All Wheel Drive Systems


All Wheel Drive Systems

All-wheel-drive systems do not give the driver the option of 2WD or 4WD. They always drive all wheels. All-wheel-drive vehicles are usually passenger cats that are not designed for off-road operation. The All-wheel-drive is designed to increase vehicle performance  in poor traction situations, such as icy or snowy roads, and in emergencies. All-wheel-drive gives the vehicle operator maximum control in adverse operating conditions by biasing the driving torque to the axle with driving traction. The advantage of All-wheel-drive can be compared to walking on snowshoes. Snowshoes prevent the user from sinking into the snow by spreading the body weight over a large surface. All-wheel-drive vehicles spread the driving force over four wheels when needed rather than two wheels. When a vehicle travels over the road, the driving wheels transmit a tractive force to the road's surface. The ability of each tire to transmit tractive force is a result of vehicle weight pressing the tire into the road's surface and the coefficient of friction between the tire and the road. If the road's surface is dry and the tire is dry, the coefficient of friction is high and four driving wheels are not needed. If the road's surface is wet and slippery, the coefficient of friction between the tire and road is low. The tire loses its coefficient of friction on slippery road surfaces, which could result in loss of control by the operator. Unlike a two-wheel-drive vehicle, an All-wheel-drive spreads the tractive effort to all four driving wheels. In addition to spreading the driving torque, the All-wheel-drive biases the driving torque to the axle that has the traction only when it is needed.

Viscous Clutch

The viscous clutch is used in the driveline of vehicles to drive the axle with low tractive effort, taking the place of the interaxle differential. In existence for several years, the viscous clutch is installed to improve the mobility factor under difficult driving conditions. It is similar in action to the viscous clutch described for the cooling system fan. The viscous clutch in AWD is a self-contained unit. When it malfuctions, it is simply replaced as an assembly. The viscous clutch  assembly is very compact, permitting installation within a front transaxle housing. Viscous clutches operate automatically while constantly transmitting power to the axle assembly as soon as it becomes necessary to improve driving wheel traction. This action is also known as biasing driving torque to the axle with tractive effort. The viscous clutch assembly is designed similarly to a multiple-disc clutch with alternating driving and driving plates.

The viscous clutch parts fit inside a drum that is completely sealed. The clutch pack is made up of alternating steel driving and driven plates. One set of steel plates is splined internally to the clutch assembly hub. The second set of clutch plates is splined externally to the clutch drum. The clutch housing is filled with a small quantity of air and special silicone fluid with the purpose of transmitting force from the driving plates to the driven plates.

Based on practical experience, vehicle operating with this clutch transmit power automatically, smoothly, and with the added benefit of the fluid being capable of dampening driveline shocks. When a difference in speed of 8 percent exists between the input shaft driven by the driving axle with tractive effort, the clutch plates begin shearing (cutting) the special silicone fluid. The shearing action causes heat to build within the housing very rapidly, which results in the silicone fluid stiffening. The stiffening action causes a locking action between the clutch plates to take place within approximately one-tenth second. The locking action results from the stiff silicone fluid becoming very hard for the plates to shear. The still silicone fluid transfer power flow from the driving to the driven plates. The driving shaft is then connected to the driven shaft through the clutch plates and stiff silicone fluid.

The viscous clutch has a self-regulating control. When the clutch assembly locks up, there is very little, if any, relative movement between the clutch plates. Because there is little relative movement, silicone fluid temperature drops, which reduces pressure within the clutch housing. But as speed fluctuates between the driving and driven members, heat increases, causing the silicone fluid to stiffen. Speed differences between the driving and driven members regulate the amount of slip in a viscous clutch driveline. The viscous clutch takes the place of the interaxle differential, biasing driving torque to the normally undriven axle during difficult driving conditions.

The viscous clutch is also used in some part-time 4WD vehicles, replacing the transfer case.

Center Differential AWD

One of the more recent AWD designs features a center differential to split the power between the front and rear axles. On the manual transmission model, the driver can lock the center differential with a switch. On the automatic transmission model, the center differential locks automatically, depending on which transaxle range the driver selects and whether or not there is any slippage between front and rear wheels.

Electronically Controlled AWD

Electronically controlled all-wheel drive is found in several import four-speed automatic overdrive transaxle designed with lockup torque converters. These vehicle drivetrains are designed with a front transaxle and two front drive shafts (each with its constant velocity universal joints). The rear drive shaft extends from the transaxle extension to the rear axle drive pinion and ring gear, two rear-drive shafts, universal joints, and driving wheels. Remember, there must be some type of interaxle differential in full-time, all-wheel drivelines.

The FWD transaxle has the same features. The torque converter is complete with impeller, turbine, stator, and lockup clutch. The turbine shaft drives the various engaged planetary controls and planetary gears to achieve the gear range selected. The interesting area of the transaxle is the output to the rear driving axle. The reduction gearset transfers torque to the front-axle drive pinion, ring gear, differential, drive shafts, and driving wheels.

At the rear of the transaxle immediately behind the reduction gearset is the multiple-disc transfer clutch. The design of this transfer clutch acts as the driveline interaxle differential, permitting the difference in front- and rear-axle speeds. The secret to the operation of this all-wheel-drive design is the method of controlling transfer clutch operation.

Strategically placed around the vehicle are sensors that monitor front- and rear-axle speeds, engine speed, and load on the engine and driveline. Information from the sensors is reported to the transmission computer unit called the transmission control unit (TCU). The TCU controls a soleniod called the duty solenoid that operates on a duty (jitter) cycle controlling the fluid flow that engages the transfer clutch. The duty solenoid pulses, cycling on and off very rapidly, which develops a controlled slip condition. Driveline windup is dissipated when the clutch pack disengages, acting like an interaxle differential to the full-time, all-wheel driveline. The result of the operation of the transmission control unit and the duty solenoid is that the transfer clutch operates like an interaxle differential to a power split from 95 percent front-wheel drive and 5 percent rear-wheel drive to 50 percent front-wheel drive and 50 percent rear-wheel drive. This power split takes place so rapidly that the vehicle operator is not aware of a traction problem.

More Four And All Wheel Drive

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