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.

emDOC: Innovative Electromagnetic Clutch for Enhanced Comfort

The emDOC electromagnetic dog clutch from HOERBIGER combines the advantages of a mechanical clutch with the precision of an intelligent electromagnetic actuator, enhanced by a position-sensing sensor. This smart 3-in-1 solution operates without wear and tear, saving space, weight, and costs.

The innovative HOERBIGER emDOC is an efficient On/Off clutch designed for safe torque transfer, ideal for use in hybrid and electric drivetrains. It can serve as a connect/disconnect application to decouple unused drivetrain components, such as a secondary axle. Other applications include multi-gear transmissions, as well as parking and differential locks, where the emDOC prevents wheel spin. The removal of external mechanical components reduces the number of parts, saving installation space, weight, and cost.

Versatile design and flexible use

The HOERBIGER emDOC stands out for its scalability and adaptability. Unlike conventional systems that require a shift fork and a gear reduction for the actuator, the emDOC offers a compact, all-in-one solution with integrated actuation. It can be customized to different voltages ranging from 12 to over 48 volts, depending on customer requirements.

Smart design and reliable performance

The emDOC system features an intelligent design for precise control. It consists of a mechanical claw system for torque transmission, combined with a mechatronic system that includes an electromagnetic actuator and a position sensor. When voltage is applied to a copper coil, it generates a magnetic field that moves the coupling sleeve into the desired position. Because the system does not use permanent magnets, fewer iron particles adhere to the components, ensuring reliable, interference-free operation.

Thanks to the full integration of the actuator into the claw system, the system switches quickly and efficiently. The contactless operation reduces wear and ensures precise positioning of the coupling sleeve over its entire lifespan. The shifting operations are performed directly on the coupling sleeve, ensuring exact positioning from the first to the last shift.

emDOC: Innovative Electromagnetic Clutch for Superior Comfort

The emDOC electromagnetic dog clutch by HOERBIGER integrates the benefits of a mechanical clutch with the precision of an advanced electromagnetic actuator, augmented by a sensor for position-sensing. This intelligent 3-in-1 solution delivers wear-free actuation while optimizing space, weight, and cost efficiency.

The HOERBIGER emDOC is a highly efficient On/Off clutch engineered for secure torque transfer, making it an ideal solution for hybrid and electric drivetrains. It functions as a connect/disconnect mechanism to decouple inactive drivetrain components, such as a secondary axle. Additional applications include multi-gear transmissions to realize different ratio stages as well as parking locks and differential locks, where the emDOC effectively prevents wheel spin. By eliminating external mechanical actuation components, the system reduces part count, conserving installation space, weight, and cost.

Versatile Design and Flexible Application

The HOERBIGER emDOC excels with its scalability and adaptability. Unlike traditional systems that rely on a shift fork and gear reduction for actuation, the emDOC provides a compact, all-in-one solution with integrated actuation. It can be tailored to various voltage ranges, from 12 volts to over 48 volts, to meet specific customer requirements.

Smart Design and Reliable Performance

The emDOC system is engineered with an intelligent design for precise and reliable control. It consists of a mechanical clutch system for torque transmission, paired with an electromagnetic actuator and a position sensor.

When voltage is applied to the copper coil, it generates a magnetic field that moves the coupling sleeve to the required position. The absence of permanent magnets prevents iron particle adhesion to components, ensuring interference-free and reliable operation.

With the actuator fully integrated into the clutch system, emDOC delivers fast and efficient shifting. Its contactless actuation prevents wear and ensures precise positioning of the coupling sleeve throughout its lifetime. Shifting operations occur directly on the coupling sleeve, guaranteeing exact positioning from the first to the final shift.

JJE advances differential locker technology JJE differential locker driven by DirectFluxTM electromagnetic clutches

Differential locker locks up differential to ensure torque output on vehicle’s wheel under low traction conditions. the axles together to provide 100% of available torque to the wheel with traction. At 2022 CTI US, JJE debuted its mono-stable and bi-stable DirectFlux differential locker (eLocker). Now this eLocker has been equipped by mid- and full-size SUVs, pickup trucks, and off-road vehicles. JJE eLocker features compact size, high torque, and high engagement speed. JJE has achieved maximum 18,000Nm locking torque in JJE’s 2-speed electric beam axle. Depending on vehicle requirement, JJE’s eLocker can be bi-stable, which is fail-safe (prevent sudden locker release and loss of traction); or can be mono-stable to achieve “default to open” function.

Ping Yu, CEO, Chief Engineer, Founder, Jing-Jin Electric
Dr Yang Cao, Transmission Clutch Team Manager, Jing-Jin Electric

DirectFluxTM electromagnetic clutch makes locking faster and safer

DirectFluxTM electromagnetic technology clutch is used to drive the locker. The clutch uses direct magnetic force, or magneticflux in the same direction as the magnetic force. JJE created unique magnetic circuit that greatly reduces flux leakage, hence utilizes the magnetic flux to generate force more effectively, generating higher electromagnetic force than conventional reluctance clutches. With the DirectFluxTM electromagnetic technology clutch, eLocker can be designed as mono-stable or bi-stable based on vehicle requirement. A mono-stable eLocker is locked by magnetic force generated by current, and is unlocked by spring force when the current is turned off. A bi-stable eLocker will only change its state when there’s an affirmative current command, which is only a pulse; otherwise, it will hold its state.

Normally, bi-stable differential locker provides higher safety level, as it won’t change state in the event of loss of control. This fail-safe feature can prevent sudden locker release due to control failure. This security enhancement is valuable for rear axle, or the “main drive axle”, for conditions such as claiming steep rock trail or pulling heavy boat out of water on slippery ramp. While for front steerable axle, mono-stable differential locker is sometimes preferred as it will default to open position in the event of control failure, which unlocks the front differential to allow easier steering at higher speed.

No speed limit, no drag loss

Conventional locking differential must limit vehicle speed very low for engagement, and also limit vehicle’s running speed after engagement. “The locker engagement should not be maintained greater than 30kph”, or “the locker only can be engaged when vehicle speed is below 5kph”. The operating speed is limited by the rotational friction between differential case and electromagnetic coil case during locker’s engagment. And the conventional locker can only be mono-stable.

JJE’s eLocker is driven by DirectFluxTM electromagnetic clutch, which has no contact or friction between moving plate and coil assembly during engagement and disengagement. Therefore, there is no mechanical wear, no drag loss, no heat generation. Therefore, JJE eLoker does not limit speed for engagement or operation.

Wide application

Differential locker is indispensable in off-road vehicles, and other high capability vehicles such as heavy duty pick-up trucks JJE’s eLocker has been used in several independent axle 3-in-1 drive units, with output torque ranging from 3000Nm to 6,000Nm. The differential locker is also adopted in JJE’s 18,000Nm electric beam axle. JJE eLocker does not limit operating speed, which enhances these vehicles’ capabilities when both locking and speed are needed, e.g., driving through sand or mud.

JJE eLocker for back-to-back dual motor drive

Back-to-back dual motor e-drive does not require a differential, but some vehicles do require a locker that mechanically locks up left and right wheels. JJE’s electromagnetic clutch is suited for such locker function. JJE has two back-to-back dual motor drive units – coaxial planetary and offset layshaft – and both drive units use bi-stable electromagnetic clutch as locker.

Fig. 4 JJE’s eLocker for back-to-back dual motor drive unit

JJE eLocker & eDisconnect combo system

JJE eLocker can also be integrated with JJE eDisconnect, which becomes a compact disconnect & locker combo. This combo is most valued for secondary drive axle, where disconnect is used for changing configuration between 4×4 and 4×2, and locker is used for traction assurance. As example, a 4×4 off-road SUV needs such flexibility.

The eLocker-eDisconnect Combo packages nearly the same as JJE’s single eLocker, thanks to the deeply integrated mechanical design and very its compact electromagnetic elements. This new Combo product will be launched in a high-end off-road SUV in 2025.

Fig. 5 eLocker & eDisconnect solution for high-end off-road SUV

eDrive application rapidly expands and now reaches almost all market segments. Users accept no compromise in vehicle’s capability, and even expect more from eDrive than ICE. JJE eLocker, eDisconnect and the Combo are designed to meet such high market expectations, and will continue to push out the envelope of technology.