water injection questions

[Tony Bryant]

> 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.
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* "Insanity is the only sane response to an insane world" *
*                 >> bryantt at fp.co.nz <<                  *
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