Researchers from Penn State and the University of Michigan have characterized the nanostructure and oxidative reactivity of soot generated by a light-duty turbodiesel engine operating under a dilute, low-temperature combustion process referred to as high-efficiency clean combustion (HECC). Their paper appears in the International Journal of Engine Research.
Earlier work by members of the team (Gregory Lilik and André Boehman) had shown that high cetane number fuel with HECC leads to reductions in all primary pollutant emissions—i.e., THC and CO as well as NOx and PM. (Earlier post.) Less established, however, is how well such dilute combustion processes influence soot formation.
HECC is achieved by operating a common rail diesel engine with a single early fuel injection event under increased injection pressure and with ~50% exhaust gas recirculation (EGR). The result is an extended ignition delay where the lean fuel–air charge is partially premixed. Subsequent combustion produces lower in-cylinder temperatures, compared to conventional combustion. An adverse effect of advanced diesel combustion is incomplete combustion that manifests as unburned hydrocarbons (UHCs) and carbon monoxide. Elevated UHC increases the fraction of organics that can adsorb and condense onto soot, the combination being classified as PM.
… A number of studies have considered the impact of combustion conditions on PM emissions and soot characteristics, including LTC and, more generally, advanced combustion strategies. … The unique feature of the work presented here is that oxidative reactivity for the LTC soot is linked to the nanostructure of the primary soot particles through quantitative analyses of high-resolution transmission electron microscope (HRTEM) images of the soot particles.—Yehliu et al.
In the study, the team used two conventional diesel combustion (CDC) modes and two HECC modes. The researchers used thermogravimetric analysis (TGA) to determine the oxidative reactivity and volatile organic fraction (VOF). Morphology and nanostructure os the soot were studied by transmission electron microscopy (TEM).
The researchers found that:
Operating in an advanced combustion mode can increase the relative VOF between two and three times that of CDC. Among the four conditions presented in the study, the highest VOF in the PM is from HECC B mode (30.6%)—3.7 times of the VOF in the PM from CDC B mode (8.2%).
With an increase in VOF from advanced diesel combustion, the soot gains an order of magnitude increase in oxidative reactivity. HECC B mode soot sample has the greatest derived rate constant for soot oxidation for the first 30 min, 1.92 x 10-6 1/Pa/min, while CDC B mode soot sample has the smallest rate constant, 4.77 x 10-7 1/Pa/min.
The HECC soot samples show disorganized fullerenic nanostructure characterized by high levels of tortuosity, which characterizes the high premixing achieved by the HECC mode.
An advanced diesel combustion mode, such as HECC, can be synergistically coupled with exhaust aftertreatment systems potentially to produce near-zero particulate emissions.
|Skeletonized images overlaid on selected regions of four TEM images: (a) HECC A, (b) HECC B, (c) CDC A and (d) CDC B. Yehliu et al. Click to enlarge.
… in this study, the nanostructure characteristics of the primary soot particles are consistent with the well reported trend that reduced order correlates with increased oxidative reactivity, and that combustion under a HECC mode yields particles with more disorder and higher reactivity in comparison with particles generated by CDC modes. These observations are also consistent with the limited amount of published work on the characteristics of PCCI and LTC soot particles. The impacts of other advanced combustion operating modes on soot characteristics remain to be explored, in addition to verifying that these trends in soot characteristics are consistent across a broad range of diesel engine designs and displacements.—Yehliu et al.
Kuen Yehliu, Gregory K Lilik, Randy L Vander Wal, Chenxi Sun, André L Boehman (2016) “Impacts of advanced diesel combustion operation on soot nanostructure and reactivity” International Journal of Engine Research doi: 10.1177/1468087416659947