Hybrid Piston Pilot Air Ratio

Harness the power of precision mixtures of pressurized flammable vapor. Safety first! These are advanced potato guns - not for the beginner.
User avatar
hectmarr
Corporal 5
Corporal 5
Argentina
Posts: 955
Joined: Sat May 07, 2016 3:49 pm
Location: Argentina
Has thanked: 167 times
Been thanked: 167 times
Contact:

Sun Jun 27, 2021 11:58 am

I have a small piston valve, and it seems to me that the piston area on the pilot side is too small, and therefore opens too early.
On the combustion chamber side, it is 6.5 mm in diameter and on the pilot side, 8 mm in diameter. the pressure of the hybrid is 8X. Do you think I can increase the area of the pilot's side more? up to how much diameter?
What is the ratio they use?
I have thought about increasing the diameter of the pilot side to 12 mm, keeping the 6.5 mm diameter of the piston pointing towards the combustion chamber constant. What do you think... :roll:
User avatar
mark.f
Sergeant Major 4
Sergeant Major 4
Eritrea
Posts: 3605
Joined: Sat May 06, 2006 11:18 am
Location: The Big Steezy
Has thanked: 43 times
Been thanked: 34 times
Contact:

Donating Members

Sun Jun 27, 2021 6:36 pm

You want the barrel seat (outlet) and the piston to be closer in size for higher opening pressures for a given pilot pressure.
To get the cracking pressure of your valve it's just P_c=(P_p*A_p)/A_c, where:
  • P_c=cracking chamber pressure
  • P_p=pilot pressure
  • A_p=pilot area
  • A_c=chamber area
So with an 8mm piston and a 6.5mm OD "barrel" (or valve seat), your cracking pressure would be (for example) 294 pounds at with a 100 PSI pilot pressure. Increasing your piston size to 12mm with the same outlet seat would actually decrease the cracking pressure to 141 PSI. Decreasing the piston size to, say, 7.5mm would increase cracking pressure to 401 PSI, but you'd get problems sealing the closer the sizes get.
All of this assuming you're building a "normal" hybrid piston valve. :mrgreen:
EDIT: (normal as in barrel-sealing)
User avatar
hectmarr
Corporal 5
Corporal 5
Argentina
Posts: 955
Joined: Sat May 07, 2016 3:49 pm
Location: Argentina
Has thanked: 167 times
Been thanked: 167 times
Contact:

Sun Jun 27, 2021 8:22 pm

Actually the valve is like in the drawing.
I did some tests, with the piston ratio 6.5 mm - 8 mm, as it is at present, but, with different pressures in the combustion chamber and the pilot, and compared.
In the first case, it is 7 bar for the combustion chamber and 7 bar for the pilot. In the second case, it is 7 bar for the combustion chamber and 9 bar for the pilot, using a pressure regulator to obtain different pressures.
The energy has increased a lot ... tomorrow I will integrate a barrel with a bolt to compare the speeds of the ammunition.
The pending question is how much more can I increase the pressure in the pilot, keeping the pressure in the combustion chamber at 7 bar, or in other words, when the valve will not be able to open ...
I think that once I have obtained the data of the highest pressure that I can use in the pilot, having the combustion chamber pressure as constant, I will be able to deduce the force exerted by the pilot, and obtain a diameter of the piston on the pilot's side, to increase diameter and work with the same pressure on both sides. We will see. Thanks for your explanation.
Válvula a pistón con piloto.png
Válvula a pistón con piloto.png (15.28 KiB) Viewed 1478 times
User avatar
mark.f
Sergeant Major 4
Sergeant Major 4
Eritrea
Posts: 3605
Joined: Sat May 06, 2006 11:18 am
Location: The Big Steezy
Has thanked: 43 times
Been thanked: 34 times
Contact:

Donating Members

Mon Jun 28, 2021 5:17 am

Ah, so you're using a chamber/outlet sealing design. That makes more sense.
So are you trying to find the right sized piston so that you can use equal (before ignition) chamber and pilot pressures and still get opening at an optimal (higher) pressure?
If so you could probably play around with GasEq and use the formula: d = 2 * sqrt( ( P_c * A_c ) / ( P_p * pi() ) ), where:
  • d = piston diameter
  • P_c = desired cracking pressure
  • A_c = chamber area
  • P_p = pre-ignition pressure/pilot pressure
For instance using an 8 atm pre-ignition and pilot pressure and picking a completely arbitrary "target" cracking pressure of 75% of the maximum adiabatic combustion pressure (which is about 57 atm), I work it out to needing a ~17.4 mm piston.
All of this just a simple back-of-the-envelope approximation so take it for what it's worth. :P
User avatar
hectmarr
Corporal 5
Corporal 5
Argentina
Posts: 955
Joined: Sat May 07, 2016 3:49 pm
Location: Argentina
Has thanked: 167 times
Been thanked: 167 times
Contact:

Mon Jun 28, 2021 8:12 am

oh yeah, I like your estimate. An exact approximation is not necessary, but in general terms, 8 mm is very small diameter. Therefore, I am going to remake the valve with a 16mm piston on the pilot side and 6.5mm on the combustion chamber side, to have more power. Thank you very much for your explanation for me. :)
User avatar
Moonbogg
Staff Sergeant 2
Staff Sergeant 2
United States of America
Posts: 1686
Joined: Mon Oct 13, 2008 10:20 pm
Location: SoCal
Has thanked: 110 times
Been thanked: 70 times

Tue Jun 29, 2021 5:43 pm

Is this a vented pilot design? I worked the numbers for an 8X mix and a 75% of maximum opening pressure for the piston. Same pressure in the pilot as the preignition chamber pressure. I get a pilot piston diameter of 16.73mm (assuming a 6.5mm piston diameter chamber side), so basically the same as Mark's work above.
User avatar
hectmarr
Corporal 5
Corporal 5
Argentina
Posts: 955
Joined: Sat May 07, 2016 3:49 pm
Location: Argentina
Has thanked: 167 times
Been thanked: 167 times
Contact:

Wed Jun 30, 2021 4:56 pm

The piston valve is the same one I used here: viewtopic.php?f=28&t=27190
It remains open after firing and closes automatically when compressed air enters the dispenser. Thank you for your input in the calculation. I'm going to make a 16mm pilot side piston, and a 6.5mm combustion chamber side.
User avatar
hectmarr
Corporal 5
Corporal 5
Argentina
Posts: 955
Joined: Sat May 07, 2016 3:49 pm
Location: Argentina
Has thanked: 167 times
Been thanked: 167 times
Contact:

Sat Jul 10, 2021 7:14 am

I built a new little 16mm piston on the pilot side and 6.5mm on the combustion chamber side.
I still have a pressure regulator sandwiched in the air supply going to the pilot area.
This is because the piston valve is a bit stiff and that force is in addition to that exerted by the pilot piston. The valve is working properly.
Last edited by hectmarr on Sun Jul 11, 2021 3:00 pm, edited 1 time in total.
User avatar
hectmarr
Corporal 5
Corporal 5
Argentina
Posts: 955
Joined: Sat May 07, 2016 3:49 pm
Location: Argentina
Has thanked: 167 times
Been thanked: 167 times
Contact:

Sat Jul 10, 2021 9:41 am

Well, the valve was tested but it has a drawback that makes it almost useless.
I am referring to this piston valve design exclusively.
The practical problem is
that it does not retain the lubrication in the small 6.5 mm piston, and for this reason,
the o'ring breaks ... Perhaps the solution is to incorporate a lubrication system,
(I use silicone), but it complicates things a lot, and I am a lover of simple things.
My opinion is that the advantage of using a front seal on the firing barrel is that it does not need lubrication to
work properly.
The verdict is: It works well but it is not practical because of the problem I explained earlier.

The option of using lubricated air is not feasible because it fouls the igniter and the hybrid often does not fire.
Another issue is to include in the calculation of the relationship between the pistons, 6.5 mm - 16 mm,
in this case, the force necessary to open the valve, because this is a force that is added to that
of the larger piston, to seal the combustion chamber.
Here is a photo of the construction of
the 1045 steel valve. The larger piston includes the o'rings so that it is automatically depressurized
after each shot. All this works excellent, but the issue of the small o'ring breakage makes it unfeasible.
Válvula a pistón 4.JPG
User avatar
hectmarr
Corporal 5
Corporal 5
Argentina
Posts: 955
Joined: Sat May 07, 2016 3:49 pm
Location: Argentina
Has thanked: 167 times
Been thanked: 167 times
Contact:

Fri Aug 13, 2021 4:46 pm

Here are the photos of how it is inside.
Válvula a pistón 1.JPG
Válvula a pistón 2.JPG
Válvula a pistón 3.JPG
Here are the photos of how it is inside.
User avatar
Moonbogg
Staff Sergeant 2
Staff Sergeant 2
United States of America
Posts: 1686
Joined: Mon Oct 13, 2008 10:20 pm
Location: SoCal
Has thanked: 110 times
Been thanked: 70 times

Sat Aug 14, 2021 12:08 am

Does the small o-ring pass over a port while under pressure? If so, it's like running the o-ring over a cheese grater. You might try a different shaped o-ring, such as the X shaped ones. They have much less friction when squeezed, but there would still be friction caused by pressure. They might work better. If the only problem is that o-ring breaking, I feel it's worth trying to find a solution to that problem. If the o-ring is catching a sharp edge of a port and getting sliced off, maybe you can chamfer the edge of that port. Maybe chamfering plus the X shaped o-rings can help with the issue.
User avatar
hectmarr
Corporal 5
Corporal 5
Argentina
Posts: 955
Joined: Sat May 07, 2016 3:49 pm
Location: Argentina
Has thanked: 167 times
Been thanked: 167 times
Contact:

Sat Aug 14, 2021 10:47 am

I understand what you tell me. Actually the small o'ring does not touch anything. It is built as in the drawing.
The o'ring fits into a 6.5 mm bronze tube with a conical mouthpiece and all well mirror polished, when moved to the left, (in the drawing), to seal the combustion chamber. When combustion occurs, and it moves to the right, it remains floating, so that the gases can pass towards the piston and accelerate the opening of the valve and let the hot air out through the gas outlet towards the firing barrel.
Reviewing what happens with the o'ring, I am suspecting that rather than breaking it, the explosion takes it out of its housing. I think I will have to try a harder o'ring because the ones I am using are common, and apparently the pressure wave forces it to come out in the direction of the axis, (piston rod), and from left to right. Just when the o'ting comes out of the bronze shirt, it has nothing to hold it from the outside. Me imagino que al forzarlo en los bordes de la ranura con violencia, lo corta, y es por eso que directamente desaparece al salir junto con los gases por el cañón de disparo... :?
Sin títulVálvula a pistón, detalle.png
Sin títulVálvula a pistón, detalle.png (6.27 KiB) Viewed 811 times
User avatar
Moonbogg
Staff Sergeant 2
Staff Sergeant 2
United States of America
Posts: 1686
Joined: Mon Oct 13, 2008 10:20 pm
Location: SoCal
Has thanked: 110 times
Been thanked: 70 times

Sat Aug 14, 2021 3:50 pm

Thanks for the explanation. Now I get it. Like you said, the o-ring isn't supported from the outside, so it stretches and just comes off the rod when you shoot it. I agree that a harder o-ring is the best experiment to try next. If all else fails, you can try to modify the valve with a sealing face that screws on to the piston rod.
User avatar
hectmarr
Corporal 5
Corporal 5
Argentina
Posts: 955
Joined: Sat May 07, 2016 3:49 pm
Location: Argentina
Has thanked: 167 times
Been thanked: 167 times
Contact:

Sat Aug 14, 2021 8:55 pm

If you're right. I'm going to try to get a harder o'ring, as a first alternative solution.
The second may be, why not? what you propose, make a thread in the place of the o'ring groove, and a small plate with some material to seal the exit of the combustion chamber. For now I have to try another harder o'ring.
Thanks colleague for the suggestions and for commenting. Greetings!
User avatar
jackssmirkingrevenge
Five Star General
Five Star General
Posts: 25966
Joined: Thu Mar 15, 2007 11:28 pm
Has thanked: 334 times
Been thanked: 193 times

Donating Members

Thu Aug 19, 2021 9:47 am

I'm impressed with the piston, it certainly was not improvised with epoxy... Did you buy a lathe or is there a friendly machinist in your neighborhood?
hectmarr wrote:You have to make many weapons, because this field is long and short life
Post Reply