Wednesday, January 19, 2011

Audi Quattro All-wheel Drive System

Audi Quattro All-wheel Drive System

Audi introduced the quattro all-wheel drive system in a production vehicle in 1980, kicking off a long-running success story. Today Audi is the world’s leading premium-segment manufacturer of passenger cars with permanent all-wheel drive. Audi currently has four different quattro technologies in its portfolio, providing the ideal solution for each vehicle concept.

Audi Quattro
The bevel-gear center differential
The superior traction of the Iltis military off-road vehicle during winter test drives in 1977 provided the inspiration for the development of the quattro permanent all-wheel drive system. A small team under the supervision of Development Director Dr. Ferdinand Piëch designed a sleek, 2+2-seater coupe over a number of development steps. The car was intended as the homologation basis for entry into the Rally World Championship.

The Audi quattro, which debuted at the Geneva Motor Show in spring 1980, was a sensation. Its permanent all-wheel drive was lightweight, compact and fast-running, thus making it suitable for high speeds.

Like the Iltis technology, the quattro principle did not need the heavy, separate transfer case and weighty auxiliary shaft to the front axle that were the standard at the time. It was the first volume-built permanent all-wheel drive system suitable for fast and sporty cars.

The stroke of genius by Audi was a 263 millimeter (10.35 in) drilled out secondary shaft in the gearbox, through which power flowed in two directions. At the rear end, the hollow shaft drove the housing of the center differential, which was flanged directly to the shaft. Designed with a classic bevel gear layout, it sent 50 percent of the torque to the rear axle via the prop shaft in every driving situation. The other half of the power was transferred to the front axle’s differential along an output shaft rotating inside the hollow shaft.

The bevel-gear center differential eliminated the stresses in the powertrain that arise because the front wheels follow a slightly larger curve than the rear wheels when the vehicle is cornering. The differential allows them to rotate faster. This proved to be something of a handicap on slippery surfaces. The amount of power that could be transferred was limited by the axle with the lesser traction. To remedy this, the driver of the Audi quattro could manually lock both the center and the rear axle differential.

The Torsen differential
With the debut of the Audi 80 quattro in fall 1986, Audi introduced a new center differential – a component that was still strictly mechanical, but highly versatile. The name Torsen was a contraction of the English words “torque” and “sensing.” The Torsen differential had already proved itself in the world of technology as a high-tech rear axle differential; Audi developed it further for use as a center differential.

The innovative feature of the Torsen differential was its arrangement of helical gears with special teeth cut at an angle to the gear axis. The two sun gears on the ends of the drive shafts leading to the front and rear axles are each in mesh with three roller-shaped satellite gears; these are arranged in pairs in a triangle formation around the sun gears and each pair is interconnected via meshing spur gears. If the wheels of one axle cannot transmit the torque supplied, this creates friction in the helical gear train. The helical gears instantly and steplessly transfer the power to the axle with the greater traction.

The standard distribution was still 50:50, but, when needed, up to 75 percent flowed to the axle with the better traction. The Torsen differential locked only when under load. The differential was unlocked as soon as the driver took his or her foot off the gas, thus the ABS was always functional when it was needed. To further improve traction when starting under extreme conditions, the driver could still lock the rear-axle differential electro-pneumatically at the push of a button.

The hydraulic multi-plate clutch
The Torsen differential is an excellent solution for a longitudinal engine and a powertrain that runs in a straight line to the back. Audi chose a completely different technology for the transverse-mounted engines in the compact models – an electronically controlled and hydraulically actuated multi-plate clutch. It first appeared in 1998 in the TT quattro and the A3 quattro.

The clutch is located at the end of the prop shaft, in front of the rear-axle differential – an installed position that also benefits the vehicle’s axle load distribution. It normally sends the majority of the engine’s power to the front axle. The controller uses a variety of data to analyze the driving conditions and redistributes the power as required.

Inside the clutch is a package of plates that rotate in an oil bath. The metal friction rings are arranged behind one another in pairs – one ring of each pair is rigidly meshed with the housing which rotates with the prop shaft, the other ring with the output shaft to the rear axle differential. The package of plates can be forced together by controlled hydraulic pressure. As the pressure increases, more torque flows steplessly to the rear axle – up to 100 percent in some cases.

Two electric pumps are used to quickly build up the oil pressure, which can reach over 100 bar. In current A3 and TT models, an accumulator that maintains the oil pressure at all times provides for the even faster redistribution of torque, which takes place within just a few milliseconds.

The self-locking center differential
In 2005, Audi ignited the next stage in the evolution of its classic quattro drive system in the second-generation RS 4. The new self-locking center differential, which is active today in many models with a longitudinal engine, remained true to the purely mechanical principle, yet represented significant progress versus the Torsen differential.

Under normal driving conditions, power is distributed 40:60 between the front and rear axles. This asymmetric and dynamic distribution of torque results in sporty handling with a rear end bias. The center differential can divert up to 60 percent of the power to the front and up to 80 percent to the rear, if necessary. If a wheel on one axle should happen to spin, the electronic differential lock EDL controls it by applying the brakes.

The self-locking center differential is configured as a planetary gear. An internal gear encloses a sun gear; rotating between these two elements are roller-shaped planet gears connected to a rotating housing. They distribute the torque asymmetrically – the somewhat larger fraction flows to the rear via the internal gear, which has a larger diameter, and the output shaft connected to it. The smaller fraction is transferred to the smaller sun gear, from where it is sent to the front axle.

If traction is reduced at one of the axles, the helical form of the gears and their oblique splines produce axial forces in the differential. These forces act on friction discs to provide a defined locking torque and to divert the power to the wheels with the better friction values.

The large Q7 SUV uses a special form of the self-locking center differential that is integrated into a transfer case. The sun gear uses a chain to drive an auxiliary shaft that runs past the gearbox to the front axle. The chain is used to transport the oil, eliminating the need for the oil pump normally used. The entire powertrain of the Q7 has shed considerable weight in the latest evolutionary stage. Regardless, the transfer case is very robust. It also allows a high ground clearance, an important trait for off-road use.

The viscous coupling
The R8 high-performance sports car occupies a special position in the Audi range – and this extends to its packaging and its drive system. The mid-mounted engine is arranged longitudinally at the rear of the car in front of the rear axle, with the gearbox right behind it. It also includes an auxiliary drive for a prop shaft running past the engine on the side and up to the front axle.

There a viscous coupling distributes the power between the front axle and the rear axle, which is equipped with a locking differential. Under normal driving conditions, the coupling sends only about 15 percent of the torque to the front axle, giving the R8 the rear bias typical of sports cars. If the rear wheels slip, an additional 15 percent almost immediately flows to the front.

The primary component of the viscous coupling is a package of round clutch discs, alternating between discs with different gearing. One is connected to the prop shaft via the housing; the next disc is connected via the output shaft to the front axle. The clutch plates rotate in a viscous fluid.

If they rotate at greatly different speeds due to a loss of traction at the rear axle, the oil becomes more viscous as a result of its internal friction. By picking up the other clutch disc of each pair, a greater torque is transferred via the drive shaft to the front axle.

The sport differential
The self-locking center differential in the classic quattro powertrain does an excellent job of distributing the power between the axles. To make driving even more dynamic, Audi introduced an additional component in the dynamic S4 sedan in late 2008 that actively splits the torque between the wheels of the rear axle – the sport differential.

The sport differential is a state-of-the-art rear differential. A superposition gear comprising two sun gears and an internal gear was added to both the left and right sides of the classic differential; it rotates ten 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 forces the higher speed of the superposition stage on the gear. Being forced to turn faster results in the additional torque required being drawn off from the wheel on the inside of the curve 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 while coasting as it is under load. It is electronically controlled and reacts within a few hundredths of a second. Audi developed the software itself. 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.

Vehicles with conventional axle drives tend to understeer in fast corners. With the sport differential, it is like riding on rails. When turning into or accelerating in a curve, the majority of the torque is directed to the outside wheel, pushing the car into the curve. The system thus nips any tendency toward oversteer or understeer in the bud.

The crown-gear differential
Exactly 30 years after the debut of the first quattro, Audi introduced a new, innovative evolutionary stage of its permanent all-wheel drive system for longitudinal front-mounted engines – the quattro drive with crown-gear differential and torque vectoring.

Inside the new center differential used in the RS 5, the A7 Sportback and the new A6 are two rotating crown gears that owe their name to the crown-like design of their teeth. The rear crown gear drives the propeller shaft to the rear-axle differential while the front crown gear drives the output shaft to the front-axle differential. 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 adjacent plate packages are pressed against one another. The resulting self-locking effect subsequently diverts the majority of the torque to the axle achieving better traction; up to 85 percent can flow to the rear. Conversely, if the rear axle has less grip, the opposite happens; up to 70 percent of the torque is correspondingly diverted to the front axle.

Thanks to this even wider range of torque distribution, the crown-gear differential surpasses its predecessors to facilitate even better traction. Forces and torques are redistributed utterly consistently and without delay. The mechanical operating principle guarantees maximum efficiency and instantaneous responsiveness. Other strong points of the crown gear differential are its compactness and low weight – at 4.8 kilograms (10.58 lb) it is roughly two kilograms (4.41 lb) lighter than the previous component.

Audi couples the crown-gear differential with an intelligent brake management software solution called torque vectoring. The software can act on each of the four wheels individually, and the new system makes cornering even more precise and dynamic.

When cornering at speed, the software uses the driver’s steering input and desired level of acceleration to calculate the optimal distribution of propulsive power between all four wheels. If it detects that the wheels on the inside of the curve, which are under a reduced load, will soon begin to slip, it marginally brakes these wheels – just slight application of the pads on the disks at minimal pressure is all that it takes.

This action by the differential enables the outside wheels to apply more torque to the road. This assistance is provided smoothly and continuously. The car remains neutral noticeably longer; understeer while turning and accelerating is practically eliminated. Last but not least, the ESP intervenes later and more gently – if any intervention at all is necessary.

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