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
NOX, 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.
