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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 » Sat Nov 17, 2007 7:09 am

jackssmirkingrevenge wrote:I would really advise against using spudguns in the classroom, mini or not, except for officially sanctioned projects.


Yeah true, I might risk getting expelled from school... But I don't wanted to shoot random people in class, but rather to show it off to friends without the hassle of torches and syringes.
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Unread postAuthor: Tsukiten » Sat Nov 17, 2007 7:11 am

wangpushups wrote:Oh my god dude you just made my day!!! :lol: In the classroom... How old are you?

Haha I'm guessing you're all 18~35 :roll: Well anyway I'm 14 (about a month to go before I hit the 15, hell yeah)
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Unread postAuthor: jackssmirkingrevenge » Sat Nov 17, 2007 7:12 am

Actually most people here seem to be teenagers.
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Unread postAuthor: wangpushups » Sat Nov 17, 2007 7:18 am

Right on... Get that pea shooter done so you can show it off.
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Unread postAuthor: Maniac » Sat Nov 17, 2007 12:14 pm

off tpic sorta but i was suprised that jsr didnt support the syringe idea

with all his weird fetishes and all :lol: :lol:
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Unread postAuthor: Hubb » Sat Nov 17, 2007 6:07 pm

Tsukiten wrote:
hubb017 wrote:Did you miss my first post in this thread?

Well not really but I gave up on experimenting because I spent ALOT of time with experimenting with clicks, seconds...It just doesn't seem to work with this chamber! :evil:


I forgot to mention. If you give a burst of butane (say, for one second) and it doesn't fire, then you inject another burst (say, two seconds) you now have three seconds (is that possible?) of butane in the cannon. Be sure to exhaust the mini after every injection, whether it was fired or not.
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Unread postAuthor: Tsukiten » Sun Nov 18, 2007 6:53 am

hubb017 wrote:I forgot to mention. If you give a burst of butane (say, for one second) and it doesn't fire, then you inject another burst (say, two seconds) you now have three seconds (is that possible?) of butane in the cannon. Be sure to exhaust the mini after every injection, whether it was fired or not.


Hey thanks for you tips :D But because counting seconds in your mind is a bit inaccurate(which I think :roll: ), I use 'clicks'. But anyway my spudgun finally fires but before that I had to fix the 2 chambers together which I think makes it easier to get a right mix and SHOULD make it stronger (read more about that below...). And second, I had to switch to another butane container because for some strange reason the old one just doesn't work :x
Here's the result:
Image

BUT there's a NEW problem...
My first one with the smaller chamber could fire trough 2 layers of cardboard or smash light bulbs. ( :twisted: )
THIS gun has a significant bigger chamber and CANNOT fire trough even 1 layer of cardboard!
Wtf :!: :?: :shock: Does it have something to do with the fuel mix (like it needs more time to mix... or just don't got the mix right), or it needs more time to combust all the gases? I don't get it! :cry:
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Unread postAuthor: DYI » Sun Nov 18, 2007 2:08 pm

The larger the chamber, the more slowly combustion occurs. Which is why you can't put a larger chamber on a combustion or hybrid to give it more power without increasing the barrel size.
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Unread postAuthor: SpudBlaster15 » Sun Nov 18, 2007 3:03 pm

DYI wrote:The larger the chamber, the more slowly combustion occurs. Which is why you can't put a larger chamber on a combustion or hybrid to give it more power without increasing the barrel size.


There is no such thing as a chamber that is too large, only insufficient ignition systems.
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Unread postAuthor: DYI » Sun Nov 18, 2007 4:17 pm

Well yes, but most people don't want to have 50 ignition points when they could just use a smaller chamber. I was thinking in terms of what was actually practical. If you can get a large chamber to have the same burn rate as a smaller chamber, the performance should increase, but for a set size and number of sparks (size so that you can't pull the 12" spark tesla coil ignition argument), I would expect that performance would drop downward after increasing the chamber size much past the most efficient C:B.

Also, even with enough ignition points to make it burn quickly, you will eventually run into the point of diminishing returns that a pneumatic runs into.
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Unread postAuthor: SpudBlaster15 » Sun Nov 18, 2007 4:55 pm

DYI wrote:I was thinking in terms of what was actually practical.


"Oversized" chambers with multiple ignition points are certainly practical. An average, $5 - $10 BBQ igniter is capable of arcing across at least 3 - 4 gaps. A $5 electronic BBQ igniter is capable of at least 4 - 5. A $20 photoflash/ignition coil circuit built from a simple disposable camera and an ignition coil salvaged from a junkyard is likely capable of arcing across at least 10 gaps. I don't know of many people who would consider any of these setups impractical, especially the ones that are already implemented on almost every combustion cannon in existence.
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Unread postAuthor: DYI » Sun Nov 18, 2007 6:26 pm

How many combustions have you seen using 5+ gaps to compensate for oversized chambers, compared to the number that opted for more efficient C:B ratios. The majority of people like to take the easier route, and a smaller chamber is easier than a massive chamber with 10 spark gaps.
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Unread postAuthor: SpudBlaster15 » Sun Nov 18, 2007 6:38 pm

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?
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Unread postAuthor: jimmy101 » Mon Nov 19, 2007 11:49 am

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.
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Unread postAuthor: jimmy101 » Mon Nov 19, 2007 11:57 am

SpudBlaster15 wrote:
DYI wrote:The larger the chamber, the more slowly combustion occurs. Which is why you can't put a larger chamber on a combustion or hybrid to give it more power without increasing the barrel size.


There is no such thing as a chamber that is too large, only insufficient ignition systems.

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

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

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?
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