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Ethanol gasoline blends - reality vs science

  1. Apr 6, 2015 #1
    Hello everyone,

    (Hope this is the right place to post this)

    I am wondering, and have for a while, if there is any "real" answers, as to why some people report much lower fuel economy running ethanol mixed gasoline fuel.
    Math says, that ethanol fuel is lower in BTU's than gasoline, so taking a fuel like "e10", where it is 10% ethanol, should never come close to netting a 10% reduction in fuel economy, but in the real world, myself, and others, have seen DRASTIC mileage improvements switching from e10 regular gasoline, to premium non ethanol gasoline.

    I realise that with real world driving, there are many variables when driving, like tire pressure, driving habits, etc. But, there seem to be some vehicles, where they seem to get 20-30% better fuel economy, sometimes claiming even higher, on non ethanol fuels.

    I have read that e10 blended fuels are made using a base gasoline 2.5 octane points lower before mixing, so if it's an 89 octane rating, it would be 86.5 octane rated gas blend, with 10% ethanol added on top, to bring it to an 89 octane rating. One could speculate that it is a low grade gasoline mix to begin with, but I'm not sure how accurate that would be.

    Gasoline is 115,000 btu/gal
    Ethanol is 76,100 btu/gal

    How does e10 end up causing such miserable fuel economy in some engines?
    Math says if ethanol had 0 btu, mileage could only be reduced by 10%, yet fuel mileage decreases of far worse seem common.

    Is there anyone that has experience with ethanol mixed fuels, that knows about potential problems, or circumstances that can provide foul burning mixtures, or unintended side effects from these fuel mixtures burning?

    I realize this isn't a really easy question to answer, as gasoline could be mixed with many different chemicals, detergents, etc depending on brand and location.
    I also know that most of you would be able to word this question better than myself, and provide better leg work.

    I calculated 3 of our vehicles miles per gallon, using the odometer, liters added, on the same trips, and premium (ethanol free, less BTU than regular) has netted massive improvements in mileage over the regular gasoline that is e10 mixed. When I say massive, I mean that the extra cost of the premium ethanol free gasoline has more than paid for itself.
  2. jcsd
  3. Apr 6, 2015 #2

    Doug Huffman

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    Gold Member

    Clearly, to me, it is not a simple linear relationship. Ethanol doped gasoline is advertised as offering many hypothetical benefits, none of higher mileage. I just bought my third diesel powered passenger vehicle.
  4. Apr 6, 2015 #3
    I have a diesel truck, and I love it, but it is a popular brand, so if it breaks, parts are easy to find. Diesel cars and SUV's are not so popular, so I stay away. That, and diesel fuel seems to be so expensive now, that I don't know if there are any savings to be had, after paying the price for the diesel vehicle, along with the added fuel cost. Maybe for a big rig, when they pull almost all of the time.

    I bought myself a 1996 caprice classic to have a car when I want. Wife has a chevy equinox. Both got around 19-20 mpg on regular e10 mixed gas, and after running premium non ethanol gas for a few tanks, shot up by about 5 mpg. This is over many trips, filling the tank to the filler cap, shaking the vehicle around to get any air bubbles out, lol. And using the odometer to track mileage, so fairly accurate.

    We also have a little japanese imported mini truck, and while I track the mileage, it's all stop and go in town driving, so can't be accurately measured. But, it seems to be going significantly longer on a tank as well. I fill it up every week, and it used to be just under half a tank, now it's just over half a tank.

    It seems like premium gasoline is marketed as a premium grade fuel, but with the ethanol blends on the table, premium is needed just to get the advertised performance out of any of these vehicles anymore.
    I wish there was more information floating around about e10 gasoline, and how to modify vehicles to burn it better. Maybe different sensors to fool the computer? Maybe different spark plugs, or different gaps? Timing?
    If the gasoline mixed with ethanol still has a higher BTU rating than the premium ethanol free gas, it stands to reason that it is not being burned very efficiently.
    That is what I'm curious about; the science behind getting an engine to burn ethanol mixed gasoline more efficiently.
    I feel like there has to be a pile of research somewhere.
    I also think that because ethanol fuel is a government funded project, many downfalls of ethanol fuel might be systematically hidden from view, and hard to find.
    Sort of like all the studies done on the emissions systems on new diesels with particulate filters, and how there are many problems known, but they are deemed acceptable, in the name of emission reduction.
  5. Apr 6, 2015 #4
    Ethanol will mix with water, and facilitate the water mixing with the gasoline. A fuel that is manufactured as 85% gasoline and 15% ethanol will absorb water and may wind up being 10% water by the time it gets to your engine, with the gasoline and ethanol portions reduced proportionally. This is worse then it sounds.

    Combustion in an engine does not happen all at once. As a combustion event begins the heat breaks the hydrocarbons up releasing significant quantities of monatomic hydrogen. Monatomic hydrogen has fewer degrees of freedom then more complex molecules, basically it can only move in X,Y and Z directions, all of these translational movements can make it bump into the piston and help push it. A water molecule by contrast can not only move in X, Y, and Z directions, but it can also rotate around 3 different axis, as well have spring-like vibration between the O and H atoms. All of these other ways of moving will absorb energy, but none of them will contribute to pressure. While it is true that the monatomic hydrogen will burn into water later in the combustion process, it will contribute to the pressure in the cylinder and the power developed before it does that.

    In a nut-shell, adding ethanol to gasoline encourages the fuel to absorb water. Water in a combustion chamber will absorb a lot of heat while making relatively little pressure compared to the other substances in the fuel.

    You can design an engine to take advantage of water in the combustion chamber. Water absorbs a lot of heat (as described above). You can use this property to reduce combustion temperatures, thereby allowing you to burn more fuel with more air in each combustion event without thermal damage to the chamber. I.E. you can make more power. To take advantage of this though, you have to be able to control the amount of water entering the combustion chamber. If you design your engine to run on a fuel that is 10% water, then you get fuel that actually IS 85% gasoline / 15% ethanol, you will damage the engine.
  6. Apr 6, 2015 #5


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    Staff: Mentor

    [QUOTE="parkland, post: 5066047, member: 211363"
    It seems like premium gasoline is marketed as a premium grade fuel, but with the ethanol blends on the table, premium is needed just to get the advertised performance out of any of these vehicles anymore.
    Premium gasoline has a higher octane rating, which allows a motor to be designed with higher compression and more spark advance. But using premium in a motor that wasn't designed to take advantage of the higher octane rating doesn't help and may even hurt, as the energy content of the fuel and the octane rating aren't especially correlated and may even be anti-correlated.

    Many modern motors have detonation sensors which allow them to adjust their spark timing and (by varying the valve timing) the dynamic compression so that they can operate as efficiently as the octane rating of the fuel allows. All else being the same, these will deliver better efficiency with higher-octane fuels - but all else is almost never the same. Ethanol is more detonation-resistant than gasoline, so you may be getting more ethanol when you buy higher-octane premium than when you buy regular, and then the increased efficiency of the motor is somewhat offset by the lower energy content of the fuel.

    However, as MrSpeedyBob points out above, the biggest issue with ethanol is its tendency to absorb water.
  7. Apr 7, 2015 #6
    I am hearing what you guys are saying.
    To clear something up, our gas here is ethanol mixed e10 regular 89, ethanol free premium 91, and mid grade is the 2 mixed together.
    I don't have anywhere to try buying regular 89 ethanol free.
    I know there are a lot of people who like premium, or thing their vehicles run better on it, but don't need it, but this is a different case, as the only reason I run premium in everything now, is because it is ethanol free, and giving obvious returns on the cost.
    There are plenty of other better places to discuss which gas belongs in which vehicle, but here, I'd like to leave it at that myself, and others, have had incredible poor fuel mileage running ethanol blended fuel.

    I know some car websites speculate that the ethanol separates, leaving the gas at 86.5 octane, while the ethanol burns separately, and not very well. I personally had thought this was not the case for me, as we go through gas rather quickly, so it shouldn't have time to separate.

    The water in the fuel doesn't seem like a feasible problem, although maybe I'm not thinking right.
    But with the small amount of fuel that is injected, and the even smaller amount of water with it, how would that even change anything, in comparison to the massive amounts of air being ingested by the engine?
    I just don't see how a tiny bit of water could change combustion temperature that much.

    I was really thinking it might have something to do with flame front travel speed, or changing the combustion pressure curve badly enough that the engine becomes much less efficient.
  8. Apr 7, 2015 #7
    You may think it's only a small amount of water in each combustion event, but it adds up.
    Suppose you have a 10 gallon fuel tank that's 10% water, you only have 9 gallons of fuel.
    Not only that, but it is liquid water, when coming out of your tail pipe it will be steam, so your're using some of that 9 gallons to boil water instead of power the vehicle.
  9. Apr 7, 2015 #8
    Air at 90*F can hold 2 lbs of water per 1000 cubic feet of air.
    A 5 liter engine running at 2000 RPM and 1/4 throttle is 1250 cubic feet of air per minute.
    ((2000/2) * .25 * 5)
    I know that my math is very vague, but I don't think water or ethanol in fuel has a snowballs' chance in hell of ever coming close to the moisture in the air already.
    Also, (I can't speak personally on this), but it seems like lots of people are running water injection with positive fuel mileage gains, not negative.

    So I don't think water or moisture going into the intake is the problem, I have a hunch it has more to do with the water being emulsified in the fuel, causing the burn rate to change or something. Then again, I suppose the problem is maybe as was already said; extra moisture being absorbed into the fuel....
    I suppose it's fair to say, that moisture IN the fuel can change the burn properties a lot more drastically than moisture or water in the air around the fuel.
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