GM to Incorporate Innovative Transmission in 2007

June 12, 2006

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General Motors Corporation (GM) announced that the new Hydra-Matic 6T70 and 6T75 front- and all-wheel drive six-speed automatic transmissions will debut in several GM models in 2007. The Saturn Aura and Pontiac G6 will offer the 6T70 as an option, and the 6T75, which is rated for higher torque capacity, will be standard in the Saturn Outlook, GMC Acadia and Buick Enclave crossovers.

According to GM, the 6T70/75's clutch-to-clutch operation and 6.04:1 overall ratio help the transmissions deliver both performance and fuel economy, enabling up to 7% improved performance and up to 4% improved fuel economy compared with current front-wheel drive four-speed automatics. Both transmissions use a very high numerical 4.48:1 first gear, which helps deliver exceptional launch feel, and a 0.74:1 overdrive sixth gear, which reduces engine rpms at high speeds, thereby reducing engine noise and vibrations. Fifth gear is 1.1 direct drive.

The final drive ratios can be tailored to the vehicle performance requirements. For the Saturn Outlook, the final drive ratio is 3.16:1, while the Aura sedan uses a taller 2.77:1 ratio.

The 6T70 is rated for engines up to 315 horsepower and 280 lb.-ft. of torque, and with its larger transfer gears, a five-pinion input carrier and stronger structural ribbing on its aluminum case, the 6T75 can handle engines up to 315 horsepower and 300 lb.-ft. of torque.

"The additional gear states are almost like having two transmissions in one," said Bob Vargo, assistant chief engineer for the new transaxle. "The low first gear provides tremendous off-the-line acceleration, but the transmission is able to use the middle gears to evenly distribute the torque and offers an overdrive sixth gear that helps deliver great fuel economy."

Because there are six gears instead of four, the difference between ratios is less than in a four-speed automatic, giving the vehicle more options for using the best ratio for speed and load conditions. For example, with the transmission in an optimum ratio on a hill, the engine will not need to downshift or upshift unnecessarily. Driver shift control further reduces shift events by allowing the driver to select the desired gear for road conditions. Automatic grade braking reduces brake pedal usage by calculating the rate of acceleration while descending a hill and retaining a lower gear in the transmission.

The electronic controls are designed so that the shift feel is calibrated specifically to either the new midsize sedans or the new midsize crossover vehicles through the timing of the shifts and torque converter lockup speeds. The space-saving hyper-elliptical narrow section torque converter uses a single-plate lockup clutch that employs GM's proprietary electronic converter clutch control (ECCC) to dampen driveline vibrations.

The transmission was co-development by GM and Ford, which means that many common components of the transmission are shared, along with the on-axis design; but the controls, calibrations and operation of the transmission are unique to each company. Costs and development time were reduced in some areas by 50% because of the co-development process.

According to GM the Hydra-Matic 6T70/75 features:

• Dimensions of 357 mm in length and 197 mm in width approach those of some manual transmissions and are among the most compact for a six-speed automatic.
• Clutch-to-clutch shift operation for all shifts except 1-2 reduces mechanical complexity and mass, and is calibrated for smooth shift feel.
• Integrated transmission electro-hydraulic control module (TEHCM) with driver shift control (DSC), auto grade braking for consistent speed in hilly terrain and while towing or fully loaded.
• Internal control module reduces powertrain complexity.
• Heat-treated gears are honed for a more precise fit, reducing noise, vibration and harshness.
• Hyper-elliptical narrow-section torque converter enhances packaging and maintains efficiency.
• Reduced number of seals for stronger input shaft design.
• Adjustable capacity vane-type fluid pump enhances fuel economy.

With its wide ratio spread and capacity for high-torque engines, the 6T70/75 has the capability to transfer more torque to the drive wheels, particularly in the all-wheel drive Outlook, Acadia and Enclave. It also helps vehicles to feel livelier at lower speeds for a given engine, particularly at launch or when accelerating from a stoplight, said GM.

The 6T70/75's advanced clutch-to-clutch operation is designed for smooth shift feel and packaging efficiency. All shifts except one (the transmission "freewheels" in first) feature clutch-to-clutch operation. The sophisticated electronic controls allow timing the clutch-to-clutch engagements and disengagements so that the transfer of power from one gear to the next is transparent to the driver. The three conventional planetary gears, with three stationary clutches and two rotating clutches, can be packaged in a smaller space without additional freewheeling mechanisms.

A sophisticated transmission electro-hydraulic control module (TEHCM) is mounted inside the 6T70/75, reducing vehicle complexity. Similar to the control system used in the Hydra-Matic six-speed rear-wheel drive transmissions, the TEHCM offers improved quality through its hard-wired connections and pre-calibration during the controller's manufacture.

The unit is located entirely within the transmission and operates while bathed in transmission fluid. Locating the controller internally facilitates the modular design and assembly strategy while also shielding the unit from the outside environment. Temperatures are consistent inside the transmission.

GM's proprietary model-based controls strategy reaches a greater level of sophistication in the new transverse six-speed transmission family, said Vargo. The 32-bit system incorporates three levels of "learning" that actually allow the components to adapt to one another.

It is not uncommon to have a transmission-control module that adapts to the specific transmission with which it is mated; however, the 6T70/75 accomplishes this with two steps: when the control module assembly is manufactured, and when it is installed inside the transmission.

Certain components within the major subsystems that make up the transmission learn from one another - via the controller software - in a unique form of self-adaptation that maximizes the interface of all the networked components. This procedure takes place as the transmission tests itself and the interaction of its internal components before it is shipped from the assembly plant.

The TEHCM also enables performance-oriented and driver-controllable shift features including driver shift control "tap shift" and auto-grade braking for the midsize SUV and Performance Algorithm Shifting (PAS) for the midsize sedan. PAS recognizes a driver's particular style of throttle and brake application and predicts which gear will be most advantageous for performance driving or economical cruising.

Source: General Motors Corp. (GM).