Research Onapplication Of Acoustic Emission In Mornitoring Valve Leakage Of Medium-speed Marine Diesel Engine

As one of the most essential part of marine power systems, diesel engine’s statusis related to the security and stability of the ship directly. A safe and reliable intakeand exhaust systems is the important guaranty of engine’s security. On the other hand,valves are one of the most critical parts of the intake and exhaust system. Engine’svalves are not only exposed to metal oxides and gas with high temperature and highpressure, which generated by combustion, but also bear the alternating stress causedby stricking with valve seats. As a result, engine’s valves became one of the mostprone to failure parts. If the valves leakage happen, it will decrease cylinder pressureand power of engine, increase fuel consumption. More seriously, it can lead todamaging of turbocharger.In this paper, the acoustic emission technology is used to diagnose valvesleakage failure of medium-speed marine diesel engine. The mechanism andcharacteristics of acoustic emission signal are researched with the methods oftheoretical and experimental research. It shows that the valve leakage fault can bedetected timely with analyzing the acoustic emission signals. This research hastheoretical and application significance. The main contents are as follows:(1) The cylinder head model of Daihstsu6DK20engine is established, thetransient excitation caused by gas when the exhaust valve is normal and defective iscalculated, with CFD software—FLUENT. With the help of fluid-structure interactioncalculation technology, the accoutic emission signals when the valve under differentstatus is calculated with ANSYS software. The results show that valve leakage willcause an increase of the signal peak and RMS, and the more serious of the leakage,the greater magnitude of the change.(2) A test bench with Daihstsu6DK20engine is built and the exhaust valve isdemaged artificially with different degree to simulate valve leakage. Acousticemission signals of the cylinder head are collected when the exhaust valve underdifferent states, as well as non-burning state. Several effective characteristicparameters are extractited. It shows that the accoutic emission signal beeween25CA and80CA(Crank Angle) after combustion TDC is sensitive to valve leakage, the amplitude and energy of this part signals will increase when exhaust valve leak;The energy in low-frequency band (0Hz-10KHz) reduct, in contrast, the energy inhigh-frequency band (20kHz-50kHz) increase when exhaust valve leak.(3) EMD (Empirical Mode Decomposition) technology is introduced to analysisthe acoustic emission signals of cylinder head. And with this method, the signalscharacterstic is distinguished effectively. It shows that IMF1is the noise component,IMF2and IMF3, which represent the high component of signal, are sensitive to valveleakage, the energy proportion of both IMF2and IMF3increase when valve leak.(4) A system used to monitor engine valve leakage is developed based onLABVIEW. This system takes acoustic emission technology into the practicalapplication of valve leakage monitoring of marine diesel engine.