Under the microscope: The emission labs: Time-to-certification has multiplied

The core of emissions certification and thus the primary element when approving a new car are emissions and consumption tests on roller dynamometers. Mercedes-Benz Cars uses considerably more than three dozen roller dynamometers on three continents in Germany, USA and China for certification and development. The introduction of the WLTP legislation with the new WLTC test cycle means that the time-to-certification has increased significantly, also because of the special requirements for plug-in hybrids and electric vehicles. The certification measurements in the emissions lab at our site in Stuttgart are not just carried out for approvals in Europe but also for almost all markets around the world. Therefore, a high five-digit number of tests on roller dynamometers takes places every year. Daimler reacted to the increased requirements in a timely manner with extensive investments in capacities.

The emissions lab in Stuttgart-Untertuerkheim is the focal point for handling certification testing. This is where the official dynamometer tests take place in the presence of a technical service both for the WLTP certification and for the certification of almost all other country variants of Mercedes-Benz vehicle models (cars and vans). In addition to operating various dynamometers, the emissions lab also assumes a leading function within the global certification work at Daimler in introducing new legal emission specifications. Apart from the dynamometers at the Stuttgart site, the roller dynamometers in Sindelfingen are also used for development scopes. From an organisational point-of-view, work on the dynamometers in vehicle development is kept strictly separate from work on the dynamometers in certification.

The majority of roller dynamometers are all-wheel dynamometers so as to fulfil the latest requirements of the certification and development work. In addition, emission roller dynamometers are operated near production sites in other German laboratories as well as at several international locations in the US and China.

Apart from the roller dynamometers, there are is other testing equipment in the emissions laboratories: the so-called SHED chamber (Sealed Housing for Evaporative Determination). This is where evaporation emissions are measured, which are currently used for the new WLTP EVAP provisions (short for Evaporation Emission; see Glossary).

Certification runs around the clock

To ensure the three-shift operation of the certification laboratory with regard to vehicle availability, the laboratory contains a large parking area for more than one hundred vehicles over several floors with an automated storage system. That is where the vehicles are preconditioned (“soaked”) at a legally defined and stipulated test temperature until they are tested.

The vehicles can be easily swapped in and out as required via a lift. The vehicles to be tested including all operating fluids (coolant, oil) are preconditioned on the particular soak spaces at the testing temperatures (-7 °C to 23 °C) defined by the certification requirements.

A majority of the emissions tests are then conducted at a dynamometer temperature of 23 °C. In addition, there are a few climate tests, for example at -7 °C, to take account of the changed vehicle behaviour at low temperatures. Furthermore, as part of the certification for the US and South Korean market, the climate tests are conducted at a test bench temperature of 35 °C to 40 °C and also with a radiation intensity of 850 watts per square metre to simulate solar irradiation in order to map the influence of the air conditioning system on emissions.

More time on the rollers: The WLTC test takes longer

The effort for certification has increased dramatically in the past years. From 2016 to 2018, the total time required for the roller dynamometer has almost tripled. A further increase is assumed in the following years. Among other things, the re-certification required for many vehicles according to the provisions of the WLTC (Worldwide Harmonized Light Vehicle Test Cycle – test cycle of the new WLTP standard) as well as other changes in international emissions certification form the background here. The WLTC test is more realistic, takes longer and presents particular challenges in terms of the parameters (see chapter: WLTP and RDE).

At least two vehicle variants are tested per vehicle family: The configuration with the least CO2 emissions (VL: "Vehicle Low") and the configuration with the highest (VH: "Vehicle High"). Although a WLTC emissions measurement on the roller dynamometer only takes a good hour including set-up and break-down, one individual cycle is not all that is involved: The WLTP certification encompasses a test programme of several weeks, from test preparation and checking the vehicle by the Technical Service to concluding all required measurements including additional RDE measurements on the road.

In addition, alongside the WLTC, in the transition phase up until 2020, the NEDC figures for all vehicles still must be determined. Background: NEDC figures are required for the CO2 fleet target calculation. To determine this computationally, the simulation tool CO2MPAS has to be used for all vehicles with a combustion engine. If this calculated value does not confirm the value reported by the manufacturer, a double test according to the NEDC can be used for validation. In addition, all hybrid and purely battery-electric vehicles are always tested twice. In contrast to the procedure for NEDC certification, the time for recertification according to the WLTP in Europe has thus increased significantly overall. This means that, depending on the vehicle type and vehicle family, certification of a vehicle model can take up to two months on the emissions dynamometer Approval processes and the documentation then follow thereafter.

Driving until standstill: Range measurement in electric vehicles

Certification tests for plug-in hybrids and electric vehicles take a particularly long time: Compared with a conventionally powered vehicle, an electric vehicle such as, for example, the Mercedes-Benz EQC (combined power consumption: 22.2 kWh/100 km; combined CO2 emissions: 0 g/km, provisional figures)[7] spends much more time on the roller dynamometer. In addition, there are the charging processes for the vehicle battery which are on the same scale as the test time itself and take place away from the roller dynamometer.

Determining the electric range of plug-in hybrids and electric vehicles is particularly time-consuming. These are tested on the same dynamometers as the conventional vehicles. Part of the certification stipulates that the test cycle has to be driven for as long as it takes for the combustion engine to start because the battery is empty (plug-in hybrids) and/or until the battery's output is no longer sufficient to follow the cycle's acceleration and speed specifications (electric vehicles). Depending on the vehicle's performance and battery capacity, this can take a long time: The EQC, for example, was on the emissions roller dynamometer for more than 15 hours continuously until its battery with an 80 kWh capacity was empty. The result: A standard range of more than 450 kilometres (according to the NEDC)[8].

This presents quite a burden for the experts behind the wheel, too: Due to labour laws, drivers have to change every two hours at the latest. This means that several employees are involved for long test procedures, for example the Mercedes-Benz EQC over three shifts in the emissions laboratory in Stuttgart. By way of comparison, the test required for a conventional vehicle can usually be driven by one dynamometer driver since it rarely lasts longer than two hours. For purely development tests, on the other hand, it is possible to conduct the test fully automated and with driving robots.

Apart from determining the electric range, driving the battery flat also serves to determine the representative electricity consumption (required charging energy in watt-hours until the battery is completely charged in relation to the distance driven in kilometres during the test).

High level of utilisation: The roller dynamometers are only idle on Sundays – but actually not really

The roller dynamometers are driven on around the clock. Regular three-shift operation runs on all roller dynamometers in the emissions laboratory from Sunday evening to Friday evening, and further shifts follow on from this until Saturday evening. The roller dynamometers in test mode are only idle on Sunday mornings and afternoons, however not in the emissions laboratory: The accuracy of the test equipment is counter tested using special measuring equipment and automated maintenance is carried out on all roller dynamometers. This is required in order to show a weekly verification of the measuring equipment incl. the required documentation, which is essential for the certification operation.

Ten steps: Europe certification of a conventional vehicle

The time and effort it takes to certify a vehicle with combustion engine can be exemplified by a short summary. The complexity of this process will continue to rise with increasing electrification of the drivetrain. If the provision for the temperature changes, there will be a longer soak period between the individual tests. The following outlines the main steps of the procedure just for emission certification on roller dynamometers (without RDE road measurements and without EVAP measurements):

  1. Certification assignment and vehicle are received
  2. Parts and vehicle check as well as documentation of the data status are checked along with a technical service, which monitors certification
  3. Further preparatory steps, for example fuelling up on standardised certification fuel
  4. ATCT test (Ambient Temperature Correction Test): Reference test for standardisation of the CO2 measurements in line with the average European temperature of 14 °C using a "family correction factor" (FCF)
  5. NEDC measurement: with different load parameters (Vehicle Low and Vehicle High)
  6. WLTP measurement: Also with low and high load parameters, optionally also a supporting point VM (Vehicle Middle)
  7. Type 2/3 measurements (idle CO and crankcase emissions) directly after a WLTP VL test
  8. Logging a vehicle's cooling behaviour following a WLTP VH test
  9. Type 6 measurement: Cold-start emissions at -7 °C
  10. Determining idle consumption

[7] Figures for power consumption and CO2 emissions are provisional and were determined by the Technical Service. The range figures are also provisional. EC type approval and a certificate of conformity with official figures are not yet available. Differences between the stated figures and the official figures are possible.

[8] preliminary data