Experimental study on the characteristics of nano-particle emissions from a heavy-duty diesel engine using a urea-SCR system

Particulate matter in diesel engine exhaust, particularly nano-particles, can cause serious human health problems including diseases such as lung cancer. Because diesel nano-particle issues are of global concern, regulations on particulate matter emissions specify that not only the weight of particulate matter emitted but also the concentration of nanoparticles must be controlled. This study aimed to determine the effects on nano-particle and PM emissions from a diesel engine when applying a urea-SCR system for NO_x reduction. We found that PM weight increases by approximately 90% when urea is injected in ND-13 mode over the emission without urea injection. Additionally, PM weight increases as the NH₃/NO_x mole ratio is increased at 250 °C. In SEM scans of the collected PM, spherical particles were observed during urea injection, with sizes of approximately 200 nm to 1 μm. This study was designed to determine the conditions under which nano-particles and PM are formed in a urea-SCR system and to relate these conditions to particle size and shape via a quantitative analysis in ND-13 mode.     

Urea-SCR system optimization with various combinations of mixer types and decomposition pipe lengths

The demand for NO_x after-treatment system has increased dramatically due to the stricter NO_x emission regulations for diesel vehicles. The urea-SCR system is one of the NO_x after-treatment methods found to be quite effective to meet the regulation requirement enforced by various authorities including the Euro-6. In order to develop an effective urea-SCR system, it is critical to establish an even distribution of reductant over the catalyst surface since this favorable distribution can increase reduction reaction and in turn, improve NO_x conversion efficiencies. In the current study, a number of design variations of the urea-SCR system which included two mixer types and three decomposition pipe lengths, were evaluated systematically using CFD analysis and experimental measurements. The purpose of the CFD analysis was to estimate the distribution of reductant within the urea-SCR system with a specific configuration and the purpose of the engine emission test was to measure the amount of NO_x reduction, respectively. The results from the systematic analysis revealed the relation between the reductant distribution over the SCR and the performance of the NO_x reduction.     

Sliding-mode observer for urea-selective catalytic reduction (SCR) mid-catalyst ammonia concentration estimation

This paper presents an observer design for SCR mid-catalyst ammonia concentration estimation using tailpipe NO_x and ammonia sensors. Urea-SCR has been popularly used by Diesel engine powered vehicles to reduce NO_x emissions in recent years. It utilizes ammonia, converted from urea injected at upstream of the catalyst, as the reductant to catalytically convert NO_x emissions to nitrogen. To simultaneously achieve high SCR NO_x conversion efficiency and low tailpipe ammonia slip, it is desirable to control the ammonia storage distribution along the SCR catalyst. Such a control method, however, requires a mid-catalyst ammonia sensor. The observer developed in this paper can replace such a mid-catalyst ammonia sensor and be used for SCR catalyst ammonia distribution control as well as serves for fault diagnosis purpose of the mid-catalyst ammonia sensor. The stability of the observer was shown based on the sliding mode approach and analyzed by simulations. Experimental validation of the observer was also conducted based on a medium-duty Diesel engine two-catalyst SCR system setup with emission sensors.     

Effect of UWS injection at low exhaust gas temperature on NO<Subscript>x</Subscript> removal efficiency of diesel engine

Urea-SCR systems have been widely used in diesel vehicles according to the strengthened NO_x (Nitrogen Oxides) emission standard. The NO_x removal efficiencies of the latest well optimized urea-SCR system are above 90 % at moderate exhaust gas temperature of 250 ~ 450 °C. However, a large amount of NO_x is emitted from diesel vehicles at cold start or urban driving conditions, when the exhaust gas temperature is not high enough for SCR catalyst activation. Although many researchs have been stuied to improve NO_x conversion efficiency at these low temperature conditions, it is still one of important technical issues. In this study, the effect of UWS injection at low exhaust gas temperature conditions is studied. This study uses a 3.4 L diesel engine equipped with a commertial urea SCR system. As a result, it is found that about 5 % of NO_x removal efficiency is improved in the NRTC test when UWS injection starts at the SCR inlet temperature of 150 °C compared to 200 °C. It is also found that urea deposits can be formed on the wall of exhaust pipe, when the local wall temperature is lower than temperature of urea decomposition.     

Model-based optimization of parameters for a diesel engine SCR system

A Selective Catalytic Reduction (SCR) system simulation model for diesel engines was established based on MATLAB/Simulink. The model includes four subsystems: signal acquisition, DEF (Diesel Exhaust Fluid) dosing control, catalytic chemistry reaction and NO_X emissions reduction. The SCR control strategy was optimized based on ETC simulation results. Simulation and test results indicate that the model developed in this paper can predict ETC NO_X emissions accurately.     

Performance of the NOx sensor based on mixed potential for automobiles in exhaust gases

The NO_x sensor based on mixed potential was made by laminating YSZ green sheets, on which electrodes including an NO_x sensing electrode, an NO_x conversion electrode, a Pt heater and thermocouple were printed and sintered. The output signal of this sensor was fairly independent of gas temperatures and the velocity of gas flows in the test gases. The engine test for the exhaust gases at around λ=1 showed that the sensor outputs changed corresponding to NO_x concentrations from the gas analyzer. It is expected that this sensor based on mixed potential can be utilized for automobiles.     

Real-time empirical NO<Subscript>x</Subscript> model based on In-cylinder pressure measurements for light-duty diesel engines

This paper proposes a real-time empirical model of NO_x emissions for diesel engines. The proposed model predicts the level of NO_x emissions using an empirical model developed based on the thermal NO formation mechanism, the extended Zeldovich mechanism. Since it is difficult to consider the exact physical NO formation phenomena in real-time applications, the proposed algorithm adapts the key factors of the NO formation mechanism from the extended Zeldovich mechanism: temperature of the burned gas, concentration of the gas species, and combustion duration where NO is generated. These factors are considered in a prediction model as four parameters: exhaust gas recirculation rate (EGR rate), crank angle location of 50 % of mass fraction burned (MFB50), exhaust lambda value, and combustion acceleration. The proposed prediction model is validated with various steady engine experiments that showed a high linear correlation with the NO_x emission measured by a NO_x sensor. Furthermore, it is also validated for transient experiments.     

NOx emission characteristics in rich–lean combustion of hydrogen

Characteristics of NO_x formation in a gas turbine fuelled with hydrogen were analyzed with both an experimental and a numerical approach. This research experimentally investigated NO_x reduction effect of rich–lean combustion in a coaxial burner. Hydrogen emits no Prompt NO even in rich mixture conditions, and can be more effective to reduce NO_x in the rich–lean combustion system than hydrocarbons. The results show that the rich–lean combustion of hydrogen successfully reduces NO_x emission compared with diffusive combustion. In the rich–lean combustion, hydrogen combustion has lower NO_x emission compared to methane combustion, especially with larger equivalence ratio of richer side mixture. Calculations of NO_x formation in the rich–lean combustion were also done employing the extended Zel’dovich NO formation mechanism.     

Evaluation and visualization of stratified ultra-lean combustion characteristics in a spray-guided type gasoline direct-injection engine

To comply with reinforced emission regulations for harmful exhaust gases, including carbon dioxide (CO₂) emitted as a greenhouse gas, improved technologies for reducing CO₂ and fuel consumption are being developed. Stable lean combustion, which has the advantage of improved fuel economy and reduced emission levels, can be achieved using a sprayguided-type direct-injection (DI) combustion system. The system comprises a centrally mounted injector and closely positioned spark plugs, which ensure the combustion reliability of a stratified mixture under ultra-lean conditions. The aim of this study is to investigate the combustion and emission characteristics of a lean-burn gasoline DI engine. At an excess air ratio of 4.0, approximately 23% improvement in fuel economy was achieved through optimal event timing, which was delayed for injection and advanced for ignition, compared to that under stoichiometric conditions, while NO_x and HC emissions increased. The combustion characteristics of a stratified mixture in a spray-guided-type DI system were similar to those in DI diesel engines, resulting in smoke generation and difficulty in three-way catalystutilization. Although a different operating strategy might decrease fuel consumption, it will not be helpful in reducing NO_x and smoke emissions; therefore, alternatives should be pursued to achieve compliance with emission regulations.     

Simplified emission test for the inspection of emission control system installed on vehicles

The objective of this study is to develop a simplified emission test for the inspection of in-use vehicles. For this purpose, a simplified chassis dynamometer (S-CHDY) was developed first. S-CHDY consists of a speed meter tester and a direct current motor system which can load electrical inertia force to the test vehicle. To examine the effectiveness of the emission test, the emission control systems of four vehicles were tampered with, and they were driven on S-CHDY with M4 mode pattern. CO, HC and NO_x concentrations in tail pipe gas were continuously measured and those mean values were evaluated. As a result, it was shown that the mean values rise according to the abnormality in the emission control system. Moreover, to directly measure the mass emissions from the test vehicles, a new type CVS method was developed.     

Development of Preview Active Suspension Control System and Performance Limit Analysis by Trajectory Optimization

The main role of the suspension system is to achieve ride comfort by reducing vibrations generated by the road roughness. The active damper is getting much attention due to its reduced cost and ability to enhance ride comfort especially when the road ahead is measurable by an environment sensor. In this study a preview active suspension control system was developed in order to improve ride comfort when the vehicle is passing over a speed bump. The control system consists of a feedback controller based on the skyhook logic and a feedforward controller for canceling out the road disturbance. The performance limit for the active suspension control system was computed via trajectory optimization to provide a measure against which to compare and validate the performance of the developed controller. The simulation results indicated that the controller of this study could enhance ride comfort significantly over the active suspension control system employing only the skyhook feedback control logic. Also the developed controller, by displaying similar control pattern as the trajectory optimization during significant time portions, proved that its control policy is legitimate.     

Study of the effects on exhaust emissions in direct injection diesel engines: Effects of fuel injection system,distillation properties and cetane number

The effect of injection nozzle, diesel fuel density (volatility) and cetane number on diesel exhaust emissions were investigated. Decreasing injection nozzle hole diameter decreases PM emission. However, a small nozzle hole increases NO_x emission and decreases the effect of fuel on PM emission. Decreasing fuel density is effective for reduction of NO_x emission. But the effect is smaller than that of nozzle hole diameter and injection pressure. Furthermore injection timing retardation decreases the effect of fuel density on NO_x emission.     

Experimental and 3D,embedded modeling for diesel engine SCR deposit

Nowadays, the selective catalytic reduction (SCR) is a promising solution to fulfill stringent nitrogen oxide (NO_X) emission standards enforced by worldwide regulation bodies for lean burning engines. However, in the practical operation, urea deposits occur under unfavorable conditions, which bring about a failure of urea dosing strategy, affect the SCR system de-NOx performance and lowering the fuel economy by increasing the engine back pressure. This paper will present 3D deposit model, which can be used to predict the deposit position. Furthermore, a model-based controls strategy and calibration are designed. The comparison test results of both engine emission tests and vehicle field tests shows there is significant deposits improvement with the embedded model.     

Development of an active front steering (AFS) system with QFT control

This paper investigates an active front steering control strategy based on quantitative feedback theory (QFT). By incorporating feedback from a yaw rate sensor into the active steering system, the control system improves the dynamic response of the vehicle. The steering response of a vehicle generally depends upon uncertain quantities like mass, velocity, and road conditions. Thus, QFT is used to design a controller with robust performance. A multi-degree-of-freedom nonlinear model is co-simulated here by MATLAB Simulink and ADAMS/CAR. The performance of the control system is evaluated under various emergency maneuvers and road conditions. The result shows that the designed robust control system has good control performance and can efficiently improve handing qualities and stability characteristics.     

Switched control of vehicle suspension based on motion-mode detection

This paper presents a study on switched control of vehicle suspension based on motion-mode detection. This control strategy can be potentially implemented via the interconnected suspension such as hydraulically interconnected suspension by actively switching its interconnection configuration in terms of the dominant vehicle body motion-mode. The design of the switched control law is developed focusing on three vehicle body motion-modes: bounce, pitch, and roll. At first, an H_∞ optimal controller will be designed for each motion-mode with the use of a common quadratic Lyapunov function, which guarantees the stability of the switched system under arbitrary switching functions. Then, a motion-mode detection method based on the calculation of the motion-mode energy is introduced. And then, the possible implementation of the control system in practice is discussed. Finally, numerical simulations are used to validate the proposed study.     

Effect of direct in-cylinder CO2 injection on HCCI combustion and emission characteristics

Fuel injection during negative valve overlap period was used to realize diesel homogeneous charge compression ignition (HCCI) combustion. In order to control the combustion, CO₂ in-cylinder injection was used to simulate external EGR. Effects of CO₂ injection parameters (injection timing, quantity, pressure) on HCCI combustion and emission characteristics were investigated. Experimental results revealed that CO₂ in-cylinder injection can control the start of combustion and effectively reduce NO_x emission. Either advancing CO₂ injection timing or increasing CO₂ injection quantity can reduce peak cylinder pressure and mean gas temperature, delay the starts of low temperature reaction (LTR) and high temperature reaction (HTR), and lower pressure rise rate; NO_x emission was reduced, while smoke, HC, and CO emissions increased. Since the combustion phase was improved, the indicated thermal efficiency was also improved. Injection pressure determines the amount of disturbance introduced into the cylinder. Generally, with the same injection quantity, higher injection pressure results in higher momentum flux and total momentum. Larger momentum flux and momentum has a stronger disturbance to air-fuel mixture, resulting in a more homogeneous mixture; therefore, larger injection pressure leads to lower NO_x and smoke emissions.     

NOx reduction characteristics of Pt-ZSM-5 catalyst with diesel engine exhaust

The platinum ion-exchanged ZSM-5 zeolite catalyst (Pt-5), which reduces nitrogen oxides (NO_x) in the presence of oxygen and hydrocarbons, was applied to actual diesel engine exhaust. Compared to the Cooper ion-exchanged ZSM-5 zeolite catalyst, the Pt-Z had higher NO_x reduction efficiency, η_NOx = 33%, and lower activation temperature, 250°C, in normal engine operation. It was found that water in the exhaust gas did not apparently affect the NO_x reduction, while the reduction efficiency was significantly affected by the aspect ratio of the catalyst reactor and by the shape of the catalyst, i.e. pellet or honeycomb.     

SCR control strategy based on ANNs and Fuzzy PID in a heavy-duty diesel engine

The paper presents an innovative method combining artificial neural networks (ANNs) with Fuzzy PID to demonstrate the advantages of this control approach for meeting both NOx emission requirements and NH3 slip targets. An ANN model was utilized to simulate the formation of NOx emissions under various engine operating conditions. Next, an effective closed-loop control strategy with a type of feedback known as fuzzy PID is adopted for on-line, real-time control of 32.5% aqueous urea dosing in the exhaust stream. The new strategy explores the benefits by simulation and testing in the environments of Matlab/Simulink and ESC/ETC, respectively. The notable achievement of considerable NOx reduction and an acceptably small NH3 slip is obtained based on this new, feasible and effective strategy.     

Learning-based traffic signal control algorithms with neighborhood information sharing: An application for sustainable mobility

This research applies R-Markov Average Reward Technique based reinforcement learning (RL) algorithm, namely RMART, for vehicular signal control problem leveraging information sharing among signal controllers in connected vehicle environment. We implemented the algorithm in a network of 18 signalized intersections and compare the performance of RMART with fixed, adaptive, and variants of the RL schemes. Results show significant improvement in system performance for RMART algorithm with information sharing over both traditional fixed signal timing plans and real time adaptive control schemes. The comparison with reinforcement learning algorithms including Q learning and SARSA indicate that RMART performs better at higher congestion levels. Further, a multi-reward structure is proposed that dynamically adjusts the reward function with varying congestion states at the intersection. Finally, the results from test networks show significant reduction in emissions (CO, CO₂, NO_x, VOC, PM₁₀) when RL algorithms are implemented compared to fixed signal timings and adaptive schemes.