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Homework Help: Steam Engine work application problem

  1. Oct 26, 2011 #1
    1. The problem statement, all variables and given/known data
    In a steam engine the pressure and volume of steam satisfy the equation PV1.4=k, where k is a constant. Calculate the work done by the engine (in ft/lbs) during a cycle when the steam starts at a pressure of 100 lb/in2 and a volume of 400in3 and expands to a volume of 1100in3.

    2. Relevant equations
    Use the fact that the work done by the gas when the volume expands from volume v1 to volume v2 is:

    W=∫V2v1 P dV

    3. The attempt at a solution

    As usual, getting hung up on where to start. So I tried to find k. Solving for k at the starting pressure of 100lb/in2 and volume of 400in3 I get a k=439,424.2

    That # seems way high. Is the problem assuming the pressure is starting at these values (100lb/in2 and 400in3)??

  2. jcsd
  3. Oct 26, 2011 #2


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  4. Oct 26, 2011 #3
    interesting.... now I'm even more confused!
  5. Oct 26, 2011 #4
    Our professor didn't say we needed any special formulas or anything - that we should know how to do this from our previous section's lessons. Where do I start? That link shows more variables than I have (I think) like lambda values, etc.
  6. Oct 26, 2011 #5
    Please, any help would be appreciated. I'm not looking for an answer, just a push in the right direction. Here's what I've tried to do to start out:

    -put integral in terms of V
    -break down Volume values and try to solve for r and h
    -ratio between starting values: 100lb/in^2 and 400in^3

  7. Oct 27, 2011 #6


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    The attached article shows how to integrate P dV when PV^gamma = k (Note: there are no lambdas present.) Work is explicitly given in terms of initial pressure and initial and final volumes. (See section "Derivation of discrete formula")
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