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:
- internal combustion engine
- clutch / torque converter
- gearbox
- differential
- 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:
- internal combustion engine
- clutch / torque converter
- gearbox
- rear differential
- rear propeller (longitudinal) shaft
- transfer case (with central differential and gear reductor (optional))
- front propeller (longitudinal) shaft
- front differential
- 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
Credit: Cadillac
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.
For any questions or observations regarding this article please use the comment form below.
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larry davidson
I recently had my car towed but they towed it with 2 wheels down in neutral it is a 2008 bmw x5 all well drive is it safe to tow any of these cars like this
Karanjoshi
Hi,your article was so good and very informative looking forward for separate and detailed article of each type,Nice article!!
Sandra Patterson
Thank you for explaining that all the parts after the engine are all part of the drivetrain. My sister has been having problems with her car as of late, but she is sure where it is coming from. Maybe the problem is coming from the driveline; I’ll forward this article to her and suggest that she find professional to check it out for her.
Anton Ayrapetov
what about Subaru technology? Can you elaborate please ?
Kamran Nazari
hi
your article was so good
looking forward for separate and detailed article of each type
well done
Lakshman
Extremely good
Tuomas Hiismäki
The quality of your article is exceptionally good. Most other articles about the subject found in internet have 20…200 major errors, yours only those I mentioned in my 2 previous comments.
Congratulation for achieving that. If you correct those issues mentioned your article will be the best by a very wide margin and deserves to be recommended.
Anthony Stark
Thanks for your constructive comments! This is exactly what I’m expecting from the readers. Only with these types of comments, the articles can be more valuable and people can learn from them.
Tuomas Hiismäki
I have an observation regarding this article:
About BMW X-drive you claim :”When the clutch is fully closed, the torque split is 50:50 between front and rear axle.”
That is incorrect. An open differential will typically have 50:50 torque split, but there are differentials designed otherwise. A locked differential will have equal angular speed on both output shafts, but the torque is hardly ever equally split. It varies with road conditions and axle weight and can be absolutely anything between 0:100 and 100:0.
That is a fact for all locked differentials, including but not limited to the X-drive.
Anthony Stark
Hi Tuomas,
The 50:50 split can happen only if the load is the same on each axle. If the vehicle is driven on a straight road, with constant speed and clutch fully closed, the split is 50:50.
I think that your description is more generic and complements what I’ve said. I’ll add it in the article. Thanks.
Tuomas Hiismäki
I have a couple of observations regarding this article:
About Torsen you claim that :”Their self-locking characteristic depends on torque difference sensing between front and rear axles or between left and right wheels.”
That is not correct. Torsen do not self lock at all. It is a limited slip device only, not a locking device.
As a passive device (not producing any mechanical energy only transmitting), it will always send more torque to the axle spinning slower, and less to the axle spinning faster. Doing the opposite would mean being a perpetual motion device making energy out of nothing, hence no limited slip device is capable of such action. That means that if while cornering the wheel on the inside has less grip and begins slipping, it will have more torque by the Torsen, untill it spins equally fast than the outside wheel. there might be some systems using individual wheel braking that can do the opposite, but a Torsen differential will make their job harder in this situation.
The direction more torque is transmitted by Torsen is therefore always dependent on difference in angular speed of the 2 output axles, but never about the amount of torque. The amount of torque difference of the 2 output shafts on the other hand, is only dependent on torque of the rotating shafts, never on the angular speeds. You should include these facts to this article (at least the fact about conservation of energy requiring direction of torque transmission to always be towards the shaft with less angular speed), or some of it to some other article more specifically about Torsen.
Anthony Stark
The “self-locking” property of the Torsen differential is used in automotive literature as well. For reference, see Automotive Transmissions, by Harald Naunheimer.
I have in plan to have a dedicated article for Torsen differentials, for which I’ll take into account your comment.