cooled battery thermal control Kakaf et.al 3) explain that the optimum temperature range in the battery pack is 25 to 45-°C. The mainstream of cooling methods for batteries is air cooled and watercooled type. Recently, oil type fluids have been studied as to immersion cooling for batteries because oils do not conduct electricity and a higher heat capacity compared to air-cooling 5) . In this study, we developed the test method assuming immersion cooling for batteries, as shown in Figure 2. This test is aimed to evaluate the heat transfer characteristics between the heating elements and test oil when heating elements are installed in the test chamber and the test oil is flowed in the chamber. Heat flux at 500 W was supplied to the heating elements with a flow rate of 7.5 L/ min. The surface temperature stabilized after 15 minutes elapsed. Comparing at 30 minutes after starting tests, lower viscosity oil shows lower surface temperature with 5-°C than the case of higher viscosity oil. The effect of flow rate and heater surface temperature was evaluated ranging flow rate from 0.5 to 7.5 L/ min. Heating element temperature shows a lower value with higher flow rate, indicating that flow control as well as oil properties would be important for cooling performance. Figure 2: Test method for cooling performance assuming immersion cooling for batteries. It is necessary to understand oil flow behaviors near the heating elements. Particle method 6) is used for fluid simulation, which represents a fluid as a collection of particles, and local flow distribution can be analyzed. Figure 3 shows the simulation results by the particle method. Physical properties of the test oil, inlet flow rate, and heat capacity of the heating element were the same as in the experimental test. The particle size of the fluid was 0.3 mm, and the analysis was performed for 900 seconds, no external force was applied to the fluid in the chamber. From the velocity distribution in the left side of Fig. 3, the inlet speed in the lower right side is higher. The temperature distribution in the right side of Fig. 3 shows that the fluids take away heat when it passes between the heating elements, resulting in a temperature increase of the fluid. The flow velocity near the heating element would be changed depending on the fluid viscosity. Figure 3: Simulation results in the test chamber Conclusion Lubricant oil impact on the cooling performance for motor and battery thermal management system applied to electric transaxle unit. As a result, lowering kinematic viscosity of lubricating oils improved cooling performance at forced convection, and this cooling speed could be greatly influenced by base oil molecular structure. Furthermore, the test method assuming immersion cooling for batteries was developed. The lower viscosity oil could improve the cooling performance with higher flow rate, indicating that flow control as well as oil properties would be important for cooling performance. Simulation using particle method for was conducted for understanding phenomenon of the fluid flow near the heating elements in the chamber. Results revealed the velocity and temperature distribution near the heating elements in the chamber, which might play a role in affecting the cooling performance. 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