18:1 AFR is "cleaner"?

[rmay635703]

Originally Posted by California98Civic

That is the kind of thing I cannot find clear answers for online. I see contradictory things... some say lean burn makes the catalyst cooler.

It runs out of fuel and saturates the catalyst over time (years) it can become brittle and fail

[oil pan 4]

Originally Posted by Piotrsko

Doesn't the cat have overtemp isses when too much air is introduced?

The only time I have ever seen a confirmed melted catalytic converter was on an extended dyno run on a car that was definitely not running lean.
The fix for the melted cat was "run a bigger cat", as apparently this is a fairly common problem.

[California98Civic]

Originally Posted by rmay635703

It runs out of fuel and saturates the catalyst over time (years) it can become brittle and fail

That would be a problem. But Hondas such as the Gen1 Insight, Civic VX, and CIVIC HX have not had that problem & they run as lean as 22:1, according to Wikipedia. 18:1 is not nearly as lean and it would produce a modest improvement in emissions and in fuel economy. I don't understand why it was ever phased out, if it was phased out, for light load cruising and moderately low engine speeds. I gotta think it is still programmed into Honda iVEC systems but not promoted.

[rmay635703]

Originally Posted by California98Civic

That would be a problem. But Hondas such as the Gen1 Insight, Civic VX, and CIVIC HX have not had that problem & they run as lean as 22:1, according to Wikipedia. 18:1 is not nearly as lean and it would produce a modest improvement in emissions and in fuel economy. I don't understand why it was ever phased out, if it was phased out, for light load cruising and moderately low engine speeds. I gotta think it is still programmed into Honda iVEC systems but not promoted.

Older cars (pre 2000) had 2 way CATS or modified 3 way
which aren’t affected by that issue, they also more importantly didn’t need to meet the more stringent NOx limits of later years so some NOx could pass a full trap without issue.


The Insight and newer had a Nox trap and would cycle lean to rich to make sure the cat stayed lit and the NOx levels didn’t increase after the NOx trap filled.

[California98Civic]

Originally Posted by rmay635703

Older cars (pre 2000) had 2 way CATS or modified 3 way
which aren’t affected by that issue, they also more importantly didn’t need to meet the more stringent NOx limits of later years so some NOx could pass a full trap without issue.


The Insight and newer had a Nox trap and would cycle lean to rich to make sure the cat stayed lit and the NOx levels didn’t increase after the NOx trap filled.

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.

[RustyLugNut]

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.

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.

[rmay635703]

Originally Posted by RustyLugNut

Look back at the graph that was posted at the start of the thread. Notice the line for HC. It is climbing quickly

In summary, NOx at ultra lean fuel mixtures is not the culprit. HC and CO are.

The cat should burn off the hc and co using the excess NOx

[California98Civic]

Originally Posted by rmay635703

The cat should burn off the hc and co using the excess NOx

Would that be true even if the lean mixture causes cooling of the catalyst to a less efficient temp? Maybe that's what you are saying. Not certain.

[rmay635703]

Originally Posted by California98Civic

Would that be true even if the lean mixture causes cooling of the catalyst to a less efficient temp? Maybe that's what you are saying. Not certain.

Yes those compounds still get burned up ,
the percentage of those compounds burned decreases though as temperatures decrease.

[oil pan 4]

I like how they just ignore increasing fuel economy as a way to fight pollution, as if the gasoline just miracles it's self into your gas tank tail pipeemission are the only way a car can damage the environment.