1 Introduction

In Europe, the first vehicle emission standards, known as Euro 1, were introduced in 1992. Since then, increasingly stringent emission standards have been a major driver of technological progress in internal combustion engine vehicles [1].

In addition to the drastic reduction of the regulated emission limits, the vehicle test conditions for type approval have been continuously tightened since then. In 2017, the rather outdated NEDC cycle was replaced by the more dynamic WLTC (see below section). Subsequently, the introduction of the Real Driving Emissions (RDE) testing under the Euro 6d-TEMP regulations [2] revealed significant differences between the NOx emission levels of diesel vehicles measured in the laboratory and under real-world driving conditions [3, 4], necessitating the implementation of new emission reduction technologies and vehicle calibration techniques. Despite these measures, major European city centres are still struggling to comply with the air quality regulations [5, 6]. At the same time, the WHO had announced even stricter air quality guidelines for 2021 [7].

Against this background, the Euro 7 regulations require the development of a new generation of vehicles capable of meeting ultra-low emissions limits under all possible real-world driving conditions [8]. To this end, individual technologies have recently been intensively investigated. These range from engine-related measures, such as the introduction of advanced injection and boosting systems [9], dynamic cylinder deactivation [10, 11] and lightweight structures [9], to powertrain related measures, such as close-coupled exhaust gas aftertreatment systems [12, 13], electric exhaust gas heaters [11, 14] and fuel burners in the exhaust system [15, 16]. However, there is very little work that addresses the holistic emission-based design of Euro 7 diesel vehicles, taking into account all aspects introduced by the upcoming regulations.

This study first aims to highlight the technical challenges for the holistic emission-based development of Euro 7 diesel vehicles in the context of the diversity of regulated RDE operating conditions. It then presents the required technological upgrades for the powertrain and the exhaust gas aftertreatment system to meet the Euro 7 NOx emission requirements compared to current Euro 6 vehicles. A mild-hybrid light-duty commercial vehicle (LCV) with a diesel internal combustion engine (ICE) and automatic (AT) gearbox is examined. LCVs combine in their operation the characteristics of a light-duty (LD) passenger car and a larger heavy-duty (HD) truck used for freight transport. This makes it possible to apply the research results to both vehicle types. The emission results presented in this study are based on simulation and are focused on tailpipe NOx emissions.

2 Emission-Based Euro 7 Design Methodology and Legal Boundary Conditions

An LCV of the N1 Class III class with a curb weight of 2.2 t and a gross combined weight of 3.5 t was selected for the investigations. The vehicle is powered by a 2-l inline 4-cylinder diesel engine with a rated power of 115 kW. The power-to-mass ratio of the fully loaded vehicle is therefore less than 35 kW/t. The considered vehicle specifications are listed in Table 5 of the Appendix. The simulative investigations were conducted in a MATLAB/Simulink environment, jointly developed by the Chair of Thermodynamics of Mobile Energy Conversion Systems (TME) at RWTH Aachen University and FEV Europe GmbH [17]. The platform allows a holistic longitudinal vehicle simulation for given driving cycles by combining existing models for all powertrain sub-components (i.e. the ICE, the exhaust gas aftertreatment system (EATS) and the vehicle). An overview of the structure and the prediction accuracy of these models can be found in [18,19,20,21,22,23,24,25]. Data from previous Euro 7 light-duty vehicle projects were used to calibrate the baseline models. These were scaled accordingly to meet the powertrain specifications considered in this work. The engine, vehicle and underfloor selective catalytic reduction (SCR) catalyst (see Sect. 4.2) calibrations were additionally validated with dedicated experimental data from comparable LCVs with the one investigated.

The Euro 7 emission-based design methodology adopts a design approach that focuses on adverse RDE scenarios for emissions formation. If compliance can be ensured in these scenarios, it can be ensured in any other realistic RDE test. Tables 1 and 2 summarize the Euro 7 emissions limits and testing conditions for normal driving proposed by the EU commission and compare them to the Euro 6e standards [2, 8, 26]. This provides a reference to the current regulations. For driving situations that do not fall under normal driving conditions, specific emission corrections should be implemented, as per the Euro 7 regulation proposal. These are summarized in Table 3.

Table 1 Comparison of the Euro 6e and the proposed Euro 7 emission limits for N1 Class III LCVs with a power-to-mass ratio of less than 35 kW/t [2, 8, 26]
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Table 2 Comparison of the RDE normal testing conditions for N1 Class III LCVs with a power-to-mass ratio of less than 35 kW/t under the Euro 6e and the proposed Euro 7 standards [2, 8, 26]
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Table 3 Emissions corrective factors for the extended RDE driving conditions according to the Euro 7 regulation proposal [2, 8, 26]
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From Tables 1, 2 and 3, the following conclusions can be drawn about the expected regulations: (a) more stringent emission standards are introduced compared to the Euro 6e standards; (b) NH3 is a new regulated emission species; (c) a minimum distance of 10 km is specified for meeting the emission requirements; (d) challenging driving situations, including short urban trips, mountain driving, extreme cold starts and trailer pulling, are part of the tested RDEs, with no strict specification of the urban, rural and motorway shares; and (e) The RDE conformity factor is no longer considered.

By considering the preceding, four critical scenarios for emission compliance are investigated for the design of the Euro 7 vehicle concept in the following. These cover a very wide range of operation and ambient conditions. They are selected to be challenging from all emission-relevant technical aspects. These scenarios are presented in Fig. 1.

Fig. 1
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Considered Euro 7 scenarios

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3 Challenges to Meet the Euro 7 NOx Emission Requirements

In Fig. 2, the profiles of the driving cycles that correspond to the Euro 7 scenarios depicted in Fig. 1 are shown.

Fig. 2
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Driving cycles that correspond to the Euro 7 scenarios [31]

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A low-speed “Cold Urban” drive during traffic jams is reflected in the first scenario (Fig. 2, top left). For this, the 9-km “London Inter-Peak” cycle developed by the local British government entity, Transport for London (TfL), is used [27, 28]. As a worst-case scenario for normal cold ambient conditions, a temperature of 0 °C has been assumed.

The second scenario (Fig. 2, top right) models “Mountain Driving”. The driving cycle depicts mountain driving with limited traffic flow, focusing solely on the uphill part [29]. Due to hairpin curves, driving is aggressive with frequent slowdown and acceleration. The road grade increases continuously, averaging 9% for the cycle. The 16-km cycle is a real-world driving profile that was derived from GPS measurements. Since vehicles usually drive up and down the mountain, a mountain downhill driving cycle of the same distance follows the mountain uphill driving cycle in the corresponding scenario presented in Fig. 1. To model a downhill driving profile, the uphill driving cycle and its corresponding slope profile depicted in Fig. 2 (top, right) were mirrored round the horizontal axis towards increasing time to mimic a return drive to the mountain bottom. The slope profile was additionally multiplied by − 1 to model the downhill road grades. Due to the considerably increased overall system mass in the 7 t variant with trailer traction, the vehicle was unable to launch or accelerate properly in certain high dynamic phases, due to lack of power. Hence, it could not satisfactorily follow the requested vehicle speed, leading to its emissions results being incomparable with these of the two other variants. The preceding indicates that in reality, this system would operate differently in such dynamic and high-slope scenarios (e.g. with less aggressive acceleration and/or vehicle speed reduction) and hence the selected “Mountain Driving” driving cycle was deemed unsuitable to model the 7 t vehicle driving behaviour in such a scenario. In normal ambient conditions, 0 °C is the worst-case cold temperature.

In the third scenario (Fig. 2, bottom left), extreme “Hot Urban” congestion is modeled during summer in southern European cities like Madrid. The 11-km cycle is generated from measurements by the Chair of Thermodynamics of Mobile Energy Conversion Systems of the RWTH Aachen University under congested situations with rapid acceleration following a protracted stop. Similar to the “Mountain Driving” scenario, the Madrid driving cycle was deemed unsuitable to model the 7 t vehicle driving behaviour in urban traffic congestion due to its very high dynamics, and is therefore not further considered in this study. A 35 °C ambient temperature is the worst-case hot temperature for normal ambient conditions.

The last scenario (Fig. 2, bottom right) investigates “High Speed Highway” driving. The driving style is aggressive with dynamic acceleration and speed requests of over 170 km/h. The scenario represents a winter highway journey in Germany at worst-case normal cold ambient conditions at 0 °C. The 103-km cycle is generated from measurements at the Chair of Thermodynamics of Mobile Energy Conversion Systems of RWTH Aachen University. For the test case “trailer towing”, the maximum vehicle speed was limited to 100 km/h as required by the regulations in many European countries [30].

To quantify the new challenges of the different cycles, their key features as well as their impact on the ICE engine-out exhaust conditions are analyzed in Table 4, Figs. 6 and 7, respectively. For reference, Table 4 compares the key features of the Euro 7 test cycles with those of the well-known NEDC and WLTC dynamometer test cycles.

Table 4 Comparison of the key-features of the Euro 7 driving cycles and the NEDC and WLTC
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Table 4 demonstrates that the Euro 7 regulations will result in more dynamic urban driving with lower average vehicle speeds and more frequent idling phases, such as the City RDE Madrid and TfL London Inter-Peak driving cycles. This will increase the formation of transient emissions while simultaneously reducing the average exhaust gas enthalpy, making it more challenging to attain the light-off temperature of the exhaust gas aftertreatment system (EATS).

Simultaneously, scenarios with very high average speeds (e.g. German Highway) or road gradients (e.g. Mountain Driving) and thus very high ICE loads must be taken into account. These result in high raw emissions, requiring a high deNOx performance, and high exhaust temperatures, potentially affecting the deNOx capability of the system. At elevated temperatures, the NH3 storage capacity of SCR systems is drastically reduced. Consequently, there is a higher risk for NH3 slip during high average load cycles. Figures 6 and 7 of the Appendix depict the time-based distribution of the ICE operating points over the engine-out NOx and exhaust gas temperature maps for the four Euro 7 driving cycles considered.

In summary, a Euro 7 powertrain must be able to handle the following technical challenges: (1) ensuring proper heating of the EATS even under conditions of very low-load cold starts, (2) reducing idling and transient emissions, and (3) being appropriately sized and controlled to achieve emissions compliance in conditions of both very low-load cold starts and very high loads. Based on these requirements, technical solutions for Euro 7 are discussed in the next chapter.

4 Solutions to Meet the Euro 7 NOx Emission Requirements

Meeting the Euro 7 emission standards requires the implementation of new powertrain technologies. These can be divided into two main subcategories: (1) measures for ICE raw emissions reduction and (2) measures to increase the deNOx efficiency.

Figure 3 shows the considered technology upgrades in the simulation model, in order to comply with the Euro 7 standards, compared to the state-of-the-art Euro 6 technology for this vehicle category [29]. As expected, the Euro 7 concept is significantly more complex than the Euro 6 reference system. The two deNOx unit layout, with a diesel particulate filter (DPF) with a selective catalytic reduction (SCR) coating (ccSDPF) followed by a second SCR in underfloor position (ufSCR), remains the same. The Euro 7 EATS volume shows an overall increase of 2.8 times, with the first deNOx unit being largely sized by 2 times and the second deNOx unit by 5 times compared to the respective Euro 6 catalysts. The powertrain is electrified with a 48 V system that provides the necessary power supply to the electric heater (E-heater) installed in front of the “close-coupled (cc)” diesel oxidation catalyst (DOC) and the electric motor (E-motor) installed in P2 position. Hence, faster heat-up of the EATS and pure electric drive become possible in the Euro 7 system.

Fig. 3
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Euro 7 powertrain and EATS concept compared to the Euro 6 state-of-the-art technology

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Compliance should be ensured also under adverse RDE operating conditions. To cover all possible payload conditions, the emission compliance of the LCV was tested when empty (2.2 t), fully loaded (3.5 t) and fully loaded with trailer towing (7 t). All simulations started with a depleted battery (State of Charge (SoC) = 15%) as an unfavourable scenario for the usability of the electrified components. In addition, a very low initial NH3 of 5% of the maximum was assumed for both deNOx units, which minimizes the deNOx capability of the exhaust gas aftertreatment system at the beginning of a cold-started cycle.

In the following, the adopted technical solutions for Euro 7 in this study are described in more detail.

4.1 Reduction of ICE Raw Emissions

Strategies to reduce the ICE raw emissions are necessary when the EATS efficiency is limited, such as during cold start or high-load operation. To lower the ICE raw emissions under these conditions, while still maintaining favourable fuel efficiency, combined HP- and LP-EGR systems have become state-of-the art [32]. Besides the proper ICE hardware selection, dedicated low-NOx operation of the ICE is essential to meet all Euro 7 emission requirements.

For the Euro 7 vehicle concept presented here, a low-NOx mode for the cold start is implemented in the model in which the stationary NOx raw emissions of the ICE are reduced by 40% compared to the baseline Euro 6 warm emissions calibration. The strategy is implemented for as long as the upstream temperature of the second deNOx unit, the ufSCR (see Fig. 3, bottom plot), remains below 240 °C. Similarly, in the high-load and high-speed range (i.e. above 3500 1/min and 80 kW ICE power), a 20% reduction in NOx emissions is considered compared to the baseline Euro 6 emissions, to address the particularly high formation of raw emissions in the considered high-power Euro 7 cycles. The NOx raw emission reductions are realized in the model by adjusting the EGR rate [18]. For this level of NOx raw emission reduction, model calculations indicate an average increase in EGR rate by 8.5% (EGR rate units) at low-load cold starts and 3% (EGR rate units) at high loads during the activation phase of the respective strategy, compared to the Euro 6 baseline. Based on available measurement data from comparable engines, the impact of the EGR rate increase on combustion efficiency worsening, and hence fuel consumption and CO2 emissions increase, lays in the range of 0.5–1% (combustion efficiency units) for the investigated levels of raw NOx emission reduction [33, 34]. Such a combustion efficiency reduction can be considered negligible. Based on the same data, an increase of soot emissions in the range of 40–100% was estimated. This can be compensated by the increased DPF volume in the Euro 7 system. The typically expected impact of the increased EGR strategy on the engine performance at high loads can be compensated by different measures, such as the boost pressure increase. Last, it is worth noting that when it comes to moderate driving conditions, the ICE measures considered are expected to be activated for a limited period of time. The baseline warm NOx emissions calibration for this concept derives from a state-of-the-art Euro 6 ICE and is depicted in Figs. 6 and 7 in the Appendix.

A significant proportion of the NOx emissions in a driving cycle is due to the transient acceleration events and the idling operation. Here, powertrain hybridization can be very beneficial. By also considering the 2030 CO2 emissions targets [35], a P2 mild-hybrid system is considered for this Euro 7 concept. The system enables higher energy recuperation capability [17] and hence a CO2 emission reduction compared to market-standard P0 concepts. Moreover, it enables pure-electric propulsion and ICE start-stop functionality. The latter contribute to a raw NOx emissions reduction [17, 22].

The implementation of the previous measures enabled a raw NOx emissions level below 300 mg/km in the classical WLTP procedure, which is characterized as minimum requirement for typical LD applications to comply with the Euro 7 standards [36].

4.2 Exhaust Gas Aftertreatment System

Regarding the catalytic system, a DOC-based system is preferred over a Lean NOx Trap (LNT)-based system due to the latter’s negative impact on fuel consumption and rapid ageing at high operating temperatures [36]. The requirement for high deNOx capability under both cold-started low-load and high-load conditions necessitates the installation of two deNOx units. In cold-started low-load cycles with low average exhaust gas temperatures (e.g. urban driving), the first deNOx unit, a ccSDPF, is mainly responsible for the NOx emission conversion. The catalyst is placed close to the ICE, in “close-coupled (cc)” position, in order to allow the catalyst light-off temperature to be reached quickly. For the HC and CO emissions oxidation, a ccDOC is placed upstream of the ccSDPF.

To further decrease the time to the first internal post fuel injections, and hence enable the faster heating of the ccDOC and the complete EATS, an E-heater is installed upstream the ccDOC. The E-heater power is 4 kW, which is the current market standard for 48 V mild-hybrid electric vehicles (MHEVs) [37, 38]. The heater provides power to the exhaust gases for as long as their temperature at the ccSDPF inlet remains below 220 °C. The implemented internal post fuel injection strategy aims at optimum operating temperature windows for both deNOx units. Post injections start when the ccDOC inlet temperature is above 200 °C and are deactivated when the exhaust gas temperature downstream of the ccSDPF is 240 °C. A 20% reduction in the targeted boost pressure for ICE loads below 25% provides additional support for preventing the EATS from cooling down during low-load operation.

In addition to the measures mentioned so far, the P2 mild-hybrid system creates improved conditions for the EATS through its start-stop and ICE load-point shift functions [22]. In this concept, pure electric driving is activated only during urban driving when the SoC level is above 20%. When pure electric driving is not possible and the battery SoC level is below 50%, a load point shift strategy is implemented to avoid operation below 15% of the maximum ICE load. This prevents the flow of cold exhaust gases through the EATS, and hence its cooling down, avoids operation in areas of poor ICE fuel efficiency and higher raw NOx emissions and also recharges the battery. For NOx emission reduction at higher loads and temperatures (e.g. mountain driving), a larger second SCR is placed in underfloor position. The catalyst is appropriately dimensioned to ensure the necessary deNOx capacity, also at these operating conditions. Its heat-up behaviour and deNOx performance were validated using vehicle measurements from a comparable LCV with the one investigated.

NH3 will become a regulated emission species in the new Euro 7 regulations (see Table 1). Therefore, an ufSCR unit with an ammonia slip catalyst (ASC) coating in its last slices is considered to avoid tailpipe NH3 slip. Combined with a robust urea dosing strategy, a universal NH3 control concept has been defined that can ensure compliance with the Euro 7 NH3 and NOx limits in all four scenarios. Figure 4 visualizes the toolchain for identifying the proper ufSCR sizing and a universal Euro 7 compliant NH3 slip strategy.

Fig. 4
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Toolchain for identifying the proper ufSCR sizing and a universal Euro 7 compliant NH3 slip strategy

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In the following, the Euro 7 emission results of the vehicle concept are discussed and analyzed in detail. In all simulations, aged catalysts (Euro 7—160,000 km equivalent) were considered.

5 Results and Discussion

After the challenges to achieve the Euro 7 NOx emission requirements were identified and solutions to address them were discussed, the specified Euro 7 vehicle concept is tested for emission compliance.

Figure 5 summarizes the tailpipe NOx emission results. The results both at 10 km and at cycle end are shown, since compliance is only achieved, if the system manages to reach the Euro 7 limits after 10 km of driving and stays below these until the cycle ends. The “uncorrected” and the “corrected” emissions results are shown in the same figure. The “corrected” results include the necessary emission corrections depending on the presence or not of “extended” driving conditions, as described in Table 3. In Tables 6 and 7 of the Appendix, a detailed explanation of the emissions correction criteria is given.

Fig. 5
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Evaluation of Euro 7 NOx emission compliance: Tailpipe NOx emission results at 10 km and at cycle end in the four driving scenarios considered

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As shown in Fig. 5, after the necessary corrections are applied, Euro 7 compliance is achieved in all scenarios. The designed system shows very good emissions performance in the Cold Urban and the High Speed Highway scenario. In both, the emissions stay well below the Euro 7 limit, even before the necessary emission corrections are applied. The only exceptions are the two cases with the trailer towing. The same is also verified by the tailpipe NOx emission course in those scenarios, which are depicted in Figs. 8 and 9 in the Appendix.

In the Hot Urban and the Mountain Driving scenarios, NOx emission compliance is also achieved. However, in these cases, fulfilling the Euro 7 emission limits is more challenging.

Focusing on the first scenario, the combination of prolonged idling phases followed by aggressive and short acceleration events creates particularly demanding boundary conditions for emissions compliance during city driving. According to Table 4, the v*apos in the selected City RDE Madrid driving cycle for the Hot Urban scenario is 2.3 times higher than in the TfL London Inter-Peak cycle of the Cold Urban scenario. This indicates that the first cycle has higher dynamics. Combined with the also higher stop share by 1.8 times, the City RDE Madrid appears more challenging for emission compliance than the TfL London Inter-Peak, which is known as a challenging emission cycle for diesel light-duty vehicles. The prolonged idling phases in such low-load cycles render the proper EATS heat-up very difficult, due to the insufficient exhaust gas enthalpy available for its heating.

The short acceleration events increase significantly the transient NOx emissions. Both phenomena appear in a worst-case combination when the LCV is driven fully loaded. However, even in this case, the advanced Euro 7 emission measures considered marginally succeed in ensuring emissions compliance after the necessary corrections are applied. This is also confirmed by the tailpipe NOx emission courses depicted in Fig. 10 for this scenario.

In the Mountain Driving scenario, on the other hand, the technical challenges arise from the high NOx raw emissions and high exhaust temperatures. The critical variant for compliance with the emission limits is also the 3.5 t LCV, which shows very high average ICE load demand during uphill driving. Indicative of this is the cumulative ICE work over the travel distance. This is found 2.4 times higher for the uphill part of the Mountain Driving scenario driven with the 3.5 t LCV than for the WLTC driven with the 3.5 t LCV while towing a 3.5 t trailer (i.e. 0.5 kWh/km and 1.19 kWh/km, respectively).

The critical Euro 7 distance of 10 km falls in the uphill portion of the Mountain Driving scenario, as shown in Fig. 11. Combined with the extended driving conditions in the first 2 km after the cold start due to the driving cycle dynamics (see Tables 6 and 7), this results in extremely challenging operating conditions for the EATS. Hence, compliance cannot be achieved at 10 km, as shown in Fig. 5. Considering that vehicles driving up a mountain usually drive it down afterwards, the vehicle certification that accounts only the emissions of the uphill part for the emissions compliance evaluation is not representative of the overall vehicle operation in such a driving situation. Therefore, the emissions at the end of the cycle, after the vehicle has descended the mountain again, were taken into account for the evaluation of compliance with the Euro 7 standard in this scenario.

As shown in Figs. 5 and 11 of the Appendix, the specified Euro 7 system achieves the NOx emissions requirement of 75 mg/km in the case of the 3.5 t LCV, when it reaches the bottom of the mountain again. In the uphill part, the tailpipe emissions clearly exceed the Euro 7 NOx limit after the first 8 km, when the average vehicle speed and slope reach their highest level. However, in the downhill part, the vehicle is mainly coasting down with the ICE switched off, thus minimizing the emission formation and compensating for the significantly high emissions of the uphill part. The previous are confirmed by the tailpipe NOx emission courses depicted in Fig. 11 for this scenario.

For a more thorough understanding of the antipollution performance of the investigated EATS concept and to further support future work on the optimal dimensioning of diesel catalytic systems, the contribution of each deNOx unit in the conversion of the engine-out NOx emissions is presented in Fig. 12 in the Appendix. In addition, in Figs. 13 and 14, the time-based engine-out conditions are illustrated for all cases investigated.

6 Conclusions

Following a holistic emission-based vehicle design approach, a novel vehicle concept was presented that can meet the NOx emission requirements of the Euro 7 standards as set out in the latest Euro 7 proposal of the European Commission. The design was based on four RDE scenarios that are particularly unfavourable for NOx emission formation. These were selected accordingly to cover all possible operating situations of a light commercial vehicle.

The specified Euro 7 system consists of a close-coupled DOC-based EATS with a 6-l ccSDPF, combined with a 4-kW E-heater, a P2 mild-hybrid system, and dedicated low-NOx ICE operation. In underfloor position, a 10-l SCR with an ASC coating in its last slices is installed. The system was able to ensure Euro 7 NOx emission compliance even in the Madrid City RDE cycle, which is significantly more challenging than the already harsh TfL London Inter-Peak cycle in terms of emissions compliance. Despite the aggressive driving style and the required vehicle speeds of up to 175 km/h, emission compliance appeared noncritical in the investigated High Speed Highway scenario.

As expected, the uphill driving scenario proved to be the most challenging, as the average ICE load demand is very high when driving uphill. With kilometre-specific vehicle certification, scenarios of permanent uphill driving could lead to emission compliance issues. However, this largely depends on the driving speed and the road gradient. If both the uphill and the downhill part of the Mountain Driving scenario are considered for the Euro 7 emissions certification, compliance does not appear critical.

Closing, the required emissions technology and, consequently, the overall vehicle costs are highly dependent on the RDE scenarios considered, and how demanding they are from the emission formation perspective. The more demanding the scenarios, the more sophisticated will be the necessary antipollution technology. The specified Euro 7 EATS system is considerably larger dimensioned than the reference Euro 6 system. Future research should explore the feasibility of decreasing the catalyst sizing while imposing dedicated, temporal ICE power limitations. Moreover, alternative EATS concepts, such as placing the ccSDPF before the ccDOC, should also be investigated, as they could prove beneficial in terms of antipollution efficiency and catalysts size reduction.

Concerning the design methodology followed, this was based on adverse RDE scenarios for emission formation. In the future, an even more precise design method should additionally consider the frequency of occurrence of such driving situations on an average day of the year and also relate the selection of the RDE testing scenarios more strongly with air quality aspects. Regarding the latter, recent studies have highlighted that although mountain uphill driving scenarios are typically very challenging for emissions compliance, they remain uncritical from an air quality point of view [29]. Adopting the previous measures would enable more justifiable emission technology upgrades and, consequently, limited increases in vehicle costs.