Immaturity of soot particles in exhaust gas for low temperature diesel combustion in a direct injection compression ignition engine
Graphical abstract
Introduction
Low temperature diesel combustion (LTC), as investigated in this study, consists of the partially premixed combustion fueled with commercial diesel in a compression ignition engine using early injection timing and an exhaust gas recirculation [1], [2]. Exhaust gas recirculation (EGR) is a promising technology to achieve LTC without significant modification since the recirculated exhaust gas, including carbon dioxide (CO2) and water vapor (H2O), replaces a portion of the fresh air in the intake, which reduces the amount of oxygen and its concentration. Along with the early injection timing, this reduced oxygen concentration prolongs the ignition delay, which gives the mixture of fuel and oxidizer sufficient time to form a partially premixed charge in the cylinder. After ignition, the low oxygen concentration and high heat capacity substances such as CO2 and H2O can support a relatively inactive combustion with gradual heat release [3]. LTC is characterized by a two-staged heat release rate, low-temperature reaction (cool flame), and high-temperature reaction (hot flame), relative to the conventional diesel combustion of four combustion phases [4], [5], [6].
With the increasing stringency of emission regulations, the most beneficial aspect of LTC is the significant reduction of nitrogen oxides (NOx) and particulate matter (PM) [2]. The low combustion temperature can allow the formation rate of NOx to be low. Likewise, the inception of soot particles is not readily permitted because the overall combustion temperature can be lower than the inception temperature, although the temperature in particular areas may be high enough to form incipient soot particles. On the other hand, the oxidation processes of hydrocarbon (HC) and carbon monoxide (CO) are deteriorated by a low combustion temperature, leading to high emissions of these gases in the LTC mode. These unoxidized substances can be related to the formation process of PM particularly.
Many researchers have researched PM in relation to engine operating conditions such as idle, low, and high load. The Argonne group has vigorously studied the structural characteristics of particulate matter such as the diameter of the primary particle, the radius of gyration, and fractal dimensions by means of transmission electron microscopy (TEM) in single-cylinder heavy duty engines [7], [8] and four-cylinder light duty engines [9], [10], [11]. Typical particulate matter images showed chain-like agglomerates with a highly crystalline structure at high load. At light load, however, their images gave the appearance of a nebulous and amorphous structure, which is regarded as the first observation of precursor particles [12]. The particulate matter was suspected to contain a significant amount of soluble organic fractions, or other liquid phase chemicals [8]. Soot particles using TEM under various engine operations were investigated [13], [14]. It was found that the dependence of the categorization on the engine load was not evident [15]. The size of the primary particle was measured using microscopy, and the size of agglomerate evaluated by radius of gyration based on the microscopy was compared to the mobility measurement [16], [17], [18], [19], [20]. Soot structures have been studied with respect to the oxidation process, usually occurring in the after-treatment system, and they reported the internal burning mode of soot at an EGR rate of 20%, which is a far faster oxidation mode than the external shrinking mode [21], [22].
Particulate matter under LTC with a large amount of EGR has been investigated comprehensively, and it was found that the size distribution of particles in LTC mode was shifted to a smaller range based on the results from a scanning mobility particle sizer [23]. To explain this shift, three possible agglomerate models were suggested: clusters with small primary particles, fewer primary particles with short chains, and very large particles. TEM-based morphology analysis supported the first model in which smaller particles form a fractal aggregate [24].
In a burner flame, highly matured soot aggregates can also be seen in the region beyond the flame front while incipient soot or precursor particles (young soot particles) can be found at low flame heights [25], [26]. However, particulate matter from internal combustion engines contains chemical compounds such as soluble organic fractions originating from the engine lubricant and fuel. These compounds, which are absent in the burner flame, can be a challenge, leading to efforts directed at removing them through solvents, evaporation, or gasification. The comparison of soot particles from burner flames with PM from internal combustion engines was performed with elemental analysis in terms of black carbon fraction along with size and morphological investigation by TEM and a mobility particle sizer [27].
It is recognized that the inhalation of the soot particles makes an impact on the potential human health [28]. Several studies have demonstrated the increased toxicity or interstitialization of ultrafine particles compared to fine particles of the same material [29]. Since these particles showed liquid-like properties, the nanometer-sized particles draw our attentions to investigate the adverse effect on human health epidemiologically. In terms of the optical constant, the absorption coefficient of these particles is larger than that of the matured soot [30]. Because the nanometer-particles show liquid-like properties and are found to be semi-transparent in TEM from diesel ignition engines, the similarity of combustion environment to form the soot particles is required to be investigated further.
In terms of soot evolution and maturity, many studies have not been performed to compare soot from the internal combustion engine, especially compression ignition engine, with soot from the burner flame. In this study, the comparison was carried out by the chemical composition as a measure of the soot maturity after separating the soot particles from the organic fraction. Two combustion modes, the conventional combustion and LTC were applied to assume the similarity of the environment for matured aggregates, and nascent or young soot particles, respectively. A temperature-based similarity is plausible, given that temperature plays a significant role in soot particle formation. In addition, thermogravimetric analysis was conducted to examine difference in the soot particle from these two combustion modes during oxidation process of volatile organic fraction and carbonaceous soot particles. TEM was also employed to investigate and compare the morphological characteristics of the particulate matter between these two combustion modes.
Section snippets
Experimental apparatus and conditions
A single-cylinder, direct injection compression ignition engine was used to investigate the characteristics of soot particles under conventional and low temperature diesel combustion. A schematic of the experimental setup is shown in Fig.␣1, and the engine specification is listed in Table␣1.
Commercial diesel fuel was pressurized through the filter with a high-pressure pump in a common-rail system, where the pressure was adjusted by a pressure controller (Zenobalti Co., ZB-1200). The timing and
Atomic ratio of carbon to hydrogen as a measure of the maturity of soot particles
Fig.␣2 shows the mass reduction and its derivative of thermogravimetry (DTG) using TGA with nitrogen as the purge gas with the temperature increased up to 400 °C at a rate of 10 °C/min. Compared with the use of ambient air during the TGA experiment, the adsorbed and condensed fractions of volatile hydrocarbon could be removed without the oxidation when nitrogen as the purge gas was applied to TGA. While the mass reduction occurred during the desorption process, DTG was gradually reduced before
Conclusions
The particulate matter (PM) from the conventional combustion and low temperature diesel combustion (LTC) was comprehensively evaluated by using thermogravimetric analysis (TGA), elemental analysis and transmission electron microscopy (TEM). With the efforts to exclude the volatile organic fractions from PM, the soot particles, which is carbonaceous core substance, are extracted by the TGA experiment with nitrogen up to 400 °C. The result of elemental analysis applied to these soot particles
Acknowledgements
Support for this research was provided by Korean Government, Ministry of Knowledge Economy, and Department of Transportation (10033440). The authors would like to thank Dr. Jungseo Park in Zenobalti Co. for knowledge and technical support to provide the highly advanced control equipment.
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