Experimental investigation of emission from a light duty diesel engine utilizing urea spray SCR system

Stringent pollutant regulations on diesel-powered vehicles have resulted in the development of new technologies to reduce emission of nitrogen oxides (NOx). The urea Selective Catalyst Reduction (SCR) system and Lean NOx Trap (LNT) have become the two promising solutions to this problem. Whilst the LNT results in a fuel penalty due to periodic regeneration, the SCR system with aqueous urea solution or ammonia gas reductants could provide a better solution with higher NOx reduction efficiency. This thesis describes an experimental investigation which has been designed for comparing the effect NOx abatement of a SCR system with AdBlue urea spray and ammonia gas at 5% and 4% concentration. For this study, a SCR exhaust system comprising of a diesel particulate filter (DPF), a diesel oxidation catalyst (DOC) and SCR catalysts was tested on a steady state, direct injection 1998 cc diesel engine. It featured an expansion can, nozzle and diffuser arrangement for a controlled flow profile for CFD model validation. Four different lengths of SCR catalyst were tested for a space velocity study. Chemiluminescence (CLD) based ammonia analysers have been used to provide high resolution NO, NO2 and NH3 measurements across the SCR exhaust system. By measuring at the exit of the SCR bricks, the NO and NO2 profiles within the bricks were found. Comparison of the measurements between spray and gas lead to insights of the behaviour of the droplets upstream and within the SCR bricks. From the analysis, it was deduced that around half to three quarters of the droplets from the urea spray remain unconverted at the entry of the first SCR brick. Approximately 200 ppm of potential ammonia was released from the urea spray in the first SCR brick to react with NOx. The analysis also shows between 10 to 100 ppm of potential ammonia survived through the first brick in droplet form for cases from NOx-matched spray input to excess spray. Measurements show NOx reduction was complete after the second SCR bricks. Experimental and CFD prediction showed breakthrough of all species for the short brick with gas injection due to the high space velocity. The long brick gas cases predictions gave reasonable agreement with experimental results. NO2 conversion efficiency was found higher than NO which contradicts with the fast SCR reaction kinetics. Transient response was observed in both cases during the NOx reduction, ammonia absorption and desorption process. From the transient analysis an estimate of the ammonia storage capacity of the bricks was derived. The amount of ammonia slippage was obtained through numerical integration of the ammonia slippage curve using an excel spreadsheet. Comparing the time constant for the spray and gas cases, showed a slightly faster time response from the gas for both NOx reduction and ammonia slippage

Investigation on new structure of Selective Catalyst Reaction coated Diesel Particulate Filter for optimising DeNOx

Selective Catalyst Reduction (SCR) is the most promising technology to reduce NOx emissions from conventional diesel engines and other lean combustion engines. Traditional after treatment system in the majority of diesel vehicles includes a Diesel Oxidation Catalyst (DOC), a Diesel Particulate Filter (DPF) and a Selective Catalytic Reduction (SCR) catalyst. As emission control by regulations is tightening every year and the demanding for better efficiency, a combination of Selective Catalytic Reduction (SCR) catalyst and Diesel Particulate Filter (DPF) have been researched for several years. The combination catalyst is actually putting an SCR coating on and into DPF’s porous walls so that this combination catalyst (SCR coated DPF or SCR-DPF) can be used to filter diesel particulates and reduce NOx at the same time. The benefit of this combination catalyst system is obvious: the compactness of SCR-DPF reduces the weight, the cost and the complexity of the after treatment system; more importantly, it reduces the engine’s back pressure losses as one of the catalysts is removed from the after treatment system. Despite those benefits, SCR-DPF has some drawbacks. There are studies claiming that the NOx conversion efficiency of SCR-DPF is lower than traditional SCR catalyst and more challenging to control NOx conversion process during the regeneration process due to high temperature and so on. In this research, a new catalyst structure is proposed to solve one of the SCR-DPF limitations. This structure provides large extra surface catalytic area for NOx conversion and it is free from ash loading effect as the extra surface islocated in the outlet channel. And based on the results, for the same NOx conversion rate, the catalyst size can be reduced to half if using new structure. This is owning to the extra surface area in the new structure. In this thesis, 3 three-dimensional catalyst models on channel scale, a Flow-Through (FT) catalyst, a Wall-Flow (WF) catalyst and a Wall-Flow catalyst with Fin (WF-Fin) in outlet channel are proposed and built in order to investigate the cause of performance difference between traditional Flow-Through SCR, Wall-Flow SCR and Wall-Flow SCR with fins. The model of Flow-Through SCR is acted as a benchmarksince it is built and validated against Olsson’s experimental research work. A later model of Wall-Flow type catalyst is modified to Flow-Through SCR model by changing the geometry. The results from the Flow-Through SCR model follows Olsson’s experiment results closely for a wide temperature range under steady and transient conditions, that indicates the successful modelling of the base model. The comparison between WF and WF-Fin model has been made from many different aspects, such as velocity, species composition and reaction rates. The WF-Fin model is focused in the later investigation, the fins bring more active site surface for SCR reactions, but it also has adverse effects to the flow in porous walls if the fin is impermeable as a solid material. The effect of different performance factors requires more studies. But based on the current results, a conclusion can bedrawn for the difference between these three types of SCR catalyst, and the possible causes of performance difference are identified

Multiphysics Diesel Aftertreatment System Modeling for Reduced Emissions from Hybrid Electric Heavy-Duty Powertrains

Hybridization of heavy-duty on-road vehicles presents an opportunity to significantly reduce internal combustion engine emissions in real-world operation. These gains can be realized through the coordination of the electric drive, engine, and aftertreatment systems. Accurate Multiphysics models of all powertrains sub-systems are required to achieve the goal of reduced emissions. This research aims to develop a model of a highly complex diesel engine aftertreatment system. This study focuses on utilizing transient data for calibration and validation of the aftertreatment system and reducing the run time when compared to real-time experiments. The calibration focuses on two physical phenomena, thermal behavior and chemical kinetics. Once a base model is set up, the calibration parameters are optimized using an accelerated genetic algorithm for factors that contribute to the reaction rates and the exhaust gas temperature. The research only utilizes data from transient engine experiments to better automate and speed-up the calibration process over traditional methodologies. The model setup ensures that it is fast-running, with ten times speed-up as compared to real-time. The model is capable of predicting and matching combined error for and concentration on a cumulative basis under 9.8% and 1% for the experimental data for cold FTP and hot FTP, respectively. The results of the model also predict close trends with the temperature profiles and have a close match with the tailpipe emission species concentration over a cumulative basis but fails to capture some transient behavior. The model results are also evaluated to identify the leading cause for the error so the model can be improved for further development. The model has the capability to generate results for the aftertreatment for further research

Advanced CIDI Emission Control System Development

Ford Motor Company, with ExxonMobil and FEV, participated in the Department of Energy's (DOE) Ultra-Clean Transportation Fuels Program with the goal to develop an innovative emission control system for light-duty diesel vehicles. The focus on diesel engine emissions was a direct result of the improved volumetric fuel economy (up to 50%) and lower CO2 emissions (up to 25%) over comparable gasoline engines shown in Europe. Selective Catalytic Reduction (SCR) with aqueous urea as the NOx reductant and a Catalyzed Diesel Particulate Filter (CDPF) were chosen as the primary emission control system components. The program expected to demonstrate more than 90% durable reduction in particulate matter (PM) and NOx emissions on a light-duty truck application, based on the FTP-75 drive cycle. Very low sulfur diesel fuel (<15 ppm-wt) enabled lower PM emissions, reduced fuel economy penalty due to the emission control system and improved long-term system durability. Significant progress was made toward a durable system to meet Tier 2 Bin 5 emission standards on a 6000 lbs light-duty truck. A 40% reduction in engine-out NOx emissions was achieved with a mid-size prototype diesel engine through engine recalibration and increased exhaust gas recirculation. Use of a rapid warm-up strategy and urea SCR provided over 90% further NOx reduction while the CDPF reduced tailpipe PM to gasoline vehicle levels. Development work was conducted to separately improve urea SCR and CDPF system durability, as well as improved oxidation catalyst function. Exhaust gas NOx and ammonia sensors were also developed further. While the final emission control system did not meet Tier 2 Bin 5 NOx after 120k mi of aging on the dynamometer, it did meet the standards for HC, NMOG, and PM, and an improved SCR catalyst was shown to have potential to meet the NOx standard, assuming the DOC durability could be improved further. Models of DOC and SCR function were developed to guide the study of several key design factors for SCR systems and aid in the development of urea control strategy for maximum NOx reduction with minimum NH3 slip. A durable co-fueling system was successfully built and tested, with the help of service station nozzle and dispenser manufacturers, for simultaneous delivery of diesel fuel and aqueous urea to the vehicle. The business case for an aqueous urea infrastructure in the US for light-duty vehicles was explored

Mathematical Methods for Design of Zone Structured Catalysts and Optimization of Inlet Trajectories in Selective Catalytic Reduction (SCR) and Three Way Catalyst (TWC)

Abgaskatalysatoren zählen zu den wichtigsten Maßnahmen, um Schadstoffemissionen von Verbrennungsmotoren zu vermindern. Mit der stetigen Verschärfung der Emissionsstandards nahm über die Jahre der Forschungsbedarf zu Abgasnachbehandlungssystemen signifikant zu. Der Fokus dieser Arbeit liegt auf der Lösung von Optimierungsproblemen im Bereich der Autoabgaskatalyse, um die Effizienz zu steigern. Dabei werden drei Problemklassen behandelt: 1) Die Light-Off-Verzögerung beim Kaltstart in Oxidationskatalysatoren, 2) Die effiziente Ammoniakdosierung bei der selektiven katalytischen Reduktion (SCR), um Ammoniakdurchbrüche zu vermeiden, 3) Die Spannungsstabilisierung der Lambda-Sonde im Drei-Wege-Katalysator (TWC) während einer Schubabschaltung. Das erste Problem wird durch eine modellbasierte mathematische Optimierung beschrieben, bei der das Beladungsprofil von gezont-strukturierten Katalysatoren auf Basis von Platingruppen-Metallen (PGM) optimiert wird. Dazu wird ein Optimierungsproblem aufgestellt, bei dem ein katalytisch aktiver Kanal in Zonen aufgeteilt wird, die mit unterschiedlichen Mengen von PGM beladen werden. Eine solche Beladung kann auch experimentell getestet werden. Die Effekte der Beladung auf Diffusionslimitierungen im Washcoat werden ebenso berücksichtigt. Ziel ist es, die axiale Verteilung der Beladung zu optimieren, wobei die Gesamtmenge an PGM konstant gehalten wird, um den Gesamtumsatz unter transienten Bedingungen zu maximieren. Dabei wird ein transientes 1D+1D-Modell mit dem impliziten Differentialgleichungslöser DASPKADJOINT numerisch gelöst und in ein nichtlineares Optimierungsproblem übersetzt, das mit einem beliebigen ableitungsbasierten nichtlinearen Optimierungslöser (NLP) behandelt werden kann. Dieses Modell wird auf zwei Beispielfälle angewandt: die CO-Oxidation auf einem Pt/Al2O3 Dieseloxidationskatalysator (DOC), um die Kaltstart-Emissionen zu minimieren, sowie die CH4-Oxidation auf Pd/Al2O3 unter Minimierung der Deaktivierungseffekte. In beiden Fällen wird beobachtet, dass bei der optimalen Lösung ein Beladungsmaximum am Kanaleingang zu einer Umsatzsteigerung führt. Die präsentierte Methode ist darüber hinaus allgemeingültig und kann auf andere Systeme mit unterschiedlicher Chemie angewandt werden, so dass auch signifikant andere Lösungen generiert werden können. Die Fähigkeit, NOx effizient durch Ammoniak zu reduzieren, ist Grundlage der SCR-Technologie für die Dieselabgasnachbehandlung. Ammoniak wird diskontinuierlich durch Zersetzung von Harnstoff-Wasser-Lösung dem SCR-Katalysator zugeführt. Bei der Anwendung im Fahrbetrieb ist es wegen hochgradig transienter Wechsel der Emissionen nicht sinnvoll, konstante Menge Ammoniak zu dosieren. Eine effiziente optimale Dosierungsstrategie ist wichtig, um einerseits hohen Umsatz zu gewährleisten und andererseits NH3-Schlupf zu vermeiden. Die Entwicklung einer optimalen Dosierungsstrategie erfordert die Anwendung einfacher, aber hinreichend akkurater mathematischer Modelle und robuster Optimierungsalgorithmen, um eine Lösung für eine große Anzahl zu optimierender Parameter zu erhalten. Mehrere Modellreduktionstechniken aus der Literatur wurden verwendet, um ein Grey-Box-Modell zu konstruieren. Die Methode der orthogonalen Kollokation über finiten Elementen (OCFE) wird genutzt, um die differential-algebraischen Gleichungen aus dem Optimierungsproblem in ein nichtlineares Programm zu überführen. Das Modell wird auf eine Simulation des WHTC-Testzyklus angewandt, um die NH3-Dosierung für jede Sekunde des Zyklus zu optimieren. Die optimale Lösung verbessert die Effizienz des Reduktion unter Einhaltung eines Schlupf-Maximums von 10 ppm zu jedem Zeitpunkt. Die präsentierte Methode lässt sich auch auf ähnliche Probleme zur Optimierung transienter Eingangsbedingungen anwenden. Im dritten Beispiel wird dieselbe Optimierungsmethode erweitert, um eine optimale Lambda-Trajektorie zu berechnen, die das Lambdasensorsignal am Katalysatorausgang stabilisiert, um Durchbrüche fetter Abgasgemische zu vermeiden. Zunächst wurde ein Beobachtermodell mit vereinfachter Kinetik entwickelt und gegen Versuchsstand-Experimente kalibriert. Direkte Kollokation auf Basis der OCFE wird genutzt, um das Optimierungsproblem in ein nichtlineares Programm zu überführen. Die optimale Lösung zeigt eine schnelle Stabilisierung der Ausgangssensor-Spannung ohne Überschwingungen. Diese Strategie verringert die Relaxationszeit der Sensorspannung signifikant, was wichtig für den Einsatz als Feedback-Controller in einem Dreiwegekatalysator wäre