摘 要：针对工质选择对热力循环系统效率有着重要影响等卡诺定理不能做出合理解释的问题，将工质吸热后的热力学能火用进一步划分为等熵膨胀〖HT5”，7〗火〖KG-*3〗用与等压换热火用，指出可被动力设备直接利用的火用只有工质的等熵膨胀火用。理论推导表明：在等熵膨胀过程中，压缩气体的膨胀功并不能全部转化为有用功（火用），气体膨胀功转化为有用功的效率取决于气体工质的内部压强和气体所处外部环境的压强。气体的内部压强越大、外部环境压强越小，气体膨胀功转化为有用功（火用）的效率越高。

Abstract:The selection of working fluid can influence the system efficiency of thermodynamic cycle,while the second law of thermodynamics falls short of directly providing a reasonable explanation to this phenomenon.In this paper,the exergy of gas after isochoric heat absorption process is divided into exergy in isentropic expansion and exergy in isobaric heat exchange.And it is pointed out that the exergy that can be used directly by power equipment is the exergy in isentropic expansion.By analyzing the thermodynamic cycle with air as working fluid,the results show that the expansion work in the pressurized gas cannot be converted to exergy entirely.The efficiency of the conversion depends on the air pressure and the ambient pressure.The higher air pressure and the lower ambient pressure，the higher conversion efficiency of the expansion work to exergy.

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