|
|
On Fri, 2005-01-21 at 09:12, Ed Anderson wrote:
The Swiss Mistral rotary folks reported that when they went to long runners
that they believed they encountered a "distillation" problem as Ernest
mentioned. Due to this problem they believed that they encounter detonation
with their turbo set up due to the "Low Octane" part being ingested at a
different time than the lighter high octane part of the fuel. I am
certainly not enough of a chemist to even know if this even sounds
plausible. However, the team did have a Chemist and that is what he
reported.
I must admit I'm a little bit skeptical of this mode as it would seem that
even if it happened you would have a continuos stream of light and heavy
elements intermixing between one injection period and the next. But, they
certainly had the resources and inclination to look into the problem and
that was their conclusion.
Ed A
It's entirely plausible, Ed. First consider gasoline. Being a organic substance, it is not a nice
even mixture of identical molecules. It is a random and often chaotic
mixture of carbon compounds. These compounds are mostly chains of
carbon atoms with hydrogen hanging off the unused bonds. Sometimes the
chains hook back on themselves, but mostly they just intertwine like
strands in a cotton ball. The shorter strands evaporate and burn easier
and faster. If I can get the order correct, the number of carbon atoms
in each molecule goes:
1)methane
2)butane
3)propane
.
.
8)octane (this one's important)
12)hexane
Now the way organic material burns is important. When exposed to oxygen
and energy (we usually use heat, but other methods are possible), the
oxidation process removes a carborn from the END of the chain, reacts it
with the oxygen to give up carbon dioxide and water. It's very
important to consider that the center carbons are safe till all the ends
are burned off, and none will burn till the ends are exposed to oxygen.
Now, how does that apply to us.
First, liquid gas does not burn except for the very surface...the part
exposed to oxygen. In the few milliseconds that a molecule will be in
the combustion chamber of an engine, it has to be exposed to air and
burn completely. If it has to wait for 1000 neighboring molecules to
burn away first, it will be halfway down the exhaust before it can even
get started.
Second, if you let the gas distill, you seperate out the short chains
from the long chains. Think of dried wheat chaff, stick, and logs. The
chaff will flash and be gone. The sticks will keep an nice fire going,
and the logs will burn all night...IF you can get them lit. For a nice
campfire, you'd want some of all of it. What Mistral experienced was
the chaff getting sucked in quickly and being burned off with some of
the sticks, and then the logs clumping up and being sucked in as a tree
trunk.
Third, the magical 'octane'. The original test for octane was to
compare the burning of a sample of a fuel in a calibrated engine. The
engine was calibrate with pure OCTANE, exactly 8 carbon atoms in every
molecule. You can burn any fuel in an internal combustion engine, you
just have to get the mixture and spark timing correct. If the fuel
burns fast like propane, you want to spark later. You'd want to spark
diesel earlier. A mixture that is either lean or rich of peak will want
an earlier spark. Higher compression calls for a later spark. In all
cases, what you're doing is compensating for how quickly the fuel burns
so that you can get maximum pressure in the cylinder at the right time. The problem that was found with the long runners was that it screwed the
mixture up. Instead of a nice clean flame front that could be
compensated for, you got a hodgepodge mixture of wheat chaff and oak
tree trunks.
This is courtesy of an overzealous organic chemistry professor from
1987. I may have forgotten a thing or two since then.
|
|