[Diy_efi] Timing Advance Curve?

[Mike]

At 10:30 AM 12/20/02 -0700, you wrote:
>Flame front speeds have more to do with AFR, density (fuel), compression
>ratio, and flow turbulence, but the slower the burn--the greater the chance
>of abnormal combustion. The faster the burn, the more chance of detonation
>or that peak cylinder pressures are occuring too soon.

We are not talking abnormal combustion, detonation is a meeting of
flame fronts, one comes from the spark ignition - the other comes
from that region of the chamber that spontaneously ignites due
to either an instantaneous rise in temperature or pressure.

Sure for any one fuel, AFR is an issue - but for the *one* engine
we are not changing CR or fuel density (of all things) are we ?

>To say that low octane and high octane burn at exactly the same "flame
>speed" would say that it wouldn't matter which you use.

You see, I interpret and infer you are thinking that detonation
is an issue of some problem with flame propogation - no, refer
the previous para and definitions of detonation.

Its when flame fronts collide and produce severe shock waves
that cause all sorts of problems.

Try to picture two situations on an engine with equal AFR, CR,
fuel density etc But where they change only due to octane...

Situation one, low octane, low enough to cause detonation:-

Spark ignites fuel in direct proximity to plug,
flame front expands outwards from this region,
simultaneously piston is moving up bore (advance timing),
Spontaneous ignition at some point away from flame front from spark,
Flame fronts collide causing the ping sound etc

Situation two, high enough octane, all else as in Sit one equal:-

Spark ignites fuel indirect proximity to plug,
flame front expands outwards from this region,
simultaneously piston is moving up bore (same advance timing)
Initial flame front spreads unhindered and collapes at walls,

In both cases above the flame front speed is the same, however
in situation one the 1st flame front collided with another flame
front from a spontaneous ignition somewhere else in the chamber,
if the chamber was clean its detonation, it if was dirty then
its pre-ignition from say a hot carbon deposit.

Also you might want to read the faq's Brian and I listed earlier
on this topic,

rgds

Mike
PS: Seems like this email went off before completion, sorry dont
know what happened (MS), this is the complete one, doh.


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