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Meeting future NO
X
emission
regulations by adding an
electrically heated mixer
P. Meruva
1
, A. Matheaus
1
, C. A. Sharp
1
, J. E. McCarthy Jr
2
,
M. Masoudi
3
*, N. Poliakov
3
and S. Noorfeshan
3
1
Southwest Research Institute (SwRI), San Antonio, TX, United States,
2
Eaton, Galesburg, MI,
United States,
3
Emissol LLC, Mill Creek, WA, United States
New regulations by the California Air Resources Board (CARB) demand a stringent
0.02 g/hp-hr tailpipe NO
x
limit by the year 2027, requiring Selective Catalytic
Reduction (SCR) catalysts to provide high NO
x
conversions even at low (below
200°C) exhaust temperatures. This work describes utilizing an Electrically Heated
Mixer System (EHM system) upstream of a Light-Off SelectiveCatalytic Reduction
(LO-SCR) catalyst followed by a conventional aftertreatment (AT) system
containing DOC, DPF, and SCR, enabling high NO
x
conversions meeting
CARB’sNO
x
emission target. The AT catalysts were hydrothermally aged to
Full Useful Life. Conventional unheated Diesel Exhaust Fluid (DEF) was
injected upstream of both the LO-SCR and primary downstream SCR. The
EHM system allowed for DEF to be injected as low as 130°C upstream of the
LO-SCR, whereas, in previous studies, unheated DEF was injected at 180°Cor
dosed at 130°C with heated DEF. The combination of unheated DEF, EHM system,
LO-SCR, and downstream SCR enabled the needed increase in NO
x
efficiency in
low exhaust temperatures, which was observed in drive cycles such as in cold-
FTP, LLC, and World Harmonized Transient Cycle (WHTC). There were several-
fold reductions in tailpipe NO
x
using this configuration compared to its baseline:
3.3-fold reduction in FTP, 22-fold in Low Load Cycle (LLC), 38-fold in Beverage
Cycle, 8-fold in “Stay Hot”Cycle, and 10-fold in WHTC. Finally, itis shown thatthe
EHM system can heat the exhaust gas, such as during a cold start, without
needing additional heating hardware integrated into the system. These results
were observed without performing changes in the engine base calibration.
KEYWORDS
heated mixer, advanced aftertreatment, reduced NOx emissions, electric heater,
increase exhaust temperature, 130°C DEF injection
Introduction
The concerning issue of pollution requires immediate action for a better global
environment. Air pollution is one of the major concerns as it directly impacts our daily
lives. One of the prime contributors to air pollution is vehicle emissions. With the ongoing
increase in on-road vehicles and the high demand for new vehicles in the future, the California
OPEN ACCESS
EDITED BY
Jinlong Liu,
Zhejiang University, China
REVIEWED BY
Ruomiao Yang,
Zhejiang Chinese Medical University,
China
Wanhui Zhao,
Civil Aviation University of China, China
Juan Ou,
Xihua University, China
Ziming Yan,
Clemson University, United States
*CORRESPONDENCE
M. Masoudi,
mansour.masoudi@emissol.com
SPECIALTY SECTION
This article was submitted to Engine and
Automotive Engineering,
a section of the journal
Frontiers in Mechanical Engineering
RECEIVED 11 July 2022
ACCEPTED 02 August 2022
PUBLISHED 17 October 2022
CITATION
Meruva P, Matheaus A, Sharp CA,
McCarthy JE Jr, Masoudi M, Poliakov N
and Noorfeshan S (2022), Meeting
future NO
X
emission regulations by
adding an electrically heated mixer.
Front. Mech. Eng 8:991579.
doi: 10.3389/fmech.2022.991579
COPYRIGHT
© 2022 Meruva, Matheaus, Sharp,
McCarthy, Masoudi, Poliakov and
Noorfeshan. 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 cited, in
accordance with accepted 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 17 October 2022
DOI 10.3389/fmech.2022.991579
Air Resources Board (CARB) has initiated the Omnibus NO
x
regulations for 2027 with a limit of 0.02 g/hp-hr. While current
regulations are at 0.2 g/hp-hr, the new NO
x
regulations for the year
2027 require a 90% reduction from the current standard (California
adopts strong new regulation to further reduce smog-forming
pollution from heavy-duty diesel trucks, 2022). This requires a
high NOx conversion on the order of 99.5% from the engine out
(EO) NO
x
on a Federal Test Procedure (FTP) cycle in combination
with the inclusion of the new Low Load Cycle (LLC). Meeting the
NO
x
standards in addition to phase 2 Green House Gas (GHG)
regulations (Greenhouse Gas Standards for Medium- and Heavy-
Duty Engines and Vehicles, 2022) is required to move forward.
The current aftertreatment (AT) development shows that
Selective Catalytic Reduction(SCR)helpsconverttheNO
x
in a
vehicle. However, it needs to be at optimum temperatures to obtain
good NO
x
conversion efficiency (Scott Sluder et al., 2005). This
causes issues at low-temperature conditions such as cold start and
Low Load Cycles (LLCs), as the SCR might not be hot enough to
convert the incoming NO
x
from the engine. When the temperatures
are too low, the possibility of Diesel Exhaust Fluid (DEF) injection
reduces as there is a high possibility of deposit formation, which can
corrode the system or even block the DEF injection. At this stage, the
AT needs additional help from the engine or an external source.
Previous work shows that the addition of passive NO
x
adsorber (PNA) in the AT system was useful in reducing NO
x
emissions during the cold start (Milovanovic et al., 2016;Berndt,
2019). However, as the PNA has a limit for the NO
x
adsorption,
this might not be very helpful, especially during an LLC where the
exhaust temperatures are too low for a prolonged duration. Sharp
et al. (2017) combined PNA with fuel burner technology
upstream of a conventional AT system to reduce NO
x
to meet
the CARB 2027 NO
x
regulations. The engine calibration was
modified in this effort.
There has been modeling work recently to predict a reduction
in NO
x
emissions to meet the 2027 regulations by using 48V
electrical systems on the engine and AT system. Dhanraj et al.
(2022) used a 48V electric technology package in the engine
modeling, which included 48V E-Turbo, 48V EGR pump,
friction reduction with down speeding, exhaust variable valve
technology, and Cylinder Deactivation (CDA) for thermal
management. This, coupled with a hydrocarbon dosing and a
48V Electrical Heater (E-Heater) on an advanced AT system
model, achieved 0.015 g/hp-hr NO
x
for composite FTP with ~1%
reduction in CO
2
emissions.
Significant work has been done in recent years to meet the
2027 NO
x
regulations and in most of those works engine
equipped with CDA projected a viable option (Joshi et al.,
2017;Ramesh et al., 2018), which can help the AT system
reach/maintain the optimum temperature to convert the NO
x
.
Work was done in the past using a CDA engine equipped with a
heated doser and a 48 V electric heater, which has helped in even
reducing CO
2
emissions by ~ 2% compared to its baseline system
(Zavala et al., 2022). A 2.4 kW electric heater (E-Heater) in this
system was proven useful in achieving 0.012 g/hp-hr on a
composite FTP (40% margin to CARB 2027).
There was another work using a fuel burner upstream of a
standard AT system (without a LO-SCR) and equipped on a
non-CDA engine where the overall FTP composite tailpipe
(TP) NO
x
was at 0.023 g/hp-hr, which was 15% above the
regulatory limit with less than 1% fuel penalty (McCarthy
et al., 2022). Later, CDA was added to this system, and the
results were quantified with and without a LO-SCR; however,
this work is not yet published.
The motivation for this work was to meet the 2027 NO
x
regulations with no additional support from a production engine
(without CDA or engine calibration change) with an external heat
source on an AT system. The recent works on the AT system to meet
the 2027 NO
x
regulations include using a LO-SCR (Kasab et al.,
2021;Matheaus et al., 2021;Sharp et al., 2021). This catalyst was
added to a conventional AT system in this work. The AT was
equipped with an Electrically Heated Mixer System (EHM system)
upstream of the LO-SCR. The EHM system combines an Electrically
Heated Mixer (EHM™) and an embedded electric heater
(E-heater™). With the addition of the EHM System upstream of
LO-SCR along with a conventional AT system coupled with a
production engine, the CARB NO
x
regulations for the model
year 2027 were achieved. This additional heat will also travel
downstream of the primary AT system to aid in raising the
catalyst temperatures. The following sections contain details
about the engine and AT configurations, a description of the
EHM system, and the AT controls, followed by the drive cycles
tested and the results and conclusions.
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
calibration. The engine retained the production air handling
system, Exhaust Gas Recirculation (EGR) system, internal
components, and fuel system. The engine, as shown in
Figure 1, is an inline six-cylinder with a bore-to-stroke ratio
of 0.8:1, displacement of 15 L, rate power of 373 kW at 1,800 rpm,
and a peak torque of 2,500 Nm at 1,000 rpm.
The engine is expected to run mainly in two different
operating modes, commonly referred to in this work as
thermal management (TM) and fuel economy (FE) modes.
The authors attributed the transition between these operating
modes of the engine to being triggered based on the downstream
primary SCR temperature. As the names suggest, the engine runs
on the TM mode when the downstream primary SCR
temperature is low. During the TM mode, the engine
generates more heat leading to low EO NO
x
and higher fuel
consumption and vice versa in the case of the FE mode.
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Aftertreatment configurations
The AT system used in this program was a Full Useful Life
(FUL) hydrothermally aged system. An accelerated aging
protocol (Zavala et al., 2020) was used to age the AT system
by thermal exposure to FUL of 435,000 miles, similar to previous
works (Harris et al., 2021;Matheaus et al., 2021;Zavala et al.,
2022). These aged catalysts are commonly referred to as
“Development Aged”end-of-life catalysts (Sharp et al., 2021).
These Development Aged catalysts were not exposed to chemical
aging such as sulfur and lubricant poisoning.
Figure 2A shows a schematic of the following production-intent
AT system in several industry architectures. The AT system consists of
a conventional DEF doser, LO-SCR, a Diesel Oxidation Catalyst
(DOC), a Diesel Particulate Filter (DPF), and a compact mixer
followedbySCRandAmmoniaSlipCatalyst (ASC). Generally,
such an advanced arrangement for the downstream conventional
portion of DOC-DPF-SCR-ASC represents a 2022 production system.
This combination of LO-SCR and conventional downstream AT is
considered the “baseline”in this demonstration (Meruva et al., 2022).
Unlike previous studies that used a heated DEF doser upstream of the
LO-SCR (Harris et al., 2021;Matheaus et al., 2021;Sharp et al., 2021;
Zavala et al., 2022), this study uses a standard DEF doser in both
locations. Catalyst specifications are shown in Table 1.
The focus of this work was to evaluate the addition of an EHM
system upstream of the LO-SCR, with the remaining AT being the
same as the baseline, as shown in Figure 2B. The actual setup is
pictured in Figure 3. In this setup, the conventional mixer upstream of
LO-SCR was replaced with the EHM system. The purpose of adding
the EHM system was to heat up and maintain the LO-SCR at an
optimal temperature for efficient NO
x
reduction, particularly for rapid
heat up during cold start and/or during low-temperature drive cycles.
EHM system
The EHM system delivers heat to DEF droplets injected in the
exhaust pipe, accelerating their conversion tothe desired ammonia
reductant. It is particularly helpful in lower exhaust temperatures
or in low-load cycles while mitigating deposit formation. The
EHM system comprises two sections: the EHM and its embedded
E-heater. The EHM is the first section of the EHM system, which
FIGURE 2
(A) Baseline aftertreatment system architecture with LO-SCR positioned close to the engine. (B) Aftertreatment system architecture with
conventional DEF dosers and EHM system.
FIGURE 1
Cummins X15 engine platform installed in the test cell.
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heats up, enabling DEF injection in exhaust temperatures as low as
130°C. AT system might need additional heat depending on several
factors such as engine calibration, cold start, or rapid heat up
strategy, AT architecture, system demand or certification target. At
these instances, the second section of the EHM system, which is the
E-heater, helps in heating the exhaust gas and the downstream
catalysts, accelerating catalyst warm-up, especially the SCR
catalysts (light-off or primary). The EHM system was
positioned upstream of the LO-SCR in this study. In this
program, along with the E-heater, the heat generated by the
EHM was also leveraged to heat the exhaust gas until the
turbo-out temperature reached a certain target, after which the
EHM would primarily heat the DEF. During the earlier state, the
EHM’s bed temperature would be higher than the latter.
Figures 4A,B show the EHM model and an EHM system
model, respectively, each schematically connected to the
controller. Figure 4C shows an iso view of a model of the
EHM system. The EHM system was designed to operate in
12, 24, or 48 V systems. The demonstration in this work was
conducted on a 48 V system, with the first section of the EHM
system (Figure 4A) providing 3.8—4 kW and the second section
(Figures 4B,C) providing nearly 2 kW, together about 6 kW.
Similar to the previous works with E-heaters (Dhanraj et al.,
2022;Zavala et al., 2022), the EHM system also contains an E-
heater embedded in it. EHM System was used to replace the
conventional DEF doser mixer and heated doser as the EHM
could heat the DEF with a conventional doser. The swirl plate at
the end of the EHM system is used to distribute the flow into the
cone upstream of the LO-SCR.
A control algorithm managed the EHM system for heating the
AT system. The controller receives signals from the CAN-bus (e.g.,
mass flow rate, exhaust temperature, DEF injection rate, and NO
x
sensor signal). The controller (i.e., in real time) readily manages the
EHM system operations to minimize the power consumption while
yielding the needed reductant concentration (ammonia), maximizing
SCR efficiency, and mitigating deposit formation. In this program,
the E-heater was controlled by the Model Based Controller (MBC)
and the EHM was controlled by a separate controller.
Figure 5 shows a representative control where the EHM and its
embedded E-heater are controlled independently to raise the LO-
FIGURE 4
(A) Electrically Heated Mixer (EHM) standalone with its
controller. (B) EHM with its embedded E-heater (EHM system) with
a controller. (C) Physical depiction of the EHM system.
FIGURE 3
Engine and aftertreatment system, including EHM system and
LO-SCR.
TABLE 1 Advanced system 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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SCR temperature to 130°C, where DEF can then be injected. Once
this is achieved, the power demand reduces. Actual power traces for
this work are shown in the test results section. In this example, the
E-heater was targeting 130°C at the SCR temperature, and it shuts off
once it is achieved. The maximum power capacity of the EHM is
4 kW, and its embedded E-heater is 2 kW.
EHM system power
The EHM system is powered up by an external power supply
in the engine test cell. The power was calculated by recording the
voltage and current transmitted to them. This total power was fed
to the dynamometer as a parasitic load onto the engine in real time,
assuming an 80% generator efficiency consistent with previous
works (Matheaus et al., 2021). This additional load was not
accounted for in the cycle work, but the emissions resulting
from this load were included in the test result analysis. Eq. 1
shows the formula used for this calculation:
Total Engine Torque (Nm)Cycle Torque (Nm)
+
EHM Power (kW)+E−heater Power (kW)
0.8p9548.8
Engine Speed rpm.(1)
Model-based SCR controller
This program used a model-based controller (MBC) for the
upstream and downstream SCR catalysts (Sharp et al., 2017;Rao
et al., 2020) to control the DEF injection, which is consistent with
the baseline work (Meruva et al., 2022). The model tracks
ammonia storage in each of the SCR bricks and has a target
ammonia storage based on temperature. This work adapted the
MBC to control the thermal management strategies to power on
the flow heating capabilities of E-heater using the average LO-SCR
temperature as the feedback.
Conventional DEF dosers were used during this work. The DEF
dosing minimum temperature was set to 180°Cthroughoutthesystem
for the baseline work (Meruva et al., 2022). The EHM system
upstream of LO-SCR allowed the DEF dosing to begin at a lower
temperature, down to 130°C for the LO-SCR. The primary
downstream SCR maintained a 180°C dosing temperature
consistent with previous work (Sharp et al., 2021). The DEF
dosing trigger temperature was set to the average gas temperature
of LO-SCR and the average gas temperature of the first primary SCR
for the respective dosing. In this work, a conventional doser coupled
with an EHM system was used to replace the heated DEF doser.
Dosing at low temperatures (130°C) can be useful for
increasing ammonia storage in the SCR catalyst early in the
cycle. It has been demonstrated on both bench testing and engine
testing (Masoudi et al., 2022a;Masoudi et al., 2022b) that EHM
strongly promotes both ammonia storage and increased SCR
NO
x
conversion efficiency.
With an advantage of better DEF atomization with the EHM
system when compared to the baseline, the DEF injection strategy
upstream of LO-SCR was modified to be more aggressive at low
temperatures and less aggressive at high temperatures, assuming
the primary SCR to be hot enough to help with the NO
x
conversion. This dosing strategy helps reduce NO
x
at lower
temperatures while reducing the risk of ammonia slip from the
primary SCR during high exhaust temperature operation.
Emission measurements
Raw exhaust measurements complying with a Code of
Federal Regulations Part 1065 were used in this work, which
included the following:
•A raw Horiba MEXA 7000 series each for the EO and TP
emissions sampling.
•An FTIR for LO-SCR out NO
x
emission measurements.
The NO
x
and CO
2
measurement variability are ~ ± 0.001 and
~ ± 2 g/hp-hr, respectively. This is true for all the test results
discussed in this work.
Drive cycles evaluated
This section describes the different drive cycles used to
evaluate the performance of the different AT configurations.
Federal Test Procedure
The FTP, a regulatory drive cycle in the United States, was
tested, also referred to as the heavy-duty transient cycle, which
includes a cold and hot cycle. The CARB 2027 NO
x
regulatory
FIGURE 5
Example of the EHM system operation during a cold start.
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standard for the FTP drive cycle drops by 90% from 0.2 to 0.02 g/
hp-hr (California adopts strong new regulation to further reduce
smog-forming pollution from heavy-duty diesel trucks, 2022).
Low Load Cycle
The LLC is a real-world drive cycle consisting of data
collected from different applications. This cycle is a new
regulatory cycle that is approximately 1.5 h. This cycle focuses
on sustained low load, short and long idles, high accelerations
after a pronounced cooling period, and low-speed cruise with
motoring (MSPROG, 2022). The CARB NO
x
regulatory limit is
at 0.05 g/hp-hr for the year 2027.
Beverage Cycle
The Beverage Cycle is a subset of the LLC. This cycle is
derived from a food service delivering truck (MSPROG, 2022).
The cycle power is less than the LLC, representing a sustained low
load condition. It is only 800 s in length, much shorter than the
LLC. As a result, this cycle was repeated four times in succession
for thermal conditioning, and the last two cycles were later
analyzed for stable results.
Stay Hot Cycle
The Stay Hot Cycle focuses on the AT performance after a
prolonged period of cooling by idling the engine for 40 min. This
cycle involves operating at a preset speed and load preceding the
long idle for thermal conditioning of the engine and the AT
system until the temperatures attain a steady state. The test then
drops to idle for a 40 min time period where the effect of the AT
cooling and NO
x
reduction can be assessed.
World Harmonized Transient Cycle
The drive cycle named the World Harmonized Transient
Cycle (WHTC) is based on the global pattern of heavy-duty
commercial vehicle usage. It is a transient engine
dynamometer cycle. Both the cold and hot tests for the
WHTC are reported.
Test results
This section shows the test results for the baseline advanced
AT system compared to the EHM system configurations using
the above-mentioned drive cycles. This section shows that a
production engine coupled with an AT system integrated with an
EHM system upstream of a LO-SCR can meet the CARB NO
x
regulations for the model year 2027.
FTP composite
Table 2 shows the FTP composite values for the baseline AT
along with the added EHM system upstream of the LO-SCR. FTP
composite brake-specific emissions are calculated based on 1/7 of
the cold test and 6/7 of the hot test. The results reveal that the
baseline AT system equipped with the EHM system reduces the
composite FTP TP Brake SpecificNO
x
(BSNO
x
) emission by 3.3-
fold (from 0.06 to 0.018) compared with the baseline AT,
complying with the NO
x
regulatory limit of 0.02 g/hp-hr for
the year 2027 California regulations, also now in consideration
with the US EPA for 2031 and beyond (Summary of EPA
proposal, 2022).
The rise in Brake SpecificCO
2
(BSCO
2
) of ~2% compared
with the baseline AT testing is due to the fuel penalty for
powering the EHM system. Further optimization is warranted
to improve the NO
x
and CO
2
tradeoff.
Cold FTP
Table 3 shows that the AT equipped with the EHM system
helps reduce NO
x
in cold FTP twofold compared with the baseline
AT with approximately a 2% fuel penalty. The higher LO-SCR out
NO
x
for the test with EHM system configuration was because of
the change in the DEF injection strategy to support better NO
x
TABLE 2 FTP Composite test results.
Config BSNO
x
(g/hp-hr) BSCO
2
(g/hp-hr)
EO LO-SCR out TP
Baseline AT 2.66 0.67 0.060 508.5
AT +EHM System 2.48 0.83 0.018 518.9
TABLE 3 Cold FTP test results.
Config BSNO
x
(g/hp-hr) BSCO
2
(g/hp-hr) EHM system energy (kW-hr)
EO LO-SCR out TP
Baseline AT 2.00 0.45 0.159 529.5 --
AT +EHM System 1.92 0.71 0.078 541.2 0.47
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conversion at low-temperature conditions and to reduce the
ammonia slip at high-temperature conditions. The EHM
system energy consumption was 0.47 kW-hr, which comprises
EHM and its embedded E-heater consuming 0.235 and 0.235 kW-
hr, respectively. Each element of the EHM system uses the same
amount of power on the Cold FTP.
The EHM was set to a maximum of approximately 4 kW, and
it tries to heat the exhaust gases until the turbine out temperature
is at 200°C. Then, it primarily heats the DEF. Its embedded E-
heater was set to a maximum of 2 kW, and it targets the LO-SCR
average temperature to reach 235°C. Once the target
temperatures were reached, the EHM system was turned off.
Figure 6 shows a graphical comparison of the baseline AT
and the AT + EHM system configurations using LO-SCR average
temperature, primary SCR inlet temperature, EO NO
x
, LO-SCR
out NO
x
,TPNO
x
, the heater power consumption, and BSCO
2
as
the parameters for the cold FTP cycle.
As observed from the average LO-SCR temperature plot, the
catalyst temperature cools down in the baseline run during an
idle event in the baseline configuration, which is avoided with the
AT + EHM system configuration. This helps the LO-SCR convert
more NO
x
during the initial 500 s of the cycle by increasing the
LO-SCR temperature; however, this also adds to the fuel penalty
for powering up the EHM system by about 2%. As mentioned
earlier, during 800–1000 s of the cycle, the DEF dosing strategy
reduces the DEF injection at the LO-SCR, allowing the primary
SCR to convert most of the NO
x
as the primary SCRs are hot
enough to convert NO
x
at these regions.
Hot FTP
Table 4 shows the numerical comparison of the baseline AT
and AT + EHM system configurations for the hot FTP cycle. The
EHM system control strategies were the same between the hot
FTP and the cold FTP cycles, except that, in the hot FTP cycle, the
heating function of the E-heater targeted 225 °C as the LO-SCR
average temperature. The table displays that the AT equipped
with an EHM system helps drop the TP NO
x
by almost 5.5-fold
(from 0.043 to 0.008) with a fuel penalty of approximately 2%.
The EHM system energy consumption for the hot FTP was
0.3 kW-hr, which comprises EHM and its embedded E-heater
consuming 0.2 and 0.10 kW-hr, respectively. About two-thirds of
the energy is used by the EHM, whereas its embedded E-heater
contributes the remaining third. Note that even the hot FTP
requires power to the EHM system, which is about 36% less than
the cold FTP.
FIGURE 6
Comparing the baseline AT to the same equipped with the
EHM system on a cold FTP cycle for the LO-SCR average
temperature, primary SCR inlet temperature, EO NO
x
, LO-SCR out
NO
x
,TPNO
x
, heater power consumption, and BSCO
2
.
TABLE 4 Hot FTP test results.
Config BSNOx (g/hp-hr) CO
2
(g/hp-hr)
EHM
system
energy
(kW-hr)
EO LO-SCR
out
TP
Baseline AT 2.77 0.71 0.043 504.9 --
AT + EHM system 2.58 0.85 0.008 515.1 0.3
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Figure 7 compares the baseline AT and the AT + EHM
system configurations using LO-SCR average temperature,
primary SCR inlet temperature, EO NO
x
, LO-SCR out NO
x
,
TP NO
x
, the heater power consumption, and BSCO
2
as the
parameters for the hot FTP cycle.
The hot FTP test results show that, unlike the baseline
configuration, the AT + EHM system configuration sustains the
LO-SCR operational temperatures during the idling events until
400 s. This heat at the LO-SCR helps the primary SCR retain the
temperature from the previous cycle enabling improved TP NO
x
conversion performance by more than fourfold or more than 80%.
The DEF injection strategy was the same across all the AT + EHM
system configuration tests, which explains the lower LO-SCR NO
x
conversion during 800—1,000 s of the cycle, such as the cold FTP.
Low Load Cycle
Table 5 shows the numerical comparison of the baseline AT
and AT + EHM system configurations for the LLC. It reveals the
AT + EHM system helps reduce the TP NO
x
emission by 22-
fold, which is a cycle-averaged TP NO
x
conversion efficiency of
99.3%, relative to 82.5% in the baseline. The EO NO
x
is higher
for the AT + EHM system configuration test as the heat
generated from the EHM system is gradually transmitted
across the AT system heating the primary SCR, and this
enables the engine to run in FE mode. A certain threshold
of primary SCR temperature helps the engine stay in FE mode
for a longer duration, reducing the CO
2
penalty and increasing
the EO NO
x
, but as the EHM system was powered on for most
of the test, the overall CO
2
penalty was still higher by
approximately 5%. The EHM system energy consumption
was 4.3 kW-hr, which comprises EHM and its embedded E-
heater consuming 3.04 and 1.26 kW-hr, respectively.
Approximately 70% of the electrical energy is put into the
EHM and the remaining into its embedded E-heater for
the LLC.
FIGURE 7
Comparing the baseline AT to the same equipped with the
EHM system on a hot FTP cycle for the LO-SCR average
temperature, primary SCR inlet temperature, EO NO
x
, LO-SCR out
NO
x
,TPNO
x
, heater power consumption, and BSCO
2
.
TABLE 5 LLC test results.
Config BSNO
x
(g/hp-hr) CO
2
(g/hp-hr)
EHM
system
energy
(kW-hr)
EO LO-SCR
out
TP
Baseline AT 4.10 2.68 0.716 614.9 --
AT + EHM system 4.64 0.70 0.032 647.2 4.30
TABLE 6 Control strategy for the E-heater component in the EHM
system for LLC and beverage drive cycles.
Average LO-
SCR target
temp. (°C)
Maximum
power (kW)
Downstream SCR1 in
temp. (°C)
225 2 0
220 2 150
210 2 190
200 2 ≥200
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The heating strategy controlled by the MBC targeted the
average LO-SCR temperature based on the inlet temperature
of the downstream SCR 1, as shown in Table 6.TheEHM
control strategy was the same as the FTP cycles, except that
it tried to heat the exhaust gas until the turbo-out was at
180°C.
Figure 8 compares the LLC between baseline AT and the AT
+ EHM system configurations. Control parameters are LO-SCR
average temperature, primary SCR inlet temperature, EO NO
x
,
LO-SCR out NO
x
,TPNO
x
, the heater power consumption, and
BSCO
2
.
The baseline results show that the LO-SCR was
ineffective for most of the cycle in converting the EO NO
x
due to the extremely low-temperature range. However, with
the AT + EHM system configuration, the average LO-SCR
temperature was mostly >200°C over the entire cycle, helping
it convert NO
x
even after long idles, which normally cools
down the AT system (as it did in the baseline run).
Furthermore, in the AT + EHM system, the heat from the
upstream catalyst was gradually transferred to the
downstream SCR, which helped reduce the TP NO
x
by 22-
fold or an additional ~96% to a net TP NO
x
efficiency of
99.3% in this configuration compared to the baseline helping
the system to maintain the TP NO
x
within the California
2027 regulatory limit.
Beverage Cycle
Table 7 shows the numerical comparison of the baseline
AT and AT + EHM system configurations for the Beverage
Cycle. The EHM system control strategy was the same as the
LLC. The AT + EHM system reduces the baseline TP NO
x
by
38-fold (from 1.669 to 0.044). It produces a cycle-averaged
NO
x
reduction efficiency of 99.2% versus 58.9% in baseline
configuration. Similar to the LLC, the engine runs a lot
more in the FE mode in the configuration with the EHM
system, which explains the drastic difference in the EO NO
x
values. The EHM system energy consumption was 1.76 kW-
hr, which comprises EHM and its embedded E-heater,
consuming 1.08 and 0.68 kW-hr, respectively. The EHM
requires 62% of the electrical energy, whereas its
embedded E-heater uses 38%.
Figure 9 compares the Beverage Cycle with baseline AT and
the same with EHM system configurations using control
parameters LO-SCR average temperature, primary SCR inlet
FIGURE 8
Comparing the baseline AT to the same equipped with the
EHM system on an LLC for the LO-SCR average temperature,
primary SCR inlet temperature, EO NO
x
, LO-SCR out NO
x
,TPNO
x
,
heater power consumption, and BSCO
2
.
TABLE 7 Beverage Cycle test results.
Config BSNO
x
(g/hp-hr) CO
2
(g/hp-hr)
EHM
system
energy
(kW-hr)
EO LO-SCR
out
TP
Baseline AT 4.06 3.60 1.669 686.3 --
AT + EHM system 5.87 0.93 0.044 754.9 1.76
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temperature, EO NO
x
, LO-SCR out NO
x
,TPNO
x
, the heater
power consumption, and BSCO
2
.
The LO-SCR NO
x
conversion increases in the AT + EHM
system configuration compared to baseline by maintaining the
temperature above 200°C for most of the cycle. This heat
transferred onto the downstream SCR helped in the overall
reduction in TP NO
x
compared to baseline. Though the Beverage
Cycle is not a regulatory cycle, the TP NO
x
emission values were
within 0.05 g/hp-hr with the AT + EHM system configuration, even
with an increased EO NO
x
. The net fuel penalty for powering the
EHMsystemwasapproximately10%. The primary SCR temperature
shown in Figure 9 is the inlet gas temperature. The actual catalyst bed
temperature is expected to be a little hotter, which might help reduce
the outlet NO
x
from the LO-SCR.
Stay Hot Cycle
Table 8 shows the numerical comparison of the baseline AT
and AT + EHM system configurations for the Stay Hot Cycle.
The cycle used the EHM to provide extra heat to the system. This
reduced fuel penalty, relying solely on the EHM to heat the AT
system. The EHM control strategy is the same as that described in
the LLC. As observed in Table 8, the AT + EHM system
configuration reduces the TP NO
x
by almost eightfold (from
0.238 to 0.031) with a fuel penalty of approximately 5%. The
EHM system energy consumption was 1.30 kW-hr, which was
consumed completely by the EHM as its embedded E-heater was
turned off during this cycle.
Figure 10 compares the baseline AT and the AT + EHM
system configurations using LO-SCR average temperature,
primary SCR inlet temperature, EO NO
x
, LO-SCR out NO
x
,
TP NO
x
, the heater power consumption, and BSCO
2
as the
parameters for the Stay Hot Cycle.
Though the EHM is mostly advantageous for converting DEF
to reductants, it was used in this Stay Hot Cycle to provide
enough heat to retain the LO-SCR temperature for long
durations to keep the NO
x
conversion active, which does not
happen in the case of the baseline, as the LO-SCR temperature
drops down to ~120°C where it likely cannot respond to any
incoming EO NO
x
.
World Harmonized Transient Cycle
The WHTC Composite is provided first, followed by details
on the WHTC cold and then the hot cycles.
FIGURE 9
Comparing the baseline AT to the same equipped with the
EHM system on a Beverage Cycle for the LO-SCR average
temperature, primary SCR inlet temperature, EO NO
x
, LO-SCR out
NO
x
,TPNO
x
, heater power consumption, and BSCO
2
.
TABLE 8 Stay Hot test results.
Config BSNO
x
(g/hp-hr) CO
2
(g/hp-hr)
EHM
system
energy
(kW-hr)
EO LO-SCR
out
TP
Baseline AT 3.14 1.37 0.238 655.9 --
AT + EHM system 3.22 0.11 0.031 687.2 1.30
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WHTC Composite
The WHTC Composite brake-specific emissions are
calculated based on 14% of the cold test and 86% of the
hot test. Table 9 shows the WHTC composite values
comparison of the baseline AT and AT + EHM system
configurations. The results show that compared to
baseline, the AT + EHM system configuration reduces the
transient TP NO
x
emissions from 0.142 to 0.014 g/kW-hr or
by 10-fold. This is 32-fold below the EURO-VI NO
x
regulatory limit of 0.46 g/kW-hr (0.34 g/hp-hr). The fuel
penalty for powering up the EHM system is only about
0.5% relative to the baseline.
Cold WHTC
Table 10 shows the numerical comparison of the baseline AT
and AT + EHM system configurations for the cold WHTC. The
EHM system control strategies are exactly like the cold FTP cycle.
The results show that both LO-SCR out NO
x
and the TP NO
x
dropped by almost threefold relative to baseline compared to the
AT + EHM system configuration. The increase in the EO NO
x
in
the AT + EHM system configuration test might be because the
engine might have switched to the FE mode at an earlier state
than the baseline after the start of the engine as the AT
temperatures had risen faster when the EHM system was
turned on. The EHM system energy consumption was
0.80 kW-hr, which comprises EHM and its embedded E-
heater, consuming 0.48 and 0.32 kW-hr, respectively.
Approximately 60% of the electrical heating occurred in
the EHM.
Figure 11 compares the baseline AT and the AT + EHM
system configurations using LO-SCR average temperature,
primary SCR inlet temperature, EO NO
x
, LO-SCR out NO
x
,
TP NO
x
, the heater power consumption, and BSCO
2
as the
parameters for the cold WHTC. Adding heat during the cold
start for NOx reduction is advantageous as the baseline LO-SCR
is too cold to convert NOx for the first 400 s of the cycle. As
observed in Figure 11, after 100 s, the AT starts to convert more
NO
x
at the LO-SCR when it is equipped with the EHM system
compared to the baseline configuration, which in turn helps
FIGURE 10
Comparing the baseline AT to the same equipped with the
EHM system on a Stay Hot Cycle for the LO-SCR average
temperature, EO NO
x
, LO-SCR out NO
x
,TPNO
x
, heater power
consumption, and BSCO
2
.
TABLE 9 WHTC Composite test results.
Configuration BSNO
x
(g/kw-hr) BSCO
2
(g/kw-hr)
EO LO-SCR out TP
Baseline AT 4.86 2.01 0.142 661.9
AT + EHM system 5.27 0.67 0.014 666.0
TABLE 10 Cold WHTC test results.
Config BSNO
x
(g/hp-hr) CO
2
(g/hp-hr)
EHM
system
energy
(kW-hr)
EO LO-SCR
out
TP
Baseline AT 3.98 1.53 0.245 678.8 --
AT + EHM system 4.28 0.56 0.091 686.5 0.80
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reduce the overall TP NO
x
emissions by ~ 63%. The overall NO
x
reduction efficiency with the AT + EHM system configuration for
this cold cycle was 98%, with a 1% fuel penalty.
Hot WHTC
Table 11 shows the numerical comparison of the baseline
AT and AT + EHM system configurations for the hot WHTC.
As observed, LO-SCR NO
x
out dropped by an additional
threefold using the AT configuration equipped with the
EHM system relative to the baseline and TP NO
x
by 125-
fold. The EHM system control strategies are exactly like
those described for the hot FTP cycle. The EHM system
energy consumption was 0.57 kW-hr, which comprises of
EHM and its embedded E-heater consuming 0.42 and
0.15 kW-hr, respectively. The EHM system uses 74% of
the electrical energy for the EHM and 26% for its
embedded E-heater.
Figure 12 compares the baseline AT and the AT + EHM
system configurations using LO-SCR average temperature,
primary SCR inlet temperature, EO NO
x
, LO-SCR out NO
x
,
TP NO
x
, the heater power consumption, and BSCO
2
as the
parameters for the hot WHTC.
Figure 12 reveals a drastic difference observed in the TP
NO
x
values between the baseline and the AT equipped with
EHM system configurations. Similar to the hot FTP cycle, the
downstream primary SCR does not lose the thermal heat from
the previous cycle in the AT + EHM system configuration as
the exhaust gases are heated up before reaching the primary
SCR helping achieve a near zero TP NO
x
on a hot WHTC with
less than 1% fuel penalty. The low fuel penalty is because the
engine was running more on the FE mode due to the AT
temperatures, increasing the EO NO
x
and compensating the
fuel penalty for powering the EHM system.
Summary of results
AnEHMsystemaddedtoaLO-SCRandconventionalAT
system was investigated on a 15 L heavy-duty diesel engine.
The technology has a flexible control strategy embedded in a
microcontroller (supplied by Emissol), providing ample
opportunities for DEF injection well below 200°C in low-
FIGURE 11
Comparing the baseline AT to the same equipped with the
EHM system on a cold WHTC for the LO-SCR average
temperature, primary SCR inlet temperature, EO NO
x
, LO-SCR out
NO
x
,TPNO
x
, heater power consumption, and BSCO
2
.
TABLE 11 Hot WHTC test results.
Config BSNO
x
(g/hp-hr) CO
2
(g/hp-hr)
EHM
system
energy
(kW-hr)
EO LO-SCR
out
TP
Baseline AT 5.00 2.08 0.125 659.2 --
AT + EHM system 5.43 0.69 0.001 662.7 0.57
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temperature operations and prolonged low-load cycles. In the
study performed here, the EHM system was used on a
hydrothermally aged Full Useful Life-aged catalyst AT
system for optimal system NO
x
conversion, targeting to
meet future NO
x
emission regulations, including California
2027 (0.02 g/hp-hr tailpipe NO
x
).
Tables 12 and 13 show the summary of all the test results for
both the baseline AT and the AT + EHM system configurations
during this work. Results with AT equipped with EHM system
along with LO-SCR indicate the following:
•TP NO
x
in the FTP cycle was reduced by 3.3-fold (to
0.018 g/hp-hr) compared to the baseline, meeting CARB
2027 target.
•Likewise, TP NO
x
in LLC was reduced by 22.4-fold.
•In Beverage Cycle, TP NO
x
was reduced by 38-fold.
•In the Stay Hot cycle, TP NO
x
was reduced by 7.7-fold.
•TP NO
x
in WHTC was reduced by 10-fold to 0.014 g/kW-
hr, likely meeting the upcoming EURO VII limit, which is
expected to be ~0.03–0.05 g/kW-hr (ICCT Comments and
Technical Recommendations on Future Euro-7/VII
Emission Standard, 2021).
Engine optimization/calibration was not performed in
this study. Therefore, the observed results could be
optimized via further calibration explorations, such as
work to reduce the CO
2
penalty. The production engine
used in this work currently switches between different
modes based on the primary SCR temperature. In the
future, the engine calibration could be modified to also
consider the LO-SCR temperature so that the engine can
stay in fuel economy mode for a longer duration, helping
reduce the fuel penalty. The NO
x
/CO
2
tradeoff could also be
improvised by combining additional technology with the
engine, such as CDA.
Future work
The current work focused on thermal management of the
AT system through external heaters. However, in addition to
the EHM system, adding CDA to the engine can help reduce
the fuel penalty for powering up the heaters because CDA
technology reduces the exhaust flow rate of the engine,
allowing the AT to stay hotter for a longer duration without
cooling down, which in turn helps achieve higher fuel
economy. This has been observed with one of the programs
in the past with a CDA engine and an E-heater (Scott Sluder
et al., 2005).
Likewise, the fuel penalty observed in some cycles could be
alleviated by synergizing CDA and higher EO-NO
x
with
FIGURE 12
Comparing the baseline AT to the same equipped with the
EHM system on a hot WHTC for the LO-SCR average temperature,
primary SCR inlet temperature, EO NO
x
, LO-SCR out NO
x
,TPNO
x
,
heater power consumption, and BSCO
2
.
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Meruva et al. 10.3389/fmech.2022.991579
engine-AT calibration optimization, largely unexplored in this
study.
Future studies could focus on exploring the following,
amongst others:
(i) Impact of the EHM system on deposit mitigation.
ii) More aggressive ammonia storage strategies in sustained
low-temperature operations consistently below 200 C as
the EHM system can form reductants “on-demand”for
improved in-use compliance.
iii) Testing Real Driving Emissions (RDE) cycles for European
applications.
iv) Using EHM (or EHM + E-Heater) for primary SCR.
v) Synergy with advanced engine technologies such as CDA.
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 were involved with planning the testing and
reviewing the results. All authors contributed discussion to the
paper, which was led by PM. All authors contributed to the
figures. PM compiled the tables. All authors participated in the
revision process.
TABLE 12 Test results summary of drive cycles.
Config Cycle BS NO
x
(g/hp-hr) BSCO
2
(g/hp-hr)
EO LO-SCR out TP
Baseline AT Cold FTP 2.00 0.45 0.159 529.5
Hot FTP 2.77 0.71 0.043 504.9
FTP Composite 2.68 0.67 0.060 508.5
LLC 4.10 2.68 0.716 614.9
Beverage 4.06 3.60 1.669 686.3
Stay Hot 3.14 1.37 0.238 655.9
AT + EHM system Cold FTP 1.92 0.71 0.078 541.2
Hot FTP 2.58 0.85 0.008 515.1
FTP Composite 2.48 0.83 0.018 518.9
LLC 4.64 0.70 0.032 647.2
Beverage 5.87 0.93 0.044 754.9
Stay Hot 3.22 0.11 0.031 687.2
FTP Composite is the composite value of the cold FTP and hot FTP in the ratio of 1:7 and 6:7 respectively as per regulatory standards.
TABLE 13 Test results summary of European Regulatory Test Cycle WHTC.
Config WHTC BS NO
x
(g/kw-hr) BSCO
2
(g/ kW-hr)
EO LO-SCR out TP
Baseline AT Cold 3.98 1.53 0.245 678.8
Hot 5.00 2.08 0.125 659.2
Composite 4.86 2.01 0.142 661.9
AT + EHM system Cold 4.28 0.60 0.091 686.5
Hot 5.43 0.69 0.001 662.7
Composite 5.27 0.67 0.014 666.0
FTP Composite is the composite value of the cold FTP and hot FTP in the ratio of 1:7 and 6:7 respectively as per regulatory standards.
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Acknowledgments
The testing at SwRI for the work was funded by Eaton in
cooperation with the SwRI team. This material is based on the work
supported by the National Science Foundation under Grant no.
1831231. The latter acknowledgment refers solely to EHM™/EHM
system™development and engineering support from Emissol.
Conflict of interest
PM, AM, and CS were employed by SwRI. JM was employed
by the Eaton Corporation. MM, NP, and SN were employed by
Emissol LLC.
The authors declare that this study received funding from the
Eaton Corporation and Emissol supplied the EHM
™
/EHM
System
™
components (including its microcontroller) and
engineering support for this investigation. Testing and
analyses were performed by the SwRI team. Eaton, Emissol
and SwRI were involved in planning the testing, reviewing the
results, the writing of this article, and the decision to submit it for
publication.
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 evaluated in this article, or
claim that may be made by its manufacturer, is not guaranteed or
endorsed by the publisher.
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