Note: This text was apparently nicked from a circa-1999 Subaru publicity site, but I can't find the original source to give it proper credit. Nevertheless, it is a terrific description of the Subaru AWD system. I've omitted most of the references to the manual version and to the 1999 improvements, which are considerable. For my source click here.
All Subaru models sold in the US feature the Subaru All-Wheel Driving System. This system consists of several subsystems working in harmony to ensure maximum traction availability without any driver input. The heart of the Subaru All-Wheel Driving System is the Subaru boxer engine, an inherently short power plant. Because the boxer engine is so short it can be mounted longitudinally in a car. This allows Subaru engineers to mount the transmission directly behind the engine down the centerline of the vehicle. This simple statement has far reaching ramifications.
The transmission is forward enough in the vehicle to allow the front drive axles to exit in a direct line to the center of the front wheels. Because the engine can be mounted low in the vehicle the drive axles are connected to the front wheels in a more horizontal plane. The significance of keeping the drive axles as straight as possible is that drive axle friction is kept to a minimum thus reducing the amount of power lost to the effort of turning the axle.
Because the engine is mounted longitudinally, axle length is equal and somewhat long. The advantage of equal-length axle shafts is the virtual elimination of torque steer. While not necessarily immediately evident, the advantage of using long axle shafts is they allow for longer suspension travel which, in turn, gives a better handling car with a smooth ride since the suspension system has more travel in which to absorb bumps.
Power to the rear wheels is sent out the back of the transmission in a straight line to the rear differential. Since the engine and transmission assembly is mounted low in the vehicle, the rear propeller shaft is kept close to horizontal and that helps keep down friction because its connecting joints can rotate in a truer plane.
Having the transmission mounted directly behind the engine also means that power flow to the front and rear wheels can be kept in a straight line. Bulky and heavy power transfer mechanisms are not needed so total vehicle weight can be kept low.
Unlike many four-wheel drive or all-wheel drive systems, the Subaru all-wheel drive system consists of power transfer mechanisms that are small enough to fit inside the transmission case. These small components not only hold down weight and power loss in the Subaru cars but also ownership costs since no extra maintenance is required for the system.
Keeping the driveline in the centerline of the vehicle helps Subaru engineers tune the suspension for excellent handling since its fundamental mass is symmetrical and low in the vehicle. Subaru vehicles have an inherently low polar moment of inertia thanks to this design. The weight of the driveline system is placed close to the centerline of the vehicle on both the "x" and "y" axis to contribute toward this low moment of inertia.
Subaru uses two types of transmissions. A five-speed manual transmission and a four-speed automatic are innovative Subaru designs that are compact and completely house the all-wheel drive transfer mechanism. Traditional four-wheel drive systems typically require a separate transfer case that is about the size of another transmission.
4-speed Electronic Automatic Transmission
First introduced in the 1987 Subaru XT, the 4-speed automatic transmission is a complex engineering accomplishment that has proven to be reliable, efficient and fun to drive. It uses a combination of sophisticated hydraulic components controlled by an even more sophisticated electronic control unit (TCU, Transmission Control Unit). The 4EAT is available in all Subaru vehicles sold in the US today.
The TCU works in harmony with the engine as management system (ECU) and, when equipped, with the anti-lock braking system to ensure predictable, sure, shifting so the vehicle is always in the right gear at the right time. The TCU also controls the all-wheel drive system allowing a greater degree of control than even the 5-speed transmission system.
A great deal of engineering effort has been put into how the transmission shifts. Shift shock receives extra special attention to ensure drivers get a firm up and down shift without undo fuss. Ways of accomplishing this formidable task include balancing shift schedules and line-pressure with load and driver demand.
One example of how shift shock is controlled is during gear activation from either Park or Neutral into Drive or Reverse. In those situations when a gear is selected and engine speed is above 1500 rpm, fuel to two cylinders is momentarily cut and the engine ignition timing is momentarily retarded to reduce driveline shock.
A system called engine torque control is used to reduce shift shock while the vehicle is being driven. It monitors the transmission's speed sensors and compares them to actual clutch/brake engagement and release operation. Based on preprogrammed variables, the TCU then signals the engine's computer to reduce engine torque (it retards ignition timing momentarily) to minimize the occurrence of shift shock.
While underway, the TCU continually monitors how the vehicle is being driven and applies it to the transmission's learning control system. By monitoring engine air intake it can determine load demands and operating altitude. It then can alter shift schedules (software maps). Up shifts can be delayed, down shifts can occur sooner or a specific gear can be selected as in the case of when ABS operation is occurring. This system also compensates for wear of the friction materials and works to reduce shift shock caused by that wear. Using a sophisticated formula, the TCU compares target clutch engagement times versus actual engagement times and then changes how it controls solenoids that allow gear engagement to occur.
Other significant inputs to the TCU include vehicle speed, gas pedal stroke, engine temperature, gear lever position and front and rear output shaft speeds. Combined, these inputs help determine which software map is to be used to determine when to lock-up the torque converter and which gear is to be selected.
Like many automatics, the Subaru 4-speed automatic uses a combination of hydraulic clutches, brake bands, one-way clutches and planetary gear sets for its operation. What sets this transmission apart from the rest is how all-wheel drive is incorporated within the transmission and how AWD is controlled by the TCU.
Improvements to the transmission (all Subaru models) have been on going and include changes to internal components to improve wear characteristics, shift quality and efficiency.
Active All-Wheel Drive
Active all-wheel drive is a term coined by Subaru to differentiate the all-wheel drive system in the automatic transmission from other "reactive" all-wheel drive systems on the market today. What makes this all-wheel drive system so special is its ability to anticipate traction needs and take action before a wheel slips.
The mechanism that transfers torque fore and aft is contained within the transmission's tailshaft. To the casual observer it looks just like a typical hydraulic clutch found in any automatic. The key difference in this clutch pack is its operation. Its designed to slip dependent on how much all-wheel drive is needed. Normally, when an automatic's clutch slips, it's malfunctioning and will burn up. But the multi-plate transfer (MPT) clutch uses a special friction material (changed for 1999 to reduce low speed judder) that easily withstands the friction loads generated during torque transfer. The MPT's operation is controlled by the transmission's ECU (Transmission Control Unit or TCU) and constantly changes dependent on how the vehicle is being driven. To get more AWD less slip occurs within the clutch pack. Less AWD calls for more slip and the TCU reduces the hydraulic pressure to the clutch.
Under normal, dry pavement operation torque split is about 90% front and 10% rear. This distribution helps to compensate for the car's weight distribution and resultant smaller effective rolling diameter of the front tires. As weight transfers to the rear of the vehicle like when under acceleration, the TCU shifts the torque split more toward the rear wheels. Under hard braking torque is directed forward which helps with shorter stopping distances. Torque distribution is changed based upon how the vehicle is being driven. Throttle position, gearshift lever position, current gear and other factors combine to influence the TCU and it, in turn, selects a software map that determines how aggressively torque split will be adjusted.
Two speed sensors are used by the TCU to detect wheel slippage. One sensor monitors the front axle set, the other the rear. Pre-programmed variables help the TCU differentiate between slipping wheels and normal wheel speed differentials as when cornering. A speed differential (front-to-rear) of up to 20% signals the TCU that the vehicle is cornering and torque is distributed to the front wheels to help increase traction during the turn. Anything above 20%, however, indicates to the TCU that wheel slippage is occurring and torque is then distributed to the rear wheels.
Another feature that sets this all-wheel drive system further apart from most is its interaction with the anti-lock brake system. When ABS is engaged, the transmission selects third gear reducing the unpredictability of engine braking and, thus, reducing the possibility of wheel lock-up. But all four wheels are still connected to the engine through the AWD system and are brought back up to overall wheel speed quicker and can, therefore, be controlled again sooner. In a two-wheel drive system if the locking wheel isn't a drive wheel, it can only be brought back up to overall wheel speed by whatever traction exists between it and the road. The quicker a wheel can be controlled, the better the stopping performance.
Mounting Systems
NVH, noise vibration harshness, is a term applied to the concept of making cars smooth, quiet and, therefore, more comfortable to drive. There are many things that contribute to NVH. Things like the shape of the vehicle, the quality of the road and tires, and the driveline components.
Subaru vehicles are fortunate to have the boxer engine, an inherently balanced/smooth running engine. But to further ensure a smooth comfortable ride, dynamic dampers are employed on strategic mounting hardware.
The front crossmember uses a damper of about two pounds to control noise and vibration generated when driving a manual transmission-equipped vehicle. Because these forces can be transmitted through the gearshift lever, the 5-speed transmission's gearshift lever receives a urethane cover to further mask any driveline noise. Finally, small dampers are installed on the rear differential-locating member to guarantee that any noise generated by the all-wheel drive system is absorbed and not transmitted to the interior of the vehicle.
The coupling and actuating devices in an automatic transmission system are fluid operated and provide a secondary benefit of absorbing vibration and, therefore, noise. The transmission's rear crossmember saddle is a one-piece design that helps hold down weight and cost.
At Subaru we are always flattered to hear people remark how our cars "don't feel like a four-wheel drive vehicle". Thanks to complex engineering we can execute a simple all-wheel system that requires little additional maintenance, gets excellent gas mileage, and is almost invisible to the driver until its traction is needed.