Search

In modern diesel engines, the exhaust soot primarily comes from the main-injection combustion and post-injection combustion. Therefore, to reduce the diesel soot emissions, it is essential to better understand the soots generated from the main-injection combustion (main-soot) and from the post-injection combustion (post-soot). This work focused on the properties of the main-soot and post-soot during the combustion process, including the primary particle size, nanostructure and soot mass. The in-cylinder soot samples were obtained using a self-developed total cylinder sampling system, and the primary particle size and nanostructure were determined using high-resolution transmission electron microscopy. The isolation of the post-soot was achieved by adding dimethyl ether to the intake gas instead of the real main-injection to create a simulated main-injection combustion environment for post-soot formation. Combustion analysis and numerical simulation results showed that the simulated combustion environment for post-soot formation generated by the DME combustion was very similar to that generated by the real main-injection combustion. During the combustion process, although the main-soot and post-soot exhibit similar variations in the primary particle size, the maximum primary particle size of the post-soot is smaller than that of the main-soot (23.38 nm for the main-soot and 20.51 nm for the post-soot). The main-soot and post-soot show almost the same trends in the nanostructure, as characterized by the fringe length, separation and tortuosity, throughout the combustion process. The introduction of the post-injection accelerates the reduction of the primary particle size of the main-soot and the increase in the structural order of the main-soot. Because a large number of the main-soot particles are oxidized during the post-injection combustion, the post-soot accounts for a considerable proportion in the engine-out soot (i.e., 42%). [ABSTRACT FROM AUTHOR]

In this work, the role of volatile organic compounds (VOCs) in soot oxidation and fragmentation was investigated in a lean, premixed CH 4 flame. An aerosol generator was used to homogeneously disperse soot with and without VOCs (termed Soot-W and Soot-WO, respectively) into the flame. Because minimal soot was formed in the flame itself, information on the fragmentation of the Soot-W and Soot-WO during the oxidation process was readily captured to determine the role of VOCs in soot fragmentation. The integrated number concentration of fine mode particles and the degree of particle aggregation show that the presence of VOCs suppresses soot aggregate fragmentation, primarily by the mitigating consumption of the neck sites between primary particles due to the decreased specific surface area. The actual oxygen diffusion route and effective pore width between the crystallite structures were determined using the image analysis technique. To determine the characteristic regime of oxygen diffusion, the distribution of effective pore width was studied using high-resolution transmission electron microscopy and image analysis. The estimated effectiveness factor indicates that the presence of VOCs lowers the propensity of internal burning via intra-particle oxygen diffusion, and reduces the extent of primary particle fragmentation, as evidenced by a relatively smaller peak of ultrafine particles. The analysis of the nanostructure of neck sites vs. the bulk discovers significantly more disordered and thus reactive structure at neck sites, explaining the prevalence of aggregate fragmentation over primary particle fragmentation. The finding of the role of VOCs in oxidation-driven fragmentation is beneficial to the optimization of existing models for predicting the number concentration of fine and ultrafine particles. Furthermore, a better understanding of this role may aid in the control of the most harmful fine and ultrafine particle emissions. [ABSTRACT FROM AUTHOR]

The properties and oxidation of in-cylinder soot from a diesel engine fueled with n -heptane were explored under the simulated exhaust gas recirculation (EGR) condition, via CO 2 addition to the intake air of the engine. A self-developed total cylinder sampling system was employed to obtain the soot samples at the EGR ratios of 0 and 25%, and this study only focused on the soot oxidation that occurred in the soot oxidation-dominated (SOD) phase. Computational fluid dynamics (CFD) simulations were performed to determine the local combustion characteristics related to soot oxidation. Soot oxidation rates were evaluated based on in-cylinder soot mass traces and CFD results. Soot properties were characterized in terms of nanostructure, carbon chemical state, and surface functional groups. Detailed analysis of the soot mass indicates that EGR addition enhances the mass-based specific rate of soot oxidation. However, CFD results show that EGR addition reduces the flame temperature and the concentrations of O 2 and OH, and consequently decreases soot surface oxidation rates by O 2 and by OH. The characterization of soot properties shows that EGR decreases the degree of structural ordering and results in the formation of more oxygenated and aliphatic C–H groups on the soot surface. This alteration in physico-chemical properties favors soot oxidation during the combustion process, and results in an increase in the mass-based specific rate of soot oxidation in the SOD phase. [ABSTRACT FROM AUTHOR]

The mechanism for the C2H3+ CH3CH2OH reaction has been investigated by the CCSD(T)/cc-pVTZ method based on the geometries of stationary points optimised at the B3LYP/6-311++G(d, p) level of theory. Five reaction channels have been identified for the production of C2H4+ CH2CH2OH, C2H4+ CH3CH2O, C2H4+ CH3CHOH, C2H3OH + CH3CH2,and C2H3OCH2CH3+ H (denoted as channels (1)–(5), respectively). The potential barrier heights of the corresponding transition states TSR/P1, TSR/P2, TSR/P3, TSR/P4 and TSR/P5 were predicted to be 12.40, 8.50, 8.11, 38.38 and 42.21 kcal/mol at the CCSD(T)//B3LYP level of theory, respectively. The rate constants and branching ratios for the three lower energy H-abstraction channels were calculated using conventional transition state theory with Eckart tunnelling correction at the temperature range 300–2500 K. The predicted rate constants have been compared with the available literature data. Both the forward and reverse rate constants have positive temperature dependence, and the tunnelling effect is only important at low temperatures. The branching ratio calculation shows that channel (2) is predominant and channel (3) is less competitive, while channel (1) is almost neglectable at low temperatures. However, all three channels are important in high-temperature region. [ABSTRACT FROM PUBLISHER]

The mechanism for the hydrogen abstraction reaction C 2 H 3 + CH 3 CHO has been investigated by the CCSD(T)/cc-pVTZ method based on the geometries of stationary points optimized at the B3LYP/6-311++G(d,p) level of theory. Two abstraction channels have been identified for the production of C 2 H 4 + CH 2 CHO and C 2 H 4 + CH 3 CO. The potential barrier heights of the corresponding transition states TSR/P1 and TSR/P2 were predicted to be 9.47 and 5.95 kcal/mol at the CCSD(T)//B3LYP level of theory, respectively. The rate constants and branching ratios for the two H-abstraction channels were calculated using conventional transition state theory with Eckart tunneling correction at the temperature range 300–2500 K. The predicted rate constants have been compared with available literature data. Both the forward and reverse rate constants have positive temperature dependence and the tunneling effect is only important at low temperatures. The branching ratio calculation shows that the channel producing C 2 H 4 + CH 3 CO remains predominant throughout the entire studied temperature range. [ABSTRACT FROM AUTHOR]

ABSTRACT The mechanism for the C2H3 + CH3OH reaction has been investigated by the Gaussian-4 (G4) method based on the geometric parameters of the stationary points optimized at the B3LYP/6-31G(2df, p) level of theory. Four transition states have been identified for the production of C2H4 + CH3O (TSR/P1), C2H4 + CH2OH (TSR/P2), C2H3OH + CH3 (TSR/P3), and C2H3OCH3 + H (TSR/P4) with the corresponding barriers 8.48, 9.25, 37.62, and 34.95 kcal/mol at the G4 level of theory, respectively. The rate constants and branching ratios for the two lower energy H-abstraction reactions were calculated using canonical variational transition state theory with the Eckart tunneling correction at the temperature range 300-2500 K. The predicted rate constants have been compared with existing literature data, and the uncertainty has been discussed. The branching ratio calculation suggests that the channel producing CH3O is dominant up to about 1070 K, above which the channel producing CH2OH becomes very competitive. [ABSTRACT FROM AUTHOR]