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Old 10-07-2008, 12:49 AM   #31 (permalink)
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As stated by others here increased power through the plugs increases wear on them. I do know you can get a short term power gain at least until the spark plugs degrade but is it worth it in the long run? I don't think its economically viable. The only thing that would really work is a double cored plug with two coils feeding it and a separate electrode for each core.


Aircraft have two plugs per cylinder. The also use magnetos that produce a longer and hotter spark. This does give them more power but it does wear the plugs out quicker. To deal with this they go through a much more rigorous inspection and maintenance schedule. I think they are checked every 100 hours of operation.

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Old 10-07-2008, 10:58 AM   #32 (permalink)
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Kit- I'm a broadcast engineer and electronics hobbyist. I've been making high voltage and arc discharge devices for around 30 years also.

I think we're probably talking at cross purposes here. The idea I had is to create an AC arc that lasts for around half an engine revolution, from the start of the combustion cycle until the exhaust valves open.

I don't see how a small capacitor can sustain an arc through a sparkplug for tens of milliseconds, given that the impedance of the arc is very, very low.

Do you still have the trace of the current sensor? I'd be interested in seeing it, if you do.

A low frequency (tens of kHz) AC signal at 10-40kV delivered by a flyback type transformer is an easy way to get an arc of arbitrary duration. I've used a similar circuit to keep a lean-burn forge burner lit, although this was at a very low power and voltage.

It's probably not even relevant, as there have been so many attempts to 'improve' the classic pulse transformer ignition system.

I've been enjoying discussing it with fellow enthusiasts, though.
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Old 10-07-2008, 12:56 PM   #33 (permalink)
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jim-frank,
I fail to follow your thinking. Once the spark is initiated, the impedance drops, a circuit component switches in a capacitor that delivers current to prolong the spark. The capacitor initial charge of 300-400V and 1uF provides the energy. I do not have a trace however visualize the side profile of a Dodge Intrepid. The back of the car is the initial spark the front bumper is the end of event.

In the flyback case a circuit designed for 10-40KV at low current, at spark, sees a drop in impedance and a voltage less than 50V. Now what! There is a huge mismatch in the circuit impedance.

ConnClarke brings up a good point multiple sparks help. They do this because the flame starts in two places. Speeding flame burn enables improved timing and centering of the energy delivered by the power stroke. If the flame is too slow and timing retarded, some combustion happens in the exhaust wasting energy. It is difficult to find the room in head for two plugs in multi valve (4) engine. Instead combustion chamber design is used to speed flame travel.

A correctly designed combustion chamber with correct mixture, and modern (electronic direct fire or COP) ignition system works quite well. A long duration spark that erodes plugs is not necessary to keep the fire burning!
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Old 10-07-2008, 02:10 PM   #34 (permalink)
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Quote:
jim-frank,
I fail to follow your thinking. Once the spark is initiated, the impedance drops, a circuit component switches in a capacitor that delivers current to prolong the spark. The capacitor initial charge of 300-400V and 1uF provides the energy.
The time constant of a 1 uF capacitor is one microsecond per ohm of spark gap resistance. If we assume an arc resistance of say 10 ohms, in fifty microseconds the capacitor will drop from 400 volts to nearly zero. That's not very long, and won't prolong the spark much, if at all.

Quote:
In the flyback case a circuit designed for 10-40KV at low current, at spark, sees a drop in impedance and a voltage less than 50V. Now what! There is a huge mismatch in the circuit impedance.
The spark will dump whatever energy is stored in the secondary winding of the flyback, and when the voltage drops below what it takes to sustain the arc, it will extinguish. It will then restrike on the next cycle of the waveform.

Quote:
ConnClarke brings up a good point multiple sparks help.
Multiple sparks in the time domain help as well, as the Mallory MSD shows.

Quote:
A long duration spark that erodes plugs is not necessary to keep the fire burning!
It's really a series of short duration sparks. Plug erosion will not occur if the energy and power per spark is below what it takes to erode metal from the electrodes.
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Old 10-07-2008, 02:23 PM   #35 (permalink)
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Do it. You have all the answers. Don't let me stop you.

I'm just an old fart with experience.

Please post you results, I always like to learn.
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Old 10-13-2008, 04:25 PM   #36 (permalink)
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You're not alone in the 'old fart' club.

I might make a model one of these days, but I really doubt that the end result will be sufficiently better than existing designs to make it commercially feasible. It would be interesting from a scientific viewpoint, though.

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