Audi has unveiled a new version of its quattro four-wheel drive system that it claims provides a critical improvement in efficiency.
The new quattro system uses an electro-mechanically operated multi-plate clutch and rear gear set that combine to eliminate mechanical drag by completely cutting drive to the rear differential, and with it the rear wheels, when the driver only requires front-wheel drive.
To be marketed under the name quattro ultra, the new system abandons the widely used Torsen torque sensing system, as well as the central differential and rear differential hardware used by Audi in one form or the other since the inception of its original quattro four-wheel drive system back in 1980.
In its place is a fully networked set-up capable of analysing the driving conditions every 100 milliseconds and automatically providing either front- or four-wheel drive depending on factors such as longitudinal acceleration, engine torque, steering angle, yaw rate, prevailing traction and driving style.
Set to make its debut on the upcoming A4 Allroad prior to being fitted to the new A5 coupé and second-generation Q5, the contemporary new quattro ultra system will initially be offered on models based around Audi’s MLB (modularen langsbau – modular longitudinal architecture) platform in combination with its new high compression turbocharged 2.0-litre four-cylinder petrol engine and seven-speed S-tronic dual clutch gearbox.
Unlike the old mechanical Torsen-based quattro system, which based its operation principally around existing traction levels, the new electro-mechanical system is influenced by a much wider range of information obtained by various sensors, as well as data relating to the road provided by the navigation system.
By combining this information, Audi says its new quattro ultra system can provide predictive operation by constantly sensing whether the driver requires front- or four-wheel drive and adapting the apportioning of drive to suit. In doing so, the new system is able to run in a more efficient state than the Torsen arrangement, which will continue to be used on more powerful Audi models, including upcoming RS models from the German car maker’s Quattro division.
Mechanical developments brought to the latest incarnation of the quattro four-wheel drive system are similar those used on the four-wheel drive versions of Audi’s transverse engine models, including the A1, A3, Q3 and TT.
They include a new electro-mechanically operated multi-plate clutch mounted on the end of Audi’s seven-speed S-Tronic dual clutch gearbox. Depending on the model, it consists of between five and seven pairs of clutch plates that rotate in an oil bath. When the plates are pressed together, four-wheel drive is activated. The contact pressure between each plate is used to distribute drive between the front and rear axles.
The front multi plate clutch works in combination with a newly developed dog gear style rear differential featuring a decoupler operated via a so-called claw clutch. When the decoupler is closed, the propshaft and differential are operated to provide drive to the rear wheels and thus provide four-wheel drive.
When running in front-wheel drive, the front multi-plate clutch disconnects the propshaft. At the same time, a decoupling device within the rear differential opens to reduce friction and associated drag losses.
Unlike the process used by the existing Torsen torque sensing quattro system, the only rear-wheel drive components engaged when running in front wheel drive are the bevel gears within the axle and gears within the differential – the latter of which compensate for differences in the speed of rotation of the wheels during cornering.
A passive torque vectoring effect is provided to the rear wheels via selective braking activated by sensors in the ABS anti-lock brake and ESC (electric stability control) systems.
The resulting reduction in mechanical drag along with revisions to the existing S-tronic dual clutch gear box are said to provide prototype versions of the A4 2.0 TFSI quattro ultra with fuel savings of up to 0.3l/100km in everyday driving conditions.
To activate four-wheel drive, the multi-plate clutch closes to operate the propshaft. In doing so, it accelerates the gears in the rear differential. When they are running at a necessary speed, the claw clutch closes via pretensioned springs to provide drive to the rear wheels, at point which an electromagnetically actuated pin disengages a locking lever.
All up, the quattro ultra system is claimed to weigh four kilograms less than the existing Torsen based quattro system.