Quote:
Originally Posted by California98Civic
So, perhaps as the mixture goes lean, the CAT cools, and its efficiency decreases. That would mean a DIY lean burn system would inevitably cough out more NOx, which was a major culprit in SoCal smog back in the bad old days of fog banks in the desert.
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Look back at the graph that was posted at the start of the thread. Notice the line for HC. It is climbing quickly. This must be dealt with if you want to run clean and lean. The problem is, as you correctly pointed out, the cooling exhaust gas leaves the catalytic converter unable to efficiently eliminate this HC, not the NOx.
Why is HC climbing? Why is it not lowered like CO in an oxygen rich environment? As the engine runs past 17:1 AFR, the combustion temperatures cool. You surmise this from the cooler exhaust temperatures. This implies that the combustion chamber event is also cooler, and it is. This results in the quench area thickness growing in proportion to the reduced combustion temperature. You can sweep the chamber walls with increased turbulence, but you can only do so much. You can also increase intake heat via larger volumes of exhaust gas re-circulation (EGR) noting that EGR in lean-burn mode has more reactivity in the form of OH ions due to free oxygen.
All the above was done by Honda to achieve 22:1 AFRs, but the cost of the system as well as it's finicky nature lead to it's abandonment.
Running leaner than 22:1 AFR leads to excessive combustion variability. This increasing Co-efficient of Variability (COV) results in increasing pollution as CO comes back into play.
In summary, NOx at ultra lean fuel mixtures is not the culprit. HC and CO are.