Evaluation of Cu-Based SCR/DPF Technology for Diesel
`Exhaust Emission Control
`
`2008-01-0072
`
`Jong H. Lee, Michael J. Paratore and David B. Brown
`General Motors Corporation
`
`Copyright © 2008 SAE International
`
`ABSTRACT
`
`Recently, a new technology, termed 2-way SCR/DPF by
`the authors, has been developed by several catalyst
`suppliers for diesel exhaust emission control. Unlike a
`conventional emission control system consisting of an
`SCR catalyst followed by a catalyzed DPF, a wall-flow
`filter is coated with SCR catalysts for controlling both
`NOx and PM emissions in a single catalytic converter,
`thus reducing the overall system volume and cost. In
`this work, the potential and limitations of the Cu/Zeolite-
`based SCR/DPF technology for meeting future emission
`standards were evaluated on a pick-up truck equipped
`with a prototype light-duty diesel engine.
`
`INTRODUCTION
`
`The selective catalytic reduction of NOx by urea (urea-
`SCR) is one of the most promising technologies for
`diesel engine NOx emission control. Compared to the
`competing lean NOx reduction technologies such as the
`NOx adsorber technology (e.g., lean NOx trap), it offers
`a number of advantages,
`including excellent NOx
`reduction efficiency over a wide temperature range and
`overall lower system cost.
`
`
`The SCR technology using ammonia as reductant (NH3-
`SCR) has been proven effective and used commercially
`for the removal of NOx emissions from stationary sources
`since the 1970s. Because of the challenges associated
`with storage, handling and transportation of NH3 on a
`vehicle, urea is being considered as an NH3 storage
`compound in the form of an aqueous urea solution for
`mobile applications [1]. When the aqueous urea solution
`is sprayed into a hot exhaust gas stream, urea is
`decomposed to release NH3, which is then used to
`reduce NOx over the downstream SCR catalyst.
`
`A typical aftertreatment system for diesel engines using
`the SCR technology consists of an oxidation catalyst
`(DOC), an SCR catalyst, and a catalyzed particulate
`matter filter (CDPF), which are placed in a specific serial
`order to achieve a desired level of emission reduction
`performance: DOC+SCR+CDPF or DOC+CDPF+SCR.
`SCR ahead of the filter (i.e., DOC+SCR+CDPF) allows
`
`the rapid warm-up of the SCR catalyst and thus the best
`NOx reduction performance during the cold-start FTP.
`The exhaust system architecture of DOC+CDPF+SCR
`[2] results in difficulty controlling the NOx emission during
`the cold start, especially from the larger engines.
`However, it is easier to regenerate the filter, which
`requires a periodic cleaning to remain effective.
`
`Recently, the 2-way SCR/DPF technology has been
`developed by some catalyst suppliers.
` Unlike a
`conventional DPF catalyzed with the usual precious
`metals such as Pt and Pd, a wall-flow particulate matter
`(PM) filter is coated with SCR catalysts as shown in Figure
`1. Thus, both NOx and PM can be removed in a single
`catalytic converter, reducing the overall system volume,
`mass and cost. In addition, both the DPF and NOx
`reduction
`functions are precious group metal
`free,
`resulting in further substantial cost savings. Compared to
`the system architectures described above, this technology
`allows rapid warm-up of the SCR catalyst during the cold-
`start period
`(vs. DOC+CDPF+SCR), and
`lower
`temperature exposure for the DOC and SCR catalysts
`during the PM filter regeneration (vs. DOC+SCR+CDPF).
`
`
`
`Figure 1. 2-way SCR/DPF Concept (Courtesy of Prof.
`C.J. Rutland, University of Wisconsin)
`
`In this project, a Cu/Zeolite-based SCR/DPF technology
`was evaluated on a pick-up truck equipped with a 4.9L
`
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`
`its emission
`for
`light-duty diesel engine
`prototype
`reduction performance during the cold-start FTP and
`US06
`tests.
` Also examined were NOx reduction
`efficiency at high temperatures encountered during the
`filter regeneration by in-cylinder post injections, and the
`catalyst durability of
`the system
`following
`the
`regeneration.
`
`EXPERIMENTAL
`
`A Chevy Silverado pick-up truck equipped with a
`prototype 4.9L 6-cylinder diesel engine was used for this
`project. As shown in figure 2, the aftertreatment system
`included a close-coupled DOC (0.85L), an under-floor
`(U/F) DOC (2.3L), and the 2-way SCR/DPF catalyst.
`Most of the catalysts used in this study were aged in the
`oven to simulate low mileage (LM) and high mileage
`(HM) aging (shown in Table 1), mainly to capture high
`temperatures encountered during the DPF regeneration
`mode. For the injection of aqueous urea solution (32.5
`wt.% urea), an airless urea dosing system was used. A
`static mixer was also used to improve the mixing of urea
`spray in the exhaust. Both the engine and the urea
`dosing system were controlled using ETAS
`INCA
`hardware and software, and all of the control algorithms
`were developed in-house using the ETAS ASCET rapid
`prototyping system. Ultra low sulfur diesel fuel (7-15
`ppm S; Cetane number = 40-50) was used for the
`vehicle calibration work and emission tests.
`
`
`
`
`
`DPF Exhaust Layout22--Way SCRWay SCR--DPF Exhaust Layout
`
`4.9L V6
`4.9L V6
`Diesel
`Diesel
`
`Supply Module w/Dosing control unit
`Supply Module w/Dosing control unit
`
`NOx Sensor
`NOx Sensor
`w/lambda
`w/lambda
`
`2-way
`SCR/DPF
`
`DOC
`
`Trans
`
`Urea Tank
`
`Mixing Zone
`Mixing Zone
`MixerMixer
`
`Pwr
`T/O
`
`ExhExh Gas Temp. Gas Temp.
`
`Sensor (3 positions)
`Sensor (3 positions)
`
`2-way SCR/DPF
`
`Fuel
`
`DOC – 0.8L
`
`Urea Injector
`Urea Injector
`
`DPF Pressure Sensor
`DPF Pressure Sensor
`
`
`
`Figure 2. Exhaust system configuration
`
`three
`included
`instrumentation
`The exhaust system
`temperature sensors for monitoring the exhaust gas
`temperature, and two NOx sensors for monitoring the NOx
`concentrations.
`
`In addition, K-type chromel-alumel
`thermocouples were installed at various locations. Figure
`3 shows the construction details of the test samples used
`in this work. For this picture, the DPF substrate had a
`small section removed to allow examination of the
`component details. As can be seen in Figure 3, the 2-way
`SCR/DPF appearance is identical to a standard DPF.
`
`All the emission tests were conducted at the GM
`Powertrain Emission Laboratories at the Milford Proving
`Ground.
` Four emission benches equipped with
`analyzers for NOx, THC, and CO were used to collect
`the modal data at the selected sampling locations, which
`
`were then used for the engine and emission calibration
`development. At the tailpipe (TP) location, the dilution
`and proportional sampling method was used with various
`analyzers to collect the bag test results to report the
`tailpipe emissions of all the species, including NOx, THC
`(CH4, NMHC), CO, CO2, PM, NH3, and N2O. In addition,
`the carbon balance technique was used to estimate the
`fuel economy during the test.
`
`Figure 3. 2-way SCR/DPF hardware
`
`
`
`Size
`
`Cell
`Density
`
`PGM
`loading
`
`Aging
`
`Aging
`Condition
`
`Table 1. Specifications and aging conditions of the U/F
`DOC and Cu/Zeolite-based 2-way SCR/DPF used in this
`study
`Sample
`ID
`U/F DOC
`DOC(1)
`
`2.3L
`(5”x8”x4”)
`
`400
`
`1:0:0/70
`
`low
`miles
`
`DOC(2)
`
`400
`
`1:1:0/120
`
`low
`miles
`
`300oC/38h +
`500oC/19h
`on the
`engine
`700oC/3h in
`the oven*
`
`8.0L
`(7.5”x11”)
`8.7L
`(7.5”x12”)
`
`300
`
`200
`
`
`
`
`N/A
`
`SCR/DPF
`LM
`2-way
`HM
`2-way
`
`*Hydrothermal aging with 10% H2O in air
`
`FTP COLD START AND US06 PERFORMANCE
`
`low
`miles
`high
`miles
`
`700oC/4h in
`the oven*
`700oC/100h
`+ 750oC/20h
`in the oven*
`
`As listed in Table 2, when the 8.0L low-mileage-aged
`Cu/Zeolite 2-way SCR/DPF (LM 2-way) was tested for
`FTP cold start, ~84% NOx conversion was obtained with
`just 5 mg/mi NH3 slip. On the other hand, when the 8.7L
`high-mileage-aged 2-way SCR/DPF (HM 2-way) was
`used, ~82% NOx conversion was obtained with 3 mg/mi
`NH3 slip. These test results were found to be very
`encouraging when compared with the NOx conversion
`performance of a 10L HM-aged Cu/Zeolite SCR catalyst
`(HM Cu/Z). When the engine-out (EO) conditions (e.g.,
`
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`HC, CO, NOx, PM, temperatures, etc.) were similar,
`~86% NOx conversion was obtained over the HM-aged
`Cu/Zeolite catalyst with 2 mg/mi NH3 slip. Therefore, it
`appears that the NOx reduction efficiency of the 2-way
`Cu/Zeolite SCR/DPF is comparable with that of a
`standard flow-through Cu/Zeolite SCR catalyst.
`
`CO
`
`PM
`
`.694
`
`.061
`
`.252
`
`.105
`
`.002
`
`.539
`
`.078
`
`.361
`
`.088
`
`.005
`
`.522
`
`.060
`
`.443
`
`.096
`
`.003
`
`.036
`
`.345
`
`.043 N/A
`
`.008
`
`.066
`
`Table 2. Summary of selected emission test results*
`Sample
`EO
`TP Emissions
`ID
`NOx HC
`NOx NH3 N2O
`FTP
`HM
`Cu/Z
`LM
`2-way
`HM
`2-way
`US06
`LM
`2-way
`HM
`2-way
`*All reported in g/mi
`
`%
`NOx
`
`86
`
`84
`
`82
`
`96
`
`93
`
`the HM 2-way system. The same engine and urea dosing
`calibrations were used for these tests. Thus, the observed
`large increase in the ammonia slip from the HM 2-way
`system can be attributed to the decreased oxidation of NH3
`to N2 over the Cu catalyst after the aging.
`
`During the US06 tests, the catalyst temperature ranged
`from 400oC to 550oC for the LM-aged system, and 450oC
`to 600oC for the HM-aged system. This increase in
`temperature may be due to the increase in back pressure.
`For example, the maximum back pressures across the LM
`and HM 2-way systems, which were observed at 81 mph
`during the test, were 27 kPa and 55 kPa, respectively.
`However, the high NOx conversions observed with both
`systems seem
`to
`indicate
`that
`the back pressure
`difference did not influence the NOx reduction efficiency of
`the 2-way SCR/DPF system significantly.
`
`As mentioned above, the 2-way system was exposed to
`high temperatures during these tests. Since NH3 can be
`selectively oxidized to N2 over a Cu/Zeolite catalyst at
`high temperatures (e.g., >350oC), urea dosing rate was
`increased to compensate for the loss of NH3 due to its
`oxidation, and thus to ensure that a sufficient amount of
`NH3 was available for the SCR reaction. This overdosing
`of urea solution resulted
`in high NOx conversion
`efficiency without excessive NH3 slip for the LM 2-way
`system. However, as indicated by the increased NH3 slip
`over the HM 2-way system, urea dosing rate must be
`adjusted to maximize the NOx reduction and to minimize
`the NH3 slip, depending on the degree of catalyst aging.
`
`EFFECTS OF SOOT LOADING ON NOX
`REDUCTION EFFICIENCY
`
`In the 2-way SCR/DPF technology, the SCR catalyst
`materials are washcoated on a wall-flow filter substrate,
`unlike the conventional flow-through urea-SCR systems.
`Thus, the effect of soot loading on the NOx reduction
`efficiency of the SCR catalyst was examined for the FTP
`and US06 tests.
`
`The NOx reduction efficiency was calculated from the
`modal engine-out NOx and tailpipe NOx concentrations,
`while the soot loading on the SCR/DPF system was
`estimated based on
`the engine-out PM and
`the
`accumulated mileage after a “passive” regeneration of
`the filter. During this “passive” regeneration, soot was
`gently removed at 450oC for ~45 min in the absence of
`EGR and urea injection. The complete removal of soot
`was assumed, although 3-4 kPa higher ΔP was often
`observed after each regeneration.
`
`When the NOx reduction efficiency was plotted as a
`function of soot loading during the FTP Phase 2 tests
`(shown in Figure 5), it was found that ~95% NOx
`reduction efficiency was obtained regardless of the soot
`loadings up to 5 g/L. No correlation was found between
`the NOx reduction efficiency and soot loading for the
`
`1.32
`
`.026
`
`2.59
`
`.048
`
`.022
`
`.144
`
`1.50
`
`.089
`
`3.16
`
`.110
`
`.419
`
`.137
`
`.054
`
`.060
`
`In addition, as shown in Figure 4, 67 out of 96 mg/mi
`tailpipe (TP) NOx occurs with the HM 2-way system
`during Phase 1 of the FTP test, with 63 mg/mi NOx
`breakthrough during the Cycle 1 and 2 alone. Thus, with
`improved engine calibration during the cold-start period,
`it may be possible to meet the Tier II Bin 5 standards
`(0.070 g/mi NOx at 120k miles) using the 2-way
`SCR/DPF technology.
`
`Phase 1 67 mg/mi
`Cycle 1-2 63 mg/mi
`
`100
`
`200
`
`300
`
`400
`
`500
`
`0.15
`
`0.10
`
`0.05
`
`0.00
`
`0
`
`-0.05
`
`NOx (g/mi)
`
`Time (sec)
`
`
`Figure 4. Cumulative NOx emission during Phase 1 of a
`cold-start FTP test with the HM-aged 2-way system
`Solid line: engine-out NOx; Dotted line: tailpipe NOx
`
`
`US06 PERFORMANCE
`
`Over 90% NOx reduction efficiency was obtained during
`the US06 tests with the 2-way SCR/DPF technology. As
`shown in Table 2, ~96% NOx conversion was obtained
`with just 22 mg/mi NH3 slip and 144 mg/mi N2O over the
`LM 2-way system, and ~93% NOx conversion was
`obtained with 419 mg/mi NH3 slip and 137 mg/mi N2O over
`
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`
`After the filter was fully cleaned of soot by the “passive”
`regeneration technique, ~2 g/L soot was loaded on the
`120k-aged SCR/DPF system. During the subsequent
`cold-start FTP test, the filter regeneration was carried out
`during the Phase 1 and 2. Compared to a previous FTP
`test conducted with a clean filter, the pressure drop (ΔP)
`across the 2-way system was high at the beginning of
`the test, but came down to the same low level at the end
`of the test, indicating the regeneration of the filter.
`
`NOX REDUCTION EFFICIENCY & NH3 OXIDATION
`DURING FTP COLD START FILTER REGENERATION
`
`The NOx reduction potential of the 2-way SCR/DPF was
`further examined using the second unit of the HM 2-way
`part, because the first unit was damaged during the post
`injection strategy development. As mentioned above, the
`inlet gas temperature ranged from 500 to 750oC during the
`FTP cold-start filter regeneration tests. Although the
`laboratory reactor data suggested over 80% NOx reduction
`efficiency for Cu/Zeolite catalysts at these temperatures, the
`NOx reduction efficiency of the HM 2-way system was only
`~56%. Due to high engine-out (EO) NOx emissions during
`the regeneration, this low NOx reduction efficiency resulted
`in high tailpipe (TP) NOx emissions (e.g., ~1 g/mi). Both the
`engine-out NOx and the tailpipe NOx emissions can be
`reduced by improving the engine calibration. However, high
`tailpipe NOx emissions during filter regeneration may
`require even higher NOx reduction performance for the 2-
`way system during the test cycles, when the inventory of
`NOx emissions is considered.
`
`FTP Phase 3 and US06 tests. Therefore, despite
`various other
`factors that may influence the NOx
`reduction efficiency, it appears that there is no clear
`effect of soot loading on the NOx reduction efficiency of
`the 2-way SCR/DPF system, at least up to 5 g/L of soot
`loading, based on the tests performed to date.
`
`3
`2
`Soot Loading, g/L
`
`4
`
`5
`
`0
`
`1
`
`99
`
`98
`
`97
`
`96
`
`95
`
`94
`
`93
`
`92
`
`91
`
`90
`
`% NOx reduction
`
`
`Figure 5. Effect of soot loading on the NOx reduction
`efficiency during Phase 2 of the FTP tests
`Closed square: LM 2-way; Open diamond: HM 2-way
`
`NOX REDUCTION PERFORMANCE DURING
`FILTER REGENERATION
`
`FILTER REGENERATION BY IN-CYLINDER POST
`INJECTIONS
`
`Although there was no evidence that the NOx reduction
`efficiency of the 2-way SCR/DPF was adversely affected
`by the level of soot loading, the 2-way system requires a
`periodic regeneration just like any catalyzed filter to
`remain effective for PM emission control. The soot
`removal performance of the 2-way system was examined
`using the in-cylinder post injection strategy.
`
`Interestingly, NOx concentrations increased between the
`engine and the SCR catalyst inlet during the filter
`regeneration. Thus, among many factors, the oxidation
`of ammonia over the mixer and the exhaust pipe, as well
`as the rear side of the hot DOC was examined by
`measuring the NOx concentrations at the turbo-out, the
`outlet of the DOC, and the inlet of the SCR/DPF catalyst.
`
`If the filter regeneration is not achieved during regular
`driving by a customer, then the regeneration should be
`forced. This “active” regeneration is typically accomplished
`by creating an exotherm over the DOC, which then raises
`the inlet temperature of a filter (e.g., ~650oC) to achieve
`faster soot oxidation rate. For the “active” regeneration of
`the 2-way system, the in-cylinder post injection technique
`was developed by modifying the existing engine calibration
`parameters. Calibration parameters, such as temperature
`settings, post injection fuel quantity, intake throttle position,
`etc., were adjusted to raise the inlet gas temperature after
`the first hill of the Cycle 2 during an FTP test. Although
`large temperature swings (between 500 and 750oC) were
`observed for the inlet gas temperature, overall, the inlet
`gas temperature remained above 600oC for ~10 min, and
`the outlet gas temperature remained above 550oC for 12-
`15 min during an FTP test.
`
`Table 3. Effect of Ammonia-to-NOx Ratio (ANR) on the
`TP Emissions during the Phase 2 (all reported in g/mi)
`
`ANR
`
`EO
`NOx
`
`Add’l NOx
`btw EO &
`2-way
`
`TP
`NOx
`
`%NOx
`Conv.
`
`TP
`NH3
`
`TP
`N2O
`
`1.6
`
`1.630
`
`2
`
`4
`
`1.614
`
`1.620
`
`.492
`
`.534
`
`.554
`
`.716
`
`.703
`
`.811
`
`56
`
`56
`
`50
`
`N/A
`
`.045
`
`.132
`
`.182
`
`.192
`
`.315
`
`As summarized in Table 3, the NOx reduction efficiency
`remained the same at ~56% regardless of the urea
`dosing rate (shown as the ammonia-to-NOx ratio) during
`the Phase 2, where the variance in temperatures among
`different tests is small. However, a significant amount of
`NOx was produced from the ammonia oxidation reaction
`with increasing ANR prior to the SCR catalyst (e.g.,
`0.492 g/mi at 1.6 to 0.554 g/mi at 4). Based on the urea
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`
`dosing rate, it was estimated that only ~5% of NH3 was
`oxidized to NOx at these temperatures. Excluding this
`additional NOx from NH3 from TP NOx, higher NOx
`reduction efficiency (84~90%) similar to the reactor test
`results was obtained. However, the TP NH3 and N2O
`emissions increased with increasing ANR.
`
`Therefore, in view of the oxidation of NH3 to NOx over
`the rear side of the hot DOC, the exhaust pipe and the
`mixer prior to the SCR catalyst, additional refinement in
`the urea dosing rate must be required to achieve the
`maximum NOx reduction and the minimum NH3 slip at
`higher temperatures.
`
`temperature and air-to-fuel ratio during the 5 regenerations
`were analyzed. During the 5 regeneration attempts of the
`HM 2-way(2), which had already been oven-aged at 750oC
`for 20h, the inlet gas temperature never exceeded 750oC,
`and both the front and rear bed temperatures remained
`below 720oC. On the other hand, the tailpipe air-to-fuel (A/F)
`ratio during the regeneration attempts was found to dip
`below 20. In particular, the A/F ratio dipped below 15 during
`one regeneration attempt. The performance degradation of
`the HM 2-way system may have been caused by the
`reducing environment at elevated temperatures, not just the
`high temperatures encountered during the regeneration
`attempts.
`
`CATALYST DEACTIVATION BY HIGH
`TEMPERATURE FILTER REGENERATION
`
`Table 4. Summary of selected emission test results
`before and after the filter regeneration
`
`%
`NOx
`conv.
`
`82
`
`76
`
`71
`
`77
`
`93
`
`54
`
`91
`
`76
`
`Sample
`ID
`
`EO
`NOx HC
`
`TP Emissions (g/mi)
`NH3 N2O
`CO
`NOx
`
`PM
`
`.493
`
`.060
`
`.443
`
`.096
`
`.003
`
`.066
`
`.008
`
`.667
`
`.091
`
`.944
`
`.161
`
`.009
`
`.074
`
`.005
`
`.595
`
`.107
`
`.988
`
`.171
`
`.012
`
`.032
`
`.005
`
`.593
`
`.060
`
`.684
`
`.135
`
`.005
`
`.048 N/A
`
`FTP
`HM
`2-way(1)*
`HM
`2-way(1)§
`HM
`2-way(2)†
`HM
`2-way(2)‡
`US06
`HM
`2-way(1)*
`HM
`2-way(1)§
`HM
`2-way(2)†
`HM
`2-way(2)‡
`*Baseline of the HM 2-way with DOC(1) before regenerations
`§ HM 2-way(1) with DOC(1) during the regeneration strategy
`development
`† HM 2-way(2) before regenerations, but with “damaged” DOC(1)
`‡ HM 2-way(2) after 5 regenerations, but with DOC(2)
`
`OXIDATION PERFORMANCE OF DOC
`
`1.50
`
`.089
`
`3.155
`
`.110
`
`.419
`
`.137
`
`.060
`
`1.41
`
`.033
`
`3.295
`
`.654
`
`N/A
`
`.167
`
`.032
`
`1.42
`
`.047
`
`4.448
`
`.126
`
`.359
`
`.122
`
`.051
`
`1.27
`
`.014
`
`.927
`
`.296
`
`N/A N/A
`
`.000
`
`NOX REDUCTION EFFICIENCY OF SCR/DPF
`
`Since the NOx reduction efficiency of a typical Cu/zeolite
`catalyst decreases rapidly with increasing temperatures
`above 550oC, NOx reduction performance of the 2-way
`SCR/DPF technology was examined during the course of
`the post injection strategy development. As summarized in
`Table 4, the NOx reduction efficiency decreased from ~82%
`to ~76% for the FTP, and from ~93% to ~54% for the US06.
`The low NOx reduction efficiency observed during the cold-
`start FTP tests can be attributed to a number of factors,
`such as inadequate test preparation, HC poisoning due to a
`misfire or use of excessive EGR, and insufficient NO2
`formation over the DOC, in addition to the deactivation of the
`DOC and SCR catalysts. On the other hand, the loss of
`NOx reduction efficiency during the US06 tests conducted
`using the same engine and emission calibrations can
`primarily be attributed to the deactivation of the 2-way
`SCR/DPF system, because
`its performance at high
`temperatures is independent of the DOC performance and
`ammonia storage. Therefore, when the HM 2-way system
`was replaced by another HM 2-way part, the NOx reduction
`performance of the 2-way system was recovered for the
`US06 test, but not for the FTP test (i.e., 54% (cid:198) 91% and
`76% (cid:198) 71%, respectively).
`
`The durability of the HM 2-way system was further
`examined by measuring the NOx reduction performance
`following 5 “active” filter regenerations. Following these
`5 filter regenerations via post injections, the U/F DOC(1),
`which had been exposed
`to high
`temperature
`excursions, was replaced with DOC(2), which contained
`a more
`thermally durable
`formulation.
` The NOx
`reduction performance of the HM 2-way system with the
`LM DOC(2) was then examined for the FTP and US06
`tests. As shown in Table 4, low NOx reduction efficiency
`was observed for both the FTP and US06 tests (i.e., 77%
`and 76%, respectively).
`
`The oxidation performance of the DOC is critical for effective
`NOx reduction over the downstream SCR catalyst, because
`HC can poison the active sites for the SCR reaction, and
`improve
`the
`low-temperature SCR reaction
`NO2 can
`kinetics. Thus, the oxidation performance of the DOC was
`compared before and after the high temperature excursions.
`
`The low FTP & US06 performance indicates that the current
`Cu/Zeolite SCR catalyst formulation may not withstand the
`periodic filter regenerations. Thus, the histograms of the
`
`Overall, the oxidation performance for HC, CO, and NO
`was all
`improved,
`following
`the high
`temperature
`excursion. For example, during Phase 2 of FTP tests
`
` SAE Int. J. Fuels Lubr. | Volume 1 | Issue 1
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`100
`
`BASF-2033.005
`
`

`
`500
`
`600
`
`700
`
`800
`
`900
`
`100
`
`80
`
`60
`
`40
`
`20
`
`0
`
`-20
`
`-40
`
`-60
`
`-80
`
`-100
`
`% NO2
`
`Test Time, sec
`
`
`Figure 7. NO oxidation over DOC during Phase 2
`Solid line: “damaged” DOC(1); Dotted line: DOC(2)
`
`CONCLUSION
`In this project, a Cu/Zeolite-based 2-way SCR/DPF
`technology was evaluated on a pick-up truck equipped
`with a 4.9L prototype
`light-duty diesel engine
`for
`emission reduction performance during the cold start
`FTP and US06 tests. The NOx reduction efficiency at
`high temperatures during the filter regeneration by in-
`cylinder post injection, and the thermal durability of the
`system following the regeneration were also examined.
`
`The NOx reduction performance of the Cu-based 2-way
`SCR/DPF technology was found to be comparable with
`that of
`the standard Cu-based
`flow-through SCR
`technology for the cold-start FTP and US06 tests, and to
`be independent of the level of soot loading in the filter
`(up to 5 g/L). Although the filter was successfully
`regenerated by in-cylinder post injections, significant
`performance degradation was observed
`following
`multiple filter regenerations, because of the deactivation
`of both DOC and SCR catalysts. Part of the deactivation
`of the 2-way SCR/DPF technology may have been
`related to air-fuel ratio control during filter regenerations.
`The NOx
`reduction performance during
`the
`filter
`regeneration was also low because of the oxidation of
`ammonia at high temperatures in the exhaust system
`prior to the SCR catalyst.
`
`2.
`
`REFERENCES
`1. G. Busca, L. Lietti, G. Ramis, F. Berti, Applied
`Catalysis B: Environmental, 18 (1998) 1.
`S. Godwin, “Bluetec – Heading for 50 State Diesel,”
`presented at the 2006 Diesel Engine-Efficiency
`and Emissions Research Conference, Detroit, MI,
`August 22, 2006.
`L. Olsson, E. Fridell, Journal of Catalysis, 210
`(2002) 340.
`S.S. Mulla, N. Chen, L. Cumaramatunge, G.E.
`Blau, D.Y. Zemlyanov, W.N. Delgass, W.S. Epling,
`F.H. Ribeiro, Journal of Catalysis, 241 (2006) 389.
`
`3.
`
`4.
`
`(U/F DOC outlet temperature ~250-300oC), the HC
`conversion improved from ~95% to ~97%, and the CO
`conversion improved from ~97% to ~98%. On the other
`hand, as shown in Figure 6, although overall the NO2
`formation was still good, the NO oxidation performance
`decreased at every acceleration, where it is most critical
`for the SCR reaction. Therefore, the loss of NOx
`reduction efficiency during the cold-start FTP tests,
`which was discussed above, could be attributed, in part,
`to the decreased NO oxidation efficiency over the DOC
`under these high space velocity conditions.
`
`15000
`
`30000
`
`45000
`Space Velocity, h-1
`
`60000
`
`75000
`
`90000
`
`
`
`100
`
`80
`
`60
`
`40
`
`20
`
`0
`
`0
`
`% NO2
`
`Figure 6. NO oxidation over DOC during Phase 2
`Diamond/dotted line: DOC(1); Square/solid line: “damaged” DOC(1)
`
`the high
`following
`rates
`increased oxidation
`The
`temperature excursion discussed above may be
`explained by the increased specific reaction rate per site
`observed over a sintered Pt/Al2O3 catalyst [3,4]. It has
`also been found that the NO oxidation commences after
`the HC and CO oxidation reactions start. Therefore,
`following
`the sintering due
`to high
`temperature
`excursions, the NO oxidation performance per site may
`have improved, but the total number of “exposed” sites
`has decreased. That is, it is likely that the DOC had
`enough sites available for the HC and CO oxidation, but
`not enough for the NO oxidation under high space
`velocity conditions encountered during the acceleration.
`
`the “damaged” DOC(1) was
`As discussed earlier,
`replaced by a LM DOC(2),
`following
`the 5
`filter
`regenerations. Unlike the US06 test, the NOx reduction
`efficiency was not recovered for the FTP tests, as shown
`in Table 4. Overall, the oxidation performance for HC,
`CO, and NO was good. For example, during the Phase
`2 of FTP tests, ~95% HC conversion and ~98% CO
`conversion were obtained. However, as shown in Figure
`7,
`the NO oxidation efficiency dropped at every
`acceleration, just like the damaged DOC(1). Therefore,
`the low NOx reduction performance observed during the
`FTP test can be attributed to both the low NO oxidation
`efficiency of the new DOC(2) under high space velocity
`conditions, and the regeneration-induced degradation of
`the SCR catalyst on the HM 2-way(2) system.
`
`
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