Differential Assembly: Description and Operation
COMPONENT LOCATION
OVERVIEW
The differential operates in conjunction with the active on-demand coupling to provide drive to the rear axle.
DIFFERENTIAL
The differential unit is a low-offset-hypoid spiral-bevel design, based on a 167mm crown wheel geared to deliver a ratio of 2.58:1.
The design of the differential gears and the 4 mountings that control the torque reaction of the unit within the rear subframe, provide a differential unit of excellent efficiency and refinement.
The torque delivered to the differential is controlled by the active on-demand coupling, mounted into the void at the front of the differential's cast-aluminum casing, which also provides the oil reservoir for the coupling.
ACTIVE ON-DEMAND COUPLING
Active On-demand Coupling
The active on-demand coupling provides the benefits of a permanent 4x4 system with the efficiency and economical benefits of a part-time system. Located between the differential and driveshaft, the coupling is a self contained unit combining mechanical, hydraulic and electronic functions to distribute drive between the front and rear axles with transparent automatic control.
The active on-demand coupling provides the following functions:
^ Electronic management of torque transfer.
^ Rapid engagement in response to traction demands.
^ Rapid disengagement to ensure its operation cannot corrupt wheel speed signals and compromise stability control system operation; this is especially important on very low friction surfaces.
^ Pre-engagement from rest to minimize the risk of wheel-spin.
^ No opposing forces when maneuvering or parking the vehicle.
^ Not sensitive to brake testing on a chassis dynamometer.
Control Module
The control module, attached to the casing of the active on-demand coupling, forms a single unit with the control valve/axial solenoid. By analyzing information from other vehicle modules and sensors the control module regulates the axial solenoid to control the hydraulic fluid pressure supplied to the clutch plates. Some of the modules and sensors the control module communicates with are listed below:
^ Hardwired: Control valve / axial solenoid Electric hydraulic pump Oil pressure and temperature sensor
^ Control valve / axial solenoid
^ Electric hydraulic pump
^ Oil pressure and temperature sensor
^ High speed CAN (controller area network): Engine control module Anti-lock brake system / traction control module Traction response switch Yaw rate sensor Steering wheel rotation sensor
^ Engine control module
^ Anti-lock brake system / traction control module
^ Traction response switch
^ Yaw rate sensor
^ Steering wheel rotation sensor
The axial solenoid constantly adjusts the control valve output using a PWM (pulse width modulation) signal. The fluid pressure delivered to the clutch plates determines the amount of torque that is delivered to the rear axle.
The active on-demand coupling has integrated oil pressure and temperature sensors to enable the control module to accurately manage the torque transfer under all environmental and operating conditions. Using these signals the control module will use strategies to protect the coupling from overheating; in extreme cases to protect the coupling from damage the coupling will disengage if the temperature of the hydraulic fluid exceeds 105 degrees C. The coupling will return to normal functionality when the temperature falls below 101 "C.
The control module has an integrated diagnostics system, which constantly monitors the active on-demand coupling system as well as its input and output signals. If the control module detects a fault a DTC (diagnostic trouble code) is stored. The DTC is accessed using the Land Rover approved diagnostic system.
Electric Hydraulic Pump
When negotiating very low friction surfaces such as wet grass, snow or ice; initial wheel-spin can cut into the surface and reduce grip. Even with an active on-demand coupling, almost 60 degrees of wheel rotation would occur before a moderate 500 Nm of torque could be transmitted through the coupling.
To counteract this Land Rover has developed a unique high-pressure pre-charge facility which energizes the hydraulic circuit as soon as the engine is started. Essentially an electrically operated hydraulic pump has been designed to maintain a potential of 500 Nm of torque pressure within the coupling.
The pre-charge capability also reduces the time taken to achieve full torque transmission if slip is detected. Full 1500 Nm (1106 ft. lbs.) of torque transmission can be attained from standstill in less than 15 degrees of wheel rotation compared to over 60 degrees in similar couplings without a pre-charge capability.
Vehicles fitted with Terrain Response also add further benefits by varying the level of pre-charge to deliver optimum traction over a range of different terrain surfaces. The level of pre-charge is varied depending on the particular terrain response mode, for example:
^ Terrain response in "Special Programs Off" mode as common with vehicles without terrain response, the coupling is programmed to transmit 500 Nm of torque to the rear axle when the vehicle moves from rest in a straight line. This strategy minimizes traction loss from a standing-start regardless of the terrain. When the vehicle accelerates the pressure in the coupling is decreased to improve fuel economy.
^ The ability to sense the steering angle allows the coupling to be programmed to provide no torque transfer through the coupling. This prevents the coupling locking when the vehicle is maneuvering at low speeds and acute steering angles.
^ In "Grass / Gravel / Snow" mode the coupling is programmed to maintain its pre-charge state until much higher speeds are obtained. The same applies even if the vehicle is traveling at low speeds and acute steering angles, as traction takes precedence over coupling lock-up on low-friction surfaces.
Mechanical Hydraulic Pump
The driveshaft is attached to the coupling's front clutch plate assembly (input), with the rear clutch plate assembly connected to the differential pinion (output). A swash-plate with 6 hydraulic rollers is also attached to the differential pinion. When there is no speed difference between the coupling's input and output, the rollers do not function.
However, when the front and rear axles start to rotate at different speeds, the swash-plate rotates relative to the rollers which generates the hydraulic pressure. This pressure is used to force the opposing clutch plates together, increasing the transmission of torque to the rear axle. As the difference in axle speed increases the hydraulic pressure pushes the clutch plates further together to increase the torque to the rear axle.
A control valve/axial solenoid controls the amount of pressure applied to the clutch plates, and hence the amount of torque transmitted to the rear wheels. Close manufacturing tolerances and exceptionally low component wear ensure torque control remains accurate throughout the vehicle's life.
By-pass Valve
On very low friction surfaces, driveline drag torque can occur, for example:
^ reverse torque from engine braking, or
^ forced movement of the driveshaft by the front wheels.
This can influence rear wheel speed, making it impossible to determine the true friction capability of the rear wheels, by distorting the wheel speed signal. To prevent this, the active on-demand coupling is designed to open immediately in response to a stability control event. This is achieved by a by-pass valve instantly reducing system pressure to nominal.
With the aid of a large Belleville spring, the clutch plates are forced clear of each other to prevent torque transmission through the coupling. Even at 0 degrees C torque transmission is reduced from 300 Nm to Zero within 10 ms.
Accumulator
The further the clutch plates have to move in order to contact each other, the longer it takes to displace the hydraulic fluid necessary to build pressure and transmit torque. To counter this, the coupling incorporates an accumulator. This retains a nominal 4 bar pressure within the hydraulic circuit. Although this is not enough pressure to cause significant torque transmission through the coupling, it forces the plates very close together so that very little fluid displacement is required to achieve full engagement and maximum torque transfer. Full torque transmission can be achieved in an impressive 150 ms.
Wet Clutch Pack
The clutch pack is made up of 7 pairs of plates; the inner discs are produced from hardened steel with the outer discs manufactured from steel with a sintered face. The clutch plates operate in transmission fluid.
Torque transmission across the clutch pack is limited to 1500 Nm (1106 ft. lbs.). This ensures the lower gears retain an element of front-wheel-drive for traction stability. Within the higher gears the coupling is theoretically capable of transmitting all the drive to the rear axle; although conditions would have to be extreme for this to occur.
PRINCIPLES OF OPERATION
An internal electronically-controlled pump provides hydraulic pre-charge pressure within the coupling. The pre-charge pressure supplies the required operating pressure to the clutch plates to eliminate initial wheel-spin as the vehicle accelerates from standstill.
In conjunction with the pre-charge pressure a mechanical hydraulic pump operates within the clutch plates to supply the coupling's main hydraulic operating pressure. The mechanical pump is functioned by the "input" and "output" of the coupling:
^ input - driveshaft connection from the front axle,
^ output - differential connection to the rear axle.
Any speed difference between the front and rear axles will start the operation of the mechanical hydraulic pump. The amount of hydraulic pressure applied to the clutch pack by the pump determines the gap between the clutch plates. For example, the greater the hydraulic pressure, the smaller the gap between the plates and subsequently the greater the torque transmitted through the coupling from the front axle to the rear axle.
This main hydraulic pressure is designed to transmit the torque for traction demands of off-road driving, and to provide lock-up as required.