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I've been working, slowly, on a "technical" combustion spud gun for several months. The Tech gun is specifically desigend for doing measurements on the firing of a combustion spud gun.
The gun, barrel, electronics etc. are all done. So far, I've only done a few measurements on the closed chamber (no barrel).
A drawing of the closed chamber and a photo of the complete system;
The chamber is constructed of 3" Sch. 40 pressure rated pipe capped on both ends with threaded plugs, volume 107in<sup>3</sup>. The chamber contains a fan, three spark gaps, a tire pressure (TP) gauge, a dial gauge and a piezo transducer. The TP gauge is used as a peak pressure recording gauge. The electrical signal generated by the piezo transducer in the chamber is recorded using the sound card of a PC. There is also a detector that records the firing of the spark so the ignition point is marked on the piezo recordings. The ignition source is a "100KV" stungun. Fuel is stoichiometric propane (measured with a syringe) in air.
When fired the closed chamber makes surprising little noise, just a faint "plunk" sound. The sound of the gauges rattling is almost louder than the sound of the combustion. The dial pressure gauge spikes to somewhere near midscale (100 PSIG) and then immediately drops back down to zero.
A screen shot of the Audacity recording window is shown below. This firing used the chamber fan and all three spark gaps. The wav file is here.
This chamber firing was done with the fan running and all three chamber spark gaps. The upper trace is the piezo signal and is related to the pressure in the chamber. The polarity of the piezo signal is arbitrary, the falling trace is a result of the polarity being reversed between the piezo and the sound card. With this wiring, a falling signal represents a rising pressure in the chamber.
The lower trace is the "spark recorder" signal. The highlighted region starts at the spark that ignited the chamber and ends at the peak maximum for the piezo signal. The status bar at the bottom of the Audacity window indicates that this time range was 25.6 mS.
As a first study I examined the piezo "pressure" signal obtained when the chamber is fired without the fan running and a single spark gap compared to the signal obtained with the fan running and all three spark gaps. A graph of the two piezo traces is shown below.
Comparison of No fan + 1 spark (1g) vs. Fan + 3 sparks (1j):
As you can see, the peak piezo signal occurs much earlier with the fan running and three sparks than it does without the fan and only a single spark. The times to the peak signals were ~25mS for the fan + 3 sparks firing and ~49mS for a single spark and no fan.
I have made a series of measurements with the closed chamber in which I varied the use of the fan and the number of spark gaps used. The data is summarized in the table below.
1. When fan is used for mixing but not during firing there was a 5 minute wait with the fan off before ignition.
2. Spark gaps used; B = breech end, C = central, M = muzzle end.
3. Time from the spark signal to the maximum signal deflection (peak) and the next zero crossing.
4. Relative to the longest time to the zero crossing point.
The "TP Gauge w/Friction" column gives the peak pressure reading obtained with the tire pressure gauge. The two shots that used both the fan and the full set of three spark gaps pegged the 100 PSIG TP gauge. The theoretical peak pressure for the combustion of propane in air at 51F is 129PSIG according to GasEq.
There are two data sets that were obtained using the same conditions. Shots #6 and 10 were both done with all three sparks and with the fan running. The timings are very similar for the two shots.
The piezo signal is not directly related to the actual pressure in the chamber. Piezo transducers produce a voltage that is proportional to the rate of change of the pressure with respect to time. In addition, the sound card also modifies the signal since the frequency is at or below the low end of the audio spectrum. The piezo signal recordings look like they are approximately the first derivative of the pressure versus time signal. In the absence of a method to convert the piezo signal to a true pressure signal we will instead just use the characteristic shape of the signal to identify a reference time for the combustion process. We could use the time to the top (actually the bottom) of the peak as the reference point. In the table above these values are given in the "Time To: Peak" column. Since the piezo signal actually looks more like a first derivative signal I believe that the zero crossing time is probably a better estimate of when the peak pressure occurred in the chamber. The zero crossing times are shown in the "Time To: Zero Crossing" column.
In the graph below the time to zero crossing is scaled by the slowest burn rate (no fan, one spark).
1. The "Relative Burn Time" is relative to the longest time to the zero crossing point.
2. "Fan: - -" is no fan for mixing or firing, "+ -" fan for mixing but not firing, "+ +" fan for both mixing and firing.
3. "Sprk - + -" is the central spark gap, "+ - -" is the breach end spark, "+ + +" is all three spark gaps.
There are several observations that can be made from this data. It appears that;
1. The pressure in the chamber caused by the combustion process lasts for only a very short time. Based on the response of the dial gauge, it looks like the pressure drops back to about atmospheric pressure in less than one second after firing.
2. Thoroughly mixing the fuel results in the fuel burning about 20% faster.
3. Having the fan running during firing increases the burn rate by an additional 15~20%.
4. Three sparks give a small increase (a few percent) in burn rate if the fuel is well mixed.
5. Running the fan during firing and having all three sparks gives another small increase of a few percent.
6. Three sparks and the fan gives an overall decrease in burn time of 40~45% compared to no fan and a single spark.
I have a permanent page with a more detail here, and an incomplete build log is here
It surely covers the expectations, wich gives a pleased feeling, and no need to worry. (Because: If the results would tell us that a chamber fan dropped performance, we would start worrying alot about that.) It proves these assumptions are right.
It is however very very interesting so see how much that improvement actually is, and you showed us that the burn time can almost be halved down (thus double the burn speed.)
Hmm I didn't expect that to happen so quickly, I thought it would take about 10-30 seconds to do that.
Its a good thing to keep this in mind when designing some sort of hybrid thing with a valve opening just after the combustion starts. Some people talked about that. Some of them believed pressure from combustion lasts forever, not knowing the pressure is directly coming from the temperature. (P=T*V)
But still I didn't expect it to drop THAT quickly.
All together in two words:
I just had another look at the results.
It did produce over 100PSI?? WHOA thats alot.
Most people over here (including me) thought that a (an advanced) combustion would never produce over 70-80psi.
This shows that using DWV in an advanced combustion is VERY VERY WRONG!!
"1. When fan is used for mixing but not during firing there was a 5 minute wait with the fan off before ignition."
If I missed something don't yell, Why wait 5 minutes before firing?
A automotive compression testor would work better then a tire gauge, they are made for testing static pressures, here is a good one. 0-1,000 PSI with push butten air release. If you plan on doing more test like this, I don't think you'll max this gauge out, lol. Nice write up by the way.
When life gives you lemons...throw them back they suck!
Hoooo there sparky. Are you sure about that? That's an awesome claim to make considering the number of people who have, and will continue, to build high-end combustions constructed with some DWV. In fact, anybody who's ever used a cleanout plug on a combustion launcher has used DWV. I think there'd have to be a little more data before people start scrapping their DWV combustions for uber-expensive new ones made of all pressure rated fittings, (especially in the 4" range most combustion chambers reach).
Jimmy, good idea to go ahead and test a closed chamber. Have you considered getting a larger tire gauge, (they sell 150 PSIG ones), to see where the mix is maxing out?
Yes, very good. A few more experiments may be in order, but this is awesome.
Mark: I think what he was getting at was the generation of pressure. Again, a lot of people have successfully used DWV for a combustion, but so have a lot used it with pneumatics.
Now, don't flame me; I have DWV on my combustion, and don't plan on changing it. And, as you stated, I think a lot more tests are needed before it is determined that DWV is unfit, totally, for the construction of a launcher.
Yes, many people have used DWV on a ADVANCED combustion without faillures. But what I mean, is that its not the right way to do it and its not 100% safe.
A pneumatic going to 100+ psi is just as dangerous.
While I know that DWV is "tolerated" on combustions these days, I still would like to mention its just not as safe as some people think.
Even if it held up for 1000 shots on a advanced combustion, who sais it will hold up for the 1001th time?
When using pressure rated pvc ONLY, then you can. Otherwise you cannot.
I dont say its forbidden to use DWV, its just that you cant be sure of your safety.
Still a good point to make. People will take their chances, though, as will I. I've seen what a failure of a combustion cannon can do, as well, so I'm still willing to hip fire most DWV combustions with some safety glasses, (and hearing protection depending on barrel size).
I would never shoulder fire a DWV cannon.
IF it would explode it would be next to your head!
Especially after this proved that such a cannon can produce 100+ PSI.
It even scares me a little!
But, on the plus side, the pressures are generated for a very short time. I would think that it would be more prone to exploding over time than it would be the first few times it's fired.
Also, since a pneumatic can explode when dropping it while its pressurized, it will be hard to drop the combustion just when its on the peak pressure.
Remember this is a closed chamber, no barrel or projectile. The theoretical peak pressure for propane in air combustion is about 120 PSIG. It is only when you have a barrel + ammo is the peak pressure down around 60 PSIG.
The safety of DWV depends a bit on how you design the gun. My personal view is that the gun should be able to withstand the maximum possible pressure. So a standard combustion should be designed to at least 120 PSIG.
In use, the gun will probably never get above 70 PSIG or so, unless the round jams in the barrel (or the round is very heavy).
I was trying to tell the difference between well mixed fuel that is not moving at the time of ignition with well mixed fuel that is moving at the time of igntion. It is believed that the fuel+air mixture will burn faster if it is moving when it is ignited since the flame front is turbulent. Another way to look at it is "should the fan be left running when the gun is fired or do you only need to run the fan for a brief time after fueling"? I don't know how long it takes, after the fan is turned off, for the gases to come to a complete stop. Maybe it only takes a couple seconds, who knows. I figure 5 minutes would certainly be enough for the gases to stop moving.
The automotive compressor testor has been mentioned before. It should work. (BTW they are not for static pressures, they are peak recording gauges, a generic pressure gauge is for static pressures.) Unfortunately, I don't have one. A compression gauge may have trouble with the gases cooling (the tire pressure gauge does not) and the compression gauge is generally pumped up by several compression strokes when it is attached to an engine.
Concerning the safety of DWV in a standard combustion...
Another thing to remember is that the typical 3" or 4" cleanout plug seals like crap, especially when it is just hand tightened.
Combustion spud gunners rarely, if ever, pressure/leak test their guns because there is now way to do it without sealing the barrel.
Pneumatics and hybrids are easily tested since they are designed to hold pressure for extended periods of time.
In working with the test chamber I found it extremely difficult to get a 3" threaded cleanout plug to seal. I even tried three different threaded plugs and checked the plugs for flashing in the threads. Teflon tape doesn't help much. The only way I've been able to get the cleanouts to seal reasonably well is by cranking the heck out of it with a big-ass wrench.
In your typical combustion gun the cleanout is installed by hand and not really tightened all that much. They probably leak like crazy when the gun is fired.
So far I've tried two gauges, 100 and 120 PSIG max. Both pegged with the fan running and multiple sparks and a bit of extra friction of offset the scale's momentum.
Both leak like crazy at pressure above 80 or so PSIG. I guess I'll have to spend more than $0.99 for a worth while gauge.
If I come across a 150 PSIG I'll give it a try.
"The automotive compressor tester has been mentioned before. It should work. (BTW they are not for static pressures, they are peak recording gauges, a generic pressure gauge is for static pressures.) Unfortunately, I don't have one. A compression gauge may have trouble with the gases cooling (the tire pressure gauge does not) and the compression gauge is generally pumped up by several compression strokes when it is attached to an engine."
I understand about the time delay before firing. Good thinking!
About the compression tester. It takes several compression strokes to reach peak pressure because of the valves opening and closing. One good push of pressure should read accurately because of no escaping of gasses. You can also get tire gauges that read up to 250psi, thats the highest tire gauge I have and seen.
When life gives you lemons...throw them back they suck!
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