EFI Mapping

[Robert Harris]

Why mapping?  Quite simply - simple.  The processor's used are not overly 
endowed with mathematical processing capability or time.  Solving complex 
equations takes thousands of instruction cycles without math coprocessors.  The 
silicon real estate for math coprocessors can easily be orders of magnitude 
larger than simple 8 or 16 bit controllers and yet would not be used in most 
applications. The manufacturers have chosen to use this real estate for more 
common and practical apps such as ROM, RAM, I/O etc.

Most algorithms used for control's can be approximated quite well with an 8 bit 
address ( 256 entry) look up table (i.e. "map") and the feedback loop can 
correct out the final (usually very small) deltas. Think of a MAF sensor 
capable of flowing 1000 cfm.  That means each bit (step) is 4 CFM or about 2 
engine revolutions per minute of full load air for a 5.0 V8.  Pretty well into 
the give a f... region. And if the sensor is biased toward the low flow region, 
the table can be closer at low fuel air flows than at WOT, where normally any 
approximation works.  Interpolation is generally not needed for automotive 
engine purposes - provided there is corrective feedback - EGO for example, and 
some self adjusting logic that "corrects" the tables over time.

As for space savings, the subroutines that have to be written to perform 
complex math are not small either.

Then of course, for the obsessive compulsive, somewhat anal retentive set ( 
which I have been occasionally found to be frequenting ) - there is always the 
possibility of using a Pentium class processor to get what you want.  But that 
is another fire storm and my asbestos panty liners need changing anyway.


If the first ingredient ain't Habanero, then the rest don't matter.
Other Obsessions: Ferro-Equinary , 1972 "Killer Whale" Mustang
Currently Interred in the Peoples Democratic Republic of California - Stalag 
Montclair
Puck da guns - ban Politicians!!!!!
Robert Harris <bob at bobthecomputerguy.com>


-----Original Message-----
From:	Tony Cooper [SMTP:tony.cooper at virgin.net]
Sent:	Friday, November 14, 1997 1:39 AM
To:	diy_efi at efi332.eng.ohio-state.edu
Subject:	Re: EFI Mapping

Alex White wrote:

>
> Why mapping?
> Surely some maths could have been used (simultaneous differential
> equations) to determine the optimum curves for these parameters, then
> a simple mathematical operation would be required to calculate the
> desired parameters. Surely this is a more conservative method (in
> terms of memory, adaptability) to perform this operation?
>

Yeah, the mapping is used to give the processor a good idea of where to
start for the fueling and ignition, then using 3D quadratics for
interpolation instead of linear, gives a far better 'in-between' value -
at least this is true in my experience.

Tony

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