| Summary: | Tailpipe emissions of a pool of 13 Euro 6b light-duty vehicles (eight diesel and five gasoline-powered) were measured over an extensive experimental campaign that included laboratory (chassis dynamometer), and on-road tests (using a portable emissions measurement system). The New European Driving Cycle (NEDC) and the Worldwide harmonised Light-duty vehicles Test Cycle (WLTC) were driven in the laboratory following standard and extended testing procedures (such as low temperatures, use of auxiliaries, modified speed trace). On-road tests were conducted in real traffic conditions, within and outside the boundary conditions of the regulated European Real-Driving Emissions (RDE) test. Nitrogen oxides (NO<sub>X</sub>), particle number (PN), carbon monoxide (CO), total hydrocarbons (HC), and carbon dioxide (CO<sub>2</sub>) emission factors were developed considering the whole cycles, their sub-cycles, and the first 300 s of each test to assess the cold start effect. Despite complying with the NEDC type approval NO<sub>X</sub> limit, diesel vehicles emitted, on average, over the WLTC and the RDE 2.1 and 6.7 times more than the standard limit, respectively. Diesel vehicles equipped with only a Lean NO<sub>X</sub> trap (LNT) averaged six and two times more emissions over the WLTC and the RDE, respectively, than diesel vehicles equipped with a selective catalytic reduction (SCR) catalyst. Gasoline vehicles with direct injection (GDI) emitted eight times more NO<sub>X</sub> than those with port fuel injection (PFI) on RDE tests. Large NO<sub>X</sub> emissions on the urban section were also recorded for GDIs (122 mg/km). Diesel particle filters were mounted on all diesel vehicles, resulting in low particle number emission (~10<sup>10</sup> #/km) over all testing conditions including low temperature and high dynamicity. GDIs (~10<sup>12</sup> #/km) and PFIs (~10<sup>11</sup> #/km) had PN emissions that were, on average, two and one order of magnitude higher than for diesel vehicles, respectively, with significant contribution from the cold start. PFIs yielded high CO emission factors under high load operation reaching on average 2.2 g/km and 3.8 g/km on WLTC extra-high and RDE motorway, respectively. The average on-road CO<sub>2</sub> emissions were ~33% and 41% higher than the declared CO<sub>2</sub> emissions at type-approval for diesel and gasoline vehicles, respectively. The use of auxiliaries (AC and lights on) over the NEDC led to an increase of ~20% of CO<sub>2</sub> emissions for both diesel and gasoline vehicles. Results for NO<sub>X</sub>, CO and CO<sub>2</sub> were used to derive average on-road emission factors that are in good agreement with the emission factors proposed by the EMEP/EEA guidebook.
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