Abstract
In a conventional diesel engine, air is gradually entrained into the fuel spray from the surrounding area. The ignition delay period is short, so combustion starts before the fuel has thoroughly mixed with the air. Consequently, the centre of the spray is overly rich, resulting in soot emission, while stoichiometric mixture is formed in the surrounding area, resulting in a high NOX concentration. Based on a conventional diesel engine concept, it is practically impossible to avoid fuel-rich and stoichiometric pockets totally. Furthermore, the formation of soot and NOX emissions are time-dependent. If the air-fuel mixing time is sufficiently small, both pollutants 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 new combustion strategies for diesel engines. One of the possible strategies is to use the cluster-nozzles. The basic idea behind the cluster nozzles is to provide a faster primary breakup and therefore a better air-fuel mixture formation caused by the smaller nozzle holes, while keeping a comparable penetration length of the vapour phase due to merging of the sprays. Decreasing the hole-size also improves mixing in the centre of the spray and therefore the soot production is lowered tremendously. Based on this experience, four cluster nozzles were investigated for the present work. The cluster nozzles differ in the number and the orientation of the holes. In this study, the cluster nozzles were tested at one specific low-load point of a single-cylinder diesel engine with CRI 3.3 piezo-injectors. In addition, the results of the cluster-nozzles are compared with a conventional seven-hole nozzle. The results indicate that the cluster nozzles have significant effect on the cylinder pressure and pollutants at exhaust. Numerical simulations using the Representative Interactive Flamelet model were also carried out to explain the observations of the engine experiments. The computed results are in good agreement with the experimentally measured data for all the tested nozzles. The computed soot history showed the influence of the orientation as well as of the number of holes of the cluster nozzles. After the validation of the model, a detailed analysis of the soot formation and oxidation for these nozzles was performed. From the results, it was found that the soot formation rates were higher for the horizontally-arranged cluster nozzles compared to the vertically-arranged cluster nozzles. At later crank angles higher oxidation rates for the horizontally-arranged cluster nozzles were the result of the flow fields, pushing the soot from the piston bowl (where most of the soot was located for all the nozzles) towards the centre of the combustion chamber. With increasing the number of holes in cluster nozzles (thus decreasing the hole diameter for a constant flow number), peak in soot formation as well as soot at exhaust valve opening was reduced.
Keywords: diesel engine, cluster-nozzles, NOX, soot, RIF