Octane vs Cetane - Page 2 - PeachParts Mercedes-Benz Forum

Elktonjohn · #16 04-23-2006, 02:51 AM

My attempt at a molecular diagram sucked but I hope it suffices for a starting point...sorry guys for the bad art

#17 04-23-2006, 03:32 AM

Lower Cetane has higher BTU's per gallon but higher cetane ignites easier.

In very cold areas like Alaska and upper Canada, the Cetane rating at the pumps can get into the 55-60+ range.

From http://www.musclecarclub.com/library/tech/gasoline.shtml
"Cetane Rating (Ether)

The delay between the time the fuel is injected into the cylinder and ignition is expressed as a cetane number. Usually, this is between 30 and 60. Fuels that ignite rapidly have high cetane ratings, while slow-to-ignite fuels have lower cetane ratings. A fuel with a better ignition quality would help combustion more than a lower cetane fuel during starting and idling conditions when compression temperatures are cooler. Ether, with a very high cetane rating of 85-96, is often used for starting diesel engines in cold weather. The lower the temperature of the surrounding air, the greater the need for fuel that will ignite rapidly. When the cetane number is too low, it may cause difficult starting, engine knock, and puffs of white exhaust smoke, especially during engine warm-up and light load operation. If these conditions continue, harmful engine deposits will accumulate in the combustion chamber.

Pressurized cans of starter fluid are available in emergencies, but are not desirable, because they tend to dry out the cylinders, and are dangerous if used improperly. There are also liquid forms of starter fluid available which can be added to the gasoline."

pmari · #18 04-23-2006, 01:39 PM

Gasoline and Diesel Fuel - The Basics
http://www.fitchfuelcatalyst.com/techinfo/aboutfuel.html
http://www.faqs.org/faqs/autos/gasoline-faq/part1/

The cetane number measures the ignition quality of a diesel fuel. Has nothing to do with BTU content.
http://www.dodgeram.org/tech/dsl/FAQ/diesel_fuel.htm

Link

I'll try to find my other link for distillation information. This isn't too bad. (OK, hit the links on the last page of this article two of my other saved links are listed)
http://science.howstuffworks.com/oil-refining4.htm

83-240D · #19 04-25-2006, 10:43 AM

Quote:

Originally Posted by andmoon

Anyone know what the equvalent cetane rating of svo would be?

from www.journeytoforever.com

cetane # is on the right.

Oils and esters characteristics
Oils and esters characteristics
Type of Oil Melting Range deg C Iodine
number Cetane
number
Oil / Fat Methyl
Ester Ethyl
Ester
Rapeseed oil, h. eruc. 5 0 -2 97 to 105 55
Rapeseed oil, i. eruc. -5 -10 -12 110 to 115 58
Sunflower oil -18 -12 -14 125 to 135 52
Olive oil -12 -6 -8 77 to 94 60
Soybean oil -12 -10 -12 125 to 140 53
Cotton seed oil 0 -5 -8 100 to 115 55
Corn oil -5 -10 -12 115 to 124 53
Coconut oil 20 to 24 -9 -6 8 to 10 70
Palm kernel oil 20 to 26 -8 -8 12 to 18 70
Palm oil 30 to 38 14 10 44 to 58 65
Palm oleine 20 to 25 5 3 85 to 95 65
Palm stearine 35 to 40 21 18 20 to 45 85
Tallow 35 to 40 16 12 50 to 60 75
Lard 32 to 36 14 10 60 to 70 65

Ralph69220d · #20 04-25-2006, 07:34 PM

Quote:

Originally Posted by Elktonjohn

Man, I would love to see Ralph's description in a molecular schematic. Thanks Ralph69220d...that actually made sense to me...damn miracle. Let me se if I have it right CH4 is easy with Carbon surrounded by 4 Hydrogen atoms but when it gets into the long chains, and am I thinking right, that you get C-C-C-C with each Carbon sharing two others and the rest of the bonding is stated top-wise with Hydrogen except at the end points...
H H H H
H C-C-C-C-C>>>
H H H H

And I guess that my next thing is to ask, with relation to octane, is when it's 87% octane and 13% other...is the 13% other...as in heptane...basically junk...or lower on the distillate scale and similar to cetane? which I see as junk on that scale and right near to fuel oil. I guess I'm trying to understand the distillate scale here. Take a gallon of crude oil, distill it into the various grades and you get...?? What. I assume you get a scale of oils from light to heavy. From jet fuel, kerosene, through high octane gasoline...101-87 octane and here I get lost...is there a source for the distillates out there somewhere? I know it ends with tar but the process is interesting and let's face it, it's sort of guiding the price of gas so has a direct impact on all of our lives. I'd like this thread to continue for a while.

I don't have much know-how regarding heavy, light, sweet crude, etc. But, basically at a gas refinery you will see quite a number of stacks of different heights and diameters. In all that mess the method of making different products from the crude is called "cracking" because larger molecules are split into smaller molecules. On calculating CH compounds I'll try to explain the structure and how to calculate the # of Hydrogens for a pure C&H chain.
CnH((n*2)+2)
Where n is a counting number of the set {1,2,3...}
So, if we look at octane with 8 Carbons our n=8
Therefore the number of Hydrogens is ((8*2)+2=18 Hydrogens
Number the Carbons from left to right. The first C has only one carbon next to it so it can either form a double bond with that C, plus 2 H's; but, in practice each carbon forms only a single covalent bond with another C. So, #1 C has a single bond with #2 C and 3 H's arranged around it at very specific angles as the H's single electron repels the single electron of the other H's (so we have no H2 or H3 bonds, as an aside). Moving right to C #2, that C forms a single covalent bond with the C to the left and right of it making 2 bonds, with the other 2 bonds being the two H's around it, again making four bonds to C #2. All the rest of the C's in the chain are just like C #2, except it's H's angular relations to the C are slightly modified by the H's of those C's to it's left and right. I say "all the rest", but in reality only to the last C which has only one C to bind to (the next to the last C in the chain) so it binds 3 H's to bring it's atomic bond total to four. Let me see if I can roughly draw it's structure (to a degree because of the angular bonding).
Let's use octane, so we have 8 carbons:
edit: the structure didn't print as I typed it, obviously there the spacing doesn't appear as I see it when I type, though you obviously already have the idea straight in your head.
C8H18: octane

H H H H H H H H
| | | | | | | |
H-C-C-C-C-C-C-C-C-H
| | | | | | | |
H H H H H H H H

So, now you see that the first & last C are bonded with one C and 3 H's and all the rest are bonded to two C's & 2 H's, never violating a basic tenet of organic chemistry (basically defined as the chemistry or carbon compounds, though other's might argue against this with good reason.
Now, I personally don't know, nor have I researched it that the number of the octane rating of gasoline is that number, say 87%, plus 13% of something else. I would say if a gas pump says 87 octane I personally would be in doubt that this means there is 87% octane by volume. As I said before, gas pumps might say octane=87, but it will also say something like (this isn't it, but I don't recall the exact formula: 87 octane computed by the RM/2 method. I have no idea what R or M represents, but when it says it is computed by a certain method, that means there is more than one method. Perhaps one method is used as a standard and by law must be used, but I don't know. But, consider this: Where does the energy come from by heating octane in the presence of oxygen (O2, molecular oxygen, not O, atomic oxygen). I won't attempt a guess at the exact chemistry, but there is potential energy in non-ignited octane. This energy is released by the breaking of the bonds in octane. It is the breaking of these bonds in a manner which is known as an exothermic reaction, meaning heat is released when the bond(s) break. There are other types of chemical reactions known as endothermic reactions that absorb heat, as a aside.
Nitrogen, the next element in the periodic table likes 3 bonds, oxygen the next element in the periodic table likes 2 bonds. Carbon, nitrogen and oxygen are the main elements in organic chemistry, though it is by no means limited to these three elements.
There are obviously a host of exceptions to C having 4 bonds; but other compounds follow the rule such as CO2.

O
||
C
||
O
Here we have CO2. As I said, C likes 4 bonds, O likes 2 bonds.
So, we have a double covalent bond between C & each O. As you can see, each O has the two bonds it likes and C has it's preferred four.
But, what about carbon monoxide?
CO: Here we can't satisfy both conditions simultaneously. As an aside, we all know that CO, say from a running car in the garage, or a fault gas heater will release CO, which is deadly if one breathes enough of it. In the lungs the red blood cells each have a heme group in the center of a large protein contained in the red blood cell. We inhale O2, CO2 (the basic waste product of cellular respiration) readily comes off the Fe++ atom in the heme protein and O2 readily binds to the same group where the CO2 once was. Suffice it to say that the molecular kinetics are such that CO2 unbinds from the Fe++ with a high rate constant and O2 binds the heme iron with a faster and longer lasting binding constants, so we don't just keep rebreathing CO2 and die as CO2 is a rather stable molecule and won't dissociate to C and O2, which would resupply us with O2. So, why does CO kill us? Because the on-rate binding constant of CO is much greater than O2 and it's off-rate binding constant is much less than that of O2. So, the area in our brainstem which measures CO2 buildup in the blood stream makes us breathe much more deeply and rapidly to get the O2 levels where they should be. But, to the brain stem CO "looks" chemically enough like O2 and binds tightly and quickly that we go to sleep forever.
Anyway, my octane drawing is correct (though it got buthcered when I submitted it, but I still can't represent the angular relations of the hydrogens. So, as we go from methane, ethane, propane, butane, etc. how do we know which has the greatest potential energy (heat) available? Well the longer the carbon chain, the more bonds, the more potential energy.
So, yes, in your drawing you do understand.
One more brief point as regards the explosiveness of an automotive fuel. The faster the reactions take place, faster as in velocity (though the molecule has less total energy), the more explosive the fuel. For example, though C-4 explosive is not a fuel, it's explosiveness comes from the fact that it's burning velocity (spread of the chemical reactions) is in excess of 25000 feet per second. Such as in nitro-methane used in Top-Fuel-Dragsters CH3NO2. Produces god-awful explosive power, yet pound for pound has less energy than gasoline (octane) C8H18. This is because the methane portion of nitromethane (CH3 has it's own nitrous oxide with it, NO2). You typically need about 15 pounds of atmospheric air (~22% O2) to burn one pound of gasoline, but you only need 1.7 pounds of atmospheric air to burn one pound of nitromethane. Then supercharge a nitromethane engine and you get 8000 to 10,000 horsepower out of the 500ci hemis they run in todays dragsters (though they are basically a hair away from hydrolock with nitromethane at the end of the run, about 4.5 seconds). When one of these engines is running full out at 10,000rpm, the supercharger is spinning at 8,000 rpm. A 426 dual quad street hemi from 1966 (about 500hp) doesn't have the power to even spin the supercharger on the nitromethane motor at the full 8,000 rpm. So, that is why I roundabout guess that 87 octane is not 87% octane, but is a formulation that has the equivalent of 87% octane. I should add that nitromethane has a slower burn velocity than octane (unlike the C4 explosive example), that is why you see massive flame from the headers of Top-Fuel-Motors, it is still burning well after the exhaust valve opens, but when you're injecting close to a gallonof nitromethane per second into a 500 ci engine, incredible hp is produced (they generally say six to ten thousand horsepower at peak). So, the 87 octane could be any mix of the various CH chains so that total energy with 1 pound fuel: 15 pounds atmospheric air, produces the energy of 87 octane, though computed by some DOE formula. So I think where the formulaic measure of octane comes from and is necessary is due to the type of crude oil the refinery has to work with and the limits they have on how they're going to crack the crude. Jeez, you asked me the time and I answered by telling you how to build at least part of a watch.

pmari · #21 04-25-2006, 08:14 PM

Errrrr, there is no octane in gasoline and there is no cetane in diesel fuel

http://www.fitchfuelcatalyst.com/techinfo/aboutfuel.html
The octane value of a fuel is an empirical measure of its ability to resist detonation and, to a limited extent, pre-ignition. Technically, octane ratings measure a fuel's ability to resist the spontaneous ignition of un-burnt end-gases under controlled test conditions.

Cetane Number

The cetane number is a measure of a fuel's tendency to ignite in the absence of an ignition source (flame or spark). In other words, a high cetane number is assigned to a fuel having a quick ignition, and a low number to a fuel that easily ignites sluggishly. This index is essentially the opposite of the octane rating, where a high octane fuel resists spontaneous ignition, and therefore does not tend to "knock." The cetane number of 100 was arbitrarily assigned to "cetane" or n-hexadecane, a straight-chain aliphatic chemical compound that burns very well, in the single-cylinder test diesel engine. Originally, the poorly burning 1-methylnaphthalene was assigned a cetane number of zero for testing, but in 1962 it was replaced for testing purposes by heptamethylnonane (also called "isocetane"), a highly branched aliphatic hydrocarbon that has a cetane number of fifteen. Both of these compounds belong to the class sometimes called "paraffins."

In general, normal (straight-chain) saturated hydrocarbons have high cetane numbers, and these numbers increase with the size (molecular weight, or number of carbons) of the compound. Cetane number decreases with "branching" of the chain, or with closing the chain into one or more rings. This trend was noted above with the dramatic reduction in cetane number from 100 to 15 by branching alone, on two isomeric molecules (that is, having the same number of carbons and hydrogens). Compounds with aromatic rings have relatively low cetane ratings, and those with two or more fused aromatic rings (like naphthalene and anthracene) have very low cetane numbers. The low cetane numbers of these aromatic compounds tell us that they won't burn easily or well, although their internal energy content per volume is high.

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Now, I personally don't know, nor have I researched it that the number of the octane rating of gasoline is that number, say 87%, plus 13% of something else. I would say if a gas pump says 87 octane I personally would be in doubt that this means there is 87% octane by volume. As I said before, gas pumps might say octane=87, but it will also say something like (this isn't it, but I don't recall the exact formula: 87 octane computed by the RM/2 method. I have no idea what R or M represents, but when it says it is computed by a certain method, that means there is more than one method. Perhaps one method is used as a standard and by law must be used, but I don't know. But, consider this: Where does the energy come from by heating octane in the presence of oxygen (O2, molecular oxygen, not O, atomic oxygen). I won't attempt a guess at the exact chemistry, but there is potential energy in non-ignited octane. This energy is released by the breaking of the bonds in octane. It is the breaking of these bonds in a manner which is known as an exothermic reaction, meaning heat is released when the bond(s) break. There are other types of chemical reactions known as endothermic reactions that absorb heat, as a aside.

Ralph69220d · #22 04-25-2006, 08:20 PM

Quote:

Originally Posted by pmari

Errrrr, there is no octane in gasoline and there is no cetane in diesel fuel

http://www.fitchfuelcatalyst.com/techinfo/aboutfuel.html
The octane value of a fuel is an empirical measure of its ability to resist detonation and, to a limited extent, pre-ignition. Technically, octane ratings measure a fuel's ability to resist the spontaneous ignition of un-burnt end-gases under controlled test conditions.

HTML Code:

Now, I personally don't know, nor have I researched it that the number of the octane rating of gasoline is that number, say 87%, plus 13% of something else. I would say if a gas pump says 87 octane I personally would be in doubt that this means there is 87% octane by volume. As I said before, gas pumps might say octane=87, but it will also say something like (this isn't it, but I don't recall the exact formula: 87 octane computed by the RM/2 method. I have no idea what R or M represents, but when it says it is computed by a certain method, that means there is more than one method. Perhaps one method is used as a standard and by law must be used, but I don't know. But, consider this: Where does the energy come from by heating octane in the presence of oxygen (O2, molecular oxygen, not O, atomic oxygen). I won't attempt a guess at the exact chemistry, but there is potential energy in non-ignited octane. This energy is released by the breaking of the bonds in octane. It is the breaking of these bonds in a manner which is known as an exothermic reaction, meaning heat is released when the bond(s) break. There are other types of chemical reactions known as endothermic reactions that absorb heat, as a aside.

Yes, there is octane in gasoline, by volume the greatest constituent. Yes, you are also correct the an "octane rating" is a measure of it's resistance to detonatioin, but it is a standard using octane as the metric. But C8H18 is the primary molecule of gasoline at the pump.

Elktonjohn · #23 04-26-2006, 02:52 AM

Thanks Ralph!! I was looking for a way to understand the chemistry of internal combustion, given my limited backround, and you gave me enough to answer most of my initial question plus enough more...I know it took you about an hour or more to write all that and man is it appreciated. Albeit I'll have to re-read it 5 more times before it all sinks in but I am getting it.
For many years I read the RM/2 rating at the pumps and wondered...still do...and still will. I figured that the Carbons...chain ends aside...just bonded with 2 other C's and 2 other H's, left right, top bottom respectively but I never considered the bond angles...they must fluctuate but average out at 90 degrees except at the chain terminus, termini rather. To my new thinking the 87% Octane represents the fire while the 13% represents the water as a certain balance must be maintained for reliable ignition over a long duration...I mean pure alcohol burns great but building an engine that can last 100,000 miles on pure alcohol would be difficult...so dilute your fuel to a managable point where power output is reasonable and the ability to cool the internal parts is do-able and you have a gasoline engine. ???? My lame attempt at reverse engineering but it makes sense. Plus you want it to run at 14.7psi on a regular basis...and add in a turbo when convenient...awesome chemistry lecture and man do I appreciate it! Thanks and if you have the time please write a bit more as I'm eating this up and I get it...just wish my last chem prof had the inclination...and I'm a biodiesel nut so am needing all the free education I can get. I wonder where Cetane rating and Octane rating meet. Given a static compression ratio obviously there is a meeting point somewhere as one promotes ignition via pressure while the other retards it. It's all in understanding the chemistry and of that I am certain...add 80% Nitrogen, 18% Oxygen and 2% other and your engine runs...GOOD STUFF!! Please feel free to practice your dissertation here...Graci`...thanks Ralph! and Pmari and 240D and FI

Ralph69220d · #24 04-26-2006, 12:33 PM

Hi ElktonJohn,
I'm glad I was somewhat helpful. The "octane rating" though does not mean, say it is rated as 87 octane, that it is 87% octane with the other 13% being inert (non-reactive) molecules. I don't recall the molecule, but it is set as the standard of 100 octane rating (though the molecule isn't octane). I'm pretty sure n-heptane has an octane rating of 0, which give's two points on the standard curve (with the 100 octane rated molecule being the other point on the curve) (years ago Sunoco sold a premium blend rated at 120 octane; it was the only fuel that wouldn't pre-ignite badly in my 11:1 compression 383). So, what I'm saying is that a fuel with an octane rating of 87 behaves in a certain manner as regards it's resistance to ignition. In addition to the simple carbon chains of the form CnH((n*2)+2), there are many additives the fuel companie's put in gasolines, such as detergents, other volatile molecules, etc. I don't have the forum member's name in front of me who provided the information on some of these more complex molecules, especially the aromatic hydrocarbons, but read that post and you'll see further the complexity of it all. Lead-ethyl was the old favorite before the pollution control hit heavy in the late 60's. It was a good heat absorber, so raised the octane rating, yet it is not a true hydrocarbon at all. Though you can mix ethanol & water and burn it in a gasoline engine (because the ethanol binds with water, preventing it from globulizing (if that's a word, lol), gasoline contains no water, per se, and your fuel filters are designed to not let water pass. As regards the bonding angles of the hydrogens the organic chemists can quantify these, but now we're delving into quantumn mechanics, a world completely non-intuitive. Just a quick example, in a single hydrogen atom with one proton as the nucleus and one electron in orbit about the proton, just where in a given instant is the electron, keeping the electron in it's ground state (lowest energy orbit)? Well, the answer is "no place, everyplace". But if we take a measurement to determine it's localization then it has a localization, but only if we take the measurement, "collapses the probability wave function" (at least according to what is known as the Copenhagen Interpretation of QM. Do a search on "quantumn mechanics probability wave collapse" if interested. Further, as Heisenberg figured out in the late 20's, the more exact we measure the electrons localization, the less we know about it's energy (and vice versa). So, if we had infinite precision as to the electrons energy (momentum) then as far as we can know is that the localization of the electron could be anyplace in the entire Universe. In addition to the angular values of the hydrogen speaking of a single CH chain, we have these chains nearing & repelling one another as they vibrate and all of that is far beyond my understanding, as even quantumn mechanics struggles with it beyond a few molecules as the computational power required quickly exceeds all the computational power of the world's computers combined. The "where do the octane/cetane curves cross", I'll just be guessing the correct answer here: It's not where they have the same defined value (a certain #). They, I think, are best thought of as inverse functions relative to one another; but, cetane rating is applied to diesel and octane to gasoline. In the intuitive sense Cetane=(1/octane), just meaning as one goes up the other goes down. But, since the two functions are derived by using different standards, it is not arithmetically correct to say if you know either the cetane or the octane rating, but not both, you cannot, for example say that if cetane = 100, then octane=0.01. Not exactly, but it's something like saying where do temperature and texture measurements meet? Though, the cetane/octane thing isn't quite so ridiculous.

Ralph69220d · #25 04-26-2006, 09:37 PM

ElktonJohn,
I noticed your interest in "just what happens" in the environment of an internal combustion engine at the point of ignition, we have the same interest. One thing that fascinated me about diesel at the point of ignition by compression is the manner in which the injected fuel cloud burns. The seemingly best picture I could come up with as to reliability of the source was some old Mercedes Manuals. This is their abbreviated picture: We're coming up the compression stroke, greatly raising the temperature; at some point BTDC the injector sprays it's cloud of pure diesel; the cloud is elongated (and obviously changing across the whole of the fuel release). This cloud has the highest density in the middle of the elongated stream and becomes less and less dense as we move outwards. So, the inner cloud is initially too rich to even ignite, so, the initial ignition takes place in multiple spots at the outer edges of the cloud. That initial ignition is totally a function of the high temps created by the compression. Some of the middle section of the cloud also ignites by compression from the rising piston and the high pressure (so temps) created by the outer cloud igniting from multiple spots. There is a precise time frame in which all this is happening, but I don't have those #'s in front of me. So, after the ignition sequence I've described up to this point, compression no longer dominates as the fuel igniter. What dominates now is the radiative heat from the burn pattern of the outer cloud and more and more of the inner cloud. The fascinating thing to me is that I had always thought of diesel engines (even though I was a diesel mechanic in the Service and went through courses at Aberdeen Proving Ground, and the civilian Detroit Diesel / Allison Automatic in Iowa) as being totally compression dominant devices. And, most certainly, a diesel with compression problems runs poorly to not at all. But, it is the engineering of the pistons and combustion chamber and fuel injector that results in the engine utilizing the two components of complete ignition, that is: 1) compression 2) radiant heat from multiple compression initiated burn sights at the outer, lean edges of the cloud. (Though the piston & initial compression mediated ignition produces, well, more compression, it is the radiant heat from burning diesel that dominates as the igniter of the initially rich inner cloud which becomes leaner and leaner as we step through the cycle).
This makes it very easy to see why we want a diesel fuel of proper cetane rating; one can go too high and one can go too low. If one is to use #2 diesel or #1 diesel or bio-diesel, or mixes, the engine sounds different dependent on which fuel it is burning, so what is going on in the combustion chamber is altered; not necessarily for better or for worse, but that same engine has something different enough going on from beginning to end of fuel burn as a function of fuel type. In light of this discussion.