Experimental Investigation on an Innovative Additive Manufacturing-Enabled Diesel Piston Design to improve Engine-out Emissions and Thermal Efficiency beyond Euro6

In recent years the research on Diesel thermodynamics has been increasingly shifting from performance and refinement, obtained thanks to advanced fuel injection and charging technologies, to ultra-low emissions and efficiency. The last two attributes are key factors for the powertrain competitive-ness in the automotive electrified future, especially in the European market where the pollutant emis-sions are approaching the Euro6D RDE step2 level and the CO2 the value of 95g/km fleet average. In this framework, the present paper describes a research study conducted in cooperation between GM and Istituto Motori, exploring how the application of the most recent additive manufacturing technologies allow complex combustion bowl features enabling optimized combustion process to a level that was not compatible with standard manufacturing technologies. An innovative highly reentrant-sharp-stepped profile featuring radial-lips has been studied and fully characterized by means of a proper designed test rig and experimental campaign. A 0.5l single-cylinder engine coupled to a state of art fuel injection sys-tem has been developed, with the objective to demonstrate the potential of optimized fuel stratification and spray separation enabled by the radial mixing zone concept. The results show an important reduc-tion of the engine-out smoke emissions in comparison to the Euro6B design with no detrimental effects on the efficiency when advanced compact injection patterns and optimized calibration are applied at partial loads. To this aim, sensitivity studies have been conducted to understand the optimal injection protrusion, spray indexing to the ribs and fuel injection parameters. Moreover, the prototype additive manufactured steel piston did not highlight durability concerns after an extensive experimental campaign at partial load conditions.