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Old 10-24-2009, 11:11 AM   #1 (permalink)
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lean burn=Lean of peak?

My understanding of air fuel mixtures comes from flying small aircraft. on takeoff the mixture is very rich as you are making lots of power and have poor cooling (due to low airspeed during climb) so extra fuel is used to cool. Once at the cruising altitude the throttle is pulled back, prop adjusted, then mixture leaned. I have been reading a bit about lean of peak running, and done a bit at high altitude (thin air means low power so not much heat created). When you pull the mixture back the EGTs go up a couple hundred degrees in some cases as extra fuel is no longer available to cool, and the mixture burning a little slower. Once you get to the peak EGT the mixture keeps coming out and the EGT and CHT temps will start dropping. So the leaner the engine is the cooler it runs. The only real limit is each cylinder will have a slightly different mixture so one will go too lean to support combustion and will have a misfire.

From what I understand the most hazardous point on the mixture continuum is just rich of peak EGT (exaust gas temp) once past that it seems pretty safe. In my plane on the ground I will lean for taxi to prevent fouled plugs when I lean enough and try to increase power over that needed for taxi I get hesitations and rough running due to the mix going to lean to burn.

Has anyone tried flow matching, intake and heads to get a leaner mixture in their cars?

http://www.avweb.com/news/pelican/182084-1.html Some interesting aircraft mixture information should be transferable to cars as an aircraft ICE is pretty similar to a ground crawler ICE.

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Old 10-24-2009, 09:27 PM   #2 (permalink)
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I'm pretty sure that if you use custom ECU programming like MegaSquirt you can map your own rich/lean ratios. But you have to be pretty smart about it - too lean and you'll burn valves etc. I bet it can be done though.

The Honda Civic VX and the later Civic HX ('96 - '05) both had lean burn capabilities coded into their ECUs. I think some of the later Hondas also had/have lean burn but I can't say which ones for certain. I think Insight Gen 1 and Civic Hybrid?? Maybe others??

My own thinking is that lean burn lets you get max power out of a given qty of fuel because you're not sending a percentage out the exhaust unburned. Standard stoich ratio leaves some unburned fuel in the exhaust deliberately to keep the cat converter temp up - it combusts in the converter. The lean burn Civics keep the cat temp up by having the cat in the exhaust manifold (at least that's what my HX has) so it tends to stay pretty hot.

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