This is WHY there is a NEW Digest!!

[Jim Conforti <lndshrk@xxxxxxxxxxxx>]

mark kibort wrote:

> With all due respect Jim.
> 
> The AFM DOES measure mass flow .

  No Mark.. it doesn't...

  It measures air VOLUME FLOW..

  The Temperature corrections and Baro corrections are specific
  to AIR DENSITY.. 

  That's why they call it an aFm.. see... the F is for FLOW as
  in VOLUME.. and HFM stands for HeissFilmluftMassenMesser

  or Hot Film Air MASS Meter.. preceded by the HLM or Hot WIRE Air
  MASS meter.. 

> JUST PLAIN SENSE.  

  No Mark.. like lots of science.. PLAIN SENSE doesn't work here..

 PLAIN SENSE says you can't put a paper card over a glass of water
 and invert it and have air pressure hold the water in!

> YES , the ECU get data from temp sensors and pressure sensors  to fine tune
> and  other parameters as you know.   (now this is your area of expertise,)
>  I imagine with a 120 degree day  your timing and fuel mixture would change
> to provide protection of pinging.   the same mass flow at a lower temp
> would not have the tendancy to detonate.  correct??

 None of this matters Mark.. 

 READ MY LIPS..

 it measures VOLUME flow.. not MASS FLOW..

 If it measured MASS flow you would not need any correction for Barometric
 Pressure or temp.. which you DO on an AFM'd car.. and don't on an HFM'd
 car...

 Example:

 E30 M3.. AFM.. Baro Sensor and Temp Sensor correct for AIR DENSITY

 E36 M3.. HFM.. No Baro.. and the Air Temp Sensor is only used for emissions
 stuff...
 
> I think you really never thought this one through

 Yeah.. you're right.. and I and all the engineers at 
 Robert Bosch are WRONG.. 

 Mark.. what you haven't figured out is that I don't open my mouth
 unless I KNOW the answer.  

> If Im wrong, well, I will owe you a million "im sorry's"    That is why
> airplanes go so fast at altitude   speed over the sensing devices are
> saying 200mph, but your going 500mph ground speed.  

  Because of DENSITY?!?  We don't have any Pitot tubes in the AFM last
  time I looked.. 

> Point being tha.t on an engine your speed is fixed to the displacement and
> the rpm.   so, air speed stays the same.  

  It does.. Hmm.. I guess I can throw all those predictive equations of 
  cylinder and manifold filling away now.. Quick, I'll get McLaren on
  the phone to tell them your discovery.

  As far as pressure differentials.. they don't always affect flow rates
  (hint: remember about CRITICAL flows.. and pressure ratios)

  Suggestion: Put down the AERO texts.. and pick up something pertinent
  like John B. Heywoods Internal Combustion Engines. (around $90)

> as far as the MAF goes, yes this a very sensitve temp sensor that needs
> constant measureing of temp , barametric pressure, and humidity (i imagine)
>  for the ecu to  calculate mass flow.

  NO.. the MAF works quite simply..

  Heat a resistor up to a temperature and flow a gas across it.. the 
  drop in temp will be DIRECTLY proportional to the MASS of the gas 
  flowing across the surface.. (good old THERMODYNAMICS)

  Now.. Stick the thing in a closed loop to keep it heated to the
  same temp always and measure the voltage across a fixed resistor
  in the circuit (indirectly measuring current) and you have a nice
  DIRECT measurement of the MASS FLOW of gas across the element.

  Put it in a fixed cross section device and you have a MASS AIR FLOW SENOR
  that can be calibrated..

  Isn't sensitive to Intake Temp, nor Baro!

  Jim

  PS: Mark.. if you havent realized by now.. you've personally
      chased some 1000 people to a NEW DIGEST now..

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