Nico

You´r right there, the voltage probably will be very low to conduct the current, I really like to know how much voltage develops in order to get the the current through. Do you have info about that? Interesting

Nico

The layer is barium oxide for cathodes with low temperature (red-hot) and Thorium for high temperature (white hot) cathodes as used extensively in transmitting tubes. But the effect will take place in vacuum where the elctron travel distance is large. At atmospheric pressure, that high temperature will lead to an easy ionisation of the hot gas around the electrode, the electron emission is of no effect. . It is the electric field that ionizes the gas (more easy when the gas is hot) and the polarity is again of no consequence. The spark plug voltage is AC as a result of the resonance of the capacitor and the inductance of the coil.

Tom Piper

I wasn't dreaming about the graphite in a lead pencil.

Read to the end of this:

http://hometown.aol.com/bgahc/01_jwh...parkplugs.html

While reading this, keep in mind, the "waste spark" coil secondaries are electrically isolated from their primaries.
But, usually, the single coil system coil is electrically connected from primary to secondary.

And, more importantly, notice the text is talking about postive or negative of the battery (low voltage source, not the high voltage causing the spark).

Plus, the "waste spark", or any multi-coil setup, does not have to worry so much about "dwell time" problems, like a single coil system does.
My guess is "waste spark" coils have a hot enough spark that polarity is not considered as a problem like in a single coil system where the spark is marginal at high rpm due to decreased dwell time.

Tom Piper

silver & red CE

I meant "engineering very low", which I always mean to be less than 10% of measured value (that's what they taught us back in the day), in this case, 10% of the the arc over voltage in the compressed cylinder . We didn't do any waste gas testing in our lab, so I never saw it work, just heard about it. Unfortunately, the guy I did this with retired over 5 years ago, so I don't have anyone to call, sorry.

silver & red CE

The voltage is ringing, with an AC component, but all the systems I've seen are direct coupled and the coil's energy can only be used up in the spark gap by flowing in one direction. The coil is really a transformer the steps up the voltage on the spark gap and the circuit is a classic flyback with the spark gap acting as a rectifier on the secondary side. If you want to simulate a simple spark gap in SPICE, use two zener diodes back-to-back. A better simulation includes some active componets to simulate the higher breakover voltage and lower conduction voltage.

LT4BUD

You are half right

1)Right----Breakdown in vacuum is a whole different animal than breakdown with the electrode surrounded by a gaseous medium......

In a vacuum it is for the most part a suface phenomena involving the contact material....think about it there is no gas surrounding the electrode to ionize...

In a combustion chamber there is a gaseous mixture of air and gasoline, under pressure, heated from both compression & the engine.....it is the gaseous mixture that ionizes into conduction...

Vacuum tube phenomona is almost totally irelevent to what goes on in a combustion chamber....

2) Wrong----Polarity does come into play.........in my previous post I said "significant"......that is an overstatement, should have said measurable difference..

Nico

This is really interesting. I like to investigate. I did design a few plasma generators and never found a difference. Most are RF powered, but also DC powered cold plasma generators are sometimes needed. If there is a polarity issue, it may have some impact in my work.
If I have some time, I am gonna measure it. I run an electronic laboratory for high frequency power electronics (RF generators, transmitters, tuners, plasma systems etc.) here and have all the means to test it.
I keep you posted.