| The natural gas engine technology development can effectively promote the national energy structure,energy-saving and emission-reduction targets in the field of internal combustion engine,and effectively solve the dual problems of energy-supply security and ecological environment protection.Foreign original equipment manufacturers have successfully developed high performance large natural gas engines,with applying lean burn combustion and air-fuel ratio control technology.This type of engines mainly includes single-fuel gas engine with pre-chamber ignition feature,and the dual-fuel engine with pilot diesel ignition feature.The domestic research mainly focused on light-duty and heavy-duty natural gas engines.The related technical research on the pre-chamber ignition and lean combustion characteristics,from basic theory to engineering application,are deficient for marine gas engines.The existing research findings cannot support the development of high-performance high power large marine gas engine;therefore,it is urgent to carry out the relevant research.In this study,the research object is an 8M23 G large gas engine with independent gas supply pre-chamber system.First of all,a comparative study is carried out to analyze the combustion characteristics of pre-chamber ignition and spark plug ignition,under different pre-mixed conditions.The CFD simulation are performed within a constant volume chamber and a combustion chamber in a marine engine cylinder respectively.The influence of lambda and initial pressure on the formation and development of pre-chamber jet flame is analyzed.The results show that,because of the multiple spherical flame jets generate large contact area,the independent gas supply pre-chamber ignition system has obvious advantage on the flame propagation,especially in lean burn condition.The formation and propagation of pre-chamber jet flame are faster with lower lambda in the pre-chamber.The lower lambda also speeds up the flame propagation in the main chamber.Furthermore,the effects of lambda in pre-chamber,lambda in main combustion chamber,ignition timing and fuel/air pressure difference on the combustion characteristics are experimentally studied with an 8M23 G gas engine.It is found that the influence of pre-chamber lambda variation on combustion characteristics is basically consistent with the optical measurement in the constant volume chamber.Within the scope of the test plan in this paper,when the lambda in the pre-chamber is lower,the formation and development rate of the turbulent jet flame is relatively higher,the ignition delay period and the combustion duration are relatively shorter,the combustion cycle variation is relatively smaller,and the cylinder head exhaust temperature is relatively lower.But the peak firing pressure and maximum pressure rise rate in cylinder will be relatively higher.This feature helps to extend the lean-burn limit of 8M23 G to more than 2.0.The lambda in the main combustion chamber has a significant effect on the combustion characteristics.Generally speaking,as the lambda decreases,the peak firing pressure and exhaust gas temperature increase,the ignition delay period and the combustion duration are shortened,the CA50 is advanced,the maximum pressure increase rate is increased,and the coefficient of variation(COV)of peak firing pressure is decreased.However,there are also cases where the trend of influence is unobvious,non-linear,and interacts with the ignition timing under a certain load and a certain test scheme.The ignition timing mainly has a significant impact on the peak firing pressure,the ignition delay period,the CA50 and the maximum pressure increase rate.The influence of the fuel/air pressure difference on the combustion characteristics is obvious and linear only at the middle and low load.It is reflected that as the fuel/air pressure difference increases,the combustion heat release slows down,the maximum pressure rising rate decreases,and the COV of peak firing pressure rises,while the peak firing pressure decreases and the exhaust temperature rises.Based on the experimental design analysis,defining the lambda in main combustion chamber and ignition timing as the dependent variables,focusing on the ignition delay period,CA50,combustion duration,maximum pressure rise rate and COV of peak firing pressure as combustion characteristics,the quantitative influence weight and the degree of influence are obtained by the main effect and Pareto analysis.It is concluded that the lambda in main combustion chamber is the maximum effect weight on the combustion duration,the maximum pressure rise rate and the COV of peak firing pressure.The causes of inflection points under some test conditions are analyzed.In contrast,the ignition timing is more pronounced for the ignition delay period,the peak firing pressure,the maximum pressure rise rate and the CA50,and the law of influence is more linear.Based on the two influence parameters of the turbulent jet flames,the lambda in main combustion chamber and the ignition timing,the Kriging model is selected and built,and the multi-objective optimization is carried out based on this model.By analyzing the results of optimization,combined with the research results of various lambdas in pre-chamber and fuel/air pressure differences,an optimization scheme of combustion characteristics that takes account of reliability and emissions is designed.The test results also show that the Kriging approximation model can accurately reflect the main influence laws of engine combustion characteristics,and can be used to carry out multi-objective optimization.Moreover,the method of building the combustion optimization scheme is effective,and it can meet the requirements of multi-objective optimization under different conditions,the thermal efficiency from 8M23 G engine test bench can reach up to 42.8%.This method is especially suitable for the case where the combustion characteristics are more complicated with more influence or limitation factors,which has high engineering application value. |
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