Tuesday, July 31, 2007

How the ACD works

This thread is dedicated to the ACD system used on Lancer Evolutions. I'm cutting and pasting a bunch of useful information from around the Internet which hopefully, someone, sometime will find useful

A significant portion of the content here comes from http://forums.evolutionm.net/showthread.php?p=2644484#post2644484 and related links.


The Mitsubishi description also says that the ACD operates in a free state when rapid steering movements are made and when the hand brake is used. This means in those conditions that the drive must be going all to the front wheels.

Here is some info I got from BTR preparations who have tested and recorded what the ACD is doing. Their website also has some useful info on the ACD.

With the original ECU the mapping is fairly tame.....but having said that there will be a noticeable effect to the handling and traction when selecting between the tarmac or gravel/snow options. I would be surprised if much difference will be felt between snow and gravel on a wet tarmac road with road tyres as these 2 maps are broadly similar.

Based on the rally car, the car works better in the gravel mode on wet asphalt conditions than on the tarmac map. This is only relevant when driving the car at a speed where the car is generating wheel speed error
across the axles (sliding or on slippery surfaces with large throttle openings)

The biggest advantage for the average road driver is the traction out of corners on a wet road, which will be better in gravel mode. On a trailing throttle there is unlikely to be a difference as the diff pressure reduces
with low throttle openings.

For normal dry road use the asphalt map is the one to use as it locks the diff under braking to a larger extent than the snow/gravel maps - reducing the braking distances by aggregating the braking force through the
transmission - a trick the earlier Evos cannot match!


How does the Active Centre Differential system work?

Mitsubishi ACD - “active centre differential system” ACD is the new major technical feature that distinguishes the previous Evo 4-6 models from the new Evo 7 ACD is an extension of the technology used in for the anti yaw control systems (AYC) employed in various previous Mitsubishi models in the rear axle position.

The ACD system comprises an electric motor, driving an oil pump which pressures an oil reservoir to a peak 16-bar pressure. This reserve of “SYSTEM PRESSURE” is fed to the piston of the ACD plate pack via a modulation (proportion) valve. The modulation valve is software controlled by the “ACD ECU (electronic control unit).

The control inputs for the ACD electronic control unit are
4 wheel speeds
G force both lateral and longitudinal
Throttle position – a variable value
Braking state - on or off
Steering angle – neutral position (straight ahead) and off centre position provided by 3 optical inputs generated by rotation of the steering wheel.
Handbrake state – on or off
System pressure – Hydraulic pressure state
Mode switch state – to select Gravel, Snow or Asphalt software strategy (maps)

We fitted a Motorsport logging system to the E7 RS test car during our 90 mile shakedown run of the new car (in the well knownYorkshire Forest complex) prior to the Network Q Rally GB. The test data from the ACD system has provided a full understanding of how the system functions and it’s efficiency.

The “Gravel” and “Snow” software strategies are similar; Gravel has the highest ultimate locking value of the two. In “Asphalt” mode a unique software strategy is employed

The system is technically far superior to the preceding Evo models VC centre differential systems. The centre differential performing well and able to limit front to rear axle “slip” on full throttle to a mean of around 0.6% on a slippery gravel surface. To put this in perspective, a good condition VC unit on a Evo 4,5 or 6 would struggle to be better than 30% aggregate slip value, which is traction going out of the window.
Pressure can be introduced and lost within the ACD system at speeds surprisingly close to WRC car standards, which has allowed Mitsubishi engineers to use a complex and sophisticated software control strategy.

Which ACD electronic control unit should I use?

The production Evo 7 comes fitted with a relatively “soft” control code within the production electronic control unit. This means the potential of the ACD system is not fully realised without fitting an electronic control unit.

The RA553681K1 has a much improved software strategy aimed primarily for high performance road use, which works the ACD system more effectively than standard. We can recommended this unit for track day and fast road applications.

The RA553681K2 has a software strategy which is very specifically for Motor Sport use, which works the ACD system more effectively and harder than standard and may have negative life and warranty implications for the mechanical parts in the ACD system. This should not be used on a road car. The FIA have been asked to clarify the legality of using this unit in Motor Sport events and for the moment the unit should be treated and is listed in the parts system as a non-GpN legal part