Surprisingly, but the fact is that many car owners do not understand the types of all-wheel drive transmissions at all. And the situation is exacerbated by automotive journalists, who themselves have difficulty understanding the types of drives and how they work.

The most serious misconception is that many still believe that the right all-wheel drive must be permanent, and categorically reject automatic all-wheel drive systems. At the same time, the automatically connected all-wheel drive is of two types, divided by the nature of the work: reactive systems (turned on by the fact of slipping of the drive axle) and preventive ones (in which the transmission of torque to both axles is activated by a signal from the gas pedal).

I will talk about the main options for four-wheel drive transmissions and show that electronically controlled four-wheel drive transmissions are the future.

Everyone roughly understands how a car's transmission works. It is designed to transmit torque from the crankshaft of the engine to the drive wheels. The transmission includes a clutch, gearbox, main gear, differential and drive shafts (cardan and axle shafts). The most important device in the transmission is the differential. It distributes the torque supplied to it between the drive shafts (half shafts) of the drive wheels and allows them to rotate at different speeds.

What is it for? When driving, in particular when cornering, each wheel of the car moves along an individual trajectory. Therefore, all the wheels of the car in turns rotate at different speeds and travel different distances. The absence of a differential and a rigid connection between the wheels of one axle will lead to increased stress on the transmission, the inability of the car to turn, not to mention such trifles as tire wear.

Therefore, for operation on paved roads, any car must be equipped with one or more differentials. For a vehicle with a drive on one axle, one cross-axle differential is installed. And in the case of an all-wheel drive car, three differentials are already needed. One on each axle, and one central, center differential.

To understand the principle of the differential in more detail, I highly recommend watching the documentary short film Around the Corner, filmed in 1937. For 70 years, the world has not been able to make a simpler and more understandable video about the operation of the differential. You don't even need to know English.

The main drawback, but rather a feature, of the operation of a free differential is known to everyone - if there is no clutch on one of the driving wheels of the car (for example, on ice or hung out on a lift), then the car will not even budge. This wheel will spin freely at twice the speed while the other will remain stationary. Thus, any 2WD vehicle can be immobilized if one wheel of the drive axle loses traction.

If you take a four-wheel drive car with three conventional (free) differentials, then its potential ability to move in space can be limited even if ANY of the four wheels lose traction. That is, if an all-wheel drive car with three free differentials is put with just one wheel on rollers / ice / hung in the air, it will not be able to budge.

How to make sure that the car can move in this case? Very simple - you need to block one or more differentials. But we remember that a hard differential lock (and in fact this mode is equivalent to its absence) is not applicable to the operation of a car on paved roads due to increased loads on the transmission and inability to turn.

Therefore, when operating on paved roads, a variable degree of differential lock is necessary (we are now talking about a single center differential) depending on driving conditions. But off-road, you can move even with all three differentials completely locked.

So, in the world there are three main types of all-wheel drive solution:

Classic all-wheel drive transmission(in the terminology of automakers referred to as full-time) has three full-fledged differentials, so such a car in any driving mode has a drive to all 4 wheels. But as I wrote above, if at least one of the wheels loses traction, the car will lose its ability to move. Therefore, such a car definitely needs a differential lock (full or partial). The most popular solution used on classic SUVs is a mechanical hard locking center differential with a 50:50 torque distribution along the axles. This allows you to significantly increase the cross-country ability of the car, but with a rigidly locked center differential, you cannot drive on paved roads. Optionally, off-road vehicles can have an additional locking rear cross-axle differential.

In the Full-time transmission, there are three differentials A, B and C. And in the part-time, the center differential A is absent and is replaced by a mechanism for hard connecting the second axle manually.

At the same time, a separate direction of mechanical plug-in all-wheel drive(Part-time). Such a scheme completely lacks an interaxle differential, and in its place is a mechanism for connecting the second axle. Such a transmission is usually used on low-cost SUVs and pickups. As a result, on paved roads, such a vehicle can only be operated with a drive on one axle (usually the rear). And to overcome difficult off-road sections, the driver manually turns on all-wheel drive by rigidly locking the front and rear axles together. As a result, the moment is transmitted to both axles, but do not forget that a free differential continues to remain on each of the axles. This means that with a diagonal hanging of the wheels, the car will not go anywhere. This problem can be solved only by blocking one of the inter-wheel differentials (primarily the rear), so some SUV models have a self-locking differential on the rear axle.

And the most universal and popular solution at present - automatic all-wheel drive(A-AWD - Automatic all-wheel drive, often referred to simply as AWD). Structurally, such a transmission is very similar to a part-time all-wheel drive, which does not have an interaxle differential, and a hydraulic or electromagnetic clutch is used to connect the second axle. The degree of clutch lockup is usually electronically controlled and there are two mechanisms of operation: preventive and reactive. About them below in detail.

There is no center differential in the transmission, two shafts come out of the gearbox, one to the front axle (with its own differential), the other to the rear, to the clutch.

It is important to understand that for the most efficient all-wheel drive transmission (regardless of whether it is full-time or a-awd), a variable locking center differential (clutch) is required, depending on road conditions (a separate discussion about wheel differentials, not within the scope of this article) . There are several ways to do this. The most popular of them: viscous coupling, gear self-locking differential, electronic lock control.

1. A viscous clutch (a differential with such a clutch is called VLSD - Viscous Limited-slip differential) is the simplest, but at the same time ineffective way to block. This is the simplest mechanical device that transmits torque through a viscous fluid. In the case when the speed of rotation of the input and output shaft of the coupling begins to differ, the viscosity of the fluid inside the coupling begins to increase until it completely solidifies. Thus, the clutch is blocked and the torque is equally distributed between the axles. The disadvantage of a viscous coupling is too much inertia in operation, this is not critical on paved roads, but practically excludes the possibility of its use for off-road operation. Also, a significant drawback is the limited service life, and as a result, by a run of 100 thousand kilometers, the viscous coupling usually ceases to perform its functions and the center differential becomes constantly free.

Viscous couplings are now sometimes used to lock the rear axle differential on SUVs, and as a locking center differential on Subaru vehicles with a manual transmission. Previously, there were cases of using a viscous coupling to connect the second axle in systems with automatically connected all-wheel drive (Toyota cars), but they were abandoned due to their extremely low efficiency.

2. The well-known Torsen differential belongs to gear self-locking differentials. Its principle is based on the property of a worm or helical gear to "jam" at a certain ratio of torques on the axles. This is an expensive and technically complex mechanical differential. It is used on a very large number of all-wheel drive vehicles (virtually all Audi models with all-wheel drive) and has no restrictions on use on paved roads or off-road. Of the shortcomings, it should be borne in mind that in the absence of rotational resistance on one of the axles, the differential remains in the unlocked state and the car is not able to budge. That is why cars with a Torsen differential have a serious "vulnerability" - with a complete lack of adhesion on BOTH wheels of one axle, the car is not able to move. It is this effect that can be seen in this video. Therefore, new Audi models now use a crown gear differential with an additional clutch package.

3. Electronic lock control includes both simple methods of braking slipping wheels using the standard brake system, as well as complex electronic devices that control the degree of differential lock depending on the road situation. Their advantage lies in the fact that the viscous coupling and Torsen limited-slip differential are completely mechanical devices, without the possibility of electronic interference in their operation. Namely, electronics is able to instantly determine on which of the wheels of the car torque is required and in what quantity. For these purposes, a complex of electronic sensors is used - rotation sensors on each wheel, a steering wheel and gas pedal position sensor, as well as an accelerometer that records the longitudinal and lateral accelerations of the car.

At the same time, I want to note that the differential lock simulation system based on the standard brake system often turns out to be not as effective as the direct differential lock. Usually imitation of blocking with the help of the brake system is used instead of inter-wheel blocking and is currently used even on cars with a drive on one axle. An example of an electronically controlled locking center differential would be the VTD all-wheel drive transmission used on Subaru vehicles with a five-speed automatic transmission, or the DCCD system used on the Subaru Impreza WRX STI, as well as the Mitsubishi Lancer Evolition with an active ACD center differential. These are the most advanced all-wheel drive transmissions in the world!

Now let's move on to the main subject of discussion - transmissions with automatic all-wheel drive (a-awd). Technically the most simple and inexpensive way to implement all-wheel drive. Among other things, its advantage lies in the possibility of using a transverse engine layout in the engine compartment, but there are options for its use with a longitudinal engine arrangement (for example, BMW xDrive). In such a transmission, one of the axles is the leading one and under normal conditions it usually accounts for most of the torque. For cars with a transverse engine, this is the front axle, with a longitudinal engine, respectively, the rear.

The main disadvantage of this type of transmission is that the wheels on the connected axle cannot physically rotate faster than the wheels on the "main" axle. That is, for cars where the clutch connects the rear axle, the proportion of torque distribution along the axes ranges from 0:100 (in favor of the front axle) to 50:50. In the case when the “main” axle is rear (for example, xDrive system), often the nominal torque ratio is set with a slight shift in favor of the rear axle in order to improve the steering of the car (for example, 40:60).

In total, there are two mechanisms for the operation of an automatically connected all-wheel drive: reactive and preventive.

1. The reactive algorithm of operation implies blocking the clutch responsible for transmitting torque to the second axle, upon the fact of wheel slip on the drive axle. This was exacerbated by huge delays in connecting the second axle (in particular, for this reason, viscous couplings did not take root in this type of transmission) and led to ambiguous behavior of the car on the road. Such a scheme has become massively used on initially front-wheel drive vehicles with a transverse engine.

In turns, the work of the reaction clutch looks like this: Under normal conditions, almost all the torque is transmitted to the front axle, and the car is essentially front-wheel drive. As soon as there is a difference in the rotation of the wheels on the front and rear axles (for example, in the event of a drift of the front axle), the center clutch is blocked. This results in sudden traction at the rear axle and understeer is replaced by oversteer. As a result of connecting the rear axle, the rotation speeds of the front and rear axles are stabilized (the clutch is blocked) - the clutch is unlocked again and the car is front-wheel drive again!

Off-road, the situation does not get better, in fact it is an ordinary front-wheel drive car, on which the moment the rear axle is turned on is determined by the slipping of the front wheels. It is for this reason that many off-road crossovers with this type of drive are completely incapable of moving in reverse. And on such a transmission, the moment of connecting the rear axle is especially well felt. At the same time, on paved roads, the car always remains front-wheel drive.

Currently, such an algorithm for the operation of an automatically connected all-wheel drive is rarely used, in particular, these are Hyundai / Kia crossovers (except for the new DynaMax AWD system), as well as Honda cars (Dual Pump 4WD system). In practice, such a four-wheel drive is completely useless.

2. A safety lock clutch works differently. Its blocking occurs not upon the fact of wheel slip on the “main” axle, but in advance, at the moment when traction is required on all wheels (the speed of rotation of the wheels is secondary). That is, the clutch lock occurs at the moment when you press the gas. Things like steering angle are also taken into account (with the wheels turned too far, the degree of clutch lockup is reduced so as not to burden the transmission).

Remember, to connect the rear axle, slipping of the front is not required! The clutch lock of the automatic all-wheel drive is primarily determined by the position of the gas pedal. Under normal conditions, about 5-10% of the torque is transmitted to the rear axle, but as soon as you press the gas, the clutch is blocked (up to a complete blockage).

A serious mistake, which has been made by automotive journalists for more than a year, is not to confuse the algorithms of the automatically connected all-wheel drive. The system of automatically connected all-wheel drive with preventive blocking constantly transmits torque to all 4 wheels! For her, there is no such thing as "sudden connection of the rear axle."

Preventive lockup clutches include Haldex 4 (my separate article on the topic) and 5th generation, Nissan/Renault clutches, Subaru, BMW xDrive system, Mercedes-Benz 4Matic (for transversely mounted engines) and many others. Each brand has its own operation algorithms and control features, this should be borne in mind in a comparative analysis.

This is what the front axle coupling looks like in the BMW xDrive system

You should also pay special attention to driving skills. If the driver is not familiar with the principles of driving a car on the road and, in particular, with how to corner (I just talked about this recently), then with a very high probability he will not be able to park the car with an automatically connected drive system sideways, while he can easily do this on an all-wheel drive car with three differentials (hence the erroneous conclusion that only Subaru can drive sideways). And of course, you should not forget that the amount of traction on the axles is regulated by the gas pedal and the steering angle (including, as I wrote above, the clutch will not completely block when the wheels are turned too far).

The scheme of operation of the 5th generation Haldex coupling, fully controlled by electronics (let me remind you, Haldex 1,2 and 3 generations had a differential pump in the design, which was driven by the difference in the rotation of the incoming and outgoing shaft). Compare to the insanely complex Haldex 1st generation clutch design.

In addition, almost always such systems are supplemented with electronic imitation of interwheel differential locks using the brake system. But it should be borne in mind that it also has its own characteristics of work. In particular, it only works in a certain rpm range. At low speeds, it does not turn on so as not to “strangle” the engine, and at high speeds, so as not to burn the pads. Therefore, it makes no sense to drive the tachometer into the red zone and rely on the help of electronics when the car is stuck. For off-road applications, hydraulic clutch systems have a higher resistance to overheating than friction electromagnetic clutches. In particular, the Land Rover Freelander 2/Range Rover Evoque can be an example of a vehicle with automatic four-wheel drive based on the 4th generation Haldex clutch and very impressive off-road capabilities.

What is the result? There is no need to be afraid of automatic all-wheel drive systems with preventive blocking. This is a universal solution for both road use and occasional off-road use of medium difficulty. A car with such an all-wheel drive system handles adequately on the road, has neutral steering and always remains all-wheel drive. And do not believe the stories about the "sudden connection of the rear axle."

Addition: A very important issue to understand is the distribution of torque along the axes. Promotional materials from automakers are often misleading and further confusing in understanding how an all-wheel drive transmission works. The first thing to remember is that torque only exists on wheels that have traction. If the wheel is hanging in the air, then despite the fact that it is freely rotated by the engine, the torque on it is ZERO. Secondly, do not confuse the percentage of torque transmitted to the axle and the proportion of torque distribution over the axles. This is important for automatic all-wheel drive systems, because. the absence of a central differential limits the maximum possible distribution of torque along the axes in the ratio 50/50 (that is, it is physically impossible for the ratio to be greater towards the connected axle), but at the same time up to 100% of the torque can be transmitted to each axle. Including connected. This is explained by the fact that if there is no adhesion on one axis, then the moment on it is equal to zero. Therefore, all 100% of the moment will be on the axle connected by the clutch, while the ratio of the distribution of torque along the axes will still be 50/50.

Now a very large number of so-called crossovers have not entirely honest all-wheel drive. It is not permanent, and even connected for a very short time (I would like to note that it is automatically connected) - we will definitely talk about whether this is good or bad in another article, but today I want to talk about “automatic connection” using a “viscous coupling” - and what is it you know? After all, this unit is now very much in demand, but unfortunately many simply do not represent the principle of its operation, although this name is on everyone's lips. Well, as usual, I figured out the topic and will try to tell you in detail what it is and how it actually works, there will be a detailed video at the end, so read on - watch ...

In fairness, I would like to note that viscous couplings are used not only in all-wheel drive systems, but also in car cooling systems and not only. To begin with, as usual, the definition.

Viscous coupling (or viscous coupling) is an automatic device for transmitting torque by means of the viscous properties of special fluids.

To put it simply, the torque is transmitted by changing the viscosity of a special fluid in the viscous coupling housing.

About the liquid inside

At the very beginning, I want to talk about the fluid that is inside the viscous coupling, what it is and what properties it has.

To begin with, I would like to say that they are poured inside - a dilatant liquid, which is based on silicone. Its properties are very interesting, if it is not strongly heated and stirred, it remains liquid. BUT if you mix it a lot and heat it up a little, it thickens and expands very much, it becomes more like a hardened glue. After mixing again becomes insignificant, it again acquires its original state of aggregation, that is, it becomes liquid.

It should be noted that the liquid is filled for the entire service life of this unit and is not subject to replacement.

Device and principle of operation

It's very much like an automatic transmission's torque converter, if you will, where torque is transmitted by oil pressure. Here, too, the transmission of torque occurs due to the liquid, however, there are global differences in the principle of operation.

There are only two main viscous devices:

• There is a closed sealed housing in which two turbine wheels with impellers rotate opposite each other (sometimes more), one is mounted on the drive shaft, the other on the driven one. Of course they rotate in our dilatant fluid. As long as the shafts rotate synchronously, there is practically no mixing of the liquid. BUT once one axle stands up and the other rotates very quickly (wheel slip), then the liquid inside begins to mix and heat up very quickly, which means it thickens. Thus, the first driving impeller engages with the driven impeller and begins to transmit torque to the second axis. After the car has coped with off-road, mixing stops and the rear axle is automatically switched off.

• The second design also has a closed body. Only on the driving and driven shafts are several groups of flat discs. Part on the slave, part on the master. They also rotate in a special fluid. While the rotation is uniform, the mixing of the liquid is minimal and it is liquid, but after one axis stands up, the second one starts to slip, the mixing is huge! It not only thickens, but also expands. Thus - very strongly pressing the disks to each other. As a result, the transmission of torque - the second axis begins to rotate.

A viscous coupling is a fairly simple and effective mechanical device, with proper use it can walk without any problems for a very long time.

Where are viscous couplings used?

Actually there are only two main applications, but now there is only one:

• Used for engine cooling. A viscous coupling with a fan was attached to the rod. It was driven from the crankshaft of the car through a belt drive. The faster the engine rotated, the more the liquid thickened and the connection with the fan became stiffer. If the speed dropped, then there was no such strong mixing, which means that there were slippages, that is, the fan rotated, it did not cool the radiator so much. Such a system is effective for the cold (winter) period, when the engine does not warm up much, but it is also cooled. Now the use of such systems on new cars is no longer to be found, it was replaced by electronic fans (with sensors in the liquid), which are powered by electricity and are in no way connected with the engine crankshaft.

• Automatic connection of a full drive. It is in this direction that viscous couplings have remained very much in demand. Practically on 70 - 80% of crossovers or SUVs, such systems are now used. True, they are gradually beginning to be replaced by fully electromechanical options, but so far they are more expensive and not so practical.

On the one hand, a viscous coupling is a very simple, cheap, practical and versatile mechanical device, on the other hand, it has a lot of disadvantages.

Pros and cons of viscous coupling

To begin with, I propose to talk about the advantages of this site:

• Simple construction. Indeed, the design is very banal, there is nothing overly complicated in it.
• Cheap. Due to its simplicity, it is not expensive at all
• Durable. The viscous coupling housing can withstand a pressure of 15 - 20 atmospheres, it all depends on the design. If initially there were no breakdowns, then this means that it can take a very, very long time.
• Practical. WHEN USED PROPERLY. Installed for the entire life of the car, does not require any attention.
• ON a dirt road or asphalt, it can also work. If you say abruptly “started” from a place or there is a slip on ice or dust. Then the rear axle will automatically connect. This gives advantages in handling even in the city.

Despite the advantages of the design, it is worth noting about its shortcomings, because there are also many of them.

• Maintainability. As a rule, it is not repaired, that is, disposable, it is not profitable to repair and it is very difficult for a simple layman. Almost always replaced with a new one.
• Connectivity. There is no linear dependence of the all-wheel drive connection, it is almost impossible to guess when the disks inside will slow down! Therefore, there is no control over all-wheel drive.
• You cannot connect the drive manually yourself.
• Low efficiency all-wheel drive. The transfer of maximum torque will be only when the front wheels will slip very much.
• Large viscous couplings are not used. Because it needs a large body, and since it hangs from below, it really greatly reduces the car's ground clearance. The use of small housings, that is, small viscous couplings, leads to a limited transmission of torque to the rear axle, because there are fewer discs and a small volume of special fluid
• A viscous coupling cannot work for a long time. This is highly undesirable! It is not designed for long-term loads, otherwise it will simply fail, it will completely jam. That is, it tells us that you can’t meddle in serious off-road! Use it as soon as possible for snowy yards and a little dirt in the country, that's all.

On many cars, all-wheel drive is pluggable. The all-wheel drive on Chery Tiggo cars is also arranged, the rear-wheel drive is connected here automatically, through an electromagnetic clutch.

The clutch is controlled by the four-wheel drive control unit. The principle of operation of an electromechanical clutch is almost the same as that of a clutch. When voltage is applied to the clutch, the discs inside the clutch are pressed against each other and torque is transmitted to the rear wheels through them.

Four-wheel drive is connected to Cherry Tiggo only at the moment of slipping of the front wheels, and approximately after the second rotation of the wheel. When the need for all-wheel drive disappears, it turns off. Also, the drive is switched off when a certain speed threshold is exceeded, because the clutch operation is not designed for high speeds.

There is a four-wheel drive test lamp on the Cherry instrument panel. When the ignition is turned on, the lamp lights up and the system performs a self-test. If everything is in order, the lamp goes out. In the presence of malfunctions, the lamp will continue to burn.

Unfortunately, there are no identification signs that the drive has turned on in the car. But you will easily understand this when you get stuck and start to slip. When the rear wheel drive is connected, you will feel a slight push, and the car will slowly start to climb out of the blockage.

Torque to the rear wheels is transmitted through the transfer case (2), front cardan (4), electromagnetic clutch (5), rear cardan (6), rear axle gearbox (7) and rear wheel drives.

Vehicle all-wheel drive transmission diagram

1 - gearbox, 2 - transfer case, 3 - front wheel drives, 4 - front cardan gear, 5 - electromagnetic clutch, 6 - rear cardan gear, 7 - rear axle gearbox, 8 - rear wheel drives.

Transfer case

The transfer case is rigidly attached to the gearbox housing. The drive for the razdatka is the differential box. The transfer box itself is two-stage. There is no center differential in the transfer case, and the redistribution of torque between the axles is performed by an electromagnetic clutch, depending on road conditions.

Shafts of cardan gears are made of thin-walled steel. The electromagnetic clutch transmits torque to the rear wheels only when the clutch is partially or completely blocked by a signal from the four-wheel drive control unit.

The all-wheel drive control unit is located under the driver's seat. The drive unit receives information from the engine control unit and, based on the data received, turns on or off the clutch, thus applying or removing torque to the rear wheels.

The block receives the following information:

- longitudinal acceleration of the car (from the acceleration sensor under the dashboard console)

- vehicle speed and wheel speed difference (from wheel sensors)

“Honest four-wheel drive” is not quite a clear but convincing term, the sacred mantra of the Internet guru. However, today the vast majority of manufacturers rely on electronics and multi-plate clutches that automatically connect the rear axle ...

It is good to have a car with a 4x4 wheel formula in case of an assault on a snow drift, and for the rest of the time - an economical monodrive. And when starting off on wet pavement, it is useful to be fully armed. But after a moment, when the speed is dialed, the extra drive axle is just a waste of fuel.

This is a 100% crossover format, and in order to make it possible to quickly or briefly turn on the second pair of driving wheels, various multi-plate clutches for their connection have appeared.

SAVING METAL AND FUEL
The inexpensive and compact multi-plate clutch, which does not cause additional vibrations and is extremely responsive, has replaced all other types of transmission in 90% of all-wheel drive vehicles today, reducing the formula of the current construction of a mass crossover to a single principle: a transversely located front motor constantly drives the front wheels, and the rear ones are connected by a clutch along needs.

Four-wheel drive, implemented in this way, is much simpler than real off-road structures. There is no transfer case, only an additional pair of power take-off gears and an output shaft remain near the front differential. Another plus: thanks to the low weight and size, it became possible to unload the weight of the clutch from the already heavy front of the car. The multi-plate clutch settled directly on the rear gearbox.

DIFFERENT
But the clutch is different. With the same principle of connecting the second bridge, the structures can have significant differences.

Initially, it was decided to somehow force the clutch to work from slippage of the front half, connected to the motor and front wheels, relative to the rear, connected to the rear wheels. I stalled in front, the difference in the speed of the halves went, the clutch blocked, the back was connected. Is it logical?

The very first clutches were used by Volkswagen Golf in its Syncro transmission. The clutch pack did not shrink in them, but was filled with silicone fluid, which thickened under heavy loads and itself transmitted rotation. It was impossible to control such a visco-clutch, the characteristics of its work left much to be desired, and it could not transmit 100% of the torque to the rear wheels. In addition, when slipping in the mud, the silicone boiled up, the clutch quickly overheated and ... burned out.

Another design found its way onto early Ford Escapes. There, the clutch discs were already compressed, but this happened purely mechanically, with the help of balls and wedge-shaped slots, at the moment the front part was rotated relative to the rear. The clutch worked more clearly, but sharper, causing unexpected blows in the most critical phase of a slippery turn.

Imagine that in a turn your car suddenly turns from front-wheel drive into a “classic”, and when you release the gas, the clutch also suddenly turns off. The consequences can be fatal.

This problem has plagued coupling manufacturers for quite some time. In order to more adequately regulate the flow of power to the rear wheels, and at the same time protect the clutch discs from overheating, an attempt was made to use hydraulics.

COMING HALDEX
The latest version of the uncontrolled clutch was the first generation of Haldex in 1998. Here, the disks were compressed by a hydraulic cylinder, the oil pressure for which was generated by a pump. The pump was mounted on one half of the coupling, and the drive to it came from the other. That is, now, with a difference in the speed of the front and rear wheels, the compression pressure increased and the clutch was blocked. Haldex worked gently and was successful.

There were two wins at once: the oil, now circulating through the hydraulic pump, cooled better, and the hydraulic drive was clearer and, most importantly, worked faster. But still, part of the drive functionality remained unused - anticipating the connection of the rear axle at the very beginning of the development of a dangerous situation, partial blocking of the clutch for cornering. Electronics could and should have coped with this.

So in 2004, the second generation of Haldex appeared, all with the same discs and a pump, but with an electronic valve, and the department in charge of all-wheel drive was introduced into the “brains” of the machine’s stabilization system.

Compact. The entire set of Haldex clutch elements is assembled into a tight block and is only slightly larger in size than a standard differential.

The system became controllable, and the torque transmitted back was no longer directly dependent on the difference in the speeds of the front and rear wheels.

FOREWARNED IS FOREARMED
Everything would be fine, but situations remained “unaffected” in which it would be nice to get a complete four-wheel drive even before the front wheels slip. In other words, the pump, powered by the difference in speed between the halves of the clutch, no longer suited the transmission engineers. After all, his saving pressure in some modes of motion was simply absent.

The solution turned out to be simple and, in general terms, is still used today in most drives implemented by means of a clutch.

The next - the fourth - generation of Haldex received an electric pump attached from the outside and the adjustment valves already familiar to us in front of the hydraulic cylinders. Now, at any time, the clutch could be fully or partially closed only by an electronic signal.

This principle has had many positive effects. There were modes of starting from a place, in which the clutch is completely blocked for a short period of acceleration. Significant blocking modes have been added in corners, when good grip on dry pavement allows you to use all-wheel drive to the fullest.

Surprisingly, all-terrain qualities have increased. After all, now it has become possible by simply pressing a button to switch the clutch operation algorithm from “asphalt” to “off-road” or to entrust this matter to automation.

Do you recognize the three main transmission modes of your crossover? Of course, you have just such a clutch in the rear wheel drive!

Just a moment. Two components of the system's speed - an electronic brain and an ultra-fast electrovalve, the opening time of which is less than 0.1 s

FURTHER MORE
It has become convenient to combine the electronic control of the clutch with both the stabilization system and the clutch's own safety program. A small thermal sensor inside the clutch now monitored the operating temperature and turned off the drive if the clutches were close to overheating. Of course, a car that has become non-driven for ten minutes can unbalance, but this is incomparably better than smoke from under the bottom and transmission failure.

In addition, the more crossovers with electronically controlled clutches ended up in the hands of the owners, the wider and more accurate the programs of all-wheel drive systems became. Today, the best of them are no longer afraid of overheating, not only in loose snow, but also with frank mud skidding. And also chemists with materials scientists did not sit idly by. New materials for discs and linings made it possible to double the emergency shutdown temperature, as well as to increase the moment transmitted by the friction clutches to values ​​that are obviously greater than the motor can give out.

Modern clutch materials, high quality oils and advanced disc lock control programs make it possible to even keep the clutch partially engaged without fear of overheating. At the same time, the car receives a torque distribution along the axes in a ratio of 10:90, or even 40:60, which for brands that gravitate towards a rear-wheel drive layout allows you to combine classic habits on the road with light all-wheel drive, sometimes almost imperceptible. And even continuously vary the degree of connection, improving the controllability of the car and helping the stabilization system to do its job.

Given the flexibility of the algorithms of work and the high degree of refinement of the design of multi-plate clutches, today this is the most massive version of the organization of all-wheel drive and it is unlikely that something fundamentally new awaits us here in the foreseeable future.

Somehow it so happened that the plug-in all-wheel drive is considered a solution that is not particularly reliable, not capable of transmitting a large moment, and generally palliative, associated with cost savings. Moreover, 9 out of 10 of my friends who know about cars firsthand are sure of this. But you must admit: the words “savings” and “cheaper” sound somehow strange when it comes to the latest X5, X6 and Cayenne, well, or about the “modest” 550Xi or Panamera. Apparently, the reason is completely different - it is hardly possible to “save” so much on a banal center differential.

If the differentials were so expensive, then instead of the interwheel, they would probably also use something else? And the well-known Torsen is clearly not worth millions. Yes, it's not the price of the differential itself. Surprises were presented by the identified nuances in setting up the handling and operation of various electronic "assistants": ABS, ESP and other active safety enhancement systems. And all this is because the requirements for active safety of cars have grown dramatically over the past decades, and the handling of even simple cars is at a level that sports cars never dreamed of in the eighties.

What is good permanent four-wheel drive? The fact that the torque is constantly present on all wheels, being distributed according to certain rules, rigidly set by the device of the mechanism. It is not possible to directly specify the distribution, but there are other ways to "teach" the machine to do what it needs. For example, the introduction of a lock, the use of brakes or something else.

It seems that there is no particular need for such “subtleties” on paved roads, because Audi Quattro, Alfa 155, Lancia Delta Integrale drove ... Due to its distribution to all four wheels, it allows to increase the lateral component of the load, which means that it is faster to take turns. In addition, you can implement engine traction on any surface. In addition, the differential is a reliable thing, it is not so easy to break it, they are made with a margin, the differential has a very high resource. In general, solid pluses.

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Unfortunately, there were some cons too. Any change in traction on an all-wheel drive vehicle causes a redistribution of mass along the axles and wheels, and a complex transmission then distributes the moment. The share of the moment will go to all four wheels, but its amount will depend on many factors. From the adhesion of each of the wheels, from the mass of transmission parts, from friction losses in the nodes, and so on. As a result, it turns out that it is difficult to predict exactly how the traction on each of the axles will change. Given the constant change in load, changes in the slip angles of the front and rear axles become almost unpredictable. Only a very experienced driver can feel all the nuances of the car's reaction to control actions and be ready for any development of events. We had to find a way out of this situation.

How is it done?

The stability of the machine can be increased by special design measures. For example, by increasing the moment of inertia around the vertical axis, distributing the load in favor of one of the axles so that it is constantly greater on one than on the other, changing the thickness of the tires or installation angles. Doesn't it remind you of anything? Of course, Audi cars. On them, permanent four-wheel drive became familiar and had at least a few features from this list.

Pictured: Audi A6 Allroad 3.0 TDI quattro" 2012–14

The motor located in front of the axle provided a large moment of inertia around the vertical axis and a guaranteed high loading of the front axle. The multi-link front suspension provides the best grip precisely on the front axle in wide load ranges.

On the Porsche 911 Carrera 4, a similar drive scheme is simply "flipped" by 180 degrees, and the layout features are the same. But on cars of other brands, this scheme somehow did not take root - the only exceptions are rare cars for "racers" and a small number of crossovers.

Pictured: Porsche 911 Carrera 4 Coupe "2015–present

Subaru's all-wheel drive scheme and layout are almost the same as those of Audi, with the exception of simpler suspensions and a more compact engine. At the same time, due to the smaller size and less overload of the front axle, the handling is much more “sporty”.

Mitsubishi, Lancia and Alfa Romeo are not even worth remembering: their layout with a transverse motor, and even on very compact cars, was not originally intended for unprepared drivers.

In the photo: Under the hood of the Alfa Romeo 156 "2002–03

It turns out that if you do not take special design measures, a car with permanent all-wheel drive has complex controllability. It can demonstrate the habits of either a front-wheel drive or a rear-wheel drive car, depending on traction, load, and a thousand other reasons. To obtain an acceptable result for a production car, you will have to spend a lot of effort on fine-tuning the handling, because the average driver does not like such surprises, he needs unambiguous behavior. Of course, it can be obtained by installing sophisticated electronic stability control systems, but this is a complicated and expensive way. It will be much easier to simplify the transmission scheme by installing a clutch that connects the second axle only if necessary. Of course, you still can’t do without electronics, but in the case of a front-wheel drive car with a transverse engine, the transmission will become an order of magnitude simpler. For example, instead of a very complex and heavy transfer case, you can get by with a simple angular gearbox.

On machines with a longitudinal engine and a classic layout, the advantages of installing a clutch are slightly less. In the mass of a significant gain, it will not work, but on the other hand, the front axle can be almost not connected, getting rid of jerks in steering traction. And you can also reduce fuel consumption, which is also important for a production car.

To connect or not to connect?

Permanent four-wheel drive is not so complicated, and it is not so expensive. And it is no coincidence that they were often equipped with permanent all-wheel drive. Why are there crossovers - remember our Niva, which turned out to be cheap and angry at the same time.

For initially front-wheel drive cars, it turned out to be really easier and cheaper to make the drive plug-in. A difference in weight of 50 kg is already very serious, and the advantages of unambiguous controllability and the possibility of easy tuning of ABS systems significantly reduced the price of “finishing” the model.

The viscous couplings used at first to connect the rear axle turned out to be not the best choice, and they were quickly changed to electronically controlled designs. True, some manufacturers, for example, Honda, held on to their specific ways of connecting all-wheel drive (we are talking about the Dual-Pump-System). But after the mass introduction of even the simplest systems with controlled connection, it became obvious that such a drive is quite enough for the vast majority of drivers. Moreover, it is enough even in the case of powerful machines and increased requirements for handling and cross-country ability.

There are also disadvantages to the plug-in all-wheel drive system. First of all, they are due to the fact that there are many nodes that are expensive. Therefore, they are constantly trying to make cheaper and simpler. The results, however, are not always encouraging.

For example, the clutch may not hold all the engine torque in first gear, but only part of it, or hold the moment only for a limited time. It may not provide the ability to work with slippage, and the connection speed may not be regulated or regulated too roughly. The clutch may not be designed for long-term operation, as a result of which it often overheats under load.

The electronics serving the connection system can also be simplified. In this case, the algorithms sometimes do not take into account some of the driving modes, reducing the ease of safe handling.

After all, the clutch always has wear parts - for example, the clutches themselves, and often also the components of the hydraulic drive or electrics.

And yet, as the cost of electronics decreases and the use of such systems on more and more expensive machines, the quality of such a connection mechanism is steadily increasing. Although in general the clutch is still much more expensive than a simple differential, and attempts to make it even cheaper do not stop.

I note that there are such connection designs, the efficiency of which exceeds all permanent all-wheel drive systems. These include almost all the latest generations of all-wheel drive transmissions with variable thrust vectoring on Subaru and Mitsubishi and premium German cars. They make it possible to directly control the torque on one or more wheels to choose from. This allows you to create cars with perfect handling and fantastic capabilities. Behind the wheel of such a car, any curve on any surface will be “registered” almost perfectly, and with minimal effort on the part of the driver. Unfortunately, these are complex and expensive systems that aim to deliver fantastic performance on the racetrack. And they are designed without regard to the cost of operation.

Do not be afraid of simpler systems. For example, much more massive cars endow with excellent handling and patency of the Haldex coupling of the last few generations. Sub-models Land Rover, Range Rover, VW, Audi, Seat and Volvo make extensive use of the brand's designs. And in operation, such systems have proven to be quite reliable.

BMW all-wheel drive cars get both excellent cross-country ability and impeccable behavior on asphalt. Since the permanent all-wheel drive on the E53 was replaced with a plug-in one, the system has been continuously improved, and the results of the progress are impressive. Even the reliability was able to increase to a completely acceptable level.

Today, even very inexpensive systems with a purely electric drive from Asian brands do not give up on the roads, and on the highway, cars with them please with excellent behavior.

What will happen next?

Another ten years - and apart from jeepers, few will remember about permanent all-wheel drive. And as ICE cars are replaced by electric vehicles, complex transmissions will die out on their own, like mammoths. And I'm afraid it's time for everyone to reconsider their attitude to permanent four-wheel drive. This is not an expensive or elite solution, but just a technology from the mid-eighties that is not particularly in demand. From the time when the capabilities of motors were far ahead of those of tires and electronics. It was then that the legend of the most complete and permanent drive appeared. Which, however, is still alive today.