This is the first test of a rocket igniter (i.e. small rocket engine) running on compressed air and propane. Using compressed air as the oxidizer is notorious for causing difficulty in keeping a rocket engine lit, as you can see here where the rocket only ignites a few times (by the spark plug) and never stays lit for any length of time. Basically, it appears like the spark just ignites what's in the ignition chamber before it all goes out. I tried some nozzles slightly smaller and slightly larger, and one just very slightly smaller seems to work best. (I also tried increasing the spark frequency from 31Hz to 122Hz, which helped slightly.) All of them seem to work best after the chamber becomes warmer after more testing. I may try running it for longer to warm it up and see if it ever stays lit for an appreciable length of time. I'm just concerned about melting the plastic tubing which brings the propellants into the chamber if I get the chamber any warmer (yeah, I probably should've tried metal plumbing there...). I may try seeing if I can water-cool it a little.
http://youtube.com/7alGMFgz52s
I originally had a lot more trouble with the spark generator producing a whole bunch of EMF which caused the microcontroller to go crazy. The relay which I was using for isolation earlier wasn't helping anymore for some reason, so I switched to an LED/photoreceiver setup which worked pretty well then had its own problems. I separated the power supplies to separate AC circuits and then used a metal mesh to shield the non-sparker side of the test stand, and that seemed to do the trick, 95% of the time, at least (good enough for me, though I can do better). I think I'm slowly wearing out the microcontroller somehow.
If I'm ever able to get it working smoothly, I will make a parts list. I need suggestions about how to keep it lit.
Here's a picture of the chamber with the nozzle unscrewed:
