I'm missing something...

[Craig Dotson]

> It is called an air flow meter for a reason...it can only measure air
> *flow*.  The units of flow are volume per time....so the AFM can tell the
> ECU, "I am measuring 20 cubic meters of air/hour."  Now you can see why
you
> would need a density correction for this, 20 m^3 is a volume and depending
> on temp and pressure, you could have a varying amount of oxygen molecules
> (which is all the ECU wants to know).  It's very likely that the vehicle
you
> were riding in does not do continous atmospheric correction and that is
why
> the a/f ratio went out of wack with the increase in elevation.

With the flapper, I am assuming that it is measuring flow by determining
angular displacement of the flapper as it pushes against some form of
torsion spring.  I could be wrong, as I've never seen such a device.  If
that is the case though, there is *some* level of density sensing inherent
to the device. How the rest of the system (including ECU programming) is
constructed will determine whether or not that level of density sensing is
capable of fully adjusting fuel maps for correct mixture at elevation.

The reason I say there is density sensing: a torsion spring has a
predictable angular displacement with regards to force applied.  If the
flapper moves a certain angle, the plate has a known cross-section impeding
the airflow, and the actual force applied (a function of the spring used and
the angle it is displaced) can be used to calculate the air flow.  For a
given flapper displacement, there are an almost infinite combination of air
densities and air velocities that will yield the same force number.  Higher
speed and lower density, or higher density and lower speed, or both between,
can all give the same force applied to the flapper.

Craig Dotson
crdotson at vt.edu
2002 VT FormulaSAE

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