In recent years the research on Diesel engines has been increasingly shifting from performance and refinement to ultra-low emissions and efficiency. In fact, the last two attributes are key for the powertrain competitiveness in the propulsion electrified future, especially in the European market where 95gCO_2/km fleet average and Euro6D RDE Step2 are phasing in at the same time. The present paper describes some of the most innovative research that GM and Istituto Motori Napoli are performing in the field, exploring how the steel-based additive manufacturing can be used to create innovative combustion bowl features that optimize the combustion process to a level that was not compatible with standard manufacturing technologies. In particular, a novel profile combining highly-reentrant sharp-stepped bowl with inner radial-lips has been studied on a 0.5l single-cylinder engine in conjunction to a state of art 2500bar fast-acting fuel injection system, with the objective to demonstrate the full potential of optimized fuel stratification and spray separation enabled by the Radial Mixing Zone (RMZ) concept. The results confirmed the improved fuel/air mixing and more complete combustion promoted by RMZ, that favors PM oxidation. Indeed, after fine-tuning of injector protrusion and spray indexing to the radial lips, excellent reduction of PM by 25-50% vs baseline Euro6B design was recorded, with no detriment to the indicated efficiency nor engine power density. The novel bowl profile was especially effective when compact injection patterns have been applied at part load. This initial study provides a good reference for further improvements that are already under investigation, including durability of the additive manufactured piston with complex geometry.
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