To satisfy future emission classes, e.g. EU6c, the particulate number (PN) of Direct-Injection Spark-Ignition (DISI) engines must be reduced. For these engines, different components influence the combustion process and thus also the formation of soot particles and deposits. Along with other engine components, the injector nozzle influences the particulate number and deposits in both fuel spray behavior and nozzle “tip wetting”.
In case of non-optimized nozzle layouts, fuel may impinge on the piston and the liner in an unfavorable way, which implies low-oxygen diffusive combustion by retarded vaporizing wall films. For the tip wetting, wall films are present on the actual surface of the nozzle tip, which is also caused by unadapted nozzles. For non-optimized nozzles, the latter effect can become quite dominant.
This paper deals with systematic nozzle development activities towards low-deposit nozzle tips and thus decreasing PN values. To reduce formerly high PN values, the fuel wall film on the nozzle tip must be made smaller. For this purpose, different geometrical designs of the inner nozzle geometry, the nozzle holes and the tip shape are compared and analyzed in detail to explain the relationship between geometrical nozzle features and both spray behavior and wall films. The focus of the study is to study the role of cavitation tendency within the nozzle and its impact on PN emissions. To achieve this a suitable measurement technique has to be found and an evaluation procedure to be developed.
By means of a systematic study, engine results are compared with optical and hydraulic spray measurement techniques. Based on the fundamental findings, the potential of and also the limitation of predicting the engine results by lab test bench measurements are presented. Overall, the gained knowledge by the geometrical study is necessary to optimize the next generation fuel systems and to achieve robust solutions for worldwide applications.