Maximum advance

[Bernd Felsche]

Greg Hermann tapped away at the keyboard with:

> >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!

Let's keep the computation fluid thermodynamics out of this. Heat
transfer to the walls is a small factor; as is heat transfer to
end-gas. The compression of the end-gas, and hence its increase in
energy, should be taken into account.

So for a first-approximation, the model is sufficient.  Else you'll
end up trying to figure out at what point boundary-layer stripping
occurs due to knock. If you manage to do that, then the boys from
Detroit will come around with some iron bars and re-arrange your
knee caps; simply because you're being an extreme smart-arse!  :-)

> 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!

Given that the pressure would "equalise" in about 0.1 milliseconds,
it's a reasonable first approximation to assume some things
"constant". Especially if you're trying to get a working simulation
of sorts within a week; compared to the 30 years required for a
detailed one.

-- 
Bernd Felsche - Innovative Reckoning, Perth, Western Australia
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