[Diy_efi] Digifant (was:This turbo assist stuff...)

[Bernd Felsche]

On Fri, Jan 10, 2003 at 02:29:23PM +0000, Mike wrote:
> I wasnt looking to argue with you about the reletive merits of
> VW's control systems strategy. I'm trying to understand your

I wasn't trying to argue either, just explain their design
compromises and objectives.

> problem and make an assesment as to what might make it rather
> worse than what you'd expect from a 'drive out the showroom'
> mode of operation. Incidentally the knock sensor on my Bosch/Hitachi
> ECU is never tested by the ECU via pushing the advance. It seems
> to be tested by virtue of noise detection within the rpm band its
> designed to be effective if and only if the ignition is already
> retarded after an initial knock detect and for my ECU its only 5deg.
> It comes up with error code 34 (I think) to report some connection
> problem or sensor problem or other problem in effect causing excessive
> knock after a retard has alread occured.

Your system is a couple of orders of magnitude more complex.

> On your system,
> You are telling me the ECU detects knock frequently enough to
> cause you some angst, even with hi octane fuel, to be a distinct
> irritation.  What I am focussing on is whether there is some
> aspect of your overall system that gives the ECU an indication of
> excessive knock vs 'slight' knock, because you are reporting the
> output falls dramatically in those circumstances and you indicate
> this is relatively frequent.

I gather it's because of the drastic rate of retard that occurs when
knock is detected. Something like 3 degrees immediately, whereas the
advance (back to "imminent" knock) is done in smaller increments
of 0.5 degrees over a longer time. Maximum retard due to knock is
supposedly 15 degrees.

Engine timing was originally for 95 Octane; though the (per)version
for Australia was supposed to tolerate 91 Octane. (ULP)

> What I am therefore trying to get a handle on is what you think
> might be causing the ECU or sensor to (in effect) see an excessive
> knock signal (which might be erroneous) vs a slight knock which
> would (in theory) only require a minor retard - not the significant
> retard you are reporting which you find so irritating.

A small perturbation results in at least 3 degrees of retard on one
cylinder. Because the timing on all cylinders is not allowed to
differ by more than 6 degrees (IIRC), the event may retard other
cylinders by a smaller amount. That's why I think the torque drops
so sharply.

> And, the most probabilistic item I am focussed on at this point has
> to do with suppression of ignition noise. I see a potential in
> the probability that your 'bosch amp' has degraded to the point
> that its inbuilt suppression cap is not functioning optimumly
> causing possible coupling to the knock sensor - hence causing
> the ECU to 'over-correct' for erroneous knock detection.

Historically speaking, as the car has exhibited similar behaviour
since Feb 1992 (when it was "new"), I don't see degradation as being
the issue. If there is excessive ignition noise causing the problem,
then it existed when the car left the factory.

As I mainly encounter pinging in dry weather when using Ultimate 98,
i.e. it greatly reduces the incidence of knock I tend to think that
a reduction in ONR by humidity control to be the more-likely
solution.

That is not to say that your suggestion is disregarded; I simply
don't have the suitably-rated bits at hand and will have to buy some
to try.

-- 
/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
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