Stoichiometry of blended fuels (gas + alcohol)
11-06-2007, 12:00 PM
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Burning Brakes
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Stoichiometry of blended fuels (gas + alcohol)
Since there are so many individuals using meth now, I thought I would copy this terrifc post by black2003cobra (Eric S.) from ModularFords.com. I'm not going to use vBulletin "quote" wrap on the post, as it deminishes the reading capability.
The original thread can be found here:
http://www.modularfords.com/forums/s...ad.php?t=71048 <---click!
Originally posted by black2003cobra:
"I had promised oldbones that I’d try to find him a graph of how the stoichiometric air-to-fuel ratio (AFRs) is affected by the addition of methanol for his new setup. Although there is published data available for a blended fuel of gasoline + ethanol, I couldn’t find anything similar for a blend with methanol. So I had to balance the equation myself, (shown below). I also did the same thing for the ethanol blend as a check, and found my results agreed with the published data, so the results for meth should be OK. I’m also including a graph of AFRs vs %meth (or ethanol) by volume. I decided to post this thinking others might be interested in the results, as well.
As with ethanol, AFRs drops with a higher percentage of methanol. The decrease in AFRs is more dramatic simply because the stoich ratio of pure meth is less than that of pure ethanol (6.47 vs 9.0). Therefore, to maintain a given equivalence ratio, (φ = FAR/FARs = AFRs/AFR), one would need to run a richer (lower) AFR, depending on how much meth is injected. For reference, for just “plain” gasoline most seem run an equivalence ratio of φ ≈ 1.25, (which translates to AFR ≈ 11.7). So if one were to run a mix of, say, 20% methanol whereby AFRs drops to ~12.9, to maintain the same equivalence ratio of 1.25, the required AFR would be 12.9/1.25 = 10.3. If one runs a leaner AFR than that, it pushes one closer to the knock limit, (which IMO, tends to defeat the purpose from the safety point of view of running methanol). On the other hand, I don’t think most do this. Perhaps that is because the higher octane that results by adding methanol, allows one to run a slightly lower equivalence ratio. However, it is worth noting that the literature suggests that although the RON increases nearly linearly in proportion to the concentration of methanol added, that the MON (the more important number) only increases up to about 15%, after which point it levels off. As a result, the fuel sensitivity (RON-MON) increases with an increasing percentage of methanol. In other words, the PON isn’t increasing as fast as the RON. I’m also attaching a graph of this (ref OEC 2002). In a private communication, James (RWTD) pointed out that if the vehicle has knock sensors, that helps get around the issue. From the power point of view, the equivalence ratio at which the (laminar) burn velocity is a maximum is slightly lower for methanol than it is for gasoline, (~1.1 vs ~1.2), so that also suggests that a slightly lower equiv ratio could/should be used, too. Nonetheless, I would highly recommend checking w/ guys like Ed or James or Jerry, et al., on this aspect of it, (i.e., what equivalence ratio to run). I know there are other members here that have some experience with alcohols, too, and they might also have some input on this aspect of it. I invite their comments. That all said, the biggest impact on knock reduction comes from any reduction in IAT2 due to the high latent heat of evaporation of the alcohols, any way. Since oldbones setup injects the meth past the intercooler, I think he may see some advantage there."
The original thread can be found here:
http://www.modularfords.com/forums/s...ad.php?t=71048 <---click!
Originally posted by black2003cobra:
"I had promised oldbones that I’d try to find him a graph of how the stoichiometric air-to-fuel ratio (AFRs) is affected by the addition of methanol for his new setup. Although there is published data available for a blended fuel of gasoline + ethanol, I couldn’t find anything similar for a blend with methanol. So I had to balance the equation myself, (shown below). I also did the same thing for the ethanol blend as a check, and found my results agreed with the published data, so the results for meth should be OK. I’m also including a graph of AFRs vs %meth (or ethanol) by volume. I decided to post this thinking others might be interested in the results, as well.
As with ethanol, AFRs drops with a higher percentage of methanol. The decrease in AFRs is more dramatic simply because the stoich ratio of pure meth is less than that of pure ethanol (6.47 vs 9.0). Therefore, to maintain a given equivalence ratio, (φ = FAR/FARs = AFRs/AFR), one would need to run a richer (lower) AFR, depending on how much meth is injected. For reference, for just “plain” gasoline most seem run an equivalence ratio of φ ≈ 1.25, (which translates to AFR ≈ 11.7). So if one were to run a mix of, say, 20% methanol whereby AFRs drops to ~12.9, to maintain the same equivalence ratio of 1.25, the required AFR would be 12.9/1.25 = 10.3. If one runs a leaner AFR than that, it pushes one closer to the knock limit, (which IMO, tends to defeat the purpose from the safety point of view of running methanol). On the other hand, I don’t think most do this. Perhaps that is because the higher octane that results by adding methanol, allows one to run a slightly lower equivalence ratio. However, it is worth noting that the literature suggests that although the RON increases nearly linearly in proportion to the concentration of methanol added, that the MON (the more important number) only increases up to about 15%, after which point it levels off. As a result, the fuel sensitivity (RON-MON) increases with an increasing percentage of methanol. In other words, the PON isn’t increasing as fast as the RON. I’m also attaching a graph of this (ref OEC 2002). In a private communication, James (RWTD) pointed out that if the vehicle has knock sensors, that helps get around the issue. From the power point of view, the equivalence ratio at which the (laminar) burn velocity is a maximum is slightly lower for methanol than it is for gasoline, (~1.1 vs ~1.2), so that also suggests that a slightly lower equiv ratio could/should be used, too. Nonetheless, I would highly recommend checking w/ guys like Ed or James or Jerry, et al., on this aspect of it, (i.e., what equivalence ratio to run). I know there are other members here that have some experience with alcohols, too, and they might also have some input on this aspect of it. I invite their comments. That all said, the biggest impact on knock reduction comes from any reduction in IAT2 due to the high latent heat of evaporation of the alcohols, any way. Since oldbones setup injects the meth past the intercooler, I think he may see some advantage there."
Last edited by RWTD; 11-07-2007 at 07:36 PM.
11-08-2007, 01:27 PM
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aka allbottle¬hrottle
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Location: Brooklyn New York The DVL and the chicken sleep with the fishes....
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St. Jude Donor '08
I feel so stupid right now ..... It looks like you put alot of work into this , what does it all mean ??? 
11-09-2007, 02:50 AM
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Burning Brakes
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Though the post may seem like it's only centered around meth, its goal was to help make aware of the importance of stoich tuning, as well as helping in determining the new equivalence ratio (provided the stoich point is known of the fuel being using).
Pump gas generally has a stoich of approx. 14.6x, and gas with 10% ethanol has a stoich of approx. 14.0x. Basically, in a nutshell, to keep the same equivalence ratio, and power and safety, you need to actually run richer with 10% ethanol. i.e. The equivalence ratio for "straight" pump gas is approx. ~1.22 (=14.64/12), therefore if at WOT you have a 12.0 afr with straight pump gas, then you would need to have an AFR of of 11.47 (=14/1.22) with 10% ethanol.
It's a *very* simple change in your current tune (1 scalar or table needs to be modified, and the pcm will adjust all fueling properly).
Regards,
James
Pump gas generally has a stoich of approx. 14.6x, and gas with 10% ethanol has a stoich of approx. 14.0x. Basically, in a nutshell, to keep the same equivalence ratio, and power and safety, you need to actually run richer with 10% ethanol. i.e. The equivalence ratio for "straight" pump gas is approx. ~1.22 (=14.64/12), therefore if at WOT you have a 12.0 afr with straight pump gas, then you would need to have an AFR of of 11.47 (=14/1.22) with 10% ethanol.
It's a *very* simple change in your current tune (1 scalar or table needs to be modified, and the pcm will adjust all fueling properly).
Regards,
James
11-09-2007, 12:11 PM
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Safety Car
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St. Jude Donor '09
That's much clearer to me. Especially since my "old" ride sees 20+ lbs of boost on crap 91 octane. The only other option I have is to load up on Torco or add 4 gallons of 100 octane to a tank of 91 to eliminate high load knock. My exhaust is so loud I don't run it hard too much here.
San
San
