[Diy_efi] Speed-density vs. MAF/MAP...

[Shannen Durphey]

Adam Wade wrote:

> 
> My understanding (especially from watching a MAP
> sensor on a motorcycle in use on an airbox) is that
> raw MAP output from the airbox at anything but high
> airflow situations leaps around like a prancing elf.


They do bounce around.  With a fast scope, in the right application, you
can see the pressure spikes created when the intake valves close.  The
trick is to filter the reading.

> ;)  How any meaningful measurement could be made from
> such an airbox measurement, I do not know.

You've used a vacuum gauge.  Some bounce more than others, but each is
somewhat useful and some are more useful.  You have to know how to read
it.


> 
> I'm not sure I understand why, as long as your fueling
> map compensates for that.  Is not some actual
> measurement of the actual air mass being ingested the
> best way to determine accurate fueling (we'll assume
> steady-state, since acceleration is a whole other can
> of worms).  If you factor in VE when making a fuel
> map, why switch?  Is there something magic about 100%
> VE?  How can someone inject a turbo, if that is the
> case?
There are many turbo/supercharger apps which use an equivalent to alpha
N fueling.  One method is to use "power enrichment" fuel tables to add
much more fuel than needed in NA.  The result is not optimum, and is
generally rich all over except at the "top" of the rpm range.  GM
Factory turbo cars use a 2bar MAP sensor to do the job correctly.

> 
> Am I correct in my assumption that a small-ranged MAP
> sensor is more sensitive to small variations in MAP,
With the GM map sensors this is true.  I do not know if anyone makes a
MAP sensor with a "segmented" voltage output to allow larger changes in
output through a small voltage range at lower pressures.  

> 
> And what about alpha-n?  Is that a bad way of doing
> things at low rpm, compared to the problems of
> speed-density?
Depends on the application.  If you drive in all kinds of weather, at
different altitudes, it may be bad.  You will have difficulty creating a
"one size fits all" fuel calibration.  The ultimate setup for many folks
is one which will correctly determine mass air ingested in all
conditions.  I guess it depends, because lotsa race cars have run on
alpha N and won.


> I guess I'm still missing why MAP has to be limited to
> 100Kpa, or any other arbitrary number.
Sensor design.  Most common sensors are good for ~ 100Kpa.  

> And from my own observations, it would seem that
> there's very little useful data in an airbox-mounted
> MAP at low airflows (small throttle openings and low
> rpms).  Thus my feeling that alpha-n would be the
> choice.

You could simulate a MAP reading at low rpm by adding intake air temp
and humidity to rpm and tps.  The result could be indexed to a lookup
table.  (The MAP reading isn't really needed in this case, but it could
simplify the amount of coding needed in the ecm.) This would be one way
to combat large MAP/MAF pulsations at idle.  

> 
> I suppose the question becomes, how would you set up
> an accel enrichment that would be effective?  that
> itself would be pretty complex, I think.

>From what point of view are you asking?  Writing the code?  Implementing
the hardware?  

One method is to add a rapid series of asynchronous injector pulses. 
Look for delta throttle to be > minimum amount, then use a lookup table
for "rpm vs throttle position" to get base pulse time.  Modify the time
by the delta tps to adjust pump shot for the amount of throttle
opening.  Add a coolant compensation table to ensure warm engines get
less AE than cold engines.  And if you're really nervous, add a delta
map table to cover sudden changes in engine load (still can't figure out
the need, but they do it).

> 
> Except at or near idle on engines with generous
> overlap (and I think even modest cam numbers for bikes
> would make a lot of car people blush).  
It would be possible write code which looks at engine rpm and tps and
applies a software filter to the MAP or MAF reading.  More filter can be
applied in conditions known to cause reversion.

 
> You mean at or near idle, with overlap?  I believe
> that's a major problem with bikes, from the scope
> traces I've watched on the airbox.

Even mild mannered 4 cylinder cars have troubles with excessive MAP
activity.  I should dig out some of the scope traces I've saved.  After
a while, I gave up being amazed at the wild ones, and started asking "is
something wrong" when I found a smooth trace.

Shannen

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