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Dual ignition & chamber fan...do they help?

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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Dual ignition & chamber fan...do they help?

Unread postAuthor: Moonbogg » Tue Nov 25, 2008 5:51 am

Building my first propane spud demon here, and I was just wondering if dual ignition (which I planned for) and chamber fan (which I didn't plan for) relaly help. Here is my guess...I would guess that a chamber fan would only shorten the time needed to allow for complete fuel/air mixing. Without a chamber fan, you would get just as much power as long as you're patient enough to let the fuel/air properly mix on its own. So acoording to my guess, a chamber fan gives you full power with much less time.
As far as dual spark ignition goes, it only helps when the fuel/air is fully mixed. If you have two sparks at opposite ends of the chamber, only one will matter if your fuel is concentrated near it, however, if you have thoroughly mixed fuel/air and two sparks at opposite ends of the chamber, then combustion will be more simultanious and thus create a little more power.
Is my thinking right? I have no chamber fan because I didn't design for one. Would it benefit me to have dual spark ignition? Or should I keep it simple and stick to a single spark...
Thanks,
-Ryan
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 25, 2008 6:09 am

Both a chamber fan and dual ignition will serve you well. To what extent this will increase power, percentages depend on the individual setup, and in some cases might not give an improvement that would not be observabale without a chrony. However if you have the resources, why not ;)
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Unread postAuthor: Moonbogg » Tue Nov 25, 2008 6:42 am

Why not indeed. I will use dual source ignition and as stated this design will have no chamber fan. Dual source ignition couldn't hurt performance. The shame is, as you mentioned, I won't know the difference without 2 identical setups, one with single ignition, the other with dual ignition, and a chrony. I will simply trust that dual ignition won't hurt performance and if anything at all, it will help it.
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Unread postAuthor: starman » Tue Nov 25, 2008 8:47 am

There is a difference and it is noticable. Go with 3 gaps if you can and run the fan while you fuel up and while you shoot. D_Hall's HGDT programmodels the number of spark gaps as well as a chamber fan and predicts performance. I suggest you load this program and model some cannonage.

I will simply trust that dual ignition won't hurt performance and if anything at all, it will help it.


.. :roll: ..I'm not sure why you would be worried that 2 would be worse than 1. Even without the HGDT models, doesn't common sense tell you that the chamber will burn out the fuel air faster if ignited simultaneously at spaced out locations? Keep your gaps in the center of your chamber and divide the chamber roughly equally with your gaps. Here's a pic of 3 - 3 gap spark strips. (you'll only need one strip per cannon chamber of course)

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Unread postAuthor: Radiation » Tue Nov 25, 2008 10:26 am

Tried both on one design. Had one spark gap in the cap of a 3" by 14" chamber. It was powerful. Added 4 spark gaps and the power was increased 3X by this change alone. Anyone who tells you more gaps does nothing has no idea what they're talking about. IMO don't stop at 2, have 3 or more if you can!

As far as chamber fan, you sort of have the right idea, it mixes the fuel inside and all designs the fan vents the gun quicker than passively venting the chamber.
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Unread postAuthor: jimmy101 » Tue Nov 25, 2008 11:37 am

I would say the fan is more important than multiple gaps.

It takes a LONG time to fully mix fuel in a typical sized combustion chamber. Depending on how the fuel is injected it could take a minute or more on a hot day. On a cold day it takes much longer.

The difference in performance between well mixed and poorly mixed fuel is huge. In my basic combustion, on a cold day, with perhaps ten seconds mixing time, and gentle metered fuel injection, the fan takes the range from ~50 feet to >300 feet with a 2" 100g spud.

The fan not only significantly boosts performance it makes the gun's performance much more consistent.
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Unread postAuthor: psycix » Tue Nov 25, 2008 11:47 am

Here I go again:
I had a basic combustion. Single spark gap, no fan.
It splattered potatos against the barn door.
I upgraded it with a fan and dual ignition.
I put a board before the barn door and shot at it.
The potato went through both the board AND the door.
Which convinced me of the power of the upgrades.


A fan does not only mix your fuel faster, it does it so fast that it also mixes better, to achieve the mix rate a fan does in 5 seconds, you would have to wait more then 10 minutes for the fuel to diffuse.
Also a fan induces turbulence which improves the burn rate and on top of that, the fan helps greatly with venting. Faster venting (and thus reload) times and more reliable shots due to the air being vented better.

Igniting at the right places can theoretically half the burn time of your spudgun when a double gap is used.

These upgrades are highly recommended and increase performance by ALOT.
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Unread postAuthor: mrbadexample » Tue Nov 25, 2008 8:11 pm

i don't think a fan is necessary, i load the spud and leave it at the end of the barrel, then add propane, then ram the spud down the barrel, which doubles the air in the chamber and mixes the gas.

i'm going to install dual sparkers in my chambers and fire them with a 150,000 volt stun gun. i plan on using the same stun gun on multiple combustion chambers with extension cord plugs. just plug the stun gun to the chamber, probably put a strip of velcro on the sides (they have great FREE velcro at the illinois tollway authority office that they use for I-PASS units, just walk in and grab a handfull :laughing3: ) should have all this stuff done in the next few days. :wink:
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Unread postAuthor: starman » Tue Nov 25, 2008 10:13 pm

mrbadexample wrote:i don't think a fan is necessary,


No of course it isn't necessary, but you won't get the same high performance without it. Also, you say you are ramming the potato down and doubling your air... :roll: unless you have a perfectly air tight seal you aren't doubling your air...and a potato will not offer an air tight seal under your ramming scenario.

The fan is a proven performance enhancer, it isn't a debateable issue.
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Last edited by starman on Wed Nov 26, 2008 11:32 am, edited 1 time in total.
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Unread postAuthor: jimmy101 » Wed Nov 26, 2008 11:14 am

Like starman said, mrbadexample method is a very bad idea.

When you ram the spud home you put about 15 PSIG in the chamber, and assuming a CB ratio of 1. A 2" barrel will have ~45 pounds of force trying to push the spud back out. So, one of two things happens, the pressure pushes the spud back up the barrel a bit or you vent the chamber a bit, either through leaks at the cleanout or as blowby around the spud. Those leaks are of incompletely mixed fuel + air. So you are messing up your fuel ratio.

The gases won't really mix all that well as you ram. They mix well enough to ignite but there is a big difference between "well enough to ignite" and "well mixed". The gun will perform better with a chamber fan.
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Unread postAuthor: jimmy101 » Wed Nov 26, 2008 11:15 am

Like starman said, mrbadexample method is a very bad idea.

When you ram the spud home you put about 15 PSIG in the chamber, and assuming a CB ratio of 1. A 2" barrel will have ~45 pounds of force trying to push the spud back out. So, one of two things happens, the pressure pushes the spud back up the barrel a bit or you vent the chamber a bit, either through leaks at the cleanout or as blowby around the spud. Those leaks are of incompletely mixed fuel + air. So you are messing up your fuel ratio.

The gases won't really mix all that well as you ram. They mix well enough to ignite but there is a big difference between "well enough to ignite" and "well mixed". The gun will perform better with a chamber fan.
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Unread postAuthor: Radiation » Wed Nov 26, 2008 12:19 pm

For the original poster, I just want to summarize unequivocally a chamber fan and multiple spark gaps are incredibly important to the performance of a combustion spudgun.

Will your spud gun work without either? Of course it will, but the performance will never be as good or nearly as consistent if you don't have a metered, chamber fanned, multiple spark gap system end of story (and some good math to go along with it of course!). It's the difference between a Jalopy and a Ferrari.
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Unread postAuthor: Moonbogg » Thu Nov 27, 2008 3:19 am

Oh CRAP then! I just built my gun with dual spark gaps and NO chamber fan. What it does have is a great ventillation fan behind a ball valve. The reason I didn't add a chamber fan was because the materials I had at hand (ABS) and common fittings, there was no way to install a chamber fan and allow it to be replaceable. And with the heat and pressures involved, I figured a PC fan may warp/deform and thus need to be replaced. My next design will surely have a list of criteria which will be met, and a chamber fan will be one of them. I just didn't have a way to make the chamber fan replaceable. Thanks very much for al your input. I am convinced a chamber fan and multiple sparks are the way to go. Now that I JUST finished my first spudder (pics coming soon) I will start work on a new one once I have adequately planned everything all out.
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Unread postAuthor: starman » Thu Nov 27, 2008 3:49 am

In a 4" cleanout/pipe chamber joint, I simply grind the corners of an 80mm computer fan and hot glue it in. Hot glue holds well enough but certainly isn't a permanent type cement in case you need to replace the fan. Configure the fan to blow in to the chamber and out the barrel.

This allows just about any chamber to be back fit with a fan, even if it wasn't designed for it to begin with.
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Unread postAuthor: Moonbogg » Thu Nov 27, 2008 10:40 pm

starman wrote:In a 4" cleanout/pipe chamber joint, I simply grind the corners of an 80mm computer fan and hot glue it in. Hot glue holds well enough but certainly isn't a permanent type cement in case you need to replace the fan. Configure the fan to blow in to the chamber and out the barrel.

This allows just about any chamber to be back fit with a fan, even if it wasn't designed for it to begin with.


Thats a good idea. What to do about ventillation then? Perhaps attach a small ball valve to the cleanout plug somehow?
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