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premixed fuel chamber.

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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premixed fuel chamber.

Unread postAuthor: SpudFarm » Thu Jul 29, 2010 4:44 am

Hey, I just made a piston hybrid out of spare parts (:D) and it wont fire.

I am using a 6l premixed chamber at 5x. The premixed chamber has a female quick disconnect fitting from a compressor and my gun has a male one.

I mix the premix chamber to a normal 5x mix with 4.5% propane just like I use in my big hybrid. Then I just connect my piston hybrid to the chamber and fill it. The reason I do this is because I can't seal the piston at the VERY low pressure required for manometric metering.

When I try to fire it, nothing happens. I don't think the mix ignites.

My question is if I need to use another mix in the premix chamber? That would actually be a good ting since it reduce the chance of combustion inside the chamber while reloading.

Thanks!
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Re: premixed fuel chamber.

Unread postAuthor: SpudBlaster15 » Thu Jul 29, 2010 5:01 am

SpudFarm wrote:I am using a 6l premixed chamber at 5x.


:shock:

...The reason I do this is because I can't seal the piston at the VERY low pressure required for manometric metering.


I'm assuming you're not using an isolated pilot design, and are fueling through the pilot chamber. I wouldn't suggest this, because you may end up with an ignitable mix behind the piston, which could potentially diesel under compression when the piston opens. Are you using any type of pilot vent?

My question is if I need to use another mix in the premix chamber?


Are you venting the chamber after firing, or simply adding 5 atmospheres of air/fuel mixture to the post combustion atmosphere? If the latter, you shouldn't need to change the ratio in the bulk tank. Otherwise, you'll need to add more fuel.
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Unread postAuthor: SpudFarm » Thu Jul 29, 2010 5:09 am

The premix chamber is the one Larda tested to 2400PSI for me so shouldn't be a problem :P

There is no pilot at all. I was just hoping to get one firing to get the feel of things. It is also remotely fired at this time. However, nothing have happened yet. I thought about adding more fuel, I almost thought my head off for some reason :P

I'll go out making an attempt with an additional mix of propane.
Thanks!

Edit: This time I could feel the cannon getting hot after each failed attempt. Now I just need to figure out how to fix the valve.
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Unread postAuthor: SpudBlaster15 » Thu Jul 29, 2010 11:32 am

SpudFarm wrote:This time I could feel the cannon getting hot after each failed attempt. Now I just need to figure out how to fix the valve.


How exactly are you keeping the piston closed and sealed when the chamber is pressurized? You say there's no pilot, are you using a mechanical spring?

Whatever the case, the piston opening pressure is clearly too high for the mix you're using. That's why you feel the chamber getting hot; the mix is igniting, but as the pressure is too low to open the valve, the thermal energy is simply lost to the chamber walls.
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Re: premixed fuel chamber.

Unread postAuthor: ramses » Thu Jul 29, 2010 1:50 pm

SpudBlaster15 wrote:
I'm assuming you're not using an isolated pilot design, and are fueling through the pilot chamber. I wouldn't suggest this, because you may end up with an ignitable mix behind the piston, which could potentially diesel under compression when the piston opens. Are you using any type of pilot vent?


Finally someone else sees the light!!! :D

The area ratio of the piston is too small, which causes the opening pressure to be too large. Unfortunately there's nothing much you can do, except either make a bigger piston, a smaller barrel port, or add O-rings to the piston and fill the pilot to a lower pressure like I do with mine.
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Unread postAuthor: psycix » Thu Jul 29, 2010 2:08 pm

On top of it, injecting a stoichiometric mix into a chamber that already has air will result in a lean fuel mixture.
If you want a mix of 4.5% you will need a higher percentage in your bulk tank, as your chamber is already filled with atmospheric air.
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Unread postAuthor: ramses » Thu Jul 29, 2010 4:20 pm

or you could simply purge through with air/fuel mixture.
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Re: premixed fuel chamber.

Unread postAuthor: c11man » Thu Jul 29, 2010 10:12 pm

ramses wrote:
SpudBlaster15 wrote:
I'm assuming you're not using an isolated pilot design, and are fueling through the pilot chamber. I wouldn't suggest this, because you may end up with an ignitable mix behind the piston, which could potentially diesel under compression when the piston opens. Are you using any type of pilot vent?


Finally someone else sees the light!!! :D

The area ratio of the piston is too small, which causes the opening pressure to be too large. Unfortunately there's nothing much you can do, except either make a bigger piston, a smaller barrel port, or add O-rings to the piston and fill the pilot to a lower pressure like I do with mine.


if you do the math the piston and seat have to be increadible close to prevent opening. iirc a 1.9 inch seat and a 2inch piston is around 600psi opening pressure at a 10x mix. so unless he has practicly the same diameter seat as piston it should open if the fuel is ignited
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Re: premixed fuel chamber.

Unread postAuthor: ramses » Fri Jul 30, 2010 10:50 am

c11man wrote:iirc a 1.9 inch seat and a 2inch piston is around 600psi opening pressure at a 10x mix. so unless he has practicly the same diameter seat as piston it should open if the fuel is ignited


nah, that would open at 1538 PSI.

the optimal area ratio would be around 10:1.5, but because you have to subtract to find the area of the "ring" the chamber acts on, there is no easy rule for the diameters.

Then you have to make allowances for friction, cooling, imperfect mix, etc.
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Re: premixed fuel chamber.

Unread postAuthor: Technician1002 » Fri Jul 30, 2010 11:57 am

ramses wrote:
c11man wrote:iirc a 1.9 inch seat and a 2inch piston is around 600psi opening pressure at a 10x mix. so unless he has practicly the same diameter seat as piston it should open if the fuel is ignited


nah, that would open at 1538 PSI.

the optimal area ratio would be around 10:1.5, but because you have to subtract to find the area of the "ring" the chamber acts on, there is no easy rule for the diameters.

Then you have to make allowances for friction, cooling, imperfect mix, etc.


Add one more item, piston opening time. The initial opening force on a close ratio piston is relatively low. This delay may permit the gas to cool (high radiant heat levels provide rapid cooling) so the ratio you want is lower than the "optimum" because of the delay in opening due to mass times force is the initial cracking of the valve when the force is low due to the small area.

Once cracked, the force is much larger and opening time is quick, but the time to crack has to be accounted for.
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Unread postAuthor: SpudBlaster15 » Sat Jul 31, 2010 12:03 am

ramses wrote:Then you have to make allowances for friction, cooling, imperfect mix, etc.


Technician1002 wrote:Add one more item, piston opening time.


All the more reason to isolate the pilot chamber, and enable yourself to adjust the valve opening force by varying the pilot pressure independently of the pressure in the combustion chamber.

The only advantage to an equal pressure system is the simplicity of the design; you only need one air fill point. However, that's not to say one can't integrate all operating functions into one system...

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Unread postAuthor: c11man » Sat Jul 31, 2010 2:42 pm

SpudBlaster15 wrote:The only advantage to an equal pressure system is the simplicity of the design; you only need one air fill point. However, that's not to say one can't integrate all operating functions into one system...


the only reason? a lower plot pressure will let the piston accelerate faster when it first starts to move becuae it will have less resistance. also the piston has less of a chance to reboud forcefully into the seat after firing becuae the amount of air behind the piston is less/ as you mentioned the simplicity is also nice. one other thing is that if you have the correct piston to seat ratio it works at many different mixes. no need to calculate the right pilot pressure to get the opening pressure right


Add one more item, piston opening time. The initial opening force on a close ratio piston is relatively low. This delay may permit the gas to cool (high radiant heat levels provide rapid cooling) so the ratio you want is lower than the "optimum" because of the delay in opening due to mass times force is the initial cracking of the valve when the force is low due to the small area.


i dont know how fast the combustion is but a good piston can open completly in under 3ms so is the delay between starting moving and open long enough to have an effect?
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Unread postAuthor: SpudBlaster15 » Sat Jul 31, 2010 5:13 pm

c11man wrote:the only reason? a lower plot pressure will let the piston accelerate faster when it first starts to move becuae it will have less resistance. also the piston has less of a chance to reboud forcefully into the seat after firing becuae the amount of air behind the piston is less


I'm not sure what you're getting at. With an isolated pilot setup, you can design the area ratio such that the valve works optimally at a lower pressure than what the combustion chamber is filled to. Your post suggests reasons against using an equal pressure setup...

And even then, if you're using any type of rapid responding vent system on the pilot chamber, these factors are insignificant.

one other thing is that if you have the correct piston to seat ratio it works at many different mixes. no need to calculate the right pilot pressure to get the opening pressure right


The optimum opening pressure for each mix is not a fixed variable. It's highly beneficial to be able to independently set the opening pressure based on the properties of the projectile, and also the fuel/oxidizer/buffer gas combination being used.

If one is incapable of calculating and memorizing an additional set of pressure values, they have no business building a hybrid cannon in the first place. As I said, the simplicity is nice, but the advantages don't extend beyond that.

i dont know how fast the combustion is but a good piston can open completly in under 3ms so is the delay between starting moving and open long enough to have an effect?


If you're setting the valve such that the net force switches direction at the instance combustion completes, then yes, there will be a fairly significant amount of heat loss before the valve undergoes initial movement and the pressure begins to act on the entire cross sectional area.

Once the net force is maximized, the valve will typically open in under a few milliseconds. However, as the initial movement is relatively slow, it needs to be accounted for. I typically set the valve opening pressure on my hybrid at ~60 - 80% of the peak theoretical combustion pressure, depending on whether I'm firing darts with very frangible foam sabots, or solid wood/metal slugs.
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Unread postAuthor: ramses » Sun Aug 01, 2010 11:59 am

The optimum opening pressure for each mix is not a fixed variable. It's highly beneficial to be able to independently set the opening pressure based on the properties of the projectile, and also the fuel/oxidizer/buffer gas combination being used.


I don't think opening at a 1-200 PSI under optimum would hurt a high-ish mix hybrid too much. Tech has converted me to the side of seat ratio/non-isolated pilot in another thread. You can still use the isolated pilot method with 1 air port if you use a self-relieving regulator with a flow restricting orifice. That way, you don't have to wait for the air/fuel to seep around the piston before each shot, in case you were trying for a high ROF or something...

I need to run a CFD simulation to ensure there won't be a good fuel/air ratio in the pilot, otherwise, it will diesel as a ~50x mix, assuming the original hybrid was around 8x.
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Unread postAuthor: SpudBlaster15 » Sun Aug 01, 2010 1:14 pm

ramses wrote:I don't think opening at a 1-200 PSI under optimum would hurt a high-ish mix hybrid too much.


HGDT seems to agree.

Tech has converted me to the side of seat ratio/non-isolated pilot in another thread.


What advantage do you see other than simplicity? Seems a bit illogical to want to downgrade your setup when you already have a working isolated pilot system.

You can still use the isolated pilot method with 1 air port if you use a self-relieving regulator with a flow restricting orifice.


It's much simpler to attach a T fitting to your air fill port, and connect one side to the combustion chamber, and the other to the pilot chamber. A ball valve on each fill line can be used to separate them and allow each chamber to be filled to a different pressure.

That way, you don't have to wait for the air/fuel to seep around the piston before each shot...


You're proposing that one inject the fuel through the pilot chamber as well? :shock: That's just begging for a violent compression heating induced explosion. Filling the system with air after adding a correctly measured quantity of Propane would probably remove a majority of the fuel from the pilot chamber, but given the very slow flow rate around the piston, some mixing would occur, and you could very well end up replacing 96% of the volume with air...

In addition, the exact quantity of fuel left in the pilot chamber would vary as a result of a myriad of factors that are not precisely controlled during the firing cycle. The most significant of these is probably diffusive mixing between the chambers, which will occur when the launcher is pressurized, and the internal fluid flows are relatively stagnant. Even if you don't experience high compression dieseling on the first few shots, there's a strong possibility that the pilot chamber would eventually reach an ignitable oxidizer/fuel proportion, and experience a violent structural failure when you least expect it.

I need to run a CFD simulation to ensure there won't be a good fuel/air ratio in the pilot, otherwise, it will diesel as a ~50x mix, assuming the original hybrid was around 8x.


Designing any system to potentially create favourable conditions for an accidental explosion is BAD engineering, period. If you're going to locate the air fill on the pilot chamber, at least use a separate port to inject the fuel directly into the combustion chamber. Although it will be more difficult to achieve proper mixing, the system will be much safer to use.
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