5.6.1.c Scheme three
Third layout: the engine can be located longitudinally or transversely. The front wheels are mainly driven constantly, and the rear wheels are connected via a viscous drive or via an electronically controlled clutch, which is called Haldex. This is one of the most popular part-time all-wheel drive layouts, which is used in almost all SUV cars (in everyday life - "SUV" or "crossover").
Figure 5.41 Torsen in the drive gear.
Note
A viscous coupling (viscous drive) is a special mechanical device, which can transmit torque through a viscous fluid. This mechanism consists of many round plates with projections and holes. These plates are located very close to each other in a sealed casing. There are two types of plates in the clutch: the plates of one of them are connected to the drive shaft, and the plates of the other are connected to the driven shaft. Plates of different types are arranged in such a way that they alternate with each other and rotate around the same axis. The casing is filled with a special fluid (called dilatant). When two types of plates rotate at the same frequency, the liquid particles almost do not mix. The viscosity of the liquid is low, in which case some torque is transmitted through the coupling. In case if one of the shafts starts rotating faster (or slower) than the other, the liquid starts mixing and its viscosity, due to its special properties, starts increasing. The liquid can become almost solid, causing the plates to almost stick together. As a result of this, the clutch can transfer more torque from one type of plate to another.
Figure 5.42 Arrangement of viscous drive.
At the moment, each of the manufacturers creates all-wheel drive systems such that it considers most suitable for a particular model.
Below are described some of the layouts that are used on some popular modern SUVs.
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