The central differential is also known as the inter-axle differential. For multi-axle drive vehicles, the drive axles are connected by drive shafts. The central differential enables each drive axle to have different input angular velocities to eliminate the sliding of the drive wheels on each axle. Types of central differentials include: open central differential, multi-plate clutch differential, Torsen differential, and viscous coupling differential.
When a vehicle is in motion, it not only travels in a straight line but also takes various turns. When a vehicle is turning, the trajectories of the four wheels are four arcs with different radii. This results in different rotational speeds of the four wheels. If the wheels could only rotate at the same speed, the vehicle would be unable to turn. Even if forced to turn, the middle axle would break due to the difference in wheel speeds. Therefore, a differential is needed to achieve differential speed, decomposing the fixed rotational speed of the engine output shaft into different speeds and transmitting them to the wheels.
When a vehicle turns, the turning radius of the front wheels is larger than that of the rear wheels on the same side. Therefore, the front wheels rotate faster than the rear wheels, and the four wheels follow completely different paths. Thus, four-wheel drive vehicles require a central differential to distribute the torque between the front and rear axles.
Types of central differentials include: open central differential, multi-plate clutch differential, Torsen differential, and viscous coupling differential.
I. Open Central Differential
An open differential is one that has no restrictions and can work normally when the vehicle is turning. The planetary gear set has no locking device. If a four-wheel drive vehicle is equipped with three open differentials (front, center, and rear), if one wheel slips, all the power of the vehicle will be wasted on this wheel, and the other three wheels will not receive any power.
Advantages: No particular advantages, as differential is a necessary condition for normal vehicle operation;
Disadvantages: In the field of off-road vehicles, an open differential can affect the vehicle's ability to get out of difficult situations on non-paved roads.
II. Multi-plate Clutch Differential
The multi-plate clutch differential generates differential torque through a wet multi-plate clutch. This system is often used as the central differential in on-demand four-wheel drive systems. It has two sets of friction discs, one set as the driving discs and the other as the driven discs. The driving discs are connected to the front axle, and the driven discs are connected to the rear axle. The discs are immersed in special oil, and their engagement and disengagement are controlled by an electronic system.
When the vehicle is traveling in a straight line, the rotational speeds of the front and rear axles are the same, and there is no speed difference between the driving and driven discs. At this time, the discs are separated, and the vehicle is basically in a front-wheel drive or rear-wheel drive state, which can save fuel. During turning, the front and rear axles have different rotational speeds, and the driving and driven discs also have a speed difference. However, since the speed difference has not reached the preset value of the electronic system, the two sets of discs remain separated, and the vehicle's turning is not affected.
When the speed difference between the front and rear axles exceeds a certain limit, for example, when the front wheels start to slip, the electronic control system will control the hydraulic mechanism to press the multi-plate clutch, and the driving and driven discs will come into contact, similar to the engagement of a clutch. Torque is transmitted from the driving discs to the driven discs, thus achieving four-wheel drive.
The engagement conditions and torque distribution ratio of the multi-plate friction limited-slip differential are controlled by the electronic system, with a fast response speed. Some models also have a manual control "LOCK" function, where the driving and driven discs can remain fully engaged, similar to the four-wheel drive lock state of professional off-road vehicles. However, the friction plates can only transfer up to 50% of the torque to the rear wheels, and high-intensity use can cause the friction plates to overheat and fail.
Advantages: Fast response and can engage instantly; most models are electronically engaged and do not require manual control;
Disadvantages: Can only transfer up to 50% of the power to the rear wheels and is prone to overheating under high loads.
III. Torsen Differential
The name "Torsen" comes from "Torque-sensing Traction". The core of Torsen is the worm gear and worm wheel meshing system. From the structural view of the Torsen differential, it can be seen that there are double worm wheels and worm gears. It is precisely their mutual meshing and interlocking, as well as the one-way transmission of torque from the worm wheel to the worm gear, that realizes the differential lock function, which limits slippage. During normal driving on a curve, the front and rear differentials act as traditional differentials, and the worm gears do not affect the different speeds of the half shafts. For example, when the vehicle turns left, the right wheel rotates faster than the differential, while the left wheel rotates slower. The worm wheels with different speeds can precisely match and synchronously mesh with the gears. At this time, the worm wheel and worm gear are not locked because the torque is transmitted from the worm wheel to the worm gear. However, when one side of the wheel slips, the worm wheel and worm gear assembly comes into play, and through the Torsen differential or the hydraulic multi-disc clutch, the power distribution is automatically and rapidly adjusted.
When the vehicle is driving normally, the differential housing P rotates, simultaneously driving the worm gears 3 and 4 to rotate. At this time, there is no relative rotation between 3 and 4, so the red 1 shaft and the green 2 shaft rotate at the same speed. However, when one side of the axle encounters greater resistance while the other side is idling, for example, the red axle encounters greater resistance, then initially it remains stationary, while the differential housing is still rotating, thus driving the worm gear 4 to roll along the red shaft. As 4 rolls, it also drives 3 to rotate, but 3 and the green axle 2 have a self-locking effect, so the rotation of 3 cannot drive the green axle 2 to rotate. Therefore, 3 stops rotating, and at the same time, 4 also stops rotating. Thus, 4 can only rotate the red axle along with the rotation of the differential housing, distributing the torque to the red axle and allowing the vehicle to get out of the predicament.
The most core device is the central torque-sensing self-locking differential, which can continuously adjust the power output between the front and rear axles from 25:75 to 75:25 according to the driving state, and the response is extremely rapid, with almost no lag (the characteristics of the torque-sensing self-locking differential have been analyzed in detail earlier), and with the support of the electronic stability program, the initiative of power distribution is further improved.
Simply put, the Torsen differential is a fully automatic pure mechanical differential, that is, it does not require manual control and is 100% reliable and has direct transmission. From a certain perspective, it is a very balanced design.
Advantages: It can provide feedback on the resistance difference between the driving wheels in an instant and distribute the torque output. Moreover, the lock-up characteristic is linear and can be adjusted within a relatively wide range of torque output.
Disadvantages: There is no two-wheel drive mode; the differential's limited-slip ability is limited, and the power cannot be fully transmitted to a certain wheel.
IV. Viscous Coupling Differential
The viscous coupling differential is a smart device for automatically distributing power in today's all-wheel drive vehicles. It is usually installed in all-wheel drive vehicles based on front-wheel drive. These vehicles usually drive in front-wheel drive mode. The most significant feature of the viscous coupling is that it can automatically distribute power to the rear drive axle without the driver's operation.
The working principle of the viscous coupling is somewhat similar to that of a multi-plate clutch. There are many inner plates on the input shaft, which are inserted among the many outer plates in the output shaft housing, and high-viscosity silicone oil is filled in. The input shaft is connected to the transmission and transfer case of the front-mounted engine, and the output shaft is connected to the rear drive axle.
During normal driving, there is no speed difference between the front and rear wheels, and the viscous coupling does not function, so the power is not distributed to the rear wheels, and the vehicle still behaves like a front-wheel drive vehicle. When a car is driving on icy and snowy roads, the front wheels may spin idly and there will be a significant difference in rotational speed between the front and rear wheels. The silicone oil between the inner and outer plates of the viscous coupling is stirred and begins to expand due to heat, generating a large viscous resistance that prevents the relative movement between the inner and outer plates and produces a considerable torque. Thus, power is automatically transmitted to the rear wheels, and the car becomes a four-wheel drive vehicle.
When the car is turning, the viscous coupling can also absorb the speed difference between the front and rear wheels caused by the inner wheel difference, functioning as a differential. During braking, it can prevent the rear wheels from locking up first.
Advantages: Compact size, simple structure, and low production cost.
Disadvantages: Slow response, small torque distribution ratio, inability to manually control engagement and disengagement, and potential failure due to overheating under high load.