The first standard specification for EV fluids by TotalEnergies

Liang XUE, Emmanuel PINOT, Emmanuel MATRAY, TotalEnergies Lubricants Technology and Product Engineering Flavio SARTI, Richard VERNAY, TotalEnergies R&D

TotalEnergies Lubrifiants developed the first standardized specifications for Electric Drive System (EDS) fluids. This new performance standard is a first in the industry for hybrid and electric vehicles.

Pioneering electrical lubrication

In 2019, TotalEnergies Lubrifiants introduced Quartz, Rubia and Hi-Perf EV Fluids, the world’s first ranges of fluids specifically engineered for hybrid and electric vehicles, covering both light and heavy vehicles as well as two-wheelers. These fluids were designed to meet the specific requirements of hybrid and electric vehicles, as well as associated electrical, thermal, and frictional constraints.

TotalEnergies Lubrifiants EV Fluids were also designed to meet the needs of automobile manufacturers and support them in developing eicient driveline systems, while maintaining the vehicles in optimum operating conditions throughout their service life.

Today, TotalEnergies Lubrifiants is once again demonstrating its commitment to innovation in hybrid and electric vehicle lubrication by developing the first standardized specifications for Electric Drive System (EDS) fluids.

This development comes at a time when no standard exists for EV fluids, unlike conventional transmission oils. This standard has been drawn up to ensure that EV fluids meet strict criteria such as viscosity, oxidation, corrosion, durability and material compatibility, while optimizing the fuel eiciency and performance of electric motors and transmissions.

A comprehensive specification

TotalEnergies Lubrifiants has taken the lead in developing this specification tailored specifically for these fluids. Leveraging its expertise and cutting-edge testing resources, TotalEnergies has introduced this new performance standard, a first in the industry for hybrid and electric vehicles. This standard is designed to provide crucial support for automobile and parts manufacturers.

This very first specification has been achieved through a selection of test procedures. Based on TotalEnergies Lubrifiants’ extensive expertise in the field of fluids, this methodological development process firstly guarantees the good physicochemical properties of Quartz, Rubia and Hi-Perf EV Fluids, as well as their compatibility with diferent materials, in particular the new materials used in electrical applications, compared with conventional transmissions. Next, the tribological properties and durability of TotalEnergies Lubrifiants EV Fluids has been verified and confirmed at component level for gears and bearings. In addition, this process has included the creation of several test benches. First, a standardized bench was created to test the eiciency of the transmission at high speed in order to classify fluids according to their ability to improve battery life. A standardized bench was then developed for drive units to classify fluids according to their thermal capacity in electric motors. Finally, a durability methodology has been designed, based on road data and implemented on powertrain test beds, to speed up the vehicle validation process by reducing the time required.

The new specification is an industry first for electric vehicles. It is designed to ensure that TotalEnergies EV Fluids deliver outstanding performance when faced with the specific challenges of electric applications. It demonstrates once again TotalEnergies’ pioneering role in the transition era of vehicle electrification and its commitment to developing cutting-edge vehicle technologies, as well as its commitment to supporting vehicle manufacturers with innovative and tailored solutions and tools. With this new EV Fluids standard, TotalEnergies Lubrifiants continues to strengthen its position as a leading innovator in electric and hybrid vehicle lubrication.

Leading the way in e-fluid developments

As automakers work to decarbonise, most are opting for powertrain electrification, an option that is driving growth in dedicated hybrid and electric vehicle transmissions. However, these systems present new performance challenges that require dedicated fluids to ensure their complete protection. As leaders in driveline additive technology and e-fluid formulation, Infineum has invested in the development of step-out technology and innovative new test methods to ensure our e-fluids deliver the required performance in critical areas.

Andrew Wood, Driveline Fluids Technologist, Infineum UK Ltd
Scott Campbell, Hitesh Thaker, Masahiro Ishikawa, Driveline Fluids Technologists, Infineum USA

The pressure to decarbonise, coming from both regulators and con sumers, means the number of hybrid and battery electric cars on our roads is growing. Which in turn means the use of reduction gearboxes and dedicated hybrid transmission systems are also increasing. This expanding electrified vehicle parc needs bespoke e-fluids, that provide not only traditional transmission fluid properties but also meet new e-specific requirements.

Fig. 1 e-fluid requirements

It’s a careful balance. As well as meeting all of the requirements in Figure 1, fluids must be formulated to optimise transmission performance and protection. And, in today’s lower viscosity environment, this is even more vital, which means careful component selection is important when formulating fluids for these applications.

These new electrified vehicle performance challenges require dedicated e-fluid technology. As leaders in driveline additive technology and e-fluid formulation, Infineum has developed dedicated e-fluids, optimised for excellent field performance. To assess these new fluids, we’ve developed innovative new tests in some of the critical areas, designed to be closer to the real-world application than currently available industry test methods. Two recent examples are a high-speed aeration test (HSAT) and an energised copper test (ECT), which have given our technologists new and exciting insights into fluid performance.

Material compatibility testing

With motors now being placed into the transmission, a number of materials are being introduced that are significantly different from those used in conventional powertrain architectures. For example, the copper wire and connections are susceptible to corrosion, which can lead to electric current leakages or to a short circuit in the transmission. These potential issues mean many OEMs see copper compatibility as an important e-fluid design parameter.

With motors now being placed into the transmission, a number of materials are being introduced that are significantly different from those used in conventional powertrain architectures. For example, the copper wire and connections are susceptible to corrosion, which can lead to electric current leakages or to a short circuit in the transmission. These potential issues mean many OEMs see copper compatibility as an important e-fluid design parameter.

Infineum’s energised corrosion test (ECT) uses a single printed circuit board with copper grids, that can be used with a covering board to provide a capillary gap (Figure 2). Board spacing can be tuned with a spacer washer and various board designs have been used to investigate trace spacing impact.

Fig. 2 The oil immersed energised corrosion test (ECT) allows us to screen technology to ensure good copper compatibility.

Tuning trace and board spacing had a significant impact on fluid performance (Figure 3). These data highlight the critical importance of tuning test conditions and set-up to screen for real world performance.

Fig. 3 Impact of trace and board spacing

The novel video imaging system has delivered new insights into copper corrosion mechanisms (Figure 4).

Fig. 4 Deposit / dendrite growth between anode and cathode

This new rig is helping us to better understand some of the parameters impacting corrosion and the ways corrosion progresses. These insights will be helpful in developing advanced e-fluids capable of delivering better material compatibility performance.

New high speed aeration test

Infineum has also developed a high-speed aeration test (Figure 5), which more closely matches the conditions found in high-speed e-motors and gearboxes vs the standard ASTM test – addressing a gap in e-fluid performance.

While ASTM D892/D6082 use airflow to generate foam in the test fluid the foam may not be representative of aeration experienced in real world electric vehicle hardware, where e-motors and gears spin at >20,000 rpm – much faster than in conventional ICEs.

In electrified applications, high speed shear from gears and bearings can lead to increased aeration of the driveline lubricant. This is a challenge since it can cause cavitation. This leads to irregular fluid film and loss of hydraulic performance, resulting in wear.

Using the existing ASTM D892 and D6082 foaming test as a starting point, our objective was to mimic the impact of parts spinning at speeds of up to 27,000 rpm and to add in a high-speed shearing and churning effect to simulate the aeration caused by parts spinning at these very high speeds. Automation and video capture help to ensure heating and timing accuracy.

The newly developed High Speed Aeration Test (HSAT) has been used to assess the impact of fluid viscosity, viscosity modifier and anti-foam selection on aeration.

Following the successful development of this new test, Infineum is pursuing options for industry standardization of the HSAT.

Fig. 5 The Infineum HSAT test set up mimics the effect of parts spinning at extremely high speeds under shearing/churning

Conclusion

As leaders in driveline additive technology and e-fluid development, Infineum has invested in developing new test methods to provide insights into critical performance areas. The deeper understanding of materials compatibility and aeration is helping us to develop step-out dedicated e-fluids with the optimal performance balance designed to protect electrified transmission systems.

Our new test methods give us a fast and effective means to screen a wide number of formulations.

Following on from these laboratory tests, our advanced e-fluids are tested in real-world conditions. We have already completed almost three million kilometres of field trials across the globe – testing our extensive e-mobility product portfolio in a wide range of hybrid and electric vehicles.

Infineum technology is setting the e-fluids benchmark with next generation products to provide performance you can rely on.

About Infineum

Infineum is a specialty chemicals company with strong research and development capabilities focused on innovative chemistry that plays a crucial role in sustainability. They provide products essential for the electrification of mobility and work towards making internal combustion engines as clean as possible. Their expertise extends to generating sustainability advantages for numerous new markets globally. The company has a rich heritage supported by leading-edge research and development activities, having been innovators of additive products for nearly 80 years. These products are used in automotive, heavy-duty diesel, and marine engine oils, diesel fuels, and specialty applications such as transmission fluids and gas engine oils. Their smart solutions have become key components of today’s most demanding applications and advanced hardware systems. The organization operates worldwide production facilities with sales representation in more than 70 countries.

A unique collaboration for the development of the next generation electric transmission fluid

Influence of lubricant on electrical drive unit

In the rapidly growing Chinese EV market, for a new EV product to stand out, it needs to provide premier performance in range, NVH and reliability under all operating conditions, regardless of drive cycle (city, rural or highway) in climate extremes.

Weiyi Wang, Engineering Manager, Li-Auto
Guodong Liu, Insulation and Lubricant Engineer, Li-Auto
Lucy Hu, R&D Group Leader, Driveline, Afton Chemical Corporation
Jon Horner, Senior R&D Engineering Specialist, Afton Chemical Corporation
Wenjun Liu, Senior CTS Specialist, Afton Chemical Corporation
Yun Zhang, Senior OEM relationship Manager, Afton Chemical Corporation

To make an EV successful, the electrical drive needs to have the highest efficiency while enduring high power density to provide outstanding reliability. To achieve this, the lubricant plays a crucial role as it is the means to remove heat from the motor while providing lubrication and protection for gears and bearings. The list below displays examples of how a lubricant can influence an EV’s drivetrain performance.

Scuffing, Pitting and Wear Protection – Poor lubrication is the primary failure mechanism for all of these phenomena. When a lubricant fails to provide proper extreme pressure or scuffing and anti-wear protection during high contact pressures and high sliding speeds, the gears and bearings can rapidly fail.

Coefficient of Friction – The gear meshing and bearing frictional losses account for 30-40% of total power loss in EDU, especially when experiencing higher power demands. All gears and bearings are lubricated with the electric transmission fluid (ETF), so lowering the coefficient of friction is one of the most effective ways to improve a drivetrain’s efficiency.

Electrical Properties – Breakdown voltage, resistivity, and permittivity can influence the electrical induced bearing damage (EIBD) behavior, which is a unique and challenging problem for all inverter driven motors due to common mode dV/dT events and circulating currents.

Understanding that the appropriately formulated ETF technology is the key to promote efficiency, durability and reliability of an EDU, Li-Auto has engaged with Afton Chemical Company to develop a customized ETF to best suit the Li-Auto in-house-designed drive unit.

Li-Auto’s Methodology on the customized ETF’s development

The next generation ETF, Advanced Performance Fluid 1.0 (APF1.0), was designed and developed specifically for Li-Auto’s next generation, in-house designed drive unit, Scalable Power Drive (SPD) (see Figure 1). This is the main drive platform for both REV and BEV models.

SPD is an 800V electrical drive unit platform. The inverter, motor and gearbox are integrated as one whole assembly, but its peak power and torque could be tuned from 200~300kW, 3500~5000Nm by adjusting power modules, stator & rotor, or geartrain as it is a modular designed drive unit. It enables SPD to suit various car models. Apart from that, SPD has boost charging function, which significantly reduces charging time but meanwhile much more heat is generated from the rotor. The system poses great challenge on cooling and lubrication, and APF1.0 needs to have great heat dissipation and hardware protection performance. On top of that, Li-Auto has focused on how this directly contributes to the vehicle range and battery cell cost, which ultimately influences the overall vehicle performance.

The ETF design is an optimum combination of chemical additives and base oil technology to deliver balanced performance. If the balance is not adhered to, the desired level of performance in the drive unit can be compromised.

The primary design target for the APF1.0 development was efficiency, while providing adequate hardware protection, especially under low lambda ratios. To improve efficiency, a geartrain power loss model was developed to calculate the contribution from sliding and rolling losses and churning losses (gears and bearings) to better guide the lubricant formula design. During the modeling work, it was found that different hardware configurations yield different power loss contributions.For example, churning loss is greatly reduced in a geartrain with intermediate shaft placed on top (no contact with sump oil, as illustrated in Figure 2) compared to that placed at the bottom (partially submerged in oil sump) and rolling frictional loss could be greatly reduced with ball bearings versus tapered-roller bearings.

*Geartrain configuration of above figure:
BTTO – Ball bearings on input shaft, Tapered roller bearings on intermediate and output shaft, intermediate shaft on top with churning from output gear only
BTTIO – Ball bearings on input shaft, Tapered roller bearings on intermediate and output shaft, intermediate shaft on bottom with churning from both intermediate gear and output gear
BBTO – Ball bearings on input and intermediate shaft, Tapered roller bearings on output shaft, intermediate shaft on top with churning from output gear only
*Power loss calculation models:
Gear mesh & churning loss is calculated per ISO14179-1;
Bearing loss is calculated per SKF’s public bearing loss empirical model

The same to hardware protection. For example, pitting life reduces exponentially as contact stress in-creases for gears and bearings. If the hardware design puts the contact stress near the limit, it will be required to have an ETF that has ultra-high-performance regarding pitting and scuffing. In this scenario a higher viscosity would be preferred to ensure oil film is sufficient. On the contrary, if the drivetrain is under lower stress level, a lower viscosity fluid could be applied to provide the greatest vehicle range.

Based on the aforementioned performance factors, Li-Auto decided that the APF1.0 design had to be customized to obtain the best performance with the SPD hardware. An off-the-shelf product will not provide the ultimate performance. Li-Auto selected Afton Chemical as cooperation partner as Afton fully understands Li-Auto’s technological capability and provides a new formulation that meets the specified requirements, providing both range extension and hardware durability.

In this cooperation project, both parties agreed that as OEM, Li-Auto’s role is to provide clear and specific evaluation methods for the lubricant based on Li-Auto’s hardware design; and as the lubricant expert, Afton’s role is to design and develop formulas that could meet Li-Auto’s requirement. During this process, it was found that the conventional lubricant evaluation methods such as FZG (spur gears) and FE8 (cylindrical roller) do not appropriately evaluate APF1.0 as related to the end application utilizing helical gears, ball and tapered roller bearings. Thus, we designed our own evaluation methods in terms of pitting prevention, friction optimization and churning loss based on our hardware design and actual operating conditions from fleet statistic data.

Performance evaluation of APF1.0

The efficiency performance of APF1.0 was evaluated by comparing it with the previous generation fluid on the same dyno and the same drive unit (a selected SPD drive unit, with baseline efficiency of 91% – CLTC drive cycle at 40°C). APF1.0 provides a 0.3% efficiency increase per CLTC drive cycle at 40°C oil sump temperature; 1.1% increase per CLTC drive cycle at -7°C oil sump temperature; 0.5% increase under 120kph highway cruising condition at 40°C. It showed significant efficiency increase at both normal and cold temperatures, both city-road and highway conditions.

Fig. 4 Efficiency gain chart of APF1.0

On the reliability side, we performed the geartrain endurance test, high temperature high speed test, power temperature cycling endurance (PTCE) equivalent to the EV life of 300,000 kilometers, covering driving intensity of 99.7% of fleet drivers. No failure was detected after any of the tests.

Before performing efficiency and endurance tests on full drive unit level, we conducted a series of lubricants screens to ensure valuable dyno stand resources were maximized for candidates. We set coefficient of friction reduction targets for the lubricant at multiple operating conditions based on the power loss simulation model and set clear requirements on the lubricant’s hardware protections. Only those formulas that met all the requirements would be selected to the drive unit level test phase.

For friction evaluations at lubricant level, MTM was used to determine the friction performance of the fluids. The friction performance was compared to the reference at different loads, entrainment speeds, slide-to-roll ratios and bulk oil temperatures. The operating conditions are derived from the drivetrain’s actual operating conditions and allows an appropriate insight as to how the fluid can influence overall efficiency as related to friction reduction.

On the hardware protection lubricant level test side, since APF1.0 is a low viscosity lubricant used in a high power density, high torque drivetrain, we are most concerned about a fluid’s pitting and wear protection capability. For pitting, we used MPR test equipment for evaluation. We specified contact stress, sliding speed, entrainment speed and run time based on one of the most aggressive drivers’ driving profiles collected from fleet data. The reference fluid pitted after 38.77 million cycles (43.7hrs) but APF1.0 was able to complete 87.83 million cycles (99hrs) without pitting.

Projection of ETF trend

We believe the items listed below will be the future of ETF development from an OEM standpoint.

Efficiency is always the first priority, as it is the key to reduce the overall cost of the vehicle and resolve EV range anxiety. New combinations of additives and base oils are currently underway to reduce coefficient at thin film conditions, and improve durability while improving EDU efficiency at lighter load duty cycles. In the future, base oil and additives with new molecular structures could potentially further bring friction down.

Viscosity should be carefully selected based on hardware protection needs. Low viscosity does not always provide higher efficiency gains in an EDU. An ETF with properly selected viscosity could allow more operating conditions to fall into the elastohydrodynamic regime (which gives lowest coefficient of friction). To achieve such, the OEM needs to build capability to perform drivetrain power loss analysis and component loss simulations with input from gear, bearing and lubricant suppliers.

EIBD is a unique phenomenon and challenging topic for BEV and REV drivetrain. Lubricant properties such as breakdown voltage, resistivity, and permittivity could result in different EIBD performance theoretically, and it has been observed from tests that different lubricants do yield different discharge event frequency and severity. It is valuable for the lubricant and powertrain industry to make further study on ETF’s influence to EIBD and provide formula that mitigates EIBD risk in the end.

Li-Auto highly encourages all of its partners in the drivetrain supply chain to join in this effort to make the next significant steps to improve EVs further in cost and performance.