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Homework Help: Calculation of Air Density

  1. Nov 22, 2007 #1

    I have the pressure value (which is less than atm) and temperature value. How to calculate Density of the air?
    I calculated but the result is not satisfactory.
  2. jcsd
  3. Nov 22, 2007 #2
    Please show what you did.
  4. Nov 22, 2007 #3
    P = Pressure at the region : in Pa
    R = Specific gas constant R : 287 J/KgK
    T = Air Temp : in K

    Density of air = P/(R*T) in Kg/m3
    Last edited: Nov 22, 2007
  5. Nov 23, 2007 #4


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  6. Nov 23, 2007 #5
    Density = mP/RT

    Rs = R/M

  7. Nov 23, 2007 #6
    Can anyone tell me the method to calculate the engine idle air flow requirement?

    Engine rpm = 1200 rpm = RPM
    Rev/cyle = 2
    Engine capacity = 395 cc = 0.395 Lit = L
    Volumetric Efficiency = 90% = VE

    Then VAF (Volume of Air Flow) is,

    VAF = (L/28.317) x (RPM/2) x VE.

    I want o know what is the meaning for 28.317
    Can anyone tell be the derivation for it?
  8. Nov 23, 2007 #7


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    It looks pretty close to the molecular mass of air.
  9. Nov 26, 2007 #8

    Molecular mass of air is 29. But this value is 28.317 which is 0.6 lesser than molecular mass of air.
    I am confused on this 28.317...
  10. Nov 26, 2007 #9


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    Molecular mass of air is generally 28.97, that's why I said it's close to that of air.
  11. Nov 26, 2007 #10
    That's the number of cubic meters in 1000 scf. This is why I cringe when there are no units shown.
  12. Nov 26, 2007 #11


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    Good point! I'm cringing too!
  13. Nov 26, 2007 #12
    Yes it is the unit conversion data only.
    But what is the purpose of the 28.317 there.
    See i am giving the Engine capacity in Litre only,

    And my end unit of Volume of air is Lit/min only.

    And also VAF = (Engine Displacement/1728) * (RPM/2) * VE

    I am still confused on the 28.317?
  14. Nov 27, 2007 #13
    That's just a unit conversion. It changes litre/minute to cubic foot/minute. I don't know why you do that, but....

    When you do problems, try to always use units. That will reduce your effort by at least half.
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