Here’s the latest reader question, along with my reply!
Josh asks: Do FWD based AWD have a center differential or a viscous coupling system, because I thought that only RWD based AWDs have a center differential or viscous coupling. Can you please send me a diagram, as well as an explanation, because I am confused on how these AWDs work.
My reply: As in a truck-type 4WD system, all AWD systems – whether based on a transverse engine/FWD layout or longitudinal/RWD layout – have an additional component to transfer power to the other pair of driven wheels (the rear wheels in a FWD-based layout or the front wheels in a RWD-based layout). It can be mechanical or viscous-type coupling, which uses silicon to engage plates in response to different wheel rotation speeds, causing power transfer to equalize rotation speed and thus control wheel slip. In some cases, the two systems are combined, to reduce lag time in between slippage and reaction of the system. 
A more detailed technical explanation can be found here.
The confusion arises because there are differences between the various manufacturer’s AWD systems whereas truck-type 4WD is fundamentally the same across the board (e.g., a two-speed mechanical transfer case that can be engaged for gear reduction and front and rear differentials, etc.)
I’ve embedded a graphic from Subaru highlighting some of the FWD/RWD/AWD vs.4WD differences.
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