  Engineering Thermodynamics has been the core course of many science and
engineering majors around the world, including energy and power, mechanical
engineering, civil engineering, aerospace, cryogenic refrigeration, food
engineering, chemical engineering, and environmental engineering, among which
gas power cycle is one of the important contents. However, many Engineering
Thermodynamics textbooks focus only on evaluating the thermal efficiency of gas
power cycle, while the important concept of specific cycle work is ignored.
Based on the generalized temperature-entropy diagram for the gas power cycles
proposed by the authors, an ideal Otto cycle and an ideal Miller-Diesel cycle
are taking as examples for the thermodynamic analyses of gas power cycles. The
optimum compression ratio (or the pressure ratio) for the maximum specific
cycle work or the maximum mean effective pressure is analyzed and determined.
The ideal Otto and the ideal Miller-Diesel cycles, and also other gas power
cycles for movable applications, are concluded that the operation under the
maximum specific cycle work or the maximum mean effective pressure, instead of
under the higher efficiency, is more economic and more reasonable. We concluded
that the very important concept, i.e., the optimum compression (or pressure)
ratio for the gas power cycles, should be emphasized in the Engineering
Thermodynamics teaching process and in the latter revised or the newly edited
textbooks, in order to better guide the engineering applications.
