What is a Four-Wheel Drive Central Differential?

[VivianFitz]

Central differential refers to the component installed on the intermediate drive shaft of a four-wheel drive vehicle to regulate the rotational speed of the front and rear wheels. When the car is turning or driving on uneven roads, it allows the left and right wheels to roll at different speeds, ensuring that both driving wheels perform pure rolling motion. Functions of the central differential in four-wheel drive vehicles: It eliminates the sliding phenomenon of the driving wheels on each axle and better distributes the vehicle's power. The central differential, also known as the inter-axle differential, is used in multi-axle drive vehicles where the drive shafts connect to each driving axle. Central differentials can be categorized into multi-plate clutch type, Torsen type, and viscous coupling type. The most common type nowadays is the multi-plate central differential, often used in vehicles with on-demand four-wheel drive systems. This differential has two sets of friction plates inside: one set is the driving plate connected to the front axle, and the other is the driven plate connected to the rear axle. Types of central differentials in four-wheel drive vehicles: The main types of central differentials include the conventional open differential, multi-plate clutch type, Torsen type, and viscous coupling type. The conventional open differential uses a standard symmetrical bevel gear structure and operates normally during turns, with no locking mechanism in its planetary gear set. If a four-wheel drive vehicle is equipped with front, center, and rear open differentials, all power will be wasted on a single slipping wheel, leaving the other three wheels without power. In off-road vehicles, open differentials can hinder performance on unpaved surfaces. The multi-plate clutch differential relies on wet multi-plate clutches to generate differential torque. This system is commonly used as the central differential in on-demand four-wheel drive systems. It consists of two sets of friction plates soaked in special oil, with engagement and disengagement controlled electronically. During straight-line driving, the front and rear axles rotate at the same speed, and there is no speed difference between the driving and driven plates, keeping them separated. The vehicle essentially operates in front-wheel or rear-wheel drive mode to save fuel. During turns, a speed difference arises between the front and rear axles, creating a speed difference between the driving and driven plates. However, if the speed difference does not meet the preset electronic system requirements, the plates remain separated, allowing normal steering. If the speed difference exceeds a certain limit, such as when the front wheels start slipping, the electronic system activates the hydraulic mechanism to engage the multi-plate clutch, transferring torque from the driving to the driven plates to enable four-wheel drive. The engagement conditions and torque distribution ratio of the multi-plate limited-slip differential are controlled electronically, with fast response times. Some models also feature a manual "LOCK" function, keeping the driving and driven plates fully engaged, similar to the locked four-wheel drive mode in professional off-road vehicles. However, the friction plates can only transmit up to 50% of the torque to the rear wheels, and excessive use can cause overheating and failure. The Torsen central differential's core consists of a worm gear and worm wheel system. Their interlocking mechanism and the one-way torque transfer from the worm gear to the worm wheel enable the differential's locking function, limiting slippage. During normal cornering, the front and rear differentials function as conventional differentials, with the worm wheel not affecting the half-shaft output speed differences. For example, when turning left, the right wheel rotates faster than the differential, while the left wheel rotates slower. The worm gears synchronize tightly with the meshing gears. The worm gear and worm wheel do not lock during this process because torque transfers from the worm gear to the worm wheel. When one wheel slips, the Torsen differential or hydraulic multi-plate clutch quickly adjusts power distribution. The viscous coupling operates similarly to the multi-plate clutch. It has many inner plates mounted on the input shaft, interspersed with outer plates inside the output shaft housing, filled with high-viscosity silicone oil. The input shaft connects to the transmission transfer case of the front-mounted engine, while the output shaft connects to the rear drive axle. During normal driving, with no speed difference between the front and rear wheels, the viscous coupling remains inactive, and no power is distributed to the rear wheels, keeping the vehicle in front-wheel drive mode. When a significant speed difference occurs between the front and rear wheels, the silicone oil between the inner and outer plates is agitated, heats up, and expands, creating high viscous resistance that prevents relative motion between the plates and generates substantial torque. This automatically transfers power to the rear wheels, converting the vehicle into all-wheel drive mode.