The new Audi A8 offers the utmost superiority and the latest technology in power transmission. All three engine versions come standard with a new, eight-speed tiptronic electronically controlled with an innovative selector lever and quattro permanent all-wheel drive. The dynamic sport differential is available from Audi as an option, and is standard with the 4.2 TDI quattro.
The eight-speed tiptronic is a torque converter transmission that has been newly developed from the ground up. Its key strength lies in the high overall gear ratio of 7.0:1 between the shortest and longest gear ratio. The A8 accelerates dynamically when starting off; it rolls along the highway with low, consumption-reducing engine speeds in the two highest gears.
The new transmission reduces fuel consumption by roughly 6 percent over the six-speed tiptronic in the previous model. Its eight speeds keep the jumps in engine speed between gears low, and the engine always runs in the vicinity of its ideal operating point. Gear changes are very gentle, almost imperceptible to the driver. They are snappy, fast, and very flexible. It is even possible to perform a kickdown by shifting directly from eight gear into fourth.
Thanks to the novel design of the gears and switching elements of the eight-speed tiptronic, it requires less space for installation than the unit used with the top two engines in the previous model and has fewer distinct individual parts. Like in many Audi models, the differential is installed in front of the torque converter. This shifts the front axle far forward, bringing the advantages of a long wheelbase and a well balanced weight distribution.
The newly developed, hydraulically damped center differential contributes to the high level of vibrational comfort on board the new A8. Its back end rests against the tunnel crossmember. It is made of ultra lightweight magnesium with the V8 TDI and aluminum with the other two engines. The front of the tiptronic and also the engine are connected to the front axle frame by means of two hydraulic bearings. Thanks to a special circuit, the damping of the four bearings is particularly soft when at idle.
Greater efficiency: the transmission heating system
The 8-speed tiptronic achieves a high level of efficiency. One of the reasons for this success is the highly efficient vane pump for supplying oil, another is the heating of the oil following a cold start. Once the engine has heated enough cooling water, a valve in the loop opens and the hot water flows on to the transmission oil heater, which reduces the friction losses due to cold oil.
The torque converter lockup clutch and the integrated damper also play an important role in the efficiency concert. The lockup clutch is closed during normal driving, connecting the transmission directly to the engine. It works with limited slip in certain situations, which in conjunction with the damper precisely tuned to the engine permits very low engine speeds without vibrations.
When the A8 comes to a stop, an internal clutch separates the transmission from the engine, even if D is still selected. The torque converter therefore is not working against a stationary turbine which is more efficient with respect to energy. The clutch reengages as soon as the driver releases the brakes.
Slightly different variants of the eight-speed tiptronic are available depending on the engine. When used with the 3.0 TDI, it also includes a small, permanently full hydraulic accumulator on board to ensure start-stop functionality. When first started, the oil that it contains – roughly 100 milliliters – is forced into the actuating elements required for startup by a spring tensioned piston. The transmission is operational within a very short time, much faster would be possible without the accumulator.
The DSP dynamic shift program that controls the transmission features a new, faster processor and is located in a small steel box inside the control unit for the tiptronic. In mode D, the DSP prefers high gears and low engine speeds in the interest of efficient driving; in the sporty S mode, it stays in the gears longer during acceleration.
The intelligent networking of the transmission control unit is a pioneering step. The DSP is always aware of the precise route because it receives a continuous stream of data from the optional MMI navigation plus. It thus incorporates upcoming curves in its gearshift strategy – it avoids unnecessary gear changes on winding roads, for example.
The redesigned selector lever on the center tunnel is used to choose the driving modes (P, R, N, D, and S). Shaped like the thrust lever on a yacht, it is one of the highlights of the sedan's elegant interior. Its grip can be optionally covered with leather or wood. Black high-gloss paint and narrow trim strips set fine accents. On the left of the lever is an LED indicator for the driving program, plus the shift pattern appears on the multifunction instrument when the driver presses the release button on the lever.
The lever communicates with the transmission electronically (shift-by-wire); the previous mechanical connection has been eliminated. The driving programs are selected by a brief tap of the lever forward or back, after which it automatically returns to its center position. The lever moves smoothly and precisely. This enables the proven operating logic of the driving modes P, R, N and D and thus the intuitive operation to be retained, while the short shift travel together with the concise increments set an additional sporty accent.
The transmission automatically sets the parking lock when the engine is shut off, relieving the driver of yet another small bit of work. Gears can also be shifted manually using the standard paddles on the steering wheel.
Permanent advantage: quattro all-wheel drive
quattro permanent all-wheel drive is a major reason for the head start that the A8 enjoys over its competitors. It provides for reliable traction and directional control, and lays the foundation for dynamic handling and uncompromising road stability under all weather conditions.
The heart of the quattro drivetrain is the mechanical center differential that reacts virtually instantly and distributes the power to both axles. With the asymmetric/dynamic standard distribution, 60 percent flows to the rear differential and 40 percent to the front differential – a sporty characteristic. The mechanical center differential senses the torque, and instantly and automatically transfers most of the power to the axle with the better traction as needed. Up to 80 percent can be directed to the rear wheels and up to 60 percent to the front wheels.
The principle of lightweight construction was also applied to the cardan shaft. It is connected to the center differential by latching into a clamp connection. This solution saves 1.3 kilograms (2.87 lb) of weight compared to the conventional flanged joints while also providing for the smooth transmission of torque and high rigidity. An auxiliary shaft runs from the front end of the center differential and along the right side of the eight-speed tiptronic to the front differential. It is inclined slightly and therefore has a very sophisticated system of teeth. Audi has also reduced the weight and internal friction of the cardan shafts for the front axle.
Cornering as if on rails: the sport differential
The housing of the rear differential and the frame that connects it to the rear axle subframe are cast from aluminum. The innovative sport differential available as an option from Audi distributes the drive power in continuously variable proportions between the rear wheels.
It is standard with the 4.2 TDI quattro and has been specially reinforced for the high available torque.
With the active sport differential, Audi elevates the driving dynamics and traction of the quattro permanent all-wheel drive system to a level previously unheard of in the luxury class. The vehicle remains sportily neutral when at higher speeds, the vehicle reacts more directly to steering input and the vehicle is stabilized in the event of load changes. This not only makes the car more fun to drive, it also allows greater lateral acceleration in curves and substantially reduces the driver’s steering effort.
The foundation for this torque distribution is a high-tech rear-axle differential. The conventional rear differential was expanded to include a superposition unit comprising two internal gear stages mounted on the left and the right.
An electrohydraulic actuator operates a multi-plate clutch running in the oil sump which interconnects the internal gears. When the clutch is engaged, the driven wheel is accelerated a maximum of 10 percent by the superposition unit and thus receives an additional torque. This is drawn from the opposite wheel via the differential. In this way nearly all of the torque can be directed to one wheel. The maximum difference between the wheels is 1,800 Nm (1,327.61 lb-ft).
When turning into or accelerating in a curve, it directs the majority of the torque to the outside wheel, pushing the new A8 into the curve. The system nips any tendency toward oversteer or understeer in the bud. With a short reaction time of less than 100 milliseconds, it is even faster than the ESP stabilization program; it reacts just as effectively while accelerating as it does when coasting and is even active at zero load.
The secret to the effectiveness and fast reaction speed of the sport differential lies in the carefully matched interaction between the software, the electronics and the mechanical components.
The software was developed in-house for that typical driving feel. The controller quickly and constantly recalculates the ideal distribution of the forces for each driving situation as a function of the steering angle, yaw angle, lateral acceleration, speed and other information.
The sport differential is integrated into the Audi drive select system; the driver can adapt its characteristic suit his or her individual wishes. The “comfort” mode prioritizes road safety and stability. “Auto” mode represents a balanced setup, whereas in “dynamic mode,” the sport differential is particularly nimble when cornering and in transition curves.
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.