Rear Differential System Description - AWD System (AWD)
Overview
The All Wheel Drive (AWD) system is adopted into the rear differential. The AWD features a built-in real-time controlling mechanism that distributes the proper torque to the rear wheels as needed depending on the rotating difference between front and rear wheels or the engine power. The hydraulic circuit of the AWD is equipped with a motor-driven oil pump and an electronically-controlled valves, resulting in a wide range of torque controls, improvements in responsiveness, reduction of excessive torque, and compact size/lightening.
Construction
The AWD Differential Assembly consists of a wet-disc clutch assembly, differential, and a hydraulic system.The main components of the wet-disc clutch assembly are as follow:
| Components | Purpose |
|---|---|
| Clutch Disc A | Clutch discs A are splined to the clutch guide. |
| Clutch Disc B | Clutch discs B are splined to the clutch hub. |
| Clutch Guide | Clutch guide is connected to the propeller shaft. |
| Clutch Hub | Clutch hub is connected to the hypoid pinion shaft. |
| Hydraulic Pressure Piston | The end plate is pressed by an oil pressure, and the clutch plates A and B are engaged. |
Hydraulic Circuit
The main components in the hydraulic circuit are as follows:
| COMPONENT | FUNCTION |
|---|---|
| Oil Pump | Operated by the rear differential pump motor, the oil pump receives the oil from the strainer and sends it to the hydraulic circuit. The rear differential pump motor is controlled by the AWD control unit. |
| 1-Way Valve | Retains the oil pressure in the hydraulic circuit. |
| Pressure Relief Valve | Operates when the oil pressure failed, and release the oil pressure into the strainer. |
| Rear Differential Solenoid Valve | Turns ON/OFF to adjust the oil pressure in the hydraulic circuit. |
| Rear Differential Fluid Pressure Sensor | The fluid pressure sensor monitors the pressure in the hydraulic circuit and reports this to the AWD control unit. |
| Rear Differential Fluid Temperature Sensor | The fluid temperature sensor monitors the fluid temperature and reports this to the AWD control unit. |
The AWD control unit receives signals from the PCM, VSA Control Module, Gauge Control Module, and steering angle sensor.These signals are used to calculate the amount of torque to be provided to the rear wheels.The AWD control unit actuates controls on the differential that generates variable hydraulic pressure in the hydraulic circuits.This pressure causes a clutch assembly inside the housing to be applied, delivering variable amounts of power to the rear wheels.
Hydraulic Control System Diagram
Operation
Driving Power Delivery
The driving power delivered from the propeller shaft is transmitted to the clutch guide and clutch plate. Since the clutch is not engaged when the oil pressure is not generated, the pinion gear is not driven by the propeller shaft. In this case, the driving power is not transmitted to the rear wheels, but it is only rotated by the vehicle movement.The generated hydraulic pressure presses the pressure piston against the end plate, compressing the clutch discs. This compression reduces the slippage between clutch discs A and B, thus rotating the clutch drum together with the clutch hub.
The drive power delivered to the propeller shaft is outputted from the clutch hub to the drive pinion gear, ring gear, differential and rear drive shafts when the clutch is applied. In this situation, the drive power is transmitted to the rear wheels.
In addition, the clutch drum and the clutch hub are not in complete contact but linked to each other with a maximum of 80% contact rate. This slippage reduces the friction caused by the rotational difference between the front and rear.
Hydraulic Control
Torque is distributed to the front and rear wheels by controlling the amount of clutch application in the differential. Clutch application force is controlled through oil pressure created by the oil pump drive motor and a solenoid valve.
By applying pressure and depressurization control, the AWD control unit sets the optimum level of pressure in stages to suit the driving conditions and road conditions. The basic control method involves the hydraulic circuit being pressurized by the motor to a level close to, but generally exceeding, the target pressure calculated from the vehicle conditions and road conditions received by the AWD control unit.
The hydraulic controls in a normal driving condition are as follows:
- In the pressurization mode, the oil pump driving motor generates a target oil pressure in the hydraulic circuit during a period of time calculated by the AWD control unit. The 1-way valve is opened by the oil pressure and the AWD control unit closes the solenoid valve (energized), thus, the pressure piston squeezes the clutch discs.
- In the retention mode, if the oil pressure reaches the pressure level set by the AWD control unit, the motor stops pressurizing. The 1-way valve closes and the solenoid valve keeps closed (energized), and the oil pressure generated in the hydraulic circuit is kept as-is. Thus, the pressure piston is fixed in a place and maintains the pressure on the clutch discs.
- In the depressurization mode, if the AWD control unit determines that it is not necessary to distribute a torque to the rear wheels, it turns on electricity to open the solenoid valve (de-energized), and releases the oil pressure from the hydraulic circuit. Thus, the pressure piston returns back and releases the clutch disc.
Basic Operation
Operating Modes
The operations of AWD are performed appropriately to meet the driving conditions. The following examples show the AWD performance against several driving conditions.
Accelerating From a Stop on Dry Pavement
When the vehicle is stopped on a dry level surface, the AWD control unit puts the system in Standby Mode.This mode pressurizes the hydraulic system and applies the clutch pressure, and distributes a maximum of 20% of torque to the rear wheels for an initial acceleration.To achieve this, the All Wheel Drive control unit receives information from the VSA system to determine the longitudinal angle of the vehicle. Upon determining the vehicle is on level ground, the AWD control unit activates the oil pump drive motor to pressurize the hydraulic system, then cycles the oil pressure solenoid on and off until the target pressure is met. Two levels of system pressurization can be created, depending on conditions. When the target pressure is achieved, the oil pressure solenoid remains closed to maintain the pressure in the hydraulic circuit until the vehicle begins to accelerate.
Cruising at Steady Speed on Dry Pavement
When the vehicle is cruising at a steady speed on dry pavement, the AWD control unit deactivates the All Wheel Drive system. The vehicle operates in 2WD mode, helping to increase fuel economy. To achieve this, the AWD control unit receives information from the PCM, VSA and MA-EPS systems to determine the stability of the vehicle.When it determines that the vehicle is traveling at a steady speed and no wheel slippage is occurring, all pressure is released from the hydraulic circuit, enabling 2WD-only operation.This mode continues until the AWD control unit determines that torque needs to be transferred to the rear wheels.
Front Wheel Slipping on Slick Surface
When the front wheels are slipping, for example on a low friction surface such as wet or icy roads, the AWD control unit receives information from the VSA and MA-EPS systems that the front wheels are slipping.When the slipping of the front wheels occur, 80% of torque will be transferred to the rear wheels to reduce the load on the front wheels, and to regain vehicle stability.This mode continues until the All Wheel Drive control unit determines that the front wheels are no longer slipping.
Accelerating Uphill From a Stop on Dry Pavement
To stabilize the vehicle while driving uphill, the AWD control unit transfers more of the available torque to the rear wheels than when driving on a flat road.To achieve this, the AWD control unit receives information from the VSA system to determine the slope where the vehicle is driving.Upon determining this, the AWD control unit activates the oil pump drive motor to pressurize the hydraulic system, then cycles the oil pressure solenoid on and off until the target pressure is reached.The level of pressurization varies depending on the calculated slope the vehicle is driving. The torque transfer to the rear wheels will be controlled at a maximum of 80% depending on driving force. This mode continues until the AWD control unit determines that the vehicle is no longer accelerating uphill.