SpudFiles

After much delay I finally found time to finish my piston valve hybrid cannon, but after 4 attempts at a test fire I can't seem to get it to work.

I've went through and looked at all the joints for leaks (w/soapy water) at much more pressure than it would see post fuel and air injection and I can't find any. I'll have another run through later to double check though.

For my mixes I've been trying 4x and 5x mixes (2.4PSI propane and 44 PSI of air with a final pressure of 46 PSI as well as 3 PSI of propane and 59 PSI of air with a final pressure of about 60 PSI). I believe I have my pressures correct and my gauges are functioning as I've tried using my metering setup on my burst disc hybrid and it worked fine. Please correct me if those numbers don't look correct though.

My ignition setup has been acting up lately with the remote switch but 9 out of 10 times it works (I'll probably try and fix, or I may just rig something up that's a little more robust). The wires leading into the chamber don't seem to be shorting anywhere and the spark gap is set up just about in the center of the chamber.

My only guess is that the fuel-air mix isn't mixing well enough in the chamber? I tried letting it sit for a minute but it still didn't fire.

Any suggestions/ideas of what I'm doing wrong?
Thanks!!!

Can't these set ups work as piston pneumatics as well? Get the piston actuating first then add the mixes would be my advice.

What are you using for ignition and how big is your spark gap?

Does it fire at 1x?

I've fired it as a pneumatic a few times to test that the piston fits and the seals are good, for ignition I'm using a dc step up module similar to this one (https://m.ebay.com/itm/DC-3V-6V-to-400k ... 2522245188).
I haven't thought to try 1x mix, I'll do that when I get the chance.

Thanks!

Your problem might be too big of a spark gap, don't forget that it behaves differently as the pressure goes up.

I forgot to mention, my spark gap is around .5-1mm before I inserted it (I don't know of it shifted when i installed it). I tried filling the chamber with 90 PSI of air and it seemed to be sparking in the chamber (at least it was sparking off the leads going into the chamber).

Juggernaut12121 wrote:it was sparking off the leads going into the chamber

There's your problem?

Oh no, I mean I have two leads coming out of the chamber for the spark gap, and while under pressure I connected one lead to one end of my module and then held the other lead ~1mm from the other end of my module to see if a spark would jump (as I assumed that it would mean I was getting a spark in the chamber).

Ah ok. Must be a mix issue then. Have you given volumetric metering a go rather than manometric or is the launcher too big for that?

It's somewhat impractical to try and fire with volumetric metering (chamber is about 2''x32'' [51mm x 810mm]). I've just tried firing it at a 1x, 2x, and 3x mix, loading the propane in first on the latter two and then the air as well as just trying to let it sit for a minute for the gases to mix, but still nothing
I've also tried out my metering setup and the same propane and air pressures on my smaller burst disc hybrid and it seemed to work fine. After each non-fire I checked the pressure in the chamber and it doesn't drop at all, so I can't imagine I have a leak substantial enough to ruin my mixture.
I'm very stumped with this one.

If the mix is right then the spark is wrong. You may have have an internal short in the spark gap.

If you have lubricated some part, it has happened to me, being excessive the amount of lubricant, the hybrid does not explode. It is interposed between the small space of the electrodes.

The piston is lubricated but its not at all volitile (it doesn't seem to be putting off any fumes that I can smell). I hooked an ohmmeter up to the spark gap leads and read some resistance so I thought that there may have been a short but once I redid the leads and their entry into the chamber it still didn't fire. There might still be some shorting? I tried popping one end of my ohmmeter onto the conductor and the other onto the insulatong jacket of the wire I was using and it still read ~1.5MΩ, so I'm not sure if my wire is just crappy. Even so I've tried it with a 3x mix and I feel that the spark should have jumped in the chamber (the pressure still being fairly low and the gap being ~1mm, it also had an easy time arcing without shorting with a 2cm gap in open atmosphere).

At any rate I have to go to campus so it'll have to wait for another 8 months or so. Thanks for all of the suggestions!