By committing to a carbon-fiber body for its 2013 MegaCity electric vehicle, BMW is driving design, process, and materials technologies that will help push the strong, lightweight composite closer to the automotive mainstream, according to an expert in lightweight material applications.
“As the [Toyota] Prius proved with hybrids, there are benefits to being a pioneer in key technologies, and it appears that is BMW’s strategy regarding carbon fiber and the MegaCity program,” said Anthony Mascarin, Managing Partner at IBIS Associates, a materials technology and applications consultancy.
IBIS recently completed a study of carbon-fiber structural concepts aimed at high-volume applications for USCAR’s Automotive Composites Consortium.
The MegaCity EV’s body-in-white, currently in development, features a monocoque “safety cell” designed for passenger protection. BMW calls the four-seater structure LifeDrive. It uses carbon-fiber-reinforced plastic (CFRP) material developed in collaboration with SGL Group. CFRP is 50% lighter than steel for a given component, with superior strength and rigidity.
According to BMW engineers, the CFRP body, in combination with aluminum chassis components, will offset up to 772 lb (350 kg) of additional mass related to the car’s 96-cell lithium-ion battery packs, wiring, and electrical components.
Cost and process cycle times are two of the major hurdles in implementing carbon fiber beyond premium-priced niche applications in the auto industry, Mascarin told AEI. Traditional processing is relatively slow and extremely labor intensive—carbon-fiber mats are joined to their thermoset-polymer resin matrices by hand.
BMW engineers are convinced that higher volumes—50,000 units per year—and physically smaller parts will reduce costs and make the MegaCity structure “only” twice as expensive as an aluminum-intensive structure. The small urban runabout will carry a premium price compared with other EVs expected to enter the market during the same time.
The fibers are composed of 50,000 filaments. SGL Group weaves the fibers into fabrics that will be molded into components. The fiber mats are oriented in specific directions in the mold to provide strength. The German company is building a $100 million plant near Seattle, WA, to produce components for the MegaCity.
The plant’s location near Boeing’s primary engineering and production facilities also is aimed at future aerospace business, Mascarin reckons.
Currently, SGL Group is known for its carbon-ceramic brake rotors and clutch plates supplied to various European OEMs’ high-performance models. Its CFRP product line carries the SIGRAFIL brand name.
Design and production of the LifeDrive monocoque are keys to minimizing the MegaCity’s inherent cost premium, Mascarin explained.
“BMW’s production of the individual parts will be highly automated,” with component consolidation in the design stage aimed at reducing the overall bill of materials, he said. Automated adhesive bonding will be employed, and Mascarin said some areas requiring enhanced structure may use aluminum elements embedded in the CFRP molds.
“Cycle times traditionally have been an issue with carbon composites, and labor is the killer here,” he said, noting that the USCAR program showed that automation is critical to performing and layup.
“We saw that molding a part in three minutes is no problem, but getting all the constituents into the mold efficiently is the challenge.” By comparison, resin-transfer molding (RTM) cycle times are approximately 10 to 30 min on average—still far slower than metal stamping on a part-by-part basis.
Composite-material cost trends are encouraging, according to Mascarin.
“Within the last six months, carbon-fiber economics have never looked better,” he noted. “Prices have come down a great deal, perhaps because of increased aircraft use by Boeing. In the USCAR program, we were talking $15-20 per pound; now prices are in the $9-per-pound range—they’re expected to go up, but not return to the $18 range.”
According to early renderings released by BMW, the MegaCity’s seating will be positioned higher than in a traditional car due to the battery's underfloor location. The rear-wheel-drive car will also likely feature a hatchback, short overhangs, and large wheels.
While CFRP’s recyclability “should not be an issue,” Mascarin noted that working out the material’s repairability in the field is still a concern to the industry.
BMW is not alone in bringing carbon-fiber materials to the emerging hybrid and EV space. Daimler has announced it will introduce the materials in Mercedes vehicles by 2013; the company has a development agreement with Japan's Toray Industries.
Toyota also used a carbon-fiber body to reduce the mass of its 1/X concept car by 67% compared with the Prius hybrid, but no official production plans have surfaced. And like the Prius, BMW will likely lose money on the MegaCity EV until significant volumes are realized.