Studies Of Vibration And Noise Signal Time-frequency Analysis Method In Gasoline Engine

At present,noise pollution,air pollution,water pollution and solid waste pollution are listed as the four main sources of pollution in the world.With the amount of car ownership keeping growing,traffic noise has accounted for more than 70% of the total city noise.More stricter laws and regulations about car noise emission limits have been successively issued both at home and abroad.As one of the main sources of automobile noise,engine's vibration and noise level will not only produce important influence on the vehicle's NVH,but also could bring damage to its related compenents.So,researches about the mechanism and optimum scheme of engine's vibration and noise have very important signifinance.As supercharging technology and gasoline direct-injection(GDI)technology are widely used,the maximum explosion pressure inside the engine cylinder has increased significantly,it also makes the proportion of engine's combustion noise increase obviously.Therefore,by studying the mechanism and transmission path of combustion noise,it proved to be very important to research and optimize the combustion noise.Based on the different exciting forces and transmission paths of combustion noise,this paper used multiple regression analysis method,solved the direct and indirect combustion noise transfer function in the frequency domain,which is only related to the structural parameters of the engine.And there is a comparation between direct and indirect noise in the frequency domin.The results show that the multiple regression model fitting degree is high at each frequency,so the direct and indirect combustion noise transfer function obtained by this method is relatively accurate.Both of the direct and indirect combustion noise transfer function exist peak at about 2000 Hz.To this gasoline engine,indirect combustion noise is higher than direct combustion noise between 25 Hz to 1250Hz;otherwise direct combustion noise is higher than indirect combustion noise between 1250 Hz to 20000 Hz.Moreover,based on a comprehensive comparison between almost of the noise source identification methods,this paper made full using of the advantages of Angle Domin Analysis and Hilbert-Huang Transform Technology to make a processing of the engine cylinder head vibration signal.According to the law of amplitude and frequency of the vibration signal with crank angle in Hilbert time spectrum and marginal spectrum,the causes of the IMFs are identified and analysised effectively.The results show that the vibration signal of the cylinder head mainly contains three sets of IMF components.The first two groups are IMF1~IMF2 and IMF3~IMF5,which the frequency range are mainly concentrated in 16000~20000Hz and 2000~8000Hz respectively.Combined with the crank angle position,the instantaneous burst pressure caused by gas combustion in the cylinder and the transient impact of the valve mechanism on the cylinder head are the main sources of these two groups of IMFs.The first set of IMFs has the largest amplitude.The low frequency components of IMF6~IMF13,whose frequency range is below 600 Hz,its amplitude is relatively small and have obvious characteristics of FM modulation.The inertial force and moment of the moving parts are the main sources of low-frequency vibration of the cylinder head when the engine is running.