Research On Optimization Of In-cylinder Combustion Process Of Low-pressure Injection Natural Gas Dual Fuel Engine

At present,the requirements for pollutant emission at home and abroad are becoming increasingly stringent,so domestic and foreign began to conduct in-depth research on the combustion technology of clean fuels（such as natural gas,etc.）,among which the more prominent is the low-temperature combustion strategy of the turbulent jet ignition system with jet flame as the ignition source,which produces higher ignition energy through the ignition fuel combustion of the pre-chamber,forms a high-momentum jet flame in the small volume of the pre-chamber,enters the main combustion chamber through the jet hole channel,and ignites the lean mixture inside the main combustion chamber.For dual-fuel low-speed ship machines using pre-chamber jet ignition mode,high ignition stability and low NOx emission are distinguishing features from conventional combustion strategies,but the knocking problems they face greatly limit the improvement of engine power.Therefore,this paper uses numerical simulation to study the combustion process of dual-fuel low-speed ship machine,analyzes the development law and influence mechanism of knocking,and explores the knock suppression path to achieve the goal of high thermal efficiency and low emission based on the multi-objective optimization design method.Firstly,a single-cylinder low-pressure injection natural gas-diesel dual-fuel low-speed ship model was established by the three-dimensional CFD software CONVERGE.Meanwhile,a new simplified chemical reaction mechanism of natural gas/diesel dual fuel was constructed based on Chemkin,and the ignition delay time and laminar flame velocity of mixed fuel under high back pressure environment were verified,and the whole machine model verification under actual working conditions was completed based on the newly constructed dual fuel mechanism.Secondly,the flow and combustion process in the cylinder of dual-fuel marine engine were analyzed,the effects of compression ratio,number of pre-chambers and pilot diesel mass on engine performance,combustion and emission characteristics were studied,the changes of various species in the combustion process were analyzed,the amplitude-frequency characteristics and influencing factors of pressure oscillation were summarized regularly,and the spontaneous combustion phenomenon of end gas under detonation conditions was analyzed.Then,the effects of different EGR rates and H₂ replacement rates on the combustion process in the cylinder were studied by exhaust gas recirculation and hydrogen blending.Finally,based on the orthogonal experimental design,the response surface analysis of the matching scheme of multiple groups of EGR rate and H₂ replacement rate is carried out,and the functional relationship between each control variable and multiple target outputs（NOx,ISFC,MAPO）is studied,to obtain the best combination of control variables and make reasonable predictions of each output parameter.Through the above research,it is concluded that the knock intensity in the double pre-chamber mode is smaller,the thermal load of the parts is lower,but the NOx emission is high,the thermal efficiency is comparable to the single pre-chamber mode,and the knocking of the single and double pre-chambers is the low-frequency pressure oscillation in the（1,0）mode.The pressure oscillation intensity in the cylinder has an exponential relationship with the unburned mass fraction corresponding to H₂O₂ maximum,and when the unburned mass exceeds 27%,the pressure oscillation intensity will increase significantly.Through the synergistic optimization of EGR rate and H₂ replacement rate,the thermal efficiency of the whole machine can be improved and the pressure oscillation can be reduced while reducing NOx emissions,and the best matching scheme predicted by the regression model is 44.1%EGR rate and 14.1%H₂ replacement rate,and the corresponding NOx emissions,fuel consumption and pressure oscillation are 6.7g/k Wh,129.5g/k Wh and 0.128MPa,respectively.