Synthetic Exhaust Gas Research Articles

Impact of operation parameters and lambda input signal during lambda-dithering of three-way catalysts for low-temperature performance enhancement

A synthetic exhaust gas bench was dynamically operated to investigate the impact of temperature, amplitude, split cycle, mean lambda, gas hourly space velocity, and oxygen storage capacity on average pollutant conversion and product selectivity of three-way catalysts in periodic operation. As temperature and amplitude increase and oxygen storage capacity decreases, the optimal frequency for maximum pollutant conversion increases. This is consistent with faster desorption of CO and O2 from the catalyst, yielding free surface sites. Regarding the formation of secondary products, the optimal frequency for maximum pollutant conversion does not always correspond to minimal N2O and NH3 emissions. The split cycle variation reveals the enhancement of C3H8 and NO conversion after both lean-rich and rich-lean switches and C3H6 and CO conversion after rich-lean switches at the optimal frequency. As periodic operation does not affect existing engine settings or operating conditions, it is a cost-effective control strategy for meeting future emission limits.

Combined Ash and Soot Monitoring for Gasoline Particulate Filters Using a Radio-Frequency-Based Sensor

Increasingly stringent emission limits have made particulate filters necessary for gasoline engines. Similar to diesel applications, gasoline particulate filters (GPFs) can be monitored by differential pressure measurement or by the radio-frequency-based filter diagnosis (RF sensor). In addition to measuring the soot loading, ash detection is critical for monitoring the GPF over the entire vehicle lifetime. Because the RF sensor detects the filter loading through a change in the dielectric properties of the GPF, it can detect not only soot but also ash. In diesel applications, the RF sensor has already demonstrated its potential for ash detection. To verify the feasibility of simultaneous ash and soot monitoring for GPFs, filters were loaded with ash on an engine test bench and measured on a lab test bench under defined synthetic exhaust gas conditions. By evaluating resonant modes, soot and ash could be clearly distinguished, as ash mainly affects the resonant frequency, while soot also changes the quality factor due to its high dielectric losses. However, higher soot loadings could not be detected by the resonant parameters, but instead by a frequency-averaged transmission signal. While the presence of ash caused an offset in this signal, its sensitivity to soot was not affected. Thus, the influence of ash can be corrected if the signal in the soot-free filter state is known, e.g., from the behavior of the resonant parameters. Therefore, even with a continuously increasing ash loading over the lifetime of a vehicle, an accurate soot detection is possible with the RF sensor.

Selective Catalytic Reduction with Hydrogen for Exhaust gas after-treatment of Hydrogen Combustion Engines

In this work, two palladium-based catalysts with either ZSM-5 or Zeolite Y as support material are tested for their performance in selective catalytic reduction of NOx with hydrogen (H2-SCR). The ligh-toff measurements in synthetic exhaust gas mixtures typical for hydrogen combustion engines are supplemented by detailed catalyst characterization comprising N2 physisorption, X-ray powder diffraction (XRD), hydrogen temperature programmed reduction (H2-TPR) and ammonia temperature programmed desorption (NH3-TPD). Introducing 10% or 20% TiO2 into the catalyst formulations reduced the surface area and the number of acidic sites for both catalysts, however, more severely for the Zeolite Y-supported catalysts. The higher reducibility of the Pd particles that was uncovered by H2-TPR resulted in an improved catalytic performance during the light-off measurements and substantially boosted NO conversion. Upon exposition to humid exhaust gas, the ZSM-5-supported catalysts showed a significant drop in performance, whereas the Zeolite Y-supported catalyst kept the high levels of conversion while shifting the selectivity from N2O more toward NH3 and N2. The 1%Pd/20%TiO2/HY catalyst subject to this work outperforms one of the most active and selective benchmark catalyst formulations, 1%Pd/5%V2O5/20%TiO2-Al2O3, making Zeolite Y a promising support material for H2-SCR catalyst formulations that allow efficient and selective NOx-removal from exhaust gases originating from hydrogen-fueled engines.

High temperature oxidation behavior of type 444 stainless steel in synthetic automotive exhaust gas

The oxidation behaviors of type 444 ferritic stainless steel (FSS) were studied in air and synthetic exhaust gas at 1000, 1050 and 1100 °C, respectively, by means of mass gain, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The oxidation rate of 444 FSS enlarges in both environments with the increase of temperature, but is faster in synthetic exhaust gas than in air under isothermal conditions. The oxidation rate becomes very fast since the occurrence of breakaway transition. Both the higher temperature and the synthetic exhaust gas facilitate the breakaway oxidation. The breakaway oxidation results mainly from the growth of oxide nodules and the spallation of oxide scale. The synthetic exhaust gas isn't conducive to the formation of SiO2 layer in comparison with the air. The oxidation mechanisms are discussed on the basis of the oxide scale characteristics.

Effect of phase formation due to holding time of vacuum brazed AISI 304L/NiCrSiB joints on corrosion fatigue properties

Whether in turbine components or exhaust gas heat exchangers, vacuum-brazed nickel-based joints are subjected to varying cyclical loads during their applications, often in corrosive environments. The microstructure of the brazed seam, which is determined by the alloy composition and the brazing process parameters, is essential for the service life. In this experimental study a modified BNi-5a foil was produced and used to braze cylindrical AISI 304L butt joints with two different holding times. Using energy dispersive spectroscopy analyses, a direct correlation of the element distribution at the brazing seam with the holding time was detected as a result of diffusion processes. Individual phases were identified, and it could be shown that the longer holding time led to a reduction of borides and silicides as well as to a more even microhardness curve through the seam. The effect of the microstructure on the corrosion fatigue properties was evaluated using multiple amplitude tests by a stepwise increase of the maximum stress amplitude in synthetic exhaust gas condensate. Thereby, improved corrosion fatigue and cyclic deformation behaviors were achieved for the more homogeneous microstructure. Afterwards, topography analyses of the fracture surfaces and cross-sectional fracture investigations enabled an understanding of microstructure-dependent damage mechanisms including fatigue crack initiation and propagation.

The influence of near service environmental conditions on the corrosion and LCF behaviour of a beta-stabilized γ-TiAl alloy

The damage behaviour of the TiAl alloy Ti-43.5Al-4Nb-1Mo-0.1B was investigated by isothermal thermogravimetric-analysis (TGA) and strain controlled low-cycle-fatigue (LCF) experiments at high temperature. Aluminium depletion and the formation of a continuous brittle layer in the subsurface zone takes place in synthetic exhaust gas. Contrary, a continuous ductile γ-phase forms in the subsurface zone in air. Since a continuous TiN layer as diffusion barrier is not formed in the early stages of corrosion, strong hardening is observed in LCF test samples in exhaust gas and a strain amplitude of 0,6 %, leading to a significant reduction of the cycles to failure.

Fatigue and Corrosion Fatigue Behaviour of Brazed Stainless Steel Joints AISI 304L/BAu-4 in Synthetic Exhaust Gas Condensate.

As brazed stainless steel components in service often have to withstand cyclic loads in corrosive environments, the corrosion fatigue properties of brazed joints have to be characterised. Application-relevant corrosion fatigue tests in corrosive media are extremely rare for brazed joints and cyclic deformation curves are barely investigated. In this study, fatigue tests of brazed AISI 304L/BAu-4 joints were performed in air and synthetic exhaust gas condensate K2.2 according to VDA 230-214. The fatigue behaviour of the brazed joints was compared to properties of the austenitic base material. Strain, electrical, magnetic, temperature and electrochemical measurement techniques were applied within fatigue and corrosion fatigue tests to characterise the cyclic deformation and damage behaviour of the brazed joints. It was found that the fatigue strength of 397 MPa at 2 × 106 cycles was reduced down to 51% due to the superimposed corrosive loading. Divergent microstructure-related damage mechanisms were identified for corrosion fatigue loadings and fatigue loadings of specimens in the as-received and pre-corroded conditions. The investigations demonstrate the important role of corrosive environments for the mechanical performance of brazed stainless steel joints.

Characterisation and modelling of the reactions in a three-way PdRh catalyst in the exhaust gas from an ethanol-fuelled spark-ignition engine

This work investigated and modelled the performance and characteristics of automotive catalytic converter formulations when subjected to a synthetic exhaust gas mixture representative of that emitted by an ethanol-fuelled spark-ignition engine. A synthetic gas reactor and exhaust gas emission analyser were used to assess the catalytic activity, the products distribution and chemical mechanisms exhibited by a commercial catalytic converter formulation when exposed to ethanol containing gas mixtures. A commercially available after-treatment modelling platform named Axisuite was used to simulate the catalyst performance. This software was used to assign the pre-exponential frequency factor and activation energy variables within the rate equations. A set of global kinetic coefficients for the relevant reactions was established and is reported.

Frequency‐dependent fatigue and corrosion fatigue assessment of brazed AISI 304L/BNi‐2 joints in air and synthetic exhaust gas condensate

AbstractSince brazed components are often cyclically loaded in corrosive environments, the corrosion fatigue behaviour of the joints has to be investigated. Fatigue tests of brazed AISI 304L/BNi‐2 joints, relevant for exhaust gas heat exchangers, were performed with specimens in the as‐received condition and after pre‐corrosion according to VDA 230‐214. Additionally, the superimposed corrosion fatigue loading in an exhaust gas condensate was realised using a corrosion cell. Corrosion‐induced and deformation‐induced microstructural changes were metallographically evaluated. The influence of the test frequency from 1 to 150 Hz on the cyclic deformation and damage behaviour was characterised. In a statistical analysis, the fatigue strength of 210 MPa at 2·107 cycles was determined for the as‐received condition with a 50% failure probability. The pre‐corrosion as well as the superimposed loading lead to a reduction of the fatigue strength down to 22%. A novel test strategy is suitable for precise fatigue and corrosion fatigue assessments.

High-Temperature Corrosion-Fatigue Behavior of Ductile Cast Irons for Exhaust Manifolds Applications

In the present study, low-cycle fatigue (LCF) tests and oxidation tests in controlled atmospheres are carried out at 800°C on two ductile cast irons SiMo51 and SiMo1000. The LCF tests are carried out in argon and synthetic exhaust gas, whereas oxidation tests are carried out in the latter atmosphere. S-N curves and weight-gain curves are presented. The crack growth mechanisms and oxidation mechanisms are investigated, as well as the synergetic effects. A surprising finding of increased fatigue resistance in oxidizing atmosphere is partly explained.

Characterisation of the corrosion fatigue behaviour of brazed AISI 304L/BNi-2 joints in synthetic exhaust gas condensate

Exhaust gas recirculation (EGR) with brazed heat exchangers is commonly used for the reduction of NOx emissions of automotive diesel engines. Since the fatigue loading of brazed EGR coolers during the operation is superimposed by corrosive attack due to aggressive exhaust gases, the corrosion fatigue behaviour of brazed joints is relevant for the component design. In the present study, brazed AISI 304L/BNi-2 joints were cyclically tested in the synthetic exhaust gas condensate K2.2, using a corrosion cell, as well as in air after pre-corrosion acc. to VDA 230–214 with ageing durations of up to 6 weeks. A significant reduction of the fatigue strength at 2 × 106 cycles down to 43% was determined for superimposed and down to 22% for successive corrosion fatigue loading. A galvanostatic anodic polarisation of the specimens tested in situ was applied successfully for an efficient reproduction of the corrosion fatigue behaviour after the long-time and cost-intensive pre-corrosion. In this context, a novel test strategy with local strain measurements, using a newly developed extensometer, as well as electrical and electrochemical measurements was established for a precise corrosion fatigue assessment of the brazed joints. Microstructure-related corrosion fatigue damage mechanisms were evaluated using scanning electron microscopy.

Storage of Nitrous Oxide (NOx) in Diesel Engine Exhaust Gas using Alumina-Based Catalysts: Preparation, Characterization, and Testing

This work investigated the nitrous oxide (NOx) storage process using alumina-based catalysts (K2 O/Al2 O3 , CaO/Al2 O3, and BaO/Al2 O3 ). The feed was a synthetic exhaust gas containing 1,000 ppm of nitrogen monoxide (NO), 1,000 ppm i-C4 H10 , and an 8% O2 and N2 balance. The catalyst was carried out at temperatures between 250–450°C and a contact time of 20 minutes. It was found that NOx was effectively adsorbed in the presence of oxygen. The NOx storage capacity of K2 O/Al2 O3 was higher than that of BaO/Al2 O3. The NOx storage capacity for K2 O/Al2 O3 decreased with increasing temperature and achieved a maximum at 250°C. Potassium loading higher than 15% in the catalyst negatively affected the morphological properties. The combination of Ba and K loading in the catalyst led to an improvement in the catalytic activity compared to its single metal catalysts. As a conclusion, mixed metal oxide was a potential catalyst for de-NOx process in meeting the stringent diesel engine exhaust emissions regulations. The catalysts were characterized by a number of techniques and measurements, such as X-ray diffraction (XRD), electron affinity (EA), a scanning electron microscope (SEM), Brunner-Emmett-Teller (BET) to measure surface area, and pore volume and pore size distribution assessments.

On the flame stability and laminar burning speeds of syngas/O2/He premixed flame

Fundamental properties such as flame structure and laminar burning speed of syngas/O2/He premixed flames have been investigated. Synthetic gas, also known as syngas, is a mixture of hydrogen and carbon monoxide that has been used in the present study. Three different mole fractions of 5%, 10% and 25% of hydrogen in the syngas were used in this research. Experiments were done in a cylindrical and a spherical chambers. Flame structure studies were made in the cylindrical vessel. The cylindrical vessel was coupled with a Z-shape schlieren system, equipped with a high speed CMOS camera, which has the capability of capturing pictures up to 40,000 frames per second, in order to study flame structure and instability. Hydrodynamic and thermo-diffusive instabilities on the flame front during propagation of flames have been observed and studied. Helium’s smaller molar heat capacity and higher thermal conductivity than nitrogen results in a more stable and smooth flames. The increase range of smooth flames can be used to validate chemical kinetics mechanisms at higher pressures and temperatures. Pressure rise data as a function of time during the combustion process was obtained through a pressure transducer in the spherical vessel was the primary input of the multi-shell model used to calculate the laminar burning speed for the smooth flames. Power law correlations over a wide range of pressures (from sub-atmospheric up to 7.3atm), temperatures (298K up to 650K), and equivalence ratios (0.6–3.0) have been developed for laminar burning speeds of smooth H2/CO/O2/He flames. At equivalence ratios corresponding to maximum laminar burning speeds, the experimental laminar burring speeds show a positive relationship with pressure. Experimental laminar burning speeds of H2/CO/O2/He mixtures have been compared with numerical values calculated by free flat flame simulation using two chemical kinetics mechanisms and some discrepancies discovered for equivalence ratios greater than 2. The effect of helium as a diluent on flame morphology and laminar burning speeds of syngas are compared with two diluents, nitrogen as well as nitrogen that is further diluted with synthetic exhaust gas recirculation (SEGR).

Exhaust gas recirculation effects on flame structure and laminar burning speeds of H2/CO/air flames at high pressures and temperatures

Experimental studies have been performed in conjunction with a novel differential based multi-shell model to investigate the flame structure and measure laminar burning speeds of H2/CO/air/diluent premixed flames at high pressures and temperatures. This paper focuses on synthetic gas (syngas) as the fuel blend, which is a mixture of H2 and CO, and investigates the effect of synthetic exhaust gas recirculation (SEGR) as the diluent on flame structure and laminar burning speed. SEGR is a mixture of 14% CO2 and 86% N2. In these experiments two different SEGR concentrations of 5% and 10% have been used. The experiments were performed in two constant volume spherical and cylindrical chambers. The cylindrical chamber was set up in a schlieren system equipped with a high speed CMOS camera, capable of taking pictures up to 40,000 frames per second, which was used to study the structure and stability of the flame. The laminar burning speed of the combustion process was calculated from the pressure rise measurement during flame propagation in spherical chamber. Power law correlations have been developed for laminar burning speeds of smooth H2/CO/air/SEGR flames over a wide range of temperatures (298K up to 450K), pressures (from sub-atmospheric up to 5.5atm), equivalence ratios (ϕ=0.6–3) and three different hydrogen concentration of 5%, 10% and 25% in the fuel mixture. SEGR lowers the laminar burning speed and has significant effect on the flame stability compared to H2/CO/air, especially for very lean and very rich mixtures. Experimental burning speeds of H2/CO/air/SEGR mixtures have been compared with available measurements as well as computed values obtained by 1D free flame simulations using two chemical kinetics mechanisms. Very good agreements have been observed with the experimental data available in the scientific literature as well as computational burning speeds.

A Parametric Study of Diesel Oxidation Catalyst Performance on CO Reduction in Diesel Dual Fuel Engine Exhaust

In this research study, a synthetic exhaust gas system is employed to simulate various exhaust conditions similar to those from conventional diesel and Dual Fuel-Premixed Charge Compression Ignition (DF-PCCI) combustion. OEM DOC is tested to compare the effectiveness of reducing CO from both exhaust characteristics. Variations of the temperature and the concentration of CO, THC, and O2 are done to investigate DOC performance on CO reductions according to Design of Experiment (DOE) concept. The results showed that in DF-PCCI exhaust conditions, DOC requires higher exhaust gas temperature as well as O2 concentration to reduce CO emissions.

Kinetic modelling of Pt and Pt:Pd diesel oxidation catalysts

This paper describes an experimental and theoretical investigation of the oxidation of carbon monoxide, nitrogen monoxide and propene, and the reduction of nitrogen monoxide by propene over two diesel oxidation catalysts. Synthetic exhaust gas mixtures are used. The catalysts were commercial monolith samples of 400 CPSI with a total PGM loading of 95g/ft3. One monolith had platinum catalyst only, and the other was a bimetallic catalyst of 4:1 by mass platinum and palladium. The bimetallic catalyst was more active, and showed a different behaviour for propene oxidation than the Pt only catalyst. Global kinetic models for the oxidation of CO, C3H6, and NO, and the selective catalytic reduction of NO to N2 and N2O are developed. A good agreement between experimental results and simulation is observed.

Hydrocarbon Assisted NO Oxidation with Non-thermal Plasma in Simulated Marine Diesel Exhaust Gases

The NO oxidation performance in a non-thermal plasma (NTP) reactor under realistic synthetic exhaust gas compositions is investigated. The gas compositions differ mainly in the NO–NO2 ratio and represent different modes of operation of a marine diesel engine. It is found that the maximum NO oxidation efficiency is independent on the NO–NO2 ratio. Up to 55 % of the NO is mainly oxidised to NO2 in all gas mixtures being analysed. However, the specific energy density needed to reach the highest NO oxidation varies with the gas composition between 15 and 60 J/L. The performance of the NTP-reactor was significantly improved by the addition of propene (C3H6) acting as an additional oxidising agent. The energy consumption for NO–NO2 conversion was found to be between 20 and 45 eV/NO, depending on the ratio of the added propene as well as the initial concentrations of nitrogen oxides.

Performance and characteristics of platinum, palladium/rhodium and palladium automotive catalysts when subjected to ethanol, acetaldehyde and a synthetic E85 exhaust gas mixture

The performance optimization of automotive catalysts has been the focus of in-depth research for many years as the automotive industry has endeavoured to reduce the emission of toxic and pollutant gases generated from internal combustion engines. Just as the emissions from diesel and gasoline combustion vary, so do the emissions from the combustion of alternative fuels such as ethanol, with variation in both quantity and chemical composition. In particular, when ethanol is contained in the fuel, ethanol and acetaldehyde are present in the exhaust gas stream, and these are two compounds which the catalytic converter has not traditionally been designed to manage. The aim of the study outlined in this paper was to assess the performance of various catalyst formulations when subjected to a representative ethanol exhaust gas mixture. Three automotive catalytic converter formulations were tested, including a fully platinum sample, a palladium/rhodium three-way catalyst sample and a fully palladium sample. Initially the samples were tested using single component hydrocarbon light-off tests followed by a set of tests with carbon monoxide included as an inlet gas to observe its effect on each individual hydrocarbon oxidation. Finally, each formulation was tested using a full E85 exhaust gas mixture. The study was carried out using a synthetic gas reactor along with Fourier transform infrared spectroscopy and flame ionization detector exhaust gas analysers. All formulations showed selectivity toward acetaldehyde formation from ethanol dehydrogenation which resulted in negative acetaldehyde conversion across each of the samples during the mixture tests. The fully platinum sample was the most detrimentally affected by the introduction of carbon monoxide into the gas feed. The palladium and palladium/rhodium samples exhibited a tendency toward acetaldehyde decomposition resulting in methane and carbon monoxide formation. The platinum sample did not form methane but did form ethylene as a result of ethanol dehydration.

Lean NO x Reduction with Various Bio-Diesels as Reducing Agents

Commercial Cu-ZSM-5- and Ag/Al2O3-based lean NOx catalysts were evaluated in a synthetic exhaust gas bench with the fuels RME, B30, B15, Agrodiesel 15, GTL, NExBTL, and MK1 as reducing agents. The influence of reductant was larger for Ag/Al2O3, albeit moderate, whereas the Cu-zeolite showed the highest NOx conversion at lower temperature for all alternative fuels tested.

ChemInform Abstract: Physicochemical Properties of Ce-Containing Three-Way Catalysts and the Effect of Ce on Catalyst Activity.

The activity of Pt,Rh,Ce/γ-Al2O3 and Pt,Rh/CeO2 catalysts has been studied in a full complement synthetic exhaust gas mixture consisting of H2, CO, C3H6, C3H8, NO, O2, N2, CO2, H2O, and SO2. Direct interaction between Pt and CeO2 was shown to lead to large improvements in catalyst performance after activation of the catalyst in the synthetic exhaust gas. Catalyst activation was shown to be due to reduction of the noble metals and surface Ce and this state of the catalyst was found to be particularly effective for CO oxidation. The key component responsible for activation was shown to be H2; even at the low H2 level present in the exhaust gas (≤.26 vol%), reduction is shown to be very easy. The degree of Pt/Ce interaction, and thus activity, after catalyst activation could be controlled by varying the CeO2 crystallite size. Decreasing the CeO2 crystallite size led to greater Pt/Ce interaction as shown by TPR and STEM analysis and resulted in greater activity for both fresh and laboratory aged catalysts. Direct Pt/Ce interaction also led to a synergistic reduction of Pt and surface Ce which we found to qualitatively correlate with catalyst performance after activation.