[Diy_efi] Timing Advance Curve?

Erik Jacobs emj14 at columbia.edu
Thu Dec 19 20:36:22 GMT 2002


> Modern fuels have no straight iso-octane in them.
> That's a reference fuel.

ok cool.

> Common sense tells you that the more short pieces are
> floating around, the more "open ends" are available to
> combine with oxygen.  Volume x of longer chains has
> fewer easily availavle binding sites for oxygen than
> volume x of shorter chains.  Thus, longer chains tend
> to burn more slowly, all other things being equal
> (which in gasoline, like in tires, they are not).

There technically are no "open ends" on a complete hydrocarbon chain.
Unless you are dealing with double and triple bonded sections, the C's and
H's are pretty happy to be where they are, are they not?

Remember that CO2 looks like this:

O = C = O

doesn't it?

H2O is like
  O
H H

Now on a LONG chain of CH, like

   HHHHHH
HCCCCCCH
   HHHHHH

There's a lot of places to bring an O up to to get it close to being
 O
HH

Aren't there?

On
   HH
HCCH
   HH

There aren't so many now are there?

True you say there are no straight chain CH groups in normal fuels anymore,
but what you are saying kinda sorta doesn't make sense to me (fewer
available binding sites)... in the case of many double and triple bonds, in
HCs aren't those double and triple bonds pretty easy to "crack"?? It would
make sense that it would be EASIER to initiate the reaction in that
insatnce, but still we're talking about ease of initiating the reaction, not
the actual SPEED of the reaction.

I guess I've forgotten the chemistry behind CH combustion... I should go
look it up somewhere.

> Not overall, but in pump gas, it has a very high
> correlation.

Well you should tell that to Motor Trend... because in an article they had
written they replied to someone saying that there is no relationship between
octane and burn rate (and this was in reference to people putting higher
octane fuels in their cars at the pump when unnecessary).

> ...or look at the pressure curve during a burn, and
> analyze what came out the tailpipe.  How do you think
> people figured out all that nifty checmical science?

Good brains?

I guess the pressure curve would tell you how fast the reaction was going,
this is true.  But it would only be a relevant comparison if you used the
same everything (chamber, atomizer, etc etc) to compare the two combinations
of fuel... and since pump gas is such a bastardized mixture of chains
anyway, I guess I'm really analyzing the wrong stuff when I think about
octane, since it's a measure of a mixture of lots of lengths of chains, of
all sorts.

Oh well, I'm getting tired of chemistry, can we go back to... wait four gas
analyzers are chemistry too... I'll just step out my window now =)


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