综述了以朗肯循环为基础的低品位热源热机的研究现状，指出采用相变工质是朗肯循环热机的最大优点。简述了热声理论，指出热声转换对工质的本质要求取决于等压热膨胀系数β０。通过对液相工质特性的讨论，指出工作于近临界状态下的液相工质热声热机是利用低品位热源的合理方案。 The current research status of low grade heat source heat engine based on Rankine cycle is reviewed. It is pointed out that the use of phase change material is the biggest advantage of Rankine cycle heat engine. The thermoacoustic theory is briefly introduced. It is pointed out that the essential requirement of working medium for thermoacoustic conversion depends on the isobaric thermal expansion coefficient β0. Through the discussion of the characteristics of liquid working fluid, it is pointed out that the liquid working medium thermoacoustic heat engine operating in the near critical state is a reasonable scheme of using low-grade heat source.