[SEL] semi OT - acetone in gas for cleaner, better running?

[Curt]

Richard,
As I understand it the problem of knocking is a burn rate problem. Too 
quick of a flame propagation/burn flexes the top of the piston and we 
hear the knocking sound.
I've been told all gasoline is the same with the differences in regular, 
mid, and premium being the additive packages that slow the burn rate.
We hear these called octane boosters. What exactly is an octane booster?
It is hard to imagine that all gasoline is perfect C(8) carbon chains. 
I'm guessing that it is a mixture of volatiles of lower carbon chains 
and oils of higher carbon chains, with the average being C(8) long. Do 
octane boosters make an attempt to tie shorter chains together to reduce 
the volatility? Or is there some other mechanism at work to reduce 
knocking/slow burn rate?

I've been trying to correlate physical properties to knocking. At first 
I was wondering if there was a relationship between flash point and 
knocking. But kero has a much higher flash point than premium gasoline 
and yet it is prone to knocking, so that kills that correlation.

I've also been wondering if there is a relationship between knocking and 
the explosive range of a fuel. For example, propane has a fairly narrow 
mixture range at which it will explode. Gasoline has a moderate range. 
On the other extreme is hydrogen. With hydrogen almost any mix ratio 
with air will result in an explosive mixture. I don't think I've ever 
heard a propane engine knock (forklifts for example).

Your thoughts? What physical properties of a volatile represent burn 
rate? If we know this, perhaps Bill's question about acetone as a decent 
non-knocking fuel can be answered.
Curt Holland
Gastonia, NC

Richard Allen wrote:

>Hi, Bill,
>
>>From a chemical standpoint, acetone is a ketone, which is a class of
>oxygen-containing compounds based on hydrocarbon molecules.
>Specifically it is methyl-methyl ketone or dimethyl ketone. Another
>well-known ketone with somewhat similar properties is methyl-ethyl
>ketone or MEK. Acetone is produced by the body naturally (called
>ketosis) during the breakdown of fats when they are metabolized for
>energy and can often be smelled on the breath and mistaken for alcohol.
>Diabetics also produce acetone when their insulin levels are low, which
>is from the forced metabolism of body fats for energy because glucose
>is not available. Acetone is a poor solvent for a lot of things in
>general, but for a few materials it is an excellent solvent, or the
>only commonly available solvent that works for things like oxidized
>gasoline gums, epoxy, polyester resin, and instant setting glue. It
>works for dissolving the gummy residues that used to build-up in
>gasoline systems and be a problem, but modern gasolines contain
>adequate oxidation inhibitors and detergents to prevent that sort of
>thing nowadays for the most part. A lot of engine performance additive
>product sales depend on older people’s memories of the gummy gasoline
>and lousy lubricants that USED to be the only products available, but
>all that has changed with additives meant to maintain fuel injection
>systems for the gasoline engines that run slightly hotter nowadays. As
>far as an octane booster goes, I don’t have any idea how well it would
>work, but I have run straight acetone in an old Honda 150 motorcycle
>with a compression ratio of 8.5 to one and it didn’t knock at all. I
>would be very hesitant about using acetone in a modern fuel system
>because of the many elastomers or plastics used in the fuel delivery
>systems nowadays. For the old fashioned copper tubing, cast iron, zinc,
>and aluminum bodied carburetors, steel parts, glass sediment bowls,
>cork gaskets, etc., acetone won’t cause a problem. I would imagine that
>its ultimate combustion characteristics are similar to ethyl alcohol
>and adding it to gasoline would affect engine performance in a similar
>manner. If a large amount of acetone is added to gasoline the
>carburetor would need to be adjusted to enrich the mixture since
>acetone and alcohol burn lean compared to gasoline. With my Honda
>motorcycle I had to raise the carburetor-metering needle up several
>notches to get the mixture rich enough for the engine to run right with
>straight acetone or ethyl alcohol. That very wide adjustment capability
>of the Kehin-Seiki carburetors makes them very versatile because of the
>very wide variety of fuels they will effectively carburet. 
>
>Richard Allen
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