In recent years a variety of end-user requirements – including improved fuel economy, increased vehicle performance, greater power requirements and enhanced driving experience – have driven tremendous changes and developments in automotive transmission systems and fluids.
But right now in the light-duty vehicle market, by far the biggest driver for transmission hardware evolution is improved fuel economy, as original equipment manufacturers work to meet ever-tightening fuel economy mandates and targets.
We are seeing a real diversification in transmission technology as OEMs take different approaches to improving fuel economy.
One approach, adopted by a number of OEMs, is to increase the number of gears in conventional stepped automatic transmissions. While six-speed transmissions have been the workhorse design for many OEMs, some are moving now to eight-, nine- and ten-speed configurations.
Here are some examples of this shift: Buyers of the new 2016 Honda Pilot can get a regular 6-speed automatic transmission, or upgrade to a fuel-saving, 9-speed shift-by-wire automatic. General Motors has stepped up to offer 8-speed transmissions on certain SUVs (Cadillac Escalade, Chevrolet Silverado, Yukon Denali) and sports cars including the Camaro and Corvette Stingray. Toyotas new Lexus RC-F coupe boasts an 8-speed automatic, too. Meanwhile, transmission builder ZF closed out production last year of 6-speed automatics in Saarbrucken, Germany, after 13 years and 7 million units; Saarbrucken now is devoted to making 8-speeds.
Other approaches include continuously variable, dual-clutch and hybrid technologies. The uptake of these systems varies from region to region, and amongst other factors, their popularity typically reflects the driving profile of each region.
Clearly with such a wide range of available hardware, the careful tailoring of transmission fluids for each OEM and each specific application is increasingly important.
Fluids and Fuel Economy
Over the last two decades transmission fluids have had more of an indirect impact on fuel economy – mainly acting as hardware enablers. In the 1990s for example, new friction-control additives were developed to provide anti-shudder durability to enable more effective slipping torque converter clutch systems. More recently, as continuously variable transmissions (CVTs) were commercialized, new fluid chemistry has been introduced to deliver high steel-on-steel friction to prevent belt slippage while maintaining paper-on-steel friction to avoid shudder.
Today automatic transmission fluids increasingly are expected to have a direct impact on fuel economy performance. The fluids viscosity for example has a direct effect on pumping, drag and churning losses. And with the oil pump thought to account for about 50 percent of the energy lost in the transmission, these sources of inefficiency cannot be underestimated or ignored. In addition, ATF viscosity and boundary film chemistries can have a direct effect on gear and bearing efficiency.
To gain the most fuel efficiency, the new formulation trend is for transmission fluids to have lower and lower viscosities over a wider range of operating conditions.
A number of OEMs have moved from conventional viscosity fluids of more than 6.8 centiStokes at 100 degrees C, to low viscosity – typically 5.5 cSt fluids – for their automatic transmissions. Plus, we are already seeing 4.5 cSt fluids in use, and there may be a demand to move to even lower viscosities in the future.
While it is possible to get more fuel economy from transmission fluids, far more than lowering viscosity at 100 C is involved – low-temperature viscosity is also critical. In addition, because there is more tendency to encounter mixed and boundary lubrication as viscosity is lowered, gear and bearing losses can also increase. However, from our research it is clear that the careful selection of friction modifying and film forming additives, combined with the most appropriate base stock, can reduce these losses.
Carefully designed research methodology can evaluate the fuel economy performance of fluids while ensuring they also deliver sufficient hardware protection.
This evaluation involves the use of a variety of bench screener tests, which are used to assess the initial impact of additive changes in a formulation. Once the best candidate oils have been selected, these are tested further in transmission hardware test beds – and ultimately in vehicles in real-world conditions.
This level of testing enables additive companies like Infineum to assess future ATFs under a variety of conditions, including a wide range of oil sump temperatures and torques. In our view, this kind of intense analysis is increasingly essential as formulations are optimized to meet the specific requirements of individual OEM transmission designs.
Heres an example of the effort it takes to differentiate performance: In a recent test program, Infineum formulated three test ATFs with different viscosities by combining a selection of base oils, viscosity modifiers and viscosity building additives. Fluid A was designed as a 5.4 cSt product, while both Fluid B and Fluid C were 4.8 cSt and used different additives.
Next, bench and screener tests were used to evaluate each fluids bearing and gear efficiency against a 5.4 cSt factory-fill reference oil. This comparison allowed the additive package to be tailored to ensure the optimum combination of viscosity and efficiency, so that maximum fuel economy performance could be delivered.
The final phase of the test program assessed the fuel economy gains using the Federal Test Procedure (also known as the FTP75 Test Cycle), and then by over-the-road field tests.
In the FTP75 Chassis Dynamometer Test all three oils demonstrated improvements in fuel economy, ranging from a gain of 0.6 percent for Fluid A to 2.3 percent for Fluid C, versus the factory-fill reference ATF. These fuel economy performance improvements were subsequently verified in 35,000 miles of on-road testing.
This extensive bench, test bed and field testing program has clearly demonstrated that the proper design of an automatic transmission fluid matched with a specific transmission design can yield significant fuel economy improvements.
In the future we expect formulation challenges to come in the form of further requirements for improved durability, longer drain intervals, and fuel economy – all in increasingly complex transmission hardware. This will put even more stress on the fluid and result in the development of even higher performing ATFs.
In our view the formulation landscape is becoming increasingly complex, and the co-engineering of the lubricant and hardware is increasingly essential to ensure we strike the right balance between fuel economy gains and friction durability to deliver continued hardware protection.
Joe Noles is global technical advisor at Infineum USA L.P. in Linden, N.J., responsible for developing factory-fill ATFs for North American and European OEMs. He joined Exxon Chemical after obtaining his Ph.D. in chemical engineering from Cornell, and in 1999 moved to Infineum. For information, e-mail him at Joe.Noles@Infineum.com.