help with ignition. trying to follow SB15's 101.

[jhalek90]

Hello, i will start with a photo of my hybrid:


I used a simply trick to find the volume of the chamber/meter.
I filled the chamber and meter with water, and weighed the water in each of those parts respectively.

chamber=81g of water or, 81cc or 4.94 cubic inches.
Meter=18g of water or 18cc or 1.1 cubic inches.

The fuel is propane.

According to the fueling 101 sticky by SB15:
http://www.spudfiles.com/forums/hybrid- ... 13602.html

Vp=4.94/0.958-4.94
Vp=0.216 cubic inches of propane.

Meter pressure =Vp/Vm or 0.216/1.1
Meter pressure= 0.196atm or 2.88 psi.

So i need 2.88 psi in my meter, correct? (for a "0x" mix)

My issue is that the cannon will not fire with the meter at 2.8, but will sometimes fire with a meter pressure of 7.5psi. The cannon will not fire at 15psi with 1bar of air added.

I would like some help checking my math, and finding why my hybrid wont ignite. Thanks.

EDIT:
The spark is very reliable, and sits in the center of the chamber, inside the T. The spark plug is a "Hubb plug" design.

[MrCrowley]

Bit busy at the moment so I wont cover the math, however;

Some tips:

I imagine your gauge reads up to ~150PSI, if it had an error of 2% F.S., that's 3PSI. So if the gauge reads 4PSI, it could be as low as 1PSI or as high as 7PSI. To be able to use a gauge like that you will need a small meter volume. If you could get it so you need closer 10PSI for a 1x mix, that would be better.

Also, I had problems with my meter once and SpudBlaster said I should have included my meter volume as part of the chamber volume in the calculations due to my meter setup. I can't remember what particular meter setup required this though, so I'm unsure if yours would need to do that.

I don't think my first hybrid ever fired as a 1x mix by the way, 2x and above and it worked pretty well. Have you tried 6PSI of fuel with 15PSI of air?

I never realised how my gauge may have been the root cause of my fuelling inconsistencies until I built a manometric meter for my other hybrid. I don't think I would ever go back to making volumetric meters, manometric works so well for me and allows me to use the same meter on multiple cannons.

So i need 2.88 psi in my meter, correct? (for a "0x" mix)

That's a 1x mix

[jhalek90]

oh. Thank you MrC.

I had never thought my gauge might be off. Also, if i Add the meter volume to the chamber volume i get 3.5 psi for a "1x" mix.

Thanks, ill update with results in a bit.

[Mr.Tallahassee]

I highly suggest putting gauges behind a ball valve as to not destroy them on hybrids. You would need two gauges though. One low pressure gauge behind the propane valve and one high pressure off of the chamber for air.

[jhalek90]

There is a valve in between the chamber and the meter... is that not good enough?

[jackssmirkingrevenge]

[jhalek90]

i would love to get the on board fueling working... but i guess a syringe may work just as well.

I made a smaller meter, and will try that first.

Now if i could only get my burst disk to seal in the union.... damn metal on metal seal..... it worked fine a few times up 100ish psi... but it wont seal with even a single layer of foil now.....

[jackssmirkingrevenge]

A syringe is useful for intitial testing.

As to the seal, chuck in a rubber washer

[MrCrowley]

Now if i could only get my burst disk to seal in the union.... damn metal on metal seal..... it worked fine a few times up 100ish psi... but it wont seal with even a single layer of foil now.....

Have you grind down the lip protruding from one half of the union like so?


As JSR suggested, you'll also need a rubber washer. I used them for years and went through three or four but recently I found that o-rings work well and so far I haven't had one shot through the barrel yet, they all (I use four) stay in place.

Last suggestion; once you've grind down the union, it helps if the grind down half is attached to the barrel. Reasoning is that it provides more surface area than the other half of the union for the disk to sit against. Since the disk is being pushed towards the barrel end, you want the half with the most surface area attached to the barrel so the disk doesn't slip out instead of bursting.

[jhalek90]

I have not yet ground down the lip... but i will this afternoon.