| Research on the noise reduction of internal combustion engines is being paid more and more attentions recently because of its importance in the essential industrial branch such as power, transportation, shipping, petrochemistry etc. The first task for noise control purposes is identifying which surfaces on his power unit will radiate noise well and the major vibroacoustical sources. Then in order to control the noise level, it is important to take the corresponding methods depending on the characteristics of major engine vibroacoustical sources. The acoustic signals obtained from a diesel engine contain many useful vibroacoustical sources information to reflect the engine operating conditions. Such information often exists in the form of short duration transients. Because of the complex multi-excitations caused by engine structural and energy transform procedure, its non-steady working process and non-liner system behavior, the traditional analytic theory and methods in acoustic signals are not always suitable to describe this complex situation. Based upon a detailed analysis of the present worldwide research status, an in-depth and systemic theory and methodology investigation is made to the some problems. Research achievements are put into the practical engineering application. Among the total, the major creative contributions and achievements are as follows: 1. A detailed theoretic analysis is made to the fundamental dynamic behavior of the reciprocating engine. Via the development and resolution of the inner-cylinder combustion gas oscillating equation, its transient oscillating behavior and changing tendency are investigated. A practical shock-induced transient response model is developed to reveal the engine dynamic response and proceed the relative research of applicable identification methodology. 2. The physical meaning of sound intensity and its application in noise field analysis and noise sources identification were discussed. A sound intensity measurement system was set up. Based on those, research on application of sound intensity in IC engine noise source identification and acoustic field analysis for a 6108 diesel engine, 493ZQ diesel engine, WD615 diesel engine and a medium bus was made. Influences of measurement methods on measurement accuracy were discussed. 3. The acoustic signals obtained from a IC engine are non-stationary transients. Unfortunately, the Fourier transform (FT) based methods are not suitable for non-stationary signal analysis and are not able to reveal the inherent information of non-stationary signals. Compared with the FT, the wavelet transform has many distinct advantages for acoustic signal analysis. In this part, the dissertation starts with a comparison among the Fourier transform, the Short Time Fourier transform and the wavelet transform, whose properties are given. Some problems about the wavelet transform are discussed in detail, including the concentration, the border distortion and the effect of the wavelet function's central frequency on the resolution of the wavelet transform. A method to determine the maximal and the minimal scale parameters in the wavelet transform calculation is put forward. 4. Based on theory and test, research on application of the wavelet transform in IC engine acoustic source identification and acoustic field analysis for a 6108 diesel engine, 493ZQ diesel engine, WD615 diesel engine was made. Some important acoustic sources are identified. 5. Although the IC engine noise sources have distinctive time instances, it is still difficult to resolve them accurately based on noise measurement. This is because the occurrences of each noise source are too close together. And some noise sources such as the injection of fuel and operation of inlet and exhaust valves produce low-level noise. Hence, the signal energy conservation based traditional methods are unable to recognize such noise, which contain relatively small energy. The independent component analysis (ICA) brings a different strategy in dealing with the problems of blind source separation (BSS). This dissertation focuses on the study of the acoustic signals generated from a test diesel engine using the ICA in an effort to identify its noise sources. The principles, properties and implementation issues of the ICA are presented. Then research on acoustic field analysis for a single engine and 493ZQ diesel engine was made and illustrates the separation results from the measured acoustic signals.
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