1/21/2012

I tested the new Augmented Spark Igniter (ASI) last weekend (see archive notes from 9/29/2011 for details). Four runs were performed, the last of which melted parts of the igniter so I'll have to make a new one. But that's not a big deal because it was fairly easy to fabricate and I want to change some aspects of the design anyway. Runs 1 and 2 were performed at the design mixture ratio (design r=1.0, actual r=1.1) but I didn't see any visible plume so I initially assumed that it wasn't working. I measured a reasonable chamber pressure but didn't see anything interesting in the video. Close examination of the still pictures for Run 2 show a slight darkening of the image in the exhaust so it appears that the igniter was working. However, the combustion reactions were probably complete before the exit due to the long nozzle so nothing was visible. After the run, as the pressures were dropping, the mixture leaned out enough where I saw a brief flash of flame. For Run 3, I adjusted the pressures to get a leaner mixture (r=1.9) which resulted in a more "normal" looking plume but was much hotter than I had designed it for. For Run 4, I leaned it out even more (r=3.8 oops!) and got a really impressive plume but it was so hot that it melted parts of the stainless steel chamber. Interestingly, all the of the damage was confined to the upstream portion of the chamber - the throat was unaffected. I think I also had a leak at the pressure port because you can see the tube glowing red hot almost immediately. For the next version, I'm going to move the throat down closer to the end of the igniter so the the body doesn't draw out all the heat from the gas flow.

It looks like the NGK ME-8 spark plug may not be the best choice for this application. The center electrode is very small and indeed, after Run 3, I noticed it was mostly gone so I changed it out prior to Run 4. The new one didn't survive Run 4 either. Of course, both Run 3 and Run 4 were much hotter than I intended so the plug might have been fine for a mixture ratio of 1.0. For the next version, I'm considering using an NGK R847 or R0045J NGK surface gap plug, both of which have larger center electrodes. They are also larger in diameter (8 and 10 mm) and have a much longer reach (19 mm) so the igniter body will have to be longer to hold the threads. The RCEXL ignition worked great so I'll keep using it.

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