[Diy_efi] Timing Advance Curve?

[Bernd Felsche]

On Sun, Dec 22, 2002 at 01:05:38PM +0000, Mike wrote:
> At 03:20 AM 22/12/2002 +0800, you wrote:

> >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.

> Now this is an interesting point, is there an asymptotic limit
> in repsect of chamber geometry variables and implementation
> which might well quell the vast majority of detonation like
> incidents in line with the paradigm you describe in your para above ?

I'm not au fait with the current state of the art. Volkswagen is one
manufacturer that's built relatively high CR engines (10.5:1 was
normal even in 1990) but they reduced the problem of knock that
is inherent with such a high CR by using slightly smaller valves
than would be deemed appropriate for peak performance, and they
angled their ports; which resulted in high gas speeds as an angle,
promoting swirl. It mostly works; except at low engine speeds.

Of course, swirl imposes other effects on the air-fuel mix in the
cylinder; if there's a combustible mix near a hot spot, then it
needs to be near that hot spot to get enough energy to ignite. By
moving the gases rapidly, you "even out" the temperature rises and
provide convective cooling to the spot while spreading the thermal
load over a greater volume of mix.

One can probably come up with some more reasonable theories about
what happens if the gas is moving prior to ignition. I wish I had
the numbers to be able to know what factors and effects are more
significant. Knowing that it works is good; knowing how can take a
long time to figure out.

As for the limiting parameters; there's the matter of the kinetic
energy of the molecules themselves. Worst case is that this will
contribute to the heat required to produce detonation in the end gas
as it's bouncing around - assume all forms of energy become heat in
the limit.

-- 
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