[Diy_efi] Speed-density vs. MAF/MAP...

[Adam Wade]

--- Klaus Allmendinger <klaus at innovate-tech.com>
wrote:

> 1. MAF sensors would be the best way to measure mass
> air flow.

Sure looks that way, since it actually determines the
mass of air entering the engine.  No tables or
estimates for VE.

> The disadvantage is that MAF sensors are flow-
> direction insensitive (the output signal is
> essentially proportional to the full-wave rectified
> flow).

Good description!  Yes, that's it exactly.

> Therefore a low-pass filter (plenum) has to be
> inserted between the AC input signal (oscillating
> air mass in intake stream) and the sensor to filter
> out the AC component of the signal and allow to
> measure the DC component (the actual flow). Like all
> filters it creates a delay which is a problem for
> fast flow changes.

This makes me wonder if the idea of four tiny MAF
sensors inside the throttle volume (on a 4-cyl) might
be feasible with a certain amount of processing of the
raw signal.  Probably makes more sense to simply use a
better method than MAF for situations where there is a
lot of reversion.

> 2. MAP measurement is flow indifferent.

In fact, it's better than that, because reversion will
create HIGH pressure in the manifold.  So even with
reversion, at lower rpms, you are (if you sample
properly) only counting air that actually enters the
cylinder.

> The actual mass-flow is calculated from
> experimentally determined VE/RPM tables and MAP,
> RPM, BP and IAT. To make even sense from the MAP
> data it must be sampled at a specific point in
> the engine cycle.

Except where the rpm is sufficient to give a
reasonably constant MAP.  But basically, that's the
gist of it.

> This point may not be fixed ove the RPM range but
may
> change because of the phase difference between the
> resonant pressure and flow of the intake tract and
> engine cycle. 

Sounds reasonable to me.

> 3. TPS/RPM inferres MAP from experimentally
> determined tables, then proceeds as above.

Yes.

> The three methods increase in workload for the tuner
> because each method requires more experimentation
> than the one before.

Very much so.  As you go down the list, fuel economy
or power (whichever you are tuning for) likely drops
as well, due to the increasing lack of precision.

> I think it would be best to have a MAF sensor that
> is flow direction sensitive.

That would be nice, but the ways I can think of would
introduce a flow restriction.

> I don't know if such an animal exists. I could
> imagine one built of two small pitot tubes (used to
> measure airspeed in airplanes) in the intake stream,
> one pointing upstream, one pointing downstream. The
> differential signal between the two would create a
> flow sensitive air-speed signal. Taking the actual
> pressure and temperature at the pitot location
> into account the mass-airflow can then simply be
> determined by integrating the signal over one intake
> period without the filter delay. Does anybody know
> if this has been done?

I'd be interested to see if it had, but isn't a pitot
tube measuring air PRESSURE?  Wouldn't that make it a
differentiated MAP sensor instead of a MAF sensor?

=====
| Adam Wade                       1990 Kwak Zephyr 550 (Daphne) |
|   http://y42.photos.yahoo.com/bc/espresso_doppio/lst?.dir=/   |
| "It was like an emergency ward after a great catastrophe; it  |
|   didn't matter what race or class the victims belonged to.   |
|  They were all given the same miracle drug, which was coffee. |
|   The catastrophe in this case, of course, was that the sun   |
|     had come up again."                    -Kurt Vonnegut     |

__________________________________________________
Do you Yahoo!?
Yahoo! Mail Plus - Powerful. Affordable. Sign up now.
http://mailplus.yahoo.com

_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org
http://www.diy-efi.org/mailman/listinfo/diy_efi