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Finding pressure in gas nailer combustion

  1. Mar 23, 2012 #1
    Problem: I need to find the new pressure inside a combustion chamber, from the burning of the fuel, and the increase in temperature, if the piston/plunger seized/
    some of this is nominal, just to get a general idea of where I'm at.

    Honestly it's best explained as a pipe bomb... what if i have a set volume cylinder, that has some propane with some room air in it.. somehow the propane reacts, then the pipe is thrown into a camp fire (oh no!) and the gas is heated via the cylinder to a relatively hot temp, what is the new pressure inside?
    I believe, I can use PV=nRT, however because of some insecurities about what actually creates the pressure difference when something burns.. I don't know how to set it up... like, when something burns is there more moles on the new side of the eqn if there is a larger sum of coefficients in the balanced eqn? .. or is it just the actually temp change of the gas that causes it to expand.
    also, why is butane more "volatile of and explosion" over propane?
    hope that makes sense.. and I'm not making a pipe bomb.. or any intentional weapon for that matter..

    Thanks,
    K
     
  2. jcsd
  3. Mar 23, 2012 #2
    You seem to be mixing something up here.

    Combustion means you are taking CnH2n+2 and oxidizing it to O2 and CO2. This is a different process from just heating something and increasing the vapor pressure above the solution.

    The former process depends on what you are combusting and under what conditions; because something like a plain old alkane would just give typical combustion products (Oxygen and Carbon Dioxide) but if there are heteroatoms around (whether in the pure substance or as impurities) you can get various other gases. This can be approximated by finding partial pressures using the ideal gas law and then finding the total pressure.

    For the latter process you can probably find tables of vapor pressures as functions of temperature, I'm not sure if calculations are possible (someone more knowledgeable should have that answer).

    As far as what makes things more or less volatile, its a function of intermolecular interactions where Van Der Waal's become the important one for plain old alkanes (you can think of as a function of size of the molecule, but the fundamental property is polarizability). AFAIK propane should be more volatile than butane (smaller molecular, less polarizibility and weaker intermolecular interactions). This situation changes when heteroatoms are present due to dipole or even ionic interactions.
     
  4. Mar 23, 2012 #3
    I am sure you meant to say water H2O and carbon dioxide CO2
     
  5. Mar 23, 2012 #4
    Ok, so to restate, yes I understand it's two separate questions; (1) how do I calculate the pressure in a fixed container after the propane and oxygen have burned (mind, this is a known amount of pure propane, and 21% of the air in the container is oxygen, and assume it burns to completion.) ? And two, if then after in the same container the temp of internal gasses was increased how do I calculate the additional pressure increase.

    My guess is, that for first question, i just adjust for the difference in moles on left of Rx eqn to the mole on right.. But that seems fishy. And for the second, I think it's a liner relatiOnship since the vol doesn't change so it's just the change in temp?
     
  6. Mar 24, 2012 #5

    Borek

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    Staff: Mentor

    I would google for the thermodynamics of the internal combustion engine, these things were researched to death.
     
  7. Mar 24, 2012 #6
    firstly, write down the chemical equation for when propane and oxygen combine to form water and carbon dioxide.

    Ideally,
    For the first question you could
    1. Determine the energy of combustion of propane - look it up on the iinternet.
    there is a higher heating value and a lower heating value HHv and LHV. Use LHV as the water will still be a vapour state.
    2. That energy of combustion is your heat Q added to your products. And your products include the nitrogen in the air that also undergoes a temperature increase.
    3. Use thermodynamics to find a temperature increase of the gases, and pressure increase.
    Temperature of products should be similar to adiabatic flame temperature - note the assumptions.
    http://en.wikipedia.org/wiki/Adiabatic_flame_temperature

    For the second part, thermodynamics again on the products , using a heat input from the bonfire!

    so there you go.
    Good luck.
    and look up Boreks suggestion for more input to your problem.
     
  8. Mar 25, 2012 #7
    Yes yes I absolutely meant water and carbon dioxide (I actually made that mistake twice and then proofread the damn post and still didn't correct it). I should be sentenced to remedial Chemistry!

    As far as the original question goes, depending on what type of precision you are looking for both approaches would work; estimation via using thermodynamics or finding some values under similar conditions through the literature. The literature values would probably be closer to reality as these are done via experiment in the real world, as opposed to idealizing everything via thermodynamics.
     
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