Maximum advance

[Greg Hermann]

>
>Beyond that, I'm almost entirely ignorant of how one would normally
>predict pressure based on the combustion process. I suppose one
>could assume an energy conversion based on the proportion of the
>mixture burnt behind the flame front and derive the gas pressure by
>perhaps also assuming adiabatic expansion. Total combustion chamber
>volume is "constrained" by the piston(*) so the gas pressure and
>temperature) would vary.  Does that seem reasonable?
>
>Assuming that the temperature doesn't vary by much behind the flame
>front, and that the pressure is uniform throughour the cylinder, a
>rough numerical model would then be feasible.
>
>The model would at least demonstrate how changing the ignition
>timing influences pressure distribution and hence engine torque with
>varying engine speeds.  Changing parameters such as bore, stroke and
>rod lengths in the model would also show their effects on timing.
>
Ah, but there are so many more fun things to be considered! Some heat goes
into (nearly) adiabatic compression (and therefore heating) of the unburnt
gas, There is radiative heat transfer of heat to the piston crown and
chamber walls (really only significant from the burning and burnt gas,
since radiative heat transfer depends on the fourth power of absolute
temperature), convective heat transfer from both the burnt and unburnt
gasses, and so on!

Also sonic effects--as in pressure changes can only equalize themselves
across the entire chamber at Mach 1. (Which of course varies with the
(changing) molecular weight of the gasses and with the (also changing)
(square root of) the absolute temp of the gasses. Nope, gas density and
pressure have no effect on Mach 1!

Greg


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