EU: Cars and Light Trucks: RDE Testing

Regulatory Background

The Real Driving Emissions (RDE) test is intended to quantify light-duty vehicle emissions using portable emissions measurement systems (PEMS) during a range of normal vehicle operating conditions. One of the key factors that prompted the development of the RDE test was the discrepancy between NOx emission levels in diesel cars measured in the laboratory (NEDC) and in real driving using PEMS. The RDE test, introduced in 2017 at the Euro 6d-TEMP stage, has been an effective tool that forced more robust NOx aftertreatment systems and strategies, contributing to a greatly increased NOx emission compliance levels in diesel vehicles under most operating conditions. The RDE test has also played an essential role in forcing the widespread use of gasoline particulate filters (GPF) on gasoline cars.

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The RDE test procedure was initially developed in Europe for European vehicles and is defined in Annex IIIA of Regulation 692/2008 [2764][5136]. RDE testing requirements based on the EU procedure have also been adopted by other countries such as China. A new Global Technical Regulation (GTR) and UNECE Regulation on RDE have been under development [5162].

The RDE test was introduced in four different packages between March 2016 and November 2018 for Euro 6 vehicles, and further revised at the Euro 6e stage:

1. The 1^st package, voted on in May 2015 and published in March 2016 (2016/427), included the basic features of the RDE test, including the trip characteristics, description of data evaluation tools, technical requirements of the PEMS equipment, and reporting obligations [3362]. The focus was on gaseous emissions, primarily NOx. Two options were available to evaluate the test data, a moving average window method and a power binning method. The 1st package was only for monitoring and reporting with no requirement to meet an NTE limit. It applied to Euro 6c vehicles (NEDC approved) starting 2016.04.
2. The 2^nd package, voted on in October 2015 and published in April 2016 (2016/646) included NTE limits on NOx equivalent to the EURO 6 limit multiplied by a conformity factor and a timetable for RDE implementation [3638]. It also included several technical features such as dynamic boundary limits (RPA and V·a_pos), a limit on the altitude gain and GPS validation. Vehicles were required to conform to a NOx Conformity Factor (CF) of 2.1 from September 1, 2017 (Euro 6d-TEMP) and 1.5 starting January 1, 2020 (Euro 6d). Cold start emissions were excluded.
3. The 3^rd package, voted on in December 2016 and published in June 2017 (2017/1154), included the introduction of a particulate number measurement protocol and associated conformity factor, procedures for including cold starts and regeneration events in the RDE test, and provisions for hybrid vehicles [3637][3819]. The same package introduced the possibility for manufacturers to declare that they emit even lower than the limit (i.e. the maximum value that any RDE trip might reach) in their Certificate of Conformity. A PN conformity factor of 1.5 was applicable from September 2017 for new models and from September 2018 for all new M1 and N1 Class I vehicles and one year later for N1 Class II and III and N2 vehicles. The introduction of the 3rd package coincided with introduction of WLTP.
4. The 4^th package voted on in March 2018 and published in November 2018 (2018/1832), formed the basis of in-service conformity (ISC) and surveillance tests along with specific provisions for light commercial vehicles [5137][5138]. The conformity factor for NOx was lowered to 1.43. The data evaluation method was simplified to so that the measured distance specific result is equal to the test result unless the distance specific CO₂ emissions during RDE testing are more than 20%-30% higher than during WLTP testing. In the latter case, a correction less than 1 can be applied.
5. The Euro 6e stage, adopted in August 2022, introduced further revisions to the RDE procedure and tightened the NOx and PN conformity factors.

Emission Limits and Result Correction

In RDE testing, vehicles are tested while operating on the road over their normal driving patterns, conditions, and payloads. Throughout the normal life of a vehicle type its emissions during the test shall not be higher than the following not-to-exceed (NTE) values:

NTE_pollutant = CF_pollutant × EURO-6 (1)

where EURO-6 is the applicable Euro 6 emission limit.

The conformity factor CF_pollutant for the respective pollutants is summarized in Table 1. The value of these is subject to annual review and will be revised as the PEMS procedure and/or PEMS technology improve.

• The 2^nd RDE package introduced a temporary (Euro 6d-TEMP) conformity factor of 2.1 and a final (Euro 6d) conformity factor of 1.5 (1 + margin of 0.5) for NOx.
• The 3^rd RDE package introduced a PN conformity factor of 1.5 (1 + margin PN of 0.5). Measurement uncertainty introduced by the PEMS equipment was accounted for with “margin” and “margin PN”.
• The 4^th RDE package lowered the final conformity factor for NOx to 1.43 (1 + margin of 0.43) before the final conformity factor of 1.5 from the 2^nd package was applied.
• The Euro 6e step introduced further revisions to the RDE test procedure and redefined the former conformity factors as PEMS error margins.

The NTE limit shall not be exceeded for the urban portion of the test and for the complete RDE test PEMS trip:

M_RDE,k ≤ NTE_pollutant (2)

where:
M_RDE,k is the final RDE distance-specific mass of gaseous pollutants [mg/km] or particle number [#/km] after correcting for the RDE/WLTP CO₂ ratio, and
k=t for total trip and k=u for the urban portion.

The measured emissions are corrected with CO₂ ratio as follows:

M_RDE,k = m_RDE,k · RF_k (3)

where:
m_RDE,k is the distance-specific mass of gaseous pollutant or particle number over the RDE trip prior to CO₂ ratio correction, and
RF_k is the evaluation factor calculated for the RDE trip and depends on the ratio (r_k) of the distance specific CO₂ emissions during the RDE test over that of the WLTP, Figure 1. For ICE vehicles, the denominator for urban CO₂ ratio (r_u) is determined from the first two WLTP phases.

Prior to the 4^th RDE package, two options were available to calculate the final test result, EMROAD and CLEAR. EMROAD is a Moving Average Window method developed by the European Commission’s JRC that divided the measured data into windows and used “normal” windows to calculate the test result. CLEAR is a power binning method developed by the Technical University Graz that adjusted the measured result according to bins corresponding to instantaneous power at the wheels.

The 4^th RDE package also includes a requirement to report the maximum NOx and PN values for the urban portion and the entire trip in the certificate of conformity. These “Declared Maximum RDE Values” are used for In-Service Conformity.

Trip Validity

Figure 2 summarizes the steps required to ensure trip validity. The three main steps are:

1. Ensure the trip complies with the general requirements, boundary conditions, trip and operational requirements, and the specifications for lubricating oil, fuel and reagents.
2. Ensure the trip complies with the dynamic and altitude gain requirements.
3. Ensure enough moving average windows have distance specific CO₂ emissions within the required margin of those from the WLTP.

In-Service Conformity

To ensure emissions compliance over the life of the vehicle (5 years or 100,000 km), in-service conformity (ISC) must be demonstrated. RDE testing is included in the ISC requirements for M and N1 class I vehicles based on types approved after 1 January 2019 and all vehicles registered after 1 September 2019. For N1 classes II and III and N2 vehicles, the additional ISC requirements apply to types approved after 1 September 2019 and all vehicles registered after 1 September 2020.

For vehicles that have Declared Maximum RDE Values reported in the Certificate of Conformity lower than the Euro 6 limits, ISC shall be checked both against the Declared Maximum RDE Value increased by the margin and the not-to-exceed limit. If the sample is found not to conform with the Declared Maximum RDE Values increased by the applicable measurement uncertainty margin, but pass with the not-to- exceed limit, the manufacturer is required to take corrective actions.

Data Transparency

Since the 1st RDE package, vehicle manufacturers have been obliged to publish RDE type-approval test data for PEMS test families. One source of this data for a number of manufacturers is through ACEA [5139]. Other vehicle manufacturers have chosen to provide the data through their own websites.

When consumers purchase a new vehicle, they receive the Certificate of Conformity (COC) which includes the RDE test results. In this way, they know how their vehicle performs in real driving situations. The COC also includes the road loads that are required for chassis dynamometer testing. Since the COC is issued by the EU, this also simplifies registration of vehicles purchased in different EU countries.

Conformity Factor Assessments

In December 2018, the General Court of the European Union ruled that the European Commission did not have the authority to introduce conformity factors via its implementing powers and that they should have been adopted through ordinary legislative procedures [5159]. The Court did not rule on the technical necessity of conformity factors but rather on how their use was introduced. In response, the European Commission introduced a legislative proposal in June 2019 to amend Regulation (EC) No 715/2007 to re-instate the conformity factors previously adopted in Commission Regulation (EU) 2016/646 and to have the legislator empower the Commission to annually review downwards the conformity factors to reflect improvements in PEMS accuracy [5157]. Parliament responded to this proposal by adopting a position in September 2020 that the conformity factor would cease to apply by 30 September 2022, after which only the raw data from RDE test would be used to determine compliance with EU emission limits. The Council of the European Union maintained that measurements by PEMS remain inaccurate and any phase-out of conformity factors for NOx under Euro 6, without scientific evidence and significant technical progress, would be inappropriate. As of April 2021, the two sides have been unable to reach an agreement [5158].

The EU’s Joint Research Center (JRC) assessments of portable emissions measurement systems (PEMS) measurement uncertainty suggests that the conformity factors could be further reduced from those shown in Table 1. Their 2020 assessment suggested the NOx margin could be lowered to 0.32 (conformity factor 1.32) [5085]. The 2021 assessment suggested the NOx margin could be further reduced to 0.23 (CF=1.23) and the PN margin reduced to 0.34 (CF=1.34) [5057].

UNECE Regulation

On 12 January 2024, UN Regulation No 168—Uniform provisions concerning the approval of light duty passenger and commercial vehicles with regards to real driving emissions (RDE)—was published in the Official Journal of the European Union [5622]. The regulation provides a worldwide harmonized method to determine the levels of Real Driving Emissions of gaseous compounds and particles from light-duty vehicles.

The regulation applies to the type approval of vehicles of categories M1 with a reference mass not exceeding 2,610 kg and vehicles of categories M2 and N1 with a reference mass not exceeding 2,610 kg and a technical permissible maximum laden mass not exceeding 3,500 kg with regard to their Real Driving Emissions. At the manufacturer’s request, type approval granted under the regulation may be extended to vehicles of categories M1 with a reference mass not exceeding 2,840 kg and vehicles of categories M2 and N1 with a reference mass not exceeding 2,840 kg and a technical permissible maximum laden mass not exceeding 3,500 kg and which meet certain conditions laid down in the regulation.

The UNECE regulation is closely based on the EU RDE procedure and introduces the same PEMS margins as the Euro 6e regulation (Table 1).

CEN Activity

CEN Workshop Agreement CWA 17379:2019 ‘General guideline on real drive test methodology for compiling comparable emission data’ provides a testing methodology and guidelines to conduct on-road testing of cars and vans to capture data on emissions of urban NOx. The Agreement builds upon the RDE methods and describes the process to test the actual emissions recorded during urban driving, with the aim of comparing the on-road emissions performance of different vehicles. Testing involves at least two matching examples of each model, during three separate journeys, including at least five 10 km trips conducted on paved roads, at an average speed between 20 km/h and 40 km/h. The proposer of this CEN Workshop was Emissions Analytics.

Acknowledgements: Karsten Mathies of TÜV Hessen has graciously provided valuable guidance and background information for this article.