O2 Sensor Open/Close Loop

[kleenair at ix.netcom.com]

RABBITT_Andrew at mv8.orbeng.com.au wrote:
> 
> > The target A/F ratio during WOT can varry from engine to engine.
> 
> this is true, but not in the way you suggest below,
> 
> >More specifically, it is related to fuel pressure, fuel injector
> >nozzle design, and injector location.
> 
> this is not true. inasmuch as the effects of these variables are quite
> small to the point of being negligible.
> 
> >The objective is to deliver the correct amount of fuel in fine
> >particles to the combustion chamber.  So theoredically, if all the
> >fuel was vaporized, you would not need a WOT enrichment.
> 
> this is also not true.  I get the idea you've confused WOT A/F with
> acceleration enrichment.  WOT is usually richer than stoich because
> this produces best power for thermodynamic reasons.  Acceleration
> enrichment (which also happens at part load) is to maintain the
> setpoint A/F by compensating for manifold wetting effects due to
> changing manifold pressures.
> 
> >Therefore, on multi port fuel injection systems with higher
> >pressures, the WOT A/F is very close to stoichiometric.  In fact,
> >some newer vehicles (1996) do not go to open loop during WOT!
> 
> given the above, I find this (very) hard to believe, however if you
> can quote sources, I'd be interested.
> 
> Andrew Rabbitt
> Orbital Engine Company
> Perth, Australia.


As I understand it, the purpose of the acceleration enrichment 
(assuming there are no inherent design flaws in the control system that 
would cause a delay in delivery of fuel) is to deliver an extra burst of 
fuel during a transient to account for the lag in fuel delivery due to 
wall wetting.

You are correct about the thermodynamic effect of rich A/F ratio during 
WOT which is very different than transient response as you indicated.  
The added fuel cools the charge as it vaporizes.  Most of my experience 
is with gasous fuels which cannot benefit from this.  Regarding what OEMs 
do on late model vehicles, I had a 1996 3.8L Buick on the dyno, which 
maintained its closed loop operation at WOT.  However, their motive may 
have nothing to do with peak power.  That may just be a way of protecting 
the catalyst by reducing excess fuel in the exhaust during extended high 
engine load periods.

Also, just because the system stays in closed loop does not necessarily 
mean actual A/F is 14.7:1.  If the system is flexible enough, you can 
make the Rich to Lean transient slower than Lean to Rich transient, in 
effect raising the average O2 sensor voltage.  Most systems can also use 
a different target O2 voltage based on MAP and RPM.