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Homework Help: Thermal Efficiency in an Ideal Diesel cycle.

  1. Nov 30, 2013 #1


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    1. The problem statement, all variables and given/known data

    Helium in an ideal Diesel cycle is compressed from 4 L to 0.25 L, and then it expands during the constant pressure heat addition process to 0.50 L. Under air standard conditions, the thermal efficiency of this cycle is:

    a) 79.5%
    b) 20.5%
    c) 61.4%
    d) 67.4%
    e) 84.3%

    2. Relevant equations

    $$r_c = \frac{v_3}{v_2}=\frac{0.50L}{0.25L}=2$$

    3. The attempt at a solution

    My main problem with this question is in the wording. "Under air standard conditions" I typically would use the k value of 1.4, but as it specifically states "Helium" I wonder if I shouldn't use k=1.667.

    k=1.667 my answer is 79.5%
    k=1.4 my answer is 61.4%

    Any help on what the standard operation with this type of wording is would be greatly appreciated.

  2. jcsd
  3. Dec 1, 2013 #2

    Andrew Mason

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    Homework Helper

    I am not sure how helium works in a Diesel engine. But you aren't supposed to worry about that.

    I suggest that you just assume that the system consists of only He connnected to hot and cold reservoirs. The cycle is comprised of a constant pressure expansion while connected to the hot reservoir, then an adiabatic expansion, then a constant volume cooling when connected to the cold reservoir, followed by adiabatic compression. Use the efficiency formula for the Diesel using the gamma for He.

  4. Dec 1, 2013 #3


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    Thank you, the gamma was based on Helium. Now I'm going to go build a Diesel Engine that runs on helium... (who writes this stuff?)

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