My understanding is the reason that lower octane fuels are more prone to
detonation is not due to faster burn, but due to a more precise light-off.
That is, detonation is premature light-off, say from a "hot spot" in the
combustion chamber, etc. as the fuel/air mixture heats from compression
during the compression stroke. With lower octane, ie. less refined, fuels,
there is a higher fraction of lower and higher molecular weight carbon
compounds--such as pentane (5 carbons) and hexane (six carbons) and 11 and
12 carbon compounds. Not only do the lower weight compounds light-off at
lower temperatures, but the more heterogenous the fuel mixture, the more
unstable it will be. *(octane ratings are, as most know, just measures of
the percentage of lower and higher carbon compounds in the gas mixture,
with the eight carbon molecule "octane" being ideal for gasoline engines.)
The reason higher octane, more refined fuels are more detonation-resistant
is they have more heptane (7 carbons), octane, nonane (nine carbons), and
less of the smaller and larger carbon molecules and are therefore more
stable at higher temperatures--therefore, they only ignite when initiated
by a spark and the whole process is much more predictable with the more
uniform the fuel mixture.
Alex Avery (a guy who still remembers his high school and college chemistry
factoids).
At 12:29 AM 9/23/00 -0400, CarterCM@aol.com wrote:
>Tom,
>I've scratched my head on this a few times myself. Question...Gas CAN be
>refined to a higher octane (anti knock) without additives. So, isn't
>detonation (explosion) just a degree of the same process of rapid burn? So
>wouldn't the higher octane 'resist' the increase in rate of burn (burn
>slower) compared to a non-additive standard, as the ignition advances and
>pressure goes up?
>
>Craig Carter
>
>In a message dated 9/22/00 7:53:20 PM Pacific Daylight Time,
>ra0618@email.sps.mot.com writes:
><< I have to admit some old myth's die hard. I had for ages believed that
>premium
>fuel burned slower. Not so, all the grades burned the same. Premium just had
>the additives to prevent knock, so you can run a higher spark advance (which
>creates a higher pressure, hence better performance). Of course you need
>higher compression to take advantage of the advanced spark to gain the higher
>combustion pressure. >>
|