| NOX and particles emitted from diesel engine have been the major sources of air pollution in China. In order to reduce NOX and particles emission and meet emission regulations, high pressure fuel injection, exhaust gas recirculation(EGR) and particle catalytic oxidation/capture technology(POC/DOC/DPF) are utilized in light vehicle diesel engine. As significant in-cylinder purification action for NOX, EGR reduces the peak combustion temperature by increasing the specific heat value and diluting oxygen concentration of the mixture gas. Nevertheless, the declines of excess air coefficient and oxygen concentration cause the increase of particles.In present research, the formation and evolution of diesel particles with introduction of EGR were well investigated. And seven chapters were included in this dissertation about the influence of the EGR rate, exhaust gas component and temperature on particles. To be specific, the impact of EGR on combustion process of diesel engines, the formation path of particle polycyclic aromatic hydrocarbons precursors were respectively studied, along with solid dynamic processes, in which particle collision and coalescence were included, were further explored. The evolution of particle functional groups, oxidative activity and micro physical characteristics were discussed particularly. A combination of bench test and numerical simulation was implemented, and in experiment, several methods were applied, namely chemical kinetics analysis, thermogravimetric analysis, SEM, X-ray near-edge absorption spectroscopy, Raman spectroscopy and X-ray small-angle scattering.The components of exhaust gas dominated the combustion process significantly. Specifically, CO2 in exhaust gas had more impacts on diesel combustion process than N2. It was a principal component blocking the combustion reaction mainly through the inhibition of H2O2 decomposition reaction and reduction of generated OH radicals. During combustion period, CO2 delayed the ignition time of diesel and further blocked the OH radical oxidation of CH2 O. Then transformation from the low temperature to a high temperature of diesel cracking oxidation process was suppressed. Finally the combustion reaction was slowed. The inhibition effect of CO2 for NOX was obviously stronger than N2, which meant it could effectively reduce NOX emission from diesel engines. In addition, N2 led to the increase of size, number concentration and mass concentration of accumulation mode particles with EGR introduced. However, CO2 followed the opposite pattern to N2.The chemical reaction mechanism of n-heptane-PAHs was established in which the formation path of aromatic hydrocarbon was included based on the combustion feature of hydrocarbon fuel. Furthermore, the numerical results of reaction mechanism, namely ignition delay, combustion and intermediate products, along with in-cylinder pressure were verified respectively. It could be observed that, with EGR rate increasing, the peak values of generation amount of four typical PAHs, namely benzene, naphthalene, phenanthrene and pyrene, were all increased. Also the peak times of those were delayed. For the same EGR rate, there was a substantial increase of the peak value of PAHs mole fraction when exhaust gas was replaced by N2. On the other hand, there was an obvious drop of the peak of PAHs mole fraction when exhaust gas was replaced by CO2. With the increase of exhaust gas temperature, the peak values of those PAHs amount were entirely increased. It can be summarized from above that the main factor in the exhaust gas increasing the amount of PAHs are N2 and the exhaust gas temperature. The change of exhaust gas components played an important role.The formation rate and sensitivity analyses were applied to ascertain the main generation path of PAHs after using EGR. It can be summed up that promotion of the benzene was resulted from acceleration of the propynyl polymerization reaction, which was the main reaction path of benzene generation. Moreover, naphthalene was mainly generated by secondary dehydrogenation and adding acetylene reaction of phenyl along with compound reaction between phenyl and vinyl acetylene. Besides, there were two reaction paths of phenanthrene. One of them was that at low temperatures, it was mainly obtained by reaction of phenanthrenyl A3-4 and H, which was mainly generated by C2H2 phenanthryl, vinyl naphthyl A2C2 HA * and A2C2 HB * reaction. And another path was that biphenyl(P2-) reacted with acetylene, which occurred at high temperature. After EGR was introduced, the reaction between phenanthryl A3-4 and H was the main path to generate phenanthrene. The sensitivity coefficient of major elementary reactions, in which pyrene was generated, was nearly not modified even with EGR and pyrene was mainly generated from the reaction between A3-4 and acetylene.Capture model between coarse and fine particles was established, in which collision and coalescence were included. In addition moment method was utilized to solve the General Dynamic Equation for particles in free molecule regime. During the particle collision and coalescence process, the higher the size dispersion was, the greater the collision frequency became. Meanwhile, when the initial particle size distribution of the in-cylinder particles widened, with the raise of the initial particle dispersion at the beginning of collision and coalescence, the decay rate of particle number increased. However, the average volume and amount of the particles followed the reverse pattern. Presumably, mature particles were reentered into the cylinder with exhaust gas, which caused increase of initial dispersion of the particle system; hence the collision frequency was increased. As a result, the decay of particle number was accelerated and the average volume of diesel particles was increased.Surface area and contact area of the particles became larger for particles of more irregular shape. Under the effect of Brown collision and coalescence, with the increase of irregular degree of particle shape, particle number declined rapidly and the rate of increase in the average volume was greater. When the mass of fine particles was lower, the number and volume of particles were influenced mainly by trapping effect of coarse particles for fine particles. With the increase of fine particle mass ratio, the collision and coalescence effect were gradually enhanced. Presumably, mature particles were reentered into the cylinder with exhaust gas causing increase of irregular degree, hence coarse particles trapping effect for fine particles was strengthened resulting in the increase of collision frequency.The near-edge energy of carbon functional groups in diesel particle was in the range of 284 ~ 292 ev, which suggested a typical graphite structure in diesel particle. The main functional groups in diesel particles were oxygen-free aromatic carbon, graphite carbon functional groups and oxygen-containing quinone, carboxyl, carbonyl, phenol/ketone functional groups, as well as aliphatic C-H functional groups. With the increase of EGR rate, the oxygen-free functional groups were gradually decreased. Otherwise, oxygen-containing functional groups and aliphatic C-H functional groups tended to increase, which meant that EGR stimulated the increase of oxidative activity particles. CO2 in the exhaust gas increased the relative amount of oxygen-containing and aliphatic C-H functional groups, reducing the oxygen-free functional groups as well. Nevertheless, N2 had minor impacts on relative amount of the functional groups. With the increase of exhaust gas temperature, the oxygen-free functional groups in particles significantly were raised, On the contrary, oxygen-containing and aliphatic C-H functional group were decreased.Thermogravimetric analyzer was employed to investigate the mass of volatile fraction and soot. In addition, oxidation dynamic parameter and ignition temperature and combustion index were calculated to study the influence of EGR on oxidative activity. With the increase of EGR rate, the mass of soot was reduced, while the mass of moisture and volatile organic compounds were increased. Also the particle volatile precipitation temperature TSOF1 was gradually decreased; When CO2 was adopted to replace the exhaust gas, TSOF1 reached the minimum value, besides, CO2 in exhaust gas was the dominant component leading to the change of soot and volatile organic compounds when EGR rate was raised; On the other hand, when exhaust gas temperature was raised, TSOF1 reached the maximum value and components of the particles have the opposite variation to EGR rate increasing. Whereas, the trends of volatiles combustion temperature TSOF2, soot ignition temperature Ti and burnout temperature Th are consistent with precipitation temperature. The particle activation energy was approximately linear to EGR rate. With the increase of EGR rate, combustion and burnout performance index of particles were gradually increased, and there was an obvious drop in activation energy which meant that less oxidation reaction energy was needed and combustion efficiency was improved. To summarize, CO2 was the most significant component for improving oxidative activity and combustion performance. On the contrary, exhaust gas temperature affected the oxidation activity and combustion performance adverselyLaser Raman spectroscopy was employed in the investigation of the influence of EGR on graphitization degree and carbon atom distribution in graphite structure. With the increase of EGR rate, the number of edge carbon atoms in graphite structure was increased, while the number of inner layer carbon atoms and the graphitization degree were gradually decreased. By comparing the influence of exhaust gas components and temperature, it can be assumed that CO2 dominated the decline of graphitization degree with EGR rate increasing, while with the exhaust gas temperature increasing, the number of inner layer and edge carbon atoms were increased and reduced respectively, also graphitization degree was promoted.Test methods of scanning electron microscopy, atomic force microscopy and X-ray small-angle scattering were employed to investigate the effect of EGR on morphology of particle. As the EGR rate and exhaust gas temperature were raised, the size of particle cluster was increased, and the statistical mean distance between particles was decreased. In addition, the interval size and aggregate number were both decreased. Meanwhile, the degree of particle density was increased and therefore the spatial structure was more compact. Structural rigidity was also enhanced. On the other hand, inter-particle agglomeration forces were increased. The force action type transferred from the liquid bridge force to a combination of liquid bridge force and van der Waals force. Gradually van der Waals forces played a major role among agglomeration forces. What’s more, compared with the exhaust gas and pure N2, CO2 had the function of reducing particle cluster size, compactness and rigidity of particle structure.The influence of EGR on internal structure of the carbon particles was researched by transmission electron microscopy along with image processing method. The layer spacing, curvature and fractal dimension were gradually raised with EGR introduced, while crystallite size was gradually decreased representing micro structure of strong oxidation ability. As a dominant component for micro structure of carbon particle, CO2 was beneficial to improve oxidation ability of carbon particles, however, the exhaust gas temperature had adverse effect. |
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