MAF vs MAP

[atsakiri at ford.com]

> >As for acceleration enrichment, we've entered a new arena.  Now
> >we're discussing A/F ratio control, not measurement of inducted 
> >air.  If the approach is to add additional fuel, without trying
> >to measure air then calculate the amount of fuel (based on 
> >measured air and desired A/F ratio), then why add a MAP sensor?
> >The throttle position sensor is already there.  Am I missing a
> >benefit of MAP-based enrichment over throttle-based enrichment?
> 
> >Anthony Tsakiris
> 
> Ah, but you forget - throttle position does not have a direct relationship
> to engine load.  You can have different manifold pressures with the same
> throttle
> position under varying engine loads.  Thus, the MAP would be giving valuable
> information that the TPS could not.  This may or may not have a drastic
> effect on mixture demands, but I'm betting that it would sure affect
> driveability and fuel consumption.  This may explain why some MAF based
> systems have MAP sensors. 

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.

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)?

Vehicle manufacturers tend not to think of these things
from a control systems perspective (i.e. command signal,
controller, plant, sensors).  They think of them, just not
in the process of designing controllers to achieve desired
behavior.  I digress.

It's a very interesting problem, made all the more difficult
by those pesky cost constraints.


Anthony Tsakiris

MAP = manifold absolute pressure
TPS = throttle position sensor
EGR = exhaust gas recirculation
A/F = air/fuel
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The opinions expressed are my own and not necessarily those of my employer.