| The widespread use of traditional internal combustion engines has promoted the vigorous development of the world economy and civilization,but its high NO_X and soot particulate emissions have also aroused concern from all sectors of society.Natural gas engines have attracted widespread attention due to their advantages of good economy and low emission pollution,but they have not been widely used due to the relative lack of power.Dual-fuel in-cylinder direct injection technology can effectively improve the power performance of natural gas engine and has good emission performance.However,due to foreign technology monopoly,there are still difficulties to be overcome.In this paper,numerical simulation of dual fuel injection and combustion process is carried out around the dual fuel in-cylinder direct injection technology.Firstly,the experiment and Simulation of dual fuel injection based on coaxial injection are carried out.Then,the influencing factors of coaxial injection in-cylinder direct injection natural gas engine are simulated and analyzed.Finally,a four-nozzle scheme is designed to realize dual fuel in-cylinder direct injection.The main research contents and results are as follows:(1)A dual fuel injection experimental system with PIV and Schlieren is adopted to visualize the process of dual fuel injection in constant volume bomb in this paper.The reliability of CFD numerical model of dual fuel spray mixing process is verified by visualization test data,and numerical simulation analysis of Diesel/natural gas dual fuel high pressure direct injection process is carried out.The results show that when the head of natural gas jet meets diesel spray,it will be disturbed,and with the entrainment and entrainment of the natural gas jet,the diesel particles will gradually distribute evenly in the natural gas jet.(2)Based on Cummins ISX natural gas engine,a coaxial injection of pilot ignited natural gas engine model is established.The model is verified and the influence of excess air coefficient on the combustion and emission is analyzed by CONVERGE software.The results show that when the starting speed is 1200 rpm,the air excess coefficient increases,the peak value of average pressure in cylinder increases,the average temperature and heat release peak in cylinder decreases greatly,the combustion rate of natural gas slows down,burns smoothly,and the higher the excess air coefficient,the better the emission.When the excess air coefficient is 3.5,the soot,NOx and CO emissions decrease by 74%,15%and 99%,respectively.Based on L23/30 natural gas engine,an engine model with two diesel nozzles and two natural gas nozzles is designed.The effects of injection strategy on combustion and emission of engine are studied by simulation at 75%load.It is found that among the three different injection times of ignited diesel fuel,when the injection time of ignited diesel fuel is-25°CA ATDC,the effect is the best,the peak value of average pressure in cylinder is high,the combustion phase is at the front,the high temperature duration is short,the indicated mean effective pressure is high,and the emission of NO_X is reduced by 15%.When the injection time of ignited diesel fuel is-25°CA ATDC and the injection interval decreases from 19°CA to 16°CA,the peak value of average pressure in cylinder increases by 0.77 MPa,the time of peak heat release rate is ahead,the soot mass decreases by 22%,and the emission of NO_X increases.When the amount of ignited diesel fuel increases,the peak pressure in cylinder increases and the emission of NO_X increases.The research results of this paper have a certain guiding effect on the performance and combustion improvement of coaxial dual fuel natural gas engine,and provide a direction for the research and development of dual fuel direct injection system of large cylinder diameter natural gas engine in China. |
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