The race to put new electric cars on American streets within two years is turbocharging efforts to reduce vehicle weight.
Carmakers are now more willing to accept higher costs for high-performing materials’ systems made of magnesium or carbon-fiber reinforced plastics because reduced weight will extend battery range or allow lower-weight batteries. Design News’ interviews with chief engineers at all three American OEMs show every pound in a car is under investigation, resulting in new materials, new processes and new assembly technologies.
Some of the more dramatic changes will be use of polycarbonate as a glazing material for car roofs and rear ends, increased use of molded plastic in car body panels, more widespread use of thinner-gauge high-performance steel and much higher use of aluminum and magnesium.
Significant incremental improvements are already evident in new cars now hitting the market. Weight-saving measures to the 2008 Ford Focus include:
1. Brakes: High-strength aluminum is replacing steel in front brake calipers. Weight savings: 7.5 lb.
2. Seat backs: The blow-molding process, which creates rigid, hollow structures, is used for the rear seat back. The seat cushions have been redesigned and lightweight side bolsters are now being used. Weight savings: 10.7 lb.
3. Trunk floor: Another alternative plastic process, compression molding, is used for the base layer of the trunk carpeting. Weight savings: 3 lb.
4. Cooling systems: The cooling system is all new, employing a new overflow bottle, a new combined radiator-and-cooling fan and combined air conditioning and transmission coolers. Weight savings: 3.6 lb.
5. Air conditioning compressor: A new approach to compressors provides a higher capacity and is also lighter. Weight savings: 1.2 lb.
6. More rigid body: The body of the 2008 Focus is more rigid and lighter, due to high-strength steel and new structural adhesives. Weight savings: 3.7 lb.
7. Power train: Engine and transmission control modules are relocated to the engine compartment and the throttle control is now electronic. New constant velocity joints are lighter. Weight savings: 6 lb.
8. Wheels: Two-thirds of the fleet use aluminum wheels, providing a classier look and weight savings of 22 lb.
9. Fascia braces: The old lower rear fascia brackets behind the rear wheels were replaced by “pencil” braces, yielding a weight savings of 1.5 lb.
“I don’t know if you’ve noticed, but our head man is from the aircraft industry and he doesn’t understand why our vehicles aren’t lighter already,” says Shawn Morgans, body structure technical leader at Ford. He was referring to Alan Mulally, who became chief executive officer of Ford in 2006 after a 37-year career at Boeing, going from engineer to executive vice president. Mulally was involved in the game-changing decision to go to all-composite aircraft bodies at Boeing.
Ford’s short-term strategy targets use of thinner-gauge, high-strength steels to reduce weight of vehicles currently under design. Steelmakers are developing new grades that will help lighten the load. For example, ThyssenKrupp Steel and JFE Steel of Japan jointly developed a new multiphase steel for automotive applications. It has a minimum strength of 780 megapascals (MPa), similar to that of the advanced ultra-high-strength steels but with up to 40 percent higher elongation.
Morgans says Ford has been a leader in recent years in increasing the use of aluminum and magnesium to reduce weight. That strategy will broaden in the next two to three years, he says. Ford is also planning increased use of aluminum for hoods and in lift gates. Morgans says more than half of Ford vehicles already have aluminum hoods. The typical aluminum part is 40 to 50 percent lighter than a comparable steel part, according to the Aluminum Assn.
Eight models that debuted at last fall’s Frankfort Auto Show make extensive use of aluminum for front and rear bumper systems, monobeam solutions, subframes and fuel filler pipes, according to aluminum supplier Hydro. The Audi A4, for example, uses aluminum for its front impact management system.
The Audi is also breaking ground in materials’ systems for the rear bumper in the R8. A newly engineered polyurethane system (based on Bayflex 180) matches aluminum for fit, in the opinion of Dieter Gaumitz, an engineer at Bayer MaterialScience AG.
Ford expects greater use of high-strength composites in front-end applications, Morgans says, but there are no plans to introduce dramatic new plastic technologies such as carbon-fiber reinforced composites. They’re too expensive right now, says Morgans. There are also technical problems with much-discussed efforts to make roofs out of polycarbonate. Scratch and weathering problems still have to be resolved, says Morgans.
Morgans also wants producers to make improvements to sheet molding compound, which Ford has used significantly for body and truck bed applications in shorter production run vehicles.
“The weight savings weren’t where we thought they should be,” Morgans told Design News. “There would have to be further technical developments in that area before we would consider it a true weight-saving strategy.”
SMC producers made a major pitch at the Detroit Auto Show in January for their materials as weight cutters.
The Automotive Composites Alliance benchmarked a prototype of a future hybrid sedan to how thermoset composites could cut weight. “We disassembled the car, weighed and measured components and discovered a lot of opportunities for mass savings,” says David Dyke, advanced engineering manager at Meridian Automotive Systems, a member of the ACA.
An analysis of the study shows that where metal parts can be integrated, plastic molding definitely offers an advantage. For example, six to eight metal stampings in a complex trunk structure could be replaced with a one-piece structural low-density composite, reducing weight 50 percent and tooling costs by 70 percent. Four metal parts could be consolidated into a two-piece composite deck lid assembly, cutting weight up to 35 percent.
The ACA study says sheet molding compound could reduce weight 30 to 50 percent. “The total cost is always cheaper overall for composites in volumes under 125,000,” says Dykes. One weight-saving strategy could be incorporation of glass bubbles into the composite compound. New grades developed by 3M boost already high weight-to-strength ratios.
Charging the Volt
The Chevy Volt concept car introduced at the 2007 Detroit Auto Show uses an all new approach for the hood: a molded thermoplastic composite. The Volt has become the poster child for GM’s electric car development, with Vice Chairman Bob Lutz promising a commercial car in two years. He also told GM engineers the car should go at least 40 miles without a charge and must cost no more than $30,000.
The new high-performance composite technology (HPPC) developed by GE Plastics (now Sabic Innovative Plastics) for the hood of the concept Volt features a sandwich of glass mat and thermoplastics made from bottle waste.
“The structure of the composite has three layers,” says Robert Butterfield, global market director of design innovation in the former GE Plastics’ automotive business, in an interview with Design News after the auto show. “The top and bottom layer has a skin and in the middle is a core. The purpose of the core is to move the compression and tensile areas away from the neutral axis to give it more strength. So what you have is a composite that you would more typically associate with aircraft or race cars.”
Mark Verbrugge, director of GM’s Materials and Processes Lab, says the Volt “is one of the most exciting projects in the company and a lot of us are losing sleep.” He also told Design News, “the materials used in the concept car are very much in play.”
All new materials technologies for electric cars are evaluated at GM on dollars per kilogram, according to Verbrugge. “Obviously, you are going to have a higher metric because you have to meet that 40-mile range. The batteries are extremely expensive. If the materials weigh more, you need bigger batteries,” he says.
Verbrugge says there’s a real horse race now between carbon-fiber-reinforced plastics and magnesium for load-bearing components in future GM cars. “Last year, we put in an industry-first die-cast magnesium alloy, AE44, for a front-engine cradle. We’re looking now at expanding that to other applications.” GM now has a demonstration project under way for a magnesium front-end application that has load-bearing requirements. While there will be some uses for CFRP in structural applications, Verbrugge thinks there will be quite significant increases in magnesium in a one-to-10 year time frame.
The cradle provides a 35 percent weight savings over the previous aluminum structure. It was die cast by Meridian Technologies, the largest global producer of magnesium die castings for the auto industry. “There are only about 10 pounds of magnesium in a car now,” says Verbrugge. “There’s a lot of bandwidth there for magnesium to go after.”
Similar to Ford, GM is exploring increased use of plastic for body panels. Will the role of SMC grow? “That’s a good question and one we ask every day,” says Verbrugge. One of the issues for auto OEMs is the cost of redoing a body shop for large-volume applications, such as Chevy Impala. “The short answer is, if you can stamp it, you want to stick with steel or aluminum,” says Verbrugge. “If you want to accentuate styling, composites are a better bet. But for the mainstream, composites are having a tough time competing with existing technologies.”
Process issues, such as requirements of an autoclave for Class A finishes, don’t bode well for use of carbon fiber composites for body panels. A compact hybrid concept car shown by Toyota at the Chicago Auto Show in February makes extensive use of CFRP through the body frame. The curb weight of the 1/X concept car is 926 lb compared to 2,890 for the Prius hybrid.
GM has used CFRP for secondary load paths, such as floor pans. Even drive shafts are a potential application because they are not critical in crash energy management. “One of the problems with CFRP for true load-bearing structural applications is the nondestructive evaluation tests,” says Verbrugge. “You get in a crash and then you have to determine if there is still structural integrity in the members. It’s not that easy with a composite.” GM is now examining tools it can use for that purpose.
Chrysler currently uses carbon fiber on the 2008 Viper ACR (American Club Racer) in the fender supports, air dam and spoiler. “Because of the cost of carbon fiber composites, we are careful in applying the material,” says Susan Yester, senior manager, Organic Materials Development at Chrysler.
The Automotive Composites Consortium within USCAR is working on a project to help reduce carbon fiber costs so the material application can be more widespread. Chrysler is not currently using carbon fiber for the frame.
For the short-term, it looks like the auto OEMs will try to work with thinner and lighter versions of proven materials, primarily metals. Farther out, look for breakthrough in new systems. One development to keep an eye is a new DuPont nanocrystalline metal/polymer hybrid technology used to manufacture extremely lightweight parts.
“The advanced electric vehicle market is going to be a phenomenal proving ground for plastics,” says David Loren, project leader, Future Business Group, Bayer MaterialScience. “The rules change when you talk AEVs.”