EGO clogging. (long)

[Dale Ulan]

> EGO=oxygen sensor
> UEGO=?????
  universal exhaust gas oxygen sensor. It is capable of sensing what
  the relative mixuture is, not just whether it's lean or rich of
  stoich.

> I thought the O2 sensor generated voltage depending on amount of O2 in the 
> exhaust.  sort of an exponential curve with stoich somewhere around where it 
> starts to rise.  Does it need to be driven externally after warm-up?

It has an 'S' type curve, where the voltage is close to 0 when the exhaust
contains exhaust from completely burned lean mixture.
In a rich mixture, the sensor reads approximately 800 mV. Right around
stoich, the sensor goes into its linear region. The height and slope of
this linear region is highly dependant on temperature, even more so
than on the mixture...

                  +----------- 800 mV
                 +
                 +
                 +
100 mV----------+

  LEAN    14.8  14.7  14.6

> I thought it already read out the A/F ratio.  What exactly are you saying 
> above?  Is the "pump" a constant current source?

The 'pump' is fed by a current source. The amount of current fed through
the 'pump' cell controls when the sensing element does the above...
for example, by feeding 5 mA into the pump cell, the trip point looks the
same (more or less), but it is shifted... I'd suggest reading SAE paper
860408, by Seikoo Suzuki, Takao Sasayama, Masayuki Miki, Minoru Ohsuga,
Shigeru Tanaka, Sadayasu Ueno, and Norio Ichikawa of Hitachi.
Also, papers 841250, 850378, and 850379 contain material on this topic.


                    +-------- 800 mV
                   +
                   +
100 mV -----------+
	 10.7    10.6   10.5


<long clip deleted>

That's what everybody thought, too...
About 3 or 4 years ago, a few people from the EPA wrote an SAE
paper on the subject of oxygen sensors. My copy of the paper
is at school, so I can't quote the names or give you the SAE
paper number. But I can summarize it, because I was suprised, too.

The EPA was going to do a study of oxygen sensor aging and break-in
periods, and quantify how this affects exhaust emissions. They
built a test setup with a heater (which would heat up the sensor and
the gas it was exposed to), and a valving system that would allow them
to purge the test system with nitrogen gas, and then give samples
of other gasses.

What they found surprised them... The most obvious test is to see what
temperature was required for the sensor to sense oxygen. So they
cranked up the O2 flow, and started heating. The O2 sensor started to
respond at about 800 or 900 degrees C. No exhaust system operates that
under normal road-load conditions.

At this point, they decided that their study should concentrate on this
lack of O2 sensor activity. What they discovered was that the O2 sensor
would respond to carbon monoxide and hydrogen. At normal operating
temperatures, they concluded that the O2 sensor is not capable of
sensing oxygen at all. Few people seem to have read this paper, though,
so most people out there think that the oxygen sensor actually senses
oxygen in a vehicle. It *can* sense oxygen, but it'll have to be
glowing pretty bright to do it.

I would suggest going to your local technical library and finding
this SAE article. It will be in one of the annual article abstract
books, somewhere between 1989 and 1993, and may be present in either
the big thick SAE publication hardcovers, and/or in 'Sensors and
Actuators', an SAE special publication series.

I haven't seen any papers arguing against their conclusions,
but if there are, I'll hopefully eventually find them and read them.


-Dale