[en] Highlights: • High instantaneous NO_x represent a small percentage of observations or driving time. • High instantaneous NO_x represent a large percentage of real-world NO_x emissions. • Constraint of high instantaneous NO_x significantly reduce NO_x emission factors. • Relationships between high instantaneous NO_x and several parameters were identified. • Conditions with more probabilities of high instantaneous NO_x were identified. -- Abstract: In real-world driving, most Euro 5 and 6 diesel passenger cars exceed the nitrogen oxides (NO_x) emission limits of type approval procedure. The emission factors of the fleet of Euro 6 vehicles show high variability, irrespective of the NO_x control technology. This comprehensive study focused on the events of high instantaneous NO_x 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 NO_x trap (LNT) and selective catalytic reduction (SCR) were tested based on the real driving emissions (RDE) regulation. The results show that high instantaneous NO_x emissions represent a large amount of total NO_x emissions, although they are produced in a small percentage of driving time. A theoretical constraint of these high NO_x emissions could reduce emission factors by 30–82%. The emission of high instantaneous NO_x 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 NO_x 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 NO_x emissions. These findings could be useful in the design of low emission policies, optimization of NO_x control strategies, and the improvement of micro/meso emission models.

[en] Highlights: • Real-world performance comparison between compressed natural gas and diesel buses. • Higher THC emissions for the compressed natural gas bus compared to the diesel bus. • Increase of 6–55% of fuel consumption with variations in the operating conditions. • Less NO_x emissions for the natural gas bus at high congestion and road grade levels. • Vehicle specific power predicts CO₂ and NO_x emissions with good accuracy. This study investigated the effects of passenger load, road grade, and congestion level on the fuel consumption and emissions from a Euro VI compressed natural gas (CNG) urban bus and a Euro V diesel urban bus. Testing was performed under real-traffic conditions in Madrid, Spain, using a portable emission measurement system (PEMS). The PEMS data also were combined with the vehicle specific power (VSP) methodology to analyse the differences between the performance of the two types of buses and develop an energy-based emission model. Between the empty and 4000 kg passenger load cases, the fuel consumption and CO₂ emissions for the diesel bus showed a significant increase by approximately 25%. With an increase in the road grade, and congestion level, the fuel consumption and CO₂ emissions of both types of buses increased, by 6–55%. Unlike in the case of the diesel bus, the NO_x emissions of the CNG bus decreased by 40–50% as the level of road grade and congestion increased. At intervals of VSP ≥ 2 kW/t, NO_x emission rates for the CNG bus were approximately 60% lower than those of the diesel bus. Finally, the proposed VSP-based model estimated the fuel consumption and the CO₂ and NO_x emission factors with relative total errors of less than 13%.