Analysis of the high instantaneous NOx emissions from Euro 6 diesel passenger cars under real driving conditions

In real-world driving, most Euro 5 and 6 diesel passenger cars exceed the nitrogen oxides (NOx) emission limits of type approval procedure. The emission factors of the fleet of Euro 6 vehicles show high variability, irrespective of the NOx control technology. This comprehensive study focused on the events of high instantaneous NOx emissions produced under real driving, to assess their impact on emission factors. Additionally, the relationships of these events with different parameters measured using portable emissions measurement system (PEMS) were determined. Three Euro 6b diesel passenger cars with exhaust gas recirculation (EGR), lean-burn NOx trap (LNT) and selective catalytic reduction (SCR) were tested based on the real driving emissions (RDE) regulation. The results show that high instantaneous NOx emissions represent a large amount of total NOx emissions, although they are produced in a small percentage of driving time. A theoretical constraint of these high NOx emissions could reduce emission factors by 30-82%. The emission of high instantaneous NOx emissions are related to characteristic speed modes of urban, rural and motorway sections, and are primarily produced in a narrow engine speed range of approximately 700 rpm. In general, the probability of producing high instantaneous NOx emissions increases as the engine speed, the exhaust gas temperature or the vehicle speed is increased. Finally, regarding driving severity, speed per positive acceleration (v-a(+)) observations above the maximum values of the new European driving cycle (NEDC) and the world-harmonized light-duty vehicle test cycle (WLTC), have a strong probability to yield high instantaneous NOx emissions. These findings could be useful in the design of low emission policies, optimization of NOx control strategies, and the improvement of micro/meso emission models.