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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]

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]

Abstract Post injection has significant benefit in the reduction of diesel soot emissions. Therefore, there is a need to understand the effect of post-injection strategy on soot physicochemical properties and reactivity because they play an important role in soot oxidation process that governs the final soot emissions. This work focuses on the impact of post injection on the physicochemical properties and reactivity of diesel in-cylinder soot using a main plus post injection (M*P) and a single injection (M) strategy. The soot was sampled by a developed total cylinder sampling system, and the dividing points of soot formation-dominant and oxidation-dominant phases were used for studying the impacts of post injection on the characteristics of in-cylinder soot. The physicochemical properties of the soot samples, including primary particle size, nanostructure, carbon chemical state and surface functional groups, were characterized. The soot reactivity was evaluated in terms of peak temperature, burnout temperature and apparent activation energy. In the oxidation-dominant phase, the M*P soot initially possesses smaller primary particle size, shorter fringe length, larger tortuosity, lower sp2/sp3 hybridization ratio of carbon atoms and higher content of aliphatic C H groups than the M soot. The beneficial influence of physicochemical properties on soot reactivity when using post injection is validated by the thermogravimetric data, which shows that the M*P soot is more reactive than the M soot at the onset of the oxidation-dominant phase. In the M*P case, the soot generated from the main-injection combustion has lower reactivity than the soot from the post-injection combustion after they experience the soot formation-dominant phase. The results indicate that the use of post injection leads to in-cylinder soot with physicochemical properties that favor reactivity. The enhancement of reactivity means that the soot will be more readily oxidized in the subsequent combustion process, and consequently contributes to a reduction in final soot emissions. [ABSTRACT FROM AUTHOR]

This paper reports the effect of post-injection on pollutant emissions from a four-cylinder, direct-injection diesel engine fueled with biodiesel. Characteristics of exhaust pollutants were measured and evaluated for different main-post intervals (MPIs) and post-injection rates (PIRs). Measurements included emissions of carbon monoxide (CO), total hydrocarbons (THC), nitrogen oxides (NO x ) and particulate matter (PM), the particle number concentration distribution, and the reactivity and graphitization degree of the soot particles. Increasing the MPI or PIR increased the emissions of CO and THC but decreased those of NO x . At both low and high MPI values, higher PM emissions and particle number concentrations were observed for the 12 and 20% PIRs. At a PIR of 4%, PM emissions and particle number concentration increased with the increase in MPI. The size distributions of exhaust particulates exhibited a trimodal character under the applied operating conditions. The particle geometric mean diameter decreased with the increase in MPI, probably as a result of an increased yield of soluble organic compounds. Post-injection significantly impacted the reactivity of emitted soot particles, as evidenced by changes in the graphitization degree of soot. [ABSTRACT FROM AUTHOR]

We investigate effects of post injection strategies on regular exhaust emissions and soot particulate matters on in-cylinder emission control. A trade-off relation indicates that CO and THC emissions increase as post injection timing delays, but NOx emission significantly decreases when crank angle intervals increase because of cylinder volume expansion and heat transfer. In PM emission, total particle concentration and particle size distributions indicate that, at early post injection timing, SOF emission is relatively low but soot emission is relatively large caused by high temperature and local lean oxygen conditions. As crank angle intervals increasing, soot emission decreases quickly and becomes insensitive to the post injection timing. When SOF emission has an obvious increasing trend, there is a lowest level for the PM emission at intermediate post injection timing. Moreover, variations for total particle concentration are similar with soot emission at different post injection strategies. Particle size distribution result illustrates that PS is insensitive to post injection timing at different post injection strategies. [ABSTRACT FROM AUTHOR]

This work compared the surface functional group (SFG) types and concentrations and sp 3 /sp 2 hybridization ratios of in-cylinder soot samples generated by a heavy-duty diesel engine when employing n -heptane and a toluene/ n -heptane mixture (20% toluene by volume) as the fuels. In-cylinder soot samples were obtained from a total cylinder sampling system, and the SFGs and sp 3 /sp 2 hybridization ratios were analyzed using Fourier transform infrared and X-ray photoelectron spectroscopy. Despite the differences in fuel formulation, both n -heptane and n -heptane/toluene soot exhibited similar trends in terms of changes in the SFGs concentrations and sp 3 /sp 2 hybridization ratios during the combustion process. However, the addition of toluene to the n -heptane was found to increase the concentrations of all SFGs as well as the sp 3 /sp 2 hybridization ratio. The C OH and C O group concentrations exhibited a bimodal distribution for both the n -heptane and n -heptane/toluene soot throughout the combustion process, with the concentrations peaking in the premixed and diffusion combustion phases, respectively. In contrast, the relative amounts of aliphatic C H groups decreased in the premixed combustion phase, increased in the early diffusion combustion phase, and then decreased in the subsequent combustion phase. The sp 3 /sp 2 hybridization ratios obtained from both fuel soot were observed to initially decrease, then to increase before a decrease during the combustion process. There was a definite correlation between the sp 3 /sp 2 hybridization ratio and the relative concentration of aliphatic C H groups. [ABSTRACT FROM AUTHOR]

This work studied the evolution of in-cylinder polycyclic aromatic hydrocarbons (PAHs) in a diesel engine fueled with n -heptane, and explored the effects of adding toluene to n -heptane on in-cylinder PAHs. In-cylinder PAHs were sampled in a direct injection diesel engine using a total cylinder sampling system. PAHs were analyzed by gas chromatography–mass spectrometry with programmed temperature vaporization in the solvent vent mode. For n -heptane, the mass of total in-cylinder PAHs ( Σ M PAHs ) increased in premixed and late diffusion combustion phases, and decreased in early diffusion and late combustion phases. Among the 16 PAHs detected, Naphthalene (Nap) was the most abundant. PAHs with two benzene rings and one five-membered ring, such as Acenaphthylene, Acenaphthene and Fluorene, were also plentiful, implying that PAHs with five-membered ring could play an important role in the growth of in-cylinder PAHs. When adding 20 vol.% toluene to n -heptane, the evolution of ΣM PAHs was varied. The Σ M PAHs for n -heptane/toluene increased in premixed and early diffusion combustion phases, and decreased in late diffusion and late combustion phases. Toluene addition reduced the ΣM PAHs in the premixed combustion phase by up to 48% but increased ΣM PAHs by 30% in the early diffusion combustion phase. In the subsequent combustion phase, the ΣM PAHs was reduced by approximately 67%. For in-cylinder individual PAHs, toluene addition generally reduced the percentages of Nap in ΣM PAHs during the engine combustion process but increased the percentages of larger PAHs, indicating that toluene addition was conducive to the growth of PAHs to form larger PAHs. [ABSTRACT FROM AUTHOR]

Highlights: [•] Reductions in CBC emissions were assessed for CFT diesel. [•] Significant reductions in emissions of total and individual CBCs were obtained. [•] The percentage reduction in the total CBCs decreased with an increase in engine load. [•] The percentage reduction in the total CBCs increased with increase in engine speed. [•] The ozone formation potential of CBCs emitted was strongly reduced with use of CFT. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports the characteristics of surface functional groups (SFGs) and oxidation reactivity of in-cylinder soot during the diesel combustion process and discusses the correlation between SFGs and the oxidation reactivity of soot. Test soot samples were obtained from a total cylinder sampling system. SFGs and the apparent activation energy for the soot oxidation were determined by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis. Results reveal that the concentrations of aliphatic C–H, C–OH and Care dependent on the combustion stages. The ratio of aromatic Chatic C–H groups rapidly increases in the premixed combustion phase, indicating significant loss of aliphatic C–H groups from the soot surface. As the combustion proceeds, the ratio of the two groups gradually increases, following an earlier drop during the diffusion combustion phase. In contrast, the concentration of both C–OH and Cshows a bimodal distribution during the combustion process, with the concentrations peaking in the premixed and diffusion combustion stages, respectively. The apparent activation energies for the oxidation of soot collected under the applied engine operating conditions are observed in the range of 122.12–158.2kJmol−1 at various crank angles, revealing a similar variation to that of aliphatic C–H groups. Aliphatic C–H groups are a more important factor in governing soot oxidation reactivity than the oxygenated SFGs. [Copyright &y& Elsevier]

Abstract: The nanostructure, fractal dimension and size of in-cylinder soot during diesel combustion process have been investigated for a heavy-duty direct injection diesel engine, using a total cylinder sampling system followed by high-resolution transmission electron microscopy and Raman scattering spectrometry. Different structural organizations of in-cylinder soot are found depending upon the combustion phase. It is revealed that both the fringe tortuosity and separation distance decrease as combustion proceeds, while the mean fringe length increases distinctly from 1.00 to 2.13nm, indicating the soot evolution toward a more graphitic structure during the combustion process. The fractal dimensions of aggregates are in a range of 1.20–1.74 at various crank angles under the applied engine operating conditions. As temperature and pressure increase, the fractal dimension decreases significantly to a minimum at the early diffusion combustion stage. The soot particles become more compact again as the fractal dimension increases during the subsequent combustion period. Primary particle sizes start small, go through a maximum in the early diffusion combustion phase and decline again as combustion proceeds. [Copyright &y& Elsevier]

Abstract: This paper presents an investigation of the carbonyl emissions from a direct injection heavy-duty diesel engine fueled with pure diesel fuel (DF) and blended fuel containing 15% by volume of ethanol (E/DF). The tests have been conducted under steady-state operating conditions at 1200, 1800, 2600rpm and idle speed. The experimental results show that acetaldehyde is the most predominant carbonyl, followed by formaldehyde, acrolein, acetone, propionaldehyde and crotonaldehyde, produced from both fuels. The emission factors of total carbonyls vary in the range 13.8–295.9mg(kWh)−1 for DF and 17.8–380.2mg(kWh)−1 for E/DF, respectively. The introduction of ethanol into diesel fuel results in a decrease in acrolein emissions, while the other carbonyls show general increases: at low engine speed (1200rpm), 0–55% for formaldehyde, 4–44% for acetaldehyde, 38–224% for acetone, and 5–52% for crotonaldehyde; at medium engine speed (1800rpm), 106–413% for formaldehyde, 4–143% for acetaldehyde, 74–113% for acetone, 114–1216% for propionaldehyde, and 15–163% for crotonaldehyde; at high engine speed (2600rpm), 36–431% for formaldehyde, 18–61% for acetaldehyde, 22–241% for acetone, and 6–61% for propionaldehyde. A gradual reduction in the brake specific emissions of each carbonyl compound from both fuels is observed with increase in engine load. Among three levels of engine speed employed, both DF and E/DF emit most CBC emissions at high engine speed. On the whole, the presence of ethanol in diesel fuel leads to an increase in aldehyde emissions. [Copyright &y& Elsevier]

A series of complex oxide La1− x Ce x CoO3 catalysts was synthesized at a low temperature through a combustion process, in which x is among 0, 0.1, 0.2, 0.4 and 0.6 corresponding to the quantity of La3+ partial substitution by Ce4+. The catalysts were characterized for phase composition using chemical analysis and X-ray diffraction. The catalytic activity of the catalysts in removal of NO x , total hydrocarbon (THC) and particulate matter (PM) from diesel exhaust gases were examined in detail using temperature-programmed reaction technique. The results show that after partial substitution of La3+ with Ce4+, the oxygen vacancy concentration increases significantly and a Co3+-Co2+ system is formed. Consequently, the catalytic activity in the removal of THC and NO x is significantly improved. But for the PM, the effect is not so obvious. The possible catalytic mechanism for this was presented. It is also worth noting that the doped catalysts showed good stability. [ABSTRACT FROM AUTHOR]

• The physical properties of CFT and ULSD soots are studied at various engine modes. • CFT produces the soot with smaller fractal dimensions and primary particle sizes. • CFT soot has more ordered nanostructure and lower reactivity than the ULSD soot. • The fringe length and tortuosity show definite correlations with soot reactivity. This study investigated the physical properties of the exhaust soot particles from a light-duty diesel engine fueled with an ultra-low sulfur diesel fuel (ULSD) and Fischer–Tropsch diesel fuel synthesized from coal (CFT). The morphology and nanostructure of the obtained soot were characterized using a high-resolution transmission electron microscopy, while the oxidation reactivity of soot was evaluated by the thermogravimetric analysis (TGA). Despite the different fuel properties, both the ULSD and CFT soot exhibits similar changes in fractal dimension, primary particle size and nanostructure parameters (including fringe length, separation distance and tortuosity) when varying the engine speed or load. However, the CFT soot has smaller fractal dimensions and primary particle sizes and a more ordered nanostructure as compared to the ULSD soot. The TGA results demonstrate that the CFT soot has the higher apparent activation energy, indicating that it is less reactive than the ULSD soot. The length and tortuosity of the fringes within the soot structure are found to have definite correlations with soot reactivity. [ABSTRACT FROM AUTHOR]