[Diy_efi] Ion Sensing project-

[Ville Vartiovaara]

> After Ignition, assuming constant current, wouldn't the voltage be 
> proportional to the no. of ions in the gap? 

There's a fundamental difference between currents that flow accross a gap,
and that is the form of charge-carriers. They can be pure electrons, like
in a cathode ray tube, or ionized particles (molecules and bigger), as in
what we are interested in in ion sensing.. 

Also the particles are divided into two groups: The ones that are ions due
to an ongoing chemical reaction (combustion -> the flame term), and the
ones that are ionized by the voltage applied accross the gap (both during
post-sensing the pressure curve, and also when ignition arc is burning).

Actually, only the dielectrical breakdown voltage (the voltage at which
the spark ignites) is proportional to pressure (no. of ions/volume), and
after that the thing becomes a mess due to the ions produced by the
reaction, and also that during combustion the particles recombine
producing new particles with different ionization properties.. 

Does combustion blow-out the arc? 

No candle-effect ;) Actually, it creates ions that would help the arc
continuing, but the power is too high that is needed to maintain the
ionized path, so the arc extinguishes.

> The spark voltage waveforms I've seen have a fast rise to the arc voltage 
> followed by a jagged, distorted, stepped, exponential fall. 

Just, guessing that the steps mey be due to that different particles have
different ionization voltages; when the voltage drops below some of these,
only a few of that type continue as charge carriers.. So the voltage
drop slows down suddenly.

Generally, the problem in spark-current measurements is that the current
is so high, that no coil can supply it constantly. Also the side-effects
of the 'plasma path' are unpredetermined.

> How about a "strain gauge head-bolt washer" to directly measure cyl pressure?
> 

I would only use it as a feedback of if there was a strong kick at all or
not. The massive head acts as a lowpass filter, giving a very nonlinear
phase response, so no reasobnable waveform can be obtained, I think.


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