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Technical Paper

Wall-scale Reaction Models in Diesel Particulate Filters

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.
Technical Paper

Wall-Flow Diesel Particulate Filters—Their Pressure Drop and Collection Efficiency

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).
Technical Paper

The Optimum Cell Density for Wall-Flow Monolithic Filters: Effects of Filter Permeability, Soot Cake Structure and Ash Loading

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.
Technical Paper

The Micromechanics of Catalytic Soot Oxidation in Diesel Particulate Filters

Despite the great effort devoted to the modeling of the operation of catalytic DPFs, even today very simple expressions are used for the soot oxidation rate. In the relevant to DPF operation case of a gas phase rich in oxygen, the structure of the soot-catalyst geometry and its evolution during oxidation determines the reaction rate. An extensive set of controlled experiments (isothermal or with linear temperature increase) using fuel borne catalysts and catalytic coatings has been performed in order to obtain corresponding soot oxidation rate-conversion curves. The shape of the resulting curves cannot be described by the typical theories for solid phase reactions posing the need for microstructural models for the micromechanics of soot catalyst interactions.
Technical Paper

Study on Catalyzed-DPF for Improving the Continuous Regeneration Performance and Fuel Economy

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.
Technical Paper

Study of a Sintered Metal Diesel Particulate Trap

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.
Technical Paper

Spatial Non-Uniformities in Diesel Particulate Trap Regeneration

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.
Technical Paper

Soot Oxidation Kinetics in Diesel Particulate Filters

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.
Technical Paper

Solid Nucleation Mode Engine Exhaust Particles Detection at High Temperatures with an Advanced Half Mini DMA

Diesel and gasoline direct injection engines emit nucleation mode particles either under special conditions or as part of their normally emitted size distribution, respectively. Currently, European legislation excludes nucleation mode particles as particle number vehicle emission measurements are limited down to 23 nm. The rationale behind such a cut-off size is based on the avoidance of significant uncertainties inherent in the sampling and measuring of sub-23 nm solid particles. However, the sub-23 nm particles have drawn increased attention since a large fraction of particles emitted by modern vehicles lies in this size range. In this study we investigate the possibility of accurate nucleation mode particles detection by using the Advanced Half Mini Differential Mobility Analyzer (HM-DMA).
Technical Paper

Simulation of Triangular-Cell-Shaped, Fibrous Wall-Flow Filters

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.
Technical Paper

Periodically Reversed Flow Regeneration of Diesel Particulate Traps

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.
Technical Paper

Optimized Filter Design and Selection Criteria for Continuously Regenerating Diesel Particulate Traps

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.
Technical Paper

Multichannel Simulation of Soot Oxidation in Diesel Particulate Filters

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.
Technical Paper

Multi-Instrumental Assessment of Diesel Particulate Filters

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.
Technical Paper

Multi-Functional Reactor for Emission Reduction of Future Diesel Engine Exhaust

Future diesel emission control systems have to effectively operate under non-conventional low-temperature combustion engine operating conditions. In this work the research and development efforts for the realization of a Multi-Functional catalyst Reactor (MFR) for the exhaust of the upcoming diesel engines is presented. This work is based on recent advances in catalytic nano-structured materials synthesis and coating techniques. Different catalytic functionalities have been carefully distributed in the filter substrate microstructure for maximizing the direct and indirect (NO2-assisted) soot oxidation rate, the HC and CO conversion efficiency as well as the filtration efficiency. Moreover, a novel filter design has been applied to enable internal heat recovery capability by the implementation of heat exchange between the outlet and the inlet to the filter flow paths.
Technical Paper

Microstructural Properties of Soot Deposits in Diesel Particulate Traps

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.
Technical Paper

Microstructural Aspects of Soot Oxidation in Diesel Particulate Filters

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).
Journal Article

Micro-Simulation of NO-NO2 Transport and Reaction in the Wall of a Catalyzed Diesel Particulate Filter

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.
Journal Article

Investigation of SCR Catalysts for Marine Diesel Applications

Evolving marine diesel emission regulations drive significant reductions of nitrogen oxide (NOx) emissions. There is, therefore, considerable interest to develop and validate Selective Catalytic Reduction (SCR) converters for marine diesel NOx emission control. Substrates in marine applications need to be robust to survive the high sulfur content of marine fuels and must offer cost and pressure drop benefits. In principle, extruded honeycomb substrates of higher cell density offer benefits on system volume and provide increased catalyst area (in direct trade-off with increased pressure drop). However higher cell densities may become more easily plugged by deposition of soot and/or sulfate particulates, on the inlet face of the monolithic converter, as well as on the channel walls and catalyst coating, eventually leading to unacceptable flow restriction or suppression of catalytic function.
Technical Paper

Growth and Restructuring Phenomena of Deposits in Particulate Filters

As use of Particulate Filters (PFs) is growing not only for diesel but also for gasoline powered vehicles, the need for better understanding of deposit structure, growth dynamics and evolution arises. In the present paper we address a number of deposit growth and restructuring phenomena within particulate filters with the aim to improve particulate filter soot load estimation. To this end we investigate the dynamic factors that quantify the amount of particles that are stored within the wall and the restructuring of soot deposits. We demonstrate that particle accumulation inside the porous wall is dynamically controlled by the dimensionless Peclet number and provide a procedure for the estimation of parameters of interest such as the loaded filter wall permeability, the wall-stored soot mass at the onset of cake filtration.