O2 sensor response times- catalyst

[Will McGonegal]

Further to my earlier post and after further investigating O2 sensor
info with my colleague Jim (Heywood book, and Bosch
Electric\Electronic Handbook), it is evident that non-wide ratio O2
sensors (regular on/off rich/lean sensors) also have a thin
gas-permeable platinum layer.  This acts as a catalyst.  My
understanding is that this is used on regular sensors to help
completely combust all the O2 left in the exhaust with a rich
mixture.  (The Bosch book states that even with a lambda of 0.95
(rich) there can be 0.2-0.3% O2 by volume).  I'm assuming that this
extra O2 must react with some fuel (if there is excess fuel) to
prevent it from misreading that the AFR is actually lean (since it
would detect this small amount of O2 that didn't get combusted by the
engine).  Similar catalyst action is used in the wide ratio sensors as
in my previous post.  Any thoughts from those who know?

Another interesting thing I read was that excessive temperature
shortens the service life.  Sustained temperature can be 850 C, 930 C
for short periods.  Aside from lead and other fuel contamination,
could this be why the wide ratio sensors don't have that long a
service life?  If they are operating in a AFR rich environment, then
the catalytic action (reacting unburned fuel) is going to generate a
lot of heat.  Maybe this causes the sensors to have a shorter life
when operating in a rich environment.  And this may partially explain
why in the Honda lean burn car the wide ratio sensor would last a long
time (it runs lean, no extra fuel to react on the sensor, therefore
the sensor runs cooler).  If this is the case, perhaps some system
could be devises to measure the sensor temperature (heater resistance
perhaps) and if the sensor starts to get too warm, get it cooled
somehow.  In 2-strokes unburned AFR mixture gets into the exhaust
through the scavenging process.  I thought this is why O2 sensors
could not be used in 2-strokes (lots of un-combusted O2 in exhaust
even with rich mixtures).  With a catalytic reaction occurring at the
sensor (wide ratio or regular) I'm curious as to why they are not
recommended for 2-stroke use.  Maybe it's because they get too hot and
don't last long?  Any thoughts on O2 sensor heating?

Just for anyone interested, in the NTK wide ratio sensor manual it
states that the O2 "pump" current varies from -12.5 to 12.5 mA, the O2
sensor is biased with a 25 uA +/- 3 uA current (the sensor output
voltage (450 mV for stoich) is read on top of that).  The heater
voltage is 10.5  +/- 0.5 volts.  I still don't know how you go from
pump current to AFR!

> > ------------------------------
> >
> > >Heywood pg 301:  "Equilibrium is established in the exhaust gases
> > >by the catalytic activity of the platinum metal electrodes."
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