I got some questions.

[PracticallySane]

So I've put something together, its a hybrid clone of my phenumatic rifle. Right now they share a barrel but it will get its own dedicated when complete.

For the time being I have had a few successful test shots at 10x using the chart from the forum. They are hard to repeat. But I have had some success.

The gun is made from hydraulic pipe with a work pressure of 3000psi I feel like I am safe to push maybe 50x??

Questions
1• how high would you go on hydraulic pipe?
2• how accurate are the gauges we use for metering?
3• can you ignight a 50x mix with a spark?
4• how to achieve reliable fueling?
5• most cases of a failure i get some smoke out of the chamber when venting, so partial ignition? Am I lean or rich? I am using a 0-300 psi gauge at the moment and it has no resolution.

Let me hear it guys? What do you think. I insist on achieving super sonic.

[PracticallySane]

Since posting i believe i have figured out my issues. I noticed that if I leave the gun sit for roughly a minute before firing it goes off reliably. I increased the chamber volume aswell.

I am in the process of creating a "venturi" fuel mixing system to mix the gasses as the chamber is filled.

[jackssmirkingrevenge]

The gun is made from hydraulic pipe with a work pressure of 3000psi I feel like I am safe to push maybe 50x??

50x in a closed chamber will likely generate less than 6000 psi, safety factor is probably more than double... personally I would probably be ok with it.

Are you using a burst disk or is there a piston? What are you hooking up to your spark plug?

Have you considered using volumetric instead of barometric fueling?

Also, since there is just over 20% concentration of oxygen in air, you cannot really go beyond about 28x with compressed air. beyond that you would have to inject oxygen.

I insist on achieving super sonic.

That should be achievable even with more modest mixes.

[Moonbogg]

50X? Remote fire it behind a blast shield. Regarding the spark at that mix, I'm not sure that will work. Too much resistance. You'd need a lot fo voltage and a tiny gap. A guy named Larda on this forum made a very high mix hybrid and I think he used a wire inside the chamber and ran current through it to make it glow red hot. That ignited the mix.

[PracticallySane]

So the gun has a piston valve.

If you have seen my other rifle, this is a clone.
The only difference being a venting pilot valve, a spark plug, and the piston does NOT have a check valve.

Because of the identical construction I can use the rifled barrel on both of my projects.

My miss fire issue is due to mixing the fuel, at least this is my conclusion.

Regarding 50x and compressed air, my understanding is that by increasing the pressure you increase the available oxygen? How does the oxygen stop getting pumped into the chamber?

I was on the forums many years ago. I credit larda for getting me into hybrids. Watching that thing punch a hole in 1/2 steel plate was something els. He did use a heated wire, something like a glow plug.


I say 50x because a 10x air/propane yelled significantly less damage than my phenumatic rifle at 3000psi. Same barrel, same ammo.

I cant have a phenumatic that out performs my hybrid

Thanks guys, this place is great!

[hectmarr]

Nice project. What do you use to give compressed air to your new hybrid?
I build "domestic" hybrids, with little mixing, and a volumetric dispenser works well for me. I am measuring with good precision, between 2 and 10 cm3 of butane. I use a manual air pump with two valves, one for intake, the other for discharge. To mix, I suck in the dosed butane, I mean the exact amount, through the suction valve of the air pump, and then it is introduced, through the outlet valve, into the combustion chamber. The air pump cylinder acts as a premix chamber.

[Moonbogg]

Also, regarding the mixing issue, I don't think the fuel/air mixture is an issue at all. I'm betting it's a fuel/air ratio issue. I thought the same about my last cannon but it turned out to be a ratio issue because I didn't measure my meter pipe volume accurately enough. It fixed it. No mixing required. The air being pumped in will mix it up nicely, especially with a cannon as small as yours.

[jackssmirkingrevenge]

I say 50x because a 10x air/propane yelled significantly less damage than my pneumatic rifle at 3000psi. Same barrel, same ammo.

Technically at the same pressure the hybrid should outperform the pneumatic due to the hot gasses, but that being said at 10x you are not generating more than 3000 psi, even if the chamber was completely sealed off.

I would give volumetric metering a go, it's very accurate so you know you have the right mix, just need to be sure to take your pump volume after the check valve into account.

Regarding 50x and compressed air, my understanding is that by increasing the pressure you increase the available oxygen? How does the oxygen stop getting pumped into the chamber?

I meant this as a limitation for volumetric metering, if you're doing it by pressure then there is no such limit.

[PracticallySane]

Eventually ill figure out how to quote, and use the forum properly, but for now...

I use a 4500 psi PCP pump for air.

It has to be a mixing problem, I personally attributed the small chamber as why it doesn't mix well. My thought is it only takes maybe 5 pumps to reach 150 psi so not much mixing actually happens.

If I try to fire immediately after filling its a no go. Come back 1 minute later and it goes off with a sharp bang. I will start "upping" the mix this weekend.

Volumetric metering math.... lol maybe ill give it a shot if I want to scratch my head for a while.

For me Xpsi + Xpsi is profoundly simpler.

What is the lowest mix i can achieve 3000psi post ignition?

[jackssmirkingrevenge]

What is the lowest mix i can achieve 3000psi post ignition?

I guessed your chamber dimensions and simulated it in HGDT with a "closed chamber":



I needed 27x before the chamber pressure exceeded 3000 psi

[KannonKing]

Since posting i believe i have figured out my issues. I noticed that if I leave the gun sit for roughly a minute before firing it goes off reliably. I increased the chamber volume aswell.

I am in the process of creating a "venturi" fuel mixing system to mix the gasses as the chamber is filled.

Without a chamber/exhaust fan, your CO2/O2 ratio will be higher than first shot, or waiting for considerable diffusion to occur! Looks good!

[PracticallySane]

I'm thinking about giving the volumetric route a try. I cant get any real consistency right now. Makes a nice bang when it does go off though.
I dont have any real way to measure the volume at the moment. So for now I'm messing with the pressures. Not sure which gauge is wrong..

[jackssmirkingrevenge]

Sorry if I'm stating the obvious but there is one thing about gauges, if an air pressure gauge reads zero at atmospheric pressure, the actual pressure is around 15 psi... did you take that into account?

[PracticallySane]

I have been using the chart posted in the forum, I haven't done any calculations.

For 10x 6.44 ~6.5 psi propane
138.74 ~ 140psi air

Are you saying I need to subtract 15 psi?

So 6.5 and 125?

[jackssmirkingrevenge]

The table you are using seems to account for this, with the air gauge at zero for 1x.

One thing that might be throwing off your mix is the volume of the pump between the launcher and the pump's check valve, can you measure the volume of the hose?

Basically once you connect the pump the propane from the chamber is going to flow into the hose and dilute your mix.