Sparkplug feedthroughs

[Jon Rosenstiel]

See Jim Lux' web site at: http://home.earthlink.net/~jimlux/hv/sparkplug.htm
for an excellent how-to originally written by Richard Hull.

You can also find more info by searching through the Songs.com and Intranets.com Fusor archives.

I have read that the peak voltage on a plug can reach 30 to 40kV. Remember that the plug has to fire under considerable pressure inside the engine's cylinder. It takes a pretty high peak voltage to "break-down", (ionize), the fuel/air mix between the plug electrodes in the cylinder.

I would say the 30 to 40kV number is correct because from testing I have done on motorcycle ignitions the coil has to produce about 40kV to reliably fire the plug under racing conditions.

Jon Rosenstiel

[DaveC]

Many new high energy automotive ingnition systems can produce up to 80 kv. The plugs are usually more prone to external flashover since that is the dirty area and ... it is at one atmosphere only. Peak pressures in the cyclinder during ignition are quite far above the compression ratio making it easier for the shorter plug nose to stand off the voltage.

Under vacuum, however, the short nose is now the weak point in the insulation. I have not made any detailed tests, but I would expect surface contamination... and... outgassing to be significant in limiting the maximum breakdown voltage. Depending on the contamination makeup, secondary emission from impact ionization produces avalanche condtions along the surface.

Vacuum levels usually need to be in the 10^ -6 Torr range to get the best stand off.


Dave Coope

[guest]

I just purchased the AC 44XLS and the Champion F121501(AKA the 220). The Champion is definately the better canidate. It is true that the vacuum side insulator is about 1 1/4" long after machining. After the ceramic ends, there is approx another 1 1/4" length of bare electrode sticking out. Lots of room to work with.