Throughout its model range, Audi offers a very wide variety of car concepts – and quattro technology is equally multifaceted.
Modular longitudinal platform: the self-locking center differential
In Audi models with a longitudinally mounted front engine, the heart of the quattro drivetrain is a self-locking center differential. It is configured as a purely mechanical planetary gear with no time lag. A hollow wheel surrounds a sun wheel. Between the two, cylindrical planetary wheels turn, which are connected to the rotating housing. In regular driving operation, 60 percent of the drive torque flows to the rear axle via the hollow wheel, which has a larger diameter, and its associated output shaft. 40 percent goes to the front axle via the smaller sun wheel. This asymmetrical, dynamic torque distribution results in sporty handling that emphasizes rear propulsion. Oblique splines produce axial forces immediately when torque is transfered by the differential. These forces act on friction discs to produce a locking torque that diverts the torque to the wheels with the better traction.
In its latest form, the center differential can divert 70 percent of the torque to the front, or up to 85 percent to the rear. The high locking values enable a clearly defined torque distribution and highly precise interaction with control systems such as the ESC and wheel-selective torque control. For even greater dynamics and driving safety, top-of-the-line Audi models based on the modular longitudinal platform can also be equipped with a sport differential. This technology is currently used in the following model series:
Modular transverse platform:the electro-hydraulic multi-plate clutch
For compact models with a transverse-mounted engine, Audi uses a quattro drivetrain featuring an electronically controlled multi-plate clutch with hydraulic actuator. It is mounted at the end of the propshaft before the rear differential to optimize weight distribution. Inside the clutch is a package of metal friction rings mounted in pairs one behind the other. One ring of each pair is permanently meshed with the clutch basket, which rotates with the propshaft. The other ring of each pair is connected to the shaft leading to the rear differential.
The all-wheel drive software constantly calculates the right torque distribution between the front and rear axles based on numerous data. When there is demand for torque, the highly efficient electric axial piston pump builds up as much as 40 bar of hydraulic pressure within just a few milliseconds. It presses the friction plates together, which enables variable transmission of the drive torque to the rear axle. The electronically controlled multi-plate clutch guarantees the best possible traction, driving dynamics and driving safety while delivering dynamic handling with actively controlled torque distribution.This technology is currently used in the following model series:
In the Audi TT, Audi S1, Audi S3, Audi RS Q3 and Audi RS 3, management of the multi-plate clutch is configured with a focus on dynamics. Its special control philosophy puts considerable emphasis on parameters relevant to driving dynamics; at the same time it is based on the Audi drive select mode that is active (TT, S3 and RS3) and electronic stabilization control (ESC). This results in more frequent and pronounced torque distribution favoring the rear axle, which further adds to sporty handling. In sport mode, or when ESC is disabled, it permits controlled drifts on road surfaces with low friction coefficients. In a more relaxed mode of driving, on the other hand, it is possible to fully disengage the clutch temporarily in the TT to save fuel. As soon as the driving state changes, the quattro drive is reactivated.
Audi R8: Actively controlled front differential
In the Audi R8, the seven-speed S tronic with its integrated rear differential and limited-slip differential is located behind the engine. The electro-hydraulic multi-plate clutch integrated in the front differential transmits torque to the front wheels via a propshaft within just a few milliseconds. It can distribute torque between the two axles fully variably. The combination of a high-performance transmission mechanism and all-wheel drive software tailored to a mid-engine sports car enables driving dynamics that attain an unprecedented balance between stability and agility.
The all-wheel drive software continuously computes the ideal distribution of torque to the axles as a function of the driving situation, driver inputs and ambient conditions. Up to 100 percent of the power can flow to the front or rear axle, for instance. quattro management is integrated into the Audi drive select driving dynamics system which offers the four modes comfort, auto, dynamic and individual. In the performance mode, it also offers programs for dry, wet and snow. These programs adapt driving dynamics parameters to the specific grip conditions of the road surface.If grip is decreasing at the rear wheels, such as on a road surface with a low friction coefficient, an electrically driven axial piston pump presses the clutch plates inside the clutch together at a pressure of up to 38 bar; these plates are made of steel and are coated with an organic material. The higher the oil pressure, the greater the force that is redirected variably from the rear to the front axle. To assure quick pressure build-up, the pump continually circulates a fixed volume of oil. The multi-plate clutch is connected to the engine’s coolant circulation system, which utilizes three large radiators at the front of the car. This solution ensures that the clutch can perform its control tasks fully and precisely under all conditions. A temperature sensor and a pressure sensor continually monitor conditions and adapt control parameters as necessary.The mechanical rear differential further improves traction and driving dynamics. It has a locking effect of 25 percent during engine traction operation and 45 percent during engine overrun. It is precisely tuned for the dynamic character of the Audi R8 and the actively controlled quattro drive. This results in turn-in behavior that practically eliminates understeer.