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Hybrid won't fire

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Hybrid won't fire

Unread postAuthor: finland66 » Sun Nov 16, 2014 1:12 pm

I got bit of problem with my hybrid, I won't get it fire. Propane burns but it won't be powerfull enought to break burstdisk. I get it fire once with 6x mix so it was 5 bars of air and 25 kpa of propane but I had no luck getting it fire after that. Burstdisk I used was few layers of aluminium foil. I'm pretty sure that my mix canculations are correct.
I use flyback coil for ignition and I'm sure that spark is good.
Here some pic's if the could help.

Gun itself
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Ignition system
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Re: Hybrid won't fire

Unread postAuthor: MrCrowley » Sun Nov 16, 2014 2:52 pm

What fuelling method do you use, can you post a picture, and can you state its volume if it is a volumetric fuel meter?
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Re: Hybrid won't fire

Unread postAuthor: DYI » Sun Nov 16, 2014 10:03 pm

Even if it isn't a volumetric meter, post the meter volume and the chamber volume. Excessive meter volume in a manometric can result in an unexpectedly rich mix. In severe cases, too rich to ignite.

A more likely scenario is poor mixing of the fuel, which can happen very easily in geometries like this. Several members here have struggled with mixing issues in small hybrids running propane/air mixes and using manometric metering. Volumetric meters tend to not suffer as badly from this issue, due to the turbulent mixing process that occurs when the pressurized meter is dumped into the chamber.
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Re: Hybrid won't fire

Unread postAuthor: finland66 » Mon Nov 17, 2014 9:06 am

I'm using manometric meter and it's volume is aprox 33 cubic centimeters including hose from propane tank.
That black/gray thing top of the propane tank is regulator. Chamber volume is aprox 400 cubic centimeters.


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Re: Hybrid won't fire

Unread postAuthor: MrCrowley » Mon Nov 17, 2014 3:44 pm

Could be a mixing issue as DYI suggested. One thing I like to do before pumping in the air is to attach the air connection to the chamber but leave the valve separating the two closed, then pump up the air line to 120PSI or so and then open the valve to the chamber, which might help stir things up a bit.

I use the same manometric meter (larger in volume than yours) on cannons with about 400cm^3 and 1400cm^3 of volume without issue. On the smaller cannon, I like the move the cannon around in the air a bit when it is fuelled and pumped up with air. Either the act of doing this or the time it takes to do it allows the fuel to mix a bit better and usually solves any issues if I don't get ignition at first.
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Re: Hybrid won't fire

Unread postAuthor: finland66 » Mon Nov 17, 2014 4:03 pm

I'm filling air from normal compressor so should I rapidly open ball valve or wery slowly to get best bossible mix. I will try that your "shaking" method tomorrow.
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Re: Hybrid won't fire

Unread postAuthor: MrCrowley » Mon Nov 17, 2014 4:15 pm

finland66 wrote:I'm filling air from normal compressor so should I rapidly open ball valve or wery slowly to get best bossible mix. I will try that your "shaking" method tomorrow.

Try both but the method I go with would be similar to rapidly opening the valve with the theory being that the air will rush in to the chamber and agitate the fuel.
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Re: Hybrid won't fire

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Re: Hybrid won't fire

Unread postAuthor: finland66 » Tue Nov 18, 2014 11:36 am

Now I got it firing with aluminium burstdisk and it did fire pretty well (I think :D ). But when I tried harder burstdisk (metal part from springless can 0.2mm thick) mixture did burn but burtsdisk didn't break. I believe that my mixture isn't powerfull enought to break anything harder than aluminium foil.
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Re: Hybrid won't fire

Unread postAuthor: MrCrowley » Tue Nov 18, 2014 3:09 pm

finland66 wrote:Now I got it firing with aluminium burstdisk and it did fire pretty well (I think :D ). But when I tried harder burstdisk (metal part from springless can 0.2mm thick) mixture did burn but burtsdisk didn't break. I believe that my mixture isn't powerfull enought to break anything harder than aluminium foil.
It all depends on the strength of the material. If you're getting consistent burnings but not bursting then your disk is too strong.

You should be able to use aluminium burst disks cut from soda cans at any mix above 4x. I use one disk at 4x for a 3/4" union and four disks at 10x for a 3/4" union. They seem to burst around 300PSI but there is some factor introduced by the shock of the gas igniting rapidly because only pumping up the disk to 300PSI with air doesn't burst it.
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Re: Hybrid won't fire

Unread postAuthor: finland66 » Tue Nov 18, 2014 3:38 pm

Okey I will give it try I will upload results here. Two layers of soda can aluminium probably makes good burstdisk for 6x on 1" union but probably best way to find good burstdisk is by testing.

But anyway thanks for helping I greately appreciate it :)
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Re: Hybrid won't fire

Unread postAuthor: finland66 » Wed Nov 19, 2014 12:59 pm

Now it works fine with aluminum burstdisk from soda can. I will post it soon to hybrid showcase.
Thanks for helping me.
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