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questions regarding propane in an attempt at a semi

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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questions regarding propane in an attempt at a semi

Unread postAuthor: F.E.A.R._Sniper » Wed Jan 13, 2010 1:47 pm

aight so i haven't kept up much as of late but i am pretty sure the hunt for a reliable semi-auto combustion launcher is still on. i have an idea that i am looking to try and build. but before i go into full build mode i have some questions regarding propane.

1. does propane self combust if it reachs a specific pressure? ( i dont see this being a problem but i just wanna know.)

2. assuming standard atmospheric pressure is in a sealed chamber, and i start to add propane into this chamber, at what psi would the proper burn mixture be at?

3. what is the energy efficiency of propane?
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Unread postAuthor: jimmy101 » Wed Jan 13, 2010 2:50 pm

1. Yes, all combustible materials will auto ignite if they get hot enough in the presence of an oxidizer (like air). The auto ignition temperature is a poorly defined characteristic of a chemical. The actual ignition event depends on both the temperature and how long the compound has been at that temperature. The "autoignition" temperature of propane is typically listed as about 470C (870F). Most organic molecules will auto ignite if they are heated above about 500C in air. (Paper supposedly will ignite at just 233C (451F) but in practice that doesn't usually happen. The paper has to be held at that temperature for a long time.)

2. 4.2% of the volume of the chamber should be added if it is a closed chamber and none of the air is displaced by the propane.

3. The question is kind of odd. The "energy efficiency" of burning propane would be darn near 100%. The efficiency of converting that energy into kinetic energy of a projectile is much less. For a typical 1X propane + air gun with a CB of about 0.8 figure it is about 10% or so. As an upper limit, it is probably not possible to do better than about ~30%. To get to ~30% you would need to do something like compress the mixture (like an internal combustion engine). IIRC, most real weapons using modern gun powders are less than 30% efficient and less than 15% is common. For a short barrel gun, like a pistol, the efficiency might be less than 5%.
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Unread postAuthor: Heimo » Wed Jan 13, 2010 3:00 pm

I do not know if you are aware of this but tippmann already makes a propane powered marker it is called the tippmann C3
here is a animation I found on how it works

Image

now I know I have not answered your questions, I just thought that this is a good direction to look at when designing a propane powered semi automatic spudgun :wink:
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Unread postAuthor: F.E.A.R._Sniper » Wed Jan 13, 2010 4:18 pm

thank you for your help gentlemen.

to jimmy,

you kind of mis understood questions 1 and 3, question 1 was meat to be like if propane reaches X psi does it automatically combust like acetylne(sp?) does. and for question 3, the energy efficiency of a specific fuel measures how much heat is used in the burning of the fuel and how much heat is spread into the surroundings. such as with gasoline, it has an energy efficiency of roughly 18% so that 18% is used in the burning of the fuel while roughly 82% of that heat is bled off into the block, pistons, etc. this is why diesel fuel is more efficient than gas since it has much higher (over 50%) fuel efficiency

and to heimo, yes painball is what inspired this idea. it is based off of an unbalanced spool valve system
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Unread postAuthor: jimmy101 » Fri Jan 15, 2010 2:57 pm

Sorry, about the answer to #1. I misread it as temperature and not pressure. Propane and butane would be similar to gasoline in their tendency to autoignite (diesel) at high pressures. Figure you would have to get to quite a bit more than 10 atmospheres (~10:1 compression ratio) for that to be even a remote possibility. Propane can be be used in many standard gasoline engines so it's tendency to diesel can't be any worse than gasoline.

As to "energy efficiency". I've never heard of anyone using that treatment when talking about fuels. The "efficiency" is generally calculated one of two ways. The first is just a comparison of the thermodynamic heat of combustion with the mechanical energy obtained from the engine. The second drops the theoretical limit by talking into account the inefficiency caused by the engine exhausting gases at temp of something other than absolute zero.

I believe the higher efficiency of diesel is basically just due to operating at higher compression ratios (15:1 or more). Unless you can build a 15X gun then I doubt the difference in "efficiency" is going to make any difference regardless of the fuel. Besides, going for optimal "efficiency" typically reduces the overall performance of the gun. Since fuel is darn near free efficiency is the least of a designers worries.

Diesel sure as heck isn't anywhere near 50% efficient in any real world device. 30% would be pretty darn spectacular, especially if it can do that over a wide range of operating conditions.

Bottom line though is that all simple combustion processes have terrible efficiencies. A home made gun like device at 1X pressure would be hard pressed to make 15% efficiency, and 10% or less is much more likely.

If you are concerned about overheating the gun when it is fired quickly then that just comes down to how fast you fire and how fast the gun can radiate heat. A plastic gun firing more than a couple times a minute will probably lead to chamber failure pretty quickly. For a metal gun it shouldn't be much of a problem in a semi-auto mode.
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Unread postAuthor: chenslee » Tue Jan 19, 2010 11:21 am

FYI

When designing my repeater cannons, I initially intended it to be semi-auto. When I did the calculations, it became clear that in order to generate enough force and stroke using a propane powered actuator, most of the propane would have to be dumped to atmosphere rather than be used for combustion.

However, I was dealing with a rather high friction reciprocating system and a relatively small combustion chamber. If you were to scale your combustion chamber to utilize the volume of propane required for racking, and solved the venting/oxidizer problem, you might be able to use the propane fuel pressure as a mechanical power source.
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