Air Flow Measurement

[Mazda Ebrahimi]

tom cloud wrote:
> I wanted a system that would compensate for altitude
> and barometric / atmospheric changes .... hence BAP.  I can see that MAP
> might could be ignored and just BAP and TPS (plus IAT) used to give a
> very workable efi system .... is that what you're saying?  NO. You need MAP.

> Just seems to
> me that BAP - MAP = delta P across a known opening (the throttle).  Why
> is that any different at WOT?


My only concern is that at near FULL LOAD, your delta P is very small (may be a few 
inches water column maximum.  Under this condition the throttle no longer controls 
(or limits) air flow, therefore it is not adequate to just measure delta P and correct 
for temperature. Also, remember full load does not necessarily mean WOT. Consider that 
at 1500 RPM, at thirty to forty percent throttle openning, you may already reach your 
maximum air flow (i.e. manifold pressure is almost atmospheric).  By contrast, at part 
throttle, since the main restriction IS the throttle blade, you can simply use a look-up 
table to calculate air flow based on TPS, MAP, and BAP, and then correct for temp.  
Notice no RPM term is involved here.

When delta P across the throttle is small, what limits the flow is essentially the flow 
through the intake valves which also relates to intake manifold dynamics (which are RPM 
related).  Under these conditions, an RPM based table is needed.  I guess you could make 
it TPS vs RPM instead of MAP vs. RPM, but using TPS for this purpose is not accurate.  
Remember that at lower RPMs you loose a lot of accuracy with the TPS, because the last 
2/3 of your throttle openning do nothing to the flow.


>  And it looks like that would work quite
> well at idle and low throttle, where the MAF sensor is at its weakest
> performance point.  Course, I'm thinking about standard manifold, too.
> I can see that runners and TPI could greatly affect the equation, as
> there might not be a _real_ definitive MAP reading (would be different
> at different places ???).

You are very correct about the definitive MAP reading part.  I have access to the logic 
of a natural gas fuel injection system, and they use an averaging scheme to dampen the 
MAP sensor input!  I wouldn't be surprised if other do the same to stabilize the MAP 
signal.

> 
> So, you're saying that speed density is _very_ adequate?  I think I knew
> that??  I wanna know what's wrong with my assumptions -- why do oem's go
> to all that trouble and expense to put in MAF if it can be calculated
> from sensors already in place?  (I understand that there might be bypasses
> that degrade the data -- but seems that could be overcome.)
> This is just my opinion, but a very good reason for using MAF from an OEMs point of view 
is that the engine calibration process becomes much much simpler.  You simply get one 
signal that tells you the air mass flow.  I can't think of an easier way of doing it.

> Tom Cloud <cloud at peaches.ph.utexas.edu>