For years, weight saving has been a subject to which the auto industry in general has paid lip service but achieved relatively little progress. But now, for reasons that span international political uncertainty, energy supply, plus environmental and legislative pressure, that progress has to be made—and quickly.
“Until just a few years ago at Lamborghini when the talk was of performance, it was top speed that was most important, followed by acceleration, and then handling. Now the order has changed: today it is handling first, then acceleration, with top speed last. The future will be less about increasing power, more about reducing weight,” said Stephan Winkelmann, President and CEO of Automobil Lamborghini, as he revealed the secrets of the company’s new Aventador LP 700-4, with its advanced technology carbon-fiber-reinforced polymer (CFRP) monocoque.
Carbon fiber is a central part of the company’s systemic lightweight design ethic.
Lamborghini is a wholly owned subsidiary of Audi AG (part of the Volkswagen Group). Both companies focus sharply on lightweight construction, Audi particularly on aluminum and multimetal hybrid materials construction solutions. (It is 17 years since the aluminum spaceframe Audi A8 entered production, to be followed by the semi-high-volume A2; although in 1913, Audi predecessor company NSU built the aluminum-bodied Type 8/24.) Lamborghini also focuses on aluminum but with greater emphasis on CFRP that can be used with greater financial confidence for a high-end, low-volume product, although chopped fiber may have high-volume potential.
There is significant technology transfer between Audi and Lamborghini on materials R&D and production, with the Italian company able to “prove” composite solutions that may be extrapolated to higher volume production.
Lamborghini’s experience with composites goes back some 30 years (prepreg technology was used in the 1983 Countach), and its command of the material’s application was demonstrated in 2010 with its extreme Sesto Elemento lightweight concept, which used forged composite chopped-fiber applications for its monocoque and parts of its suspension—scaling only 1000 kg (2205 lb) and with a claimed 0-100 km/h (62 mph) time of 2.5 s.
At 2.9 s, the production CFRP-intensive Aventador is not far behind.
“We have developed improvements in materials research and in manufacturing technology,” explained Lamborghini R&D Director Maurizio Reggiani.
Partnerships are particularly significant, notably with Boeing regarding chopped-fiber development and simulation using the aerospace company’s “building block” approach that facilitates effective modal prediction for a given weight, allowing safe weight reduction during development. Callaway Golf Co. also works very closely with Lamborghini, as do Dassault, Huntsman, IBM, EDAG, University College London, and the U.S. Federal Aviation Administration (FAA).
Vital to its work is Lamborghini’s center of research, the Advanced Composite Structures Laboratory (ACSL) at the University of Washington, Seattle. “Work there allows us to produce and use the right technology in the right way—and to do so in-house,” said Reggiani. The ACSL serves as the hub of all Lamborghini’s cooperation partners in the U.S.
The monocoque and body-in-white (BIW) of the new car are built at Lamborghini’s Sant’Agata headquarters. There, a new 5400-m2 (58,125-ft2) production facility has automated systems to complement its traditional craftsman skills. The facility further strengthens Lamborghini’s determination to keep as many production aspects as possible in-house; this now covers everything from receipt of the fiber mat rolls from its supplier to completion of the paint-ready bodyshell.
The facility has a five-line layout: on the first, the prepreg parts are made, and on the second, parts and subassemblies are created using RTM-Lambo (Resin Transfer Molding patented by Lamborghini; curing is in the mold in a heated chamber, not an autoclave) and the prepreg and epoxy foam parts integrated into the monocoque structure.
On the third line, epoxy foam stiffening components are produced for assembly as inserts into the prepreg and RTM process; on the fourth, the whole monocoque structure and roof are machined, assembled, and measured for total accuracy; and on the fifth line, the finished monocoque is fitted to the aluminum front and rear subframes.
Carbon fiber mats (prepreg) are preformed and impregnated with a precise amount of resin. They are then heat-cured—while the part is in the mold—in an autoclave. The prepregs are pre-injected by the supplier with a thermosetting liquid resin before being laminated in molds and cured under heat and pressure in an autoclave. Class A surface quality is achieved.
Lamborghini’s patented RTM-Lambo system incorporates very light, carbon-fiber tooling instead of a heavy, complex metal piece. It obviates the laborious use of hand lamination and autoclave, making the process faster, more flexible, and more efficient. Because the process needs only low injection pressure, a cost saving is also made.
Tripling carbon fiber use
Lamborghini’s total use of carbon fiber, including product, tooling, and R&D, will triple from its 2009 figure to reach more than 300 t (330 ton) by 2013.
As well as a new production facility, Lamborghini also has its equally new Advanced Composite Research Center (ACRC) at Sant’Agata. It operates closely with the ACSL in Seattle, where behavior of materials developed at the ACRC is researched and tested (including for crash performance).
Work is under way at the ACRC into applications of forged composite—a heated 1000-t (1100-ton) press is part of an extensive range of equipment—for likely future automotive applications. Chopped fibers of about 20 to 50 cm (7.9 to 19.7 in) are used instead of the traditional long interwoven fibers. More than 500,000 braided fibers/in2 create a material that has only a third the density of titanium, yet is considerably stronger, explained Luciano De Oto, Head of the ACRC.
Thanks to an innovative forging process, forged composite can be formed very efficiently and to the highest levels of precision. Lamborghini’s partner in forged composite, Callaway Golf, uses the technology for club heads.
An alloy insert into CFRP has also been achieved without the risk of corrosion, said De Oto.
Super stiffness for a supercar
Achieving ultimate practical torsional stiffness has been a design driver for the Aventador, which is more than 150% better in that respect than the Murcielago that it replaces. Weight of the whole monocoque is 147.5 kg (325 lb), and that plus front and rear aluminum frames scales 229.5 kg (506 lb) and achieves a torsional stiffness of 35,000 N·m/° (25,815 lb·ft/°).
This has given major gains in crash performance, particularly with regard to side impacts, to meet U.S. legislative requirements. The floor of the tub does not just carry the seats and support the gearbox holder but also absorbs energy. Braiding is produced via RTM.
The carbon-fiber weave technology is based on textile industry experience and is applied to the production of tubular components for specific areas of the monocoque, including structural roof pillars and rocker panels. The woven components are created by diagonally weaving the fiber in several layers.
Epoxy foam components are used as spacers between composite layers in the Aventador’s monocoque, adding stiffness and damping NVH. Aluminum inserts are laminated into the front and rear monocoque surfaces to more easily facilitate connection with the car’s aluminum front and rear subframes.
Lamborghini’s technology cross-linking with Audi is strong, benefiting both companies. Its most important base material alongside carbon fiber is aluminum, so it benefits from the leading lightweight-engineering expertise possessed by the Audi brand, Winkelmann stresses.
The new Audi A6 is an example of the company’s use of aluminum in the areas and amounts it deems necessary to achieve its targets. Depending on engine, the new A6 consumes up to 21% less fuel than its predecessor (it also handles better) with lightweight construction accounting for about a fifth of that figure. Generally speaking, Audi’s figures indicate that taking 100 kg (220 lb) out of a vehicle reduces fuel consumption from between 0.3 and 0.5 L/100 km.
For Audi, aluminum is just a starting point—albeit the major one—for vehicle dieting. As weight diminishes, more compact brakes, a smaller engine, and lighter exhaust system can be used; and as fuel consumption reduces, the fuel tank becomes smaller and lighter, particularly when full. Lightweight gas springs can support a lightweight rear hatch or hood. And a lighter vehicle means less kinetic energy is generated, which brings benefits in deformation crash performance.
New alloys understood to be in development by Audi may have up to 70% more strength than those in production use today. CFRP sandwich applications are also in prospect for wider use; Audi has 50 specialists working in its FRP Technical Center.
But as both Lamborghini and Audi demonstrate, as well as bringing fuel consumption and emissions benefits, light weight is also about improved performance and handling. Audi made the point by inviting AEI to track-test the one-off aluminum and CFRP (doors, hood, trunk lid) A5 technology demonstrator for the Audi quattro show car seen at Paris last year and a potential successor to the 1984 Audi Sport quattro.
The aluminum body is hand-built based on ASF (Audi Space Frame) technology. The car has a five-cylinder, 2.5-L TFSI turbo engine producing 300 kW (402 hp) and weighs 1400 kg (3086 lb). Take off another 100 kg if the concept becomes a production reality, which looks possible.
The car felt nimble and in some respects like a responsive sports coupe of a couple of decades ago but with modern chassis capability.
Audi also provided a TTS coupe technology platform, which in general terms points to the next generation of the car. It has an 8.2-kg (18-lb) CFRP hood, which has demonstrated satisfactory pedestrian safety performance (it cracked to provide necessary energy absorption), and seats (32-kg [71-lb] reduction for the two) that, together with other components, save more than 80 kg (176 lb), again adding dynamic pluses to fuel and emissions savings.
Fiber-reinforced plastic (aramid and/or carbon) is a miracle weapon, reckon the Audi researchers, but there will not be total reliance on carbon. Eventually, Audi specialists believe that against comparable current models, weight could be honed by some 40%—but cost:benefit ratios may dictate a different story until convincing economies of scale are achieved.
Joining technology is also part of the general weight-saving equation at Audi. Friction element welding has been developed although it is not yet in use, and flow drill screw technology can penetrate FRP and aluminum mixed-material applications without causing delaminations.
At Audi and Lamborghini, heavyweight expertise and R&D programs are being applied to achieving lightweight solutions that are set to eventually change the way cars will be designed and built. And the question, “What’ll it do flat out?” may soon become, simply, “What does it weigh?”