Under the microscope: The PEMS measurement: No two journeys are the same
As part of WLTP certification, proof of compliance with the limits in the so-called RDE (Real Driving Emissions) road test must also be demonstrated. At Mercedes-Benz Cars these drives take place long before actual certification and are a fixed component of vehicle development. The development vehicles are equipped with Portable Emission Measurement Systems (PEMS) for this purpose. The measurement system is on the trailer coupling or in the boot. During the real measuring drive, the measuring tools analyse the content of gases as well as the particle number (PN).
Whether a spacious E-Class estate with trailer coupling, a two-seater smart or a G-Class with side pipes: The PEMS measurement system has to be accommodated either in the boot or fixed to the rear of the vehicle. If there is no trailer coupling then complex assembly must be selected. Mercedes-Benz uses PEMS devices from various manufacturers. Depending on the model, it weighs up to 75 kg, can cost more than 100,000 euros and is the size of a large suitcase or cardboard box used for moving house. Integrated lithium-ion batteries provide the PEMS device with power while in operation on the road. This makes the device self-sufficient of the on-board electrical system.
The preparations are extensive: A second number plate just like for a rear-mounted bike rack is obligatory for external installation. Calibration of the system before every journey as well as regular maintenance and leak checks are part of Mercedes-Benz's quality assurance procedures. In addition, the PEMS device is validated against the dynamometer measuring equipment before each series of measurements. Driving on the road can only commence when the results of the PEMS device correspond to those from the emissions roller dynamometers or are within certain tolerance limits. For their personal safety, drivers take a small gas detector on board for each measuring drive so as to be able to detect a possible gas leak in time.
An adapter and stainless steel pipes are attached to the exhaust tailpipes. Individual solutions are required for each vehicle in this case, too. At the same time, it is important to create as little counter-pressure as possible for the engine via a favourable route for the pipe system in terms of aerodynamics. Piping made purely of plastic or silicon should not be used since these can lead to a false or increased particle number value. The pipes culminate in a horizontal measuring tube, where the exhaust mass flow is measured to determine the exhaust gas flow for each moment of the measuring drive. A so-called exhaust flow meter (EFM) is used here. Environmental information such as GPS, temperature, pressure and humidity data from sensors also play a role here. This enables a more precise correlation between driving situation and emissions results to be formed at a later stage.
Behind the EFM, a small part of the exhaust gases are removed via a probe. This exhaust gas partial flow is continuously fed to the analysis device via a heated tube.
The measuring tools analyse the carbon monoxide content (CO), carbon dioxide content (CO2), nitrogen oxide content (NOx) - formed from the sum of nitrogen monoxide (NO) and nitrogen dioxide (NO2) - as well as the particle number (PN). The device is calibrated before each measuring drive with test gases of known concentrations. The measuring drive then takes place and thereafter a renewed check of the system with the same test gases. A so-called "analyser drift" occurs if the analysers do not attain the values previously calibrated. If the drift of an analyser exceeds certain limits then the measuring drive is declared invalid.
Detailed information on how measurements are conducted
The gaseous emissions and the particle number are analysed using various procedures. An optical procedure in the infra-red range is used to determine the CO and CO2 concentration. Depending on the system, nitrogen oxides are determined with various measuring principles. In part, an optical procedure in the ultra-violet range or a chemiluminescent detector (CLD) is used. A chemical reaction produces light that is transformed into an electrical measuring signal by a photocell. Also, depending on the manufacturer, differing measuring procedures are used to determine the particle number.
The PEMS device logs all measured values as raw data. This raw data is then subsequently evaluated with special software and the emissions for the routes covered are shown in a result log.
The PEMS drive is conducted in real-life everyday traffic, naturally in compliance with the road traffic regulations. Mercedes-Benz uses various routes for the PEMS measurements, which the test drivers drive using the routes saved in the navigation device.