| The combustion noise plays an important role in the whole noise of DI dieselengine. As we know, the transient changing of conditions reflect the changing of thework process, in order to study the phenomena that the noise change acutely in thetransient conditions, it is necessary for us to result the effect mechanism of thetransient conditions on combustion noise. In this article, a single cylinder DI diesel engine is chosen as the research object,by using the wavelet packet analysis and coherence analysis methods to study therelationship between the cylinder pressure and surface vibration and the combustionnoise, the results show that the noise made by the piston slapping mainly presents atthe frequencies of 4000 hertz and 7200 hertz. In order to separate the noise of pistonslapping from the combustion noise and identify the combustion noise, a newexperiment method is designed to measure the spectrum of combustion stimulate andthe spectrum of the combustion noise, the transfer function of the combustion noisewhich reflects the structure attenuation of engine can be obtained. From the point of view of hydrokinetics the cylinder pressure mathematicmodel is established and its spectrum character is studied in detail. With the relationinvestigation of aerodynamic load and combustion noise through the analysis ofcylinder pressure, rate of pressure rise, heat release rate and combustion noiseobtained by design a time window, it is found that the rate of pressure rise is themain dynamic load factor that affects the combustion noise. By using the soundmodel experiment and model analysis with method of sound response and Sysnoisesound software, the high-frequency stimulation mechanism of combustion noise isstudied. With the influence of chamber volume and temperature on sound model ofcombustion chamber, it further affects the high-frequency oscillation of combustionpressure. With FEM analysis method the sound pressure response is calculatedwhen stimulation by a point sound source in the combustion chamber, and thefrequency of pressure oscillation rather violent is obtained when combustionchamber is stimulated by combustion. With the analysis of cylinder pressure andcombustion noise energy in each frequency range, it is found that thehigh-frequency oscillation pressure's energy is only 5% of the total energy of the IIcombustion pressure; however, the combustion noise's energy inspired by it is 80%of the whole combustion noise energy. The results show that the structure of theengine determines the transfer character of combustion noise. Combustion noise'smain component concentrates in the frequency range of 400~8000 Hertz and it isdetermined by the rate of pressure rise and high-frequency oscillation ofcombustion pressure working together. By developing testing technique and method of transient conditions, thestimulation mechanism in transient conditions of internal-combustion engine isstudied in detail. By using the method of multi-regression analysis, the first-levelinfluence model of dynamic load, high-frequency of pressure oscillation andcombustion noise can be established. The relation of second-level influence factorsof combustion noise in transient conditions is investigated too. It will result indifference of the ignition delay, start point of combustion, and fuel injectionquantity in transient conditions compared with steady condition, because thecombustion chamber wall's temperature, fuel injection pressure, maximum ofneedle lift and unseal standing time of needle lift in transient conditions are higherthan steady conditions in the same load and rotation speed. As a result, it'll causethe difference of combustion pressure, rate of pressure rise and oscillationfrequency and amplitude of high-frequency oscillation of combustion pressurebetween transient and steady conditions, and make difference with combustionnoise.
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