The G-Lader was invented in 1905 by a Frenchman called Le Creux. Materials technology at the time prevented his design being a viable option. In the eighties once supercharging (as opposed to turbocharging) was decided upon as their route to performance Volkswagen decided to follow through with Le Creux's original design. Over 400 designs were produced before the current unit was settled on. The reliability of the current unit meets the following minimum requirement: Full boost at 10,000 rpm for 800 hours continuous. As a measure of what this means in real terms consider that one of the designs that fell by the wayside could only last 4 hours on the test bed under these conditions, but would still last 20,000 miles in a VW test car.
The lader (charger) unit has two air inlets and one outlet. One inlet is direct from the air cleaner box, the other recirculates unwanted compressed air. The lader is always spinning in proportion to engine revolutions (factor is 1.6 times crank speed). The unit is driven via two belts, which also drive the alternator. Outgoing air is driven through the intercooler (air cooled, using the same principle as the radiator, but without a fan) and then on to injection system. On the back end of the injection system is a bypass valve, which distributes the compressed air between the cylinders and the second lader inlet. The bypass valve is directly connected to the throttle. Only at full throttle opening is all the charge air directed to the cylinders, at other openings the air circulates, meaning the engine operates in pretty much a normally aspirated mode.
Internally the lader is a double spiral shaped chamber. Air is pulled into the outer spiral then forced inwards via a magnesium displacer, which rotates eccentrically, using a similar principle to the Wankel engine. The hot outgoing air is driven through the intercooler, which by cooling the air (by about 130 degress F) compresses it.