A Review of Current Understanding of the Underlying Physics Governing the Interaction, Ignition and Combustion Dynamics of Multiple-Injections in Diesel Engines

This work is a comprehensive technical review of existing literature and a synthesis of current understanding of the governing physics behind the interaction of multiple fuel injections, ignition, and combustion behavior of multiple-injections in diesel engines. Multiple-injection is a widely adopted operating strategy applied in modern compression-ignition engines, which involves various combinations of small pre-injections and post-injections of fuel before and after the main injection and splitting the main injection into multiple smaller injections. This strategy has been conclusively shown to improve fuel economy in diesel engines while achieving simultaneous NO_X, soot, and combustion noise reduction - in addition to a reduction in the emissions of unburned hydrocarbons (UHC) and CO by preventing fuel wetting and flame quenching at the piston wall. Despite the widespread adoption and an extensive literature documenting the effects of multiple-injection strategies in engines, little is known about the complex interplay between the underlying flow physics and combustion chemistry involved in such flows, which ultimately governs the ignition and subsequent combustion processes thereby dictating the effectiveness of this strategy. In this work, we provide a comprehensive overview of the literature on the interaction between the jets in a multiple-injection event, the resulting mixture, and finally the ignition and combustion dynamics as a function of engine operational parameters including injection duration and dwell. The understanding of the underlying processes is facilitated by a new conceptual model of multiple-injection physics. We conclude by identifying the major remaining research questions that need to be addressed to refine and help achieve a design-level understanding to optimize advanced multiple-injection strategies that can lead to higher engine efficiency and lower emissions.