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