23rd International Colloquium Tribology - January 2022 173 Enhanced Gear Lubricity for Lubricant Oils Applied to Transaxles in HEVs and EVs Keiichi Narita Idemitsu Kosan Co.,Ltd./ Lubricants Research Laboratory, Ichihara-shi, Japan Corresponding author: keiichi.narita.0440@idemitsu.com 1. Introduction Numbers of hybrid electric vehicles (HEVs) and electric vehicles (EVs), which provide an excellent fuel economy and reduced carbon dioxide emission, are increasing. 1) E-Axle (transaxle for electric vehicles) is an electric drive unit that integrates a motor, an inverter and reduction gears, which is highly versatile and has high fuel efficiency. E-Axle is being developed by various manufactures. For further improvement in motor performance and dedicated utilization for EVs, these units are expected to become a downsized transaxle in future. In addition, for street use with frequent starts and stops, around half of motor power loss is caused by copper loss, which is affected by the coil temperature. 2) Heat transfer property for motors, which is called as motor cooling is an important issue for E-Axle to improve the efficiency of driving motors. Automatic transmission fluids (ATFs) are used in some cases as a lubricant for E-Axle. 2) The main performance requirements for E-Axle fluids are (1) cooling ability for motors (2) durability for gears and bearing components. In addition to (1) and (2), also required properties are oxidation stability, anti-forming and compatibility for elastomers, similar to conventional ATFs. ATFs have complex compositions designed to provide lubrication and friction control for shift devises, and are not always optimal as E-Axle fluids. Lubricant additives providing excellent lubricity may generally tend to reduce electric resistance on the condition that the motor coil is immersed in axle fluids. This lubricant aspect involves the ability of the oil limit corrosion of copper elements, mainly copper wiring and electronic sensors. 3) At present, the lubricant properties on the cooling performance have not been systematically studied. An effective oil for E-Axle may be expected provide high performance in electrical compatibility at extremely low viscosities, at higher speeds in an operating condition. In this study, we report the results aimed mainly at improving cooling ability by base oil properties and extending durability for gear components. 2. Cooling performance for E-Axle fluids Excellent cooling can be obtained for oil-cooled system because oil is highly insulating and the motor is immersed directly into it. It is reported that a three-phase squirrel-cage induction motor is operated at the maximum temperature of 150 °C. 1) Therefore, it is important to focus on the heat transfer behaviour from 100 to 150 °C. A laboratory test method for the cooling at a forced convection condition was originally designed. The test oil is supplied by the oil pump from the oil tank, and is controlled at a constant flow rate of 0.5 kg/ min. and at 70 °C temperature. Oil bulk temperature in oil pan may be around 70-80 °C in normal use. The test oils are flowed through the rectangular section, the copper plate attached the heater is set up. Heat flux at 160 W was supplied to the copper plate to reach 150 °C. The change in the copper plate temperature was monitored by thermocouples and then the cooling speed was calculated through differentiating the temperature by time. Figure 2 shows the effects 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 with 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 hydrocarbon rather than naphthene base oil. Thermal vibration energy will come down to the main chain, and such energy transferred through collisions with neighbouring molecules is promptly propagated to the end of the main chain through intermolecular heat transfer, leading to a higher heat conductivity. 174 23rd International Colloquium Tribology - January 2022 Enhanced Gear Lubricity for Lubricant Oils Applied to Transaxles in HEVs and EVs Figure 1: Effects of base oil on the cooling performance 3. Gear lubricity for E-Axle fluids Applying a transmission fluid with extremely lower viscosity to E-Axle system would potentially give an advantage for a better motor cooling. It is known that low friction in gears is achieved by using lower viscosity gear oils. 3) However, it is necessary to consider a negative impact on the durability of gear components. The gear pitting fatigue life by lubricants was evaluated by using a FZG (Forshungsstelle fur Zahnrader and Getriebebau) gear test rig. Gear type C is operated at a pitch line of 8.3 m/ s speed in torque stage 302Nm at 90 °C oil temperature. The pitting life is defined as the number of load cycles when the mostly damaged gear flank exceeds about 5 mm 2 . The test oils were prepared by blending a hydrocracked mineral base stock and three kinds of additive formulation. Figure 2 shows the effects of test oil viscosity and additive formulation on the FZG pitting life. First, the viscosity of test oils was varied from 4.0 to 6.3 mm 2 / s at 90 °C with the same additive formulation A (circle plot in Figure 3). The gear fatigue life decreased with lowing viscosity in proportion to the viscosity 0.7 , which could be influenced by the difference in the oil film thickness calculated under EHD lubrication regime. Figure 3 shows the optical images of the post-test gear flank surface at the 19 million load cycles from the test oil with 4.0 mm 2 / s viscosity and additive formulation A. Macropitting was observed on the overall gear flank surface. This damaged area shows the shape of an inverted triangle, which seems to be due to a typical surface originated rolling fatigue. There are many micropitting under the pitch line, which could promote a rapid growth of macropitting. In the upper part of dedendum area, there are many micropitting. Significant adhesion wear occurred in the lower part of dedendum. The high stress and sliding action of the gear tends to result in removing material and ploughing material toward the pinion root. This adhesion wear in the case of lower viscosity oil was more significant than that of higher viscosity. Secondly, the additive formulation was found to be a great influential impact on the fatigue life. The sample with higher viscosity 8.6 mm 2 / s at 90 °C and additive formulation B (rhombus plot in Figure 3) shows a shorter fatigue life than the case of additive formulation A. Interestingly, the oil containing additive formulation C (square plot in Figure 3) demonstrates a longer fatigue life in spite of the lowest viscosity of all tested samples. Combining X-ray photoelectron microscopy (XPS) analysis with sputtering by ion irradiation enables depth-profiling investigation of interfaces with multi-layer films. The XPS results revealed that the film composed of calcium and phosphorus species was formed with 100-200nm thickness in case of longer life sample. This tribofilm could minimize adhesion in the contact regions in the pinion dedendum region, leading to prevent from generating micropitting, with a longer fatigue life. We propose that lower viscosity fluid with appropriate additive formulation would give an advantage for motor cooling and gear lubricity for E-Axle. Figure 2: Effects of oil viscosity and additive formulation on the gear pitting life Figure 3: Images of the post-test gear flank surface at FZG 19 million load cycles from the test oil with 4.0 mm 2 / s viscosity and additive formulation A 4. Conclusions We investigated the lubricant impact on the motor cooling performance and the gear lubricity. Lowering kine- 23rd International Colloquium Tribology - January 2022 175 Enhanced Gear Lubricity for Lubricant Oils Applied to Transaxles in HEVs and EVs matic viscosity of lubricants improved cooling performance in forced convection condition, and this cooling speed could be greatly influenced by base oil molecular structure. The gear fatigue life decreased with lowering viscosity of oils. Interestingly, a sort of additive formulation could extend the gear pitting life. Tribofilm derived from anti-wear agent could play a role in minimizing adhesion wear, leading to a longer fatigue life. 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