The design and development of pure electric vehicle gearbox lubricating oil will become an important development direction

December 28, 2023

The design and development of pure electric vehicle gearbox lubricating oil will become an important development direction

The development of vehicle electrification has put forward higher requirements for parts suppliers and new challenges for lubricants. At present, the transmission system of pure electric passenger models is mainly divided into water-cooled motor + single-stage gearbox, oil-cooled motor + single-stage gearbox, oil-cooled motor + two-stage gearbox, and water-cooled motor + single-stage gearbox is the most widely used in the market at this stage. The structure and operating condition of the single-stage gearbox are different from the transmission structure of the traditional automobile, so the use of lubricating oil also puts forward new requirements. This paper will focus on the performance of lubricating oil of single stage gearbox.

Lubrication requirements and performance evaluation of pure electric passenger car gearbox

Most of the single-stage gearbox uses a fixed speed ratio, which can meet the requirements of small and medium-sized pure electric vehicles. The equipment has the following characteristics:

◇ More compact structure.

◇ High input speed. The motor drives the gearbox directly, because the input speed of the motor is high, the speed of the single-stage gearbox can reach 12 000 ~ 20 000 r/min. The maximum speed of a conventional engine is 6 000 r/min.

◇ High operating temperature of oil products. Due to the high speed and high load in the gearbox, the oil temperature can reach 100 ° C, up to 150 ° C. The operating temperature of the manual transmission, which is traditionally driven in the form of gears, is below 90 ° C and the maximum is not more than 120 ° C.

◇ Pay more attention to transmission efficiency. In order to maximize the energy saving of the vehicle and extend the driving range, the electric vehicle gearbox pays more attention to the transmission efficiency.

The structure of the water-cooled motor + single-stage gearbox refers to the relative independence between the motor and the gearbox, the motor is equipped with a traditional coolant, and the gearbox is lubricated with drivel oil. Combined with the performance characteristics of the gearbox, the lubricating oil mainly focuses on the following properties:

Kinematic viscosity and load bearing properties

At present, 75W-90 viscosity grade oil is more commonly used in China. For the industry's concern about the transmission efficiency of pure electric vehicles, it can be improved from the level of optimizing the design of the gearbox and improving the processing accuracy of the hardware, and the change of the lubricating oil also helps to improve the transmission efficiency. Therefore, increasing the fluidity of the oil and reducing the viscosity of the reducer oil have become an idea of oil development. In order to achieve good transfer efficiency, the oil motion viscosity (100 ° C) can be reduced from 13.5 to 18.5 mm2/s to 6.0 mm2/s or even lower. Because the oil needs to ensure the lubrication of the gear and bearing at a higher speed, the bearing capacity of the oil needs to be ensured at the same time of low viscosity, which poses a certain challenge to the pure electric passenger car gearbox oil.

Low temperature fluidity and shear stability

In order to ensure the normal use of the vehicle in winter, especially when the vehicle starts, the oil can effectively lubricate all parts, and the pure electric vehicle gearbox oil is required to have good low-temperature flow performance. On the basis of the same viscosity, the low temperature flow performance of synthetic oil and semi-synthetic oil is better than that of ordinary mineral oil, but the disadvantage of synthetic oil is that the cost is generally higher. Therefore, if mineral oil products want to make up for this defect, it is necessary to choose a suitable Base Oil, Viscosity Index Improver and point depressant.

Due to the high speed and high load of the gearbox of pure electric passenger cars, the gearbox oil will be subjected to strong shear force in actual work. The strong shear force will cause the decrease of oil viscosity, resulting in the decrease of oil film thickness, resulting in wear. Therefore, it is necessary for the oil to have good shear resistance while ensuring low temperature performance.

The comparison of viscosity and shear stability data between the developed pure electric passenger car gearbox oil and the commercially available reference oil is shown in Table 1. The developed sample and the commercially available oil are mineral products. Compared with the oil in the market, the developed sample can better meet the requirements of the operating conditions in terms of viscosity, shear stability and low temperature performance, and the performance is comparable to that of similar semi-synthetic schemes and synthetic schemes. Because the semi-synthetic scheme and the synthetic scheme are not dominant in terms of economy, this article will not discuss the above two schemes too much.

Oxidation stability

The reducer oil is vigorously agitated in the work, and it will be in contact with air, metal, impurities, etc., and the temperature in the reducer box is high, which is easy to oxidization and deterioration to lose the original performance, so the oil needs to have good oxidation stability. The comparison of oxidation properties of pure electric passenger car gearbox oil and commercially available products is shown in Table 2.

Figure 1 to Figure 6 shows DKA oxidation performance test based on CEC L-48-A-00 test method.

In addition, combined with the gearbox operating conditions, pure electric passenger car gearbox oil anticorrosive capacity, foam performance and other properties should also be paid attention to.

Development and verification of pure electric passenger car reducer lubricants

According to the above oil performance requirements, the product suitable for pure electric passenger car reducer (water-cooled motor) is developed, and the typical data are shown in Table 3. The product has passed a number of performance verification tests and the test results are good, indicating that the product has application value in the water-cooled motor + single-stage gearbox pure electric vehicle.

Efficiency test

The efficiency test is to evaluate the efficiency by calculating the total energy consumption under the same working condition using different gearbox oil products. Efficiency test results show that the pure electric passenger car gearbox oil developed in this study plays a positive role in improving efficiency.

Transmission fatigue endurance test

The fatigue durability test of gearbox is to cycle the gearbox according to the rated input torque at a certain speed and a certain oil temperature, and investigate the fatigue durability of the whole gearbox, that is, gear, shaft, bearing and housing. Transmission fatigue durability is a must-do item of transmission bench. This method can also be used to investigate the effect of oil in gearbox. The pure electric vehicle gearbox oil developed in this study has passed the fatigue and durability test of typical gearbox, and uses the highest torque of single-stage gearbox in China, which has a demonstration effect in the industry. In addition, the oil also passed the differential durability test and other bench tests, and the test results are good. The fatigue endurance test of oil is shown in Table 4.

Pure electric passenger car gearbox oil has excellent performance in transmission efficiency, low temperature performance, shear stability, oxidation stability and load bearing performance, and has application prospects in the domestic market.

At present, the use of pure electric passenger car gearbox oil on the market mostly follows other types of driveline oil, such as manual transmission oil, transaxle oil and automatic transmission oil. Various Oems (original equipment manufacturers) have different requirements for the use of oil products, although they have paid attention to the characteristics of the equipment, but from the perspective of the overall industry there is still a lack of targeted lubrication products and relatively unified lubrication specifications. With the increase in the market share of pure electric passenger vehicles, the introduction of relevant industry standards will help further regulate the filling and aftermarket oil products in the future.

With the development of pure electric passenger vehicles, many Oems at home and abroad have begun to design and develop gearboxes, and the types of gearboxes are also different. Among them, the matching of the oil-cooled motor and the gearbox, the design of the two-speed transmission will become an important development direction in the future industry, and the supporting transmission oil should also pay attention to the electrochemical performance, thermochemical performance, and friction performance. In particular, the compatibility of electrochemical properties, thermochemical properties, frictional properties and traditional properties is also an important topic of current research.

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