Explaining vehicle emissions

A new report by the European Environment Agency (EEA) provides a non-technical guide that describes why, for certain pollutants, vehicles can emit substantially higher emissions on the road than official emissions tested in laboratories. It gives a simplified explanation of the often complex information available on road transport emissions as well as the technologies to reduce them.

Standardised measurements are made in laboratories to check that vehicles meet the official requirements for exhaust emissions. However, the testing procedures currently used in the EU are not representative of real driving conditions. For example, emissions of nitrogen oxides (NOx) from new diesel cars can be more than seven times higher in real driving conditions than in official tests. Moreover, new vehicles can emit up to 40 per cent more carbon dioxide (CO₂) than official measurements would indicate. And this gap has increased in recent years.

The report outlines three main reasons for these discrepancies:

  • The EU is using an outdated test procedure that does not reflect real driving conditions;
  • Car manufacturers exploit the permitted flexibilities in the current test procedure to “optimise” certain testing conditions, thereby achieving lower fuel consumption and emission values;
  • Various in-use factors which are driver-dependent (e.g. driving style) or independent (e.g. environmental conditions).

Work is ongoing in the EU to improve future consistency between official vehicle emissions and real driving performance. This includes changing the outdated official test procedure to one that is more representative of real driving emissions, and introducing a procedure for measuring the real driving emissions of vehicles on the road.

The European Commission is planning to introduce a new test cycle (known as the WLTP) in the EU with a focus on improving CO₂ emissions testing – the timing of this is still to be agreed.

A new real driving (RDE) procedure will measure emissions of NOx, and later also particle numbers, using portable emission measurement systems (PEMS) attached to the car. The new protocol will require the real driving emissions from cars to be lower than the legal limits multiplied by a “conformity factor”. This factor expresses the ratio of on-road PEMS emissions to the legal limits. The NOx conformity factor has been set at 2.1 (i.e. 110% above the Euro 6 limit) from 1 September 2017 for new models and two years later for all new vehicles. In a second step, it will be reduced to 1.5 (i.e. 50% above the Euro 6 limit) from 1 January 2020 for new models and one year later for all new vehicles. 

The report “TERM 2015, Evaluating 15 years of transport and environmental policy integration” (Dec. 2015) can be downloaded at: http://www.eea.europa.eu/publications/term-report-2015

In this issue