In September 2017, on-road emissions testing of cars was introduced in the European Union. This was in response to the growing realisation that pollution caused by vehicles was generally much worse than laboratory test results suggested. Today we look at the significant improvements achieved by the introduction of Real Driving Emissions (RDE) requirements and how vehicle emissions can be further improved.
Conducted on the road, the RDE tests complement the laboratory test to ensure that pollutants emitted by cars are measured and certified in real-world conditions which cannot be replicated in the controlled environment of a laboratory. This includes driving at different altitudes, up and down hills, at a wide temperature range (between -7 and 35 °C) and on urban and rural roads as well as motorways.
How RDE tests work
Tests are conducted by type approval authorities as well as independent technical services. With the introduction of the Real Driving Emissions legislation, independent third-party testing laboratories can also perform these tests using portable emissions measurement systems (PEMS) and driving on routes that have been validated as representative of the conditions required.
These PEMS units are attached to the car and provide real-time measurement of the vehicle’s nitrogen oxides (NOx) and particulate number (PN) emissions. The emissions data collected is then processed as prescribed in the European regulation to calculate the permitted emissions, which need to be compliant both in the urban part of the test route and also on the total trip including urban, rural and motorway driving.
The introduction of the RDE test with the Euro 6d-temp legislation – for new models from September 2017 and subsequently for all cars registered from September 2019 – has led to significant NOx and PN emissions reductions from pre-RDE cars. The charts below show the NOx emissions in diesel vehicles and the PN emissions from gasoline vehicles. Both charts show the progression of real-driving emissions of cars certified to the latest standards.
A conformity factor (CF, expressed as “CF = 1 + margin”) is currently applied to provide a distinction between what manufacturers should achieve at the exhaust tailpipe and what current PEMS technology should achieve.
The “one” means manufacturers should achieve the applicable emission limit at the exhaust tailpipe and the margin recognises the variation in measurement accuracy between the laboratory and PEMS. The current margins in Euro 6d are 0.43 for NOx and 0.5 for PN. The CF for NOx was initially set at 2.1 in the Euro 6d-temp standard.
Generally, emissions over normal RDE tests are found to be well below the Euro 6 NOx limits but it is the worst-case combination of RDE boundary conditions that is the challenge for manufacturers. It should be noted that discussions are ongoing to consider a further lowering of the CFs based on demonstrated improvement in PEMS measurement accuracy.
How to improve RDE tests
Despite these positive results, the RDE regulation has its limitations, one of which being that it only covers a certain range of driving conditions. Improvements to the measurement framework are therefore needed to take account of emissions beyond the current scope. For example, greater focus could be put on short trips in urban conditions. The current RDE trip prescribes that the urban share should cover at least 16km. This is a significant distance, considering most of the trips usually covered in the city are shorter.
A new Euro 7 regulation provides the opportunity to address the areas not currently covered in RDE legislation with the following recommendations:
- legislation should cover all conditions commonly experienced in populated areas, including road gradient changes, real acceleration rates and vehicle loads
- all data from each emissions test should be used and reported without exclusion or modification
- the remaining emission peaks should be properly controlled by designing the testing protocol to apply appropriate averaging of emissions over the emissions test
- actual measured data should be reported without applying normalisation for a variation in the driving conditions, as they are under the Euro 6 regime
- these variations should form a range of emission levels according to the route, traffic, weather and driver influence
- measurement framework should represent realistic city driving distances
In the end, the RDE test has been a powerful tool to analyse vehicle emissions in real-world conditions and give a more realistic picture of how these vehicles pollute depending on different driving scenarios.
Achieving low levels of NOx and PN emissions in real-word operation has only been possible by integrating advanced catalysts, filters and adsorbers with the engine and emission control system. AECC has demonstrated that advanced emission control technologies can enable ultra-low emissions limits to be achieved in all driving conditions.
It is therefore reasonable that emissions should be measured in all conditions, building on the progress that has already been made and ensuring that best available technology is used to achieve improvements in air quality.