World Harmonized Transient Cycle Research Articles

The impact of energy flow on catalytic behavior in advanced DOC+SCR emission control systems

To further improve the catalytic efficiency of the aftertreatment system in exhaust conditions, the exhaust decoupling system is employed to investigate the effect of energy flow on aftertreatment catalytic systems (SCR & DOC+SCR) at wide engine load conditions (10–100 %). The physicochemical properties of particulate matter (PM) are analyzed by transmission electron microscopy (TEM). Furthermore, the energy flow and catalytic performances under the World Harmonized Transient Cycle(WHTC) are revealed. The research results exhibited that the energy distribution of exhaust in “DOC+SCR” was 10–15 % higher than that in “SCR” under steady state conditions, the provision of additional activation energy enhances the catalytic reaction, thereby improving the rate of the reaction, particularly in terms of NOx catalytic efficiency. On the other side, under transient conditions, the energy distribution of exhaust in “DOC+SCR” was 8.72 % higher than that in “SCR”. The NO/NO2 ratio remains stable value after the exhaust passing through DOC, thereby promoting the formation of rapid SCR reaction conditions. By revealing the PM microstructure, DOC enhances PM’s microcrystalline tortuosity and reduces particle size, thereby improving its oxidation reaction characteristics through combined catalysis. Simultaneously, this study demonstrates that the optimized combined catalytic method holds great potential for complete PM treatment.

Development of the two-stage SCR control strategy to satisfy ultra-low NOX emission regulation for heavy-duty diesel engine

The emission regulations for heavy-duty diesel engines regarding nitrogen oxide (NOX) are becoming increasingly stringent, particularly in relation to cold start cycles. While the two-stage selective catalytic reduction (SCR) has the potential to achieve ultra-low NOX emissions, several challenges remain, including the accurate prediction of ammonia (NH3) storage mass and the co-control of the two-stage SCR. The first step in this study involved the establishment of a rapid control prototype platform to facilitate the development and validation of a two-stage SCR control strategy. Secondly, an initial method for predicting the NH3 storage based on the mass conservation law was proposed, which was subsequently improved by filling and emptying experiments. The third step involved the development of a two-stage SCR co-control strategy, including obtaining the steady-state NH3 storage target value, dynamic correction for NH3 storage target value, regulation of NH3 storage, and control of the close-coupled SCR urea injector state. Finally, the two-stage SCR urea injection control strategy was certified under the world harmonized transient cycle (WHTC). The results demonstrate that the composite value of engine outlet NOX emissions under cold and hot start WHTC cycles is 13 g/(kW·h). Meanwhile, the composite value of tailpipe NOX emissions under cold and hot start WHTC cycles is 0.065 g/(kW·h), representing only 14% of the EU VI limit value of 0.46 g/(kW·h). Thus, the findings demonstrate that integrating an accurate NH3 storage prediction method with the two-stage SCR co-control function is crucial for heavy-duty diesel engines to achieve ultra-low NOX emissions.

A temporal discretization and spatial integration SCR model with dual temperature-related parameters

Developing an accurate and rapid Selective Catalytic Reduction (SCR) model is crucial for advanced control strategies, which is challenging due to the complex physicochemical processes. In this paper, a novel control-oriented SCR model is proposed, which exhibits faster and more accurate estimation performance for both NOx and NH3 emissions. Unlike traditional modeling methods, this research develops a temporal discretization and spatial integration (TDSI) SCR model with dual temperature-related parameters. By introducing NH3 recycling, the spatial integration model is improved, leading to an enhanced accuracy in model estimation. Moreover, A time discretization method based on current state can effectively address the issue of sampling time sensitivity. Additionally, dual temperature-related parameters can significantly enhance model accuracy, particularly in high-temperature regions. A simplified and clear flowchart is provided for parameter identification based on a limited number of engine bench test results. In a cold-start World Harmonized Transient Cycle (WHTC), the TDSI model can achieve NOx and NH3 accuracies of 92.26% and 92.52%, respectively, with a computation time of 1.47 s. Through comprehensive comparisons, the proposed model exhibits comparable accuracy to the GT model while significantly reducing computation time by 98.39%. Additionally, compared to the Continuous Stirred-Tank Reactor (CSTR) and θT models, there is an approximate 3% improvement in NOx and NH3 accuracies. This TDSI model provides a foundation for designing urea injection control strategies and offers new perspectives for simplifying the model design.

Thermal Management of Diesel Engine Aftertreatment System Based on Ultra-Low Nitrogen Oxides Emission

To achieve diesel engine ultra-low nitrogen oxide emission, light-off selective catalyst reduction (LO-SCR) has been suggested for better performance with lower exhaust temperature. An electric heater upstream of the exhaust aftertreatment system was applied to significantly decrease the NOx emission at a low exhaust temperature. With a 7.2 kW electric heater coupled with LO-SCR, the NOx emission during 200~500 s of the world harmonized transient cycle (WHTC) decreased from 282.6 ppm to 61.5 ppm, which is a decrease of 45%. Application of an upstream diesel oxidation catalyst (DOC) decreased the NOx emission by 63% at the same interval at the cost of worse cold-start performance. The urea input was also adjusted to avoid NOx emission during the latter part of the WHTC.

A Comparative Investigation of the Emissions of a Heavy-Duty Diesel Engine under World Harmonized Transient Cycle and Road Spectrum Cycle

In the present study, detailed comparative experiments on a heavy-duty diesel engine used in the world harmonized transient cycle (WHTC) and road spectrum reversely deduced cycle (RSRDC, which was derived from a road test) were carried out. Fuel consumption and gaseous and particulate pollutants, along with some engine operation parameters, were measured transiently; thus, specific emissions can be calculated. Results showed that the BSFC of WHTC and RSRDC was 201.8 and 210 g/kW·h, respectively, because the real road driving cycle (RSRDC) had wider operating point distributions and more points located in the low-efficiency zone relative to WHTC. Thus, WHTC operations exhibited higher raw CO (abundant CO formation needed a specific temperature threshold) and NOx but lower HC. Furthermore, with aftertreatment, all pollutants met the newest China regulation limit. Finally, transient emissions were analyzed in detail. Although the specific emissions of some pollutants were similar in value for both cycles, transient processes may largely be different. Therefore, the current study is meaningful, and we not only provide broad and detailed information but also directly compare two types of operations (one is a real road driving cycle) in the laboratory: this is rarely discussed in the literature.

Emission Characteristics of Heavy-Duty Vehicle Diesel Engines at High Altitudes

The aim of this study was to accurately quantify the emission characteristics of pollutants at different altitudes. We used an intake and exhaust altitude simulation system that could simulate the intake and exhaust pressures of a national sixth vehicle diesel engine at different altitudes. Experimental research was conducted on the World Harmonized Transient Cycle (WHTC) and World Harmonized Steady State Cycle (WHSC) of the diesel engine. The results showed that carbon monoxide (CO) emissions increased with the altitude at full load, but their rates were significantly reduced at low speed (800 rpm), increasing by 0.0084–0.665 ppm/m. Hydrocarbon (HC) emissions showed an initial decreasing and then increasing trend, with a rise of up to 30%. Nitrogen oxides (NOx) showed a linear decreasing trend, especially at low speed. With the increase in altitude, the cycle work of the diesel engine decreased in a non-linear manner, and the decrease became more pronounced above 3000 m. The raw emission results of the WHTC and WHSC tests also revealed that CO increased exponentially, NOx decreased slightly and then increased rapidly, HC increased linearly, and the emissions of all pollutants deteriorated significantly above 3000 m. The exhaust emission results of the WHTC and WHSC tests showed that the CO emission showed an initial decreasing and then increasing trend with the elevation of the altitude, approximately 15 ± 5 mg/kWh. HC emissions showed an increasing trend, with HC emissions of 3 – 6 mg/kWh for the WHTC and 1 – 2 mg/kWh for the WHSC. NOx emissions did not follow any obvious rule, while the particulate matter (PM) tended to increase and then decrease with the elevation of the altitude. In relation to the current emission standards, the limit value margin for CO and HC exhaust emissions is greater than 95% and the limit value margin for PM emissions is greater than 88% at an altitude of 4000 m. The NOx emission limit is greater than 87% (within 3000 m), but there is a risk of exceeding the limit above 3000 m. The second sampling data from the WHTC and WHSC showed that the raw emissions of the engine were higher in the high-altitude area than in the low-altitude area, but the change law of the exhaust emissions was not obvious, and the levels of both emissions were low.

Prediction of the transient emission characteristics from diesel engine using temporal convolutional networks

In order to predict the transient emission characteristics from diesel engine accurately and quickly, a novel prediction model, based on temporal convolutional networks (TCN) that incorporates the dilated convolutions and residual connections, was presented in the paper. Firstly, 1800 samples from the World Harmonized Transient Cycle (WHTC) were employed to train and validate the model. A Random Forest algorithm was used to select six top important variables as inputs to reduce the data dimensionality. Then the effect of model hyperparameters on the prediction performance was discussed and the optimal hyperparameter combination was obtained by a particle swarm optimization (PSO) algorithm. The optimized TCN model showed a coefficient of determination value (R2) above 0.972 for training dataset and 0.941 for validation dataset, respectively. The root mean squared error (RMSE) and the mean absolute error (MAE) were relatively low. Finally, the measured data from World Harmonized Steady Cycle (WHSC) was used to test model, and the average R2 value of 0.936 demonstrated that TCN model has excellent robustness and generalization. Moreover, a comparative investigation between TCN model and other advanced algorithms, including BP, GBRT, XGBoost, RNN, LSTM and Transformer, was also conducted. The result showed that TCN model has not only higher accuracy, but also has less computing time. This demonstrates that it is a promising method to predict the emission characteristics of diesel engine.

A new transient NOx prediction model for diesel engine based on neural network model with Pelican Optimization Algorithm

The World Harmonized Steady Cycle (WHSC) and the World Harmonized Transient Cycle (WHTC) are used to simulate the real driving conditions of a vehicle, which provides an opportunity for researchers to predict engine emissions through the use of models and thus investigate engine emission technologies or technology optimization to meet regulations. In this study, a Back-Propagation neural network (BPNN) is used to build an engine NOx prediction model, determine the number of nodes in the hidden layer, and optimize its weights and thresholds using the Pelican Optimization Algorithm (POA). NOx emissions from diesel engines are predicted by using the WHTC. The results show that the proposed model has a high R coefficient value of 0.9641. The results of the cold WHTC cycle NOx emission prediction shows that the mean absolute error (MAE) for the three cycles is about 32–35 ppm. The MAE was approximately 2.8% of the maximum NOx emissions in all cycles. These results indicate that the model has high accuracy in predicting transient NOx emissions.

Experimental investigation on exhaust emissions of a heavy-duty vehicle powered by a methanol-fuelled spark ignition engine under world Harmonized Transient Cycle and actual on-road driving conditions

Compared to the conventional fossil fuels, methanol is a renewable, environment-friendly, and inexpensive oxygenated fuel. In this work, to explore the feasibility of using methanol as a substitute for diesel or gasoline in internal combustion engines, the exhaust emissions of a heavy-duty vehicle powered by the methanol spark ignition (SI) engine are tested under the World Harmonized Transient Cycle (WHTC) and actual on-road driving conditions. The results indicate that the brake specific CO, NOx, PM, and non-methane hydrocarbon emissions of the heavy-duty engine fuelled with methanol under the WHTC are lower than those of the engine fuelled with diesel and diesel-gasoline. However, the NH3 emission of the heavy-duty engine fuelled with natural gas is much higher than that of the methanol-fuelled engine, and exceeds the China VI emission standard. Under the actual road test, CO emission decreases once the exhaust temperature of the test vehicle reaches up to 250 °C, while NOx emissions are maintained at a relatively low level and then sharply increase at the initial cold-start of the road test. However, NOx emissions increase with further increase in the exhaust temperature.

Thermal management strategy for active regeneration of diesel particulate filter

The method of diesel oxidation catalyst (DOC) assistance is an effective way to achieve active regeneration of diesel particulate filter (DPF). Therefore, an appropriate DOC inlet temperature is the essential boundary condition for this regeneration process. In this paper, the thermal management measures and a novel strategy based on the requirements of DPF active regeneration have been proposed and studied through experiments. Results show that intake throttling can increase DOC inlet temperature by 45% at high speed and 13% at low speed. However, its effect becomes significant only after the throttle closure exceeds 65%. Near post-injection is a more effective method than intake throttling to increase the DOC inlet temperature, and is suitable for situations where the DOC inlet temperature differs greatly from the target value. Under the premise of economic consideration, the optimal value of near post-injection time is always 20°CA after top dead center (TDC). The near post-injection quantity has a greater effect on DOC inlet temperature than the near post-injection time. However, too large near post-injection quantity can also lead to a sharp deterioration in fuel economy. Meanwhile, a novel thermal management strategy based on engine working zone division is proposed according to the ability of different thermal management measures and the distribution law of original DOC inlet temperature. With this strategy, the DOC inlet temperature in whole engine operating range increases significantly. In steady state, more than 80% of the operating points can reach the target value of 450 ℃. In world harmonized transient cycle (WHTC), the average DOC inlet temperature is also increased by 28.8%.

Application and Efficiency of a Series-Hybrid Drive for Agricultural Use Based on a Modified Version of the World Harmonized Transient Cycle

Off-road vehicles and transportation are vital for agricultural economics, yet the transition to green energies is challenging. To make this transition easier, a tool that enables the testing of heavy-duty off-road vehicles in various scenarios was created. Based on the methods of the World Harmonized Transient Cycle (WHTC), a new Hybrid Operational Cycle (HOC) that reflects the features of agricultural work was created and applied in a graphical model simulation. This was a newly developed methodology. The cycle and the model were based on gathered research data. A numerical model of a medium-power tractor with an internal combustion engine and a series-hybrid setup was created, and simulations were performed in Matlab and AVL Cruise. Both diesel and hybrid vehicles were compared in terms of their power production, fuel consumption, and efficiency in fieldwork and transportation scenarios. The results showed that a series-hybrid transmission can achieve an efficiency similar to that of a tractor with a continuously variable transmission (CVT), but because it uses an electric powertrain, it still provides the opportunity to exploit energy regeneration during transportation and under low-load conditions. The designed model may also be used to develop control algorithms for hybrid drives and improve their efficiency.

Prediction Model for Transient NOx Emission of Diesel Engine Based on CNN-LSTM Network

In order to address the challenge of accurately predicting nitrogen oxide (NOx) emission from diesel engines in transient operation using traditional neural network models, this study proposes a NOx emission forecasting model based on a hybrid neural network architecture combining the convolutional neural network (CNN) and long short-term memory (LSTM) neural network. The objective is to enhance calibration efficiency and reduce diesel engine emissions. The proposed model utilizes data collected under the thermal cycle according to the world harmonized transient cycle (WHTC) emission test standard for training and verifying the prediction model. The CNN is employed to extract features from the training data, while LSTM networks are used to fit the data, resulting in the precise prediction of training NOx emissions from diesel engines. Experimental verification was conducted and the results demonstrate that the fitting coefficient (R2) of the CNN-LSTM network model in predicting transient NOx emissions from diesel engines is 0.977 with a root mean square error of 33.495. Compared to predictions made by a single LSTM neural network, CNN neural network predictions, and back-propagation (BP) neural network predictions, the root mean square error (RMSE) decreases by 35.6%, 50.8%, and 62.9%, respectively, while the fitting degree R2 increases by 2.5%, 4.4%, and 6.6%. These results demonstrate that the CNN-LSTM network prediction model has higher accuracy, good convergence, and robustness.

Development of a model based strategy for catalyst screening to control NOx and N2O emissions in NH3-SCR process towards Eu7 readiness

Transient kinetic models were used to compare different commercial NH3-SCR catalytic monoliths under World Harmonized Transient Cycle (WHTC) conditions for NOx abatement performance, N2O formation and NH3 slip. Towards this goal, kinetic models for four Cu-zeolite, two Vanadia and one Fe-zeolite formulation were developed using experimental data of NH3 adsorption, desorption, NH3 oxidation, NO oxidation, Standard SCR and Fast SCR reactions. Models were in very good agreement with experimental data. Cu and Fe-zeolite formulations showed high deNOx activity with cycle efficiencies > 90% and > 95% respectively, for cold and warm WHTC runs, whereas Vanadia formulations outshined with negligible N2O formation. Simulations were performed to assess the variation of tailpipe NOx as a function of reactor volume. A plateau in deNOx performance was observed for all formulations as a function of the reactor volume which was around 30 and 50 L for cold and warm WTHC runs, respectively. This was found to be due to the decreasing temperature and fractional coverage of adsorbed NH3 along the axial distance of the reactors. Models were also used to develop hybrid reactor in series designs. Successive SCR reactors consisting of different formulations were simulated. The most successful design in terms of high deNOx performance and low N2O formation was with a reactor using a Vanadia catalyst followed by a reactor with Cu-zeolite catalyst. This resulted in a very similar deNOx performance but with a significantly lower N2O emissions as compared to a reactor with the same total volume consisting of Cu-zeolite.

Research on fault diagnosis and signal reconstruction technology of diesel engine NOx sensor based on deep learning algorithm

Modern diesel engines are commonly equipped with NOx sensors for precise and efficient selective catalytic reduction (SCR) reductant injection control, but due to the limitations of operating characteristics and environmental conditions, the output signal of NOx sensors may show abnormalities such as drift, deviation, or even distortion. Therefore, the evaluation and adjustment of the NOx sensor signal is essential to achieve intelligent control of the engine SCR system. In this study, an engine NOx calculation model is designed using deep neural network (DNN) to predict the upstream NOx emission concentration of engine SCR and provide reference values for sensor fault diagnosis, evaluate the sensor fault status by calculating the relative integration error index (IE), and reconstruct the sensor signal using the model prediction signal after the fault is confirmed. In addition, this paper uses the firefly algorithm (FA) to find the appropriate diagnostic threshold (IEthre) and integration period ( T) to meet the requirements of diagnosis response speed. Finally, based on the data collected from World Harmonized Transient Cycle (WHTC) bench test of a commercial high-pressure common rail diesel engine, the IEthre = 12.76% and the T = 7.57 s. The root mean square error (RSME) of the NOx concentration signal predicted by the engine NOx calculation model was 37.35 ppm, and the normalized cumulative NOx emission was 98% of the actual signal. The performance of the designed algorithm was tested using the synthesized bias fault and drift fault signals. The sensor signal RSME was reduced from 108.46 to 29.66 ppm for the bias fault, and from 106.92 to 31.30 ppm for the drift fault.

Experimental study on pollutant emission characteristics of diesel urea-based selective catalytic reduction system based on corrugated substrate

In this study, a corrugated substrate of diesel engines for urea-based selective catalytic reduction (urea-SCR) with V2O5-WO3/TiO2 (V-SCR) and Cu-based (Cu-SCR) catalysts is proposed and the effects of the ammonia-to-nitrogen ratio α on the NOx emission, NH3 slip, and N2O emission were experimentally investigated on a diesel engine test bench with the World Harmonized Transient Cycle (WHTC). The volume ratio of the catalyst to the engine capacity is 1.6. With the increase of α, the NOx reduction reaction of V-SCR performed better NOx emission characteristics than that of Cu-SCR. When α increased from 1.0 to 1.2, the NOx emission of V-SCR and Cu-SCR decreased from 0.44 to 0.26 g/kW·h and from 0.63 to 0.36 g/kW·h, respectively. However, the NH3 slip of Cu-SCR was less because of its absorption capacity of NH3. The NOx conversion efficiency β and N2 selectivity δ of V-SCR outperformed those of Cu-SCR. At α = 1.1, β and δ of V-SCR were 1.07% and 3.25% higher than those of Cu-SCR, respectively. The dynamic response of V-SCR to NH3 showed high competitiveness compared to Cu-SCR. It was concluded that, based on the corrugated substrate, the V-SCR showed better pollutant emission characteristics than did the Cu-SCR.

NH3 and N2O emission durability of the heavy-duty diesel engine with DOC, DPF, SCR, and ASC through the accelerated aging method

A general accelerated aging method for aftertreatment devices of Heavy-duty Diesel Engine (HDDE) is proposed in this paper, and three sets of aftertreatment devices were aged according to this method. The error between the World Harmonized Transient Cycle (WHTC)-NOx emission factors of conventional aging and those of accelerated aging fitted by equivalent thermal accumulation is −4.12 to 3.13 %, which proves that the two methods have good equivalence. Meanwhile, the NH3 and N2O emission characteristics and durability were tested by Fourier Transform Infra-Red Spectroscopy (FTIR) equipment. The results show that NH3 and N2O emissions do not deteriorate linearly with the intensification of thermal aging like NOx. Except that the NH3 emission of AT03 exceeds the limit of China VI emission regulation (the average and maximum NH3 concentration in the WHTC test are 35.25 ppm and 350.30 ppm, separately), NH3 emission of other aged devices meet the requirements. Combined with the equivalent durability mileage of 1130000 km of AT02, the test China VI HDDE can effectively control NH3 emissions and has a large margin with the limit requirements. However, the deterioration rate of N2O emission gradually slows down with the intensification of accelerated aging. The accelerated aging tests also indicate that N2O generation is closely related to NH3 in SCR, consistent with previous studies conclusions of low-temperature and high-temperature N2O generation mechanisms. Equally important is that the N2O emission emitted from test China VI HDDE is significantly higher than the emission limit of the Environmental Protection Agency (EPA). The N2O emission factors under the WHTC test are as high as 0.15 to 0.35 g/kWh, which should draw the attention of Chinese environmental regulatory authorities, who should start to formulate N2O emission limits and put forward complementary strategies to reduce N2O emissions from HDDEs.

Emission characteristics of a diesel engine with an electrically heated catalyst under cold start conditions

The high emissions of diesel engines at cold start conditions have attracted more and more attention, because the exhaust gas temperature at this condition is too low to light-off the aftertreatment system. The electrically heated catalyst (EHC) has a high potential for increasing exhaust gas temperature and thus lowering emissions from diesel engines under cold start conditions. In this study, based on a diesel engine with an aftertreatment system that an EHC was installed in the inlet of a diesel oxidation catalyst (DOC), the engine was cold started to a low-load operating point (1000 r/min, 56 N m) and the World Harmonized Transient Cycle (WHTC). The influence of the EHC on the exhaust gas temperature and emission characteristics under the two cold start conditions is investigated. When EHC-on as opposed to EHC-off, the time required for the exhaust gas temperature at post-DOC to reach 138°C was shortened by 767 s during the low-load cold start process, and the time required to reach 200°C was reduced by 418 s during the WHTC cold start process. The total conversion efficiencies of CO and HC increased by 92.8% and 50.8% respectively in the warm up process of the low-load cold start, and the increases were 39.8% and 38.5% respectively in the urban driving cycle of the WHTC cold start process. It was found that the particulate number (PN) concentration at post-DOC increased when EHC-on, and the proportion of nucleation-mode particulates also increased.

Meeting future NOX emission regulations by adding an electrically heated mixer

New regulations by the California Air Resources Board (CARB) demand a stringent 0.02 g/hp-hr tailpipe NOx limit by the year 2027, requiring Selective Catalytic Reduction (SCR) catalysts to provide high NOx conversions even at low (below 200°C) exhaust temperatures. This work describes utilizing an Electrically Heated Mixer System (EHM system) upstream of a Light-Off Selective Catalytic Reduction (LO-SCR) catalyst followed by a conventional aftertreatment (AT) system containing DOC, DPF, and SCR, enabling high NOx conversions meeting CARB’s NOx emission target. The AT catalysts were hydrothermally aged to Full Useful Life. Conventional unheated Diesel Exhaust Fluid (DEF) was injected upstream of both the LO-SCR and primary downstream SCR. The EHM system allowed for DEF to be injected as low as 130°C upstream of the LO-SCR, whereas, in previous studies, unheated DEF was injected at 180°C or dosed at 130°C with heated DEF. The combination of unheated DEF, EHM system, LO-SCR, and downstream SCR enabled the needed increase in NOx efficiency in low exhaust temperatures, which was observed in drive cycles such as in cold-FTP, LLC, and World Harmonized Transient Cycle (WHTC). There were several-fold reductions in tailpipe NOx using this configuration compared to its baseline: 3.3-fold reduction in FTP, 22-fold in Low Load Cycle (LLC), 38-fold in Beverage Cycle, 8-fold in “Stay Hot” Cycle, and 10-fold in WHTC. Finally, it is shown that the EHM system can heat the exhaust gas, such as during a cold start, without needing additional heating hardware integrated into the system. These results were observed without performing changes in the engine base calibration.

Experimental study on the performance and diagnosis of hydrothermally aged selective catalytic reduction over a copper-based catalyst

The conversion of nitrogen oxides (NOx) by hydrothermally aged copper (Cu)–based selective catalytic reduction (SCR) is an irreversible phenomenon. Tail exhaust NOx emission control and on-board diagnostics (OBD) monitoring are difficult with hydrothermally aged Cu–based SCR because the on-board NOx sensor has cross sensitivity to NH3 in the exhaust. In this study, the performance and diagnosis of hydrothermally aged SCR under real engine exhaust were studied. The influence of hydrothermally aged SCR on NH3 storage, tail exhaust NOx, urea injection, and OBD diagnosis were quantitatively analyzed under the World Harmonized Transient Cycle (WHTC). The results show that hydrothermally aged SCR had a great influence on the NH3 storage capacity. In terms of SCR conversion efficiency and OBD detection, compared with fresh SCR, hydrothermally aged SCR was more sensitive to space velocity than to temperature. Under different OBD monitored release conditions, the NOx conversion efficiency was greatly different; hence, the occurrence of a false diagnosis can be avoided through the selection of appropriate release conditions. These research results provide a reference for SCR size design, NOx control, and diagnostic strategies.

The effect of ammonia gas and UWS injection on the N2O formation in diesel engine after-treatment systems under steady and transient conditions

In this study, the N2O emission formation from heavy-duty diesel engine was studied by injecting ammonia gas and urea water solution (UWS) in Cu-zeolite SCR. Furthermore, the effect of exhaust gas temperature and NH3/NOx (α) ratio on the N2O formation were investigated. The ammonia was supplied using an ammonium carbamate (AC) based ammonia generating system. Experiments were performed under the European stationary cycle (ESC), world harmonized transient cycle (WHTC), and non-road transient cycle (NRTC). Ammonia gas injection produced a higher amount of tailpipe N2O than UWS injection for all test conditions. Compared with UWS an increment of 27% in N2O formation for ammonia gas was observed during the ESC test. The N2O emissions of ammonia gas injection in the WHTC and NRTC were increased compared with that of UWS during the cold phase by 63% and 55%, respectively, and during the hot phase by 57% and 74%, respectively. Meanwhile, NO and NO2 emissions were decreased by injecting ammonia gas compared with UWS. The higher NH3/NOx (α) ratio led to higher tailpipe N2O and lower NO and NO2 emissions. For each reductant test, the concentration of N2O formation was high at the exhaust gas temperature of 200–250 °C. These presented results provide useful information about the N2O formation from after-treatment systems of diesel engines when ammonia gas is used.