4WD systems: Types, Peculiarities and Principles of Operation

The design of a four-wheel drive vehicle ensures the capacity of transmitting torque from the engine to all four wheels of the vehicle. Structural arrangements of 4WD systems allow realizing the full power potential of the car, its road handling, active car safety. The four-wheel drive car can be abbreviated as 4x4, 4WD, or AWD. In this article we will review different types of 4WD (AWD) car systems, peculiarities of their structural arrangement, transmission layout in four-wheel drive cars.

The main design feature of the 4WD (AWD) vehicle in comparison with front-wheel drive and rear-wheel drive variants is the presence of an additional unit in the transmission system – the transfer case. In fact, this unit is responsible for torque distribution between two axles of the car. Another design difference between 2WD and 4WD transmissions lies in the fact that the 2WD output shaft is longer than the 4WD (AWD) version.

Learn more about 4WD systems

In general, the 4WD design includes the following components:

• Manual or automatic transmission;
• Transfer case or multiple-disc clutch;
• Interaxle/center differential;
• Cardan drive;
• Rear and front differentials.

Drive Types

We will consider 4 types of drives:

• Full-time 4WD;
• On-demand 4WD;
• Part-time 4WD;
• Selectable 4WD.

Full-time 4WD

This drive system differs from others in that torque is constantly distributed to all wheels simultaneously. This type of drive can be implemented on different types of vehicles (SUVs, sedans, hatchbacks) regardless of the engine positioning (longitudinal or transverse). At the same time, the transfer case in this system has a reduction gear, which can be activated with the help of an electronic actuator (the driver simply selects the required mode via the selector and the servo mechanism performs switching).

Audi Quattro

The presence of a center differential is a distinctive feature of the transfer case in the full-time 4WD. This system design has a locking mechanism for the center differential to use the full potential of 4WD. The self-locking differential can distribute power between axles in different amounts for maximum efficiency. The center differential locking in modern 4WD cars is implemented automatically by means of a viscous coupling, self-locking differential Torsen, or multiple-disc friction clutch.

BMW xDRIVE

Mercedes 4Matic

It should also be noted that full-time 4WD of modern cars are mainly controlled by electronics. The most known full-time 4WD systems are the following: Audi Quattro, BMW xDrive, and Mercedes 4Matic. Electronic systems in cars with permanent 4WD receive different signals from numerous sensors (for example, wheel speed sensors), after that the system almost instantly changes the ratio of transmitted power. Moreover, the electronic system also takes into account road conditions, possible wheel slippages, etc. Cross-axle differentials in full-time 4WDs are also equipped with locking mechanisms, but not always (they are not usually used on sedans and hatchbacks). Cross-axles are not necessarily to be installed on two axles; frequently such mechanism can be used on only one axle.

Principle of operation of full-time 4WD

First of all, the torque from the engine is transmitted to the gearbox and then to the transfer case. In the transfer case, the torque amount is distributed between front and rear axles. Subsequently, the torque is transmitted to the main gear through cardan shafts, and then to the center differentials of both axles. From the differential the torque is transferred to the drive wheels. If wheels of one of the axles start slipping, then the differential locking mechanism is actuated. This type of 4WD is considered to be one of the best solutions available on the market, because this system can significantly increase active safety and dynamics of the car.

The main drawbacks of this system are increased fuel consumption and higher loads on all 4WD transmission components, which reduce the resource of the main components, especially under the condition of aggressive driving.

On-demand 4WD

4Motion system from Volkswagen

This four-wheel drive system is activated only under certain conditions: when the wheels of the constantly operating axle start slipping. The rest of the time, the car with this 4WD system is either front-wheel drive (with transverse engine positioning) or rear-wheel drive (the engine is positioned longitudinally). Almost all leading automakers include this type of 4WD in their model line-ups. The most popular on-demand 4WD solution is 4Motion from Volkswagen. This system has some design peculiarities. The transfer case in this drive type has a simplified structure and it has no reduction gear, but at the same time On-demand 4WD ensures constant distribution of torque between axles. There is also no center differential, but this system has a smart mechanism for actuation of the “true” 4WD. It is noteworthy that this mechanism uses the same components as the center differential – viscous coupling or friction clutch with electronic control.

Principle of operation of On-demand 4WD

Torque from the engine is transmitted via the clutch, gearbox, rear-axle drive and differential – to the front axle of the car. Through the transfer case and cardan shafts torque is also transmitted to the friction clutch. In the normal position, the friction clutch has a minimum compression, at which up to 10% is transmitted to the rear axle. When the front axle wheels start slipping, the electronic control unit actuates the friction clutch and transmits torque to the rear axle. The amount of torque transmitted to the rear axle can vary within certain limits. This type of 4WD is considered by many specialists as one of the most promising solutions: under normal operating conditions you use only 2WD without putting additional load on powertrain components and save more fuel, but when needed the 4WD system is activated with the help of the transfer case.

Part-Time 4WD

If you need a reliable 4WD solution intended mainly for off-roading rather than driving on urban roads, the Part-Time 4WD system will be the best choice. This system ensures a rigid connection between the front and rear axles and torque transmission in the ratio of 50:50, therefore it can be considered as a great off-road option. Moreover, the part-time 4WD design is much simpler than the abovementioned versions: there is no center differential and locking mechanisms, there are no excessive pneumatic or hydraulic components. The rear axle in the system is the leading one, and the front axle is actuated by the driver. Torque is transmitted to the front axle when the driver engages the transfer case with the help of the lever and/or buttons.

This system will not allow you to drive at high speeds on solid surfaces, because equal torque distribution leads to slippages of wheels, which, in turn, entails extra loads powertrain components, rapid wear of tire, deterioration in road handling at high speeds on solid road surfaces. Therefore, nowadays this system is rarely used by car manufacturers.

Selectable 4WD

It should be noted that there are also combined 4WD versions, which include features of several drive systems mentioned above and they are designated as Selectable 4WD. In other words, the 4WD mode in these systems can be actuated manually and automatically (at the same time you can disengage any of the axles, if needed). The same applies to locking mechanisms in center and cross-axle differentials.

In this category we can point out the following models: Mitsubishi Pajero (Super Select 4WD), Jeep Grand Cherookee (SelecTrac), Nissan Pathfinder (All-mode 4WD). These systems can be considered as Full-Time 4WD with the capacity of forcible disengagement of the front axle. In Pajero, for example, you can select one of the following modes of operation: 2WD, 4WD with automatic locking of the center differential (analog to Full-Time 4WD), 4WD with a rigidly locked differential (analog to Part-Time 4WD), and low range Part-Time 4WD.

There is also an electromechanical 4WD option. In this case, all torque is transmitted to one axle only, while the second axle is equipped with electric motors, which are activated only in the automatic mode of operation. Thanks to the rapid development of environment-friendly technologies in the automobile industry, this solution becomes more and more popular. In the classic sense, it can hardly be called as the 4WD system, but rather a hybrid drive version.

Pros of 4WD

The key strengths of 4WD are traction and power. If you are driving on a steep hill or off-roading you will certainly need additional power that comes with 4WD.

• 4WD enhances traction parameters in hazardous driving conditions, such as snow, ice, rocks, and other cases that can cause trouble for a driver. When torque is distributed to both axles, traction and car handling gets much better.
• Extra weight facilitates to enhanced road-holding capacity.
• Hard acceleration without slippages of the wheels, while in case with 2WDs such accelerations are likely to result in slippages.

If you often drive on road surfaces with low traction, or if you enjoy off-roading, you will greatly benefit from four-wheel drive.

Cons of 4WD

• Enhanced fuel consumption;
• Expensive repair and maintenance;
• Additional components (differentials, transfer case, etc.) make this system much more complex and consequently there are more elements that may fail;
• Operation under severe conditions adds extra load on the powertrain components, thereby increasing their wear;
• Owners of 4WD vehicles can get carried away with driving too much under severe conditions and eventually get stuck even with 4WD.

If you drive mainly on urban roads under good weather conditions, then it is no use for you to spend more money on the 4WD car, but is more reasonable to stick with the 2WD option.

4WD Tips

• 4WD cars operate in the most efficient manner, when they are regularly used and serviced in accordance with recommendations provided by the automaker. If you do not engage the 4WD system for a long period of time, the seals can get dry. It is necessary to keep the system lubricated by engaging it at least once every few months.
• It is desirable to engage 4WD when you really need it to save as much fuel and money as possible. Driving 4WD under normal road conditions can be damaging for the front axle, differential gears, and other powertrain elements. Stick to 2WD on dry pavement.
• If you get stuck, activate 4WD and slowly release the gas pedal to get yourself out.