4 wheel drive cars

5.6.1 4 wheel drive cars

Transmitting engine thrust to four rather than two wheels (in case of a two-axle vehicle) has always been the best way to implement all the power and torque of an engine. The improved cross-country ability is accompanied by improved handling, of course, provided that the driver has good driving skills.

There are several ways of transferring torque from the engine via the gearbox to the second axle. One of them is the installation of a transfer case (the principle of operation of which we have already considered above). There are also layouts without the use of such an assembly as a transfer case, or rather, without the form in which we represent it.

Figure 5.40 The most popular layouts of all-wheel drive cars with various blocking mechanisms.

Note
It is worth immediately dividing all four-wheel drive vehicles into two families: with permanent all-wheel drive and with actuated all-wheel drive.

Below we will describe the layouts of the all-wheel drive of passenger cars, presented in Figure 5.40.

5.6.1.a The first scheme

First layout: longitudinally located engine, after which there is a gearbox, behind which a transfer case is installed. From the latter, cardan gears transmit traction to the rear and front drive axles. A differential is installed inside the transfer case. It can be symmetrical and asymmetrical, as well as blocking and non-blocking. Blocking can be carried out using a viscous drive. In addition, a self-locking differential can be installed.

Note
A differential is called symmetric, if it distributes (divides) the thrust from the engine between the axles of the car equally (50:50). An asymmetric differential distributes (divides) the thrust from the engine between the axles of the car unevenly (for example, 40:60)..

5.6.1.b Second scheme

Second layout: permanent four-wheel drive without transfer case. Audi Company presented the best solution to the world. The gearbox contains a Torsen limited slip differential, which, under normal operating conditions, divides the thrust from the engine between the front and rear axles of the vehicle in half. As soon as one of the axles starts slipping, the torque will instantly start redirecting to the axle, the wheels of which have the best grip (are in the best grip conditions).

Note
Torsen (Torque Sensing) is a limited slip differential, which allows the torque to be redirected exclusively mechanically to one or another axle/wheel, depending on the operating conditions. It can be symmetrical and asymmetrical.

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.

5.6.2 BMW X1

This car has a longitudinally located engine with a gearbox. Behind the gearbox there is a transfer case with a chain drive (a Morse chain is installed). From it a cardan gear is brought to the rear axle, where there is a regular symmetric differential. The engine thrust is transmitted to the front axle through an electronically controlled multi-disk clutch. Thus, under normal conditions, the car is rear-wheel drive, and as soon as there is a need for the front axle, the electronics "give the command", the clutch discs start closing and the traction is transmitted to the front axle.

5.6.3 Mini Countryman

The engine is installed transversely. Under normal conditions the car is front-wheel drive. The thrust is transmitted to the rear axle from the gearbox through a pair of gears (it can be conventionally called a transfer mechanism). But a viscous drive is installed directly in front of the rear drive gear. As soon as there is wheel slip on the front axle, the clutch starts transmitting part of the torque from the engine to the rear wheels. In case if there is no need for traction on the rear wheels, the clutch discs open - and the car again becomes front-wheel drive.

5.6.4 Nissan Juke

This car is a vivid example of a non-standard approach to solving a problem. As in the above example (diagram three), the engine is installed transversely. The thrust from the engine is transmitted to the rear axle through the cardan transmission, but there is no differential in the rear axle! Instead, there are two multi-disk clutches on both sides of the drive gear, that are electronically controlled. While using two clutches, engineers have achieved imitation of a full differential lock. Operation of a free symmetrical differential, or a complete disengagement of the rear axle if it is not needed

5.6.5 Subaru Forester

Subaru cars are "all-wheel drive to the bone". There are simply no single-axle vehicles in model lineup of this company. Moreover, until recently, the developers of the Subaru transmission preferred the Torsen limited slip differential, which was mentioned above. Now everything is different: a longitudinally located powertrain, after it there is a transfer case, from it - a cardan gear back and a cardan gear forward. Traction is driven through a symmetrical differential, which can be locked using an electronically controlled multi-disk clutch.