September 6, 2007
Source: Audi AG
- Innovative four-wheel-drive system for even greater driving pleasure and safety
- Variable tractive-force distribution between the rear wheels
- Enhanced agility, greater stability
With its active sport differential, Audi elevates the road dynamics of cars with quattro permanent four-wheel drive to a new level. An innovative regulating system distributes torque from the engine in continuously variable proportions between the rear wheels. This greatly enhances agility, driving pleasure and active driving safety. The controlled power flow enables the car to take corners even more directly and responsively, and to retain its directional stability considerably longer. Understeer becomes a thing of the past. This new high-tech device from the inventors of quattro permanent four-wheel drive will be available from 2008 onwards.
This sport differential extends Audis Vorsprung durch Technik even further with regard to road dynamics, says Michael Dick, Head of Technical Development at AUDI AG. With it, we redefine agility in a new, fascinating way and at the same time achieve a previously unmatched dimension in safety. Our sport differential is a logical step towards enhancing the driving pleasure so typical of an Audi still further. By integrating this four-wheel-drive technology into the Audi drive select system, which itself combines refinement and dynamism in a totally new way, we have created a uniquely superior road dynamics system that for the first time can be perfectly matched to the drivers individual wishes.
A synonym for supreme driving dynamics
Since 1980, quattro has been a synonym for cars with outstanding driving dynamics. In its basic setting, the torque-sensing centre differential delivers 40 percent of the tractive force from the engine to the front axle and 60 percent to the rear axle. Since the latest Audi RS 4 was introduced in 2005, this degree of rearward bias has been standard on all new Audi models; it guarantees a high level of driving pleasure. The torque-sensing differential redistributes engine output without delay if road surface conditions change, and is thus the most advanced technical principle in this competition area.
This exceptionally good traction puts the quattro system at an extremely high level as far as dynamics in a forward direction are concerned. But quattro technology also has distinct advantages in terms of lateral dynamics. By distributing traction among all four wheels, each wheel has greater potential to resist lateral forces, and tyre grip is maintained for longer. The action of the electronic differential lock (EDL) and the typical Audi settings adopted for the ESP electronic stabilisation system also add to driving pleasure and a high level of stability.
Lateral dynamics redefined
With the sport differential, Audi has now raised the bar to a decisively new level. The high-end version of the quattro system also distributes input torque variably between the two rear wheels, with almost the entire tractive force reaching one wheel if this becomes necessary. The torque input can be used to influence the cars self-steering behaviour in a controlled manner, and even give it entirely neutral road behaviour. The sport differential takes effect not only under load, but also on the overrun; its torque distributing action even enhances the cars agility when the clutch is disengaged.
Both when the steering wheel is turned and when accelerating in a corner, vehicles with conventional drive to the axles tend to understeer for as long as the tyres have adequate grip. The laws of physics cause a moving body to resist any change of direction initially.
When a vehicles steering is turned away from a straight line, the tyres and suspension pivots absorb some of the initial movement before the direction changes. When accelerating in a corner, the front axle load is reduced and therefore less lateral force can be transmitted to the tyres: the vehicle understeers and is forced out on to a larger-radius curve.
As if on rails
With the active sport differential driving the rear wheels, the car quite literally takes every corner as if on rails. Depending on steering angle, lateral acceleration, yaw angle, road speed and other signals, the control unit calculates the most suitable distribution of torque to the wheels for every driving situation. When the steering wheel is turned, for example, or the car accelerated in a corner, power is redirected in a controlled manner to the outer rear wheel. This has the effect of forcing the car into the corner so that the angle of the front wheels is followed accurately. The difference in tractive force between the left and right wheels also exerts a steering effect, so that the usual corrections by the driver at the steering wheel are no longer needed. Understeer, that is to say the tendency for the car to run wide at the front, is completely eliminated.
The car follows the successive bends when driven briskly along a typical country road in a manner similar to the precise track formed by carving skis in the snow. The sport differential redistributes the power input continually with great rapidity and counteracts any tendency for the car to understeer or oversteer before either of these effects can build up. For the driver, this means reduced effort at the steering wheel and higher maximum lateral acceleration.
A notable safety bonus
Close to the cars handling limits, the sport differential acts like ESP, but with the principle reversed: corrective movements are not initiated solely by altering the engine settings or applying the brakes, but also by controlled redistribution of tractive force. The cars progress is distinctly smoother and free-flowing; its actual road dynamics and those sensed subjectively by the driver are both enhanced, and driving pleasure benefits too, since ESP comes into action much less frequently.
Since its reaction time is extremely short less than 100 milliseconds the sport differential takes effect even more rapidly than ESP. Furthermore, ESP can only react to a discrepancy between the cars steering angle and its actual body rotation, whereas the sport differential influences the cars dynamic behaviour before any such discrepancies occur.
Active torque distribution is based on an entirely new design principle for the rear-axle differential. So that the new high-tech unit can also divert torque to the outer wheel on the axle, which revolves faster than the inner wheel when cornering, the classic rear-axle differential has superimposed stages at the left and right, and a multi-plate clutch running in an oilbath. The clutches are activated by an electro-hydraulic actuator.
Superimposed stages for acceleration
The superimposed output stage rotates ten percent faster than the half-shaft that normally drives the wheel; the two drivelines can be connected together by the multi-plate clutch. If the clutch is engaged, the wheel is accelerated by the superimposed stage, which is running at a higher speed, and thus receives an additional torque input. The differential reduces the tractive force reaching the opposite wheel accordingly. In most driving situations, almost the complete input torque can be diverted to one rear wheel in this way. The maximum difference in torque between the wheels is 1,800 Newton-metres.
The sport differential is even capable of completely eliminating the classic load-reversal reactions when the accelerator is lifted or the brakes applied suddenly while cornering; the driver can specify this function by selecting comfort mode. If the accelerator pedal is released, torque distribution between the wheels is adjusted almost instantaneously. Unlike other systems, the Audi sport differential is just as effective when the engine is driving the wheels as it is on the overrun or when the car is coasting freely. In simplified terms, here too one wheel is braked and the other accelerated with the aid of the superimposed drive stages. This diverts the torque to the wheels and the vehicle is given a precisely calculated impulse in the required direction.
A technically superior solution
The task of distributing a high level of torque with minimum losses by way of a transmission that is compact and light in weight was an extremely challenging one. The solution arrived at by Audis engineers is as ingenious as it is innovative. A new transmission stage, consisting of two sunwheels and a ring gear, performs the task in a very simple fashion. The ingenuity lies in the fact that the ring gear is displaced in relation to the sunwheels. The resulting generous gear tooth overlap enables torque transfer to take place using a minimum number of components. This is the first time that this principle has been used in a road-vehicle driveline.
The system needs no maintenance throughout its life, and operates at a high level of efficiency. The additional fuel consumption needed for the active sport differential is so small that it is hardly measurable. The device takes effect extremely quickly and has a very high level of control accuracy.
Thanks to its control unit, the sport differential is an intelligent unit. Audi developed the necessary software, which uses a unique operating principle, in-house. The computer obtains the information it needs from the steering angle, wheel speed, lateral acceleration and yaw-rate sensors and from current engine data. Twin yaw-rate and lateral acceleration sensors are installed for the sport differential.
This explains the systems extremely high reaction speed: data from both sensors are continuously compared in the control unit, so that reliable results are available more rapidly.
Audi drive select the unique high-tech package
The quattro driveline with active sport differential is part of Audi drive select, the innovative dynamic driving system premiered on the new Audi A4. As part of an optimal combination with Audi dynamic steering and electronic shock absorber control, the quattro permanent four-wheel driveline and the sport differential demonstrate their immense capabilities with regard to driving dynamics.
Audi drive select integrates the technical components that determine the quality of the driving experience: engine, transmission, steering, shock absorbers and the sport differential. Drivers can adjust the characteristics of these systems at any time to suit their personal preferences. Audi drive select is a successful synthesis of comfort and convenience with dynamic sportiness at a previously unattained level.
Buttons on the centre console enable drivers to vary the operation of Audi drive select in three stages ranging from comfortable to sporty, or to put together a profile that suits their personal preferences. The engines throttle response characteristic can be determined first, followed by that of the servotronic speed-dependent power steering and, if the car has automatic transmission, its shift points.
Three operating modes for the sport differential
The functions of the sport differential can also be varied to suit the drivers individual wishes. In the comfort mode, driving safety and the cars stability have absolute priority, with optimal damping of load reversals. In the auto mode the program achieves a harmonious balance between all the functions. Finally, the dynamic mode emphasises the functions contributing towards agility, so that the action of the sport differential is most easily felt. In this case, response to load reversals is both agile and easily controllable.
Audi drive select with the sport differential can be combined with electronic damping control. The gas-filled hydraulic shock absorbers have an additional valve with a continuous opening action. At a cycle frequency of 1000 per second, the control unit computes the optimal damping force. The electromagnetically controlled valve selects a damping characteristic to match the current driving situation at any given moment. Higher damping force may be needed to resist body movement when cornering rapidly or braking, whereas a lower damping force will be appropriate on potholed roads and a moderate setting on poor country roads.
Audi dynamic steering the perfect partner
Audi drive select becomes the ultimate dynamic driving system in conjunction with Audi dynamic steering. It is the perfect partner for the quattro driveline with sport differential. Its effect when combined with damping control and dynamic steering opens up a whole new dimension in safe, sporty driving.
The dynamic steering system uses a superimposed zero-play transmission to vary the effective steering ratio according to road speed. It is integrated into the steering column and combined with an electric motor. Here again, the Audi engineers have come up with a brilliant solution: axial transmission is extremely compact, light in weight and torsionally rigid. It operates without any play and is therefore extremely precise, with low friction. It can transmit very high torques and operates at a high level of efficiency.
Broader operating range: up to twice the ratio
In accordance with the vehicles road speed and the chosen Audi drive select mode, the dynamic steering system can vary its ratio by almost 100 percent. When the car is being parked, the steering is extremely direct, whereas when driving in a straight line at high motorway speeds a more indirect steering ratio and reduced power assistance help to maintain the cars outstanding directional stability.´
The dynamic steering system can also correct a small to moderate angle of float as rapidly and effectively as a rally driver could ever react, by automatically turning the wheels slightly in the opposite direction. Brake applications are normally only needed if the angle is greater, and then in most cases only to exert a damping effect.
With regard to road dynamics and safety, the dynamic steering system, damping control and sport differential work closely together with the ESP. They reduce the burden on the stabilisation program because the corrections take place considerably earlier via the input torque, the damping force or the steering angle. These rapid, efficient interventions avoid the need for brake applications in many cases, so that the car can be driven more dynamically and smoothly.