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Help with diagnosing first hybrid cannon

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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Help with diagnosing first hybrid cannon

Unread postAuthor: LutzNutz » Sun Dec 12, 2010 4:23 pm

I'm attempting my first hybrid cannon after making a few spray and prays, and using Burnt Latke's plans and fuel calculator. using fuel tool 1.5 and ratcalc (and my own water measurements), I built what it says was the proper size chamber and meter pipe. It's firing, but it's not consistently firing. So I have a few questions:

1. the cannon is firing very sporadically, and I've been up and down the PSI range. One time it'll fire at 40 psi, and the next time, nothing. Same thing at 35 and 45. If I open the chamber, it'll spark right up. I know this means my fuel air ratio is off, but why would it fire once and then nothing? Should I put a ball valve on the back of the chamber so I can recharge the chamber with air?

2. I'm using a electric grill sparker from one of my S&Ps, with the spark being about 5 inches from where the injector dumps into the combustion chamber. I've got the spark gap set to about 3/16. I know a stun gun is ideal (and that's a near future upgrade), but I'm wondering about proper placement of the electrodes. Should they closer to the dump point, or further away?

3. the regulator next to the tank started to frost over after being adjusted up and down a few times. It's about 60 degrees today in Florida. It passed the leak test during assembly. It's an pressure regulator like what you'd use with an air compressor. It'll hiss and vent when adjusting down, to obviously relieve pressure. Should I be using a different regulator?

Thanks for the help. I've been lurking for a while but hit a snag with this thing.
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Unread postAuthor: ramses » Sun Dec 12, 2010 4:56 pm

Welcome to Spudfiles.

You may have mixing issues, exhausting the chamber to atmosphere may mix it enough to combust I have the same issue with my hybrid, and the ignition is off in a corner. You're probably fine with your current ignition placement. Try bleeding a bit of mix before firing (eg. fill for 4x and bleed to about 3x) Also try injecting the air as quickly as possible or letting it sit for a while after you inject air.


You may not be getting a spark due to compressed gas being harder to spark through than gas at atmospheric pressure. Try making the gap as small as possible. People run around 1mm quite frequently. Your ignition may be arcing over somewhere else. To test this, make a tiny gap outside the chamber in series with the inside gap, fill the chamber to whatever pressure with air only, and look/listen for a spark. Electronic BBQ ignitors are fine, and they hurt slightly less than stunguns if you are shocked by them. (don't ask :wink: )
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Unread postAuthor: MrCrowley » Sun Dec 12, 2010 4:58 pm

Could you post pictures of the cannon and meter? There are several things in your metering setup which could be causing the problems so it's important we know what configurating the meter is in.

It would help if you told us the chamber and meter volume as well so we can double check your calculations.
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Unread postAuthor: Technician1002 » Sun Dec 12, 2010 6:13 pm

Should I put a ball valve on the back of the chamber so I can recharge the chamber with air?


The fuel/air ratio for a proper burn is assuming Oxygen as part of the air.

Full replacement of spent gas with fresh air is required.

Ball valve, purge line, whatever, get the burnt gas out and fresh in.
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Re: Help with diagnosing first hybrid cannon

Unread postAuthor: omniscient » Sun Dec 12, 2010 11:10 pm

LutzNutz wrote: 1. the cannon is firing very sporadically, and I've been up and down the PSI range. One time it'll fire at 40 psi, and the next time, nothing. Same thing at 35 and 45. If I open the chamber, it'll spark right up. I know this means my fuel air ratio is off, but why would it fire once and then nothing? Should I put a ball valve on the back of the chamber so I can recharge the chamber with air?


The correct amount of air pressure for firing a hybrid, is as follows:

1x mix (Atmospheric Pressure)
2x mix (15psi - which is 1 more "atmosphere")
3x mix (32psi - which is 2 more "atmospheres" plus the 1.8psi from the fuel... Rounded off to the next number)

LutzNutz wrote:
2. I'm using a electric grill sparker from one of my S&Ps, with the spark being about 5 inches from where the injector dumps into the combustion chamber. I've got the spark gap set to about 3/16. I know a stun gun is ideal (and that's a near future upgrade), but I'm wondering about proper placement of the electrodes. Should they closer to the dump point, or further away?


The electric grill sparker only has enough energy to jump a gap that large when firing in atmospheric pressure. In a pressurized environment,
the gap needs to be approximately 0.030" - 0.035" (as ramses has already suggested; 1mm)

A stun gun is ideal, mostly because of the "plug & play" factor.


LutzNutz wrote: 3. the regulator next to the tank started to frost over after being adjusted up and down a few times. It's about 60 degrees today in Florida. It passed the leak test during assembly. It's an pressure regulator like what you'd use with an air compressor. It'll hiss and vent when adjusting down, to obviously relieve pressure. Should I be using a different regulator?


Cranking the regulator too high and then allowing to vent (several times) can cause it to ice up. Just to be sure though; - Check the regulator for leaks (at the fittings) with a soapy solution of water.

As Mr.Crowley suggested; It would help if you told us the chamber and meter volume as well so we can double check your calculations.
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Unread postAuthor: mark.f » Sun Dec 12, 2010 11:28 pm

Are you sure you're building a hybrid? It sounds to me like you're describing a 1 atm combustion cannon.

A hybrid uses an air/fuel mixture at pressure, which is held in by a burst disk or special valve before ignition.
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