Four-wheel drive transmissions have various designs. Together, they form all-wheel-drive systems. The following types of all-wheel-drive systems are distinguished: permanent connection, automatically connected and manually connected.
Different types of all-wheel-drive systems have, as a rule, different purposes. At the same time, the following advantages of these systems can be distinguished, which determine the scope of their application:
Permanent all-wheel-drive system (another name – Full Time system , translated as “full time”) provides a constant transmission of torque to all wheels of the car.
The system includes structural elements that are characteristic of an all-wheel-drive transmission, namely: clutch, gearbox, transfer case, cardan gears, final drives, small-wheel differentials of the rear and front axles, as well as wheel axles.
Permanent four-wheel drive is used both on cars with rear-wheel drive layout (longitudinal arrangement of the engine and gearbox), and on cars with front-wheel drive layout (transverse arrangement of the engine and gearbox). Such systems differ mainly in the design of the transfer case and cardan gears.
Famous permanent all-wheel-drive systems are Quattro from Audi, xDrive from BMW, 4Matic from Mercedes.
The clutch provides a short-term disconnection of the engine from the transmission when shifting gears, as well as protecting the transmission elements from overloads. The gearbox serves to change the torque, speed and direction of movement of the car. In an automatic transmission, the clutch function is a torque converter.
The transfer case is designed to distribute torque along with the axles of the car and increase it if necessary. The modern transfer case includes a chain gear (gear transmission), which provides torque transmission to the front axle, downshifts in the form of planetary gear (in separate designs) and an interaxle differential.
The presence of the center differential is a distinctive feature of the transfer case of a permanent all-wheel-drive system. For the full realization of all-wheel-drive capabilities in the system design, a centre differential lock is provided.
A differential-lock can be done automatically or manually. The modern designs of automatic locking of the centre differential are viscous coupling, Torsen self-locking differential, multi-plate friction clutch .
Manual (forced) differential lock is made by the driver using a mechanical, pneumatic, electric or hydraulic actuator. On some designs of the transfer case, the functions of both automatic and manual locking of the center differential are provided.
Cardan drives transmit torque from the secondary shafts of the transfer case to the shafts of final drives. The main gear is used to increase the torque and its transmission on the axle shaft of the wheels.
The cross-axle differential provides the distribution of torque between the drive wheels and allows the axles to rotate at different angular speeds. In all-wheel-drive systems, the cross-axle differential is used on the front and rear axles.
To implement all-wheel-drive capabilities, one or both differentials have the ability to lock. The cross-axle differential can be locked manually or automatically (viscous coupling, Torsen differential). On modern cars, an electronic differential lock is used.
Torque from the engine is transmitted to the gearbox and then to the transfer case. In the transfer case, the moment is distributed along the axes. If necessary, the driver can be engaged in a reduction gear. Further, the torque is transmitted through the driveshafts to the final drive and the center differential of each of the axles. From the differential, the torque is transmitted through the axles to the drive wheels. When the wheels slip on one of the axles, the interaxle and interwheel differentials are automatically or forcibly locked.
The all-wheel drive system is automatically connected (another name is the On demand system , translated “on demand”) is a promising direction for the development of all-wheel drive cars. This system provides the connection of the wheels of one of the axles in case of slipping of the wheels of the other axle. Under normal operating conditions, the car is front or rear-wheel drive.
Almost all leading automakers have cars with automatically connected all-wheel-drive in their lineup. Volkswagen’s famous 4Motion auto-plug-in system is automatically connected.
The design of the all-wheel drive system plug-in is automatically similar to a permanent four-wheel drive. An exception is the presence of a rear axle coupling.
The transfer case in an automatically connected all-wheel drive system is usually a bevel gear. Downshift and center differential are missing.
As a rear axle coupling, a viscous coupling or an electronically controlled friction clutch are used. A well-known friction clutch is a Haldex clutch, which is used in Volkswagen’s 4Motion all-wheel drive system.
Torque from the engine through the clutch, gearbox, final drive and differential is transmitted to the front axle of the car. Torque through the transfer case and cardan shafts is also transmitted to the friction clutch. In the normal position, the friction clutch has minimal compression, in which up to 10% of the torque is transmitted to the rear axle. When the wheels of the front axle slip, the friction clutch is activated by the command of the electronic control unit and transmits torque to the rear axle. The amount of torque transmitted to the rear axle may vary within certain limits.
Manual all-wheel-drive system (another name – Part Time system , translated as “partial time”) is currently practically not used, because is ineffective. At the same time, it is this system that provides a rigid connection between the front and rear axles, transmission of torque in the ratio of 50:50 and therefore is truly off-road.
The design of a manual all-wheel-drive system is generally similar to a permanent all-wheel-drive system. The main differences are the lack of an interaxle differential and the ability to connect the front axle in the transfer case. It should be noted that in some designs of permanent all-wheel drive, the front axle shut-off function is used. True, in this case, disconnecting and connecting is not the same thing.
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