The objective of this study is to investigate the effects of a swirl nozzle injector on spray behavior and the effect of swirl flow induced by a swirl nozzle injector on the mixture formation and soot reduction in a CI engine under the different injection pressure and the different start of energizing timing. In addition, it was conducted to investigate the effect of various cylinder geometries, which were designed to reduce the rich fuel mixture and the soot emission in a CI engine with a swirl nozzle injector. The test engine was used as a single-cylinder engine based on a small common-rail diesel engine. ECFM-3Z model was applied for combustion analysis as sub-models.
The applied swirl nozzle injector was set for tilting at 20° in a tangential direction from the nozzle outlet, and the injection pressures were 40 and 80MPa. A break-up model was used as the wave model, and the results were compared and analyzed in terms of spray tip penetration, Sauter mean diameter (SMD), and swirl ratio characteristics.
As a results, the swirl nozzle injector proposed in previous studies has been reported that it has a 5% increase in injection quantity under the same injection condition. Also, it is also expected a high possibility of soot reduction due to the swirl which is about 6 times higher than the conventional nozzle injector. It is reason that the air-fuel mixture continuously mixes with air in the center of the cylinder as a result of the high swirl ratio of the swirl nozzle injector.
Based on these advantages, this numerical analysis study was carried out to investigate the effect of swirl nozzle injector on the possibility of soot reduction under the different injection conditions of CI engine. However, as compared swirl nozzle injector with conventional injector, when the start of energizing timing was changed from BTDC 12 deg to TDC under the same injection conditions, the swirl nozzle injector decreased 10 % for IMEP value, and 46 % for NO generation amount however, soot generation increased compared with the conventional nozzle injector. The reason is that the combustion performance of the swirl nozzle injector deteriorated owing to the wall film phenomenon, and residual fuel formed at the center of the piston. In addition, it was sure that the swirl nozzle injector has a lower combustion performance than the conventional injector. However, when the start of energizing time was retarded, the fuel was injected into the center and bottom of the piston bowl, and the amount of generated soot decreased owing to the collision effect. Through these results, four cylinder geometries was applied to improve combustion performance and reduce soot emissions. Four cylinder geometries increase the diameter of the piston bowl to expand the combustion zone or increase the length of the piston bowl to increase the break-up distance of the injected droplets. In addition, piston rim length changed to investigated the effect of squish flow.
As a result, in case of the large bowl, the soot was rapidly oxidized due to the wide combustion zone by the increased bowl diameter, so the ISSoot value decreased about 21%. In case of the small bowl, which was reduced the diameter and expanded the piston bowl length, it had the reduced ISSoot value by 16.5% than the conventional cylinder geometry because the residual fuel decreased since enough break-up length was secured and the formed soot in the piston bowl was rapidly oxidized.
Therefore, this study proposed four cylinder geometries which was designed to cause a reduction of generated soot by the reduction of residual fuel. Large bowl and small bowl according to the diameter of the piston bowl were judged to be suitable for the purpose of this study. In addition, the large bowl had the smallest value of ISSoot and ISNO. So, it was determined that the large bowl cylinder geometry was optimized for a swirl nozzle injector.