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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
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Ash Small wrote ...
Proud Mary wrote ...
My point, not perhaps well made, is that as the LOPT frequency increases, the amount of filtration required to turn the output into 'pure' DC decreases, and this is one of the reasons that makes using these much higher frequencies attractive to circuit designers. At 150 kHz, I would expect the amount of filtration provided by stray L and C would tend to produce a 'raw DC' waveform - no clearly separate current pulses as you are hoping for.
Well, that might not be a problem. If we put a capacitor accross the spark plug, it will charge up until breakdown is achieved. Current will continue to flow at a reduced voltage after breakdown. This is pretty much the ideal scenario. Thanks PM.
Doesn't the spark have to happen at a very precise point on the stroke? That wouldn't happen if you left it up to the spark gap to decide when it wanted to break down - far too many variables, like erosion and/or deposits on the elctrodes, cylinder pressure, ionized gas flow around the electrodes, and so on.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The book actually includes schematics for a few electronic ignition circuits (one based on tubes!) and has a discussion of CDI vs. ordinary ignition. He mentions that the spark from a CDI system is better at overcoming dirty plugs, but somewhat less effective at igniting weak mixtures, because of its short duration.
It is an old book, but Ricardo was well ahead of his time.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
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Posts: 4245
Proud Mary wrote ...
Doesn't the spark have to happen at a very precise point on the stroke? That wouldn't happen if you left it up to the spark gap to decide when it wanted to break down - far too many variables, like erosion and/or deposits on the elctrodes, cylinder pressure, ionized gas flow around the electrodes, and so on.
Yes, but we're talking ~6uS, I think, between pulses, 360 degrees @ 6000RPM = 0.01S. 1 degree is ~30uS, so even if it takes five pulses before ignition, that would only be one degree, which won't be critical. In reality it should only vary by a pulse or two (I'd expect), so that shouldn't be an issue here.
This engine probably won't be efficient at 6,000RPM anyway, it will delelop most power at around half those revolutions per minute.
The spark plug has ~10pF anyway, so additional capacitance may not be required anyway. Voltage will just increase until breakdown occurs, then current will flow at reduced voltage.
That pretty much resembles some of the waveforms I've seen, so maybe I'm not so far off target?
EDIT:
Proud Mary wrote ...
My point, not perhaps well made, is that as the LOPT frequency increases, the amount of filtration required to turn the output into 'pure' DC decreases, and this is one of the reasons that makes using these much higher frequencies attractive to circuit designers.
Do you have any idea how they drive them at these frequencies? Any 'tricks' they use?
Steve Conner wrote ...
He mentions that the spark from a CDI system is better at overcoming dirty plugs, but somewhat less effective at igniting weak mixtures, because of its short duration.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
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Posts: 4992
I'd always thought that the production of microsecond pulses in the range 25 - 50 kV at some power, as you have proposed, required circuit topologies and techniques rather different to the ones that seem to interest you - such as transmission lines, which I mentioned near the beginning of this thread, or even pseudo-spark switches.
And then again, what advanage would be gained by supplying the sparking plugs with 50 kV?
It's all a bit too deep for me, I'm afraid, but there will be others here who have a clearer idea how you might best proceed with this.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
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Proud Mary wrote ...
I'd always thought that the production of microsecond pulses in the range 25 - 50 kV at some power, as you have proposed, required circuit topologies and techniques rather different to the ones that seem to interest you - such as transmission lines, which I mentioned near the beginning of this thread, or even pseudo-spark switches.
And then again, what advanage would be gained by supplying the sparking plugs with 50 kV?
It's all a bit too deep for me, I'm afraid, but there will be others here who have a clearer idea how you might best proceed with this.
You may well be correct, PM. I'm approaching this by reading the claims the existing units on the market, and looking at the claimed waveforms. Also, monitor flybacks operate at these frequencies, although at lower power levels, without the kind of matching networks I assume you are hinting at.
The leads to the plugs appear to be conventional, un-suppressed (copper) HT leads. I've come accross no mention of matching transformers, spark tanks, etc.
The point you make about the individual pulses 'merging' into one long pulse interests me though, as this could/would explain some of the waveforms I've seen. The 'stray' capacitances in the plug, etc. would allow the voltage to rise until breakdown, then the discharge would continue at a lower voltage.
I also can't be certain what peak voltages these units are achieving. Claims of up to 50kV have been made, although breakdown usually occurs at lower voltages,
I guess I'm just going to have to try it and see what happens.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Perhaps it would be helpful to first define the needs of the application, and then consider the various ways in which those needs can be met.
What is an ignition system but a pulse generator with external sync?
What is the pulse amplitude, duration, and repetition frequency required by the application?
Once you have defined these basics - adding the fast burst mode pulsing of your particular experiment - you will be in a much better position to choose among the different means of achieving these ends.
So turn off your mind, relax and float down stream, until you come up with the best solution, rather than trying to pin your hopes on a monitor flyback from the outset.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
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Posts: 4245
I've not pinned my hopes on anything yet
The flyback topology is one option. There are two others, one is to just produce one spark, and try to get it to oscillate accross the gap, the other is to use push-pull topology, at the resonant frequency of the stray capacitances and inductances, and ring it up until breakdown, which is what I believe some of the other units do.
I appreciate that, due to the capacitance of the plug, etc. that it might take several pulses from a flyback before the voltage is high enough for breakdown to occur, and that it won't be a clearly set burst of pulses, as they will tend to merge into one,
I'm sure there are plenty of (more complicated) ways to achieve this, possibly 'borrowing' from Tesla coil technology, but the 'flyback topology', due to it's simplicity seems a good place to start, especially since we know it works in monitors at these frequencies.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I've found a paper on high voltage flybacks with resonant elements, which I'm currently reading: ]1-jpe09133.pdf[/file]
EDIT I think, from reading another paper (I'll try to post it later), that, if the output pulses 'merge' into one, then that constitutes continuous mode operation. The above paper clearly relates to discontinuous mode operation, because the output current drops to zero before the primary switches on, so the above paper is irrelevant here.
damn!! I was gonna mention silicon chip article by J Clarke.
I thought nissan skylines had ferrite cores and the coil is direct on the spark plug so theres no inductance of the leads etc
I few weeks ago I was reading IH article on polarity of sparks using a pencil = old school tricks
what about the guy who holds the record using cadillac coils , hes up around 100Kv . he might have a few ideas
I reakon youll come up with something interesting, weve been using coil ign for over 100 years . also Ford model T had the trembler coil that oscillated each "spark" not just once
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Electronic ignition thread
