Post in 'The Gear' started by EatenByLimestone, Oct 9, 2013.
This is a new one to me. In what way does the engine "burn ethanol more efficiently" ?
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This is what I do as well. I see a bit of drop in vehicles but not enough to matter and they run it fine. You also get a cleaning effect with it.
In my saws weed eater and boat motor if it will sit in the latter case I buy real gas.
Yes, I think that is the way most people measure mpg... they have a 'full' tank then travel some distance and re-fill the tank. On the re-fill, they record the distance traveled and fuel it took to fill the tank, then divide to get mpg.
Where you start to run into problems - if traveling cross country, you most likely aren't at the exact same pump, so maybe one or the other is more sensitive to exactly when it shuts off. Maybe one gas station has a slight slope so the fuel in the tank seems fuller or emptier, the terrain or weather is rarely the same, (I always get better mpg heading south out of Minnesota in the winter with a load of E10, than I do driving up there on a tank of E0 ... I wonder if it's the 30mph north wind practically pushing me down the highway ...or acting as a barrier when headed north). Maybe the first tank was slightly fuller or emptier than you thought, etc.
The 'fuel measuring' problem is worse with higher mpg readings because it is even more critical to measure fuel volume: Consider:
Motorcycle at 50 mpg with a 5 gallon tank: In a perfect world that would get you 250 miles down the road when the tank goes bone dry. Suppose you start with the tank overfull by just 0.2 gallons...you now go 260 miles when the tank runs out. On the refill, you under fill by just 0.2 gallons because the pump kicks off a bit early. So you 'think' you went 260 miles and put 4.8 gallons went in the tank. So 260mi / 4.8 gal = 54+ mpg. Now you have 'gained' over 4mpg from two tiny errors in measurement of less than a quart.
I could cite papers and go on for days, but the two main effects are:
1) Ethanol has a higher 'latent heat of vaporization' - fancy way to say it absorbs more heat from the intake air and gives you a cooler / denser intake charge. This means more power for the same reason racers ice their intakes/fuel lines, or in a street car, it allows you to go the same speed with slightly less throttle and/or the computer can introduce higher levels of exhaust gas recirculation and/or more advanced ignition timing - further increasing mileage
2) Ethanol burns faster than gasoline. This means once the spark plug fires, cylinder pressure rises faster, generates more torque and the piston has a longer time to extract energy from the combustion mix.
At higher ethanol levels, compression can be increased substantially which is a further benefit to efficiency. There was one outfit a few years ago, 'Ricardo' I believe, who developed a turbocharged engine specifically for E85. I believe it had about the same torque, power and mileage compared to it's diesel counterpart, but the E85 was about $1.50 cheaper per gallon compared to diesel.
I see your point makes total sense. No pumps shut off identical, I agree.
Im aware of that of that but what I'm not seeing is how exactly it relates to efficiency.
What you are describing, combined with the lower stoichiometric A/F ratio for ethanol (9:1) vs. gas (14.7:1) is why you can cram more fuel into the same volume of air and make more power from the same displacement on alcohol than you can with gas. But you are burning a lot more fuel to do it.
I think of efficiency as the % of energy in the fuel that turns into energy driving the crank - i.e. BSFC. Your alcohol engine is burning more fuel and making more HP, sure - but it isnt turning a larger % of the energy in the fuel into energy turning the crank. What Ive read is that in tests the difference in BSFC between E0 and E10 is within the noise but as you go up from there BSFC steadily drops for higher ethanol blends.
I fill the gas tank on the bike until it won't take any more. I get as much in there as I can. Travelling across the country in states & on roads that I'm unfamiliar with, I may need as much as possible. I know that I don't go to the same pump at each fill up, but I don't rely on the pump to stop automatically, either...Cite your chemical engineering skills all you want. I know what I know. My eyes tell me when I can't put more in the tank... I'm saying I got way better mileage with the E-Free stuff. Period. I wasn't alone. Everybody else on the trip (7 bikes) had nearly identical results. On pure gasoline, our Harleys get better mileage. YMMV
Yes, I think you have it, the difference is the AFR is related to the fuel by chemical composition. It's the same for a given fuel regardless of what engine the fuel is in. The BSFC can and does change even though you are on the same fuel. ie - consider a low compression ratio engine and a high compression ratio engine - both on the same fuel. The high CR engine extracts energy more efficiently so has a lower BSFC. Really, BSFC even changes as you drive down the road...open the throttle, that removes intake restrictions, lets more air into the cylinder and raises the compression pressure at which the fuel burns...suddenly you get a better BSFC. At closed throttle, the engine may be operating at 22" vacuum, very little air in the cylinders and bad BSFC.
The same can be true if the fuel changes and the engine stays the same. This is very similar to the way a stove will burn hotter and put out more heat on dry wood compared to wet. (and similar to ethanol, the same way you will never convince some old timers burning dry wood is the way to go when they can smoulder a load of wet wood for 3 days straight). Both logs may have the same energy, but the dry wood burns in a way where the stove can extract more usable heat into the room.
Overall, I'm not saying you use less fuel, instead - of the volume of fuel you do use, more ethanol is converted to usable energy compared to gasoline. As you say, the E0/E10 is generally 'down in the noise'. It's 3% less btu 'on paper', but 1% or so is offset by added efficiency of combustion, 'straight gasoline' probably varies 5% in btu depending on seasons and location, and you need pretty top notch laboratory equipment to get repeatability of fuel efficiency measurements of 1-2%...there is just so much to measure...fuel density and temperature, fuel flow, fuel BTU content, engine power output, engine temperature, atmospheric pressure, temperature and humidity...all of these and probably 1/2 dozen more all work to change fuel efficiency - and that is just for steady state running!
just like running your engine on propain will work, ethanol works but is less dense per pound/gal, it gives less energy in the same engine.
an engine designed to run at higher compression specifically for ethanol will work fine, look at all the race cars that run on ethanol based race fuels (methane). you would ruin them with regular gasoline.
if you want ethanol free gas, your local airport sells it, 100LL. It does have lead in it though, it will ruin your catalytic converter (plug it up solid) and electronic emissions sensors on a car, but in small engines, boats snomobiles, motorcycles, etc. racers love it. I run it in my chainsaws, mowers, generator.
it is stable and doesn't gum up.
The methane derived fuel used in racing (drag racing and Indy cars pre 2006, etc) is methanol, not ethanol. Methanol has even less energy density than ethanol but runs richer Even than ethanol so is at least if not more prowerful, at the expense of fuel economy. Methanol is also the primary ingredient of model engine fuel if you have ever flown an rc plane or 'gas' rc car or had one of those little Cox engines you know its distinctive smell.
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