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HOT AND CHARGED: Developing Fluids for Electric Gearboxes



Automotive OEMs are making the turn from internal combustion engines toward electric vehicles, and they’re bringing the lubricant industry along for the ride.


Opportunities and technical challenges associated with the transition to EVs were highlighted in a special session at the annual meeting of the Society of Tribologists and Lubrication Engineers in Nashville, Tennessee.


Torsten Murr of Shell Global Solutions told his audience at the May meeting that electrification is expected to reach 50 percent of global vehicle production by 2030,while sales of ICE vehicles will decrease significantly, according to IHS Markit.


The size of the global electric vehicle fleet is accelerating from around 3 million in 2017 to 5 million in 2018, reaching 130 to 250 million in 2030, according to the International Energy Agency, a Paris-based intergovernmental organization. These numbers include battery electric vehicles and plug-in hybrids.


Projections for global annual sales are between 23 and 43 million electric cars in 2030. In the IEA’s more modest of two scenarios, EVs’ share of new vehicle sales are expected to be greatest in China at 28 percent (57 percent including two- and three wheelers). EV sales in Canada will reach 29 percent of the market, followed by Europe (26 percent) and Japan (21percent). The United States lags behind at just 8 percent.


Last year, the most electric cars were sold in China, Europe and United States at1.1 million, 385,000 and361,000 units, respectively. Electrification is a general term that refers to the use of electric power to replace other power sources. In the automotive industry, electrification currently entails using battery-powered electric motors to power vehicles. The automotive industry is embracing different forms of electrification, from hybrids, which have both electric motors and ICEs, to fully electric vehicles.


Full hybrids have an electric motor with batteries that are recharged inside the vehicle by regenerative braking and a generator powered by the ICE; they cannot be plugged in. Regenerative braking recycles kinetic energy otherwise lost in the braking process. The ICE extends the vehicle’s driving range beyond what is provided by the battery storage capacity.



The next step in electrification is the plug-in hybrid, which has an on-board charger used to recharge the batteries by means of the electric power grid. The ICE only propels the vehicle as a back up to the electric motor. For example, the Chevrolet Volt has an estimated e-motor driving range of 53 miles between charges, and a total range of 420 miles.


Full electric battery vehicles such as the Tesla Model 3, Nissan Leaf and Chevrolet Bolt have estimated e-motor ranges of 325 miles, 120 to226 miles and 238 miles, respectively. These vehicles rely completely on their electric motors for propulsion and use the electric power grid and regenerative breaking to recharge their batteries.


According to Murr, lubrication technology can make a critical contribution by helping to extend battery electric vehicle range. Automotive hardware developments are driving a sea-change from lubricants developed for ICE vehicles to new formulations specialized for EV transmissions and other power train elements.


The Heat Is On


Arup Gangopadhyay, technical leader of power train tribology at Ford, told his audience that developments in EV powertrain designs are creating new performance targets for lubricants. A power train produces power in an ICE or electric motor and delivers it through a transmission and other hardware to the wheels of the vehicle. As OEMs move from dry e-motors (with the motor outside the gearbox) to wet e-motors (with the motor inside the gearbox of a wet transmission), a new opportunity arises for the lubricant industry.


“EVs will have a gearbox—also called an e-axle—to transmit power from the electric motor to the wheels.[Automatic transmission fluid]is used today to lubricate gearboxes. Unlike typical axles used with ICEs, the electric motor is inside the gearbox along with the gears in an EV,” Gangopadhyay explained.


“The e-motor gets very hot when it operates, and the copper windings in the e-motor are cooled by the ATF inside the gearbox,” he continued. A drop of 15 degrees Celsius in winding temperature allows an e-motor to deliver more torque to the wheels and increase vehicle performance.


“Ideally, we would like to have a lubricant that will help take away heat from the copper windings faster, allowing the electric motor to run cooler while maintaining other lubricant performance attributes, such as wear protection, aeration, oxidation and elastomer compatibility,” Gangopadhyay said.


New fluids for e-axles and wet electric motors should have all of the properties of traditional ATFs, except clutch friction durability, he noted. In addition, these fluids will need improved thermal conductivity, higher temperature stability (up to 150 C), improved oxidation stability and copper corrosion resistance. They’ll also have to be compatible with laminates, insulators and rare earth materials in electric motors.


In search of an ATF formulation better suited for electric motors, Ford investigated the effects of base oils on motor winding temperature.“We found that thermal conductivity is more important than specific heat to achieve cooling of the windings,” Gangopadhyay reported. “The thermal conductivity is similar for mineral oils used for ATF formulations and 10 to 15 percent higher for polyalphaolefins and polyalkylene glycols.


“However, we need 50 percent higher thermal conductivity than mineral oils, and therefore, we may need to look at API Group V oils. In the future, it may be possible to take base oil that already has high thermal conductivity and enhance its thermal properties by adding nanoparticles,” he concluded.


Base Oil Effects


Afton Chemical’s Adam Banks, drive line marketing manager, and Yungwan Kwak, driveline research and development group leader, agreed with Gangopadhyay about the importance of thermal properties for lubricating EV drive trains.


“In simple terms, thermal conductivity is how quickly heat can be transferred. In lubrication, this determine show quickly heat can be taken away from a surface,” Banks explained. “Specific heat is how much thermal energy a fluid can hold,” which means a smaller amount of fluid with higher specific heat can take away more energy.


“The ability of the molecules to move around each other and their different ways to move and vibrate will determine these properties. This means that the molecular weight distribution, branching, saturation and purity will all have an influence,” he continued.


Kwak added, “Different base oils have their own chemical and physical properties, such as molecular structures, compositions and densities. Base oils that have higher molecular weight and relatively linear structure would provide higher thermal conductivity. Other base oils that have higher density and certain chemical functionality or structure that can absorb more heat energy by molecular motions would provide higher specific heat capacity.”


In a comparison of mineral and synthetic base oils, Afton found that in general, synthetics like polyalphaolefin have better thermal properties such as high thermal conductivity, specific heat capacity and thermal diffusivity, Kwak reported. “However, when viscosity was considered, it became a different story: 2centistoke mineral base oil from the additive, but between one additive or another, this is small. However, additives can help prevent oxidation, which allows the fluid to resist degradation from heat overtime, allowing it to retain good thermal properties that could otherwise be diminished,” he observed.


On the other hand, Banks noted, “Additives have a large effect on the lubricant electrical conductivity. This is because many of the additives are polar and surface-active to do their job. We have found in their laboratory, connected it to an AC drive and DC absorbing dynamometer, and measured torque output and rotor speeds. Motor efficiency was comparable for two commercial ATFs, possibly because their viscosities were similar. Viscosity index seemed to be less significant in these e-axles than in ICE transmission gearboxes.


Sensors revealed other, much more significant differences. At five locations in the gearbox, operating temperatures were between 3 and 5 degrees C cooler with Fluid 2 had more efficient cooling performance than 6 cSt PAO, as viscosity played an important role for cooling performance.”


Additive effects tend to be less influential than base oil when it comes to thermal properties. According to Banks, “Thermal properties are a bulk effect. Most of a finished fluid is the base oil with only 5 to 15 percent additive package, which means this is much more influential in thermal performance. “There will be some effects that different additives, even within the same component class, can have a significantly different effect.”


ATF Studies in a Bolt


Peter Lee, staff engineer –chief tribologist, fuels and lubricants research division with Southwest Research Institute, measured performance differences of ATF fluids using a gearbox from the fully electric Chevrolet Bolt passenger car.


The team at SwRI put a Bolt gearbox on a test stand versus Fluid 1 due to differences in thermal conductivity and specific heat capacity.


Lee’s team noted a sweet spot in ATF electrical conductivity versus temperature. Too much conductivity can be dangerous, as the electrical power of the motor could transmit to the case of the gearbox, potentially causing damage to the gearbox or other parts. Too little electrical conductivity could result in static buildup in the gearbox, which could suddenly discharge. This sweet spot has not yet been defined.


Electric charge is another parameter unique to e-motors. Lee’s project included electorheology, which is the study of changes in fluid viscosity due to the presence of an electric charge. “Electro-rheology test capability has been around a long time, but it has not been used before for engine and drive train lubricants as they do not operate in the presence of an electric charge in ICEs,” Lee explained. “However, the lubricant in an e-axle is exposed to charge, so we investigated what effects, if any, charge has on lubricants.”


Lee’s team discovered that new ATF had a higher viscosity than used fluid, both when charged and when uncharged. But the viscosity of the used ATF increased slightly when charged, while the viscosity of the new ATF dropped slightly when charged. “This was just one test, so these results might not be representative of all ATFs, but these new results show an interesting effect that is worth investigating further,” he noted.


“Our team at SwRI believes that not all ATF fluids behave the same in an e-axle, and that there is need for more in depth investigation of [the effects of] formulation differences as well as e-motor operation on fluid performance in order to understand the most important factors and optimize these fluids for e-axles,” Lee concluded.