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Old 06-13-2015, 05:15 PM   #4 (permalink)
Isaac Zackary
Full sized hybrid.
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Join Date: Apr 2015
Location: Colorado
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Suzy - '13 Toyota Avalon Hybrid XLE
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Originally Posted by Daox View Post
You can try it, but I don't think you'll have that great of success. If your engine can only run 18:1 in lean burn, you're not even going to get to 18:1 if you start injecting other stuff. The reason that the engine starts to miss at 18:1 is the charge is diluted so much that it simply doesn't propagate the flame front smoothly and evenly. There simply isn't enough fuel to support smooth even combustion. If you start adding water or EGR to the mix, its going to be even further diluted and/or quenched.

There are ways around this. Honda did pretty much all of these with their lean burn engines. Designing the head and combustion chamber for lean burn is one. This means lots of swirl in the cylinder and a higher compression ratio.

Water/EGR will definitely help if, for example, you're running 16:1 and hitting high EGTs. At this point adding water or EGR will help because you've still got good flame propagation.
Thanks! That makes plenty of sense. So basically if an engine has a lean limit of 18:1 then that's an air+EGR+water to fuel ratio. In other words an 18:1 AFR plus 15% EGR is actually a 20.7:1 of everything to fuel ratio, which is beyond the limit. Interesting. Whereas a 16:1 AFR plus 15% EGR is 18.4:1.

Well, I'm going from a 7.5:1 CR to a 9.5:1 CR. That should help (or hidder). Also I'm shooting for a tight quench. I can put dual spark plugs, that is, two in each cylinder. Would that help? How do I get more swirl?
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