Different kinds of automatic transmissions have different requirements that transmission fluids must satisfy.
High-friction coefficients, clutch materials made from paper or carbon and extreme pressure agents are examples of unique issues with automatic transmissions.
With new models of electric cars, transmissions and transmission fluids are designed concurrently.
Floor it and off
that older car goes—but not smoothly. A jerky motion and a drop in the engine’s pitch reveal a gear change. But then overdrive hits and cruising begins.
In use since the 1940s (1
), automatic transmissions have come a long way. They’ve largely met (or surpassed) the fuel efficiency of manual transmissions (2
). And in the last 20 years, they have evolved into different kinds of automatic transmissions that accelerate faster and much more smoothly than conventional models. On top of that, transmissions are finding themselves harboring electronic components and in an entirely new environment in electric vehicles.
Besides the old-fashioned automatic transmission, called the step or planetary gear transmission, newer cars employ the continuously variable transmission found in Japanese makes such as Nissans, and the dual clutch transmission, found in cars of European descent, such as Volkswagens.
“The new transmissions all operate inside of a vehicle, and all the driver knows is that there’s two pedals,” says senior customer technical service specialist Claire Whitton at Afton Chemical Corp. in Richmond, Va. “But they have no idea that these three transmissions actually work and look different, right? The driver just sees two pedals and says, ‘Oh good! It’s an automatic; I can drive this!’”
Those different styles of automatic transmissions mean the fluids that create hydraulic pressure, dissipate heat and protect the metal gears and other parts are functioning differently too. Other changes are happening to transmissions as well.
“When we look at the recent predominant changes in the transmission industry, a couple things really jump out,” says senior engineer Dr. Burak Gecim, anti-friction components specialist at Allison Transmission in Indianapolis, Ind. “One is an increased number of speed ratios, which gives you better fuel efficiency and a potentially smoother shifting between each gear ratio. The other is the electrification of the systems.”
Consumer demand and rising fuel efficiency requirements are driving technological improvements to both transmissions and the transmission fluids they work with. Not surprisingly, different kinds of transmission hardware require different kinds of transmission fluidware.
Clutch of the matter
The whole point of a transmission is to take the constant force generated by the spinning crankshaft of an engine (called torque instead of force because it’s rotating), convert it into the right amount of power at any given speed using gears and deliver it to the wheels through the driveshaft.
A car starting from stop requires more energy to move and so takes a lower, bigger gear to “slow” the speed of the engine and give that initial push. A car cruising down the highway doesn’t need as much power to maintain a steady speed, so it can use a high, smaller-diameter gear to transfer or upgrade all those engine RPMs into speed. A truck pulling a trailer up a mountain pass needs more power, hence a lower gear, than when it is cruising along on level roads.
The clutch makes those power shifts happen (see To Let Go of Shifting or Not to Let Go
). In the big-picture view, a vehicle’s clutch (known as a friction clutch) ultimately serves as a go-between for the engine crankshaft, which can only turn at one speed, and a selection of gears that convert the engine’s one speed to the wide range of speeds of a moving vehicle. (In a technical view, multiple shafts and gears accomplish this seemingly simple task.)
To let go of shifting or not to let go
Experts refer to the jerk of an old-style automatic accelerating as shift shock. It occurs due to the drop in power during gear changes—after disconnecting the engine from the wheels and causing a drop in RPMs. But a CVT’s continuously variable gear design doesn’t have big steps between gears, so it shouldn’t have a pronounced shift shock.
“It’s a very different driving sensation when you don’t feel that jerk,” says Whitton. “But the American market is kind of funny. Most people today can’t drive manuals, but they still feel that performance is equivalent to feeling that shifting. And if they don’t feel that shifting, then they think maybe they really don’t have a performance vehicle.”
So some car manufacturers have purposely engineered in shift shock to attract consumers (3
). “There’s a computer module in some of these transmissions that will give the driver a sensation of a gear shift,” says Whitton.
Computer-controlled shift shock might be unexpected given CVT design, but some CVT vehicle manufacturers have the ability to shift front and center—on the steering wheel. Vehicles outfitted with paddle shifters let drivers shift gears without using a clutch (4
Pushing the paddle shifter skips to a different place on the cones electronically. Since the cars are fully automatic, the engine will eventually shift to that gear ratio regardless. But skipping to a different gear ratio leads to a sudden gear change, along with a touch of shift shock (5
When the clutch is engaged, it allows the crankshaft to, through various connecting parts, turn a gear that turns the wheels. When disengaged, the clutch separates the wheels from the crankshaft. The engine is still turning the crankshaft but the wheels are getting no power.
At this point, the car (or person, if it’s a manual) can switch to a lower or higher gear, as needs dictate, before letting the clutch contact the flywheel mounted on the end of the crankshaft. How the clutch handles gear shifts indicates whether it’s a conventional automatic transmission, a continuously variable transmission (hint, the clutch is missing) or a dual clutch transmission (another hint, there’s two).
Same but different
In reality, transmissions are much more complicated, have many more gears and types of gears, have more than one clutch even though there’s never more than one clutch pedal and are bathed in a transmission fluid for protection and performance.
In spite of their differences, the various automatic transmissions have similar requirements for their transmission fluid, says Whitton. First, they all need wear protection that prevents the transmission parts from wearing out. They need a fluid to keep the various parts clean and to prevent corrosion. They all need to cool the moving parts because the transmission can really heat up as it is operating.
“And the biggest thing that a transmission needs is friction control. That’s where automatic transmission fluids differ the most from other lubricants, in the friction control,” says Whitton. “Even so, for our three transmissions, the conventional AT, the CVT and the DCT, the frictional appetite is different for each.”
Let’s start with a conventional transmission and some weird math. Gear ratios are defined by the number of teeth on the driven gear (which turns the wheels) divided by the number of teeth on the driving gear (which the engine turns). First gear provides a low gear ratio between the driveshaft and the engine—say 3:1, where the engine is spinning three times faster than the driveshaft. The gears are cutting the engine’s higher speed and turning it into power. A high gear provides a high gear ratio, say 0.8:1, where the car is in overdrive and the driveshaft is spinning faster than the engine (6
). Regardless, the denominator—the engine’s gear size—stays the same.
(If you are wondering, like I did, why 3:1 is low and 0.8:1 is high, think of it from the engine’s point of view. Gear ratio 3:1 makes the engine’s RPM lower
. It’s also called a reduction gear. A ratio of 0.8:1 makes the engine’s RPM higher
CVTs turn wheels at different speeds with fewer single-sized gears and more gear ratios (7
), where variable-diameter pulleys provide infinite gear ratios between them. In low gear, a belt between the pulleys connects the smaller circumference end on the engine’s pulley (with a circumference value of, say, 1) to a fatter circumference on the driveshaft pulley (with, say, a value of 3) to provide a low gear ratio (3:1).
Simultaneously, the pulleys slide over to position the belt at their other ends, moving the belt toward the fatter end of the engine’s gear (pumping its value from, say, 1 to 2) and toward the driveshaft’s thinner end (dropping its value from 3 to 1.6). Although the gear ratio is the same as in the previous example (1.6:2, or 0.8:1), the denominator value changes. These changing gear circumferences allow the ratio to vary infinitely. This design also cuts out the need for a clutch.
The way these types of transmissions change gears indicate the kinds of frictional appetite they have. For one thing, the CVT’s belt is metal, and there is no intervening non-metal clutch, so CVTs have metal on metal to consider.
“To keep that belt from slipping or wearing, you have to have just the right amount of friction,” says Whitton. “Steel-on-steel friction has a friction co-efficient higher than what you would put in a step automatic transmission.”
In addition to the gear teeth interacting, the conventional step AT has clutch plates made out of a number of different kinds of materials. “The design is specific to the original equipment manufacturer,” says Whitton.
The clutch plate materials are usually paper, a paper composite or carbon fiber composite. Each of those options works differently with friction modifiers in transmission fluid, and their compatibilities don’t necessarily overlap. But the friction occurring in the step AT is lower than that found in a CVT.
Another kind of clutch, the dog clutch, has been working its way into conventional transmissions and some electric motor-powered transmissions, says Gecim. Normally the purview of manual transmissions (it’s the clutch that locks the output to the stick shift-controlled gear of choice) (8
), a dog clutch has interlocking teeth and doesn’t require friction at all to engage (9
“There’s a challenge with dog clutches in that their shifting is not as smooth as a friction clutch. But that’s an engineering challenge to work out,” says Gecim. “But elimination of the friction clutch helps from the lubricant standpoint because now your additive pack can be tuned in a different way. An OEM probably wouldn’t need a lot of research into the friction modifiers and clutch-friction-paper compatibility that will come with the friction clutches. So that’s an added benefit.”
And then there’s the DCT—the dual clutch transmission, which is basically two manual transmissions running in parallel, says Whitton, figuratively speaking. One “transmission” handles the odd-numbered gears and the other handles the even ones. When the car is in an odd gear, the transmission is in the ready position to go to the even gear (10
). Because of the type and configuration of the gears, and the way those gears mesh together, DCTs require stronger extreme pressure agents than other kinds of transmissions.
Extreme pressure agents in a dual clutch transmission may contain active sulfur, which has the ability to chemically bond with metal. The extreme pressure in the DCT forces the sulfur to react with metal and form a protective layer.
The extreme conditions found in DCTs don’t exist in step ATs, so step ATs don’t need that active sulfur. Also, active sulfur can corrode “yellow metal” parts. Yellow metal is an alloy with high amounts of copper, used in ATs. A transmission fluid designed for a DCT, then, could ruin a step AT.
A transmission fluid used in a CVT, on the other hand, must provide really good wear protection due to all of that metal on metal activity, compared to other ATs.
“So, different automatic transmissions, different types of appetites,” says Whitton. “Do not confuse your automatic transmission fluids.”
Fluid manufacturers use additives to customize their product for these different needs—for example, to protect metals from corrosion, wear and improper amount of friction in the transmission environment. Another component will make a fluid flow at very low temperatures and not get too thin at very high temps. Today wires and computers the size of cell phones are finding their way into transmissions.
“In conventional transmissions, copper and its alloys are used mostly as structural components, and accordingly conventional automatic transmission fluids are tested for basic copper protection,” says Anouska Norman, the commercial manager for Automatic Transmission Fluids at The Lubrizol Corp. in Wickliffe, Ohio. “But now, in addition, copper is used as electrical conductors in electric motors and control electronics. Damage to this copper can lead to open or short circuits that would be catastrophic for the transmission. It is important that fluids for electrified transmissions and eAxles (see Figure 1
) incorporate the most advanced copper protection technology and are more thoroughly tested for such.”
Figure 1. This cutaway of an eAxle reveals electronics that help reduce friction. (Figure courtesy of The Lubrizol Corp.)
To design new transmission fluids, manufactures start with a group of chemicals that has the desired properties.
“This is [fluid manufacturers’] bread and butter research, and a lot of it is trade secrets and proprietary,” says chemical engineer Byron Wall, also at Allison Transmission. “Some amount of what they’re doing is taking technologies that previously worked in terms of providing antiwear protection and just adding more of it. In other cases they’re actually pursuing more novel chemistry or variations on chemistry that optimize for a more robust antiwear package.”
And then researchers experiment with the additives and the transmission’s components to see if the additives behave as they are expected to. Sometimes the fluid might need to be adjusted, and sometimes the mechanics of the transmission itself might need to change.
“The key is we have to balance all the additives we’re putting in there so they don’t interfere with each other’s action too much,” says Whitton.
Lost in transmission
Engines have a small range of RPM in which they are running most efficiently. Changing gears keeps the engine in that range over the many speeds it must attain. So better gas mileage in newer automatics is partly due to an increase in the number of gear ratios.
“OEMs consider a standard automatic transmission layout and by adding more content, planetary gears and clutches, change the power flow and squeeze more gear ratios out of it,” says Gecim.
“Overall you have a wider ratio from the lowest to highest speed ratio, and shifting from each to the next one becomes smoother. Fuel economy benefits, and the driver gets a smooth uninterrupted power flow,” he says.
Another way to improve gas mileage is to improve the efficiency of the mechanical system as a whole. Transmissions suffer from what engineers call spin loss. Basically, it is the amount of torque lost in the transmission: the torque that enters the transmission minus the torque that leaves it, getting burned up as heat.
This loss comes from drag. A more viscous transmission fluid is going to introduce a greater amount of drag as rotating components are either interacting with the fluid, potentially submerged in the fluid or just generally rotating with the fluid on them. Decreasing the viscosity of the fluid should, in theory, decrease the mechanical losses that the fluid is introducing through viscous drag.
“That’s an area that folks are generally pursuing,” says Wall. “You see it across the board with some of the various new fluids that are being introduced and the specifications that are backing those. The general trend, transmission and engine oil, is toward lower viscosity.”
One way to get around gas mileage entirely is with electric vehicles, but EVs have their own issues with automatic transmission fluids. Early EV transmissions were designed to use conventional transmission fluid.
“We believe they did that by designing the hardware around the fluid, which means the hardware isn’t as efficient as it could be,” says Norman.
“But when you start looking at next-generation designs, I think we’re going to see OEMs reaching out for more efficient designs, which means they will have to pay attention to the lubricant as well,” she says.
Whitton agrees. “Whenever an OEM is designing a new transmission, they should be considering the fluid as a component in their transmission, just like a gear,” she says.
While friction is a main concern with the various internal combustion transmissions, electric vehicles have fewer moving parts. “For example, in a conventional automatic transmission fluid, you have to consider clutches that are in the torque converter and clutches that are used for shifting gears. But in many EVs, you don’t have the clutches for shifting gears. Often you don’t have the torque converter. So those frictional requirements are not necessary,” Norman says.
Gecim says EV manufacturers are getting away from frictional requirements by using a different kind of clutch. “Especially with e-motor powered transmission systems, switching from friction clutches to dog clutches seems to be a trend,” he says.
Another way OEMs are reducing friction is through the expanded use of eAxles (11
) (see Figure 2
). These are essentially motors on axles that give a power boost right at the wheel. eAxles use electronics to convert high gears to low.
Figure 2. An eAxle is essentially a motor on the axel. (Figure courtesy of www.vehicleservicepros.com).
Although driving electric motors in EVs run around 4,000-6,000 RPM, engineers are working on motors that could go up to 20,000 RPM.
“At those speeds they’re going to generate more heat,” says Norman. “We also hear that OEMs are going to be integrating e-motors into the transmission more, meaning the motors will be exposed to the oil. So that means that the oil is going to come into contact with much higher heat and it’s going to need to dissipate that heat and it’s going to need to be more robust than it is today. The current automatic transmission fluids may not be up to the challenge of the future electrified transmission.”
Another challenge specific to EVs is that they could exercise much higher acceleration and deceleration rates, says Gecim. “When you support the shaft that’s coming out of an electric motor with the conventional rolling element bearing, you have to realize that now that bearing could be subjected to very high acceleration or deceleration rates than before.”
Just as Whitton suggests treating the transmission fluid as a component in the transmission itself, Wall suggests treating tribologists as part of the design team.
“When you’re trying to tackle all of these concerns from the fluid point of view, I would say it’s essential to have a tribologist in the mix,” says Wall.
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Mary Beckman is a freelance science writer based in Richland, Wash. You can contact her at email@example.com