http://www.theicct.org/sites/default/files/publications/ICCT_PEMS-study_diesel-cars_20141013.pdf

On average, real-world NO_x emissions from the tested vehicles were about seven times higher than the limits set by the Euro 6 standard. If applied to the entire new vehicle fleet, this would correspond to an on-road level of about 560 mg/km of NOx (compared to the regulatory limit under Euro 6 of 80 mg/km). This is compelling evidence of a real-world NO_x compliance issue for recent-technology diesel passenger cars, for both the EU and US test vehicles.

In most cases the exceedances found could not be attributed to “extreme” or “untypical” driving. Instead, they were due to transient increases in engine load typical of everyday driving (e.g., going up a slight incline), or to normal regeneration events in the normal diesel exhaust aftertreatment systems.

Performance differences among the vehicles tested indicate that the technologies for real-world clean diesels already exist. Some of the tested vehicles had average emissions below Euro 6 emission limits, suggesting that the technologies to achieve that level of performance are available, but that policies are not yet in place that can force manufacturers use these technologies and calibrate them to effectively control emissions over the majority of in-use operating conditions, not just those covered by the test cycle.

The European Commission’s Real Driving Emissions (RDE) working group is preparing for the introduction of on-road PEMS testing as part of the passenger-car type-approval process in the EU. The results of this study strongly support this approach to improving type-approval testing for new passenger vehicles.