what _IS_ a good cruise A:F ??

Gregory R. Travis greg at indiana.edu
Wed Mar 26 16:43:47 GMT 1997


On Tue, 25 Mar 1997, Tom Cloud wrote:

> I've been told by someone that my term "detonation" is incorrect,
> that "pre-ignition" is the term I should be using.  Wish I knew  8^)

I've got, what I think is a pretty damn good, definition and distinction
between the terms on my Web page at:

	http://gtravis.ucs.indiana.edu/Engines/octane.txt

If it matters, >I< think we're talking about detonation although extremely
lean mixtures CAN lead to preignition if the longer burn times heat up
components (eg. the exhaust valves) enough that they can ignite the mixture.

> 
> It was common in days of yore to run lean for cruise/economy
> and rich (terms relative of stoich) for power.

It's still common.

> Now, I am confused.  The tech books show curves that give
> max economy to the lean side and power to the rich side of
> stoich. It's been done for years.  I've read that open
> cylinder and domed piston design increase the likelihood
> of detonation, but now I don't know for sure if we're all
> using the same terms ?????

Combustion chamber shape and piston design have profound effects
on the OCTANE REQUIREMENTS (again, see the referenced article above)
of a given engine at a given compression ratio and power output.

>  .. what is it that causes the big-end bearing to get smushed
> and commonly destroys the top ring gland.  Whatever that is
> called is what I had.  I did experience what I consider to
> be normal clatter, on rare occasions, going up inclines at mid
> to WOT that could be cured by putting higher octane fuel.

This is detonation, known on the street as "pinging."  Mild cases of
it often cause no harm whatsoever.  Severe cases of detonation up to
and including detonation combined with preignition will hammer the
bearings in the manner you described.

>  .. I don't see how low-octane "pinging" can cause the above
> problem.

It often doesn't if the "pinging" is mild, of short duration, and/or
occurs at very low power levels.

"Pinging" at high power levels, which is conceptually the same
thing that you experienced, can lead to preignition and runaway
detonation.  That will destroy pistons, bearings, etc.  It's all just
a matter of degree.

  To me, that is "pre-ignition", which is what I was
> calling detonation.  I guess if I was to make up terms, I would
> call it "self-detonation" or an "uncontrolled ignition event"
> if I were a political spin doctor (an oxy-moron), whereas
> "pinging" is the rapid banging of the explosion caused by
> the ignition spark.

In the simplest terms:

	Detonation/Pinging is the spontaneous ignition of the ENTIRE
	charge in the cylinder.  This is in opposition from the normal
	combustion event in which there is a clearly defined flame front
	in the cylinder which progresses in a linear fashion across the
	combustion chamber.  Detonation is caused by uniformly HEATING
	the entire mixture (i.e. by compression) to the point at which
	it spontanously explodes.

	Preignition is simply the term used when the mixture ignites,
	for any reason, prior to the sparkplug firing.  The most common
	cause of preignition are localized hotspots in the cylinder such
	as glowing carbon or small pieces of metal (such as the spark plug
	electrode).

	Preignition can lead to detonation AND vice versa.

	The type of "pinging" that most of us are familiar with doesn't
	involve preignition at all.  It involves the spontanous explosion
	of the mixture AFTER the sparkplug fires.  

> So, I'm back to the question:  if lean running has worked
> for so long, what did I do wrong?  Was it something else?
> I'd like to know before I finish assembling this high $$
> (for me) engine and put it back in.

Lean running primarily causes the following:

	1.  The mixture takes longer to burn (assuming normal combustion).

	2.  The mixture burns hotter for the given amount of fuel.


With extremely lean mixtures, it's actually possible for the charge to
be still burning as the exhaust valve begins to open.  This raises the
temperature of the exhaust valve SIGNIFICANTLY.  The hot exhaust valve,
in turn, can become a source for preignition.

Likewise, the hotter burn of the mixture can raise the temperature of
the combustion chamber or pieces of it enough to lead to detonation and/or
preignition.

The possibility of either of these events happening is a function of the
fuel type, the degree of leaning, ignition timing, and combustion chamber
design and materials.

greg


greg		greg at indiana.edu	http://gtravis.ucs.indiana.edu/




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