All-wheel drive vehicles in our country are honored and respected, but at the same time, such a coveted 4x4 scheme can be implemented in different ways. Consider the advantages and disadvantages of circuits with mechanical center lock and locking by means of an electronically controlled clutch.
Historically, the first scheme appeared all-wheel drive, in which to the transmission rear wheel drive car they added a transfer case, and from it to the front (now also driving) axle they stretched their cardan shaft. At the same time, the connection front axle carried out out of necessity and "hard". According to this scheme, the transmissions of many "professional" off-road vehicles are still made. Among the domestic ones, you can name the entire UAZ family. A lot of imported ones - from compact Suzuki Jimny before legendary Land Rover Defender.
And if on the road such "rogues" have no equal, then in the city, you will agree, it is not very easy to cope with them. Therefore, the designers proposed a more convenient and practical technical solution. This is an all-wheel drive scheme, in which the torque was transmitted to both axles through the differential. Typical representatives - domestic Lada 4x4 and Chevrolet Niva.
Permanent four-wheel drive with lockable center differential
Chevrolet Niva's four-wheel drive is permanent - the torque from the engine is always transmitted to both axles (the bridges are not turned off). Such a scheme increases the vehicle's cross-country ability, while simultaneously reducing the load on the transmission units, but slightly increases fuel consumption.
The front and rear axles are connected through a center differential, which allows the front and rear wheels to rotate at different speeds. angular velocities depending on the trajectory and driving conditions. The center differential is located in transfer case. It is similar to the interwheel differentials in the front and rear axles, but unlike them, the interaxle differential can be forcibly blocked. In this case, the drive shafts of the front and rear axles become rigidly interconnected and rotate at the same frequency. This significantly increases the car's flotation (on slippery slopes, in mud, snow, etc.), but worsens handling and increases wear on transmission parts and tires on surfaces with good grip. Therefore, the differential lock can only be used to overcome difficult sections and at low speeds.
You can turn on the lock while the car is moving, if the wheels do not slip. But this will not eliminate the danger of “diagonal hanging”, when one of the wheels on each axle loses traction with the ground - in this case, you will have to add soil under the suspended wheels or dig it under the rest. To increase the torque supplied to the wheels, the lowest gear in the transfer case is used, its gear ratio - 2,135. Top gear, designed for normal driving conditions, has a gear ratio of 1.20.
All-wheel drive transmission with electromagnetic clutch for connecting the rear wheels
However, progress did not stand still - the designers proposed an idea that was ingenious in terms of ease of execution and profit making: to create on the basis of front wheel drive car crossover. The recipe for all automakers is similar. Let's consider such a scheme in detail with an example Renault models Duster.
The engine and gearbox (mechanics or automatic) are installed transversely relative to the car. All shafts inside the gearbox, respectively, too. And the torque must be transferred to the rear axle. To do this, they used an angular gearbox in front and a cardan shaft, which, in turn, is connected to a clutch. The leading part of the coupling in conjunction with cardan shaft always rotate when the front reduction gear rotates. The driven part of the clutch is splined to the main gear drive shaft. The body of the electromagnetic clutch is also attached to the main gear housing: an angular gearbox combined with a differential. From the differential, the drives transmit torque directly to rear wheels. The clutch is equipped with an electronic control unit, which, in turn, depends on the transmission mode switch on the instrument panel console. This is how the all-wheel drive scheme of most modern crossovers with a transverse arrangement of the power unit.
To control the compression force of the clutch discs, a cam mechanism is used that changes the clamping force. Voltage applied to the clutch solenoid causes the clutch discs to close and engage rear axle. The amount of transmitted torque is regulated by the adhesion force of the friction discs in the clutch. So, if the voltage supplied to the electromagnet is reduced, the clutch will provide an incomplete circuit and will be able to rotate with a small moment. However, even when fully energized, a closed clutch can transmit torque limited by the friction forces in the clutch.
For the clutch to operate, at least a small “lag” is needed. rear wheels from the front. The most interesting thing is that there are no temperature sensors in the clutch, and it is turned off “by overheating” when the control unit through ABS sensors for some time it fixes that with full tension on the clutch, the rear wheels do not rotate, and the front wheels rotate at a considerable speed. So in most cases, electronics are simply reinsured.
What to choose?
In both schemes, all drive and cardan shafts rotate constantly, so there is no difference in terms of fuel consumption. A hard-lock clutch arrangement is preferred in rough off-road applications, as clutches with electronic control are capable of transmitting only a limited moment, and when slipping friction clutches, they are prone to rapid “overheating”, albeit often virtual. Unexpectedly for the driver, the automatic engagement of the clutch during cornering can sometimes be dangerous.
From personal experience
Owning a car with an electromagnetic clutch for connecting the rear axle, I can tell you which modes I use. In the summer, on paved roads, the 2WD mode is always on, in the mud I use all the potential and turn off the system dynamic stabilization ESP. In winter, the AUTO mode is always on. First of all, not to lose the spikes on the front wheels. Tests show that the loss of studs is especially great when the drive wheels slip. If in winter you need a sharp acceleration, and under the wheels the coating is of poor quality, for example, tiles tram tracks, then turn on the LOCK mode. And if necessary, get out of the snowdrift - LOCK mode and turn off the ESP.
Was in use and Niva. So, if necessary, start on slippery surface turned on the lock, and in dead traffic jams crawled on a reduced one - so the load on the clutch is less.
Many people think that a four-wheel drive car is designed to overcome difficult off-road conditions. That is, all-wheel drive increases only the vehicle's cross-country ability. This is not entirely true. Yes, four-wheel drive improves cross-country ability, but it can also be used on passenger cars mobiles. But no one has yet taken it into his head, for example, to storm a dirt road soaked with rain on an Audi A4 ... Why does a passenger car need four-wheel drive? It's simple, to improve security.
A car with all-wheel drive is more stable on slippery road, it is safer to drive smooth long turns. Therefore, many automakers produce and all-wheel drive cars. Not all potential car owners are ready to buy a car with all-wheel drive. Maintenance of such a car is more expensive than usual, and fuel consumption is somewhat higher.
Therefore, automakers have found a compromise between efficiency and safety. These are cars with automatic all-wheel drive. By default, the car is front-wheel drive or rear-wheel drive, but when the drive wheels slip, the electronics connects the second drive axle.
Many crossovers use just such a scheme. Ground clearance on crossovers more than on passenger cars. Therefore, very often they are equated with SUVs. Potential buyers do not delve into the design and buy cars with this layout. And of course they exploit their iron horse, How real SUV. This naturally leads to a breakdown of the all-wheel drive connection system.
Principle of operation
The all-wheel drive connection system is quite reliable. But you must always remember and understand that a crossover cannot and should not move off-road. He is contraindicated in severe road conditions. And if the driver still got into unpleasant situation, you need to correctly use the capabilities of all-wheel drive. On vehicles with such a system, there is a control button. The button is usually installed on the dashboard of the car and allows the driver to select auto mode or turn on all-wheel drive.
In automatic mode, the control unit itself “decides” when to connect the all-wheel drive. When manually engaged, the four-wheel drive works all the time, that is, the second drive axle clutch is locked (on). To protect units and mechanisms from large overloads, it is provided automatic shutdown forced blocking. Shutdown occurs when a certain speed is reached during acceleration. But the shutdown does not occur completely, the system goes into automatic mode.
Device
The all-wheel drive clutch is installed on the GP gearbox. On the one hand, a cardan is connected, going from the RK to rear axle, and the output shaft of the coupling engages with the HP shank.
When the car is moving, the cardan rotates, but the bridge itself does not work. GP rotates from feedback wheels with the road at idle, torque from the gearbox is not transmitted to the wheels. When switched on, the magnetic coil of the clutch is supplied with electricity. Under the action of a magnetic field, a package of special friction discs is compressed. Due to friction, the entire package becomes a single body and the rotation is transmitted to a special unit, which, in turn, mechanically compresses another package of friction discs. Now the rotation is transmitted to the shank of the GPU and further to the wheels. The clutch housing is filled with oil.
Attention! GP oil and clutch oil do not mix during operation. It is poured into the GP transmission oil, and in the clutch - a special hydraulic oil with enhanced friction properties. Such oil simultaneously lubricates the entire mechanism and improves the adhesion of the friction discs to each other. It is forbidden to pour ordinary gear oil into the clutch.
Breakdowns
If used incorrectly, the clutch cannot cope with the increased load and fails. In automatic mode, a non-constant voltage is applied to the electromagnet winding. The control unit, depending on the conditions, supplies a pulsed current. The more torque is required to be transmitted, the longer current pulses are applied to the winding. Friction discs at the same time they are compressed, then released. At the moment of adjoining of disks to each other there is their intensive wear.
At the same time, the node that compresses the second clutch pack perceives variable loads and also wears out. The second clutch package acts as a damper, smoothing out sharp clutch engagement due to slipping of the friction discs. It is necessary for more long service the GPU gearbox itself.
When the clutch is turned on and off, due to the friction of the clutches, the entire mechanism heats up. Strong heating can lead to boiling of oil in the clutch cavity, resulting in increased pressure inside.
The seals are starting to "snot". Also, when the pressure increases, the control clutch pack (which is activated by an electromagnet) is compressed without electricity, and the clutch does not turn off. It's hardly noticeable in a straight line. But when the car turns, the friction disc packs cannot cope with the increased load, the discs begin to slip, making a grinding sound. There is intensive wear of both packages.
With very high heating, an interturn circuit is possible in the winding of the electromagnet. If the driver complies with all operating rules, it is enough to monitor the oil seals to avoid oil leakage. If oil leaks, the clutch will remain without lubrication and heat up. The result of overheating is described above.
How to avoid clutch breakage
It is possible to avoid or at least extend its service life. The less often the car is used off-road, the longer the clutch will last. When overcoming small difficult sections, full blocking should be included. You should not rely on the automatic mode, in such conditions it is not optimal. While driving, you do not need to sharply press the gas, sharply brake. Even when completely blocked, such actions adversely affect the life of the clutch. Should move to low gear. There are situations when on city roads there are difficult conditions. The front axle of the car is on ice, and the rear axle is on dry pavement. Constantly pressing the button is not very convenient, but you need to move off in such conditions as smoothly as possible.
As often as possible, visually inspect the clutch housing for oil leaks. Little oil is poured, so if it leaks, it will flow out very quickly and this will lead to breakage. At the first symptoms of wrong work clutch must be stopped immediately. A timely stop will help to avoid serious damage. If possible, deliver the car to the place of repair on a tow truck. Towing is not recommended.
Clutch repair
No matter how correctly and competently the driver does not operate his car, the all-wheel drive clutch can still fail. Dealerships change the clutch assembly, as it is very problematic to find spare parts. The most common failure is the jamming of the clutch in the on state. This happens more often due to overheating.
When repairing, you need to disassemble the mechanism, visually inspect all parts for wear. If the parts are in satisfactory condition, thoroughly rinse and blow through compressed air. Check the bearing for play and noise when turning by hand. If the bearing has play, noise during rotation, it should be replaced. An analogue can be selected by size.
At high mileage auto it is desirable to change the oil seals. Their service life is pretty decent, but still not worth the risk. Seals can be selected by size and marking. It is imperative to change the sealing ring of the clutch cover, lubricate it during installation and make sure that the edges do not lift up. If you damage during installation sealing ring, it is possible to mix the GP oil and the clutch during operation, which is not permissible.
The same applies to the inner oil seal, which is installed on the HP side. Fill with new oil before installing the cap. Insert the assembled clutch into the housing, while adjusting the gap between the movable plate and the housing. It is important that when the electromagnet is turned on, the plate does not touch the clutch housing.
Elastic cardan coupling
Another common failure is the hum while driving. The clutch bearing usually hums. When replacing it, carefully inspect all parts of the coupling for wear. It is advisable to change the oil at each disassembly in order to prevent wear products from entering the mechanism.
Rarely fails the winding of the electromagnet. It is possible to check its work directly on the car. Apply 12 V voltage to the connector pins, a click should be heard. And if you take the clutch with your hand, then at the moment of switching on you can feel a slightly noticeable knock inside the clutch. This indicates the health of the electromagnet.
All-wheel drive clutches Hyundai Tucson And KIA Sportage are identical. They differ only in the outer case, depending on the year of manufacture of the car. Also different catalog numbers. In the event of a breakdown, it must be completely replaced. But if desired, the clutch can be repaired on its own and at a lower cost. The most pressing issue in self repair looking for spare parts.
good roads and good luck with the repair!
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 boost systems active safety. 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, by introducing a lock, using brake mechanisms 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 very experienced driver can feel all the nuances of the machine'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 ranges loads.
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 machines of other brands, this scheme somehow did not take root - the only exception is 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 identical to those of Audi, with the exception of simpler suspensions and more compact motor. 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 transverse motor, and even on very compact cars, it was not originally intended for unprepared drivers.
Pictured: under the hood 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. In order to obtain an acceptable production car result, it will take a lot of effort to fine-tune controllability, because the average driver does not like such surprises, he needs unambiguous behavior. Of course, it can be obtained by setting complex electronic systems stability control, 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 for stock car is also important.
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 weight difference of 50 kg is already very serious, and the advantages of unambiguous controllability and the ability to easily adjust 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 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. And it is enough even if powerful machines and increased requirements for handling and maneuverability.
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.
Electronics, serving the system connections, too, can 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 for more and more expensive cars the quality of this connection mechanism is steadily improving. 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 cost effort on the part of the driver. Unfortunately, these are complex expensive systems, which are aimed at obtaining fantastic performance on racing tracks. And they are designed without regard to the cost of operation.
No more fear simple systems. For example, much more massive cars endow with excellent handling and patency of the Haldex coupling of several latest generations. Junior Land models Rover, Range Rover, VW, Audi, Seat and Volvo use the brand's designs extensively. And in operation similar systems have proven to be quite reliable.
All-wheel drive BMW 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 pure 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 about the most complete and permanent drive. Which, however, is still alive today.
A viscous coupling or viscous coupling is a device that transmits torque from one shaft to another due to viscous properties. special liquid located inside the clutch. This mechanism has become widespread in technology, but it is more familiar to motorists as a device in a car transmission. This is a simple and inexpensive mechanism that is capable of providing both automatic differential lock and automatic all-wheel drive in most modern crossovers. Consider the principle of operation, design, as well as the advantages and disadvantages of the popular transmission mechanism.
The principle of operation of the viscous coupling
The viscous coupling is hermetic case, inside which are perforated discs and a dilatant liquid (a material based on silicone, having high viscosity). One part of the disks is rigidly connected to drive shaft, the other - with a differential housing.
General view of the viscous coupling
When the vehicle is moving on a level road surface, the differential and drive shaft rotate synchronously. The perforated discs also rotate as a whole. If the car begins to slip, the wheels of one axle begin to rotate rapidly, and the other axle becomes stationary. At this point, the discs connected to the drive shaft begin to rotate rapidly and mix the dilatant fluid. As a result, the silicone substance quickly thickens and hardens, blocking the differential. The torque is transmitted to the second axle, thereby “connecting” the all-wheel drive, which helps the car cope with off-road. After overcoming an obstacle, the silicone fluid returns to its original state, the viscous coupling is unlocked, and the rear axle is turned off.
Device and main components
Viscous coupling diagram: 1 - driven hub; 2 - clutch housing connected to the drive shaft; 3 - driven disk; 4 - leading disk. The main components of a viscous coupling are flat perforated discs, a dilatant fluid and a sealed housing.
The package of discs with holes is divided into two groups: one group is connected to the drive shaft, the other to the driven shaft. All disks are at a minimum distance from each other, while the master and slave alternate.
Dilatant filling fluid inner space viscous couplings, is an organic substance based on silicone. With active stirring and heating, the substance thickens and passes into a solid state. After the silicone material expands and hardens, the pressure on the perforated discs increases greatly, due to which they are pressed against each other. It is after this that the rear axle of the machine is put into operation.
Advantages and disadvantages
First, about the advantages of viscous coupling:
- the simplest design;
- durable case that can withstand pressure up to 20 atmospheres;
- affordable price due to the simplicity of the design;
- does not require maintenance, it is usually operated without breakdowns during the entire life of the vehicle.
The main disadvantages of viscous coupling:
- impossibility of repair (if the viscous coupling is broken, it is replaced with a new one);
- danger of overheating long work;
- there is no possibility of manual blocking;
- incomplete automatic blocking;
- belated operation;
- incompatibility with ;
- lack of control over all-wheel drive;
- large clutches greatly reduce the clearance.
Viscous coupling application
The viscous coupling is mainly installed on vehicles with cross-country ability as automatic lock center differential(e.g. cars Jeep Grand Cherokee and range rover HSE). However, a viscous coupling can also be used in conjunction with a gear free differential, acting as an auxiliary automatic locking mechanism.
Note that a dilatant fluid coupling is the easiest and cheapest way to connect both axles of a car. The efficiency and accuracy of this mechanism is in most cases sufficient to prevent the front wheels of the machine from slipping relative to the rear wheels on a normal pavement. However, now automakers are increasingly refusing to install viscous couplings due to their incompatibility with the ABS system.
transmissions four-wheel drive vehicles have various designs. Together they form all-wheel drive systems. There are the following types of all-wheel drive systems: permanent connection, automatically connected and manually connected.
Different types All-wheel drive systems have, as a rule, different purposes. At the same time, the following advantages of these systems can be distinguished, which determine the scope of their application:
Permanent all-wheel drive system
Permanent all-wheel drive system (another name - full time system, in translation " full time”) provides a constant transmission of torque to all wheels of the car.
The system includes structural elements typical for all-wheel drive transmission, namely: clutch, gearbox, transfer case, cardan gears, main gears, small-wheeled differentials of the rear and front axles, as well as wheel axles.
Permanent four-wheel drive is used both on vehicles with a rear-wheel drive layout (longitudinal arrangement of the engine and gearbox) and on vehicles with a front-wheel drive layout (transverse arrangement of the engine and gearbox). Such systems differ mainly in the design of the transfer case and cardan gears.
Well-known permanent all-wheel drive systems are the Quattro system from Audi, xDrive from BMW, 4Matic from Mercedes.
The differential lock can be automatic or manual. Modern designs automatic locking center differential is a viscous coupling, limited slip differential Torsen, multi-plate friction clutch.
Manual (forced) differential lock is performed by the driver using a mechanical, pneumatic, electric or hydraulic drive. On some designs of the transfer case, the functions of both automatic and manual locking of the center differential are provided.
The principle of operation of the permanent all-wheel drive system
Torque from the engine is transmitted to the gearbox and then to the transfer case. In the transfer case, the moment is distributed along the axes. If necessary, the driver can turn on a downshift. Further, the torque is transmitted through the cardan shafts to main gear and center differential of each axle. From the differential, torque is transmitted through the axle shafts to the drive wheels. When the wheels of one of the axles slip, the inter-axle and inter-axle differentials are automatically or forcibly locked.
Automatic all-wheel drive system
All-wheel drive system connected automatically (another name - on-demand system, translated "on demand") is promising direction all-wheel drive development cars. This system provides connection of the wheels of one of the axles in case of slippage of the wheels of the other axle. Under normal operating conditions, the car is front- or rear-wheel drive.
Almost all leading automakers have in their model range vehicles with automatic all-wheel drive. A well-known all-wheel drive system connected automatically is 4Motion from Volkswagen.
The design of the all-wheel drive system connected automatically is similar to the permanent all-wheel drive. The exception is the presence of a rear axle coupling.
The transfer case in the automatic all-wheel drive system is, as a rule, a bevel gear. Downshift and center differential are absent.
As a coupling for connecting the rear axle, a viscous coupling or an electronically controlled friction clutch. A well-known friction clutch is Haldex coupling, which is used in the Volkswagen 4Motion all-wheel drive system.
The principle of operation of the all-wheel drive system connected automatically
Torque from the engine, through the clutch, gearbox, final drive and differential is transmitted to the front axle of the car. Torque through the transfer case and cardan shafts is also transmitted to the friction clutch. In the normal position, the friction clutch has a minimum compression, at which up to 10% of the torque is transmitted to the rear axle. When the wheels of the front axle slip on command electronic block control, the friction clutch is activated and transmits torque to the rear axle. The amount of torque transmitted to the rear axle can vary within certain limits.
All-wheel drive system connected manually
All-wheel drive system connected manually (another name is system Part Time , translated as "partial time") is currently practically not used, because. is inefficient. At the same time, it is this system that provides a rigid connection between the front and rear axles, transmission of torque in a ratio of 50:50 and therefore is truly off-road.
The device of the manually connected all-wheel drive system is generally similar to the permanent all-wheel drive system. The main differences are the absence of a center differential and the ability to connect the front axle in the transfer case. It should be noted that in a number of permanent all-wheel drive designs, the front axle disengagement function is used. Truth in this case unplugging and plugging in is not the same thing.
