Although a fuel cell-powered Chevrolet Volt is still a ways off, progress on the plug-in E-Flex model continues apace at General Motors with production timing set for the end of 2010. Progress was reaffirmed recently when GM announced that in-vehicle testing of the critical lithium-ion (Li-ion) battery pack will commence sometime in April.

Continental's battery pack

The Volt’s 6-foot-long, 375-pound T-shaped Li-ion battery pack will soon begin real-world evaluation in a modified 2005 Chevrolet Malibu E-Flex test mule.

“We’re working with incredible speed,” declares Frank Weber, global vehicle chief engineer for the Chevrolet Volt and E-Flex systems. “Production timing of the Volt is directly related to our ability to predict how this battery will perform over the life of the vehicle.”

Numerous engineering innovations were required to maximize the Volt’s 40-mile electric-only range while minimizing the use of its range-extending internal combustion engine. The E-REV (extended-range electric vehicle) Volt is expected to have a driving range in excess of 400 miles between fill-ups. (Current plans call for a gasoline- or E85-fueled IC engine/generator unit, although the bio-diesel version has been shelved for the time being, according to GM.)

"The challenge is predicting 10 years of battery life with just over two years of testing time,” Weber explained. “The battery team is able to utilize human and technical resources around the globe to reduce testing time.”

To compress the battery testing into the program timeline, GM battery engineers developed a new computer testing algorithm to help them evaluate the durability of the Li-ion battery pack. This advanced computer program duplicates real-life vehicle speed and cargo-carrying conditions, and compresses 10 years (equal to 150,000 miles) of comprehensive battery testing into the Volt’s brisk development schedule.

Cycle testing

Prototype Li-ion battery packs undergo life-cycle testing using greatly accelerated charge and discharge cycles, as well as extreme temperature operation in test cells such as the one shown above.

Battery cycling equipment is used around the clock at GM’s test facilities in Warren, Michigan and in Mainz-Kastel, Germany. The equipment charges and discharges the full-size prototype battery packs based on the Volt’s approximately 40-mile electric-only drive cycle. While this test data will help predict the long-term durability of the battery, evaluation of the prototype battery packs has yet to include vibration testing, which will be accomplished during the recently announced in-vehicle tests.

“Extensive analysis in our battery labs is an important step in proving this technology,” Weber pointed out. “We expect to further validate these batteries when they are integrated into engineering development vehicles -- where the battery is exposed to shaking, moisture and rapidly changing temperature conditions -- which are much more extreme than the controlled settings of the lab,” he added.

The prototype Li-ion battery packs, which are sourced from two suppliers (Continental and CPI), have so far been evaluated only in the laboratory. Because of this, an E-Flex test mule has had to suffice with nickel-metal hydride (NiMH) pack. Although there are no plans to use the lower-power density NiMH battery in the production Volt, this existing battery technology has allowed the E-Flex powertrain engineers to test and refine the balance of the Chevrolet Volt’s electric drive system.

“We were able to create a setup at the Milford Proving Grounds where we could actually simulate the entire vehicle, all of the controls around the vehicle, all of the interfaces in the vehicle, yet never have the (Li-ion) battery,” explained Michael Bly, director of hybrid vehicle integration and controls at GM’s Milford Proving Grounds.

“We’ve been running (the test mule) around the proving ground since October without a real E-Flex battery,” said Bly, whose team is anticipating the up-fit of the first Li-ion battery pack this month. Once installed, on-board evaluation of the E-Flex battery pack can begin. This work will continue in parallel with the laboratory testing to determine the effects of real world driving on state of charge estimation, thermal management, propulsion development and battery health.

Bly’s team also evaluates the plug-in charging system for the Volt at the proving grounds facility in Milford. “A critical feature of the E-Flex system is the ability to charge up within a certain amount of time. We have a facility that can simulate every condition from 110 volt to 220 volt, brown-outs, black-outs; all those things that you have to deal with.”

For the next two years, cycle testers such as the one shown above will expose the Volt’s battery packs to 10 years’ worth of operation through continuous charge and discharge cycles.


Revised Exterior Design

“We all like the shape a lot, and we all like what the Volt is saying,” said Weber referring to the original Volt concept vehicle. “The design team has the challenge to somehow achieve both: the spirit and the character of the Volt, the efficiency of the vehicle, and to give it an aerodynamic shape that makes the whole concept credible in an energy sense.”

However, for those hoping that the Volt would be a Saturn Sky for the electric set, be prepared for somewhat of a disappointment. While the original Volt concept vehicle did have four doors, it looked more the part of a sporty coupe. Unfortunately for electric sports car fans, the final production Volt will be slightly taller than the concept car with more of a sedan shape.

“It’s important to note that the Volt is a five-door, four-passenger sedan, not a two-plus-two,” said Tim Grieg, design manager for the Volt. “Two-plus-two suggests that for styling or aerodynamics’ sake we wanted to compromise the rear seat package and that’s not what this is all about,” he said adding that they could not fit a fifth passenger into the vehicle due to the battery pack running down the middle of the car.

Photos of the production Volt have yet to be released, and journalists were not allowed to view the full-size clay exteriors during this visit to GM’s Warren Tech Center. At this point in time the final design is difficult to predict, however the inclusion of a rear hatch could offer some hope that the production vehicle will maintain at least some of the sporty aesthetics found in the concept vehicle.

The E-FLEX studio

“The promise of the show car was that it be sporty, personal and dynamic,” said Bob Boniface, design director for the E-Flex Systems Design Studio and the Chevrolet Volt. “I don’t think people are only going to buy this car because of its efficiency. I think it’s a personal statement, a fashion statement; and the car has to look good.

Indicating the efficient exterior packaging work, Boniface explains that the production Volt will have a tight look. “We’ve put a very snug upper on the car for frontal area and aerodynamic performance. We shrink-wrapped this upper around the occupants while meeting all the rollover standards and airbag packaging.

Starting with the Volt, GM’s new E-Flex Systems Design Studio will develop a variety of vehicles using the E-Flex propulsion system.

The Volt has spent more time in the wind tunnel than almost any other project currently running at GM, according to the project team. “There’s no fat on this car. You can’t wring anymore water out of it, and we still can accommodate a six-foot-two male very comfortably in both the front and rear,” added Boniface, who was also the lead exterior designer for the Volt concept vehicle and who now directs a hand-picked team of about 45 creative designers, sculptors, design engineers, scientists and administrative staff.

Reduced Drag More Important than Reduced Mass

The Volt development team has spent a great deal of time refining the vehicle’s aerodynamics, due in large part to aerodynamic drag accounts for about 20 percent of the energy consumed by an average vehicle. According to GM engineers, aerodynamic factors matter more than mass, especially in an electric vehicle with regenerative braking.

In an electric-powered vehicle mass is secondary, according to Weber, even with a weight difference of 400 pounds. In the city, an all-aluminum Volt gets about 43 miles on a single battery charge, yet a 400-pound heavier vehicle would get 41 miles per charge. “There’s even less sensitivity on the highway side,” explains Weber. “It is 40 miles and 39 miles, just to illustrate.

A GM windtunnel

“This is not to say that mass is not important to us,” Weber says. “Mass is important, but you have to balance it with other criteria in the vehicle. What you learn quickly is that aerodynamics is true waste. On the mass side you can recuperate and regenerate the energy back into the vehicle. With aero, what’s lost is lost,” he says.

“Because you can regenerate (power in an electric car), the impact of aerodynamics is dramatic, even in the city cycle, which was a surprise to us,” said Weber pointing out that on the highway the impact is even greater.

As wind tunnel testing continues, the production Volt will receive additional refinements. According to Boniface, the benefits will be measurable. “For example, we’re tuning the head of the sideview mirror now and we’re getting improvements of three to four counts (of aerodynamic drag). Ten counts of aero in the city means I can go an extra quarter of a mile; ten counts on the highway is an extra half a mile. So, it really gets to the importance of tuning the shape of this car for aerodynamic efficiency.”

The Volt design team continues to use the GM Tech Center’s wind tunnel to refine the aerodynamics of the vehicle for maximum efficiency.


APRIL 2008