First look at all-new Voltec propulsion system for 2G Volt; “the only thing in common is a shipping cap”

29 October 2014

Cutaway of the new power electronics unit, which is much smaller than in gen 1, and which is now integrated into the drive unit housing. The power electronics and the motors use separate cooling: water for the PE and ol for the motor unit. Click to enlarge.

The second-generation Volt, which makes its world debut in about 10 weeks at the North American International Auto Show in Detroit, features a clean-sheet, all-new Voltec propulsion system—new battery, new electric drive unit, new power electronics and new range-extending engine. At an introductory media briefing on the new powertrain held at the Warren Transmission Plant in Michigan, where the new drive unit will be built, Larry Nitz, GM Executive Director, Transmission and Electrification, noted that the only common part between the gen 1 and gen 2 drive units was a little yellow plastic intra-plant shipping cap for the manual selector.

The battery cells, with a tweaked NMC/LMO chemistry from LG, increase storage capacity by 20% volumetrically when compared to the original cell. The drive unit features a large number of changes: new roles for the two motors, two clutches instead of three, and a smaller power electronics unit integrated into the housing among them. (No more big orange high-voltage cables underneath the hood.) The new direct-injected 1.5 liter engine with cooled EGR features a high compression ratio and is optimized to function in its range extender role.

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The second-generation Voltec propulsion system with range-extending engine and integrated power electronics and battery pack. The high-voltage (orange) cables from battery to drive unit are the only high-voltage cables in the system; there are no cables connecting the power electronics to the motor unit under the hood. Click to enlarge.

Overall the engineering team increased efficiency and reduced weight; the drive unit is up to 12% more efficient in operation—although GM has yet to quantify publicly what that means in terms of range or fuel economy—and 100 lbs (45 kg) lighter. The battery system, with fewer, albeit larger, cells (192 vs. 288) is nearly 30 lbs (13.6 kg) lighter, but offers more capacity (also unspecified at this point) than its predecessor.

The overall goal of the redesign was to give the customers more of what they want, the GM team emphasized over and over: more electric range, better fuel economy, and more power, and lower noise from the engine when it runs.

In a five-year period, I can’t think of any other product that we have that we have gone through a complete, a complete reengineering.

The second-generation battery pack retains the T shape, but increases capacity and reduces weight with almost 100 fewer cells (192 vs. 288). Click to enlarge.

Battery. Although it maintains the external form factor of its predecessor, the battery pack for the second-generation Volt features a number of significant changes, including revised cell chemistry, developed in conjunction with battery supplier LG Chem, which also provides the current generation cells for the Volt.

The Gen 2 battery was also a clean-sheet design, said Bill Wallace, Director, Battery Systems; there are only 9 carry-over products in the new battery.
The cell is still a prismatic pouch, but redesigned. The engineers went to a 3P design instead of a 2P, allowing them to increase individual capacity in cells by slightly more than 50%. There are mechanical changes as well, for a more efficient package.

In terms of chemistry, Wallace said:

We are still NMC and LMO. We changed the ratio a little bit—a little more NMC and a little less LMO. NMC is a high surface area modified product—this is a brand new class of NMC. We’ve changed things like binder materials, electrocoat and ionic conductivity, and we were also able to drive our electrode count down and our coating weights up. That helps us get more energy. We were able to improve volumetic energy density 20% at a cell level.

It’s not a radical change in cell chemistry, but it is absolutely the most modern NMC/LMO material. We also made some changes on the graphite side to improve performance and life.

Wallace noted that approximately 20 million battery cells have been produced for the more than 69,000 Chevrolet Volts on the road today with industry-leading quality levels of less than two problems per million cells produced (2 ppm).

The battery system continues to use the Volt’s active thermal control system that maintains electric range over the Volt’s life.

Based on a GM study of more than 300 model year 2011 and 2012 Volts in service in California for more than 30 months, many owners are exceeding the EPA-rated label of 35 miles (56 km) of EV range per full charge, with about 15% surpassing 40 miles (64 km) of range. Current generation Volt owners have accumulated more than 600 million EV miles.

EV range estimates will be revealed in January at the North American International Auto Show in Detroit.

GM will manufacture the Volt battery pack at its battery assembly plant in Brownstown, Mich.

The second-generation Voltec drive unit. Click to enlarge.

Drive unit. Like the battery system, the next-generation Volt’s drive unit was reengineered with a focus on increased efficiency and performance, improved packaging and reduced noise and vibration characteristics. The two-motor drive unit operates approximately 5 to 12% more efficiently and weighs 100 pounds (45 kg) less than the current system.

The gen 1 Volt drive unit comprised two motors—a 111 kW main traction and 63 kW (at 4800 rpm) generator motor (55 kW generator output)—as well as three clutches and a planetary gear set tucked in the end of the traction motor.

Although the gen 2 system also uses two motors, the motors are new (one being rare-earth element free, the other with lower use of rare earth elements) and their roles have changed. Rather than designating Motor A as the generator motor and motor B as the traction motor, the two motors in the gen 2 system share both roles, providing more flexibility. GM has reduced the number of clutches to two from three.

Further, the ability to use both motors helps deliver more than 20% improvement in electric acceleration.

If we do it [provide traction] with two motors rather than one, we can spread the ratio. So now we have a spread torque band that’s actually wider and more responsive with a two-motor configuration. I like to call it a twin torque path. How those motors actually during the EV drive work together to give you that interval response is one of the key characteristics that our experts said really will make this be a better drive feel than we had before.

When you add that [two-motor] flexibility to your system, you increase efficiency—we’re up about 12% in some of the driving modes,—but you also make it more responsive. The whole twin torque path spread ratio requires a lot of software and a lot of optimization.

The Traction Power Inverter Module (TPIM), which manages power flow between the battery and the electric drive motors, has been directly built into the drive unit to reduce mass, size and build complexity while further improving efficiency.

Top: The integrated motor/TPIM drive unit. Bottom: The drive housing being prepared to receive and receiving the TPIM module; the motors are already in place, but hidden by the housing. (The briefing including a hands-on build of two complete drive units.) The high-voltage connectors linking power electronics to motors are seen at the bottom of the housing in both pictures. Click to enlarge.

The two motors are all-new GM motor designs, although they are built for GM by Hitachi. They are bar-wound interior permanent magnet type motors, and feature common tooling but with a reduced total physical size. The motors feature a mass reduction of more than 33 lbs. (15 kg), while offering a greater total power capability of 4% and with peak motor efficiencies increased by 2%.

While the stators are twins, there are differences in the rotors, Savagian noted.

Design for the motors was begun several years back when concerns over rare earth metals pricing were quite high, noted Pete Savagian, General Director, Electric Drive. Accordingly, Motor A uses no rare earth magnets at all, featuring instead a GM multi-barrier Ferrite magnet design. Motor B uses a reduced dysprosium-type grain boundary diffusion magnet technology.

Total rare earth magnet mass reduction in the new system is 60%—from 7 lbs (3.2 kg) to 2.6 lbs (1.2 kg). The reduction of heavy rare earths is more than 80%—from 0.6 lb (282 g) to 0.08 lb (40 g).

New 1.5L range extender. Energy for extended-range operation comes from an all-new, high-efficiency 1.5L 4-cylinder engine. The engine features a direct injection fuel system, high-compression ratio of 12.5:1, cooled exhaust gas recirculation and a variable displacement oil pump. The Voltec range extender also runs on regular unleaded fuel. Its use in the Voltec system is the first production application of the engine in North America, GM said.

We had the idea in first gen that we would have a full-size battery and a half-size engine. We did all our simulation and analysis, and thought we had the tiny engine that can. But if you drive the car hard, in more strenuous situations, the full-size battery and the half-size engine get behind, and the engine gets loud. It wants to make power, has to make power. But our customers want the quiet EV experience in extended range. So we developed the next gen volt with a 60% size engine. Just a little bit more.

On an average basis, the engine runs at lower speed, delivers more torque, is quieter. So making the engine bigger is actually giving our customers exactly what they want. They want the feel of an electric car even when they get into range extension. And that’s what we’re going to give them.

The 1.5L engine will be manufactured at GM’s Toluca, Mexico engine plant for the first year of production, then shift to the Flint, Mich. engine plant.

GM engineers are in the process of preparing of number of SAE papers on the new Voltec system, getting deeper into design, power flow and the like. These will be presented beginning in February.

The second-generation Volt is due to go on sale in the second half of 2015.