Types of four-wheel drive (4WD) and all-wheel drive (AWD) systems

Vehicle acceleration on a flat road is possible thanks to two systems: powertrain and driveline (drivetrain).

The powertrain is the system which generates power (torque and speed). In most of the cases it’s an internal combustion engine, but it can also be an electric motor or a combination of both (in case of a hybrid electric vehicle).

The driveline is the sum of mechanical components placed between the wheels and the powertrain. All the components after the engine, which transmit the power to the wheels, are part of the driveline. These components are: clutch/torque converter, gearbox, propeller shaft, differential and drive shafts. The driveline has multiple roles:

• allows the engine to run even if the vehicle is stationary
• allows a smooth vehicle launch from standstill
• converts engine torque and speed to match the road conditions
• allows the vehicle to move backwards, for the same direction of rotation of the internal combustion engine
• allows the drive wheels to rotate with different speeds during vehicle cornering

Image: Front-wheel drive (FWD) and rear-wheel drive (RWD) driveline architectures

Legend:

1. internal combustion engine
2. clutch / torque converter
3. gearbox
4. differential
5. propeller (longitudinal) shaft

The drive wheels are the wheels of a vehicle’s axle which are receiving the engine power, thus performing the traction. Depending on which axle contains the drive wheels, we can have:

• front-wheel drive (FWD)
• rear-wheel drive (RWD)
• four-wheel drive (4WD) / all-wheel drive (AWD)

Front-wheel drive (FWD) vehicles contain both the engine and the drive wheels on the front axle. This is the most common powertrain and driveline arrangement for small and compact vehicles, because of the advantages in terms of space and efficiency.

Rear-wheel drive (RWD) vehicle usually have the powertrain on the front axle and the drive wheels on the rear axle. This is also called the “classical” driveline arrangement, because this is how the first road vehicles were configured. Most of the luxury sedans and sport cars have rear-wheel drive configuration.

Both FWD and RWD vehicles are two-wheel drive (2WD) vehicles because the power is transmitted only through two wheels.

There are some vehicle architectures which have both the engine and the drive wheels on the rear axle (e.g. Porsche 911 classic, Renault Twingo 3).

Image: All-wheel drive (AWD) and four-wheel drive (4WD) driveline architectures

Legend:

1. internal combustion engine
2. clutch / torque converter
3. gearbox
4. rear differential
5. rear propeller (longitudinal) shaft
6. transfer case (with central differential and gear reductor (optional))
7. front propeller (longitudinal) shaft
8. front differential
9. coupling device (viscous, electromagnetic)

When the engine power is distributed to all wheels the vehicle is all-wheel drive (AWD) or four-wheel drive (4WD). There is no clear distinction between AWD and 4WD, but usually 4WD vehicles contain a transfer case, which has a central differential and an optional two-gear reductor (LO-low and HI-high).

In case of a AWD or 4WD vehicle, both front and rear axles need to be equipped with a differential, due to the fact that all wheels transmit power and they need to rotate with different speeds during vehicle cornering.

AWD/4WD vehicles are also called “four-by-four” (4×4) vehicles. The numbers come from the vehicle driveline formula:

$2 \cdot \text{TotalNumberOfAxles x } 2 \cdot \text{TotalNumberOfDriveAxles}$

For a vehicle with two axles, if only one axle has the drive wheels, the formula becomes “4×2“. If both axles have the drive wheels, the formula is “4×4“.

A permanent/full-time all-wheel drive vehicle has a permanent torque split between the front and rear axle, it can not be disabled by the driver or by an electronic control module (ECM).

An AWD/4WD vehicle can have a 2WD mode because the ECM (or the driver) can disconnect one of the axles from being driven. In modern vehicles, the switch between 2WD and 4WD mode is usually done without the driver noticing.

Vehicle manufacturers use different AWD/4WD technologies. Some of them are proprietary driveline systems, some use dedicated components, from Tier 1 suppliers.

Torsen®

Torsen comes from Torque Sensing and it’s a limited-slip mechanical differential. This type of differential was manufactured by the Gleason Corporation. They can be used as front / rear differential or as central (inter-axial) differential.

Image: Audi automatic transmission with Torsen center differential
Credit: Audi

Torsen differentials are fully mechanical, with satellites and helicoid gears. Their self-locking characteristic depends on torque difference sensing between front and rear axles or between left and right wheels.

Examples of vehicles equipped with Torsen AWD systems: Audi Quattro, Alfa Romeo Q4.

Haldex®

Haldex AWD systems are based on a central coupling device with a wet multi-disc clutch. They are manufactured by Haldex Traction AB group, currently owned by BorgWarner. Haldex systems are usually used as rear axle limited-slip differential.

Image: Cadillac SRX all-wheel drive (AWD) with Haldex electronic limited-slip differential

The Haldex limited-slip differential is controlled by an electronic control module (ECM). Through the multi-disc clutch position (open, closed, slipping), the vehicle can be operated as a FWD vehicle or AWD vehicle. The torque split between the front and rear axles is variable, depending on the clutch position. The system is controlled through an electro-hydraulic actuation system.

Haldex AWD systems are widely used in automotive industry, for example in the vehicles: Audi Q3, Skoda Octavia 4×4, VW Tiguan, SEAT Alhambra 4, Lamborghini Aventador LP 700-4, Bugatti Chiron, Volvo V60 AWD, Volvo XC90 AWD, Ford Kuga, Land Rover Range Rover Evoque, Opel Insignia, Buick Lacrosse, Cadillac SRX, etc.

BMW xDrive®

xDrive is the BMW proprietary 4WD technology. First BMW equipped with xDrive was X5 in 2004. The main component of the xDrive system is the transfer case. The purpose of the transfer case is to split the power coming from the gearbox between front and rear axles.

Image: BMW X-drive (4WD) driveline
Credit: BMW

The torque control between front and rear axle is performed through a wet multi-disc clutch inside the transfer case. The clutch position is actuated with an electric motor by an electronic control module. When the clutch is fully closed, the torque split is 50:50 between front and rear axle.

Mercedes 4MATIC®

4MATIC is the AWD/4WD technology developed by Mercedes-Benz. In consists in a central planetary differential which splits the torque between front and rear axles. The first generation of 4MATIC was using an electronically controlled central differential, a rear limited-slip differential and a front open differential. The latest generation of 4MATIC system is using three open differentials (front, rear and central).

Image: Mercedes S350 Bluetec 4-MATIC all-wheel drive (AWD) system
Credit: Mercedes

EMCD AWD systems

EMCD comes from Electro-Magnetic Control Device. It consists wet multi-disc clutch controlled by an electro-magnetic actuator. The EMCD system is manufactured by GKN Driveline. It is acting as a limited-slip central differential, controlled by an electronic control module (ECM).

Image: GKN Electro-Magnetic Control Device (EMCD)
Credit: GKN

The vehicles equipped with an EMCD are working in nominal mode as a FWD vehicles. The AWD capability is “on-demand” depending on the vehicle and road conditions. The driver has the option to completely lock the clutch, for permanent AWD capability, but in AUTO mode the ECM takes the decision.

Examples of vehicles with EMCD AWD system: Nissan Quashqai, Nissan X-Trail, Dacia Duster, Fiat Sedici.

Visco-coupling system

These are the simplest 4WD technologies. The front and rear axles are linked together through a viscous self-locking coupling device. The visco-couple contains several circular plates with tabs and perforations. These are immersed in a viscous, silicone-based fluid.

 Image: Visco-coupling device Image: Visco-coupling device cutaway

The visco-coupling technology was usually used on small vehicles. The front axle is the nominal drive axle, the rear axle was pulled, without torque being transferred through the visco-couple. If the front axle was spinning, due to loss of adhesion, the visco-couple started to lock, transferring torque to the rear axle.

Example of vehicle with visco-coupling: Fiat Panda, Renault Scenic RX4.

The advantage of the visco-coupling technology is simple construction at low cost. The disadvantages are low efficiency and slow reaction time.

Each of the above AWD/4WD technologies will be described in detail in separate articles.

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