GM’s new 8-speed automatic may enable 30-mpg Corvette in 2015

  • 29-Jan-2014 03:14 EST
2015 8L90 8spd auto.jpg

GM has moved to an integral, die-cast aluminum bell housing/main case for the new 8L90 for reduced NVH. Note "squashed" torque converter. The unit is lighter overall and fits in the same physical space as the incumbent 6-speed.

General Motors engineers are quietly optimistic that their all-new 8L90, the automaker’s first 8-speed planetary automatic transmission developed in house, will be the kicker that finally pushes the Chevrolet Corvette Stingray into an EPA-certified 30-mpg highway fuel efficiency rating, when the new transmission enters production later this year.

“No comment—but we’re giving the vehicle team a ratio spread of seven that’s wider than the current 6-speed and will be useful in improving overall efficiency,” Kavoos Kaveh, GM Powertrain’s Global Chief Engineer for 6- and 8- speed Automatic Transmissions, said with a chuckle. Added Tadge Juechter, Corvette Executive Chief Engineer: “We’re very close to 30-mpg with the 6-speed car, so two extra ratios is something we like very much.”

The 2014 Stingray equipped with the 6L80 6-speed carries an EPA city/highway rating of 16/28 mpg, and 20 mpg combined.

GM claims the 8-speed improves the Corvette’s powertrain efficiency by up to 5% versus the 6L80. Better fuel economy is enabled by lower top-gear N/V (ratio of engine speed to vehicle speed capability) relative to the 6-speed it replaces. A deeper first-gear ratio improves launch capability. The new 8-speed is over 8 lb (3.6 kg) lighter than the 6-speed while being equivalent in overall size. The transmission controller executes commands 160 times/s, enabling WOT upshifts that are up to .08 s quicker than the DCT-equipped Porsche 911, engineers said. They added that the wider ratio spread also improves overall NVH attenuation.

Sixth gear is 1:1 direct. Seventh and eighth gear are overdrive--.85:1 and .65:1, respectively.

Five-clutch arrangement

With a rated turbine torque capacity of 1000 N·m (738 lb·ft), the 8L90 is more than capable of handling the estimated 635 lb·ft (861 N·m; SAE ratings not yet available) of the new supercharged 6.2-L LT4 V8 that will power the 2015 Corvette Z06.

In addition to the Corvette applications, the new 8-speed will also go into select Cadillac and other rear-wheel-drive models across GM’s lineup. It will be built at GM’s Toledo, OH, transmission plant which received a $204 million retooling to support the program.

The transmission was approved during the period of global economic recession and GM’s bankruptcy, and the program was delayed by approximately one year—Kaveh’s team received Beta hardware in 2010. Because of the delay, GM decided to outsource Aisin’s TL-80SN 8-speed (http://articles.sae.org/11942/) as a stopgap for use in the 2014 Cadillac CTS.

In April, SAE International will publish Technical Paper 2014-04-01 on the 8L90, authored by GM Powertrain engineers James Michael Hart, Tejinder Singh, and William Goodrich. It will be available through www.sae.org.

To establish a new transmission’s power flow, GM Powertrain’s advanced engineering team starts with a sophisticated and proprietary computer program. “They use this tool to go through thousands of different power flows,” Kaveh said, “eventually arrive at the optimum one that can transfer torque, be fuel efficient, then be able to package efficiently in the same space as the existing unit, in this case the 6-speed.”

Once the package is done, the team “painstakingly goes through every structural member to remove mass and make it more efficient, while meeting our durability requirements,” he said. “Then we go after parasitic losses by various means. Depending on the different subsystems, we do a lot of FEA for structural integrity while simultaneously running CFD for optimum power flow.”

The 8L90’s die-cast aluminum case features an integral bell housing for increased stiffness and reduced NVH. Within it are a new “squashed” (compact) torque converter, four close-coupled planetary gearsets, two braking clutches, and three rotating clutches.

The rotating clutches are located ahead of the gearsets to minimize the length of oil feeds, said Assistant Chief Engineer Bill Goodrich, one of the authors of the 2014 SAE tech paper. He said the 258-mm (10.1-in) torque converter is based on the 6-speed unit, with modifications to take advantage of moving the main fluid pump off axis, from the center of the housing (as on the 6L80) to the side, located low in the case inside the valve body.

Switching to magnesium

The pump is a small pressure binary-vane type (two inlets, one discharge per revolution), which effectively serves as two pumps occupying the space of one. “When demand is such that the high flow isn’t needed, we can actually recirculate the oil and only run half the pump at a time, to reduce pumping losses and improve efficiency,” Goodrich noted.

He reckons the pumping arrangement is an industry first for automatic transmissions. As to its location, “If it was located in the center, we’d then have to size it to the shaft that runs in the center,” Goodrich explained. “That would’ve made the pump significantly larger. So we were able to shrink the drive mechanism and optimize the dynamics.”

He said the overall design helps reduce parasitics and optimizes manufacturability of the converter’s turbine shaft, and enables faster shift response.

One reason the 8-speed is lighter than the 6-speed is the use of magnesium castings for the clutch piston and valve body, Kaveh said. Another is the use of synthetic lubricant, developed by GM Powertrain’s lube team and a supplier. Its properties “help reduce the ‘footprint’ on the controls, giving us another mass savings,” he explained.

Kaveh noted that the 8L90’s the transmission controller module is externally mounted. “We’re using our next-gen controls, with three sensors, in this transmission,” he said. “They allow us to have very quick shifts, quicker than a DCT on WOT." The electro-mechanical element is inside the case, he added. Shift response is also enhanced by a simplified lubrication circuit, compared to that used on the 6L80.

Corvettes equipped with the 8L90 will feature paddle-type shifters for manual control.

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