Numerical Research On Combustion Mechanism And Control Strategy Of Diesel/Natural Gas Dual Fuel Engine

With the energy crisis and environmental pollution becoming more and more serious,natural gas has attracted wide attention because of its large proven reserves and low CO₂ emission.Diesel/natural gas dual-fuel engine has gradually become a research hotspot in the field of natural gas engine due to its advantages in thermal efficiency,low harmful emission and combustion controllability.At present,the control of combustion heat release rate of diesel/natural gas dual-fuel engine under high load condition is important to improve thermal efficiency.Therefore,the research on the combustion mechanism of diesel/natural gas dual-fuel engine and the proposed cooperative optimization control of high load combustion are of great engineering significance for the development of diesel/natural gas dual-fuel engine with high thermal efficiency and reliability.Firstly,this paper used CONVERGE to build a three-dimensional numerical model of natural gas port injection dual-fuel engine.The influence of compression ratio on the combustion process and IMEP was studied under low speed and high load conditions.On this basis,the potential of single-injection strategy and double-injection strategy to improve the combustion process were explored.The results showed that the maximum pressure rise rate could be effectively reduced when the compression ratio decreased from 16.8 to 14.8,however IMEP decreased slightly;Based on the combustion chamber with a compression ratio of 14.8,when the diesel fuel injection strategy was adopted,the indicated thermal efficiency could be effectively improved by increasing injection pressure under the condition of high natural gas substitution ratio,and indicated thermal efficiency of 48.1%with 85%natural gas substitution ratio could be achieved by cooperative control of injection pressure and injection time;The double-injection strategy improved the control of the combustion process,so that the natural gas substitution ratio could be further increased to 90%,and the indicated thermal efficiency could be increased to 48.4%.The model of natural gas direct injection dual-fuel engine was further built based on calibration model.The potential of combustion improvement with both natural gas post-injection strategy（diesel ignites firstly+final gas injection）and natural gas partial pre-injection strategy（part of natural gas inject firstly+diesel fuel intermediate injection ignition+final gas injection）were studied,and the mixture distribution characteristics of four strategies were analyzed.The results showed that the maximum pressure rise rate increased when the injection time of diesel and natural gas of natural gas post-injection strategy is closer or farther from each other and the indicated thermal efficiency of 48.15%was achieved through the coordinated control of the injection time of these fuels and the injection pressure of natural gas;When the natural gas partial pre-injection strategy was adopted,delaying natural gas pre-injection time and increasing the proportion of natural gas preinjection would increase the maximum pressure rise rate,and the indicated thermal efficiency of 49.38%is achieved through the collaborative control of time of natural gas pre-injection,proportion of natural gas pre-injection and natural gas post-injection time;Compared with the single-injection strategy,the double-injection strategy could improve the mixture stratification at the time of the main injection time,thus could control the combustion process;The mixed gas stratification of natural gas post-injection strategy was too high,which affected the mixing rate of natural gas and air,indicating that the improvement of thermal efficiency was limited;The the mixture stratification of natural gas partial pre-injection strategy was higher than diesel single-injection strategy and double-injection strategy but lower than natural gas single post-injection strategy,indicating that it was beneficial to improve the indicated thermal efficiency and reduced the maximum pressure rise rate.In conclusion,it can control the mixture concentration and active stratification,then controlled the combustion heat release rate effectively by controlling the injection mode of natural gas and diesel,and then the pressure rise rate and detonation pressure can be inhibited while the thermal efficiency can be improved.