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Airsoft hybrid

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Airsoft hybrid

Unread postAuthor: Demon » Tue Apr 13, 2010 5:00 pm

I havent really tough to any kind of diagram yet, but i think it could be cool to make a mini-hybrid, and i have one question :

If i have always the same quantity of air and propane, but i compress it, will i get better perfomances ? Without the extra combustion chamber space needed at 0x, i think it could give out more psi due to less dead space .

What do you think?
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Re: Airsoft hybrid

Unread postAuthor: Bluetooth » Tue Apr 13, 2010 5:06 pm

Demon wrote:I havent really tough to any kind of diagram yet, but i think it could be cool to make a mini-hybrid, and i have one question :

If i have always the same quantity of air and propane, but i compress it, will i get better perfomances ? Without the extra combustion chamber space needed at 0x, i think it could give out more psi due to less dead space .

What do you think?


I'm not sure (I'd have to research), but I would say it would still give you the same amount of force.
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Unread postAuthor: Demon » Tue Apr 13, 2010 5:11 pm

Yes, it would give me the same amount of energy, but as the chamber is smaller, there is less deadspace.

no?
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Unread postAuthor: DYI » Tue Apr 13, 2010 6:21 pm

I'm not sure (I'd have to research), but I would say it would still give you the same amount of force.


In the simplest case, force is directly related to pressure; F = pressure*base area. In this case, it depends on the pressure the gas can apply to the base of the projectile (which may be limited by the gas' particle speed and SOS as well as the actual chamber pressure). Force isn't the most useful yardstick of performance, as work done is equal to F*d. Pressure (and thus force) will drop off more rapidly for a higher mix (assuming the same amount of fuel/air for any mix, with a varying chamber volume), but will start at a higher value. Trying to figure out the optimal combination is why we have programs like GGDT and HGDT.

Also, I'd just like to add that a 0X mix would have no fuel and no air. Atmospheric pressure combustion is, by the generally accepted standard here, just barely less than 1X (0.96X, if you're being picky).

In this case, higher mix and lower volume will be superior in terms of performance. With a quick check on HGDT, I see that, using the same mass of propane (11.5mg) with an 18" long barrel, a 0.3g BB will reach 710fps at 1X, 997fps at 2X, 1318 at 4X... all the way up to 2485fps at 64X. As this is a constant 536J of chemical energy release, efficiency goes from 1.3% at 1X, to 16.1% at 64X. This is a vast improvement. In any build where muzzle energy is important, one should always use the highest pressure feasible.
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Unread postAuthor: Demon » Wed Apr 14, 2010 8:40 am

Thank you a lot, so approx 2x is all what i need to get a .43 to 520 fps
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Unread postAuthor: POLAND_SPUD » Wed Apr 14, 2010 9:57 am

good thinking... I think I will take the same route...

ohhh btw do you think this could be used as a reg for both air and propane ??

http://www.sfs-fluidsysteme.com/datenblaetter/E018.pdf
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Unread postAuthor: Demon » Wed Apr 14, 2010 11:15 am

I will try when i will be completly done with my carbine, but i tough of a l-96 style bolt action that would compress the air to a 2x mix manually and then vent it with a mini-fan installed in the bolt .
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Unread postAuthor: Ragnarok » Wed Apr 14, 2010 3:16 pm

DYI wrote:As this is a constant 536J of chemical energy release, efficiency goes from 1.3% at 1X, to 16.1% at 64X. This is a vast improvement.

Strictly speaking, you should include the energy stored in the air pressure as well if you're assessing efficiency.
Yes, if you count only the chemical energy, then it is ~16% efficient, but if you take the stored pneumatic energy into account as well (which, at 64 atmospheres, is not insignificant!), it is less so.

Atmospheric pressure combustion is, by the generally accepted standard here, just barely less than 1X (0.96X, if you're being picky).

Personally, I don't see that it's sensible to have a standard that makes an atmospheric combustion anything other than 1X.

For propane, I use the unit X as being equivalent to a propane density of 1.6715 mol m<sup>-3</sup> (or, if you prefer, 73.705 g m<sup>-3</sup>).
By that definition, a combustion at atmospheric pressure and 20<sup>o</sup>C is a 1X mix to more decimal places than would matter for any practical purpose.

It's a well defined unit that handles the difference in gas densities between temperatures and with different oxidisers. I can explain how it was derived later, but I've got other things that need doing right now.
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Unread postAuthor: DYI » Wed Apr 14, 2010 10:10 pm

Personally, I don't see that it's sensible to have a standard that makes an atmospheric combustion anything other than 1X.


I think it quite sensible. If one has a launcher with a chamber full of air at atmospheric pressure, seals it, and adds the correct amount of fuel for stoichiometric combustion, that is 1X by my standard. Combustion at atmospheric pressure involves displacing some of the starting air out of the chamber, which isn't generally done in hybrids. Since the X notation is the exclusive domain of hybrids, it makes little sense to base it on a practice which they almost never employ.

As to the efficiency issue: I was answering the OP's question, as regards efficiency of chemical energy. Obviously, a more in depth analysis would be required to find the actual potential energy efficiency.
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