Misc ?? EFI

Scot Sealander Sealand at clarityconnect.com
Sat Nov 21 22:50:22 GMT 1998


thergen at svn.net wrote:

> Your statements make sense to me. 

Good.


> Just to add a couple more cents...
> When an ECM is in PE and open loop, it can still be using the info it
> learned about the engine during closed loop operation.

This is very true.  And in fact it usually does, except for some
calibrations.  Sometimes if the ECM learned to remove fuel during cruise,
it won't let it remove during PE.  An example may be someone who raises
fuel pressure a considerable amount.  The ECM will compensate, and
remove fuel.  But it may not remove fuel that it learned to remove 
while in PE.


> So, while it can't use the O2 sensor to try to maintain an AFR of say 
> 12.5 (PE and open loop), it can extrapolate from what it learned during 
> closed loop and make adjustments to the fuel delivery that ECM 
> programmer thought would result in an AFR of 12.5.

Exactly.

> Depending on how good the programmer was, the asumptions made, and 
> engine mods, the resulting AFR may end up close to 12.5.

True.

Take this example.  If you know the amount (mass) of air that is in the
cylinder and divide that by the desired AFR, you get the amount of fuel 
needed for that cylinder.  Multiply the result by the injector constant, 
and you get the amount of time needed for the injector to deliver the
required fuel.

Now when in closed loop, the desired AFR is 14.7 to one.  The O2 sensor
just happens to work perfectly for us at that AFR. The calculated fuel PW 
is dithered slightly rich/lean, and the O2 sensor lets the ECM know if
it is actually moving rich/lean.  If not, appropriate CL action is taken.

If CL control takes the integrator a certain amount away from a neutral 
value, over time the BLM will move to correct the fuel PW, returning the
Int to neutral.

So the fuel equation may be:

AirInCyl/AFR * injector constant * Int * BLM.

If everything is perfect, then commanding a richer AFR will take the
correct proportion of fuel, and it will be the newly commanded AFR.
Notice that to be right, the AirInCyl term has to be right, and the
BLM and Int should be neutral.  Also the injector should be maintained
with the same pressure drop across it (fuel pressure to manifold
pressure, assuming port injection), so that it remains linear across it's 
commanded pulse widths.

I have seen people say that the commanded AFR has nothing to do with
reality, but that is only true if the terms in the equation are not
being accurately measured/used, or the injector is seeing varying fuel
pressure deltas across it.

Scot Sealander    Sealand at clarityconnect.com




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