Study of injection strategies on a Diesel engine: Experimental evaluation of a split injection combustion and proposal of a new combustion chamber design

The present work’s goal is to experimentally define the advantages in terms of NOx and Particulate emissions and thermal efficiency of a split injection, namely an unconventional combustion strategy implemented on a diesel engine, here called Partially Premixed Diesel Combustion (PPCI). The hardware of the experimental setup consists of a single-cylinder, four-stroke, supercharged, direct injection, diesel engine. The software utilized for the computational simulations of the combustion process is AVL Fire, while engine-out data is reprocessed with the help of an Excel code based on the heat balance equation. Initially, a comparison of the Partially Premixed Diesel Combustion with the Conventional Diesel Combustion is carried out at three different engine loads (low, medium and high). A pilot-pilot-main injection strategy is selected for the Conventional Diesel Combustion. A triple injection is implemented for the Partially Premixed Diesel Combustion, resulting in a two-stage heat release: the first one typically produced by a premixed flame, the second one by a diffusive flame. Thermal efficiency, noise and emissions are monitored along with the change of the combustion phase and with the implementation of Exhaust Gas Recirculation. Finally, simulations are run on a new proposed combustion chamber design which is coupled with a double-injector system and has the goal of spatially separate the premixed flame and the diffusive flame, promoting a better in-cylinder air utilization. Improvements in terms of emissions and thermal efficiency are generally achieved by the PPCI only at low and medium loads, while noise levels remain comparable with the conventional case. Simulations on the new combustion chamber suggest that air utilization improvement locally takes place.