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Problems w/ Onboard Fuel Injection

Discuss miniature cannons built from things such as pill bottles and pen caps.
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Unread postAuthor: Tsukiten » Mon Nov 19, 2007 1:07 pm

jimmy101 wrote:It'll help if you actually would spend ten or fifteen seconds figuring out what you are trying to do.

Like others have posted, calculate the volume of the chamber. Water displacemnt is overkill since filling by counting seconds isn't going to be all that accurate anyway.

New chamber: pi*1cm*1cm*8cm = about 25.1 square cm

Your units are wrong. The chamber volume is 25.1 cm<sup>3</sup>

The correct volume of butane is about 3.2%. (Accurate enough since your meter is not very accurate to begin with).

3.2% of 25.1cm<sup>3</sup> is 0.8cm<sup>3</sup>.

From your photos it looks like you have a small hose from the lighter to the chamber.

Disconnect from the chamber. Run the hose up into an inverted measuring cup filled with water and submerged in sink. Press the lighters valve and bubble say 2 seconds of butane into the measuring cup. Note the volume of gas in the cup. Divide the volume by the number of seconds you bubbled in the fuel. That gives you the approximate numbers of cm<sup>3</sup> this particular lighter delivers per second. Figure out how many seconds you need to get the 0.8cm<sup>3</sup> the chamber requires.


The only thing I've got to say is: Brilliant :shock: But will take a helluva lot of time to set up... By the way what formula/percentage of the chamber do you use to calculate how much cubic cm gas I need?

And yeah it was a typo back there, of course I ment 'cubic cm' instead of 'square'.
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Unread postAuthor: SpudBlaster15 » Mon Nov 19, 2007 7:19 pm

jimmy101 wrote:It has been proven without a doubt that, for a single spark, a too large chamber will decrease performance.


Well yes, of course, a chamber that is "too large" will be over sized for a reason, but that's not to say that the most efficient chamber volume is the dividing line between increasing and decreasing performance. I tend to believe that it is much more complicated than that. The chamber that yields the greatest performance is obviously going to be the one that produces the highest average barrel pressure, and I have my doubts that this is also the chamber that yields the most efficient (zero acceleration at the muzzle) C:B ratio in most cases. Usually it will depend greatly on the geometrical orientation of the chamber, and the mass of the projectile (longer barrel dwell times will allow more time for combustion to complete).

jimmy101 wrote:Multiple sparks may increase the performance of an oversized chamber but as far as I know, nobody has ever actually measured the affect.


Within a certain C:B range, I doubt that multiple ignition points would increase performance, unless the chamber was extremely long, and small in diameter. Just as a pneumatic launcher's valve has a window of opening time within which the launcher's performance is constant, a combustion launcher is likely to have a "valve" opening time window as well. However, I am entirely certain that 10 - 15 ignition points within a 10ft long chamber of a relatively small diameter will perform many times more effectively than a single ignition point in said chamber.

jimmy101 wrote:If the optimal CB for a single spark is 0.8. What is the optimum CB for two sparks, or 4 sparks, or 1,2375,832 sparks?


Keep in mind that Latke used 2 spark gaps in his test launcher. As much as I doubt the use of a single gap in his relatively small chamber would make a difference in his findings, it is something to consider.

I believe that the optimal C:B ratio is dependent on far too many variables to be used as a general "guideline" for combustion launcher construction. When one asks about C:B ratios, the response should not be "Use 0.8:1, it is the best for all combustion guns". Rather, "We will need to know the mass of your ammo, your chamber dimensions, and your spark gap positioning and quantity in order to make an approximation" would be appropriate.

However, most of this is just my speculations and theories. I think A LOT more quantitative testing needs to be done in the realm of combustion spudguns in order to gain a complete understanding of what goes on inside these launchers.
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Unread postAuthor: jimmy101 » Tue Nov 20, 2007 12:20 pm

Spudblaster12, all very good points.

Efficience versus maximum muzzle velocity is a very interesting question. One for which there is very little hard data for spudguns.

In gasoline engines the efficience and power versus engine RPM graphs are usually very similar. That is, the most efficient RPM is usually also very very close to the most powerful RPM. In high performance engines there is a very small window where you can trade efficience (fuel savings) for power (performance). Stock cars, Indy cars etc. usually can switch between the most efficient fuel/timing settings (to maximize distance on a tank of fuel) and the most powerful fuel/timing settings (to outrun the other cars on the last couple laps of a race if enough fuel is left in the car). Of course, an ICE has a fixed chamber/barrel volume and the only things that can be tweaked are the fuel mixture and timing.

I completely agree about the 0.8 CB. Heck, looking at Latke's data the window is really pretty wide for the "optimal" CB. It looks like any CB between 0.6 and 1.0 will behave the same. If you work that out on a typical gun it is a pretty broad range of barrel lengths for a fixed chamber, or chamber lengths for a fixed barrel.

I believe that the optimal C:B ratio is dependent on far too many variables to be used as a general "guideline" for combustion launcher construction. When one asks about C:B ratios, the response should not be "Use 0.8:1, it is the best for all combustion guns". Rather, "We will need to know the mass of your ammo, your chamber dimensions, and your spark gap positioning and quantity in order to make an approximation" would be appropriate.

I think the last sentence should be "We will need to know the mass of your ammo, your chamber dimensions, and your spark gap positioning and quantity in order to make an approximation which is probably so inaccurate and unreliable that you might as well just build to a CB of 0.6~1.0." :D
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Unread postAuthor: jimmy101 » Tue Nov 20, 2007 3:42 pm

Tsukiten wrote:The only thing I've got to say is: Brilliant :shock: But will take a helluva lot of time to set up... By the way what formula/percentage of the chamber do you use to calculate how much cubic cm gas I need?

Really shouldn't take all that long to setup and measure the gas delivery rate. Got a kitchen? Everything you need should be in it already.

The volume of fuel needed for a particular chamber depends on the volume of the chamber and the fuel being used. (And, to a lesser extent, on exactly how the fuel is injected into the chamber.)

For a particular fuel you need to know what is in the fuel and you need the balanced chemical equation for the fuel's combustion.

Easiest thing is to just check the SpudWiki page on fuels.
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Unread postAuthor: peeeto » Sat Feb 09, 2008 3:38 pm

SpudBlaster15 wrote:The combustion gun I am working on (should be completed sometime in January) will have a 10ft long chamber, and a 15 gap spark strip powered by a photoflash circuit and 2 ignition coils wired in anti-parallel. It will also be hand held, capable of 500fps with a 1lb projectile, and capable of reloading/refueling times of less than 30 seconds. Sounds quite practical to me. Why should efficiency be a concern when fuel gas is cheap, and one desires maximum performance?


good lord, what would the recoil be like with a 1-lb projectile!?

how are you coming along with that one? i can't wait to see a vid of it!
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Unread postAuthor: TwitchTheAussie » Sun Feb 10, 2008 5:18 am

Glue stick chambers have never worked for me.
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