The Real Driving Emissions (RDE) test measures the pollutants, such as NOx, emitted by cars while driven on the road. RDE does not replace the WLTP laboratory test, but complements it. RDE ensures that cars deliver low emissions over on-road conditions. Europe is the first region in the world to introduce such on-road testing, marking a major leap in the testing of car emissions.
Under RDE, a car is driven on public roads and over a wide range of different conditions. Specific equipment installed on the vehicle collects data to verify that legislative caps for pollutants such as NOx are not exceeded.
Conditions include:
- 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 are fitted with Portable Emission Measuring Systems (PEMS) that provide a complete real-time monitoring of the key pollutants emitted by the vehicle (ie NOx).
The PEMS used for regulated emissions are complex pieces of equipment that integrate advanced gas analysers, exhaust mass flow meters, weather station, Global Positioning System (GPS) and a 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.
- RDE step 1 (with a NOx conformity factor of 2.1) applies since 1 September 2017 for new car types. It will apply to all types as from September 2019.
- RDE step 2 (with a NOx conformity factor of 1.0 plus an error margin of 0.5) will apply in January 2020 for new types and then from January 2021 for all types.
A conformity factor is defined as a ‘not to exceed limit’ that takes into account a margin for error, which is present simply because the 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. PEMS – or Portable Emission Measurement Systems – are installed on the back of the vehicle to measure real-world pollutant emissions and are complimentary 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 for a diesel car 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 only reflects 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 worst-case scenarios of driving are very rare.
For RDE nearly all diesel vehicles have to be fitted with selective catalytic reduction (SCR) systems and some also with lean-NOx systems. This implies additional costs for car manufacturers and smaller cars may not be able to accommodate the fitting of SCR equipment, while prospective owners may be turned away by the additional costs.