O2 sensor response times-wide ratio catalyst

[Will McGonegal]

We have an NTK wide ratio O2 sensor here at our lab.  We will have to
be using it for an upcoming project on the dyno, so we wanted to
determine how it functioned.  My colleague Jim have been looking into
this and he dug up some info this morning that explained a lot and it
indicates that there can be a catalytic reaction taking place in the
sensor.

As discussed in previous posts, the NTK wide ratio sensor sensor has
three parts, heater, O2 sensor and O2 "pump".  The O2 pump can move O2
towards or away from the sensor cell.  The amount of current run
through the pump (positive or negative) determines how much O2 is
pumped to or away from the sensor.  The idea is to keep the sensor
outputting 0.450 V (stoich, complete combustion).  If the engine is
running lean (excess O2) the sensor output starts to fall and the O2
pump removes the excess O2 from the small amount of exhaust in the O2
sensor cell, in an attempt to maintain a 0.450 V sensor voltage.  How
much O2 you remove (you know this by the current running through the
pump) can be used to determine the AFR.  If the engine is running rich
(excess hydrocarbons), the sensor output voltage rises so the pump
moves O2 towards the sensor.  There is platinum in there to promote a
reaction between the unburned HC and O2.  How much O2 is added
(pumping current) to maintain the sensor 0.450 V stoich setting is
used to determine what the AFR is.  Oh, you also need to factor in the
Hydrogen to Carbon ratio of the fuel and whether the fuel is
oxygenated (e.g.: alcohol) to get accurate AFR readings.  So yes,
there is a catalytic reaction taking place in the sensor when the
engine is running rich.  Essentially the O2 pump is used to keep the
exhaust stoich (450 mv) in the tiny region of the sensor.

That's my understanding of it.

Will

> ------------------------------
> 
> Date: Mon, 27 Mar 2000 18:27:41 -0800
> From: garwillis at msn.com (Garfield Willis)
> Subject: Re: O2 sensor response times
> 
> On Mon, 27 Mar 2000 17:35:30 -0800 (PST), Orin <orin at diy-efi.org> wrote:
> 
> >Heywood pg 301:  "Equilibrium is established in the exhaust gases
> >by the catalytic activity of the platinum metal electrodes."
> >
> >So yes, there are other reasons.
> 
> Yes, but that's in a place where contact of the platinum with the
> exhaust gases catalyzes/oxidizes the HC and CO gas components (namely,
> in a CAT). Are you saying that this effect is part of the operation of
> the O2 sensor? If so, it's never mentioned in any of the papers AFAIK.
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