Optimizing the performance of transmission fluids for EVs and HEVs
By Dr. Raj Shah, Dr. Simon Tung, and Nathan Aragon
Electric vehicles (EVs) and hybrid electric vehicles (HEVs) are growing in popularity because of their energy efficiency and positive impact on greenhouse gas emissions; though, several challenges still need to be addressed, including optimising the functional performance of transmission fluids. More effective transmission fluids can significantly enhance the overall performance of EVs and HEVs.
Performance characteristics of EV/HEV transmission fluids
Transmission fluids for EVs and HEVs differ from traditional internal combustion engine (ICE) vehicle fluids. They still require excellent mechanical and tribological properties to maintain optimal driving and fuel economy performance. However, in many HEV or EV advanced drivetrain system designs, transmission fluids have come into contact with electric motors (e-motors), batteries, electrical insulators, and thermal management devices, necessitating additional electrical and thermal properties.
In hybrid configurations, since transmission fluids can come into contact with the electric motor, superior electrical properties, including conductivity, dielectric constant, and dielectric strength are required. Electrical conductivity must be carefully balanced with other electrical properties. If conductivity is too high, short-circuiting, or dangerous current leakages can occur. If conductivity is too low, static charges can build up in the system. This can degrade the fluid by oxidation due to electric arcing.[1] Dielectric strength and breakdown voltage also require management so that the fluids can withstand the additional electrical field during vehicle operation.[1, 2]
Recently EV/HEV electrification components, including e-motors and regenerative brake components, have been integrated into this advanced configuration with a thermal management system for cooling applications. In HEVs, an important function of the transmission fluid is to provide thermal cooling for other components, such as electric motors and e-drive configuration. The cooling fluids need to handle a very wide temperature range and require a high heat transfer coefficient to provide for efficient thermal cooling. The fluid must also withstand sudden temperature changes and sharp temperature gradients. In addition, driveline fluids for the electrified propulsion system need to function as an effective lubricant, providing corrosion protection to copper windings, composites, and rare earth magnetic materials — while protecting against wear and oxidation.
Compatibility with new materials is also important. Copper is often used in next-generation vehicles because of its thermal and electrical conductivity properties. Transmission fluids for EVs/HEVs must be resistant to copper corrosion. Modern vehicles also utilize lightweight and polymeric materials, which require further compatibility considerations.
EVs and HEVs generate enormous amounts of torque. Transmission fluids need to handle high loads at low speed. Viscosity should be low — but not too low — so that the lubrication film does not become excessively thin. These driveline fluids also should have a high flash point so as not to be highly flammable. Further, their pour point should be low, so as to operate at a wide temperature range. They should be water resistant as electro wetting can damage the driveline components, potentially leading to destabilization of the fluid.
Frequent start-stop in an HEV engine results in a lower overall operating temperature, which can cause water to leak into the lubricant and could result in sludge buildup. We can address this issue by using a lubricant with substantial hydrophobicity.[2,3] Hydrophobicity is the physical property of a compound that allows it to be repelled by a mass of water. Electric motors in both HEVs and EVs also produce large amounts of heat. Transmission fluids have been transitioning towards lower viscosity formulations, not only for better fuel economy but because thinner viscosity fluids can provide more efficient heat transfer and higher energy efficiency.[3, 4, 5]
OEMs are looking for durable “fill-for-life” transmission fluids to reduce long-term fluid degradation and oxidation. At present, power transmissions are integrated with e-axles or dual clutches. Multiple speed transmissions and regenerative brakes have also been introduced for high-performance hybrid vehicles. In these new applications, driveline component efficiency and durability are receiving considerable attention, at both a system and component level.
Bearing components, gears, clutch friction durability, protection of seals, and oil spin loss are among the major areas where the specific fluid performance characteristics need to be evaluated when designing driveline fluids for electrification components in electric and hybrid powertrains.
Electrical considerations
Most recent research focuses on optimizing the electrical properties of transmission fluids. A key issue is fluid degradation caused by higher voltages and currents. The introduction of free radicals into the fluid can attack oxygenated components, leading to the formation of peroxides and potentially carboxyl groups. Strong currents can affect various additives by causing them to agglomerate in certain areas. Overheating of a charged environment in the fluid can lead to the formation of micro-bubbles and further destabilization. This can start a dangerous cycle as fluids containing micro-bubbles are more inclined to degrade from electrical breakdown.
Non-polar fluids can be affected by electro wetting phenomena when the interfacial tension between the fluid and a metal surface increases significantly. A build-up of electrostatic pressure can destabilize the emulsion formed by two immiscible phases within the fluid.[4] Likewise, the electrical conductivity of the fluid must be carefully balanced with the formulation of functional additives. Ionic liquids are proven to be effective at reducing the conductivity of transmission fluids. They also can provide better wear resistance, but are currently expensive. Other additives known to lower conductivity are stearic acid, phospholipids, and calcium salicylates; however, it is exceedingly difficult to know exactly how a certain additive will affect the system without lab tests [2, 3]. More research needs to be conducted to determine the surface interaction between the electrified components and driveline lubricants.[4, 5]
Thermal Considerations
New applications in the thermal management for EV/HEV[5] are helping extend the driving range and lifetime of electric and hybrid vehicles. Thermal cooling has become one of the major focuses for advanced propulsion systems, including microelectronics, battery materials, e-motors, and advanced electrification components. The higher working temperatures of hybrid drivetrains can generate extensive wear in higher loading parts compared with conventional driveline components. This will create a loss in fuel efficiency and ultimately seize the driveline components. To prevent this problem, there is a need for advanced thermal cooling systems for EVs and HEVs.
Recent changes in the configuration of EVs/HEVs have resulted in the shift from mechanical to electrification components. Established fast cooling methods apply increased surface areas such as fins and micro-channels for heat dissipation. Both thermal conductivity and thermal capacity are primary characteristics for driveline fluids, and they should be optimized for effective thermal management. “Nanofluids” with nanoparticles suspended within the base fluids have shown much promise for thermal cooling applications. [5] Using metal- oxide nanoparticles mixed with the base fluid plus dispersant additives has shown better thermal cooling performance. The nanoparticles and dispersants in nanofluids have increased the surface area of nanoparticles within the nanofluids, contributing to better thermal conductivity and convective heat transfer of the nanofluids.
With the proper application of specific dispersants to prevent the agglomeration of the nanoparticles, nanofluids hold much potential for extending fluid life. Recent research has demonstrated that using nanoparticle additives, such as silica nanoparticles and carbon nanotubes, has increased both thermal conductivity and heat capacity in neat base oils, as well as improved wear resistance. Nanofluids, such as nanographene, can be ideal for the thermal cooling of EV/hybrid components. Using nanofluids or nanographene in EV/hybrid applications have been shown to improve almost 35-40% of the thermal cooling efficiency of the fluid, compared with traditional coolants such as polyethylene glycol or other synthetic polyether coolants.[5]
The major challenge moving forward is to develop well-balanced formulations that combine the most important electrical properties with the thermal properties and appropriate material compatibility. We should conduct more joint research programs between OEMs and lubricant formulators, which could generate high potential applications for an all-in-one transmission fluid for EVs and HEVs.
REFERENCES
[1] https://360.lubrizol.com/2018/EVs-Need-Different- Lubes
[2] Chen Y., Jha S., Raut A., Zhang W., and Liang H. “Performance Characteristics of Lubricants in Electrical and Hybrid Vehicles: A Review of Current and Future Needs.” Frontiers in Mechanical Engineering, October 14, 2020.
[3] “Lubrication Challenges of Hybrid and Electric Vehicles.” F+L Magazine, October 3, 2018.
[4] He, F., Xie, G., and Luo, J. “Electrical Bearing Failures in Electric Vehicles.” Friction, 2020, 8, 4-28.
[5] Tung, S. C., Woydt, M., and Shah, R. “Global Insights on Future Trends of Hybrid/EV Driveline Lubrication and Thermal Management.” Frontiers in Mechanical Engineering, October 20, 2020.
