what _IS_ a good cruise A:F ??

[Gregory R. Travis]

On Thu, 27 Mar 1997, Evert Rosseel wrote:

>> 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).

Knock can occur at relatively low power levels.  For example, most
cars with EGT systems will knock at highway cruise speeds (which typically
demand only 12-30% of the engine's maximum power) if the EGT system is
defective.  My own truck, a Toyota, will knock (detonate) at 40MPH at a
steady cruise on a flat road.  This is at approximately 20% of its
maximum power output.

> > 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.

I would like to see some evidence of this.  I have flame trace photographs
which CLEARLY show the onset of detonation after only 40-50% of the mixture
has been consumed.

I don't see how this can possibly be any kind of law.  For example, knock
(detonation) can occur in a diesel engine before virtually any of the
mixture is consumed if the fuel's cetane rating is too low (causing the
delay period to be too high).

> > 	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).

  "By preignition is meant ignition of the charge BEFORE the ignition spark 
   occurs.  This type of ignition is caused by a hot surface.  In practice,
   the chief sources of preignition are overheated spark-plug electrodes
   or very hot carbon deposits.  When preignition occurs, it is the
   equivalent of an advanced spark and may cause detonation.

   Conversely, when detonation is severe and long-continued, it may heat
   up the spark-plug points or carbon particles to the point where
   preignition occurs. ..."

		Charles Fayette Taylor, _The Internal Combustion Engine
					 in Theory and Practice_ Vol 2, p. 84

  "Surface ignition is ignition of the fuel-air mixture by a hot spot
   on the combustion chamber walls such as an overheated valve or spark plug,
   ... It can occur BEFORE the occurrence of the spark (PREIGNITION) or after
   (postignition)."

		John B. Heywood, _Internal Combustion Engine Fundamentals_
				 p. 450

greg

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