S-AWC is not simply the name of a new all-wheel drive system, but an advanced vehicle dynamics control network that reads and reflects driver intent in real time. This new system regulates drive torque at each wheel by controlling a network of dynamic handling technologies, including: Active Center Differential (ACD), Active Yaw Control (AYC) rear differential, Active Stability Control (ASC), and Sport ABS brakes.
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Super All-Wheel Control in the Lancer Evolution will be consistent in all world markets. Integration of Active Stability Control (ASC) is new for all markets and also provides a significant advance in both dynamic capability and safety. S-AWC's integrated control system provides better overall stability and performance compared to systems in previous Lancer Evolution models that controlled ACD and AYC (in markets where offered) independently.
It is important to note, too, that the new global platform that underpins the 2008 Lancer (and Outlander) was designed from the start for the highest-performance variant. The super-stiff structure, optimized chassis systems, wider use of aluminum for the engine, body, and chassis components, as well as greater use of high-tensile steel, all play important roles in the Super-All Wheel Control concept.
[Mitsubishi applies the All-Wheel Control philosophy in its Outlander SUV, although in that vehicle AWC controls a completely different array of components and systems designed specifically for all-weather traction performance.]
An Evolution of Mitsubishi Four-Wheel Drive Performance
Super All-Wheel Control is the culmination of more than 20 years of Mitsubishi road and rally four-wheel drive innovation and experience. In the U.S., some of the most memorable four-wheel drive road cars in recent times have worn the Mitsubishi badge: the first Eclipse GS-X 17 years ago; the incomparable 3000 GT VR-4 in the 1990s; the Galant VR-4 that achieved instant cult status, and of course the Lancer Evolution that brought Mitsubishi's rally-conquering technology and performance to the showroom.
By introducing Super-All Wheel Control for the 2008 Lancer Evolution, Mitsubishi is once again advancing four-wheel drive performance technology. As a result, the new Lancer Evolution offers an extraordinary level of dynamic control at each wheel, going well beyond the capabilities of other all-wheel drive systems.
Following is a description of each part of the Super-All Wheel Control system:
Active Center Differential (ACD) Overview
The heart of any four-wheel drive system is the means of power distribution, which is a key to establishing the vehicle's handling behavior. Many vehicles today offer all-wheel drive systems, and most of these are designed to enhance traction in slippery road conditions.
Some are as simple as a viscous coupling that passively transfers torque away from slipping wheels in reaction to wheel spin; others are more complex and designed to meet specific performance goals. Lancer Evolution models have always featured a performance-oriented full-time four-wheel drive system.
First employed on the Japanese-market Evolution VII model, Mitsubishi's Active Center Differential (ACD) made its North American debut in the Lancer Evolution models. Combined with a helical limited-slip front differential (also used in the 2008 model) ACD helped raise the previous-generation Evolution's already stellar handling performance to a new level.
How ACD Works
The ACD splits torque up to 50:50 between the front and rear wheels using an electronically controlled hydraulic multi-plate clutch. The ACD's electronic control unit (ECU) optimizes ACD clutch cover clamp load for different driving conditions, regulating the differential limiting action between a free state (where torque is split equally between front and rear wheels) and a locked state to optimize front/rear wheel torque split and thereby produce the best balance between traction and steering response.
The maximum limited-slip torque of the ACD multi-plate clutch is about three times that of a conventional viscous coupling. The hydraulic unit housed in the engine compartment regulates the hydraulic pressure of the multi-plate clutch within the range of zero to 145 psi.
The S-AWC computer takes data input from various sensors to continuously calculate the ACD's limited-slip torque. Steering wheel angle, throttle opening, wheel speeds, and the vehicle's longitudinal and lateral movements are constantly measured to determine the vehicle's path of travel. Using this data, S-AWC determines whether limited-slip torque should be increased or decreased at any given time.
As the previous Lancer Evolution did, the new model offers three driver-selectable traction modes for the ACD, changeable while the car is moving using a switch on the dash: “Tarmac” for dry, paved surfaces; “Gravel” for wet or rough surfaces, and “Snow” for snow-covered surfaces.
In each mode, S-AWC adjusts center differential locking behavior to suit the road conditions. The car's other dynamic handling systems respond to the road conditions and driver input. The multi-information monitor, located between the tachometer and speedometer, displays the selected ACD mode and also provide status indicators for ACD and AYC operation. The driver can see at a glance how each differential is acting.
Active Yaw Control (AYC) Overview
The 2008 Lancer Evolution is the first U.S. market Evolution model to incorporate the innovative AYC rear differential. The first component of its type, AYC was first used in the Lancer Evolution IV model in 1996. In 2003 Mitsubishi switched from using a bevel gear to a planetary gear differential, doubling the amount of torque AYC was able to transfer. The new version of AYC in the 2008 Lancer Evolution retains the planetary-gear differential.
The AYC differential uses a torque transfer mechanism to control rear wheel torque differential for different driving conditions, enhancing cornering performance by limiting the yaw moment acting on the vehicle. AYC also acts like a limited-slip differential by controlling rear wheel slip to improve traction. The AYC differential in the 2008 Lancer Evolution model now adds yaw feedback control using a yaw rate sensor and also gains braking force control via the Active Stability Control (ASC) system.
By controlling the amount of torque transmitted to the rear wheels when there is less traction, or a difference in grip on the road surface, AYC also helps to improve acceleration and stability on slippery roads. Again, it is aided in the 2008 Lancer Evolution model in this respect by the integration of ASC. Mainly, though, AYC was designed as a performance-enhancing system to enable greater stability during high-speed cornering and, ultimately, higher cornering grip.
How AYC Works
The AYC differential actively splits torque between the right and left rear wheels, effectively changing the yaw moment of the vehicle as needed for a given cornering situation. Like ACD, AYC is controlled by the S-AWC computer, using inputs from steering wheel angle, throttle opening, individual wheel speeds, and longitudinal and lateral movements.
The AYC features two sections – the rear differential and the left/right power split control section. Within the power-split section are two hydraulic clutches – one each for the left and right axles. During cornering maneuvers, the right or left clutch – taking input from the S-AWC computer – will act on the differential to increase torque to the outside rear wheel and reduce the torque level to the inside rear wheel. This action changes yaw movement of the vehicle, influencing it to steer inwards and reducing the amount of slip on the front tires, thus reducing understeer.
Active Stability Control (ASC) Overview
Integrated management of the ASC and ABS systems allows S-AWC to effectively and seamlessly control vehicle dynamics when accelerating, decelerating or cornering. The ASC system, which includes stability control and traction control, helps to maintain optimum traction by regulating engine power and the braking force at each wheel. ASC helps the driver follow a chosen line more closely by comparing the car's path (as determined from yaw rate sensor data) to the desired path (as determined from steering inputs) and applies individual wheel braking or throttle control to correct any divergence. ASC also enhances vehicle stability by helping to prevent wheel spin on slippery surfaces, and helping to prevent sliding as the result of sudden steering inputs.
How ASC Works
It is important to note that S-AWC does not use stability control to vary torque distribution, as some other all-wheel drive systems have done: It is S-AWC that actively controls front-to-rear torque distribution through the ACD, and rear wheel side-to-side torque distribution through AYC. A helical limited-slip front differential reacts to torque input to govern side-to-side torque distribution at the front wheels.
With ASC activated, S-AWC can exert even more control over vehicle behavior in on-the-limit driving situations. Increasing braking force on the inside wheel during understeer and on the outer wheel during oversteer situations, ASC works in concert with torque transfer regulation to allow higher levels of cornering performance and vehicle stability.
Using engine torque and brake pressure information in the regulation of the ACD and AYC components allows S-AWC to determine more quickly whether the vehicle is accelerating or decelerating.
ASC is programmed to allow performance driving and can be turned off for track driving; turning off ASC does not compromise operation of the car's ACD or AYC differentials. Of special note to track racers, however, holding down the “ASC off” button for three seconds will also disengage the AYC brake-control function.
Sport Anti-lock Braking System (Sport ABS)
ABS can help the driver to maintain steering control and help keep the vehicle stable by preventing the wheels from locking under heavy braking or when braking on slippery surfaces. The addition of yaw rate sensors and brake pressure sensors to the Sport ABS system has improved braking performance through corners compared to the previous-generation Lancer Evolution. (See Chassis section in this press kit for details on braking system.)
Helical Front Differential
The helical-type limited slip front differential constantly biases torque to the wheel that has more traction. This type of differential reacts to torque input – it is not actively controlled like ACD or AYC. Under straight-line acceleration, power remains evenly split between the front wheels. While cornering or accelerating out of a turn, the helical LSD directs power away from the inside wheel and toward the outside wheel, allowing the driver to begin accelerating earlier and exit the turn at a higher speed, without losing traction.
The helical front differential can also compensate for loss of traction when the front wheels are on slippery surfaces, biasing torque to the wheel with the best traction. Because the front differential uses gears, it is stronger and more durable than other types of LSDs, with no plates or clutches that can wear out and require replacement.
