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So I began work on my first hybrid a few months ago. It's designed for 20x mixes, made of DOM steel, and has been hydrotested to 3500 psi. Unfortunately, it doesn't fire half the time .
I've looked all around this forum, read the wiki, and read Hybrid Fueling 101. First I tried using the chamber pressure method. Assuming a 2x mix (the mix at which I have thus far carried out my testing do to noise concerns) chamber pressure after addition of propane would be:
((100/.958)-100)*2 = 8.76 ci
(8.76/100)*14.7 = 1.28 psi
then adding 1 atm of air:
14.7+1.28 = 15.98 psi
Using this method I got rather sporadic ignition, and was only able to use at most 6 layers of foil if I was lucky.
After this I thought that maybe my gauge wasn't accurate enough for such a method, so I decided to make a meter pipe. My meter pipe volume is 6.1 cubic inches, and my chamber volume is 24.2 cubic inches
to achieve a chamber pressure of 1.28 psi after adding propane I'd need:
then I would add air until the chamber was at about 16 psi as above. Still got sporadic ignition.
After some reading, I decided I should move my spark plug to the middle of the chamber, so I did so and tried both fueling methods again. Still no consistency, so now I'm thinking that the problem lies with the fuel not mixing well enough to ignite. To remedy this I am going to try and add a small orifice in the delivery pipe to increase gas entry velocity, and hopefully create turbulence to mix the fuel. If that doesn't work, then I guess I'll have to wait until i can afford some new gauges
I know the spark circuit isn't the issue, as the ignition coil has plenty of juice. Besides fuel mixing and gauge accuracy, I'm not sure what could be wrong. I'm at my wits end. Any ideas would be greatly appreciated
Fuel/air mixing could indeed be the problem. The good news is that this is a very easy thing to check for. Rather than just fill the gun and fire, try filling the gun and waiting 20 minutes before you fire. If your reliability goes up, then yes, that's a very good indicator that your fuel and air are not mixed.
The beauty is that you can test this without changing anything on your gun.
I also had this problem after I took my hybrid apart to clean it. When I put it back together I was getting sporadic ignition. Sometimes I would get ignition when I shouldn't (not taking in to account chamber pressure of fuel when injecting air (assuming chamber pressure equals 0 when injecting air pressure)).
The problem ended up being I forgot to include the meter volume in my formula but since you inject air directly and not through your meter you don't need to do this. Still, I wouldn't get ignition if I took in to account chamber pressure after fuel injection. Now, it is such a PITA to cut burst disks I hardly use it anymore haha.
No, it won't separate. Ever.
Once mixed, gases stay mixed. Were it not so, we'd all be in a hurtin' way. After all, CO2 is heavier than air....
hm, ok i'll try that tomorrow and post results. I still haven't added the orifice. Also the times that i have been able to achieve combustion, there was always smoke in the chamber. Does this indicate a rich mix?
Another possibility is a problem with the spark. High mixes take much higher spark voltages than do lower mixes. At 10x it'll take roughly 10X higher voltage, or a 10X smaller gap, to get a spark.
You might consider adding an external gap, wired in series, to the internal gap. If the external gap shows a spark then the internal gap should have sparked as well (unless there is a short in the internal gap). If you have a small xenon flash tube (salvaged from a disposable camera flash unit) then wire that in series with the internal gap. The tube will spark just like a regular gap but it'll do it at a pretty low voltage (well a couple KV anyway). The tube will consumes fairly little power from the ignition circuit and even though the gap is 0.5 inch or so it'll spark at a relatively low voltage because of the Xenon gas in the tube. You only get a faint spark in the tube, it won't give a bright flash like a flash circuit.
Also remember that since the break down voltage goes up with higher pressures a gap that works fine at 1X may be shorting (through the insulation, to the chamber ...) at higher mixes. For example, at 1X your gap sparks ("breaks down") at 5KV, the wire insulation is good to 10KV, so you're good since the gap breaks down first. Now you go to 3X mixture, the gap break down is now 15KV so the wire insulation breaks down first and you don't get a spark at the gap.
Tried a couple of 2x shots and so far all have ignited, but I still can't use any more than 6 layers of foil. I'm going to try and tune the mix to see if I can get better combustion. Also there is still quite a bit of smoke left over in the chamber after firing, so that is another hint that my mix isn't quite right yet. It seems that my problem was inadequate chamber venting, and meter pipe venting.
@jimmy101 I sincerely hope that this is not the problem, otherwise i am screwed. I know that DYI has successfully used an "ignition coil" type device up to 20x, so i don't think that should be too much of an issue.
Well, finally got some consistency Five 2x shots in a row. Still only 6 layers, but I'll take what I can get. Gunna try some higher mixes when I can find a huge field far away from my city house
You're lucky to break all 6 layers, as each layer of foil is good for about 40psig. The smoke you've experienced in the chamber could be due to the fact that you're just barely breaking the 6 layers of foil... The spent gases have little velocity left, after barely popping the foil and will stay mostly in the chamber.
... Did a LOT of closed chamber testing, years ago, as well as a LOT of aluminum foil layer testing.
Might I suggest trying the same 6 layers of foil... 2pisg of propane and then pressurize with compressed air, until the gauge reads 32psig?
( 3x mix: .045% propane + 2 atm = 31.4psig )
... Be sure to wear hearing protection
Oh I don't skimp on hearing protection. I'm studying to be an audio engineer, so my ears are a huge asset. I'm actually worried that even with adequate protection the sonic boom from higher mixes will deal me some permanent damage.
I'm not sure this makes sense, where does the pressure go? I see your point in that it could be compared to throwing a baseball at a piece of glass: the glass breaks, and the ball slows down. But combustion doesn't stop when the disks break. The disks simply allow the gasses to reach a higher pressure before pushing the projectile down the barrel.
Many people seem to miss the fact that I filled with oxygen and propane for the 20X shot (I did start with atmospheric air in the chamber, but the other 19X was pure O<sub>2</sub> ). The chamber would have been at around 70psig. I did use an ignition coil / capacitor circuit up to 10X air/propane, but I made an effort to get the electrodes to closer than 0.5mm for reliable ignition and avoidance of arcing elsewhere.
Your pressure gauge will be more accurate in some pressures ranges than others. What is the middle of its range? Take a look at the current thread on Spudfarm`s small hybrid, as many of the problems involved may be similar. Upping the mix and oxygen content would be a quick solution, if doing so is feasible. At 30X oxy/propane, I had consistent ignition even on a chamber of 2 cubic inches that leaked noticeably.
Although larger projectiles in supersonic flight (read: golf balls) may make some entertaining sound effects, the muzzle blast will almost always present a much greater risk to hearing. Only with VERY large or fast projectiles (by our standards, at least) would in-flight noise become a real hazard to the operator of the launcher.
Spudfiles' resident expert on all things that sail through the air at improbable speeds, trailing an incandescent wake of ionized air, dissociated polymers and metal oxides.
I'm sorry, I should have read your posts more thoroughly.
Anyway, right now I'm using the 0-30 psi gauge that came with my lpg regulator. (I think it's probably the most accurate gauge I have now, but I plan to get some better ones) Thus I am limited to 2x mixes.
I do have a spare welding cylinder or two (yes another, more powerful hybrid is in my future) So I could try oxy mixes, but I'd need a new regulator (for oxygen service), and I have no way of knowing the chamber pressures, because HGDT doesn't work with oxy mixes...
If your gauge reads in 5 pound increments, then it's useless... If it reads to only 30psig, then it's useless.
You'll need to get yourself a 0-60psig gauge, that reads in 1 psig increments (inexpensive and not too hard, to find).
You'd be better off, just making a separate metering pipe, for filling the chamber with propane... it'll be a lot more accurate, then your current setup.
Do you still have a problem, making sense of the "smoke" issue, or did you figure it out?
It has numbers every 5 psi, major gradations every psi, and then minor gradations every half psi. Also I have been using my meter pipe for the latest tests, but I think the meter pipe method actually adds more inaccuracy to your measurement system because it necessitates a volume measurement, whereas the manometric system isn't dependent on volume. I think some gauges just work better at higher pressures, and that is why the meter pipe seems to work well for low mixes.
Also, the smoke has lessened. It was the worst when the disks didn't break. Some of it might have been water vapor as well.
@everyone, thanks for all the help As soon as I get some new gauges I'll post up in the hybrid section with plenty of damage pics
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