MAP map or MAF map

[Steve Baldwin]

> 
> Bryan Zublin:
> > I thought that one of the benefits of the MAF was that correction for 
> > barometric pressure (altitude) was NOT required.  I do not have practical 
> > experience with MAFs, I am merely repeating what I have read in the 
> > literature (mainly SAE papers).  Regarding the air temp compensation, isn't 
> > this inherent in the MAF because of the reference temp sensor that is 
> > upstream of the hot wire?
> 
> Are the air-flow sensors that use a flapper and a potentitiometer and the
> ones that use a counterbalanced disk in a cone doing essentially the same
> thing as a hot-wire MAF?
> 
> I've often wondered about this.
> 
> If they are, then at least in practice you can get by without measuring the
> barometric pressure with a MAF since most of the above-mentioned systems do
> not.
> 
> AT least two of the above systems _do_ measure temperature, however one
> measures engine-block temperature, the other measures cylinder-head
> temperature. I assume this is for mixture adjustment as the engine warms
> up.
> 

They aren't quite doing the same thing. The flapper type transducer
itself measures volumetric flow while the hot wire anenometer output is
directly proportional to the mass flow rate. Thus, the flapper flow
meter requires additional information (density) to determine the actual
mass flowrate.
The density can be determined from the temperature of the incoming air
(somebody's gas law applies).
The upstream temp sensor in the hot wire sensor isn't there to measure
the actual temperature of the incoming air but to determine the change
in temperature between the heating element and the detection element.
A simplified mass flow meter would consist of two sensors, one upstream
and one downstream of the heating element. You would expect the down
stream element to be warmer than the upstream one because of the (heat
carrying) mass going in that direction. 
So the difference in heating at the sensor is proportional to the mass
flow rate.

Steve.