A NOx Emission Model Incorporating Temperature for Heavy-Duty Diesel Vehicles with Urea-SCR Systems Based on Field Operating Modes

Abstract

The selective catalytic reduction (SCR) is the most commonly used technique for decreasing the emissions of nitrogen oxides (NOx) from heavy-duty diesel vehicles (HDDVs). However, the same injection strategy in the SCR system shows significant variations in NOx emissions even at the same operating mode. This kind of heterogeneity poses challenges to the development of emission inventories and to the assessment of emission reductions. Existing studies indicate that these differences are related to the exhaust temperature. In this study, an emission model is established for different source types of HDDVs based on the real-time data of operating modes. Firstly, the initial NOx emission rates (ERs) model is established using the field vehicle emission data. Secondly, a temperature model of the vehicle exhaust based on the vehicle specific power (VSP) and the heat loss coefficient is established by analyzing the influencing factors of the NOx conversion efficiency. Thirdly, the models of NOx emissions and the urea consumption are developed based on the chemical reaction in the SCR system. Finally, the NOx emissions are compared with the real-world emissions and the estimations by the proposed model and the Motor Vehicle Emission Simulator (MOVES). This indicates that the relative error by the proposed method is 12.5% lower than those calculated by MOVES. The characteristics of NOx emissions under different operating modes are analyzed through the proposed model. The results indicate that the NOx conversion rate of heavy-duty diesel trucks (HDDTs) is 39.2% higher than that of urban diesel transit buses (UDTBs).Atmosphere 2019, 10, 337 2 of 17 trucks collected in 2018 using GPS), urban transit buses frequently operate at a lower speed range than heavy-duty trucks, while heavy-duty trucks run fairly smoothly at a high-speed range.Atmosphere 2019, 10, x FOR PEER REVIEW 2 of 17 heavy-duty buses and heavy-duty trucks collected in 2018 using GPS), urban transit buses frequently operate at a lower speed range than heavy-duty trucks, while heavy-duty trucks run fairly smoothly at a high-speed range. Figure 1. Field trajectories of an urban transit bus and a heavy-duty truck.The differences in operating modes for different source types of HDDVs result in the uncertainty of the emissions estimations. For the HDDVs equipped with SCR systems, the NOx emissions are affected by the temperature of the exhaust gas [8]. Even for the same operating speed or vehicle specific power (VSP), there is a significant variation in NOx emissions caused by the different exhaust gas temperatures. However, current studies about NOx emission estimations do not consider the temperature of the exhaust gas, and the emission rates are directly calculated using the field data, which leads to an inaccuracy of the emissions measurement.In order to accurately calculate NOx emissions of HDDVs, it is necessary to understand the working mechanism of the SCR system. For HDDVs, the preferred reductant is typically an aqueous solution of nontoxic urea, which is d...

NOx Emissions from Euro 5 and Euro 6 Heavy-Duty Diesel Vehicles under Real Driving Conditions

“…Furthermore, as they consider real driving on urban roads, rural roads, and motorways in the Seoul Metropolitan Area, they can reflect the realistic conditions in a more effective manner. It is expected that reliability and reality could be enhanced by using enormous quantities of real driving test results for emission factor calculations [21,33]. Figure 5 shows a comparison of NOx emission characteristics under various average vehicle speed conditions between Euro 5 and Euro 6 HDVs.…”

“…The NOx emissions in the range of 30-35 °C were slightly higher than in the normal test temperature range (15-25 °C). Although the number of measured vehicles was insufficient, the NOx emissions at 5-10 °C were slightly higher than at normal test temperatures (15-25 °C) [33,34].…”

“…During the testing period, the ambient temperature ranged from 5 to 37 • C. Some N2 category vehicles with a GVW of 7 tons or less showed high NOx emissions at an ambient temperature of 30-37 • C. The NOx emissions in the range of 30-35 • C were slightly higher than in the normal test temperature range (15-25 • C). Although the number of measured vehicles was insufficient, the NOx emissions at 5-10 • C were slightly higher than at normal test temperatures (15-25 • C) [33,34].…”

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