How Does Adding a Gap in HV Cables Enhance Spark Reinforcement?

[CHICAGO]

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Hi all.

Some time ago I read that old competitions car drivers realized that by inserting a gap in the HV cables, the spark in the plug was "reinforced", like getting a more energetic discharge and, consistenly, a better fuel ignition.

This is a drawing about what I am talking about. The gap was obtained by inserting a isolating button and threading the cable tips throu two opposite button holes.

[PLAIN]http://img97.imageshack.us/img97/7996/button1engl.jpg
By chicago49 at 2010-04-23

Is there an explanation for this spark strengthen by adding this gap to the HV circuit?.

Thanks in advance for your reponses.

 

[pallidin]

Well, the button would function as a high dielectric capacitor(assuming plastic) with nearly zero capacitive storage.

Not sure how it would help, though, in this case. Seems like it would reduce rather than increase anything!

 

[jack action]

Although I never heard of this trick, I imagine that it would act as a transformer. By putting more turns on one winding on one side than the one on the other side, you could create a higher voltage, hence a better spark. Some sort of "secondary coil". It must have been done at a time when ignition technology was at its beginning and not very reliable.

 

[Gordianus]

Sorry guys but the trick neither rely on dielectric properties nor is a transformer.
Old ignition systems generated high voltage pulses with a rather long rise time. This poor rise time generated "weak" sparks, specially at high rpm. The button provided a way to delay the onset of the discharge until the voltage was very high. This way a stronger spark was achieved. Nowadays we have electronic ignition circuits that deliver high energy, fast rise time pulses, so the days of buttons are gone.

 

[pallidin]

Sorry guys but the trick neither rely on dielectric properties nor is a transformer.
Old ignition systems generated high voltage pulses with a rather long rise time. This poor rise time generated "weak" sparks, specially at high rpm. The button provided a way to delay the onset of the discharge until the voltage was very high. This way a stronger spark was achieved. Nowadays we have electronic ignition circuits that deliver high energy, fast rise time pulses, so the days of buttons are gone.

That doesn't make any sense.
If the button delayed the eventual spark until it "built-up" there would be serious timing issues that would need to work precisely with the rest of the piston spark plug ignition.

Perhaps someone can show legitimate literature showing this was used for sustained benefit, or any literature showing this was used at all.

 

[Antiphon]

Sorry, that's the right explanation. The rise times are long in electrical, not mechanical terms. We're talking about rise times in the 10's of microseconds. The culprit in old cars were the mechanical contacts that had to break the primary circuit on the induction coil. Those old points took milliseconds to open and during that millisecond you get a weak output pulse. Modern ignition systems use semiconductor switches that open the induction primary in nanoseconds.

 

[pallidin]

Sorry, that's the right explanation. The rise times are long in electrical, not mechanical terms. We're talking about rise times in the 10's of microseconds. The culprit in old cars were the mechanical contacts that had to break the primary circuit on the induction coil. Those old points took milliseconds to open and during that millisecond you get a weak output pulse.

And this would be improved by a shirt button? Prove it!
I want to see actual literature or other evidence. Not just that "someone say's it's true"

Can you do that?

 

[Antiphon]

No I can't. All I can offer you is 7 years of experience designing high voltage coils and the circuits that drive them for the purpose of creating strong sparks (for lighting hot arc lamps.)

do you have anything other than deep conviction to support your view?

 

[CHICAGO]

Thanks for your answers, Antiphon and Gordianus. To me it sounds convincing.

The usual old points-coil ignition systems had an L-C resonant frequencies in the range of 5 to 10 khz. That means a long rise time of the first cycle and the spark breakdown voltage would have an uncertainty value especially at high RPMs.

So, by inserting this kinda air gap, we need a higher breakdown voltage but the "trigger" is better defined without loosing the necessary coil-energy to keep the spark ignited for the 1 or 2 milliseconds we need for a total combustion.

Is this more or less what you argue?.

Thanks a lot.

 

[jack action]

The way I understand the explanation of Antiphon is that the ignition system needs a very large electrode gap at the spark plug to function properly, but increasing the distance between the electrodes would be useless since the distance from the central electrode to the side of the spark plug is the largest gap possible. By putting this additional gap on the circuit, the two resistances add up, and the ignition coil works accordingly.

As for the timing issue - if there is one - last time I've check, initial timing is adjustable on every engine.

 

[pallidin]

OK, well there seems to be enough people here that claim it's true, so I guess it's another "I learn something new everyday"
I wasn't belittling anyone, I had just never heard of it before and was pressing for facts that PF often requires.
Anyway, interesting!