Newsgroups: rec.autos.tech
Subject: Gasolines and oxygenates
From: srgxbhh@grace.cri.nz
Date: Mon, 11 Jan 93 02:51:14 GMT
Organization: Industrial Research Ltd., New Zealand.
NNTP-Posting-Host: grv.grace.cri.nz


Jgd@dixie.com writes

>Chlorine and Bromine will also oxidize hydrocarbons too but that's
>not called "burning" is is?

Your strategy of throwing irrelevant material into the discussion
makes responding not really worthwhile. I mentioned an element
beginning with F ( Fluorine ), NOWHERE did I suggest Chlorine and
Bromine were alternatives to Oxygen. Do you know the alphabet?.

>>In general , if people would realise that the oxygenated fuel
>>additives are really just partially burned hydrocarbons, they
>>would realise they are paying for oxygen that can not provide
>>energy. On a mass basis one molecule of en is equivalent
>>to 1.33 carbons or 16 hydrogens, so significant amounts of
>>potential fuel energy are lost.

>Then you would assert that nitromethane (4CH3NO2) is oxygen paid
>for that can not provide energy?  After all, it carries TWO oxygen
>atoms.

Yup, consider Nitromethane CH3NO2 has a heat of combustion of 163
kcal/mol and  Methylamine  CH3NH2 has a heat of combustion of 223
kcal/mol. I prefer to compare apples with apples, and the inclusion
of Nitrogen results in significant differences, and the above is
the nearest I can think of, even if it isn't the most appropriate.
Compare also Nitrobenzene C6H6NO2 (713 kcal/mol) and Aniline C6H5NH2
(787 kcal/mol).

>Octane rating has NOTHING to do with combustion characteristics.  Period.

Really?, let me know how you got an Octane Rating without combusting
the fuel. The two most common Octane ratings are determined on CFR
engines ( usually variable compression ratio ) using very clearly
defined operating conditions ( ASTM D 2699 ( Research ON ) and D 2700
( Motor ON )).

>Octane rating is the measure of a very specific parameter, the ability
>of a fuel to resist explosive combustion.  This characteristic has to
>do with how slow the combustion precursor reactions proceed.  When
>they are kept sufficiently slow either from "strong" molecules or
>the catalytic action of TEL, no knock occurs.

Funny, you now mention " explosive combustion ". :-)

"Knock" or "detonation" occurs when the unburnt gases ahead of the
flame front spontaneously ignite, giving rise to an excessive rate
of pressure rise, thereby causing the characteristic knocking or
"pinking" sound. Under knocking conditions, ignition of the mixture
and the initial burning process proceed quite normally, but the
"end gas", i.e. the part of the charge ahead of the flame front, is
compressed and heated by the advancing flame front until it reaches
the point of self ignition. I may have mis-interpreted what you wrote
above, but your definition of what Octane Ratings measure doen't match
mine, as I consider Octane Numbers measure the "knock" resistance.

>As far as using oxygenates for improving octane of "inferior stock", no.
>It is cheaper to reform or isrize low quality feedstocks to higher octane
>components than it is to synthesize ethers and blend.  Ether IS used to
>push the octane above that reasonably possible through reforming but the
>base stock surely is NOT inferior.

Refer Hydrocarbon Processing Feb.1992 p41-44. " Ethers help gasoline
quality ". From the text ( specially selected to support my case :-) )

" Until recently, MTBE was only used in the US as a high octane gasoline
blending component. The price of MTBE has traditionally been closely
related to its octane blending value,".... "As oxygenated gasolines and
reformulated gasoline become required by law and MTBE supplies tighten,
refinerd may have no choice but to pay a large premium above MTBE's
octane value. Consequently, many refiners are rushing to install their
own MTBE production capacity."
" The total gasoline octane also increases by 3.1 because of the higher
MTBE and alkylate volumes. Higher Octane obtained from the FCC/MTBE/
Alkylation complex will allow refiners to reduce the reformer severity.
This change is highly desirable for producing reformulated gasoline"...

Please cite your reference for saying that oxygenates are not used for
improving octane. If octane is improved, surely the base stock IS
inferior to the finished produced, or are you suggesting the oxygenates
are added to superior product to make it inferior?.

>The only "combustion characteristic" that differs is the stoichoimetric
>point is richer, thus causing the overall mix to be leaner.

Really?. Care to provide a generally available reference?. I consider
Octane rating (see above) a "combustion characteristic".

>>I'm not certain of the US definition of "specific energy" but here
>>it's defined in MJ/kg of the fuel. Typical values for fuels are

>Then you are misusing the term.  The definition of specific energy
>accng to SAE and API is the actual fuel energy delivered to the
>combustion chamber per unit mass of air.  The theoretical specific
>energy is computed by dividing the lower heating value by the stoich
>air/fuel ratio.  The actual SE is computed from the actual heat of
>combustion and the actual air/fuel ratio.  Indeed this terminology
>is used throughout the literature in my library.

Once again, please cite a generally available reference. Obviously
our conventions are different. I am aware of Specific Energy
Consumption ( SEC ) but I thought that was derived from the Heating
Value and other parameters. Regardless, the Heat of Combustion
(ASTM D 2382) or Heating Value, is the revelant property for the
original comparision of gasoline and oxygenate fuels, as I haven't
seen a US gasoline specification that uses "specific energy".

Bruce Hamilton     srgxbhh@grv.grace.cri.nz