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I was rereading some emissions test data I posted last year on my 87 Renix and it reminded me of something that had been bothering me for some time regarding the so called 14.7:1 air to fuel ratio O2 sensor stoichiometery that is bantered about in many threads here and elsewhere.
There have been some nearly heated debates as to what stoichiometry really means in this case, and what lean and rich means, etc. In the world of chemistry a balanced stoichiometric ratio means complete consumption of both reactants. Also in the automotive world (or combustion in general) a lean ratio would mean excess air, and a rich ratio would mean excess fuel.
I have been told repeatedly that 14.7:1 is slightly rich, not the true exact (if there is such a thing with the variations in fuel chemistry we see) balanced reaction, but a compromise value that minimises NOx emissions, and is a little under the peak fuel economy ration(so it is slightly rich to satisfy the NOx emissions gods) , and a little under the peak power curve, but then again the computer compensates for peak power needs when you stomp on the gas peddle.
OK, now that most of you are baffled, confused, etc, here is my problem or concern, and it finally hit me looking at this data I posted in the RenX files thread last year. This data was from a Texas TCEQ operated computer emissions test analyser.
Getting sort of back on topic here, I got this beast (87, 4.0, Wagoneer, 248,000 miles) emission inspected in one of the tough Texas counties (Houston) earlier this week.
Here are the results:
High Speed: 2441 RPM
HC (ppm) was 8 and the limit was 220 to pass the test
CO % 0.04 and the pass limit was 1.2 %
CO2 % was 14.9
NOx was 0.00 ppm WOW!!!!! :eyes:
O2 % was 5.1
at 698 RPM (Idle) low speed results were:
HC (ppm) 28 with the pass limit being 220
CO % was 0.13 % with the pass limit being 1.20 %
CO2 was 14.7 %
O2 was 5.4 %
NOx once again was 0.00 PPM :eyes:
Not bad for a 20 year old jeep with 248,000 miles on it.
Now, here is the problem, if 14.7:1 is nearly a reaction rate stoichiometric ratio, and since my Jeep passed the inspection, and since it had at the time of the test a new working O2 sensor and Renix Computer controlled fuel injection system, stock, OEM, with a very clean exhaust, and a new Cat converter,
Why is the HC (unburned Hydrocarbon fuel on the 698 rpm low speed test) only 28 ppm, while the unused O2 is 5.4% (roughly about 50,000 PPM!)
To me that sounds typical of other results I have seen before, but how can that be called stoichiometric or even rich with all that excess O2 in the exhaust???
I am guessing there is some kind of excess O2 fudge factor in all this that no one ever mentions to us. Or in other words the 14.7:1 ratio has a huge unused excess O2 reactant value hidden in the 14.7:1 so called stoichiometric ratio that no one ever admits to.
Anyone know what the real truth and or history of this is? I suspect it is partly related to needing a certain amount of excess O2 in the exhaust to avoid lots of CO and to insure somewhat complete combustion of the fuel.
IT also begs the question of what is the O2 sensor actually measuring. One other thing that dawned on me is that an O2 sensor can not measure a lack of or negative amount of oxygen, so a wideband sensor even in the very rich range must still be able to see unused O2 in order to make a measurement of O2 concentration, so once again 14.7:1 is not a true stoichiometric reaction ratio. It is some kind of suedo BS!:laugh3:
I would love to know what it really means and what the history is behind it.:rattle:
There have been some nearly heated debates as to what stoichiometry really means in this case, and what lean and rich means, etc. In the world of chemistry a balanced stoichiometric ratio means complete consumption of both reactants. Also in the automotive world (or combustion in general) a lean ratio would mean excess air, and a rich ratio would mean excess fuel.
I have been told repeatedly that 14.7:1 is slightly rich, not the true exact (if there is such a thing with the variations in fuel chemistry we see) balanced reaction, but a compromise value that minimises NOx emissions, and is a little under the peak fuel economy ration(so it is slightly rich to satisfy the NOx emissions gods) , and a little under the peak power curve, but then again the computer compensates for peak power needs when you stomp on the gas peddle.
OK, now that most of you are baffled, confused, etc, here is my problem or concern, and it finally hit me looking at this data I posted in the RenX files thread last year. This data was from a Texas TCEQ operated computer emissions test analyser.
Getting sort of back on topic here, I got this beast (87, 4.0, Wagoneer, 248,000 miles) emission inspected in one of the tough Texas counties (Houston) earlier this week.
Here are the results:
High Speed: 2441 RPM
HC (ppm) was 8 and the limit was 220 to pass the test
CO % 0.04 and the pass limit was 1.2 %
CO2 % was 14.9
NOx was 0.00 ppm WOW!!!!! :eyes:
O2 % was 5.1
at 698 RPM (Idle) low speed results were:
HC (ppm) 28 with the pass limit being 220
CO % was 0.13 % with the pass limit being 1.20 %
CO2 was 14.7 %
O2 was 5.4 %
NOx once again was 0.00 PPM :eyes:
Not bad for a 20 year old jeep with 248,000 miles on it.
Now, here is the problem, if 14.7:1 is nearly a reaction rate stoichiometric ratio, and since my Jeep passed the inspection, and since it had at the time of the test a new working O2 sensor and Renix Computer controlled fuel injection system, stock, OEM, with a very clean exhaust, and a new Cat converter,
Why is the HC (unburned Hydrocarbon fuel on the 698 rpm low speed test) only 28 ppm, while the unused O2 is 5.4% (roughly about 50,000 PPM!)
To me that sounds typical of other results I have seen before, but how can that be called stoichiometric or even rich with all that excess O2 in the exhaust???
I am guessing there is some kind of excess O2 fudge factor in all this that no one ever mentions to us. Or in other words the 14.7:1 ratio has a huge unused excess O2 reactant value hidden in the 14.7:1 so called stoichiometric ratio that no one ever admits to.
Anyone know what the real truth and or history of this is? I suspect it is partly related to needing a certain amount of excess O2 in the exhaust to avoid lots of CO and to insure somewhat complete combustion of the fuel.
IT also begs the question of what is the O2 sensor actually measuring. One other thing that dawned on me is that an O2 sensor can not measure a lack of or negative amount of oxygen, so a wideband sensor even in the very rich range must still be able to see unused O2 in order to make a measurement of O2 concentration, so once again 14.7:1 is not a true stoichiometric reaction ratio. It is some kind of suedo BS!:laugh3:
I would love to know what it really means and what the history is behind it.:rattle:
