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Journal Article

Visualization of Oxidation of Soot Nanoparticles Trapped on a Diesel Particulate Membrane Filter

Through microscopic visualization experiments, a process generally known as depth filtration was shown to be caused by surface pores. Moreover, the existence of a soot cake layer was an important advantage for filtration performance because it could trap most of the particulates. We proposed an ideal diesel particulate filter (DPF), in which a silicon carbide (SiC) nanoparticle membrane (made from a mixture of 80 nm and 500 nm powders) instead of a soot cake was sintered on the DPF wall surface; this improved the filtration performance at the beginning of the trapping process and reduced energy consumption during the regeneration process. The proposed filter was called a diesel particulate membrane filter (DPMF). A diesel fuel lamp was used in the trapping process to verify the trapping and oxidation mechanisms of ultrafine particulate matter. Thus, the filtration performance of the membrane filters was shown to be better than that of conventional DPFs.
Technical Paper

Visualization Study of PM Trapping and Reaction Phenomena in Micro-structural Pores through Cross Section of DPF Wall

Trapping of diesel particulates and phenomena of chemical reaction in regeneration were investigated by visualization through the cross-sectional area of a diesel-particulate-filter wall, using a digital-microscope with a high focusing depth. Herein, SiC-DPF walls were polished up to make a uniform height and to create a mirror-like surface on each SiC-particle-grain. At the beginning of the trapping process, it was observed that large particulates were trapped once in the small pores inside the wall, and then, since the flow-pattern was changed drastically, the trapped particulates were pushed out and blown off again, and finally, trapped in a region further downstream. As time passed, image analysis disclosed that since fine particulates were deposited around the SiC-particle-grain surface, the flow-channels became increasingly narrow.
Technical Paper

Soot Oxidation Characteristics of SiC Nanoparticle Membrane Filters

A diesel particulate membrane filter (DPMF) has good trapping efficiency of soot and reduces the pressure loss through the soot accumulation process on the diesel particulate filter wall. The activation energy reduction effect of the soot oxidation reaction by DPMF was clarified. The membrane consists of SiC nanoparticles with a diameter of 10-100 nm. A thin oxide layer is formed on the SiC particle surface, and nanoscale noble metal particles are distributed on the surface. The reduction mechanism for the activation energy was investigated in detail. Nanoscale soot was accumulated on DPMF from a diesel lamp. Furthermore, the soot oxidation in the regeneration process was observed using an optical microscope. An Arrhenius plot was made from the change of the concentration of the product gases CO and CO₂ with respect to time. The performance and the temperature dependence of oxygen desorption on the oxide layer was measured by thermal desorption spectroscopy (TDS).
Technical Paper

Simultaneous Measurements of Temperatures of Flame and Wall Surface in a Combustion Chamber of Diesel Engine

In order to investigate the combustion phenomena in a combustion chamber of the diesel engine at transient operations, the simultaneous measurements of temperatures of flame and wall surface in a combustion chamber were conducted. The new technique for simultaneous measurements of flame temperature and wall surface was developed. Laser-Induced phosphorescence was used for the measurement of wall surface temperature which was coupled with the flame temperature measurement by a two-color pyrometry. The NOx and soot emissions were also measured simultaneously in transient operations. The relation between the temporal changes of emissions and temperatures of flame and surface wall are discussed. The results show that the temporal change of NOx emission during transient operation is similar to that of the average gas temperature in a chamber. On the other hand, the temporal change of soot emission is similar to neither that of flame temperature nor that of average gas temperature.
Journal Article

Simultaneous Measurements of Aromatic Hydrocarbons in Exhaust using a Laser Ionization Method

A simultaneous multi-composition analyzing (SMCA) resonance enhanced multi-photon ionization (REMPI) system was used to investigate gasoline engine exhaust. Observed peaks for exhaust were smaller mass numbers than those from diesel exhaust. However, large species up to three ring aromatics were observed suggesting that soot precursor forms even in the gasoline engine. At low catalyst temperature condition, the reduction efficiencies of a three-way catalyst were higher for higher mass numbers. This result indicates that the larger species accumulate in the catalyst or elsewhere due to their lower vapor pressures. To evaluate the emission of low volatility species, the accumulation should be taken into account. In the hot mode, reduction efficiencies for aromatic species of three-way catalyst were almost 99.5% however, they fall to 70% in the cold start condition.
Technical Paper

Simultaneous 2-D Imaging of OH Radicals and Soot in a Diesel Flame by Laser Sheet Techniques

The OH and soot in an unsteady flame, which was achieved in a rapid compression machine, were visualized simultaneously by the laser-induced fluorescence and laser-induced scattering techniques. The fuel mixture consisting of 90% paraffin hydrocarbon (reference fuel) and 10% polypropylene-glycol was used to reduce the optical attenuation caused by dense soot cloud. The simultaneous images of the fluorescence from OH and scattering from soot show that the soot and OH exist separately from each other in the leading portion of the spray flame, and the OH is formed earlier than the soot in the near field region of spray flame.
Technical Paper

Scanning Electron Microscopic Visualization of Transition from Surface Pore Filtration to Cake Filtration Inside Diesel Particulate Filter Walls

Surface pores that are open to the inlet channel below the surface play a particularly important role in the filtration of particulate matter (i.e., soot) inside the walls of a diesel particulate filter (DPF); they are closely related to the pressure drop and filtration efficiency through the DPF as well as the performance of the regeneration process. In this study, a scanning electron microscope (SEM) was used to dynamically visualize the soot deposition process at the particle scale as “time-lapse” images corresponding to the different increases in the pressure drop at each time step. The soot was first trapped at the deepest areas of the surface pores because the porous channels in this area were constricted by silicon carbide grains; soot dendrite structures were observed to grow and finally cause obstructions here.
Journal Article

Scanning Electron Microscopic Visualization of Bridge Formation inside the Porous Channels of Diesel Particulate Filters

Time-lapse images of particulate matter (PM) deposition on diesel particulate filters (DPFs) at the PM-particle scale were obtained via field-emission scanning electron microscopy (FE-SEM). This particle scale time-series visualization showed the detailed processes of PM accumulation inside the DPF. First, PM introduced into a micro-pore of the DPF wall was deposited onto the surface of SiC grains composing the DPF, where it formed dendritic structures. The dendrite structures were locally grown at the contracted flow area between the SiC grains by accumulation of PM, ultimately constructing a bridge and closing the porous channel. To investigate the dominant parameters governing bridge formation, the filtration efficiency by Brownian diffusion and by interception obtained using theoretical filtration efficiency analysis of a spherical collector model were compared with the visualization results.
Technical Paper

Quantitative Measurement of Fuel Vapor Concentration in an Unsteady Evaporating Spray via a 2-D Mie-Scattering Imaging Technique

The cross-sectional distribution of fuel vapor concentration in an evaporating spray was measured quantitatively by a new scattering imaging technique, silicone particle scattering imaging method, which was proposed in a previous paper[1]. When fuel containing silicone oil injected into a nitrogen environment at high temperature, the volatile base fuel in the droplets vaporized rapidly, leaving behind small droplets of silicone oil suspended in the vapor-gas mixture. The silicone oil droplets were illuminated by a thin laser sheet, and the scattered light was imaged by a CCD camera. The cross-sectional distribution of vapor concentration was estimated from the scattering image of the silicone oil droplets by Mie scattering theory. The results demonstrated clearly the inhomogeneity of the fuel vapor concentration. The distribution of vapor concentration was discontinuous, and islands of rich mixture with a scale of several millimeters existed in the center region of the spray.
Technical Paper

Pyrene-LIF Thermometry of the Early Soot Formation Region in a Diesel Spray Flame

In order to investigate early soot formation process in diesel combustion, spectral analysis and optical thermometry of early soot formation region in a transient spray flame under diesel-like conditions (Pg2.8 MPa, Tg620-820K) was attempted via laser-induced fluorescence (LIF) from pyrene (C16H10) doped in the fuel. Pyrene is known to exhibit a temperature\-dependent variation of LIF spectrum; the ratio of S2/S1 fluorescence yields, from the lowest excited singlet state S1 and the second excited singlet state S2, depends on temperature. In the present study, pyrene was doped (1%wt) in a model diesel fuel (0-solvent) and the variation of LIF spectra from the pyrene in the spray flame in a rapid compression machine were examined at different ambient temperatures, ambient oxygen concentrations, measurement positions and timings after start of fuel injection.
Technical Paper

Physical Characterization of Biodiesel Particle Emission by Electron Microscopy

Nanostructures of diesel and biodiesel engine particulate matters (PMs) were investigated by using a Transmission Electron Microscopy (TEM). The average single particle sizes of biodiesel and diesel PMs are approximately 30-40 nm and 50-60 nm, respectively. Image processing process was used to estimate each carbon platelet length by using TEM image. The average carbon platelet length of biodiesel and diesel PMs are in the range of 0.1-7.0 nm. Moreover, carbon atoms per cubic volume of PMs are approximately 500-900. The result shows that engine load and fuel property are strongly impact on the size of single particle and carbon atom density of particle. This is one of interesting behaviors need to be investigated for better understanding. The results of this research would be used as basic information for design and develop removing process of PM emitted from engine combustion which using in diesel and biodiesel fuels.
Journal Article

Particulate Matter Trapping and Oxidation on a Catalyst Membrane

Particulate matter (PM) trapping and oxidation in regeneration on the surface of a diesel particulate catalyst-membrane filter (DPMFs) were investigated in detail using an all-in-focus optical microscope. The DPMF consists of two-layer sintered filters, where a SiC-nanoparticle membrane (made from a mixture of 80 nm and 500 nm powders) covers the surface of a conventional SiC filter. Using a visualization experiment, it was shown that PMs were trapped homogeneously along fine surface pores of the membrane's top surface, whereas in the regeneration process, the particulates in contact with the membrane may have been oxidized with some catalytic effect of the SiC nanoparticles. A soot cake was reacted continuously on the nanoparticles since pushed by a gas flow. The oxidation temperature of particulate trapped on the SiC-nanoparticle membrane was about 75 degrees lower than that on the conventional diesel particulate filters (DPF) without a catalyst.
Technical Paper

Particulate Formation and Flame Structure in Diesel Engines

The present paper describes the particulate formation in diesel flames considering the flame structure and its similarity to that of gaseous turbulent diffusion flames. A comparison of spatial variations of soot concentration, equivalence ratio and flame temperature between diesel flames and turbulent diffusion flames reveals the facts that soot particles are mostly farmed in a region where the equivalence ratio is near stoichiometric and the flame temperature is the highest in both flames, and that in diesel flames this region exists generally near the flame tip. A close inspection of high speed photographs of diesel flames suggests the three major routes of soot emission from diesel engines: quenching of flamelets detached from the flame tip due to 1) the flame impingement onto the wall; and 2) cooling of the flamelets by the bulk air; and 3) survival of soot containing flamelets inside the flame.
Technical Paper

Numerical Simulation of Turbulent Dispersion of Fuel Droplets in an Unsteady Spray via Discrete Vortex Method

The turbulent dispersion of particles in an unsteady two dimensional particle-laden jet was simulated by a discrete vortex method coupling with a model of gas/particles interaction. Numerical analysis of a spray yielded the distributions of vorticity, fuel mass concentration and local Sauter mean diameter (SMD) of droplets in a spray. The predicted distribution of local SMD of droplets in a spray demonstrated that the size of droplets in the spray periphery is larger than that of droplets in the center region of spray. This trend of distribution of drop size coincided with that of measured one. The predicted distributions of drop size and vorticity revealed that the larger droplets are easily centrifuged to the periphery of the spray. The effects of the pattern of injection rate on the mixing process in a transient spray were also investigated.
Technical Paper

Mixing Enhancement in Diesel-Like Flames via Flame Impingement on Turbulence-Generating Plates

Soot concentration is very high in the periphery near the head of an unsteady spray flame which is achieved in a quiescent atmosphere in a rapid compression machine. To reduce soot concentration in this region, it was intended to improve fuel-air mixing by letting the flame impinge on a turbulence-generating plate. Two types of turbulence-generating plates, one donut-type, the other cross-type, were tested. Soot concentration in the flame was imaged using the laser shadow technique. The effect of injection pressure on soot reduction by the flame impingement was also investigated. The overall soot concentration is reduced significantly in the case when the flame impinges on the cross-type turbulence-generating plate at 50 mm (333 nozzle diameters) from the nozzle exit. The flame impingement on the cross-type turbulence-generating plate at 333 nozzle diameters makes soot reduction little dependent on injection pressures.
Journal Article

Microscopic Visualization of PM Trapping and Regeneration in Micro-Structural Pores of a DPF Wall

Trapping and regeneration processes in a SiC wall-flow diesel particulate filter (DPF) without a catalyst were investigated in detail through microscopic visualization. By microscopic observation of the cross section and surface, the transition from depth filtration to surface filtration could be observed clearly. The open pores on the wall surface were strongly related to the filtration depth of diesel particulate matter (PM). During the regeneration process, after the soot cake was burnt out, the particulates trapped inside the surface pores were oxidized. As a result, the particulate trapping and oxidation behaviors were strongly dependent on the microstructural surface pores.
Technical Paper

Measurement of Excitation-Emission Matrix of Shock-heated PAHs using a Multi-wavelength Laser Source

Measurements of Excitation-Emission Matrix (EEM) of shock-heated vapors of polycyclic aromatic hydrocarbons (PAHs) at high temperature (750-1500K) and high pressure (0.3-1.3MPa) conditions were conducted using a multi-wavelength excitation laser in order to demonstrate the potential of the single-measurement EEM fluorometry for investigation of soot precursors. Argon-diluted vapors of naphthalene and pyrene, as PAH model compounds, were heated in an optically accessible shock tube. The PAH vapors were excited by a coherent multi-wavelength “rainbow” laser light generated by converting the 4th harmonic (266nm) of a pulsed Nd:YAG laser using a Raman cell frequency converter filled with high-pressure (2MPa) methane-hydrogen mixture.
Technical Paper

Low Temperature Starting Techniques for Ethanol Engine without Secondary Fuel Tank

The present study aims to investigate the parameters affecting cold start characteristics of ethanol at low temperature, and suggest a solution to avoid cold starting problem without the installation of second fuel tank. The testing engine is a 125cc volume displacement, single-cylinder four strokes SI engine with fuel injection and ignition timing system controlled by ECU (electronic control unit). The cold starting performance tests were extensively conducted with different percentages of ethanol blends, surrounding temperatures, heating inside combustion chamber, heater injector, pre-cranking without fuel injection, and amount of fuel injection. From the experimental results, when using ethanol fuel in conventional engine, the problem of cold starting was observed at surrounding temperature lower than 20°C for ethanol. Increasing of injection duration can lower the possible cold start temperature of neat ethanol.
Technical Paper

Lattice Boltzmann Simulation on Particle Transport and Captured Behaviors in a 3D-Reconstructed Micro Porous DPF

In this study, particle transport and captured behaviors in a Diesel Particulate Filter (DPF) was investigated with Lattice Boltzmann Method. LBM calculation was performed to a 3D-reconstructed micro porous DPF substrate, which was obtained by micro-focus 3D X-ray technique. Simulating advection-diffusion behaviors of diesel particulates in micro porous channel, we adapted a LBM method used for high Peclet number flow, simulating flow conditions in DPFs. We investigated flow behaviors in a wide variety of inlet velocity. LBM simulation has clearly shown that non-dimensional flow field is similar in wide range of flow conditions in the DPF, because flow Reynolds number in the micro porous substrate is sufficiently low, dominated by laminar flow regime. It was also revealed that less than 40% pore channels was responsible for more than 80% volume flux in the porous substrate without particle loading.
Journal Article

Investigation of Soot Oxidation Carried out on Membrane Filters Composed of SiC Nanoparticles

The diesel particulate membrane filter (DPMF) is a good solution to the problem of high pressure drop that exists across diesel particulate filters (DPFs) as a result of the soot trapping process. Moreover, DPMFs that have a membrane layer composed of SiC nanoparticles can reduce the oxidation temperature of soot and the apparent activation energy. The SiC nanoparticles have an oxide layer on their surface, with a thickness less than 10 nm. From the visualization of soot oxidation on the surface of SiC nanoparticles by an environmental transmission electron microscope (ETEM), soot oxidation is seen to occur at the interface between the soot and oxide layers. The soot oxidation temperature dependency of the contact area between soot and SiC nanoparticles was evaluated using a temperature programmed reactor (TPR). The contact area between soot and SiC nanoparticles was varied by changing the ratio of SiC nanoparticles and carbon black (CB), which was used as an alternative to soot.