Developing EV-specific Lubes

As electric-vehicle technology accelerates, dedicated e-fluids are necessary to maintain proper levels of drivetrain performance.

Many OEMs have an increasing interest in electrifying the transmission, but an integrated e-motor presents significant tribological challenges.

Electric vehicle (EV) technology is transforming the world of transportation at an increasingly fast pace, with a range of options including full hybrids, plug-in hybrids or all-electric vehicles. Most existing OEMs are pursuing electrification in some form, and there have been a variety of new market entrants.

As the variety of EVs proliferates, lubricant manufacturers must keep pace to ensure proper fluid management. It has become increasingly clear that high-performance fluids and lubricants are even more necessary than ever, and a wide variety of non-traditional performance characteristics must be accounted for. Electrified drivetrains have different demands; therefore, new fluid formulations are a necessity for longevity and protection.

Dr. Michael Gahagan, technology manager of driveline at Lubrizol.

Many OEMs who are developing electrified drivetrains have an increasing interest in electrifying the transmission, and for good reason: Electrified dual clutch transmissions (e-DCTs) can fit with traditional hardware platforms and can reduce the manufacturing complexity, setting up a cost advantage for hybrids. It is also a good example of some of the inherent challenges of electrification.

Compared to entirely electrical vehicles, a vehicle equipped with an e-DCT can have a smaller battery and hybrid-electric powertrain. It also eliminates the starter motor, which again lowers its complexity. e-DCTs are highly efficient, save on energy use and can be made up of helical gears, synchronizers, clutches, axles and integrated e-motors.

All these components are increasingly expected to be lubricated by a common fluid. For traditional transmissions, this required certain performance attributes. The addition of the integrated e-motor, however, presents significant tribological challenges and a range of new performance characteristics that an e-fluid must be able to provide, including corrosion control, thermal properties, material compatibility, friction performance, electrical properties, and prevention of noise, vibration and harshness (NVH).

Copper compatibility

Because e-fluids interact with charged components in an electrified drivetrain, a properly formulated e-fluid must maintain the right conductive characteristics. Traditional lubricants are already very good electrical insulators (most maintain electrical conductivity of around 0.0000000001 Siemens/cm), but for the purposes of allaying safety concerns, e-fluids generally are expected to maintain even higher insulative properties than traditional fluids.

The e-motor can be integrated into the DCT box and cooled by the transmission lubricant. This approach allows for lower development complexity and higher production volumes.

Issues may arise, however, if an e-fluid’s conductivity is too low. A properly formulated e-fluid should maintain a slight level of conductivity in order to prevent the buildup of static electricity. Achieving this balance of conductive properties is essential to prevent issues in the transmission.

An e-fluid’s interaction with electrical componentry also brings with it concerns around material compatibility and how it interacts with an increasing amount of wiring and electronics. Modern EVs incorporate a significant amount of copper and power electronics, as well as a range of plastics used as insulation or in place of traditional metal. In many cases, electrical componentry may be entirely immersed in a fluid and must remain operational.

A copper corrosion test shows the “robust protection” provided by the customized e-fluid.

Therefore, an e-fluid must be compatible with and be able to protect a broader range of materials. Copper, specifically, poses a critical challenge, as it is highly susceptible to liquid and vapor phase corrosion. To demonstrate this phenomenon, Lubrizol used a copper corrosion test, in which a bare copper piece was placed in a conventional fluid as well as an e-fluid. The two samples were heated to 80°C (176°F) over a period of 168 hours, and the results were striking.

The copper portion above the conventional oil demonstrated significant vapor corrosion damage, whereas the specifically-formulated e-fluid provided robust protection against the same phenomenon. This has implications for the protection of sensitive copper-based components.

Lubrizol also developed an in-house test to measure a fluid’s ability to prevent conductive deposits and corrosion. In this more sophisticated test, a team immersed printed copper circuit boards in a fluid heated to 150°C (302°F) for a period of 1,000 hours with or without voltage applied and monitored the process. The conventional fluid used as reference showed high levels of corrosion and deposit formation, whereas the e-fluid demonstrated outstanding protection. These tests clearly demonstrate the importance of a dedicated fluid that has been formulated to interact with and protect copper electronic elements that are essential to the functionality of EVs.

Additionally, e-fluids increasingly come into contact with different plastics in newer EVs. For example, some OEMs have replaced traditional metal bearing parts with high-performance polymer in thrust bearings. A traditional fluid may have the tendency to compromise the strength of these polymers. For example, Lubrizol tested a polyamide (PA66 GF-35) sample submerged in conventional fluid at 150°C for 1,000 hours and found the sample had lost nearly 45% of its strength. A dedicated e-fluid subjected to the same test provided robust protection. E-fluids must be formulated with this type of performance in mind.

Thermal, NVH considerations

Dedicated fluids formulated to interact with and protect copper electronic elements are essential to the functionality of EVs.

Lubricants and fluids for use within EVs also must have increasingly effective thermal-management properties to handle some severe operating conditions. Unlike traditional internal-combustion engines, the temperature differentials in EV engines often spike at high temperatures and are generally less consistent. For these and other reasons, lubricant manufacturers must pay closer attention to the e-fluid’s heat transfer and thermal stability characteristics.

These characteristics include dynamic viscosity, or how easily the fluid flows; density, or the weight of fluid in a given volume; specific heat capacity, or how much heat the fluid can store; and thermal conductivity, or how easily heat travels through the fluid. A properly formulated e-fluid that balances each of these characteristics can deliver significant benefits. For example, if an e-fluid can help an e-motor run cooler during peak times of operation, the e-motor will operate more efficiently. Additionally, an e-fluid’s ability to prevent extreme temperatures can better protect the entirety of the driveline.

NVH is another critical consideration. An e-DCT may experience clutch shuddering, gear and bearing noises, torque ripple and electrical whining, all of which detract from the overall driving experience. To combat NVH, it’s important to use the proper driveline fluid formulation that provides enhanced friction performance. (For more on lubricant “tuning” for EVs, see here).

Lower-viscosity lubricants and oils are just as popular in EV transmissions as they are in traditional ICE hardware; thinner fluids mean less resistance to moving parts. Manufacturers must find the right balance between this demand and all others.

Ultimately, what does this mean for lubricants and lubricant manufacturers? It means that EVs are increasingly having their own standards for lubricants instead of just borrowing standards from the ICE industry. As transmission designs for EV and hybrid vehicles evolve, manufacturers are developing specialized e-fluids for a range of e-transmissions. As the market continues to move in the direction of higher EV adoption, e-fluids will become more critical to the function and reliability of these vehicles.

Dr. Michael Gahagan, technology manager of driveline at Lubrizol, wrote this article for Truck & Off-Highway Engineering. His most recent SAE Technical Paper on this topic can be found here .