Implementation of Portable Emissions Measurement Systems (PEMS) for the Real-driving Emissions (RDE) Regulation in Europe

1. Select the Vehicle

1. For type approval purposes, choose a representative vehicle from a "PEMS test family." Families are considered to be vehicles with the same technical characteristics (i.e., propulsion type, fuel, combustion process, number of cylinders, engine volume, method of engine fueling, cooling system, after-treatment devices, and exhaust gas recirculation). For details, see Chapter 4 and Appendix 7¹³.
2. For any other purpose (e.g., comparison of laboratory versus on-road emissions), choose a vehicle that suits the experimental objectives.

2. Prepare the Vehicle

1. Prepare the PEMS.
   NOTE: See Appendix 1 of the Regulation⁸ for the PEMS equipment.
   1. Use (at least) CO and NO_x analyzers to determine the concentration of pollutants in the exhaust gas. Use a CO₂ analyzer to determine the driving severity of the test (aggressiveness), during the verification and calculation steps.
   2. Use one or multiple instruments or sensors, such as an exhaust mass flow meter (EFM), to determine the exhaust mass flow.
   3. Use a global positioning system (GPS) to determine the position, altitude, and speed of the vehicle.
   4. If applicable, use sensors and other appliances that are not part of the vehicle (e.g., a weather station) to measure factors such as ambient temperature, relative humidity, air pressure, or vehicle speed.
   5. Use an energy source independent of the vehicle to power the PEMS. For passenger cars, 12 V or 24 V batteries are typically used.
   6. Optionally, use other auxiliary equipment, like battery chargers, a personal computer to remotely check the PEMS status, straps for installation of the PEMS inside of the car, or metal platform for installation on the tow bar outside of the car.
2. Install the PEMS.
   1. Install the PEMS main and control units outside of the vehicle (e.g., on a tow bar by means of a dedicated platform) or in the boot/trunk (Figure 1). If the PEMS is installed in the cabin, fix it well using straps and vent excess gases outside the car, such as by using polytetrafluoroethylene (PTFE) tubes.
   2. Install at least CO₂, CO, and NO_x analyzers (and upon approval of the third RDE package, a particle number analyzer) with their heated sampling lines. Follow the instructions of the PEMS manufacturer and the local health and safety regulations.
   3. When the PEMS is not provided with its own batteries, mount a 12 V battery in the vehicle cabin, for instance, behind the co-driver's seat. Fix it well with straps.
   4. Using magnets, attach the weather station and GPS directly onto the vehicle chassis (e.g., on the roof of the vehicle). Connect the GPS signal cables to the PEMS main unit signal input port.
   5. Whenever an EFM is used, ensure that the measurement range of the EFM matches the exhaust mass flow rates expected during the test. Consult the manufacturer's specification sheet for the EFM. An example is given in Table 2.
   6. Adapt the vehicle tailpipe to the EFM using hose clamps and flexible connectors or welding metal tubes. Use connectors that are thermally stable at the exhaust gas temperatures expected during the test in order to avoid the generation of particles. Avoid decreasing the inner diameter of the tailpipe using smaller tubes or decreasing the cross-section by adding many sampling probes at the same position.
      1. If in doubt, check that the installation and operation of the PEMS does not unduly increase the static pressure at the exhaust outlet. Measure the pressure with a pressure sensor (accuracy better than 0.1 kPa) in the exhaust outlet or in an extension with the same diameter, as closely as possible to the end of the pipe.
         NOTE: If no pressure limits are given by the vehicle manufacturer, the addition of PEMS or any probes should not cause the static pressure at the exhaust outlets on the vehicle to differ by more than ± 0.75 kPa at 50 km/hr or more than ± 1.25 kPa at 120 km/hr from the static pressures recorded when nothing is connected to the vehicle exhaust outlets.
   7. Fit the sampling probe(s) at least 200 mm upstream of the exit point of the exhaust outlet in order to minimize the influence of ambient air downstream of the sampling point (Figure 2). If an EFM is used, install the sampling probes downstream of the EFM, respecting a distance of at least 150 mm to the flow sensing element (Figure 2). The probes should have an appropriate length that permits sampling from the centerline. Probes with lengths equal to the inner diameter of the tailpipe can also be used if they have multiple holes along their lengths.
   8. Ensure that the maximum payload is respected (i.e., <90%). The PEMS plus a co-driver are around 150 kg, so the maximum load of the car is not reached. Add extra weights if the 90% limit must be reached.
   9. After the installing the PEMS, perform a leak check by following the instructions of the PEMS manufacturer. Block the probe tip with a soft plastic cap, turn the PEMS sample pumps on to draw a vacuum, and then shut them off. The pumps may be controlled by connecting the PEMS to a PC via an Ethernet cable. If this is not possible (e.g., the probe is installed in the exhaust stack), then perform the leak check from the sample inlet of the analyzer.
      NOTE: The PEMS software communicates with the main unit and controls the pumps once the leak check procedure has begun. Monitor the vacuum pressure. The pass/fail pressure limit loss is specified by the PEMS manufacturers.

Table 2: Example of typical flow meter characteristics. For each flow meter, the dimensions and the maximum flow rates at different exhaust gas temperatures are given. The data comes from Sensors' High Speed Exhaust Flow Meter.

Figure 1: PEMS from different manufacturers. In these examples, the PEMS are installed outside of the vehicle on a support or on the tow bar. Please click here to view a larger version of this figure.

Figure 2: PEMS installation. The gas analyzers are located inside the vehicle. The minimum required distances before and after the EFM are also given in the figure. Note that no elastomer connectors were used in this setup. Please click here to view a larger version of this figure.

3. Validate the PEMS installation.
   NOTE: This sub-step is optional. However, it is recommended to validate the installed PEMS once for each PEMS-vehicle combination by running a test on a chassis dynamometer over a cycle similar to the one used for type approval, either before or after the on-road test.
   1. Put the vehicle with the PEMS on a chassis dynamometer. Prepare the PEMS as in step 4 (see below) for conducting a trip.
   2. Drive a type approval test over a World Harmonized Light-vehicles Test Cycle (WLTC), following as far as feasible the requirements of the in-force laboratory regulation (see Appendix 3)¹³.
   3. Measure the pollutant emissions with the PEMS, in parallel with the laboratory equipment used for the type approval of vehicles.
   4. Calculate the PEMS emissions per second (as in step 4). Sum the calculated real-time emissions to get the total mass of pollutant emissions (g) and then divide it by the test distance (km) obtained from the chassis dynamometer.
   5. Compare the PEMS total distance-specific mass of pollutants (g/km) with the reference laboratory system calculated according to the regulation. The difference has to fulfil specific requirements for each pollutant (e.g., for NO_x, ± 15 mg/km or 15% of the laboratory reference, whichever is larger).

3. Design the Trip

1. Design the trip based on street maps. Have the executed trip fulfill the requirements specified in Tables 3 and 4.
2. Ensure that the trip starts with an urban (U) part (speed ≤60 km/hr), continues with a rural (R) part, and ends with a motorway (M) part (speed >90 km/hr).
3. Ensure that the shares of urban, rural, and motorway driving are equal. For the purpose of trip design, the definition of urban, rural, and motorway operation is defined on the basis of the instantaneous vehicle speed and takes into account the topography of the testing location.
   During the definition of the motorway trip portion, pay attention to the presence of restrictions, such as toll stations, which will limit the actual speed.
   NOTE: Electronic maps may provide additional information on local speed limits, trip duration, trip distance, and local elevation with respect to sea level.

Table 3: Boundary conditions of a valid RDE test¹². The boundary conditions refer to the initial conditions that have to be respected before and during the test trip. For each condition, the limits and some comments are given.

Table 4: Operational requirements for a valid RDE test¹². The operational requirements refer to the conditions that have to be respected during the test trip. For each condition, the limits and some comments are given.

4. Conduct the Trip

1. Switch on the PEMS and let it stabilize for about 40 min, according to the specifications of the PEMS manufacturer.
   1. To avoid humidity condensation and to ensure appropriate penetration efficiencies of the various gases, ensure that the sampling line(s) have reached a minimum temperature of 60 °C, with or without cooler, for the measurement of gaseous pollutants. For particles, the minimum temperature is 100 °C.
   2. Confirm that the PEMS is free of warning signals and error indications. In case of warning messages, refer to the PEMS manual troubleshooting section.
2. Choose the calibration gases to match the range of pollutant concentrations expected during the trip (i.e., the calibration range should cover at least 90% of the concentration values obtained from 99% of the measurements of the valid parts of the emissions test). For CO₂, a range of 10-14% is recommended, while for NO_x, around 1,500-2,000 ppm is recommended. The true concentration of a calibration gas has to be within ± 2% of the stated figure.
3. Perform zero and span calibration adjustments of the analyzers using the calibration gases.
   1. Connect the zero gas (N₂) or synthetic air or use the ambient air as zero gas.
   2. Prepare the software (e.g., Sensor's Tech). Select Test → Session Manager → Give a name → Open (a session) → Pre Test options: Zero.
   3. Disconnect the zero gas.
   4. Connect the span gas bottle to the PEMS at a pressure of 1 bar.
   5. Prepare the software. Select Test → Session Manager → Pre Test options: Span.
   6. Insert the concentrations of the gases in the bottle in the PEMS software (under the zero/span graphic user interfaces). The PEMS software automatically detects the analyzer response and compares it with the bottle value. The system automatically adjusts the response of the analyzer to the span value.
   7. Disconnect the span gas and connect the next one.
      NOTE: The user has the option to use one span bottle with all relevant gases (at least CO₂ and NO_x) or separate gas bottles.
4. When everything is ready, start the sampling measurement. Create a file name in the "Test name" tab.
   1. Before starting the engine, start recording the parameters by pressing "Start" in the Session Manager via the PEMS software already installed on the PC. To facilitate time alignment, start recording the parameters either in a single data recording device or with a synchronized time stamp.
      NOTE: Commands to start and stop sampling and to start and stop recording are available in the PEMS software, which was previously installed on a PC and connected via an Ethernet cable to the PEMS main unit. Different software and graphic user interfaces are adopted by the PEMS manufacturers.
5. Conduct the mapped trip by following the instructions of a navigation system. The trip should last 90-120 min. Drive normally, avoiding excessively timid or aggressive driving. Respect all local and national road safety rules. The air conditioning system or other auxiliary devices can be operated in a way that is compatible with their possible use by the consumer.
6. Continue sampling, measuring, and recording the parameters throughout the on-road test. The engine may be stopped and started, but the emissions sampling and parameter recording must continue. Measurement and data recording may be interrupted for less than 1% of the total trip duration, but for no more than a consecutive period of 30 sec, solely in the case of unintended signal loss or for the purpose of PEMS system maintenance.
7. Document any warning signals suggesting malfunction of the PEMS.
8. At the end of the trip, switch off the combustion engine. Continue the data recording until the response time of the sampling systems has elapsed (approximately 20 sec). Press "Stop" in the Session Manager.
9. At the end of the test and before the analyzers are turned off, check the drift of the analyzers, which measured the zero and the span, using the calibration gases that were used before the test, as follows. Follow the procedure in step 4.3, with the difference of selecting Zero and Span from the "Post Test" window.
   1. Measure the zero level of the analyzer(s). Check that the difference between the pre-test and post-test results complies with the requirements specified by Appendix 1⁸. For example, for NO_x, the permissible zero drift is 5 ppm.
   2. Measure the span level of the analyzer(s). It is permissible to zero the analyzer prior the span drift verification, if the zero drift was determined to be within the permissible range. Check that the difference between the pre-test and post-test results complies with the requirements specified by Appendix 1⁸. For example, for NO_x, the permissible zero drift is 5 ppm and the permissible span drift is 5 ppm, or 2% of the reading (whichever is larger).
   3. If the difference between the pre-test and post-test results for the zero and span drift is higher than permitted, void the test results and repeat the test.

5. Verify the Trip

1. Export the recorded data to a spreadsheet file. In "Data Files," upload the file created before the tests. Then, in "Data Analysis," choose "Process the file."
   NOTE: In the "Settings" tab, ensure that the settings are correct; if in doubt, use the default values from the manufacturer. In the "Output" tab, select the signals you want to export (typically all of them).
2. Check that (i) the parameter recordings reached the required data completeness of more than 99%, (ii) the calibrated range of the analyzers accounts for at least 90% of the concentration values obtained from 99% of the measurements of the valid parts of the emissions test, and (iii) less than 1% of the total number of measurements used for evaluation exceed the calibrated range of the analyzers by up to a factor of two. If these requirements are not met, the test must be voided.
3. Based on the exported data, check the compliance with the boundary conditions (Table 3).
   1. Verify the conformity to the boundary conditions for ambient temperature and altitude, as specified in Table 3, by checking the instantaneous ambient humidity and temperature data, respectively.
   2. Check that the trip duration is between 90 and 120 min.
   3. Check the shares of urban, rural, and motorway driving; the maximum vehicle speed; the average speed; and idling shares of urban driving and confirm that they comply with Table 3.
   4. Verify the excess or absence of driving dynamicity, as specified by the product of instantaneous vehicle speed and positive acceleration (v∙a+), and the Relative Positive Acceleration (RPA) (see Chapter 5 and Appendix 7a)^13,14.
   5. Verify the realized altitude profiles (i.e., the trip cumulative positive elevation gain and the difference in elevation between the start and end points of a trip) (Chapter 6 and Appendix 7b)^13,14.
4. Based on the exported data, check the compliance with the operational requirements (Table 4). Verify that a sufficient coverage of normal dynamicity was achieved during the test (Table 4), applying the moving averaging windows (MAW) and/or the power binning methods on the basis of composite parameters, such as CO₂, that encompass the effects of road grade, wind, driving dynamics, (e.g., accelerations, decelerations), and auxiliary systems upon vehicle energy consumption and emissions (see Appendices 5 and 6¹³).

6. Calculate the Emissions

1. Calculate the RDE emission result for all events within the boundary of normal driving dynamics using the MAW and/or the power binning methods. For Sensor's Tech PC software, this is done automatically if, in the "Settings" tab, the "Window" method was selected.
2. Calculate the ratio of RDE emissions to the emission limit of the specific pollutant. A vehicle passes the test if the pollutant emissions remain below the applicable conformity factor (see Chapter 2)⁸ using at least one of the two methods (MAW or power binning). For NO_x, this factor is 2.1 from 2017-2019 (new type approvals/new registrations) and will be lowered to 1.5 in 2020-2021.
   NOTE: At the end of the trip, most calculations and emissions reports are done automatically, as most PEMS manufacturers offer suitable calculation software. Alternatively, the free software EMROAD (for MAW) or CLEAR (for power binning) can be used to execute step 5 (verify the trip).