b) Electronic Shift-on-Fly AWD
The shifting process can be automated using a frictional clutch (e.g. multi-disk clutch). Fig. 3-35 shows the transfer case of such an all-wheel drive system consisting of a spur-gear planetary differential and two multi-disk clutches.
In case of the four-wheel drive "3-Matic" of Mercedes-Benz, the vehicle is driven on pure rear-wheel drive in normal operation. The differential is locked by the first spring-loaded multi-disk clutch while the opened second multi-disk clutch separates sun wheel and front-wheel drive. Based on demand, three operating steps which add on to each other may be recalled.
Step 1: The front-wheel drive is engaged:
Using electronics the first multi-disk clutch which locks the differential is hydraulically released, while the second one is closed. In this way, the differential distributes the driving torque to the front and rear axles at a definite torque ratio (35:65).
Step 2: In extreme circumstances, the differential can additionally be locked by closing the first clutch. If the hydraulics fail, the spring load guarantees a functional RWD.
Step 3: The rear-axle differential is locked.