Effects of Specific Heat Models on the Performance of the Irreversible Over-Expansion Cycle

Since specific heat variation of working fluid has a great influence on the performance of the engine cycle, it is indispensable to evaluate the effect of different specific heat models on the cycle performance. Different specific heat models including linear, logarithmic, and polynomial ones had already been applied to calculation of the power and efficiency of internal combustion engine cycle, of which polynomial specific heat reflects its detailed variation with temperature. This study aims at deriving the analytic equation with respect to ecological function of the irreversible Dual-Miller cycle (DMC) by using the specific heat model of polynomial, and comparing results of performance calculation based on constant, linear, logarithmic specific heat models with that of polynomial specific heat model. In order to reflect the characteristics of DMC engine in more detail, the cycle involving heat transfer loss, friction loss, and internal irreversible loss of heat expansion as well as variation of specific heat is considered here. In addition, the effects of cycle parameters such as compression ratio and miller cycle ratio, cut-off ratio on the maximum power output and the maximum efficiency conditions are analyzed. The results indicate that logarithmic specific heat model is simpler than polynomial one, but it gives nearly approximate result to polynomial one in calculation of the performance of DMC and AC (Atkinson cycle). The presented models and results are expected to provide guidelines for the design and optimization of DMC engines.