SpudFiles

Is it possible to use a spark plug and ignition coil running off a small 12v battery as an ignition source? I was thinking of doing that for my first propane cannon.

yes, but you need some method to rapidly turn the current to the iggy coil on and off. this is often done with a relay, hooked up so that when energized, it cuts it's power. The coil is wired in series with that, with a capacitor in parallel to keep the inductive kick from burning the contacts on the relay too quickly.

If you want something quick and dirty, you could dump a camera flash capacitor through the coil, although the recharge time is ungodly slow, and you will get one heck of a shock if you make a mistake.

ramses wrote:yes, but you need some method to rapidly turn the current to the iggy coil on and off. this is often done with a relay, hooked up so that when energized, it cuts it's power.

Alternatively, it can be done (read: I've done it) with something as simple as a beefy momentary toggle switch. Just "goose" the switch and you're good.

Hmm well I tested it out last night with a pair of 9v batteries and a switch. I also had an earth lead going from the plug to the negative side of the coil as well as the normal ignition lead and it sparked every time I flicked the switch on OR off. Which confuses me a bit.

cfb_rolley wrote:Hmm well I tested it out last night with a pair of 9v batteries and a switch. I also had an earth lead going from the plug to the negative side of the coil as well as the normal ignition lead and it sparked every time I flicked the switch on OR off. Which confuses me a bit.

It's the change in magnetic field which cause the sparks. The field is created when you complete the circuit then collapses when you disconnect the battery.

Oh ok. So if I use a button in place of the switch it would be fine as a trigger then?

yes. I would recommend placing a capacitor in parallel with the batteries, as that will give you a higher peak current (which is what you want for a more energetic spark) You could even put a resistor between the capacitor and battery to make the battery last longer. You wouldn't need to tap the trigger, either. Instead simply press it, and when the capacitor is discharged, the field would collapse, yielding a spark.

ok sweet. what size capacitor are we talking about? and would i benefit from using a higher voltage or would the coil prevent it from being higher than 12v anyway?

To properly use a regular ignition coil with a 12 volt battery, a small high voltage cap needs placed across the switch. In this configuration, the switch is referred to as the points. A momentary push button switch will do the job.It provides the main spark on button release.

Schematic is all over the net.
http://en.wikipedia.org/wiki/Ignition_system

Note in the diagram shown, the one side of the points and one side of the cap are connected to ground. In s spudgun, this would be the battery minus, the cold side of the spark strip, one side of the momentary switch and one side of the capacitor.

A better drawing showing the components connected is this one.


Typical condensor values range from 0.01 to 0.03 mfd.

So I built my new cannon and the spark plug ignition failed. I'm not sure if the voltage was high enough or not, but I had a working spark every time and no fire, which sucks. This leads me to building my own circuit, or using some other form of electronic ignition available from bunnings. Any suggestions?

what about glow plugs ? they have some advantages and tehy might be especially useful for semiauto and full auto hybrids

Hmm that'd be a good one to look in to

the issue is the mixing. Almost any spark will ignite a fuel/air mixture, as long as the mixture is good near the spark.

Try a chamber fan, and propane if you're not already using it. If your spark gap is in an isolated area of the chamber, a good mix might not get to it. I had that problem with my hybrid.

Well my cannon is pressure rated to 140psi, so I was intending to hook it up to metered propane. The chamber is 55mm I.d x 500mm and the spark is at the back at the moment so maybe longer chambers need a fan no matter what? Is big spark only really needed in a hybrid?

You never need a "big" spark. In a hybrid the high pressure increases the breakdown voltage of the gap so you need a higher voltage source, but not a higher current source, so the power required really doesn't change all that much.

In a 1X gun any visible spark should ignite the mixture. Chances are a no-fire situation is caused by an improper mixture or poor mixing. If you suspect the mixture is OK then you can rule out mixing as a problem simply by letting the gun sit for a few minutes after fueling. That gives the fuel time to mix by diffusion alone.