Wall-flow Diesel Particulate Filters Research Articles

Experimental study of hexagonal and square diesel particulate filters under controlled and uncontrolled catalyzed regeneration

Although diesel engines have high thermal efficiency, large amounts of PM (particulate matter) including soot are emitted. A wall-flow DPF (diesel particulate filter) is one of the most important technologies for diesel emission control. However, the soot accumulation inside the DPF causes an increase of pressure loss. Then, the accumulated diesel soot needs to be burned, which is called a filer regeneration process. In this study, we have investigated the soot combustion on bare and catalyzed DPFs under controlled and uncontrolled regeneration. Two types of DPFs with conventional square and hexagonal cells were used. Results show that, in comparison with the bare DPF, the regeneration efficiency of the catalyzed DPF is clearly higher, indicating a marked effect of catalysts. Independent of regeneration temperature, a greater increase in the regeneration efficiency of the catalyzed DPF was confirmed under controlled regeneration. On the other hand, under uncontrolled regeneration, the maximum temperature of the catalyzed DPF is higher than that of the bare DPF, and it is reached shorter times. Interestingly, by comparing the conventional square cell DPF, the soot oxidation of the hexagonal cell DPF is promoted under controlled and uncontrolled regeneration.

Packed bed of spherical particles approach for pressure drop prediction in wall-flow DPFs (diesel particulate filters) under soot loading conditions

The soot loading process in wall-flow DPFs (diesel particulate filters) affects the substrate structure depending on the filtration regime and produces the increase of pressure drop. Deep bed filtration regime produces the decrease of the porous wall permeability because of the soot particulates deposition inside it. Additionally, a layer of soot particulates grows on the porous wall surface when it becomes saturated. As soot loading increases, the pressure drop across the DPF depends on the porous wall and particulate layer permeabilities, which are in turn function of the substrate and soot properties. The need to consider the DPF pressure drop influence on engine performance analysis or DPF regeneration processes requires the use of low-computational effort models describing the structure of the soot deposition and its effect on permeability. This paper presents a model to describe the micro-scale of the porous wall and the particulate layer structure assuming them as packed beds of spherical particles. To assess the model's capability, it is applied to predict the DPF pressure drop under different experimental conditions in soot loading, mass flow and gas temperature.

A study on the cell structure and the performances of wall-flow diesel particulate filter

As for the recent PM regulations, a diesel particulate filter (DPF) has been one of the important aftertreatment technologies. Although the square cell structure of DPF is a generally worldwide standard, several cell designs have been proposed to reduce the pressure loss due to the soot loading as well as the ash deposition in DPF. In this study, we focused on the cell geometry using a hexagonal cell DPF and a conventional square cell DPF. In the engine test bench under nearly real conditions, these DPF performances were evaluated. Results show that, in comparison with square cell DPF, the particle number concentration of the hexagonal cell DPF decreases more rapidly, and the filtration efficiency is higher. In addition, in DPF regeneration test, independent of the inlet temperature, the regeneration rate of the hexagonal cell DPF is higher. Between two DPFs, the aperture ratio of inlet/outlet cells is different. Thus, the superior DPF performance of the hexagonal cell DPF could be explained by the difference of exhaust gas flow and soot deposition region.

Particle Number Emissions of Nonroad Diesel Engines of Various Ages

Over the last two decades, gaseous and particle mass emissions of new diesel engines have been reduced effectively and progressively in response to the emissions legislation and due to the applied new technologies. There is, however, increasing concern about whether the engine modifications, while improving combustion and reducing emissions, have increased the number emissions of ultrafine and nanoparticles. So far, emissions regulations have solely been based on particulate matter (PM) mass measurements, not on particle number. Nanoparticles, however, form a major part of the PM emissions, but they do not considerably contribute to the PM mass and cannot be seen as a problem, if only PM mass is determined. Therefore, there is increasing interest in expanding the scope of the regulations to also include particle number emissions, e.g., Euro VI for on-road engines. The PM number limit will also be enforced for nonroad engines slightly later. Thus, more information is required about the particle number emissions themselves, but also about the effects of the engine technology on them. Wall-flow diesel particulate filters reduce the particle number very effectively within the entire particle size range. Nevertheless, in order to keep the filter as small as possible and to lessen the need for regeneration, the engine-out PM number should also be minimized. If the diesel particulate filters (DPFs) could be left out or replaced by a simpler filter, there would be greater freedom of space utilization or cost savings in many nonroad applications. This might be realized in installations where the engine is tuned at high raw NOx and a selective catalytic reduction (SCR) system is adopted for NOx reduction. However, it is not self-evident that new engine technologies would reduce the PM number emissions sufficiently. In this study, particle number emissions were analyzed in several nonroad diesel engines, representing different engine generations and exploiting different emissions reduction technologies: four- or two-valve heads, exhaust gas recirculation, different injection pressures and strategies, etc. All engines were turbocharged, intercooled, direct-injection nonroad diesel engines. Most engines used common-rail fuel injection technology. Comparisons were, however, also performed with engines utilizing either a distributor-type or an in-line fuel injection pump to see the long-term development of the particle number emissions. In this paper, the PM number emissions of nine nonroad diesel engines are presented and compared. Gaseous exhaust emissions and fuel consumption figures are also provided.

Heat transfer modelling in honeycomb wall-flow diesel particulate filters

Heat transfer in wall-flow monoliths has gained in interest because of the widespread adoption of these systems by automotive industry to fulfil soot emission regulations and the importance of heat exchange on the regeneration process control to avoid damaging the monolith. This paper presents a heat transfer model for wall-flow diesel particulate filters coupled with an unsteady compressible flow solver. The heat exchange between the gas and the solid phase is based on a bi-dimensional discretisation of the porous medium both in axial and tangential directions. The monolith can be discretised in the radial direction to account for the heat fluxes towards the environment through the monolith and the canister, which is also coupled with the inlet and outlet ducts of the filter. The model is validated against experimental data obtained in a flow test rig. A test campaign under non-reacting conditions has been conducted to show the capability for thermal response prediction. Tests cover clean and soot loaded monolith, continuous flow under steady and transient thermal conditions, and pulsating flow. In this case, the characteristics of the pressure waves in amplitude and frequency are similar to those that the monolith can undergo depending on its location along the exhaust line.

Experimental–theoretical methodology for determination of inertial pressure drop distribution and pore structure properties in wall-flow diesel particulate filters (DPFs)

Wall-flow particulate filters have been placed as a standard technology for Diesel engines because of the increasing restrictions to soot emissions. The inclusion of this system within the exhaust line requires the development of computational tools to properly simulate its flow dynamics and acoustics behaviour. These aspects become the key to understand the influence on engine performance and driveability as a function of the filter placement. Since the pressure drop and the filtration process are strongly depending on the pore structure properties – permeability, porosity and pore size – a reliable definition of these characteristics is essential for model development. In this work a methodology is proposed to determine such properties based on the combination of the pressure drop rement in a steady flow test rig and two theoretical approaches. The later are a lumped model and a one-dimensional (1D) unsteady compressible flow model. The purpose is to simplify the integration of particulate filters into the global engine modelling and development processes avoiding the need to resort to specific and expensive characterisation tests. The proposed methodology was validated against measurements of the response of an uncoated diesel particulate filter (DPF) under different flow conditions as cold steady flow, impulsive flow and hot pulsating flow.

Development and validation of a model for wall-flow type selective catalytic reduction system

The two-way diesel particulate filter (DPF)/selective catalytic reduction (SCR) emissions-reduction system has recently been extensively studied due to its potential cost savings and packing flexibility. Unlike a conventional emissions-reduction system that consists of a DPF and an SCR for reduction of both particulate matter and nitrogen oxides (NO x), this two-way system incorporates SCR function into a wall-flow DPF as a single device by coating SCR catalysts into the filter wall of the DPF. Such a combinatorial system has the advantage of lower system volume and cost, since exhaust gas undergoes soot filtering and NO x reduction reaction simultaneously as it passes through the channels and filter wall. In this work, the mathematical formulations which govern the physical and chemical phenomena in the two-way DPF/SCR are derived and a computer simulation model is developed based on these formulations. The model has been validated and tested with experimental data from a reactor flow bench in a systematic manner, which captures ammonia adsorption/desorption and NO x reduction of the blended two-way DPF/SCR device.

Particulate Matter in New Technology Diesel Exhaust (NTDE) is Quantitatively and Qualitatively Very Different from that Found in Traditional Diesel Exhaust (TDE)

Diesel exhaust (DE) characteristic of pre-1988 engines is classified as a "probable" human carcinogen (Group 2A) by the International Agency for Research on Cancer (IARC), and the U.S. Environmental Protection Agency has classified DE as "likely to be carcinogenic to humans." These classifications were based on the large body of health effect studies conducted on DE over the past 30 or so years. However, increasingly stringent U.S. emissions standards (1988-2010) for particulate matter (PM) and nitrogen oxides (NOx) in diesel exhaust have helped stimulate major technological advances in diesel engine technology and diesel fuel/lubricant composition, resulting in the emergence of what has been termed New Technology Diesel Exhaust, or NTDE. NTDE is defined as DE from post-2006 and older retrofit diesel engines that incorporate a variety of technological advancements, including electronic controls, ultra-low-sulfur diesel fuel, oxidation catalysts, and wall-flow diesel particulate filters (DPFs). As discussed in a prior review (T. W. Hesterberg et al.; Environ. Sci. Technol. 2008, 42, 6437-6445), numerous emissions characterization studies have demonstrated marked differences in regulated and unregulated emissions between NTDE and "traditional diesel exhaust" (TDE) from pre-1988 diesel engines. Now there exist even more data demonstrating significant chemical and physical distinctions between the diesel exhaust particulate (DEP) in NTDE versus DEP from pre-2007 diesel technology, and its greater resemblance to particulate emissions from compressed natural gas (CNG) or gasoline engines. Furthermore, preliminary toxicological data suggest that the changes to the physical and chemical composition of NTDE lead to differences in biological responses between NTDE versus TDE exposure. Ongoing studies are expected to address some of the remaining data gaps in the understanding of possible NTDE health effects, but there is now sufficient evidence to conclude that health effects studies of pre-2007 DE likely have little relevance in assessing the potential health risks of NTDE exposures.

Influence of Operative Parameters on Microwave Regeneration of Catalytic Soot Wff for Diesel Engines

Wall-Flow Diesel Particulate Filters (DPFs) are considered the most effective devices for the control of diesel particulate emissions; however they need a periodic continuous regeneration. Recently, there has been a lot of interest in using MW energy for DPF regeneration due to its instantaneous and selective heating process. So the objectives of this study were to conduct experiments to: 1. prepare a specifically catalysed SiC DPF with MW absorption properties; 2. demonstrate the efficiency of the microwave regeneration.

Evaluating the efficiency of Diesel Particulate Filters in high-duty vehicles: Field operational testing in Portugal

The higher fuel economy allowed by diesel engines in comparison with Otto cycle based engines drove them to a progressive lead in vehicle fleet shares, especially within high-duty fleets, during last years. However, diesel engines have also other less welcome properties such as considerable tailpipe Particulate Matter (PM) emissions that contribute to adverse health effects and also to ecosystems and built heritage degradation. This burden requires straightforward mitigation measures and one of the most frequent in Europe has been the operational use of particle traps in a great diversity of fleets, in particular captive ones. These captive fleets account for an important percentage of annual mileage in urban areas. Nevertheless there’s a lack of information on filters efficiency under real operational conditions as also of an affordable and easily accessible method for doing this evaluation, since the typical methodology for testing particle traps is used only for homologation purposes. Aiming to operationally test Diesel Particulate Filters (DPF) performances using different engine types and emission technologies (EURO standards) tests were implemented in a total of 12 vehicles equipped with 4 different commercially available wall-flow DPF. These tests were done in the two largest Portuguese metropolitan areas, Lisbon and Oporto. Several opacity measurements using the free acceleration test were performed with and without the filter installed for at least 200 running hours per vehicle. Other relevant data (such as fuel consumption) were recorded for evaluating also other possible secondary effects on the vehicle performance. Results indicate an average reduction of 92% in opacity, while no significant changes in fuel consumption and engine power were registered. Conclusions highlight not only the adequacy of the filters tested for PM reduction as well as an inexpensive methodology with potential for ensuring its efficiency in high-duty vehicle fleets real-world operation.

A fluid dynamic model for unsteady compressible flow in wall-flow diesel particulate filters

The use of particulate filters (DPF) in Diesel engines has become in recent years the standard technology for the control of soot aerosol emissions. Once emissions reduction through the management of filtration and regeneration aspects has reached its maturity, the effect of the system location on engine performance and acoustics are key topics to be addressed. In this paper, a fluid dynamic model for wall-flow monolith filters is described in which non-homentropic one-dimensional unsteady compressible flow is considered. The good agreement with experimental data confirms that the model is able to describe the mechanisms contributing to the pressure drop across the whole filter under steady and impulsive flow conditions. The approach of the flow governing equations provides a reliable evaluation of the contributions to the pressure drop with axial resolution in the description of the flow field properties. In addition, the frequency response predicted by the model confirms its ability to evaluate the dynamic response and acoustic potential of the DPF.

Multi-dimensional computation of compressible reacting flows through porous media to apply to Internal Combustion Engine simulation

In this work, a new multi-dimensional Finite Volume (FV) solver of partial differential equations (PDEs) for compressible and reacting flows through porous media has been developed. The solver makes use of a pseudo-staggered arrangement, in order to preserve sharp value changes in pressure and velocity fields across Diesel Particulate Filter (DPF) porous walls; the resulting form of the pressure correction equation is able to achieve a fast convergence at very low permeability of the medium, also when it is associated with strong grid non-orthogonality. A description of the theory adopted for the implementation of a highly efficient C++ object oriented dynamic library is presented first. The library has been applied to the new compressible solver for the multi-dimensional simulation of the hydrodynamics of full-scale wall-flow Diesel Particulate Filters. Code validation has been performed against experimental data available from the published literature.

Experimental investigation of soot deposition in diesel particulate filters

The present paper highlights the features of soot deposition inside the channels of wall-flow diesel particulate filters (DPFs). This investigation is tailored to understand the behaviour of soot loading in DPFs, since the subsequent regeneration step is strongly affected by the cake profile in the inner channels of the filter. In addition, the experimental data here reported are useful to validate detailed mathematical models for the prediction of pressure drop, filtration efficiency and filter loading. The investigation regards the differences in soot deposition profiles of two geometries for the filter housing: the first one was responsible for an uneven distribution of the flow at the inlet of the channel, closer to real cases in the exhaust pipes, while the other one was designed to minimize this effect. The two housings were tested in the same operating conditions, with lab-scale filters and synthetic soot, in order to achieve a better reproducibility of the inlet conditions, rather than with real diesel soot from a diesel engine. The evolution of soot deposition profiles at different loading times was monitored as well. Furthermore, the filtration efficiency was evaluated through measurements of particle size distributions before and after the filter. Some conclusions regarding the most appropriate mathematical model to simulate these phenomena are reported at the end of the paper, with reference to our past modelling activity.

Micro- and macroscopic visualization of particulate matter trapping and regeneration processes in wall-flow diesel particulate filters

Particulate matter trapping and regeneration processes in wall-flow diesel particulate filters (DPFs) without catalysts were investigated through micro- and macroscopic visualization experiment. The vertical walls of a small DPF were polished using a lapping process to create a mirror-like surface on each ceramic particle grain. Using an all-in-focus optical microscope, micro-scale flow pores inside the DPF wall could be clearly observed from the polished surface to a depth of 100 μm. Furthermore, a real-time video with a speed of 30 frames per second could be sharply recorded. Through the microscopic cross-sectional view, transition from depth filtration to surface filtration could be observed clearly. Only surface pores opened on the wall surface were related to the filtration depth, i.e. the penetration depth. During regeneration of the DPF without catalyst, after a particulate (soot) cake was burnt out, the particulates trapped inside the surface pores were oxidized. On the other hand, using a half-cylindrical-shaped, wall-flow DPF, the overall trapping phenomena and regeneration process were clarified through a long-distance focusing lens camera. Diesel particulates were trapped almost uniformly over the entire surface of an inflow channel of the DPF in the direction of the channel flow, while the trapped particulates were not necessarily oxidized uniformly since there was a large temperature difference between the inlet and the outlet of the flow channel. The regeneration patterns were strongly dependent on the initial particulate mass and the inlet temperature of the working gas, including the microscopic phenomena in each location. Consequently, microscopic surface pores played a significant role in the regeneration process as well as in the beginning of trapping. Furthermore, at a macroscopic level, a uniform temperature and wall-flow distributions were found to be significant for quick regeneration.

Nanoparticle motion trajectories and deposition in an inlet channel of wall-flow diesel particulate filter

A two-dimensional gas–particle two-phase flow model in an inlet channel of diesel particulate filter (DPF) has been developed. The interaction between the gas and the particle is treated as one-way coupling due to the extremely dilute particle concentration. The drag force, the Brownian excitation, and the partial slip are included in the particle motion equation. The particle motion trajectories for various particle source locations and various particle sizes are presented. The dependences of the velocity field of the channel on wall permeability and upstream velocity are evaluated. The effects of particle size, upstream velocity, wall permeability, Brownian motion, and partial slip on the particle deposition are investigated. The three dominant mechanisms, which are drag force, Brownian motion, and particle inertia, are analyzed deeply. These results demonstrate that the drag force always plays the most primary role in determining the particle motion and deposition. On the other hand, the effects of Brownian motion and particle inertia vary with Pe number and Re p * number, respectively. As Pe number and Re p * number increase, the effect of particle inertia enhances, while the effect of Brownian motion lowers. The predicted velocity field is compared with the results obtained from the classical one-dimensional model, and the computational pressure loss of DPF is compared with the experimental values. The reasonable agreements of the two cases are both observed.

Active Regeneration of Diesel Particulate Filter Employing Microwave Heating

Wall-flow diesel particulate filters (DPFs) are considered the most effective devices for the control of diesel particulate emissions. A requirement for the reliable operation of the DPFs, however, is the periodic and/or continuous regeneration of the filters. While microwave heating has been considered a potential active regeneration method for the DPFs, past studies on the technology have identified several technical problems leading to filter failure. The problems are mainly associated with the use of inappropriate filter materials for the microwave system and the generation of local hotspots due to uneven microwave heating, resulting in the physical damage to the filters. The objective of this study was to develop and demonstrate the technology employing a microwave-absorbing filter material coupled with an effective waveguide design for the reliable regeneration of DPFs. In this study, a well-equipped diesel emission control laboratory was established to conduct the experiments. The experimental facilities included a 6-kWe diesel generator, an exhaust flow control system, a diesel particulate filter system, a microwave energy supply system, a soot sampling system, a differential-pressure measurement system, and a temperature measurement system. The DPF was a silicone carbide wall-flow monolith filter enclosed in a quartz filter holder. A commercial 1.4-kWe microwave oven was modified to accommodate the quartz holder and a waveguide was engineered to evenly supply the microwave energy to the enclosed filter to achieve filter regeneration. In the experiments, the diesel engine exhaust was lined up to flow through the filter with a fixed flow rate. The microwave regeneration was triggered after a specific amount of soot loading was reached based on the differential pressure drop reading. The results have indicated that the designed system has been able to achieve uniform temperature profiles both in the radial and the vertical DPF positions. The off-line regeneration of DPF by microwave energy has been observed to be highly efficient in terms of energy consumption and regeneration efficiency. The DPM filtration efficiency has remained comparably high after 150 cycles of filtration/regeneration with no apparent physical damage to the DPF being observed. The on-line microwave regeneration of the DPF, however, is not as efficient as the off-line regeneration due to the insufficient oxygen concentration in the engine exhaust stream.

Secondary Effects of Catalytic Diesel Particulate Filters: Conversion of PAHs versus Formation of Nitro-PAHs

Diesel particulate filters (DPFs) are a promising technology to detoxify diesel exhaust. However, the secondary combustion of diesel soot and associated compounds may also induce the formation of new pollutants. Diesel soot is rated as carcinogenic to humans and also acts as a carrier for a variety of genotoxic compounds such as certain polycyclic aromatic hydrocarbons (PAHs) or nitrated PAHs (nitro-PAHs). Furthermore, diesel exhaust contains considerable amounts of nitric oxide (NO), which can be converted to more powerful nitrating species like nitrogen dioxide (NO2), nitric acid (HNO3), and others. This mix of compounds may support nitration reactions in DPFs. Herein we report effects of two cordierite-based, monolithic, wall-flow DPFs on emissions of genotoxic PAHs and nitro-PAHs and compare these findings with those of a reporter gene bioassay sensitive to aryl hydrocarbons (AHs). Soot combustion was either catalyzed with an iron- or a copper/iron-based fuel additive (fuel-borne catalysts). A heavy duty diesel engine, operated according to the 8-stage ISO 8178/4 C1 cycle, was used as test platform. Emissions of all investigated 4- to 6-ring PAHs were reduced by about 40-90%, including those rated as carcinogenic. Emissions of 1- and 2-nitronaphthalene increased by about 20-100%. Among the 3-ring nitro-PAHs, emissions of 3-nitrophenanthrene decreased by about 30%, whereas 9-nitrophenanthrene and 9-nitroanthracene were found only after DPFs. In case of 4-ring nitro-PAHs, emissions of 3-nitrofluoranthene, 1-nitropyrene, and 4-nitropyrene decreased by about 40-60% with DPFs. Total AH-receptor (AHR) agonist concentrations of diesel exhaust were lowered by 80-90%, when using the iron- and copper-based DPFs. The tested PAHs accounted for < 1% of the total AHR-mediated response, indicating that considerable amounts of other aryl hydrocarbons must be present in filtered and unfiltered exhaust. We conclude that both DPFs detoxified diesel exhaust with respect to total aryl hydrocarbons, including the investigated carcinogenic PAHs, but we also noticed a secondary formation of selected nitro-PAHs. Nitration reactions were found to be stereoselective with a preferential substitution of hydrogen atoms at peri-positions. The stereoisomers obtained are related to combustion chemistry, but differ from those formed upon atmospheric nitration of PAHs.

A finite-volume-based two-dimensional wall-flow diesel particulate filter regeneration model

Many existing diesel particulate filter (DPF) models do not sufficiently describe the actual physiochemical processes that occur during the regeneration process. This is due to the various assumptions made in the models. To overcome this shortcoming, a detailed two-dimensional DPF regeneration model with a multistep chemical reaction scheme is presented. The model solves the variable density, multicomponent conservation equations by the pressure implicit with splitting of operators (PISO) scheme for inlet and outlet channels as well as the porous soot layer and filter wall. It includes a non-thermal equilibrium (NTE) model for the energy equation for porous media. In addition, for the first time, experiments on the DPF were conducted to determine the interstitial heat transfer coefficient inside the DPF porous wall. The results compare well with an in-house one-dimensional model and subsequently this was used in the new two-dimensional model. By using this detailed two-dimensional model, some interesting observations of the DPF regeneration process were revealed. These included flow reversals and asymmetry in the filter channels.

Influence of Pre Turbo Catalyst Design on Diesel Engine Performance, Emissions and Fuel Economy

Influence of Pre Turbo Catalyst Design on Diesel Engine Performance, Emissions and Fuel Economy

Effects of diesel particulate filter structural parameters on filtration performance of different size particles

In this study, microcosmic capturing mechanism of single unit on diesel engine particles is firstly studied,then, macroscopical filtration model of single porous medium bed and trap model of monolithic wall-flow diesel particulate filter (DPF) on particulate matter (PM) are developed and formulated. Effects of main DPF structural parameters on filtration performance of different size particles from diesel engines are studied. The results show that the trap efficiency of 10 nm diameter particles is highest, and the efficiency descends while the particle diameter increases. The trap efficiency will be lowest near 500 nm diameter particles, and the trap efficiency ascends with particles diameter increasing to 1μm continuously. This indicates the complex effects of diffusion, inertia and interception mechanisms on particle trap. With increasing cell density, wall thickness, diameter and length of filters, the filtration efficiencies of different size particles ascend with different extents. The trap efficiencies rapidly increase with decreasing porosity and mean pore size of filter substrates. These results are valuable for matching DPF structural parameters and improving filtration performance of DPF.