Muhammar ([Only registered users see links. ]) wrote:
: This got me thinking:
: there is a correlation between anti-knock efficiency of
: octane-boosting additives and their capability to form stabilized
: (=lazy) radicals. Tetraethyl-led, branched alkanes, TBME etc.
: If scavenging the radicals in the chain-reaction of the combustion
: process is the way to get octane number iboosted, then one can perhaps
: use common radical scavengers known from organic synthesis, food
: industry, polymer processing. For example, tert-butylated
: hydroxytoluene is cheap, non-toxic and highly efficient as a
: scavenger. Only a tiny bit would suffice and it would not extract into
: groundwater from leaking fuel storage tanks.
Ethanol is much cheaper, and all you need is some grain, yeast, and
high-country spring wate (okay, DI water will do just fine).
William "Dave" Thweatt
Robert E. Welch Postdoctoral Fellow
Houston, TX [Only registered users see links. ] [Only registered users see links. ]
On 25 Nov 2003 13:07:19 -0800, [Only registered users see links. ] (Muhammar) wrote:
The octane rating of hydrocarbons is determined by the structure of
the molecule, with long, straight hydrocarbon chains producing large
amounts of easily-autoignitable pre-flame decomposition species, while
branched and aromatic hydrocarbons are more resistant. This also
explains why the octane ratings of paraffins consistently decrease
with carbon number.
In real life, the unburnt "end gases" ahead of the flame front
encounter temperatures up to about 700C due to compression and radiant
and conductive heating, and commence a series of pre-flame reactions.
These reactions occur at different thermal stages, with the initial
stage ( below 400C ) commencing with the addition of molecular oxygen
to alkyl radicals, followed by the internal transfer of hydrogen atoms
within the new radical to form an unsaturated, oxygen-containing
These new species are susceptible to chain branching involving the HO2
radical during the intermediate temperature stage (400-600C), mainly
through the production of OH radicals. Above 600C, the most important
reaction that produces chain branching is the reaction of one hydrogen
atom radical with molecular oxygen to form O and OH radicals.
The addition of additives such as alkyl lead and oxygenates can
significantly affect the pre-flame reaction pathways. Antiknock
additives work by interfering at different points in the pre-flame
reactions, with the oxygenates retarding undesirable low temperature
reactions, and the alkyl lead compounds react in the intermediate
temperature region to deactivate the major undesirable chain
MTBE works by retarding the progress of the low temperature or
cool-flame reactions, consuming radical species, particularly
OH radicals and producing isobutene. The isobutene in turn consumes
additional OH radicals and produces unreactive, resonantly stabilised
radicals such as allyl and methyl allyl, as well as stable species
such as allene, which resist further oxidation.
In contrast to oxygenates, the alkyl lead interferes with hydrocarbon
chain branching in the intermediate temperature range where HO2 is the
most important radical species. Lead oxide, either as solid particles,
or in the gas phase, reacts with HO2 and removes it from the available
radical pool, thereby deactivating the major chain branching reaction
sequence that results in undesirable, easily-autoignitable
As you can see from the above, it's not that simple.
Very unlikely - it would decompose in the flame reaction and wouldn't
be available to scavenge. If the fuel evaporated the BHT would remain
as a residue, blocking jets. etc.