The seven-speed S tronic transmission in the new Audi RS 4 Avant harmonizes perfectly with the high-revving character of the 4.2 FSI and is designed for 9,000 rpm. Compared with the volume model, targeted modifications have been made to certain aspects of the dual-clutch transmission. The seventh gear is relatively long to reduce fuel consumption.
The seven-speed S tronic consists of two sub-units, whose multi-plate clutches actuate different gears. The large K1 clutch located on the outside directs the power via a solid shaft to the gear wheels for the odd-numbered gears 1, 3, 5 and 7. A hollow shaft rotates around the solid shaft. It is connected to the smaller K2 clutch, which is integrated into the inside of the K1 clutch, and which controls the gear wheels for the even-numbered gears 2, 4 and 6, as well as the reverse gear.
Both transmission sub-units are continuously active, but only one is powered at a time by the engine. For example, when the driver accelerates in third gear, the fourth gear is already engaged in the second transmission section. The shifting process takes place as the clutch changes – K1 opens and K2 closes. Shifting gears takes only a few hundredths of a second and is completed with practically no interruption of traction. It is so dynamic, fluid and smooth that it is hardly noticeable.
The driver of the new Audi RS 4 Avant can use the seven-speed S tronic in a number of different ways. The fully automatic mode offers the D (Drive) and S (Sport) programs. The management system also considers the mode selected in the Audi drive select dynamics system.
If the driver changes gears manually, either via the gear selection lever or the paddles behind the steering wheel, the seven-speed S tronic switches into a particularly sporty mode. It no longer upshifts when accelerating, for example. If the dynamic Audi drive select mode is active, the transmission management system double-clutches when downshifting.
Another specialty of the seven-speed S tronic in the RS 4 Avant is the launch control function, which enables extremely dynamic starts. The driver needs only to press the accelerator to the floor. The system manages the engagement of the clutch so that all of the power of the 4.2 FSI is delivered to the road via the quattro permanent all-wheel drive system with perfectly controlled tire slip.
Latest evolutionary stage: the quattro powertrain
In its latest evolutionary stage, the quattro powertrain features the self-locking crown gear center differential and torque vectoring. The two crown gears rotating inside the center differential get their name from their tooth geometry.
The front crown gear acts on the output shaft to the front axle differential, the rear gear on the prop shaft to the rear axle differential. A lightweight plug construction provides the connection. The crown gears mesh with four rotatable pinion gears. They are arranged at right angles to each other and are driven by the differential’s housing, i.e. by the transmission output shaft.
Under normal driving conditions, the two crown gears rotate at the same speed as the housing. Because of their special geometry, they have specifically unequal lever effects. Normally 60 percent of the engine torque goes to the rear differential and 40 percent to the front differential.
If the torques change because one axle loses grip, different speeds and axial forces occur inside the differential, and the integrated plate packages are pressed together. The resulting self-locking effect now diverts the majority of the torque to the axle with the better traction; up to 85 percent can flow to the back. If the rear axle has less traction, up to 70 percent of the torque flows to the front axle.
With this extremely broad torque distribution range, the crown-gear center differential surpasses its predecessors. Forces are redistributed without any time lag and absolutely consistently. The mechanical operating principle guarantees maximum efficiency and immediate response. Other strong points of the crown-gear differential are its compactness and low weight – at 4.8 kilograms (10.58 lb) it weighs roughly two kilograms (4.41 lb) less than the previous unit.
In the new RS 4 Avant, the crown-gear differential works together with an intelligent software solution in the brake management system so that torque vectoring acts on all four wheels. When cornering at speed, its control unit uses the driver’s steering input and the position of the gas pedal to calculate the optimal distribution of power. If it detects that the wheels on the inside of the curve, which are under a reduced load, are about to slip, it marginally brakes these wheels – just slight application of the pads on the disks at minimal pressure is all that it takes.
Torque vectoring works smoothly and continuously. The Audi RS 4 Avant remains neutral for an extremely long time at the handling limits; the slight understeer when turning into curves and when accelerating is essentially offset. The ESC stabilization control intervenes later and more gently – if it is even necessary at all.
High-tech rear differential: the sport differential
As an additional option, Audi supplements the quattro powertrain with the sport differential, which actively distributes the power between the rear wheels. When turning into or accelerating in a curve, it directs the majority of the torque to the outside wheel and pushes the RS 4 Avant into the curve, nipping the tendency to oversteer or understeer in the bud.
With the sport differential, a superposition gear comprising two sun gears and an internal gear is mounted on both sides of a conventional differential. It turns 10 percent faster than the drive shaft. A multi-plate clutch in an oil bath and operated by an electrohydraulic actuator provides the power connection between the shaft and the superposition gear.
When the clutch closes, it steplessly imposes the higher speed of the superposition stage on the outside wheel. The additional torque required is drawn away from the inside 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).
The sport differential is just as effective when coasting and rolling as it is under load, reacting electronically in just a few hundredths of a second. Audi has developed special software for use in the RS 4 Avant. The controller 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 equipment and data specified in this document refer to the model range offered in Germany. Subject to change without notice; errors and omissions excepted.