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I'm looking for any tips on a simple and cheap means of generating a regular spark for ignition of propane in air, could be electrical or other but needs to be simple and fairly reliable.

Criteria are :

  • needs to remotely ignite/re-ignite a propane jet through a 6 mm OD copper pipe, will be burning in open air (not an enclosed furnace etc)
  • it is a prototype so doesn't need to be packaged as a consumer product and will be closely monitored so needs not be completely foolproof but equally shouldn't be inherently unsafe.
  • Can have bulky remote elements via wires etc but the working end needs to be reasonably compact.

This is the prototype

https://youtu.be/zagkIKq17x0

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  • $\begingroup$ A piezo electric unit but if fully auto an auto transformer with a temperature driven switch. $\endgroup$ – Solar Mike Aug 21 '17 at 5:30
  • $\begingroup$ "As he [the Emperor Alexios I] knew that the Pisans were skilled in sea warfare and dreaded a battle with them, on the prow of each ship he had a head fixed of a lion or other land-animal, made in brass or iron with the mouth open and then gilded over, so that their mere aspect was terrifying. And the fire which was to be directed against the enemy through tubes he made to pass through the mouths of the beasts, so that it seemed as if the lions and the other similar monsters were vomiting the fire." Greek fire The quote is from the Alexiad $\endgroup$ – Phil Sweet May 12 '18 at 0:57
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How about a car ignition coil and sparkplug? They are intended to do almost exactly what you want.

The ignition coil is really a auto-transformer. You apply 12 V between the tap and the low end (the primary). This builds up current over a few milliseconds. When you suddenly interrupt that current, the energy in the magnetic core is released by forcing current out the top of the coil (the secondary). That can be 10s of kV if necessary.

I'd keep the ignition coil near the sparkplug, like it is in a car. The two wires to the ignition coil primary can be long, with the power source and switch at the remote end. All you need at the remote end is a 12 V source and a pushbutton that can handle the current of 12 V divided by the DC resistance of the ignition coil primary winding.

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For spark ignition methods, I would recommend using an already existing product if possible. You can modify the length of the high voltage wire(s) and spark points into something more attractive as needed.

Often there is only one wire and the chassis is used for the return. This would be favorable in your design, since one wire is less clutter than two. They make some good small diameter high voltage wire, that you could probably chase up the inside of one of your pipes. Just make sure the voltage rating of the wire is higher than the voltage required for the spark gap. The dielectric strength of air is 3MV/m, so for 15kv wire the spark gap needs to be less than 5mm.

If you make your own spark points you will want them to be out of high temperature oxide resistant metal, or over time (100+ ignitions) the surfaces of the sparking materials will erode to the point of failure.

A pizeo igniter generates a high voltage spark by a push button spring striking a pizeo crystal. This obviously requires mechanical input, but wires can be extended to where you need them and no batteries are required.

An electric bbq igniter uses a AA or AAA battery, some internal electronics to create a pulse through a transformer to get the high voltage. Beyond the unit, the high voltage wire and ceramic insulated spark points are the same.

Another option to consider is a nicrome hot wire (especially if your device turns on and off frequently or has variable flow rates). You run low voltage DC electricity through the hot wire till it glows red and it will ignite the gas. Looks like someone on instructables used it for a similar application. This requires access to more electricity than the other options, but it makes your controls simpler since you would only need to modulate the gas. It is silent and doesn't make spark noises. It would also keep low gas flows ignited beyond the point of stable flame.

You would only need 5 mm of 20gauge nicrome and 5 volt at a couple amps would probably do the trick. You may need to put it just to the side of the gas stream so it doesn't cool down too much in a rapid on/off situation.

Pretty cool project, good luck as you move forward on it!

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Go to a home & garden supply store that sells replacement pushbutton piezoelectric igniter units for propane BBQ grills and buy one.

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