Abstract
One of the most important parameter that affects diffusion controlled combustion process in diesel engine is air fuel mixing which is mainly controlled by injection. Based on the diesel concept, it is practically impossible to avoid fuel rich and stoichiometric pockets, but the formation of soot and NOx are also time dependent. If the mixing time is sufficiently small, both pollutant could be reduced simultaneously without getting into the well known soot-NOx trade off. In order to develop a low emission engine, research is necessary to come up with a new combustion strategy for diesel engine which includes the use of cluster nozzle. The concept of cluster nozzle is that it is possible to improve the condition of air fuel mixture by changing nozzle configuration. The cluster nozzle, grouping multi smaller holes together, has the potential to improve air fuel mixing in the center of spray. The cluster nozzle is characterized by high number of degree of freedom relate to spray figure, number of spray holes and configuration of holes in a group. Three cluster nozzles were tested with a Bosch CRI 3.3 piezo injector in a single-cylinder Duramax engine and compared with a conventional nozzle. The experiment was conducted under four test points and various rail pressures were applied to find an optimal rail pressure at each test point. Compared to conventional nozzle, ignition delay of cluster nozzle is shorter and the initial rate of heat release and the maximum rate of heat release of diffusion combustion are higher. Also cluster nozzles combined with high injection pressure shows better fuel consumption rate and emission level. The cluster nozzles achieved improved smoke-NOx trade off while maintaining same NOx level with higher EGR. And three different mean swirl ratios were tested because the swirl is an important factor of spray formation and combustion process in combustion chamber. Swirl ratio variation was conducted to find an optimal swirl ratio for cluster nozzle at each test point. Among three different swirl ratio, 0.5, 1.5 and 3.0, better smoke emission was achieved with 1.5 and 3.0 swirl ratio compared with 0.5 swirl ratio. Between 1.5 and 3.0 swirl ratio, even though both resulted similar in smoke emission, 1.5 swirl ratios has lower BSFC and HC level than 3.0 swirl ratio. The findings form those experiments could be helpful for conceptualizing cluster nozzle for application in the whole operation range of diesel engine.
Keywords: Cluster nozzle, Rail Pressure, Swirl, PCCI, Soot.