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General Motors Six-Speed Transmissions: A History

By Joel Arellano | October 17, 2007
PONTIAC, Mich. - Transmission engineers at General Motors Corp. have given new meaning to the term "overdrive." By using high-tech computerized tools, they launched nine new fuel-saving six-speed transmission models - the industry's largest lineup - in just four short years. Furthermore, their use of this technology has shaved as much as six months and $15 million from the typical transmission development process. GM transmission engineers use sophisticated math modeling, among other advanced tools, to not only design the transmission components, but also to predict and test their reliability, analyzing functions such as oil pressure and flow, lubrication distribution and shift quality. "We are using computer-based tools in powertrain development for our global engine and transmission programs and seeing terrific results," said Tom Stephens, group vice president, GM Powertrain and Quality. "On our six-speed transmissions alone, we launched nine models in four years whereas we previously averaged three or four transmissions every decade." Stephens said this pace of product development and first-time quality is having an impact in GM's gasoline and diesel engines, where analytical models also are used early in the process to improve design and functional performance. "On our new 4.5L V-8 diesel, for example, analytical models were used to design and evaluate nearly 500 different piston shape designs during a 10-day period before we picked the best one. To evaluate this many designs without computer tools would have been unrealistic in terms of timing and cost to do with actual parts," said Stephens. GM's computer modeling and use of common parts on the four rear-wheel-drive (RWD) transmission models reduced development time as much as 50 percent. In fact, 47 percent of the parts are shared among all of the RWD models; and the 6L90 transmission (used in GM's full-size pickups) shares 75 percent of its components with the 6L80 transmission (used in GM's full-size sport utility vehicles, the Chevrolet Corvette, Cadillac XLR-V and STS-V, among other models). GM engineers use the high-tech tools early in the development process to identify whether the computer analysis is sufficient and if physical parts still need to be fabricated and tested. In some areas, simulated tests took only 48 hours versus several weeks, helping to eliminate, in some cases, the need for costly and time-consuming prototypes. Development of the six-speeds began in 2003, with the first of the RWD models being launched in 2006. GM's extensive use of computer math modeling tools has enabled the company to develop the various six-speed models in parallel. "Our use of computer tools has improved the first-time quality of our transmission prototypes by five times over past programs," said Jim Lanzon, executive director of GM Powertrain's Transmission Engineering. "We've eliminated one of three pre-production hardware builds - essentially a complete set of prototype transmissions - and the associated physical tests." Lanzon said the new 6T40 front-wheel-drive six-speed model launching in GM Daewoo's Tosca facility this month was developed by GM Powertrain's Transmission Engineering in Ypsilanti, Mich., and the math model was sent to GM Daewoo in Korea for final development and manufacturing. "The 6T40 transmission was built in Korea from our math models, placed in the first vehicle and performed beautifully," said Lanzon. Among GM's first six-speed models were RWD transmissions introduced in 2006 in the Chevrolet Corvette, Cadillac XLR-V and STS-V. All of the six-speed automatics for RWD and FWD/AWD were designed for maximum application flexibility, allowing them to be used in the greatest number of vehicles in GM's vehicle portfolio. Shared traits among the new six-speed automatics reduce complexity, size and mass, including clutch-to-clutch operation that enables the six-speed to be packaged into approximately the same space of a four-speed automatic. The transmissions also share an overall 6.04:1 gear ratio, which contributes to their signature balance of performance and fuel economy. # # # GM'S COMPUTER TOOLS SHORTEN POWERTRAIN PRODUCT DEVELOPMENT Fast turnaround GM's recent development and launch of nine six-speed transmissions included two basic families of six-speeds: rear-wheel-drive (RWD) and front-wheel-drive (FWD) and all-wheel-drive (AWD). Within the families, four six-speed models were designed for RWD applications and five for FWD/AWD applications. Each family underwent more than 250 different computer analyses via Computer Aided Engineering (CAE) math models - in areas such as fluid dynamics and finite element analysis - contributing greatly to the reduced development time and cost associated with the program. The math tools were used to design the transmissions' components, predict part and system behavior and identify performance variations in the production vehicle and manufacturing process. In some areas, computers were able to simulate tests in 48 hours that previously took several weeks. "Together, the collective simulations saved months of development time and reduced the number of physical tests and hardware builds needed to develop and validate the transmissions," said Jim Lanzon, executive director of GM Powertrain's Transmission Engineering. "The time savings enabled us to examine the best options for each application and more precisely tune each transmission for maximum efficiency." One of the most dramatic examples of the math models' effectiveness was their effect on fluid dynamics, said Lanzon. "The math helped determine the correct fluid pressure distribution for lubrication throughout the transmission approximately eight months prior to production," he said. "Previously, we would have built early prototype transmissions to develop the correct lubrication balance in the lab, but the data from the models told us how to do it right the first time, contributing to a three-month savings in development time." Similar math modeling and analysis contributed to numerous other time-saving developments, including: * Analysis determined the best placement for the fluid inlet and outlet ports of new FWD transmissions' gear pumps prior to the first hardware build * Models proved the viability of the "compressed torus" torque converter design that enables a more compact package in the FWD transmissions * Analysis models of the transmissions in each proposed current and future vehicle were built and run to predict and optimize fuel economy, based on numerous factors such as shift points, engine power, vehicle dimensions, etc. * Valve body sealing tests were reduced from several weeks in hardware to two days execution in the computer * Park system strength simulations were accomplished in two days, rather than the previous 1.5 weeks Collectively, the simulations and analysis enabled the transmissions' production design and processes to be incorporated during the first hardware build. In fact, the accuracy of the components developed with the math models - and the fact that almost no trial-and-error tests were required - allowed engineers to reduce the typical pre-production hardware builds from the previous three to only two. Flexible design Reduced complexity, shared design traits and a common electronic control strategy allow GM to use the six-speeds in a wider range of vehicles and put them into production sooner. Indeed, the first of the six-speed models was introduced in a production vehicle within three years of the start of development. The RWD models, for example, comprise three primary components: the bell housing, which mounts the transmission to the engine; the main case, which contains the gears, shafts, and clutches; and the case extension, which contains the link that connects to the driveshaft. Use of these three separate components enables broader use and quicker implementation into different vehicle applications. The independent main case allows engineers to use the transmission in almost any application, as a specific bell housing or case extension can be easily and quickly designed for different vehicles. Many other automatic transmissions use a case with an integrated bell housing and/or case extension, which can present packaging challenges or preclude their use altogether. This modular design strategy also contributed to the six-speed transmissions' reduced development time and cost, as well as reduced manufacturing complexity. In the RWD family, the number of unique parts between the models was reduced by 90 percent when compared with a comparable four-speed automatic. The base transmission electronic hydraulic control module contains basic software and algorithms shared among all the six-speed models, including RWD, FWD and AWD. The module is located inside the transmission, which reduces manufacturing complexity, and requires only vehicle brand and model-specific calibration to the core program. "There was tremendous dedication and drive to produce the new six-speed automatics," said Lanzon. "The promise of six-speed transmissions is improved fuel economy with excellent performance characteristics and that's exactly what these transmissions are delivering in more than 40 GM vehicles around the world." Press release via General Motors
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