RDE: On-road measurement of exhaust emissions: Realistic verification of lab values
As the football saying goes: "Performance on the field is what matters". When measuring emissions, the same applies to performance on the road. For this reason, according to the Euro 6d-TEMP standard and the even more far-reaching Euro 6d standard, the lab measurements according to the WLTP are supplemented by the so-called RDE test (Real Driving Emissions). Pollutant emissions (including nitrogen oxide and particles) are measured in vehicles directly on the road and compliance of limits with conformity factors is checked.
The additional RDE test is intended to allow the pollutant emissions to be tested under real driving conditions. In contrast to a lab test, the RDE test does not follow a set driving cycle. Instead, the emissions are tested under real driving conditions with legally defined permissible ambient conditions. For the RDE test, the vehicles are equipped with a so-called PEMS (Portable Emissions Measurement System) device for mobile emissions measurement (for details, see chapter “Under the microscope: The PEMS measurement”).
For vehicles undergoing an RDE test as part of the Euro 6d-TEMP or Euro 6d emissions standard, the emissions limits of the Euro 6 standard plus so-called conformity factors must be complied with. For nitrogen oxide, this conformity factor in Phase 1 of RDE (new passenger car approvals are possible up until 31 December 2020) is 2.1, for the particle number 1.0 +0.5. The value of 0.5 for the particle number corresponds to a measuring tolerance of 0.5 which illustrates variations in the measurement technology. The vehicle must technically comply with the particle limit of 6*1011 per km in an RDE drive.
In RDE Phase 2 (Euro 6d), which applies to new models from 1 January 2020 and/or to all newly approved cars from 1 January 2021, the factor for nitrogen oxide is 1.0 + 0.43, the 0.43 value corresponds to the measuring tolerance. On December 13th, 2018, the General Court of the European Union (EGC) made a first decision concerning the power of the EU Commission to adopt specific provisions on RDE tests. It is currently unclear whether the European Commission will appeal this ruling to the European Court of Justice (ECJ). If the judgment becomes final, it may lead to a fundamental reassessment of the conformity factors. The legally defined permissible range for an RDE test covers a wide field of application, e.g. it allows speeds of up to 160 km/h, temperatures down to -7 °C and driving in mountainous terrain.
The limits with the above-mentioned conformity factors are to be observed. Since the limits have to be fulfilled under all possible combinations of the parameters, emissions in real-life customer operation are generally significantly below the prescribed limits.
Independent technical services take on the certification drives
Car manufacturers currently have to have at least 50 percent of the drives carried out by technical services for RDE certification. Mercedes-Benz has transferred all of these tests to external independent organisations. As part of field validation, official test institutes as well as e.g. private environmental organisations can in future conduct their own RDE tests within the previously mentioned parameters.
The RDE measurements normally take place in parallel to the WLTP dynamometer tests. WLTP system authorisation via the model testing authority is only granted if the manufacturer can present valid RDE measuring drives and can confirm compliance with the RDE requirement.
Parameters include (examples):
- Driving time between 90 and 120 minutes
- Speed-based split of driving into 34 percent urban (but at least 29 percent of the route), 33 percent extra-urban and 33 percent motorway. To allow for changing traffic conditions, a tolerance of +/- 10 percent is permitted for these values.
- Each route (urban/extra-urban/motorway) must be at least 16 km long.
- The speed ranges are
- urban: 0 to 60 km/h (average speed 15-40 km/h); several stop phases of 10 seconds or longer (max. 300 seconds) are permitted; the stop phases may account for 6 – 30 percent (time-based).
- extra-urban: 60 to 90 km/h
- motorway: between 90 and max. 160 km/h.
- The elevation difference between the start and end points of the route must be no more than 100 metres; the cumulative elevation gain per 100 km must be no more than 1200 metres; the maximum absolute elevation is 1300 metres.
- The weight of the vehicle may be max. 90 percent of the total of the "mass of the passengers" and the "payload".
- The ambient temperature must be between -7°C and +35°C.