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.

Modern diesel and petrol cars

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.

Differences between real-world and test cycle emissions

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 This gap is the result of various factors, such as outdated laboratory-based testing procedures, the optimisation of permitted flexibilities by vehicles during vehicle testing, as well as differences in driver behaviour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).

Similar to CO₂ emissions, there is also a discrepancy between real-world NOx emissions and that of the legislative laboratory limits. Several studies have demonstrated that despite the stricter emission limits, NOx emissions in real-world conditions have not decreased as expected. On the contrary, many Euro 5 diesel cars in particular were found to emit several times above the respective limit, as tested by independent laboratories. Although this gap was known to the scientific community for several years, it only received public attention after the ‘dieselgate’ scandal was revealed.

[1] The image shows that for the latest technology Euro 6d-temp and Euro 6d cars this gap has been eliminated, while it was still substantial in the early Euro 6 cars (Euro 6b and Euro 6c).