The U.S. seems to have reached the tipping point on electric vehicles (EVs), with General Motors’ announcement that it will cease production of internal combustion engine light vehicles by 2035, and the company will be carbon-neutral by 2040. Electric motors of the size needed for a typical automobile tend to run at peak efficiency at around 15,000-18,000 rpm, while automobile wheels rotate at close to 1,000 rpm at 100 km/hr (about 60 mph). To accomplish this difference, EVs use some type of reduction gear, typically around 10 to 1 reduction. To maximize efficiency, some auto makers are incorporating additional gear ratios. Such transmissions need to be lubricated, and the challenges are quite different from their internal combustion engine counterparts.
From a mechanical standpoint, the most significant difference is that electric motors generate maximum torque at zero rotational speed, meaning the gears must be able to manage high stress even before they begin to rotate. Since gear rotation is the usual mechanism for distributing the lubricant, this start-up condition is a limiting factor of the gearset. While there is much research taking place on gear materials and coatings, it has become obvious to the industry that new lubricants will be required.
The challenges for the gear oils in EVs don’t end there. In addition, the lubricant for an EV transmission usually also lubricates the electric motor. Therefore, the oil must be compatible with the large amount of copper in the coils, as well as advanced polymers used for sensors and seals. The motors and gears also generate a significant amount of heat, so the gear oil also must function as a coolant. (Virtually all liquid lubricants provide some cooling, but in the case of EVs, this coolant function is particularly critical).
The oil also is exposed to very high electric and magnetic fields, so the fluid’s insulating and dielectric properties need to be managed. Of course, it should not be a good electrical conductor, as short-circuits through the fluid would drain power from the system. However, a perfect insulator would allow charge to build up within the system, and when static charges reach a critical threshold, dielectric breakdown of the fluid allows sparking to occur, with accompanying damage to the materials. So the conductivity of the fluid should be very low but not quite zero.
As I review the literature on EV lubricants, it appears that vehicle manufacturers are working with the lubricant suppliers to develop new chemistries for their individual applications. I wonder, however, if this is the best approach. While each company’s vehicles are unique, the physical principles and limitations are similar. Is this the time for an industry-wide organization to develop standards and specifications for EV lubricants? The Society for Automotive Engineers (SAE), the American Petroleum Institute (API) and the International Lubricant Standardization and Approval Committee (ILSAC) spring to mind.
Even if it is not possible to come up with one set of specifications for EV gear lubricants (analogous to ILSAC GF-6, for example), the industry should at least develop some standardized tests specifically for these fluids to allow for direct comparison of the different formulations.