A New Wave of ATFs

Government policies, fuel taxes and greenhouse gas emissions targets direct these trends, but technological advances enable their progress. For more than a century, ICEs have dominated automotive performance and design, including transmissions and other hardware that convert energy to transportation.

EV Flavors

Electric motors are simply not drop-in replacements for ICEs. Innovations in automotive engineering and lubricant formulation for EVs were highlighted at the Society of Tribologists and Lubrication Engineers’ May 2019 meeting in Nashville, Tennessee.

As highlighted by Torsten Murr, the global technology manager of transmission fluids for Shell Global Solutions based in Germany, electrification is expected to reach 50% of global vehicle production in 2030 while sales of ICE vehicles will decrease significantly, based on a market analysis by IHS Markit.

Electrification refers to the use of electric power to partially or fully replace vehicles’ propulsion requirements. Batteries store and deliver electric power in most electrified vehicles. Drivers may be unaware that their vehicle has a battery-powered unit that unobtrusively assists acceleration after start/stops and recharges automatically. Streamlined hybrid EVs – both plug-in and non-plug-in types with quiet electric motors supplemented by ICEs – no longer turn heads.

Consumer acceptance of plug-in HEVs and pure battery EVs will depend whether these cars can at least replicate the performance of ICEs and non-plug-in HEVs in terms of range or distance traveled between battery charges.

Murr explained that enhancing e-vehicle range will depend critically on lubrication technology to increase efficiency, as well as battery performance. The current generation of automotive lubricants were optimized for use in ICE and diesel engines and transmissions. Formulation targets are shifting to new criteria for EVs. Moreover, many original equipment manufacturers are commercializing unique designs for EV powertrains.

ICEs are lubricated with engine oils that reduce friction and wear between moving parts such as cylinders and liners. In addition, combustion byproducts accumulate in engine oils where additives neutralize and disperse them. Coolant, based on a mixture of water and glycol, is circulated through the ICE and pumped to a radiator to release heat. Transmissions are filled with ATFs formulated primarily to protect gears from friction and wear. ATFs limit foam formation, enhance efficiency and have good thermal stability for filled-for-life gearboxes.

The latest HEVs are configured with a “dry” e-motor attached to a transmission and lubricated separately. An e-motor produces heat, but no combustion byproducts. Unlike an ICE, an e-motor has coils of copper wire, sometimes covered with insulation. It is necessary to cool e-motors without damaging the insulation and copper wires.

Most hybrids have a continuously variable transmission, or CVT, that uses pulleys and steel belts as well as gears. CVTs are lubricated for life with ATF based on synthetic base oil and appropriate friction modifiers, antiwear additives and viscosity modifiers. An HEV also has an inverter/converter to convert AC current produced by the transmission to DC current that can be stored in the vehicle’s batteries. An inverter generates significant amounts of heat and has its own dedicated cooling system (coolant, radiator or heat exchanger).

A sea change is in the works as OEMs design next generation “wet” e-motors that will be installed inside transmission gearboxes and immersed in new ATF formulated specifically for this unique application. The advantages of wet over dry configurations include weight reduction, more compact design, lower power losses, more efficient cooling management, fewer parts and lower material costs, according to Murr.

Hybrid ATFs

Murr explained that new ATFs for wet e-motors will be hybrid fluids with all the functionality of current ATFs plus improved thermal stability to withstand greater thermal stress from the e-motor. Additionally, they will be expected to cool e-motors while providing electrical insulation without corrosion or incompatibility with clutch materials.

This presents a conundrum because formulations for typical ATFs, coolants and transformer oils share little in common. Murr proposed using high-quality API Group III and IV base stocks for their thermal properties and cost relative to esters and PAGs. He noted that lighter viscosity synthetic oils reduce frictional losses and operating temperatures in gearboxes and improve efficiency, but have higher electrical conductivity than heavier oils.

Murr also showed data for undesirable effects of additives, temperature, and oxidation byproducts on electrical conductivity. Moreover, some ATF additives corrode copper, the metal of choice for windings on rotors in e-motors. But copper wire windings in wet e-motors are exposed directly to the oil, or insulated with lacquer or other materials that can crack.

Successful commercialization of new ATFs for wet e-motors involves screening many formulations. For example, Murr discussed the importance of obtaining a balance between scuffing protection (the FZG or ASTM D5182 test) and copper corrosion (ASTM D130). While D130 is used widely as a quality control check, it relies on visual evaluation of copper strips. For better accuracy, Murr’s team measures milligrams of copper in the fluid after the test. Additionally, they developed a technique to immerse copper wire in test fluid at 130 degrees Celsius for 240 hours and then study the chemical composition and microscopic details of the wire surface using a scanning electron microscope and an energy dispersive x-ray.

Murr concluded that formulating successful new e-fluids will require investments in test methods and alternative additive chemistries and base stocks.