[Diy_efi] Timing Advance Curve?

[Bernd Felsche]

On Sun, Dec 22, 2002 at 12:09:24AM +0000, Mike wrote:
> At 10:38 PM 21/12/2002 +1100, you wrote:
> >According to [my understanding of] Charles Fayette Taylor the
> >peak cylinder pressures resulting from the meeting of two flame
> >fronts coming from two spark plugs are essentially the same as a
> >single spark point (in comparison to pressures of detonation).

> Well thats one description, probably a bit simplistic but it
> stands to reason that if there is one spontaneous ignition point
> there are likely to be others and this may well contribute to the
> recognition of a particular phenomena, there may be degrees of
> this at many combinatorial levels and prior to that which is easy
> to recognise. ie. The issue of so called 'silent detonation' where
> the chamber conditions are marginal.

> In any case a flame front is not likely to be a simple event,
> there's a heap of dynamics going on so who really knows the
> details.

As I understand it; the gas movement prior to ignition adds to that
of the flame front; hence high "swirl" rates appear to reduce
detonation because they "outpace" the thermal transfer to the
end-gas so that it's less likely to detonate. The flame-front is
"swept" through the combustion chamber at greater speed (up to 80
metres/second IIRC); about the same order of magnitude of the flame
front velocity.

Keeping in mind that the "secondary compression" of the end-gas due
to the combustion process occurs at the speed of sound - whatever
that may be (depending on temperature), a lower pressure at a
"catalytic site" by the time the flame front reaches that site as
it's swirled, would tend to reduce the propensity to knock.

For detonation to occur at a particular "catalytic site", the general
conditions of combustion must still apply; there must be suitable
oxygen and fuel molecules, and temperature/pressure conditions under
which those molecules can react.

> There must have been people who have used a transparent chamber,
> high speed camera, feul additives and spectral analysis to
> show up instantaneous pressure events through the whole gamut
> of detonation like phenomena, but I aint seem em and know little
> of their interpretation :(

There is a great deal of visualisation research going on,
unfortunately proprietary, so published details are sketchy.
One could spend a lot of money attending conferences or buying SAE
papers on the matter... older research, which is still valid
ground-work, is available freely through NASA sites.

> >The photographs of combustion in his book seem to show that there
> >is no such thing that one could describe as a flame front when
> >detonation occurs - the unburnt remainder ignites throughout. I
> >must reread the relevant chapters, but I think to say that
> >detonation is a meeting of flame fronts is innacurate.

> Well it would be an interesting bit of probability to find several
> million molecules all deciding to grab their nearest partner at
> the same time, no mexican wave... Seems a little bit far fetched,

I'm sceptical of the "spontaneous" explosion inference. More than
likely; the supersonic detonation process is either simply too fast
to be captured or is too destructive on the flame front.

Back in 1977, when I last studied chemistry, nobody even mentioned
how long it takes for a molecular reaction; so that sort of
knowledge is pretty specialised.

The only thing I can't explain right now is why I'm writing this at
3 a.m. on a Sunday. :-(

-- 
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