In Europe, the average amount of nitrogen oxides (NOx) present in exhaust emissions from modern diesel passenger cars is more than double the levels from modern trucks and buses, according to a new paper by the independent research organization International Council on Clean Transportation (ICCT).

The difference is attributable in large part to differences in how light-duty and heavy-duty vehicle emissions are regulated, and the contrasting performance highlights the importance of an upcoming decision on strengthening the real-driving emissions (RDE) test for passenger cars in the EU, according to the report.

The ICCT paper reported data for 24 Euro VI buses and trucks, some tested on a chassis dynamometer by the Technical Research Centre of Finland (VTT), and others tested on-road using portable emissions testing equipment by the German type-approval agency KBA. On average, NOx emissions of the heavy-duty vehicles tested were approximately 210 mg/km. Currently, NOx emissions of Euro 6 diesel passenger cars under real-world driving conditions are approximately 500 mg/km, as determined by testing carried out by KBA and other European type-approval agencies. This means that NOx emissions of diesel cars are more than double those of trucks and buses.

“The simple average difference in NOx emissions per kilometer between light-duty and heavy-duty vehicles is startling enough,” said Rachel Muncrief, ICCT researcher and author of the paper. “But on a fuel-consumption basis, taking into account the higher engine-load demands for trucks and buses, the NOx emissions of diesel cars are 10 times higher than the corresponding measurement values for heavy-duty vehicles.”

The significantly lower NOx emission levels of trucks and buses are most likely a result of differences in regulation, according to the ICCT. Official testing requirements of light-duty vehicles remain limited to laboratory measurements of carefully prepared prototype vehicles.

“In contrast, for measurement of NOx emissions from trucks and buses, mobile testing devices became mandatory in 2013. As a consequence, randomly selected vehicles can be tested under real-world driving conditions,” said Peter Mock, Ph.D., Managing Director of ICCT in Europe.

Similar tests with mobile devices, called portable emissions measurement systems (PEMS), will be introduced for passenger cars beginning in September 2017, as part of the European Real-Driving Emissions (RDE) regulation. That should bring about a significant improvement in the NOx emission levels of diesel cars. But further improvements in the light-duty vehicle testing protocols will be needed to truly measure and control NOx emissions.

“According to the current status of the RDE regulation, vehicle manufacturers will still be allowed to carefully select special prototype cars for emissions testing,” said Mock. “Instead, it would be much better to measure the emissions of ordinary mass-production vehicles, obtained from customers who have had been driving them in an ordinary way, and not from the manufacturers.”

The European Commission, despite resistance from some vehicle manufacturers and EU member states, planned to bring forward for discussion a package of possible revisions to the existing RDE regulation as part of a working group meeting scheduled for January 17 in Brussels. The package would include, among other changes, provisions for testing in-use, privately owned cars by independent third parties.