Short question topic

[Moonbogg]

I think I see what you are saying. The gas uses energy to move the piston, and once that work is done, that energy has been transferred and you can't get it back to do any work on the projectile. Would it be the same situation with a burst disc? It requires a lot of energy just to break the disc, but once it breaks, the projectile is gone. It's a good thought exercise. It gets kind of confusing because the gas is springy.

[hectmarr]

Yes, that's what I was thinking. My interest is to know how much of the total energy is invested in moving the piston, or breaking the disc. I see that it depends exclusively on the specific design, (areas, driving force, stroke), and the chosen method, (such as piston or rupture disc), to let hot air escape into the barrel.
Variables like damn dead spaces where compressed air by temperature can expand a little, lowering its pressure and losing energy, is another aspect that is undoubtedly negatively influencing the tests. The fact of having relatively long transfers, also dares, lost by friction.
I think a solution is to increase the amount of fuel, and the air age, (and consequently, the pressure of the hybrid), to compensate for these losses, especially when they are designed without any kind of calculation, just "by eye".

[jackssmirkingrevenge]

I feel that such losses are probably not that significant. Pistons typically have a large surface so they do not require a high pressure to move. Added to that, their travel is usually very limited so the increase in chamber volume relative to the total is minimal.

[hectmarr]

These are the two cases I was testing.
In case "A", the retention corresponds to an o'ring, and the power is much higher than the other case.
In "B", the "captive ball valve", when combustion occurs, the 6.35mm mole moves to the left and passes the flow through the transfer into the firing barrel. This green ball is the one that is thrown. The idea is to be able to make a very small restraint system to fire lead pellets or other ammunition. The two bb`s are the same. Different color is just for reference use. .

sistema bola cautiva.png (5.36 KiB) Viewed 28829 times

[Moonbogg]

Based on the sketches the first thought that comes to mind is that you may not have enough chamber volume to move the piston and also the projectile. Maybe I'm missing something, but if I reduced the chamber volume of my piston cannon to be very tiny, then I'd expect most of the energy to go into moving the piston and not much would be leftover. It might just be a volume issue. The chambers in the sketches look pretty small. I feel like I'm missing something here.

[hectmarr]

Based on the sketches the first thought that comes to mind is that you may not have enough chamber volume to move the piston and also the projectile. Maybe I'm missing something, but if I reduced the chamber volume of my piston cannon to be very tiny, then I'd expect most of the energy to go into moving the piston and not much would be leftover. It might just be a volume issue. The chambers in the sketches look pretty small. I feel like I'm missing something here.

Sorry for my drawing, and for not clarifying that it is not at all to scale.
I have left constant the volume of the combustion chamber, (which in "A" works wonderfully), the same firing barrel length, the same ammunition, and the same execution pressure.
With the "A" system it has at least twice the energy of the trip than with the "B" version, which has the small valve incorporated.
I mean that the weapon is the same in all respects in "A" and "B". So my question.
In practical terms in "A" the ammunition cleanly passes two cans of soup. In version "B", only one side of one of these cans. So are the facts.
I have tried to raise the execution pressure of the weapon in version "B", to see how much more energy is needed to achieve the same results as in "A" on the target. It seems to make up for much of what I explain to you earlier. I increased it from 5x to 7x, and they are almost the same. I think that starting from the retention the bb, in case "B" it takes 2 bar at least, to say it in an informal, unscientific and simple way.

[Moonbogg]

I wonder if the shape of the B version could also have something to do with it. At least you can solve it by making up for it with more fuel and air. The A version is more like a burst disc configuration. I'd like to have two identical cannons, one using a burst disc and the other using a piston, and then compare the performance of the two to observe any losses caused by the piston. That would take time, effort, and money though. Besides, you sort of just did that experiment anyway. It looks like burst discs win.

[hectmarr]

I wonder if the shape of the B version could also have something to do with it. At least you can solve it by making up for it with more fuel and air. The A version is more like a burst disc configuration. I'd like to have two identical cannons, one using a burst disc and the other using a piston, and then compare the performance of the two to observe any losses caused by the piston. That would take time, effort, and money though. Besides, you sort of just did that experiment anyway. It looks like burst discs win.

Yes I agree with you. At least in this size, the rupture discs or equivalent, (like a retaining o`ring), use less energy than an o'ring`s used as a small piston.
Surely size influences all of this, so I would personally attribute a partial value, which is true for small sizes, and not necessarily for much larger or much smaller sizes.

[farcticox1]

Playing with the idea of a piston actuated semi automatic airsoft, there will be a spring behind the piston to return it to the home position which will allow another to drop into position, detent may be needed to stop it rolling out. Not sure what will happen to the piston as it reaches full stroke as it will have equal(ish) air pressure on both sides so the spring will try and re seat it, the valve is a 24vdc solenoid so I can pulse it with a timer and actually get a crude full auto

piston.jpg (11.86 KiB) Viewed 28636 times

Progress so far, piston is from a pneumatic cylinder, the rod is 6mm so is perfect for an airsoft barrel.

[hectmarr]

Does anyone know how to safely depressurize this type of gas damper? It must have a high pressure gas inside because it takes a lot of force to compress it, about 40 or 50 kg. They are those that are used in doors and hoods of automobiles. I could not find images of their internal construction to safely depressurize them, and the mechanic who gave them to me, warned me that I must be very careful when opening them and he did not explain anything else ... any data will be very useful to me.

[farcticox1]

071a09762c40fc2c4579d64aa7e8a016.png (7.66 KiB) Viewed 28496 times

[hectmarr]

071a09762c40fc2c4579d64aa7e8a016.png

THANK YOU. I am watching the video now.

[mrfoo]

His explanation of how it works is dead wrong, the pressure in the two chambers will always equalise. The principle at work is exactly the same as that of a QEV, the force exerted on the piston at the "bulb" end is gas pressure * piston cross sectional area, and at the strut end it's gas pressure * (piston cross sectional area - piston rod cross sectional area). That's why the force is more or less constant, there is rise in pressure, and thus force, due to the displacement of the piston rod, but that's all.

[hectmarr]

I managed to unzip without problem. A 1mm hole on the opposite side of the stem and all the nitrogen escaped, quite violently After measuring the inside diameter, and finding out the area, approximately knowing the force necessary to press the stem to the bottom, I calculated the pressure it should have inside, between 20 to 25 bar.

[jackssmirkingrevenge]

Playing with the idea of a piston actuated semi automatic airsoft, there will be a spring behind the piston to return it to the home position which will allow another to drop into position, detent may be needed to stop it rolling out. Not sure what will happen to the piston as it reaches full stroke as it will have equal(ish) air pressure on both sides so the spring will try and re seat it, the valve is a 24vdc solenoid so I can pulse it with a timer and actually get a crude full auto

A variation of the blow-forward bolt