In USA, on-road measurement is done since 2005 to evaluate heavy-duty vehicles emissions; for LDV, real world emission is used for monitoring (EPA 2005; Engeljehringer 2019). The European Union developed a procedure for vehicle’s real-world measurement called Real Driving Emissions (RDE), which it was made effective after 2016 for monitoring and since 2018 for regulatory proposals (EU 2016a). RDE is being also adopted, even partially, in many other countries, like South Korea, Japan and Australia (Engeljehringer 2019). China is preparing itself to apply a similar RDE test like Europe in a regulatory phase called China 6, that will take effect after July/2020 (He and Yang 2017) and a RDE international procedure, based on the European and Japanese methods, is under development by the United Nations Economic Commission for Europe (UNECE) (UNECE 2020). As European and UNECE RDE are very similar and the international method is still on the way to be implemented, in this study the EU procedure was adopted as reference for the RDE test.
2.2.1 EU RDE procedure
When Europe defined Regulation 715/2007 (EU 2007), it pointed to the need of ensuring that real world emissions correspond to the measurements in laboratory tests, recommending the introduction of a specific procedure to this. The European RDE procedure have a combination of trips that represents a regular drive at local and express roads and highways (EU 2016a, b), the vehicle is coupled to a Portable Emission Measurement System (PEMS), that is able to measure CO2, CO, NOx, PN, PM and HC (AVL 2016; EU 2016a; Horiba 2016).
According to the EU RDE requirements (EU 2016a, b), the test must have three phases, divided by speed: urban, up to 60 km/h, rural, between 60 to 90 km/h, and motorway, over 90 km/h; each one driving at least 16 km; the cumulative positive elevation gain limit is 1200 m / 100 km; altitude difference between the start and finish must be lower than 100 m; moderate environment conditions are when altitude is below 700 m and temperature between 0 to 30°C.
The RDE Brazil is based on EU RDE and will be made effective after 2022, in a new type-approval phase called PROCONVE L7 (CONAMA 2018). The main difference between EU and Brazilian procedures is that, in Brazil, there are just two driving phases, urban and rural; the urban trip is about 30 to 40 km long and rural trip has about 16 to 20 km.
The traveled distance and engine data come from vehicle on-board diagnosis (OBD) and the PEMS built-in GPS provides both horizontal and vertical position which results also in distance, as well as speed and altitude. The GPS accuracy in the horizontal plan is about 3 meters; however, vertical measurement is subject an inherent higher dispersion (Garmin 2020); for example, Drosos and Malesios (Drosos and Malesios 2012) notice an error up to 7 meters in a free field study.
EU RDE accepts data from GPS for evaluation of positive cumulative altitude gain, but due to the noise in altitude the values must be smoothed, where a two-step interpolation is done over 400 meters segments (EU 2016a). According to Zhang and Frey (Zhang and Frey 2006) and Sandhu (Sandhu and Frey 2013), a single-step interpolation on 160 meters segments is enough to represent accurately the road grade; on the other hand, the 400 meters criterion is accepted worldwide, even though it produces a less variable profile and enlarges the confidence intervals.
There are other two alternative methods to measure altitude that are cost-effective and with good precision: by digital elevation maps (DEM) and by barometric altimeter. Other costly options are the use of LIDAR in an airplane traveling over the region to be analyzed, inertial sensors and high frequency GPS (Zhang and Frey 2006). Many DEM are available in the internet, providing altitude values for latitude/longitude coordinates with a horizontal resolution between 30 to 90 meters (GPSvisualizer.com 2019). For the Brazilian topography, for example, the DEM is based on the Shuttle Radar Topography Mission (SRTM-3), developed by NASA that gather altitude data between parallel 56°S and 60°N at 90 meters resolution for all South America (NASA 2021).
Barometric altitude can be come from an atmospheric air pressure sensor and calculated by the Eq. 1 (Grimm 1999). Usually, all PEMS models have this sensor available.
Z = 8*(1013 - P) (1)
Where Z is the altitude (m) and P is the atmospheric pressure (hPa or milibar).
2.2.2 RDE dynamic parameters
Since the driver’s behavior has direct effect on emission, EU RDE settles limits for vehicle’s dynamic (EU 2016b). There are two parameters under control: Relative Positive Acceleration (RPA) and v*a[95]. RPA, in m/s2, is measured every time when the vehicle accelerates more than 0.1 m/s2; instants with acceleration lower than 0.1 m/s2 or negative are not taken on account. The EU RDE procedure defines a minimal value to RPA; to be under this limit means that the driver is requiring such a low power from the vehicle that will introduce a bias in the results, because the vehicle will have a very low consumption and pollutants emissions than in comparison to the laboratory test.
In a similar way, v*a[95], measured in m2/s3 or W/kg, is the percentile 95 of vehicle speed times positive acceleration for which one-second time steps (EU 2016b). This parameter represents how much kinetic energy is required to accelerate the vehicle; a v*a[95] above the limits shows that the driver is requiring more power than compared to laboratory test cycle and it also may distort the emissions results (Fontaras et al. 2014; Wyatt et al. 2014; Pouresmaeili et al. 2018).