By Jeanna Van Rensselar, Senior Feature Writer

Electric vehicles (EVs) are not new. They are as old as automobiles. According to STLE Fellow and Past President Dr. Edward Becker, P.E., president of Friction & Wear Solutions, LLC, in Brighton, Mich., electric vehicles outnumbered gasoline-powered vehicles in the U.S. before 1900. However, development of the modern electric vehicle began in earnest around 1990 as rising petroleum prices, stricter environmental regulations and increasing fuel economy standards put pressure on automakers to investigate other forms of propulsion, including alternative fuels and EVs.

STLE-member Arup Gangopadhyay, technical leader, Powertrain Research and Advanced Engineering, for Ford Motor Co. in Dearborn, Mich., defines terms upfront. “I would like to clarify an important distinction between electric vehicles and vehicles with electrified powertrains,” he says. “A purely electric vehicle has no ICE (1). The power is supplied by the battery. There is no transmission either. There is usually a simpler eAxle (2) or differential to help divert power to individual axles and wheels. An electrified powertrain will have an engine and a transmission as we see them today, but they will be assisted by electric motors. These are basically hybrid electric vehicles.”

STLE-member Dr. Farrukh Qureshi, technical fellow for The Lubrizol Corp. in Wickliffe, Ohio, cautions that the term electric vehicle itself is open to interpretation. “EV is a general term that may mean different things to different people rather than a specific form of vehicle,” he explains. “EV architecture is still evolving and depends on the choice of the specific OEM. Additionally, the degree of electrification would vary and may range from a mild hybrid vehicle to a vehicle in which all systems will be electrical.”

In this article, only purely electric vehicles are referred to as electric vehicles or EVs unless otherwise noted. Vehicles equipped with a driveline containing both battery and ICE power sources will be referred to as hybrids.

Gangopadhyay sees four key consumer concerns regarding EVs:

1. Driving range between charges. Today’s consumers are accustomed to driving 300-400 miles between fill ups, and they expect the same from EVs.
2. Charging time. It takes minutes to fill a gas tank but hours to charge a battery.
3. Difficulty accessing charging while traveling. Not knowing if there are enough charging stations in the area so that the vehicle can get charged within a reasonable driving distance.
4. Cost of the vehicle.

Fanghui Shi, GM technical fellow, Global Propulsion Systems, Engine Hardware General Motors in Pontiac, Mich., believes that consumer education is key for adoption. “We know that, at the start, there is a lot of education needed about the experience of an EV, including how to charge and what to expect,” he says. “Cost also is an important consideration, and General Motors is working hard to build EVs that meet the functionality and affordability our customers are looking for, as shown with the Chevrolet Bolt EV.”

Current technological challenges are many, but bringing down certain costs is paramount. Gangopadhyay says, “The cost of the battery is still relatively high and needs to come down to a more affordable level. The second cost concern is the cost of rare Earth materials used in electric motors.”

Shi says that General Motors is working to address key technological challenges of battery energy density, costs and charging to accelerate the adoption of EVs.

“Current lithium-ion battery energy density is about one eightieth of that for fossil fuel,” he explains. “Even after considering the high efficiency of the electric motor (in the 90% range) versus the lower efficiency in the ICE (in the 30% range), and the mechanical system in the EV being simpler, EVs still have weight and range disadvantages. Although charging at home offers the advantage of waking up to a ‘full tank’ via a charged battery, high-voltage DC fast charging does not yet match the speed of refueling at the gas station. Thermal management and battery life balance are among the challenges.”

There is no question that tribological considerations still will be key performance factors in both hybrid and completely electric vehicles. STLE Past President Dr. Ali Erdemir, Argonne Distinguished Fellow, Argonne National Laboratory, Applied Materials Division in Argonne, Ill., explains, “In hybrid EVs, tribology will still play an important role in the efficiency, emissions and durability of such vehicles, but in pure EVs the concerns over emissions will diminish and nearly all remaining efficiency and durability issues will be burdened on remaining moving parts and, hence, their tribology.”

Before starting his own consultancy, Becker spent 30 years as a GM engineer working mainly on a variety of engines and transmissions. He says that by reducing friction in components such as wheel bearings and gears, tribology can contribute to extending the range of EVs. However, EV accessories (power steering, AC) use greased, sealed for life electric motors rather than motors driven off the crankshaft of an ICE. “Lubricants for EVs will need to be compatible with new materials such as copper wires and windings, and advanced polymers, as well as compatibility with electric currents and magnetic fields,” he says.

Shi explains the basic differences between lubricating ICE-equipped and EV vehicles: “In an ICE vehicle, the primary function of the lubrication, in addition to cooling, is to provide hydrodynamic load-carrying capacity to separate metal surfaces in load-carrying devices such as rings, pistons and all kinds of bearings. However, the normal loads or radial loads that are intrinsic to the crank-slider mechanism no longer exist in electric motor supporting structures. The function of such devices changes from load-carrying to torque-transferring. NVH (3) (noise, vibration, harshness) concerns or NVH-induced durability concerns will rise as the primary challenges as compared to the durability concerns in ICEs. Control and rotor dynamics for high-speed rotor and air lubrication in the high-speed compressor will become popular research topics.”

Unlike an ICE, an electric motor typically generates maximum torque at zero speed and maximum efficiency at around 90% speed. Given this, Becker explains that to provide good launch characteristics and allow the motor to operate near peak efficiency more often, all current production EVs use a reduction gear (typically around 8-1) to achieve higher efficiency over their speed range. “These gears are subjected to very high torque at low speed,” he says.

Shi says that due to the simpler mechanical interactions in the EV powertrain and drivetrain, the absolute friction loss is reduced. The percentage of power to wheel is significantly increased from 20% for ICE-based vehicles to 80% for EVs. Therefore, the drive to reduce friction for fuel economy gain is less substantial.

Becker concludes that tribological advances can realistically only assist with range.

Qureshi explains that the choice and type of fluid would depend on the degree of electrification, the configuration and design of the vehicle and location of the electric motor/components. “Fluid properties will be dictated by the potential operating environment of the fluid and whether the electric motor or other electrical components are to be wetted by the fluid,” he says. “Some of the hybrid/EV architectures would use existing engines, transmissions and axles. As a result, conventional lubricants may suffice.”

He adds that if an electric motor is used for propulsion in addition to an ICE, which is a common configuration for prevalent hybrid vehicles, an existing engine oil could be used for lubricating engine components.

“Care would be needed to make sure that the lubricant can provide protection to engine components for more frequent stop/start events compared to a conventional ICE,” Qureshi says. “Depending on drive cycle, the ICE may operate at a lower temperature for a short time duration. This may result in water condensation leading to emulsion formation in the fluid. Further studies would be needed to understand the impact of emulsion on fluid performance.”

Regarding required fluid properties for EVs, Qureshi says that, in general, performance criteria of electrical conductivity, thermal transfer properties, copper corrosion protection (4) and elastomer compatibility would be of most importance.

“In some cases, new base fluid as well as additives might be needed to provide the desired combination of performance attributes for hybrid/EV vehicles,” Qureshi explains. “Some of these performance levels may be determined by conventional test methods; others would require development of new test methods to meet the criteria needed to provide long service life of EV systems for mobility. Whether a new additive chemistry would be needed would depend on whether the required criteria can be met with current base fluids and additive chemistries or not.

“Several drivetrain configurations would exist depending on vehicle configuration and design. If the motors are embedded in the transmission or axle where they are immersed in oil, the fluid’s electric and thermal conductivity will be of importance. However, there is lack of consensus among OEMs on targets for electric or thermal conductivity. Friction and wear protection of gears, bearings and seals also will be important. Motor output speeds may exceed 25,000 rpm, which would require that associated bearings, seals and other components are properly lubricated.”

There are other engine oil considerations. Gangopadhyay says, “Hybrid engines tend to operate at load/speed conditions related to high efficiency points to maximize fuel efficiency. Also, the engine does not operate all the time due to the introduction of start/stop technology, fuel shut off during deceleration, etc. Thus, engine oil tends to run cooler than a normal engine. This offers the opportunity to use lower viscosity grade engine oil, which can further improve fuel economy while maintaining comparable minimum oil film thickness. Many high-volume engines today use SAE 0W-20 oils, but one can now go to SAE 0W-16 or even lower provided durability, oil consumption, etc., are not compromised.”

Qureshi indicates that The Lubrizol Corp. scientists are working hard to solve potential issues for fluids and EVs and also cautions that developing, registering and manufacturing new chemical components is an arduous, time-consuming and expensive process.

Chris Marks, senior engineering specialist for The Timken Co. in North Canton, Ohio, says that the usual number of bearings depends on whether the vehicle is mild hybrid, full hybrid (5) or pure EV. “With a mild hybrid, these vehicles typically contain the same number of bearings as today’s designs,” he says. “However, a full hybrid that uses an electric motor to propel the vehicle may have more bearings in the powertrain, depending on the integration architecture. 

“For pure EVs, where an electric motor is the only vehicle propulsion source, the overall number of bearings in the powertrain probably will be reduced compared to a conventional powertrain. Typical powertrain architectures use about eight bearings between the electric motor and the gearbox. The number of wheel bearings in the vehicle stays the same.”

Early hybrid gearboxes were often designed around existing production bearings to bring an EV powertrain solution to the market faster. However, this may not offer the best long-term solution in terms of vehicle power density and overall efficiency.

“Timken offers power-dense, fuel-efficient bearing solutions that allow for more compact powertrain designs, providing an overall vehicle weight reduction, lower bearing operating temperatures and improved powertrain efficiency,” Marks says. “Additionally, these bearing solutions can help reduce NVH by eliminating the clearance in the bearings and preventing the backlash motion when going from drive to coast conditions encountered during regeneration. New lubricant formulations also are likely to arise in the market to meet the EV application requirements, including lubricant characteristics such as foam control, low conductivity, cooper corrosion and thermal stability.”

Chris Shamie, director of Advanced Automotive Development, Americas-Schaeffler Group USA Inc. in Livonia, Mich., is convinced that bearings are key to the success of EV vehicles in the future, especially with respect to one of the biggest hurdles faced by EVs in the race to gain widespread acceptance, what he calls “consumer range anxiety.” He says, “While improvements in battery energy density are, of course, needed, driving range also can be improved through reduced parasitic losses—which Schaeffler innovations, such as high-speed motor bearings and ultra-low friction bearings, can help overcome.”

Shamie continues, “Today’s ICE vehicles utilize as many as 11 gear states to maximize powerpack efficiency. As we move to pure electric vehicles, one or two speeds will likely be sufficient in the near term, based on the larger efficiency island of an electric motor as compared to an ICE. With a reduction in the number of gear states comes a reduced demand for bearings. This is actually good news for Schaeffler, as this transition allows us to focus our resources on delivering E-mobility solutions such as complete electric axles, P2 (6) modules, actuators and fully dedicated hybrid transmissions products for which there is strong market demand.”

Shamie explains that bearings used with brushless eMotors and IGBT (7)-type power electronics present a special challenge. As electricity flows through the eMotor’s rotating shaft to the machine frame, the current fluctuates at a high frequency. This can result in electrical discharge machining, which, in turn, can lead to premature bearing failure.

“Schaeffler is developing a shunt bearing that bypasses this current via a secondary conductor, thereby eliminating electrical discharge machining,” he says. “In addition, new lubricants that can lower the power of these electrical discharges are being developed for EVs. Schaeffler also is collaborating with the manufacturers of these lubricants to optimize the system’s robustness.”

Schaeffler’s bearings also must operate without making much noise under high speed (see Figures 1 and 2). While EV motors can spin at speeds of up to 20,000 rpm, they will no longer have an ICE to mask background NVH. To address this potential issue, Schaeffler has developed special high-precision bearings. It also is working on a new bearing coating that can reduce bearing NVH at high speeds (see Advanced Coatings/Materials for EVs).

New driving regimes combined with new oil/grease formulations with much reduced viscosity will call for the use of more advanced surface technologies, coatings and materials. STLE Past President Dr. Ali Erdemir, Argonne Distinguished Fellow, Argonne National Laboratory, Applied Materials Division in Argonne, Ill., says, “In the long run, the goal will be fill-for-life lubrication in most if not all EVs, and this can only be achieved by the use of more advanced materials and coatings.”

Regarding the potential for adding coatings to EV bearings, Chris Marks, senior engineering specialist for The Timken Co. in North Canton, Ohio, says, “Typically surface coatings are not used on bearings in automotive applications today. It is possible that some bearing manufacturers may use surface coatings to enhance the performance (e.g., fatigue life, electrical resistance). Long term this is probably not a viable high-volume manufacturing process, especially if EV production volumes increase. Usually coatings are applied in a batch-type process. If coatings are applied through a continuous process, it can lower the cost, making this option more attractive. A coating could be used to offset thin film bearing conditions that occur as a result of using lighter weight oils. In addition, coatings can help prevent surface adhesion.”

Marks continues, “There are several applications where bearing coatings can be beneficial to our customers. With the high speeds and loads that accompany eMachines, the bearings sometimes move slightly inside their housings (also known as bearing creep). This can cause damage to the bearing or the housing itself. To that end, Schaeffler has developed a special coating to prevent creeping inside the bearing housing.

“Also, as mentioned earlier, we need to be mindful of the potential for electrical discharge machining that can sometimes occur in high-speed eMachine bearings. In addition to the shunt bearing concept, which moves the current flow path away from the rolling element raceways, Schaeffler has developed a specialized coating to prevent electrical discharge thorough the bearing. Used on electric train bearings for years, this coating can be applied to the bearing’s inner race, outer race or to the rolling elements themselves.

“In addition, EVs will no longer have an ICE to help mask the bearing noise,” he says. “Consequently, the bearings themselves will have to run more quietly, which is achievable with a special bearing coating. Schaeffler has invested heavily in coating technologies for many years to ensure that the above-mentioned problems can be solved.

The automotive industry is moving toward lighter weight oils for overall efficiency improvements, regardless of the vehicle propulsion type. “These lighter weight oils must provide adequate lubrication to the bearings under a range of application conditions to meet bearing performance expectations (e.g., fatigue life) and to aid in the removal of heat from the drivetrain (e.g., gearbox),” Marks says. “Lubrication requirements in an EV may have a different set of challenges compared to a conventional axle depending on its interaction with the electric motor and surrounding components (e.g., is it cooling the motor and the gearbox?).”

Qureshi indicates that The Lubrizol Corp. is currently working on fluids with desirable attributes of thermal management, efficiency and durability in high-speed driveline systems.

Shamie believes that lubrication and tribology challenges for bearings inside EVs will depend on the region where the vehicles are sold.

He explains that in the U.S., electric vehicles will sell when they are marketed on the basis of their improved function and performance attributes regarding comparable vehicles with ICEs. Many of these high-performance EVs will use direct-spray cooling for the eMachine, and the runoff from this process can then be used to cool and lubricate the attendant bearings.

In Europe and Asia, the need for CO2 reduction will be the driving force behind the growth of E-mobility. One of the easiest ways to lower CO2 emissions is to reduce the mass of the vehicle. “Products like eAxles and the P2 module clutch could push optimization targets for lubrication and heat extraction around bearings to the edge of robustness (see Figures 3 and 4),” Shamie says. “Many of these designs could use water-jacket cooling for the eMachine, which would require the use of sealed, greased bearings.

“Schaeffler is in the unique position of being a full-service supplier of both the eAxle as well as the P2 system. This allows us to look at the overall heat transfer and lubrication delivery so we optimize not only the bearings but the complete modules, too. The eAxles will contain high-speed motors with large reduction ratios that will present challenges to bearing design. Since these electric motors will be turning at speeds in excess of 20,000 rpm, robust bearings are needed here.”

Another factor that must be considered is the need to control oil aeration at these high speeds,” Shamie says. “The overall system must be carefully designed to ensure that splash lube features do not cause excessive aeration at high speeds.

Shamie explains, “Because EVs use many parts made from copper, the biggest challenge for lubricant manufacturers is to ensure that the EV lubricant does not corrode copper components. Such a corrosive lubricant could dissolve the copper and deposit it on areas that need to be electrically insulated. Because certain existing powertrain lubricants are corrosive on copper, they will need to be reformulated.”

When selecting and analyzing bearing solutions for an EV, The Timken Co. leverages its vast application knowledge across multiple industries whether that is a high-speed machine tool spindle or a rotor craft gearbox application to provide bearing solutions that meet customer requirements for EV applications.

“The operating conditions and environment of an EV gearbox application are vastly different from a conventional axle and offer many new bearing and lubrication challenges along the way,” Marks says. “Bearing fatigue performance is subject to impact from the harsher thermal environment along with the lighter weight lubricants. OEMs must work to validate the EV system solution and balance all of their ‘e-requirements’. Extensive testing must drive the development of EV powertrains to establish acceptable component performance such as bearing fatigue life.”

As vehicles become more and more sophisticated, parts, including bearings, will have to work harder. “When we get to ‘robo-taxi’-type conveyances (an example of which is Schaeffler’s recently introduced ‘Mover’ concept car), the vehicle’s duty cycle will be greatly increased. This means that bearings, gears and lubrication systems will need to be more robust as well,” Shamie says. “It is realistically conceivable that these components would need to last 10 times longer than what today’s mechanical systems are engineered for. We will likely see the advent of cloud-based condition monitoring to make sure these vehicles stay on the road instead of inside the repair shop, which is why Schaeffler has invested heavily in digitalization.”

Currently there is a limited amount of grease used in EV gearboxes. Grease is typically found in sealed ball bearings, which have been used in some EV gearboxes or in electric motors. “Bearing positions using sealed solutions in high-speed and high-temperature applications bring challenges for long-term performance,” Marks says. “The reason is that the grease cannot be serviced over the life of the vehicle; as a result it has a finite working life under these conditions. In addition, there might be challenges with the interaction of the gearbox lubricant. At The Timken Co. we draw on our machine tool industry experience to help determine if it is appropriate to apply greased, sealed bearing solutions for these types of demanding applications.”

Shamie says, “Depending upon the customer’s architecture preferences, grease-filled bearings are often favored for eMotor bearings in eAxle applications that use a water jacket (as opposed to splash lube),” he says. “Working with our partners, Schaeffler has developed a special grease for these high-speed motor bearings.”

A power split transmission, with increased torque transfer capability, is currently used for hybrid vehicles, which is simpler than a conventional step transmission (6-speed/8-speed/10-speed).

Gangopadhyay explains, “In this transmission, the power from the engine passes through a set of planetary gears set to the ring gear through a final drive gear. The ring gear feeds power to the wheels through the transmission output shaft. There are two electric motors inside the transmission housing: the integrated motor generator and the traction motor. The traction motor can directly supply power to the wheel when the engine is not running. The motors get very hot, and they are cooled by the transmission fluid dripping on them.

“The electric machines are more efficient if they can be kept cool. That is where the opportunity lies. There is a need to develop a transmission fluid with improved heat transfer capability to take heat away faster to keep the motors from getting hot. This can allow more current to pass through the motor offering more torque, which customers desire. There are no friction clutches in this transmission, therefore, some of the additives used in today’s ATF (8) may not be necessary and this may offer more degrees of freedom for formulation. However, durability of planetary gears, bearings, etc., must be maintained.

“Since the transmission fluid comes in contact with copper windings, insulations, laminates and rare Earth materials in motors, adequate corrosion protection becomes highly important. In addition, it is desirable to increase operating temperature of the fluid to enable passing even more current through the copper windings. Therefore, higher oxidative stability would be very much desired.

He adds that while the above requirements are more like additional requirements for a traditional ATF used today, other ATF requirements such as antiwear protection, contamination control, even fill-for-life, etc., continue to apply.

Full hybrid vehicles will still rely on an ICE and transmission in addition to the electric motor in its integrated drivetrain. Currently in the light vehicle market, EVs typically employ architectures with a single-speed gearbox. Two-speed gearbox designs are more attractive to the commercial vehicle market to accommodate city versus highway driving conditions.

“The two-speed designs may offer a solution for the larger vehicles in the light vehicle market, such as the light truck and large SUV market, if those vehicle types adopt EV powertrains,” Marks says. “In EV gearboxes there is a trend toward using ATF as the gearbox lubricant versus the traditional lubricant packages used in conventional axle center applications. One driver for the usage of this lubricant type is EV gearboxes generally use helical gearing instead of hypoid pinion and ring gears, which have significantly more sliding.”

Shamie agrees that for pure electric vehicles one- or two-speed gearboxes will likely be sufficient for the immediate future. “Hybrid/electric vehicles will either use dedicated hybrid transmissions or P2 systems that utilize the OEM’s existing planetary automatic investments by locating an eMachine between the ICE and gearbox,” he says. “In each of these cases, reducing parasitic loss is crucial to extending EV driving range, so low-viscosity oils such as automatic transmission fluid will be utilized. Most of the building blocks for these transmission types will be the same as those used for conventional planetary automatics. However, just as with eMotor bearings, they will have to handle higher speeds due to speed-reduction ratios of 15:1 or higher.”

Erdemir explains that fundamental research can help in two ways:

1.  Development of a new breed of ultra-low viscosity lubricants and greases specifically tailored toward the special driving requirements or regimes of EVs, and, more importantly,
2.  Development of lubricious coatings and/or tribomaterials that are fully compatible with these low-viscosity lubricants so that they work together to provide high resistance to wear and scuffing and, hence, long life (if possible fill-for-life) under the much harsher operating conditions and severe lubrication regimes, which will be triggered by the use of increasingly lower viscosity lubricants.

“Despite increasingly harsher operating conditions, these new lubricants and greases will have to work harmoniously with the material side and increase efficiency and, hence, need to be formulated differently than what is currently used in current ICEs,” Erdemir says. “When and if heavier (steel, cast iron, etc.) tribomaterials are replaced by lighter weight alternatives, then the R&D for a new breed of lubricants compatible with such new materials also may be very important.”

He adds that most, if not all, current funds are supporting advanced battery R&D in private, academic and public institutions. “It will be very worthwhile to consider some of these funds for the development of more advanced lubricants and materials that will impact efficiency and durability in EVs as well,” he says. “My lab performs very extensive R&D work for the development of next-generation batteries/EVs, but there is not much work going on at present in the development of more advanced lubricants or materials tailored or directed to EV uses.” 

Gangopadhyay, Shi and Qureshi agree that industry-wide fluid specifications for EVs would be beneficial. “At this stage it is highly desirable and recommended that OEMs, tier suppliers, oil companies and additive manufacturers work in developing system solutions in unison,” Qureshi says. “This collaborative effort would be most time efficient with maximum return on investment for all the stakeholders.”

Shi explains that General Motors has shared a vision for a world with zero crashes, zero emissions and zero congestion. “The development of EVs is a key contributor to reduce CO2 emissions,” he says. “Additionally, with the drop in battery price and increase of energy density, the cost of EV manufacturing is approaching the traditional ICE-based vehicles. Lower maintenance and refueling costs are among the two major cost differentiators between EV and ICE-based vehicles. Finally, General Motors believes EVs provide the right platform for future autonomous and connectivity technology to enable future mobility.”

The driving force behind development of hybrid/electric vehicles is to reduce CO2 emission and air pollution. Gangopadhyay explains, “The strict legislation introduced by many countries is accelerating the development. Some European countries are considering a zero-emission zone in some cities while China is mandating certain percentages of vehicles sold must be battery electric.

“It is projected that there will be increased penetration of hybrid vehicles in the next 15-20 years. The EV penetration is expected to be slower than full hybrid vehicles. Therefore, ICEs are going to be around for quite some time.”

“Currently most non-conventional vehicles are hybrid and will remain so for a long period of time,” Qureshi concludes.

References
1. Internal combustion engine.
2. The eAxle is a cost effective and compact electric drive solution for electric vehicles and hybrid applications. The electric motor, power electronics and transmission are combined in a compact unit directly powering the vehicle’s axle. Available here.
3. Noise, vibration, harshness.
4. Some fluids can corrode copper components.
5. The primary difference between a full hybrid and mild hybrid is a full hybrid has a battery that can power the vehicle alone for at least several miles at moderate speed. A mild hybrid is able to accelerate the vehicle from a stop but can’t run the vehicle alone. Available here.
6. The P2 module combines a high-voltage electric traction motor, engine disconnect clutch, clutch control module and dual mass flywheel into a single package. It rests between the engine and transmission.
7. An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch that combines high efficiency and fast switching. Available at https://en.wikipedia.org/wiki/Insulated-gate_bipolar_transistor.
8. Automatic transmission fluid.

Jeanna Van Rensselar heads her own communication/public relations firm, Smart PR Communications, in Naperville, Ill. You can reach her at [email protected].

Key Concepts
• Electric vehicles alter but don’t diminish the application of tribology.
• Concerns consumers have about EVs include high vehicle cost, limited driving range and battery-charging issues.
• The development and adoption of hybrid vehicles for the consumer market will continue to be robust, while in the near term all electric vehicles will continue to evolve.

Reprinted with permission from the 01.07.2019 issue of Tribology & Lubrication Technology (TLT), the monthly magazine of the Society of Tribologists and Lubrication Engineers (STLE), an international not-for-profit professional society headquartered in Park Ridge, Ill., www.stle.org.