Wall-flow Particulate Filters Research Articles

Comparative study on potassium poisoning of Cu-CHA catalysts for NH3-SCR: Stability and transformation of Cu2+ ions

Cu-chabazite based catalysts (Cu-SSZ-13 and Cu-SAPO-34) may suffer from chemical deactivation due to the deposition of alkali impurities on the surface of wall-flow particulate filters. In this work, the resistance of two Cu-CHA catalysts to potassium poisoning was compared, and the key factors affecting preservation and evolution of Cu2+ ions were ascertained based on the experimental characterizations and DFT calculations. The feasibility of atom (Cu and H) at various sites (6MR and 8MR) in different types of zeolites (SSZ-13 and SAPO-34) upon K+ attacking was compared. The stability of isolated Cu2+ ions in the zeolite framework and the aggregates of detached copper species determined the catalytic performance of the poisoned catalysts.

Fuel economy benefits in internal combustion engines due to soot restructuring in the particulate filter by water injection

Wall-flow particulate filters are key elements to control particulate matter emissions. The stricter emission standards also for non-road mobility machinery makes this device essential to improve the air quality in the short to medium term. However, their high filtration efficiency brings with it an increase in backpressure. This effect becomes more damaging as particles get accumulated in the filter and in hybrid vehicles where exhaust temperature are lower due to more frequent cold starts. Pre-filter water injection is a proven method to reduce the impact of soot load on the pressure drop avoiding the fuel consumption increase. In this paper, the effect of pre-filter water injections is analysed in engine and flow test rig environments. After verifying the impact of consecutive injection events on fuel consumption, the filter was loaded and divided into quarters. These were studied one at a time in flow test rig to separate soot mal-distribution from water drag effects. A wide range of conditions were tested to assess the change in pressure drop generated by a single injection. With this reference, the soot restructuring pattern was analysed employing optical techniques. These provided evidences of the way the soot fragments got released from the particulate layer and moved towards the inlet channels rear end. Additionally, a closer look into the porous wall micro-structure provided insights explaining the lack of effect on filtration efficiency. These results provide a basis for synergistic removal of vehicle condensates for use in fuel consumption reduction.

Fuel efficiency optimisation based on boosting control of the particulate filter active regeneration at high driving altitude

The air pressure change as a function of the driving altitude affects negatively the engine performance and emissions as well as the efficiency of the exhaust aftertreatment systems (ATS). These detrimental effects have driven the emissions standards to account for the altitude as boundary condition in type-approval tests. The new requirements demand to balance the recovery of the engine performance as the altitude increases by means of control strategies avoiding negative effects on tailpipe emissions, i.e. combination of engine-out emissions and ATS performance. In this context, this study identifies experimentally the need of optimisation of the active regeneration strategies applied to wall-flow particulate filters for altitude driving. Firstly, a particulate filter was subjected to active regenerations at altitudes ranging from sea-level to 2500 m for a variety of initial soot loads. As boundaries, the engine worked according to the series calibration, which was found to be a function of the altitude for the boosting system actuation. The results evidenced a noticeable deterioration of the soot oxidation rate as the altitude increased. In fact, some operating conditions showed an early balance between filtration and oxidation that avoided the completeness of the regeneration process. The root causes of the soot depletion rate reduction, which involved thermal and transport phenomena, are reasoned from the combined analysis of experimental and modelling approaches. The conclusions provided a guide to successfully conduct the turbine actuation for an optimum exhaust thermal management during the regeneration event at altitude.

Simulation of particulate matter structure detachment from surfaces of wall-flow filters applying lattice Boltzmann methods

Wall-flow particulate filters are used for particulate matter removal from exhaust in aftertreatment systems of combustion engines. Inside these filters, the gas flow passes through a porous wall between oppositely arranged inlet and outlet channels. Due to continuously introduced solid material, the filters have to be regenerated resulting in a breakup of continuous layers deposited on the walls into individual fragments. Those can re-arrange and impact operational filter performance significantly.This process is governed by the interaction of hydrodynamic and adhesive forces, which can only be accurately modelled by taking the surfaces of individual layer fragments into account. With surface resolved simulations, those forces can be evaluated and detailed investigations of detachment and rearrangement processes can be performed.The present work, therefore, presents a lattice Boltzmann approach for the simulation of particulate matter structure detachment capable of capturing the time dependent evolution of the flow field during the filter’s regeneration. First, a numerical model of a single wall-flow filter channel is presented and its validity for the particle-free case is substantiated by showing super-linear grid convergence and good agreement with a reference solution. Then, the model’s capability is shown by applying it to the transient simulation of the rearrangement process.

Prediction Capabilities of a One-dimensional Wall-flow Particulate Filter Model

This work is focused on the formulation of a numerical model for prediction of flow field inside a particulate filter. More specifically, a one-dimensional mathematical model of the gas flow in a particulate trap-cell is deduced and solved numerically. The results are given in terms of velocity, pressure, and filtration velocity. In addition, the dependence of the pressure drop on the main governing parameters has been investigated. More specifically, the permeability of the porous medium and the hydraulic diameter play a fundamental role in the pressure drop.

Filtration efficiency and pressure drop modelling of particulate filters with rear plug damage

A model suitable for wall-flow particulate filters with partial rear plug damage is developed and experimentally validated in this work. A ceramic filter with 16% of the rear plugs mechanically removed is tested at steady-state conditions on the engine bench and transient driving cycle conditions on the chassis dynamometer. After decanning of the monolith, destructive analysis is conducted to identify deposit loading variations and scanning electron microscopy is used to study the deposit structures in the channels. It is shown that channels without rear plugs develop distinct deposit structures in the entry zone. Hence, a local pressure loss coefficient is applied to model the effect of entrance flow constrictions, taking also into account deposit restructuring phenomena at higher flow rates. In addition, a deep-bed filtration submodel is used to capture the effect of non-uniform wall velocities on deposit accumulation in the wall. The modified model is first fitted to the engine bench data and then validated in a wider range of conditions using the driving cycle tests. With the exception of prolonged steady-state loading conditions, good pressure drop and filtration efficiency predictions are obtained throughout the tests in conjunction with correct deposit property profiles. Notably, the cold-start worldwide harmonized light vehicles test cycle shows that the current European on-board diagnosis threshold limit for particulate mass is too relaxed to trigger a malfunction indication for moderate filter faults. In conclusion, the model can be applied in damaged particulate filter studies for the assessment of leaked particulate mass, the specification of more effective legislation limits and the development of rigorous on-board diagnosis systems and algorithms.

Insights into Automotive Particulate Filters using Magnetic Resonance Imaging

Understanding the manufacture and operation of automotive emissions control particulate filters is important in the optimised design of these emissions control systems. Here we show how magnetic resonance imaging (MRI) can be used to understand the drying process, which is part of the manufacture of catalysed particulate filters. Comparison between a wall-flow particulate filter substrate and a flow-through monolith (FTM) has been performed, with MRI giving spatial information on the drying process. We have also used MRI to study the fluid dynamics of a gasoline particulate filter (GPF). Inlet and outlet channel gas velocities have been measured for a clean GPF and two GPF samples loaded with particulate matter (PM) to understand the effect of PM on the filter flow profiles and porous wall permeability as soot is deposited.

Numerical Simulation of a Wall-Flow Particulate Filter Made of Biomorphic Silicon Carbide Able to Fit Different Fuel/Biofuel Inputs

To meet the increasingly strict emission limits imposed by regulations, internal combustion engines for transport applications require the urgent development of novel emission abatement systems. The introduction of biodiesel or other biofuels in the engine operation is considered to reduce greenhouse gas emissions. However, these alternative fuels can affect the performance of the post-combustion systems due to the variability they introduce in the exhaust particle distribution and their particular physical properties. Bioceramic materials made from vegetal waste are characterized by having an orthotropic hierarchical microstructure, which can be tailored in some way to optimize the filtration mechanisms as a function of the particle distribution of the combustion gases. Consequently, they can be good candidates to cope with the variability that new biofuel blends introduce in the engine operation. The objective of this work is to predict the filtration performance of a wall-flow particulate filter (DPF) made of biomorphic silicon carbide (bioSiC) with a systematic procedure that allows to eventually fit different fuel inputs. For this purpose; a well-validated DPF model available as commercial software has been chosen and adapted to the specific microstructural features of bioSiC. Fitting the specific filtration and permeability parameters of this biomaterial into the model; the filtration efficiency and pressure drop of the filter are predicted with sufficient accuracy during the loading test. The results obtained through this study show the potential of this novel DPF substrate; the material/microstructural design of which can be adapted through the selection of an optimum precursor.

Late Fuel Post-Injection Influence on the Dynamics and Efficiency of Wall-Flow Particulate Filters Regeneration

Late fuel post-injections are the most usual strategy to reach high exhaust temperature for the active regeneration of diesel particulate filters. However, it is important to optimise these strategies in order to mitigate their negative effect on the engine fuel consumption. This work aims at understanding the influence of the post-injection parameters, such as its start of injection and its fuel quantity, on the duration of the regeneration event and the fuel consumption along it. For this purpose, a set of computational models are employed to figure out in a holistic way the involved phenomena in the interaction between the engine and the exhaust gas aftertreatment system. Firstly, an engine model is implemented to evaluate the effect of the late fuel post-injection pattern on the gas properties at the exhaust aftertreatment system inlet in different steady-state operating conditions. These are selected to provide representative boundary conditions of the exhaust gas flow concerning dwell time, exhaust temperature and O 2 concentration. In this way, the results are later applied to the analysis of the diesel oxidation catalyst and wall-flow particulate filter responses. The dependence of the diesel particulate filter (DPF) inlet temperature is discussed based on the efficiency of each post-injection strategy to increase the exhaust gas temperature. Next, the influence on the dynamics of the regeneration of the post-injection parameters through the change in gas temperature and O 2 concentration is finally studied distinguishing the pre-heating, maximum reactivity and late soot oxidation stages as well as the required fuel consumption to complete the regeneration process.

Numerical and experimental studies of gas flow in a particulate filter

Predicted gas flow fields in wall-flow particulate filters, used for vehicular emissions abatement, from both one- and three-dimensional numerical models have been validated for the first time. This has been achieved using experimental measurements made using magnetic resonance imaging (MRI). Two-dimensional MRI velocity images of isotropic spatial resolution 140 µm px−1 were acquired perpendicular to the filter channels at multiple points, allowing measurement of the gas axial velocity in both the inlet and outlet channels along the length of the filter. Consideration of the mass balance between data points permitted the through-wall (i.e. filtration) velocity to also be calculated. These velocity data were acquired at six flow rates, corresponding to channel Reynolds numbers in the range 102–1251, and compared with the predictions from both an open source three-dimensional CFD code and a recent one-dimensional model. Good agreement was observed for both models at low flow rates, but the three-dimensional model outperformed the one-dimensional model at higher flow rates. The validation of the 3D CFD method allowed its use to probe local gas hydrodynamics, specifically calculating the variation of the momentum convection correction factor and wall friction factor. Comparisons of the calculated factors and literature correlations showed the effects of flow development are significant and have not previously been considered. When these correlations were used with the 1D model, the velocity predictions were coincident with those of the 3D CFD. Hence, with the appropriate flow descriptors, the 1D model can be used to give accurate gas velocity predictions but at a fraction of the computational time required to run CFD code.

Impact of Particulate Size During Deep Loading on DPF Management

Wall-flow particulate filters are a required emission control device to abate diesel emission in order to comply with current regulations. DPFs (diesel particulate filters) are characterized by high filtration efficiency—but in order to avoid deterioration of power and performance, they are required to cause low values of backpressure. The periodical oxidation of collected particle allows for the reestablishment of the ideal flow conditions. Studies highlighted that the regeneration event has an important impact on engine emission, since it is responsible for the emission of a large number of smaller particles. From these considerations, the importance of optimizing the DPF management for what concerns both filtration and regeneration mechanisms arises. The present paper focuses on the loading process of the filter. A filtration model was implemented, based on the ‘unit-collector’ and fluid-dynamic approaches, known as valid modelling techniques. The model was used to predict trapped mass and filter backpressure evolution with time during loading processes, in which soot particle sizes varied, with the aim to analyze how particulate size affects the filter pressure drop rise. A wall-flow filter was investigated, and the behavior of clean material was evaluated by a parametric analysis in which particle diameter varies in the filed 20–1000 nm, that is the typical range of soot sizes in diesel engine exhaust. The results demonstrate that soot size has a great influence on the initial deep bed loading process. Moreover, it defines the value from which the linear pressure drop shape during cake filtration starts, not only when the initial loaded is completed, but also each time the regeneration event is concluded. This outcome provides an important guideline to define the most appropriate strategy for the initial DPF loading in order to establish the regeneration event based on the estimation of trapped mass accounting for the filter backpressure and on the time interval between two successive regeneration.

Internal pore diffusion and adsorption impact on the soot oxidation in wall-flow particulate filters

The automotive industry is driven its efforts to cleaner internal combustion engines. As a result, the engine has become conditioned by the exhaust aftertreatment systems. The regeneration of wall-flow particulate filters (PFs) evidences such an interaction. The PFs prevent the soot emission whereas, as a counterpart, the fuel consumption increases. Consequently, passive and active regeneration strategies are needed to clean the filter back and limit the penalty in CO2. In this context, modelling tools play a key role to achieve a comprehensive understanding and control of the regeneration. In this work, a regeneration model coupled to a one-dimensional compressible unsteady flow solver for PFs is presented. The importance of the main physical and chemical steps related to the soot oxidation is discussed. The influence of the diffusion of gaseous reactants inside the primary soot particles is firstly addressed. The inclusion of this step into the definition of the reaction rate provides temperature dependence to the soot specific surface. Next, the reactants adsorption is analysed. This step leads to define a surface coverage, which behave as an equivalent reaction order. It allows figuring out the influence of the gaseous reactants concentration on the reaction rate and its dependence with the temperature.

Measuring velocity and turbulent diffusivity in wall-flow filters using compressed sensing magnetic resonance

Gas-phase compressed sensing magnetic resonance methods have been used to image gas flow velocity and turbulent diffusivity in wall-flow particulate filters. Two-dimensional magnetic resonance velocity imaging was used to observe the local distribution of gas velocity in the direction of superficial flow (z) in the entrance and exit regions of the filter at an in-plane spatial resolution of 140 µm (x) × 140 µm (y) and 140 µm (x) × 390 µm (z) perpendicular to and parallel with the direction of superficial flow, respectively. Images were acquired in 14 min. Three-dimensional images of the turbulent diffusivity were acquired at a spatial resolution of 280 µm (x) × 280 µm (y) × 1250 µm (z) for channel Reynolds numbers, Rec, of 210, 360, 720 and 1140. These data provide evidence of regions of turbulence inside the filter that has not been predicted by earlier numerical simulations. For Rec = 1140, a three-dimensional velocity image was also obtained at the same spatial resolution as the image of turbulent diffusivity; the data acquisition time was 2 h. Co-registration of these two images enables visualisation of the spatial extent and magnitude of these two characteristics of the flow field.

Gasoline direct injection engine soot oxidation: Fundamentals and determination of kinetic parameters

Current emissions legislation for road transport vehicles, including modern gasoline vehicle fleet limits the mass and the number of Particulate Matter (PM) emitted per kilometre. The introduction of a gasoline particulate filter (GPF) is expected to be necessary, as was the case for diesel vehicles, the traditionally recognised source of PM in transportation. Therefore, for the design of efficient GPFs and the regeneration strategies, soot oxidation characteristics in gasoline must be understood.Extensive research has been carried out mainly to investigate the oxidation of diesel soot, however, in the most cases soot were collected on microfiber filters and the activation energy was calculated with the logarithm method assuming mass and oxygen reaction orders equal to one. Identified limitations that lead to inconsistent and inaccurate trends and results are presented in this paper. As a consequence, a novel methodology to accurately obtain the oxidation kinetic parameters for soot emitted from a Gasoline Direct Injection (GDI) engine has been developed and presented in this paper. The particles collected in a silicon carbide wall-flow particulate filter are directly exposed to oxidation conditions in a thermogravimetric analysis (TGA) without the use of microfiber filter.The significance of more accurate and consistent calculations of soot oxidation kinetic parameters as a result of this methodology will aid modelling and experimental work of the aftertreatment systems and will lead in improving the GPF regeneration process in modern GDI vehicles. Avoiding high peak temperatures during regeneration and large thermal stress gradients and thus increasing the operating life of the filters is amongst the benefits can be seen.

On the Impact of Particulate Matter Distribution on Pressure Drop of Wall-Flow Particulate Filters

Wall-flow particulate filters are a required exhaust aftertreatment system to abate particulate matter emissions and meet current and incoming regulations applying worldwide to new generations of diesel and gasoline internal combustion engines. Despite the high filtration efficiency covering the whole range of emitted particle sizes, the porous substrate constitutes a flow restriction especially relevant as particulate matter, both soot and ash, is collected. The dependence of the resulting pressure drop, and hence the fuel consumption penalty, on the particulate matter distribution along the inlet channels is discussed in this paper taking as reference experimental data obtained in water injection tests before the particulate filter. This technique is demonstrated to reduce the particulate filter pressure drop without negative effects on filtration performance. In order to justify these experimental data, the characteristics of the particulate layer are diagnosed applying modeling techniques. Different soot mass distributions along the inlet channels are analyzed combined with porosity change to assess the new properties after water injection. Their influence on the subsequent soot loading process and regeneration is assessed. The results evidence the main mechanisms of the water injection at the filter inlet to reduce pressure drop and boost the interest for control strategies able to force the re-entrainment of most of the particulate matter towards the inlet channels’ end.

Filtration modelling in wall-flow particulate filters of low soot penetration thickness

A filtration model for wall-flow particulate filters based on the theory of packed beds of spherical particles is presented to diagnose the combined response of filtration efficiency and pressure drop from a reliable computation of the flow field and the porous media properties. The model takes as main assumption the experimentally well-known low soot penetration thickness inside the porous wall. The analysis of soot loading processes in different particulate filters shows the ability of the proposed approach to predict the filtration efficiency as a function of the particle size distribution. Nevertheless, pressure drop and overall filtration efficiency are determined by the mode diameter of the raw particulate matter emission. The results reveal the dependence of the filtration efficiency in clean conditions on the sticking coefficient. However, the dynamics of the pressure drop and filtration efficiency as the soot loading varies is governed by the soot penetration thickness. This parameter is closely related to the porous wall Peclet number, which accounts for the porous wall and flow properties influence on the deposition process. The effect of the transition from deep bed to cake filtration regime on the pressure drop is also discussed underlying the importance of the macroscale over microscale phenomena.

The effects of filter porosity and flow conditions on soot deposition/oxidation and pressure drop in particulate filters

Particulate filters are widely used in the automotive industry to reduce PM (particulate matter) produced by internal combustion engines. Wall-flow particulate filters trap PM while exhaust gas passes through the porous walls of the filter, with the pore microstructure of the porous walls affecting soot deposition, oxidation, and pressure drops during filtration and regeneration. In this study, soot deposition/oxidation behaviors were visualized in relation to the pressure drop, and the pressure drop characteristics of two particulate filters having different porosity were compared based on the results of the visualizations. It was found that the oxidation rate of the soot cake on channel walls is slower than that of the soot in the pores, due to limited oxidizer diffusion into the soot cake, which causes three-stage regeneration under the controlled regeneration regime. The high-porosity filter offered a lower pressure drop at the same amount of soot loading, faster pressure drop recovery, and higher regeneration efficiency during controlled regeneration.

Analysis of fluid-dynamic guidelines in diesel particulate filter sizing for fuel consumption reduction in post-turbo and pre-turbo placement

Wall-flow particulate filters are in the present days a standard aftertreatment system widely used in diesel engines to reduce particle emissions and meet emission regulations. This paper deals with the analysis of the macro- and meso-geometry definition of the DPF monoliths from a fluid-dynamic modelling approach. Focus is driven to the analysis of the influence on pressure drop and hence on engine fuel economy.The influence of the DPF volume on the engine performance is analysed with a gas dynamic software including both post-turbo and pre-turbo placement under clean and soot loading conditions. A swept in cell density is also considered for different thermal integrity factors. This approach allows analysing the trends in pressure drop and cell unit geometric parameters defining the monolith thermal and mechanical performance. A discussion considering constant specific filtration area and constant filtration area is performed providing a comprehensive understanding of the DPF and engine response as volume and cellular geometry are changed. Results are leading to rigorously justify known but usually empirical guidelines for DPF design in post-turbo applications. A discussion on the potential for monolith volume reduction in pre-turbo applications with respect to the post-turbo baseline is addressed. This is based on the very low sensitivity of fuel consumption and pressure drop both to volume reduction and soot and ash loading with pre-turbo DPF configuration.

Analysis of Asymmetric and Variable Cell Geometry Wall-Flow Particulate Filters

Analysis of Asymmetric and Variable Cell Geometry Wall-Flow Particulate Filters

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.