Research On The Effect Of Exhaust Gas Recirculation On Combustion And Emission Of Diesel Fuel Spray

With the exploding of automotive vehicles, emission regulations of the internal combustion engine are getting stricter. Exhaust gas recirculation (EGR) technique is widely applied to reduce NO_x emission in diesel engine. However, the introduction of EGR increases the emission of PM, especially in heavy load. So it is necessary to research on the effect of exhaust gas recirculation on combustion and emission of diesel fuel jet.Firstly, a diesel injection and combustion experiment was carrried out on the constant-volume bomb to acquire pressure, photos data of the combustion process. The heat release rate formula for the combusiton of diesel fuel jet in constant-volume bomb was deduced and the temperature of flame fringe is caculated to be 2257K, based on the theory of black-body radiation and with help of Photoshop cs2.0 and Matlab software.Then a multi-dimensional numerical model of constant-volume fuel injection system was established with the KIVA-3V program, and the soot formation and oxidation model codes were written and compiled. Simulation results show: the agreement between experiment combustion pressure data and simulated combustion pressure is very good, and the flame propagation and development in experiment conform to that of simultion basically.In this paper, the mass fraction of CO₂ and H₂O is changed to simulate the effect of CO₂ and H₂O, which are main ingredients of exhaust gas recirculation, on combustion and emission of diesel fuel jet. Results indicate: dilution effect is the key factor of EGR. Dilution effect increases the ignition delay, decrease the heat release rate peak and flame temperature, but enlarges the combustion zone. With the increase of dilution rate, the combustion heat efficiency increases at first and then decrease. There is a dilution rate on which the combustion heat efficiency reaches its maximum. With the increase of dilution rate, the emissions of CO and NO_x decrease, and the emissions of HC and SOOT increase. The decrease of flame temperature and O₂ concentration contribute to the significant decrease of NO_x together. The increase of SOOT is mainly due to reduction of oxidation of SOOT at low O₂ concentration. When dilution rate is higher than 9%, the decrease in NO_x is minor, while causing substantial increase in SOOT.Compared to dilution effect, the chemical and thermal effect of CO₂ and H₂O is minor. The chemical effect of CO₂ reduces burning heat release rate and mean combustion temperature, increases the emission of CO, and decrease the emission of NO_x to a certain extent. The thermal effect of H₂O increases the ignition delay of combustion, reduces combustion efficiency and flame temperature, and decrease the emission of CO and NO_x to some extent. The effect of H₂O to decrease NO_x and to increase SOOT is more obvious than that of CO₂. Under different EGR rate, the mass of NO_x is more sensitive to flame temperature, while the mass of SOOT is more sensitive to O₂ concentration.