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

[Adam Wade]

--- Shannen Durphey <shannen at grolen.com> wrote:

> 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.

Kawasaki's old GPz750 turbo seemed to use a pressure
sensor in the plenum, and a TPS...  Since airbox
pressure would not change appreciably with throttle
changes in some cases, I would expect there would be
some combination of alpha-n and speed density in such
a case.

>> 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.

I suppose we should assume a barometric sensor for
speed density, but I don't know of many systems with a
relative humidity sensor.  ;)

> You will have difficulty creating a "one size fits
> all" fuel calibration.

I know.  I'm just wondering exactly where advantages
and disadvantages lie, and what I learned (that
started the thread) seemed backwards from what I
expected.

> 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.  

That was my thought.  Although if you could find a
crank position where a reliable, useful MAP reading
could be taken, it would be trivial to capture it
there, thus making MAP a much more attractive
proposition at low rpm.

>> I suppose the question becomes, how would you set
>> up an accel enrichment that would be effective?

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

Implementing the hardware shouldn't take anyting, I
don't think.  More from the standpoint of getting the
most acceleration without wasting fuel.

> 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.

You could also add a gear position modifier so you
would be able to figure in air resistance as it
relates to delta rpm.  That would be neat.  ;)

> 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).

Not sudden changes, but changes, perhaps.  If the
engine "spools up" quickly, you would want to deliver
your AE more quickly, maybe a smaller total volume?

> 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.

I like the way you think.  ;)

=====
| Adam Wade                       1990 Kwak Zephyr 550 (Daphne) |
|   http://y42.photos.yahoo.com/bc/espresso_doppio/lst?.dir=/   |
| "It was like an emergency ward after a great catastrophe; it  |
|   didn't matter what race or class the victims belonged to.   |
|  They were all given the same miracle drug, which was coffee. |
|   The catastrophe in this case, of course, was that the sun   |
|     had come up again."                    -Kurt Vonnegut     |

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