Air Flow Meter discussion (WAS: "I'm missing something...)

[rob files]

Yes, Brian, I think you got it.

To summarize; Air velocity entering the engine creates the pressure
difference across the flapper door.  The amount the door opens is the
cross-sectional opening the air flows through.  If you have velocity Y
feet/min and it passes through an opening of X ft^2 then you calculate Y *
X, you get a flow of f^3/min (CFM).  Notice that this measurement is
density *independant*.  That is why AFM measurements requires density
correction in order to calculate the air mass entering the engine.


-Air Flow Meter (AFM or flapper/vane sensor):  measures air flow and must
have density correction to calculate air mass.
-Manifold Absolute Pressure sensor (MAP):  measures manifold pressure and
must have density plus other correction and some assumed constant values to
*guess* at the air mass.
-Mass Air Flow sensor (MAF or hotwire):  measures the mass of the air
directly and does NOT require any corrections by the ECU.
-Karmon Vortex:  I believe these also measure flow and must correct for
density.  I am a little sketchy on how these work; something about a
honeycomb structure that creates turbulence in the air flow stream that can
be measured (at least in the Toyotas).


-Rob

    From: Brian Dessent <brian at dessent.net>
Reply-To: diy_efi at diy-efi.org
To: diy_efi at diy-efi.org
Subject: Re: I'm missing something...
Date: Mon, 12 Nov 2001 16:25:59 -0800

<SNIP>

The way the pressure difference is created is
    from the velocity of air moving across it.  (I guess technically there
could be a pressure difference with zero velocity, but that would be a
transient state and not a steady-state since as soon as the pressure is
not equal on both sides the flapper would begin to open and there would
be flow, hence velocity.)  Hence the amount of force on the flapper is
proportional to the velocity of the flow.  Conveniently, the amount of
force on the flapper directly controls the cross-sectional area through
which the flow passes, so now we know the volumetric flow rate once we
have velocity and area (it's the product of the two.)



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