GM Goes Clean Sheet in Engineering Chevy Volt’s Second-Gen Powertrain

WARREN, MI – General Motors is upsizing the engine for its next-generation Chevrolet Volt, but overall the plug-in hybrid’s powertrain package will get smaller and more efficient thanks to an aggressive re-engineering of what already was a groundbreaking system when launched five years ago.

“We took a clean sheet to Gen II – do it over, do it better,” Larry Nitz, executive director-transmission and electrification, says during a media backgrounder here this week at the plant that will build the Volt’s transmission. “I can’t think of any other product (at GM) where we had a complete re-do in five years.”

There’s only a single carryover part in the powertrain package, a small thimble-shaped plastic piece that provides protection during shipping.

The result is visually stunning when the old and new powertrains are placed side by side. Despite a small increase in the displacement of the gasoline engine from 1.4L to 1.5L – many had expected the next Volt to have a smaller, not bigger 4-cyl. – the overall package is reduced 18% in volume and has shed about 130 lbs. (59 kg). GM officials say all of that savings will translate directly to the new ’16 Volt, which will be at least that much lighter than the current model.

Much of that size and weight reduction is a result of some clever integration of the power electronics and inverter, which now are mounted directly on the transmission, and the neatly packaged 2-motor electric drive system. Like in some other plug-in hybrids, such as the Honda Accord, the two motors serve as the car’s transmission, varying their speeds to create different drive ratios.

One thing that appeared to alarm some electric-vehicle purists when the first Volt arrived was the news that under certain circumstances the gasoline 4-cyl. – mainly used to maintain battery charge – would power the drive wheels directly.

In the second-gen powertrain, there actually will be more, not fewer, events where the internal-combustion engine is asked to carry the drive load, a concession to customers who were asking for more power under certain driving conditions.

“There will be times when the engine is connected to the wheels more than in the first generation,” concedes Pam Fletcher, executive chief engineer. “But we’re going to drive the wheels most efficiently (at all times).”

Volt Buyers Asking for More Power, Range

The 1.5L engine is the first application in North America of GM’s new-generation Ecotec family. It currently is available in 1.4L and 1.0L turbo versions on Cruze models sold in China, and more applications are coming in the U.S., as well. The Volt engine initially will be imported from Mexico, but output will begin sometime in 2015 at GM’s Flint, MI, plant.

The 1.5L features direct injection, a 12.5:1 compression ratio, a cooled exhaust-gas-recirculation system, variable displacement oil pump and, unlike the outgoing 1.4L, an aluminum block. Importantly, it runs on regular unleaded gasoline, unlike the current model, which requires premium.

GM did look at smaller engines, but ruled out the new Ecotec 1.4L and 1.0L turbo because they couldn’t operate as efficiently as the 1.5L in the Voltec application.

“In the first-gen system we had a fullsize battery and half-size engine,” Nitz says. “Now we have a 60% size engine. It runs at lower speed on average, produces more torque, is as smooth as (Gen I) and even quieter.”

The current Volt engine can become loud during hard driving, but the more powerful 1.5L should solve that, Nitz says.

“Customers want that (quiet) EV experience, even when in extended range (with the engine running),” he says. “So making the engine bigger goes with what the customers want.”

The automaker is saving specifics on power and range for the Gen II system until January when it unveils the revamped Volt at the North American International Auto Show in Detroit. But it says the powertrain will be up to 12% more efficient overall and offer longer electrical range, a longer extended range between fill-ups and more power, with about 20% better acceleration in low-speed ranges.

GM engineers say they relied heavily on customer input in designing the new powertrain. Some 60% of the more than 69,000 Volt drivers agreed to provide anonymous data on their driving through the onboard OnStar telematics system.

First-gen system (left) presents much larger package.

“This has been one of the most studied populations of vehicle owners out there,” Nitz says. “They’re our secret weapon. We didn’t have that when we did Gen I.”

In addition to more power, buyers want to operate in electric-only mode longer. Engineers say they were surprised to find Volt owners drove 80% of the time on battery only, covering 650 million miles (1 million km) in EV mode, and that they tend to recharge their cars more than once a day.

The improvements should allow most buyers to cover 90% of their driving in EV mode on average, Nitz indicates.

GM and cell supplier LG Chem have stepped up battery performance, as well. There are only nine carryover parts in the Gen II lithium-ion battery pack, which still is T-shaped and packaged into the floor, but sits about a half-inch (13 mm) lower to improve performance dynamics, and the overall system is about 30 lbs. (14 kg) lighter.

The new pack contains fewer cells, 198 compared to the current Volt’s 288, and while engineers decline to be specific, they say energy storage capacity is up 20% by volume. GM looked at dozens of alternative module designs that were not liquid-cooled, but decided to stick with the Volt’s current thermal-management strategy.

Charging times are expected to stay about the same with the new model: four hours on a 240V line, 10-16 hours on a 120V line.

With the new design comes stepped-up North American sourcing. In addition to a cost-saving shift of transmission production from Mexico to the Warren plant, the Volt’s Hitachi-supplied electric motors now will come from a U.S. facility, rather than Japan. In all, U.S.-Canada content on the powertrain will rise to 70%.

Some $240 million was spent on the 56-year-old Warren Transmission Plant to refurbish and tool 300,000 sq.-ft. (27,870 sq.-m) of unused space to produce the new Voltec electric-drive transmission. About 160 jobs were retained at the facility as a result of the new work.

Fletcher believes this clean-sheet redo of the Volt powertrain gives GM a leg up in the EV movement.

“We lead in electrification and have no intention of giving that up,” she says.

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