Audi AGIngolstadt, Germany

Audi A8 aluminum luxury sedan

Doug Bartholomew, Samuel Greengard, Glenn Hasek, John Jesitus, Scott Leibs, Kristin Ohlson, Robert Patton, Barb Schmitz, Tim Stevens, and John Teresko contributed to this article. In automotive history, innovation tends to proceed incrementally and rarely does technology -- especially materials technology -- radically influence the way cars are made and perform. The 1997 Audi A8 with its aluminum space-frame body technology indicates what is possible when the status quo in materials is challenged in automotive design. Introduced in the U.S. as a 1997 model, the luxury sedan delivers a new standard in weight savings, structural integrity, safety, performance, and comfort. For example, the Audi A8 achieves a 40% reduction in weight and a 40% increase in rigidity. In addition, the energy-absorbing characteristics of aluminum plus the lighter weight of the vehicle greatly increase collision safety for both the occupants of the Audi A8 as well as for the occupants of the other vehicle. Acting as a safety cell, the aluminum space frame's structural members absorb energy better in relation to their weight than steel. Less weight also translates into lower kinetic energy levels, so a collision in an Audi A8 would in theory involve less deformation. Under collision loads, the long members in the front and rear are designed to crumple in accordion-like fashion, absorbing tremendous energy, and sparing the remaining central structure the full force and deceleration of the impact. The rigid space frame also enhances comfort by suppressing low-frequency vibrations. To achieve those benefits, the designers did much more than simply substitute one material for another. Instead, they sought to optimize the structural potential of aluminum. The result is a space-frame type of design in which every component surface is integrated as a structural entity. It is formed of mainly box-sectioned, straight and curved extrusions that are usually joined to complex die-cast components at the highly stressed corners and connection points, says Heinrich Timm, manager of fundamental concepts at Audi AG in Ingolstadt. Aluminum also posed a challenge to manufacturing, notes Klaus Gugisch, manager of manufacturing planning. "Completely new production techniques are required since the aluminum body structure consists of extrusions and die castings, while sheet aluminum is used for the complete outer skin and various interior parts such as the floor, the rear partition panel, the plenum chamber, and wheel housings." The aluminum-alloy outer panels are said to be even more dent-resistant than steel. Audi's pursuit of the advantages of an aluminum space frame began as an early 1980s R&D initiative that became a joint-venture development program with Aluminum Co. of America (Alcoa). The space frame took 10 years to develop and is the result of 40 new patents, seven new aircraft-grade aluminum alloys, and extensive design analysis via supercomputers. Audi sources parts and subassemblies from Alcoa's plant in Soest, Germany. The design effort translates into a full-size luxury vehicle with curb weights of 3,682 lb with the 3.7-liter engine and 3,902 lb for the 4.2-liter version. Audi says that compares with the Mercedes Benz S-Class (S320 SWB) at 4,480 lb and S420 at 4,650 lb. Both Audi engines are 32-valve, aluminum V-8s of double-overhead-cam design. The 4.2-liter engine produces 300 hp at 6,000 rpm with 295 ft lb of torque at just 3,000 rpm and can zoom from zero to 60 in just 6.9 seconds. The 3.7-liter V-8 produces 230 hp at 3,500 rpm with a maximum torque of 235 ft lb at 2,700 rpm. And to dramatize how the strength and lightness of the aluminum space frame affects the engines, Audi makes this comparison: "The A8 3.7 propels only 16 lb with each available horsepower compared to the Mercedes-Benz S320 and S420 or Jaguar XJ6 that have a ratio of 19.6, 16.9, and 16.6 lb/hp, respectively." What does the A8 portend for the future of automaking? Alcoa's Pete Bridenbaugh, executive vice president-automotive, observes a steady pressure to build lightweight cars utilizing aluminum-space-frame technology. "Another one that Alcoa is involved with will be introduced next year." He says more is involved than weight, fuel economy, or environmental benefits. "It is a desire for more usable volume -- bigger cars and minivans -- and more space for the same weight, as well as safety. Our reduced-cost program is aimed at learning how to build high-volume cars with aluminum -- and not just in the luxury class."

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