Exhaust Gas Aftertreatment Systems Research Articles

Challenges and Solutions to Meet the Euro 7 NOx Emission Requirements for Diesel Light-Duty Commercial Vehicles

AbstractThe improvement of air quality requires a further reduction of pollutant emissions, especially in urban areas. The Euro 7 regulations aim at the development of a new generation of internal combustion engine vehicles capable of achieving ultra-low pollutant emissions under demanding, real-world operating conditions. They introduce new technical challenges in the holistic design of a vehicle’s powertrain and emission control system. To identify these, four real-world Euro 7 driving scenarios are investigated, covering demanding urban, highway and mountain driving situations. Technical solutions are then presented to address these challenges and ensure compliance with the Euro 7 emission requirements as set out in the latest regulation proposal of the European Commission. The study focuses on the NOx emissions of an N1 Class III light commercial vehicle with 3.5 t mass and a P2 diesel mild-hybrid powertrain. To ensure emission compliance, a Euro 6e exhaust gas aftertreatment system with enlarged catalysts is combined with NOx raw emission improvements. For low-load cold starts, a 4-kW electric heater in the exhaust system is considered in addition to a 2-l DOC and a 6-l DPF with SCR coating. For high-load cycles with high raw emissions, a 10-l underfloor SCR is considered to ensure the necessary deNOx performance.

An experimental study on aging effects of fuel-cut events including sound optimized torque reduction on modern three-way catalysts

Continuously increasing country-specific emission standards for passenger vehicles demand additional development steps in complex exhaust gas aftertreatment systems for a more effective reduction of harmful gases. The efficiency of the aftertreatment system declines over its lifecycle due to specific load profiles and other boundary conditions, resulting in increasing emissions over vehicle mileage. As a result, the durability of the exhaust system becomes more important. Fuel cut is an intended, temporary interruption of the fuel supply of modern combustion engines of passenger cars to reduce fuel consumption and emissions. During this fuel reduction event, unconsumed oxygen of rich and cool fresh air is introduced to the combustion chamber and to the exhaust system. Besides a catalyst cool down, oxidation effects are provoked by enhanced oxygen reactions processes within the catalyst’s washcoat and surface layers. In this publication, specific aging cycles with a variation of fuel-cut programs were examined on their effects on a modified modern eight-cylinder turbo-charged engine, especially on their oxidation effects correlating to catalyst aging. Light-off curves and conversion heat maps were used to evaluate possible damaging impacts of fuel-cut events on the aging behavior of the catalyst systems. Results indicate that higher frequency of fuel-cut events results in a reduced catalyst conversion efficiency, whereas thermal sintering might be reduced due to overall lower temperature load. Temporary oxidation processes on catalyst’s surfaces occur only in a short timespan after changing the air fuel ratio to lean, resulting in a weaker thermal sintering during longer fuel-cut events. Sound optimized torque reduction functions, used in sports cars for a better drivability and powertrain response, exhibit a stronger aging influence due to additional thermal shock effects on the catalysts front face, as demonstrated via optical infrared sensor technology during catalyst aging cycles on the modified engine bench setup. A theoretical calculation of aging processes as a combination of thermal sintering by Arrhenius equation as well as oxidation process is discussed to describe an aging classification induced via oxidation processes. Additional physisorption measurements of different catalyst probes support the results which indicate the direction of future experimental approaches.

Active regeneration particulate filter operating cycle theoretical justification

Introduction (problem statement and relevance). Particulate filters or traps used in diesel vehicles to catch suspended particles require continuous and/or periodic regeneration in order to maintain operability. The order of diesel particulate filter loading and its regeneration can be presented in the form of an algorithm. Specific features of particulate filter functioning both in regular conditions and during active regeneration have been studied in a number of scientific papers, and the results obtained in them can be considered when developing the algorithm.The purpose of the work is development of the algorithm of the operating cycle of the diesel vehicle particulate filter including modes of filter loading, active regeneration and troubleshooting.Methodology and methods. Information from scientific literature regarding specific features of diesel vehicle exhaust gas aftertreatment systems operation, namely regarding particulate filters, has been collected under this study. The study assumes a noncatalytic diesel particulate filter that is contained in the system together with a diesel oxidation catalyst, nitrogen oxide reduction catalyst and ammonia oxidation catalyst. Particulate matter sensors, temperature sensors, pressure sensors, exhaust-gas flow sensors and flow sensors for fuel to burn the soot accumulated in the filter have been used as measuring instrumentation of system indicators.Results. The created algorithm of the particulate filter operating cycle included the loading mode with definition of parameters for switching to active regeneration, definition of the transition moment between the stages of deep filtration and filtration in the soot layer. Active regeneration consisted of a sequence of two controlled fuel injections for combustion of soot in the filter and occurrence of the temperature impulse between the injections with selection of the most suitable level of exhaust gas flow rate as well as the control over the filter temperature and content of oxygen. The problems of malfunctions of the treatment system were solved by controlling the fuel supply during regeneration.Scientific novelty. The peculiarities of processes in the particulate filter during active regeneration were considered in the algorithm development, including ensuring high efficiency of soot removal from the filter.Practical significance. The algorithm can be used to develop a procedure to perform bench and driving tests of diesel vehicles as well as it can be applied in a diesel engine during vehicles operation

Two-dimensional calculation model for hazardous substances formation in combustion products of a spark-ignition engine

Introduction (problem statement and relevance). Improvement of environmental parameters of internal combustion engines is one of the most relevant tasks of the domestic transport engineering. Calculation support allows reducing the time and raising the efficiency of works for refining of the bench work process in order to meet the required parameters. The paper is devoted to development of a two-dimensional calculation model for hazardous substances formation in combustion products of spark-ignition engines based on the non-equilibrium kinetic models of formation of nitrogen oxides and carbon monoxide.Methodology and research methods. Computational methods for differential equations (ordinary and partial ones) are applied. Non-equilibrium kinetics of hazardous substances formation in combustion products is considered. The work process is calculated based on a combination of the mesh control-volume method and determination of the observable flame rate based on the experimental data (taking into account the development stage of the flame source).Scientific novelty and results. The new calculation method for flame propagation in the spark-ignition engine is suggested. The developed model combines such advantages as good predictability and short time of calculation.Practical significance. The model can be applied, in particular, to preliminary planning of three-dimensional calculations of the work process or their check in the absence of experimental data (at the stage of preliminary/concept design). The calculation results can be used as initial data for modeling of the exhaust gas aftertreatment (additional purification) system.

Results of recurrent in-service exhaust gas measurements with an EU stage IV forest harvester fuelled with rapeseed oil within the emission durability period

The real driving emissions of an EU stage IV forest harvester were measured four times within five years to monitor long-time emission behaviour. In this period, the harvester worked 7650 h in total, thereof 6300 h with pure rapeseed oil fuel DIN 51605 (R100) and 1350 h with conventional diesel fuel initially. Data analysis according to relevant EU regulation 2017/655 shows that the emission behaviour complies with the legal requirements of exhaust gas stage IV within the five years under consideration. According to EU regulation 2016/1628 the achieved 7650 operating hours nearly correspond to the emission durability period of 8000 h. However, between the single measurements some differences in emission results are clear evident. They are primarily caused by different working profiles, and unavoidable random events. Detailed analysis of the results showed that the measured nitrogen oxides (NOX), carbon monoxide, and hydrocarbons remain at the same level over time at comparable operation conditions. Thus, the operation time had no major impact on the emission behaviour of the harvester. During cold start and non-working events higher nitrogen oxides (NOX) concentrations are observed in the exhaust since the exhaust aftertreatment system is not within its operation temperature. When the exhaust gas aftertreatment is within its operating range, exhaust emissions are at a very low level indicating an efficient, clean combustion. It can be concluded that the operation of the harvester with R100 did not affect the emission behaviour and functionality of the exhaust gas aftertreatment system.

Achieving Zero-Impact Emissions with a Gasoline Passenger Car

The Euro 7 legislation and the Zero-Impact Emissions concept aim at significantly improving air quality. Technologies that reduce pollutant emissions beyond current gasoline passenger cars have already been intensively investigated, but a holistic system layout considering extended boundary conditions is missing so far. This paper therefore develops technical solutions to achieve a Euro 7 scenario and Zero-Impact Emissions for a 2030+ vehicle. First, challenging test scenarios are identified to develop compliant vehicles. The scenarios cover extreme conditions in real-world driving, such as hot and cold ambient conditions, stop-and-go in rural areas or high speed and steep gradients on highways. Different technology options are discussed and selected for the investigations. An empirical–physical simulation model for the exhaust gas aftertreatment system is extended with new technologies, such as an electrical heater disc in front of the catalyst or a burner in the exhaust system. In addition to stoichiometric engine operation and increased catalyst volume, the results show that the expected Euro 7 regulations can be achieved in all extreme scenarios by combining additional exhaust gas heating with engine power limitation or pre-heating. Moreover, even Zero-Impact Emissions are achieved in most cases with the same technology options.

Characterization from Diesel and Renewable Fuel Engine Exhaust: Particulate Size/Mass Distributions and Optical Properties

Combustion of fossil fuel produces emissions and is one of the major environmental problems leading to climate change. Diesel engines are highly efficient but produce particulate emissions. These particulate emissions are considered dangerous to human health because inhaling particulates may cause respiratory and heart disease. Substituting fossil diesel fuel with renewable diesel fuel and using diesel particulate filters is one possibility to meet stringent legislative requirements. With this motivation, the present experimental investigation aimed to evaluate the particle size distribution (PSD), optical properties of particulate matter (PM) emitted, and the outcome of using an after-treatment system comprising of a diesel particle filter (DPF). This investigation aimed to make a comparative analysis of particulate emission upstream and downstream of the DPF with and without ultraviolet (UV) light (405 nm and 781 nm wavelength) turned on/off. Experiments were performed at (a) engine idle with a torque of 6 Nm at 750 rpm, IMEP of 1.35 bar and power of 0.5 kW, (b) engine at part load with a torque of 32 Nm at 1200 rpm, IMEP of 8.5 bar and power of 4.5 kW. Diesel engine was operated on two fuels (a) Diesel and (b) EHR7. Results showed that as and when UV light was turned on, a distinct nucleation mode that dominated the number concentration for both test fuels were observed. Downstream of the filter had relatively higher AAE values which show the contribution to climate change. Present experimental research is important for renewable fuel industries, industrial innovation's future, and the exhaust gas after-treatment system (EATS) community. The results contribute to knowledge for occupational exposure, human health, and the environment.

Reducing Air Pollutant Emissions and Optimizing Fuel Economy by Controlling Torque and Catalyst warm-up in mild HEV via Cascaded MPC

In this paper, Cascaded MPC (Model Predictive Control) including Supervisory MPC, Heating MPC and Torque MPC is applied to mild HEVs (Hybrid Electric Vehicles) in order to improve fuel economy and reduce NOx emissions. There is a difference in the timescale between Heaitng MPC and Torque MPC, which are connected via the battery. Supervisory MPC treats the power and temperature systems comprehensively and calculates reference values for NOx emissions to be tracked in Heating MPC. In Heating MPC, the optimal amount of heating from the battery to the catalyst in the exhaust gas after-treatment system is determined. The amount of heating must be determined in a way that the catalyst can optimally purify NOx emitted from the engine. At Torque MPC, the optimal torque distribution for the engine and motor is determined. Finally, the effectiveness of our proposed method is demonstrated by simulations.

Reducing NOx Emissions and Optimizing Fuel Economy by Controlling Torque and Catalyst Warm-up in Mild HEVs

In this paper, hierarchical model predictive control is applied to mild HEVs (Hybrid Electric Vehicles) in order to improve fuel economy and reduce NOx emissions. There is a difference in the timescale between the temperature control (high-level) and the torque control (low-level), which are connected via the battery. In high-level of the hierarchical control, the optimal amount of heating from the battery to the catalyst in the exhaust gas after-treatment system is determined. The amount of heating must be determined in a way that the catalyst can optimally purify NOx emitted from the engine. At the low-level of the hierarchical control, the optimal torque distribution for the engine and motor is determined. The controller at both levels then applies model predictive control to consider the future behavior of the HEV's components. Fuel consumption, battery SoC and NOx emissions from the engine are taken into account. Finally, the effectiveness of our proposed method is demonstrated by simulations.

Impact of unintentionally formed CH2O in oxygenated fuel exhausts on DeNOx-SCR at different NO2/NOx ratios under close to real conditions

Formaldehyde emissions of vehicles with combustion engines, burning oxygenated fuels are a major challenge for exhaust gas aftertreatment systems. This study shows the impact of such emissions for NH3-SCR, with a high NO2-ratio.

Advanced strategies to reduce harmful nitrogen-oxide emissions from biodiesel fueled engine

Advanced techniques have shown an effective approach to reducing nitrogen oxide (NOx) emissions from biodiesel-fueled engines. The primary concern is higher NOx emissions using biodiesel due to its different physio-chemical fuel properties and higher in-cylinder combustion temperatures than diesel. This study aims to scrutinize the existing literature on NOx forming origins from biodiesel and present the state-of-the-art literature on reduction technologies, identify research gaps and introduce further developments to the research opportunities. Accordingly, the present review has organized the technologies into three categories: pre-combustion, combustion, and post-combustion techniques. In addition, the pros and cons of each technique are presented by identifying their role using combustion characteristics. The study identifies that ignition delay and combustion duration are the major factors to consider in reducing NOx emissions. Applying innovative tactics to modify the fuel, chamber, and injection parameters prolongs the ignition delay and combustion duration, thereby reducing NOx emissions. Furthermore, exhaust gas after-treatment systems with novel catalytic convertors have shown a strong capability of reducing NOx emissions. The findings also indicate significant reductions in NOx emissions from biodiesel with provided technologies; however, a clear roadmap is needed to integrate these technologies for sustainable application. The study concludes that emulsion techniques, low-temperature combustion strategies, retarded injection timing, and exhaust gas after treatment have shown significant NOx reduction through reduced peak temperature rates. The study recommends further investigation on reactive controlled combustion techniques as they directly impact NOx emission, and their results comparisons will give a clear understanding for deriving better commercial options.

Exhaust Gas After-Treatment Systems for Gasoline and Diesel Vehicles

Review Exhaust Gas After-Treatment Systems for Gasoline and Diesel Vehicles Liye Bao 1, Jihua Wang 2, Lihui Shi 2, and Haijun Chen 1, * 1 College of Electronic Information and Optical Engineering, Nankai University, Tianjin 300350, China 2 Valiant Co., Ltd., Yantai 264006, China * Correspondence: chenhj@nankai.edu.cn Received: 5 October 2022 Accepted: 10 November 2022 Published: 25 December 2022 Abstract: Exhaust gases released from vehicle engines have been a major cause of air pollution, and the emission limits have become much stricter in recent years due to a worldwide concern about the impact of air pollution on public health. These regulations have been complied to minimize the emissions of carbon monoxide (CO), hydrocarbons (HCs), nitrogen oxides (NO x) and particulate matter (PM) from gasoline and diesel vehicles engines. Different after-treatment systems (ATS) have been developed for the treatment of exhaust gases from gasoline and diesel engines, respectively. The ATS for gasoline engine based on the three-way catalysts (TWCs), as well as the ATS for diesel engines including diesel oxidation catalysts (DOC), selective catalytic reduction (SCR), diesel particulate filters (DPF) and ammonia slip catalysts (ASC), are summarized in this mini-review.

Decomposition of Heavy Diesel SCR Urea Fluid Adsorbed in Cu/HZSM-5 SCR Catalysts Studied by FTIR Spectroscopy at Ambient Conditions

A method is presented to study the decomposition of urea deposited on Cu/HZSM-5 SCR catalysts and therewith the ability of the Cu/HZSM-5 SCR catalyst to be regenerated when being overdosed with SCR urea fluids during operation. This straightforward laboratory method could speed up calibration of exhaust gas aftertreatment systems. As an example, the removal of adsorbed urea to the SCR substrate due to dosage malfunction is studied. To study the removal of adsorbed urea on the catalyst substrate, FTIR experiments have been conducted to investigate the state of the catalyst. Besides Cu/HZSM-5 also HZSM-5 and CuOx were studied as model compounds to provide more inside on the processes occurring at the Cu/HZSM-5 surface upon urea decomposition. To simulate exposure of the SCR catalyst to overdosing of the urea solution, samples were impregnated with a 32 wt% urea solution, which correlates to that of commercial heavy duty diesel urea solutions. After impregnation, the samples were heated at various temperatures in the 133–400 °C temperature region, typically the operation window of a SCR catalyst. After heating, the samples were cooled to room temperature and measured in FTIR. The obtained spectra were compared with various literature reports to correlate the observed absorption bands to urea, urea related compounds and decomposition compounds. The concentration of these adsorbed species decreases at increased thermolysis temperature and is no longer visible at temperatures >250 °C. Extended heat treatment at 200 °C revealed only minor loss of adsorbents after 6 h and were still observable in the FTIR spectra after 24 h. Urea derived adsorbents were completely removed when simulating catalyst regeneration under SCR operation conditions under continuous air flow with a humidity of 10% and at elevated temperatures (400 °C).

Transient Flow Uniformity Evolution in Realistic Exhaust Gas Aftertreatment Systems Using 3D-CFD

To precisely control a vehicle powertrain to minimize emissions, accurate and detailed models are needed to capture the spatio-temporal variability of the variables of interest. The aim of this work is to analyze flow and temperature fields in a geometrically realistic — and thus complex — exhaust gas aftertreatment system under transient conditions. The spatio-temporal response of these fields to upstream step changes is predicted using three-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) kappa - omega simulations where the catalytic converter is described as a porous medium. A catalytic converter geometry with a 90^{circ }-bend and a partially dead volume is used to demonstrate the effects of time-resolved flow maldistribution on the profiles of velocity and temperature. Two sets of transient simulations in terms of step changes in velocity and temperature are performed. Uniformity indices are used to characterize the distribution and variability of the different catalyst channels under transient conditions. The evolution of the uniformity indices as functions of time and axial distance into the catalyst are calculated at different cross-sectional planes. The results show that the evolution of the temperature uniformity is rate controlling, continuously modulating the otherwise much faster flow uniformity response via the fluid properties. The temperature uniformity time scale is determined by the balance of flow, thermal inertia, and the heat losses from the system. The interplay between pressure drop and heat losses governs the transition to the new steady state in uniformity. These types of transient simulations and analyses can contribute essential information when developing reduced-order engineering models to represent the spatio-temporal variability in exhaust aftertreatment systems, in particular during rapid events such as cold start.

Operando QEXAFS Study of Pt–Fe Ammonia Slip Catalysts During Realistic Driving Cycles

Bifunctional Fe–Pt ammonia slip catalysts were studied by operando quick-scanning extended X-ray absorption fine structure spectroscopy (QEXAFS) under conditions mimicking rapid temperature variations that occur in an automotive exhaust gas aftertreatment system during real driving. Two catalysts, Pt/Al2O3 and Fe-ZSM-5, were tested individually, as mixtures and in dual bed arrangements. Applying QEXAFS allowed to track changes of active metal state with high time resolution. It uncovered a strong dependence of the active metal state on reaction conditions and catalyst bed layout. For example, proximity to platinum stabilized iron species in their more active oxidized state and led to higher Fe-ZSM-5 activity. On the contrary, isolated iron species were more susceptible to overreduction by ammonia which led to deactivation and low selectivity. The use of transient conditions uncovered the influence of non-equilibrium phenomena on catalytic performance under industrially relevant conditions. Specifically, the effect of ammonia storage on the increase of activity was shown. This was also accompanied by elevated N2O production not observed during tests with gradual heating. Additionally, unusually high NOx selectivity was detected for Fe-ZSM-5 under these conditions. Lastly, tracking catalyst state under dynamic reaction conditions disclosed that Fe-ZSM-5 activity did not grow directly with temperature increase but rather depended on the oxidation state of Fe and surface concentration of ammonia.

On the Suitability of NOx-Storage-Catalysts for Hydrogen Internal Combustion Engines and a Radio Frequency-Based NOx Loading Monitoring

Hydrogen combustion engines can contribute to CO2-free mobility. However, they produce NOx emissions, albeit only to an extremely small extent when operated very leanly. One approach to reduce these emissions even further is to use exhaust gas aftertreatment systems like NOx storage catalysts (NSC). So far, they have mainly been used in diesel or gasoline applications. This contribution shows that under conditions such as those prevailing in hydrogen engines, the NSC can achieve not only a higher storage capacity for nitrogen oxides (NOx) but also a higher conversion. To ensure permanently high conversion rates, the amount of stored NOx has to be monitored permanently to prevent NOx breakthroughs. Conventional NOx sensors may not be accurate enough due to the very low NOx emissions. The functionality of the radio frequency (RF) sensor, which enables a direct determination of the NOx loading, is demonstrated for operation under hydrogen conditions. Furthermore, the influence of rich exhaust gas on the RF signal, which is relevant for a correct NOx loading determination during regeneration, is analyzed.

Electric Heater for Rapid Heating of the Exhaust Gas Aftertreatment System

Electric Heater for Rapid Heating of the Exhaust Gas Aftertreatment System

Experimental analysis of engine out emissions of a light-duty diesel engine during warm-up under cold start conditions

Emission targets of diesel engines are achieved by high-performance exhaust gas aftertreatment systems (EATS). Due to the light-off temperature, the system does not work as desired under cold operating conditions. This is especially challenging with the introduction of Euro 7 because engine warm up is getting more critical, as not only emission limits are getting tighter, but also the range of ambient temperatures is widened. Especially cold start conditions will be challenging: On one hand engine out emissions cannot be converted by the EATS before light-off which is extended by a greater temperature deficit. On the other hand, engine out emissions are affected by poor combustion boundary conditions. This paper focuses on the impact of temperature on emission formation by carrying out the first five km of three different driving cycles under cold start conditions. Based on these cycles, representative stationary operating points for each emission species were defined. A method was developed to reproduce thermal boundary conditions from engine warm up. This method allows to investigate the impact of thermal conditions on emission formation in steady state. Therefore a light-duty diesel engine was instrumented with a high number of temperature sensors in the cylinder liner wall and cylinder head as well as in the oil, gas, fuel and coolant path. The results were analyzed using 0D/1D simulation models to isolate the thermal impact on the emission formation. The obtained results serve as a basis for following measures on a similar single-cylinder research engine to develop innovative cold start measures.

Optimal control of real driving emissions

Real driving emissions (RDE) testing is an important legislative tool to reduce the gap between type-approval and real-world emissions of internal combustion engines. Since low fuel consumption and low nitrogen oxides (NOx) emissions are conflicting objectives, the setpoints of a Diesel engine and its exhaust-gas aftertreatment system have to be optimised and controlled accordingly. To this end, several authors have developed a so-called supervisory controller. In order to comply with RDE legislation, most approaches use a conservative tuning and thus sacrifice performance. In this paper, we explicitly include the RDE requirements in the optimal control problem, which we then solve using direct multiple shooting and nonlinear programming. Using the optimal solution as a benchmark and comparing it to a conservative strategy, we find that fuel savings of up to 3% are possible in simulation. Furthermore, as a first step towards an online, RDE compliant, and optimal supervisory controller, we use Pontryagin’s minimum principle to derive the fact that the optimal costate related to the tailpipe NOx emissions is piecewise constant.

Effects of Catalyst Structural Parameters on the Performance of Exhaust Gas Aftertreatment System of Diesel Engines

Effects of Catalyst Structural Parameters on the Performance of Exhaust Gas Aftertreatment System of Diesel Engines