Taking place on real roads, the RDE test complements lab tests by measuring that a car delivers low pollutant emissions on the road.
Real driving emission (RDE) tests will measure the pollutants, such as NOx, emitted by cars while driven on the road. RDE will not replace laboratory tests, such as the current NEDC and the future WLTP but it will be additional to them. RDE will ensure that cars deliver low emissions over on-road conditions. Europe will be the first region in the world to introduce such on-the-road testing, marking a major leap in the testing of car emissions.
Under RDE, a car will be driven on public roads and over a wide range of different conditions. Specific equipment installed on the vehicle will collect data to verify that legislative caps for pollutants such as NOx are not exceeded.
- low and high altitudes
- year-round temperatures
- additional vehicle payload
- up- and down-hill driving
- urban roads (low speed)
- rural roads (medium speed)
- motorways (high speed)
To measure pollutant emissions as the vehicle is being driven on the roads, cars will be fitted with Portable Emission Measuring Systems (PEMS) that will provide a complete real-time monitoring of the key pollutants emitted by the vehicle (ie NOx).
The PEMS used for regulate emissions are complex pieces of equipment that integrate advanced gas analysers, exhaust mass flow meters, weather station, Global Positioning System (GPS) and connection to the vehicle networks.
All parties – including approval authorities – must learn the proper use of PEMS systems. There is no ‘standard’ PEMS equipment and equipment manufactured by different suppliers will always deliver slightly different results. The collected data is analysed to check that the RDE trip boundary conditions were achieved and that the emissions were within acceptable levels.
The EU has adopted a step-by-step approach to introducing RDE, splitting the legislation over a total of four separately-approved ‘packages’. The first two packages are now adopted, enabling manufacturers to start preparing for the introduction of the new emissions test. However, two key packages are still missing and are urgently needed to complete the legislation. The Commission has the aim of presenting the third package to the member states by October 2016 and the final part should be presented by mid-2017.
The Real Driving Emissions (RDE) test will be implemented in September 2017 for new types of cars and will apply for all registrations from September 2019.
Compliance with the Euro 6 standard limit of 80mg of NOx per kilometre is verified using NEDC, the test specified for that purpose in Regulation 715/2007.
Manufacturers comply with NEDC, but cannot verify compliance with the legal limits ‘on the road’ without a clear and fixed protocol to do so. Otherwise this would lead to widely different – and therefore incomparable and meaningless – results.
A conformity factor is defined as a ‘not to exceed limit’ that takes into account a margin for error, which is present simply because PEMS equipment does not deliver exactly the same results for each test. For example, PEMS are not as accurate as a full laboratory system so they will not measure to the same level of repeatable accuracy as a lab test. In practice, car manufacturers must set their design objectives well below the legal limit to be certain of complying.
In October 2015, the European Commission’s regulatory committee (TCMV) agreed a final conformity factor applicable from 2020 of 1 plus a margin for PEMS measurement error of 0.5. A PEMS – or Portable Emission Measurement Systems – are installed on the back of the vehicle to measure real world pollutant emissions and are a complimentary measurement to laboratory testing of pollutants. A ‘conformity factor’ then is the cap by which the Euro 6 NOx emission limit would be allowed to be exceeded in real world driving – measured in the RDE test – effectively providing a ‘not-to-exceed limit’.
This margin of error exists simply because PEMS equipment has various inaccuracies. For example, PEMS are not as accurate as a full laboratory system so they will not measure to the same level of repeatable accuracy as a laboratory test. In addition, PEMS are affected by altitude and ambient temperature, as well as by the tolerances and errors of various sensors and instruments taking signals from the vehicle and elsewhere necessary for the PEMS systems to work, by condensation in the exhaust sample line, by slippage and drift in the calibration of the PEMS system over a two-hour RDE trip.
A conformity factor of 1 means the Euro 6 NOx limit of 80mg/km must be met over an RDE test. This is fully in line with the request of the European Parliament to have a conformity factor that is as low as possible and which reflects only measurement error.
In practice, car manufacturers must set their design objectives well below the legal limit to be certain of complying and to account for the risk that PEMS on any particular day may have an even higher error margin.
This means that emissions that are the product of the ‘normal driving’ conditions that the average driver will encounter on a day-to-day basis, will be far below this ‘not to exceed limit’, because in real life the worst case scenario of driving is very rare.