[Diy_efi] RE: Throttling intake air -- references

[Mike]

At 03:48 PM 15/1/2003 -0000, you wrote:
>Going back to controlling boost via throttle position. I know its possible,
>no doubt there, but can I ask:

I have seen occasional reference to this method but many years ago
when ECU's werent explicity set up to do it compounded by the
ready availability of casting which included a wastegate causing
a market 'critical mass' and hence momentum for that approach
and probably influenced by the feedforward aspect of the control
systems complexity...

>Trying to control boost at 2500 revs using throttle only is more difficult
>than at 5000 on the same engine, as the boost rises and falls over a much
>smaller throttle movement? What sort of (boost controlling) linkage would
>you use that could control boost via the (blow through) throttle plate, but
>alter the 'sensitivity' of the throttle so as the boost came and went over
>the same amount of pedal travel?

Oh yes, I can appreciate that point and its well put. I've not yet
considered the linkage issue but should (I expect) be containable
via various leverage mechanisms, PID stability plays a major part.

These days with extremely powerful 25mips processors for USD$5 and
development in sensors and control electronics it might be feasible to
revisit the issue armoured with much better implementation tools
capable of handling what you well describe - vis a vis the positional
nonlinearity or rather angular nonlinearity issue in respect of
control dynamic for boost pressure via existing throttle body mechanics.

>For the same engine, is the exhaust backpressure in the manifold the same
>when running wide open throttle and wastegate part open as it is when
>running wastegate shut and throttle part open?

Well I have recalled, from a magazine sponsored article some years ago,
that once the wastegate opens (and stays open) the higher pressure on
the pre-turbine side is vented to the post-turbine side, therefore
its logical to interpret that the EBP must be higher. These days its
easy to measure and when i get time I'll set up a pair of gauges
to report what I find.

Its common sense to some major degree, if the wastegate needs to
be opened and stays open to dump pressure one would expect the
pressure post-turbine to be higher - for some other posters to suggest
it might be the opposite is ludicrous, sure the turbine will spin
down a tad *but* if the need is still there to bypass the turbine
then the region post-turbine must be seeing pressures that exist
on the pre-turbine side as the wastegate is open and this must
be higher as its not all going through the damn turbine <sigh>

I also think there is tradition in seeing the wastegate as a control
(proportional) element when its really more akin to a digital
relief valve, hence is discontinuous in its reponse to dumping
exhaust energy and therefore should be logical to control the fuel/air
more precisely some time before its combusted. Historically this has
been difficult and more expensive, these days it should be much
easier if people can intellectualise the control systems
algorithms and that was the impetus for this thread in the first
place,

;-)


rgds

mike


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