Page 106 - Entrophy Analysis in Thermal Engineering Systems

Irreversible engines—Closed cycles 99

Fig. 7.7 The normalized power output, thermal efficiency, and normalized entropy
generation rate of the irreversible Diesel cycle varying with the compression ratio,
γ ¼1.4, η com ¼0.85, η exp ¼0.90, T R ¼4, and π ¼5.

1
2
γ 1
ð CRÞ _ Φ min ¼ CRÞ _ W max π ðÞ
ð
which is the same result that we found for the Atkinson cycle; see Eq. (7.41).
Fig. 7.7 depicts an illustrative example where the normalized power output,
thermal efficiency, and normalized entropy generation rate of the Diesel
cycle are plotted against the compression ratio. Evident from Fig. 7.7 is that
a design based on minimization of entropy generation rate would be imprac-
tical, consistent with the observations we made previously for the Brayton,
Otto, and Atkinson cycles.

7.6 Isentropic compression and expansion
A special case worthy of discussion from a theoretical perspective is
that the isentropic efficiencies of the compression and expansion processes
are assumed to be 100%, i.e., η com ¼η exp ¼1. In this case, the irreversible
models of the Brayton, Otto, Atkinson, and Diesel cycles reduce to their
endoreversible designs presented in Chapter 5. The only irreversibility
would then be due to the heat exchange between the thermal reservoirs
and the engine. In this case, the thermal efficiency would then be deter-
mined using Eqs. (5.9), (5.14), (5.22), and (5.27) for the Brayton, Otto,
Atkinson, Diesel cycles, respectively.