Post Injections for Soot Reduction in Diesel Engines: A Review of Current Understanding

Abstract

This work is a technical review of past research and a synthesis of current understanding of post injections for soot reduction in diesel engines. A post injection, which is a short injection after a longer main injection, is an in-cylinder tool to reduce engine-out soot to meet pollutant emissions standards while maintaining efficiency, and potentially to reduce or eliminate exhaust aftertreatment. A sprawling literature on post injections documents the effects of post injections on engine-out soot with variations in many engine operational parameters. Explanations of how post injections lead to engineout soot reduction vary and are sometimes inconsistent or contradictory, in part because supporting fundamental experimental or modeling data are often not available. In this paper, we review the available data describing the efficacy of post-injections and highlight several candidate in-cylinder mechanisms that may control their efficacy. We first discuss three in-cylinder mechanisms that have been frequently proposed to explain how post injections reduce engine-out soot. Thereafter, to provide a foundation for interpretation of past research, we briefly review basic soot formation and oxidation chemistry, and soot/fluid processes in fuel sprays and engine flows. Next, we provide a comprehensive overview of the literature on the efficacy of post-injections for soot reduction as a function of engine operational parameters including injection duration and dwell, exhaust-gas recirculation, load, boost, speed, swirl, and spray targeting. We conclude by identifying major remaining research questions that need to be addressed to help achieve a design-level understanding of the mechanisms of soot reduction by post injections.

Towards Identifying Flame Patterns in Multiple, Late Injection Schemes on a Single-Cylinder Optical Diesel Engine

“…The indicated mean effective pressure (IMEP) was found in the range of 2.1 -2.6 bar for all the post injection cases, while the base case was found to be approximately 1.8 bar (due to the overall less fuel amount per cycle). The low-load, low-speed conditions of the present work are in line with the literature, where is has been suggested that such conditions are suitable for the study of soot reduction strategies since, in general, post injection is less effective as a strategy for reducing soot at higher loads (O'Connor and Musculus, 2013a). Moreover, the PPC-like operation mode is believed to have better applicability in the lower part of the engine load range, while retaining the use of conventional diesel fuel with current fuel injection equipment of passenger cars (Bakker et al, 2014).…”

Transient Flame Development in a Constant-Volume Vessel Using a Split-Scheme Injection Strategy

“…Post-injections can (re-)introduce high-temperature reactions in a region near the injector or increase local mixing again, promoting complete oxidation of the soot formed earlier [11]. The scheduling of post-injections is particularly important and seems to be hardware dependent, but reduced engine-out soot levels have been reported, without necessarily compromising on fuel economy [7]. 4.…”

“…With an optimized configuration of injection and dwell timing, injection of small quantities prior to the principal fuel delivery has been proven capable of reducing fuel consumption, particulate matter, and CO and NOx emissions in different load regimes [1][2][3][4][5]. Supplementary injections after the main delivery, known as post-injections, may significantly reduce engine-out soot and unburnt hydrocarbon (UHC) emissions [3,6], but the reported efficacies of this differ significantly [7].…”

Effects of injector valve movement on diesel spray characteristics under short full valve opening

“…A combination of ultraviolet-visible optical diagnostics was applied to the combustion chamber of a diesel engine in order to study the multiple fuel injection (Mancaruso, et al, 2008). The understanding of post injections for soot reduction in diesel engines was reviewed (O'Connor and Musculus, 2013).…”