Thin. Not Too. Thick, Around the world. What Viscosity Index Improvers Can Do for ATFs. November/December 2008 Number 10. By Bill Dimitrakis

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1 November/December 2008 Number 10 Not Too Thick, Not Too Thin What Viscosity Index Improvers Can Do for ATFs By Bill Dimitrakis Around the world there is increasing focus on conserving energy and reducing greenhouse gas emissions, and the focus is growing in all parts of society. Consumers are demanding more efficient vehicles in response to rising fuel prices. Businesses are reducing energy consumption and their carbon footprint to reduce their overall environmental impact. And governments continue to tighten regulations requiring better vehicle fuel economy and lower carbon dioxide emissions. It should not be a surprise that vehicle manufacturers have made it a top priority to find ways to improve energy efficiency and fuel economy. One of the approaches they have taken is to design and build more efficient engines and drivetrains. Examples include the development of stepped automatic transmissions with six or more speeds and the development of dualclutch transmissions. Manufacturers are also working with the lubricant industry to develop more efficient lubricants. An outcome that clearly resulted from this work was the development of lower viscosity lubricants. In the past, automatic transmission fluids generally had kinematic viscosity that averaged more than 7 centistoke at 100 degrees C. Newer factory- and L n G Europe Middle East Africa November/December

2 Not too thick, not too thin service-fill fluids now average 6 cst at 100 degrees C. Original equipment manufacturers are considering even lower viscosity fluids for the future. There is a trade-off, however, when it comes to raising or lowering viscosity a trade-off between efficiency and lubrication or, seen another way, between performance at the two extremes of the system s temperature range. A lubricant must be viscous enough to maintain a fluid film to protect equipment surfaces, but not so viscous that it creates too much resistance. Moreover, it must retain a proper balance throughout its operating temperature not becoming too thin at high temperatures or too thick at the low end. There may be a practical limit to how low viscosities can drop and still be acceptable. With the trend in ATFs, auto manufacturers have expressed some concern about the long-term ability of verylow-viscosity fluids to adequately protect transmissions in the field. There may be a practical limit to how low viscosities can drop and still be acceptable. The issue is especially pertinent in view of new and looming regulations requiring fuel economy to be measured at cold temperatures. 2 L n G Europe Middle East Africa November/December 2008 For example, the U.S. Environmental Protection Agency requires measurement at minus 7 degrees C. Such scrutiny at low temperatures could have a negative effect on a vehicle s overall fuel economy rating. Faced with this conundrum, ATF formulators have looked for other ways to improve efficiency and fuel economy while ensuring proper equipment function and protection. One approach is to work with viscosity index, or VI, a measure of how a fluid s viscosity changes with temperature. Fluids with higher VI undergo less viscosity change as temperature increases or decreases. Thus, high VI fluids that have adequate viscosity at high temperatures will be less viscous at low and normal operating temperatures than fluids that have equal hightemperature viscosity but a lower VI. In other words, a high VI fluid can improve efficiency at cold start and under normal operating conditions without sacrificing efficiency and protection at high temperatures. A number of factors affect a finished fluid s VI, from the base stock to the type and amount of viscosity modifier it contains. With VIs of approximately 120 and 140, respectively, API Group III and Group IV base stocks have inherently greater VI than Group I or Group II oils. This yields a higher VI in finished fluids that contain them. The disadvantages of Group III and Group IV are higher cost and more limited availabil-

3 Best of Both Worlds Viscosity Cold Start Normal Use Fuel Economy 0-60 C C 100 C Extreme Service Temperature Fluid Durability Current Fluid Reduced Viscosity Fluid Higher viscosity index means less friction at low temperatures, more fluid film protection at higher temperatures. ity. Group IV stocks, which by definition are polyalphaolefins, are significantly more expensive than mineral base stocks and are in tight supply. Group III stocks also have constrained supply in some markets, although openings of new plants will alleviate that concern. Viscosity modifiers (VMs), or VI improvers, boost finished fluid viscosity and VI. VMs are oil-soluble polymers that alter the base oil s viscosity-to-temperature relationship and allow creation of multigrade lubricants. A multigrade lubricant meets the high-temperature viscosity requirements of a heavier viscosity grade while having the low-temperature fluidity of a lighter viscosity grade, hence the term multigrade. Virtually all modern automotive lubricants are multigrade because they must perform well year-around in climates where temperatures vary widely throughout the year. The choice of VM chemistry depends on the shear stability and the viscometric requirements of the application. Commonly used chem- Seeking Greater Fuel Economy, Less Pollution Many governments are trying to encourage less fuel consumption by automobiles, but they are taking a variety of approaches. The European Union is coming from a perspective of impact on climate change. To help reduce greenhouse gas emissions and meet its Kyoto Protocol targets, the EU has proposed that average carbon dioxide emissions from new passenger cars by 2012 should not exceed 120 grams per kilometer. The current voluntary standard calls for an average of 140 g/km by 2008 for European manufacturers. Improvements in motor technology would have to account for most of the reduction. Complementary measures, including ef- ficiency improvements for car components such as the drivetrain, would contribute the balance. Longer term, the EU is advocating research efforts aimed at further reducing emissions from new cars to an average of 95g CO2 per km by Through the car labeling directive and other measures, the EU also is promoting the purchase of fuel-efficient vehicles. Another example is the U.S. National Highway Traffic Safety Administration proposal to increase the U.S. corporate average fuel economy (CAFE) requirement for U.S. passenger cars and light trucks by 4.5 percent per year over five years from 2011 to For passenger cars, the proposal would increase fuel economy from the current 27.5 miles per gallon (11.7 km/l) to 35.7 mpg (15.2 km/l) by For light trucks, the proposal calls for increases from 23.5 mpg (10.0 km/l) to 28.6 mpg (12.2 km/l) in Another recent change is the new U.S. EPA fuel economy test procedures required for all vehicles sold in the U.S. market, beginning with the 2008 model year. The revised procedures provide a more accurate estimate of the fuel usage consumers can expect under real-world driving conditions. These changes include testing at higher speeds, more aggressive acceleration and deceleration, and at both hot and cold temperatures. The EPA expects the new procedures to reduce vehicle fuel economy ratings by approximately 10 percent. Heavy equipment manufacturers, especially in Japan, are actively working to address fuel consumption concerns in off-road equipment. The Japanese Construction Mechanization Association has developed protocols to test fuel consumption using simulated work cycles. Protocols have been developed for tractor bulldozers, wheel loaders and hydraulic excavators. Bill Dimitrakis L n G Europe Middle East Africa November/December

4 istries include olefin copolymers or OCPs, styrene block copolymers, polyolefins and polyalkyl methacrylates. The polymer chemistries differ in thickening efficiency, lowtemperature fluidity and the degree to which they increase VI. The polymer s molecular weight also affects thickening efficiency, shear stability and VI increase characteristics. Specifically, there is a tradeoff between performance in these three parameters. The developer s job is to balance the various performance characteristics to achieve the lubricant properties needed. The effects of different polymer chemistries including different molecular weights are well Under all the speedload combinations the transmission ran more efficiently with the higher VI fluid. established, as are their limitations. Polymer architecture has received less attention, but recent research shows that it offers an alternative way to improve performance. Conventional polymerization produces polymer chains with random structure. Controlled polymerization allows creation of polymer chains with a block or star structure, which changes the way the polymer interacts with the oil. Tests by Lubrizol showed that VMs produced by controlled polymerization can increase VI more than similar VMs made through conventional processes. There are practical limits to the VI that can be achieved in a finished lubricant. These depend on target viscosity for the finished fluid, the desired after-shear viscosity and the VM chemistry. The majority of a vehicle s energy loss occurs in the engine, and most of the efficiency gains required to meet new government regulations are expected to come from there. By some estimates, the drivetrain accounts for 5 percent to 6 percent of total vehicle energy loss. Nevertheless, gains in drivetrain efficiency achieved through the lubricant still are worthwhile because vehicle and equipment OEMs are interested in these improvements and are actively working with the lubricant and additive industry to develop new high-efficiency fluids. Much of that activity is focused on reducing viscosity, but OEMs also are interested in high VI fluids. Lubrizol conducted a study to learn if efficiency at low temperatures can be improved through use of a high VI fluid. Two transmission fluids having different VI characteristics were prepared. One had a VI of 183, which is typical for a highquality service-fill ATF. The other had a VI of 251. Both fluids were formulated to 7 cst at 100 degrees C and contained the same performance package chemistry. The low VI fluids contained a conventional polymethacrylate VM, while the high VI fluid contained the star structure polymethacrylate. Cold Transmission Efficiency Test (Sample Results) Light Load Running at 40 km/h Heavy Load Cruising at 60 km/h Heavy Load Running/Towing L n G Europe Middle East Africa November/December 2008

5 Test Conditions Step Description Gear Position/Input rpm/input Torque 1 Heavy load running/towing 1 st gear/3600 rpm/71 Nm 2 Idling 1 st gear/800 rpm/8 Nm 3 Light load running at 40 km/hour 2 nd gear/2500 rpm/13 Nm 4 Normal cruising at 60 km/hour 4 th gear/1800 rpm/40 Nm Test time: Until stable sump temperature was reached. To evaluate the fluids, a transmission efficiency test rig measured relative torque losses in a six-speed passenger car automatic transmission unit. The transmission was driven at different input revolution/output torque combinations to measure the effect of viscosity modifier on torque loss and converter slip at several different speeds and loads. The rig was modified to allow both the transmission and the fluid to be cooled to the minus 20 degrees C low-temperature starting condition. Fluid temperature was allowed to rise as the test progressed. Under all the speed-load combinations the transmission ran more efficiently with the higher VI fluid. The cold-start region of each condition showed the largest gains, approximately 3 percent to 4 percent on an absolute basis. This reflects the larger differences in viscosity of the fluids at those temperatures. As the transmission approaches higher temperatures, the difference in efficiency between fluids decreases. It isn t possible from this rig testing to predict gains in a vehicle s overall fuel economy. However, achieving this extent of increased mechanical efficiency solely by changing the fluid viscometric characteristics will help maximize the fuel economy improvement. OEMs are hoping to improve total vehicle efficiency by 1 percent or more through their work to develop highefficiency fluids. More research is being done to correlate the VI differences of the transmission fluid to actual fuel efficiency gains in vehicles. The results discussed here indicate that it may not be necessary to reduce fluid viscosity at 100 degrees C to the extent being considered today and that significant improvements in efficiency are possible with high VI fluids. In addition, new star-structure polymers are becoming available for formulators to use in developing the new fluids. Bill Dimitrakis is business manager for specialty viscosity modifiers at Lubrizol Corp. in Cleveland, Ohio, USA. L n G Europe Middle East Africa November/December