•PN emissions of Euro 6 gasoline and CNG vehicles in the size ranges 10nm and higher.•Comparisons at different sampling locations.•Analysis of particle emission in different operating conditions ...including cold start.•Comparisons among DI and MPI injection systems in terms of particle emissions.•Correlation of the particle emissions to the rated power displacement ratio of the engines.
Current particle number (PN) emission limits set by regulation, involve counted particles with a dimension >23 nm. The measurement procedure is specified and involves the dilution of the exhaust gas in a so-called constant volume sampling (CVS) device. Research efforts are concentrating in the further development of existing measurement techniques in order to capture smaller particles down to 10 nm, given their higher health threatening potential.
In the present study, six state of art, Euro 6, gasoline vehicles and in addition, one compressed natural gas (CNG) light duty vehicle, have been measured on the chassis dynamometer during different test cycles. Three particle sampling lines have been used, two in parallel at the CVS, counting particles >23 nm and 10 nm, and a third one directly at the tailpipe of the vehicle. The results allow a detailed evaluation of the emitted PNs. In addition, differences in the emissions patterns of the direct fuel injection (DI) and multi point port injection (MPI) gasoline vehicles could be identified. During cold starts, particles have been separated in distinctive size classes in order to obtain relevant number-size distributions.
Counting particles >10 nm resulted in roughly doubling the PN emissions in respect to those when counting particles >23 nm. This relation holds for all examined driving cycles. PNs measured at the CVS where significantly higher than at the tailpipe, especially when capturing also the smaller particles. The CVS could be identified contributing to the increase of the registered particle numbers during cycle parts with no or very low engine particle emissions.
Lowest PN emissions have been measured in combination with the CNG vehicle. The differences between DI and MPI gasoline vehicles have been significantly lower than expected from previous studies. While the MPI gasoline vehicles have been identified to emit more PN during cold start, the DI vehicles emit larger numbers during high engine loads. During cold starts, higher emissions of smallest particles have been evident.
The increased rated power to displacement ratio of modern engines, based on the current “downsizing” trend, shows a good correlation with the PN emissions.
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•An integral design approach for new generation catalysts’ is proposed.•Numerical study of mass transfer in different polyhedral lattices is performed.•Open cell polyhedral lattices ...have higher mass transfer than honeycombs.•The optimal cell is Cubic45 with low strut diameter and high porosity.•Tests in a gas flow reactor confirm numerical simulations results.
Polyhedral open cell lattice catalyst substrates are proposed based on results of numerical simulations and recent advances in Additive Manufacturing (AM) techniques.
Detailed simulations have compared different polyhedral structures in terms of mass transfer (aiming at optimal reactivity in the mass transfer limited domain) and flow through resistance. The simulations have taken into account dimensional limits given by the possibilities of AM techniques. Comparisons with state of art honeycombs have also been used in order to identify the optimal shape.
Substrates with these optimal characteristics have been manufactured out of Al2O3 with Stereolithography. Subsequently, these substrates have been coated and used for measurements of C3H6 oxidation in a model gas reactor. Measurements have focused in determining oxidation efficiency at different gas hourly space velocities as well as light-off behaviour.
Simulation results show that the optimal open cell structures are comprised by a cubic elementary cell rotated by 45° so that one spatial diagonal of the cube is aligned to the main gas flow. Higher porosities and smaller strut diameters improve the reactivity to pressure drop trade off. However, given the current manufacturing limitations, it is not possible to produce structures with strut diameter lower than 0.5 mm. This results in high porosity but low specific surface area (i.e. ε = 0.95 and Sv = 400 m2/m3). Thus, reaching a target conversion requires higher overall catalyst volume. The simulations show that for a series of geometrical parameters the open cell structures can reach identical conversion in respect to the honeycombs with only a fraction of the overall surface area and thus a fraction of the noble metals, while the overall dimensions are in the same order of magnitude and the pressure drop can reach lower levels.
Measurements in the model gas reactor confirm the mass transfer advantages of the polyhedral structures as predicted by the simulations. Measurements also show that the polyhedral lattices have very similar light-off behaviour in spite the four times lower surface area.
Brake wear particles constitute a significant part of the non-exhaust related particulate matter (PM) associated to traffic. These particles derive from various components of the vehicles’ brake ...system and were recognized as an important pollution source only recently. In the present study, electron microscopy was applied to elucidate brake wear particle morphology and associated chemical composition. Sampling was carried out on a brake test bench. Particles were collected on 13 successive polycarbonate foil bags and a backup filter by means of a Dekati Low Pressure Impactor (DLPI), based on the equivalent aerodynamic particle diameter. The results revealed a broad size range from the micro-to the nano-scale, as measured on electron microscope images, including coarse (2.5–10 μm), fine (0.1–2.5 μm) and ultrafine ones (<0.1 μm), down to a few nanometers. It cannot be implied with certainty to what extent the particle size spectrum identified under the experimental conditions of the present study strictly applies to real-world conditions. The particles occur typically in form of aggregates, also in the ultrafine scale; single, i.e. non-agglomerated particles are more common in the ultrafine fraction than at larger sizes. Imaged particles have commonly rounded outlines probably due to friction and associated abrasion during braking; melting/evaporation at high temperature braking phases, associated with the rate of the generated frictional energy of braking, and subsequent rapid solidification can be invoked to interpret the nearly spherical shape of nanometer size particles. The individual constituents of the aggregates have various sizes and chemical composition. Energy dispersive X-ray analysis of the imaged aggregates and of their constituents showed that Fe is prevalent in all size fractions but is less pronounced in the smaller sizes, where Ca, occasionally in combination with S and/or P becomes more frequent. Other frequently analysed elements are Al, Sn, Mg, Si, Cr, Ti, K and W; less frequent are Ni, Zn, Zr, Ba, S, C, P, F, Mn and rarely Bi and Sb.
•Brake wear particles include a wide size range from the micro-to nano-scale.•The particles occur mainly as polydisperse, heterogeneous aggregates.•Nanoscale particles are often nearly spherical and occur also as single particles.•The particles' chemical composition is dominated by Fe and in smaller sizes by Ca ± Fe ± Ti.•Other commonly analysed elements include Al, Sn, Mg, Si, Cr, Ti, K ± W.
Investigation of ash PM deposited in a diesel particulate filter (DPF) operating on a light truck by means of SEM and TEM reveals the following: ash inside the DPF occurs in form of chemically very ...inhomogeneous, mostly brittle agglomerates accumulated at the plugged ends of inlet channels and deposited directly on the inlet channel walls all along the filter length. Ash agglomerates occur within pores of the channel walls. A minor part of ash PM may escape to the atmosphere. The individual ash phases are mostly crystalline with round outlines and sizes between ca. 170 and 60 nm, down to 7–12 nm, that is far below the breathable size range PM10. Aggregation of the predominantly finest fraction of ash particles leads to densification, which may translate to fewer breakouts from the DPF. EDX mapping and chemical analyses of the bulk ash reveal that ash consists mainly of Ca, Mg, P, Zn, S, O and minor Fe, Al and Si. Based on TEM diffraction data of ash single phases, combined with data on their chemistry, the diversity of ash phases is higher than previously presumed. Comparison with the ash particles of a heavily used DPF from a passenger car operating with Fe-based fuel-borne additives reveals characteristics very similar to those found for the light truck DPF with a tendency to generally lower sizes of the participating phases, mostly between 30 and 60 nm. 4–40 nm large, locally abundant Pt particles deriving from the coating material of the diesel oxidation catalyst (DOC) upstream the DPF occur within ash agglomerates of both DPFs. Ash collected from the exhaust gas at the exit of the light truck DPF under normal engine operation reveals that some fine particles, as well as a few of the larger (200–600 nm) ash-bearing agglomerates escape filtration. Very fine ash particles are reaching the atmosphere also attached onto soot agglomerates.
► Ash agglomerates, up to a few μm large, escape filtration and reach the ambient air. ► The size of individual ash phases goes down to a few nanometers, far below PM10. ► Part of the finest ash particulates form larger aggregates and may thus be trapped by DPFs. ► Ash microscopic characteristics are independent of aftertreatment conditions and engine loading. ► Pt particles (4–40 nm) deriving from the oxidation catalyst surface may reach ambient air.
•We describe the implementation of a CFD model for the simulation of reacting flows through catalyzed porous substrates.•We validate the model on the basis of previous experimental works available in ...the literature.•The numerical model has been applied to investigate the physical phenomena occurring at the micro-scale of open-cell foams.•Light-off curve for CO combustion was computed in case of foam-type reactor under different operating conditions.
Open cell foams are regarded with interest for applications as catalytic substrates for combustion, reformers and after-treatment converters for the pollutant emissions control. In this context, CFD represents a reliable and convenient tool for investigating and understanding the physical phenomena occurring at the micro-scale, in order to design and optimize these substrates. A CFD model for the simulation of the catalytic reactions occurring over the surface of open-cell foams is implemented and validated. The approach is based on a coupled finite-volume/finite-area strategy capable to describe the fluid-dynamic and the chemical phenomena occurring in both the fluid phase and solid phases. The adsorption/desorption of the reactants on the active sites and the surface reaction is modeled on the basis of a Langmuir–Hinshelwood mechanism. The model is able to describe the reactants conversion under both kinetics and diffusion control, allowing to predict the light-off curve characterizing the catalyst-coated foam substrate.
Aircraft soot has a significant impact on global and local air pollution and is of particular concern for the population working at airports and living nearby. The morphology and chemistry of soot ...are related to its reactivity and depend mainly on engine operating conditions and fuel-type. We investigated the morphology (by transmission electron microscopy) and chemistry (by X-ray micro-spectroscopy) of soot from the exhaust of a CFM 56-7B26 turbofan engine, currently the most common engine in aviation fleet, operated in the test cell of SR Technics, Zurich airport. Standard kerosene (Jet A-1) and a biofuel blend (Jet A-1 with 32% HEFA) were used at ground idle and climb-out engine thrust, as these conditions highly influence air quality at airport areas. The results indicate that soot reactivity decreases from ground idle to climb-out conditions for both fuel types. Nearly one third of the primary soot particles generated by the blended fuel at climb-out engine thrust bear an outer amorphous shell implying higher reactivity. This characteristic referring to soot reactivity needs to be taken into account when evaluating the advantage of HEFA blending at high engine thrust. The soot type that is most prone to react with its surrounding is generated by Jet A-1 fuel at ground idle. Biofuel blending slightly lowers soot reactivity at ground idle but does the opposite at climb-out conditions. As far as soot reactivity is concerned, biofuels can prove beneficial for airports where ground idle is a common situation; the benefit of biofuels for climb-out conditions is uncertain.
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•Soot produced by conventional kerosene Jet A-1 at ground idle is most prone for reaction.•Biofuel leads to soot reactivity reduction at ground idle but not at climb-out conditions.•Soot reactivity decreases with increasing engine thrust.•Considering soot reactivity, biofuel may prove beneficial for ground idle; the benefit for climb-out is uncertain.
Conventional kerosene (Jet A-1) at ground idle produces the most reactive soot; biofuel can prove beneficial for ground idle; its advantage for climb-out is uncertain.
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•Warm up during cold starts inside an open cell catalytic converter is analyzed.•CFD simulations of the transient heat and mass transfer launched in OpenFOAM.•Open cell polyhedral ...lattices have higher mass and heat transfer than honeycombs.•Optimal converter design for faster warm up.•Dimensionless model of the thermal response of the open cell catalytic converter.
Additive Manufactured (AM) Open Cell Polyhedral lattices are novel substrates for automotive catalytic converters due to promising properties. The present investigation focuses on heat and mass transfer with chemical reactions during cold starts, based on numerical simulations in OpenFOAM and dimensionless analytical analysis. The numerical model consists of a multi-region approach with overlapping meshes for fluid and solid regions, in order to simulate the presence of porous substrates. Experimental results from first vehicle-size AM catalysts are used as a basis. The catalyst heat-up is characterized by two distinguished phases: the initial phase where heat is convected from the inflowing gases to the catalyst and the following phase which is governed by the heat released by the chemical reactions. The impact of different operating parameters, lattice and converter geometries has been quantified. The introduction of dimensionless temperature, time and space, evidences the similarity of the initial warm-up phase.
Experiments were carried out to investigate the performance of different fuels used in a internal combustion engine: gasoline, methane and fuel blends containing methane with 5%, 10% and 15% hydrogen ...by volume, respectively. A two-litre naturally aspirated bi-fuel engine with port fuel injection was used. The engine was operated stoichiometrically. For each fuel the spark advance for best efficiency was determined. Experiments were conducted at 2000
rpm and 2
bar brake mean effective pressure. A heat release analysis and a loss analysis were performed for all fuels. The main findings are that increasing the hydrogen fraction of the methane hydrogen fuel blend decreases the overall burn duration. This decrease is predominantly achieved by a shortened duration of the fist stage of combustion (ignition to 5% mass fraction burned). The faster combustion comes along with an increase in fuel conversion efficiency. The different losses for gasoline and pure methane operation interact such that equal fuel conversion efficiencies result. However, care has to be taken when comparing fuel conversion efficiencies among the different fuels as the relative error in fuel conversion efficiency for the gaseous fuels is 0.2% at most, whereas it is about 1% for gasoline.
Reticulated ceramic foams are widely used for industrial applications such as metal filtration, exhaust gas and air purification, catalyst support and others. In this work, the compression strength ...and specific surface area of reticulated foams have been improved, while at the same time maintaining a high level of permeability in the final foam structure. In particular, a vacuum infiltration step by using a suitable slurry, followed by a pre-sintering cycle was adopted for filling up the hollow struts, generated due to the burnout of the PU foam. Furthermore, various mixtures of fine and coarse-grained alumina as well as in combination with zirconia, were utilised with the aim of controlling the foam properties such as compression strength, specific surface area and permeability. The compression strength was improved by a factor of two for alumina foams by infiltrating the hollow struts, and by a factor of four when infiltrating the struts of ZTA foams, with the composition 70
mol% Al
2O
3 and 30
mol% ZrO
2. The weight gain resulting from the vacuum infiltration process was in the order of 10
wt%.
Abstract
A mathematical model is developed for the simulation of the chemical and transport phenomena in three-way catalysts for natural-gas vehicles, with honeycomb and foam substrates. The chemical ...phenomena are modelled on the basis of a simplified approach, which assumes that pollutant conversion is governed by oxygen storage and release dynamics. The transport phenomena, i.e. heat transfer, gas-phase mass transfer, washcoat diffusion, and pressure drop, are modelled on the basis of correlations adapted from the literature. The model is validated using lambda scan tests and a cold-start driving cycle, carried out with a honeycomb catalyst and a ceramic foam catalyst. The experimental data show a decline in methane conversion for lean mixtures, which is modelled by an inhibition term of methane oxidation from nitric oxide. The proposed model may capture the performance of both substrates with sufficient accuracy, both under cold-start and under hot-mode conditions. The model-based comparison of the two substrates shows that the ceramic foam presents inferior carbon monoxide conversion in the extra-urban part of the cycle. Although the gas-phase mass transfer in the ceramic foam is faster than with the honeycomb monolith, diffusion in the washcoat pores is much slower. As a result, the mass-transfer-limited conversion efficiency of the foam is lower. This is primarily attributed to the higher washcoat thickness of the foam sample tested.
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