Octane vs. Ethanol (or the lack thereof)
04-08-2010, 10:39 PM
#12
Registered User
I'm no automotive engineer, but my guess is that the ping/knock from lower octane fuel would be much more harmful than the effects of 10% ethanol. Plus, I further guess that the retarded timing to deal with the lower octane fuel would hurt power output more than the lower energy density of the ethanol.
I've gotten gas in the middle of nowhere (well, in Childress while driving out west) and even those pumps had the "May contain blah blah blah" sticker on them. Which surprised me because I would have thought they'd have no smog requirements or whatever. So maybe something else is going on at the point of distribution?
(altiain, I'm quite the nerd - I enjoy thinking, discussing and calculating this sort of stuff.
)
I've gotten gas in the middle of nowhere (well, in Childress while driving out west) and even those pumps had the "May contain blah blah blah" sticker on them. Which surprised me because I would have thought they'd have no smog requirements or whatever. So maybe something else is going on at the point of distribution?
(altiain, I'm quite the nerd - I enjoy thinking, discussing and calculating this sort of stuff.
)
04-09-2010, 03:23 AM
#13
Registered User
This isn't answering your question, but rather my opinion of why I prefer normal gas over ethanol.
e10 spoils, rather it has an expiration on it that bothers me quite a lot. If you leave e10 in a can, don't expect to use it in your lawn mower after the winter when you grass starts growing again.
I don't know if this site is right, but it says e10 has a shelf life of 3 months, which I thought it was more like 6-8 months.
http://www.fuel-testers.com/expirati...hanol_gas.html
e10 spoils, rather it has an expiration on it that bothers me quite a lot. If you leave e10 in a can, don't expect to use it in your lawn mower after the winter when you grass starts growing again.
I don't know if this site is right, but it says e10 has a shelf life of 3 months, which I thought it was more like 6-8 months.
http://www.fuel-testers.com/expirati...hanol_gas.html
04-09-2010, 05:01 AM
#14
IIRC, ethanol doesn't require as much oxygen to combust, so even though it doesn't have as much energy content per gallon as regular gasoline, it can deliver more power per combustion cycle by dumping more ethanol into the cylinder per cycle. What I don't recall is whether a typical ECU will know to deliver the extra fuel ...
04-09-2010, 08:30 AM
#15
Mark - it all depends on location. The station at the entrance to MSR used to advertise "100% gas - no ethanol", but I believe Parker county was recently added to the EPA smog counties in Texas, and now they sell E10. I would always fill up leaving the track, and used to get some of my best mileage with that fuel - actually managed 23mpg once. 
Greg - ethanol does have a different stoichiometric ratio than gasoline. For gasoline, the stoichiometric ratio (ratio of air to fuel that provides theoretical complete combustion) is 14.7:1. For pure ethanol the ratio is 9:1. most modern engines can adjust air/fuel ratio within a certain spectrum, but only vehicles labeled FlexFuel (E85 compatible) will be capable of adjusting as low as 9:1 without modification. Running that rich on a low ethanol blend like E10 or E15 would foul plugs and catalytic converters very quickly, and both of those components are mandated 100,000 mile life per the EPA, so manufacturers are very careful not too run any richer than they have to.
However, the biggest reason people can make more power on high percentage ethanol blends (like E85) is primarily dependent on the octane rating. E85 is ~104 octane, so you can advance timing (and in forced induction engines you can increase boost pressure) compared to a 93 octane gasoline. Again, a 93 octane E10 blend is not going to provide any more power than a 93 octane straight gasoline, because both have the same relative knock resistance. You could get similar power levels as E85 out of unleaded 104 octane race fuel… but corn is heavily subsidized, so E85 is a lot less expensive (and easier to find). That’s what makes it attractive to tuners.
In an NA engine, running E85 would be a waste, because you can only do so much to advance the timing. You could greatly increase the compression ratio of an NA engine (to something approaching 18:1) and make more power on E85, but it would be an expensive proposition.
Greg - ethanol does have a different stoichiometric ratio than gasoline. For gasoline, the stoichiometric ratio (ratio of air to fuel that provides theoretical complete combustion) is 14.7:1. For pure ethanol the ratio is 9:1. most modern engines can adjust air/fuel ratio within a certain spectrum, but only vehicles labeled FlexFuel (E85 compatible) will be capable of adjusting as low as 9:1 without modification. Running that rich on a low ethanol blend like E10 or E15 would foul plugs and catalytic converters very quickly, and both of those components are mandated 100,000 mile life per the EPA, so manufacturers are very careful not too run any richer than they have to.
However, the biggest reason people can make more power on high percentage ethanol blends (like E85) is primarily dependent on the octane rating. E85 is ~104 octane, so you can advance timing (and in forced induction engines you can increase boost pressure) compared to a 93 octane gasoline. Again, a 93 octane E10 blend is not going to provide any more power than a 93 octane straight gasoline, because both have the same relative knock resistance. You could get similar power levels as E85 out of unleaded 104 octane race fuel… but corn is heavily subsidized, so E85 is a lot less expensive (and easier to find). That’s what makes it attractive to tuners.
In an NA engine, running E85 would be a waste, because you can only do so much to advance the timing. You could greatly increase the compression ratio of an NA engine (to something approaching 18:1) and make more power on E85, but it would be an expensive proposition.
04-09-2010, 11:56 AM
#16
Registered User
Thread Starter
Good comments everyone. Iain, logically what you've said makes complete sense to me. I just don't get why I've never had this problem with 91 octane before. My car only requires 91, so it should be fine. Maybe it's time for a tuneup?
I'm sure I'd normally have more to say, but I just got to Manila and it's 4am. Needless to say, I'm ready for bed. I'm not sure how much time I'll have to be on the forum over the next two weeks while I'm here, but I'll try to pop in every now and then.
I'm sure I'd normally have more to say, but I just got to Manila and it's 4am. Needless to say, I'm ready for bed. I'm not sure how much time I'll have to be on the forum over the next two weeks while I'm here, but I'll try to pop in every now and then.
04-19-2010, 06:12 PM
#18
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Originally Posted by altiain,Apr 9 2010, 08:30 AM
However, the biggest reason people can make more power on high percentage ethanol blends (like E85) is primarily dependent on the octane rating. E85 is ~104 octane, so you can advance timing (and in forced induction engines you can increase boost pressure) compared to a 93 octane gasoline. Again, a 93 octane E10 blend is not going to provide any more power than a 93 octane straight gasoline, because both have the same relative knock resistance. You could get similar power levels as E85 out of unleaded 104 octane race fuel… but corn is heavily subsidized, so E85 is a lot less expensive (and easier to find). That’s what makes it attractive to tuners.
I converted the Evo to E85 last year. With the same boost (28psi) I'm making 400ftlb of torque and 375 horsepower. It added about 40-50 ft/lb or torque and hp. Pretty significant change.
Oh. I pay 10-30 cents LESS per gallon than 87 octane.
My fuel economy has dropped from about 240 miles on a tank (11.5-12 gallons) to 200 on a good day.
All the Evo needed was a new fuel filter and a line in the gas tank (other than the much larger injectors and fuel pump). All other pieces of the stock system are fine so far with E85.
Before I converted, I sought out the E0 93 octane gas. I got better fuel economy and my "seat of the pants-o-meter" said that I was making smoother power. I once had a guy in Oklahoma tell me that they made "91" (the max you can get there) by mixing 87 with more Ethanol in order to get the octane rating up. Apparently there was a big shit storm on the news about it when gas got really $$ they started going up to E50 (illegally) and it was causing a ton of problems.
