premixed fuel chamber.

[SpudFarm]

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!

[SpudBlaster15]

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[SpudFarm]

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

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

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.

[SpudBlaster15]

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[ramses]

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!!!

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.

[psycix]

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.

[ramses]

or you could simply purge through with air/fuel mixture.

[c11man]

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!!!

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

[ramses]

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.

[Technician1002]

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.

[SpudBlaster15]

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[c11man]

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?

[SpudBlaster15]

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[ramses]

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.

[SpudBlaster15]

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