In a day-long briefing at the GM Tech Center in Warren, Michigan, the GM team responsible for the Volt provided a snapshot of the current state of development of the extended range electric vehicle (earlier post)—which is tracking to production in November 2010—to an international group of journalists.
The accelerated development of the Volt is pushing GM to design, to develop and to test in parallel the powertrain and the vehicle. Key topics addressed during the briefing were the cycle life and calendar life testing of the lithium-ion battery pack; the coming integration this month of the first packs into the mules; other powertrain and exhaust system issues; aerodynamic modifications to support the 40 mile all-electric range; and design issues.
The battery packs and the powertrain. GM has received initial packs from both CPI and A123Systems. General parameters of the packs are an energy capacity of 16 kWh; weight of about 170 kg, length of about 1.8m. The packs, which are based on prismatic cells from each company (the prismatic cell is GM’s go-forward design choice for energy batteries and perhaps even power batteries, according to Denise Gray, Energy Storage Devices and Strategies Director) need to:
Provide sufficient power to accelerate the vehicle from 0 to 60 in 8.5 seconds; deliver passing capability; and deliver “predicted driveability”.
Provide sufficient energy to support a 40-mile all electric range under city and highway driving conditions.
Have a cycle life that supports 150,000 miles under mixed electric vehicle (EV) charge depleting and extended range (ER) charge sustaining modes of operation.
Have a calendar life that is more than 10 years under real world conditions.
GM and its suppliers have already undertaken the cell level testing that has assessed the capability of each type of cell (CPI uses a manganese spinel chemistry, A123Systems a nano-phosphate chemistry) to achieve the required vehicle performance along with accelerated life testing and abuse behavior.
GM is currently engaged in pack level testing (cycle life, calendar life, temperature and vibration). The third major test set will be the dynamic testing of the packs in the mules and then in the production version of the Volt under road conditions.
GM also plans to test charge integration under a variety of simulated conditions (110V, 220V, presence of appliances such as washer/dryer or freezer in the garage, etc.), brownouts, blackouts, and so on.
The dynamic testing will begin following the integration of the first packs into E-flex mules (based on the Malibu) later this month. That will proceed in parallel with the pack-level tests in the battery lab that will continue for almost two more years—the shortest amount of time into which GM could compress testing to validate the required 10-year, 150,000-mile lifespan of the packs.
|GM’s Cycle Test for the Volt Packs|
For the cycle testing, GM is using a two-hour combination of the US urban, highway and the more demanding US06 cycles for the discharge; re-charging for three-hours; and simulating parking for 3 hours. This pattern will run 24x7 for two years on each of the packs. Although the packs are designed to be liquid cooled, the testing on the packs is being done without the cooling. The battery cycling testing is being done at the GM test facilities in Warren and in Mainz-Kastel, Germany.
Although the pack is sized overall at 16 kWh, GM plans to use only 8kWh in operations—i.e., a state of charge depletion window of around 50% (the pack won’t be charged fully to 16 kWh, because with a full pack, the first regen event would begin overcharging the battery). That 8 kWh needs to be deliverable at the end of the battery’s 10-year life, therefore GM is building in a buffer with the 16 kWh pack.
With future iterations of the powertrain, GM may begin expanding the SOC window, and/or begin implementing minor changes in the batteries.
Although the sizing of the pack is conservative, GM’s projected consumption of 200 Wh per mile (8 kWh over 40 miles) is somewhat aggressive. The team is confident in that figure, according to Frank Weber, Global Vehicle Chief Engineer, Chevrolet Volt and E-Flex Systems, because of the testing, modeling and simulation work that they have already done. Nonetheless, the sizing of the pack provides a buffer.
The E-Flex mules that will receive the Li-ion packs for battery testing have already been in operation at the Proving Grounds for 6 months, according to GM, with NiMH packs as the energy storage system. The focus to date with the mules—which implement full E-Flex systems—has been bringing them up to the state of readiness to accept the Li-ion packs and begin dynamic testing. Work on the mules has included:
Software function and integration testing;
Preliminary proof of concept; and
Calibration of the regenerative braking system.
The mules (one of which was on a lift with the Li-ion pack stationed beneath it) have been in operation 6 days a week for two shifts a day. GM expects to have a fleet of Li-ion equipped mules in operation by the summertime.
Over the next six months, GM will be focused on three primary areas with the powertrain: battery state estimation; thermal development; and propulsion development. The latter area includes refinement of the all-electric drive mode, hardware testing in EV mode, and further development of the power electronics.
GM has yet to finalize its selection of the combustion engine component of the powertrain, although the engine will be from its Family 0 of European small-displacement engines (1.0-, 1.2- and 1.4-liter).
GM is opting for a smaller fuel tank than originally conceived to reduce vehicle mass. The tank under consideration will still provide a 400-mile combined range, and GM decided to do as much as possible to ensure its 40-mile all electric range rather than “go overboard” in other areas, such as extended vehicle range. In recognition that the Volt (ideally) will operate for long periods of time without using the engine, GM has modified the fueling system to be able to eliminate evaporative emissions.
Aerodynamics. The inflexible design point of delivering 40 miles all-electric range under a variety of conditions with sprightly performance have forced GM to put a great deal of effort into the refinement of the exterior shape of the Volt. The impact of incremental improvements to the coefficient of drag makes a significant difference at highway speed—much more so than the weight of the vehicle.
The electric range of the Chevrolet Volt is most sensitive to improvements in aero, which is in contrast to a traditional vehicle program in which mass typically plays a larger role.—Frank Weber
Changes to the shape of the production Volt compared to the show car that GM was prepared to discuss at the briefing included a longer front overhang; more rounded front corners, rather than the sharp corners of the show car; a slightly higher roof; and some modifications to the rear.
GM showed a 1/3 model of the production design of the Volt in the wind tunnel at the company’s aerodynamics lab in Warren, and uncovered small sections (front corner, rear corner) of full-size clay models of the production Volt in the design studio.
The interior. The shape and placement of the battery has pushed the occupants outboard, or to the sides of the vehicle, so the design team worked with the sections of the roof structure and doors to enable aerodynamics and provide adequate head room. The interior will accommodate a 6-foot 2-inch (99th percentile) male comfortably in the front and rear seats.
GM says it is putting a great deal of effort into the development of the human-machine interface for the Volt, although no examples of that were shown during the briefing.
We are looking for an i-Phone like experience.—Frank Weber
Green Car Congress attended the briefing at GM’s expense.