Geyservillebob

I'm thinking about O2 sensors and hit on something that is making my brain hurt.

An O2 sensor is supposed to produce a voltage inversely porportional to the O2 content of the exhaust gases relative to the O2 content of the atmosphere. The voltage ranges from ~.1 (lean) to ~1 V (rich) with ~.45V representing 14.7:1 A/F ratio. 14.7:1 ratios are called stoichiometric and supposedly all the air and HC is used up and the byproducts are ideally CO2 and H2O.

OK, since ideally 14.7 mean ZERO O2 in the exhaust, wouldn't A/F ratios lower than this theoretically produce the same output voltage since all the O2 is gone at 14.7?

I'm pretty sure this is a matter of definitions. Clearly for the O2 sensor to work as it does there is O2 present in the exhaust even in so called rich mixtures, but I have not found an explaination of this. I'd like to understand this better.

In otherwords, I'd like to know how come we say we can measure stoichiometric with an O2 sensor when in fact there is still O2 in the combustion products. And, what are the expected or accepted amounts of O2 in the combustion products of a stoichiometric mixture, why are they there, etc.

Frank

RBob

iTrader: (1)

The quick answer is that NB O2 sensors have a platinum coating. This is a catalyst which burns hydrocarbons. And
in doing so reverses the O ion exchange within the ceramic sensing cone.

RBob.

vernw

iTrader: (2)

In other words.....

"Stoich" does not mean the presence of NO oxygen, it just means the presence of the CORRECT amount of oxygen (aka, correct kind as well).

That's why it will still always read SOME oxygen. But at "stoich" it's reading the correct amount for the cats to work properly...

Make sense now?

Geyservillebob

Vernw, By all correct definitions stoichiometric combustion means combustion where all the oxygen atoms in the oxidizer and all the atoms in the reactant react chemically to appear in the products.
I have several thermodynamics and internal combustion texts that all define stoichiometric this way. So yes, stoichiometric combustion means just the right amount of oxygen and therefor there should be zero left if combustion is ideal (which it isn't).

The CAT wants very little or no input oxygen to work best (reduce NOx), oxygen is liberated to oxidize remaining hydrocarbons and in two bed convertors additional air is pumped into the oxidizing bed after the reducing bed. The CAT also does not want to see excessive HC because that will lead to convertor overheating. True stoichiometric combustion, or close to true, is best for the CAT.

RBob, could you provide a slightly longer version of your answer? Are you saying that the O2 sensor actually senses excess HC?

What I think is going on here is that stoichiometric combustion simply does not take place even when the intake mixture is stoichiometric. The O2 sensor stoichiometric voltage has been chosen based on calculations or imperical data to correspond to an O2 content typical for combustion of a stoichiometric mixture given combustion imperfections. The O2 sensor has a pretty limited A/F range from just slightly lean to just slightly rich, in this range because of typical combustion inefficiency even the rich mixtures contain some O2 after combustion. What do you think?

vernw, maybe this is what you were trying to say but I was just being too literal with the terms?

Frank