I use ethanol in my car, not to heat my home. In my car it is used to push against a piston using pressure created by the combustion process. This pressure, not the heat created, drives the piston and engine. This pressure drives the turbines in my turbochargers. I understand that according to the ideal gas law, pressure and temperature are related, and therefore temperature difference is often used to determine how well energy has been used. If the combustion process with E85 creates more exhaust volume, the pressure can be higher with a lower temperature, and actually do more work, with a lower btu content.

 

interesting point. someone needs to look up the molar volume of the products of gasoline and e85 combustion.

 

I've been reading this question since it was first posted, but I can't seem to come up with an intelligent response. Now I wish I had paid closer attention in science classes.

BTU is primarily a measurement of heat output. However, a gasoline engine is primarily driven by the expansion of burning gases more so than just the heat alone. I guess that is why engine output is measured in HP and not in BTUs.

And since your shiny Lennox is not licensed for street use, it is not rated in HP.

I think the answer is: this is simply an apples to oranges comparison. Unrelated, but fruity nonetheless.

 

I did the chemistry and posted it in some other thread a while back. E85 combustion definitely has more combustion products than gasoline. Let me see if I can find my post.

 

Here is my old post.

Granted, this is for complete combustion, but if my chemistry and math is right, then it helps show what is going on.

Here it is quoted, so you don't have to click on the link:

 

Too much for me lol .

 

i now how to put on my tank and works so fine ...

 

simple exp is it takes more BTU to burn ethanol(e85) alcohol burns quicker with a lower temperature produced, you need more E85 to complete one combustion cycle just because it burns a lot quicker. That's as simple exp as it can get with no formula.

the only reason 85 or any type of alhohol is better is because it burns cooler(less prone to detonation) so you can run more timing and
boost but the downside is you need more of it.

 

If it's all about BTUs per gallon, how does adding water injection give better mileage? Water has zero btus. A comparison of BTU per gallon is not a good indicator of relative mileage.

 

The key point that is missing from this thread is Gay-Lussac's Law which states that pressure is proportional to temperature. The idea that the heat produced by combustion isn't relevant -- only the pressure matters -- ignores that the two are perfectly correlated.

 

A typical low end optimized for gasoline flex fuel car today will typically operate at 80% of distance on a gallon of E85 vs the 73% that the btu content differences would imply. (My small fleet runs at this 80% number). Remember that at the beginning of combustion the beginning temp was lower due to the charge cooling and while the max temp of combustion may be lower -the total temp rise is equal or greater thus expansion may be as great or greater. Also remember that a gas engine is only 25-28% efficient with btu vs work-- the majority is lost thru heat loss to the coolant and exhaust--if your top combustion temp is a little lower- less btu will be lost.

Some FFVs are terrible and operate at 73% of range and yet others can easily operate at 86% of range. A design change can do better yet and still allow for better than 90%-- but it just has not been a priority for automakers to do this.

The EPA did testing a number of years ago and published data with a 19:1 methanol/ethanol test engine- pretty impressive results that would exceed gasoline mileage-- but I doubt that was very "streetable" at that level of compression.

 

GM engineers claim a mid single digit percentage difference in the e85 mileage of their Buick Regal Turbo. They have also said with more work they may be able to match the gas mileage.

 

I tried to reference Gay-Lussac's Law when I mentioned the Ideal Gas Law. Let me ask the question this way:

Is it possible to start at a lower temperature and end at a lower temperature with the same starting pressure and same ending pressure. This way you create the same pressure difference and work done, with a fuel having fewer btus.