This thesis investigates the physiochemical characteristics such as size, shape, composition, nanostructure, and oxidative reactivity of the soot particles produced in the diesel engine under various operating conditions. Such characteristics are of importance with regard to regulations, health effects, environmental impacts, performance of diesel after-treatment system and fundamental understanding of the soot formation mechanisms in diesel engines. The experiments were carried out in two different apparatus. First, soot samples were collected from the exhaust of a heavy duty diesel engine. Then, for a closer investigation of soot formation/oxidation mechanisms, direct sampling from a diesel spray flame was performed in a constant volume chamber. A wide range of analysis methods were employed to characterize the soot. The size and the morphology of the soot aggregates were studied by image processing performed on images acquired through high resolution transmission electron microscopy (HRTEM). Image processing was also used to study the nanostructure of primary soot particles. Other techniques such as X-ray diffraction and Raman micro-spectroscopy were also applied to investigate the nanostructure of soot particles. Thermogravimetric analysis (TGA) was carried out to assess the oxidative reactivity and the volatile organic fraction (VOF) of the soot particles.
The results of the study are reported in chapter 3 to chapter 6. The focus of the chapter 3 in on the conventional diesel combustion and the effect of the operating conditions such as fuel injection pressure, injection timing and engine speed on the size and the morphology of the soot particles. It is found that as injection timing is advanced the soot particles become smaller and more compact. The increase in injection pressure, and decrease in engine speed also lead to a similar effect on the soot aggregate size and shape. To investigate the underlying mechanisms, all the studied cases were compared in pairs (totally 10 pairs) and relative dominance of various in-cylinder conditions such as mixing quality, in-cylinder temperature and residence time were discussed. A conclusion was drawn that the size and the morphology of the soot aggregates primarily depend on the quality of mixing. The secondary influential factor is found to be the effectiveness of the in-cylinder late cycle oxidation which is influenced by in-cylinder temperature and the soot residence time.
The soot produced in low temperature combustion (LTC) regime is the subject of the chapter 4. The soot particles produced in partially premixed charge compression ignition (PCCI) regime, which is characterized by high level of exhaust gas recirculation (EGR) and early injection timing is studied, and compared to the soot produced in the conventional diesel combustion regime. The results indicate that the overall morphology of the PCCI soot is similar to the conventional soot, being in the form of chain-like aggregates. The average size of PCCI soot was found to be smaller compared to the conventional soot, and the share of very small particles (smaller than 100nm) which are particularly harmful for human health were significantly higher in PCCI regime compared to the conventional regime (51% as opposed to 27%). The effect of the operating parameters such as injection timing, engine speed and EGR level on PCCI soot was also studied. It was found that when EGR is increased from 45% to 60% the morphology of the soot particles is fundamentally changed. The aggregates in this case were significantly larger and were composed of aggregation of large round masses of carbonations material with nebulous boundaries.
The chapter 5 deals with the nano-structure of the primary soot particles, and its relation with the oxidative reactivity of the soot. For this purpose, Five different injection strategies including single injection and multiple injections with various pilot injection amounts and dwell times were tested with and without EGR (totally 10 cases) while combustion phasing, engine speed, and fuel injection quantity was matched for all cases. Results indicate that for the soot produced under EGR effect, nano-structural order can explain the soot reactivity. However, in the absence of EGR, the reactivity trend cannot be explained by the structural order. It is discussed that a possible reason can be a higher level of in-cylinder oxidation in non-EGR cases indicated by higher level of surface functional groups noticed in XRD analysis. It is suggested that in-cylinder oxidation roughens the soot surface, and enhances the oxidation by increasing the specific soot surface area. Therefore in non-EGR cases the structural order is not the dominant factor governing the oxidative reactivity. The surface roughening also make the surface more accommodating for VOF condensation, which explained the correlation between higher VOF level and higher reactivity noticed in this study.
Chapter 6 aims to investigate how morphology and nano-structure of the soot particles produced in a diesel spray flame evolve due to the rise in temperature/pressure caused by the piston motion. For this purpose, soot sampled from the exhaust line of the diesel engine were compared to the soot directly sampled from the spray flame of the same injector, injecting in an ambient condition identical to the in-cylinder condition, but inside a constant volume chamber where the temperature/pressure rise due to piston motion did not exist. Results show that soot aggregates grow in size, become longer and more fractal along the spray axis. However, the soot particles become smaller, shorter more compact and less fractal when emitted from the engine. This is discussed to be due to the oxidation enhancement by temperature rise caused by piston motion, and the consequent oxidation induced fragmentation of the soot aggregates. Also due to the in-cylinder oxidation, the engine-out soot was found to have smaller primary particles as well. The nano-structural analysis of two samples showed that the lamella size in the engine-out soot is increased compared to the soot originally produced in the spray flame under the effect of temperature rise caused by piston motion.
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