Numerical Simulation Of Combustion Process, Soot And NO_X Emissions In A Vehicle Diesel Engine

Increasing worldwide concern over energy crisis and environmental protection triggers the demand for the great improvement on performance and emission level, which are enormously influenced by the combustion process of the diesel engines. Due to its low cost, short period, abundant information and convenient optimization three-dimensional numerical simulation has been widely adopted to investigate the combustion process of the diesel engine. In this thesis, the combustion process of a turbocharged inter-cooled diesel engine was simulated, meanwhile soot and NOX emissions were preliminary analyzed.A numerical simulation model was established in Star-CD, a CFD software. Comparison was made between the simulation calculation and the data obtained from the total cylinder sampling experiment. Based on the model, the flow pattern of the in-cylinder charge, the distribution of temperature and the formation of soot and NOX were studied, and further analysis were carried out on the major factors influencing the formation of the pollutants. In addition, the effects on the combustion process and the emissions, including swirl ratio, start of injection, diameter of injector nozzle and pressure of common rail, were investigated.The in-cylinder pressure and the fromation of soot emssions from simulation agree well with the experimental data. Therefore, it has been proved that the working model is reasonable. Simulation results show that the peak amount of soot occurs around 17CA ATDC, and then most soot is oxidized as the combustion continues. And almost NOx forms before 25CA ATDC, followed by a"frozen"that the amount of NOx remains constant. It is discovered that lager swirl ratio and smaller injector nozzle tends to promote the movement of the in-cylinder charge, extend the high-temperature region and reduce soot emissions. While at the same time NOx emissions were increased. Altering the spray angle and the position of injector has little effect on the in-cylinder pressure, but dominate the position of the spray jet. If the position is too high, most fuel will be sprayed to the squish, leading to poor combustion and more soot, while in the opposite, the spray reaches the bottom of the combustion chamber, resulting in soot increased because fuel droplets adhered to the chamber. Advancing the injection or increasing the common rail pressure cause mixing improved, peak pressure and temperature raised. And thus NOx increased and soot reduced are observed. Higher intake pressure remarkably raises the in-cylinder pressure, and consequently reduces soot emissions.