Automatic Transmission Fluids for modern automatic transmissions
To reduce vehicle fuel consumption, all Automatic Transmissions (AT) now have lock-up clutches bypassing their torque converters, and the smooth engagement of these clutches is critical to eliminate shudder. The interaction between the Automatic Transmission Fluid (ATF) and the friction materials of these clutches is central to shudder prevention. As Bill Abraham, Driveline Technology Manager at Lubrizol, explains, “We need to maintain high torque capacity, but still not cause any shudder on gearshifts. If the fluid is not designed specifically for the torque converter lock-up clutch, the driver will feel a shudder, a vibration when the transmission shifts between gears. In the past eight years, transmission manufacturers have upgraded the torque converter materials from a heat-treated cellulose-based material to a more thermal stable material consisting of more aramid polymer, carbon fiber, and/or graphite particles. Material manufacturers are also adjusting other material properties such as material porosity, material thickness, and waffle patterns in order to maximize the efficiency of the clutches and torque converter.”
Trend to increase the number of ratios
Another major change, and perhaps the most visible improvement as far as the customer is concerned, is the dramatic increase in the number of ratios or speeds. Again with the aim of improving both fuel efficiency and smoothness, the typical speed count of a state-of-the-art transmission has gone up from five in the 1990s to seven or eight. ZF is now about to present its new nine-speed for front-drive applications and Korea’s Hyundai has recently confirmed it will launch a ten-speed unit. Inevitably, says Abraham, along with the increasingly sophisticated internal workings, the transmission’s friction materials have become even more sophisticated, too. “Take a new eight- or nine-speed stepped transmission. First, OEM’s are purposely slipping the torque converter continuously regardless of shifting speeds in order to reduce energy losses for better fuel economy. This places a major emphasis on material integrity along with fluid design. Consequently, torque converter friction materials will be more advanced and quite different than the ones used eight or more years ago, as well as the first two or three clutch packs, which have to cope with high levels of energy than before. These clutch materials, like the newer torque converter materials, will be more thermally stable and more expensive,” Abraham explains.
In addition, adds Abraham, “every time the transmission engineer designer reduces the size of a clutch pack, the fewer clutches will experience even more energy, further demanding the clutch material and the ATF to be of better quality.”
Given that transmission makers’ need to keep a firm hold on the pricing of their products, it is thus easy to see why a modern automatic transmission contains such a wide variety of different friction materials, each demanding a specific set of characteristics from the ATF and each having to be compatible with the other. Today’s eight-speed units, for instance, use four or five different types of friction material, several of them advanced graphitic and woven carbon fiber materials that require a totally different approach as far as transmission fluid composition is concerned.
Transmission manufacturers are now opting for reduced viscosity fluids to cut down on churning and pumping losses, while putting more power and torque through the transmissions and expecting equal or even extended anti-shudder durability and long-lasting protection for gears, bearings and seals.
Single Automatic Transmission Fluid
This entire spectrum of capabilities must be delivered by a single fluid that must last for six-figure miles, underlining the challenge faced by the fluid formulator. “The older fluids used by pre-2004 transmissions are simply not good for these new transmissions at all; they were not designed with all of these new friction materials in mind,” says Abraham. “When you design an ATF for these new and more complicated transmissions, you’ve got to ensure that one fluid can provide good friction protection with all of those materials, not just one or two of them. You have to have good chemistry to do that.”
Other factors, such as chemical compatibility with the seals, valve bodies and solenoids, need to be considered as part of the formulation equation. Anti-wear performance also needs to be optimized because of the higher speeds of the six-, seven- and eight-speed transmissions and their twin sun-planet gear sets. Gear and bearing wear performance needs to be exceptional, too, says Abraham. “Needle bearings are often a problem as fluid viscosities drop, and though this drop means lower churning losses and better efficiency, the gain in performance means you begin to tax the fluid’s durability. So you need the appropriate additive chemistry to offset that loss.”
Continuously Variable Transmissions (CVTs) pose a unique set of different challenges versus ATs such as requiring higher shear stability and maintaining the appropriate amount of metal-metal friction while having enhanced anti-shudder performance; Dual Clutch Transmissions (DCTs) exhibit even more energy through the main start-up clutch than an AT torque converter, thus placing further demands on the lubricant. Both of these transmission types in addition to the high speed AT discussed above require the latest advancement in additive fluid technology in order for the equipment to be lasting.
Requirements on transmission fluids are getting more and more stringent. I wonder how trade-offs are done when formulating transmission fluid? Are these the same than for engine lubricants formulation?