EGO clogging. (long)

[Steve=Ravet%Prj=Eng%PCPD=Hou at bangate.compaq.com]

I didn't understand a lot of this post.  Can someone enlighten me?
Maybe someone (me?) should put together a digest with common accronyms,
the difference between SD, MAP, MAF, etc.


Dale Ulan <ulan at ee.ualberta.ca> Wrote:
| 
| 
| > Aaahh!! Neat!
| 
| Note I said 'most'. Some ECM's actually control the temperature,
| notably cars using UEGO sensors...
EGO=oxygen sensor
UEGO=?????


| 
| > So the 'trip point' is much the same but the overall voltage is 
| lower at
| > lower temperatures.. The curve is 'squashed down' if you will.
| > So, with a thermocouple and an EGO you could (with some 
| processing) find
| > out the A/F ratio.
| 
| Yes and no. You could find it out right around stoich. Generally, an O2
| sensor doesn't like sitting at one voltage very much. That's why the O2
| sensing algorithms always oscillate the sensor a bit.

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?

| 
| > How does a UEGO work?
| 
| It contains an additional 'pump' cell. Since the normal O2 sensor
| responds to hydrogen and other gasses (except oxygen, of course),
| the reverse process (applying current to the cell) will move these
| same ions through the ceramic.
| By combining an O2 sensor and a 'pump' cell, the O2 sensor will operate
| with a shifted centre point. By pumping, say, 10 mA of current through
| the cell in one direction, the trip point will go from, say, 14.7:1 to
| 10:1. etc. With some creative electronics, the sensor can be made to
| read out the actual A:F ratio.

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

| 
| After a bit of testing, people have found that the O2 sensor does not
| actually respond to oxygen ions until the cell reaches 800-900 deg. C.
| Few exhaust systems run that hot, but at the lower temperature of
| 400-500 deg. C., the cell responds to other gasses, such as hydrogen.
| The net result is correct, but the cell doesn't actually respond to oxygen.

Below is a post I got off r.a.t explaining O2 sensors.  The poster seems to 
know what he is talking about, and is the first real explanation that I've 
seen:



>Please excuse a question from a novice...

>Just exactly what does the oxygen sensor do (please don't say it's senses
>the oxygen level), i.e. what is done with the info it produces.  Also,
>why should it be replaced every 30,000 miles?  What's wearing out?

>Thanks!



This guy obviously sounds like he wants some insight into the oxygen sensor 
so......strap yourselves in........it's gonna be a long one.....

Well.........from my knowledge the oxygen 
sensor is a device made of zinc oxide or titanium oxide mostly, that tends 
to attract oxygen ions and accumulate them on its surface. Now the thing is 
oxygen ions have a net negative charge and the sensor material has a net 
positive charge thus the attraction. However the magnitude of oxygen ionic 
charge concentration is slightly higher than that of the sensor material. 
Now the sensor is mounted half in and half out of the exhaust manifold. 
Obviously the nose is exposed to exhaust gases and the boot is open to the 
atmosphere. The atmospheric oxygen ions (of which air is made up of 21% O2) 
are accumulated on that end of the ZnO2 wafer. Any oxygen present in the 
exhaust gas accumulate on the other side of the ZnO2 wafer. When the sensor 
achieves > 300 degrees celcius, the wafer becomes conductive and the 
ionic difference in oxygen concentrations between outside air and exhaust 
oxygen produce an electric field across the wafer, current flows and a 
potential difference or voltage is measured across the platinum terminals of 
the sensor. 

Now if you think about it......if your engine is running lean, then the 
excess oxygen which is unburnt in the combustion chamber appears as part of 
the exhaust gas. This oxygen concentration (usually 2% of exhaust gas for 
lean conditions) is collected at the sensor and the difference in its 
concentration with respect to atmospheric oxygen concentration produces a 
small voltage which gets smaller as fuel mix becomes leaner, since for very 
lean conditions more oxygen appears in the exhaust and the difference 
between concentrations of exhaust oxygen and air oxygen is smaller thus 
sensor output falls. Typically values are around 0.1 - 0.3 volts DC for very 
lean to slightly lean, respectively. Alternatively a slightly rich mix 
reduces oxygen present in exhaust and concentration difference is larger 
thus output voltage increases right up until a very rich mix with no oxygen 
in exhaust and maximum voltage is produced from the sensor. Typically 0.6 to 
0.9 volts for slightly rich to very rich mix.........Now, when air/fuel 
ratio is correct (14.7 parts airmass to 1 part mass of fuel) combustion is 
fully completed with (theoretically) very little or no air AND fuel left. 
Exhaust gas is at its cleanest when this ratio is achieved, pollutants are 
minimised (check your chemistry on this one) and power and torque are 
maximised, which is what the controller in your engine management system is 
trying to achive. An optimum Air Fuel Ratio (AFR) produces about 0.45 volts 
from the sensor.

	Now what does the controller do with this sensor input I hear you 
ask. Well the Electronic Control Unit (ECU) accepts the sensor voltage and 
it knows from the voltage it recieves whether AFR is too lean, too rich or 
just right. If AFR is too lean then the ECU knows it has to supply more fuel 
to bring the mixture to 14.7:1 AFR. Thus the ECU opens the injectors for 
slightly longer in this case. If the sensor supplies a voltage which 
indicates too rich then the ECU will open the injectors for a shorter time 
allowing less fuel to balance the mix back to 14.7:1. Now the ECU will 
always react to the oxygen sensors DC output and adjust injector open time 
to keep the sensor voltage output around 0.4 - 0.45 volts which indicates 
the optimum mixture. Simple really! This feedback goes on continuously with 
sensor checks and injector open time adjustments SO LONG AS THE OXYGEN 
SENSOR TEMPERATURE IS ABOVE 300 DEGREES CELCIUS. If its lower than this the 
ECU will disregard the sensor until it warms up and becomes conductive 
enough to supply valid results. A cold or cool sensor will NOT work properly.

	Now why does it have to be replaced regularly I hear you ask. That's 
because for a number of reasons. Firstly exhaust gas contains a lot of shit 
in it. Pollution, contaminants and stuff all exist in this gas which tends 
to accumulate on the oxygen sensor ZnO2 wafer. Now accurate measurements in 
oxygen difference between air and exhaust cannot be made if the exhaust end 
is covered in muck. This stops most of the oxygen in the exhaust 
accumulating on the wafer surface thus output signal from the sensor does 
not indicate the true exhaust gas oxygen concentration from which the ECU 
can base its adjustments. Sometimes you can clean the crap off the 
device by deliberately running the engine lean at high speeds for 
an hour or two and then RESET AFR to its proper value afterwards. Lean 
running tends to burn and blow off the contaminants restoring sensor life 
and keeping it clean. However if your car runs really shit or burns 
oil then you can't save the sensor and you'll have to replace it 
regularly. Secondly (not as critical) operating at temperatures of more than 
300 degress usually about 450 degress every time you drive your car for a 
number of years takes its toll on the poor sensor. All the cooling down at 
night heated the next morning stresses the material, although this should 
not be an immediate problem since nowadays they are built to take it. But 
after many years you never know. Changing one every 30,000 miles is not 
something I would do. I'd check it at regular intervals, keep it clean and 
replace when it is REALLY at the end of its life. I'd be wary of service 
technicians who immediately tell you that they have to replace the oxygen 
sensor without having tried to clean it using a lean mixture for a while or 
without explaining what EXACTLY is going on. In Australia Oxygen sensors 
sell for about 200 dollars a pop not the sort of device you change on a 
technicians hunch or whim without getting a second opinion, just like 
visiting the doctor.......

........Sorry about the length of the explanation....hope you guys are still 
awake.......

Anyway.........hope this helps..........

			Keep Drivin'............

				The Metallian.