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Figure 3: Average and Maximum Temperatures for the different regions of e-motor calculated for the two tested fluids [11]
TotalEnergies performed a study on their water-based fluid focusing on the direct cooling strategy for an electric vehicle drive [11]. They found a significant temperature reduction for different parts of the e-motor using water-based fluid as shown in Figure 3 and confirmed better cooling efficiency provided by their WBL compared to oil cooling. Therefore, the water-based fluid cooling strategy can be helpful for OEMs to design future electric drive trains with high power density.
Main areas to focus on: corrosion, additive chemistry, film formation
A major drawback associated with WBLs is the increased potential for the lubricant to induce corrosion in machinery, particularly in applications such as gearboxes. The goal of ongoing research and development is to identify the best way to deal with the corrosion problem. Chemical activation stands out as a key avenue for tackling corrosion problems. Studies have been conducted to determine the ideal percentage of additives capable of delivering the highest performance and enhancing material properties.
Ionic liquids are one type of additive that shows a lot of promise in improving the water-based lubricant’s ability to reduce corrosion. The amino acid ionic liquids can be a potential additive for corrosion resistance. Yang and his team [13] conducted a comprehensive study that yielded conclusive evidence that the integration of this additive into the system resulted in minimal corrosion, effectively preventing the manifestation of visible rust even under standard viewing conditions. Bio-inspired graphene-based coatings [14] can be introduced as an additive that initiates a cross-linking effect within the intricate microstructure of the hybrid coating. This unique process leads to the development of a densely stacked lamellar coating to enhance the reinforcement of corrosion resistance. The interconnected layers
12 LUBE MAGAZINE NO.179 FEBRUARY 2024
formed through the cross-linking structure create a robust protective barrier, fortifying the coating against corrosive elements and thereby elevating its overall resistance to degradation. PEI-RGO nanosheets emerge as a nano-additive for water-based lubricants, as indicated in reference [15]. This particular additive demonstrates the capability to effectively double both anti-wear and anti-corrosion properties, particularly when applied to WBLs utilised with steel materials. The PEI-RGO nanosheets counteract these processes by forming a protective deposited film. This film serves as a barrier, preventing oxygen molecules from infiltrating the wear scars that would typically develop during the wear process. The additive’s ability to create this protective shield contributes significantly to its efficacy in enhancing the overall durability and anti-corrosive performance of WBLs in steel applications. An additional potential solution to the corrosion issue with water-based lubricants has been demonstrated by the Fraunhofer IWM [16]. They have developed the method on sintered slide bearings by forming a galvanically produced protective layer on which the shaft can slide. This technology can be valuable for applications such as electric motors.
The fluctuation of viscosity under pressure poses a significant challenge to the performance of lubricants within a system. Elevated pressure in a confined lubricant typically leads to an increase in viscosity, particularly noticeable in conditions such as mixed and elastohydrodynamic lubrication. This phenomenon is often seen in non-conformal and heavily loaded contacts like ball bearings and gears, where contact pressures can exceed atmospheric pressure by several orders of magnitude. Hence, it is crucial in practical terms to evaluate and quantify the pressure-viscosity behaviour of lubricants to estimate their effectiveness within a system under diverse operating pressure conditions.
Conventional oils are highly pressure sensitive and can become semi-solid as the contact pressure increases in a confined non-conformal contact. This pressure-viscosity response can be characterised using a pressure viscosity coefficient(α). Usually, mineral oils have high-pressure viscosity coefficient values in the range of 15 to 20 Gpa-1
. However, water-based
lubricants are not highly sensitive to pressure and do not show the piezo viscous response like mineral oils. Recently, a comprehensive study was conducted at Luleå Technical University on several water-based lubricant technologies that consist of glycol, glycerol
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