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本文提出了在建立发动机故障方程时应当引入故障因子的概念。文中指出，根据原封不动地保留着部件正常特性方程（即不引入故障因子）的数学模型所得到的线性模型仅仅是小偏差方程而不是故障方程。这种小偏差方程不能适用于故障诊断，并且还会导致用较少测量参数可以诊断出较多的同时发生的独立故障的错误结论。文中给出了建立故障方程的正确方法，并以不可压流的管口出流问题为例加以说明，并且对个别相当有影响的文献在这一问题上的错误观点进行了详细的分析。
In this paper, the concept of fault factor should be introduced in establishing engine failure equation. It is pointed out in the paper that the linear model obtained from a mathematical model that holds intact normal component equations (ie, without introducing fault factors) is only a small deviation equation and not a failure equation. This small deviation equation can not be applied to fault diagnosis, and can lead to more wrong conclusions that more simultaneous faults can be diagnosed with fewer measurement parameters. The correct method of establishing fault equations is given in this paper. The case of incompressible outflow of nozzles is taken as an example to illustrate the misunderstandings on this issue in some influential documents.