Performance demands for some lubricants are no different in EVs than in conventional vehicles. Those include break and shock absorber fluids, as well as greases for doors, locks and wheels. Requirements differ for other lubes and fluids. In some cases, a lubricant is used in an EV but not in an ICE-powered car, or vice versa. Some products are used across all vehicle types, but the equipment differs enough to require differences in lubricants. And in some cases the equipment is the same, but operating conditions differ enough to affect lube requirements.
Several issues are emerging as the main sources for differences in performance requirements – and as the main areas of current focus for the industry’s research efforts. These include the challenges of thermal management for e-motors and batteries; exposure to higher levels of electrical current in EVs; and the risk of copper corrosion in e-motors. Viscosity, material compatibility, friction management and performance life are also areas that need to be addressed for EV lubricants and fluids.
Today, BEVs and PHEVs are largely using off-the-shelf products developed for ICE engines, but there is consensus that OEMs would like to have more optimized products today and customized solutions will be necessary in the future.
Increasingly, the trend across the board for lubricants is toward lower viscosities. Viscosity is a key parameter that the lubricant industry and OEMs understand well. In the ICE motor oil segment, lower viscosities have enhanced engine efficiency leading to improved fuel economy. Lower viscosities also provide improved low-temperature performance, but there is always a balance between lowering viscosity, hardware protection and volatility. As viscosities go down, oil volatility goes up, which can be detrimental to the performance of fluid and equipment life.
Peak performance needs to be maintained throughout the lifecycle of the oil. Advances in base stocks to provide lower viscosities without negatively impacting volatility will be important, and this rules out conventional base stocks as we look at advanced API Group III, polyalphaolefins and maybe even esters.
The issue of electrical compatibility overarches all EV developments and many of them are associated with hybrids. The power electronics of EVs operate at hundreds of volts, compared with the 12-volt systems in conventional ICE-powered vehicles. Motors are usually powered by alternating current, so the direct current generated by the battery must be inverted to drive them. This all brings new requirements into play.
Electrical conductivity could have a significant effect on formulation and the risks are clear. If conductivity is too high, current will leak with the risk of electric shock and short-circuits in the motor. Conversely, if a fluid is too effective an insulator, static buildup can occur, leading to discharges that could damage equipment.
There is plenty of evidence in other areas of lubrication that static discharge can damage equipment, so this must be addressed. Currently, ISO 6469-3:2018 specifies electrical safety requirements for protection of persons against electric shock and thermal incidents but does not lead to a definition of what is “too low” or “too high” electrical conductivity in EVs.
Research by additive companies has determined that lubricant and fluid conductivity can be affected by several factors, including viscosity, chemical characteristics of additives and lubricant aging. The effect of fluid viscosity is well understood, in that lower viscosity leads to higher conductivity, because it is easier for a charge carrier to migrate through the fluid. With OEMs asking for lower-viscosity fluids, this could be a significant issue in the future. Increases in temperature also increase conductivity.
Lubricant additives generally increase conductivity, but Afton Chemical researchers concluded that the magnitude of impact varies depending on polarity. Low polar components, such as dispersants and detergents, have relatively small impacts, while others such as friction modifiers, antioxidants and anti-wear agents cause larger increases in conductivity. The clear implication then is that formulators will need to strike the right balance between conductivity and performance.
The conductivity of an oil increases with use. Lubrizol examined some ATFs during a field trial and concluded that oil oxidation and reduced viscosity led to increases in conductivity. This confirms much first principles conjecture: Oxidation products are more polar, so they either carry charge themselves or solvate the charge carriers, and lower viscosity allows charge carriers to move faster. However, Lubrizol demonstrated that other feasible causes of higher conductivity, such as the presence of wear particles or dissolved metals, were not important.
Since not all lubricants are the same with regards to oxidation rates, the additive package and base stock are key factors in managing conductivity, and formulators should understand how to balance this.
Interaction between these factors and their relative impact also matter, and there is a lack of consensus on that as well as whether the conductivity of existing fluids is problematic. Afton compared the effect of viscosity and additives when considering the effect of temperatures changes on conductivity.
Afton and Lubrizol both concluded that chemical additives have a greater impact on conductivity than fluid viscosity. In its 2016 SAE paper on electrical conductivity of new and used ATFs, Lubrizol concluded that “the differences in conductivity between these ATFs, although small, is due to differences in the amount and type of additives used and not to viscosity differences.” While showing that the detergents used contributed much more to conductivity per unit mass of additive in one ATF, the Lubrizol team stressed that, once typical treat rates were taken into account, all additive types impart a significant fraction of the total conductivity.
No conclusion is drawn over whether existing lubricants are too insulating and all products that were tested are in the dissipative range of conductivity. It is also unclear whether conductivity will be an issue. If it is, however, how much can formulators affect conductivity with existing additives and base stocks?
“Our research shows that electrical properties can be influenced in both new and used fluids. We are able to significantly dial in the conductivity contribution from the additive based on the types of individual components used and their relative treat rates. The choice of base oils and the viscosity [of the fluid] also have a big effect,” said Chris Cleveland, R&D manager at Afton Chemical.
Additive companies, including Lubrizol, have investigated electrical conductivity to determine how much of a concern this is. How much does electrical conductivity impact efficiency, components and life of the lubricant, and can the lubricant pose a shock hazard?
To investigate, work was done on actual transmissions. If conductivity is too low, could it lead to static build-up and arcing causing damage to components? Lubrizol found that “lubricants are static dissipative,” so low conductivity generally is not a problem.
Corrosion is, of course, an issue common to all vehicles, but there are specific challenges in hybrids and plug-ins. Preventing corrosion of ICEs in hybrids can become a bigger challenge than in non-electric cars because in the former, the ICE is used less and therefore will less often reach operating temperatures necessary to burn off fuel and water that collect in engine oil sumps. Oils in hybrid ICEs can therefore become diluted by water and fuel and contaminated by acids, leading to increased wear and corrosion.
Corrosion takes on an additional dimension, however, in electric motors that are integrated into transmissions. This design puts the copper wire windings of the e-motor in direct contact with the lubricant. If the copper is allowed to corrode, two problems can result. First, it could compromise performance of the wires, which create an electro-magnetic field key to the operation of the motor. Second, corrosion can create copper sulfide like the white dust that forms on terminals of conventional car batteries. There is a range of pure copper sulfides with varying ratios of copper to sulfur, many of them electrically conducting. If these collect in certain areas they can bridge insulating gaps leading to electrical shorts.
Formulators can improve corrosion protection by reducing sulfur and phosphorus levels. Unfortunately, sulfur and phosphorus compounds have long been popular agents for protection from scuffing. That suggests a need to balance anti-wear performance with protection from copper corrosion. However, not all sulfur-containing additives create copper sulfide. To date, additives suppliers have been very careful to not mention those additives that cause problems.
To address these issues, the industry will need tests that gauge the level of copper corrosion protection that a lubricant provides. The ASTMD130 copper strip test is a common fresh oil test used that measures corrosion, but some question its relevance in modern transmissions that are exposed to electrical currents during operation. Lubrizol therefore developed a charged wire resistance test to help evaluate corrosion in transmission fluids. The company says this test is a more realistic simulation of the workings of an EV than submerging a copper strip in fresh oil. The test can also be run at realistic temperatures and can be completed in a short period of time.
Some EVs use as much as double the amount of copper as an ICE vehicle, in wiring and connectors. This makes copper corrosion a greater concern for OEMs and lube formulators, who need to balance sulfur and phosphorus content in lubes that promote corrosion with scuff protection.
New test tools such as this one can examine new concerns, such as conductive deposits that can form copper sulfide and can bridge conductor circuits, as well as stray voltage, which can promote corrosion that would not occur without an electric current. How significant a problem this is, is still to be determined, but experience should lead to optimization of a transmission fluid’s ability to tolerate significant electrical current.
At first glance, the combination of copper contained in electrical circuits or as a component of alloys, voltage and sulfur in lubricants additives would seem like a recipe for trouble. If copper sulfide is formed inside the e-motor and builds up in a similar manner to copper sulfate on battery terminals, then this could be a serious problem.