Re: "As The Lug Nut Turns"

[Bruce Murray <bmurray@domain.elided>]

>Joe Elliott <jee@domain.elided> writes
>
>
>While I fear that I may sound like a stooge, I'm going to take this
>opportunity to ask people to call heated-wire type airflow meters
>"heated-wire type airflow meters."  I'm of the opinion that the term "mass
>airflow sensor" doesn't exist and was made up by the same breed of idiot
>that works behind the counter at AutoZone.  The term simply does not make
>sense!  Perhaps "air mass sensor" would be appropriate since, between
>volume and temperature, that's what an AFM tries to measure, but since
>there's nothing mass(ive) about the airflow, the term "mass airflow sensor"
>is entirely nonsensical.  Perhaps that would be an appropriate name for a
>component on a jet engine, but I'd have nothing to do with an airplane that
>had anything to do with L-Jetronic or goonies from AutoZone...
>Anyway, can we agree not to call a heated-wire type airflow meter (or a
>flap-type airflow meter for that matter) a mass airflow sensor, mass air
>sensor, or MAS ever again?  If you must, I suppose I could live air mass
>meter or air mass sensor, but since the real word is Airflow Meter can we
>stick with that?  I realize this rant makes about as much sense as
>insisting that Spider be spelled with an "i," but I thought I'd give it a
>go anyway.
>
>Joe Elliott
>(did I mention that I'm a double-major in mechanical engineering AND English?)

Well I am sorry, but I think Joe is wrong and I for one will continue to 
call is a mass flow sensor.  The heated wire airflow meter DOES measure the 
mass of the airflow passing into the engine. In the ones I am familiar 
with, the hot wire runs at a nominally constant temperature.  The current 
is passed through the wire at a sufficient rate to maintain a constant 
temperature at all air mass flow rates.  As the mass flow (volume X 
density) changes, so does the heat extracted from the wire. A control 
system senses the change of wire temperature and adjusts the current to 
bring the temperature back to nominal.  The amount of current passing 
though the wire is thus a direct indication of the mass flow entering the 
engine.  It is very clever because it gives a linear output with mass flow.

A flap type sensor is not a mass flow sensor but is effectively a dynamic 
head sensor (one half rho vee squared).  A separate temperature sensor is 
needed to establish the value of rho and hence the mass flow.  The flap 
sensor output is non linear with flow and thus it is more complex to deal 
with its output in the ECU.  It is however more rugged which is why it was 
used until they developed ruggedized hot wire meters.

(did I mention I am a principal in mechanical engineering AND an Englishman 
to boot?)

Bruce Murray
Principal Engineer
BBN Technologies
A Verizon Company
70 Fawcett Street
Cambridge, MA 02138
Ph    617 873-3356
Fax  617 873-2918

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