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A new cycle model of an irreversible quantum harmonic heat engine is established in this paper, which is composed of two isothermal processes, an adiabatic process and a constant frequency process. Based on the quantum master equation and semi-group approach, the general optimal performance characteristics of the cycle are investigated. Expressions for several important performance parameters such as the efficiency, the power output and the ecological function are derived analytically in high temperature limit. By numerical calculation, a set of performance characteristic curves is plotted, such as the optimal curves between the power output and efficiency, the ecological function and efficiency. A comparison of the corresponding efficiencies at the maximum ecological function and power output is also presented.

An irreversible Braysson heat engine cycle model, in which the heat transfer obeys a linear heat transfer law between the working fluid and the external heat reservoirs, with heat resistance, heat leak and internal irreversibility is established in this paper. The analytical expressions of power output, efficiency and ecological objective function are derived. These performance parameters are maximized with respect to the cycle temperatures along with the isobaric temperature ratio. The performance characteristic curves are obtained by numerical calculation. The optimal operating regions of the important parameters are determined.

An irreversible model of the air-standard Diesel heat engine cycle is established, in which the primary irreversibilities, such as the heat leak loss through the cylinder wall, friction loss, internal irreversibility and temperature-variable heat capacities, are considered. The analytic expressions of the power output and efficiency are derived. The relations between the power output and the ratio of the highest to the lowest pressure of the working fluid, between the efficiency and the ratio of the highest to the lowest pressure of the working fluid, as well as the optimal relation between the power output and the efficiency of the Diesel cycle are shown by numerical example. Moreover, the influence of these primary irreversible factors on the performance parameters of the Diesel heat engine is discussed. The results obtained here may provide some significant guidelines for the optimal design and operation of the practical Diesel heat engines.