December 10, 2012

RS5 Cabriolet - Drivetrain
Source: Audi Media

The seven-speed S tronic makes an important contribution to the efficiency of the Audi RS 5 Cabriolet – its long seventh gear is designed to improve fuel economy, while its six lower gears feature sporty gear ratios. Engineers made specific modifications to the dual-clutch transmission compared to the standard production model, focusing on such areas as the transmission fluid supply and clutches.

The seven-speed S tronic consists of two subunits, and two multi-plate clutches control the gears. The large K1 clutch located on the outside transfers torque via a solid shaft to the gear wheels for 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 inside the K1 clutch, and which controls gear wheels for gears 2, 4 and 6, as well as reverse.

Both transmission subunits are continuously active, but only one is connected to the engine at any given time. For example, when the driver accelerates in third gear, the fourth gear is already engaged in the second transmission subunit. The shifting process involves switching the clutches – K1 opens and K2 closes. This takes just a few hundredths of a second with virtually no loss of tractive force; the process is dynamic, smooth and comfortable.

The seven-speed S tronic in the Audi RS 5 Cabriolet can be used in a variety of modes. Fully automatic modes include D (Drive) and S (Sport). The mode selected in Audi drive select is also integrated in the management. If the driver shifts manually using the selector lever or the paddles behind the steering wheel, the operating mode is extremely sporty – when accelerating, the S tronic no longer upshifts by itself. In dynamic mode in Audi drive select the downshift is accompanied with a dose of double-declutching.

State-of-the-art technology: the quattro drivetrain
The Audi RS 5 Cabriolet features the very latest version of the quattro drivetrain – with the self-locking crown-gear center differential and torque vectoring. At its heart lie two crown gears which rotate inside the differential; they owe their name to the crown-like design of their teeth. The front crown gear drives the output shaft to the front differential, the rear crown gear drives the propshaft to the rear axle. The two crown gears intermesh with four rotatable pinion gears, which are arranged at right angles to each other.

Under normal driving conditions, the crown gears rotate at the same speed as the differential housing, i.e. the transmission output shaft. Because of their special geometry, they are designed to have 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, this leads to different speeds and axial forces inside the differential and the integrated sets of plates are pressed together. The resulting self-locking effect diverts the majority of the torque to the axle with the better traction; up to 85 percent can flow to the rear axle. If the rear axle has less traction, up to 70 percent of the torque is diverted to the front axle.

With this extremely broad torque distribution range, the crown-gear center differential surpasses its predecessors. Forces are redistributed virtually without any time lag and absolutely consistently; the entirely mechanical operating principle guarantees maximum efficiency and instantaneous response. Other advantages of the technology are its compactness and low weight of 4.8 kilograms (10.58 lb).

Intelligent software solution: torque vectoring
The crown-gear differential in the Audi RS 5 Cabriolet works together with an intelligent software solution in the brake management system – the torque vectoring system, which acts on all four wheels. If during fast cornering the control unit detects that the wheels on the inside of the curve, which are under a reduced load, are about to slip, it brakes these wheels slightly - all that it takes is minimal pad pressure on a brake disc.

Torque vectoring works smoothly and continuously. The RS 5 Cabriolet maintains neutral handling for a long time even at performance limits. If the ESC stabilization control still needs to intervene, it does so later and gently.

As a complement to the quattro drivetrain, the optional sport differential is available, which actively distributes torque between the rear wheels. When turning into or accelerating in a curve, the majority of the torque flows to the outside wheel and pushes the four-seat convertible into the curve, counteracting the tendency to oversteer or understeer early.

With the sport differential, a superposition gear is added to each side of the classic rear differential; it comprises two sun gears and an internal gear which turns ten percent faster than the drive shaft. A multi-plate clutch in an oil bath and operated by an electrohydraulic actuator makes the power connection between the shaft and the superposition gear.

When the clutch engages, it steplessly imposes the higher speed of the superposition stage on the outside wheel. The additional torque required is obtained from the inside wheel via the differential. In this process, 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 works just as efficiently in overrun as it does under load and when coasting; it is electronically controlled with response times of just a few hundredths of a second. The controller calculates 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.

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