MAF vs MAP

[Darrell Norquay]

At 08:42 AM 6/6/96 -0400, Anthony Tsakiris wrote:

>Agreed, but the MAP sensor has the same problem, doesn't it?
>You can have the same MAP under varying engine loads due to 
>varying EGR flow rates.

I don't really agree, here.  If you ignore atmospheric pressure variations
for a moment, I think MAP is much more "closely coupled" to engine load
variations than you suggest.  I don't think the EGR actually represents a 
large volume compared to the volume of air inducted into the manifold.  Few 
percent maybe.  I don't think this is enough to alter manifold pressure 
significantly.  In any case, all the EGR would do (purely from an AFR
standpoint) would be to "dilute" the incoming air slightly in terms of O2
content.  This could easily be compensated by reducing the injector DC 
slightly.  Closed loop operation would take care of this, but the computer 
also "knows" the EGR setting and could compensate.

>Also, varying altitude may complicate a strictly MAP-based 
>approach.  The MAP sensor can surely sense differences due
>to ambient pressure, but does a MAP measure of 0.6 bar 
>indicate part-throttle at sea-level (and therefore imply
>stoichiometric A/F ratio) or wide-open throttle at high
>altitude (and therefore imply non-stoich A/F)?

Most MAP sensors are really differential pressure sensors with one port open
to atmosphere.  This automatically compensates for any differences due to 
altitude or Barometric Pressure, and essentially measures the difference 
between atmospheric and manifold pressure.  Some systems (Cadillac springs to 
mind) have a separate BP sensor as well as the MAP, and compensate that way.  
Some systems use an absolute pressure sensor (referenced to a hard vacuum) and 
simply take a reading before the engine starts to determine current BP, and 
remove the BP as an offset.  This obviously doesn't work too good if you 
drive up a mountain without stopping the engine once in a while to "catch up".

What do you say, Ed? (By the way, your editor works perfectly...  I now have
a "real" mail system anyway, (Eudora), and it doesn't care.

regards
dn


MAP = Manifold Absolute Pressure sensor
TPS = Throttle Position Sensor
EGR = Exhaust Gas Recirculation
AFR = Air/Fuel Ratio
BP  = Barometric Pressure
DC  = Duty Cycle