The Chemistry of Combustion & Use of Oxygenated Fuel (Gas vs Alcohol)

[Devil69]

Hello Everyone,

        This is my first post and I see that I have already mistakenly found out that by hitting tab you can accidentally post what you're typing too soon, and that apparently you cannot delete a post you just put up by mistake.  I would like to reply to Autoholic's post regarding his question of the difference between using gas vs alcohol in his topic "Fuel Related Performance" http://fepower.net/simplemachinesforum/index.php?topic=3477.0.  I figured that I would make my response its own topic with its own title so that this might be easier to other users in the future to search for this information again.

        In order to obtain my final credit hour during my senior year at University of Illinois in Mechanical Engineering, I did an independent study for one of my professors who taught the engineering of off-road vehicles.  The topic of my study was the forced induction of internal combustion engines.  In that paper I wrote about the chemistry of combustion and use of oxygenated fuels, amongst other topics.  I figured that I would post the relevant pages here for others to read in case people found this useful.

        I have to admit after re-reading it, that I should make the following qualifying statements:

1.    The intended audience for this paper was my college professor, and as such it is somewhat heavy on the technical side, and some elements that are referenced in the paper are expected to have been known ahead of time and are not explained (for example what the 'lower heating value' of a fuel means).  I will try to answer any questions regarding such things if anyone would like.

2.    The analysis I conducted is based purely in the theoretical, mathematical, and scientific realm, and definitely not from a position of experience or measured data on a dyno for instance.  I believe that Autoholic is more interested in that kind of data to support or contradict his own analysis (correct me if I am wrong here with that assumption).

With that here are the images of the pages (I have never imbedded images before so if it didn't work please let me know).



After reading this first page I figure I will add a few statements that may make the information more understandable or perhaps more relatable, that some readers may have questions on.
1.        Gasoline (as well as other fuels like Diesel, kerosene, natural gas and propane) consists entirely of molecules that only contain hydrogen and carbon atoms, hence the term 'hydrocarbon fuel'.  Since the amount of carbon and hydrogen atoms differs based on the fuel, a term is used to encompass all the fuels that fall into this category which is a 'general hydrocarbon' and is shown as a molecule with 'a' number of carbon atoms and 'b' number of hydrogen atoms: C_aH_b

2.        For this analysis 'gasoline' that you would purchase at the pump is considered a 'practical fuel' which has too much variation for these equations to hold valid all the time.  As such the practical fuel is reasonably approximated by a particular 'Pure Hydrocarbon' called Iso-Octane which has the molecular structure of 8 carbon atoms and 18 hydrogen atoms: C₈H₁₈

3.        The stoichiometric (ideal combustion) equation (1.2) for Iso-Octane then becomes the following:



4.        The stoichiometric air to fuel ratio of Iso-Octane then becomes the following:



This shows that 15.0666 moles of air is required to stoichiometrically combust exactly 1 mole of Iso-Octane.  This is slightly leaner than that of actual Gasoline where other sources reference the stoichiometric A/F ratio as around 14.7 to 1.



The important information in page 2 here (numbered three in the actual document) is in the top and lower paragraphs.  Understanding the technical definition of 'lean' vs 'rich' mixture is somewhat useful, but the really important information is in the concluding paragraph.  Here I basically sum up that in a closed volume with a relatively fixed ratio of fuel to air (meaning that you can't stray too far from the calculated stoichiometric ratio and still achieve combustion), that the limiting factor on the amount of fuel that can be burned (and thus work and also power that can be generated) in that closed volume is the total amount of oxygen available.  Once this is established a list is given of the three available ways to increase the amount of oxygen in the closed volume (ie. the cylinder).

The entire preceding two page analysis basically boils down to stating a very simple conclusion really, which is that you can't burn as much fuel as you want in the cylinder to increase power, and that the amount of fuel you can burn is limited entirely by how much oxygen you can get in there.


Now moving onto the comparison to alcohol fuels...



This first page is useful in that it defines what an alcoholic (oxygenated) fuel is, and how to go about calculating the stoichiometric air to fuel ratio depending on the chemical make up of that fuel.  The importance of that information though is in the table on the next page...



In table 3.4 I have simply taken the stoichiometric A/F ratio of Iso-Octane (gasoline) and divided it by the stoichiometric A/F ratio of the listed alcoholic fuels.  In comparing gas to nitro-methane for example I am showing that for the same mass of air being burned (lets say 1 slug **yes I just used the slug mass**....ok fine lets say 1 pound) that you can burn 8.91 times more nitro than gas.  This DOES NOT mean however, that by switching to Nitro that you can expect to produce 8.91 times more horsepower.

Why?

Well if you look back at equation 3.2 you see that the in the combustion of these fuels that you not only get all these different molecular products like carbon dioxide or carbon monoxide etc. but that what makes the power is the ENERGY released from the combustion, and the amount of energy released is not equal between different fuels being burned.

So the actual comparison of the power difference has to incorporate both the energy release as well as the A/F ratio of the fuels being compared...



The conclusion seen here is that between gas, Diesel, Ethanol, and Methanol, that there is actually pretty little difference in how much energy can be released by the same mass of air, with the exception of Nitro.  Again it isn't 8.91 times more powerful than gas, but actually 2.32 times more powerful.

In fact Jay stated in the topic created by Autoholic, that his experience shows that switching to methanol will net you approximately a 5% increase in power over gas (sorry but I haven't figured out how to quote yet), and based on the equations for the energy released by these different fuels the methanol would net a 5.5% increase in power compared to gas:

3.103/2.940 = 1.05544 = 5.544% increase.

This of course is only comparing if you were to take your existing engine and did not change any parameter to it (like compression ratio for instance) other than what fuel you burn in it and this is the approximate power gain you will see.  The potential however is there to gain much more than that simple 5% from methanol for instance due to the fact that it can cool the inlet charge much more than gas can...



This means that by using an alcoholic fuel, I not only can get an immediate power increase from the energy release of the fuel itself, but I could also now allow myself to increase the compression ratio, or increase the boost pressure without pre-igniting the fuel. 

I hope that my edits over the course of the day has helped some readers understand some of this a little better.

Thank you for welcoming me into your forum,
Calvin

[ChiefDanGeorge]

That was a riveting dissertation! Ah now there is some text there!

[ChiefDanGeorge]

Nitromethane has some serious energy release!

[cjshaker]

I think I'll go have a beer now and pet my cat.

[fekbmax]

Make that two beers Mr Smith, .

[machoneman]

Jay, congrats! Your site is helping raise a couple of real automotive Rhodes Scholars, no foolin'!

Bet you never thought this would be a welcome outcome when you started this site.

[Devil69]

I don't know if edits automatically alert people to changes in the thread on the forum or not, but for those that may have read this topic earlier, I have added more information to try to help folks understand what I was saying a little better.

Does anyone on the forum (like Jay for instance) have dyno comparisons on the same engine using gas vs alcohol to help Autoholic with his research?  To see how much of the theory holds to the reality?  That kind of info would be really interesting to see.

Calvin

[Autoholic]

Calvin, the purpose behind my research falls in line with what you are saying but for a different purpose. The goal of my research is to dispel the myth that the reason why alcohol fuels have a lower fuel economy is due to a lower energy density. Those that have really looked at fuels know this but the myth is out there. So I have to take the chemical engineering out of the conversation, without actually removing it, so that anyone can understand it. I would still look at the energy released, the same way you actually did. Energy density per liter of air.

I won't be diving into the molecular make up of the fuels as this isn't necessary and you don't really use iso-octane fuel that often. I'll be looking at petrol, E10, E85, methanol and nitro-meth. Also, I don't want to cause any readers to shy away from reading it because they start seeing complex reaction equations and freak out. So I'm pretty much doing what you did, but doing it in a way that is easy to read and understand for non-engineers. Which can be difficult to do sometimes when you're an engineer, knowing when you've gone above your audience's head is hard to gauge.

[Devil69]

I'm pretty much doing what you did, but doing it in a way that is easy to read and understand for non-engineers. Which can be difficult to do sometimes when you're an engineer, knowing when you've gone above your audience's head is hard to gauge.

I definitely agree that keeping the content easy to understand for the intended audience is paramount and that is why I made sure to include my qualifying statements when I first made my post, and then added further explanations when I got time during the day (although I freely admit that even with the added explanations the content may not be any more relatable).  I had actually initially thought about rewriting the whole paper recognizing that this forum would probably look at it as too abstract, but I decided that that would have taken me quite a bit of time to do right.  Perhaps you can use my paper as a good reference for how not to write your own.

The goal of my research is to dispel the myth that the reason why alcohol fuels have a lower fuel economy is due to a lower energy density.

I have to apologize that I must have overlooked that that was your goal, and I hope my reply hasn't derailed the focus of your questions to the forum.  I do have a question though just for clarification, in that you are not trying to make the argument that alcohol fuels are more fuel efficient that what is popularly believed, but you are really trying to make the argument that although it is given that alcohol fuels are lower fuel economy, that the source of that lower fuel economy is not solely because of the lower energy density, and that there are other factors at play that account for the lower fuel economy.  Do I have that right?

Calvin

[Drew Pojedinec]

On another note, yall should leave diesel out of this, it operates in a totally different way then the rest of the fuels listed

[Qikbbstang]

Drew you're prejudice! LOL
"..... leave diesel out of this, it operates in a totally different way then the rest of the fuels listed"

 Other than compression heat in a diesel leading to ignition rather than a spark I can't see any significant differences in the combustion of any fuel in an internal combustion engine or what am I missing?.............I throw out they toss in propane and also can start adding up the boost by sequential turbo's as power adders to diesels........more oxygen professor!

[Drew Pojedinec]

Prejudice?  nah, you can't compare an engine that lacks a throttle and runs lean by nature to one that does not.
If a diesel isn't smoking it's running lean enough to melt most gasoline engines.

[Autoholic]

Calvin,

The reason why an engine consumes more fuel when running ethanol is only due to the required AFR for a given lambda. Energy density has nothing to do with it, yet people often say that the reason why the engine requires more fuel is due to a lower energy density. Energy density is how much energy you'll get from a given amount of fuel, the exact amount of fuel present though has to do with the AFR and displacement.

[Devil69]

Calvin,

The reason why an engine consumes more fuel when running ethanol is only due to the required AFR for a given lambda. Energy density has nothing to do with it, yet people often say that the reason why the engine requires more fuel is due to a lower energy density. Energy density is how much energy you'll get from a given amount of fuel, the exact amount of fuel present though has to do with the AFR and displacement.

Exactly.  The A/F ratio of ethanol for instance is 8.966 to 1, whereas the A/F ratio of gas is often referenced as 14.7 to 1.  This would mean that for the same volume of air you would have to burn about 64 percent more fuel using pure ethanol (14.7/8.966 = 1.6395 = ~64% increase).  For an engine of the same displacement, converting from gas to ethanol would net you approximately a 2 percent power increase but also the 64% increase in fuel useage.  For an engine of the same power output, you could reduce the overall displacement by that same 2%, but you would still be burning 62% more fuel with ethanol.

With E85, where 15 percent is gas and 85 percent is ethanol, the A/F ratio would become roughly 9.8 to 1 (.15 x 14.7 + .85 x 8966 = 9.826).  For this fuel you could expect to burn roughly 50% more fuel than regular gas with an engine of the same displacement (14.7/9.826 = 1.496 = ~ 50 percent increase).

Now if the energy density of ethanol was much much greater than gasoline (at least 64% greater), then you could design an overall smaller displacement engine that would burn less fuel than gas and still produce the same or greater power, in spite of the 'richer' A/F ratio.  But that isn't the case.