KEY CONCEPTS
• Different materials with different interactions demand different requirements for EV lubricants.
• While friction, wear and oxidative stability are important, there are additional objectives, such as environmental friendliness, for EV lubricant development. 
• Many traditional ASTM tests do not apply to EV lubricants. This leaves developers to devise tests on their own. While many of these tests appear to be helpful, they lack the universal acceptance of standardization.

Unexpected gains in the adoption of electric vehicles (EVs) in the European market, closely followed by the U.S. market, are a challenge to oil and additive suppliers. The key challenges, now and going forward, involve lubricants and test methods. Much of this is due to the complexity of EV powertrains, which require lubricant formulations that:
• Reduce friction and wear
• Meet aeration and oxidation requirements
• Act as coolants
• Are electrically insulating
• Are compatible with EV-specific materials.

In order to determine the compatibility and performance of the lubricants, new test methods need to be developed. As with every challenge there is opportunity, in this case for OEMs and oil and additive suppliers.

Increasing demand for EVs and compatible lubricants
Dr. Timothy Newcomb, strategic technology manager, The Lubrizol Corp., predicts short-term and long-term implications. “The EV market will continue to expand, and the need to increase range will drive the development of new EV lubricants,” he explains. “In the short term, we will see efforts to develop low viscosity lubricants with superior corrosion protection, adequate hardware protection and balanced electrical properties. These lubricants will reduce windage losses and increase, through more effective cooling, the operational efficiency of the motor, leading to greater vehicle range. In the long term, even lower viscosity lubricants will be needed, and these will need to meet the sustainability expectations of end-users.”

Greg Seman, eMobility OEM liaison, Americas/BP Castrol, notes that Europe and China continue to lead the U.S. in the transition to EVs due to legislation and proactive action by organizations and manufacturers. Because of this, many advancements in EV technology and supporting industries are happening in Europe and China. “However, the lag in other regions doesn’t necessarily mean that those regions will need to progress through the same development cycles that Europe did,” he says. “All of the latest technology will be available globally, and all regions will continue to develop next-generation technologies. This is true for both EV hardware and fluids used in EVs.”

Impact of lubricants on EV performance
According to STLE member Arup Gangopadhyay, power unit tribology technical leader, Ford Motor Co., the impacts of an advanced lubricant on EV performance could be increased efficiency and cooling. “Improved efficiency resulting from reduced friction will contribute to reduction of overall energy loss,” he says.

“Cooling functionality of lubricants is something we did not pay much attention to before,” he adds. “Under high torque conditions, copper coils in electric motors heat up very fast and require cooling before they start derating. There are various cooling strategies used today including spraying lubricant. This works well, but it would help more if the lubricant itself has the ability to remove heat faster. This will require improved thermal properties of the lubricant, i.e., higher thermal conductivity and higher specific heat. Lower viscosity also helps in carrying heat away from the affected areas.”

Attributes that will drive innovation in EV lubricants 
Given that EVs have an electric drive system with an electric motor, an inverter, a reduction gear and an electronic control panel, STLE member Dr. Hyun-Soo Hong, driveline advisor and EV fluids development lead, Chevron Technical Center, explains that there are fundamental variations within the design. “As of now, the electric drive system can be modular or fully integrated, but there is a trend to integrate these parts. Therefore, key attributes of a new EV driveline lubricant can vary depending on the types of electric drive unit designs.

“Group III and IV base oils are generally used in EV driveline lubricants. Any well-designed lubricant based on these synthetic base oils should meet most of the performance requirements that are needed for an EV driveline lubricant. In addition, there are ways to responsibly manage the impact of EV driveline lubricants on the environment. One way is to lower the carbon intensity of EV driveline lubricants by using base oils, such as renewable base oils, with lower carbon intensity than the Group III and IV base oils and especially PAOs, which generally show higher carbon intensity.”

According to Hong, there are two pillars of performance that are required for a new EV driveline lubricant.

1. Durability related properties. 
Examples of these properties are:
• Wear protection (e.g., to accommodate lower viscosities of an EV driveline lubricant compared to commercially available automatic transmission fluids [ATFs])
• Copper corrosion resistance (e.g., to protect motor windings, etc.)
• Electrical conductivity (to avoid any detrimental impact)
• Material compatibility (e.g., coating protection) and oxidation resistance (e.g., to provide a fill-for-life drain interval)

These properties are needed to guarantee durability of EVs. Some of these properties also are needed for conventional driveline lubricants, but key requirements such as frictional properties due to the use of clutch packs in ATFs or synchromesh performance in manual transmissions are not needed.

2. Efficiency-related properties.
These include:
• Traction coefficients of fluids
• Lower viscosity
• Thermal conductivity.

Lower traction coefficients decrease frictional loss, thus increasing the efficiency of the EV driveline system. Lower viscosity is generally needed to improve efficiency, but good wear protection must address durability issues. Good thermal conductivity will be beneficial for lowering oil temperatures, especially for the integrated EV driveline system, to avoid an impact on motor performance.

“Any improved efficiency provides OEMs with another tool for developing a better EV driveline system,” Hong says.

Attributes Gangopadhyay believes are important are electrical resistivity and material compatibility. “We have not paid attention to electrical resistivity properties of lubricants in the past,” he says. “But with EVs, we need to keep an eye on it. Generally, lubricants have high enough electrical resistivity to prevent any leakage current passing through bearings. But if we have higher resistivity than the current formulation, it will help in system robustness. Material compatibility is another important attribute. Current ATFs are compatible with seals, elastomers and solenoids, but electric motors have different types of materials that come in contact with lubricants such as polyamide imide coating on copper, insulating paper, polyester fibers, varnish, etc. The next generation EV driveline fluid should be compatible with all these different materials.”

Opportunities for oil and additive suppliers
Currently, ATFs developed for stepped transmissions are used in EV drivelines, but Gangopadhyay thinks oil/additive companies can do better. “This is because traditional ATFs contain additive components to address clutch friction durability,” he explains. “However, in most of the EV drivelines, there is no clutch. Therefore, eliminating those additive components opens up the formulation space to deliver additional performance attributes.”

Newcomb believes that the role of developers and suppliers is to work closely with OEMs in order to understand the direction they are taking and determine how to create lubricants that will enable new EV designs with greater energy efficiency and range.

“Future generations of EV lubricants will rely on new base oils and advanced additive formulations,” he observes. “Work is already in progress to develop the next generation of base oils to meet the future criteria we have been discussing. We, as an additive supplier, are actively developing chemistries to ensure these new oils will provide the necessary hardware protection. Additionally, we are determining how these new chemistries and oils can be merged to create lubricants that will provide the best value to our customers. It has never been more critical for lubricant and mechanical engineers to work together for a sustainable energy efficient world.”

The role of OEMs in EV lubricant development
Some OEMs work closely with additive/oil companies to develop next generation fluids for EVs. Gangopadhyay explains, “We communicate the desired performance attributes and the performance target levels. The additive/oil companies have a wealth of knowledge on how to formulate to meet our desired needs. They also generate a lot of data with their formulations and share those with us to demonstrate how well these meet our targets. We also direct them to a third-party test stand for some critical performance evaluations. Once everything looks good and we select a fluid, it then goes through a battery of in-house durability tests for validation.”

Newcomb recalls that one of the first recognized challenges for OEMs and lubricant developers was ensuring that the lubricant would not provide a potential pathway for leakage current from the busbar¹ to the housing. “This depends both on the electric drive unit design as well as the electrical characteristics of the lubricant,” he says. “As the hardware packaging is reduced to improve range, the electrical conductivity (or volume resistivity) of the lubricant becomes more important. The trick is minimizing electrical conductivity to prevent harmful leakage current without creating an environment for tribo-charging and static discharge. This can be a problem with conventional lubricants used in new electric drive unit designs. Therefore, understanding the lubricant’s electrical characteristics is now more important.”

He continues, “Another challenge is stepping up corrosion protection. Faults within the electronics can lead to disruption of vehicle operation, and as reliance on sensors and other electronics increases, the need for assured corrosion protection increases.

“However, electric motors add another concern as the copper windings can reach very high temperatures, which enable corrosion processes that do not occur at normal operating temperatures. Breaks in the insulating material (magnet wire insulation, weld coatings) can lead to the formation of conductive deposits that can short the motor, leading to motor burn out. Conventional ATFs in current designs have provided acceptable protection, but we have seen these lubricants lead to burn-out failures during validation of new designs.”

Lubricant developers also need to consider that electric motors come with new materials that the lubricant must be compatible with, including the magnet wire insulation, which can be made of complex polymer films, flexible insulation materials and structural plastics. Other concerns lubricant scientists are challenged with relate to wear and air entrainment at high speeds.

“What I have described so far are the challenges that lubricants must meet to protect the next generation of EV hardware,” Newcomb clarifies. “However, beyond this, engineers are looking for the lubricant to play an even greater role in improving vehicle range by increasing system efficiency.”

Newcomb adds that direct cooling of the electric motor by the lubricant already improves the operational efficiency of the motor. But it can be improved further by changing the lubricant’s physical properties—most notably viscosity. “While it’s possible to tweak properties like thermal conductivity and specific heat capacity, the property that really can improve cooling is viscosity,” he says. “Super low viscosity fluids can reduce motor temperature by several degrees centigrade, and this is sufficient to significantly extend range. Also, the motors themselves are sensitive to windage losses, and so these too are reduced. The challenge is how to lower viscosity more than it has been before, while maintaining hardware protection and low volatility.”

Hong emphasizes the importance of creating environmentally responsible lubricants for the rapidly growing range of EVs that OEMs are developing. “Globally, governments are mandating more strict carbon-neutral policies to reduce greenhouse gases and address worsening climate issues,” he says. “To meet these ever-increasing requirements, most OEMs have announced that they will manufacture vehicles with at least one form of electrification by the year 2030. As a part of these efforts, these OEMs are working on developing electric drives to improve efficiency and achieve the maximum benefits out of their electric drive designs. Lubricant technology is an enabling technology and gives OEMs and end-users the benefit of developing and implementing their new equipment and designs. The oil industry has made great progress in emission control and fuel economy over the years. I think we are still at an early stage of EV development, but the oil industry will rise to the challenge and help the OEMs’ innovative designs become reality.”

Developing test methods for new formulations
Since electric motors have significantly higher speeds than internal combustion engines, unique tests and test rigs need to be developed in order to simulate the on-road use of the hardware. “This requires a significant financial investment as new hardware (motors, inverters, dynamometers, etc.) is procured, installed, validated and calibrated,” Seman points out. “What is unique in the EV arena is that there are not yet any industry standard tests; many test rigs and procedures have been developed behind closed doors. Some tests such as the FZG² have been adopted from combustion-powered testing to evaluate EV fluids, but many key differentiating tests are unique and confidential to the test facility that developed them.”

Gangopadhyay says there also are test applicability challenges. “The first one is the copper corrosion test. Traditionally the industry worked with the ASTM D-130 test, which is an immersion test and is still being used for EV fluid development. However, this test is only partially relevant for this application,” he says. “Because copper coils come in direct contact with liquid lubricant, no current is passed through the test sample, which may accelerate corrosion. The other aspect is corrosion induced by lubricant vapor present in the transaxle housing.”

Gangopadhyay explains that there are three types of laboratory bench tests that have been developed:
1. A copper wire corrosion test where an electric current is passed through a thin copper wire while immersed in the lubricant
2. A conductive deposit test where current is passed through a circuit board with copper strips partially immersed in lubricant and partially exposed to air inside an enclosed vessel
3. A strip of copper hung inside an enclosed vessel where it is partly immersed in lubricant and partly exposed in air in the absence of any current passing through it.

These tests are in different stages of development, and there is no agreement on which test best represents actual applications. While none of these are currently ASTM standard tests, that may change.

Gangopadhyay continues, “There is a need for a high-speed gear wear test because the trend is toward higher-speed electric motors. Today the electric motor speed is about 18,000 rpm, but it could go as high as 30,000 rpm. Today we do not have a gear test for the current motor maximum speed. Again, there is a need for an ASTM test method.

“Foaming is another area in need of a better test method. Currently, the industry uses ASTM D892, but the limitation is the absence of any fluid shearing. Development of a test method is needed, and ultimately an industry standard test is desired.”

Per Hong, EV driveline fluid evaluation either in an e-drive and/or in a hub-dyno vehicle test method has not received much attention. “This may be due to the lack of easily available EV driveline systems,” he suggests. “The bench tests that are commonly used provide good indication on durability related properties but don’t provide much insight on efficiency related properties.”

Hong reports that it is possible to successfully evaluate efficiencies of candidate fluids compared to commercially available ATFs. Efficiencies of e-drives were measured in an e-drive and a hub-dynamometer (dyno) vehicle test stand using a commercially available EV from North America. The e-drive test stand was developed using an e-drive that is connected to an inverter and a dynamometer. “The e-drive test is an excellent tool to check any efficiency differences between fluids with different viscosities,” Hong says. “However, the operating ranges are narrow due to instability of the tester at low speed/low torque and very high speed/very high torque conditions. A hub-dynamometer vehicle tester uses a vehicle instead of only an e-drive, so a wide range of speed/torque can be easily accommodated without any instability issues.”

According to Newcomb, though new to the lubricants industry, adequate tests do exist that measure the electric characteristics of lubricants. “We are beginning to see these tests included in new OEM specifications,” he observes. “However, there is a clear need for better corrosion tests. For the most part, our industry relies, as do many others, on a version of the copper strip test described in ASTM D130. We had seen an electric motor burn out during validation testing using a conventional ATF. This ATF had been considered to have good corrosion protection as it passed the OEM’s criteria specified for corrosion using the strip test. Interestingly, the lubricant that replaced the conventional ATF passed the OEM’s criteria with the same strip test result. This suggests that current practices do not distinguish between lubricants that do well or fail in these applications from ones that can cause severe motor damage.”

Lubricants for EVs need to accommodate the interaction of the lubricant with copper for a wide temperature range. “This would be a very cumbersome undertaking with strip tests,” Newcomb notes. “A few years ago, a wire corrosion test was introduced that easily characterizes how the lubricant interacts with copper, or any other metal that can be drawn into a wire, over a wide range of temperatures. Many laboratories have adopted this methodology, and there is work underway to standardize it. Though the method provides an actionable assessment of the lubricant’s corrosion prevention characteristics, it does not address the potential problems caused by

These byproducts are mainly copper sulfides which are semi-conductors and can create pathways to electrically short motors. At least two methods were put forward to address this. One method incorporates a printed circuit board as a test platform and has gained popularity. “The insights gained are very complimentary to the wire corrosion test, and I think both tests will be needed for validating new EV lubricant formulations,” Newcomb says.

Traditionally, lubricant developers have been careful to ensure that the formulations will not damage elastomers used in seals. But the electric motor introduces other polymers, of which the magnet wire insulation is of critical importance. “Motor manufacturers run various validation tests to confirm the lubricant will not damage the wire insulation; however, a corresponding lubricant evaluation test does not exist,” Newcomb observes. “Though much of the current magnet wire insulation is made from very lubricant-resistant films, this may well change under pressure to reduce cost. So, there is a need to evaluate this compatibility during lubricant development, and this is part of joint work we are conducting with other organizations.”

“From an oil supplier perspective, we think the industry should put some urgency in developing a specification with clearly defined performance limits,” Hong says. “The SAE Focus Group on e-drive fluids is in the last stages of finalizing SAE J3200. SAE J3200 is a compilation of recommended and available tests that will help researchers interested in developing an EV driveline lubricant, but it is still a challenge without specific performance limits. Considering other ongoing and just started industry activities, I think the industry will be able to address these challenges in the next few years.”

Conclusions
The automotive industry is in the early development of EVs with different designs and innovations being introduced daily. “Until an overall industry direction is established, the lubricant solution to enable optimum performance will need to be tailored; one size does not fit all,” Newcomb concludes.

REFERENCES
1. A busbar is an electric conductor and ground plane separated by an insulator. It transfers electrical energy from the battery pack to the inverter for conversion to the AC electricity needed by the electric engine.
2. FZG tests are generally used to determine the micropitting load capacity of gear lubricants and also scuffing.

Jeanna Van Rensselar heads her own communication/public relations firm, Smart PR Communications, in Naperville, Ill. You can reach her at jeanna@smartprcommunications.com.