Hybrid Piston Pilot Air Ratio

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hectmarr
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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:
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mark.f
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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)
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hectmarr
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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 532 times
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mark.f
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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
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hectmarr
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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. :)
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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.
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hectmarr
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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.
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hectmarr
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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.
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hectmarr
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Sat Jul 10, 2021 9:41 am

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