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Old 04-02-2018, 11:04 AM   #1 (permalink)
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Run lean for emissions?

Given that even NOx emissions can be much lower at an AFR 18:1, why don't we see it more often? Or do we? Are OEM cars currently tuned to run lean under light load? 18:1 does not seem very radical. Wasn't lean burn in the Civics 20 or 22 to 1? Would you even need a knock sensor if it were strictly for light load, like freeway cruising? How much hotter would a block get? How much hotter would the CAT get?



Just curious.

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Last edited by California98Civic; 04-02-2018 at 12:10 PM.. Reason: Spelling error in title
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Old 04-02-2018, 06:31 PM   #2 (permalink)
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That is a good question that has people often mistakenly thinking that running lean is a bad thing for emissions.

The graph you put up is a classic used in many tomes on combustion. It does not take into account modern engines and controls. The graph does show clearly the problem. It's not the production of NOx at these lean ( 18+) AFRs but the increase of HC and CO. In the Honda lean burn engines that ran at 22:1 air/fuel mass ratios, the combustion became erratic and HC and CO would skyrocket. But, the engine actually runs COOLER past 18:1 AFR resulting in very poor catalytic converter efficiency. This is why lean burn is limited in the North American market in gasoline spark ignition vehicles. Natural gas engines on the other hand, have inherently better CO and HC profiles for combustion and it would be interesting to see more development of lean burning gaseous fueled engines.
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Old 04-02-2018, 08:19 PM   #3 (permalink)
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Up until about 17.5:1, according to this basic image, only O2 is rising from stoich. CO continues a slow decline and only HC begins to rise after roughly 18:1. So seems a mild leaner burn could be accomplished at light cruise and 18:1 with a net polution benefit, unless oxygen is somehow more menacing than I realize.
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See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.



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Old 04-03-2018, 01:14 AM   #4 (permalink)
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See stratified charge systems on Honda CVCC engines and Curtiss-Wright rotary engines.
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Old 04-06-2018, 03:43 PM   #5 (permalink)
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This thinking has been followed before.

Quote:
Originally Posted by California98Civic View Post
Up until about 17.5:1, according to this basic image, only O2 is rising from stoich. CO continues a slow decline and only HC begins to rise after roughly 18:1. So seems a mild leaner burn could be accomplished at light cruise and 18:1 with a net polution benefit, unless oxygen is somehow more menacing than I realize.
I remember working on a 78 Dodge Maxiwagon Van that had a lean burn system as it utilized a spread bore carburetor with the primaries tuned to a lean mix of 18:1 AFR and the vacuum secondaries tuned rich. Heavy use of EGR during the primary operation reduced NOx and a two way catalyst took care of the residual CO and HC. It was a somewhat finicky system that took some skill to tune correctly otherwise rough running became noticeable. We were able to trade out the Carter Thermo Quad for a Holley Spread Bore with vacuum secondaries so we could use the extended tune-ability of the Holley. This allowed us to richen the secondaries for better acceleration and lean the primaries even more with an early Edelbrock digital water injection unit replacing the EGR. Our early Hawk sensor could only read fuel mixes to about 21:1 AFR but the engine ran flawlessly with the inadvertent inclusion of a zinc bolt plugging the EGR crossover entrance underneath the carb. The water injection would hit this hot zinc and the zinc would become zinc oxide and release small amounts of hydrogen. Once the heavy zinc coating was used up, the engine ran rough ( within a few hundred miles ). I machined a plug bolt that accepted a slug of zinc and we could then run smoothly for at least three tankfulls or so ( over a thousand miles ). This lead me down the path of experimenting with electrolytically generated hydrogen, on board. This was two decades before the HHO hysteria broke out. Small amounts of hydrogen were all that was needed to stabilize combustion to extend the lean limits of the engine.

Modern computer controlled engines are far easier to tune for lean burn if you have a programmable electronic control suite. PfgPro and others have done it. If you can improve the combustion so that even extended lean burn does not result in increased COV ( co-efficient of variability ) you will not have the increased CO and HC output and thus will not need the help of the three way catalyst to meet emissions as NOx is very small. The problem is when you need increased engine output and the system goes back to stoichiometric, the catalyst will be relatively cool and unable to clean the gasses.

Continued research in catalysts is resulting in materials that operate in lower temperature regimes. A close coupled CAT ( tucked right up against the engine head output ) utilizing lower temperature catalysts will allow lean burn operation again.

The stratified charge engines have a slew of problems that are more akin to a diesel. That is another discussion and is not applicable to most Ecomodders as we cannot design and build engine heads for the time being ( maybe soon with 3D printing ). We can play with lean burn on most gasoline engines available to us now.
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Old 04-06-2018, 09:20 PM   #6 (permalink)
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Quote:
Originally Posted by RustyLugNut View Post
... Modern computer controlled engines are far easier to tune for lean burn if you have a programmable electronic control suite. PfgPro and others have done it. If you can improve the combustion so that even extended lean burn does not result in increased COV ( co-efficient of variability ) you will not have the increased CO and HC output and thus will not need the help of the three way catalyst to meet emissions as NOx is very small...
I have toyed with the idea of a second ECU that I might chip or otherwise hack, then swap out for SMOG testing. But I always have too many projects, so I wait. Still, it seems a 17.5:1 is a pretty modest lean burn and ought to be accomplishable on a car like mine, without a knock sensor, harmlessly. Would be fun to see what FE numbers would result in my aeromodded 98 DX with its tall '93 VX/CX transmission.

Quote:
Originally Posted by RustyLugNut View Post
... Continued research in catalysts is resulting in materials that operate in lower temperature regimes. A close coupled CAT ( tucked right up against the engine head output ) utilizing lower temperature catalysts will allow lean burn operation again.
Are there publically available research papers on such catalyst materials? I would be interested in attempting a read but I have failed to idetify good search terms or something.

Much of the time I would think I could minimize the somewhat cooled CAT problem at the end of a long LB cruise by going into neutral and coasting with the engine on for maybe 10 or 15 or more. The catalyst would warm up quickly but without the heavy load on it at the same time.
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See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.



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Old 04-06-2018, 11:08 PM   #7 (permalink)
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