is there octane in Diesel fuel? - PeachParts Mercedes-Benz Forum

sixto · #1 01-11-2004, 03:39 PM

I was thinking of a license plate frame for the SDL that reads, "Look Ma, no octane." But is there octane in Diesel fuel?

Thanks,
Sixto
95 S420
87 300SDL

Gilly · #2 01-11-2004, 03:53 PM

Octane isn't something that is "IN" anything, that's like saying is there any "Richter" in an earthquake, or any "Geiger" in something radioactive, it's just a "rating" for Gasoline. Diesel you are looking for different qualities, it uses what is called a "Cetane" rating, not Octane. I'm sure an Octane number could be established for Diesel fuel, but you could do the same to turpentine, or a Coca-cola......or water for that matter.
How about "Look ma, no sparkplugs" or better yet , make it look like you don't really CARE what your mother looks at.

Gilly

sixto · #3 01-11-2004, 04:00 PM

Well that made me look smart.

Thanks Gilly

I always thought of octane as some configuration of carbon and hydrogen atoms and guessed that such a configuration existed in the primary constituents of gasoline and not in the primary constituents of Diesel fuel. Just as some lighter fluids are called butane and there are fluids called ethane, methane, etc., I thought there was a substance called octane. Is there no such substance or fluid called octane independent of whether or not it's in gasoline?

Sixto
95 S420
87 300SDL

coachgeo · #4 01-11-2004, 04:03 PM

Quote:

Originally posted by sixto
Well that made me look smart.

Thanks Gilly

... Is there no such substance or fluid called octane independent of whether or not it's in gasoline?

Sixto
95 S420
87 300SDL

Nope. no such substance. Think of Octane and Cetane like Pounds or Kilograms. They are just units of measure.

Gilly · #5 01-11-2004, 04:24 PM

You GOT a computer, use it!

http://chemistry.about.com/library/weekly/aa070401a.htm

Here is a link I added to find info on Cetane ratings:
http://www.buyduralt.com/cetane.html

Gilly

sixto · #6 01-11-2004, 04:49 PM

Gee, Gilly, next you'll be telling me to use my brain too

Sixto
95 S420
87 300SDL

Gilly · #7 01-11-2004, 04:53 PM

I'll go ahead and make that request at this time to avoid the rush.:p

Gilly

ericnguyen · #8 01-11-2004, 06:43 PM

coachgeo: You are wrong. Octane is a saturated hydrocarbon with 8 carbon atoms. It belongs to the ALKANE family (no double bonds)

sixto: OCTANE rating is a measure of the anti-knock index (the slowing of the burning) for gasoline, while CETANE rating is a measure of the ease of the ignition of diesel fuel.

Gasoline is a mixture of light hydrocarbons (n is small, as described below),while diesel fuel is a mixture of heavier hydrocarbons (n is bigger).

C has a valence of 4 (tetravalent), so it can form 4 bonds with its surrounding atoms (including other carbon atoms). Because the carbon and hydrogen atoms can form bonds at different positions, there are MANY possible 3-dimensional structures (isomers) for the same formula. For example, for the alkane C4H10 (n=4), we have several different isomers:

Butane (or n-butane = straight): H3C-CH2-CH2-CH3

Isobutane (branched) : CH3-CH2-CH3
|
CH3

As n becomes greater, the number of isomers (different 3-D chemical structures of a same generic formula) will exponentially increase. One way to increase the OCTANE rating of gasoline (besides adding ethanol or illegal lead compounds) is to INCREASE the number of BRANCHED hydrocarbons, which may require catalyst-based reactions to convert straight hydrocarbon molecules into branched ones.

Here are the major families of hydrocarbons. Alkanes, Alkenes, Alkynes are much more naturally occurring hydrocarbons than allenes or alkadienes.

If you find any incorrect info, please point it out. My memory is not as good as when I was younger.

---------------------------------------------------------------------------------
- Alkanes CnH(2n+2), n is equal to or greater than 1
Alkanes are also called "saturated hydrocarbons" because they contain neither double bond nor triple bond. An organic compound (hydrocarbons are organic) containing only single bonds is SATURATED while the ones with at least a double bond or triple bond is UNSATURATED. BTW, aimal lipid (fat) is saturated while vegetable lipid (oil) is unsaturated.

The most popular alkane is methane CH4 (n=1).

There's another group of saturated hydrocarbons which is called "Cycloalkane", whose generic formula CnH2n is the same as that for Alkenes. However, in order to form a ring, "n" for cycloalkanes must be equal to or greater than 3. Thanks to the ring structure, cycloalkanes (with 2 less hydrogens) do not have any carbon-carbon bond, so they are still saturated hydrocarbons.

---------------------------------------------------------------------------------
- Alkenes CnH2n, n is equal to or greater than 2
Alkenes are UNSATURATED hydrocarbons that contain a single carbon-carbon DOUBLE bond.

An older name for Alkenes is Olefins.

---------------------------------------------------------------------------------
- Alkynes CnH(2n-2), n is equal to or greater than 2

Alkynes are UNSATURATED hydrocarbons that contain a single carbon-carbon TRIPLE bond. The most popular alkyne is ethyne C2H2 (the common name is acetylene). Acetylene is great for welding because it's widely available in natural gas reserves and especially because its TRIPLE bond contains lots of chemical energy, which will be converted into thermal energy (heat) for welding purposes.

---------------------------------------------------------------------------------
- Allenes CnH(2n-4), n is equal to or greater than 3

Allenes are UNSATURATED hydrocarbons that contain 2 carbon-carbon DOUBLE bonds. The 2 double bonds in allenes must be adjacent to each other. Note the difference between Allenes and Alkadienes (same generic formula) described below.

BTW, don't confuse "Allenes" with "Alleles" :-) An allele represents one of the many possible forms of a same gene in a chromosome. We are talking about chemistry, not biochemistry.

--------------------------------------------------------------------------------
- Alkadienes CnH(2n-4), n is equal to or greater than 4

Alkadienes are UNSATURATED hydrocarbons that contain 2 carbon-carbon DOUBLE bonds. The 2 double bonds in alkadienes must NOT be adjacent to each other. This is different from the relative positions of 2 double bonds in Allenes.
---------------------------------------------------------------------------------

Here are some examples of alkanes, alkenes, alkynes, allenes and alkadienes.
---------------------------------------------------------------------------------
1) Examples of alkanes, and their state at standard condition of 25 degrees Celcius.

methane CH4 gas
ethane C2H6 gas
propane C3H8 gas
butane C4H10 gas
pentane C5H12 liquid
hexane C6H14 liquid
heptane C7H16 liquid
octane C8H18 liquid
nonane C9H20 liquid
decane C10H22 liquid
undecane C11H24 liquid
dodecane C12H26 liquid
eicosane C20H42 solid
..
...
---------------------------------------------------------------------------------
2) Examples of alkenes (olefins), and their state at standard condition of 25 degrees Celcius.

Ethene (or Ethylene) C2H4 gas
Propene (propylene) C3H6 gas
Butene (butylene) C4H8 gas
Pentene (pentylene) C5H10 gas
....
....
---------------------------------------------------------------------------------
3) Examples of alkynes, and their state at standard condition of 25 degrees Celcius.

Ethyne (or Acetylene) C2H2 gas
Propyne C3H4 gas
Butyne C4H6 gas
...
...
---------------------------------------------------------------------------------
4) Examples of allenes, and their state at standard condition of 25 degrees Celcius.

propyl allene: H2C=C=CH2 gas
...
...
---------------------------------------------------------------------------------
5) Examples of alkadienes, and their state at standard condition of 25 degrees Celcius. The simplest alkadiene is: (1,3 denotes the relative positions of the 2 double bonds).

1,3-butadiene H2C=CH-CH=CH2
....
---------------------------------------------------------------------------------

Hope this helps.

Best regards,

Eric

wolf_walker · #9 01-11-2004, 06:51 PM

That is so, so beyond my understanding of chemistry.. But it was interisting to read..

Bet Gilly dosent have to tell you to use YOUR brain..

coachgeo · #10 01-11-2004, 07:02 PM

Eric,

New info to pack into my puny brain cells. Thanks for the correction. I am trying to get a good knowledge of this cause it helps me understand the ins and outs of using WVO and biodiesel.

So you are saying there is an actual structure called Octane but when it comes to the measureing tool of anti knock in gasoline it has nothing to do with the structure of the same name; it is just a measure? I ask this cause I read that in fuels for a diesel engine you want a high Cetane measure and a low octane measure. Is there a "cetane" chemical structor as well?

notlostmaybe · #11 01-11-2004, 07:03 PM

in practical use, octane rating is a percentage of the time that a gas engine will not ignite prematurely, based on a v8 engine. the
slowing of ignition in gas used to be regulated through the addition of lead and additional refinement, now other additives are used with better refinement. v8s will prematurely ignite more readily than than sixes, sixes more so than fours. this is how i remember it to be.

don

coachgeo · #12 01-11-2004, 07:25 PM

Just read Gilly's link. Interesting.

brief synopsis as I see it. Probably technicaly wrong but it might get the jist across.

Octane number is based on a scale derived from two of the "...anes" Eric listed above. One of the "...anes" apparently creates NO knocking and thus it was said to be on a scale of 0-100 a 100 perfect awsome stuff LOL. Then I guess they decided to use the worst of the "....anes" when it comes to causing knock and gave it a 0 score. From this new scale of 0-100 they have something to compare too (a measuring stick so to speak) and whalla.... you have a octane rating.

Why they chose to use the name Octane for the scale is not explained in the article. Also, what engine they fired up and listened for a knock with to see what "...ane" did not knock and which one did was not mentioned in the article. Maybe it was the V8 as suggested in this thread?

Does this make us all right LOL

Gilly · #13 01-11-2004, 08:27 PM

From the above link:

Quote:

The octane number is determined by comparing the characteristics of a gasoline to isooctane (2,2,4-trimethylpentane) and heptane. Isooctane is assigned an octane number of 100. It is a highly branched compound that burns smoothly, with little knock. On the other hand, heptane is given an octane rating of zero. It is an unbranched compound and knocks badly.

There is nothing in gasoline called "Octane", but isooctane is where the word "Octane" is derived from for grading fuel.

Gilly

Lycoming-8 · #14 01-12-2004, 12:05 AM

Additionally, the engines that are used today to test gasoline formulations for its "octane level" are highly specialized single cylinder engines produced by only one manufacturer. They are run according to an ASTM test procedure and are usually computer controlled and monitored. These are usually found at refineries and truck loading terminals. Not really sure how the diesel fuel "cetane number" test is run; but for the non chemists here, there is an alkane with that name as well.

ericnguyen · #15 01-12-2004, 04:22 AM

Gilly: gasoline is a mixture of many different hydrocarbons (different "n" and different isomers), and I'm sure octane (n=8: C8H18) is one of them. Analyzing and expanding the generic formula C8H18 of octane will show that as many as 18 octane isomers may exist. Among them, only n-octane is a straight-chain octane isomer, the remaining ones are branched-chain ones.

Octane (or n-octane): straight chain octane
2-Methylheptane
3-Methylheptane
4-Methylheptane
2,2-Dimethylhexane
2,3-Dimethylhexane
2,4-Dimethylhexane
2,5-Dimethylhexane
3,3-Dimethylhexane
3,4-Dimethylhexane
3-Ethylhexane
2,2,3-Trimethylpentane
2,2,4-Trimethylpentane (or isooctane)
2,3,3-Trimethylpentane
2,3,4-Trimethylpentane
2-Methyl-3-ethylpentane
3-Methyl-3-ethylpentane
Tetramethylbutane

As I said in my previous post, one of the current ways to increase the OCTANE rating of gasoline is to increase the amount of BRANCHED hydrocarbons such as isooctane (2,2,4-trimethylpentane) or many other branched hydrocarbons in gasoline. The reason why branched hydrocarbons burn smoothly with little knock can be explained based on the conceps of Standard Enthalpy (H), Entropy of Formation (S), Free Energy (G), and Activation Energy (E). I don't want to delve into explaining what these concepts are, because our purpose is to find out why OCTANE rating (anti-knock or no early combustion) of a specific type of gasoline will increase if there are more branched hydrocarbons in it.

Change in (G) = Change in (H) - T * Change in (S)

For a chemical process to occur spontaneously, the resulting change in Free Energy must be NEGATIVE. From the above formula, you can see that the change in Free Energy depends on the temperature (T in Kelvin), and becomes negative if T is large enough. However, it also requires that the Activation Energy (E)
supply is enough to bring the reactants (hydrocarbons and oxygen) into various transition states before they actually come into contact and react with each other. But we can temporarily ignore (E) for now, even though it's quite important to include it in some cases. For an internal engine combustion process, the Activation Energy (E) comes from piston compression heat and sparks.

The above formula implies that an early combustion (causing knocking) process may never occur if the temperature is low enough, but will occur spontaneously if the temperature is high enough to render the change in Free Energy NEGATIVE. Piston compression of the gasoline/air mixture produces heat (and higher temperature) in addition to sparks from spark plugs. Higher compression ratio is MB gas engines (such as 10:1) produces more heat (thermal energy -> activation energy), and higher temperature (change in (G)), thus gasoline with high octane rating is needed to prevent knocking in all MB gas engines.

It has been found that the changes in Standard Enthalpy and Entropy of Formation for most branched hydrocarbons during combustion process (and at a certain temperature) are in such a range of values that help keep the change in Free Energy POSITIVE (not occurring spontaneously), while straight-chain hydrocarbons with similar generic formulas more easily cause the change in Free Energy to be NEGATIVE (occurring spontaneously). Of course, when the temperature is high enough (thanks to high compression, sparks), the change in (G) for branched hydrocarbons will also become NEGATIVE (and Activation Energy (E) is high enough), and these hydrocarbons will burn right away (but delayed enough to prevent knocking).

BTW, T(in Kelvin) = 273.15 + T(in Celcius)

There are some other ways to increase the OCTANE rating via altering the change in (G) and activation energy (E) by adding Tetraethyl Lead Pb(C2H5)4 or Ethanol C2H5(OH) etc... in gasoline. Pb(C2H5)4 was banned from US gasoline for more than 20 years ago because lead is too poisonous and highly detrimental to the environment (BTW, I'm a "tree hugger"

.) There are quite a few countries which are still using leaded gasoline.

I'm sorry if my explanations are confusing and ambiguous. English is my ESL. I wish I could explain it to you guys in Vietnamese

Best regards,

Eric