OEMs Want More Efficiency, Better Heat Management from Electric Driveline Fluids
Electric vehicles are still in an early stage of their evolution, but there are already more than 42 million on roads around the world. These vehicles largely use lubricants developed for conventional autos powered by internal combustion engines – though some lube suppliers have begun introducing products marketed specifically for EVs.
But the scenario is changing, industry observers say, and more importantly auto manufacturers say it needs to change. At a recent trade event, OEM representatives said they need new lubricants and fluids better tailored to the particular needs of models that they are designing now and want to design in the future. Among their most urgent needs, they said, are products with new chemistries that allow reduced mechanical drag in drivetrains and that provide better heat management.
They also advised formulators about conducting tests to provide the type of performance documentation that automakers seek.
Global sales of electrified cars have been ramping up fast for about a decade and reached 13.8 million in 2023, up from 2.1 million in 2018 according to the Paris-based International Energy Agency. Today’s numbers are still far from the levels estimated in coming years. German engineering firm FEV forecasts that global EV sales will reach 58 million by 2040, assuming a medium level of progress on factors that encourage electrification.
As the numbers increase so do the variety of drivetrain designs. Some of the differences stem from the range of electrification. Cars running only on battery power have an electric drive unit consisting of the electric motor – which drives wheels directly through an axle or half-shaft – and a gearbox. Plug-in hybrids have both an electric drive unit and an internal combustion engine.
Whether BEV or PHEV, models may have a single electric motor that drives either the front or rear axle. Most models have more. There may be two – one each for front and rear axles; three – one for the front or rear axle and two for a transaxle driving the other set of wheels; or four – one for each wheel, for example on vehicles designed for off-road use.
Variety also comes from how the main units are housed. Drive units for any of these can be two-in-ones, containing a gearbox and electric motor, or three-in-ones, also containing an electrical inverter. If the motor and inverter are housed together and the gearbox separately, then the former may be cooled with a water-glycol and the latter with a reduction gear fluid. If the motor and gearbox are housed together by themselves, then they will be lubricated and cooled with a transmission fluid while a water-glycol cools the inverter.
If all three are housed together they need to be lubricated and cooled with a fluid that combines heat transfer capabilities with high-performance lubrication, and there is no role for a water-glycol.
Whatever the design, OEMs are seeking performance improvements that exact requirements from lubricants and e-fluids. Driving range is one of the biggest concerns for potential BEV purchasers, so extending range is one of the main priorities for manufacturers. They are seeking to do this by improving battery performance but also by improving efficiency, which means reducing energy losses.
Figure 1. Configuration and Cooling Options for Electric Driveline Units
Notes: symbols from left to right are the electrical inverter, e-motor and gearbox
Source: FEV
Speaking at the Lube Expo conference in Detroit in March, FEV North America Executive Vice President for Drivetrain, E-Mobility & Vehicle Engineering Kiran Govindswamy said 24% of all losses occur in electrical drive units, making them the second-greatest source, following aerodynamic drag. EDUs lose energy in a number of ways, several of which are involved with the generation, transmission and converting of electrical current. Automakers can also improve efficiency by optimizing the types and designs of e-motors they choose as well as the drive unit configuration.
But there are two other types of losses in EDUs where OEMs are looking for help from the lubricants industry: mechanical losses and losses influenced by temperature. Mechanical losses stem partly from fluid drag and are a familiar topic for lube formulators. As they have with ICE vehicles, automakers are asking lube suppliers to reduce fluid viscosity, Govindswamy said, and also to improve lubricity.
The heat management issue in drive units is different than the one for EV batteries, which is also garnering much attention. The large batteries that power EVs generate large amounts of heat, and much effort is being devoted to develop coolants that can move it.
In drivelines, temperatures are rising because of efforts to improve efficiency. To help extend the distance EVs can travel per kilowatt, OEMs are minimizing package sizes to save weight and space, said Troy Muransky, lead organic materials engineer for American Axle & Manufacturing, but this increases power density, raising temperatures – both locally and overall – and increasing loads and stresses on bearings and gears. In addition, rotational speeds in driveline units are trending upward – to more than 25,000 revolutions per minute for motor shafts – which further tends to raise temperature and increases potential for aeration.
The piling on of these developments stands to raise temperatures to a level that reduces efficiency, which would be the opposite direction OEMs want to go.
“Improving EDU efficiency is a key component of maximizing the overall range of electric vehicles,” Govindswamy said. “Higher motor temperatures can lead to lower motor efficiency, so cooling plays an important role for motor efficiency.”
Govindswamy, Muransky and others described characteristics that electric driveline fluids will need: ultra-low viscosity to maximize efficiency and minimize energy losses; at the same time, wear protection to ensure durability for the defined drive cycle and lifetime duration; and cooling properties to transfer out the heat generated by friction and electronics. They will also need corrosion protection against water and humidity; cold-temperature pumpability down to minus 40 degrees C; excellent foam control and prevention of aeration; and compatibility with materials ranging from copper to seals that are in use.
All in all, the lubricant industry still has much work to do to provide the lubricants and fluids that EV manufacturers will need and want.
“These new compact, high power-dense EDUs are going to be very demanding and need these fluids to do many new critical tasks,” Muransky said. “Electric drive fluids are not off-the-shelf products, and rather bespoke fluids must be developed for each unique design.”
Speakers at Lube Expo also advised lube suppliers about the level of proof that OEMs want to see of product performance, emphasizing the need for thoroughness. Govindswamy underscored that OEMs want proof that products will minimize energy losses in EDUs at the component level, along with heat management ability. Michael Berhan, a powertrain gear and bearing expert in Ford Motor Co.’s Research and Advance Engineering department, said suppliers should provide test results for multiple combinations of speed and load.
“Often suppliers show just one load and speed point and say, ‘See, you can gain X% efficiency across the board,’” he said. “That’s not good enough on a system level. A benefit at one operating point can hurt at another. Vehicle differences – such as loading, tire size, et cetera – can also make the same change help one application but hurt another.”
Berhan also suggested lubricant companies should not shy from getting help in areas beyond their expertise.
“If your firm doesn’t have experience working with those drive cycles, automotive consulting firms can run those for you,” he said.
Tim Sullivan is base oil executive editor for Lubes’n’Greases. Contact him at Tim@LubesnGreases.com