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Meeting future NO
X
emissions
using an electric heater in an
advanced aftertreatment system
Prathik Meruva
1
, Andrew Matheaus
1
*, Christopher A. Sharp
1
,
James E. McCarthy Jnr
2
, Thomas A. Collins
3
and Ameya Joshi
3
1
Southwest Research Institute (SwRI), San Antonio, TX, United States,
2
Eaton (United States), Galesburg,
MI, United States,
3
Corning Inc., Corning, NY, United States
Engine and aftertreatment solutions are being identified to meet the upcoming
ultra-low NO
x
regulations on heavy duty vehicles as published by the California
Air Resources Board (CARB) and proposed by the United States Environmental
Protection Agency (US EPA) for the year 2027 and beyond. These standards will
require changes to current conventional aftertreatment systems for dealing
with low exhaust temperature scenarios. One approach to meeting this
challenge is to supply additional heat from the engine; however, this comes
with a fuel penalty which is not attractive and encourages other options.
Another method is to supply external generated heat directly to the
aftertreatment system. The following work focuses on the later approach by
maintaining the production engine calibration and coupling this with an Electric
Heater (EH) upstream of a Light-Off Selective Catalytic Reduction (LO-SCR)
followed by a primary aftertreatment system containing a downstream Selective
Catalytic Reduction (SCR). External heat is supplied to the aftertreatment system
using an EH to reduce the Tailpipe (TP) NO
x
emissions with minimal fuel penalty.
Two configurations have been implemented, the first is a Close Coupled (CC)
LO-SCR configuration and the second is an Underfloor (UF) LO-SCR
configuration. The CC LO-SCR configuration shows the best outcome as it
is closer to the engine, helping it achieve the required temperature with lower
EH power while the UF LO-SCR configurations addresses the real-world
packaging options for the LO-SCR. This work shows that a 7 kW EH
upstream of a LO-SCR, in the absence of heated Diesel Exhaust Fluid (DEF),
followed by a primary aftertreatment system met the 2027 NO
x
regulatory limit.
It also shows that the sub-6-inch diameter EH with negligible pressure drop can
be easily packaged into the future aftertreatment system.
KEYWORDS
electric heater, advanced aftertreatment, light off SCR, heavy-duty emissions, reduced
NOx, FTP, light load cycle, WHTC
OPEN ACCESS
EDITED BY
Jinlong Liu,
Zhejiang University, China
REVIEWED BY
Yan Yuchao,
Zhejiang University, China
Meiyao Sun,
Zhejiang University, China
Christopher Ulishney,
WVU, Morgantown, United States
Ziming Yan,
Clemson University, United States
*CORRESPONDENCE
Andrew Matheaus,
andrew.matheaus@swri.org
SPECIALTY SECTION
This article was submitted to Engine and
Automotive Engineering,
a section of the journal
Frontiers in Mechanical Engineering
RECEIVED 27 June 2022
ACCEPTED 03 August 2022
PUBLISHED 06 September 2022
CITATION
Meruva P, Matheaus A, Sharp CA,
McCarthy JE Jnr, Collins TA and Joshi A
(2022), Meeting future NO
X
emissions
using an electric heater in an advanced
aftertreatment system.
Front. Mech. Eng 8:979771.
doi: 10.3389/fmech.2022.979771
COPYRIGHT
© 2022 Meruva, Matheaus, Sharp,
McCarthy, Collins and Joshi. This is an
open-access article distributed under
the terms of the Creative Commons
Attribution License (CC BY). The use,
distribution or reproduction in other
forums is permitted, provided the
original author(s) and the copyright
owner(s) are credited and that the
original publication in this journal is
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academic practice. No use, distribution
or reproduction is permitted which does
not comply with these terms.
Frontiers in Mechanical Engineering frontiersin.org01
TYPE Original Research
PUBLISHED 06 September 2022
DOI 10.3389/fmech.2022.979771
Introduction
The issues of global warming and ozone depletion urge a
need to reduce our environmental emissions, and vehicle
emissions are a major contributor to it, especially heavy-duty
vehicles which constitute a significant portion of the on-road
vehicles. The majority of the diesel vehicle emissions are
comprised of oxides of nitrogen (NO
x
), which are linked with
several health and environmental risks. Historically, there have
been several rounds of emission standards designed to reduce
NO
x
emissions from heavy duty on-road vehicles (Heavy-Duty
Warranty Cost Study Report, 2019). The most aggressive recent
standard is the Low NO
x
Omnibus regulation adopted by the
California Air Resources Board (CARB) to push the limits
further, requiring a 90% reduction in NO
x
from the current
0.20 g/hp-hr standard to 0.02 g/hp-hr starting model year (MY)
2027 while also meeting the existing requirements under Phase
2 Green House Gas (GHG) regulations to improve carbon
dioxide (CO
2
)(California Air Resources Board, 2019;
California Air Resources Board, 2020).
There have been several advances to reduce NO
x
emissions
from heavy-duty engines (Milovanovic et al., 2016;Berndt, 2019;
Zavala et al., 2020). The main area where the aftertreatment
system struggles to reduce NO
x
is during low exhaust
temperature conditions, when the Selective Catalytic
Reduction (SCR) catalytic activity is limited (Scott Sluder
et al., 2005). Electric heaters are one of the technologies that
has been investigated to provide additional heat required for the
aftertreatment system to stay active and convert Engine Out (EO)
NO
x
emissions even during low exhaust temperatures.
There has been a significant work in the past to evaluate the
need and performance of an Electric Heater (EH) towards
reducing tailpipe (TP) NO
x
. Given the concurrent need to
minimize fuel consumption, it is important to manage any
fuel penalty associated with the EH operation. Kasab et al.
(Kasab et al., 2021a) found a fuel penalty of ~1% when
adding an EH upstream of a close-coupled SCR. The resulting
emissions were 0.018 g/hp-hr on Federal Test Procedure (FTP),
meeting the 2027 CARB standards with a 10% margin. Webb
et al. (Webb et al., 2021) measured emissions from a 2017 13L
engine. The addition of a 48 V EH was found to be necessary to
meet the Low Load Cycle (LLC) limit, but a significant fuel
penalty of ~7% was recorded at 4 kW EH power.
Aftertreatment temperatures during a cold start has been
improved by some recent work using a driven turbocharger with
a turbine bypass to heat the aftertreatment system faster. Brin,
J. (Brin et al., 2021) used the turbine bypass on a production
engine with a mechanically driven turbocharger. This technology
measured an increase in the aftertreatment temperature by 50°C
or higher during the first 400 s of a cold FTP cycle with a reduced
fuel consumption. It helps in achieving faster light off
temperature for a SCR for better NO
x
conversion and could
be combined with the EH to get a better NO
x
–CO
2
trade-off.
Similar combination of technology for a better NO
x
–CO
2
trade-off is the use of Cylinder De-Activation (CDA) with the
EH. Matheaus et al. (Matheaus et al., 2021) measured emissions
from a 15 L engine modified to include CDA, and an advanced
after-treatment including a 48 V EH upstream of an LO SCR
with the maximum power of 5 kW. The use of an EH alone
(without CDA) gave substantial NO
x
reduction on a composite
FTP however, it was accompanied with an undesired fuel penalty
of ~1.5% vs. the CDA baseline. Additional work was performed
on this experiment. Zavala et al. (Zavala et al., 2022) found that
similar excellent NO
x
control can be achieved with a maximum
of 2.4 kW heating power. Reducing the heater power aids in
reducing CO
2
since the engine is required to generate the
electricity used to power the EH.
While this paper addresses electrical heat, testing with another
heat source was performed on a similar non-CDA X15 with a more
conventional setup (Harris et al., 2021;McCarthy et al., 2022). A
fuel burner was placed upstream of the conventional AT system.
Composite TP NO
x
results for the FTP were 0.018 g/hp-hr with
less than 1% increase in Brake SpecificCO
2
(BSCO
2
). TP NO
x
results over the LLC was 0.006 g/hp-hr with a 9% increase in
BSCO
2
. A portion of the engine generated CO
2
was traded for the
contribution of CO
2
by the burner.
Most of the ongoing research works to achieve 2027 NO
x
regulations are with the addition of Light-Off Selective Catalytic
Reduction (LO-SCR) to the current production aftertreatment
systems (Kasab et al., 2021b;Sharp et al., 2021;Zavala et al.,
2022). The LO-SCR gains a benefit of reaching the light off
temperature faster than the primary SCR by staying closer to the
engine. In practical applications, packaging of the LO-SCR near
the engine compartment is a difficult task; however, this work has
the LO-SCR positioned at 42 inches for the Close Coupled (CC)
LO-SCR configuration (shown in Figures 1C,2A) and 8 feet
downstream of the turbocharger for the Underfloor (UF) LO-
SCR configuration (shown in Figures 1D,2B) which offers many
packaging options. This work focuses on addressing the
packaging issue by using UF LO-SCR to reach the
2027 CARB NO
x
standards while showing the benefitof
having an CC LO-SCR configuration. The program is focused
on using an EH upstream of LO-SCR with a maximum power of
7 kW in combination with a production engine and
aftertreatment system representative of 2022 production. The
EH heaters are analyzed on different aftertreatment
configurations and compared with the baseline aftertreatment
system to analyze the NO
x
–CO
2
trade off.
Experimental setup
Engine platform
The test engine used for this program was a production
2018 model year Cummins X15 engine with a 500 hp production
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calibration similar to the engine utilized in the CARB Stage 3 on-
road baseline testing (Sharp et al., 2021). The engine retained the
production air handling system, Exhaust Gas Recirculation
(EGR) system, internal components, and fuel system. The
engine is as shown in Figure 3. The baseline aftertreatment
system was representative of the catalysts in production for
the 2022 model year which will be discussed later.
Electric heater (EH)
During the 1990’s, EH concepts were explored to improve the
cold start emissions (Reddy et al., 1994;Weiss et al., 1995;
Hampton et al., 1996). These heaters were based on thin discs
of a high temperature steel honeycomb. The resistance of the
FIGURE 1
(A) Baseline 1 aftertreatment system schematic. (B) Baseline 2 aftertreatment system schematic with LO-SCR. (C) Aftertreatment system
schematic with Close-Coupled (CC) LO-SCR and EH. (D) Aftertreatment system schematic with Underfloor (UF) LO-SCR and EH.
FIGURE 2
(A) Fully insulated aftertreatment system with CC LO-SCR
and EH. (B) Fully insulated aftertreatment system with UF LO-SCR.
FIGURE 3
Cummins X15 engine platform installed in test cell.
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heater disc was adjusted to a target value by adding slits (in the
element) to extend the electrical path length. The packaging of
the heater disc used support rings or analogous structures,
resulting in a self-supporting design, requiring no further axial
support. Examples from the original work are shown in Figure 4.
An electrical potential (voltage) is applied during the “get
hot”mode to the two electrodes resulting in an increase in heater
body temperature through Joule heating. The industry is
considering moving over to 48 V systems to power electric
heaters and other accessories. In this work, a 48 V system to
power a 7 kW EH results in a reasonable current limit of
145 amps. Much of the work in this paper had a maximum
power setting of 5 kW which drops the maximum current to
105 amps. This is a reasonable current level for the time in which
the EH is turned on.
Heater discs have cell densities of 300–450 Cells Per Square
Inch (CPSI), which provide an excellent ratio of surface area for
heat transfer to the gas and are 5–10 mm thick. The high
efficiency and compact design make these honeycomb-based
heater components very attractive. The product design is very
flexible and allows design of the heater to any resistance, within
the practically relevant range (currently ca. 200–500 mOhm).
The details of the catalyst heater used in this study can be
found in Table 1. It should be noted that while it is possible to put
a catalyst on the heater, the heater was not catalyzed in this work,
so it heats the gas stream transferring the heat to the downstream
catalyst components via convection. The heater can be
considered in its simplest and cheapest form.
The sample was packaged in the “Prototype”canning with
the insulating material and support rings (Anderson et al., 2021),
within a stand-alone can (no downstream catalyst directly
attached), enabling placement within the exhaust pipe before
the entrance cone to the LO-SCR. The setup had a bed
thermocouple on the EH for monitoring purpose. The control
algorithm utilized gas thermocouples for feedback temperature
control which would be the intent for production applications.
FIGURE 4
(A) Drawings of an electric catalyst heater (Anderson et al., 2021). Item 10 represents the heater disc, 22 and 24 the support rings, and 20 the
insulation packaging material. (B) Image of the packaged heater.
TABLE 1 Catalyst heater specifications.
Parameter Value
Element diameter 143.8 mm
Thickness 5.08 mm
Mass 173.5 g
CPSI 400
Web thickness 0.15 mm (6 mil)
Resistance 320 mOhm
Support ring ID 133.6 mm
Can length 51 mm
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Aftertreatment
The aftertreatment system has two distinct baselines.
Baseline 1 is a primary aftertreatment system consisting of
Diesel Oxidation Catalyst (DOC), Diesel Particulate Filter
(DPF), compact mixer, Selective Catalytic Reduction (SCR)
and Ammonia Slip Catalyst (ASC) and is representative of a
2022 model year. This system has a conventional a Diesel
Exhaust Fluid (DEF) injector mounted on the mixer.
Schematic representation of this system is shown in
Figure 1A.Table 2 shows the catalyst volumes for this
2022 production like aftertreatment system.
Baseline 2 is a system that builds on Baseline 1 by adding a
LO-SCR and another conventional DEF injector. There have
been many relative studies that show the merit of adding a LO-
SCR for cold start operation as this is a smaller catalyst that can
heat up quickly when positioned closed to the engine. An
illustration of the Baseline 2 aftertreatment system is shown in
Figure 1B. There is a mixer located after the DEF injector.
Previous work (Sharp et al., 2021)utilizedaheateddoser
in front of the LO-SCR. A heated doser allows DEF dosing
down to 130°C exhaust temperature instead of the normal
dosing temperature of 180°C for most advanced SCR systems.
This study utilized a conventional doser (non heated) in front
of the LO-SCR. With conventional dosing, the temperature
required to begin dosing is 180°CtoensureproperDEF
vaporization and prevention of urea deposits. However,
later in this work, with the use of an EH, the LO-SCR
temperature rises very fast so the use of a heated doser is
considered not necessary. The difference between 130 and
180°C during a cold FTP is approximately a 5 s delay in dosing
DEF. For reference, the same temperature difference at the
downstream primary SCR is an 84 s delay and occurs much
later in the cycle due to the thermal masses involved. The
catalysts specifications, for Baseline 2 and EH tests are listed in
Table 3, which includes the addition of an upstream LO-SCR
catalyst system.
This program focuses on utilizing a LO-SCR and external
heat supply upstream of the LO-SCR system shown in Table 3 as
depicted by Figure 1C. The EH was placed ahead of the LO-SCR
to enable NO
x
reduction as soon as possible. Heater control
targeted the average gas temperature (inlet and outlet) of the
LO-SCR.
There were two approaches to the LO-SCR aftertreatment
configuration in this program that is relevant to packaging on a
Class 8 Truck: 1) Close-Coupled (CC) LO-SCR and 2)
Underfloor (UF) LO-SCR. The first approach was to represent
a similar aftertreatment configuration as the Stage 3 Low NO
x
program (Sharp et al., 2021) and the second approach was to
consider a real-world issue of packaging the LO-SCR on the
chassis rail. As mentioned earlier, Figure 1B shows the baseline
configuration of the LO-SCR. This setup is referred to as close-
coupled, because the LO-SCR is as close to the turbine outlet as
possible. This was kept constant in this study while the
configuration of EH and LO-SCR was allowed to vary from in
the close coupled and underfloor configurations. However, the
distances are representative of placing the LO-SCR directly under
the passenger steps.
The electric heater was similar in size to exhaust system and
was easily integrated into the system. The mixer was located
between the DEF doser and the electric heater. The illustration of
the heater location is shown in Figure 1C. A photograph of the
setup is shown in Figure 2A.
Exhaust insulation will likely be required to meet
2027 emissions regulations. Current one-box systems already
reduce exhaust cooling and provide insulation. The work in this
paper utilized exhaust blankets to retain heat similar to the Stage
3 work (Sharp et al., 2021). Limiting the cooling of the exhaust
system will help to reduce power consumption of the EH or
energy consumption of other heating strategies and should be
considered for serial production.
The UF LO-SCR scenario was accomplished by moving the
LO-SCR further away from the engine and closer to the DOC of
the primary downstream system. A photograph of the setup is
shown in Figure 2B. The EH stayed positioned in front of the LO-
SCR while the doser and mixer remained close to the turbine exit.
Hence, the only aftertreatment system that was moved was the
EH and LO-SCR. An illustration of the system is shown in
Figure 1D. A true UF LO-SCR configuration would be
approximately six feet. In the test cell, the distance was longer
to fully capture the appropriate worst-case distance. Hence, eight
feet, as tested, would be an extreme length of UF configuration.
Table 4 provides the distances between the engine turbo out,
LO-SCR and DOC for the different aftertreatment
configurations. Baseline 1, which included only the primary
TABLE 2 Baseline 1 system catalyst specifications.
Component D × L CPSI Volume (L)
DOC 13″×5″400 11
DPF 13″×7″300 15
SCR 13″×6″600 13
SCR-ASC 13″×6″600 13
TABLE 3 Baseline 2 and EH testing catalyst specifications.
Component D × L CPSI Volume (L)
LO-SCR 13″×6″400 13
DOC 13″×5″400 11
DPF 13″×7″300 15
SCR 13″×6″600 13
SCR-ASC 13″×6″600 13
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AT system, was configured with 93 inches from the engine turbo
out to the DOC. Baseline 2, which included the LO-SCR;
configuration lengths are the same as the CC LO-SCR values.
The aftertreatment system used in this work was
hydrothermally aged by utilizing an accelerated aging protocol
on a burner based aging platform. The aging protocol targeted
the aftertreatment system with the equivalent amount of thermal
exposure for a Full Useful Life (FUL) system i.e., 435,000 miles or
9,800 h of service accumulation time similar to previous works
(Matheaus et al., 2021;McCarthy et al., 2022;Zavala et al., 2022).
The catalysts at the end of this aging cycle are commonly referred
to as “Development Aged”end of life catalysts. The difference
between “Developed Aged”and “Real World Exposed to
Chemical Poisoning”is characterized in previous works
(Sharp et al., 2021).
A model-based controller (Sharp et al., 2017;Rao et al., 2020;
Sharp et al., 2021) was being used in this program to control the
DEF dosing similar to the baseline work (Meruva et al., 2022)and
also the thermal management strategies to power on the EH. The
model tracks ammonia storage in each of the SCR substrates and
has a target ammonia storage based on temperature. The DEF
doser configurations were the same between the tests with the EH
and their respective baseline tests (Meruva et al., 2022). The model-
based control is discussed in Meruva, et al. (Meruva et al., 2022).
Electric heater (EH) power
The EH was powered up by an external power supply in the
same way as previous work (Zavala et al., 2022) and similar to
representative burner work (McCarthy et al., 2022). The control
algorithm was Proportional Integral Derivative (PID) control as a
basis and varied the voltage output of the power supply from 0 to
48 V. In this work, the feedback control for the PID was based on
the average LO-SCR temperature. The average LO-SCR
temperature was taken from the inlet and the outlet gas
temperatures of the LO-SCR. These were production
thermocouples that protrude 2 inches from the wall. This is
consistent with other EH works in the past (Matheaus et al., 2021;
Zavala et al., 2022). The voltage and current measurements were
recorded at the EH and used to calculate the electrical power
supplied to it. In real-time, a parasitic load was applied to the
engine assuming an 80% generator efficiency. This is assuming a
mild hybrid system with efficient power generation. Alternator
efficiencies are known to be lower than 80%. The engine was not
given credit for this parasitic load in the cycle work. However,
CO
2
and NO
x
emissions due to this additional load were included
in the analysis. The formula used for the real-time addition of
load is show in Eq. 1.
Total Engine Torque (Nm)Cycle Torque (Nm)
+
EH Power (KW)
0.8p9548.8
Engine Speed rpm(1)
Emissions measurements
Emission testing in this work were performed on a motoring/
absorbing engine dynamometer test cell utilizing raw exhaust
measurements complying with a Code of Federal Regulations
Part 1065. The measurement equipment used included the
following:
•A raw Horiba MEXA 7000 series each for the engine out
and TP emissions sampling
•An FTIR for LO-SCR out NO
x
emission measurements
The variability of NO
x
measurement is ~ ± 0.001 g/hp-hr and
CO
2
measurement is ~ ± 2 g/hp-hr. This is true for all the results
discussed in this work.
Test cycles evaluated
Multiple key test cycles were evaluated using the engine and
aftertreatments systems described above. These include the
Heavy-Duty FTP and the LLC. Additional real world driving
cycles evaluated were the Beverage Cycle and the Stay Hot Cycle.
Finally, the European World Harmonized Transient Cycle
(WHTC) was evaluated.
Federal test procedure (FTP)
United States Environmental Protection Agency (US EPA)
and CARB use the US Heavy-Duty FTP standard regulatory
drive cycle for evaluation of emission standards. The regulatory
TABLE 4 Change in length for UF aftertreatment.
Distances CC LO-SCR (inches) UF LO-SCR (inches)
Turbo-out to LO-SCR 42 96
Between LO-SCR and DOC 51 18
Turbo-out to DOC 93 132
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FTP composite NO
x
limit for 2027 has been set to 0.02 g/hp-hr.
Composite results are calculated as 6/7 of hot FTP and 1/7 cold
FTP. This is a well-known cycle, so it is not shown here.
Low load cycle (LLC)
The LLC is a composite of multiple real-world driving cycles.
Data was collected over various applications and combined to
form this cycle. A problem with the FTP as an evaluation cycle is
that it has higher loads than many real-world situations. Due to
the higher-than-normal loads, the exhaust temperatures are
much hotter, and the SCR system stays active. The LLC cycle
was developed to be more realistic of use and has been approved
as a regulatory cycle. These lighter loads force the exhaust
temperatures to be much lower and NO
x
control becomes a
challenge. The cycle is 5535 s in duration and is preceded by an
FTP cycle with a 20-min soak period between them. The
regulatory NO
x
limit for LLC cycle for 2027 has been set to
0.05 g/hp-hr.
Beverage cycle
The Beverage cycle is a subset of the LLC and is derived from
a service truck delivering Beverages or packages. This cycle has
idle sections that are longer than 1 min and several transient
ramps. The average load is 7.1%. Four Beverage cycles (800 s
each) are connected to form a test. The initial two cycles were for
thermal conditioning. The emission values were quantified for
the final two cycles only. The authors have found the Beverage
cycle to be an excellent cycle for controls optimization as is much
shorter (~4 cycles in less than 1 h) than the LLC (~1.5 h) while
the load factors are nearly identical.
Stay hot
The Stay Hot drive cycle involves conditioning the engine
and the aftertreatment system at a preset speed and load till the
temperatures attain steady state. This is succeeded by a 40-min
idle period before returning to the previous load and speed
conditions. This cycle focuses on how a long idle impacts the
NO
x
conversion efficiency of the aftertreatment system during
the idle and immediately after a return to service.
World harmonized transient cycle (WHTC)
The WHTC drive cycle is based on the global pattern of real
heavy-duty commercial vehicle usage. It is a transient engine
dynamometer cycle. The United Nations Economic Commission
for Europe, Working Party on Pollution and Energy group
developed the Global Technical Regulation group which
covers a worldwide harmonized heavy-duty certification
procedure for engine exhaust emissions. Composite WHTC
values are calculated by 14% cold and 86% hot. This is also a
well-known cycle, so it is not shown here. Typically, a European
engine would be tested for the WHTC while for this work, a US
production engine was tested for this cycle. As such, these results
give an indication of what may be possible while no effort was
spent to truly represent a European engine calibration.
Results and discussion
The section shows the test results performance comparison
of the CC LO-SCR and the UF LO-SCR configurations using
different drive cycles which are FTP, LLC, Beverage, Stay Hot and
the WHTC. These results demonstrate that using an EH with
maximum power of 7 kW upstream of a LO-SCR in combination
with a production engine and an aftertreatment system
representative of 2022 production will be able to achieve NO
x
standards for the MY 2027.
This project used two baseline aftertreatment systems. The
baseline 1 was to represent the current 2022 production
aftertreatment system while baseline 2 added the upstream
LO-SCR to baseline 1 configuration. Baseline 2 is
important for this work as the LO-SCR was used with all the
EH results.
Baseline 1: Representative of
2022 aftertreatment system (without
LO-SCR)
Baseline 1 (shown in Figure 1A) emissions are shown in
Table 5. Please note that the SCR is larger than what would be in
production for this engine and model year. Therefore, these
emissions are better than what a 2018 engine + aftertreatment
system would yield. The FTP composite Brake SpecificNO
x
(BSNO
x
) value is 6.5 times higher than the limit of 0.02 g/hp-hr.
For the LLC, the baseline 1 value is 18 times the limit of 0.05 g/
hp-hr. Even with the larger aftertreatment system, additional
technology is required to reduce NO
x
emissions, primarily at
lighter loads.
Baseline 2: Added LO-SCR to baseline 1
The baseline 2 (shown in Figure 1B) test results for both the
above-mentioned configurations are the same as our previous
works for a non-CDA engine with a conventional DEF doser
(unheated DEF) for the LO-SCR (Meruva et al., 2022) and the
summary of those results are as shown in Table 5. The composite
FTP NO
x
value is three times the limit. The LLC value is more
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than 14 times the limit. The baseline is also not within the EURO
VI regulations of 0.46 g/kW-hr for the composite WHTC. Just
adding a LO-SCR does not immediately remedy the high NO
x
values. Engine calibration/optimization would improve on these
values. However, additional technologies are still required to
meet 2027 emissions standards.
Composite FTP
Table 6 shows the FTP composite values of both the CC
LO-SCR (shown in Figure 1C)andtheUFLO-SCR(shownin
Figure 1D)configurations, with the EH upstream of the LO-
SCR, as these are compared to both baseline results. The CC
configuration is within the NO
x
regulatory limit of 0.02 g/hp-
hr for the year 2027. The UF configuration exceeds the limit;
however, the UF configuration was tested without any changes
to the calibration and the authors believe that tuning the
model will reduce the TP NO
x
values.Thisisstillasignificant
finding as there is considerable additional distance and metal
surface area. Both the EH configurations had a NO
x
conversion efficiency of over 99%. Observing the LO-SCR
NO
x
out values, the LO-SCR reduced 75% of the EO NO
x
for
Baseline 2. The LO-SCR reduced 84% of the EONO
x
in the CC
configuration.Finally,intheUFconfiguration, the LO-SCR
reduced 87% of the EO NO
x
.Withthissaid,theLO-SCRisnot
specifically designed to handle all the engine out NO
x
.Thereis
room to optimize LO-SCR size.
TABLE 5 Baseline 1 and Baseline 2 emission test results
Configuration Baseline 1 Baseline 2
Cycle Units BSNO
x
BSCO
2
BSNO
x
BSCO
2
EO TP EO LO-SCR out TP
Cold FTP g/hp-hr 2.18 0.209 524.6 2.00 0.45 0.159 529.5
Hot FTP g/hp-hr 2.70 0.116 503.1 2.77 0.71 0.043 504.9
Composite FTP g/hp-hr 2.60 0.129 506.2 2.66 0.67 0.060 508.5
LLC g/hp-hr 4.00 0.918 619.2 4.10 2.68 0.716 614.9
Beverage g/hp-hr 4.08 2.147 698.2 4.06 3.60 1.669 686.3
Stay Hot g/hp-hr 3.20 0.428 644.0 3.14 1.37 0.238 655.9
Cold WHTC g/kW-hr 4.28 0.232 676.8 3.98 1.53 0.245 678.8
Hot WHTC g/kW-hr 5.34 0.093 654.1 5.00 2.08 0.125 659.2
Composite WHTC g/kW-hr 5.19 0.112 657.3 4.86 2.01 0.142 661.9
TABLE 6 FTP composite, cold FTP, and hot FTP test results
Cycle Configuration BSNO
x
(g/hp-hr) BSCO
2
(g/hp-hr) Integrated heater power
consumption (kW-hr)
EO LO-SCR Out TP
Cold FTP Baseline 1 2.18 —0.209 524.6 —
Baseline 2 2.00 0.45 0.159 529.5 —
CC LO-SCR 1.98 0.31 0.075 542.5 0.67
UF LO-SCR 2.10 0.29 0.095 546.6 0.79
Hot FTP Baseline 1 2.70 —0.116 506.2 —
Baseline 2 2.77 0.71 0.043 504.9 —
CC LO-SCR 2.69 0.44 0.010 512.7 0.40
UF LO-SCR 3.20 0.41 0.012 512.4 0.48
Composite FTP Baseline 1 2.60 —0.129 506.2 —
Baseline 2 2.66 0.67 0.060 508.5 —
CC LO-SCR 2.59 0.42 0.019 517.0 —
UF LO-SCR 3.04 0.40 0.024 517.3 —
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The composite results in Table 6 are shown graphically in
Figure 5.AsobservedinTable 6, UF LO-SCR seems to slightly better
NO
x
conversion than the CC LO-SCR as the UF LO-SCR is closer to
the downstream aftertreatment system which helps in preserving
thermal inertia of the catalyst, which is lower in the case of a CC LO-
SCR. The EH cases reduce TP NO
x
significantly with a modest
penalty in CO
2
. The heater control strategy for FTP was the same
between both the configurations with a maximum power
consumption of 7 kW and the average LO-SCR target gas
temperature of 235°C (average LO-SCR T = (T
in
+T
out
)/2). The
control temperature on the LO-SCR was 10°Chigherthanthe
optimal value found in (Zavala et al., 2022); however, it is good to see
that the values are very close between 225 and 235°C.
Cold FTP
The cold FTP results are shown in Table 6.TheCC
configuration provided the lowest TP NO
x
emissions with
only 0.67 kW-hr of electrical energy used. The UF
configuration required 0.79 kW-hr of energy and had
higher TP NO
x
emissions than the CC variant. Ideally,
the cold FTP NO
x
emissionstargetis0.08g/hp-hrtohave
enough margin for the composite calculation. Increasing the
distance of the aftertreatment system from the engine
inherently allows the exhaust to cool down further so
increasedheaterpowerisexpected.Also,theUF
configuration may benefit from a different heater control
logictoreduceTPNO
x
.
Requiring the engine to generate the power for the EH shows
up in higher CO
2
numbers. Naturally, the higher electrical
consumption yields higher CO
2
. Less than 1 kW-hr of energy
is required to reduce the cold TP NO
x
emissions in the range to
meet 2027 emissions limits. The energy is spent during the first
600 s of the FTP.
Figure 6 shows a graphical comparison of the CC LO-SCR
and the UF LO-SCR configurations to Baseline 2 for the cold FTP
cycle. The heater is required for the first half of the FTP because
this is the lighter loaded portion. After 600 s, the engine
generates enough heat to keep the SCR in an optimum
temperature range.
Observing the LO-SCR average temperature, the EH does a
great job of raising the exhaust temperature above 200°C by 105 s
for the CC, and 138 s for the UF. A large portion of the NO
x
emissions is emitted during the first acceleration (~45 s). This
can be seen in the cumulative TP NO
x
graph. Since the CC
system reaches more optimum temperature faster, it can start
reducing NO
x
on the first acceleration. The UF does not reach
temperature as quickly, so it matches Baseline 2 for the first
acceleration. At the second accelerations (218 s), both CC and UF
are reducing a large portion of the NO
x
emissions. Baseline 2 is
still below optimum temperature. By the third acceleration
(382 s), both CC and UF configurations are effectively
reducing NO
x
.
At approximately 200 s, the heater control logic on
the CC configuration starts to reduce power. This can be
observed on the instantaneous heater power graph as
it starts to pulse. For the UF configuration, the heater stays
at maximum power for a longer period. The impact on
electrical consumption is shown on the cumulative heater
power graph.
Hot FTP
Table 6 shows the numerical comparison of both the
configurations along with the baselines for the hot FTP cycle.
There are some variations in EO NO
x
due to test-to-test
variation. The engine is expected to operate on two different
modes; assumed as thermal management mode and fuel
economy mode. The engine switches between these modes
based on the aftertreatment temperature. In the case of the
UF configuration, as the aftertreatment system temperature is
slightly higher than other configurations the engine is expected to
stay on fuel economy mode for a longer duration which is
expected to be the cause of a higher EO NO
x
. Both EH
configurations reduce TP BSNO
x
to levels appropriate to meet
2027 emissions.
The EH BSCO
2
values are 1.4% higher than baseline values
because the engine is required to generate the electricity
consumed by the EH. This is not a fair comparison because if
the baseline engine was calibrated in a manner to meet
2027 emissions standards, the BSCO
2
will likely be higher
than reported here. One such technique, retarding injection
FIGURE 5
BSCO
2
and BSNO
x
shown for composite FTP for all
configurations.
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timing, is known to increase fuel consumption and CO
2
emissions.
A graphical comparison of the CC and UF configurations
to the baselines are shown in Figure 7.SincethehotFTP
follows the cold FTP with a 20-min soak, there is enough
thermal heat retained in the substrates that the NO
x
generated in the first acceleration is reduced in all
configurations. The LO-SCR substrate cools down during
the idle portions of the FTP. This shows up as increased
TP NO
x
emission for the second and third accelerations on
the cumulative TP NO
x
graph. The EH configurations have
NO
x
fully under control by the third acceleration. The EH
controllerinbothconfigurations start reducing power early
in the cycle. The integrated electrical consumption is 0.40 and
0.48 kW-hr for the CC and UF, respectively. The hot FTP uses
40% less electrical power than the cold FTP, irrespective of
the configuration.
Low load cycle (LLC)
Results for the LLC cycle are shown Table 7. The EO NO
x
emissions are higher for the heater configurations indicating
that the engine was operating in a more fuel-efficient
mode. Aftertreatment temperatures are the primary basis
for determining the mode of operation. The engine is
allowed to operate more efficiently when the aftertreatment
system is hot.
FIGURE 6
Comparison of the CC LO-SCR and UF LO-SCR to Baseline 2 on a Cold FTP cycle for the LO-SCR average temperature, emissions, and heater
electrical power consumption.
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The heater control strategy for the LLC cycle was set to
target 225°C as the average LO-SCR temperature with 5 kW
max heater power. Electrical energy consumption is much
greater for the LLC than seen on the FTP tests. The LLC is
1.5 h long compared to the 20-min FTP. Overall lower loaded
cycle with a longer test duration yields an increased electrical
demand which directly increases CO
2
.TheUFconfiguration
had a lower TP NO
x
but a higher electrical energy consumption
than the CC configuration.
The cycle results for the LLC are graphically shown in
Figure 8. The NO
x
regulatory limit for this cycle is 0.05 g/hp-
hr. The dramatic reduction in BSNO
x
is apparent when utilizing
the EH. Also apparent is the increase in BSCO
2
. However, the
baseline CO
2
numbers are at a much higher NO
x
value. The
actual CO
2
numbers for a non-EH 2027 solution is not known at
this time. It can be assumed that NO
x
reduction comes at a cost of
CO
2
unless a technology such as CDA is utilized.
The comparison graph for the LLC is provided in Figure 9.
The LO-SCR average temperature for both EH cases is
considerably higher than Baseline 2, which accounts for
the excellent NO
x
reduction (see cumulative TP NO
x
graph). Both configurations had an increased average
temperature of 52°C when compared to Baseline 2. Both
configurations operated with the same heater strategy
which was 5 kW maximum heater power and an average
LO-SCR setpoint of 225°C. The UF configuration
consumed 14% more power than the CC configuration,
mostly on the first half of the cycle.
FIGURE 7
Comparison of the CC LO-SCR and UF LO-SCR to Baseline 2 on a Hot FTP cycle for the LO-SCR average temperature, emissions, and heater
electrical power consumption.
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Beverage cycle
Table 8 shows the numerical comparison of both the
configurations along with the baselines for the Beverage cycle.
This is a good “in-use”evaluation of delivery trucks such as UPS,
Coca-Cola, and Amazon. This is also a good representation of
highly congested traffic areas such as Brazil. Both EH cases show
over 99% NO
x
conversion efficiency with similar heater power
consumption. On average, the EH cases had 11% higher CO
2
than the baseline cases.
Figure 10 shows the graphical comparison for the Beverage
cycle. The heater control strategy was the same as the LLC cycle:
225°C average LO-SCR target with 5 kW max heater power. The
average LO-SCR for Baseline 2 over the cycle was 163°C. This still
allowed for a 59% NO
x
conversion efficiency due to ammonia
storage. However, additional NO
x
control is required to meet
emissions requirements. The EH in the CC configuration had an
average temperature increase of 54°C. The EH in the UF
configuration had an average temperature increase of 52°C.
The cumulative heater power values are nearly identical for
the two cases, slightly over 2 kW-hr.
Stay hot
The Stay Hot test results are shown in Table 9.BothEH
cases show a 99% NO
x
conversion efficiency. Similar TP NO
x
are achieved between the two cases with similar heater power
consumed. These emissions are within the 2027 in use
standards.
Multiple heater strategies were tested, and the best NO
x
/CO
2
tradeoffs are presented in this paper. For the CC LO-SCR, the
best strategy was a maximum heater power of 5 kW and a single
setpoint of 225°C targeting the average LO-SCR temperature. The
heater strategy for the UF LO-SCR configuration is different and
is shown in Table 10. Since the UF configuration is much closer
to the primary AT system, the most efficient strategy tried used
the primary SCR inlet temperature as feedback. Since these are
closer together, the EH also impacts the primary SCR
temperature.
The multi-tier thought process is to minimize the electrical
power needed. One way to minimize fuel consumption and CO
2
increase is to limit the maximum heater power. In some cases,
however, the limited power does not allow for enough heat
generation and there is NO
x
break through. The multi-tier
strategy was developed so that when the target catalyst is
colder, then a higher maximum electrical power and a higher
setpoint temperature is allowed. As the target catalyst increases in
temperature, the maximum power allowed and the setpoint
temperature are reduced to minimize the electrical power
consumption and thereby minimizing CO
2
. The multi-tier
approach was applied in different ways by targeting different
catalyst temperatures.
The graphical comparison for the Stay Hot tests is provided
in Figure 11. The EH for the CC case turns on 239 s after drop to
idle. The EH turns on for the UF case at 388 s after drop to idle.
For the UF case, the strategy is limiting maximum EH power
due to the temperature of the primary SCR. At 1250 s, the
primary SCR is cooling off and the maximum heater power is
increased slowly over time. There is a difference in average LO-
TABLE 7 LLC test results.
Configuration BSNO
x
(g/hp-hr) BSCO
2
(g/hp-hr) Integrated heater power
consumption (kW-hr)
EO LO-SCR out TP
Baseline 1 4.00 —0.918 619.2 —
Baseline 2 4.10 2.68 0.716 614.9 —
CC LO-SCR 4.81 1.25 0.049 660.8 5.26
UF LO-SCR 4.77 0.88 0.014 656.5 5.99
FIGURE 8
Cycle results for the LLC.
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FIGURE 9
Comparison of the CC LO-SCR and UF LO-SCR to Baseline 2 on the LLC cycle for the LO-SCR average temperature, emissions, and heater
electrical power consumption.
TABLE 8 Beverage cycle test results.
Configuration BSNO
x
(g/hp-hr) BSCO
2
(g/hp-hr) Integrated heater power
consumption (kW-hr)
EO LO-SCR out TP
Baseline 1 4.08 —2.147 698.8 —
Baseline 2 4.06 3.60 1.669 686.3 —
CC LO-SCR 6.03 1.62 0.019 762.5 2.03
UF LO-SCR 5.89 1.67 0.033 775.6 2.15
Note that the in-use limits set by CARB in the low NO
x
rule are higher than the certification lab test cycle limits by a factor of 2 for model years 2024 through 2029. Accordingly, while the
NO
x
limit on the HD-FTP cycle is 0.020 g/bhp-hr for MY 2027, the corresponding in-use limit is 0.040 g/bhp-hr (for intermediate life of 435,000 miles). It is seen that the above results on
the Beverage cycle are within these in-use limits with 50% margin for the CC LO-SCR case.
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FIGURE 10
Comparison of the CC LO-SCR and UF LO-SCR to Baseline 2 on the Beverage cycle for the LO-SCR average temperature, emissions, and heater
electrical power consumption.
TABLE 9 Stay Hot test results.
Configuration BSNO
x
(g/hp-hr) BSCO
2
(g/hp-hr) Integrated heater power
consumption (kW-hr)
EO LO-SCR out TP
Baseline 1 3.20 —0.428 644.0 —
Baseline 2 3.14 1.37 0.238 655.9 —
CC LO-SCR 3.67 1.23 0.021 679.9 1.80
UF LO-SCR 3.93 0.29 0.016 687.8 1.51
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TABLE 10 Multi-tier heater control strategy based on downstream SCR temperature.
Primary SCR Avg T
(°C)
Max heater power (kW) LO-SCR Avg setpoint T
(°C)
07 235
150 7 235
190 5 220
200 3 210
≥210 2.4 200
FIGURE 11
Comparison of the CC LO-SCR and UF LO-SCR to Baseline 2 on the Stay Hot cycle for the LO-SCR average temperature, emissions, and heater
electrical power consumption.
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TABLE 11 Composite WHTC, cold WHTC and hot WHTC test results.
Cycle Configuration BSNO
x
(g/kW-hr) BSCO
2
(g/kW-hr) Integrated heater power
consumption (kW-hr)
EO LO-SCR out TP
Cold WHTC Baseline 1 4.28 —0.232 676.8 —
Baseline 2 3.98 1.53 0.245 678.8 —
CC LO-SCR 4.20 1.35 0.095 692.4 0.88
UF LO-SCR 4.41 0.56 0.145 691.3 1.19
Hot WHTC Baseline 1 5.34 -- 0.093 654.1 —
Baseline 2 5.00 2.08 0.125 659.2 —
CC LO-SCR 6.10 2.15 0.031 666.1 0.66
UF LO-SCR 5.85 0.90 0.007 661.1 0.87
Composite WHTC Baseline 1 5.19 -- 0.112 657.3 —
Baseline 2 4.86 2.01 0.142 661.9 —
CC LO-SCR 5.83 2.04 0.040 669.8 —
UF LO-SCR 5.65 0.86 0.026 665.3
FIGURE 12
Comparison of the CC LO-SCR and UF LO-SCR to Baseline 2 on the cold WHTC for the LO-SCR average temperature, emissions, and heater
electrical power consumption.
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SCR temperature during the maximum power limited period
(800–1250 s). Afterward, the average LO-SCR temperatures
converge. The benefit of the multi-tier strategy shows up as
a reduced cumulative integrated power. Observing the
cumulative TP NO
x
graph, Baseline 2 starts to have NO
x
breakthrough at 2000 s. As the engine returns to service,
there is a spike in TP NO
x
. Both EH cases keep the NO
x
breakthrough at a minimum.
Composite WHTC
The composite WHTC results are shown in Table 11. The
baseline cases show a 97% NO
x
reduction. The WHTC is a high
enough loaded cycle that the baseline configurations do well. The
EH cases have over 99% NO
x
reduction. The increase in BSCO
2
is
minimal because the electrical power required is small. The EH
control algorithm for all WHTC cases were targeting the LO-SCR
average temperature to be 235°C and allowing the full 7 kW
heater power, if needed.
Cold WHTC
The cold WHTC results are shown in Table 11. The NO
x
conversion efficiencies for the baseline cases are in the low 90%
range. The EH allows for higher NO
x
conversion efficiencies
(97%). The increase in CO
2
is 2% on average.
FIGURE 13
Comparison of the CC LO-SCR and UF LO-SCR to Baseline 2 on the hot WHTC for the LO-SCR average temperature, emissions, and heater
electrical power consumption.
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The graphical comparison for the cold WHTC is
provided in Figure 12.TheidleperiodsfortheWHTC
are roughly 200–360 s, 710–775 s, and 1150–1200 s. The
EH is on from the start of the cycle to 650 s and from
1000 to 1200 s. It is not utilized much in other areas of the
cycle. For the EH cases, most of the TP NO
x
is generated in
the first 100 s.
Hot WHTC
The hot WHTC results are shown in Table 11. The hot
test uses 25% less electrical energy than the cold test. The EH
cases cut TP NO
x
by more than 60% when compared to
baseline. The results are shown graphically in Figure 13.The
EH controller maintains the average LO-SCR at or above
235°C. For the EH cases, the cumulative TP NO
x
is under 1 g.
As before, the UF configuration used more electrical energy
than the CC configuration.
Summary/conclusion
The test results from this program show that the CARB
proposed 0.02g/hp-hr NO
x
target for the year 2027 can be
achieved with a current production engine without any
modifications to it. The following conclusions can be made
from the above test data:
•A small EH coupled with a LO-SCR and downstream
primary aftertreatment system can reduce the TP NO
x
emissions and reach an FTP composite of 0.02 g/hp-hr
while also maintaining the NO
x
emissions within 0.05 g/
hp-hr for an LLC cycle. A composite FTP NO
x
of
0.019 g/hp-hr was achieved with hydrothermal end-
of-life aged catalysts. Using the same catalysts, TP
NO
x
of 0.049 g/hp-hr for the close coupled
configuration and 0.014 g/hp-hr NO
x
for the
underfloor configuration was achieved for the LLC.
The CC LO-SCR has an added benefitofbeingclose
to the engine which yields higher temperature for the
catalyst helping in reducing the heater power required to
begeneratedtokeeptheaftertreatment at the required
temperature
•Considering that there are packing difficulties for the CC
LO-SCR, even the UF LO-SCR that is positioned further
downstream can help in achieving the 2027 NO
x
emissions
with a minimal additional fuel penalty
A summary of all TP NO
x
and CO
2
results is shown in
Table 12 that includes the FTP, LLC, Beverage, Stay Hot and
WHTC with the LO-SCR in both the close coupled (CC) and
underfloor (UF) configurations. The test data results prove
that an EH with a maximum power capacity of 7 kW should a
good viable option in reducing the NO
x
emissions and its
maximum power capacity might be required only on few
tests, especially during the cold starts but most of the hot
cycles might just need a maximum power capacity of 5 kW.
Future work
CDA technology seems to be a viable option to increase the fuel
economy and improve the aftertreatment thermal management as
observed in previous works (Joshi et al., 2017;McCarthy, 2017;
Ramesh et al., 2018;McCarthy, 2019). Future work could include
adding CDA technology to the engine in combination with the EH
technology to provide a better NO
x
vs. CO
2
trade off. This future
TABLE 12 Summary TP BSNO
x
and TP BSCO
2
test results.
TP BSNOx TP BSCO2
Cycle Units Base 1 Base 2 CC UF Base 1 Base 2 CC UF
Cold FTP g/hp-hr 0.209 0.159 0.075 0.095 524.6 529.5 542.5 546.6
Hot FTP g/hp-hr 0.116 0.043 0.010 0.012 503.1 504.9 512.7 512.4
Composite
FTP
g/hp-hr 0.129 0.060 0.019 0.024 506.2 508.5 517.0 517.3
LLC g/hp-hr 0.918 0.716 0.049 0.014 619.2 614.9 660.8 656.5
Beverage g/hp-hr 2.147 1.669 0.019 0.033 698.2 686.3 762.5 775.6
Stay Hot g/hp-hr 0.428 0.238 0.021 0.016 644.0 655.9 679.9 687.8
Cold WHTC g/kW-hr 0.232 0.245 0.095 0.145 676.8 678.8 692.4 691.3
Hot WHTC g/kW-hr 0.093 0.125 0.031 0.007 654.1 659.2 666.1 661.1
Composite
WHTC
g/kW-hr 0.112 0.142 0.040 0.026 657.3 661.9 669.8 665.3
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work is planned as a follow-up to previous work (Matheaus et al.,
2021) using a smaller diameter EH in front of the LO-SCR while
removing the heated DEF injector in front of the LO-SCR. The
authors believe that adding the EH in this location will allow a
standard, unheated DEF doser, to be used.
Data availability statement
The original contributions presented in the study are
included in the article/Supplementary Material, further
inquiries can be directed to the corresponding author.
Author contributions
All authors participated in developing the test plan and the
outline of the paper. JM wrote abstract, introduction, and
summary. TC and AJ wrote section about the EH. PM and
AM conducted the testing and wrote about the test results,
including the analysis. All authors participated in
editing the final draft and adding additional information to
tell the story.
Acknowledgments
The authors would like to acknowledge the support of Eaton,
Corning and SwRI in this work.
Conflict of interest
Author JM was employed by Eaton (United States). Authors
TC and AJ were employed by Corning Inc.
The remaining authors declare that the research was conducted
in the absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
Publisher’s note
All claims expressed in this article are solely those of the authors
and do not necessarily represent those of their affiliated
organizations, or those of the publisher, the editors and the
reviewers. Any product that may be evaluatedinthisarticle,or
claim that may be made by its manufacturer, is not guaranteed or
endorsed by the publisher.
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