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Old 08-18-2009, 06:16 AM   #11
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No matter whether it's stoich @ 14.7:1 or lean at 16:1, 100% efficient combustion will leave less oxygen than 95% efficient combustion.

Try an experiment, get a small sealed room and a propane fired bunsen burner, see if you pass out quicker running it on yellow flame or blue flame.
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Old 08-18-2009, 06:49 AM   #12
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Let's say you do achieve 100% combustion, what is the exhaust composed of then?
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Old 08-18-2009, 07:08 AM   #13
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100% combustion at 14.7:1 AFR or richer would leave no free oxygen in the exhaust upstream from the converter.

One would have some oxygen left over in the exhaust if there was a misfire/no ignition (even at 14.7 to 1) or if you had 100% fuel combustion but were running leaner that 14.7:1- although running lean with high combustion temps might make at least some of the excess oxygen combine with atmospheric nitrogen to form Nox.
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Old 08-18-2009, 07:08 AM   #14
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100% combustion yields CO2 and H2O.

Unfortunately, in the real world, you end up with a little NOx and CO in there too.
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Old 08-18-2009, 09:00 AM   #15
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Since it is an OXYGEN sensor and a better burn leads to less oxygen then you end up with an engine that is now reading rich, even if it isn't, and the computer will actually remove fuel as a result of the better burn trying to alleviate what it sees as a rich condition.
You don't understand how an O2 sensor works.




Peak power happens at the maximum AFR where there is no O2 left in the exhaust aka 12.5-12.9 AFR.

Peak fuel economy happens at 15.4-16 AFR which also happens to be the nox peak.
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Old 08-18-2009, 09:06 AM   #16
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Hmmm how so? O2 sensors really only measure O2 balance between outside air and exhaust stream from this they infer mixture, if everything is working right, but when it comes down to it, all they measure is the oxygen content. Thus if you have a misfire, your fuelling just goes richer, nice huh?
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Old 08-18-2009, 09:18 AM   #17
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In California, when you have your car smogged they give you a detailed chart of all the gasses coming out your exhaust. Usually a properly functioning car running at stoich (14.7afr) has 1% unburned O2 in the exhaust. That correlates perfectly with the chart above. A car running rich will have near 0% O2 and a car running lean will be over 1% O2
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Old 08-18-2009, 09:19 AM   #18
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Is the yellow shaded range (HC) hydrocarbons? if so, why do they increase along with oxygen when you go really lean (say 18:1)? Is it misfiring at that point?
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Old 08-18-2009, 09:25 AM   #19
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Is the yellow shaded range (HC) hydrocarbons? if so, why do they increase along with oxygen when you go really lean (say 18:1)? Is it misfiring at that point?
Yes. Some engines like honda Vtec-E are designed to create swirl in such a way to not misfire at super lean AFRs like 18+. Most other engines have a hard time running at 18+ AFR. You get 'lean surge' where you feel the car surge back and forth as it periodically misfires.

I'm in the process of megasquirting my Mercedes using a wide band O2 sensor. The engine is perfectly happy around cruising around at 15.4 AFR. It'll even run around 16-17 afr. But as soon as it goes leaner than that it starts to surge. Idle is at 14.7 afr (from the stock 13.5) and wide open throttle is set to between 12.6-13afr. Best of both worlds.


This is an old version of my AFR target table. The new one has a lot of the 12.9s at 4000rpm+ changed to 12.6. The 20KPA row is irrelevant above 1200rpm because the injectors will be turned off on over run.
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Old 08-18-2009, 09:29 AM   #20
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You don't understand how an O2 sensor works.

Peak power happens at the maximum AFR where there is no O2 left in the exhaust aka 12.5-12.9 AFR.

Peak fuel economy happens at 15.4-16 AFR which also happens to be the nox peak.
While those are good, and true, points. What does that have to do with me not understanding how the O2 sensor works?

The O2 sensor reads O2. If you get a more complete combustion O2 stats fall which the computer reads as rich, just like your chart shows.
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