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The Hot and Cold of E85 vs Gas: A Theoretical Analysis
The general consensus is ethanol burns “cooler” than gas. Technically that is correct but what does that really mean? There’s a lot more to it and I’m throwing out my analysis (btw, this is not copied from any textbook or external resource--just my own back-of-the-envelope calcs so I make no assurances).
The burning question in my mind has been how does cooler burning correlate to cylinder temps and power? That is, how can a fuel that burns “cooler” generate more power? How much “cooler” does it burn? Are the EGTs cooler? To explain this, I did some calcs--the results seem reasonable so I thought I would post them here with explanation.
If you take one mole (6.02 x 10^23 molecules) of octane (gasoline) and one mole of ethanol and combust them in air, octane would produce 5460 KJ of heat compared to 1368 KJ for ethanol (eqs 1 and 2). The heat given off from the combustion reaction can be experimentally measured to a very precise number. In this case, more heat is evolved from combusting one mole of octane than one mole of ethanol, in fact, about 4 times as much.
C8H18 + 12.5O2 → 8CO2 + 9H2O DH (heat released) = -5460 KJ/mol [eq 1]
C2H5OH + 3O2 → 2CO2 + 3H2O DH = -1368 KJ/mol [eq 2]
(note: negative sign just means heat is released; more negative = more heat)
So in this sense, ethanol burns cooler.
Now, let’s take a look at what happens inside the cylinder when the oxygen content is the limiting factor. For this, we need to look at the combustion equations where the oxygen content is set as the limiting reagent (i.e. 12.5 moles). For every 12.5 moles of O2, you can burn 1 mole of octane and 4.16 moles of ethanol. So you can see in this case, you actually get more heat from ethanol per unit oxygen (approximately 4.4% more) [eq 3].
So how does ethanol burn cooler, yet produce more power? Well, power is a result of cylinder pressure. For that, we need to take into consideration the total number of moles of combustion products from octane and ethanol. For octane, you get a total of 17 moles of combustion products (eq. 1). For ethanol, you get a total of approximately 20 moles of combustion products (eq 4). That corresponds to approximately 18% more moles of exhaust products from burning ethanol at the same thermal energy level as octane.
Since P=nRT/V, pressure is proportional to not only T (temperature) but also n (# of moles of total exhaust products) at constant V. Well, if EtOH produces 18% more moles of gas at the same thermal energy level as octane, then the temp can drop by 18% to produce the same cylinder pressure. Hence, ethanol can burn cooler to give the same pressure as burning octane because it produces a greater amount of combustion products.
Here’s the rub--everyone wants max power, so you end up burning as much ethanol as there is O2 in the cylinder which thereby produces 4.4% more heat than octane with even greater cylinder pressures. So while ethanol can burn cooler and produce more power, in reality, we end up burning as much ethanol as possible to get max power. This results in higher cylinder temps than gas (but more power too ).
TTdude, I feel like im sitting in my college chemistry class. Rep points !! You and Avogadro are the mannnn. Have you been boosting your e85 monster yet?
Thanks guys. I thought it was interesting since we all know E85 produces more power but it also produces more heat when you max it out, at least theoretically. One would need to measure EGT at same hp level for E85 set-up vs gas to verify experimentally keeping all other variables the same.
Things came to a stall but I'm back on track now. Getting my crankcase tapped for larger studs at the moment. Thinking about going 3.8 too. Will revive my build thread once I start making more progress.
Methanol produces a lot less energy per mole than octane and 47% less energy than ethanol.
So in this sense, methanol burns way cooler.
However, if you equate the above combustion reactions with the same amount of O2 present in the cylinder regardless of what fuel you use the situation changes:
You get about an 11% increase in thermal energy. You also get the added advantage of the greater number of total combustion products (25 vs 17) which will further increase cylinder pressures and produce more power.
If we reduce the thermal energy of eq 3 by 90.7% (which now equals the thermal energy from combusting octane) then eq 3 becomes:
So for methanol, you get 22.6 (=7.5+15.1) moles of combustion products compared to 17 from combusting octane. This corresponds to a 22.6/17 = 32.9% increase in cylinder pressure at the same thermal energy. If you max it out (eq. 3) and use up all the 12.5O2, you would get a 10.7% (=6047/5460) increase thermal energy plus 47% (=25/17) more of combustion products. This would lead to a minimum of 58% more power.
Interestingly, a molecule nitromethane has about the same enthalpic energy as a molecule of methanol. Yet why does it produce so much more power? See below.
As you can see, for an equal amount of oxygen (12.5 moles) nitromethane produces 2.17 times more heat than octane.
Importantly, you also get 50 moles of combustion products (16.7CO2+25H2O+8.3N2) from nitromethane compared to 17 from octane. That’s a factor of 2.9. So the combustion products are important as the more combustion products you get, the greater the cylinder pressure will be.
Thus, the higher thermal energy (217%) plus the additional combustion products (290%) affords a theoretical 5.1x the power of octane. Now you see how Top Fuel cars can achieve 7000 hp. Note: this does not take into any cooling effects of the fuel as also would be seen with ethanol and methanol which would provide even more power (theoretically).
i find this thread interesting as i am looking more in to E85 fuels....
I do find the conclusion contradicting to what i have read elsewhere... you mention this at the bottom of your post
"Here’s the rub--everyone wants max power, so you end up burning as much ethanol as there is O2 in the cylinder which thereby produces 4.4% more heat than octane with even greater cylinder pressures. So while ethanol can burn cooler and produce more power, in reality, we end up burning as much ethanol as possible to get max power. This results in higher cylinder temps than gas (but more power too )."
I have read elsewhere that differs somewhat in regards to Heat and the effects. and i will copy the article i read below.
The Four Primary Characteristics of Racing Fuels
OCTANE is a measure of a fuel’s resistance to pre-ignition, pinging, and detonation. There are three octane ratings for motor fuels; Research Octane Number (RON), Motor Octane Number (MON), and the average of the two (R+M/2). Of these three ratings, MON is the most useful to race engine builders because it is tested under load and higher RPM conditions. High MON ratings allow the use of higher compression ratios and more advanced spark timing. However, there are other fuel characteristics that influence the ability of a particular fuel to resist knocking.
BURNING RATE is the speed at which a fuel burns and releases its heat energy. At higher RPM’s there is less time for fuel to burn, so racing fuels tend to work better if they have a rapid burning rate. If a fuel can be almost completely burned by the time the crankshaft is 20 degrees after TDC (Top Dead Center) on the engine power stroke, peak horsepower and engine efficiency are realized.
LATENT HEAT of VAPORIZATION is the ability of a fuel to cool the intake charge and the combustion chamber. It is often expressed as BT U’s/gal (British Thermal Units per gallon). A fuel with a high latent heat value will do a better job of removing heat. This makes the intake charge more dense and packs more energy per volume into the engine. The cooling effect also helps control detonation and reduces the te mperatures of engine and oiling system components.
ENERGY VALUE is an expression of the total heat energy contained in a given amount of a fuel, expressed as British Thermal Units per pound (BTU/lb). The total amount of heat energy that is available to make horsepower depends on the Net Energy Value of the fuel. This is found by taking the raw energy value of the fuel and then multiplying that by the amount of fuel that can be burned. The ideal air/fuel ratio for a fuel is called its stoichiometric. The lower the stoichiometric, the more fuel is burned and the more power can be
Fuel Characteristic Rankings
The chart below reflects how various fuels rank in each of the four previously discussed categories. Remember that the last column, Net Energy Value is the most important with regard to power making capabilities:
Advantages of Ethanol-Enriched Racing Fuels
HORSEPOWER: Because Ethanol contains oxygen, it has a very low power stoichiometric when compared to gasoline fuels (6.5 compared to 12.5). Ethanol must be run at much richer mixtures than gasoline, more than offsetting the lower energy per unit volume. The net energy released per cycle is higher and this results in more horsepower. For example, if a pound of gasoline is burned at its preferred max power air fuel mixture of 12.5/1, it will release approximately 19,000 BTU’s of energy, where ethanol run at its preferred power stoichiometric of 6.5/1 will release approximately 24,400 BTU’s. By comparison, methanol releases slightly more, about 27,650 BTU’s. The more ethanol there is in gasoline, the more powerful it is as a motor fuel. Typically, you can expect at least 5% more horsepower at the rear wheels of a vehicle running on E-85 than one burning gasoline only.
INCREASED ENGINE LIFE:
Ethanol has a very high MON octane rating, allowing engine builders to run higher compression ratios without fears of destructive detonation. It also has a very high Latent Heat of Vaporization, so the engine is cooled far better than one running on gasoline. This lowers bottom end and oiling system temperatures substantially.
REDUCED EMISSIONS:
Although reducing emissions usually will not directly affect the on- track performance of a race car, engine parts like pistons and valves tend to stay cleaner. More importantly, there are serious health conc erns with many of the octane boosting additives that must be added to racing gasoline in order for them to be compatible with higher compression racing engines. Ethanol fumes are also non-toxic.
FIRE SAFETY:
While many factors enter into the causes of fuel fires, ethanol does enjoy certain advantages over gasoline fuels. The flame temperature for ethanol is lower, 1920°C compared to 2030°C for gasoline. Because it contains less heat energy than gasoline per volume, less total heat is released with ethanol when burning a given volume of fuel. The auto ignition temperature of ethanol is lower than that of many gasoline, 360° compared to up to 460° for gasoline. Liquid gasoline has vapors present at all temperatures, down to or exceeding -40° C. Ethanol has little or no vapor pressure below 13° C. While pure ethanol fires have little or no color, even slight amounts of gasoline added to denature (poison) the mix create bright colors and dark smoke.
Why Not Methanol?
ETHANOL IS LESS CORROSIVE:
Ethanol is less corrosive to most fuel delivery system components than methanol. This is due in part to the higher oxygen content of methanol. However, certain materials that may be used as sealers or glue in methanol-compatible fuel cells may not be compatible with ethanol. There are solutions to this problem that are being developed for the IRL (Indy Racing League) series that will be running pure ethanol in the 2007 season.
ETHANOL IS NON-TOXIC:
Ethanol is really grain alcohol—the kind present in all alcoholic beverages. In order to be legally transported, it must contain a “denaturing agent” (poison) which is usually ordinary gasoline. By comparison, methanol (wood alcohol) is toxic and is harmful or fatal if swallowed. When blended with gasoline, methanol releases formaldehyde in passenger cars until the catalytic converter warms up. No auto manufacturer recommends concentrations of methanol higher than 5% in their cars. However, all car manufacturers have approved ethanol-enhanced gasoline, up to 85% ethanol content in “Flexible Fuel Vehicles”.
What is Required to Run Ethano l Fuels in a Racing Application? COMPATIBLE FUEL SYSTEM COMPONENTS: Because ethanol contains oxygen, it can form corrosive agents. Any water that enters the system can promote the formation of formic acid. Although this process takes a significant amount of time before damage occurs, the tank, pump, and lines should be either stainless steel or coated with a plastic material that is ethanol-compatible. All natural rubber parts that could be in contact with ethanol must also be replaced with synthetic and other materials. These are all readily available from manufacturers of racing carburetors and racing fuel system components. In some cases it may be desirable to modify the carburetor so that it can handle the required increased liquid fuel flow when converting from a gasoline application. When considering fuel pump compatibility, it must also be understood that gasoline is an insulator but ethanol does conduct electricity. This should not be an issue for race cars, as they nearly always have a pump that is mounted outside of the tank. However, the pump must be internally ethanol/methanol compatible and must be able to keep up with the increased fuel delivery rates. For information on mechanical pumps, you will have to contact the manufacturer to be sure the pump you plan to run is alcohol/ethanol compatible.
MANAGING FUEL VAPOR PRESSURE:
Ethanol does contribute to increased vapor pressures when the underhood temperatures ar e very high. This issue should be easily resolved if it becomes a problem by adding a cooler to the fuel lines.
STORING ETHANOL-ENHANCED FUELS:
All alcohols attract water and should therefore be stored in UL-approved fuel containers that are ethanol-compatible and that limit exposure to outside air. However, it takes a very large amount of water in the fuel (about 4 tsp. per gallon) before “phase separation” between the mixed gasoline and ethanol occur. Water in smaller amounts than that will result in phase separation of pure gasoline.
I know its gives us a lot more power but does it lower IAT'S?
I'm no scientist but i'm not sure how the fuel that's being injected into the cylinder can have an affect on intake air temp...considering they don't meet until there is an explosion.
but i can tell you (not that everyone doesn't already know this) that one can run substantially more boost on ethanol. So regardless of whether it's on IAT's or a higher octane rating or magical powers it's a proven fact it's the best bang for the buck race fuel on the planet.
I'm no scientist but i'm not sure how the fuel that's being injected into the cylinder can have an affect on intake air temp...considering they don't meet until there is an explosion. but i can tell you (not that everyone doesn't already know this) that one can run substantially more boost on ethanol. So regardless of whether it's on IAT's or a higher octane rating or magical powers it's a proven fact it's the best bang for the buck race fuel on the planet.
Its not that you can run more boost its that you can more timing without detonation that allows for much more fuel and therefore more power
05-26-2012, 11:28 AM
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You make me wish I paid more attention in Chemistry class, though I am at least pretty happy I remember what a mole was 
!!! amazing information 
