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  • Pneumatic engine valve springs

    Discussion in 'The main mechanical design forum' started by JC_Biggs, Mar 22, 2011.

    1. JC_Biggs

      JC_Biggs Active Member

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      Im SOOOO stuck on my newest engine build. (motorcycle drag racer here) im going for formula 1 technology. 20k plus rpm etc. i hope to make 47% of the power of a formula 1 car with 47% of the displacement they have, but thats wishful thinking.

      anyways. im done with everything in the bottom end of the engine. and now im on to the valve springs. fortunately actually makeing a mechanism fit my engine is easy enough. i am having problems with the control side of it. in some top secret hard to come across formula 1 designs, there is a inlet regulator, but also an outlet regulator. Im having trouble understanding why you would need the outlet. I assume that its ONLY for absolute last minute evacuation of way to much psi. (ie, anything over 120.) but is that all? you put in 80 or so psi, the area is made smaller when the valve opens ( the pressure goes up a bit) and then the spring effect pushes the valve back up, getting back to the normal 80 psi. now why cant i just be ok with only the 80 psi inlet regulator? i havent done the math but i am assuming that the pressure wont exceed "to high" (i guess i should be sure though) .. i really need a mentor on this, im a designer, not a full flegged engineer, and your help would be greatly appriciated, and you'll def get the mention when i start winning races. :) if anyone is a gear head, and is really interested, feel free to pm me, i am also free to talk over the phone. thanks
       
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    3. Dana

      Dana Well-Known Member

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      I'm not familiar with pneumatic valve springs for engines, but but I've done some work with pneumatic springs for force control in other applications.

      Generally, there are two kinds of regulators. "Non relieving" regulators let air in up to the specified pressure, but don't let it back out again if the pressure goes higher. "Relieving" regulators let air in up to the set pressure, but also let air back out again if it goes higher. Naturally, there is some hysteresis (deadband) so it might fill to 78 psi, and start venting back out at, say, 82 psi. As in everything, you get what you pay for; there are low precision general purpose regulators and more expensive precision regulators.

      Think of a relieving regulator as two regulators in one package; one to let air in up to one pressure, and the other to let it back out at the higher pressure. Separating the two may allow for a lower cost, faster acting, lighter, or more reliable system.

      In a pneumatic spring application, if you're using a non relieving regulator, the pressure will increase as the volume changes (not the area; the area does not change). In this way, it acts like a conventional spring. Depending on what you want, this may or may not be desirable. With a relieving regulator, air bleeds off as the spring compresses, so the pressure (and thus force) stays constant. Actually, even with a relieving regulator, the pressure will increase slightly until it exceeds the hysteresis in the regulator.

      In this case, I'm guessing that the pressure isn't critical, and there's a non relieving regulator to quickly let in and hold the minimum (80 psi), and the higher pressure regulator (just a simple safety relief valve, really) to vent excess pressure as the engine warms up and the air in the system expands. You would need fast inlet flow to quickly pressurize the system as soon as the engine starts (if it's not pre-pressurized) and compensate for any leakage, but only a small flow outlet to vent the slow pressure increase as it warms up.
       
    4. JC_Biggs

      JC_Biggs Active Member

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      wow.. thanks for the very informative reply. I had done a bit of research and basically came to the conclusion you gave, but i wasnt sure. and yours was much more eloquent. lol.

      Heres my biggest issue.. SPACE!!!! (with weight running a close second). i have a 4 cylinder 16 valve engine. 2 intake and 2 exhaust per cylinder. since those pairs move together, we can hook those up as 1. so basically, i need 8 inlet regulators and 8 outlet regulators. i just dont see any way to physically fit that into the motorcycle frame. so what i was thinking was, why not use one high flow non relieving regulator on the inlet side with a distribution block .followed by a check valve at each spring pair, and then on the exhaust side JUST a one way check valve at each spring pair.? or course the exhaust side would would have to be a pressure activated check valve so that it doesnt let ALL the air out. only the pressure over the max. unless there is some extremely small high flow regulators i dont know about. i think this is the most compact way, but i dont know if it will work. the biggest thing is i need to do the math and see how much pressure is going to be released at 20k rpm and see if the regulator is large enough to supply that back times 8 at only 66 psi.

      also as far as spring rates, my design calls for 66PSI (average race bike is 60) at seated (closed) valve position. at full open i would like for the pressure to be no more than 95 psi. This will ensure that the valve moves out of the way of the piston extremely quickly, but closes at only 66 psi which wont damage the valves or seats. also i worry anything higher than 95 psi will require new heavier duty cam shafts.

      also, sitting here thinking , it may be possible to design it so that the inlet tubes and distribution block are the proper sizes to allow pressure increases in the tubes, and not just in the spring. eliminating the need for an exhuast refulator at all. then i would only need a check valve separating the individual spring pairs, and would simplify for tube routing and mounting as only 1 side of each pair would have to be regulated...the only issue i see to this is the lack and exit should pressure get to high due to heat, but i am seriously considering using a 4500psi nitrogen source anyways. (no power needed to turn a air pump and plenty of pressure to make more than a few drag passes considering it seals properly.


      .....sorry that was alot of ideas all at once, but i had to get it all out before i forgot. if you guys are anything like me, you know what its like to have 15 projects going on at once... :)

      thanks again
       
    5. JC_Biggs

      JC_Biggs Active Member

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      OH.. one more thing.. there is an internal mechanical spring in the design of approx 10 pounds of force at installed height.. 13 at full open.. this is so the valves dont fall and hit the piston when the pressure source is removed. do i need to subtract this amount of force from my 66 psi of force generated by the air pressure?

      (this spring may eventually be removed as i dont think i can find one thats going to hold up or KEEP up at 84hz )
       
    6. Dana

      Dana Well-Known Member

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      If you don't need the force to increase to 95 lbs when the valve is depressed, some extra air volume between the regulator(s) and the valve cylinders will keep it more constant, since the volume change due to valve movement is proportionally less. This will smooth things out, too, since if they're all running off one regulator, one valve is opening as another is closing, which could cause weird pressure spikes if there's not much extra volume in the system.
       
    7. Pneumatic systems use conventional cams operating the valve (4) via a shim\bucket or finger follower, the valve spring pocket is replaced with a chamber (28) pressurised with nitrogen (held within a cylinder in the sidepods) that runs at a constant pressure to return the valve when the cam timing retards.
       

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