GM unveils more efficient 2016 Volt powertrain

  • 29-Oct-2014 01:49 EDT
Lead Image 2016 Voltec unit.JPG

Electro-mechanical heart of the Gen-2 Volt powertrain is the all-new Voltec electric transaxle, shown here in sectioned view with direct-mounted TPIM. Note twin electric machines. For many more images, click on the small arrow at the upper right corner of this image. (Lindsay Brooke photo)

“I can’t think of a powertrain we’ve re-engineered more extensively within a five-year period than this one,” observed Larry Nitz, GM’s Executive Director, Transmission and Electrification, speaking about the range-extender hybrid system in the second-generation 2016 Chevrolet Volt. Major changes to the car’s battery, Voltec electric drive system, and gasoline-engine generator—all are essentially new—are expected to increase its overall driving range and energy use by up to 12% compared with the current Volt, he said. The 2015 Volt’s driving range is rated at 380 mi (611 km) after depleting both the 16-kW·h battery and 9.3-gal (32-L) gasoline tank.

Nitz and key Volt program engineers unveiled the new powertrain to Automotive Engineering and other media at GM’s Warren, MI, transmission plant, where the all-new 5ET50 electric drive unit will be manufactured. The 2016 Volt will debut at the North American International Auto Show in January; SOP is scheduled for late summer 2015. At the Detroit show, GM will announce range estimates along with other changes aimed at increasing the new Volt’s efficiency and performance.

Nitz and the other technical leaders on hand—including Electric Drive Engineering Director Pete Savagian, Executive Chief Engineer Pam Fletcher, Chief Engineer Andrew Farah, Electrification General Director Tim Grewe, and GM’s Battery Systems Director Bill Wallace—are veterans of the first-generation Volt program. They noted other benefits resulting from the 2016 improvements, including 20% stronger low-speed acceleration and quieter, smoother operation when in “range extender” mode with the gasoline engine engaged. These are a result of redesigned electric motors (two per drive unit) and clutch array, a new, higher torque 1.5-L four-cylinder combustion engine that replaces the previous 1.4-L unit, and a revamped TPIM (traction power inverter module).

The new Voltec system also is 130 lb (59 kg) lighter than the first-gen system, thanks in part to the ICE’s aluminum cylinder block (the incumbent 1.4-L block is iron), the direct mounting of the TPIM to the transmission case which eliminates the heavy orange 400-A cables, and an overall focus on systems integration. Farah, who was disappointed in the original Volt’s 3781-lb (1715-kg) curb weight, indicated that the 2016 car will be lighter overall as a result of these and other measures.

New Voltec system

Greater range (particularly in EV mode), fuel efficiency, and power were the top three requests from owners of 2011-2013 Volt models, Nitz said. He explained that 60% of Volt buyers sign on to a GM program that funnels customer-use data directly to GM Engineering—“our secret weapon,” Nitz chuckled. Volt owners do more than 80 percent of their driving in EV mode, and his team was surprised to find that owners charge their cars’ batteries on average more than once per day, rather than the expected overnight charge. This indicates greater access to curbside charging at their destinations and a general desire for more battery energy-storage capacity.

“They want greater EV operating capability” than the typical 35 to 40 miles per charge in the first-generation car, Nitz noted.

Many observers of Volt’s development, including the author, expected GM to use its recently introduced 1.0-L global Ecotec three-cylinder gas engine as the second-gen Volt’s range-extender generator. But a triple “presents some NVH challenges, particularly at start-up, in an electrified vehicle where noise and vibration are not welcome in the drive experience,” explained Nitz. While the team’s strategy with the original car was ‘full-size battery with a half-size engine’, sticking with naturally-aspirated four-cylinders and adding 100 cc of displacement provide the optimum balance of refinement and the additional torque customers crave, at moderate cost.

GM claims the noise intensity of its new 1.5 L four is up to 50% quieter than Volkswagen’s EA211 1.4-L four and up to 25% quieter than Ford’s 1.0-L turbo three-cylinder. The Ecotec features 12.5:1 compression, cooled EGR, and a new variable-displacement oil pump. It is certified to run on 87 octane regular-grade gasoline, according to Grewe.

In creating the Gen-2 Voltec drive system, engineers achieved a 60% volume reduction in the power electronics and inverter, not counting the rest of the hardware, noted Savagian, compared with the existing 4ET50 electric transaxle, along with a 5-12% efficiency gain overall and a 2% increase in motor efficiency when the electric machines are operating as generators. There was also a significant focus in reducing the use of rare-earth materials.

Pointing to a sectioned TPIM on display, Savagian noted the electronics use double-sided cooling on a custom automotive-grade package for the IGBTs (insulated gate bipolar transistors). “In past technologies we used something more akin to industrial-type IGBT packages,” he explained. “And making the unit smaller means it’s also more rigid which is good because the transmission-mounted environment is more severe—up to 6 times greater vibration frequencies than previously.”

The twin electric machines, designed by GM and manufactured at Hitachi’s Kentucky facility, use the bar-wound rectangular wire technology and tooling common to GM’s other e-motors developed in-house. The motors are oil cooled, and together they use 10% less steel than is used in a single motor of the Gen-1 Voltec system. Total mass reduction in the motors is 33 lb (15 kg), Savagian said.

While the stators of both motors are in identical, their rotors are different. “We began considering the second-gen system a little before the launch of the first-gen Volt,” Savagian revealed. “And in that 2010-2011 time period there was a huge spike in the cost of rare-earth materials, which alarmed everyone in this business—that the consumption of rare-earth metals was something we should consider minimizing.”

So the work of Savagian’s team resulted in a reduced-dysprosium-type grain boundary diffusion magnet technology for the Gen-2 Voltec’s Motor B: rare-earth content dropped from 282 g (10 oz) to 40 g (1.4 oz). Motor A is rare-earth-free. It uses a GM proprietary ferrite multi-barrier magnet technology, developed from the ferrite magnets commonly used in high end industrial and automotive (starter motor) applications. The two new motors together reduce total rare-earth content from 3.2 kg (7 lb) on the Gen-1 system to 1.2 kg (2.6 lb)—“We’re pretty proud of that,” he noted.

Clean-sheet battery

Overall the 2016 Volt’s propulsion system carries over only a few parts from the current model, engineers said. Besides the new ICE, the 300-V battery pack is still T-shaped and serves as part of the vehicle structure. It carries over only nine parts—“it’s a clean-sheet design with the emphasis on performance, efficiency and quality,” Wallace said. Its cells retain the prismatic-pouch form factor and feature an all-new internal architecture.

The cell chemistry, developed by the incumbent Volt cell supplier LG Chem, enabled engineers to reduce the number of cells in the pack from 288 to 192 (still 96 cells in parallel), while reducing pack mass by 30 lb (13.6 kg). The larger cells also are positioned .5-in (12.7-mm) lower in the pack which in turn lowers the new Volt’s center of gravity.

“From that big customer data stream, we learned a lot about how our customers use their vehicles—how they charge, plug-in in harsh environments, we learned about thermal management. And we learned that our batteries are durable,” Wallace said. “We’re proud of the fact that capacity-fade on our batteries is exceptionally low.”

Wallace said that LG Chem is producing the Volt cells at its Holland, MI, plant at quality rates under 2 ppm. He said the entire Gen-2 pouch was totally redesigned from a three-parallel design (three cells arranged in parallel) to a two-parallel design which allowed engineers to increase energy capacity of each cell by “somewhat more than 50%―precise numbers will come later,” Wallace said.

He said this team worked with LG all the way down to the component material and cell-design levels. "Our new electrode-face area is slightly larger, and our chemistry is still NMC (Nickel Manganese Cobalt)/LMO (Lithium Manganese Oxide); we changed those ratios a bit. We even changed binder materials to improve our electric connectivity. And we were able to drive our electrode count down and our coating weights up; that helps us get more energy in a small space. We were able to improve our volumetric energy density by 20% at a cell level, and we use a new type of graphite on the anode side that improves its performance and life."

In terms of cell control—“lithium cells can be a bit temperamental,” Wallace said—GM engineers went to a centralized system versus the more electrically-complex distributed system. They improved BSE (battery state-of-charge estimation) incrementally, and the pack’s mechanical architecture reduced seal count by 33%.

Many more details on the new Volt and its vastly improved powertrain will be forthcoming. According to Savagian and Grewe, GM is preparing at least five SAE Technical Papers for publication at the 2015 SAE World Congress; highlights will be presented at the 2015 SAE Hybrid & Electric Vehicle Symposium, February 10-12 in Los Angeles; go to http://www.sae.org/events/hybridev/

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