Speed Density vs MAF

[robert dingli]

Brian Warburton brings up an age old debate,

> 
> Flap AFM's or for that matter all AFM including the later MAF
> hotwire types have one great advantage over the MAP (speed-density)
> type system, AFM's give an indication of the actual mass of air
> being "consumed" by the engine, the MAP type system gives you the
> data to allow you to try and infer the amount of air being consumed.
> The direct value is almost always more accurate than the indirect. 
> Trying to meet serious emissions with a speed-density system is not
> to be considered by anyone with a life, not unless they're starting
> out with a very good base-engine design which exhibits excellent
> feed-gas before bolting any electronics on it ......... 

There are a few fundamental flaws in this argument.  MAF sensors
(assuming that they are robust, work with repeatable precision and 
have negligble response times - of which I am not yet convinced) measure 
the air flow at the sensor, which is normally before the throttle plate.
The important issue for port fuel injection control is to know the air flow 
into the cylinder, after throttling and manifold filling dynamics.
(Estimating the fuel flow through the intake is another matter entirely.)
I would argue that a MAP sensor measuement is, in fact, more direct than
a MAF sensor measurement.

Most of the research into AFR control since about 1992 has dealt with 
transient fuelling behaviour.  Non-linear observers, model reference
and other state estimation techniques generally use manifold pressure 
as a state variable since it is directly used for air flow calculations
into and out of the inlet manifold.  From the work that I have reviewed,
MAP sensor data is considered more significant for the control 
technologies that we will see further down the track.

I'm not suggesting that there isn't scope for incorporating MAF data
into a control structure, just that MAP data (and correspondingly
speed-density control systems) have quite a future and that those 
working in the field do, in fact, have a life.

Robert Dingli
-- 
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             Robert Dingli           r.dingli at ee.mu.oz.au

Power and Control Systems                 Thermodynamics Research Lab
Electrical Engineering                    Mechanical Engineering
   (+613) 9344 7966                          (+613) 9344 6728
  University of Melbourne, Parkville, 3052, Victoria, AUSTRALIA
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