SpudFiles

OK, this isn't the liquid to gas expansion question, its the 4% mixture question: How much does the burning mixture expand?? I've looked everywhere to find an answer, but they're either telling me about the liquid to gas expansion or the answer is hidden in the PhD mumbo jumbo with diagrams, charts, prerequisite reading, safety aspects and cudos to foreign universities. So, does anyone have a 'safe' ballpark or window guestimate as to how much this burning gas expands?

kenbo0422 wrote:So, does anyone have a 'safe' ballpark or window guestimate as to how much this burning gas expands?

The gas' pressure will increase to a peak of around 5 to 7 bar - but I've had to rephrase your question slightly to be able to answer it correctly.

if you ignore temperature, that would give an expansion ratio of 6:7. With temperature, it's to the ideal gas law, but you don't know the temperature propane burns at in your exact gas mix. HGDT says ~100 psi for the whole thing, from 1 atmosphere. So 100/14.5=~6.8

so at peak, a ratio of 6.8, after complete cooling, 7/6.

Thats exactly what I was looking for. I'm in the midst of trying to design a 25cal (or possibly larger) sniper type rifle using combustion as the means of propulsion. The design will incorporate a hollow bolt with a pressure relief (kinda like a burst disk) if I should go to higher than atmospheric chamber pressure to start with. The bolt is the chamber. When retracted to load the next round, it envelopes a solid piston which allows a cross ventilation to take place in the open area that the closed bolt was occupying. A secondary valving system will load the 4% propane as the bolt is closed. The 'metering' valve system (the secondary valving) is based on the semi auto valve seen on the Nerf Haven site. The design should be easily modified to adjust the chamber size and/or use the piston I mentioned above to 'compress' the chamber. The spark will be located on the end of the piston, facing the chamber. This is hopefully going to give me a compact design that will allow it to be made into a 'real' looking rifle. Hope it works out....

Oh, what's the HGDT??

I think you need to do a bit more studying so you can ask a lucid question.

So, does anyone have a 'safe' ballpark or window guestimate as to how much this burning gas expands?

The gas will expand indefinetly (in a vacuum), or not all (in a closed chamber), or somewhere in between for other setups. It all depends on the conditions. Besides, you really don't care a lot about how much the gases expands, your pipe isn't rated in "gas expansion units" it is rated in pressure. So, I assume you want to know what the pressure is in the gun.

You can use GasEq to calculate the expansion of the combustion gases for propane + air. The actual expansion depends on how much heat is lost to the chamber and, of course, how much the projectile moves.

According to GasEq, for stoichiometric propane in air, in a closed chamber;
Final pressure = 9.28 ATM (122 PSIG)
Final temperature = 2631K (4276F)
This would be the worst case scenario in terms of what the gun must be able to withstand. In an actual functioning gun the temperature and pressure will not get this high.

For propane in a closed chamber, if you let the gases cool off the pressure will drop back to basically atmospheric within a second or so.

HGDT is a simulator for combustion and hybrid guns, made by a member of the forum. We worship it like an idol and it gives approximations of the performance of our guns in return.

Anyway, you will most likely have to go with a starting pressure above 1 atmosphere to achieve sniper rifle like power out of .25". I suggest you look at the hybrid canon showcase. I suggest not using the bolt to compress your chamber, as you loose performance and have to close the bolt harder. You should take a look at Fnord's Piston Hybrid for a great idea in containing the pre-combustion pressure. With enough money, patience, fine motor skills, and machining facilities, you could build a semi automatic rifle, based on any of the semi ideas floating around this website(especially the GB semi)

Depending on your budget, you could use a 3 way solenoid valve to meter propane, a normal solenoid valve and a check valve for air, and a stungun, 555 timer and various electronic components to fire and control the fueling for your creation.

I wish you luck

Ps: don't forget regulators. you would one for propane, and one for air.