With on-going public concern relating to vehicle emissions in the wake of the VW group ‘dieselgate’ issue of September 2015, attention has turned to the measurement of vehicle emissions associated with real-world (as opposed to laboratory test) driving behaviours.
Europe is leading the charge with the RDE (Real Driving Emissions) test. RDE has been designed to measure NOx and other pollutants emitted by vehicles in practical driving scenarios. It complements rather than replaces the WLTP (Worldwide Harmonised Light Vehicle Test Procedure), a laboratory-based test.
In an RDE test, a regular production vehicle is driven on the public highway in a range of realistic and widely differing conditions. Test measurement equipment on-board captures data for subsequent analysis in determining that emissions do not exceed those specified in the relevant legislation.
Some of the key conditions in which the RDE test is conducted include:
- A range of temperatures, reflecting seasonal changes and a range of geo-locations
- Varying altitudes
- High speed motorway driving
- Intermediate speeds to reflect those experienced on rural roads
- Low speed and stop-start driving, typically experienced in busy urban locations
- The vehicle’s weight is also adjusted to reflect varying passenger and luggage load conditions
- Downhill and uphill driving
The RDE test utilises PEMS (Portable Emission Measuring Systems). These are complex systems which include GPS (Global Positioning System); mass flow meters to measure exhaust output; a weather station; and advanced gas analysers.
PEMS is a generalised term which applies to this class of equipment. In practice, there is no single standard version of the equipment and a variety of PEMS systems are available from various suppliers.
This inevitably results in some variation in results between different equipment and testing bodies. The legislation allows for RDE results to be higher than those measured under laboratory conditions by applying a Conformity Factor to the official results. A Conformity Factor of 2 means that RDE measured emissions may be up to twice the maximum level permitted for laboratory test conditions.
Once the data has been collected, it is analysed to ensure that all emission characteristics fall within acceptable bounds.
There are two key deadlines or ‘steps’ for RDE implementation:
- RDE step 1 (which requires a NOx conformity factor of 2.1 or double the longer-term objective) has been in force since the beginning of September 2017 for all new car types. It is to be applied to ALL car types from September 2019.
- RDE step 2 (which mandates a full NOx conformity factor of 1.5) is to be applied in January 2020 for all new car types.
- RDE Step 2 will apply to ALL car types from January 2021.
A conformity factor can be defined as, essentially, a limit which must not be exceeded. Some margin of error is allowed to accommodate the absence of uniformity and thus marginal discrepancies associated with different PEMS equipment. (replace this para with the earlier one?)
The mobile nature of PEMS equipment further renders it susceptible to some results variance in contrast with the static lab equipment used in WLTP.
In practice, vehicle manufacturers will be aiming for emissions targets significantly within the guidelines to allow for these variances.
Whilst manufacturers are likely to design their vehicles around typical mid-market fuels, the regulators may use any fuel they wish in conducting RDE tests, so long as they conform to EN590 for diesel or EN228 for petrol.
Given qualitative variability in fuel, an unfortunate car maker with a vehicle close to the boundaries on an RDE test with a tank of low quality fuel could experience test failure.
For this reason, manufacturers have asked the experts at Coryton to produce ‘near worst case’ fuels for the purposes of internal testing and to pre-empt the above failure scenario.