Differential Assembly: Description and Operation
System DescriptionSH-AWD System
Outline
This vehicle is equipped with a Super Handling All-wheel Drive (SH-AWD) rear differential system. This system uses electromagnetic clutches, an acceleration system, and the SH-AWD control unit to control front to rear torque distribution and independent left to right rear torque distribution. This allows the system to function both as a limited slip differential and as a torque distribution system to improve standing start performance, straight-line stability, climbing, and cornering ability.
SH-AWD features include
- Independent torque distribution to the front and rear wheels, and to the left and right wheels for neutral handling when cornering.
- Controls differential (inside/outside) wheel speed in a turn to increase cornering stability.
- Limited slip differential function for better acceleration.
Operation
In a normal turn the radius of the front wheels is shorter that the radius that the rear wheels travel. If the front and rear wheels are driven at the same speed, full power is not transmitted. The SH-AWD system monitors the speed of the front wheels and increases the speed of the outside rear wheel proportionally to accelerate it around the larger radius. This improves stability and steering response. On slippery surfaces this provides the improved traction of a 4-wheel drive system.
Construction
1. The differential assembly has a conventional hypoid ring and pinion gear set.
2. The acceleration system consists of the high clutch, the one-way clutch/low clutch, the planetary gear set, and the oil pump. When accelerating straight ahead, the one-way clutch applies the driving force to the rear wheels. When turning, the high clutch connects the acceleration planetary gear to the differential. This speeds up the rear wheels to compensate for the different paths the front and rear wheels take. When decelerating, the differential drives the oil pump to generate hydraulic pressure.
3. The direct electromagnetic clutch system consists of the coils, the planetary gear sets, and the multi-plate clutches that are installed in the side cases on either side of the differential carrier. When driving straight ahead, both rear wheels rotate at the same speed. When turning, current is applied to the appropriate outside wheel main coil to engage the clutch. The amount of current passed through the electromagnetic clutch controls the amount of driving force to the outside wheel.
SH-AWD 4WD Control System
Control System
The SH-AWD control system consists of a SH-AWD control unit, the PCM, the VSA control unit, and various sensors and switches that detect the steering angle, the lateral G force, the yaw, and the vehicle speed. An oil temperature sensor and a speed sensor are located on in the rear differential. The control units exchange information via the CAN communication lines. The SH-AWD control unit has a self-diagnostic function. If a malfunction is detected, the SH-AWD control unit turns on the SH-AWD indicator in the gauge assembly and the system goes into fail-safe mode. When in fail-safe mode, the vehicle disable the SH-AWD differential. The vehicle defaults to front wheel drive, and the PCM reduces available engine output.
Driving Force Control
The driving force control distributes the power based on the driver's inputs. It distributes torque to the front and rear wheels based on the throttle opening and the available engine torque output. When turning, torque is applied to the rear wheels independently based on the lateral G input and the direction of the turn. This generates an inward yaw movement to help steer the vehicle around the turn.
Feedback Control
In low traction conditions, where a wheel will I spin or the vehicle tends to skid, the amount of torque is distributed to match the traction available.
Acceleration System
Construction
The acceleration system consists of the one-way clutch/low clutch, the high clutch, the planetary gear, and the oil pump.
Operation
Acceleration and deceleration in a straight line
1. The one-way clutch/low clutch connects the propeller shaft to the planetary gear carrier. The high clutch is disengaged by spring force.
2. The propeller shaft, the planetary gear carrier, and the pinion gear all rotate at the same speed.
Operation
Acceleration system in a corner
1. Hydraulic pressure from the solenoid valve engages the high clutch.
2. The high clutch locks the planetary gears to the propeller shaft, and the driving force is increased to both rear wheels.
3. The high clutch mechanically releases the low clutch.
Acceleration System Hydraulic Pressure Control
Construction
The hydraulic pressure system consists of the solenoid valve, the oil pump, the reverse one-way valve, the regulator valve, the shift valve, the accumulator, and the orifices.
Operation
Driving in a straight line
The pressure regulator valve regulates the hydraulic pressure generated by the oil pump. Because the solenoid valve is off, hydraulic pressure is not applied to the high clutch piston, and it remains disengaged. If the hydraulic pressure overcomes the pressure regulator valve, the pressure is relieved from the pump circuit to reduce pump drive losses.
Cornering
The solenoid valve is energized, applying hydraulic pressure to the high clutch piston via the shift valve to engage the high clutch. An accumulator and an orifice are installed between the shift valve and the high clutch piston to reduce the fluctuations in hydraulic pressure.
When backing up
The reverse rotation of the propeller shaft causes the oil pump to rotate in reverse, This opens the reverse one-way valve and allows the oil to circulate in the pump and valve body without draining the system, This prevents aeration of the oil which can delay the response of the differential.
Electromagnetic Clutch System
Construction
The electromagnetic clutch system is incorporated into the side case assemblies on both sides of the differential case. The system consists of the multi-plate clutches, the planetary gears, the pressure guides, the solenoid coil assembly (the search coil and the main coil), the armature assembly (the slider and the armature), and the yoke.
Operation
The SH-AWD control unit receives inputs from the PCM and the VSA control unit on the driving conditions. via the CAN. The SH-AWD control unit sends signals to the main coils, which engage the electromagnetic clutches that connect the planetary gear set. This increases the torque applied to the rear wheels. The amount of current sent to the clutch is continuously varied as driving conditions change.
Search Coil Detection System
As the clutch disks wear, the air gap between the pressure guides and the multi-plate clutches increases. The increase in air gap changes the magnetic field generated by the main coil and the yoke. The search coil detects these changes in the magnetic field, then the SH-AWD control unit uses predetermined maps to compensate for the changes to maintain precise control and smooth clutch engagement.
SH-AWD SYSTEM OPERATION LIST:
- When turning to the right, the magnitudes of 3 and 4 are transposed when compared with the left turn shown.
- Conditions of turn (direct coupling): For the turns with a lateral G of less than 0.075G when turning at a vehicle speed less than 30 km/h or over 120 km/h.
- Conditions of turn (speed acceleration): For the turns with a lateral G of more than 0.075G when turning at a vehicle speed between 30 to 120 km/h.
- Soft: 0 - 392 Nm (0 - 40 kgf-m)
- Medium: 392 - 785 Nm (40 - 80 kgf-m)
- Hard: 785 - 981 Nm (80 - 100 kgf-m)
Electric Control System
SH-AWD Control Unit Electrical Connections:
SH-AWD Control Unit Inputs and Outputs
SH-AWD Control Unit Inputs and Outputs:
SH-AWD Control Unit Inputs and Outputs:
SH-AWD Control Unit Inputs and Outputs:
The SH-AWD control unit terminal voltages and the measuring conditions for the SH-AWD control system.
SH-AWD Control Unit Connector Terminal Locations