Euro 7 proposal assessment of a Euro VI parallel hybrid electric bus

Full text

Transportation Research Part D 129 (2024) 104125
Available online 6 March 2024
1361-9209/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
Euro 7 proposal assessment of a Euro VI parallel hybrid
electric bus
Natalia Fonseca Gonz´
alez
b
,
c
,
*
, Ricardo Suarez-Bertoa
a
,
*
, Barouch Giechaskiel
a
,
Anastasios Melas
a
, Roberto Gioria
a
, Fabrizio Forloni
a
, Tommaso Selleri
a
,
1
,
Adolfo Perujo
a
a
European Commission, Joint Research Centre (JRC), 21027 Ispra, Italy
b
Department of Energy and Fuels, Mining and Energy Engineering School, Universidad Polit´
ecnica de Madrid, c/ Rios Rosas 21, 28003 Madrid,
Spain
c
University Institute for Automobile Research (INSIA), Universidad Polit´
ecnica de Madrid, Campus Sur UPM, 28031 Madrid, Spain
ARTICLE INFO
Keywords:
Heavy duty vehicles
NOx emissions
SPN
10
Unregulated emissions
N
2
O emissions
ABSTRACT
In 2022, the European Commission introduced the Euro 7 regulation proposal for motor vehicle
type-approval, aiming to elevate environmental protection standards. This study aimed to eval-
uate a Euro VI-C Diesel parallel hybrid city bus within the framework of this proposal. Utilizing
the World-Harmonized heavy-duty Vehicle Cycle (WHVC) across temperatures ranging from
−7◦C to 35◦C, the research sought to ascertain the extent to which Euro VI heavy-duty vehicles
deviate from the proposed limits.
The results showed that the new limits for NOx would be the most challenging, even for hot
start conditions for the vehicle studied. The emission factor for particle number larger than 10 nm
(SPN
10
) is on average more than two times the proposed limit for hot start conditions. Other new
pollutant emissions included in the Euro 7 proposal were much lower than the proposed limits,
except in the case of N
2
O emissions.
1. Introduction
Concern for air quality has been the driving force behind the gradual tightening of vehicle emission regulations worldwide. With
the implementation of the Euro VI standard (EU regulation 595/2009), several changes were incorporated into the regulations for
Heavy-Duty Vehicles (HDVs) and engines, including a more representative cycle, the World-harmonized Heavy-duty Transient Cycle
(WHTC), for engine bench tests, and vehicle testing on-road using Portable Emissions Measurement Systems (PEMS), as well as vehicle
In-Service Conformity (ISC). Since then, the Euro VI standard has gone through stepwise modications to have on-road tests more
representative of the vehicles’ use while improving alignment to the engine test, including most of the pollutants regulated in the
laboratory tests.
The tightening of the European Union (EU) emission standards for HDVs and Light Duty Vehicles (LDVs) has been shown to have
contributed to making new vehicles cleaner (Dimaratos et al., 2019; Samaras et al., 2022; Valverde Morales et al., 2020), reducing
* Corresponding authors.
E-mail addresses: natalia.fonseca@upm.es (N. Fonseca Gonz´
alez), ricardo.suarez-bertoa@ec.europa.eu (R. Suarez-Bertoa).
1
now at European Environment Agency (EEA), 1050 Copenhagen, Denmark.
Contents lists available at ScienceDirect
Transportation Research Part D
journal homepage: www.elsevier.com/locate/trd
https://doi.org/10.1016/j.trd.2024.104125
Received 18 November 2023; Received in revised form 1 February 2024; Accepted 17 February 2024

Transportation Research Part D 129 (2024) 104125
2
pollutant emissions as shown by Mulholland (Mulholland et al., 2021). For example, reductions of more than 70% in NOx emissions
and 90% in particulate emissions have been demonstrated for HDVs Euro VI compared to Euro V vehicles and reductions around 90%
in NOx and PN emissions have been shown for the newest Euro 6d LDVs compared with Euro 5 vehicles (European Commission,
2022b).
However, it has been found that still there is room for improvement for the current testing procedures for Euro 6/VI standards, as
these do not cover some critical operation conditions resulting in an underestimation of actual emissions (Bishop et al., 2013; European
Commission, 2022b; Giechaskiel et al., 2021; Kotz et al., 2016; Ragon and Rodríguez, 2021; Rodríguez et al., 2021; Samaras et al.,
2022). These critical conditions include, among others, cold start and low load operations (Kotz et al., 2016; Posada et al., 2020; Tan
et al., 2019). This is one of the reasons why, in November 2022, the European Commission presented the Euro 7 proposal. The proposal
aims to simplify the regulations while overcoming the limitations found in the current Euro 6/VI standards in order to ensure a high
level of environmental and health protection in the European Union (European Commission, 2022a; Mulholland et al., 2021).
The Euro 7 proposal aims at making road vehicles cleaner under more diverse driving conditions and over an extended operational
lifespan (Samaras et al., 2022). Euro 7 proposes lower emission limits and, like Euro VI, it is technologically neutral because these
limits are equal for all types of vehicles regardless of the type of engine they incorporate (spark ignition or compression ignition
engines) or the fuel used (e.g., Diesel, Compress Natural Gas, hydrogen, etc). In addition, the Euro 7 proposal includes new regulated
emissions for HDVs, namely: SPN
10
, N
2
O and HCHO (European Commission, 2022a; Samaras et al., 2022).
Despite the fact that the test methodology that would be used to verify compliance with the new limits proposed is nowadays still
under discussion, the proposal for HDVs includes two different limits for the engine type approval, at cold and hot start. The same
limits apply for 100th and 90th percentile of moving windows distribution obtained during on-road vehicle tests using PEMS, where
the accumulated work is more than 3×WHTC reference work.
A new “budget” limit for shorter real driving emissions tests, that will apply for all trips shorter than 3×WHTC long has also been
proposed, where the emission factors are calculated as the total emission emitted during the trip divided by three times WHTC work at
engine type approval. This approach is based on the consideration that the worst condition for exhaust emission of certain pollutants,
including NOx, for current technologies often is at cold start and that typical HDV trips contain at least three WHTCs. It should be noted
that it has been proposed that type approval and market surveillance authorities can test the vehicles under both short and/or long
tests.
In the Euro 7 proposal, Real-Driving Emissions (RDE) tests are designed to avoid a xed test composition (regarding urban, rural,
motorway shares), with no specic requirements for distance, time, or hot/cold start. In addition, these tests can be performed at any
“normal” or “extended” driving conditions, with ambient temperatures between −10 and 45◦C and below 1800 m above sea level
(masl). A correction factor, called an “Extended Driving Divider” in the Euro 7 proposal is applied when the driving conditions are
“extended” (between −10 and −7◦C and between 35 and 45◦C and between 1600 and 1800 masl). Moreover, in cases where more than
one “extended” condition occurs simultaneously, the emissions during those events will be excluded from the nal emissions
calculation.
Hybrid vehicles have been promoted over conventional ones, because electrication (including hybridization) is considered to
contribute to meeting the goals of the Paris Agreement and CO
2
EU-emission standards (European Commission, 2023), allowing energy
recovery during braking, optimization of the thermal engine operation, and increases in the energy efciency of the vehicle and
therefore, reducing fuel consumption and potentially tailpipe emissions (Benajes et al., 2019; Wijeyakulasuriya et al., 2022). Some
studies, such as those conducted by Benajes et al.(Benajes et al., 2019), indicate that hybrid technology not only has the potential to
reduce total fuel consumption and local air pollution but also contributes to decreasing total CO
2
emissions. Through simulations of a
LDV with the two different powertrain congurations, they demonstrated that Parallel Full Hybrid Electric Vehicles (HEV) can achieve
a 35% reduction in CO
2
emissions compared to conventional diesel vehicles, while Mild HEVs can promote a more modest reduction of
up to 16%. Taymaz and Benli (Taymaz and Benli, 2014), through road simulations, demonstrated a theoretical potential for CO
2
savings and fuel consumption of up to 30% in parallel hybrid light-duty commercial vehicles, for both diesel and gasoline options, in
comparison to conventional vehicles, respectively. Additionally, contemporary combustion technologies, such as Reactivity Controlled
Compression Ignition (RCCI), implemented in a medium-duty series hybrid truck using conventional diesel and gasoline, have
demonstrated, through experimentation and simulation, a reduction of more than 10% in CO
2
Tank-to-Wheel emissions in addition to
being able to meet current Euro VI regulations without an after-treatment system (García et al., 2022). However, the electrication of
HDVs has lagged behind that of LDVs due to high costs and the difculty of conguring them given the wide variety of uses and
operating conditions that exist for this category. Therefore, currently, only in specic applications, such as urban buses or urban-refuse
collecting trucks, have hybrid electric vehicle become more widespread (Kozak et al., 2022; Witt, 2010).
Making HDVs compliant with the Euro 7 proposal implies that manufacturers may have to develop and implement new engine
control techniques and innovative aftertreatment systems (Boger et al., 2022; Szpica, 2023; Ximinis et al., 2022a,b), such as those
presented by Ragon & Rodríguez (Ragon and Rodríguez, 2021), that allow emissions to be reduced in conditions that until now have
been not covered by the Euro VI regulation (Bishop et al., 2013; Ragon and Rodríguez, 2021; Samaras et al., 2022). Therefore,
evaluating the critical operating conditions where high emissions occur in currently Euro VI HDVs and the current level of the so far
unregulated emissions, will shed light on areas that may need to be further developed.
Recently, some research works have shown advances in engine and catalyst technologies that provide emission reductions,
especially at cold start conditions (Reiter and Kockelman, 2016). On the one hand, new engine control techniques are focused mainly
on reducing engine-out NOx emissions and rapidly increasing exhaust gas temperatures to levels where the aftertreatment is effective
(Ragon and Rodríguez, 2021), which can also reduce vehicle energy efciency (Ximinis et al., 2022a). Maci´
an et al. worked on a Dual-
Mode Dual-Fuel (DMDF) combustion strategy that is a Low Temperature Combustion (LTC) concept based on Reactivity Controlled
N. Fonseca Gonz´
alez et al.

Transportation Research Part D 129 (2024) 104125
3
Compression Ignition (RCCI) using blends of poly-oxymethylene dimethyl ether (OMEx) with gasoline for heavy duty applications
(Maci´
an et al., 2021). Cylinder de-activation and burner technologies that heat the exhaust gas upstream of the Aftertreatment Systems
(ATS) have been studied by Harris et al. (Harris et al., 2021). They found that among those technologies, the most fuel-efcient option
is the mini-burner technology. Pre-chamber combustion system for a dual-fuel heavy duty engine studied by Shin et al. (Shin et al.,
2022) demonstrated a reduction of NOx emissions by 50% in a Diesel mode.
Other engine-based techniques, as presented by the Italian National Research Council CNR-STEMS (Guido et al., 2022; Napolitano
et al., 2023), focused on Solid Particle Number (SPN) reduction. In this case, the research was for Compressed Natural Gas (CNG)
powertrains for HDVs, showing that improving piston ring pack design, lubricant oil quality, and CNG particle lters can be powerful
tools in sub-23 particle abatement, which would make it possible for this technology to comply with the PN limits of the Euro 7
proposal.
On the other hand, novel ATS are focused on development of alternative materials and/or changing ATS architectures, in order to
extend the effective operating temperature range of the catalytic conversion, and to specially reduce the light-off temperature and
time. Ballinger et al. (Ballinger et al., 2009) demonstrated that advanced Cu-zeolite Selective Catalytic Reduction (SCR) catalysts
coated on particle lters provided >90% NOx conversion over a wide operation temperature window, allowing NOx conversion even
during Diesel Particulate Filter (DPF) regeneration. Resitoglu et al. (Resitoglu et al., 2020) studied the effects of Fe
2
O
3
based Diesel
Oxidation Catalysts (DOCs) and SCRs, obtaining reductions of over 80% of NOx emissions. Mendoza et al. (Mendoza Villafuerte et al.,
2021) investigated the performance of a novel exhaust aftertreatment conguration, incorporating a close-coupled Diesel Oxidation
Catalyst lter (ccDOC) with a modied catalyst arrangement for an N3-category HDV. Their study demonstrates the ability to maintain
NOx emissions consistently at very low levels across a diverse range of driving conditions. However, among all new technologies, the
one that seems to be the most cost-effective is electrically preheated catalysts (Gao et al., 2019; Reiter and Kockelman, 2016). Ximinis
et al. presented a concept that uses a Thermoelectric Aftertreatment Heater (TATH) fed by an Automotive Thermoelectric Generator
(ATEG) that allows the recovery of waste heat from exhaust to convert to electricity to be used to preheat the catalyst (Ximinis et al.,
2022b). Mendoza et al. (Mendoza Villafuerte et al., 2022) introduced an emissions control system that integrates a ccDOC with an
Electrically Heated Catalyst (EHC). Their research reveals that this aftertreatment conguration enables ultra-low NOx emissions and
effectively manages secondary emissions, even under demanding conditions, with a minimal impact on CO
2
emissions.
Some studies have carried out assessments of Euro 7, but, as shown latter, very few have assessed the behavior of current vehicles
from the Euro 7 perspective and even less, if any, for hybrid HDVs. Some studies have investigated the best available technologies in
order to comply with the Euro 7 standard. Among these studies, most include experimental tests with ultra-low emissions ATS pro-
totypes for HDVs and LDVs (Mendoza Villafuerte et al., 2022; Selleri et al., 2022a; Ximinis et al., 2022a,b; Mendoza Villafuerte et al.,
2021), and others use model simulations (Samaras et al., 2022; Wijeyakulasuriya et al., 2022). Selleri et al. tested an advanced
demonstrator prototype vehicle developed to meet post-Euro VI standards that was equipped with a ccDOC +ccSCR/ammonia slip
catalyst (ASC) in addition to the Euro VI standard ATS (Selleri et al., 2022a). Napolitano and Guido’s team worked on particular lters
for HD natural gas engines to meet post-Euro VI standards (Guido et al., 2022; Napolitano et al., 2023). Samaras et al. use simulation
models to see the behavior of different ATS congurations, revealing that larger exhaust aftertreatment devices, together with opti-
mized thermal management can achieve very low emission levels (Samaras et al., 2022). Simulations for health impact assessment
have been done by Mulholland et al. (Mulholland et al., 2022). Gunja et al. made a study based on engine/ATS/vehicle simulations in
order to analyze the extent to which new On-Board Monitoring system (OBM) could inuence the function of other control systems
(Gunja et al., 2022). Ragon and Rodriguez developed cost estimates for diesel emissions control technology in order to comply with
forthcoming Euro VII standards (Ragon and Rodríguez, 2021).
Other research studies dealing with emissions under critical conditions for Euro 6/VI vehicles have been carried out, such as a study
made by Samaras et al. (Samaras et al., 2022). In this study, more than 70 LDVs were analyzed, tested by CLOVE (Consortium for ultra
Low Vehicle Emissions) partners. They found that cold starts, low ambient temperatures, and high engine power events/periods are
critical conditions where some vehicles exhibit very high emission factors. Giechaskiel et al., found similar results in a Euro VI step E
HDV (Giechaskiel et al., 2022b) and with a Euro 6d-TEMP Gasoline Direct Injection (GDI) vehicle (Giechaskiel et al., 2021). Suarez-
Bertoa et al. studied formaldehyde emissions for a Euro VI-E HDV and some Euro 6d LDVs, nding non-negligible emission factors for
gasoline and diesel vehicles (Suarez-Bertoa et al., 2022).
In consideration of these factors, the aim of this paper is to evaluate pollutant emissions of a Euro VI step C parallel hybrid Diesel
HD city bus from a perspective of meeting the proposed Euro 7 regulations. In particular, one of the main focuses was to analyze how
distant this technology and emission stage are from the new proposed limits and to nd the critical operation conditions for pollutant
emissions using cold and hot WHVC tests at temperatures from −7 to 35◦C. This study addresses three fundamental questions:
•What are the CO, THC, NOx and SPN
23
emissions of the vehicle tested, from the Euro VI point of view at temperatures from −7 to
35◦C?
•What are the current NH
3
, CO, NMOG, NOx, HCHO and SPN
10
emissions of the vehicle tested, from the proposed HDV-Euro 7
perspective at temperatures from −7 to 35◦C?
•What are the key operational conditions inuencing pollutant emissions in the studied parallel hybrid Diesel HDV?
Since proposed Euro 7 for HDV includes: changes in methodology, new emission limits and new regulated pollutants, and, as shown
above, there are very few published studies about how far current HDVs (even fewer if any for hybrid HDV) are from meeting the
proposal. This experimental study sheds light on the challenges to be overcome to comply with the forthcoming regulation, as the main
novelty of this paper.
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The paper is organized as follows. Section 2 describes the experimental campaign, the instrumentation used, the data processing
and calculations. Section 3 presents the ndings and discussion of the current research, while Section 4 offers key conclusions and
outlines directions for future work.
2. Methods
2.1. Test vehicle
The tested vehicle was a full-parallel type hybrid Euro VI step C M3 Class II equipped with a turbocharged and intercooled in-line
Diesel engine with overhead valves and electronically-controlled direct injection, and an emissions control and aftertreatment system
that included an EGR (Exhaust Gas Recirculation) +DOC +DPF +SCR/ASC. This vehicle is equipped with a high-power electric motor
(120 kW) and 600 V lithium-ion batteries (19 Ah) that let the vehicle operate in pure electric mode at low speed conditions, as shown in
Figs. 6, 10, 12, 15 and 17, where the engine coolant temperature is shown as zero when the thermal engine is off. The primary technical
specications of the vehicle and the engine are detailed in Table 1 and Table 2, respectively.
2.2. Test campaign
The vehicle was tested in a chassis dynamometer laboratory following the World-Harmonized Vehicle Cycle (WHVC), which is the
one developed with the same speed prole used for the development of the World Harmonized Transient Cycle (WHTC) dened by the
Global Technical Regulation (GTR) No. 4 used for heavy duty engines and vehicles emission type approval procedure. The vehicle was
tested at temperatures between −7 to 35◦C, as is shown in Table 3, at VELA7 heavy duty test cell of the European Commission Joint
Research Centre in Ispra, Italy, described by Selleri et al. (Selleri et al., 2022a). VELA7 is a climatic test chamber designed to operate
within a temperature range of −30◦C to 50◦C, featuring a 4-wheel-drive (4WD) 2-axis rolling dynamometer The vehicle was tested in
4WD mode. Given the substantial differences obtained for the NOx emissions during the two hot tests performed at −7◦C, the results of
these two tests have been presented and discussed individually in the following sections. The tests are labeled as −7◦C (test a) and
−7◦C (test b) or in the text for simplicity as −7◦C (a) and −7◦C (b).
All WHVC tests were done on the roller test bench using a reference mass (w
ref
) equal to 16,020 kg, that correspond to 57% of
payload. This payload was selected to have a total engine work around the reference work (at type approval WHTC) of the engine,
showed in Table 2.
M3 Class II vehicles are vehicles conditioned for interurban transport. Hence, it allows standing passengers in some circumstances
and therefore it incorporates a speed limiter. This is why the speed of the tested vehicle was limited at 70 km/h, as shown in Fig. 1.
Thus, WHVC refers to the cycle with a maximum speed of 70 km/h in this paper.
The roller bench was congured based on the road coefcients F
0
=891.802 N, F
1
=0 N/m/s and F
2
=2.50124 N/(m/s)
2
with a
mass of 16,020 kg (57% payload).
2.3. Instrumentation
VELA7 is equipped with different analyzers and instruments used to determine emissions factors, as shown in Table 4 and Table 5.
Details of the analyzers and the scheme of the measurement setup used are presented by Selleri et al. (Selleri et al., 2022a).
A diluted analyzer, AVL AMA i60, is the reference analyzer. Likewise, the FTIR (Fourier Transform Infrared) spectrometer AVL
SESAM was consider as the reference analyzer for NH
3
and N
2
O. The Advanced Particle Counter (APC) was furnished with a 350◦C
evaporation tube to eliminate volatile particles in accordance with regulatory requirements. Subsequently, the 10 nm Condensation
Particle Counters (CPC) was connected to the APC outlet for the measurement of particles exceeding 10 nm in size. The APC had no
catalytic stripper, thus volatile artifacts were possible (Giechaskiel et al., 2020). For that reason, the signals were carefully checked to
ensure that no volatile artifacts took place.
Table 1
Technical specications of the vehicle.
Parameter Value
Vehicle category M3 class II
Model year 2018
Technically permissible mass (full load) (kg) 19,000
Empty mass (kg) 12,085
Length (m) 12.6
Height (m) 3.3
Maximum vehicle speed (km/h) 70
Gearbox Automatic
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Table 2
Powertrain technical specications.
Engine Parameter Value
Emission standard Euro VI step C
Engine injection system Common rail direct fuel injection
Number of cylinders 4 – line
Total displacement (cm
3
) 5,132
Fuel type Diesel
Compression ratio 17.3:1
Maximum power (kW@rpm) 177@2,200
Engine peak torque (Nm@rpm) 918@1,200 to 1,600
Idle speed (rpm) 730
Work reference (at type approval WHTC) (kWh) 16.09
CO
2
reference (at type approval WHTC) (g) 11,649
Maximum electrical motor output (kW) 120
Torque maximum (ISO 1585) (Nm) 800
Continuous electrical motor output (kW) 70
Continuous torque (ISO 1585) (Nm) 400
Energy storage system type Lithium-ion battery with cooling
Battery size (Ah) 19
Battery voltage (V) 600
Battery capacity (kWh) 11.4
Table 3
Tests performed on chassis dyno during test campaign.
Test-Cycle # Tests Payload (%) Temperature (◦C)
WHVC (Cold +Hot) 2 +2 57 35
WHVC (Cold +Hot) 2 +2 57 22
WHVC (Cold +Hot) 1 +1 57 0
WHVC (Cold +Hot) 2 +2 57 −7
Fig. 1. Speed prole during WHVC test.
Table 4
List of analyzers used during the test campaigns. More details at (Selleri et al., 2022a).
Analyzer NOx NO NO
2
CO CO
2
THC CH
4
NH
3
PM SPN
23
SPN
10
N
2
O HCHO O
2
Diluted analyzer AVL AMA i60 D D D D D
Tailpipe analyzer AVL AMA i60 R Rd Rd R R Rd
FTIR spectrometer AVL SESAM R R R R R R R R
Tracer CO
2
D
HORIBA OBS-ONE-XL (IRLAM) R R
AVL particle counter 23 nm APC 489 D
TSI CPC 3010 at APC D
Measured in wet conditions in diluted (D) and raw (R), and raw at dry conditions (Rd).
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2.4. Data processing
2.4.1. Synchronization of signals
As the measurements were conducted using various devices with distinct initial recording times and sampling rates, a standard-
ization process to achieve a uniform 1 Hz time-base was implemented. All signals were synchronized using a correlation function and
aligned with the WHVC schedule from the automation system, which had time aligned signals. The synchronization order used was:
1. Dyno signals, using dyno vehicle speed and WHVC speed prole, because the last was considered as the reference.
2. Engine Control Unit (ECU) signals, using ECU vehicle speed and synchronized dyno speed.
3. CVS exhaust ow meter signal, using synchronized engine positive power (calculated using ECU data).
4. FTIR spectrometer signals, using the CO
2
concentration from FTIR and synchronized CVS exhaust ow. All FTIR signals had the
same response time and delay, because its measuring principle guarantees the synchronization of all FTIR measured signals.
5. Diluted and tailpipe (raw) analyzers AVL AMA i60 CO, CO
2
, NOx, NO, NO
2
, CH
4
, and Total Hydrocarbon (THC) signals, using
diluted or raw concentrations and synchronized FTIR concentrations. Each diluted and raw signal was synchronized independently
since each diluted and raw analyzer has its own response time and delay.
6. Tracer CO
2
signal, using synchronized CO
2
FTIR concentrations.
7. SPN
23
and SPN
10
signals, using synchronized ECU fuel consumption.
8. N
2
O and NH
3
signals from HORIBA OBS-ONE-XL (IRLAM), using synchronized N
2
O and NH
3
FTIR concentrations, respectively.
9. Test cell ambient signals, using cell temperature and synchronized ECU ambient temperatures.
2.4.2. Exhaust ow and mass emission calculations
Dry/wet correction was done for CO, CO
2
and O
2
concentrations measured at the tailpipe, following the methodology set forth in
UNECE Regulation 49 which is based on carbon balance and ideal combustion (Giechaskiel et al., 2019). Exhaust ow (ExhFM) was
calculated using the CVS total ow (CVS) and dilution air ow (AFM) signals as shown in Eq. (1); and through CO
2
tracer method
(Wiers and Schefer, 1972) as shown in Eq. (2), where the synchronization of CO
2
diluted wet concentration (
χ
CO2_Dil
) and CO
2
raw wet
concentration at tailpipe (
χ
CO2_raw
) is crucial. A comparison between these two methodologies to calculate exhaust ow is shown in
Fig. 2, where it can be observed that, in general, the behavior of the two signals is similar, although the exhaust ow calculated through
CO
2
tracer method shows articial spikes where the dynamicity of the engine was high due to fuel cut-offs.
ExhFMm3
min=CVSm3
min−AFMm3
min(1)
ExhFMm3
min=
χ
CO2−dill ⋅CVSm3
min−
χ
CO2−background ⋅AFMm3
min
χ
CO2−raw
(2)
Additionally, the raw concentrations at tailpipe (
χ
Raw_calcDil
) of: CO, CO
2
, NOx, NO, and NO
2
, were calculated from diluted AVL AMA
i60 measured signals (
χ
Dil
), through a mass conservation equation for the CVS system shown in Eq. (3), taking into account the
Table 5
Other measurement instruments employed throughout the testing campaigns.
Measurement device Variables measured by
JRC automation system (M.A.R.T.A.) OBD signals: speed (km/h), n (rpm), actual and friction torque (%)
Constant Volume Sample (CVS) owmeter Exhaust ow (m
3
⋅min
−1
), total ow rate (m
3
⋅min
−1
)
Weather station Ambient pressure (kPa), humidity (%) and temperature (◦C)
Fig. 2. Exhaust ow calculated. Comparison between ExhFM (green line) and CO
2
tracer (blue line) based methodology. WHVC test at 22◦C.
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Transportation Research Part D 129 (2024) 104125
7
background concentration (
χ
background
) and assuming constant pressure and temperatures through out the CVS system.
χ
Raw calcDil ⋅ExhFMm3
min+
χ
background ⋅AFMm3
min=
χ
Dil ⋅CVSm3
min(3)
Instantaneous mass emissions, using both CVS exhaust ow and tracer exhaust ow were calculated for the emissions measured at
the tailpipe and diluted at the CVS, as shown in Fig. 3 for CO
2
emissions.
Total mass emissions for each species (Tm
i
in g) were calculated by numeric integration of the calculated instantaneous mass
emissions (m
i
), as shown in Eq. (4); and using an average diluted concentration (
χ
i Dil), average CVS total ow (CVS), average dilution
air (AFM) and the density of each species (
ρ
i
), as shown in Eq. (5), where t is equal to 1800 s, namely, the length of WHVC test. Total
particle number for SPN
10
and SPN
23
were calculated similarly to Eqs. (4) and (5), but without density and using an appropriate unit
conversion factor as particle number is measured in #/cm
3
.
Tmi raw =mi raw =
χ
Raw[ppm]⋅1
106ppm⋅ExhFMm3
s⋅
ρ
ig
m3(4)
Tmi raw =t[s]⋅
ρ
ig
m3⋅1
106ppm⋅(
χ
iDil)[ppm]⋅CVSm3
s−
χ
i background[ppm]⋅AFMm3
s(5)
Relative air/fuel ratio, lambda (λ) was calculated stoichiometrically through wet O
2
and CO
2
concentrations at the tailpipe (
χ
O2 and
χ
CO2 respectively), as shown in Eq. (6), when the elementary fuel composition (C
a
H
b
O
c
) is known. The elementary fuel composition
used in this work is the one dened at UNECE Regulation 49 for Diesel B7, that is: CH
1.86
O
0.006
. It should be noted that due to the low
CO and THC emissions of Diesel vehicles, their contribution, after oxidation on the DOC, to the CO
2
emissions is negligible.
λ=


a
a+b
4−c
2
⋅
χ
O2
χ
CO2


+1(6)
2.4.3. Power, and engine work calculation
Effective engine power (PICE in kW) was calculated using engine speed (n in rpm), actual and friction torque (
τ
a and
τ
frict respec-
tively in percentage) read from Engine Control Unit (ECU), and engine maximum torque (Tref in Nm) as shown in Eq. (7).
PICE =2 ⋅
π
⋅n
60 ⋅
τ
a−
τ
frict
100 ⋅Tref
1000 (7)
Total engine work (W
ICE
in kWh) for each WHVC test was calculated as the numerical integral of the effective engine power signal
(PICE) through the cycle, considering only positive values.
2.4.4. Emissions factors calculations
For every test, the emission factors for each pollutant (i) were calculated by dividing the total mass emitted of a given gaseous
pollutant (Tm
i
) by the total work performed during the same test (W
ICE
), as shown in Eq. (8).
EFi actual =Tmi
WICE
(8)
However, in order to assess the current emissions status of the tested vehicle, from the point of view of Euro VI standard, emission
factors (EFi EuroVI) were calculated similarly the denitions in used UNECE Regulation 49, as shown in Eq. (9). Notice that UNR49 uses
weighted total mass emissions measured at hot and cold WHTC tests only at 22◦C, but in this case, the same equation was used to
Fig. 3. CO
2
instantaneous mass emissions. Comparison between all procedures.
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Transportation Research Part D 129 (2024) 104125
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calculate Euro VI emissions factors at the other temperatures, using at each temperature the actual values of total mass emissions (Tm
i
)
and engine work (W
ICE
).
EFi EuroVI =0.14 ⋅ Tmicold WHVC +0.86 ⋅ Tmihot WHVC 
0.14 ⋅ WICEcold WHVC +0.86 ⋅ WICEhot WHVC (9)
To evaluate how far off the vehicle tested is from the proposed Euro 7 emissions limits (European Commission, 2022a), emission
factors were calculated as follows:
•To compare, together with the Euro 7 “Cold” and “Hot” engine emissions limits, emission factors were calculated at WHVC cold/hot
(respectively) tests using Eq. (8), as if it was an engine test.
•To compare with the Euro 7 “budget” emissions limits, emission factors were calculated as presented in Eq. (10). As said, this
methodology is used when the actual work accumulated during an on-road test is lower than three times the reference work
measured at WHTC test (3×WHTC). In this methodology the emission factors should be calculated as the total mass measured (Tmi)
divided always by three times the reference work (3 ⋅WREF), regardless the actual work performed, as presented by the European
Commission at the Advisory Group on Vehicle Emission Standards (AGVES) on 23th February of 2023 (Thedinga et al.,2023).
Hence, six hypothetical cases can be constructed using individuals WHVC test results (cold and hot start) as a building blocks.
EFi Euro7budget =Tmi
3 ⋅ WREF


whenWICE≤3⋅WREF
(10)
In order to make the analysis easier, an Emission Ratio (ERi) has been used, calculated as shown in Eq. (11). This emission ratio is used
to compare the emission factors calculated (EFi actual,EFi EuroVI ) with the emissions limit for each pollutant (Limi) that corresponds to the
emission standard being evaluated in each case (i.e., Cold and Hot). Table 6 presents the emission limits of Euro VI and proposal Euro 7
respectively.
ERi=EFi
Limi
(11)
Note that the NH
3
limit for Euro VI, see Table 6, is expressed in parts per million (ppm), determined as the average raw con-
centration during both World Harmonized Transient Cycle (WHTC) and World Harmonized Steady Cycle (WHSC) for compression
ignition (CI) engines, and during WHTC for positive ignition (PI) engines. Thus, the Emission Ratio (ER) Euro VI of NH
3
was calculated
by dividing the weighted, cold (14%) and hot (86%), average concentration of NH
3
in ppm at the tailpipe during the test by the Euro VI
limit. Similarly, ER of NH
3
at WHVC cold and hot tests were calculated by dividing the average concentration of NH
3
at tailpipe during
the test by the Euro VI limit. The instruments’ intra-test comparison was performed using as reference the NOx, CO and THC emission
factors calculated using the average concentration measured from the diluted exhaust and the CVS ow. Similar to the measurements
performed using bags. In the case of N
2
O and NH
3
, the FTIR, which measured at the tailpipe, was used as reference instrument.
3. Results and discussion
To fulll the objectives of this investigation, this section is structured into four parts: (a) emission assessment according to the Euro
VI standard, to nd out the current emissions status of the vehicle according to the in force emissions regulations, in section 3.1; (b)
emission assessment according to Euro 7 proposal for Hot and Cold proposed limits analyzed in section 3.2, (c) analysis of intra-test
emission factor variability due to instrumentation used, presented in section 3.3, and nally (d) the analysis of the critical operation
conditions for Euro 7 compliance, presented in section 3.4.
3.1. Emission assessment according to the Euro VI standard
Since the WHVC tests have a similar total workload as the WHTC reference work, WHVC tests have been considered as engine tests
to be compared with the Euro VI emission limits. It provides an understanding of the state of the vehicle emissions, at the time that it
was tested, before analyzing it from the point of view of the Euro 7 proposal. It is important to observe that the Euro VI Emission Ratios
are calculated using the weighted EF Euro VI, as shown in Eq. (9).
In Fig. 4, it is shown that, at 22◦C, Euro VI emissions factors (calculated as Regulation UNECE 49 – Eq. (9) are below Euro VI
Table 6
EU emissions limits (mg/kWh) for heavy duty engines and vehicles.
Emissiom Standard NOx CO THC CH
4
NH
3
NMOG PM SPN
23
* SPN
10
* N
2
O HCHO
Euro VI emissions (Diesel) 460 4000 160 – 10** 10 6.0e+11 – – –
Euro 7 proposal – Cold emissions 350 3500 – 500 65 200 12 – 5.0e+11 160 30
Euro 7 proposal – Hot emissions 90 200 350 65 50 8 2.0e+11 100 30
*SPN
23
and SPN
10
limits are in #/kWh.
**NH
3
limit at Euro VI emission standard is in ppm.
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Transportation Research Part D 129 (2024) 104125
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emission limits for CO, THC and NH
3
emissions, being almost negligible even for cold start tests. NOx and SPN
23
emission factors are
closer to their corresponding limit (ER: 0.69 and ER: 0.86 for NOx and SPN
23
, respectively), but are still below the limits. Although
expected, it is important to notice that the contribution of the cold start test is high, resulting in emissions of more than three times the
Euro VI limit for NOx. Differences between hot and cold start for SPN
23
emissions are small, with higher emissions for the cold start
test. The high inter-test SPN
23
difference between the two cold WHVC cycles, showed in Fig. 4, may be attributed to the different DPF
ll state, as was given by the ECU.
Analyzing WHVC tests at temperatures from −7ºC to 35ºC (no gure shown), showed that CO emissions increase as the ambient
temperature decreases. Nevertheless, the Euro VI Emission Ratios do not exceed 2% of the limit. The maximum CO Emission Ratio was
12% for the cold start test performed at −7◦C. THC emissions remain almost constant with the temperature, not showing large var-
iations between hot and cold start, although cold start emissions seem to be a slightly higher. The maximum THC Emission Ratio was
less than 20% of the Euro VI limit, measured for the cold start test performed at 22◦C. For its part, NH
3
emissions, in ppm, do not
exceeded 5% of the emission limit (10 ppm) at any of the temperatures tested.
Fig. 5 illustrates the Euro VI NOx Emission Ratios for temperatures ranging from −7 to 35◦C. NOx emissions remain below the
emission limit at high temperatures, but at low temperatures the emission factors exceed it. These results agree with those presented by
Giechaskiel et al., for a Euro VI step E HDV (Giechaskiel et al., 2022b). In addition, similar results with LDVs at low temperatures have
been presented by Suarez-Bertoa and Astorga (Suarez-Bertoa and Astorga, 2018), Samaras et al. (Samaras et al., 2022) and Selleri et al.
(Selleri et al., 2022b). At 0◦C and −7◦C (a), NOx weighted emissions exceed the Euro VI emission limit by around 40% and at −7◦C (b)
they rise even more, reaching almost 6 times the emission limit. It should be noticed that at −7◦C (b) test, besides the high emissions
during cold start, there is also a signicant contribution from the hot start test.
In Fig. 6, it is possible to see that even though engine temperatures are similar, comparing −7◦C (test b) with −7◦C (test a) hot start
tests, at −7◦C (test b) SCR NOx emissions were higher than at −7◦C (test a) for approximately the rst 1400 s, indicating that SCR was
not effectively controlling NOx emissions. The reasons for this difference are not clear, and due to the limited information available
from the ECU, it was not feasible to further investigate them. In any case, it is noteworthy in Fig. 6 that in these two tests, the thermal
engine behavior is similar, except around 300 s, a period in which NOx emissions were practically negligible. Therefore, the high NOx
Fig. 4. Euro VI related Emission Ratios at WHVC tests performed at 22
◦C. Error bars represent the inter-test variability (min–max of two tests or of
two pairs cold/hot tests).
Fig. 5. Emission Ratio (ER) relative to Euro VI NOx emission limit at WHVC tests performed at temperatures from −7 to 35
◦C. Error bars represent
the inter-test variability.
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Transportation Research Part D 129 (2024) 104125
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emissions observed for test (b) performed at −7◦C are not attributable to the vehicle’s hybridization.
Fig. 7 plots SPN
23
emissions for the WHVC performed at different temperatures. Weighted SPN
23
emissions were below the Euro VI
WHTC limit at all examined temperatures, but were in some cases very close to them (at 35◦C and at −7◦C (a)). There was no clear
trend with testing temperature. SPN
23
emissions for the cold WHVC were higher than those for the hot WHVC at 22◦C, −7◦C (a), and
−7◦C (b) but lower at 35◦C and 0◦C. It is noteworthy the high emissions of SPN
23
from the WHTC cold start test at −7◦C (a). Possible
reasons are the different DPF ll state (the day before different pre-conditioning cycles were followed) and the different engine out
emissions due to the lower ambient temperature in one of the test (-9◦C vs. −7◦C for 7◦C (a) and −7◦C (b), respectively).
3.2. Pollutant emission assessment according to proposed Euro 7
As it was discussed above, WHVC tests have been considered equivalent to WHTC tests. Fig. 8 depicts, for tests at 22◦C, the Emission
Ratios calculated in relation to the Euro 7 limits for cold and hot WHTC respectively. All the emission factors for CO, NMOG, CH
4
, NH
3
and HCHO are well below 15% of the corresponding proposed Euro 7 limits for both cold and hot start tests. Notice the very low NMHC
and HCHO emission factors (mg/kWh) under most conditions which can be attributed in part to the low oxygen content of B7 Diesel
fuel. NMOG have been assumed to be equal to NMHC. N
2
O emission factors are around the 50% of the corresponding limit, being
higher for the hot start tests, where the high inter-test variability comes near to the emission limit proposed. NOx and SPN
10
emissions,
on the other hand, go beyond the respective limits, especially for the cold start for NOx, and hot start for SPN
10
, by factors around 4 and
3.5 times higher than the proposed Euro 7 limit respectively. It should be emphasized that, as shown in Tables 6 and 7, different limits
apply for cold and hot engine start tests.
Furthermore, as shown in Table 7, from −7 to 35◦C, a temperature range dened as normal conditions, the emission factors for CO,
NMOG, CH
4
, NH
3
and HCHO remain well below the proposed Euro 7 limits. In addition, total HCHO emissions, which are generally
very low for diesel engines (Suarez-Bertoa et al., 2022), are of some relevance for cold starts, especially at low temperatures, reaching
an emission factor equal to 11.8% of the proposed Euro 7 limit (~4 mg/kWh) at the −7◦C cold start WHVC test. Regarding N
2
O
emissions, it can be seen in Table 7, that they exceed the Euro 7 emission limits for hot start conditions at high temperature (35◦C), but
Fig. 6. NOx mass emissions at hot start WHVC tests. Comparison between tests performed at −7⁰C. The engine coolant temperature corresponding
to each test is shown in dotted lines and it is shown as zero when the thermal engine is off.
Fig. 7. SPN
23
Euro VI related Emission Ratios (ER) at WHVC tests performed at temperatures ranging from −7 to 35◦C. Error bars represent the
inter-test variability.
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Transportation Research Part D 129 (2024) 104125
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are below the limit for the other conditions. N
2
O formation has been linked to different reaction pathways taking place on the
aftertreatment systems, including the DOC, the SCR and/or the ASC (Mendoza Villafuerte et al., 2022).
NOx emissions exceed the proposed limits for all conditions except for the 35◦C hot start tests, as shown in Fig. 9. At −7◦C (test b)
for hot start, as discussed in section 3.1, NOx emissions increase excessively during the rst 1400 s showing an efciency of the SCR
that is lower than expected. In addition, it is important to notice than even though the emission ratios at −7◦C (test a) for cold and hot
start tests are quite similar, being more than 4 times the Euro 7 proposal limits, the emission factor for the cold start is much higher
(1677 mg/kWh) than that for the hot start test (380 mg/kWh), as can be seen in the NOx instantaneous mass emission graph shown in
Fig. 10.
Fig. 8. Proposed Euro 7 related Emission Ratios at WHVC tests performed at 22
◦C. Error bars represent the inter-test variability. Note that different
limit applies for cold and hot start tests (see Table 6 and Table 7).
Table 7
Euro 7 proposal related Emission Ratios at WHVC tests performed at temperatures from −7 to 35 ◦C. Emission Ratio equal to 100% correspond to an
emission factor equal to the corresponding Euro 7 limit.
Pollutant WHVC test Euro 7 limits (mg/kWh) 35◦C 22◦C 0◦C −5◦C −7◦C
CO Cold start 3500 5.1 % 6.6 % 7.7 % 11.1 % 13.5 %
Hot start 200 15.8 % 12.0 % 0.7 % 1.8 % 3.1 %
N
2
O Cold start 160 39.0 % 36.3 % 21.5 % 38.2 % 36.3 %
Hot start 100 109.8 % 68.5 % 47.5 % 55.6 % 22.9 %
NMOG Cold start 200 1.6 % 8.2 % 1.3 % 1.0 % 1.2 %
Hot start 50 0.8 % 1.7 % 0.6 % 0.0 % 0.8 %
CH
4
Cold start 500 0.2 % 0.2 % 0.2 % 0.2 % 0.2 %
Hot start 350 0.2 % 0.3 % 0.2 % 0.2 % 0.4 %
NH
3
Cold start 65 1.4 % 1.7 % 0.9 % 1.8 % 1.2 %
Hot start 65 1.3 % 1.1 % 1.1 % 1.0 % 0.9 %
HCHO Cold start 30 2.7 % 8.4 % 6.8 % 7.7 % 11.8 %
Hot start 30 2.0 % 4.6 % 3.2 % 3.7 % 8.8 %
Fig. 9. NOx Euro 7 proposal related Emission Ratios (ER) at WHVC tests performed at temperatures from −7 to 35
◦C. Error bars represent the inter-
test variability.
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Transportation Research Part D 129 (2024) 104125
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In Fig. 10, during the cold start test, it is observed that the thermal engine operates continuously, unlike the hot start test, which
exhibits several periods of operation in pure electric mode. Notably, regarding NOx emissions, the shutdown of the thermal engine
during pure electric operation does not negatively affect these emissions, most likely due to the fact that the engine coolant tem-
perature does not decrease during the periods when the engine is off. The gure also shows that under normal conditions with the SCR
system working, very high NOx emissions occur only at the beginning of the cold start test while the aftertreatment system warms up (i.
e., below the operative SCR light-off temperature). At cold start at −7◦C (a) it takes more than 500 s before emissions similar to those
emitted at hot start test are seen. That is why for the cold start the acceleration of the catalyst heat-up may be essential to meet the NOx
Euro 7 limits (Boger et al., 2022). Aftertreatment systems such as exhaust gas heaters (Ximinis et al., 2022a), electrically heated
catalyst (EHC) (Mendoza Villafuerte et al., 2022) and Thermoelectric Aftertreatment Heater (TATH), have been shown to reduce NOx
emissions for the cold start below proposed Euro 7 limits. Nonetheless, those studies focussed only on NOx emissions.
It is clear that improvements in the aftertreatment system are required, not only for cold start conditions, but also for hot con-
ditions, especially at low ambient temperatures, potentially necessitating the adoption of new aftertreatment system architectures such
as those presented by Selleri et al. (Selleri et al., 2022a), Boger et al. (Boger et al., 2022) and tested by Mendoza Villafuerte et al.
(Mendoza Villafuerte et al., 2022). Furthermore, behaviours such as the ones observed during the −7◦C (b) test are what is expected to
be avoided with the Euro 7 proposal, by guaranteeing that the aftertreatment system operates even when a vehicle operates over
extended driving conditions and ambient temperature ranges (i.e., from −10 to −7◦C and from 35 to 45◦C), unless a technical
justication is sanctioned by the type approval authority.
SPN
10
Emission Ratios, unlike NOx emissions, have their most challenging condition at hot start operation. Even more, at high
ambient temperatures with emission factors between 3 and 4 times the Euro 7 Hot limit, as shown in Fig. 11. Under cold start con-
ditions, the behavior does not seem to have a clear trend for this vehicle, although the Emission Ratios are generally lower than those
found for the hot start tests. It is important to notice that Euro 7 Hot start SPN
10
emission limit proposal (2e+11 #/kWh) is lower than
the Cold limit (5e+11 #/kWh) while in general, for the tests carried out with the parallel hybrid diesel city bus, SPN
10
emission factor
for the hot start are higher than for the cold start, making compliance with the new proposed limits even more difcult.
Fig. 12 shows SPN
10
instantaneous emissions for WHVC tests at 0◦C. It can be seen that during the rst part of the cold start test, the
SPN
10
emission peaks are lower than for hot start test. The observed difference cannot be attributed to the effect of thermal engine
Fig. 10. Comparison of NOx mass emissions for cold and hot start during WHVC test −7
◦C (a) performed at −7◦C. The engine coolant temperature
corresponding to each test is shown in dotted lines and it is shown as zero when the thermal engine is off.
Fig. 11. SPN
10
Euro 7 proposal related Emission Ratios (ER) at WHVC tests performed at temperatures from −7 to 35◦C. Error bars represent the
inter-test variability.
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Transportation Research Part D 129 (2024) 104125
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hybridization but rather to the engine temperature, which exhibits higher particle emissions at higher temperatures. As it can be seen,
when the coolant temperature is higher than 70◦C, SPN
10
emissions for cold and hot start tests are quite similar, being higher than
those shown when the engine is cold. It is important to notice that on average this research found the emission factor of SPN
10
to be
35% higher than SPN
23
, ranging from 16 to 43%, and being around 40% higher for most of hot start tests. This difference may be due to
the injection pressure, since higher injection pressures lead to the formation of smaller particles, so the SPN
10
/SPN
23
ratio depends on
actual engine injection pressure. These ratios are bigger that those reported by Giechaskiel et al (Giechaskiel et al., 2017) in 2017 for
light duty Diesel vehicles. They are in the same range with other heavy duty engines with relatively high emissions (i.e., near of at the
emission limit), as seen in studies by Giechaskiel (Giechaskiel, 2018) and by Bir´
o and Kiss in 2023 (Bir´
o and Kiss, 2023), however, they
are much lower compared to other studies, where differences of >100% are reported (Mamakos et al., 2022). In these studies, the
contribution of urea-related particles (byproducts of urea decomposition) was signicant (around 1e+11 #/kWh).
Regarding to the Euro 7 proposal “budget” limits that applies for all trips less than 3×WHTC long, in all cases studied, the CO,
NMOG, CH
4
, N
2
O and NH
3
Emission Ratios would be well below the “budget” limit, being similar to those obtained in the hot and cold
Euro 7 assessment study, discussed previously.
However, the SPN
10
emissions using the Euro 7 “budget” concept for short tests (1×WHVC) yields a “compliant” result (ER: 0.74
and ER: 0.87) when in fact this vehicle exceeds both the Euro 7 limits Hot and Cold, as shown before. Moreover, in the case of NOx
emissions, a “budget” test performed under hot conditions would result in a “compliant” result (ER from 0.27 to 0.8) even though the
vehicle is not compliant with either the Euro 7 Hot nor Cold limits when the budget is not considered. Thus, “budget tests” need to be
designed considering a wider context of the potential vehicle operating conditions, including, but not limited to, the work performed.
Nonetheless, it should be noted that, as stated in the Introduction, a Euro 7 vehicle can be tested under long and/or short tests that meet
its normal use, and will have to meet the proposed limits under those conditions.
3.3. Intra-tests emission factors variability analysis
Owing to the presence of different analyzers (as shown in Table 4), and two alternative ways to measure the exhaust gas ow (Eqs.
(1) and (2)), it has been possible to determine the intra-test variability due to the instrumentation used. The intra-test variability for
each test (i) was calculated as the standard deviation of the total mass calculated with the different methods, divided by the total work
performed during the same test (W
ICE
), similar to Eq. (8). The number of available total mass emission values (Tm
i
) used to calculate
the intra-test variability in each test is shown in Table 8. Notice that, for each pollutant, the number of total mass emission values (Tm
i
)
used is larger than those indicated in Table 4. This is because, for each emission measured at the dilution tunnel, the total mass per test
(Tm
i
) was calculated in two different ways: 1) Using average values, as shown in Eq. (5), considered as the reference value, and 2) Using
Fig. 12. SPN
10
emission at WHVC tests performed at 0◦C for cold and hot start tests. Engine coolant temperature is shown as zero when the thermal
engine is off.
Table 8
Intra-test variability attributed to instrumentation, which includes analyzers and exhaust ow. For NOx, it encompasses both the direct NOx mea-
surement and NOx resulting from the measurement of NO +NO
2
, where NO mass is calculated using the molecular weight of NO
2
as prescribed by the
regulations.
CO
2
g/kWh CO mg/kWh NOx mg/kWh THC mg/kWh CH
4
mg/kWh N
2
O mg/kWh NH
3
mg/kWh HCHO mg/kWh
Number of Analyzers 4 3 4 1 2 2 2 1
Number of Tm values 8 6 8 2 4 4 4 2
Maximum 25.94 76.94 66.14 2.06 1.55 8.84 0.32 0.21
Average 11.88 16.22 35.76 0.61 1.10 3.38 0.14 0.08
Stand Dev 6.87 19.79 15.67 0.48 0.28 2.32 0.11 0.05
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Transportation Research Part D 129 (2024) 104125
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Eq. (4) with the raw concentration signal calculated from the diluted signal (
χ
Raw_calcDil
) through Eq. (3), and the exhaust gas ow
calculated with Eq. (1). Additionally, the total mass (Tm
i
) for each emission measured at the tailpipe was calculated using Eq. (4) with
exhaust gas ow determined by both Eqs. (1) and (2). Taking CO as an example, ve instantaneous mass emission signals and the “bag-
like” CO total mass were used (yielding six values). For the diluted signal (measured by the Diluted analyzer AVL AMA i60), the total
mass emissions were obtained both using Eq. (4) and by integrating second-by-second the raw concentration signal calculated from the
diluted signal (
χ
Raw_calcDil
). For the two raw measurements (using the Tailpipe analyzer AVL AMA i60 and FTIR spectrometer AVL
SESAM), two different values were obtained due to the two ways of calculating the exhaust ow, as explained earlier.
In Table 8, the maximum, average, and standard deviation of intra-test variability were determined based on values obtained from
all conducted WHVC tests. Each intra-test variability value was calculated as previously described.
It is worth noting that the maximum intra-test variability found (refer to Table 8) in the calculated emission factors for CO, SPN
23
and THC is consistently less than 2% of the Euro VI emission limits. However, in the case of NOx, both the maximum and average
variabilities were found to be less than 15% and 8% of the Euro VI limit, respectively. This discrepancy is primarily due to some cold
start tests where the calculated emission factors exceeded the Euro VI limit by up to 6 times. Thus, the comparison involves the intra-
test variability of a signicantly high emission factor against a relatively low limit.
Upon comparing the various NOx emission factors with the reference value (calculated using the average diluted concentration of
NOx, determined by Eq. (5) - equivalent to the diluted gas measurement from the Tedlar bag), it becomes evident (refer to Fig. 13) that
the primary source of variability arises from the FTIR used. During hot start tests, where NOx emission factors are lower, the disparity
between FTIR emission factors and the reference value is more pronounced. Conversely, during cold start tests, their emissions factors
exhibit greater similarity.
When examining intra-test variabilities specically for cold start tests alongside the corresponding Euro 7 limits, it was observed
that all emission factors exhibited a variability of less than 2% of the proposed limit, except for NOx. The average variability of NOx
surpassed 10% of the proposed Cold limit, primarily due to the inuence of the FTIR analyzer, as mentioned earlier, and the raw
concentration signals from the tailpipe AMA i60. In contrast, for hot start tests, the average variability of NOx remained below 30% of
the proposed Euro 7 Hot limit. This discrepancy can be attributed to the substantially lower Hot limit compared to the Cold one, and
the fact that the tested vehicle’s emission factors exceeded this limit by a considerable margin.
These intra-test variabilities align well with those found by Giechaskiel et at (Giechaskiel et al., 2022a) for HDVs, who showed a
variability of ±20 mg/kWh for NOx emission factors below 200 mg/kWh and a variability of ±100 mg/kWh for NOx emission factors
around 1000 mg/kWh. Similar results were also presented by Valverde et al (Valverde et al., 2023) for LDVs.
3.4. Critical operation conditions for Euro 7 compliance
NOx and SPN
10
were the most critical emissions for the full-hybrid parallel-type electric-diesel bus tested, when compared to the
Euro 7 proposal. Therefore, a deeper analysis has been made to understand which are the critical operation conditions where high
instantaneous emission events (emission peaks) occur for these pollutants. In order to do so, the instantaneous emissions of NOx and
SPN
10
, considering the data from all the tests, were arranged in decreasing order. Cumulated emissions were then calculated starting
from the highest instantaneous emission until the chosen percentage of cumulated emissions was obtained, as done by Mera et al (Mera
et al., 2019). These results are shown in Fig. 14 for NOx.
As already mentioned, the problem of NOx emissions is not only during cold starts or hot starts at low temperatures, where the SCR
system is below the operating temperature range, or shows unexpected behavior, but also in hot start conditions where the emission
factors exceed, for ambient temperatures less than or equal to 22◦C, the Hot limit of the Euro 7 proposal, as shown in Fig. 9. For this
reason, the study of the critical operating conditions for NOx was done only for hot start conditions, taking into account the instan-
taneous mass emissions from all hot WHVC tests performed (excluding the test −7◦C (b)). The ndings of the investigation indicate
that the highest NOx emission events at the tailpipe (where 20% of the total NOx is emitted) take place in less than the 1% of the time.
These events typically take place at slightly lean burn conditions near to stoichiometric combustion with lambda around 1.6, mostly at
conditions with lambda between 1.54 (25 percentile) and 1.72 (75 percentile), as shown at Fig. 14 and Fig. 15. This coincides with the
conditions where NOx engine out emissions are higher (Heywood, 1988), which corresponds mainly to the operation points of
maximum power. The results suggest that controlling the NOx emissions when the engine is working with lambda values below 2
would lead to reductions in NOx emissions at hot start conditions, potentially meeting the Hot Euro 7 limit at ambient temperatures
around 22◦C. Nonetheless, this strategy alone would not be enough at lower temperatures.
In the case of SPN
10
, as illustrated in Fig. 16, it can be observed that up to 80% of the total SPN
10
is emitted when the engine
operates at lambda values around 2.0. This condition occurs less than 36% of the time, with the remaining 64.4% characterized by
minimal SPN
10
emissions, as the engine primarily operates at lambda values exceeding 3.35. The latter condition corresponds to very
low or no fuel injection. The highest 20% emission events typically occur under conditions ranging from lambda 1.86 (25 percentile)
and 2.11 (75 percentile). Previous studies (Bertola et al., 2001) have suggested that under these conditions, where some small fuel
drops may fail to reach oxygen, losing hydrogen and transforming into carbon particles that lack sufcient time to react within the
engine cylinders. The particle size is directly linked to injection pressure, decreasing as injection pressure increases (Bertola et al.,
2001).
As depicted in Fig. 16, the boxplots exhibit signicant overlap, indicating that not all SPN
10
originated in the engine, as carbon
particles formed during combustion. Other particles were produced in the SCR, as demonstrated by Robinson et al. (Robinson et al.,
2016), which are independent of the thermal engine’s operational conditions. However, analyzing hot WHVC tests at −7◦C (test a) and
−7◦C (test b), where NOx emissions were previously illustrated in Fig. 6 and again in Fig. 17 alongside SPN
10
and fuel injection, reveals
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Fig. 13. NOx Intra-test Variability: Comparison of all available signals in relation to “bag-like” NOx calculated using diluted measurement (lab-
oratory reference). Error bars represent 1
σ
standard deviation.
Fig. 14. Critical conditions for NOx emissions at hot start WHTC tests.
Fig. 15. Example of NOx emission prole and lambda at hot start WHTC test.
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Transportation Research Part D 129 (2024) 104125
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a robust correlation between fuel injection and SPN emissions. Notably, spikes is fuel injection correspond to SPN
10
spikes, aligning
with expectations for engine-out emissions but not for DPF-equipped vehicles (Mamakos et al., 2022). The observed behavior in Fig. 17
suggests that the ltration efciency of the DPF may be lower compared to other modern DPFs, where the SPN emission signal exiting
the exhaust pipe does not exhibit a correlation with the fuel consumption signal (Mamakos et al., 2022).
However, in Fig. 17, the contribution of urea-related particles can also be assessed, considering that low tailpipe NOx emissions are
Fig. 16. Critical conditions for SPN
10
emissions.
Fig. 17. Comparison of NOx emissions (top panel), SPN
10
emissions (central panel), fuel injection and coolant temperature (bottom panel) for hot
WHVC tests −7◦C (test a) and −7◦C (test b). Engine coolant temperature is shown as zero when the thermal engine is off.
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Transportation Research Part D 129 (2024) 104125
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expected when Diesel Exhaust Fluid (DEF) is injected. In the −7◦C (test a) hot WHVC test, NOx emissions remained consistently low, as
depicted in the top panel of Fig. 17. In contrast, for the −7◦C (test b) hot start test, NOx emission peaks were similar in magnitude to
those observed during the cold start (refer to Fig. 10). This behavior for the −7◦C (test b) is likely due to the absence of DEF injection
during episodes of high NOx emissions. Estimating the contribution of urea-related particles involves comparing segments of the cycle
with high NOx emissions to those with low emissions. For instance, during the 800–1200 s interval, it appears that no DEF was injected
for the −7◦C (test b) (resulting in high NOx), while DEF was injected for the −7◦C (test a) (resulting in low NOx). The SPN
10
levels for
the −7◦C (test a) are higher, suggesting that the difference in SPN
10
levels between the two test sets, as shown in the center panel of
Fig. 17, is likely due to urea-related particles. It cannot be ruled out that the difference may also be attributed to variations in the
ltration efciencies in the DPF on the two days, possibly due to different DPF ll states. According to the ECU, the DPF ll state was
approximately 50% for both tests.
4. Conclusions
A Euro VI-C full hybrid intercity Diesel bus underwent testing within a climatic test cell for Heavy-Duty Vehicles (HDVs), featuring
a 4-wheel-drive (4WD) 2-axis roller dynamometer at the VELA7 laboratory at the Joint Research Centre (JRC), Ispra site. World-
Harmonized heavy-duty Vehicle Cycle (WHVC) tests were conducted at varying ambient temperatures (from −7 to 35◦C) with the
objective to assess pollutant emissions under the proposed Euro 7 perspective, analyze how far current vehicle emissions are from the
new proposal, and identify the critical operation conditions where more improvements have to be done to comply with the new
emissions regulation proposal.
The main conclusions obtained through this research have been:
•The vehicle, equipped with a high-power electric motor and lithium-ion batteries, demonstrates efcient propulsion in pure electric
mode at low speeds, maintaining a constant engine coolant temperature when the thermal engine is off. This means that NOx
emissions are not negatively affected by engine starts after periods of inactivity. Likewise SPN
10
emissions do not seem to be
affected by vehicle hybridization.
•The tested vehicle had emission factors below the Euro VI limits at 22◦C, which is the temperature range at which WHTC engine
type approval tests are carried out (20–30◦C). However, it was demonstrated that NOx emissions were much higher at ambient
temperatures below or equal to 0◦C, which are within the range of temperatures (-7 to 35◦C) of the In-Service Conformity test with
PEMS required for the vehicles, which also includes other relevant requirements for the inclusion of data for the analysis (e.g.,
coolant temperature, time after test start, the need to cover between 4 and 8 times the WHTC reference work, among others).
•For one of the tests performed at −7◦C, the SCR aftertreatment system showed a low deNOx efciency, not controlling NOx
emissions even when the engine temperature was at a normal (hot) operation condition. Since the relevant data was not available
from the OBD port, it was not possible to further investigate the reasons for this behavior.
•The biggest challenge that the tested HDV would have to overcome to meet the Euro 7 requirements was the NOx emission control
at cold start conditions, since the emission factors were more than 4 times the proposed Euro 7 “Cold” limit.
•The NOx emission factors at hot start conditions exceeded the proposed Euro 7 “Hot” limit by more than 4 times at sub-zero
temperatures.
•Solid particle number >10 nm (SPN
10
) emissions were on average 35% higher than SPN
23
.
•The most challenging condition for SPN
10
emissions was at high load hot-start conditions, especially at high ambient temperatures,
having an emission factor more than 2 times the proposed “Hot” limit at all tested temperatures.
•Under cold start conditions, SPN
10
emissions were also of concern, but less than for the hot start tests, because the “Hot” limit is
more stringent than “Cold” one. SPN emissions were not dependent on the system temperature (engine and aftertreatment).
•The emission factors for CO, NMOG, CH
4
, NH
3
and HCHO were well below proposed Euro 7 emission limits at all tested ambient
temperatures.
•N
2
O emissions were below proposed Euro 7 limits, except for the hot-start condition at 35◦C when it was exceeded.
•In case of the short test evaluated using the Euro 7 “budget” limit, it was argued that the chosen test distance and condition at the
beginning of the test (cold or hot start) affect signicantly the outcome of the analysis.
Future research will focus on assessment Euro 7 proposal at long on-road tests, more than three times the reference WHTC work and
applying the moving window methodology.
CRediT authorship contribution statement
Natalia Fonseca Gonz´
alez: Conceptualization, Methodology, Software, Formal analysis, Data curation, Writing – original draft,
Visualization, Writing – review & editing. Ricardo Suarez-Bertoa: Conceptualization, Methodology, Investigation, Resources,
Writing – review & editing, Supervision. Barouch Giechaskiel: Investigation, Writing – review & editing, Supervision. Anastasios
Melas: Investigation, Writing – review & editing. Roberto Gioria: Investigation, Writing – review & editing. Fabrizio Forloni:
Writing – review & editing, Software, Investigation, Data curation. Tommaso Selleri: Investigation, Writing – review & editing.
Adolfo Perujo: Writing – review & editing, Supervision, Resources.
N. Fonseca Gonz´
alez et al.

Transportation Research Part D 129 (2024) 104125
18
Declaration of Competing Interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to
inuence the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgments
The author Natalia Fonseca acknowledges the nancial support from European Union within the program “NextGenerationEU”
through the Spanish “Ministerio de Universidades”, Real Decreto 289/2021 reference UP2021-035, granted through the Universidad
Polit´
ecnica de Madrid for the development of a post-doctoral stage at the Joint Research Centre at Ispra, Italy.
Moreover, we also wish to thank Andrea Bonamin and Mauro Cadario for their contribution in performing the vehicle tests.
DISCLAIMER.
The analysis and conclusions presented in the manuscript are those of the authors and under no circumstance should they be
considered to represent an ofcial position of the European Commission. Mention of commercial products are not endorsement nor a
recommendation by the authors neither by the European Commission.
References
Ballinger, T., Cox, J., Konduru, M., De, D., Manning, W., Andersen, P., Sae, S., Journal, I., Manning, W., Andersen, P., Matthey, J., 2009. Evaluation of SCR catalyst
technology on diesel particulate lters linked references are available on JSTOR for this article : you may need to log in to JSTOR to access the linked references.
Evaluation of SCR Catalyst Technology on Diesel Particulate F 2, 369–374.
Benajes, J., García, A., Monsalve-Serrano, J., Martínez-Boggio, S., 2019. Optimization of the parallel and mild hybrid vehicle platforms operating under conventional
and advanced combustion modes. Energy Convers. Manag. 190, 73–90. https://doi.org/10.1016/j.enconman.2019.04.010.
Bertola, A., Schubiger, R., Kasper, A., Matter, U., Forss, A.M., Mohr, M., Boulouchos, K., Lutz, T., 2001. Characterization of diesel particulate emissions in heavy-duty
DI-diesel engines with common rail fuel injection inuence of injection parameters and fuel composition. SAE Tech. Pap. 110 https://doi.org/10.4271/2001-01-
3573.
Bir´
o, N., Kiss, P., 2023. Euro VI-d compliant diesel engine’s Sub-23 nm particle emission. Sensors 2008.
Bishop, G.A., Schuchmann, B.G., Stedman, D.H., 2013. Heavy-duty truck emissions in the south coast air basin of California. Environ. Sci. Technol. 47, 9523–9529.
https://doi.org/10.1021/es401487b.
Boger, T., Rose, D., He, S., Joshi, A., 2022. Developments for future EU7 regulations and the path to zero impact emissions – a catalyst substrate and lter supplier’s
perspective. Transp. Eng. 10, 100129 https://doi.org/10.1016/j.treng.2022.100129.
European Commission, 2023. CO₂ emission performance standards for cars and vans. https://climate.ec.europa.eu/eu-action/transport/road-transport-reducing-co2-
emissions-vehicles/co2-emission-performance-standards-cars-and-vans_en (accessed 11.14.23).
Dimaratos, A., Toumasatos, Z., Doulgeris, S., Triantafyllopoulos, G., Kontses, A., Samaras, Z., 2019. Assessment of CO2 and NOx emissions of one diesel and one bi-
fuel GASOLINE/CNG euro 6 vehicles during real-world driving and laboratory testing. Front. Mech. Eng. 5, 1–16. https://doi.org/10.3389/fmech.2019.00062.
European Commission, 2022a. Euro 7 proposal. Com 2022, 0365.
European Commission, 2022b. Euro. VI Evaluation Study 6. https://doi.org/10.2873/933978.
Gao, J., Tian, G., Sorniotti, A., Karci, A.E., Di Palo, R., 2019. Review of thermal management of catalytic converters to decrease engine emissions during cold start and
warm up. Appl. Therm. Eng. 147, 177–187. https://doi.org/10.1016/j.applthermaleng.2018.10.037.
García, A., Monsalve-Serrano, J., Lago Sari, R., Martinez-Boggio, S., 2022. Energy sustainability in the transport sector using synthetic fuels in series hybrid trucks
with RCCI dual-fuel engine. Fuel 308, 122024. https://doi.org/10.1016/j.fuel.2021.122024.
Giechaskiel, B., 2018. Solid particle number emission factors of euro vi heavy-duty vehicles on the road and in the laboratory. Int. J. Environ. Res. Public Health 15.
https://doi.org/10.3390/ijerph15020304.
Giechaskiel, B., Vanhanen, J., V¨
akev¨
a, M., Martini, G., 2017. Investigation of vehicle exhaust sub-23 nm particle emissions. Aerosol Sci. Technol. 51, 626–641.
https://doi.org/10.1080/02786826.2017.1286291.
Giechaskiel, B., Zardini, A.A., Clairotte, M., 2019. Exhaust gas condensation during engine cold start and application of the dry-wet correction factor. Appl. Sci. 9
https://doi.org/10.3390/app9112263.
Giechaskiel, B., Melas, A.D., L¨
ahde, T., Martini, G., 2020. Non-volatile particle number emission measurements with catalytic strippers: a review. Vehicles 2, 342–364.
https://doi.org/10.3390/vehicles2020019.
Giechaskiel, B., Valverde, V., Kontses, A., Suarez-Bertoa, R., Selleri, T., Melas, A., Otura, M., Ferrarese, C., Martini, G., Balazs, A., Andersson, J., Samaras, Z., Dilara, P.,
2021. Effect of extreme temperatures and driving conditions on gaseous pollutants of a euro 6d-temp gasoline vehicle. Atmosphere (basel). 12 https://doi.org/
10.3390/atmos12081011.
Giechaskiel, B., Jakobsson, T., Karlsson, H.L., Khan, M.Y., Kronlund, L., Otsuki, Y., Bredenbeck, J., Handler-Matejka, S., 2022a. Assessment of on-board and laboratory
gas measurement Systems for Future Heavy-Duty Emissions Regulations. Int. J. Environ. Res. Public Health 19. https://doi.org/10.3390/ijerph19106199.
Giechaskiel, B., Selleri, T., Gioria, R., Melas, A.D., Franzetti, J., Ferrarese, C., Suarez-Bertoa, R., 2022b. Assessment of a euro VI step E heavy-duty vehicle’s
aftertreatment system. Catalysts 12, 1–15. https://doi.org/10.3390/catal12101230.
Guido, C., Di Maio, D., Napolitano, P., Beatrice, C., 2022. Sub-23 particle control strategies towards euro VII HD SI natural gas engines. Transp. Eng. 10, 100132
https://doi.org/10.1016/j.treng.2022.100132.
Gunja, R., Wancura, H., Raser, B., Weißb¨
ack, M., 2022. Euro 7/VII concepts in the interaction with OBD/OBM. MTZ Worldw. 83, 44–51. https://doi.org/10.1007/
s38313-022-0852-2.
Harris, T.M., Muhleck, M., Scherer, B.-P., 2021. Thermal management as key to euro VII emissions compliance. Atzheavy Duty Worldw. 14, 40–43. https://doi.org/
10.1007/s41321-021-0416-4.
Heywood, J., 1988. Internal combustion engine fundamentals. McGraw-Hill Education.
Kotz, A.J., Kittelson, D.B., Northrop, W.F., 2016. Lagrangian hotspots of in-use NOX emissions from transit buses. Environ. Sci. Technol. 50, 5750–5756. https://doi.
org/10.1021/acs.est.6b00550.
Kozak, M., Lijewski, P., Walig´
orski, M., 2022. Exhaust emissions from a Hybrid City bus fuelled by conventional and oxygenated fuel. Energies 15. https://doi.org/
10.3390/en15031123.
Maci´
an, V., Monsalve-Serrano, J., Villalta, D., Fogu´
e-Robles, ´
A., 2021. Extending the potential of the dual-mode dual-fuel combustion towards the prospective EURO
VII emissions limits using gasoline and OMEx. Energy Convers. Manag. 233 https://doi.org/10.1016/j.enconman.2021.113927.
N. Fonseca Gonz´
alez et al.

Transportation Research Part D 129 (2024) 104125
19
Mamakos, A., Rose, D., Besch, M.C., He, S., Gioria, R., Melas, A., Suarez-Bertoa, R., Giechaskiel, B., 2022. Evaluation of advanced diesel particulate lter concepts for
post euro VI heavy-duty diesel applications. Atmosphere (basel). 13 https://doi.org/10.3390/atmos13101682.
Mendoza Villafuerte, P., Demuynck, J., Bosteels, D., Wilkes, T., Mueller, V., Recker, P., 2022. Future-proof heavy-duty truck achieving ultra-low pollutant emissions
with a close-coupled emission control system including active thermal management. Transp. Eng. 9, 100125 https://doi.org/10.1016/j.treng.2022.100125.
Mendoza Villafuerte, P., Demuynck, J., Bosteels, D., Aisbl, A., Brussels, D.-I., Wilkes, T., Robb, L., Sch¨
onen, M., 2021. Demonstration of Extremely Low NOx Emissions
with Partly Close-Coupled Emission Control on a Heavy-duty Truck Application Darstellung extrem niedriger NOx-Emissionen an einem Heavy Duty Truck durch
Einsatz einer anteilig motornahen Abgasanlage und optimie.
Mera, Z., Fonseca, N., L´
opez, J.-M., Casanova, J., 2019. Analysis of the high instantaneous NOx emissions from euro 6 diesel passenger cars under real driving
conditions. Appl. Energy 242, 1074–1089. https://doi.org/10.1016/J.APENERGY.2019.03.120.
Mulholland, E., Miller, J., Braun, C., Jin, L., Rodríguez, F., 2021. Quantifying the long-term air quality and health benets from Euro 7/VII standards in Europe.
International Council on Clean Transportation Brieng. June 2021. Available at: https://theicct.org/sites/default/les/publications/eu-euro7-standards-health-.
Mulholland, E., Miller, J., Bernard, Y., Lee, K., Rodríguez, F., 2022. The role of NOx emission reductions in euro 7/VII vehicle emission standards to reduce adverse
health impacts in the EU27 through 2050. Transp. Eng. 9, 100133 https://doi.org/10.1016/j.treng.2022.100133.
Napolitano, P., Di Maio, D., Guido, C., Merlone Borla, E., Torbati, R., 2023. Experimental investigation on particulate lters for heavy-duty natural gas engines:
potentialities toward EURO VII regulation. J. Environ. Manage. 331, 117204 https://doi.org/10.1016/j.jenvman.2022.117204.
Posada, F., Badshah, H., Rodriguez, F., 2020. In-use NOx emissions and compliance evaluation for modern heavy-duty vehicles. In Europe And The United States.
Ragon, P.-L., Rodríguez, F., 2021. ICCT Working paper 2021-20 : Estimated cost of diesel emissions control technology to meet future Euro VII standards. White Pap.
Int. Counc. Clean Transp. (ICCT), Washingt. DC 1–27.
Reiter, M.S., Kockelman, K.M., 2016. The problem of cold starts: a closer look at mobile source emissions levels. Transp. Res. Part D Transp. Environ. 43, 123–132.
https://doi.org/10.1016/j.trd.2015.12.012.
Resitoglu, I.A., Altinisik, K., Keskin, A., Ocakoglu, K., 2020. The effects of Fe2O3 based DOC and SCR catalyst on the exhaust emissions of diesel engines. Fuel 262,
116501. https://doi.org/10.1016/j.fuel.2019.116501.
Robinson, M.A., Backhaus, J., Foley, R., Liu, Z.G., 2016. The effect of diesel exhaust uid dosing on tailpipe particle number emissions. SAE Tech. Pap. 8–9 https://
doi.org/10.4271/2016-01-0995.
Rodríguez, F., Badshah, H., (ICCT), I.C. on C.T., 2021. Real world NO
x
performance of Euro VI-D trucks and recommendations for Euro VII. Work. Pap. 22p.
Samaras, Z.C., Kontses, A., Dimaratos, A., Kontses, D., Balazs, A., Hausberger, S., Ntziachristos, L., Andersson, J., Ligterink, N., Aakko-Saksa, P., Dilara, P., 2022. A
European Regulatory Perspective towards a Euro 7 Proposal. SAE Tech. Pap. https://doi.org/10.4271/2022-37-0032.
Selleri, T., Gioria, R., Melas, A.D., Giechaskiel, B., Forloni, F., Villafuerte, P.M., Demuynck, J., Bosteels, D., Wilkes, T., Simons, O., Recker, P., Lilova, V., Onishi, Y.,
Steffen, M., Grob, B., Perujo, A., Suarez-Bertoa, R., 2022a. Measuring emissions from a demonstrator heavy-duty diesel vehicle under real-world
conditions—Moving forward to euro VII. Catalysts 12. https://doi.org/10.3390/catal12020184.
Selleri, T., Melas, A., Ferrarese, C., Franzetti, J., Giechaskiel, B., Suarez-Bertoa, R., 2022b. Emissions from a modern euro 6d diesel plug-in hybrid. Atmosphere (basel).
13 https://doi.org/10.3390/atmos13081175.
Shin, J., Choi, J., Seo, J., Park, S., 2022. Pre-chamber combustion system for heavy-duty engines for operating dual fuel and diesel modes. Energy Convers. Manag.
255, 115365 https://doi.org/10.1016/j.enconman.2022.115365.
Suarez-Bertoa, R., Astorga, C., 2018. Impact of cold temperature on euro 6 passenger car emissions. Environ. Pollut. 234, 318–329. https://doi.org/10.1016/j.
envpol.2017.10.096.
Suarez-Bertoa, R., Selleri, T., Gioria, R., Melas, A.D., Ferrarese, C., Franzetti, J., Arlitt, B., Nagura, N., Hanada, T., Giechaskiel, B., 2022. Real-time measurements of
formaldehyde emissions from modern vehicles. Energies 15. https://doi.org/10.3390/en15207680.
Szpica, D., 2023. Combustion systems and fuels used in engines—A short review. Appl. Sci. 13 https://doi.org/10.3390/app13053126.
Tan, Y., Henderick, P., Yoon, S., Herner, J., Montes, T., Boriboonsomsin, K., Johnson, K., Scora, G., Sandez, D., Durbin, T.D., 2019. On-board sensor-based NOx
emissions from heavy-duty diesel vehicles. Environ. Sci. Technol. 53, 5504–5511. https://doi.org/10.1021/acs.est.8b07048.
Taymaz, I., Benli, M., 2014. Emissions and fuel economy for a hybrid vehicle. Fuel 115, 812–817. https://doi.org/10.1016/j.fuel.2013.04.045.
Thedinga, B., Suarez, R., Gioria, R., 2023. Euro 7 HDV implementing regulation. AGVES Meeting 24 February.
Valverde, V., Kondo, Y., Otsuki, Y., Krenz, T., Melas, A., Suarez-Bertoa, R., Giechaskiel, B., 2023. Measurement of gaseous exhaust emissions of light-duty vehicles in
preparation for euro 7: a comparison of portable and laboratory instrumentation. Energies 16. https://doi.org/10.3390/en16062561.
Valverde Morales, V., Clairotte, M., Pavlovic, J., Giechaskiel, B., Bonnel, P., 2020. On-road emissions of euro 6d-TEMP vehicles: consequences of the entry into force of
the RDE regulation in Europe. SAE Tech. Pap. 22–23 https://doi.org/10.4271/2020-01-2219.
Wiers, W.W., Schefer, C.E., 1972. Carbon dioxide (CO2) tracer technique for modal mass exhaust emission measurement. SAE Tech. Pap. 81, 441–455. https://doi.
org/10.4271/720126.
Wijeyakulasuriya, S., Kim, J., Probst, D., Srivastava, K., Yang, P., Scarcelli, R., Senecal, P.K., 2022. Enabling powertrain Technologies for Euro 7/VII vehicles with
computational uid dynamics. Transp. Eng. 9, 100127 https://doi.org/10.1016/j.treng.2022.100127.
Witt, M., 2010. LANL debuts hybrid garbage truck. United States. https://www.osti.gov/servlets/purl/1305041.
Ximinis, J., Massaguer, A., Massaguer, E., 2022a. NOx emissions below the prospective EURO VII limit on a retrotted heavy-duty vehicle. Appl. Sci. 12 https://doi.
org/10.3390/app12031189.
Ximinis, J., Massaguer, A., Massaguer, E., 2022b. Towards compliance with the prospective EURO VII NOx emissions limit using a thermoelectric aftertreatment
heater. Case Stud. Therm. Eng. 36, 102182 https://doi.org/10.1016/j.csite.2022.102182.
N. Fonseca Gonz´
alez et al.