Optimization Of Combustion System For Low-speed Two-stroke Dual Fuel Engine

Under the background of the increasingly prominent problems of global petroleum resources shortage and environmental pollution,the marine power industry also urgently needs to develop towards a cleaner,more efficient and energy-saving direction.Premixed combustion mode dual-fuel Marine engine with natural gas low-pressure injection can meet the emission regulation requirements of IMO Tier III without installing other post-treatment devices,which has become one of the most promising technologies.Premixed combustion mode low-speed dual-fuel engine's own "low speed,large bore,long stroke" characteristics will increase the risk of abnormal combustion of premixed gas in the cylinder,so it is usually necessary to adopt lean combustion technology.However,when lean combustion is applied to practice,there will be problems such as low combustion efficiency and instability,and even occur misfire,and the large bore and long stroke characteristics of the low-speed two-stroke engine will further exacerbate the possibility of such problems.In order to ensure the highly efficient lean combustion of in-cylinder premixed gas,this paper improves the open combustion chamber of the original diesel engine,and adopts the pre-combustion chamber to improve the ignition energy to ensure the rapid and stable combustion of lean premixed gas,and to improve the combustion efficiency while inhibiting the deflagration.The paper optimizes the pre-combustion chamber combustion system and provides some reference for the design and optimization of similar engines.Based on the 6EX340 EF low-speed two-stroke marine diesel engine,a premixed lean-burn mode dual-fuel engine with low-pressure gas injection was designed.The 3D numerical simulation calculation of the dual-fuel engine working process was completed by using the CFD software AVL FIRE,then the reliability and effectiveness of the simulation model was verified by comparing the cylinder pressure and the heat release rate curves obtained from similar engine tests in the literature.The arrangement of the natural gas injection nozzles on the cylinder liner was adjusted,then the influence of the arranged angle of the nozzles and the number of nozzle on the flow field and the formation of the mixture were discussed.Finally,when two nozzles were evenly arranged around the cylinder liner with 10° inclination angle,the distribution of natural gas in the whole combustion chamber could be relatively uniform before the injection of ignition oil.Based on the design of the adjusted nozzle,the 3D CFD numerical simulation analysis of the complete working process of the dual-fuel engine was completed,then the scavenging characteristics,natural gas injection mixing characteristics,combustion and emission characteristics of the dual-fuel engine were analyzed in detail.Based on the results of numerical simulation analysis,the parameters of the pre-combustion chamber combustion system were adjusted and optimized.The effects of the pre-combustion chamber design parameters(pre-combustion chamber channel structure,channel arrangement angle,channel diameter)and diesel injection nozzle diameter on the flame propagation process,combustion and emission characteristics were studied.The dual-fuel engine optimized except for a slight loss of power performance under 100% load,the other loads can basically maintain the same power output as the original diesel engine,and NO emissions both have been significantly reduced,which has met the emission requirements of IMO Tier ?.