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?
http://ecomodder.com/forum/member-ca...-4gaschart.gif Just curious. |
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. |
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 stratified charge systems on Honda CVCC engines and Curtiss-Wright rotary engines.
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This thinking has been followed before.
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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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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. |
I like the new curb stomped version of the EPA.
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