Info on EFI systems

Jonathan R. Lusky lusky at
Fri May 20 23:37:13 GMT 1994

Steve=Ravet%Prj=Eng%PCPD=Hou at writes:
> | > Ithink a closed loop system using feedback to continuously 
> | modify timing is a 
> | > better solution than static lookup tables, but what do you use 
> | for feedback?  
> | > Exhaust temp/composition?
> | Exhaust temp and composition vary widely with a bunch of factors besides
> | spark timing, so I don't think either are appropriate.  I think the only
> | effective way to do closed loop spark is with a pressure sensor, a crank
> | angle sensor, and calculating the location (degrees ATDC) of peak
> | cylinder pressure.  You'd still want to have a static map beneath 
> | this I think.
> Well, how about this:  it takes a certain amount of time for the fuel to
> completely burn.  This should be relatively constant regardless of rpm.
> You want most of the burn to happen at max compression.  So the timing
> has to be advanced with increasing rpm to give the burn a "head start" with
> respect to compression, which happens faster at higher rpms.  This would
> imply that once you know how fast the flame front travels, you could
> calculate the advance for any rpm.  But this doesn't take into account
> the type of fuel, etc.  so there must be something else.  Do higher octane
> fuels burn faster/slower?
Actually, while I believe thats the typical non-technical explanation of
why spark advance is necessary, its very wrong according to Heywood
(someone posted a summary of the correct explanantion of
within the past few days...  I really ought to pick up a copy of Heywood
sometime).  Burn rate increases with RPM, but ignition time remains
constant, thats why you need advance.  For a feedback system, you aren't
really concerned with the details I don't think, you just want to make
sure peak pressure is at the appropriate place (~19deg ATDC???) and that
the area under the pressure vs crankangle? curve is maximized.

> actually, re-reading your reply above is causing light to dawn.  As the fuel
> burns, the pressure in the cylinder increases, right?  for max torque, you
> want this pressure peak to occur at a certain angle atdc, right?  so if you
> have a pressure sensor in the cylinder, you can measure pressure vs. crank
> angle, and vary the timing to keep the peak where you want it.  is this
> basically correct?  so are there pressure sensors that can survive in the
> cylinder, and from where can they be had?
Yep, that exactly what I meant.  There are sensors designed for this,
but I'm not sure of their cost or durability.

> on another note, incoming mail is addressed to 'diy_efi at whatever'
> when i use reply, it gets addressed to 'diy_efi-owner at whatever',
> causing john much grief.  if anyone has a suggestion for why
> this is happening, i'd like to fix it.  

Sounds like your mail program is ignoring the Reply-To: line in the
header and sending replies to the address in the From: line.

Jonathan R. Lusky  --  lusky at
 "Turbos are nice but I'd rather be blown!"
   89 Jeep Wrangler - 258 / For Sale: $8000 obo
       80 Toyota Celica - 20R / 5spd

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