Wall-flow Diesel Particulate Filters Research Articles

In situ growth of ZnO nanorods on monolithic diesel particulate filters and supporting potassium for catalytic soot combustion

Soot particulate is one of the most important pollutants emitted from diesel engines. A monolithic wall-flow diesel particulate filter (DPF) is a commercial technology for the removal of soot via filtration and subsequent combustion with/without catalysts. Herein, in situ grown ZnO nanorods on DPFs are applied to improve both soot and potassium (K)-based catalyst dispersion efficiency and thus enhancing the activity of DPF regeneration. The pretreatment and hydrothermal synthesis procedures for the homogenously covered ZnO nanorod arrays were specifically investigated on aluminum titanate (Al2TiO5) and silicon carbide (SiC) and compared with cordierite (Mg2Al4Si5O18). Potassium carbonate (K2CO3) is further supported on these monolithic substrates with in situ grown ZnO nanorods acting as catalytic active components for soot combustion. The coexistence of in situ grown ZnO nanorods and K active species on the three monoliths leads to much better soot combustion activity than those bare monoliths. The ZnO nanorods can not only trap soot to improve the contact efficiency between soot and catalysts, but also disperse the active K species. This demonstrates a promising strategy to develop highly active catalytic DPF for diesel soot removal.

Experimental characterization of ultrafine particle emissions from a light-duty diesel engine equipped with a standard DPF

A detailed experimental characterization of the particle emissions of a EURO 5 light-duty Diesel engine, equipped with a wall flow Diesel Particulate Filter (DPF), is presented. Particle Size Distributions (PSDs) in the range from 4.5 up to 160 nm have been measured at the engine exhaust by means of a Scanning Mobility Particle Sizer, during both DPF accumulation and regeneration phase. During accumulation, the size-dependent DPF removal efficiency has been evaluated by PSDs measurements at both inlet and outlet of the filter. The maximum efficiency was found in the range 10-40 nm. Nevertheless, the DPF ensures a strong reduction in the emissions for the sub-23 nm nanoparticles too, with an efficiency between 91–95%. Unless for the warm-up phase, the engine operating conditions do not have a great impact on the particles emitted due to the high DPF removal efficiency. The measurements carried out during the regeneration phase evidenced that particle emissions strongly depend on DPF temperature. Actually, once the soot burnout temperature is reached, soot oxidation starts and a great amount of particles in terms of PN is released. Overall, PSDs showed an increase in particle number concentration up to two order of magnitude with respect to the emissions measured during the accumulation phase.

Experimental Study and Numerical Analysis on Pressure Drop and Temperature Field Characteristics of Asymmetric Cell Technology Diesel Particulate Filter for Silicon Carbide

ABSTRACT Due to its high filtration efficiency, wall-flow diesel particulate filter (DPF) has become the mainstream purification device for particulate matters (PM) of diesel engine all over the world. China VI emission regulations further tightens PM emissions, which puts out higher demands on all aspects of DPF performance. In the paper, experimental and numerical study were conducted on a common-rail diesel engine for the pressure drop and temperature field characteristics of asymmetric cell technology (ACT) diesel particulate filter for silicon carbide (SiC-DPF) with high cell density and thin wall. The results show that the application advantage of ACT DPF is significant under the conditions of higher soot loading and higher engine speed while SCT DPF is more suitable for the conditions of lower soot loading (less than 4 g/L). When ash loading exceeds 30 g/L, the pressure drop of ACT DPF is remarkably lower than that of symmetric cell technology (SCT) DPF. Compared with SCT DPF, the gas flow velocity is faster at the channel outlet and the particulate concentration deposited along the axial direction is more homogeneous for ACT DPF. Therefore, pressure drop is reduced efficiently with the increasing soot loading. Moreover, the regeneration temperatures inside the ACT DPF are uniform and stable, and the peak temperature does not exceed 750°C during active regeneration with the soot loading of 8 g/L, thus regeneration is safe.

Numerical study on flow and pressure drop characteristics of a novel type asymmetric wall-flow diesel particulate filter

In order to reduce the pressure drop and regeneration cost of diesel particulate filters (DPFs), and improve the power and economic performance of the diesel engines, a novel type of asymmetric channel wall-flow filter was proposed, including the regular hexagonal and triangular channels, as well as the rectangular channels. A one-dimensional mathematical model was built to study the flow characteristics, especially pressure drop performance, for this type of particulate filter, as well as a square symmetric and a square asymmetric channel filter for comparison. The effects of exhaust gas flow rate, cell density, wall thickness, and aspect ratio were numerically accessed. The results show that the novel asymmetric channel filter can effectively reduce the lifetime pressure drop by increasing the utilization ratio of the filtration wall and the filtration area. Specially, with the novel asymmetric channel structure, the wall pressure drop is considerably reduced owing to a lower mean wall penetration velocity. Furthermore, the depth layer and soot layer pressure drops of the novel type filter are much lower than those of the square symmetric channel filter owing to the smaller penetration velocity and thinner thickness of the soot layer, nearly the same as those of the square asymmetric channel filter. Additionally, the filter with a smaller aspect ratio can slightly reduce the pressure drop, so it can be practically applied if the distribution of the inlet gas is uniform. The novel asymmetric channel filter can meet the current and future demand for miniaturization and light-weighting of particulate filters, and has excellent market adaptability.

Numerical simulation of the particle filtration process inside porous walls using lattice Boltzmann method

A domain decomposition-based lattice Boltzmann-cell automation probabilistic model (DDLB-CA model) has been developed to investigate soot particle filtration process in wall-flow diesel particulate filters. In order to obtain useful information for optimization of the porous structure, non-virtual porous walls are considered. Nine different porous walls are generated by a self-developed reconstruction scheme based on the pore size distribution (PSD) and porosity. The DDLB-CA model is validated with the results of previous studies. A clear pressure gradient and a spatial inhomogeneous velocity distribution can be seen for each porous wall. For the porous wall with the smallest mean pore size and the largest porosity, a lower initial pressure gradient and a better initial homogeneous velocity distribution can be achieved. Particles tend to deposit at the front of the porous wall with a PSD of a smaller mean pore size. Besides, particles also have an obvious tendency of depositing on the surface of narrow porous channels. Particle capture probability is obviously affected by the PSD. Therefore, adjustment of the PSD is recommended for optimization of the particle distribution and filtration efficiency. The solid nodes composed of deposited soot particles appear on the surface of narrow porous channels first and then form dendritic structures. Finally, the dendrite structures construct a bridge and block the narrow porous channel. The distributions of solid nodes are affected obviously by the structure of porous media. The locations of solid nodes affect the distribution of pressure and the uniformity of velocity distribution. The subsequent particles are more inclined to deposit at the front of the porous wall and the particle deposition efficiency η increases after the formation of solid nodes. For the porous wall with a PSD of a smaller mean pore size, the solid nodes in front of porous walls (x/Lw < 0) are more concentrated, which means the cake layer will form more easily.

Performance enhancement of microwave assisted regeneration in a wall-flow diesel particulate filter based on field synergy theory

In this work, a three-dimensional mathematical model is established to investigate the flow and the heat transfer of the combustion process and the distribution characteristics of temperature field in the wall-flow in porous media of the wall-flow diesel particulate filter (DPF) based on the Field Synergy Theory (FST). The results indicate that there is the smallest temperature uniformity coefficient in wall-flow DPF when the inlet velocity is 36 m/s. And at first, the microwave assisted regeneration temperature increases and then decreases as the exhaust temperature rises which is affected by the exhaust temperature. The data indicate that the inlet pressure can change the angle between velocity vector and temperature gradient, which leads to the change of synergy degree of the velocity field and temperature field. When the inlet pressure is 0.08 MPa, there is an optimal synergy degree with the maximum average temperature.

Particle number measurements in the European legislation and future JRC activities

The solid particle number method was introduced in the European Union (EU) light-duty legislation for diesel vehicles to ensure the installation of the best-available technology for particles (i.e., wall-flow diesel particulate filters) without the uncertainties of the volatile nucleation mode and without the need of large investment for purchasing the equipment. Later it was extended to gasoline vehicles with direct injection engines, heavy-duty engines (both compression ignition and positive ignitions) and non-road mobile machinery engines. Real Driving Emissions (RDE) testing on the road with Portable Emissions Measurement Systems (PEMS) for particle number (and NOx) during type approval and in-service conformity testing was recently (in 2017) introduced for light-duty vehicles, and is under discussion for heavy-duty vehicles in-service conformity testing. This paper will summarize the existing legislation regarding solid particle number and discuss the on-going activities at EU level. The main focus at the moment is on improving the calibration procedures, and extending the lower detection size below 23 nm with inter-laboratory exercises. In parallel, discussions are on-going to introduce testing at low ambient temperature, regeneration emissions in the light-duty regulation, a particle limit for other technologies such as gasoline port-fuel injection vehicles, and the feasibility of particle measurements to L-category vehicles (mopeds, motorcycles, tricycles and minicars). A short overview of periodical technical inspection investigations and the situation regarding non-exhaust traffic related sources with special focus on brakes and tyres will be described.

Experimental measurement of the filtration efficiency and pressure drop of wall-flow diesel particulate filters (DPF) made of biomorphic Silicon Carbide using laboratory generated particles

Biomorphic Silicon Carbide (bioSiC) has been recently introduced in the scope of porous ceramic substrates for hot gas filtration applications, where it has demonstrated to have good thermal and mechanical properties, and a high potential to meet the requirements for current Diesel Particulate Filters (DPF). In this experimental study, a small wall-flow bioSiC diesel filter was characterized using a soot generator, the particle size distribution of which being similar to the one generated by a diesel engine. The bioSiC samples were manufactured from Medium Density Fiberboard (MDF) following a general manufacturing procedure for bioSiC ceramics, but paying special attention in the mechanizing stage to the geometry and optimal design of the honeycomb structure required for diesel engine applications. The samples had a cell density of 57.59 cell/cm2 (371.6 cpsi), a square cross section of 9.2 × 9.2 mm, and a length of 31 mm. To generate the particle laden stream and perform the filtration tests, a synthetic Soot Generator (SG) was used. Tests were performed under controlled and reproducible conditions, with a fixed gas flow rate of 5 LPM and a soot mass flow rate of 4 mg/h. The filtration efficiency was determined with the aid of a Scanning Mobility Particle Sizer (SMPS) from the measurements of the particle concentration upstream and downstream the filter samples. During the soot loading process, the pressure drop was also monitored. The results show that, in the initial stage (clean filter), bioSiC wall-flow DPFs may have a filtration efficiency between 0.7 and 0.85 and a pressure drop of around 2 kPa for a normalized wall velocity of 0.01 m/s at ambient temperature. The filtration performance of wall-flow bioSiC particle filters showed in this work can help us to better understand their real potential for automotive applications.

Lattice Boltzmann simulations for wall-flow dynamics in porous ceramic diesel particulate filters

Flows through porous filter walls of wall-flow diesel particulate filter are investigated using the lattice Boltzmann method (LBM). The microscopic model of the realistic filter wall is represented by randomly overlapped arrays of solid spheres. The LB simulation results are first validated by comparison to those from previous hydrodynamic theories and constitutive models for flows in porous media with simple regular and random solid-wall configurations. We demonstrate that the newly designed randomly overlapped array structures of porous walls allow reliable and accurate simulations for the porous wall-flow dynamics in a wide range of solid volume fractions from 0.01 to about 0.8, which is beyond the maximum random packing limit of 0.625. The permeable performance of porous media is scrutinized by changing the solid volume fraction and particle Reynolds number using Darcy’s law and Forchheimer’s extension in the laminar flow region.

Optimized microwave susceptible catalytic diesel soot trap

The wall-flow diesel particulate filter (DPF) is currently the usual post-treatment system in diesel engines for soot particle abatement in passenger, heavy-duty and non-road vehicles. It consists in alternately plugged parallel square channels, in order to force the exhaust gases to flow through the porous inner walls: soot is so collected on the surface and in the porosity of the channel walls, progressively blocking the pores, making mandatory the filter regeneration. In this work we realised a specific pre-treatment procedure in order to modify the porosimetric characteristics of the bare support, with the aim of increasing the active species load and simultaneously keeping the pressure drop and the filtration efficiency of the final catalytic system in the same range of an uncatalysed DPF. The pre-treatment is based on a controlled chemical erosion of the SiC porous structure with a HF:HNO3 acid solution. The results showed that, if compared with the analogues prepared without acid treatment, the pre-treated samples resulted with a modified pores distribution, with an increased average pore diameter. This important result allowed us to obtain new catalytic filters with higher catalyst load, higher regeneration activity and simultaneously keeping the pressure drop at similar values of the uncatalysed filters. As a consequence of these improved performances, a decrease in the frequency and duration of the regeneration step will be possible without affecting the filtration efficiency, as verified from the deposition and on-line microwave assisted regeneration tests. In particular, the results showed that the simultaneous porosity modification and the higher catalyst loading (30%wt of copper ferrite) allowed to decrease the threshold catalyst temperature to about 350°C, but more importantly allowed to decrease the regeneration step duration from about 22 to about 15min. Moreover, the addition of K to the catalyst formulation had the important effect of further decreasing the regeneration time to about 8min, even if it had no effect on the threshold temperature. So, very positive effects on the performances of the final catalysed filter prepared on the carrier with a modified porosity with the proposed procedure are observed on the energy needed to regenerate the filter: the comparison of the microwave energy supplied, considering the same filter volume (about 0.35liters), during the microwave assisted regeneration phase of all the filter typologies considered in this work, compared to the traditional fuel post-injection, showed that just the addition of the catalyst allows an energy saving of about 60%, that increased up to 75% after the K addition to copper ferrite to a filter with modified porosity.

A control-oriented modular one-dimensional model for wall-flow diesel particulate filters

A comprehensive modular one-dimensional physics-based mathematical model is developed for non-isothermal compressible flow, pressure drop, and filtration and regeneration processes in wall-flow diesel particulate filters. Employing a modified orthogonal collocation method and symbolic computation in Maple™, the governing partial differential equations are reduced to a control-oriented model governed by ordinary differential equations which can be solved in real time. Numerical examples are provided to indicate the accuracy and computational efficiency of the developed model and to study the different behaviors of wall-flow diesel particulate filters.

Experimental and computational approach to the transient behaviour of wall-flow diesel particulate filters

The implementation of tight vehicle emission standards has forced manufactures to use aftertreatment systems extensively. In addition to pollutant emissions abatement, these devices have a noticeable impact on the wave pattern. This fact affects the muffler design criteria. All monolithic aftertreatment devices produces a damping effect because of the honeycomb structure and the narrow channels. However, this response is more marked in wall-flow diesel particulate filters (DPF) because of the alternatively plugged ends and the dissipative properties of the porous substrate.The main goal of this paper is to assess the transient fluid-dynamic behaviour of wall-flow DPFs using experimental and modelling techniques. The experimental data were gathered in clean and loaded conditions. The DPF was subjected to a variety of pressure excitations to characterise its transient behaviour in the time and frequency domains. Afterwards, the DPF response was evaluated under engine-like operating conditions in an unsteady flow gas stand. Once the main characteristics of the response were known, a non-linear gas-dynamics model was proposed for analysis and prediction. The model accounts for space and time gradients, combining the thermo-and fluid-dynamic solution with a model based on a packed bed of spherical particles that defines the meso-structure of the loaded substrate.

Numerical simulation on trapping efficiency of steady filtration process in diesel particulate filter and its experimental verification

Taking wall-flow diesel particulate filter (DPF) as the research objective and separately assuming its filtering wall to be composed of numerous spherical or cylindrical elements, two different mathematical models of steady filtration for wall-flow diesel particulate filter were developed and verified by experiments as well as numerically solved. Furthermore, the effects of the macroand micro-structural parameters of filtering wall and exhaust-flow characteristic parameters on trapping efficiency were also analyzed and researched. The results show that: 1) The two developed mathematical models are consistent with the prediction of variation of particulate size; the influence of various factors on the steady trapping efficiency is exactly the same. Compared to model 2, model 1 is more suitable for describing the steady filtration process of wall-flow diesel particulate filter; 2)The major influencing factors on steady trapping efficiency of wall-flow diesel particulate filter are the macro-and micro-structural parameters of filtering wall; and the secondary influencing factors are the exhaust-flow characteristic parameters and macro-structural parameters of filter; 3)The steady trapping efficiency will be improved by increasing filter body volume, pore density as well as wall thickness and by decreasing exhaust-flow, but effects will be weakened when particulate size exceeds a certain critical value; 4) The steady trapping efficiency will be significantly improved by increasing exhaust-flow temperature and filtering wall thickness, but effects will be also weakened when particulate size exceeds a certain critical value; 5) The steady trapping efficiency will approximately linearly increase with reducing porosity, micropore aperture and pore width.

Impact on vehicle fuel economy of the soot loading on diesel particulate filters made of different substrate materials

Wall flow DPFs (Diesel Particulate Filters) are nowadays universally adopted for all European passenger cars.Since the properties of the filter substrate material play a fundamental role in determining the optimal soot loading level to be reached before DPF regeneration, three different filter material substrates (Silicon Carbide, Aluminum Titanate and Cordierite) were investigated in this work, considering different driving conditions, after treatment layouts and regeneration strategies.In the first step of the research, an experimental investigation on the three different substrates over the NEDC (New European Driving Cycle) was performed. The data obtained from experiments were then used for the calibration and the validation of a one dimensional fluid-dynamic engine and after treatment simulation model. Afterward, the model was used to predict the vehicle fuel consumption increments as a function of the exhaust back pressure due to the soot loading for different driving cycles. The results showed that appreciable fuel consumption increments could be noticed only in particular driving conditions, and, as a consequence, in most of the cases the optimal filter regeneration strategy corresponds to reach the highest soot loading that still ensures the component safety even in case of uncontrolled regeneration events.

Analytic Solution for the Flow Distribution and Pressure Drop of Ceramic Partially-Plugged Wall Flow Diesel Particulate Filters

Analytic Solution for the Flow Distribution and Pressure Drop of Ceramic Partially-Plugged Wall Flow Diesel Particulate Filters

Pre-DPF water injection technique for pressure drop control in loaded wall-flow diesel particulate filters

Wall-flow type diesel particulate filter (DPF) is a required aftertreatment system for particle emission abatement and standards fulfilment in Diesel engines. However, the DPF use involves an important flow restriction, especially as the substrate gets soot and ash loaded. It gives as a result the increase of the exhaust back-pressure and hence a fuel consumption penalty. The increasing damage of fuel consumption with DPF soot loading leads to the need of the regeneration process. Usually based on active strategies, this process involves an additional fuel penalty but prevents from excessive DPF pressure drop and ensures secure soot burnt out.Under this context, new solutions are required to improve the state of the art DPF soot loading to pressure drop ratio. This paper presents a novel technique based on pre-DPF water injection to reduce the DPF pressure drop under soot loading conditions by disrupting its dependence on soot/ash loading. It provides benefits to engine fuel economy and also higher flexibility for DPF regeneration and maintenance. The work covers a test campaign performed in a passenger car turbocharged Diesel engine equipped with a wall-flow DPF. The main objective is to describe the technique, to provide a figure of its potential for pressure drop control and fuel consumption reduction. The results of the experiments also confirm soot and ash loading capacity increase and demonstrate the lack of negative effects on filtration efficiency and active and passive regeneration.

A methodology for estimating the permeability of a soot deposit in a wall-flow diesel particulate filter

An estimation of the realistic permeability of a soot deposit layer accumulated in a wall-flow diesel particulate filter was performed in this study. A unique contribution of this work is that the current methodology is highly applicable to the real-world design and development processes of diesel particulate filters. To conduct baseline simulation, a one-dimensional numerical model describing the behaviours of the exhaust gas and soot within a diesel particulate filter was constructed, employing the well-known single-channel approach. On the basis of this in-house model, the microstructural properties of a soot deposit layer were calibrated by finding the best fit to the measurement data adopted from the open literature. For the optimal applicability of the proposed methodology, the permeability of the deposit is modelled as a variable which depends on the temperature and the average wall velocity, while the porosity is constant. The current model and methodology were validated by comparison with the measurement data for pressure drops through diesel particulate filters. In particular, to boost the practical implementation of the current methodology, a map of the permeability of the soot deposit was created as a function of the operating conditions. In the last part of this study, the pressure drop characteristics through a diesel particulate filter under various operating conditions were examined using an analysis of the breakdown of the individual pressure drop mechanisms.

Simulation of flow in diesel particulate filter system using metal fiber filter media

Metal fiber is introduced as a new filter media in wall-flow Diesel Particulate Filter (DPF) system. This technology has high temperature durability which is required for filter regeneration, and can maintain the mechanical strength even in the extreme exhaust-related vibrations of vehicles. However, the regeneration near the wall (outer layer) is more difficult because of the heat loss and reduced gas flow near the wall. In this study, a flow is simulated to determine the flow control method for the more uniform flow in all filter layer. By using Star CCM+ commercial software, we obtain local velocity, streamline, and pressure distributions in the filter, which are typically difficult to obtain from measurements. The major control factors are the filter porosity, size and location of the distribution plate, and the number of blades of the swirler. By placing the distribution plate in front of the filter, the flow velocity near the wall was increased. The optimum location and size of the flat plate were chosen. By attaching the blade on the plate the flow velocity near the wall was increased more. Therefore, the regeneration efficiency is increased by using the swirler-type metal fiber DPF system.

Particulate emissions from diesel engines: correlation between engine technology and emissions

In the last 30 years, diesel engines have made rapid progress to increased efficiency, environmental protection and comfort for both light- and heavy-duty applications. The technical developments include all issues from fuel to combustion process to exhaust gas aftertreatment. This paper provides a comprehensive summary of the available literature regarding technical developments and their impact on the reduction of pollutant emission. This includes emission legislation, fuel quality, diesel engine- and exhaust gas aftertreatment technologies, as well as particulate composition, with a focus on the mass-related particulate emission of on-road vehicle applications. Diesel engine technologies representative of real-world on-road applications will be highlighted.Internal engine modifications now make it possible to minimize particulate and nitrogen oxide emissions with nearly no reduction in power. Among these modifications are cooled exhaust gas recirculation, optimized injections systems, adapted charging systems and optimized combustion processes with high turbulence. With introduction and optimization of exhaust gas aftertreatment systems, such as the diesel oxidation catalyst and the diesel particulate trap, as well as NOx-reduction systems, pollutant emissions have been significantly decreased. Today, sulfur poisoning of diesel oxidation catalysts is no longer considered a problem due to the low-sulfur fuel used in Europe. In the future, there will be an increased use of bio-fuels, which generally have a positive impact on the particulate emissions and do not increase the particle number emissions.Since the introduction of the EU emissions legislation, all emission limits have been reduced by over 90%. Further steps can be expected in the future. Retrospectively, the particulate emissions of modern diesel engines with respect to quality and quantity cannot be compared with those of older engines. Internal engine modifications lead to a clear reduction of the particulate emissions without a negative impact on the particulate-size distribution towards smaller particles. The residual particles can be trapped in a diesel particulate trap independent of their size or the engine operating mode. The usage of a wall-flow diesel particulate filter leads to an extreme reduction of the emitted particulate mass and number, approaching 100%. A reduced particulate mass emission is always connected to a reduced particle number emission.

Filtration Efficiency and Pressure Drop Performance of Ceramic Partial Wall Flow Diesel Particulate Filters

Filtration Efficiency and Pressure Drop Performance of Ceramic Partial Wall Flow Diesel Particulate Filters