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AYC - Active Yaw Control

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PostPosted: Wed Jun 18, 2008 10:45 pm    Post subject: AYC - Active Yaw Control Reply with quote

AYC stands for Anti Yaw Control.

Yaw is pitch or sideways movement which in the case of a road car means sliding right or left. The AYC system controls the amount of Yaw angle by an electro hydraulic torque distribution system in the rear diff. The AYC ECU is provided with information from various parts of the car; speed, acceleration, braking, lateral G (rate of sideways acceleration) and processes this to change the torque distribution between the rear wheels to restrict rate and amount of slide angle.
This enables Evo to achieve cornering acrobatics of an incredible nature which endows even modest drivers with superhuman ability, ultimately it means it eats twisty roads easily.
All GSR based models are equipped with AYC. The RS, RS2 and GT versions do not have it.
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.
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.

The AYC system can be split into four main sections and they consist of the following components:

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.

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 AYC 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.

Copyright: Buyers Guide
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