I disagree on two points (possibly typos?), one is the theoretical zero O2. I suspect it takes a good bit of excess O2 to reach the 14.7 stoich number, since that is not an excessively high rich burn number. I have also seen test data showing multiple percent excess O2, but that may have been tailpipe 6X dilution numbers on the O2, so the data is not always on equal footing. I could be wrong on that?
The other I think is a typo: " CO2 is going to be fairly low because it's almost naturally low enough to be considered a "trace gas".
The O2 is replaced by CO2, so if O2 goes from 21% to zero, the CO2 will go way up as the O2 is replaced by CO2. I have seen CO2 numbers average around 14% in many reports, which would suggest 6% remaining O2, but there is a huge increase in H20 gas that dilutes them all that is not measured or reported, and I have seen O2 numbers reported well below 1% on many reports (which is why the O2 numbers running from 6% to under 1% have always puzzled me so far. Only thing I can figure is the O2 reported is sometimes a diluted O2 due to tester differences, and the huge increase in CO2 and H20, from fuel combustion, dilutes the N2 and remaining O2....)
Hm.
As I understand it, "stoichiometric combustion" means there's enough oxygen to combust all fuel, there's enough fuel to consume all oxygen, and there's none of either left. Most of the "ash" from combustion of hydrocarbons is actually water vapour (there's far more hydrogen than carbon in a hydrocarbon chain - although it has been some time since I took O chem...) and measuring humidity of exhaust gas RT ambient air would be illustrative. Theoretically, you should have zero oxygen remaining in an unaltered hydrocarbon combustion system, running properly with an effective feedback loop in place.
And, that addresses both the O2 and CO2 (IIRC, seeing 12-18% CO2 is a marked increase over general atmospheric content - but it's still going to be relatively low, else we'd have asphyxiated ourselves years ago as a result of combustion of hydrocarbons in general.)
Looking at the composition of the atmosphere (Wiki chart -
http://en.wikipedia.org/wiki/Atmospheric_chemistry) it can be noted that CO2 is indeed a trace constituent of the biosphere - Hell, there's more
argon in the atmosphere than there is CO2 (and argon is and remains chemically inactive, being a noble gas.) (For those who don't want to read through the link, or lack intermediate chemistry to understand the numbers, here's a link directly to a graphic representation of the issue -
http://upload.wikimedia.org/wikipedia/commons/1/14/Atmospheric_composition_Langley.svg. It also features a raw percentage breakdown - CO2 comprises not quite 0.04% of ambient air...)
So, reading CO2 of over ten percent in the exhaust stream very much
is a marked increase in content, and makes sense.
However, if combustion in the engine is truly stoiciometric (~14.7:1AFR) and the feedback loop is effective, accurate, and the information followed, then it stands that remaining %m O2 should be zero, and %m HC should be likewise zero (stoichiometry simply refers to the combustion of hydrocarbons and the oxygen available in relation to that - CO is partial combustion, and therefore still "combustion"
in se. Ergo, CO is not measured as a byproduct to indicate that stoichiometry is not followed, it is measured to ensure that combustion is total.)
The provision of excess O2 available to combustion means that combustion will
not follow stoichiometry, by definition, that results in a "lean burn" condition (there's not enough fuel available to the quantity of oxygen provided,) and can result in elevated combustion temperatures. This relationship will continue until the LBL (Lean Burn Limit) is reached, and the chain reaction of combustion can no longer be supported (I don't recall the LBL for gasoline, but I think it's up around 20-22:1) Lean burning is typically characterised by an absence of HC and CO, the presence of exhaust gas O2, and an increase in NOx (due to elevated combustion temperatures)
Going the other way, we run into having excess fuel available, which is a "rich" burn condition. This has the effect of lowering combustion temperatures, and excessively rich combustion can cause "washdown" where the oil remaining after the oil control rings wipes the sides of the cylinder bore gets literally washed off of the metal. This accelerates ring wear (in addition to the increased HC and CO emissions.) This will continue until the RBL (Rich Burn Limit) is reached - again, I don't recall exactly, but I'm thinking it's about 7-8:1. Rich burn conditions are indicated by the absence of O2, increased HC and CO, and depressed NOx (excess heat is absorbed in finishing the job of vapouorising the fuel.)
It's interesting to note that, despite all of the fuss over reaching stoichiometric burning, it's actually
not an "ideal state" for engine operation!
"Best cruise operation" is typically slightly lean, running at about 15.2:1.
"Best power operation" is typically slightly rich, usually about 12.8-13.0:1
Go figure.