A rather interesting destinction. To my mind there are several systems.

The conventional old fashioned 4WD uses a transfer box that will send 50% of the power to the front wheels when inserted but is not suitable for on road use because it does not have a differential between front and rear axles. These systems usually have front wheel hub locks so that the front differential etc. Is not rotating when not in use. In this system provided both wheels of at least one axle have grip the vehicle will move. I have had the experience of being blocked when the left front and the right rear wheel were off the ground. Most 4WD vehicles would also have a low range gearbox which would allow the vehicle to move at crawling pace if necessary.

A more sophisticated version allows on road use when 4WD is engaged because it has a centre diff. Unless this centre diff has some limited slip arrangement the vehicle would be blocked if only one wheel completely loses traction. I would say that most modern 4WD vehicles would have some such system.

An AWD (or constant 4WD) was developed to give better traction in all conditions and came to fame for its use in rally cars, the Lancia Delta Integrale for example and later the Subaru WRX. To be effective these systems need some sort of limited slip arrangement in the differential between the front and rear axles and ideally both the front and rear axles should have limited slip differentials. Provided nothing is slipping these vehicles would have 25% of the power going to each wheel. The downside is that they have high consumption of fuel and tyres. The little AWD Subaru Justy that Alis has just replaced with a Polo had attrocious fuel economy. There are more sophisticated systems that apportion a different percentage of power to the front and rear wheels in normal use but this can change if the wheels of one axle start to slip.

The more modern AWD systems use computerised traction control on all wheels and will drive only the rear wheels until some slip is detected at that point power is transfered to the other wheels. This has now been developed to the point that vehicles equipped with these systems and winter tyres can perform surprisingly well on snow but maintain excellent fuel economy on dry roads.

Peter

[Linked Image]