| Energy shortage and ecological environment deterioration are two major problems facing human society in the 21st century.Natural gas has become one of the research hotspots because of its rich production and good characteristics.Compared with the traditional internal combustion engine,the ordinary natural gas engine has the disadvantages of low inflation efficiency,insufficient power,difficult cold start and high HC emission.Therefore,the popularization and application of natural gas engines are limited.In-cylinder diesel-nature gas high pressure direct injection technology can realize in-cylinder gas diffusion combustion,maintain the power and economy of natural gas engine,while reducing harmful emissions,so it has broad application prospects.The purpose of this paper is to reveal the essence of in-cylinder mixture formation,ignition and combustion through basic research on the formation mechanism and combustion characteristics of oil-gas high pressure direct injection engine.It is of great theoretical significance and engineering practical value for enriching combustion mechanism of internal combustion engine,improving fuel economy and reducing emissions of oil-gas high pressure direct injection engine.In this paper,the partition structure of transient gas jet and the characteristics of gas jet in cylinder are studied.Multidimensional flow model,gaseous particulate injection model,phenomenological and multidimensional mixing model are studied and compared.Phenomenological model and multidimensional mixing model are used to model uns Table vortices in the front hemisphere and quasi-s Table jets at the back.Combining the model of ignited diesel fuel injection with the model of natural gas injection,an oil-cylinder high-pressure injection model is es Tablished to simulate and analyze the characteristics of diesel-natural gas injection pressure,injection timing and injection flow rate.By means of experimental research,the characteristics of high-pressure jet of diesel and natural gas were studied in a constant volume combustion bomb.The jet penetration distance,cone angle and volume of direct injection natural gas in cylinder were studied by morphological and image processing methods.The diesel and natural gas injection models were validated respectively.The components and main reaction paths of detailed chemical reaction mechanism of methane and n-heptane combustion are studied and analyzed.A new simplified model of diesel-natural gas dual fuel is proposed by combining the main path analysis method with the main component analysis method.The model contains the simplified mechanism of diesel/natural gas dual fuel with 112 components and 658 basic reactions.Based on the multi-dimensional numerical simulation software KIVA-3V,a three-dimensional numerical simulation platform for the flow and combustion of an oil-gas high pressure direct injection engine is es Tablished in this paper.The Pa SR turbulent combustion model is embedded to realize the coupling between the multi-dimensional model and the detailed chemical reaction kinetic model,considering the effect of turbulence on the chemical reaction.The three injection parameters of the engine are systematically optimized by introducing NSGA-II.To coordinate the relationship between the three objectives.Based on the developed multi-dimensional simulation calculation platform for in-cylinder combustion of oil-gas high pressure direct injection engine,the formation of engine mixtures and combustion characteristics in-cylinder were simulated and calculated.The main conclusions are as follows:The in-cylinder high pressure direct injection of natural gas can significantly increase the velocity field and enhance the turbulent kinetic energy.The turbulent kinetic energy can reach more than 15 000 m~2/s ~2.The enhancement of turbulent intensity and turbulent kinetic energy can enhance the mixing process of in-cylinder fuel,thus accelerating the combustion speed of natural gas fuel in the engine cylinder.The change of hydrogen ratio in gas fuel has little effect on the compression ignition and combustion process of diesel oil after hydrogen is added into oil-gas high pressure direct injection engine.With the increase of hydrogen blending ratio,the heat release rate of gas fuel is ahead of schedule.The peak heat release rate of premixed combustion of natural gas is slightly higher when the hydrogen blending ratio is more than 20%,but the change is small in general.Three main injection parameters with crucial influence on the engine performance including the injection time of the diesel fuel,time interval between natural gas injection and diesel injection and the circumferential offset angle between the diesel nozzle hole and natural gas nozzle hole are optimized by the NSGA-II code and the optimized parameters are obtained with simultaneous reduction of NOx and soot in the dual fuel engine.Furthermore,the lowest ISFC(or high NOx emissions)condition is achieved when the injection interval between the diesel injection and gas injection is 1.38?A. |
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