Not every car on the road emits the same amounts of pollutants. Different cars may have very different emission levels, but they all have to comply with the respective emissions limits set by EU legislation. The picture below shows the variability in NOx emissions measured for several Euro 6 cars and the big improvement for post-RDE cars compared to pre-RDE.
[1] Average real-world NOx emissions of diesel passenger cars (g/km)
The evolution of emission control technologies remained a significant contributor to the improvement of the ‘post-RDE Euro 6’ vehicles emissions. In modern petrol cars, particulate filters with direct injection are implemented, while in modern diesel cars a combination of NOx control technologies in a compact design implemented, in contrast with the earlier Euro standards. The emission performance of the latest diesel cars is approaching that of petrol vehicles, while there is still room for improvement. The NOx emissions of diesel Euro 6d-temp were a large improvement relative to the previous Euro classes, with further improvements made in the NOx performance of diesel Euro 6d vehicles. Furthermore, modern diesel vehicles are equipped with diesel particulate filters meaning that the particulate matter emissions of diesel passenger cars are lower than that of petrol.
Modern diesel cars can however emit ammonia (NH3). NH3 emissions appear to be lower than those of petrol cars, but the exact magnitude of the emissions is still unknown as NH3 is an unregulated species. Some modern diesels have been found to have significant NH3 emissions, nevertheless, it is not yet known whether this problem occurs in all modern diesels. In general, there are strong indications that NH3 emissions coincide with dynamic driving behaviour, and possibly a cold engine. Despite the efforts of the EU to promote cleaner vehicles through the introduction of stricter regulatory emission limits, it seemed that laboratory results were not always realistic. Still today, for certain pollutants, there is a gap between laboratory emissions and real-world emissions, measured on the road. Several studies have investigated the differences between official CO2 emission values and real-world emissions, and they found these differences to be in the order of 30 to 40%. This gap is the result of various factors, such as outdated laboratory-based testing procedures, the optimisation of permitted flexibilities by vehicles during vehicle testing, as well as differences in driver behaviour under real driving conditions. Furthermore, there is increasing evidence that reductions in laboratory fuel consumption values are coming from test optimizations and not from fuel-saving technologies (Fontaras et al., 2017).
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