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Following the successful market introduction of diesel particulate filters (DPFs), this class of emission control devices is expanding to include additional functionalities such as gas species oxidation (such as CO, HC and NO), storage phenomena (such as NOx and NH3 storage) to the extent that we should today refer not to DPFs but to Multifunctional Reactor Separators. This trend poses many challenges for the modeling of such systems since the complexity of the coupled reaction and transport phenomena makes any direct general numerical approach to require unacceptably high computing times. These multi-functionalities are urgently needed to be incorporated into system level emission control simulation tools in a robust and computationally efficient manner. In the present paper we discuss a new framework and its application for the computationally efficient implementation of such phenomena.

The present study investigates the pressure drop and filtration characteristics of wall-flow diesel particulate monoliths, with the aid of a mathematical model. An analytic solution to the model equations describing exhaust gas mass and momentum conservation, in the axial direction of a monolith cell, and pressure drop across its porous walls has been obtained. The solution is in very good agreement with available experimental data on the pressure drop of a typical wall-flow monolith. The capture of diesel particles by the monolith, is described applying the theory of filtration through a bed of spherical collectors. This simple model, is in remarkable agreement with the experimental data, collected during the present and previous studies, for the accumulation mode particles (larger than 0.1 μm).

As demand for wall-flow Diesel Particulate Filters (DPF) increases, accurate predictions of DPF behavior, and in particular their pressure drop, under a wide range of operating conditions bears significant engineering applications. In this work, validation of a model and development of a simulator for predicting the pressure drop of clean and particulate-loaded DPFs are presented. The model, based on a previously developed theory, has been validated extensively in this work. The validation range includes utilizing a large matrix of wall-flow filters varying in their size, cell density and wall thickness, each positioned downstream of light or heavy duty Diesel engines; it also covers a wide range of engine operating conditions such as engine load, flow rate, flow temperature and filter soot loading conditions. The validated model was then incorporated into a DPF pressure drop simulator.

A major challenge in the development of diesel filter systems is the selection of the appropriate filter medium in terms of its geometric configuration (cell density, wall thickness) and its physical properties (porosity, pore size). This selection aims to achieve minimization of the filter pressure drop as well as more efficient filter regeneration. The aim of the present work is to provide engineering criteria to support the design and selection of suitably sized wall-flow monolithic filters for diesel particulate control.

The pore structure of DPF (Diesel Particulate Filter) is one of the key factors in contributing the fuel consumption and the emission control performance of a vehicle. The pressure loss of mini samples (1 in. in diameter, 2 in. in length) with various pore structures was measured at relatively low filtration velocity (< 5 cm/sec). Then the obtained data were evaluated by using an index of “permeability”. As a result, among the parameters which characterize the pore structure, it was found that the size of the pore diameter and the sharpness of pore distribution were the most contributing factors in reducing pressure loss which in turn is related to the fuel consumption performance when the cell structure was fixed. On the other hand, it was found that the gas permeability was not affected significantly by any parameter when the catalyst was coated because the coating caused a broadening of the pore distribution.

It is a big challenge how to satisfy both the purification of exhaust gas and the decrease of fuel penalty, that is, carbon-dioxide emission. Regarding the Diesel Particulate Filter (DPF) applied in the diesel after-treatment system, it must be effective for lowering the fuel penalty to prolong the interval and reduce the frequency of the DPF regeneration operation. This can be achieved by a DPF that has high Particulate Matter (PM) mass limit and high PM oxidation performance that is enough to regenerate the DPF continuously during the normal running operation. In this study, the examination of the pore structure of the wall of a DPF that could expand the continuous regeneration region in the engine operation map was carried out. Several porous materials with a wide range of pore structure were prepared and coated with a Mixed Oxide Catalyst (MOC). The continuous regeneration performance was evaluated under realistic conditions in the exhaust of a diesel engine.

This paper describes work supporting the development of a new Diesel particulate trap system for heavy duty vehicles based on porous sintered metal materials that exhibit interesting characteristics with respect to ash tolerance. Experimental data characterizing the material (permeability, soot and ash deposit properties) are obtained in a dedicated experimental setup in the side-stream of a modern Diesel engine as well as in an accelerated ash loading rig. System level simulations coupling the new media characteristics to 3-D CFD software for the optimization of complete filter systems are then performed and comparative assessment results of example designs are given.

Diesel particulate trap regeneration is a complex process involving the interaction of phenomena at several scales. A hierarchy of models for the relevant physicochemical processes at the different scales of the problem (porous wall, filter channel, entire trap) is employed to obtain a rigorous description of the process in a multidimensional context. The final model structure is validated against experiments, resulting in a powerful tool for the computer-aided study of the regeneration behavior. In the present work we employ this tool to address the effect of various spatial non-uniformities on the regeneration characteristics of diesel particulate traps. Non-uniformities may include radial variations of flow, temperature and particulate concentration at the filter inlet, as well as variations of particulate loading. In addition, we study the influence of the distribution of catalytic activity along the filter wall.

Direct catalytic soot oxidation is expected to become an important component of future diesel particulate emission control systems. The development of advanced Catalytic Diesel Particulate Filters (CDPFs relies on the interplay of chemistry and geometry in order to enhance soot-catalyst proximity. An extensive set of well-controlled experiments has been performed to provide direct catalytic soot oxidation rates in CDPFs employing small-scale side-stream sample exposure. The experiments are analyzed with a state-of-the-art diesel particulate filter simulator and a set of kinetic parameters are derived for direct catalytic soot oxidation by fuel-borne catalysts as well as by catalytic coatings. The influence of soot-catalyst proximity, on catalytic soot oxidation is found to be excellently described by the so-called Two-Layer model, developed previously by the authors.

In the present work we apply a computational simulation framework developed for square-cell shaped honeycomb Diesel Particulate Filters to study the filtration, pressure drop and soot oxidation characteristics of recently developed triangular-cell-shaped, high porosity wall-flow filters. Emphasis is placed on the evaluation of the applicability and adaptation of the previously developed models to the case of triangular channels. To this end Computational Fluid Dynamics, asymptotic analysis, multichannel and “unit-cell” calculations are employed to analyze filter behavior and the results are shown to compare very well to experiments available in the literature.

DPF design, system integration, regeneration control strategy optimization and ash ageing assessment, based on a traditional design of experiments approach becomes very time consuming and costly, due to the high number of tests required. This provides a privileged window of opportunity for the application of simulation tools and hence simulation is increasingly being used for the design of exhaust after-treatment systems with a Diesel Particulate Filter (DPF). DPF behavior depends strongly on the coupling of physico-chemical phenomena occurring over widely disparate spatial and temporal scales and a state-of-the-art simulation approach recognizes and exploits these facts introducing certain assumptions and/or simplifications to derive an accurate but computationally tractable DPF simulation tool, for the needs of industrial users.

Diesel particulate filter regeneration (through oxidation of the collected soot particles) is not currently possible under all engine operating conditions without additional external thermal energy. The exploitation of the autothermal properties of the reverse flow reactor has been suggested to reduce further the soot ignition temperature and hereby is studied for the periodically reversed flow regeneration of soot particulate filters, with the aid of a mathematical model for the regeneration process, validated against experimental data. The numerical results confirm the capability of the new technique to effectively succeed where conventional regeneration fails, extending thus the operating limits of already practiced regeneration techniques (thermal or catalyst-assisted) and setting the stage for the construction of an industrial prototype.

Current progress in the development of diesel engines substantially contributes to the reduction of NOx and Particulate Matter (PM) emissions but will not succeed to eliminate the application of Diesel Particulate Filters (DPFs) in the future. In the past we have introduced a Multi-Functional Reactor (MFR) prototype, suitable for the abatement of the gaseous and PM emissions of the Low Temperature Combustion (LTC) engine operation. In this work the performance of MFR prototypes under both conventional and advanced combustion engine operating conditions is presented. The effect of the MFR on the fuel penalty associated to the filter regeneration is assessed via simulation. Special focus is placed on presenting the performance assessment in combination with the existing differences in the morphology and reactivity of the soot particles between the different modes of diesel engine operation (conventional and advanced). The effect of aging on the MFR performance is also presented.

Improved understanding and compact descriptions of the pressure drop evolution of Particulate Filters (both for diesel and gasoline powered vehicles) are always in demand for intelligent implementations of exhaust emission system monitoring and control. In the present paper we revisit the loading process of a particulate filter focusing on a parametric description of the deep bed-to-cake transition in the light of recent progress in the understanding of soot deposit structure, growth dynamics and evolution. Combining experimental data, simulation models and information theoretic concepts we provide a closed-form representation of the entire evolution of pressure drop (from the initial clean state up to the evolving linear cake growth regime) parameterized in terms of the physical parameters of the system (filter and particle structure/geometry and flow properties).

Upcoming (2005) particulate matter standards for diesel powered vehicles are likely to require the deployment of aftertreatment devices, such as particulate filters to ensure emissions compliance. A major challenge in the development of diesel filter systems has been the achievement of filter regeneration by the oxidation of the collected particulate matter in a reliable and cost-effective manner. Recently the emergence of the so-called continuously regenerating trap (CRT™) in conjunction with the future availability of very low-sulphur diesel fuel, represents a promising solution to the diesel particulate control problem. In the present study, design and selection criteria are devised, regarding the sizing of wall flow diesel particulate filters for application in CRT™ systems, employing a range of analytical and 3-D CFD tools validated against experimental data.

In recent years advanced computational tools of Diesel Particulate Filter (DPF) regeneration have been developed to assist in the systematic and cost-effective optimization of next generation particulate trap systems. In the present study we employ an experimentally validated, state-of-the-art multichannel DPF simulator to study the regeneration process over the entire spatial domain of the filter. Particular attention is placed on identifying the effect of inlet cones and boundary conditions, filter can insulation and the dynamics of “hot spots” induced by localized external energy deposition. Comparison of the simulator output to experiment establishes its utility for describing the thermal history of the entire filter during regeneration. For effective regeneration it is recommended to maintain the filter can Nusselt number at less than 5.

As different Diesel Particulate Filter (DPF) designs and media are becoming widely adopted, research efforts in the characterization of their influence on particle emissions intensify. In the present work the influence of a Diesel Oxidation Catalyst (DOC) and five different Diesel Particulate Filters (DPFs) under steady state and transient engine operating conditions on the particulate and gaseous emissions of a common-rail diesel engine are studied. An array of particle measuring instrumentation is employed, in which all instruments simultaneously measure from the engine exhaust. Each instrument measures a different characteristic/metric of the diesel particles (mobility size distribution, aerodynamic size distribution, total number, total surface, active surface, etc.) and their combination assists in building a complete characterization of the particle emissions at various measurement locations: engine-out, DOC-out and DPF-out.

As demand for wall-flow Diesel particulate filters (DPF) increases, accurate predictions of DPF behavior, and in particular of the accumulated soot mass, under a wide range of operating conditions become important. This effort is currently hampered by a lack of a systematic knowledge of the accumulated particulate deposit microstructural properties. In this work, an experimental and theoretical study of the growth process of soot cakes in honeycomb ceramic filters is presented. Particular features of the present work are the application of first- principles measurement and simulation methodology for accurate determination of soot cake packing density and permeability, and their systematic dependence on the filter operating conditions represented by the Peclet number for mass transfer. The proposed measurement methodology has been also validated using various filters on different Diesel engines.

Diesel Particulate Filter (DPF) behavior depends strongly on the microstructural properties of the deposited soot aggregates. In the past the issue of the growth process of soot deposits in honeycomb ceramic filters has been addressed under non-reactive conditions and the influence of the filter operating conditions has been defined in terms of the dimensionless Peclet number. In the present work appropriate soot cake microstructural descriptors are studied under reactive conditions for different oxidation modes. To this end the effect of deposit microstructure on the soot oxidation kinetics is investigated. Different microstructural models for the reacting soot deposit are examined in a unified fashion and a generalized constitutive equation is obtained, describing several modes of microstructure evolution (shrinking layer, shrinking density, discrete columnar and continuous columnar).

Catalyzed Diesel Particulate Filters (CDPFs) continue to be an important emission control solution and are now also expanding to include additional functionalities such as gas species oxidation (such as CO, hydrocarbons and NO) and even storage phenomena (such as NOx and NH3 storage). Therefore an in depth understanding of the coupled transport - reaction phenomena occurring inside a CDPF wall can provide useful guidance for catalyst placement and improved accuracy over idealized effective medium 1-D and 0-D models for CDPF operation. In the present work a previously developed 3-D simulation framework for porous materials is applied to the case of NO-NO2 turnover in a granular silicon carbide CDPF. The detailed geometry of the CDPF wall is digitally reconstructed and micro-simulation methods are used to obtain detailed descriptions of the concentration and transport of the NO and NO2 species in the reacting environment of the soot cake and the catalyst coated pores of the CDPF wall.