Lean NOx Trap System Research Articles

Characteristics of NOx emission of light-duty diesel vehicle with LNT and SCR system by season and RDE phase

As the regulations on vehicle emissions have become more stringent internationally and real-driving emissions (RDE) have been established, the on-road characteristics of emissions have gained importance in vehicle research and development. The results of the fuel consumption levels and emissions from on-road tests are affected by many factors, such as driving conditions, routes and environmental conditions. Therefore, more research and analysis are needed for the effects of environmental factors and driving conditions according to RDE phase on the NOx emissions. In this study, RDE tests were conducted by season to analyze the on-road NOx emission characteristics of lean NOx trap (LNT)- and selective catalytic reduction (SCR)-equipped diesel vehicles corresponding to the Euro 6b regulation prior to the application of the RDE regulation. The purpose of this study is to analyze the effects of seasonal factors and phases of the RDE routes on the NOx emission and NOx conversion efficiency of catalyst. In spring/autumn and summer, the engine-out and tail-pipe NOx emissions were higher 1.3–5.9 times for vehicle A and 1.3–28.4 times for vehicle B in the urban phase than in other phases. In the urban phase, the engine bay temperature was probable to rise owing to frequent stops and low-speed driving, leading to a high intake air temperature, which causes excessive NOx emission, particularly in summer. The average air filter temperature in urban phase was 11–15 °C higher than the environment temperature for vehicle A. The NOx conversion efficiency of the LNT was highest at 54.1% on motorway and the efficiency was dependent on the phase of the test route. The NOx conversion efficiency of the SCR, which is dependent on the catalyst temperature, was highest at 98.7% in spring motorway and the efficiency was affected by the combined factors of season and phases.

MPC-Based Optimal Control for Diesel Engine Coupled with Lean NOx Trap System

In this paper, an on-board optimal control problem for diesel engines with lean NOx trap (LNT) is investigated. First, a two-order LNT model based on mass conservation and energy conservation is co...

Synergistic Effect of Cu/CeO2 and Pt-BaO/CeO2 Catalysts for a Low-Temperature Lean NO x Trap.

A lean NO x trap (LNT) catalyst has been widely used for removing NO x exhaust from lean-burn engines. However, the operation range of LNT has been limited because of the poor activity of LNT catalysts at low temperatures (≤300 °C), especially in urban driving conditions. To increase NO x removal efficiency during lean-rich cycle operation, a Cu/CeO2 (CC) catalyst was added to a Pt-BaO/CeO2 (PBC) catalyst. In comparison to PBC- or CC-only catalysts, the physical mixture of PBC and CC catalysts (PBC + CC) exhibited a significant synergy for both NO x storage and reduction efficiencies. In particular, low-temperature activity below 200 °C was greatly enhanced. A Pt-BaO-Cu/CeO2 (PBCC) catalyst, which was synthesized by depositing Pt and Cu together on a ceria support, showed poorer NO x removal efficiency. The origin of the synergistic effect over PBC + CC was investigated. Under lean conditions, the CC showed much better activity for NO oxidation, allowing for faster NO x storage on PBC. Under rich conditions, H2 was generated in situ on the CC by a water-gas shift reaction then accelerated the reduction of NO x, which had been stored on PBC, with a higher selectivity to N2. This simple modification in the catalyst can provide an important clue to enhance low-temperature activity of the commercial LNT system.

Investigation on Spray Behavior and NOx Conversion Characteristic of a Secondary Injector for a Lean NOx Trap Catalyst

Lean NOx trap (LNT) catalyst has been used to reduce NOx emissions from diesel engines. The LNT absorbs NOx in lean condition and discharges N2 by reducing NOx in rich conditions. Thus, it is necessary to make exhaust gas lean or rich conditions for controlling LNT system. For making a rich condition, a secondary injector was adopted to inject a diesel fuel into the exhaust pipe. In the case of secondary injector, the behavior of spray is easily affected by high temperature (i.e., 250 ∼ 350 °C) occurred in the exhaust manifold. Therefore, it is needed to investigate the spray behavior of diesel fuel injected into an exhaust manifold, as well as the conversion characteristics for a lean NOx trap of a diesel engine with LNT catalyst. The characteristics of exhaust emissions in NEDC (New European Driving Cycle) mode were analyzed and spray behaviors were visualized in various exhaust gas conditions. The results show that as the exhaust gas mass flow increases, the spray cone angle becomes broad and the fuel is directed to the flow field. Besides, the cone angle of spray is decreased by centrifugal force caused in exhaust gas flow field. In addition, the effects of nozzle installation degree, injection quantity, and exhaust gas flow on NOx conversion performance were clarified.

Modifying the Classical One-Dimensional Catalyst Model to Include Axial Conduction and Diffusion

Lean NOx trap (LNT) catalytic aftertreatment devices are one potential option for the reduction of oxides of nitrogen (NOx) in the exhaust of compression ignition engines. They work through a controlled modulation between a storage phase that captures NOx over an alkali earth metal and a regeneration phase that reduces the stored nitrates on the surface using a rich pulse of injected fuel or via stoichiometric engine operation. This rich phase has an associated fuel penalty while being relatively difficult to control through temperature and chemical species. In order to improve system efficiency, a number of researchers have proposed dual leg LNT systems using two LNTs, one of which is always storing while the other is undergoing regeneration. The majority of the exhaust flows through the storage LNT while only a small fraction (low space velocity) advects across the regeneration LNT. This increases the regeneration residence time, improving effectiveness and decreasing the amount of fuel used. From an LNT simulation standpoint, most researchers utilize the classical one-dimensional (1D) aftertreatment model constructed from the Euler equations of motion that neglect axial conduction and diffusion. This paper explores the applicability of this model under low flow situations prevalent in a dual leg LNT system through a carbon monoxide light-off experiment. The authors chose this type of experiment in order to focus purely on fluid mechanics and not the choice of LNT reaction mechanism. The results suggest that a Navier–Stokes (N–S) version of the 1D aftertreatment model is preferred for the regeneration leg of a dual LNT system. Moreover, the authors provide the solution of such a model within this paper.

Two-Level Nonlinear Model Predictive Control for Lean NOx Trap Regenerations

This paper describes a two-level nonlinear model predictive control (NMPC) scheme for diesel engine lean NOx trap (LNT) regeneration control. Based on the physical insights into the LNT operational characteristics, a two-level NMPC architecture with the higher-level for the regeneration timing control and the lower-level for the regeneration air to fuel ratio profile control is proposed. A physically based and experimentally validated nonlinear LNT dynamic model is employed to construct the NMPC control algorithms. The control objective is to minimize the fuel penalty induced by LNT regenerations while keeping the tailpipe NOx emissions below the regulations. Based on the physical insights into the LNT system dynamics, different choices of cost function were examined in terms of the impacts on fuel penalty and tailpipe NOx slip amount. The designed control system was evaluated on an experimentally validated vehicle simulator, cX-Emissions, with a 1.9 l diesel engine model through the FTP75 driving cycle. Compared with a conventional LNT control strategy, 31.9% of regeneration fuel penalty reduction was observed during a single regeneration. For the entire cold-start FTP75 test cycle, a 28.1% of tailpipe NOx reduction and 40.9% of fuel penalty reduction were achieved.

Modelling of a Lean NOx-Trap system with NO/NO2 differentiation and sulfur poisoning

One of the challenges in the automotive industry is to develop new vehicles and new technologies with minimal costs. In this context, modeling becomes an important tool for the design of future technologies by reducing the number of tests needed to develop a new exhaust system. With the emergence of future European standards [1], which are more restrictive on NOx and takes account of the differentiation between NO and NO2 emissions, European manufacturers have to describe precisely the formation and the behavior of NO2 in the after-treatment systems. The aim of this study is to improve the one dimensional after-treatment models developed by Renault by introducing the NO2 contribution from the engine to the tail pipe. The first part of this study focuses on the adaptation of after-treatment systems models in order to differentiate NO and NO2. Thus different global kinetics models for the Lean NOx-Trap System were studied. Different runs were carried out either at Renault or at the Laboratoire de Reactivite de Surface for understand the phenomena involved in NO2 formation and consumption. Then, the calibration of the model has been performed. Part of this paper focuses on the choice of an automated optimization method for this calibration step. By the end, the final model was validated versus engine bench tests performed on full size monoliths. The second part of the study describes more precisely the integration of the sulfur poisoning effect in the Lean NOx-Trap Model including the integration of NO2 in the model.

A Methodology for Fault Diagnosis of Diesel NOx Aftertreatment Systems

Diesel engines are today considered leading candidates for the new generations of passenger vehicles due to their fuel efficiency and drivability. One of the key elements for the future acceptability is the compliance with emission standards (particularly on nitrogen oxides), which will require precise control of the aftertreatment system. Furthermore, in light of OBD-II regulations, considerable research must be devoted to the design of fault diagnosis algorithms. The definition of fault diagnosis strategies is a complex process that involves thorough studies of the system behavior in healthy and faulty conditions. Such studies can be done in multiple ways, including experimentation and mathematical modeling. In both cases, a thorough knowledge of the system components, sensors and actuators is required.The proposed paper presents an approach to model-based fault diagnosis of Diesel NOx aftertreatment systems. The proposed methodology is based on a functional and structural analysis of the system, at the level of individual components and assemblies. This facilitates the mapping and characterization of system faults through FTA and FMEA methods, allowing for the design of control-oriented models to be used for fault detection and isolation. In this paper, the outlined approach is applied to a Lean NOx Trap system.

Model-Based Fault Diagnosis of a NOx Aftertreatment System

The Lean NOx Trap (LNT) is an aftertreatment device used to attain a reduction in nitrogen oxide emissions for Diesel and lean burn engines. The LNT is typically used as a storage device, capturing NOx during lean engine operation. The trap can be regenerated by controlling the exhaust air-fuel ratio to create a rich gas mixture. Under rich conditions, the stored NOx is released and catalytically converted. This way, tailpipe emissions can be significantly reduced by properly modulating the lean (storage) and rich (regeneration) periods. To maintain the LNT operate with high conversion efficiency, an optimized control of the regeneration scheduling is required. In addition, LNT systems require fault diagnostic schemes to detect and isolate faults, typically related to sulfur and thermal damages. This paper deals with the design and validation through simulation of a model-based fault diagnosis scheme for a LNT system. The mathematical model of the subsystem, based on the physics of the processes involved, consist of time- varying nonlinear ODEs. The proposed diagnostic approach is based on the generation of residuals using system models and through comparison of the predicted and measured value of selected variables, including the AFR, catalyst output temperature and the NOx concentration at the output of the LNT. The paper focuses on detection and isolation of controller faults and LNT parametric faults related to sulfur and thermal damage. The model utilized in the diagnostic scheme, which includes sulfur poisoning and thermal deactivation processes, has been experimentally validated from data collected on a Diesel LNT system and integrated with a quasi-steady engine and vehicle simulator to estimate tailpipe emissions during standard driving cycles.