KEY CONCEPTS
• Electric vehicle OEMs have developed integrated e-modules that require a use of a single fluid. 
• Single fluids require energy efficiency and heat transfer in addition to other standard requirements for lubricants/fluids.
• Novel base stocks/chemistries with excellent balance of low viscosity, low volatility and superior tribological properties can enable future technology advancement and maximize the performance and range of electric vehicles.

In recent years, significant progress has been made in improving electric vehicle (EV) performance, especially efficiency and the battery range by optimizing and innovating multiple components used and developing new solutions. The fluids used in EVs depend on the choice and availability of materials, hardware designs, their lifetime performance—among other things. The automotive industry transition toward zero emissions, sustainability and efficiency also are driving the lubricants industry to continuously improve and innovate related lubricants and fluids. This article gives a brief overview on the single fluid designed for EV gears, bearings, including thermal management for e-motors, and other e-components. The article discusses related testing and developmental challenges.

Single EV fluid
As the automotive industry is moving toward integrated e-modules, for many years OEMs and Tier 1s have been exploring a single EV coolant and transmission fluid as an option for gears and e-motors. The EVs integrated design approach provides more efficiency, but for fluids it means that their formulation needs to be carefully balanced—tribological and rheological lubricant properties with electrical and thermal properties are required. 

STLE member Dr. Yungwan Kwak, senior R&D scientist, Afton Chemical Corp., indicates that while theoretically possible to use a single fluid as a lubricant for gears and motors, as well as for cooling the battery and electronics, it would depend on the specific requirements of each component and their operating conditions. He says, “Lubricants for gears and motors typically have different viscosities, fluid and additive chemistry and performance characteristics than those used for battery cooling systems. As a result, any attempt to use a single fluid for multiple purposes would require careful consideration of the composition and chemistry of the fluid to ensure optimal performance for all applications. Additionally, collaboration with OEMs would be necessary for efficient hardware design to maximize the fluid’s performance.” 

STLE member Dr. Peter Lee, institute engineer and chief tribologist, Southwest Research Institute (SwRI), says, “The fluid properties required to lubricate, reduce friction and protect components from wear are very different to those required to cool. Both the lubricant viscosity and additives required are different. Additionally, the lubricant needs to work where there are known to be stray electrical currents, so it must be a dielectric fluid. Coolants are typically a mix of water and petroleum-based products (i.e., propylene glycol). To increase cooling efficiency, the viscosity of coolant is lower than dielectric fluids.” 

Requirements 
Lee also indicates that optimizing a fluid is a challenge in itself before then trying to optimize for very different requirements. He says, “One example where a fluid is used both as a coolant and lubricant is in a metal-removal machining operation. Interestingly, here, the metal-removal fluid combines both a lubricity and a cooling function. The lubricity aspect is acting to reduce the heat generation in the first place by reducing the friction between the rake surface of the cutter and the chip. Cooling is provided through the use of water, which is very effective due to its higher heat capacity, however, it is not a good lubricant as it has low lubricity and has a bad habit of causing corrosion. For this reason, many metal-removal fluids are formulated with lubricity additives such as esters and either contain water or are diluted with water prior to use. In this way, the metal-removal fluids combine lubricity with cooling to meet the requirements of machining. Although metal-removal fluids act as a lubricant, there are some significant differences in requirements between these and gear lubricants. The challenge is to find a lubricant with low viscosity to reduce pumping losses as it is pumped around the battery and electronics, while still being able to protect the gears from wear while, at the same time, acting as a dielectric lubricant. If this can be found, the added advantage of the low viscosity will be reduced frictional losses in the gears. This will likely become a larger challenge as the coolant requirements increase with the move to ultra-fast charging.” Lee shares: “If such a cooling and lubricating fluid can be developed, there would be huge advantages in simplification of the system and reduction in weight with only having one pumped controlled system. Personally, despite all the challenges, I believe it will be possible.” 

STLE Past President and Fellow Dr. Edward Becker, president, Friction & Wear Solutions, LLC, explains: “The use of a single fluid is possible. Although fluids in automobiles are characterized as ‘lubricants’ or ‘coolants,’ the difference is the primary function of the fluid. In fact, every automotive fluid cools something and lubricates something as well. Engine oil cools the pistons, turbocharger bearings and other components. The engine coolant lubricates the water pump bearing. Air conditioning refrigerant cools and lubricates the compressor components. Even the fuel cools and lubricates the fuel pump and fuel injectors. Each fluid, however, has a primary function, and the properties of that fluid are optimized to perform that particular function (cool or lubricate), and the properties that make a good lubricant are quite different from those of a good coolant. Specifically, good coolants need high values of thermal conductivity and heat capacity, while good lubricants generally need low viscosity, good oxidation and corrosion resistance, high temperature resistance and others. Generally speaking, lubricants are oil based, and coolants are water based.” 

Composition 
Kwak indicates that the potential compositions of fluids that could be used for a single-use purpose include synthetic oils, mineral oils or even water-based coolants with lubricant additives that provide both lubrication and cooling performance. “Specialty fluids, such as polyalkylene glycols (PAGs) or ester-based fluids, have potential to be used in EVs due to their ability to provide both cooling and lubrication properties. However, any fluid used for these purposes must have well-balanced additive chemistry to provide both efficiency and durability,” he says. 

Lee states that in order to develop a single-use fluid for EVs, numerous considerations need to be accounted for: “Water is normally considered a good coolant due to its higher heat capacity, however, is not a good lubricant as it has low lubricity. Oils can act as coolants and are used as part of the cooling strategy in internal combustion engines, but the oil itself then needs to be cooled before it can be re-used to remove heat. This does not make it the optimum choice but does, considering the previous comments on water and its propensity to cause corrosion, make it the only practical one.” Lee continues: “In my opinion, a synthetic lubricant with as low a viscosity as possible for low pumpability requirements, high specific heat capacity for heat removal and with an additive package focused on load carrying capacity for the gears and with dielectric properties would be the best option.” He adds: “This is a comprehensive set of requirements, but fortunately such a synthetic lubricant exists. They are esters, which are derived from carboxylic acids and provide a potential solution. Esters have excellent thermal and good tribological properties. Since esters are fully synthetic, they can be formulated with specific properties including dielectric, rheological and tribological properties. This means there may be the possibility to develop a single lubricant with low viscosity that can both cool the battery, electronics and motor while also lubricating the gears, all while operating in an electric environment.”


Consumers’ expectations of rapid EV charging and extended vehicle ranges are leading to challenges in fluids formulations for EVs.

Benefits
Becker says that the problem, as indicated previously, is finding a single fluid that can function as primarily both a coolant and lubricant. “The most likely approach, at this time, is to increase the heat transfer properties of an oil-based fluid (rather than the lubricating properties of a water-based fluid). One of the most promising approaches is the addition of suspended nanoparticles to conventional oils to improve thermal conductivity,” he says.

A single fluid aids EV performance efficiencies. Lee says, “There is a need to reduce friction across the entire vehicle. Every interacting set of parts experiences friction, which, in turn, is a waste of energy (with the exception of the brakes). The more energy that can be saved, the more that energy can be used for moving the vehicle. Rolling resistance of tires and resistance of wheel bearings, shaft seals and even the interaction of the air with the paint finish can all add to frictional losses. In the drive unit, which encompasses the electric motor and reduction gears, savings can be made by using optimized lubricants both with respect to the base lubricant and additive package used. Lower viscosity lubricants save energy loss through the gear teeth and reduce pumping losses in the pumps. EV drive units do not experience churning losses since they do not run flooded but are drip lubricated at the essential locations.”

Saving also can be achieved through the use of coatings and careful consideration of surface finishes on the gear teeth. The type of gears used also affects efficiency of the gearbox. Lee shares: “Most of the drive units I have seen use the most efficient and quietist gears—helical gears—but occasionally one will come through with some straight cut gears within it. With respect to the electric motor, efficiency can be improved primarily by keeping it at its optimal operating temperature, although seal and bearing friction also are a consideration. Most of the motor efficiency is set at manufacture and depends upon thickness of the copper windings (thicker being better) and thickness of the laminator (thinner being better). There is a trade-off between having a highly efficient motor and both the cost and weight additions for that. Good thermal design of the motor can result in reduced clearances and optimized cooling giving increased efficiency of the magnetic circuit. This results in lower operating temperatures, so less cooling required and a longer service life. There are now some good computer models to help at this design stage,” Lee says.

Lee shares: “What potential improvements in efficiency can be gained from the drive unit is an easy question to ask and not easy to answer. From what I have seen, I would suggest with a combination of gear coatings, surface finishes, lubricant optimization, improved seal materials and motor design optimization, linked with perfected cooling strategies, between 4% and 8% is achievable.” “Generally anything that can be done to save friction will allow reduction in battery size for the same original vehicle range. This, in turn, means less vehicle weight and therefore a reduction in component sizes and, hence, weight, creating a positive weight reduction loop,” Lee explains.

Kwak agrees: “The benefits of a single fluid might be obtained with continued material advances and combinations of these technologies.”

Becker states: “Today’s electric motors are already around 90% efficient, and much of the loss is due to electrical resistivity of the components. So, barring the development of room-temperature (and above) superconductors, there just isn’t much room for improvement in the efficiency of EVs.”

Aside from fluids, currently heavy batteries are another concern of EV manufacturers. Kwak explains: “To accommodate the weight of a heavy battery, changes to the body and chassis material would be necessary, such as the use of lightweight materials like carbon fiber or aluminum. These materials would help offset the weight of the battery and maintain the overall weight of the vehicle. Additionally, optimizing the placement or position of the battery to improve weight distribution would be crucial to maintaining the
vehicle’s handling, stability and performance. Furthermore, integrating the battery pack into the structure of the vehicle, such as the floor, can help reduce weight, improve efficiency and enhance the overall center of gravity.”

Lee says: “There are already materials available in motorsport that are physically lighter while having equivalent or better durability. These materials include aluminum alloys, manganese alloys, titanium, poly composites and carbon fiber. However, motorsport has very little consideration to expense, and if EVs are to become mainstream and use these materials, both the cost of the materials and the machining costs of these materials needs to reduce. There are initiatives underway to do this. In the interim, aluminum and high strength steel are replacing steel in many areas of the vehicle. Combinations of materials also are being explored, such as steel combined with carbon fiber, and carbon fiber reinforced plastic using acrylonitrile butadiene styrene (ABS) or biobased composite resins made from flaxseed fibers and polylactic acid (PLA).”

“Reducing weight of the body and chassis will result in a smaller, lower cost battery being needed on a vehicle to keep the same vehicle range, so a win-win with further reduced weight. However, with a view to sustainability, it is not just about the weight of the vehicle if the repair costs for such materials increase such that more vehicles are scrapped after accidents and/or the cost of recycling is too high, impossible or energy intensive,” Lee adds. “Other options exist for vehicle weight reduction including using the battery itself as a structural part of the chassis and developing battery technology that is lighter. EVs also have larger batteries than needed, and hence weight, in order to ensure the batteries perform at the required levels through the end of the warranty. Improved computer modeling of battery cycling and aging could reduce the need for such large safety factors,” Lee shares. “Overall vehicle weight also comes down to consumer expectation. In many modern vehicles there are buttons for operating motors—removal of these buttons and replacement with manual operated levers would reduce weight. Think about the number of motors in the average driver’s seat! Perhaps we should all go back to ‘winding’ the window down?” Lee says.

Becker agrees: “Aluminum has already replaced steel in many chassis’ components. Aluminum body panels could reduce vehicle weight further, but advanced polymers (such as carbon fiber composites) are more likely to see expanded use.”

Summary
Consumers’ expectations of rapid EV charging and extended vehicle ranges are leading to challenges in fluids formulations for EVs. Although the number of moving components is greatly minimized, fewer parts don’t necessarily translate into easier lubricant formulations. In addition, OEMs are moving toward simplifying the vehicle design and combining the electric motor, reduction gear and power electronics into a compact integrated electric drivetrain unit—which means a need for a single fluid that has to be carefully balanced to lubricate gears, bearings, etc.—as well as thermal management for electric motors, windings and other electrical components.¹ Low viscosity synthetic base stocks formulated with well-balanced additive chemistry provide the best opportunity to achieve these objectives. Sustainability and biodegradability are additional aspects that add to the challenge of designing a single fluid, and based on the knowledge shared by industry experts in this article, both lubricants and automotive industries are well-positioned to find optimal solutions in the near future.

REFERENCE
1. Canter, N. (2022), Tribology and Lubrication for E-Mobility: Findings from the Inaugural STLE Conference on Electric Vehicles, A White Paper Sponsored by STLE. Available here.

Dr. Yulia Sosa is a freelance writer based in Peachtree City, Ga. You can contact her at dr.yulia.sosa@gmail.com.