Quote:
Originally Posted by California98Civic
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.