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Ridiculously simple Pressure question

  1. Jul 16, 2012 #1
    Hey guys. First off, let me say that I am in no way a science expert. I like to read about new things, and stay up on the lastest happenings. I have a good general understanding of physics and chemistry, but not where i would like to be.

    anyways here is my question.

    Ive always read that pressure increase was uniform though-out a object. (in my case a gas)

    It would seem to me that each atom would have to progressively push on the atom next to it, so while pressure may seem to increase uniformly, there might be a "slight" delay as the pressure moves.

    in other words, if you had a basket ball filled with 10 lbs of ...lets say natural gas... and you shrink that ball, then the pressure goes up.. ok basic physics, but what happens if you shrink that ball VERY VERY FAST, at the same time the temperature in the ball is above the boiling point of the gas, so you have pressure increase there as well.

    now if all this happens fast enough, shouldn't there be some point in which the "shrinkage" of the basket ball can actually "out run" the pressure equalization through out the ball?

    and if this happens, wouldnt it be logical that the center of the mass would recieve the pressure last, and therefore could continue its phase change, effectively super cooling that center section of the gaseous mass.?
     
  2. jcsd
  3. Jul 16, 2012 #2

    CWatters

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    You mean shrink the ball faster than the speed of sound in the gas.

    Thinking on a larger scale...You frequently get parts of the earths atmosphere at different pressures. Takes a few days for the pressure to propagate from one place to another. Plenty of time for water to continue to evaporate or condense out :-)
     
  4. Jul 16, 2012 #3
    ..sure, if thats what its called.

    so is it possible to raise the pressure and reduce the volume fast enough to out run that speed?

    more specifically, what would happen to the gas that didnt get the pressure distributed THROUGH it, but is surrounded by it...

    (also this situation would only occur for a few milliseconds before the volume starts to rise again. )
     
  5. Jul 16, 2012 #4
    i think there is a reaction happening.. (well actually stalling) for a brief period and i think that is causing an issue. so im trying to figure out if my theory is possible. im not really sure how to word things like they make sense in my brain...
     
  6. Jul 17, 2012 #5

    CWatters

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    I think you are going to have to tell us more about the exact problem and set up.

    Wrap explosives around a football and I imagine you could blow it up without melting a lump of ice in the middle "first".
     
  7. Jul 17, 2012 #6
    ...ok, well I guess I can do that..

    The problem is in a high RPM internal Combustion Engine, with nitrous oxide.

    You have to know a little about how nitrous works in an engine, but basically it increase the burn speed and supplies more oxygen to the mix, meaning you can burn more fuel, faster. the Nitrogen left over from the split of the O from the N2 is what buffers the burn and keeps it from being explosive.

    what normally happens is that you have to removing timing advance in the engine to keep the peak pressure at the proper point in the crank rotation. if you dont reduce the advance then you move peak pressure forward which increases heat and eventually leads to detonation and engine damage.

    the problem that im running into, is that I have reached a point in the level of nitrous that im no longer having to retard the timing, but Im having to advance it. at the same time Im not picking up as much power as you would think for each increase in jet size, but the heat is increasing. now i would think that if your making more heat, your getting more work done, but the horsepower of the engine inst increasing at the same scale as the heat.

    my theory is that I'm spraying SOO much nitrous that its super cooling the fuel/air mix. That super cold fuel and air is then absorbing heat in the cylinder, but not enough of it. Now normally an increase in pressure increases temperature, which should offset the cooling effect of the phase change, but i think the piston rises and compresses so fast that the "center" of the fuel/nitrous mixture doesn't have a chance to reheat, and only the outer 40% or so of the gas charge heats up substantially enough to allow the oxygen to split from the nitrogen. I also think its possible that its so cold and the pressure is so high in the "center" of the nitrous charge is actually reverting back to its liquid form, and if that's not happening, then at least the "split" is stalled and the oxygen is not available for burning the fuel.

    i don't think there is any chance that liquid N2O could make it to the combustion chamber to begin with. and im also concerned that if the conditions are right, is it possible the nitrogen could be combining with something other than pure oxygen, creating some form of un burnable compound, or perhaps and explosive one. (it could pick this up from the vaporized fuel in the chamber.


    (btw.. what is the technical term for when the nitrogen and oxygen molecules split)


    sorry its so long, but thats where im at, and thats about the extent of what i think is occuring.
     
  8. Jul 18, 2012 #7

    CWatters

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  9. Jul 18, 2012 #8
    well thanks for the advice, but im way beyond what either of those have to offer... not so sound cocky, its just that those sources dont go deep in to the physics of whats happening at the level of nitrous im using. not to mention that most of the information out there is just plain wrong.
     
  10. Jul 20, 2012 #9
    As you add more nitrous you have to add more fuel. The oxygen is released as it heated in the process of combustion. It is very easy to lean the mixture to the point that you lose power - which is what I believe you are encountering. There is too much heat in the engine to re-liquify NO2 so that is not hapening. You can, however, add so much fuel and NO2 that you hydraulically lock the cylinder which will cause a loss of power too. (and result in eventual damage)
    Check with the manfacturer of your system and ask them about how much fuel and NO2 is too much for the combustion chamber and compression that you have.
     
  11. Jul 20, 2012 #10


    My theory is that there is not to much heat in the engine. you have super cold fuel and super cold nitrous that absorb alot of that heat, but im using ALOT of nitrous which means im using ALOT of fuel. which means there is ALOT of cold going in the chamber. also this is a 14krpm bike engine. I know there SHOULD BE more than enough heat to phase change the liquid, and there SHOULD BE enough pressure to split the molecule, but I believe the combustion event is happening SO fast at those engine speeds that he cold gaseous mass and fuel dont have time to absorb the heat. In turn losing efficiency. Not only is there not enough nitrous disociation, but the fuel doesnt vaporize properly either...you also have to remember that the intake path has been super cooled at that point so there is no heat left in the metal there.

    Im not putting enough fuel to hydrolock the engine, also there is no wear on the bearings and wristpins... basically what this boils down to is SOMETHING is messing up the burn. and i cant figure out what it is. if i turn the nitrous down JUST A HAIR it works fine. Turn it back up, and nothing i do helps it pick up any more power. the ONLY explanation i can come up with is that the fuel is frozen, or the nitrous has re condensed, which i beleive to be more likely..

    im really at a loss.
     
  12. Jul 21, 2012 #11
    You cannot compress NO2 enough to liquify it. As you compress it heat is generated. You are not super-cooling your engine either. If you were getting frost on the outside of your cylinder head then that might be a consideration.
    Since you are running at 14000 rpm there may be a geometrically limiting factor in the mechanics of the engine but I would bet that the nitrous/fuel mix is not right, you have ignition trouble or the fuel you are using is not meant to burn at the speed your engine is running. Racing fuels are typically designed to be harder to ignite but burn considerably faster than gasoline.
    Paul
     
  13. Jul 21, 2012 #12


    the physics of how n2o is compressed is what i want to get into. how much pressure "would it take" to make it a liquid.. just curious really. cylinder pressures could easily surpass 5000 psi. And believe it or not, there is frost on the throttle bodies and lower intake runner after a pass.. i should probably also note im using water/meth injection to help keep temps in check, but that shouldnt be causing whats happening. (and i beleive the water meth is the only reason it hasnt blown up yet)

    i can run it with my max jet settings, and it runs perfect. it goes down track with a .85 lambda, timing is perfect.. everything is right. then i go up .010" on the jets, and adjust the fuel to compensate. nothing.. as soon as the nitrous is 100% flow, the bike stops pulling just stays at 9-10k rpm and goes no higher.

    I've tried varying the fuel delivery from what the "math" calls for. the simple fact of the matter is, it just will not use any more nitrous. BUT at the same time, its not a physical delivery problem. The sensors on the bike indicate that the cylinder is supercharged from ram air, (normal, and pushing plenty of nitrous in) and temp sensors show normal, indicating the nitrous is not backing into the intake. the cam timing has been moved and adjusted, and played with, and moved some more... even tried different variations of cams to no avail. tried different exhaust, coating the exhaust for better flow, adjusting the ports for better scavenging. I cant get a good plug reading for timing because the engine essentially dies, and i have to back out before something breaks. i have a sneaky suspicion that the heat in the chamber is rising as well, but because i back out, i cant get a good accurate EGT reading. Ive gone lower on the fuel octane, ive tried other fuels, I've tried EVERYTHING. the only thing i haven't tried is lowering the compression, which I'm going to try this weekend. and if that helps it, it will confirm my theory.


    I'm 100% sure that its something "physics related" that's causing this problem. I have been over EVERYTHING with a fine tooth comb and there is NOTHING out of spec, nothing out of the norm, nothing broken.. the problem is confined STRICTLY to using more nitrous. it just will not do it.. im at a loss... whats really got me is that ive use MUCH large shots of nitrous on bigger engines with no problems. this one though.. its racking my brain.
     
  14. Jul 22, 2012 #13
    ...so in other words, you guys don't have a clue..

    thanks.
     
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