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Fueling a very high mix 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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Fueling a very high mix hybrid

Unread postAuthor: SpudFarm » Thu Feb 09, 2017 6:14 pm

Hello again Spudfiles, I don't know if any of you can remember me as I have not been active on these forums for years! However I am lurking from time to time, but not nearly enough to catch up. Now I am back with a question for you to answer.

Presuming you would like to build a high mix hybrid that is going to be fueled with propane/air, you would have to deal with one major problem; propane's low vapor pressure. I can in my climate only pressurize propane to about 7-8 atm before it starts condensing and turning liquid. With that pressure you get to about 120x, right?

My question is as follows:
Lets say my chamber is 100ml and I want to fire a 200x mix, the ratio of air/propane by volume for a stoichometric mix would be 23.9/1 IIRC.
With this we can calculate the following (And please look through this to correct me if my math is of):

Volume of propane needed for a 1x mix (atmosphere): 100/23.9= 4.184ml
Volume of propane needed for a 200x mix: 4.184*200= 836.8ml

So now we know the volume of gas (836.8ml) we need to get into the chamber before we start pumping in the air. The only method I have been able to come up with for practically getting this amount into the chamber without heating the whole setup is as follows.

I either get or make a syringe like apparatus that can hold and accurately measure this amount of gas and connect it to the inlet of a pump capable of the pressure required and then proceed to pump until the syringe or canister is empty. Maybe even make a pump of clear tube that is graded with volume marks and pump it straight into the chamber in one pump before a valve is closed and the pump removed.

This would get the right amount of propane into the chamber regardless of the propane's state as a liquid, and it would if I am correct evaporate in the chamber as air pressure is applied.

Could someone just think a little about it and tell me if I am wrong?

Thanks!
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Re: Fueling a very high mix hybrid

Unread postAuthor: D_Hall » Fri Feb 10, 2017 11:33 am

Not really my specialty so I'm willing to be wrong and corrected, but I don't think the propane will evaporate as air pressure is applied.

That said, if you're prepared to jump through all these hoops, why not just move to methane?
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Re: Fueling a very high mix hybrid

Unread postAuthor: SpudFarm » Fri Feb 10, 2017 11:50 am

As Ladra made it to 190x mix (i dont recall if he used pure oxygen for that shot though) it should be possible as his mixes did not liquify. I do think I have shot mixes with a higher preignition pressure then the vapor pressure of propane in the past, but I am not sure of this. To long out of the game I guess?

I lay sleepless tonight thinking about this and i think I got the best solution now.

I will premix propane/air on one side of a cylinder of a considerably larger volume then the chamber, in the cylinder I will have a piston and I will drive this piston forward by hydraulic pressure on the other side of the piston in the same cylinder. This will force the premixed gasses into the chamber where a valve is closed and the gun removed and ready to fire.
What do you think of this Dave?

I dont use methane as I have much easier access to propane, and its uncharted territory wich I find interesting
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Re: Fueling a very high mix hybrid

Unread postAuthor: jimmy101 » Fri Feb 10, 2017 1:58 pm

I must be missing something since I don't see a problem. You take the 1x air filled chamber and connect it to the propane tank via a valve. A meter (also protected by a valve) is attached to the chamber. You open the propane (and meter) valve and pressurize the chamber to 0.84 ATM (gauge pressure) with the propane. You close the propane valve and have 0.84 liter propane in the 1 liter chamber. You then pressurize the chamber to 200x with air. What am I missing?
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Re: Fueling a very high mix hybrid

Unread postAuthor: SpudFarm » Fri Feb 10, 2017 3:13 pm

Hello Jimmy!
Yes you are missing something, if you have a chamber with a volume of 100cc and you pressurize it up to 0.84 atm you have 84cc of gas in the chamber, that would equate to a 20x mix.
When you are adding 836ml you are actually putting in eight times the chamber volume, hence you will get a pressure of 8,36 atm, at which the propane will condense to a liquid.

I hope this cleared it up, I am no math/chem genius by any mean but I think it is QUITE accurate.
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Re: Fueling a very high mix hybrid

Unread postAuthor: Ragnarok » Fri Feb 10, 2017 3:36 pm

D_Hall wrote:Not really my specialty so I'm willing to be wrong and corrected, but I don't think the propane will evaporate as air pressure is applied?

That would also be my thoughts. From my general understanding, the phase change is based on partial pressure, and the partial pressure of propane will be the same regardless of how much air is added to the chamber.

SpudFarm wrote:As Ladra made it to 190x mix (i dont recall if he used pure oxygen for that shot though)

Larda didn't use the standard we now would for quoting his HyGaC mixes. He quoted an Oxy/Propane mix as 40x because it was pressurised to 40 atmospheres, despite this being the same quantity of propane as in a 190x Propane/Air mix.

There's two possibilities here:
- It was simply warmer when/where Larda did his tests. (Propane's vapour pressure changes a lot with temperature).
- Some of the propane in the chamber may have been liquid, making for a lean ignition. However, after ignition, the rapid rise in temperature, and the conversion of some of the gaseous propane to CO2 and water would probably mean that much or all of the liquid propane will vaporise and ignite.

What do you think of this Dave?

Compressing an explosive gas mix is how diesel engines work. There's a nasty chance that could auto-ignite.
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Re: Fueling a very high mix hybrid

Unread postAuthor: SpudFarm » Fri Feb 10, 2017 6:05 pm

Good points Ragnarok. I have decided to go with the hydraulic method as this would enable me to do fueling remotely, and the compression will happen very slowly so I doubt dieseling should be an issue.

I do not want to involve pure oxygen, so I will simply try and see if it works. I would think that even if the propane liquefies it would ignite even as a liquid as there is plenty of oxygen to make it start burning. Pressure and heat would build quickly and the burst disk will hold it back so it gets some time to evaporate.
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Re: Fueling a very high mix hybrid

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Re: Fueling a very high mix hybrid

Unread postAuthor: D_Hall » Tue Feb 14, 2017 1:04 am

SpudFarm wrote:I dont use methane as I have much easier access to propane, and its uncharted territory wich I find interesting


Methane is uncharted to a large extent in the hobby world, but it has been done many times over in the professional world for (to my knowledge) the exact reasons being discussed: It doesn't liquify at high pressures. And FWIW, the largest currently operational gun that I'm aware of is in Meppen, Germany, uses methane, and fires 2,000 pound bombs as projectiles. Quite an impressive toy, actually.

That said, if methane is hard for you to get, it's hard for you to get. Say no more.

As for the dieseling issue, yeah, bringing up pressure VERY slowly and doing it remotely should reduce the risk and make it a non-safety issue.
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Re: Fueling a very high mix hybrid

Unread postAuthor: SpudFarm » Fri Feb 17, 2017 2:57 am

Methane as you say is a more suited fuel for this kind of gun, but as I said I dont know of any way to get my hands on it over here.
I think I conclude that propane burns even as a liquid when sufficent oxygen is present. The dieseling issues is not really a concern to me, I cant imagine alot of heat building up when compression happens over a course of a few minutes.
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Re: Fueling a very high mix hybrid

Unread postAuthor: SpudFarm » Tue Feb 21, 2017 1:33 pm

Double post so someone may look. Is it just me or have the forum lost alot of activity over the last years? If so, does anyone have a good explanation as to why? It's a shame..

I am now done drawing the plans, and would like to share the chamber design for your input. I am thankful for any improvements you can point me in the direction of.

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Re: Fueling a very high mix hybrid

Unread postAuthor: matti » Tue Feb 21, 2017 11:45 pm

SpudFarm wrote:Double post so someone may look. Is it just me or have the forum lost alot of activity over the last years? If so, does anyone have a good explanation as to why? It's a shame..

I am now done drawing the plans, and would like to share the chamber design for your input. I am thankful for any improvements you can point me in the direction of.


I feel same about the activity, but maybe it's because I have been here so long time :lol:

About the chamber, as a professional machiner first thing I noticed was the coarse threads. You could use much finer thread pitch like 2mm or so. Thats what they mostly use in threaded hydraulic cylider ends.

http://fullerfasteners.com/tech/basic-m ... m1-m100-2/
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Re: Fueling a very high mix hybrid

Unread postAuthor: SpudFarm » Wed Feb 22, 2017 2:47 am

I remember a few years ago, this forum was booming.. A thread could have ten-twenty replies by different people each day. Something must have happened. Maybe teenagers today are to caught up in drinking and partying, atleast that is the case in Norway.

Thank you for your input, I will consider it especially for the barrel side, should give me more clamping force on the burst disk
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Re: Fueling a very high mix hybrid

Unread postAuthor: matti » Wed Feb 22, 2017 11:36 am

SpudFarm wrote:Thank you for your input, I will consider it especially for the barrel side, should give me more clamping force on the burst disk


Exactly, it's easier to tighten them because you don't need so much torque to get them tight.

For the other thing.. yes that, and I think all the "smart" systems like arduino, robots, 3D printing and smartphone apps etc. are taking over.. and at least in Finland I feel like making anything related to guns or shooting is considered bad :roll:
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Re: Fueling a very high mix hybrid

Unread postAuthor: Ragnarok » Wed Feb 22, 2017 6:23 pm

SpudFarm wrote:Is it just me or have the forum lost alot of activity over the last years? If so, does anyone have a good explanation as to why? It's a shame..

At a guess, part of it is that people have become more paranoid in recent years. Once, a teenager (or pseudo-teenager) who bolted bits of plumbing together in their spare time was weird, now they're a possible terrorist.

I also wouldn't be surprised if the continued growth of sites like Youtube and Facebook had an impact. Many people just stick in their own little bubbles of the internet, and prefer to ask badly phrased questions in comment sections rather than actually bothering to use Google.
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Re: Fueling a very high mix hybrid

Unread postAuthor: SpudFarm » Thu Feb 23, 2017 12:44 am

I think you both have good points, i think the problem might be the access to smart phones etc from early age. Kids learn that that is where fun comes from and it results in them not eating mud or doing anything practical to build up their creativity and curiosity.
I think future generations is going to lack many qualities like disipline and good work ethics because of it and other reasons.
Kids need to fall out of trees and tip their bike to learn.
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