what _IS_ a good cruise A:F ??

[Evert Rosseel]

> Knock can occur at relatively low power levels.  For example, most
> cars with EGT systems will knock at highway cruise speeds (which typical=
ly
> 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.

(From previous post EGT means EGR).

This is true. Because of the dilution of the charge, with EGR the 
combustion proceeds significantly slower. This means that to 
compensate for this, the ignition advance must be increased. The 
ignition system of your vehicle does just that (minimising fuel 
consumption). (Combustion phased about symmetrically around 
8=B0 after TDC is optimal).
This larger ignition advance will lead to knock IF NO EGR IS USED. 

(Of course, if you do something really wrong, you can get knock at all 
conditions).

> > 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 photograp=
hs
> which CLEARLY show the onset of detonation after only 40-50% of the mixt=
ure
> has been consumed.

True. If only 10% would knock, nothing would be visible in the 
pictures (as the burnt gas expands because of the high temperature 
the end gas volume fraction is smaller than the mass fraction).
For visualisation experiments very extreme conditions are used.

For practical occurence these extreme conditions are not relevant.
Typically, for just audible knock only about 2% of the cycles has 
knock (due to cyclic variability).
BTW, if 40% of the mixture would knock for a significant time, the 
engine would be destroyed VERY fast.
(Usually the problems start with moderate knock, which can gradually 
increase in severity until the engine is destroyed).


Diesel knock is completely different, as autoignition is necessary to 
start combustion in the diesel engine. There, large ignition delays 
cause to much of the injected fuel to burn at once.


Evert
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*    Dr. ir. Evert Rosseel                                             *
*    Laboratory for Machines                                   *
*    Department of Mechanical and Thermal Engineering          *
*    University Gent                                           *
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*    Email : Evert.Rosseel at rug.ac.be                           *
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