WPSLPGExhaust Gas Oxygen (EGO) sensor Information

[Bruce Plecan]

----- Original Message -----
From: "Steve Schwartz" <ses at home.com>
To: <diy_efi at diy-efi.org>
Sent: Thursday, August 03, 2000 11:24 AM
Subject: RE: WPSLPGExhaust Gas Oxygen (EGO) sensor Information


> Sorry, Greg, but I must disagree.
>
> The 'bang-bang' control you refer to would be fuel_on - fuel_off.  The
fact
> that the EGO can only accurately report RICH or LEAN, has no bearing on
the
> control algorithm.  The ECU maintains an injector DUTY CYCLE, which is
> constantly adjusted, in small increments, up and down, to keep the EGO
> crossing the stoich point.  PROPORTIONAL control, possibly with an
INTEGRAL
> term, under some conditions.  That is the ONLY way it can know that the
> AVERAGE mixture is NEAR stoich.  The control algorithm has nested loops,
if
> you will.  I suppose you COULD look at TPS response as a DERIVATIVE term,
> but we digress...
>
> New and interesting info:
>
> > three way CAT PERFORMS BEST if the gas coming through it
> > alternates between
> > a bit rich and a bit lean.
>
> I didn't know this!  But it has a ring of truth.

Read the archives, and it will ring louder
G
>
> Some of this is splitting hairs, and watching dozens of angels dancing on
> the head of a pin...
>
> Best,
> Steve
>
> >
> > The reasons why the oem ecu's oscillate across stoich:
> >
> > It allows use of a very cheap (in terms of ecu capability) bang/bang
> > control strategy.
> >
> > It also allows a very cheap way of checking sensor performance (I
believe
> > this is an EPA requirement), by counting the frequency of crossings.
> >
> > By using such a crude control strategy, it also plays into the fact that
a
> > three way CAT PERFORMS BEST if the gas coming through it
> > alternates between
> > a bit rich and a bit lean.
> >
> > Nothing at all to do with sensor performance.
> >
> > Greg
> >
> > >At 11:59 PM 8/2/00 -0500, Tom Meagher wrote:
> > >
> > >>Oscillation duty cycle, averaged with a low pass filter to produce a
DC
> > >>voltage is evidently the operative parameter.  Perhaps you
> > could sample the
> > >>signal with the PIC and do FFT's on it to produce additional
> > information to
> > >>better characterize AFR.
> > >
> > >If all you wanted was an average AFR over a few seconds, go ahead.
> > >It's a waste of time imho. It still won't be any more *accurate*
> > >than an instantaneous reading due to the sensor's output being so
> > >nonlinear, and varying with EGT, backpressure, etc, none of which
> > >are compensated for by any amount of averaging over time. You'd be
> > >averaging over many many cycles, slowing response time greatly,
> > >and gaining nothing in the bargain.
> > >
> > >
> > >>Does anyone know the physics behind the oscillating output?
> > >
> > >There's no physics of it, that's the ECU adjusting AFR to try and
> > >maintain stoich, aka closed loop operation. Look at the timebase
> > >on the scope traces. The sensors can show some interesting things
> > >at much higher bandwidths, but those traces are not showing any
> > >of them.
> > >
> > >Really this has all been beat to death in the archives. People
> > >are almost lucky Gar is away for the moment. I can easily see
> > >him dispensing a brace of new bodily orifices when he returns.
> > >(And if you think I'm exaggerating, you haven't been reading
> > >the archives enough.)
> > >
> > >   Chris C.
> > >
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