An Experimental Study of Cluster Nozzles for DI Diesel Engine

In a conventional Diesel engine, air is gradually drawn 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 center of the spray is overly rich, resulting in smoke, while a stoichiometric mixture is formed in the surrounding area, resulting in a high NOx concentration. Based on the Diesel concept it is practically impossible to totally 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 pollutants could be reduced simultaneously without getting into the well known soot-NOx tradeoff. In order to develop a low emission engine, research is necessary to come up with a new combustion strategy for Diesel engines, which includes the use of cluster nozzles. Conditions for low raw particulate emission are: suitable start of injection, lean air/fuel mixture, sufficiently high temperatures in the combustion chamber and a sufficient retention time with these conditions. This could be achieved by use of cluster nozzles and a careful tuning of the air/fuel mixing parameters of the engine piston geometry. Decreasing the hole-size improves mixing in the center of the spray and therefore the soot production goes down tremendously. It has been observed, that an optimum number of holes exists (6 to 8) and that a certain swirl level is necessary. Based on this experience the cluster concept was developed. The cluster nozzles can also be used for improved homogenization of in-cylinder charge so as to enable a partly homogenous mode of Diesel combustion. In this study, three cluster designs are investigated through engine measurements. They were tested in a single-cylinder engine with CRI 3.3 piezo injectors under part-load conditions for a partly homogenous mode of Diesel combustion, and also under high load conditions for conventional Diesel combustion. The experimental studies give some detailed information on how cluster nozzles could help in lowering soot formation and thereby provide more freedom to reduce other pollutants.