Another nembie Q:

[Ballistica7]

Hey gang,
Newbie here. Have been playing around with spray n prays for a while, sick of the smack, its gas injection time.
My question though is; igniiton.
Iv built a powerful ign system (we cant get stun guns in OzLand) but am wondering bout ignition points. My understanding was that combustion cannons are deflagration, not detonation [youtube][/youtube] (taken from SF.com wiki)
...
though wouldn't dual, simultaneous flame fronts, caused by twin (or three) ignition sources create detonation?
I come from a aftermarket automotive scene, where the overpressure spikes from detonation are your mortal enemy.

PS: If 'yall' hadn't seen the new pulse detonation engines yet, bet that blew your mind

[Hubb]

Do research on DDT. There, you will find your answer.

Welcome to Spudfiles.

[starman]

It is deflagration in most normal combustion chambers. The longer and thinner chambers are thought to come closer to inducing DDT. Multiple spark gaps are considered an advantage and would tend to have a chamber shortening effect.

[Hubb]

What starman means is this:

[SpudFarm]

to put it this way, if you get DDT in a PVC gun you are dead.

nice avater, i have a fetish for APFSDS rounds

[starman]

What starman means is this:

Thanks Hubb! Here's a 3 gap solution that puts the flame fronts only 2" away from each other or the chamber wall in a 4" pipe. See it as 3 - 4" diameter simultaneous burn zones....very fast complete chamber burn...no chance for DDT...noticable gun performance.

[jimmy101]

to put it this way, if you get DDT in a PVC gun you are dead.

SpurFarm should know better than this.

DDT is only a problem if it occurs early enough in the combustion process. If it takes the burning of 95% of the fuel to get the last 5% to DDT then that 5% is not going to be a problem for PVC. For DDT to be a problem it has to involve a significant amount of the fuel load.

[SpudFarm]

jimmy: you really got me to calm down on the golf ball hybrid now!

i want to do test's on tis so badly, but i have no time and money when i am working on the hybrid and to get a lathe to advance my designs..

[Ballistica7]

Hey chaps,

I'm flattered to get so many responses! Thanks!!

Do research on DDT. There, you will find your answer.
Welcome to Spudfiles.

Iv done a little research today, and found nothing conclusive; I will def keep on looking. I take this (and all) my hobbies very seriousaly, specially those that involve serious energy levels.

It is deflagration in most normal combustion chambers. The longer and thinner chambers are thought to come closer to inducing DDT. Multiple spark gaps are considered an advantage and would tend to have a chamber shortening effect.

As I mentioned, I'm into very powerful performance cars, and know multiple points of ignition are advantageous for complete combustion (especially in adverse mixture conditions) See commercial motors, small / medium sized aeronautical piston motors, alfa's etc.
Iv read the sample data from [http://www.inpharmix.com/jps/] on the speed increase of complete combustion of multi point ignition setups.
The Ign setup I've built creates a 20mm spark, which when split between 2 or 3 points, will do the job.
The issue is though, that in the auto world, (minus poor fuel, overly hot inlet temps { from bad inter cooling, turbos ran past their efficiency zone or just lots of ignition advance}) detonation occurs from secondary hot points igniting the mixture, ala diesel styles. So its these "twin points of ignition" that is confusing me a little.
Clearly, it isn't as serious as i thought, as its considered a "good thing", to have multi point. I guess my question revolves around the "when does twin point cause an issue"
As with many other guys that end up here, it BLEW my mind to find such a detailed, quality information source revolving around combustion devices. Id been playing around for 10 years; if only i knew

to put it this way, if you get DDT in a PVC gun you are dead.
nice avater, i have a fetish for APFSDS rounds

Again, my main experience is from the automotive areas. From memory, det induced a 30% increase in chamber pressures. I'm using SCH40 which i rated to around 300psi i thought? I read here that combustions are lucky to get 80-100psi;... so wouldn't a detonation occurance only seeing less then 150psi?
APFSDS? - Oh dude, dont get me started, Iv got a pretty good design in mind now, i just need the device to fire it Big ups.

What starman means is this:

Thanks Hubb! Here's a 3 gap solution that puts the flame fronts only 2" away from each other or the chamber wall in a 4" pipe.

Hey man, thanks for responding!!
Iv seen your setup around, and multi ign is def the go in my mind. Honestly though, your setup looks slick, but i cant see the advantage with just using normal sharpened screw, protruding from the inside wall of the chamber. Screws is what i will use, as the are easier to service.

SpurFarm should know better than this.
DDT is only a problem if it occurs early enough in the combustion process. If it takes the burning of 95% of the fuel to get the last 5% to DDT then that 5% is not going to be a problem for PVC. For DDT to be a problem it has to involve a significant amount of the fuel load.

Cheers man. Again i think i need to research more about DDT. My previous understanding was that ANYTIME theres two flame fronts that collide, you get det. Clearly this isnt the case.

Well, i been looking at SF.com enough to know, Iv got some serious hitters commenting here, seems the consensus is that, its HARDER to get det then i thought, specially in the propane / atmospheric environment i envisage.
Ill def do more research.
Man, scouring the forums, theres NOTHING like what im planning, it'll blow your minds :0

[DYI]

The bottom line is, DDT usually doesn't happen unless you're trying for it. Even at almost 3000 psi pre-ignition pressure, Larda's hybrid still wasn't getting any noticeably sized detonations. Unless you have a very strangely shaped chamber, or induce detonation directly with a powerful electric pulse or high explosives (like FAEs do), you really don't need to be worrying.

If you had asked here a year before now, when all this was theory, you would have got a very different set of answers. It's really only been fairly recent testing that has convinced people that DDT isn't nearly as much of a problem as it was imagined to be by the creators of the first hybrids.

[jimmy101]

The Ign setup I've built creates a 20mm spark, which when split between 2 or 3 points, will do the job.
The issue is though, that in the auto world, (minus poor fuel, overly hot inlet temps { from bad inter cooling, turbos ran past their efficiency zone or just lots of ignition advance}) detonation occurs from secondary hot points igniting the mixture, ala diesel styles. So its these "twin points of ignition" that is confusing me a little.
Clearly, it isn't as serious as i thought, as its considered a "good thing", to have multi point. I guess my question revolves around the "when does twin point cause an issue"

It's a bit iffy to compare a standard combustion spud gun with an internal combustion engine (ICE). The ICE is really a ~10x hybrid and the fuel mixture is pretty darn hot even before it is ignited (due to the compression). Plus the cylinder walls are hot. The hot fuel mix makes it much easier to get multiple ignition points from hot spots, soot, dust etc. in the cylinder.

In a standard combustion spud gun you don't have the heat and high pressure that you do in an ICE. The efficiency of a combustion spudgun is quite a bit lower than an ICE and there is a heck of a lot of waste. I believe that an "advanced" propane+air at 1X spud gun is usually 10%, or less, efficient.

Add in the fact that the spud typically starts to move when only ~10% of the fuel has been burned (chamber pressure ~10 PSIG) and the peak pressure is only about 60 PSIG, and the chances of getting to DDT (or any other type of predetonation or "pinging") is pretty slim. If a spud gun actually did "ping" it wouldn't be a big deal, the gun is fired so infrequently that the "ping" vibration is not a big deal. In an ICE it's the repetitive "pinging", several thousand times a second, that beats the heck out of the engine.

There is a lot to be learned from ICEs that is directly transferable, at least qualitatively, to spud guns. Proper fuel ratio is important, motion of the fuel mixture during combustion is important, the fuel burn speed is coupled to the movement of the projectile (piston), multiple sparks are good, thorough venting of spent gases is important, etc. There is even more similarites between ICEs and hybrid spudguns.

[Ballistica7]

Again, thanks for replying, SF.com seems so much more advanced since i was last here!!

Unless you have a very strangely shaped chamber, or induce detonation directly with a powerful electric pulse

How do you class powerful? My ign system draws on a 12v 300amp battery.

...or high explosives (like FAEs do), you really don't need to be worrying

FAE's induce det? My understanding was that after deploying the gaseous volitile mixture, it was ignited from a flare charge?

If you had asked here a year before now, when all this was theory, you would have got a very different set of answers. It's really only been fairly recent testing that has convinced people that DDT isn't nearly as much of a problem as it was imagined to be by the creators of the first hybrids.

I love the fact his place keeps on evolving, tis why I came back

[It's a bit iffy to compare a standard combustion spud gun with an internal combustion engine (ICE). The ICE is really a ~10x hybrid and the fuel mixture is pretty darn hot even before it is ignited (due to the compression). Plus the cylinder walls are hot. The hot fuel mix makes it much easier to get multiple ignition points from hot spots, soot, dust etc. in the cylinder.
In a standard combustion spud gun you don't have the heat and high pressure that you do in an ICE. The efficiency of a combustion spudgun is quite a bit lower than an ICE and there is a heck of a lot of waste. I believe that an "advanced" propane+air at 1X spud gun is usually 10%, or less, efficient.
Add in the fact that the spud typically starts to move when only ~10% of the fuel has been burned (chamber pressure ~10 PSIG) and the peak pressure is only about 60 PSIG, and the chances of getting to DDT (or any other type of predetonation or "pinging") is pretty slim. If a spud gun actually did "ping" it wouldn't be a big deal, the gun is fired so infrequently that the "ping" vibration is not a big deal. In an ICE it's the repetitive "pinging", several thousand times a second, that beats the heck out of the engine

I see what your saying, and its correct, a ~85C combustion chamber that gets fired many time / second is very different from your typical combustion cannon. The issue is (not yet, but soon) that I've designed a ballistic device that fire once per second.
A fan stuck in the chamber wont be enouh to mix... im thinking the mixing device "could" cause det..
Mixing is my biggest concern atm....
Again, THANKS to all who have bothered to help me!! xox

M





Jimmy, thanks for reply, respect...

[starman]

Honestly though, your setup looks slick, but i cant see the advantage with just using normal sharpened screw, protruding from the inside wall of the chamber. Screws is what i will use, as the are easier to service.

Screws can be more difficult than you think to line up...drilling the holes in the right position/angle is crucial...and getting the screw lengths to work out right is difficult. Also, you want to drill as few holes in your chamber as possible. So, if you are going for a multi-gap system, a spark strip of some sort makes more sense. You also want to avoid drilling holes in only one layer of your PVC chamber, thus making a center chamber gap typically more difficult to obtain with screws.

The strip I show there has only dremel cutting disk width gaps so you could easily support several more gaps cut in there with stun gun ignition. Also, you can choose to place the gaps wherever you want on the strip...just cut em in. And finally, it is easily serviced and replacable if needed.

[jimmy101]

Starman is right, getting the screws to line up correctly can be an arbeit. There are some things that you can do to fix it if they don't align. Just reach into the chamber with something stiff and bend one or both screws a small amount. Now when you screw the screws in and out the tip of the screw swings in a circle and it is usually possible to get the gap you want. If you are lucky enough to have the right size bit that is a 1/2" longer than the diameter of the chamber then it's easy to get perfect alignment since you can just drill clean through both sides of the chamber in one go.

For a 1X gun the number of small holes in the chamber is really pretty irrelvant. Eight 1/8"D holes (for 4 spark gaps) is really not a big deal and won't weaken the chamber enough to cause a problem.

BTW, there is a small drawback with a spark strip (Latke called it a "chamber short"), it takes up some of the volume of the chamber. Not just the volume of the PVC strip and screws but also pretty much the entire volume between the strip and the chamber wall. The flame front is not going to migrate behind the strip very well, it'll be slow and a lot of heat will be lost. Chances are the spud is long gone before that volume gets burned. The area behind the strip will also mix slowly, even with a chamber fan.

[starman]

The flame front is not going to migrate behind the strip very well, it'll be slow and a lot of heat will be lost. Chances are the spud is long gone before that volume gets burned. The area behind the strip will also mix slowly, even with a chamber fan.

At least in the case of my strip, it being mounted in the center of the chamber, the open gaps will allow the flame front to travel equally well under the strip as it will on top of the strip. It may not be apparent from the picture but the gaps are sawn completely through the PVC opening the bottom of the strip to the gap space. The loss of chamber cubic inches is really miniscule ( <2 ci).

Latke's strip, especially mounted on the chamber edge, does have some less that optimal design problems....some of which I have tried to mitigate with my design.