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ender
Joined: 12 May 2006
Posts: 15
Location: minneapolis
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 Posted: Sun Nov 05, 2006 1:27 pm Post subject: lean burn cruising |
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| just curious if anyone has tried tuning an ECU to run lean burn code in open loop for cruising to try to improve gas mileage. i may give it a try when i get my isht together, but i have to figure out a way to lock the ECU out of closed loop operation. interested to hear everyone's thoughts... |
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WR^2X
Joined: 14 Aug 2006
Posts: 29
Location: Geneva, IL
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| Posted: Sun Nov 05, 2006 7:30 pm Post subject: |
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| I've thought about doing it via an ECU reflash with the Enginuity/OpenECU stuff, but am leaving it for the Spring. I'd start with 15:1 just to see what (if anything) that accomplishes. |
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Revision
Joined: 14 Sep 2005
Posts: 194
Location: Carol Stream, IL
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| Posted: Mon Nov 06, 2006 8:47 am Post subject: |
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Watch your timing and EGTs when you lean it out. I believe leaning can increase the EGTs if you are not careful. Check out the E85 laminar burn chart in one of the threads as to why. As you lean out, the burn rate increases, so you will need to remove timing at some point.
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cessna
Joined: 06 Nov 2006
Posts: 52
Location: Iowa
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| Posted: Mon Nov 06, 2006 1:01 pm Post subject: |
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Revision, This is my first post. Glad to see info like the laminar burn vel. chart. I have a "92" Toyota 4x4 with 22RE that I have been running 50/50 fuel since 2001. I have a Split Second mixture meter tapped into the O2 sensor and it dithers nicely between 12.5 to 17 air fuel ratio. Do I take it that a very lean mixture acts like detonation with the quick burn? I see you guys talking about reflashing the ECU--- is there a way to do my Toy so I can go all the way to E-85?
Thanks, Marty |
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hotrod
Joined: 19 Apr 2005
Posts: 872
Location: Colorado
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| Posted: Mon Nov 06, 2006 8:38 pm Post subject: |
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No a very lean mixture does not burn anywhere near as fast as detonation. In fact if you look at the chart as the mixture goes leaner than 1.153 EQ ratio combustion slows down. Max burn speed is just on the rich side of stoichiometric mixture. What that equivalence ratio number means is the percentage rich of stochiometric fueling. ie an EQ ratio of 1.153 means the engine is getting 15.3% more fuel than it needs for stoich combustion.
Since this chart is in equivalence ratio, you need to do some conversions to make sense of the info compared to normal gasoline mixtures.
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Fuel .................... AFRst ..... FARst .... Equivalence Ratio .... Lambda
Gasoline stoichiometric 14.7 0.068 1 1
Gasoline max power rich 12.5 0.08 1.176 0.8503
Gasoline max power lean 13.23 0.0755 1.111 0.900
E85 stoichiometric 9.765 0.10235 1 1
E85 max power rich 6.975 0.1434 1.40 0.7143
E85 max power lean 8.4687 0.118 1.153 0.8673
E100 stoichiometric 9.0078 0.111 1 1
E100 max power rich 6.429 0.155 1.4 0.714
E100 max power lean 7.8 0.128 1.15 0.870
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If your readout is giving readings calibrated to gasoline's stoich mixture you need to divide those numbers by about 1.5 to get the true AFR's for 100% E85, since you are running a 50/50 mix your numbers should be about midway between the 100% gasoline and 100% E85 numbers. If possible it is best to work with lambda or equivalence ratio if your meter will read out in those units as those readouts don't care what fuel your running.
The max burn speed in the that graph roughtly corresponds to the max power lean mixture in the above chart for E85 (equivalence ratio 1.153).
If your getting fuel air mixtures of 12 -17 to one, your on the lean side already. A good target mixture for pure E85 in my experience is about 11.5:1 gasoline AFR.
Since you are running a mixture your similar mixture will be about halfway between 11.5:1 for pure E85 and 12.5:1 commonly used for gasoline, so if we simply split the difference you should target mixtures near 12:1 under open loop fueling (ie WOT). When in closed loop fueling, ie light throttle cruise you should run gasoline calibrated AFR's of about 14.7 on gasoline and around 9.7 (true AFR) on pure E85 which would be a gasoline calibrated AFR of 14.6 or so. If we again split the difference your closed loop numbers would be about 14.7-14.6 on a gasoline calibrated sensor. If your hitting 17:1 your very lean (of course this could be due to injector cut off during off throttle deacceleration.)
Now some folks have had good experience running leaner than theoretical mixtures so all this is a pure theory, but for what it is worth. If my numbers are correct, you are on the lean side of that burn speed hump most of the time. As the mixture goes leaner than 1.153 EQ, it also slows down combustion speed ( sort of a useful feed back process that might be why folks get good results on slightly leaner than ideal mixtures).
At this point we really don't have solid information on where the limits are for E85 regarding mixture, boost and timing so eveyone is still very much in the experimental stage.
As soon as I finish the fuel system upgrade on my WRX I will be able to probe some of these issues as I will have significant control over mixture and boost and timing so with luck will be able to give you better info in a few months, but that is my view at the moment.
Larry |
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Revision
Joined: 14 Sep 2005
Posts: 194
Location: Carol Stream, IL
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| Posted: Tue Nov 07, 2006 1:56 am Post subject: |
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Thanks for clearing that up! 
I've been reading that chart backwards. 
Time to rewrite my theories again... |
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hotrod
Joined: 19 Apr 2005
Posts: 872
Location: Colorado
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| Posted: Tue Nov 07, 2006 3:45 am Post subject: eq and lambda |
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| Quote: | Thanks for clearing that up!
I've been reading that chart backwards. |
Don't feel bad I mix these up all the time too.
Here is the easy way to remember for me:
Lambda as expressing oxygen availability.
ie .8 lambda means you have 80% of the air you need for a stoich mixture
Equivalence ratio is expressing fuel availability:
ie EQR of 1.153 means you have 15% too much fuel for stoich mixture.
Gasoline and E85 both develop max power in approximately the same mixtures on the lean end of best power, around 11% for gasoline and 15% rich of stoich for E85, but E85 can still make power at much richer mixtures than gasoline at 40% rich compared to only 18% rich for gasoline.
If you divide the stoich mixture by the equivalence ratio number you get the true AFR of that mixture.
For example max power rich for gasoline is 1.176 EQR, so 14.7 divided by 1.176 = 12.5 which is the commonly accepted max power rich mixture.
14.7 / 1.111 = 13.23 AFR
That is how I developed this chart of comparable mixtures using the common methods of expressing it.
I like the EQR because it best expresses what we are usually doing. ie we're usually messing with the amount of fuel available for a fixed amount of air.
Most of us know that max power is on the rich side of stoich so it is pretty logical if you work with it often, but it still puzzles me occasionally if I haven't tinkered with the numbers for a while and I use my own mixture chart to keep myself honest more often than I would like to admit 
One of the problems is in casual conversation we all say "fuel air ratio" when we should say "air to fuel ratio".
ie max power rich for gasoline is an air to fuel ratio of 12.5 parts air to 1 part fuel. When you start poking around on the NACA studies of high performance aircraft engines they use the Air to fuel ratio properly and refer to mixtures as FAR = 0.08 and of AFR = 12.5:1 --- same mixture
Larry |
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