Big, strong, stable and yet amazingly lightweight – the aluminum body of the new A8 once again sets new standards. Its low weight of 231 kilograms (509.27 lb) is a key factor of the high driving dynamics and exemplary efficiency of the new A8. In steel, the body would weigh about 40 percent more. Including its standard quattro all-wheel drive system, the A8 4.2 FSI weighs in at only 1,835 kilograms (4,045.48 lb), well less than its competitors. Audi, the lightweight design pioneer of the automotive industry, once again demonstrates its leadership role.
Fifteen years ago, Audi introduced the technology of the Audi Space Frame (ASF) into production – with the first A8 generation. ASF has proven its merit outstandingly; and now Audi is again using it in a much more advanced form on its new model.
The structure of the ASF body follows bionic principles, i.e. it incorporates ideas from nature. Its frame comprises extruded sections and pressure diecast parts of aluminum; the aluminum panels – such as the roof panels and the side panels – are joined by friction connections. Like the bones of a human skeleton, all components combine optimal function with low weight. The material is used only where necessary and always in a tailored configuration. Audi uses 13 different aluminum alloys in the new A8.
The combined weight of all the castings, most of which are made of advanced alloys, is three kilograms (6.61 lb) less than with the previous model. A particularly large component connects the side sills and the rear longitudinal member. Its complex geometries and wall thicknesses derive from the know-how that Audi has gleaned over the years in which it has developed its large lead; its ribs are also based on bionic principles.
The vacuum diecast components are used wherever high forces are induced locally and there is a need for versatility and design freedom. The A-pillar node is one such multifunctional component. It connects the longitudinal member, the windshield crossmember, the roof frame, the strut mount and the omega bracket in front of the footwell. Most of the 25 castings in the new A8 are manufactured using the very high-precision vacuum casting process.
3.20 meters (10.50 ft) long: extruded section as roof arch
The extruded sections also stand out with their design flexibility, with each one optimized precisely for its intended purpose. A classic example is the roof arch of the new A8. It is produced by hydroforming: The section is shaped by a liquid forced into it at high pressure. Its cross-section changes multiple times with smooth transitions over the 3.20 meter (10.50 ft) length.
Audi has increased the strength of the higher-strength body components in the new A8 by as much as 25 percent, thereby reducing both material thickness and weight by up to 20 percent. This achievement is due in part to an innovative composite material for the aluminum sheets called a fusion alloy, which alone saves 6.5 kilograms (14.33 lb). 15 fusion panels are used for the load-bearing parts of the structure – the center tunnel, the cross bracings in the floor, the windshield crossmember and in the area of the rear seats.
The core layer of the new material is made of an alloy having a tensile strength of more than 250 newtons per square millimeter – five complete A8s could be suspended from a strip of this material only 13 millimeters (0.51 in) thick and 30 millimeters (1.18 in) wide. There is a cover layer on both sides, and each of the cover layers represents approximately ten percent of the total thickness. With a conventional aluminum sheet, this is roughly 0.2 millimeters (0.01 in) for both together. The cover layer helps to ensure that the panels can be shaped easily in the press despite the high-strength core.
The most recent stage in the development of the ASF principle is the composite construction with extremely high tensile strength steel to further improve passive safety. Audi uses it in the new A8 in the area of the B-pillars, which are made of hot-shaped steel. The blanks are heated in a furnace to roughly 900 degrees Celsius during production and are fed into a hydraulic press immediately thereafter. Cooling tubes through which cold water flows are cast into the die of the press, and the panel is cooled to approximately 200 degrees Celsius. The martensitic structure that this produces has an extremely high tensile strength – 1,500 Newtons per square millimeter in the upper section of the pillars; somewhat lower in the lower section because this is where most of the energy is dissipated in the event of a side impact collision.
Many new methods: body construction
With the ASF skeleton of the A8, the steel B-pillars cannot simply be welded into the aluminum body. The two materials exhibit different degrees of expansion when heated, plus there would be a risk of contact corrosion at the joints.
Instead, self-tapping screws (flow drill screws) are used for a secure hold and the utmost precision. Installed by a high-speed robot working with high axial forces, the screws melt the material lightly and tap their own threads. A structural adhesive further improves the strength of the joint while simultaneously forming a separating layer and thus solving the problem of contact erosion.
The hot-shaped B-pillars make up 8 percent of the weight of the A8 body’s material matrix, with most of the rest being aluminum. This breaks down as 35 percent panels, 35 percent castings and 22 percent extruded sections. The joining of these wrought components and the 251 individual parts comprising the body of the A8 is a high-tech process that was intensively refined for assembling the body of the A8.
Audi uses a wide variety of mechanical and thermal joining methods: 1,847 self-piercing rivets, 632 self-tapping screws, 202 weld points, 25 meters of MIG-welded seams (MIG = metal inert gas) and 6 meters of laser-welded seams hold the body together. Many of the joints are also reinforced with adhesive – there are 44 meters (144 ft) of bonded joints.
Laser welding is a special Audi domain. Because the seams exhibit high strength and rigidity, it can be used to join large panels to the structure. One area where laser welding is used is the 1.8 meter (5.91 ft) aluminum invisible seam on both sides between the roof and the side of the car. The finished seam is then smoothed with brushes. Unlike many of its competitors, the new A8 has no roof trim strips or visible joints – a major element of the elegance and feeling of quality typical of Audi.
The laser seam is an area at the focus of Audi's extraordinary precision during the design and body assembly processes – the maximum tolerance for the outer skin is only 0.1 millimeters (0.004 in).
The developers not only shaved every unnecessary gram from the ASF structure, they did the same with the add-on parts. The total weight of the front and rear bumpers and the radiator grille declined by 300 grams compared to the previous model. The lower crossmember of the front end is made of a novel matrix of fiber reinforced plastic reinforced by three aluminum panels embedded it in. Weighing just 5.4 kilograms (11.90 lb), the component weighs 84 grams (0.19 lb) than a solid aluminum solution and saves 2.3 kilograms (5.07 lb) over steel. Even the doors are particularly lightweight, thanks to a new, fully integrated concept for incorporating the window frames.
Superior acoustics: vibrational comfort
The ASF body demonstrates its superiority in every field of technology. The static torsional stiffness has increased by 25 percent compared with the predecessor, already a leader in the field; it forms the basis for the precise, dynamic handling. The dynamic torsional rigidity of the body improved 15 percent, while its lightweight quality – the relationship between weight, torsional stiffness and size – improved 20 percent.
The ASF principle ensures comfortably low levels of vibrations and impressive quietness on board for that typical Audi feeling. The development engineers for the A8 targeted and minimized all vibration levels at the contact points between the passengers and the body – the floor panel, the seats, the steering wheel and the inside mirror. They also isolated the eigen frequencies – all of which are below 40 hertz – of the large components like the axles and the engine from one another to avoid resonances.
Considerable attention was focused on the sound radiation of the large sheet metal panels and on local rigidities. All points at which force inputs taken place when the car is in motion were reinforced to the necessary extent.
The front axle subframe, for example, distributes excitation from the wheels through a framework structure of members and sections, thus significantly reducing tire noise in important zones. Special bulkheads, coverings and foam layers ensure that the large cavities of the body do not begin to vibrate, and fine seam seals close joining sites and thus seal out airborne sound. Very sophisticated insulation measures in which a new, lightweight microfiber fleece plays an important role were also applied to the interior.
Slicing through the wind: aerodynamics
The new A8 glides calmly, sleekly and quietly through the wind. The 4.2 FSI version has a drag coefficient of 0.26, with the frontal area measuring 2.41 m2 (25.94 square ft). Laborious fine tuning reduced the drag coefficient compared to the previous model sufficiently to offset the increase in size. Low lift coefficients at the front and rear axles ensure confident stability at highway speeds.
A key portion of the development work took place in Audi’s aeroacoustics wind tunnel. It is a high-tech workplace. Its rotors measure five meters (16.40 ft) in diameter and accelerate the wind to 300 km/h (186.41 mph). Details such as the form of the exterior mirror can be traced back to here: A groove on the top of the housing and a deflector edge on the bottom ensure good airflow around the mirror and repel water drops so that the mirror stays largely clean when driving in the rain.
Another focal point was the flow of air through the engine compartment. The area around the grille is thoroughly sealed so that the inflowing air reaches the radiator with virtually no losses instead of becoming turbulent. Another major area of work was the underbody, the wheels and the wheel wells. A car produces 40 to 50 percent of its total air resistance in this area. The aeroacoustic wind tunnel with its rolling floor and the four small treadmills on which the wheels can turn provided insightful and detailed analyses.
The development engineers opted for a nearly full underbody panel that leaves only the exhaust system and the rear axle exposed. A NACA vent directs cooling air from the slipstream to the transmission oil cooler. At the rear of the car, a cover plate and the mufflers form a diffuser surface that rises as it extends rearward.
Fine tuning of the underbody improved the drag coefficient by somewhat more than 0.03, which corresponds to 13 percent of the total drag. The plastic underfloor panels also protect the sheet metal of the bodyshell and the mechanical assemblies against salt, moisture and stone impact, and eliminate the need for a conventional PVC underbody seal.
Quiet at any speed: wind noise comfort
The luxury sedan also once again leads its class in wind noise comfort. At a cruising speed of 120 km/h (74.56 mph), the noise of the slipstream is already the loudest sound source in the cabin. The Audi engineers adopted numerous detailed solutions to reduce it. A water-catching strip with a rounded leading edge, a complex door seal concept with three sealing lines and an acoustically highly comfortable sunroof all work together to reduce wind noise. The sunroof’s wind deflector extends in two stages as a factor of driving speed to suppress whooshing noises and booming attributable to dynamic pressure.
The windshield of the new A8 also comes standard with special acoustic properties. It is made of composite safety glass, and a special intermediate film provides further insulation against noise. Noise-insulating side windows of dual-pane acoustic glass are available as an option. These reduce the interior noise level in the wind noise frequency range by up to 6 dB(A). The insulating acoustic glass (option) has an additional metal vapor coating that reflects the infrared component of sunlight, thus reducing heating of the interior.
The time-consuming fine-tuning in the wind tunnel greatly enhanced the efficiency of the new Audi A8. Compared to the first draft design, the aerodynamics engineers were able to reduce the drag coefficient by 0.05. In the EU driving cycle, this improvement saves nearly 0.2 liters of fuel per 100 km, which corresponds to a good 4 g of CO2/km (6.44 g/mile). The effect is much more pronounced in everyday driving. At an average highway cruising speed of 130 km/h (80.78 mph), it is good for savings of 1.3 liters per 100 km.
Audi regards the development of a new vehicle as a holistic mission, and efficiency and sustainability were also a central focus during the development of the new A8. One tool used by the engineers was the environmental balance sheet, in which the ecological effects of each process step were quantified in detail.
The overall energy balance sheet reveals that the new A8 achieves better results than its predecessor for the criteria greenhouse effect, summer smog and CO2 emissions. This balance sheet covers the entire life of the vehicle, from production of the materials to the recycling of the car. The critical segment is the usage phase, which accounts for 70 percent of the total CO2. It is primarily the new engines and the technologies from the modular efficiency platform that have a positive effect here.
The lightweight aluminum used in the body is also far greener than steel with respect to CO2. Although it consumes more energy during primary production, its major weight advantage offsets this disadvantage after only 50,000 kilometers (31,068.56 miles) of driving. When the vehicle reaches the end of its life, all of the aluminum components can be recycled using just a small amount of energy. The body of the new A8 now comprises 38 percent ecologically produced secondary aluminum.
The equipment, data and prices specified in this document refer to the model range offered in Germany. Subject to change without notice; errors and omissions excepted.