| The diesel engine is the core component of marine power plant,the security and economical efficiency of ship operation are strongly influenced by the status of diesel engines.The reliability of intake and exhaust system is the basic guarantee of the oparetion of diesel engine,the valves are one of the most critical parts of intake and exhaust system,the valve leakage fault is more likely to occur because of extremely poor working condition.Therefor,it has important significance to research the diesel valve leakage fault diagnosis technology.The target was to improve the applicability of diagnosis technology of valve leakage fault for diesel engines based on acoustic emission(AE),according to aeroacoustics equations,characteristics between the valve leakage excitation and AE were analyzed theoretically,the static and dynamic experiments of valve leakage fault simulation based on three different diesel engines were studied,the extractiom method of signal analysis interval based on quasi-periodic AE of cylinder head and the extraction method of valve leakage fault characteristic parameter were researched,it can be taken as a technical reference for engineering application of valve leakage fault for marine diesel engines based on AE,the main contents and conclusions in this thesis are as follows:1.According to Lighthill aeroacoustics equations,the valve leakage associated with the AE generated by the leakage,and then the application of theoretical model was verified by the static experiment of valve leakage fault simulation on 4120 SG diesel engine.It has show that the AE energy excitated by valve leakage increased when the valve leakage degree and inlet pressure increased,therefore,it can be taken as a valve leakage fault characteristic parameter to represent the severity of valve leakage when in-cylinder pressure variation was limited in scope.2.The dynamic experiment of valve leakage fault simulation was studied on a test bench based on WP10-240 diesel engine,the extractiom method of signal analysis interval based on the cylinder head AE,and the extractiom method of the valve leakage fault characteristic parameter in the frequency domain extraction method were researched based on fault simulation experiment.The main conclusions are as follows: The conbustion section AE signal was sensitive to valve leakage fault,the extractiom of signal analysis interval should combine with the type of diesel engines and the structure of cylinder heads,to avoid the interference excitation of other cylinders,the AE band excited by valve leakage in signal analysis interval was different from combustion,the AE band energy excited by valve leakage was sensitive to inlet pressure and leakage degree,which can be taken as a fault characteristic parameter to represent the degree of valve leakage,it can be more accurately to diagnose the severity of valve leakage which combined with load of diesel engine.3.Because of the AE band energy excited by valve leakage was associated with the load of diesel engine,in the time-frequency domain,the valve leakage fault characteristic parameter extraction method of the cylinder head AE was researched based on Hilbert-Huang Transform.The main conclusions are as follows: AE energy was mainly distributed in the vicinity of the combustion top dead center under the normal condition,and the AE band was single,AE energy distribution became more variable in the time domain and frequency domain under the valve leakage condition.Therefore,the time-frequency entropy which represent the uniformity of energy distribution can be taken as fault characteristic parameter,it wasn't effected by load of diesel engine.4.The extractiom method of signal analysis interval and the characteristic parameters were verified by experiments on 4120 SG high-speed diesel engine and 6DK20 medium-speed diesel engine.On LabVIEW software platform,the valve leakage monitoring system based on AE technique for diesel engines was developed based on the research in this thesis. |
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