It is no secret that fluids used in electric vehicles will inevitably edge out engine oils as EVs continue to gain in popularity. Because of this, companies that are currently developing and manufacturing engine oils will need to invest their efforts in EV fluids. And like every good lubricant, the choice of base stock used to formulate these electricity-compatible lubricants is vital.
“Dielectric fluids have been around for a long time,” Ken Hope, global PAO technical services manager with Chevron Phillips Chemical Co., said at STLE’s Tribology and Lubrication for E-mobility Conference. But new applications—like those in EVs—have emerged in a big way, and lubricant companies around the globe are working to develop finished products that address the needs of these new technologies.
To do so, “good base oil has become a more critical component of the formulation,” Babak Lotfi, product development technologist with ExxonMobil, said at the same event.
So what are the properties that these new lubricants must possess?
Hope cited low viscosity as an important property. “Low viscosity is good,” Hope said. “It takes less energy to move a fluid that is low in viscosity. It is also beneficial for heat transfer.” Chevron Phillips Chemical has mostly been working with 2- and 4-centistoke polyalphaolefins, he said.
Viscosity is ever more important because it can affect the way that a fluid displays other properties. For instance, with PAOs, “the dielectric constant doesn’t seem to vary much at all with a change in viscosity,” Hope said. And while the specific heat and thermal conductivity also vary only slightly with different viscosity grades, biodegradability increases in lower-viscosity PAOs.
However, there are challenges associated with low-viscosity fluids. For instance, the lower the viscosity of a PAO, the lower the flash and fire points are. “Lower viscosity goes the opposite direction for safety when you look at flash and fire points,” Hope said.
Oxidative stability, oil life and thermal conductivity are all valuable properties for EV lubes as well, because they relate to cooling efficiency and heat flow.
Another vital property is specific heat. “Specific heat is how rapidly that oil is going to absorb that heat from where the heat is being generated,” Hope said. “That’s one of the first steps—to be able to transmit that away from where it’s generated.”
Another must-have property is hydrolytic stability. “Water—as we know—is everywhere,” Hope said. “Water is a contaminant, especially for dielectric fluids. Water and dirt and other things like that will degrade the dielectric strength, and that could be problematic. Also, it’s important that the fluids resist hydrolysis. Oftentimes when things hydrolyze, the resulting products then can be flammable or corrosive and cause electric problems as well.”
To round out the list, Hope explained that material compatibility is another vital property, along with shear stability, high flash and fire points, density, low freezing point and biodegradability.
What Is the Base Stock of Choice?
Are polyalphaolefins a base stock of choice for many EV applications? While other options exist, many lubricant industry players seem to think that they are.
“We have a lot of different potential materials here, but how do they relatively compare?” Hope said.
He cited silicone esters as a potentially useful base stock for electric applications, albeit one that seems to fall short of the level of performance offered by PAOs. Silicone esters tend to “have an issue with hydrolysis,” he said. “Aromatics, biodegradation and toxicology can be a concern, too.”
It is also possible to use traditional mineral and hydrocracked oils in electric applications, but PAOs generally offer better performance in electric applications.
Why is that? Hope cited their superior dielectric properties as one reason. “The military had issues with dielectric coolants that were developed for the SR-71 Blackbird,” he said. “Originally, they were using a silicone ester, and that was fine until you had the computers that were actually sitting in a vat of this oil to keep them cool and prevent air ice. What they ran into was the fluid was hydroscopic—it was picking up moisture. When that moisture came in contact with the fluid, it would form a gel. Even though the fluid was a good dielectric, when that gel floated past a couple electrical contact points, you would get some arcing, and that arcing would carbonize and you’d get a black spot. They actually termed this the Black Plague because they knew when they saw these black spots floating in the fluid that the computer was going to fail shortly after. That was an indication that something was going on.”
Because there was an obvious need to replace the fluid with one that would perform better in the application, they launched a study. The fluid needed to have high thermal conductivity and high flash and fire points. One of the first fluids tested was a hydraulic fluid that was formulated using a naphthenic mineral oil base fluid. To test it, a 50-caliber armor-piercing incendiary round was fired at a one-gallon metal can filled with the hydraulic fluid. The goal was for the fluid to avoid forming a “ball of fire,” Hope said.
Fortunately, the polyalphaolefin-based replacement fluid did just that and managed to produce only smoke during the test, meeting the military specification MIL-H 87257. “This is due to the differences in the flash and fire points” between the silicone ester-based fluid and the PAO-based fluid, Hope said.
The fluid was also put through a series of other tests, like a flame propagation test in which “you take a wetted wick and light it on fire and see how long it takes it go from point A to point B,” Hope said. “The one that moves slower is the better fluid. That’s what they ended up going with.”
The military’s fluid requirements may be a bit different from those of electric vehicles, though. “These electric vehicles right now are not flying around at 30,000 feet, so that’s not a concern,” Hope said. “Maybe some of the extremely low viscosity requirements at low temperatures are really not that much of a concern, either. But many of the other properties are.”
PAOs Get the Job Done
According to Hope, the ability to reduce friction is the primary duty of an EV lubricant, while “the second duty is to get rid of the heat, because wherever the heat is generated, it must be absorbed and then transmitted away,” he said.
Lotfi agreed but added a third item to the list: increasing energy efficiency. “The base oil is really the key component that impacts the energy efficiency and heat transfer properties,” he said.
So how do PAOs contribute to finished products that can accomplish these goals?
“The base oil is really the key component that impacts the energy efficiency and heat transfer properties.”
– Babak Lotfi, ExxonMobil
Regarding the heat removal capabilities of PAOs, Hope cited a 2002 paper written by researchers at Volkswagen and Professor Wilfred Bartz. The paper discussed the ways in which the base fluid used to formulate gear oils can affect the specific heat—the heat required to raise the temperature of the unit mass of a fluid by 1°C. The heat removal performance of a 4-cSt Group III oil and an 8-cSt Group III oil was compared to those of a 4-cSt PAO and an 8-cSt PAO. It was determined that the gear oils formulated with the PAOs had higher specific heat, allowing them to keep the gearbox 7°C-17°C cooler than hydrocracked oils did and 19°C cooler than mineral oils. A higher specific heat is indicative of the fluid’s increased ability to absorb heat.
As for energy efficiency, Hope explained that a test, commonly referred to as the ARKL EOTT, has been used to predict transmission efficiency and the ability of a fluid to control energy loss. In this test, 40 milliliters of a test fluid are placed in an insulated 4-ball apparatus. Over a period of two hours, with certain rpm and load requirements, the temperature of the fluid rises over time. When performed on lubricants formulated with Group I, Group II, Group III and Group IV oils, the Group IV PAO was observed to produce an end-of-test temperature that was significantly lower than the other oils. In fact, the PAO yielded a temperature that was 6.6°C cooler than the closest performing fluid, which was a Group III+ oil. Each formulation used the same additive package and a 5.5-cSt grade of each base fluid.
The results of the test pointed out that “you can see a difference in the friction—in the heat generation—between these different fluids,” Hope said. “Friction is important; that’s the bottom line.”
Furthermore, traction curves generated by a mini traction machine concluded that the tested PAO yielded lower traction coefficients at both 40°C and 100°C than a Group III oil of the same viscosity. This result indicates that PAOs can offer advantages across a range of slide-to-roll ratios and lubrication regimes at 40°C and 100°C, Hope said.
Everything considered, PAOs have demonstrated that they have good frictional properties—as demonstrated by the ARKL EOTT and Mini Traction Machine tests. Thermal conductivity is also important, and PAOs have proven that they have better thermal conductivity than hydrocracked and mineral oils. Lastly, PAOs have sufficient dielectric properties, as they are stable and have the ability to shed water. These properties, when taken together, indicate that PAOs are an ideal base fluid for formulating finished EV lubes with increased fluid life and reduced energy consumption.
Sydney Moore is managing editor of Lubes’n’Greases magazine. Contact her at Sydney@LubesnGreases.com
For more information about the interface between lubricants and the EV market visit Electric Vehicles InSite.