| With the increasing environmental pollution caused by the high NO_X and particulate emissions of traditional internal combustion engines,the research and development of natural gas engines has received great attention.The diesel-piloted natural gas direct-injection engines has become a hot research topic due to their higher power performance and lower emission than those of traditional diesel engines.This paper uses Cummins ISX natural gas engine as model engine and conduct simulation studies to investigate the effect of different nozzle structure(aperture,number of nozzle holes,etc.)and injection strategies on the combustion and emission characteristics.The simulated result provides more insight into the optimization and improvement of thermal efficiency and emissions.The CFD software CONVERGE was used in the simulation of the combustion chamber of a diesel-piloted natural gas direct injection engine.The selected calculation model is verified by comparing the simulation results with the experimental results.By comparing the simulation data under different maximum grid numbers,the grid settings of the combustion chamber model were optimized,to improve calculation efficiency and calculation accuracy.In diesel-piloted natural gas direct injection engines,the structure of natural gas injection holes has a significant effect on the fuel distribution and mixing process.This paper analyzes its effect on engine combustion and emissions by changing the number and diameter of natural gas injection holes.The results indicates that When the diesel nozzle is controlled to maintain 8 holes,the number of natural pores is 4 or 5 holes,and the diameter of natural gas nozzle is 1.00mm and 0.89mm,the natural gas combustion presents a single peak value of natural gas heat release rate,and the combustion duration is shortened.However,due to the long ignition retardation period,the combustion phase is late.When the number of natural pores is between 6-10 holes and the diameter of natural gas orifice is less than 0.82mm,the heat release rate of natural gas presents the characteristics of double peak.the mixture of natural gas and air is better,the ignition effect is good,the ignition retardation period is short,and the combustion phase is advanced.For dual-fuel coaxial injection engines,the matching of ignition diesel and natural gas orifice has a great influence on the combustion of fuel in the engine.It is found that increasing the number of diesel nozzles can make the high-temperature ignition zone formed by diesel ignition more widely distributed,and the gap between the zones is smaller,which is equivalent to the relative Angle between the diesel nozzles and the natural gas nozzles,so as to shorten the ignition delay period of natural gas.Increasing the number of natural gas nozzles can also shorten the ignition delay period of natural gas.In this paper,the engine performance is optimized by increasing the gas injection pressure,shortening the gas injection duration,accelerating the fuel and air mixing rate and increasing the fuel premixed combustion ratio.The research shows that when the duration of natural gas injection is shortened to 5°CA,the mixing effect of fuel and air is better,the combustion rate of natural gas is significantly improved,and the thermal efficiency of engine is significantly improved.At the same time,under the condition of not changing the gas injection pressure,the influence of changing the time of gas injection on the combustion and emission of the engine was studied.When the time of natural gas injection is-5°CA ATDC,the duration of natural gas combustion can be shortened,so as to improve the indicator power of the engine. |
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