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I have a confusion about something in the following question.

An ideal diesel engine has a compression ratio of 20 and uses air as the working fluid. The state of air at the beginning of the compression process is 95 kPa and 20°C. If the maximum temperature in the cycle is not to exceed 2200 K, determine (a) the thermal efficiency and (b) the mean effective pressure. Assume constant specific heats for air at room temperature.

When I'm calculating here $q_{\mathrm{in}}$, I know: $$q_{\mathrm{in}}=h_3-h_2=c_p (T_3-T_2)$$

However, when I get the values from the table about ideal gas and calculate $h_3-h_2$, I get different answer  compared to $c_p(T_3-T_2)$.

Why is it like that? Mustn't them be equal? Or am I missing an important point about the equation I wrote? I'd be grateful if you could help me clear up my confusion here. Looking forward to hearing from you

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  • $\begingroup$ First law for an open system and in specific form reads: $\delta q= dh - vdp$, if we discard the last part we get: $\delta q= dh = c_p dT.$ So your equation is correct, you might have read the tables incorrectly. $\endgroup$
    – User198
    Commented Feb 13 at 18:19
  • $\begingroup$ I'm sure I read it correctly .Here is the appendix ,you can go to table A-17 dropbox.com/s/tuqy5e8657ysoda/… $\endgroup$
    – Yigidocan
    Commented Feb 13 at 18:30
  • $\begingroup$ @Yigidocan Which table are you looking at in the link $\endgroup$
    – Bob D
    Commented Feb 14 at 13:13

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