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Chamber Venting with compressed air.

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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Unread postAuthor: inonickname » Tue Jul 21, 2009 7:24 pm

Tech;

And add warnings to Winnebago adds after a lady gets up while driving to make a coffee and crashes.
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Unread postAuthor: Davidvaini » Tue Jul 21, 2009 8:03 pm

As a general statement to the thread in general, can we get back on topic?
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Fuel and air delivery.

Unread postAuthor: Technician1002 » Tue Jul 21, 2009 9:31 pm

OK, Back on topic..

Fuel and air, delivery.. If fuel is injected and then air is added to purge the spent gas, the fuel will most likely be vented with the spent gas it is mixed with. It would be best to add air first to vent and then add fuel, or add fuel and air metered together to vent and provide a well proportioned mix. I had a thread on this a while back.

http://www.spudfiles.com/forums/metering-fuel-and-air-t17918.html
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Unread postAuthor: Davidvaini » Tue Jul 21, 2009 9:36 pm

so replacing a chamber fan with a nozzle shooting compressed air would work better would it not?
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Unread postAuthor: Technician1002 » Tue Jul 21, 2009 11:08 pm

Davidvaini wrote:so replacing a chamber fan with a nozzle shooting compressed air would work better would it not?


A nozzle shooting air would rapidly mix fresh with stale exhaust. I was thinking more along the lines of an aquarium stone that would diffuse clean cold air in the bottom of the chamber pointed up while the hot gas is expelled out the barrel. With a Propane mix at the same time, the new gas would even be heaver than just being colder.

I've been toying with how to make a high flow rate difusion baffle that won't be damaged by the shooting. After my current projects, my first combustion is on the drawing board, a full auto or semi auto Gumball Machine gun. There is no way to complete it in time for the contest.

I will be planning on full auto.
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Unread postAuthor: starman » Wed Jul 22, 2009 2:15 am

Davidvaini wrote:so replacing a chamber fan with a nozzle shooting compressed air would work better would it not?


I don't know about better, maybe just as good. There is real benefit to having turbulence happening during the ignition, not just to get a proper premix. Also, keep in mind multiple, properly placed spark gaps can also make some incremental improvement in combustion speed.

I know you're talking about efficient chamber evac here. That is but one piece of the overall puzzle of applying the available chamber energy as efficiently, quickly and as early as possible to the projectile....very similar to finding the fastest possible valve in a pneumatic.
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Unread postAuthor: chenslee » Wed Jul 22, 2009 2:17 pm

Why not evacuate the whole thing with a copious amount of pre-mixed, air and propane. Even if you vent some fresh fuel and air out with the stale garbage, when you shut your vent hole/valve you are guaranteed to have the correct F/A. It's not like propane is that expensive.
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Unread postAuthor: GalFisk » Tue Jul 28, 2009 4:02 am

How about placing a piston inside the chamber, which moves through the entire chamber and pushes out the combustion products? (It can even be powered by a CO2 piston ;) ). Some kind of system is needed to ensure that you don't just push the combustion products into the barrel and then suck them back into the chamber again, but there are several ways to solve this problem. The piston could also be connected to a bolt action loading mechanism.
I have some drawings (including a venting piston powered by the combustion itself, and one powered by recoil) I can post later.
I like the carburetor idea. Venting with proper fuel/air mix (which is what a 2-stroke engine does, more or less) is also a nice idea, providing that you have a simple system for creating this mix.
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Unread postAuthor: Technician1002 » Tue Jul 28, 2009 4:12 am

GalFisk wrote:How about placing a piston inside the chamber, which moves through the entire chamber and pushes out the combustion products? (It can even be powered by a CO2 piston ;) ). Some kind of system is needed to ensure that you don't just push the combustion products into the barrel and then suck them back into the chamber again, but there are several ways to solve this problem. The piston could also be connected to a bolt action loading mechanism.
I have some drawings (including a venting piston powered by the combustion itself, and one powered by recoil) I can post later.
I like the carburetor idea. Venting with proper fuel/air mix (which is what a 2-stroke engine does, more or less) is also a nice idea, providing that you have a simple system for creating this mix.


The easy way to do this is with a dual meter set up. Meter the propane and air. Open both outlets at the same pressure to a third vavle. Open the third and as both propane and air expand the same, but metered by volume, they would mix on delivery in proportion.
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Unread postAuthor: chenslee » Wed Jul 29, 2009 9:30 am

GalFisk wrote:I like the carburetor idea. Venting with proper fuel/air mix (which is what a 2-stroke engine does, more or less) is also a nice idea, providing that you have a simple system for creating this mix.


I have an incredibly simple method of creating this mix. It's a 1/2" hole with a torch head jammed in it. Note the air intake holes on the OUTSIDE of the cannon.

Image
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Unread postAuthor: ryno » Mon Aug 10, 2009 1:34 pm

My combustion used to be set up to where I'd have my fan on and have an air compressor hooked up at the other end of the chamber and I'd use the fan as an exhaust fan to get rid of the stale air. Worked great
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Unread postAuthor: ramses » Tue Aug 11, 2009 3:54 pm

GalFisk wrote:I have some drawings (including a venting piston powered by the combustion itself, and one powered by recoil)


I wouldn't mind seeing your take on it. is it too late for you to enter the contest with it?
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Unread postAuthor: GalFisk » Wed Aug 12, 2009 4:56 am

I don't have the time, tools or materials now, my next project will probably be a big combustion cannon built around a fiberglass tank I got from a water purifier. It may be recoil operated, but that will be a simpler mechanism than what I have drawn up.
And I need to get the valved hybrid working properly, I did things to solve the inaccurate fueling issue a couple of years ago, but haven't done the testing yet. My regulator/gauge may simply not be precise enough for the small fuel meter...
I've been lazy about posting the drawings (and too excited about my skydiving progress and new parachute), I'll try to remember posting them when I get home from work. If someone here can get inspiration from them and build something cool and innovative, I'm happy.
Chenslee: that looks nice :) How do you keep the combustion gasses form escaping through the vent when firing, do you use some kind of check valve?
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Unread postAuthor: chenslee » Wed Aug 12, 2009 10:26 am

GalFisk wrote:Chenslee: that looks nice :) How do you keep the combustion gasses form escaping through the vent when firing, do you use some kind of check valve?


Short answer: I don't.

Long answer: It's top secret :wink: :twisted:
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Unread postAuthor: GalFisk » Thu Aug 13, 2009 4:24 pm

Image
This is an idea for a venting piston actuated by the combustion pressure. I made the animation long ago, I can see a few ways to improve upon this thing.
Operation: the chamber itself is to the right. when the cannon is fired, the blue piston is pushed to the left, taking the yellow venting piston with it. Between the chamber and venting cylinder is a check valve, the venting piston also needs a check valve.
After the combustion pressure has disappeared, the blue piston will be returned by the big spring, but the venting piston will be held back with magnets (far left). The small center spring will not be strong enough to pull it loose form the magnets, but when the blue piston has almost returned to its initial position, the force of the big spring will be put on the venting piston, it will be pulled loose from the magnets and the center spring pulls it all the way back, forcing fresh air into the chamber.

This was ment as a solution to the problem illustrated here with a simpler design someone else posted: http://www.hostanyimage.com/files/71byr ... hzp3lk.gif

Problems: the forces of two springs and a set of magnets must be balanced for this to work. the center piston is also far too big and will have very high forces acting on it while taking energy away from the projectile. In reality, the piston could probably have a diameter smaller than the barrel. If made to travel further than in the drawing of the simple design, or if the diameter of the venting cylinder was made larger than the chamber, the simple design would work. The problem with this design is that the venting cylinder takes up at least as much space as the chamber itself.

I had designs for recoil-operated venting, but I can't seem to find the drawing or 3D model I made and I don't recall the exact mode of operation. It did involve some of the principles of the first valve though, letting the entire cannon slide backwards from the recoil, then have it move forwards (using a spring or counterweight) while a piston inside the chamber is held back by a catch. This makes the piston move through the chamber, venting it. When the cannon returns to its original position, the catch is released and the piston returned by a spring.
The advantage to doing things like this is that the recoil force only cocks the mechanism, while the actuation is done by the predictable and adjustable forces from springs, counterweights and/or bungee cords. This principle of operation can be used for any mechanism actuated by combustion pressure or cannon recoil, including loading, venting and fueling mechanisms.
Hope this can inspire someone :)
We also need research on something that was mentioned in another thread, which is venting through reed valves in the rear of the chamber, using the momentum of the combustion gasses (or if that's not enough, using vacuum from the projectile in an overly long barrel).
PS: Why doesn't phpSpell work? It says I need Opera 6+, I have Opera 9.63.
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