Use of a Multi-Zone Combustion Model to Interpret the Effect of Injector Nozzle Hole Geometry on HD DI Diesel Engine Performance and Pollutant Emissions 2005-01-0367

A major challenge in the development of future heavy-duty diesel engines is the reduction of NO_x and particulate emissions with minimum penalties in fuel consumption. The further decrease of emission limits (i.e., EPA 2007-2010, Euro 5 and Japan 05) requires new, advanced approaches. The injection system of DI diesel engines has an important role regarding the fulfillment of demands for low pollutant emissions and high engine efficiency. One of the injection system parameters affecting fuel spray characteristics, fuel-air mixing and consequently, combustion and pollutant formation is the geometry of the nozzle hole. A detailed experimental investigation was conducted at UPV-CMT using three different nozzle hole types: a standard, a convergent and a divergent one to discern the effect of nozzle hole conical shape on engine performance and emissions. According to the experimental findings, an increase of soot and decrease of NO_x was observed for the divergent nozzle hole compared to the other two. Conventional heat release rate analysis did not show any significant effect of nozzle hole geometry on the combustion mechanism. However, the use of a modified procedure to account for the differences of fuel injection rate between the three nozzles, revealed a slower combustion rate for the divergent nozzle. The results of the experimental analysis motivated the present group to conduct a computational investigation using a multi-zone combustion model to interpret the mechanism behind the different behaviour of divergent nozzle compared to others. The model was used as a diagnostic tool to capture the effect of the three nozzle hole geometries on engine performance and emissions. For this reason an automatic calibration procedure has been developed and applied to estimate model constants to predict engine performance. The only parameter that had to be modified between the three nozzle geometries examined was the air entrainment rate inside the fuel jet. Thus, it was concluded that for the divergent nozzle a lower fuel-air mixing rate occurs compared to the other two nozzle configurations. Using the estimated model constants, an endeavour was made to assess computationally its ability to capture the effect of nozzle hole geometry on emissions. Hence, predictions for soot and NO tailpipe values were made for the three types of nozzles at various engine-operating conditions using the multi-zone model. The analysis revealed that predictions for pollutant emissions are in agreement with corresponding experimental data for all cases examined confirming the fact that nozzle hole geometry affects the mixing rate of injected fuel with surrounding air. Furthermore, the usefulness of the phenomenological model to identify the underlying mechanisms of the different combustion phenomena was acknowledged providing a reasonable explanation for the observed effect of nozzle hole geometry.