## 1 Introduction  
Pollutant vehicle emission and carbon dioxide limits are becoming increasingly restrictive. In Europe, the certification limits of carbon monoxide (CO), nitrogen oxides, particulate matter (PM), and the sum of NO\({}_{\mathcal{X}}\) and total hydrocarbons (THC) for diesel engines have been reduced in large proportions since Euro 1 [1] to the current Euro 6d [2], as Table 1 shows. Nowadays, the world is facing the impact of climate change, the last six years being the warmest on record [3]. To achieve the challenges proposed in the climate change conference held in Paris 2015 [4], the concept of carbon footprint is of great importance, especially for greenhouse gases (GHG) emissions management [5]. Major developments in renewable energies [6] have been occurred to reduce GHG emissions although additional efforts are \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{1}{c}{**Date**} & \multicolumn{1}{c}{**CO**} & \multicolumn{1}{c}{**NO\({}_{\mathcal{X}}\)**} & \multicolumn{1}{c}{**PM**} & \multicolumn{1}{c}{**THC + NO\({}_{\mathcal{X}}\)**} \\  & & **[g/km]** & **[g/km]** & **[g/km]** & **[g/km]** \\ \hline Euro 1 & 07-1992 & 2.72 & - & 0.14 & 0.97 \\ Euro 2 & 01-1996 & 1 & - & 0.08 & 0.9 \\ Euro 3 & 01-2000 & 0.64 & 0.5 & 0.1 & 0.56 \\ Euro 4 & 01-2005 & 0.5 & 0.25 & 0.05 & 0.3 \\ Euro 5 & 09-2009 & 0.5 & 0.18 & 0.05 & 0.23 \\ Euro 6 & 09-2014 & 0.5 & 0.08 & 0.05 & 0.17 \\ \hline Reduction from Euro 1 to Euro 6 * & \(-81.6\%\) & \(-84\%\) & \(-64.3\%\) & \(-82.5\%\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Pollutant vehicle emissions from Euro 1 to the last Euro 6. * In the case of NOx, since the first limit was established (Euro 3). required from key sectors such as transportation. In this regard, the European Union (EU) has established stringent standards which have been reflected in the 443/2009 [7] regulation. This directive establishes that the EU fleet-wide average emission target for new cars must be 95 g CO\({}_{2}\)/km from 2021. Later on, the EU decided to reduce this limit by an additional 15% from 2025 and 37.5% from 2030 through regulation 2019/631 [8]. Along with increasingly restrictive CO\({}_{2}\) emission limits, a change in the type-approval cycle has also been set and the New European Driving Cycle (NEDC) became obsolete [9]. Instead of NEDC, two new cycles were established to represent real driving conditions accurately. First, the World Light-Duty Test Cycle (WLTC) was designed from \"real world\" driving data [10], and second, the Real Driving Emissions (RDE) cycle appeared for the first time in 2016 [11]. Both WLTC and RDE cycles reflect greater accelerations than NEDC, which produce an increment in the engine operating range [12; 13], entering the high torque and speed region of the engine map. Concerning the methodology to perform the tests, a chassis dyno is used to perform WLTC [14], while RDE must be carried out on road [15]. The newest RDE regulation [16] dictates a series of guidelines to carry out the test. In case any of the requirements were not fulfilled, the cycle would be considered invalid. Although the regulation is very extensive and detailed, the number of possible cycles is infinite so different tests can have an important dispersion in terms of pollutant and CO\({}_{2}\) emissions. Therefore, factors such as traffic, weather conditions, altitude, route typology, road grade, and driver behavior [17; 18] have a great influence on the cycle results [19; 20]. Even repeating the same trip, large differences between different cycles can be found [21]. Moreover, some studies combine experimental on-road data with numerical analysis to assess the impact of the stop/start system, vehicle mass, drag coefficient, and different aftertreatment system layouts [22]. One methodology to analyze engine operation in RDE-like conditions consists of performing cycles in a dynamic engine test bench instead of directly in the vehicle.