AFM measurement/backpressure (was Turbo speed sensor)

[Mike (Perth, WA)]

At 10:28 AM 7/03/2000 +0800,  Bernd Felsche <bernie at perth.dialix.com.au>
wrote:

>Similarly, to accurately measure the airflow, you need a fast
>pressure measurement at the inlet as well. There are pulsations in
>the airflow even with (turbo-)supercharging. Transducers will
>typically not give you enough information at high speed.

mmm - not entirely understanding of this AFM/turbo backpressure sideline.

Surely given these facts:-

a.	Instantaneous speed changes are rare
	(idle to 5000rpm in 0.5sec and likewise - my definition etc;)
b.	AFM some distance upstream from inlet valve/turbo pulsations
	will average out flow and even if there are pulsations at the
	hot wire - the average wire current will measure average AF.

Then, surely what the heck difference does the backpressure make other
then reduce the flow of air into the engine. I mean in a previous post
its suggested the ECU needs to know turbo backpressure so it can correct
for AFM signal to determine how much air actually fills the chambers.

Well, correct me if I am wrong, but doesn't the AFM do this anyway ?

If the cylinders are not filling because the engine revs are high but the
turbo hasn't yet spooled up then the AFM will reflect this *and* as the
turbo does spool up the air flow will increase, AFM will show that.

Taking fact (a) above you have an average AFM over all cylinders and
a change in engine speed much slower then the data acquisition cycle
and algorithm time through the ECU. And (b) handles the total/average
of all chambers - given they should be the same :)

So, in my book (and brain) you don't need to worry about instantaneous
pulsations to find instantaneous air flow changes to meter for air flow
because the single AFM will do that and the chambers not filling due
to backpressure will be reflected by an altered AFM reading.

I'm looking for a satisfactory rationale as to why its perceived the AFM
doesn't measure changes in cylinder filling due to turbo backpressure ?

Tah

:) Mike
Perth, Western Australia

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