Q: What is Active Yaw Control (AYC)?
A: From the Evo 4 onwards (GSR models) Active Yaw Control was introduced. It is a type of active rear differential that helps to provide maximum traction to individual rear wheels according to sensed forces on the car and the drivers input.
Q: What are the benefits?
A:
Due to the nature of the design of normal 'open' type differentials they will transfer torque to the wheel offering least resistance. You can see the effect when one wheel of an axle fitted with an 'open' diff is in mud and the other wheel is on tarmac. The wheel in the mud (low grip, least reistance) will just spin away while the one on the tarmac (high grip, high resistance) does nothing! You can also often see this process happening on the track, especially on the front axle of normal road going FWD cars. When entering a corner the outside wheel becomes heavily loaded due to weight transfer while the wheel on the inside of the corner becomes unloaded. With an 'open' diff the inside wheel can spin as torque is transferred to the wheel offering the least resistance, which is the inside wheel. To stop this torque loss a Limited Slip Differential is often fitted, especially to high performance cars. Limited Slip Differentials (LSD) come in various designs but their one aim is to limit the torque loss ('limit' the 'slip' of the wheels) and to transfer it to the wheel with the most grip and therefore providing better traction. Standard LSD's are totally mechanical in design and can only provide torque transfer in one direction proportional to the amount the 'unloaded' wheel is spinning. Mitsubishi's Active Yaw Control system builds on this principle and adds electronic control of the torque transfer.
AYC has several advantages:
- It can help equalise the loading of all four tyres and therefore provide the maximum cornering potential.
- Understeer when cornering is reduced as a Yaw moment can be set-up by torque transfer at the rear wheels.
- Sharp corners can be taken with smaller steering angles than normal due to a Yaw moment set-up by torque transfer at the rear wheels.
- When driving or pulling away with the left and right wheels on surfaces with different friction levels the AYC can transfer torque to the wheel with the most grip.
Q: How does it work?
A:
- The AYC System
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- The AYC system can be split into four main sections and they consist of the following components:
- Sensors
Wheel Speed Sensors
These are the same sensors that are used with the ABS. On ABS equipped cars the speed signal is sent to the
AYC ECU after passing through the ABS ECU.
Steering Wheel Sensor
Detects the rate of turning of the steering wheel and whether the steering wheel is centred or not.
Throttle Position Sensor
Shared with the Engine ECU and determines the throttle position.
Longitudinal G Sensor
This is the same acceleration sensor that is used with the ABS. On ABS equipped cars the sensor is shared with
the AYC ECU and the ABS ECU.
Lateral G Sensor
The same sensor as above but mounted at 90° to sense lateral G forces.
Stop Lamp Switch
This is the same switch that illuminates the stop lamps. It is used to determine if the brakes are being applied.
AYC ECU
The AYC ECU evaluates the inputs from the various sensors and controls the output of the hydraulic unit
accordingly. The AYC ECU also has a diagnosis function to identify faults in the system. If a fault is found it will stop the AYC functioning and illuminate the AYC warning lamp.
Hydraulic Unit
Electric Pump
This is activated intermittantly by the AYC ECU to maintain hydraulic fluid pressure stored in the Accumulator
(hydraulic fluid used is ATF-SPII).
Accumulator
A vessel which is used to store hydraulic fluid under pressure.
Hydraulic Switch
Informs the AYC ECU when the Accumulator has reached the required storage pressure.
Proportioning Valve
Varies the pressure output to the Clutch packs.
Direction Control Valve
Directs the pressure output to either the left or the right Clutch pack.
Torque Transfer Differential
Differential Mechanism
A standard differential, allows the left and right wheels to turn at different rates during cornering. Lubricated by
Hypoid gear oil.
Speed Increasing/Decreasing Gears
The speed increasing and decreasing gears increase the speed of the right hand clutch disc and decrease the
speed of the left hand clutch disc, relative to the right hand wheel. The speed increasing and decreasing gears are lubricated by AYC fluid (ATF-SPIII).
Wet Multi-Disc Clutches
The wet multi-disc clutches transfer torque from the faster wheel to the slower wheel when operated by the
hydraulic unit. When the left-hand clutch operates, the speed-decreasing gear causes a torque transfer from the
right wheel to the left. When the right-hand clutch operates, the speed-increasing gear causes a torque transfer
from the left wheel to the right. The clutch packs are lubricated by AYC fluid (ATF-SPIII).
How it works
On starting the car the AYC ECU checks the state of the system and operates the Electric Pump to charge the Accumulator. The Hydraulic Switch informs the AYC ECU when the
correct operating pressure is reached and the Electric Pump is switched off. This cycle is continously maintained all the while the car is running with the pump running intermittently to keep the
pressure correct.
When torque transfer is called for by the AYC ECU due to the sensed inputs and the AYC programming a signal is output to the Proportioning Valve and the Direction Control
Valve to send the correct pressure to the correct Clutch Pack in the Torque Transferring Differential.
Torque Transferring Differential
Principle of Operation of the Torque Transfer Differential
When two clutch plates of differing rotational speed are pressed against each other torque is transferred from the faster disc to
the slower disc. The transfer of torque is proportional to how hard the discs are pressed together. This is the basic principle behind the Torque Transfering Differential.
The speed increasing/reducing gearing is permanently meshed. The input gear of the speed increasing/reducing gearing is
connected to the differential case. The speed increasing gear is connected to the right-hand clutch and the speed reducing gear is connected to the left-hand clutch. This means the right-hand
clutch moves faster and the left-hand clutch moves slower relative to the differential case. The housing of the left and right clutches is connected to the right-rear drive shaft so activation of
the clutches permits torque transfer towards either side.
Flow Of Torque With The Left-Hand Clutch Activated
Due to the speed reducing gear the left-hand clutch moves slower relative to the right hand drive shaft so when the left-hand clutch is activated the right-hand drive shaft transfers some
torque back towards the differential case and onto the left drive shaft. A rightward yaw moment is therefore created.
Flow Of Torque With The Right-Hand Clutch Activated
Due to the speed increasing gear the right-hand clutch moves faster relative to the right-hand drive shaft so when the right-hand
clutch is activated the differential case transfers more torque to the right hand driveshaft. A leftward yaw moment is therefore created.
