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Old 09-30-2008, 06:01 PM   #11 (permalink)
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I've thought about this for some time, but never got around to trying it. My concept was to use a low frequency RF/high frequency audio signal to produce an AC spark instead of the single pulse that is produced by conventional ignition coils.

The proposed benefit was to have a spark that would start at the same time as a conventional one, i.e, around 8 degrees before TDC, but which would then persist until the exhaust valves open. This continuous spark would insure that any combustible mixture that passed the spark plug's electrode region would be ignited.

The frequency would be something like 100kHz or so, or whatever was most efficient for the transformer.

As far as putting capacitors in parallel with the sparkplug gap, there is a product called Pulstar (sp?) plugs that do exactly that. The key to avoiding ignition timing problems is to limit the capacitance to 80-100 pF and using non-resistor wires.

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Old 09-30-2008, 08:07 PM   #12 (permalink)
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I think you might need a big amplifier.

Spark advance is often more than just 8 degrees. Maximum advance of 30-50 degrees is typical. It depends on many factors, including RPM, load and temperature.
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Old 10-01-2008, 04:42 PM   #13 (permalink)
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Quote:
Originally Posted by KitCarlsonEMS View Post
I think you might need a big amplifier.

Spark advance is often more than just 8 degrees. Maximum advance of 30-50 degrees is typical. It depends on many factors, including RPM, load and temperature.
It doesn't take much voltage or power to strike and maintain a spark across a gap of .04-0.1 inches. 500 watts is easily obtainable with a standard H-bridge driver circuit, and should be more than plenty.

The advance I mentioned was just off the top of my head. You get the idea, anyway...
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Old 10-01-2008, 05:08 PM   #14 (permalink)
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At 100KHz? I would first try 10KHz, that might be the near spark resonant frequency. You are correct, once the spark ionizes the gap, the voltage requirement is low.
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Old 10-01-2008, 07:41 PM   #15 (permalink)
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Quote:
Originally Posted by jim-frank View Post
As far as putting capacitors in parallel with the sparkplug gap, there is a product called Pulstar (sp?) plugs that do exactly that.
...And they don't appear to have any real effect on fuel efficiency, as tested by an EcoModder forum member... (Perhaps they would help ignite a very lean mixture? But that would require more mods than just swapping spark plugs.)

-soD
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Old 10-02-2008, 05:18 PM   #16 (permalink)
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100 kHz was off the top of my head, as well. That's not critical by any means, but you'd want to avoid frequencies that could interfere with radio communications. LIke submultiples of 455 kHz, for example.

One factor would be to choose a frequency that would have at least several full cycles during the spark 'ON' time. For example, an engine at 6000rpm turns at 100 revolutions per second. If the spark was designed to operate for just over half a revolution, that's 1/200th of a second (5 milliseconds). I guess at 10kHz that would be 50 cycles, well over what I'd think would be a minimum, so that would work. The transformers are pretty efficient over a wide frequency range, so you'd have a lot of latitude.

I would definitely use coaxial cables for the spark plug wires in such a system, to prevent RFI and to transfer energy as efficiently as possible.

As far as 'spark resonant frequency' goes, I can't imagine the system would have a high enough Q to make resonance effects significant. The spark plasma itself is a fairly lossy high power resistor, and that would load the circuit enough so that resonance would be pretty broad.

Unless you have something else in mind by that term?

The Pulstar plugs evidently cause a faster growth of the flame kernel due to the intensity and speed (risetime?) of the spark. That's what the pictures at the manufacturer's website seem to show.

Supposedly the faster flame front means more complete combustion. If your engine is already burning the mix thoroughly due to good turbulence, homogenous mixture, and so on, this plug might not make a noticeable difference. I bet my old iron head Harley would pick up some benefits...
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Old 10-02-2008, 07:55 PM   #17 (permalink)
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I love spending time in the lab, bringing reality to ideas.
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Old 10-03-2008, 10:41 AM   #18 (permalink)
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Quote:
Originally Posted by KitCarlsonEMS View Post
I love spending time in the lab, bringing reality to ideas.
Me too!

It would be fun to work up this idea to a testable unit. I can make a MOSFET H bridge driver for a flyback transformer. I made a similar unit for an experimental induction heater a year or two back. A timing/triggering unit shouldn't be too hard to design, either.

Maybe a signal tapped from the cam position sensor, and some minor computing to derive an adjustable spark timing? How much overlap between cylinders (for the spark cycle) is there? Would you be able to do it with two transformers, or would you need one for each cylinder?

( I guess I could figure the answers to these questions out myself, but I'm feeling lazy this morning... )

Actually a lawn mower might be a better test bed, so the engine that blows up on version one isn't that expensive to replace.


Kit- e-mail me at engr@coloradoradio.com if you want to discuss this in a side conversation. Use the subject 'flyback transformer' so I can spot it through my spam filter.
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Old 10-03-2008, 12:06 PM   #19 (permalink)
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I don't think they heard you some_other_dave

Maybe if we replace the fuel pump with a 10,000 psi pump?
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Old 10-03-2008, 12:19 PM   #20 (permalink)
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Hey, the transformer/lots of sparks thing sounds interesting and different. All I was commenting on was the specific "Pulstar plugs" product, not the whole notion of improving the ignition.

I suspect that any gains would be very minimal, but definitely worth (someone who is not me) checking into!

-soD

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