Problems w/ Onboard Fuel Injection

[Tsukiten]

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 cm³

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

3.2% of 25.1cm³ is 0.8cm³.

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 cm³ this particular lighter delivers per second. Figure out how many seconds you need to get the 0.8cm³ the chamber requires.

The only thing I've got to say is: Brilliant 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'.

[SpudBlaster15]

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[jimmy101]

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."

[jimmy101]

The only thing I've got to say is: Brilliant 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.

[peeeto]

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!

[TwitchTheAussie]

Glue stick chambers have never worked for me.