Im usually a real laid back person. But on occassion someone gets my attention. Well I did this write up this morning and I figured it would be decent enough to post up here. I'm not entirely sure where it fits the best so I put it in here.

Feel free to comment and update any info as needed.

First thing's first, compression is king of internal combustion engine performance. Compression is what dictates the overall efficiency of the Otto cycle:
Notto = [1-(1/(Cr)^k-1] x 100
This is overall efficiency of the entire system, so compression does have the widest effect on an internal combustion engine. Period. Now one of the greatest limiting factors of compression is the fuel's resistance to detonation, knock, pre-ignition, etc... This is usually all wrapped up by one number. Its rated octane. So as you can see it is a rather important number to consider. And seeing as power production is the entire purpose for the internal combustion engine it is also of great importance to consider.
You referred to the Carnot cycle. Which while a valid cycle for thermodynamics is not normally applied to any actual working model I know. The Rankine cycle is, and usually we try to refer to working medium and use either a gas or vapor cycle.

It is worth saying there are limits to the gains from raising compression. As you continue to increase it you will see less and less returns on the increase.

Now back to the fuels being discussed. First need to update some of the “facts” in this thread to more accurately reflect collected data. I'll use Gasoline, Methanol, and Ethanol for comparison
Gasoline
High heat value:
20,460
Low heat value:
19020
stoichiometric ratio:
14.7:1
Lbs/Gallon:
6.00

Methanol
High heat value:
9,760
Low heat value:
8,580
stoichiometric ratio:
6.4:1
Lbs/Gallon:
7.60

Ethanol
High heat value:
12,780
Low heat value:
11,550
stoichiometric ratio:
9.0:1
Lbs/Gallon:
6.70


Now with respect to the amount of BTU. Yes it is British thermal units. You got that part right. However you seem to have failed to take into account the stoichiometric ratio of the different fuels. If you would have taken that into account you would have discovered that given the same amount of air Ethanol produces 409.74 more btu's in the total heat scale. Using the high heat values for both fuels given above. So Yes ethanol fuels can produce the same amount of heat as gasoline.

This leads me to the next point, mpg. Now while you can get the same amount of heat, to generate the same amount of power, you WILL burn more of your fuel. Meaning your mpg will go down. However, this argument is entirely based of the assumptions that you use the same engine without any modification beyond bigger injectors and fuel pump.
Now if you were to optimize he engine for the alcohol fuel you would start to close the gap on the mpg difference. An engine can be made to run more efficiently on Ethanol. You can increase the compression and run a tighter amount of timing. This is the flip side of the coin. An engine that is made for good performance and reliability with ethanol will barely run, if it can run at all, on pump gas. You would need to run premium for the comparison obviously.

I have no local gas station with prices for e85 to compare, so I have no input on the cost portion of this thread.

Some other things to take into consideration.
1)Look at the weight of the fuels. To run a full tank that lets say holds 12 gallons you would be carrying around 72 lbs of gas. If you filled the same tank with ethanol, you would be carrying 91.2 lbs of alcohol. So be aware that for those concerned with mpg there is a weight penalty for using ethanol.
2)Ethanol is a more stable fuel, less Volatile. Volatility is defined as the tendency of a liquid to evaporate. So it is harder for ethanol to evaporate in air, it takes more ambient heat. Which can be a good thing, or a bad thing. If you are running some sort of forced induction this is a good thing. It will take more heat out of the incoming air charge. IF you live in a cold area though, you most likely will notice it is harder to start your car with ethanol.
3)There is less carbon present in the alcohol fuels, this means that you'll have less carbon build up and far less carbon in your oil. This makes for a rather clean engine.

The only real be all end all for the argument on ethanol eating our fuel lines would be for some one to put some ethanol from a gas station in a fuel line from one of our models, plug it. Let it sit and check it periodically to see how much it softens and if it ever leaks. I don't have a gas station near me with the stuff so I can't run this test but I would love to see the results.

Ethanol, is not pollution free. The main pollutants we look for now are HC's (unburnt hydrocarbons) CO (carbon monoxide) NOx (Oxides of Nitrogen) While I could see the HC go down and the CO would also be reduced, Ethanol still produces the same amount NO x. And thats one of the hard ones on the environment.
From Wikipedia:



NOx is a generic term for the various nitrogen oxides produced during combustion. They are believed to aggravate asthmatic conditions, react with the oxygen in the air to produce ozone, which is also an irritant and eventually form nitric acid when dissolved in water. When dissolved in atmospheric moisture the result can be acid rain which can damage both trees and entire forest ecosystems.
In an internal combustion engine, a mixture of air and fuel is burned. When the mixture is tuned so as to consume every molecule of reactant (in this case fuel and oxygen) it is said to be "running at stoichiometry". When this burns, combustion temperatures reach a high enough level to drive endothermic reactions between nitrogen and oxygen in the flame, yielding various oxides of nitrogen, the results of which can be seen over major cities such as Los Angeles, California in the summer in the form of brown clouds of smog.
Not to be confused with the term NOS which is used to refer to nitrous oxide in the context of its use as a booster for internal combustion engines

If any of this information is found to be wrong I'll gladly be corrected and apologize. However I would ask that you cite some sort of actual reference when you post.

For the record I used:
“Thermodynamics and heat power 7th edition.” By Irving Grant, and Maurice Bluestein
“The Internal combustion Engine in Theory and Practice” volumes 1 & 2 by Charles Fayette Taylor
And of course Wikipedia

Basicly the guy tried to come off as being better than everyone on the boards, cause he thought he knew all about thermodynamics and didn't feel that anyone else ever posted solid info. So I corrected his assumption.