Modern all-wheel drive systems can be designed differently, depending on the conditions the vehicle is intended for. Some cars are designed for asphalt and light off-road, others for serious challenges when you need to reach the most remote point.
It is the operating conditions that determine how complex and reliable the design will be, how it will behave on different surfaces and, ultimately, how much it will cost.
All existing all-wheel drive schemes can be reduced to two main types: permanent (FullTime) and plug-in (PartTime). The rest is just variations on this theme.
Let's start with the simple:
PartTime scheme. A vehicle where one axle is driven (usually the rear, but sometimes the front), and the second is connected when needed. There is no center differential here.
This is the simplest version of all-wheel drive. For heavy conditions, the transfer case has a low range. Engaging all-wheel drive can be mechanical (via lever) or using electric or pneumatic drive. To save fuel on the highway, you can disconnect the drive shafts from the wheels—for this, manual free-wheeling hubs ("hubs") are used. They are controlled manually or by electric drive.
The main advantages of this scheme are simplicity and reliability. The disadvantages are that all-wheel drive can only be used off-road, on ice, sand, or snow. If you drive with all-wheel drive engaged on asphalt at high speed, it will lead to increased wear of the transmission and tires.
In some designs, to simplify the connection of the front axle and eliminate hubs, one of the half-shafts of the connected axle is disconnected. This way, rotation from the wheels is not transmitted to the driveshaft.
This all-wheel drive scheme is used on SUVs: Great Wall Haval, Jeep Cherokee (standard Command Trac transfer case), Jeep Wrangler, SsangYong Kyron, SsangYong Rexton (with manual transmission), Suzuki Jimny, UAZ (all modifications).
FullTime 4WD is a true permanent all-wheel drive. Here there are three differentials: center, front, and rear axle differentials.
The center differential can distribute torque in different ways. The classic is 50/50. In modern cars, asymmetric options are more common, for example, 30:70 or 40:60. To improve off-road qualities, various systems for locking the center differential are used. The most common are hydromechanical clutches with electronic control and viscous couplings. The Torsen self-locking differential stands out—a purely mechanical device that does not need control systems. If the car is initially designed for serious off-road, it provides forced locking of the center differential and, often as an option, the rear one.
Permanent all-wheel drive is used on such vehicles: Land Rover Discovery, Land Rover Defender, Lada Niva, Jeep Grand Cherokee/WK (with NV245 transfer case), Mercedes G-class (since 1989).
Pseudo-permanent all-wheel drive. Widely used in crossovers, which manufacturers call vehicles with permanent all-wheel drive, although structurally it is not so. In essence, this is a PartTime scheme, but the connection occurs automatically.
One of the first systems on the market were those where the second axle was connected via a viscous coupling. Toyota was a pioneer among manufacturers. Using its example, we will analyze the principle of operation.
The system was called V-Flex Fulltime 4WD. As already mentioned, it can only be conditionally called full-fledged all-wheel drive. There was no center differential, and the transfer case was a simple angle gearbox connected to the driveshaft. In front of the rear axle, there was a viscous coupling (V-Flex II). When the front wheels slipped and the rotation speed of the front and rear axles differed, the coupling locked and connected the driveshaft to the input shaft of the axle. If the speeds were equal, the car remained front-wheel drive. The design was cheap but had drawbacks: the viscous coupling engaged with a delay, could not fully lock, was dangerous during active driving, and the coupling itself was not durable.
Facing such problems, manufacturers began to look for a solution, and systems appeared where a hydromechanical clutch with electronic control was responsible for engaging all-wheel drive. The connection principle changed.
Torque is transmitted through a pack of friction discs compressed by hydraulics. By compressing the pack, you can change the transmitted torque up to 100%. The control unit allowed connecting the rear drive not only during wheel slip but also under other driving conditions. The clutch could proportionally distribute force between the axles. Before the system engages, the car remains single-axle drive. But it was not possible to completely get rid of the drawbacks: with frequent engagements or heavy loads, the clutch can overheat and temporarily shut off.
An example of such a design is the Haldex clutch, one of the most popular in the world. It is constantly evolving, and today the fifth generation has already appeared.
The operating principle of Haldex clutches of different generations is the same; only the method of creating oil pressure and control differs. In the first and second generations, pressure was created by a pump driven by the difference in rotation speed of the front and rear axles, while in the fourth and fifth, it was an electric pump.
The transmitted torque is adjusted by the control unit, which analyzes data from many sensors. The difference in wheel rotation speeds is no longer the main condition for engaging all-wheel drive. The clutch can engage in advance, for example, during sharp acceleration. The system works so smoothly that the moment of engaging and disengaging all-wheel drive is almost imperceptible. The car remains all-wheel drive when needed.
All-wheel drive with hydromechanical clutches is the optimal solution in terms of design and price. It allows turning any front-wheel drive car into an all-wheel drive one with minimal costs. This is a good option for cars that mostly drive on asphalt, and the most serious off-road for them is a dirt road to the dacha. All-wheel drive here is needed more to improve handling on slippery surfaces. Serious obstacles are beyond such a car's capabilities. During prolonged driving on bad roads, the clutch overheats, and safety sensors turn it off. Then the car goes from all-wheel drive to single-axle drive, which can be an unpleasant surprise in the middle of a puddle or snow (we found this out when driving on snow in a Ferrari FF). Moreover, even in working condition, the clutch does not transmit 100% of the torque. For serious SUVs, other solutions are needed.
This all-wheel drive scheme (Haldex or similar proprietary developments) is used on vehicles: BMW X5 since 2004 (XDrive system), Chevrolet Captiva, Ford Kuga, Infiniti EX/QX/FX35, Honda CR-V, Hyundai Santa Fe (since 2007), Hyundai Tucson, Kia Sportage (since 2004), Nissan X-Trail (permanent front drive, rear engages when front slips)
One of the best, but technically complex, is the Super Select system from Mitsubishi. The Japanese were the first among their compatriots to use a "switchable" all-wheel drive scheme with a center differential on SUVs.
The design turned out to be so successful that Toyota borrowed it. It made minor changes and presented it under the name MultiMode.
The ability to drive on permanent all-wheel drive (thanks to the center differential) and turn it off to save fuel made this scheme one of the best for the modern driver. On serious off-road, it is enough to engage the forced locking of the center differential, and the car turns into a "tank".
By combining all options in one system, the designers gave the driver complete freedom of choice. Such a design easily transforms from PartTime to FullTime 4WD and back.
All-wheel drive according to this scheme is installed on: Mitsubishi Pajero Sport (latest generation), Mitsubishi Pajero, Lexus / Toyota Land Cruiser (some modifications)
We have provided general information about all-wheel drive systems. Perhaps the reader will think about what to choose. The first and most important thing is to think about where and how you will drive.
Speed lovers will suit a passenger car with permanent drive, preferably with electronic control. If you drive calmly and all-wheel drive is needed as insurance in winter, a car with plug-in all-wheel drive (manual or automatic) may be your choice. For those who love active recreation or fishing, it is better to look at cars with "hard" engagement of all-wheel drive, center differential lock, and a low range in the transfer case. Remember, an all-wheel drive car is always more expensive than a single-axle drive analogue. Weigh all the pros and cons before making a final decision.
