Investigation Into The Condition Monitoring Of In-cylinder Combustion Behaviours Of Diesel Engines Based On Time-frequency Analysis Of Vibration Signals

Monitoring and improving the combustion conditions in the diesel engine chamber helps to improve the operational reliability and combustion efficiency,and reduce emissions of the diesel engine.Using piezoelectric sensor to obtain combustion pressure signal in chamber is the most direct combustion state monitoring approach.However,the mechanical installation mode of the sensor,which requires high processing precision,will destroy the structure strength of the combustion chamber.In addition,the cost of this sensor is expensive.In a word,the pressure sensor will not be widely used for application.The use of structure vibration signals for non-intrusive in-cylinder combustion condition monitoring is an attractive alternative.Vibration sensor has the advantages of tiny size,cheap price and convenient installation,and so on.The work of identifying the incylinder combustion state based on vibration signal analysis has been favored by increasingly researchers.However,the actual collected vibration signals often contain non-combustion excitation response information such as background noise and vibration events of various mechanical components,which makes the accuracy of combustion state monitoring using vibration signals always unsatisfactory.The main excitation sources of diesel engine vibration peak event are combustion pressure shock and piston side impact excitation,and the phase of peak is close to each other,which is difficult to distinguish.Some works have been done to distinguish the combustion shock event from the vibration peak.Firstly,response distribution characteristics of liner under the action of combustion shock and piston side impact,respectively,based on the transient dynamics analysis of the diesel engine cylinder assembly.And then,the correlation between surface vibration signal and in-cylinder pressure signal measured by bench test is analyzed in time domain,frequency domain and timefrequency domain.Finally,an approach based on signal time-frequency spectrum structure similarity analysis and two-dimensional filter construction based on operating condition adaptation is proposed to realize the reconstruction of cylinder pressure rise acceleration using measured vibration signals.The proposed approach can efficiently reconstruct the in-cylinder pressure rise acceleration information with a wide frequency band(0.1~15 kHz)and accurately determine the combustion condition in the cylinder of diesel engine.Firstly,based on the generation mechanism of diesel engine surface vibration signal,the main excitation source of diesel engine,structural vibration response and their corresponding relations are studied.Through the finite element analysis,the time-frequency distribution range of the structure vibration response caused by the combustion pressure shock and the piston side impact excitation is obtained,and the vibration response characteristics of the cylinder under the separate action of different excitation events are clarified.Then,the bench test system of single-cylinder diesel engine and fourcylinder diesel engine were designed to analyze the correlation between measured cylinder pressure signal and vibration signal of different measuring points.It is found that the vibration signals of different measuring points have different time domain and frequency distribution characteristics due to the different frequency response characteristics of the structure.The correlation between the in-cylinder combustion pressure and the vibration response of the structure surface is studied by using the approach of the time-domain correlation coefficient,the frequencydomain coherence analysis and the divided block of time-frequency domain respectively.It is found that the time-frequency analysis can reveal the correspondence between two kinds of signals in time domain and frequency domain.By comparing the phase difference between the inflection point and the peak point between the vibration acceleration curve and the cylinder pressure rise acceleration curve,it is found that the phase difference between the engine head vibration signal and the in-cylinder pressure signal is the smallest compared with the engine block and liner vibration.Then,the structural similarity index analysis is carried out on the time-frequency matrix of the engine head vibration signal and the pressure rise acceleration signal,and the reconstruction of the time-frequency diagram of the combustion pressure rise acceleration is realized by using the approach of morphological reconstruction approach.The structural similarity index change law between two kinds of signals under different operating conditions and different fuels was analyzed.The influence form of the operating conditions on the structural similarity index matrix is obtained,and then it can be proposed according to the operating conditions adaptation.The approach of constructing structural similarity index matrix adaptively according to the working condition is presented,which be conducive to preliminarily realize the reconstruction of in-cylinder pressure rise acceleration based on vibration signal.Finally,the time-frequency relationship between the engine head vibration signal and the in-cylinder pressure measured by the four-cylinder supercharged diesel engine bench test is studied.The applicability of the proposed timefrequency structure similarity index analysis and the second derivative reconstruction approach of in-cylinder pressure rise acceleration in the monitoring of multi-cylinder diesel engine combustion state is preliminarily verified.