ignition issues on a hybrid project

[taaafit]

Hi everyone!
I'm attempting to build my first hybrid but i'm going into some ignition troubles...

Firstly the design:
The canon will be a mix of a piston and a burst disc hybrid. In fact it has an O-ringed piston in the chamber that closes the barrel hole when it's down and a piece of pretty much every thing breakable holds the piston down while under pressure until it breaks.
At this point I have build the piston and the chamber but wanted to see it works before going further... and obviously it doesn't work...

the chamber

I have a lighter piezzo hotglued in the chamber wich makes nice sparks between the wire and the inside of the steel chamber. So I think that is not the problem...
The chamber can be used as a full pneumatic when gas ball valve closed so i tried to pressurise it and it holds 7 bar easily without leaking... so I think it's not the problem either... (works fine on pneumatic mode)
But when I try it on combustion mode (1X mix for the beginning), I add 4mL (125cc chamber water/syringe metered) of butane through the ball valve with a syringe, close the ball valve, clicks a hundred times the piezo but nothing happens...

What did I wrong?
Tks for your help!

[jackssmirkingrevenge]

Welcome/Bienvenue!

A common problem with hybrid ignition, especially if you are using a piezo, is the size of your spark gap, to be be effective it should be less than 1mm.

Also, how are you pressurizing to 15 psi for 1x? did you take the dead volume of your pump into account?

[taaafit]

Actually I tried a 1X mix so i didn't pressurize, i just let it at atmospheric pressure. That's why i thought that if i see the sparks when empty, i should also have the sparks when using it at 1X mix (because i've read the sparks are difficult to obtain when pressure rises but since i put no more pressure...). Am i wrong thinking that way?
Is the dead volume of the pump really important even if i use a presta valve to pressurize?

[jackssmirkingrevenge]

With a piezo the size of the gap is still important - you want the space between the electrodes to be about 4-6mm for an unpressurized mix.

Is the dead volume of the pump really important even if i use a presta valve to pressurize?

Yes, while pumping the gas can flow past the presta up to the pump check valve. It's not that important to factor in if you have a significantly large chamber but for smaller ones it can make a huge difference.

[taaafit]

Thank's taking time to answer!

With a piezo the size of the gap is still important - you want the space between the electrodes to be about 4-6mm for an unpressurized mix.

So do I have to adjust the gap for each mix? I assume I can't keep a small gap for low mixes because the tension would be to low and therefor the spark to weak... Is that the reason?

Yes, while pumping the gas can flow past the presta up to the pump check valve. It's not that important to factor in if you have a significantly large chamber but for smaller ones it can make a huge difference.

I don't really understand why... I was thinking the presta is already a check valve so if the gas can't pass through the pump check valve it would also not pass through the presta... But since you are probably much more experienced than me, i will just believe you How can i measure the dead volume then?

And finally i was thinking it's more a problem of the gas leaking through the ball valve while fueling (i actually directly fuel with the syringe through the second ball valve and not through the presta). Do you think I should first look at this ? if yes I will try to adapt my syringe for a presta fueling and keep the second ball valve forever closed or as a pressure release valve just in case..

[jackssmirkingrevenge]

So do I have to adjust the gap for each mix?

If you want to keep using the small piezo ignitor, then yes - for a 10x more example, you spark gap has to be paper thin, literally less than 0.5mm for example - and even like this it won't really work beyond 15x. This small gap won't work for 1x though.

I don't really understand why... I was thinking the presta is already a check valve so if the gas can't pass through the pump check valve it would also not pass through the presta...

The presta is held shut by pressure, and while pumping, the pressure on both sides of the presta is the same, so gas can pass in both directions. For the pump check valve however, while pumping there is greater pressure on the pump side than on the launcher side, so gas cannot flow past it back into the pump.

How can i measure the dead volume then?

Fill it with water or just guestimate based on the volume of the pipe from the pump to the valve, it will be accurate enough.

And finally i was thinking it's more a problem of the gas leaking through the ball valve while fueling (i actually directly fuel with the syringe through the second ball valve and not through the presta). Do you think I should first look at this ? if yes I will try to adapt my syringe for a presta fueling and keep the second ball valve forever closed or as a pressure release valve just in case..

One problem is that your chamber is long and thin, which doesn't promote good gas mixing at 1x.

Can you post an internal diagram of what your setup looks like?

Thank's taking time to answer!

No problem, the world needs more hybrids!

[jackssmirkingrevenge]

http://www.alloutdoor.com/2015/08/25/ha ... till-rock/