Re: Oxygen sensors in general

[MEL1523@ACS.TAMU.EDU]

;   Sorry that this is not totally Quattro-related, but we were talking 
;about different types of hydrocarbon combustion reactions in chemistry 
;class today. Our teacher pointed out that gas companies added 'oxygen' to 
;the gas to reduce carbon monoxide. Then, I began to explain to him the 
;way that most modern cars have O2 sensors. He knew absolutly nothing 
;about this, and I was having a hard time explaining it to him. I told him 
;that 'there is an o2 sensor in the exhaust system of most modern cars, 
;and the computer will adjust fuel/air ratios based on this reading', but 
;he still didn't quite get it. It confused him even firther when I 
;explained that the additive (MTBE) would work ok on older cars, but on 
;newer cars, it would fool the O2 sensor to some degree, and make the car 
;run rough, and in some cases, create more pollution.

They also add ethanol which screws up the vapor pressure curve (RVP) and 
cold start performance.  But, MTBE reduces pollution another way even on a 
feedback controlled (O2 sensor) car....Back in the mid 70's, the EPA made 
refiners take out the lead (tetra-ethyl lead) to reduce lead levels in the 
air.  It worked.  The problem was that now the refiners had to find another 
source of "octane" (not the molecule itself, but the ability to raise the 
anti-knock properties of their gas).  They found it in aromatics...benzene, 
toluene, xylenes, ethylbenzene, etc.  They made these from reformers...units 
that aromatized the cyclic parafins that used to just be blended in.  The 
extra hydrogen was just a bonus.  Problem then became...these aromatics 
don't burn well...i.e. HC's and CO went up.  Not particularly a problem for 
those cars with oxidizing catalytic converters and the loose tailpipe specs 
of the time.

Nowadays, the cat gets pretty much everything it can, so they're back to 
reducing the raw exhaust emissions.   MTBE and other oxygenates help 
combustion more than just by leaning out a car.  Even O2 controlled cars run 
cleaner.  Some people complain about MTBE causing their car to run bad in 
cold weather.  MTBE is not the culprit here...RVP (vapor pressure) specs are 
going down just about every year...So when the cool weather hits, and the 
low RVP gas is still in "the pipeline" the cars run bad for a month or so, 
and everyone blames the MTBE.  By letting refiners use less reformate 
(aromatics) at the same octane level, MTBE cleans up gas without screwing up 
vapor pressure (like alcohols).  These ethers have very high blending octane 
numbers.  I think MTBE is like 110.  Look it up...I forget the exact 
numbers.

Yes, there is actually a technical reason refiners prefer ehters like 
MTBE and its C5 sister TAME for mogas blanding...it's not a 
conspiracy against farmers.

;   Anyway, he wanted me to find some text on the matter, so I thought I 
;would ask people on the quattro list, tending to be more knowlageable 
;than the average joe. I was wondering when O2 sensors got widespread 
;acceptance - I assume about the time we switched from carberators (sp?) 
;to Fuel injection..but I am not sure, nor do I know the exact dates.
;I was also just looking for any general info on the subject, it doesn't 
;matter too much if it doesn't go into too much depth, I just need to show 
;the guy how it works, at least in laymans terms.

Generally speaking, in the late 70's.

An O2 sensor is technically an electrochemical concentration cell.  The 
probe is a ceramic of ZiO2 (zirconium dioxide) that is selectively permeable 
to O- ion.  This ion is present in low levels is all hot gasses that contain 
oxygen.  The ceramic is coated on both sides with Platinum.  When there is 
extra air in the exhaust gas, there is O2 on both sides of of the probe...no 
chemical potential...low voltage.  When the mixture is rich and there is no 
O2 in the exhaust, there is a concentration gradient, and you get a small 
voltage which can be determined by the concentration term in the Nernst 
equation (there is no "reaction", just a chemical potential gradient that is 
measured by the two platinum electrodes).  Everything works as long as it's 
hot (above about 575F).  Due to the combustion chemistry and the logarithmic 
term in the Nernst equation, the sensor basically makes a volt when you are 
rich (lambda<1) and nothing when lambda>1...like a switch more than a 
measurement device.

;    (obligitory audi content...)
;  When did Audi first use the O2 sensor? I would imagine a while ago - 
;1980? 1982? way before?

In 1980 CA Audi's had them and 49 states didn't.  My 49 state 1981 4000 had 
one.  They called it the K-lambda system or K-jetronic with lambda.  5000's 
got them all in '80.

;   I get about 23-24 mixed highway/city miles per gallon.. I do like to 
;hit the gas a lot, but that figure seems low - I talked w/ an Audi 
;mechanic, and he said I should get ~28-30 mpg highway....Could my O2 
;sensor be bad on this car? I don't think it has ever been replaced, it is 
;at ~71k miles.

Problem is...when they die, they make no voltage...which your car assumes 
means lean.  Basically there is no way to tell if it's bad without 
richening up the engine and knowing you're rich by other means (like exhaust 
gas CO or something) and if it still reads lean (0 volts)..it's bad.  You 
can also look for cycling of the freq valve or DP regulator to see if it's 
alive.

Most cars go by mileage only.  Every 30k mi or 60 k mi, a little light pops on 
telling you it's time.

By far the best book on the subject of engine management systems is 
(ironically) a Bently book called "Bosch Fuel Injection & Engine Management" 
by Charles O. Probst.  It's a little out of date now...it was published in 
1989, but has lots of good explanations of fuel injection systems and 
sensors.

 - Mitch Loescher