PAO Blend Steps into the EV Thermal Management Fray

The smooth operation of an internal combustion engine vehicle depends on multiple fluids and different circuits of lubrication, and the shift towards electrification changes the whole environment, Sven Meinhardt, marketing technical support chemist for ExxonMobil’s synthetics business unit, told an industry event. 

Critical factors for EVs’ electric motors and batteries are cooling and thermal management. E-motors, inverters and cooper wire wound around the motor core all generate heat during operation. This heat can be dangerous if not contained. In recent years, there have been several cases when on-road accidents caused thermal runaway and fire in lithium-ion batteries, risking the safety of their owners.  

Meinhardt, who is based in ExxonMobil’s Chemical Central Europe facility in Koln, Germany, said one approach the automotive industry should take to solve these and other critical issues, such as pumping efficiency, as well as wear and drag in the mechanical systems, is to try to combine lubrication and cooling in one. 

“This means you need to have a fluid that lubricates the gears, the bearings and the clutches and at the same time is able to cool the water, electronics and battery,” he told the TAE’s Tribology International Colloquium in January. 

This could be a tall task for lube formulators because existent cooling fluids are typically water-glycol formulations, and the shift should either include changing the material of the lubricating parts to suit the water system, which is a challenge, or the vehicle features battery cooling based on some kind of water-hydrocarbon formulation, Meinhardt said.  

Meinhardt calls these new formulations “e-module” fluids in order to contrast them with conventional driveline fluids. Conventional driveline fluids must have certain tribological properties that deliver energy efficiency, durability, oxidative stability, wear protection and material compatibility – this is challenging enough. Meanwhile, e-module fluids also must perform thermal management and have the right conductivity properties, as well as most of the tribological properties of a conventional driveline, all at higher speeds.  

A balance of low viscosity and wear protection is essential for improved efficiency while lubricating and cooling an integrated system, such as the gearbox or e-motor, according to Exxon. Meinhartd also observed that which base oils are used for the formulation of e-module fluids is critical for EVs. 

“It needs to be a base fluid with a low viscosity … But if you lower the  viscosity, you have higher volatility, and that is something you need to avoid at the same time,” he said, adding that e-module fluids typically have higher treat rates as they are lower viscosity than conventional formulations. 

By experimenting with hydrocarbons on a molecular level, Exxon says it has succeeded in formulating a blend of polyalphaolefins and alkylated naphthalene API Group V base stocks that can provide the desired low volatility and lower viscosity, in addition to the properties sought after in conventional driveline fluids. 

“Use of two-box synthetics leaves not much space for the additive systems … But when you think about these additives, you need other technology as well, as they are acting in different environments, with electrical flow, and so on,” Meinhardt said. The Exxon blends use a viscosity modifier, pour-point depressant and an additive package. 

Exxon suspects that thermal management and pumping efficiency could be due to long greater number of pumps and joint pushing fluid through longer pipes supplying the e-motor and battery cooling systems. Its research showed that this can be mitigated by using a blend of PAO and Group V base stocks. The company also claims that this novel PAO blend can provide significantly lower friction and coefficient and torque loss compared with a Group II+ and Group III+ blend, for example, which can result in improved energy efficiency in mechanical systems and extended range. 

These blends can also “provide higher film thickness compared to a Group II+ and Group III+ blend, which can result in better wear protection in mechanical systems like gears and bearings. Also, it can provide excellent clutch durability, including longer life and anti-shudder characteristics. It also has a desirable frictional gradient which enables smooth torque transfer and more energy transfer.”  

These types of base stocks have shown further improvement when compared with conventional PAOs, as well. 

“Ultra-high pressure/temperature viscosity results in Group II+ and Group III base oils suggest possible phase transition at lower temperatures, which may have unanticipated consequences, like mechanical damage or efficiency loss in the systems like bearings and gears,” the company said.  

Exxon’s novel PAO blend shows normal pressure-viscosity behavior and excellent thermal and dielectric properties, energy efficiency, film thickness and clutch performance, the company claims. 

All of these properties are essential for smooth functioning of EVs even in high summer temperatures. Indeed, Exxon also says its novel PAO base oil blends show excellent properties for EV operation in the summer and have excellent dielectric properties. 

“This can be a stepping point for further development of these formulations,” Meinhartd said.