Performance analysis of an air-standard Miller cycle with considerations of heat loss as a percentage of fuel's energy, friction and variable specific heats of working fluid

Abstract

This study is aimed at examining the effects of heat loss characterized by a percentage of fuel's energy, friction and variable specific heats of working fluid on the performance of an air-standard Miller cycle under the restriction of maximum cycle temperature. A more realistic and precise relationship between the fuel's chemical energy and the heat leakage that is constituted on a pair of inequalities is derived through the resulting temperature. The variations in power output and thermal efficiency with compression ratio, and the relations between the power output and the thermal efficiency of the cycle are presented. The results show that the power output as well as the efficiency where maximum power output occurs will increase with the increase of maximum cycle temperature. The temperature-dependent specific heats of working fluid have a significant influence on the performance. The power output and the working range of the cycle increase while the efficiency decreases with the increase of specific heats of working fluid. The friction loss has a negative effect on the performance. Therefore, the power output and efficiency of the cycle decrease with increasing friction loss. Comparison of the performance of air-standard Miller and Otto cycles are also discussed. Miller cycle has larger power output and efficiency than Otto cycle does, i.e., Miller cycle is more efficient than Otto cycle. It is noteworthy that the effects of heat loss characterized by a percentage of fuel's energy, friction and variable specific heats of working fluid on the performance of a Miller-cycle engine are significant and should be considered in practice cycle analysis. The results obtained in the present study are of importance to provide a good guidance for the performance evaluation and improvement of practical Miller engines.