Study On In-cylinder Injection Mixing And Combustion Characteristics Of Direct Injection Natural Gas Engine

In recent years,many countries have focused on the research of new alternative fuel engines.Gas-fueled engines,represented by natural gas,have great potential and broad application prospects.In-cylinder direct injection is becoming a powerful technology to improve the power and emission characteristics of natural gas engines.However,the in-cylinder direct injection natural gas engines face the problems of mixture deterioration and combustion instability.Therefore,the research on injection mixing mechanism and flame propagation characteristics of in-cylinder direct injection natural gas engine is becoming a hot topic in the field of natural gas engine.Multi-dimensional numerical simulation of mixing and combustion characteristics of in-cylinder direct injection natural gas engine was carried out.Present work clarifies the evolution of large-scale vortices and the mixture formation mechanism before ignition,and reveals the species distribution rule and the internal relationship between the key influencing factors and the flame surface development in the combustion process of the engine,and conducts further study on combustion characteristics and mechanism of engine alternative fuels,which will provide theoretical basis for the design and development of clean and efficient in-cylinder direct injection natural gas engine,and deepen the basic theoretical research on combustion characteristics of gas fuels in internal combustion engine.A simulation analysis model of methane under-expanded injection including virtual injector model and gaseous injection model was established.A virtual injector model of under-expanded methane injection was established based on the under-expanded gaseous jet theory.Based on the large eddy simulation method,coupled with Lagrange particle tracking method and thermophysical parameter model,a gas phase injector model of methane was established.The visualization experiment of under-expanded methane injection was carried out in a constant volume combustion bomb,and the simulation analysis model of under-expanded methane injection was experimentally validated.The sensitivity analysis of simulation analysis model of under-expanded methane injection was carried out.The detailed chemical reaction mechanism of methane with 61 species and 666 reactions was proposed.The detailed mechanism was reduced by combining DRG,DRGEP,SA and CSP,and hence a skeletal methane chemical reaction mechanism with 44 species and 267 reactions was proposed.The skeletal mechanism was validated by the experimental results of laminar combustion speed,ignition delay time and concentration distribution of some species.The main reaction pathways of NOx formation in methane/air flame at different mixture equivalence ratios,ambient temperatures and pressures were studied by reaction sensitivity analysis and rate-production analysis of species.The results show that,in the ambient temperature and ambient pressure lean-burn flame,the thermal reaction pathway dominates the formation of NOx,followed by the N2O and the prompt reaction pathway.In the ambient temperature and high-pressure lean-burn flame,N2O reaction pathway becomes the dominant path of NOx formation.As the equivalence ratio is unity,the the thermal reaction pathway dominates the formation of NOx under various initial pressures.The computational model and algorithm for turbulent combustion process of in-cylinder direct injection natural gas engine were studied.The DTF-LES turbulent combustion model coupled with ISAT method was established.In the model,a real-time calculation method of flame artificially thickening factor based on local grid element size and laminar flame thickness was proposed,and a real-time calculation method of flame winkling factor using reaction process variable calculation model accoriding to oxygen atom equilibrium was proposed.In the model,methane skeletal mechanism was compled,and ISAT method was coupled into the turbulent combustion model for efficient solution of chemical reaction kinetics in combustion process.An automatic generation method for three-dimensional geometric model and mesh of engine combustion system was proposed.The vertex motion algorithm of unstructured polyhedron mesh and mesh volume stretching algorithm were coupled,and an engine mesh movement method was proposed.The numerical simulation of injection mixing and combustion characteristics of in-cylinder direct injection natural gas engine was carried out.The distribution characteristics of in-cylinder large-scale vortices at different stages of intake and compression stroke were studied.The evolution rule of in-cylinder large-scale vortices at different fuel injection timing was revealed.The internal relationship between the in-cylinder mixture formation process and the fuel injection timing was studied.The in-cylinder distribution of some species during combustion was studied,and the distribution and variation of characteristic parameters of in-cylinder flame development,such as flame surface density,flame stretch rate,flame surface curvature,tangential strain rate and flame displacement velocity,were obtained.The factors influencing combustion characteristics of in-cylinder direct injection natural gas engine were analyzed.The analyses show that,1.The formation of large-scale vortices in cylinder is mainly caused by the jet-wall impingement.As the fuel injection is delayed,the process of methane jet penetration is delayed,and the process of formation,movement and break-up of large-scale vortices is delayed.A large number of in-cylinder combustible mixtures begin to form after jet-wall impingement.As the excess air coefficient is unity,with the delay of fuel injection timing,the distribution of equivalence ratio of mixture broadens and the degree of heterogeneity increases at ignition time.2.In the cylinder,NO is mainly converted by NO2 with temperature below 1800 K,otherwise NO is mainly generated by the thermal reaction pathway.Across the in-cylinder flame surface,from the unburned side to the burned side,there is a peak value of flame surface density.The flame displacement velocity and the tangential strain rate increase.The flame surface curvature decreases.The change trend of flame surface stretch rate is dominated by the propagation rate term.3.Under the theoretical excess air coefficient,the distribution of mixture equivalence ratio near the ignition position varies greatly with different fuel injection timings or ignition positions,which results in a great difference in-the combustion mode and the flame displacement velocity at the initial stage of flame propagation,and then leads to obviously different combustion pressure and the rate of fuel consumption.