The aluminum body of the new Audi A8 L is big, strong, sturdy and amazingly light. Its weight of 241 kilograms (531.31 lb) is a central factor for the sedan’s high dynamics and exemplary efficiency. Including its standard quattro all-wheel drive system, the A8 L 3.0 TFSI (minus driver) weighs in at only 1,880 kilograms (4,144.69 lb), much less than its competitors. Audi, the lightweight design pioneer of the automotive industry, once again demonstrates its leadership role.
The Audi Space Frame (ASF) technology results in a roughly 40 percent weight advantage over a comparable steel body. The layout of the ASF body follows bionic principles. 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 in a skeleton, all of the components combine optimal function with low weight. The material is used only where necessary and always in a tailored configuration. Audi uses 13 different grades of aluminum in the new A8 L.
Castings are made primarily from advanced alloys. They are used wherever high forces are induced locally and there is a need for versatility and design freedom. The A-pillar node, for example, 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 are manufactured using the very high-precision vacuum casting process.
The extruded sections also stand out with their design flexibility, with each one optimized precisely for its intended purpose. The roof arch of the new A8 L, for instance, is hydroformed. Its cross-section changes multiple times over its length, with smooth transitions.
Audi has increased the strength of the higher-strength body components in the A8 L 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 panels 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 most recent stage in the evolution of the ASF principle is composite construction with steel to further improve crash safety. The B-pillars of the A8 L are mode of hot-shaped steel. The blanks are heated in a pass-through furnace to roughly 900 degrees Celsius during production and fed into the hydraulic press directly thereafter.
Cold water flows through cooling tubes cast into the die of the press, cooling the panel to approximately 200 degrees. This produces a structure with 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.
Innovative processes: body assembly
With the ASF skeleton of the A8 L, the steel B-pillars cannot simply be welded into the aluminum body. Self-tapping screws are used for a secure hold and the utmost precision. The flow drill screws (FDS) are installed by a high-speed robot working with high axial forces. The screws melt the material slightly and tap their own threads. A structural adhesive further improves the strength of the joint.
The hot-shaped B-pillars make up 9 percent of the weight of the A8 L body’s material matrix, with most of the rest being aluminum – 35 percent panels, 34 percent cast components and 22 percent extruded sections. The joining of these wrought components and the individual parts comprising the body of the A8 L is a high-tech process. Audi uses a wide variety of joining techniques – punch riveting, self-tapping screws, spot welding, MIG welding (MIG = metal inert gas), laser welding, combined bonding and spot welding.
Laser welding is an 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 aluminum invisible seam between the roof and the side of the car. This is an area at the focus of all of Audi's precision during the design and body assembly processes — the maximum tolerance for the outer skin is only 0.1 millimeters (0.004 in). The A8 L has no roof trim strips or visible joints.
The development engineers have also trimmed every unnecessary gram of weight from the add-on components. The lower crossmember of the front end, for instance, is made of a novel matrix of fiber-reinforced plastic reinforced by three embedded aluminum panels. Weighing just 5.2 kilograms (11.46 lb), it is significantly lighter than a comparable solution of solid aluminum. Even the doors are particularly lightweight, thanks to a new, fully integrated concept for incorporating the window frames.
That peaceful Audi feeling: vibrational comfort
The ASF body is not only lightweight, it is also very rigid. Its static torsional stiffness has increased by 25 percent versus the predecessor, which was already a leader in the field. Dynamic torsional rigidity of improved by 15 percent, while lightweight quality – the relationship between weight, torsional stiffness and size – improved by 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 L 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 rearview mirror. Considerable attention was focused on the sound radiation of the large sheet metal panels and on local rigidities. All points at which force inputs take place when the car is in motion were designed specifically to meet requirements.
The front axle subframe, for instance, distributes excitation from the wheels through a framework structure of members and sections, thus significantly reducing tire noise in important zones. Acoustic bulkheads, insulation and foam layers prevent the large cavities of the body from vibrating, and fine seam seals close joining sites and thus seal out airborne sound. The cabin is also carefully insulated; a new, lightweight microfiber fleece plays an important role here.
Slicing through the wind: the aerodynamics
The new A8 L glides calmly, sleekly and quietly through the wind. The drag coefficient of the 3.0 TFSI quattro is just 0.26, with the frontal area measuring 2.41 m2 (25.94 sq ft). Low lift coefficients at the front and rear axles ensure confident stability at highway speeds.
The flow of air through the engine compartment also had a high priority in the requirements specification. The area around the single-frame grille is thoroughly sealed so that the inflowing air reaches the radiator with almost no losses instead of becoming turbulent. Another major area of work was the underbody, the wheels and the wheel wells, an area in which a car produces 40 to 50 percent of its total air resistance.
The nearly full underbody panel developed by Audi leaves only the exhaust system and the rear axle exposed. A NACA vent directs the slipstream to the transmission fluid cooler. At the rear of the car, a cover plate and the mufflers form an upwardly angled diffuser surface. Fine tuning of the underbody improved the drag coefficient by somewhat more than 0.03 – 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.
Fine-tuning in the wind tunnel greatly enhances the efficiency of the new Audi A8 L. 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 0.15 liters of fuel per 100 km – a good 4 g of CO2/km (6.44 g/mile). The effect is much greater in everyday driving. At an average highway cruising speed of 130 km/h (80.78 mph), it is good for a saving of 1.3 liters per 100 km.
The Audi A8 L is also at the head of its class in wind noise comfort. The Audi engineers came up with numerous solutions in the details to reduce the noise of the slipstream. 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 standard windshield of the new A8 L also offers acoustic advantages. It is made of composite safety glass, and a special intermediate film provides further insulation against noise. If desired, Audi can also provide noise-insulating side windows of dual-pane acoustic glass, with tinting optional. The insulating acoustic glass (standard in the W12) has a metal vapor coating that reflects the infrared component of sunlight, thus reducing heating of the interior. This option can be combined with a heated windshield.
Good environmental balance: recycling
Audi regards the development of a new vehicle as a holistic mission, and efficiency and sustainability were also an important focus during the development of the new A8 L. During the development process, the engineers prepared an environmental balance sheet in which the ecological effects of each process step were described.
The overall energy balance sheet reveals that the new A8 L 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 recycling. The usage phase 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 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 L now comprises 38 percent ecologically produced secondary aluminum.
The equipment and data specified in this document refer to the model range offered in Germany. Subject to change without notice; errors and omissions excepted.