[Diy_efi] Timing Advance Curve?

[Adam Wade]

--- Erik Jacobs <emj14 at columbia.edu> wrote:

*snip good chemistry*

Perhaps I have my molecular length thoughts backwards.
 I'll be the first to tell you my chemistry bites, and
that I've always been more interested in other aspects
of tuning than in exactly what makes a fuel behave the
way it does.  Somewhere in my brain I have a file card
for a petroleum engineer or two in case I ever needed
to know more on the subject, and so far they are
gathering dust in my brain. *hack, cough*

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

Sounds like you know it better than me, but I don't
know it very well, and should perhaps shut my yap and
read what others of more experience have to say on the
subject.  ;)

>> 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).

This was in answer to a letter?  I've found the "Q&A"
sections of hobbyist magazines to be rife with
technical errors over the years...  And would have a
lot more faith in an article based on them
intervieweing someone from a fuel manufacturer.

Regardless, I was told some time ago by a chemist for
a fuels and lubricants company (perhaps early to mid
90s) that, while it did not hold true outside of pump
gas, that WITH PUMP GAS ONLY, there was a pretty
direct correlation between octane number and burn
speed.  The reason for this has to do with the
requirements for stability and transportability with
pump gas, as well as keeping volatiles to a minimum. 
He did go on to say that with other non-pump-gas
fuels, it went right out the window; there was no
correlation at all (as someone noted with diesel,
which actually NEEDS detonation to burn in the engine
at all, AFAIK).

>> How do you think people figured out all that nifty
>> checmical science?

> Good brains?

Haha!  ;)  I'll tell you, it took me many years of
using science before it occurred to me to think of how
people came up with the science in the first place.

Basic science is easy; you observe a phenomenon, model
it, and see if your model is repeatable under other
conditions, and if it predicts other conditions with a
high level of accuracy.

But when it comes to things like "how do hydrocarbons
burn in a combustion chamber?", we're into some
territory where basics of chemistry and physics DO
work, but are near impossible to use alone to
determine outcomes.

In cases like that, people had to observe to build new
models.  They didn't just take basic chemistry, in
most cases, and push forward brute-force until they
got an answer.  They started with their basic chem,
and then took their observations on the other end, and
tried to make them meet in the middle, in most cases.

So, no, we didn't deduce what was going on in there
based on our past knowledge and then observe, and lo
and behold, we had the right answer!  ;)  At least not
AFAIK!  Observation was, and is, part of the equation,
which is why we get things like those nifty fiber
optic pressure sensors someone posted yesterday.  To
"look inside" a little more while the combustion event
is happening.

> I guess the pressure curve would tell you how fast
> the reaction was going, this is true.

Well, it'll tell you how much reaction is going on,
and how much energy it is releasing.  If you keep
things like the composition of the fuel as consistent
as possible, then you've eliminated a lot of
variables...

> 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...

Like, running them in the same engine with the same
fuel delivery mechanism?  ;)

> 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.

Well, yes and no...  Oftentimes people will observe a
correlation in effect when there is not an easy way to
deduce that observed effect from known concepts.  Then
the question becomes, "Is there a new concept here, or
just new understandings of old ones and how they fit
together?  And, for our use, does it matter, and is it
worth spending the energy to find out?"

> 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 =)

Haha!  ;)  There's chemistry, and there's chemistry,
old boy.  :D

=====
| Adam Wade                       1990 Kwak Zephyr 550 (Daphne) |
|   http://y42.photos.yahoo.com/bc/espresso_doppio/lst?.dir=/   |
| "It was like an emergency ward after a great catastrophe; it  |
|   didn't matter what race or class the victims belonged to.   |
|  They were all given the same miracle drug, which was coffee. |
|   The catastrophe in this case, of course, was that the sun   |
|     had come up again."                    -Kurt Vonnegut     |

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