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

[J. Creech]

> 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
>
Karmann Vortex MAS: I don't know how the Toyota sensor works, but the
Mitsubishi uses the honeycomb to straighten out the flow of air, remove the
turbulence (kinda like GM sensors, I believe). I believe that the honeycomb
was put in there mainly for low-flow situations, I.E. idle and light cruise
(to produce a laminar flow). After the honeycomb, it has a small "wing" in
the airstream, designed to introduce little ultrasonic "whorls" into the
airstream. Just past that, there are 2 Piezo microphones that listen for
these whorls, counting them. The sensor then outputs a square-wave signal
directly proportional to the number of whorls - more whorls = more air
flowing throught the MAS. The earlier Mitsu designs would start to lose
count somewhere near 1600hz, while the newer style is supposed to be good to
over 2200hz. The Mitsu MAS also uses a seperate Barometric Pressure sensor
and an Ambient Air Temp sensor, mounted on the outside of the MAS body, in
the airfilter can.

Scott C.

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