Air Flow Measurement

[RABBITT_Andrew at mv8.orbeng.com.au]

>
>Everything you wrote makes sense to me.  Good stuff!  So... what 
>are the disadvantages of speed-density (in your experience)?
>
>--
>Chuck Tomlinson
>

I've never played with a speed-density system, but from what I've 
picked up in the industry, the following brief thoughts may help:

 - MAP is cheap
 - MAP is calculated from several measurements, therefore the errors 
accumulate during calculations, whereas MAF is measured directly.  
It's debatable (I think) whether MAF is superior in this respect since 
temperature and MAP are easier to measure.
 - MAF requires manifold filling compensation, where MAP doesn't
 - MAP can't handle EGR since this increases the MAP without an 
increase in airflow
 - MAP relies on VE data which must be measured and which changes over 
the life of a engine, and from engine to engine.  This can be 
corrected though using closed-loop adaption/learning techniques.

I think if you look around, there are enough cars around still using 
speed density systems to prove it's still a viable technology.  In 
industry, most things are set up on an engine dynamometer, so 
measuring what you need (MAF or MAP) is not a problem.  For the DIY, 
it's a different kettle of fish.  

If I were doing it DIY (one day!) I would use MAP and adaptive 
learning of VE and force the engine to learn it's VE map at the 
appropriate resolution by carefully driving the car around.  Then I'd 
calibrate the open-loop areas (mainly WOT).

Andrew Rabbitt