Friction Modifier Differences
May 17, 2013
It is often assumed that lubricants are a commodity product and that there is little real difference between brands – or even between the types of lubes needed for vehicle components such as the engine, transmission, axles and couplings.
Of course, nothing could be further from the truth. Although they may be formulated from similar base oil stocks, transmission oils are completely different than lubricants designed for the engine. Aside from the most obvious difference – transmission oils are significantly thinner and lower in viscosity – their additives are also totally different and using the wrong oil can have disastrous consequences.
Although both engine and transmission oils contain a whole spectrum of additives and many of these additives have the same names, a given additive may have a different formulation and operate in a completely different way in an engine versus a transmission. For example, both engine and transmission oils contain corrosion and oxidation inhibitors. However, these components are especially important in the engine environment, with its blow-by of combustion products. Dispersants and foam inhibitors are also common to both types of oils, with the latter being even more important in transmissions, especially continuously variable transmissions (CVTs).
Particular confusion surrounds the operation of one key additive – the friction modifier. In an engine oil, the task of the friction modifier is to make the internal friction of the engine as low as possible. William Abraham, Driveline Technology Manager at Lubrizol, explains: “We use different chemistries to achieve these different results, and in an engine the objective is to reduce the friction in the boundary lubrication sections of the engine – the piston ring/liner area is the majority, and the other area is the valve train.”
This is all about reducing metal-to-metal friction while controlling wear and heat, observes Abraham, and most engine oils require only one, possibly two, friction modifier ingredients to reduce sliding and rolling friction.
Transmission oils, by contrast, are much more sophisticated products. This is not to say that formulating engine oils is a simple matter. However, transmission oils which have to withstand higher pressures and, in modern light vehicle applications, are designed to last for the entire operating warranty life of the vehicle unlike engine oils. For the friction modifier additive system, the task is even more complex. With automatic transmission fluids, the additive package must reduce friction in planetary gear meshes and bearings for better fuel economy. At the same time, it also needs to generate a carefully measured increase in friction (torque capacity) between specific key components, such as the torque converter lock-up clutch and the several sets of steel-composite clutches, without causing shudder (where the driver feels a shaking of the vehicle at a given speed, like driving over rumble strips). It is these steel-composite clutches that are responsible for the gear ratio changes in the transmission. The optimization of the friction characteristics for maximum energy transfer with minimal gear shift time is critical for enhanced vehicle fuel efficiency. Furthermore, these clutch component interfaces generally consist of a steel plate and a friction material such as paper composite, woven carbon fiber, graphitic paper or other synthetic compound. A 6-speed transmission may contain up to 4 different material types, adding to the frictional balance complexities – very different from simply reducing metal-to-metal friction in engines.
“The phenomenon that is most important in the stepped transmission is providing the appropriate friction characteristics for each of the different material-on-steel combinations being applied in the transmission,” observes Abraham. “There is dynamic friction, low-speed dynamic friction, static breakaway friction – many different regimes of the spinning of the [clutch] plate or the torque converter, or the wet start clutch. You need to provide high torque capacity for maximum efficiency, but not cause shudder during shifting. Severe shuddering can cause damage to friction plates and can also be unnerving to the driver. In essence, you want the highest level of friction possible between spinning, engaged clutches, but reduced friction as the engaged clutches are almost at a stop for shifting.”
Modern, stepped ATs with up to nine ratios can have three to five different types of friction material, says Abraham. With each of the several clutch packs having its own energy and inertia characteristics, the composition of the fluid has to be very finely balanced to ensure the desired smooth, efficient, long-lasting performance.
Principles of Key Additive Component Operation
Although friction modifiers are key additive components, they are only present in small quantities in the fluid. Most friction modifiers act by creating a smooth, associative film across the surface to reduce sliding and/or rolling friction. Others are more chemically active, with a strong affinity for the metal and/or composite surface, bonding with the surface to form a slippery coating. The lubrication needs must be carefully balanced, for friction modifiers must operate in the presence of other additives such as antiwear agents, which tend to increase the friction. Other surfactant-like additives will also compete with the friction modifier(s) at the surface, affecting friction performance and making new additive package development challenging. Needless to say, the desired frictional characteristics for lubricants must be preserved and consistent throughout the life of the fluid which, as the trend for longer drain intervals continues to intensify, further enhances the challenge.
What Lies Ahead
As the design of both engines and transmissions continues to evolve toward still greater efficiency and fuel savings, lubricant requirements will evolve, too. Lubricant design is likely to become a little easier for engine oils as power units are being downsized and down-speeded. There will be more electronic intelligence control of cooling and lubricant flow, as well as the switching back and forth between engine-run and motor-run hybrid vehicles. Conversely, transmission fluid design is undergoing the opposite effect, with even more challenges ahead.
For example, nine- and ten-speed automatic transmissions will be the latest new designs over the next 7 years. These newest automatic transmissions are becoming smaller and lighter, while handling more power than any previous AT. In addition, original equipment manufacturers are lowering lubricant sump volumes, causing higher running temperatures and thus placing even more demand on the lubricant.
It is small wonder, then, that formulating the automatic transmission fluid is the most daunting of the fluid design tasks on the entire vehicle. “If you want to have a premium, top-performing, optimum fluid for your 6-speed and higher automatic transmission,” says Bill Abraham, “the most difficult fluid to formulate is an ATF.”
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© 2013, The Lubrizol Corporation