文章指出现有各种自发过程的判据都是在指定的约束条件下才能应用，缺乏普适性是自发过程定义多样化的引发原因。在无约束条件下将热力学第一定律代入总熵判据得出并分析讨论了总熵判据的另一种形式，结合自发过程的特点总结出了热力学变化过程中能量变化的本质，给出了自发过程的通用定义。进一步指出原总熵判据只能分辨可逆与不可逆，不能分辨自发与非自发。文章给出的总熵判据的另一种形式——封闭系统任意过程的做功能力判据具有分辨自发与非自发的能力。通过理论研讨和实际应用表明，做功能力判据与总熵判据完全等价，在相应约束条件下可还原为当前热力学中各类方向判据。填补了常见的变温过程和变压过程在以前的教科书中无自发和非自发判据的空白。以前教科书中由于自发过程定义和解释的混乱而出现的一些疑难问题，在通用定义和做功能力判据面前都能得到满意的解答。%Current criteria for spontaneity are used with the confined conditions. The various definitions of spontaneous change show the lack of universality. Here, the first law of thermodynamics was put in the total entropy criterion without any constraints. Then another form of entropy criterion was obtained and discussed. The nature of energy change in thermodynamics processes was summarized based on the characteristics of spontaneous process. As a result, the universal definition of spontaneous process was put forward. The original total entropy criterion can distinguish between the reversible and irreversible processes, but not the spontaneous and non-spontaneous processes. Another form of the entropy criterion, i.e., the exergy criterion for the arbitrary process in the closed system, was proposed. This criterion can distinguish between the spontaneous and non-spontaneous processes. Through the theoretical derivation and the actual examples, we found that the exergy criterion is equivalent to the total entropy criterion. The exergy criterion can be returned to al kinds of direction criteria when using the corresponding limited conditions. In the paper, the spontaneity criteria for the processes with variable temperature or pressure were also given, which have not appeared in the current textbooks. In summary, the universal definition and the exergy criterion can solve the problems arisen from the confusing conception and explanation related to spontaneous processes.
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