Under ideal conditions power is split 50/50 front to rear. But in extreme cases up to 67% of the engine's power can be directed to a single wheel. The benefits to the driver are safety, performance, power, and control in copious amounts.When cornering, all four wheels of a car must cover varying distances. The task of the center differential is to compensate for the differences between front and rear axles, and to distribute engine power between front and rear wheels. The center differential is the heart of Audi's permanent all-wheel driveline. Depending on the driving situation and road conditions, the system automatically regulates the distribution of power within milliseconds. This takes place by means of either (depending on model) the Torsen® differential, Haldex™ clutch, or hydraulic multi-plate clutch. Influencing parameters in the control process include engine speed and torque, wheel spreads, and longitudinal and lateral acceleration.
Thanks to the continuous and situation-related distribution of power via the center differential, quattro® all-wheel drive ensures excellent propulsion at all times. Advantages result in all driving situations, whether the driver is on snow and ice, towing a trailer, or on an ideally surfaced road in optimal weather, quattro truly sets new standards in terms of driving safety and handling.
The locking function of the center differential and the Electronic Differential Lock make sure that an Audi with quattro® all-wheel drive can still pull away with only one wheel able to transfer engine power to the road.
The principle is simple: in the same way that four brakes ensure better deceleration, four driven wheels enable better acceleration and cornering stability. Legendary Audi quattro® all-wheel drive is the systematic application of this basic physical principle.
But that's not all. Depending on the driving situation and road surface, quattro technology also distributes drive power continuously between the front and rear axles. Especially on slippery surfaces, this means better traction and safe progress, even in conditions in which two driven wheels of a vehicle are no longer able to grip.
The division of propulsive power between all four wheels is the basis of the excellent handling and safety of Audi models with quattro. The potential of this drive concept is further optimized by a series of control systems for brake and engine management:
The Anti-lock Brake System (ABS)
Electronic Brake-force Distribution (EBD)
Electronic Differential Lock (EDL)
Anti-Slip Regulation (ASR)
All of the above assist in increasing traction when accelerating or braking. Furthermore, when cornering, the standard Electronic Stabilization Program (ESP®) increases directional stability by comparing target and actual physical driving forces.Here's how it works: If one of the wheels on an axle loses grip and starts spinning, propulsive power has to be diverted to the other wheel by the axle's differential. Previously, a mechanical differential lock enabled this to take place. Now, the Electronic Differential Lock has replaced the mechanical lock, effectively transferring the excess power of the spinning wheel to the other wheels with better traction.
On an Audi with quattro, if both wheels on an axle start spinning, the continuously controlled locking of the center differential ensures that most of the torque is transmitted to the other axle. In this way, propulsion is guaranteed in almost all situations.
The intelligent all-wheel drive system xDrive delivers maximum driving dynamics and traction by distributing engine power between the front and rear axles variably and instantaneously. The central element is an electronically controlled multi-plate clutch which reacts in a split-second to any change in road conditions.
The higher pressure on this clutch, the more power is transferred to the chain-driven front axle; the lower the pressure, the more distance there is between the clutch plates, and more power is transferred to the rear axle.
This variable power distribution system has a positive influence on the self-steering behaviour of the vehicle equipped with xDrive. In contrast o conventional all-wheel drive systems, xDrive uses the sensors of the Dynamic Stability Control (DSC) system to constantly gather important information about the vehicle's performance, such as the rotation speed of the wheels, the angle of steering or position of the accelerator. This information is processed immediately, enabling xDrive to ascertain if your vehicle is about to over- or understeer, and it takes corrective action if necessary.
If xDrive identifies understeering on a bend, i.e. the front wheels of the vehicle are beginning to drift towards the outer edge of the bend, the system reduces the amount of power reaching the front axle and raises the amount reaching ystem reduces the amount of power reaching the front axle and raises the amount reaching the rear wheels. If a vehicle is oversteering, i.e. it could spin around completely because the angle of steering is too acute, xDrive channels power away from the rear axle to the front axle.
The result? Increased directional stability, better roadholding, more accurate steering and easy control even in extreme situations. DSC is activated only when xDrive cannot maintain stability by itself: DSC cuts engine output and activates individual wheels as necessary.
When travelling on surfaces with contrasting road-holding conditions, e.g. alternating between packed ice and loose snow, drive power is variably transferred to the axle with better traction. This ensures the best possible performance and excellent handling regardless of the prevailing conditions, and optimum control is guaranteed even on slippery surfaces.
Both system utilize brakes and engine management (ESP/DSC) the real difference is Audi's ability to always transfer power, not just when slipping, this aids in cornering and traction when starting off. Audi can also put the power to one wheel if necessary. Audi has no electronic contol mechanism for the actual AWD, BMW relies on slipping electronics and is chain driven. Oh yeah and the AUDI is NOT FRONT BIASED, it is constantly adjusting !!!
Hope that helps!