what _IS_ a good cruise A:F ??

[Evert Rosseel]

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

Knock usually only occurs near full load (gas pedal to the floor), 
but also depends on engine speed. (See further).
 
> "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.

Knock is indeed only dangerous if it occurs for a longer period. In 
that respect high speed knock is most dangerous (high speed meaning 
high rpm). Al high rpm the mechanical (and driving) noise is higher, 
which masks the phenomenon to the driver who often doesn't notice 
anything untill it is too late.
At low rpm, full load also means fast acceleration. Therefore the engine 
moves to higher rpm usually before damage can occur.
 
> 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.

This is WRONG. Knock always occurs after the flame has burnt most of 
the mixture (more than 90%). Only the remainder (less than 10%) burns 
nearly instantaneously.

The normal combustion causes the pressure rise in the cylinder, which 
compresses the unburnt gas (end gas). The corresponding temperature 
rise increases chemical reaction rate until autoignition occurs (if 
the flame doesn't arrive before).

> 	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).

This is early preignition.
In some cases (late preignition) the parasitic ignition can occur 
after the spark (but before the flame has burnt the whole charge).

Preignition leads to a "normal" flame propagation, just as the spark 
does. At the end of combustion it can lead to knock (esp. early 
preignition) just as a too large spark advance can.
 
> 	Preignition can lead to detonation AND vice versa.

Very true.
 
> 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.

2 is only correct for mixtures that are very lean. For moderately 
lean mixtures this is not true (because of the larger quantity of air 
that has to be heated).
 
> 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.

In pratice two other things can happen too :
1) the spark may be unable to ignite the mixture
2) the flame can be extinguished before everything has burned

This increases exhaust HC enormously of course.

Evert
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*    Dr. ir. Evert Rosseel                                             *
*    Laboratory for Machines                                   *
*    Department of Mechanical and Thermal Engineering          *
*    University Gent                                           *
*    Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium            *
*    Tel : ++32 9 264.33.06                                    *
*    Fax : ++32 9 264.35.86                                    *
*    Email : Evert.Rosseel at rug.ac.be                           *
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