Critical Properties of Electric Vehicle Drivetrain Fluids

Battery-powered electric vehicles (BEV) are becoming more prominent in the automotive industry. As the prevalence of these vehicles increases, performance expectations are also increasing. OEMs and lubricant manufacturers have realized that providing electric vehicle drivetrain fluids (EDFs) designed to meet EV needs is critical to maintaining vehicle performance and meeting customer expectations. The most important EDF properties for maintaining EV performance include low viscosity, good gear protection, low foaming tendency, excellent heat transfer, protection against corrosion and deposits, good compatibility with polymeric materials, prevention of electrical discharge, and good oxidation resistance [1,2].

Drivetrain Fluid Viscosity and EV Efficiency

Consumers are choosing electric vehicles because they perceive the promise of lower carbon emissions and reduced operation costs. Most electricity is produced by the combustion of fossil fuels at power plants, so vehicle efficiency is critically important to consumers trying to reduce their carbon footprint. Efficiency is also important to those trying to reduce the cost of driving their vehicles and to maximize the distance they can drive with current battery technology. BEVs have 3.4 times the efficiency of vehicles with internal combustion engines (ICEs); however, the drive system creates a greater portion of the total losses in a BEV [1].  These losses due to friction can be reduced by using lower-viscosity fluids. Traditional tests such as kinematic viscosity (ASTM D445), Brookfield viscosity (ASTM D2983), and high-temperature, high-shear viscosity (ASTM D4683) therefore continue to be critical for EV drivetrain fluids.

Gear Protection for EVs

Traditionally, viscosity has been the main means of protecting gears.  Higher-viscosity fluids provide a substantial protective layer between gear teeth that prevents contact and reduces wear.  For EV drivetrains with low-viscosity lubricants to improve system efficiency, that layer is thinner and less effective. Another fluid-film problem comes from the nature of electric motors. Unlike in an ICE, the torque is at its maximum when the motor is starting from rest, and gears are exposed to high loads at low speeds [3]. High speeds are necessary to maintain an adequate protective fluid film. Therefore, additional means of gear protection are necessary.  Gear protection is often provided by anti-wear (AW) or extreme pressure (EP) additives in the lubricant.  The performance of these additives can be tested by several tribological tests that vary load and speed under lubricated conditions, such as the FZG Gear Test (ASTM D5182 or ISO 14635). 

Reducing Foam from High-Speed EV Components

For any lubrication system, air in the fluid is a problem.  It can interrupt the protective fluid films and impair the fluid’s load- and heat-transfer properties. Foam is considered a concern in EV fluids due to the high rotational speeds, propensity for fluid shearing in the gears, and the lubrication and cooling techniques common to EV drivetrain systems that promote fluid churning and splashing [1]. The tendency of a fluid to foam and the time it takes for the foam to collapse are critical. This is often controlled with silicone-based anti-foam additives. While they do not exactly replicate conditions in an electric vehicle, the foam test (ASTM D892 and ASTM D6082) and air-release test (ASTM D3427) may be used to assess the effectiveness of anti-foam additives in EV drivetrain fluids. 

Maximizing Heat Transfer with Fluid Properties

Lubricating fluids always play a role in transferring heat away from the site of its generation. This role is more important for the electric drivetrain fluids than other fluids. In addition to the heat generated by friction, there is a significant amount of electrical heat generated in the motor.  Many designs intentionally bring the lubricant into direct contact with the motor windings and other electronic components to cool them [4]. A number of fluid properties, such as density, thermal conductivity, specific heat capacity, and viscosity, affect a fluid’s heat transfer effectiveness [4]. For improved heat transfer, density, thermal conductivity, and specific heat capacity should be increased while viscosity is reduced. Each of the fluid properties can be determined with the appropriate test: specific heat capacity (ASTM E1269, ASTM D7896), thermal conductivity (ASTM D2717, ASTM D7896), density (ASTM D1298), and viscosity (ASTM D445).

Preventing Corrosion and Conductive Deposits

Reducing corrosion and conductive deposits is important in electric drivetrain fluids.  Any damage to electrical or structural metallic components, such as circuitry or gears, is problematic [1,2,5].  Conductive deposits can create shorts that quickly do severe damage to the motor windings [1,2]. Even non-conductive deposits reduce the ability of the fluid to remove heat from components [4]. The tendencies for corrosion and deposit formation often depend upon fluid additives. Traditional corrosion tests ASTM D130 and ASTM D665 give a basic assessment of a fluid’s ability to protect it from copper corrosion or rust. The wire corrosion test (SAVLAB EV-WCT, ASTM designation in-process) and the conductive deposit test (SAVLAB EV-CDT, ASTM designation in-process) give a better indication of problems with copper corrosion or conductive deposits in EDFs because they simulate conditions specific to EVs [1]. The Oxidation Stability Test (CEC L-48) indicates the tendency for fouling by deposits, regardless of whether they are conductive or not [4].

Assuring Fluid Compatibility with Polymeric Materials

Just as the metallic components of the drivetrain system need protection, so do the polymeric components like structural plastics, seals, and insulating materials.  Contact with fluids can cause swelling, changes in hardness, and changes in tensile strength for polymers [2]. These changes in insulating materials can cause cracks, chipping, thinning of coatings, or permeation of coatings. If the insulation is sufficiently damaged, the possibility of damaging shorts occurs when electrical components are spaced closely or conductive contaminants, such as corrosion and wear byproducts, enter the fluid. One means of assessing material compatibility is to measure tensile strength changes after exposure to the fluid. This technique can be applied to hard plastics or to lower tensile-strength materials as in elastomer compatibility tests (ASTM D4289, ASTM D7216, CEC L-112, ISO 1817).

Protection from Stray Currents and Damaging Electrical Discharge

Electrical discharges have a negative impact on corrosion, electrical component function, and gear and bearing wear [1]. Not only does the fluid need to avoid being electrically conductive, which would allow stray currents and could create safety hazards, it must avoid allowing static charge to build up to the point where a harmful discharge can take place [2].  This means the fluid’s electrical conductivity must be within the electrically dissipative range. There are several tests specifically for measuring the conductivity or resistivity of insulating transformer fluids.  A related method, ASTM D2624, was developed for aviation and distillate fuels and gives similar information.

Maintaining EV Fluid Performance with Oxidation Resistance

 Any effective electric drivetrain fluid will only be effective as long as it maintains its properties. The primary means for lubricant degradation in most applications is oxidation.  Oxidation can be caused by exposure to oxidizing chemicals and elevated temperatures. Fluids in BEVs do not need to contend with the reactive combustion byproducts that ICE lubricants must withstand.  However, the fluid is exposed to elevated temperatures when contacting motor components [2]. Furthermore, lubricants are exposed to other materials that could catalyze the oxidation process, such as copper.  Finally, any electrical discharge creates localized high temperatures that can cause oxidation [1]. Oxidation stability tests such as CEC L-48 or direct measurements of oxidation products using FTIR after aging or service (ASTM D7214) can be used to determine a fluid’s resistance to oxidation.

[1] Canter, Neil. “Tribology and Lubrication for E-Mobility: Findings from the 2^nd STLE Conference on Electric Vehicles.” STLE, Oct. 2023.

[2] McGuire, Nancy. “Test methods for evaluation of electric vehicle drivetrain fluids.” STLE. TLT Webinar October 2023. https://www.stle.org/files/TLTArchives/2023/10_October/Webinar.aspx, accessed Oct. 30, 2023.

[3] Van Rensselar, Jeanna. “The Tribology of Electric Vehicles” Bearing News. June 7, 2019. https://www.bearing-news.com/the-tribology-of-electric-vehicles-2/, accessed Nov. 3, 2023. 

[4] Gahagan, Michael, “Managing Heat Through E-fluids.” A Virtual Conference On EV Engineering, Charged Electric Vehicles Magazine, Oct. 5, 2023. https://chargedevs.com/oct-2023-session/managing-heat-through-e-fluids/, accessed Oct. 30, 2023.

[5] TotalEnergies. “TotalEnergies last-generation EV Fluids validated under real-life conditions.” 27/06/2023 – NEWS. https://lubricants.totalenergies.com/news-press-releases/totalenergies-last-generation-ev-fluids-validated-under-real-life-conditions, accessed Nov. 3, 2023.