water injection questions

[Robert Yorke]

Tony:  As I read this, I believe you are correct.  I've  been trying to
refer to my little "Bible" regarding automotive gasoline (The FAQ
"Automotive Gasoline" by Bruce Hamilton; posted to several usenet groups on
a monthly basis) in an effort to either refute you or agree with you, and I
can't find his definition of knock!  However, I believe you're right...
WAIT, I just found it:

"The octane rating of the fuel reflects the ability of the unburnt end gases
to resist spontaneous autoignition under the engine test conditions used.
If autoignition occurs, it results in an extremely rapid pressure rise, as
both the desired spark-initiated flame front, and the undesired autoignited
end gas flames are expanding.  The combined pressure peak arrives slightly
ahead of the normal operating pressure peak, leading to a loss of power and
eventual overheating.  The end gas pressure waves are superimposed on the
main pressure wave, leading to a sawtooth pattern of pressure oscillations
that create the knocking sound"

You are correct; I was wrong... 


At 01:29 PM 7/24/96 +1200, you wrote:
>
>> Donald:  You are correct; I was referring to the end consequence of octane
>> rating procedure and rationale.  The resistance to pre-ignition/detonation
>> (They're NOT the same) is closely associated with burn rate, which in turn
>> is associated with combustion (Test) chamber configuration, which affects
>> flame front propagation as well as a host of other factors (ad nauseum). The
>> real world  use of octane rating for fuels is as a gauge for determining
>> burn time to allow for successful advance curve determinations.  Cutting
>> through all this crap I've tried to throw up, I'm trying to say that even
>> though octane is a measure of "knock" resistance, it as a consequence, is a
>> measure of burn rate! 
>> 
><snip>
>
>To my knowledge, knocking is not a fast burn. The burning 
>process takes some time (for the flame front to travel accross the
>cylinder). As this burning is happening the pressure in the cylinder 
>rises dramatically. The pressure is also rising (BTDC) due to the 
>piston compressing the mixture. If these two pressure rises combine
>(due to over-advancing the timing), the resulting pressure = internal 
>energy of the gases may exceeded the activation energy of the 
>combustion reaction. This energy will be present at all points in the 
>unburnt gas, so all molecules of unburnt gas are likely to react, and 
>when the do the pressure rises very suddenly (around TDC) with shock 
>damage resulting to your precious lump o' steel.
>
>Knock resistance is a measure of the activation 
>energy required to combust (react) the mixture. Slowing the 
>burn rate can be achieved a number of ways, but its net effect is
>that the peak unburnt gas energy density is lower because the 
>actual peak will occur when the piston is somewhat past TDC.
>
>Knocking is not burning, so knock resistance cannot be a measure of 
>burn rate.
>***********************************************************
>* "Insanity is the only sane response to an insane world" *
>*                 >> bryantt at fp.co.nz <<                  *
>***********************************************************
>
>
2 "88 GTs, both autos!