Developing the Hydra-Matic 9T50 Nine-Speed
Chris Meagher, executive director, Transmission and Electrification Hardware Engineering, points out that on a global basis the company is having to address CO2 and fuel economy requirements that means that the work he and his colleagues do has to improve the performance and efficiency of their products, in this case, automatic transmissions. At the same time, however, Meagher says that there is the non-trivial issue of customer delight—after all, someone who is going to buy a car or crossover doesn’t want to have something that may meet engineering goals but seems to be a compromise in terms of performance and refinement.
GM has been making a six-speed automatic in its San Luis Potosi Complex for about a decade. Meagher says that transmission, based on independent testing and internal benchmarking, has proved to be world-class in terms of such things as shift quality, reliability, dependability and durability.
And so upon that transmission and all of the learnings that went into it, they’ve developed the Hydra-Matic 9T50, a nine-speed automatic. Right now it is being offered on the 2017 Chevy Malibu. Then it will roll out in the 2017 Cruze diesel and the 2018 Equinox crossover. By the end of 2017 GM plans to have the nine-speed offered on 10 vehicle models.
This is not, Meagher emphasizes, a niche product. It is a high-volume transmission.
As for the requirements? Meagher explains that the goal is to keep an engine performing in its optimal range. By having more gears, this is more readily achievable. So he says that they’re getting about a 2 percent improvement in fuel efficiency over the six-speed.
(It is worth noting that the nine-speed in the 2017 Malibu with a 2.0-liter turbo is contributing to the vehicle's EPA-estimated 33 mpg highway, which is a 3 percent increase over the 2016 model with a six-speed.)
As for the concern of meeting the customer perceptions, he says that by having shorter steps between the gears, there is a smoothness that speaks to the need for refinement in performance.
While it might be simple to create a bigger transmission to accommodate the three additional forward gears, Scott Kline, assistant chief engineer on the program, says that the goal was to maintain the approximate size of the six-speed. Rather than providing additional space for the powertrain, they reckoned that it is a better idea to provide it to the passengers. There is about an inch difference, on length, between the nine- and six-speeds. In terms of mass, the new automatic is about 10 kg heavier.
The 9T50 features five planetary gear sets, four stationary clutches and three rotating clutches.
One reason why they were able to maintain a compact size for the nine-speed is because they’re using an on-axis design, meaning that the gears are in line with the crankshaft.
But another reason that Kline cites as being key is the use—an industry-first use, he says—of a selectable one-way clutch. This is used in place of a plate clutch pack and a diode clutch. Depending on the operating mode, the clutch can freewheel (e.g., during the 1-2 shift) or hold the torque for engine braking and reverse.
Because an increasing number of vehicles are being offered with start-stop systems, this means that hydraulic pressure necessary for the transmission can be lost (the engine isn’t running so the transmission pump isn’t performing), so they’ve devised an accumulator that is charged with hydraulic pressure while the engine is running and remains pressurized when the engine is stopped. Then, when there is engine start, there is pressure readily applied for the forward clutch.
It’s worth noting that contributing to the smooth start is the orchestration provided by the control software developed for the transmission; there is a 32-bit transmission control module used. Kline says that by having hardware and software engineers working in close proximity allowed them to have better performance from the transmission than were they to have sourced the software from a third party.
Another contributor to smooth launches whether there’s start-stop or not is a torque converter that features a hyper-elliptical oval cross-section. Not only does this provide refined takeoff performance, but the design is one that is comparatively thin, thereby contributing to packaging benefits.
Other features of the new transmission that Kline cites as being beneficial are the use of linear force solenoids, electronically based active oil management and an electronic range selector.
The 9T50 is four-wheel-drive-compatible. It offers tap up/down shift capability.
Although they used the six-speed as a starting point, they have more than 60 patents for the nine-speed.
The nine-speed was developed with an “alliance partner,” Ford, with which it worked on the six-speed front-drive transmission. (The two companies also jointly developed a 10-speed automatic, which is currently available from GM in the 2017 Camaro ZL1.)
Kline says that his team built about 800 prototypes. Sometimes during these builds they’d discover that they’d need a part, like a clip or a spring. So they’d contact their opposite numbers at Ford and meet in a Walmart parking lot—because it was about half way between the two development centers.—GSV