- #1
MacLaddy
Gold Member
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Homework Statement
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%
Homework Equations
$$\eta_{TH}=1-\frac{1}{r^{k-1}}[\frac{r_c^k-1}{k(r_c-1)}]$$
$$r_c = \frac{v_3}{v_2}=\frac{0.50L}{0.25L}=2$$
$$r=\frac{v_1}{v_2}=\frac{4}{0.25}=16$$
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.
Thanks,
Mac