General Motors' new Powertrain Engineering Center in Pontiac MI will be the hub of development for many of the company's upcoming advanced propulsion systems, including the E-Flex system for the Chevrolet Volt. At the dedication of the center, Green Fuels Forecast spoke with Larry Nitz, executive director of hybrid powertrain engineering. Nitz oversees the development teams producing all of GM's hybrid and extended range electric drive systems.

When the Chevrolet Volt concept was first revealed at the 2007 Detroit Auto Show, the specifications listed a 1.0L turbocharged three cylinder to drive the generator for the range extension capability. Over the course of production development for the past year, the E-Flex engineers have decided to replace the three cylinder with a slightly larger 1.4L naturally aspirated (NA) four cylinder. 

Nitz explains that the 1.4L NA four has better brake-specific fuel consumption than the 1.0L turbo when used in steady state mode, as it will be in the Volt application. The 1.4L comes from same Family 0 small engine architecture used in other GM small cars.

E-Flex cars like the Volt are electrically driven, with the internal combustion engine starting up only when the battery state of charge drops below a certain threshold. Nitz tells GFF "the objective is to keep the engine off and when the engine comes on, you don't want to know it's on. You want it really smooth and a four cylinder is smoother than a three."

The power, efficiency, NVH and lower cost of the four cylinder make it advantageous over the original design. When the plumbing necessary for the turbocharging and the balance shaft are added to the three, the overall package envelope works out about the same with the four being lighter. The four cylinder is somewhat "longer across the car" so GM had to "reconfigure and re-look at the packaging which is turning out very nice" explains Nitz.

One criticism from detractors of the E-Flex architecture has been the engine won't maintain vehicle performance while operating in charge sustaining mode, requiring lower speeds or "turtling." At the original introduction of the concept, GM officials acknowledged this was one of the issues with the EV1 when its battery charge level dropped too low. 

Regarding the Volt, Nitz says that this criticism "is not true to start with." The Volt will be equipped with a 16kWh battery pack that will nominally operate between a 35 and 85 percent state of charge. Vehicles typically do not use the full performance capabilities at all times. When cruising at a relatively constant speed on the highway or even in around town driving, only a small fraction of the performance capability is being used. The full capability typically is only used for transient conditions, such as accelerating along a highway on-ramp or passing another vehicle.

"The range extender does not have the full power to do the dynamic response that the electric side can so you do have to depend on the battery," Nitz says.

The range extender will provide approximately 50kW while the electric drive provides 100kW.

"That vehicle will never use more than 50kW on a continuous basis," Nitz adds.

Even if the battery is at the level where the range extender is engaged, it still has a significant amount of charge left and can provide full power to drive the vehicle in those transient conditions.

"Zero to sixty, passing maneuvers, you'll be fine, the ability to actually use more than about 50kW doesn't exist very frequently," says Nitz.

The battery can be drawn below the 35 percent charge level briefly to support these driving conditions. When steady state driving resumes, the demand on the battery falls below the output of the range extender - allowing the charge to be replenished faster than it is consumed. 

"It's designed to be able to go up Baker Hill," a steep hill between Los Angeles and Las Vegas as well as other mountain grades at reasonable speeds "just with the engine," Nitz explains.

 Nitz also discusses the plug-in hybrid (PHEV) Saturn Vue program. In a recent post on the General Motors Fastlane Blog, Nitz described the status of the program.  There are currently 11 prototype vehicles running with lithium ion battery packs. The PHEV Vue is based on the Two-Mode hybrid version of the Saturn crossover that will debut late this fall. The PHEV version adds a plug-in charging system and replaces the nickel metal hydride battery with a lithium unit.

When CEO Rick Wagoner first announced the PHEV Vue at 2006 Los Angeles Auto Show, he indicated GM hoped to have the vehicle on sale by late 2009. GM has not yet announced an official on-sale date, although company sources have repeated that late '09 time frame. In spring 2007, Cobasys/A123 Systems and Johnson Controls - Saft were awarded development contracts to supply battery packs for the program. Nitz explains that a significant amount of lab testing has been completed on the batteries and the charging integration is working and is now being refined.

A production supplier for the batteries has not been announced at this point. Nitz tells GFF that Bob Kruze, executive director of Global Vehicle Engineering Hybrids, Electric Vehicles and Batteries, is working on that and the selection criteria are complete.

"We are doing a lot of negotiation over who is going to do what," Nitz says. "We are quite involved in the battery. All lithium batteries are new and energy batteries (as required for longer electric range plug-in vehicles) are the newest. It will be very exciting for us."

 The core of the plug-in Vue program is Two-Mode hybrid system that debuted in buses in 2003 and then in the Chevy Tahoe / GMC Yukon SUVs in late 2007. With each successive iteration, the system has gotten smaller and lighter, but still remains too large and expensive for smaller cars that would compete with the likes of the Toyota Prius. 

Earlier this decade, Nitz led the team at Allison Transmission that originally devised the Two-Mode system. Nitz tells GFF that a second generation Two-Mode system is currently under development. The current variants are used on the full-size GMT900 trucks and the GMT319 Saturn Vue. "We're looking at some additional applications there and lots of applications under consideration with the new fuel economy initiative," says Nitz. "We're going to see a lot hybrids in a lot of different applications."

The second generation system "will get a lot more applications" and will likely launch in the 2012-13 time-frame. Cost reduction will be the "No. 1" effort for the Gen 2 system along with "mass reduction, size/volume reduction from a battery, power electronics even a motor perspective, it's looking pretty good."

He adds: "That side of the business is still undergoing a pretty good amount of technical advances."

With automakers facing a corporate average fuel economy requirement of at least 35 mpg by 2020, GM will require all of these advances in order to stay competitive.

JULY 2008