Abstract
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 (NOX), particle number (PN), carbon monoxide (CO), total hydrocarbons (HC), and carbon dioxide (CO2) 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 NOX 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 NOX 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 NOX than those with port fuel injection (PFI) on RDE tests. Large NOX 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 (~1010 #/km) over all testing conditions including low temperature and high dynamicity. GDIs (~1012 #/km) and PFIs (~1011 #/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 CO2 emissions were ~33% and 41% higher than the declared CO2 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 CO2 emissions for both diesel and gasoline vehicles. Results for NOX, CO and CO2 were used to derive average on-road emission factors that are in good agreement with the emission factors proposed by the EMEP/EEA guidebook.
Highlights
Road transport is the second largest source of emissions of greenhouse gases (GHG) in the European Union (EU-28), accounting for 20% of total GHG emissions in 2016 [1]
The tailpipe emissions from the combustion of diesel and gasoline fuels in passenger cars are an important source of air pollutants: 39% of nitrogen oxides (NOX ), 20% of carbon monoxide (CO), and 10% of particle matter emitted in the EU-28 in 2016 originated from road transport [3]
Emission factors derived from laboratory and road-testing conditions provide complementary information which is useful to fully characterize tailpipe emissions of given vehicles
Summary
Road transport is the second largest source of emissions of greenhouse gases (GHG) in the European Union (EU-28), accounting for 20% of total GHG emissions in 2016 [1]. In the EU-28 there are 76,000, and 391,000 yearly premature deaths attributable to NO2 and PM2.5 exposure, respectively [3] and exposure to those pollutants is high in urban areas with dense traffic. Considering both climate change mitigation and air quality challenges, emissions from road transport have become a relevant item in the agenda of policy-makers from a European to a local level, in particular after the diesel emission scandal [6,7]. Despite the fact that the EFs in the guidebook were obtained using realistic test cycles, emission modellers in Europe claim that emission inventories need to take into account laboratory measurements and real-world observations [14,15] since many studies have found differences in the emissions between real-world and laboratory conditions [16,17]
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