What the hell did I blow up this time? Lousy spark at higher pressures > Paschen's Law

[SJ]

I think I have figured out the answer to the question > Why does higher
compression result in higher resistance to spark?

The answer is Paschen's Law.
"Paschen's Law reflects the Townsend breakdown mechanism in gases, that is,
a cascading of secondary electrons emitted by collisions in the gap."

Electrons need a bit of room to accelerate and ionize gas which results in a
path for the spark. Speed and collisions are required to ionize the gas. If
you have more atoms in the way(high compression), the electrons keep getting
slowed down by collisions before reaching a speed that will result in
ionization.
The main electrical spark always follows the ionization path.

It follows that if you  have a wimpy ignition system it will work ok at low
loads. But it will not have the required energy(voltage) to work correctly
at higher loads(boost pressures).

So, now we know how that works. Thanks to all those that participated in
this discussion.

This was such a pleasant diversion. I guess I have to go and do some real
work now. Sheeeeeet!

SJ
88 5kq
90 100q


> On Wed, 30 Oct 2002, SJ wrote:
>
> > I have often thought about this problem . . and how to diagnose it.
> > Similar problems occur when you have old(high resistance) sparkplug
wires.
>
> Yup, that was my first guess.  On my other turbo car, high-boost miss is
> nearly always bad wires.  High boost race cars go with really big coils to
> mitigate the problem.
>
> > it does not fire. But . . . .one would think that a richer air fuel
> > mixture(under load) would result in a lower resistance for the spark. So
why
> > does the plug fire ok at low loads and miss at higher loads??  What am I
> > missing here?
>
> Something about high pressure from forced induction increases resistance
> rather than reducing it- dunno why, but it always happens.
> --


> >You are missing the scientific fact that an arc is more easily produced
at
> >low gas pressure than at high gas pressure.
>
> this is nicely demonstrated by high voltage spark gap switches. These
consist of
> two electrodes in a pressurized chamber. You put in a know gas (N2 or
something
> else such as SF6 (sulfur hexafluoride) at a know pressure and they will
arch at
> a specific voltage you pull off of a table given the gap. I played with
them
> while working at a plasma physics fusion research lab (lots of high
voltages
> there)
>