The how and why of automatic transmission fluids
Stuart F. Brown, Contributing Editor | TLT Webinars July 2014
These unique lubricants provide critical functions for delivering better fuel economy and durability in today’s vehicles.
KEY CONCEPTS
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Most cars and trucks have three types of automatic transmissions: step-ratio, continuously variable or duel-clutch.
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A lubricant’s additive package includes more than 10 types of ingredients.
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Friction modifiers are important to the operation of automatic transmission fluids.
MEET THE PRESENTER
This article is based on a Webinar originally presented by STLE University. “Automatic Transmission Fluids” is available at
www.stle.org: $39 to STLE members, $59 for all others.
Chintan Ved is a technical expert for Ford Motor Co.’s Transmission and Driveline Fluid Development group where he focuses on technical development, specifications and publications of transmission and driveline lubricants and chemicals. He holds bachelor’s and master’s degrees in chemical engineering, with a focus on tribology, as well as a master’s degree in product development from the University of Detroit Mercy. You can reach Chintan at
cved@ford.com.
Chintan Ved
ALMOST EVERY CAR AND LIGHT TRUCK SOLD IN THE U.S. comes equipped with an automatic transmission these days. As such, there are more varieties of automatics out there than ever before. Almost all of them do their work while immersed in a bath of the indispensable lubricant known as automatic transmission fluid or ATF.
Traditional step-ratio automatic transmissions have long been produced with four, five and even six speeds, but now automakers chasing improved fuel economy are using transmissions with as many as nine or even 10 discrete ratios in their vehicles. Hundreds and hundreds of parts must be protected from wear and assured of long, trouble-free lives within these complex devices.
At the same time, the growing trend is toward continuously variable transmissions (CVTs) using belts or chains connecting variable-diameter pulleys to provide an effectively infinite continuum of drive ratios between the engine and the driving wheels. In addition, dual-clutch transmissions or DCTs have arrived on the scene, some of them using wet clutches that depend on a lubricant bath to control heat and ensure durability of friction materials. New jobs for ATF seem to be popping up everywhere, even in power-steering systems and four-wheel-drive transfer cases.
Chintan Ved, an STLE member and technical expert for Ford Motor Co.’s Transmission and Driveline Fluid Development group, discussed these topics and more in an October 2013 STLE University Webinar on the challenges facing formulators of automatic transmission fluids. Following are the key points from Ved’s presentation.
INGREDIENTS OF ATF
ATF is hard-working stuff. In step-ratio transmissions—by far the most popular type—all of the engine’s power is transmitted between the two halves of the torque converter by a swirling vortex of ATF. Essentially a pair of fans facing each other in a fluid bath—a torque converter—applies shear forces to the ATF to transmit power from its driven side (connected to the engine) to the output side, which leads to the clutch packs and planetary gear sets and, from there, the road. ATF pressurized by a pump also powers hydraulic servos that engage and disengage the various clutches and gear sets as the transmission goes through its speed ratios out on the highway. Transmission designers want the ATF to get these jobs done while minimizing pumping losses that reduce fuel efficiency.
Like engine lubricants, ATF formulations begin with mineral or synthetic base oils, which account for 70 percent or more of the finished product’s volume. Base oils provide a means of physically separating the mating surfaces of moving parts, enabling heat transfer away from friction components such as clutches and acting as the carrier for a suite of additives serving many purposes inside a transmission. The base oils also help impart the desired viscosity to an ATF formulation. Roughly speaking, “the performance of an ATF is determined about 55 percent by the additive package and about 45 percent by the choice of base oils,” says Ved.
A lubricant’s additive package can have 10 or more types of ingredients, including friction modifiers, extreme-pressure additives, antifoam components, antiwear additives and more.
Antiwear additives work by reacting with component surfaces to form a solid protective layer that prevents metal-to-metal contact, the path to disaster for mating parts. Formation of this reaction layer is a four-step process in which parts in a new transmission are helped to break in and form smoothly sliding surfaces where they contact one another. The additive is chemically adsorbed on the part surfaces where a reaction takes place, forming a protective layer that prevents surface wear.
Extreme pressure (EP) additives react with metal surfaces to form a sacrificial wear layer. As with antiwear additives, EP additives go through a four-step process when they operate inside a new transmission. First, the additives help the moving parts break in. Next they become adsorbed onto the surfaces of the parts where they react and form a sacrificial layer with a lower shear strength than the underlying metal. The so-called reaction layer prevents metal-to-metal contact and, most important, prevents moving parts from welding together under the effects of force and pressure.
Friction modifiers are a category of additives that is particularly important to the operation of automatic transmissions. Smooth shifting plays a major role in the car customer’s perception of how well the transmission works and how satisfying the vehicle is to drive. The lubricant formulator wants to provide enough slipperiness to impart a smooth shift feel while avoiding loss of grip inside such components as clutch packs. At the other extreme, too little slipperiness can result in shifts that feel harsh to the driver. Squawking, chattering and vibration are other evils friction modifiers can eliminate by ensuring that stick-slip conditions do not occur at mating surfaces. Finally, it’s essential that a friction modifier does not degrade in performance over time, lest the car owner feel that the vehicle shifts differently at 100,000 miles than it did at 500 miles.
One of the challenges engineers have faced in recent years is the increasing operating temperature of transmissions. Heat has a huge impact on the rate of lubricant degradation, keeping lubrication specialists busy developing formulations that can stand up to the rigors of fill-for-life service. Standards vary between vehicle OEMs, but at Ford Motor Co. “life” is considered to be 150,000 miles of driving under normal conditions. Aside from replacing any fluid lost during service procedures, the original factory fill of ATF is expected to remain on duty the entire time.
TYPES OF TRANSMISSIONS
There are approximately 500 to 600 components in a traditional stepratio automatic transmission. While a number of these parts are made from the same materials, there are many that are in some way unique. And the ATF must keep them all in good health. If life were very simple, all automatic transmissions would contain just one metal alloy, one friction material and one type of seal. But that’s not the case. In reality, we find that many different materials must be used in metallic and friction parts, as well as in seals and O-rings to meet specific performance and durability requirements. Engineering in compatibility between these diverse parts and the surrounding lubricant is essential.
Continuously variable transmissions (CVTs) look like they are here to stay. These mechanisms require ATF formulations containing significant amounts of sulfur, phosphorus and other antiwear and extreme pressure additives to handle the heavy loadings and pressures where metal belt edges contact the movable pulley sheaves. But there’s a fly in the ointment. Such additives can have negative effects on elastomer seals and other nonmetallic components, requiring the ATF formulators to select additives that balance the sometimes-conflicting lubrication and durability needs of the transmission’s many metal and elastomer components. Preventing unwanted swelling of elastomer seals and O-rings is particularly critical.
Engineers and mechanics may wonder why so many different fluids are specified for different vehicle systems. “We make a conscious effort to reduce the complexity because we know everyone would like to have just one product on their shelf. But we can’t always accomplish that,” Ved says. The reason is the widely varying requirements of the different types of automatic transmissions used in diverse vehicle applications and duty cycles. In a CVT, for example, conditions are harsh where the belt or chain contacts the sides of conical pulleys, which need to maintain a certain surface roughness to function properly. Preventing wear at these interfaces requires a higher level of extreme pressure additive when compared to the ATF used in a step-ratio transmission. Foaming can also occur in a CVT’s internal mechanical environment, requiring an ATF with especially strong antifoaming properties.
Dual-clutch transmissions (DCTs) are actually manual gearboxes that have automatic controls managing the shifting chores for the driver. Using two clutches gives snappy gear changes that performance-minded motorists appreciate. DCTs are built in two variants, wet and dry. In the dry version, the launch clutch operates without lubrication, but the wet type runs the launch clutch in a bath of custom-formulated ATF. The gearbox itself is usually filled with traditional manual-transmission lubricant.
A lot goes into the development and testing of an ATF formulation that will deliver good shift quality and durability throughout a long service life. It’s a process that can take two or three years. The fluids destined for a step-ratio transmission, CVT or a DCT must of necessity have different additive packages suited to their individual mechanical architectures. Still, an uninitiated person might be forgiven for asking: are the frictional requirements of Mercon SP fluid the same as those for a Mercon LV fluid? “No they are not. And if you could replace one specification with another, we would have,” Ved says. “The reason we haven’t is that there are performance differences between the two types.”
The proliferation of transmission designs can create challenges for the service technician. Some drivers don’t realize that their car actually contains a CVT. The brochure may only mention an automatic transmission. With CVTs still relatively uncommon in vehicle fleets, the unique fluid they require may not be readily available to a local garage used to working on step-type automatics. But this is no excuse for a potentially costly error. “The fluids are optimized for each application and they are distinctly different,” Ved warns. “You cannot add CVT fluid to a step-type transmission and vice versa.”
Looking down the road, automakers will naturally want to keep the number of different ATF formulations they use at a minimum for inventory and convenience purposes, while also tailoring the properties of the fluids to wring the best efficiency and drivability out of every vehicle they build.
As the endless pursuit of better fuel economy unfolds in the years ahead, Ved predicts that ATF is likely to continue being produced with lower viscosity to reduce pumping losses. And as lighter-weight rotating components made from new materials come along, ATF recipes will require ongoing tweaks to accommodate their particular attributes.
Stuart F. Brown is a free-lance writer who can be reached at www.stuartfbrown.com.