EDL's-The short course

[ScharfR@aol.com]

dmiller@iea.com asked, "EDLs - what are they?"

EDL is simply Audi's name for a form of brake-intervention traction control.
 It uses much of the controller and hardware of the ABS system to cycle the
brake on a powered wheel when wheel spin is detected on that wheel.  It's a
low-speed only system (to avoid overheating the brakes).  Here is the short
course on EDL.

The front and rear differentials (conventional open designs) do not
physically "lock."  As you probably know, an open diff will always divide
torque (NOT rpm) evenly between the two output shafts.  When one wheel spins
due to near-zero traction, it is transmitting near-zero torque.  The
non-spinning wheel on the same axle has the same near-zero torque applied to
its half shaft, but that isn't even enough to rotate the wheel.  Hence, on
most 2WD cars, if you spin one wheel you go nowhere (same will happen on many
4WD vehicles, or a first-generation quattro if no diff locks are applied).

EDL selectively applies the brake to the spinning wheel, thereby increasing
the torque necessary to continue powering that wheel's output shaft. The same
increased torque is applied to the other output shaft and Viola! you can
transmit enough torque to the wheel with better traction to achieve
drive-away.  As you observed in your driveway test, this would be
accomplished with one wheel still spinning (albeit not wildly out of control)
and the other wheel on the same axle providing the drive-away capability.
 The system will continue to cycle the brake on the wheel with least traction
until the rpm delta between the two wheels on that axle is reduced below the
triggering threshold (or until a road speed of ca. 25 mph is attained).  EDL
can also switch the brake-cycling from side-to-side in cases where you are
driving off on extremely variable surfaces.

It's pretty elegant engineering, in that you gain quite a lot of functional
capability with only a very minor cost and developemnt penalty over the price
of a contemporary ABS system

On today's front drive Audis, EDL applies to the front wheels only.  On the
quattros it applies to the front and rear.  In combination with the Torsen
center differential, a modern quattro can distribute drive torque from
side-to-side at both the front and rear axles (at low speeds), and
front-to-rear over the entire speed range.

As an aside, there is no fundamental difference in the performance of Audi's
EDL system and that used by VW on the VR6-equipped GTI/Jetta/Passat models,
or by Volvo on the 850 (and by a number of other cars).  In addition, the
_new_ Mercedes-Benz 4-Matic system will use an essentially similar EDL
capability without, however, a torsen center differential.