Special Issue Article

International J of Engine Research

2019, Vol. 20(1) 155–163
Ó IMechE 2018
High-pressure versus low-pressure Article reuse guidelines:
sagepub.com/journals-permissions
exhaust gas recirculation in a Euro 6 DOI: 10.1177/1468087418817447
journals.sagepub.com/home/jer

diesel engine with lean-NOx trap:

Effectiveness to reduce NOx emissions

Magı́n Lapuerta , Ángel Ramos, David Fernández-Rodrı́guez and

Inmaculada González-Garcı́a

Abstract
Exhaust gas recirculation can be achieved by means of two different routes: the high-pressure route (high-pressure
exhaust gas recirculation), where exhaust gas is conducted from upstream of the turbine to downstream of the com-
pressor, and the low-pressure one (low-pressure exhaust gas recirculation), where exhaust gas is recirculated from
downstream of the turbine and of the aftertreatment system to upstream of the compressor. In this study, the effective-
ness of both exhaust gas recirculation systems on the improvement of the NOx-particulate matter emission trade-off
has been compared on a Euro 6 turbocharged diesel engine equipped with a diesel oxidation catalyst, a lean-NOx trap,
and a diesel particulate filter. Emissions were measured both upstream and downstream of the aftertreatment system, at
different combinations of engine speed and torque (corresponding to different vehicle speeds), at transient and steady
conditions, and at different coolant temperatures as switch points to change from high-pressure exhaust gas recircula-
tion to low-pressure exhaust gas recirculation. It was shown that low-pressure exhaust gas recirculation was more effi-
cient than high-pressure exhaust gas recirculation to reduce NOx emissions, mainly due to the higher recirculation
potential and the lower temperature of the recirculated gas. However, such a differential benefit decreased as the cool-
ant temperature decreased, which suggests the use of high-pressure exhaust gas recirculation during the engine warm-
up. It was also shown that the lean-NOx trap storage efficiency decreased more rapidly at high engine load than at
medium load and that such reduction in efficiency was much faster when high-pressure exhaust gas recirculation was
used than when low-pressure exhaust gas recirculation was used.

Keywords
Exhaust gas recirculation, lean NOx trap, NOx emissions, diesel engine, particulate emissions

Date received: 17 October 2018; accepted: 14 November 2018

Introduction production engines was HP-EGR, modern turbo-

charged Euro 6 diesel engines often include both
NOx emissions from diesel engines have harmful effects routes, either alternatively or simultaneously,3 not only
on human health1 and on the environment2 and are in two-stage turbocharging but also in simple-stage tur-
therefore limited by stringent regulations. To satisfy bocharging configurations.4
such limits, internal measures must be combined with In the usual calibration strategy, HP-EGR is active
aftertreatment systems. One of the most effective inter- from the engine start to a given coolant temperature
nal measures is exhaust gas recirculation (EGR). EGR below the nominal switch temperature.5 This EGR
can be achieved by means of two different routes: in
the high-pressure route (high-pressure exhaust gas
recirculation (HP-EGR)), exhaust gas is conducted Escuela Técnica Superior de Ingenieros Industriales, Universidad de
from upstream of the turbine to downstream of the Castilla-La Mancha, Ciudad Real, Spain
compressor. In the low-pressure one (low-pressure
exhaust gas recirculation (LP-EGR)), exhaust gas is Corresponding author:
Magı́n Lapuerta, Escuela Técnica Superior de Ingenieros Industriales,
recirculated from downstream of the turbine and of the Universidad de Castilla-La Mancha, Edif. Politécnica. Av., Camilo José
aftertreatment system to upstream of the compressor. Cela, Ciudad Real 13071, Spain.
Although few years ago the usual practice in Email: Magin.Lapuerta@uclm.es
156 International J of Engine Research 20(1)

route is often uncooled to help the engine to warm up. (DOC), a lean-NOX trap (LNT), and a regenerative
From this point onwards, the HP-EGR is disabled and wall-flow-type diesel particulate filter (DPF). A scheme
the cooled LP-EGR is activated instead. HP-EGR pro- of the installation is shown in Figure 1.
vides faster response and better transient control than Only one of the EGR loops, either LP-EGR or HP-
LP-EGR due to smaller volume and length of the EGR, can be activated at the same time. Two para-
ducts,6–8 but has a number of drawbacks, namely, it meters are used to control the type of EGR loop. These
has lower recirculation potential as a consequence of two parameters are the coolant and intake air tempera-
thermal throttling;9,10 it involves higher concentration tures. HP-EGR is activated at low coolant or low
of pollutants (which leads to deposits in the EGR duct intake air temperatures in order to prevent water con-
and cooling system, if there is any); it permits less pre- densation at the intake pipe, whereas LP-EGR is acti-
cise and less linear control through lambda sensors vated when a minimum coolant temperature is reached
due to non-homogeneities in the air-exhaust gas and the intake air temperature is high enough, as shown
mixture;11,12 and it provides lower efficiency in com- in Figure 2. When the LNT purge is operating, HP-
pressor and turbine since they work closer to the surge EGR is also activated, regardless of the coolant tem-
line.7 In addition, the recirculated flow can be exter- perature. The EGR mass flow rate is controlled with a
nally enhanced by partially closing a valve located single valve for the HP-EGR loop and with two valves
downstream of the aftertreatment system in order to in the case of the LP-EGR loop. In this latter case, the
increase the back pressure. EGR mass flow is controlled in two steps: for low mass
Among the aftertreatment systems, Euros 6 engines flow rates, the system opens the low-pressure EGR
use lean-NOx trap (LNT) and selective reduction cata- valve (Figure 1) while the back-pressure valve is fully
lyst (SCR)13 or combination of both.14 In any case, opened. For high EGR mass flow rates, the system
stringent NOx and particulate matter (PM) emission closes partially the back-pressure valve to increase the
limits, together with very high retention efficiency of back pressure, keeping the low-pressure EGR valve
particulate filters, have led to engine designers to pro- fully opened and consequently increasing the EGR
pose aggressive use of EGR,15 as a means to reduce mass flow rate.
NOx concentration previously to the aftertreatment The engine was coupled, with a rotating shaft, to an
system. Such high EGR ratios lead to limited intake air asynchronous electric dynamometer (Schenck Dynas
dilution and thus to reductions in the combustion effi- III LI 250), which controls the engine speed and tor-
ciency and to enhanced emissions.16 In this context, an que. The control system of the dynamometer includes a
evaluation of the efficiency of the different EGR routes road load simulation (RLS) system from Horiba, which
to reduce NOx emissions is helpful for engine is able to simulate the dynamics of a vehicle (with a
calibrations. particular transmission, gearbox, aerodynamics, tires,
In particular, combining EGR with LNT in Euro 6 etc.). In this work, a Nissan Qashqai 1.5 dCi vehicle
engines requires detailed knowledge of the inter-effects was simulated during the tests.
between them. LNT adsorbs NOx at lean conditions
and moderate temperature (200–400 °C) and further
Communication with control unit
desorbs at rich conditions, which requires additional
fuel injection and higher temperature. In the mentioned The INCA PC software and the ETAS ES 591.1 hard-
range for NOx absorption, it has been proved that ware were used for the communication between the user
increasing temperature increases the absorption effi- and the electronic control unit (ECU) of the engine.
ciency from less than 30% to more than 80% except in Among the original settings of the engine mapping,
the case that LNT is close to saturation.17 Sudden lean- only those related to the EGR management were modi-
ing after injection has also shown to be beneficial for fied. This was made in order to use the two types of
further recombination of residual stored NOx.18 Other EGR (HP-LP) independently of the original manufac-
effects of the EGR route, such as residence time and turer operation settings. Therefore, operating para-
pressure, might also affect the LNT retention efficiency. meters such as fuel injection strategy, among others,
were not externally controlled during the tests. The inlet
air mass flow rate and the fuel consumption were mea-
Experimental installation and sured with the original engine sensors and registered
instrumentation with the INCA PC software. The fuel consumption
measurement was previously calibrated with an AVL
Engine test bench 733s fuel gravimetric system.
A Euro 6 Nissan 1.5 dCi engine (model K9K) was used
in this study. This is a four-cylinder turbocharged,
intercooled, direct-injection diesel engine, which is Emissions instrumentation
equipped with double EGR system, one low-pressure Particle size concentrations were determined with an
cooled EGR (LP-EGR) and another high-pressure Engine Exhaust Particle Sizer Spectrometer (EEPS)
non-cooled EGR (HP-EGR). In addition, the after- model 3090 from TSI. The EEPS sampling point was
treatment system includes a diesel oxidation catalyst placed upstream of the diesel particle filter (DPF). As
Lapuerta et al. 157

Figure 1. Scheme of the experimental setup.

gas is introduced in the evaporating tube of the

ASET15-1 where the temperature is increased to
300 °C. After that, the aerosol flows into a mixing
chamber for the second dilution in order to cool down
the aerosol temperature and to reduce the thermo-
phoretic losses. The RD temperature was set at 150 °C
to avoid hydrocarbon condensations. Dilution factors
and thermophoretic and diffusion losses were taken
from the calibration certificates provided by Matter
Engineering AG. Primary dilution factor at RD was
64.73:1 and secondary dilution factor at the TC was
6.18:1, leading to a total dilution factor of 400:1.
Total hydrocarbon (THC) emissions were sampled
through a heated line, pump, and filter (190 °C) and
were measured with a flame ionization detector
Graphite 52M-D. Carbon monoxide and carbon diox-
ide emissions were measured with a NDIR detector
Figure 2. Control strategy for HP-EGR and LP-EGR. MIR 2M. NOX emissions were measured using a che-
miluminescence Topaze 3000 analyzer. Gaseous emis-
sions were also sampled upstream of the aftertreatment
the EEPS instrument needs special temperature and system. All these emission analyzers are integrated in a
dilution conditions of the inlet gas, the sample gas was modular system purchased from Environnement, which
first diluted with a rotating disk diluter (RD) model also includes the electro-valves and software necessary
MD19-2E using dilution air supplied by a thermal con- to commute between sample gas, zero gas, and calibra-
ditioner (TC) model ASET15-1. The diluted exhaust tion gases.
158 International J of Engine Research 20(1)

Table 1. Fuel properties. efficient in emissions reduction and which is the

most appropriate cooling temperature to switch from
Property Method Diesel HP-EGR to LP-EGR.
Density at 15 °C (kg/m3) EN ISO 3675 829
Kinematic viscosity at 40 °C (cSt) EN ISO 3104 2.92
Lower heating value (MJ/kg) UNE 51123 42.91 Results and discussion
C (wt%) 86.2
H (wt%) 13.8 Compared efficiency of HP-EGR and LP-EGR
Water content (mg/kg) EN ISO 12937 24.23
T10 (°C) ASTM D86 229
Stationary modes were chosen from the NEDC. Table 2
T50 (°C) ASTM D86 261.5 shows the speed and torque associated with each station-
T90 (°C) ASTM D86 293 ary condition as well as other parameters such as fuel
Stoichiometric fuel/air ratio 1/14.63 and air consumption. Torque and speed (which are quite
CFPP (°C) EN 116 –24 °C precisely controlled with the brake) and intake air mass
Cloud point (°C) EN 23015 –22.2 °C
Lubricity (WS1.4) (mm) EN ISO 12156-1 332 flow rate were maintained independently of the EGR
Derived cetane number EN 16715 55.93 used (HP or LP). Only in the case of 100 km/h, it was
impossible to keep the intake air flow rate constant
CFPP: cold filter plugging point. because of the increase in fuel consumption observed
when HP-EGR was selected. At this condition, the
equivalence ratio was kept constant instead of the
Fuel air flow rate to avoid excessive equivalence ratio for
The diesel fuel used was supplied by Repsol (Spain) HP-EGR. All emissions shown in this section were
and it fulfills the European standard EN 590. This is a measured upstream of the aftertreatment system,
first-fill diesel fuel, with no biodiesel content, and with since the aim was to compare the internal efficiency
enhanced cold-flow properties, which makes it useful of the different EGR loops, regardless of the effi-
for arctic climates. Its main properties are shown in ciency of the aftertreatment system.
Table 1. Differences in CO2 emissions (Figure 3, left) between
EGR routes are the consequence of the differences on
brake efficiency. No significant differences in CO2 emis-
Experimental matrix sions were observed for 32, 50, and 70 km/h operating
All tests were based on the NEDC (New European modes, but the difference at 100 km/h was around 8%,
Driving Cycle). NOx, CO, and THC emissions were as a consequence of the higher efficiency at this mode
measured both upstream and downstream of the after- when LP-EGR was used due to lower pumping losses.
treatment system, while particle emissions were only On the contrary, the EGR ratio (defined as the ratio
measured upstream, due to the high retention efficiency between recirculated mass flow rate and total exhaust
of the DPF. First, tests were made with full HP-EGR flow rate) was always higher with LP-EGR because of
and with full LP-EGR at different combinations of the higher boosting of the EGR flow derived from the
engine speed and effective torque (corresponding to dif- partial closure of the back-pressure valve and because
ferent vehicle speeds), at steady conditions selected of the lower gas temperature (Figure 3, right and
from NEDC. Second, tests were made along the LNT Table 2).
loading process, in order to check the loss of efficiency Since the EGR flows (and thus the air dilutions)
in NOx retention. Third, tests were also made cover- were different for HP-EGR and LP-EGR, and since
ing the engine heating process during the whole the sampling point is located upstream of the branch
NEDC, to compare which of the EGR routes is more extracting flow toward the LP-EGR, the comparison

Figure 3. CO2 tailpipe emissions (left) and EGR ratios and EGR mass flow rates (right) measured with LP-EGR and HP-EGR routes.
Lapuerta et al. 159

Table 2. Engine parameters during steady HP-EGR and LP-EGR tests.

Stationary Speed Torque Power Fuel Air Equivalence EGR flow

velocity (km/h) (r/min) (N m) (kW) consumption consumption ratio rate (g/s)
(g/s) (g/s)

HP-EGR 32 2072 6.5 1.4 0.253 17.24 0.213 6.413

50 1392 20.6 3.0 0.266 10.84 0.356 4.406
70 1515 38.1 6.0 0.439 13.50 0.471 3.882
100 1859 74.7 14.5 1.072 22.92 0.678 4.661
LP-EGR 32 2072 6.2 1.3 0.259 17.24 0.218 11.330
50 1392 20.1 2.9 0.270 10.84 0.361 8.638
70 1515 38.7 6.1 0.437 13.50 0.469 8.494
100 1859 74.5 14.5 0.970 20.73 0.679 13.433

EGR: exhaust gas recirculation; HP-EGR: high-pressure exhaust gas recirculation; LP-EGR: low-pressure exhaust gas recirculation.

Figure 4. THC and CO concentrations (left) and particle concentrations (right) upstream of the aftertreatment system with
LP-EGR and HP-EGR routes.

between the HP and LP routes was made in concentra-

tion (ppm) rather than in absolute flow rate (g/s).
Therefore, differences will be qualitatively indicative of
engine emissions (if no aftertreatment system is
installed). Figure 4 shows that, despite LP-EGR is not
beneficial for unburned hydrocarbons and carbon
monoxide emissions (Figure 4, left) and for particle
number emissions (Figure 4, right), it is very beneficial,
with respect to HP-EGR, for NOx emissions (Figure 5).
This trend is in agreement with that shown in other
studies.3,9,19 Reductions in NOx emissions for LP-EGR
range between 40% at low loads (32 km/h) and 80% for
high loads (100 km/h). This benefit is even higher for
nitrogen monoxide (directly affected by the reduction in Figure 5. NOx concentrations (including NO and NO2)
combustion temperature, which inhibits the thermal upstream of the aftertreatment system with LP-EGR and
mechanism20) than for nitrogen dioxide (which is only HP-EGR routes.
indirectly affected). It can also be observed from the
particle size distributions (Figure 6, left) that such
increase in the particle number when LP-EGR is used is The higher effectiveness on NOx suppression with
not associated with any decrease in particle mean dia- increasing engine load (and thus vehicle speed), as well
meter. These trends can be explained by a combination as the associated increase in mean diameter, has been
of two factors: the increased EGR ratio with LP-EGR widely reported in the literature.21 Temperature reduc-
(Figure 3, right) and the lower temperature of the recir- tions of the recirculated gas and increases in fuel–air
culated gas, which is consequence of the longer route ratio have both been identified as effect enhancers for
and of the EGR cooler. EGR.10,22
160 International J of Engine Research 20(1)

Figure 6. Particle size distributions (left) and mean particle diameters upstream of the aftertreatment system (right) with
LP-EGR and HP-EGR routes.

Figure 7. NOx emissions upstream and downstream of the LNT for LP-EGR and HP-EGR routes (left) and LNT efficiency
(right) for two engine modes corresponding to 70 km/h (above) and 100 km/h (below).

LNT loading process at different vehicle speeds the EGR route: LP-EGR operates under lower tem-
NOx adsorption in the LNT occurs at lean conditions perature (as a consequence of EGR cooling and longer
route), with lower residence time (as a consequence of
as mentioned above. At such lean conditions, the stor-
higher gas flow rate, since LNT is inside the loop), and
age efficiency of the LNT was investigated for both
slightly higher pressure (as a consequence of partial clo-
EGR routes at two engine operation modes, corre-
sure of the back-pressure valve).
sponding to 70 and 100 km/h. A DPF regeneration pro-
Figure 7 shows the time-resolved NOx emissions in
cess and LNT purge were made as a preconditioning
g/s upstream and downstream of the LNT, as well as
procedure before starting the test. NOx concentrations
the storage efficiency, which was obtained as unity
were measured alternatively upstream and downstream
minus the ratio of both NOx measurements.
of the aftertreatment system.
Measurements were made downstream for most of the
To compare the effect of the EGR route on the stor-
test time, and upstream measurements were made at
age efficiency of the LNT, it must be considered that
the start of the test and then periodically. Solid lines
LNT works under different conditions depending on
represent real measurements while dashed lines are
Lapuerta et al. 161

Figure 8. Instantaneous and accumulated emissions during NEDC for switches from HP-EGR to LP-EGR at different coolant
temperatures.

interpolations between real measurements when data Effect of engine coolant temperature as a switch
was not available as a consequence of the need to share point from HP-EGR to LP-EGR
the same gas analyzer. The engine conditions did not
After analyzing the effect of the EGR route under
change during the tests.
As a consequence of the higher NOx emission rates steady conditions, the engine was tested simulating the
for HP-EGR (and even higher NOx concentrations, NEDC by means of the RLS system (see section
although not shown in Figure 7), the storage efficiency ‘‘Engine test bench’’). In all cases, pollutant concentra-
decays strongly in a shorter time. For HP-EGR at tions were multiplied by the flow rate at the tailpipe
100 km/h mode, a sharply decreasing efficiency is (which in the case of LP-EGR is not equivalent to the
observed from the beginning, and only 10% storage flow rate through the aftertreatment system) in order
efficiency is finally achieved after 200 s. This implies to simulate real engine-out emissions assuming that
that, in case of using HP-EGR at this condition, the there is no aftertreatment system.
LNT storage capacity would be clearly undersized. Engine coolant temperature is one of the parameters
Such a decrease in the efficiency is not so fast at 70 km/h, selected to manage the shift between EGR loops during
showing that, as far as enough active sites remain (during the engine warming up (Figure 2). European certifica-
the first 500 s of the test), NOx adsorption is not influ- tion procedures for commercial vehicles23 must follow
enced by the EGR route. Furthermore, residence time a cycle emission test from cold start. This means that
and temperature are not playing an important role since vehicle emissions were evaluated including the engine
LP-EGR has the same storage efficiency than HP-EGR warming up. In this study, tests were made taking dif-
despite the shorter residence time and the lower ferent coolant temperatures as switch points to change
temperature. from HP-EGR to LP-EGR.
Taking the 70% as the minimum of efficiency Figure 8 shows the instantaneous and accumulated
required to activate the purging process, HP-EGR emissions for all the cases, and differences in average
would lead to 7.5 times higher purging frequency than specific emission between the different tests are shown
LP-EGR for the 100 km/h mode and twice higher pur- in Figure 9, together with average specific fuel con-
ging frequency than LP-EGR for 70 km/h. This fact sumption. Results show a trade-off between particle
penalizes the fuel consumption by post-injection during number and NOx emissions. For all tests except full
the LNT purging process. HP-EGR, the extraurban stage (with highest load) is
162 International J of Engine Research 20(1)

1.8 0.4
50 CO (g/km)
1.6 THC (g/km) 0.35
45
40 1.4
0.3
Fuel consumpon (g/km)

35 1.2
0.25

THC (g/km)
CO (g/km)
30 1
0.2
25 0.8
0.15
20 0.6
15 0.1
0.4
10 0.2 0.05
5 0 0
0 Full LP-EGR Switch 35°CSwitch 50°C Switch 64°C Full HP-EGR
Full LP-EGR Switch 35°C Switch 50°C Switch 64°C Full HP-EGR

0.60
9E+13

Parcle number emissions (#/km)

0.50 8E+13

7E+13
0.40 6E+13
NO X (g/km)

5E+13
0.30
4E+13

0.20 3E+13

2E+13
0.10 1E+13

0
0.00
Full LP-EGR Switch 35°C Switch 50°C Switch 64°C Full HP-EGR
Full LP-EGR Switch 35°C Switch 50°C Switch 64°C Full HP-EGR

Figure 9. Average specific fuel consumption and emissions along the NEDC for switches from HP-EGR to LP-EGR at different
coolant temperatures.

entirely made with LP-EGR, and this stage is the most condensation in the intake manifold and contributing
contributing one to NOx emissions. This explains the to shorten the time with highest emissions and lowest
much higher emissions for full HP-EGR with respect engine efficiency.
to the rest of tests. In the extraurban stage, the benefits
of the LP-EGR are more evident. The lower NOx emis-
sions for LP-EGR can be explained by the lower tem- Conclusion
perature for LP-EGR and by the higher EGR ratios The main conclusions of this study are listed below:
and are consistent with results shown in section
‘‘Compared efficiency of HP-EGR and LP-EGR.’’  LP-EGR shows clear benefits in the trade-off
On the contrary, CO and THC emissions decrease between NOx and particle emissions. NOx emis-
slightly and linearly with increasing coolant tempera- sions can be reduced with LP-EGR up to 80% with
tures as switch points, because the time with LP-EGR respect to those with HP-EGR for high loads.
becomes shorter. The decrease in particle emissions is  The LNT storage efficiency decays strongly when
even sharper for early switches than for late ones. The the LNT is close to saturation, no matter if LP-
combination of the mentioned benefits in NOx emis- EGR or HP-EGR is used. As far as enough active
sions and drawbacks in CO, THC, and particle emis- sites remain, NOx adsorption is not affected by the
sions, with no significant effect on fuel consumption, EGR route. Residence time and temperature do
suggest that switching from HP-EGR to LP-EGR at not have a primary effect.
an intermediate coolant temperature would be the most  HP-EGR would need up to 7.5 times purging fre-
interesting strategy. This optimal switch should be quency than LP-EGR at high loads, and twice
made, in any case, before the start of the extraurban higher purging frequency at moderate loads.
mode.  During the engine warm-up, switching from HP-
In addition, the use of HP-EGR during the initial EGR to LP-EGR at an intermediate coolant tem-
part of the engine warm-up is helpful for a faster perature is beneficial for emissions. Switching from
warm-up, since the recirculated gas enters hotter into HP-EGR to LP-EGR at cold engine conditions has
the combustion chamber, thus avoiding water no significant benefits in NOx emissions, while it
Lapuerta et al. 163

has penalties in the rest of emissions (particles, CO, gas recirculation loops for improved fuel economy and
and THC), and could lead to water condensation in reduced emissions in high-speed direct-injection engines.
the intake manifold. Int J Engine Res 2012; 14(1): 3–11.
10. Ladommatos N, Abdelhalim S and Zhao H. The effects
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Declaration of conflicting interests 12. Asad U, Tjong J and Zheng M. Exhaust gas
The author(s) declared no potential conflicts of interest recirculation—zero dimensional modelling and character-
with respect to the research, authorship, and/or publi- ization for transient diesel combustion control. Energ
cation of this article. Convers Manage 2014; 86: 309–324.
13. Praveena V and Lenus Jesu Martin M. A review on vari-
ous after treatment techniques to reduce NOx emissions
Funding in a CI engine. J Energy Inst 2018; 91: 704–720.
14. Kang W, Choi B, Jung S and Park S. PM and NOx reduc-
The author(s) received no financial support for the
tion characteristics of LNT/DPF+SCR/DPF hybrid sys-
research, authorship, and/or publication of this article.
tem. Energy 2018; 143: 439–447.
15. Zheng M, Reader GT and Hawley JG. Diesel engine
ORCID iD exhaust gas recirculation—a review on advanced and
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Magı́n Lapuerta https://orcid.org/0000-0001-7418-1412
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