In a number of all-wheel drive systems there is a special clutch, with which the level of torque transmission to the vehicle axle is regulated.
By the way, clutch failure is becoming one of the common causes of all-wheel drive failure. The clutch can fail if it is not maintained in a timely manner:
- do not change the oil in the clutch;
- Ignore bearing noise.
4Motion system and Haldex coupling
The technology began to be used two years before the Millennium. Prior to this, the work of all-wheel drive of German cars was based on viscous couplings.
The use of the Haldex coupling has become a revolution in the field of all-wheel drive. This clutch:
- friction;
- has a large number of disks;
- operated electro-hydraulically.
Its application made it possible to create cars with automatically connected all-wheel drive. By the way, the Haldex coupling is now being installed not only on German cars, but also on cars from other European manufacturers.
Principle of operation
In the first generations of couplings, the pump worked due to the difference in the rotation of the axles. He created the necessary oil pressure. And already under the pressure of the oil, the clutch discs were compressed. The valves and the control unit regulated the level of oil pressure.4th generation clutch
On modern all-wheel drive vehicles, a 4th generation clutch is installed. The principle of its operation is similar to the principle of operation of couplings of previous generations. However, the device already has an electronic pump. The speed difference is now of secondary importance, the operation of the clutch is carried out on the basis of the exchange of signals between various sensors and the control unit.Thus, it can be noted that a modern all-wheel drive clutch is a fairly effective device that makes it possible to appropriately distribute the torque between the axles automatically, without human intervention.
A significant disadvantage of such couplings is that, under heavy loads, they can fail. And replacing or repairing them is expensive.
How to change a wheel drive clutch bearing
One of the characteristic diseases of couplings is bearing noise. Moreover, this is relevant both for old viscous couplings and for modern electrically controlled ones. If the bearing begins to ring, then it must be changed so that there are no more serious consequences. You can do this at home as well. The main thing is to have certain theoretical knowledge and direct hands. Of course, the repair technology is somewhat different, depending on the make and model of the car. But the general principle is this:
- It is necessary to drive the car into a pit or hang it on a lift.
- Identify cardan and gearbox under the bottom of the machines. The clutch itself is attached to the gearbox. Often, a number of operations are also carried out to disconnect the elements of the all-wheel drive system from each other. Such manipulations facilitate the removal of the coupling. At the same time, you can carry out prevention and other elements of the system.
- Just in case, drain the oil from the gearbox.
- Dismantle the coupling and remove the bearing.
- Remove in all accessible places all rust that has formed during the operation of the old bearing.
- Install the new bearing in its proper position, orienting it correctly.
- Carefully assemble everything in the correct order and seal.
What oil to fill in the all-wheel drive clutch
Depending on the make and model of the car, it is necessary to change the oil in the all-wheel drive clutch after 30 and 60 thousand mileage, in some sources there is a figure of 100,000 kilometers. But it's better not to delay. The process of changing the oil itself does not cause serious difficulties. The coupling has a drain hole and a filler neck. The oil change process is quite typical:
- open the drain hole, drain the oil;
- pour fresh oil into the filler neck;
- make sure there is enough oil.
It is worth emphasizing that the most common Haldex couplings are located in the final drive. There have been cases when, during maintenance of a car, servicemen confused the filler and drain holes of the clutch itself and the gearbox, which led not to fatal, but to unpleasant consequences.
Of course, those who are serviced in official car services should not puzzle over finding the necessary oil for the clutch.
As for the rest, those who love and want to service the car with their own hands, the following options are recommended:
- stop by an official car service and find out what kind of oil local specialists use;
- go to the forum dedicated to a particular make and model of car and ask a question there;
- contact the developers of a particular coupling and clarify information with them.
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 four-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 such a 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 solutions:
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 rigid 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, and 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 ratio of the moment along the axes 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 take turns (I just talked about this recently), then with a very high probability he will not be able to put 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 Haldex 5 generation coupling, fully controlled by electronics (I remind you that 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 4th generation Haldex-based automatic all-wheel drive 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 complexity. 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 a ratio of 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 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.
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 controllability, 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.
Now crossovers have become very popular in the automotive market. They have both full and monodrive. It is connected using a device such as a viscous coupling. The principle of operation of the unit is further in our article.
Characteristic
So what is this element? A viscous coupling is an automatic mechanism for transmitting torque through special fluids. It is worth noting that the principle of operation of the all-wheel drive viscous coupling and the fan is the same.
Thus, the torque on both elements is transmitted using the working fluid. Below we will look at what it is.
What is inside?
A silicone-based fluid is used inside the clutch housing. It has special properties. If it is not rotated or heated, it remains in a liquid state. As soon as torque energy comes in, it expands and becomes very dense. As the temperature rises, it looks like a hardened glue. As soon as the temperature drops, the substance turns into a liquid. By the way, it is flooded for the entire period of operation.
How does it work?
What is the principle of operation of a product called "viscous coupling"? According to the algorithm of actions, it is similar to the hydraulic transformer of an automatic transmission. Here, too, the torque is transmitted by fluid (but only through gear oil). There are two types of viscous couplings. Below we will consider them.
First type: impeller
It includes a metal closed case. The principle of operation of a viscous coupling (including a cooling fan) consists in the action of two turbine wheels. They are located opposite each other. One is on the drive shaft, the second is on the driven. The housing is filled with silicone-based liquid.
When these shafts rotate at the same frequency, mixing of the composition does not occur. But as soon as slip occurs, the temperature inside the case rises. The liquid becomes thicker. Thus, the driving turbine wheel engages with the axle. Connected As soon as the car left off-road, the speed of rotation of the impellers is restored. As the temperature drops, the density of the liquid decreases. The car is turned off all-wheel drive.
Second type: disk
Here, too, there is a closed case. However, unlike the first type, there is a group of flat disks on the driving and driven shaft. What is the principle of operation of this viscous coupling? The discs rotate in silicone fluid. As the temperature rises, it expands and presses these elements.
The clutch begins to transmit torque to the second axle. This only happens when the car has stalled and there are different wheel speeds (while some are standing, the latter are slipping). Both types do not use automatic electronic systems. The device is powered by rotational energy. Therefore, the viscous coupling of the fan and all-wheel drive has a long service life.
Where is it used?
First, let's pay attention to the element that is used in the engine cooling system. The principle of operation of the viscous fan coupling is based on the operation of the crankshaft. The clutch itself is attached to the rod and has the higher the speed of the crankshaft, the more heated the liquid in the clutch. Thus, the connection became stiffer, and the element with the fan began to rotate, cooling the engine and radiator.
With a drop in speed and a decrease in fluid temperature, the clutch stops working. It should be noted that the viscous fan coupling is no longer used. Modern engines use electronic impellers with a coolant temperature sensor. They are no longer connected to the crankshaft and operate separately from it.
Four-wheel drive and viscous coupling
Its principle of operation is the same as that of the fan. However, the part is not placed in the engine compartment, but under the bottom of the car. And, unlike the first type, the all-wheel drive viscous coupling does not lose its popularity.
Now it is installed on many crossovers and SUVs with switchable drive. Some use electromechanical counterparts. But they are much more expensive and less practical. Among the worthy competitors, it should be noted that the mechanical blocking, which is on the "Niva" and "UAZ". But due to urbanization, manufacturers have abandoned the real lock, which rigidly connects both axles and increases the vehicle's cross-country ability. The driver himself can choose when he needs all-wheel drive. If you need to overcome the off-road "SUV", it will quickly get stuck and after slipping, the rear axle will work for it. But it will not help him get out of the strong mud.
Advantages
Let's look at the positive aspects of the viscous coupling:
- Simplicity of design. Inside, only a few impellers or discs are used. And all this is powered without electronics, by physical expansion of the liquid.
- Cheapness. Due to the simple design of the viscous coupling, it practically does not affect the cost of the car (if it concerns the “all-wheel drive” option).
- Reliability. The coupling has a durable housing that can withstand pressure up to 20 kilograms per square centimeter. It is installed for the entire service life and does not require periodic replacement of the working fluid.
- Can work in any road conditions. It does not slip on mud or when driving on snow. The outside temperature does not matter for heating the working fluid.
Flaws
It is worth noting the lack of maintainability. The viscous coupling is permanently installed.
And if it is out of order (for example, due to mechanical deformations), then it changes entirely. Also, motorists complain about the inability to connect all-wheel drive on their own. The clutch engages the second axle only when the car is already "buried". This prevents the machine from easily climbing mud or snow obstacles. The next disadvantage is low ground clearance. The node requires a large case. And if you use a small viscous coupling, it will not transmit the desired torque force. And the last drawback is the fear of overheating.
You can’t skid for a long time at full drive. Otherwise, there is a risk of damaging the viscous coupling. Therefore, this type of "dishonest" drive is not welcomed by off-road lovers. Under prolonged loads, the node simply jams.
Conclusion
So, we found out how the all-wheel drive viscous coupling and fan work. As you can see, the device, thanks to a special fluid, can transmit torque at the right time without involving additional sensors and systems. This is very
Many lovers of outdoor activities and frequent trips out of town choose crossovers and SUVs, which use all-wheel drive, as a vehicle. Such cars are distinguished by increased ground clearance and all-wheel drive, which provides good cross-country ability.
But not always such cars are able to overcome even average off-road, not to mention serious dirt. And the reason for this may be the same all-wheel drive, or rather its design features. Therefore, the presence of all driving wheels does not mean that the machine is capable of conquering strong mud.
The main components of the transmission
Four-wheel drive means the transmission of torque from the power unit to the wheels of both axles, which increases the mud throughput.
The main design feature of this type of drive over others (front, rear) is the presence of an additional unit in the transmission - a transfer case. It is this node that ensures the distribution of rotation along the two axes of the car, making all wheels drive.
In general, this auto transmission consists of:
- clutch;
- gearboxes;
- transfer box;
- drive shafts;
- main gear of both bridges;
- differentials.
All-wheel drive transmission design option (automatically connected)
Despite the use of the same components, variations and designs of transmission - a lot.
Design and operational features
It is worth noting that on many cars all-wheel drive is not always carried out. That is, only one axis is always leading, while the second one is connected only when necessary, and this can be done both automatically and manually. But there are also variations of the transmission, in which the axle is not disabled.
Transmissions with a design that provides rotation to all wheels are used on cars with both a transverse installation of the power unit and a longitudinal one. In this case, the layout predetermines which of the leading axles functions constantly (the exception is permanent all-wheel drive).
The system that provides all-wheel drive can work with both manual transmission and any automatic transmission.
The principle of operation of the system is quite simple: from the motor, rotation is transmitted to the gearbox, which provides a change in gear ratios. From the gearbox, the rotation enters the transfer case, which redistributes it to two axles. And then, through the cardan shafts, the rotation is transmitted to the main gears.
But the general concept of the all-wheel drive system is described above. Structurally, the transmission may differ. So, as a rule, on a car with a transverse arrangement, the main gear of the front axle and the transfer case simultaneously enter into the design of the checkpoint.
But in a car with an engine installed longitudinally, the transfer case and the main gear of the front axle are separate elements, and the rotation on them comes from the drive shafts.
There are a number of design features that directly affect the patency of the car. First of all, this concerns the transfer case. In full-fledged SUVs, this node necessarily has a reduction gear, which is far from always available in crossovers.
Differentials also affect off-road performance. Their number may be different. Some cars have a center differential included in the transfer device. Thanks to this element, it is possible to change the ratio of the distribution of torque between the axles, depending on the driving conditions. In some cars, to increase cross-country ability, this differential is also blocked, after which the distribution of rotation over the bridges is done in strictly specified proportions (60/40 or 50/50).
But there may not be an interaxle differential in the design of the system. But cross-axle differentials installed on the main gears are present on all cars, but not all have their locks. This also affects driving performance.
The drive mechanisms are also different. In some cars, everything is done automatically, in others, the driver uses electronic systems for this, in others, the connection is completely manual, mechanical.
In general, the all-wheel drive system used on a car is not as simple as it initially seems, although the principle of its operation on all cars is the same.
The most famous systems are:
- 4Matic by Mercedes;
- Quattro from Audi;
- xDrive from BMW;
- 4motion of the Volkswagen Group;
- ATTESA at Nissan;
- Honda's VTM-4;
- All wheel control developed by Mitsubishi.
Types of drive used on cars
Three types of all-wheel drive have been used on cars, differing from each other both structurally and in terms of operation features:
- Permanent four-wheel drive
- With auto bridge
- With manual connection
These are the main and most common options.
Types of all-wheel drive
permanent drive
Permanent four-wheel drive (international designation - " full time”), perhaps the only system that is used not only on crossovers and SUVs, but also on station wagons, sedans and hatchbacks. It is used on cars with both types of power plant layout.
The peculiarity of this type of transmission is that the mechanism for disabling one of the axles is not provided. In this case, the transfer case can have a downshift, the inclusion of which is forced by an electronic drive (the driver simply selects the desired mode with the selector, and the servo switches).
Selector for selecting low gear and traffic intensity depending on the terrain
Its design uses a center differential with a locking mechanism. In different types of transmission, blocking can be carried out by a viscous clutch, a friction-type multi-plate clutch, or a Torsen differential. Some of them perform blocking in automatic mode, others - forcibly, manually (using an electronic drive).
Cross-axle differentials in the permanent all-wheel drive system are also equipped with locks, but not always (usually not on sedans, station wagons and hatchbacks). Also, it is not necessary to have a lock on two axles at once, often such a mechanism is installed on only one of the axles.
Drive with automatically connected axle
In a car with an automatically connected bridge (designation - " On Demand”), all-wheel drive is activated only under certain conditions - when the wheels of a constantly working axle began to slip. The rest of the time, the car is front-wheel drive (with a transverse layout) or rear-wheel drive (if the engine is located longitudinally).
Such a system has its own design features. So, the transfer case has a simplified design and there is no reduction gear in it, but at the same time it provides a constant distribution of torque along the axes.
There is also no center differential, but there is a mechanism for automatically connecting the second axle. It is noteworthy that the same components are used in the design of the mechanism as in the center differential - a viscous coupling or an electronically controlled friction clutch.
A feature of the drive with automatic connection is that the distribution of torque along the axes is done with a different ratio, which changes under different driving conditions. That is, in one mode, the rotation is distributed in a proportion, for example, 60/40, and in the other - 50/50.
At the moment, a system with automatic connection of all-wheel drive is promising and is used by many automakers.
Manual transmission
Transmission with all-wheel drive in manual mode (designation - " Part Time”) is now considered obsolete and not commonly used.
Its peculiarity lies in the fact that the connection of the second bridge is carried out in the transfer case. And for this, both a mechanical drive (by means of a control lever for a transfer case installed in the passenger compartment) and an electronic one (the driver activates the selector, and the servo drive connects / disconnects the bridge) can be used.
In such a transmission, there is no center differential, which provides a constant ratio of torque distribution (usually in a 50/50 ratio).
Almost always in cross-axle differentials, locking is used, and forced. These design features provide the highest cross-country ability of the car.
Other options
It is worth pointing out that there are combined transmissions that have the design and operational features of several types of systems at the same time. They were designated " Selectable 4WD» or multi-mode drive.
In such transmissions, it is possible to set the operating mode of the drive. So, the connection of all-wheel drive can be carried out both in manual and automatic mode (moreover, it is possible to disable any of the bridges). The same applies to differential locks - interaxle and interwheel. In general, there are many variations in the operation of the transmission.
There are more interesting options, for example, electromechanical all-wheel drive. In this case, all the torque is supplied to only one axle. The second bridge is equipped with electric motors, which are activated automatically. Recently, such a transmission has become more and more popular, although it cannot be called a full-fledged system, in the classical sense. Such vehicles are hybrid systems.
Positive and negative sides
Four-wheel drive has a number of advantages over other types. The main ones can be distinguished:
- Efficient use of power plant power;
- Ensuring improved controllability of the car and its directional stability on different types of coverage;
- Increased vehicle traffic.
The advantages are counterbalanced by such negative qualities as:
- Increased fuel consumption;
- The complexity of the drive design;
- Large metal transmission.
Despite the negative qualities, cars that have all-wheel drive are in demand and are very popular even among motorists who almost never leave the city.
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