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DIESEL-EXHAUST GAS AFTERTREATMENT HEADING TO SULEVIAV and Jaguar Land Rover have jointly developed a diesel exhaust aftertreatment concept for SULEV30
compliance and have tested it on a demonstrator vehicle based on a Jaguar XF with a four-cylinder diesel engine.
The system chosen consists of a close-coupled electrically heated oxidation catalytic converter, an integrated
SCR/DPF system and an underfloor SCR catalyst.
10
COVER STORY EXHAUST GAS AFTERTREATMENT
Exhaust Gas Aftertreatment
CHALLENGE FUEL ECONOMY, GREENHOUSE GASES AND EXHAUST EMISSIONS
US legislation requires a continuous reduction of fleet fuel consumption and diesel powered vehicles are receiving increased attention for their excellent long-distance and low fuel consumption performance. It is predicted that the die-sel share in the North American market will grow to meet the rising CAFE (Cor-porate Average Fuel Economy) fleet fuel consumption requirements introduced by NHTSA. From a legislative perspec-tive, meeting the LEVIII standard requires reduction of emissions of nitrogen oxides (NOx), non-methane organic gasses (NMOG) and selected greenhouse gases such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O).
CONCEPT SELECTION
Development of SULEV30 emission con-cepts requires a dedicated process that takes into account interactions of compo-nents both at the system (vehicle) and subsystem (powertrain) level. A key tool for the concept development process is a flexible vehicle simulation environment able to efficiently evaluate the design and layout of the selected emission re -duction methods. In this study, a vehicle longitudinal dynamics simulation model was built and calibrated with the relevant vehicle, powertrain, and aftertreatment parameters to predict and optimise the tailpipe emissions as well as the impact of the emission reduction systems on fuel economy. A Jaguar XF powered by a four-cylinder diesel engine with an eight-speed automatic transmission was chosen as representative of an E-segment vehicle.
Various architecture options were grouped into three category types. Type 1 features an SCR catalyst, positioned downstream of a DPF. In type 2, an SCR catalyst is located upstream of a DPF. Both types combine conventional mass production technologies. Type 3 uses a close-coupled SCR/DPF [1, 2] component followed by an SCR downstream. The detailed CAE-based exhaust aftertreat-ment selection process yielded several architectures with the potential to reduce the NOx emissions below 20 mg/mi. How -ever, most of the systems either use ex -cessive active warm-up and thus increase fuel consumption (type 1) or employ
AUTHORS
DR. RER. NAT. LUTZ KRÄMERis Head of Department Diesel
Aftertreatment at the IAV GmbH in Berlin (Germany).
DIPL.-ING. FRANK BUNARis Senior Technical Consultant Diesel
Aftertreatment at the IAV GmbH in Berlin (Germany).
DIPL.-ING. KEN FRIIS HANSENis Principal Engineer, Powertrain
Research and Technology, Advanced Diesel and Gasoline Emissions at the Jaguar Land Rover Limited in Whitley
(United Kingdom).
DR. JONATHAN HARTLANDis Research Engineer, Powertrain
Research and Technology, Advanced Diesel and Gasoline Emissions at the Jaguar Land Rover Limited in Whitley
(United Kingdom).
11 01I2014 Volume 75
Exhaust Gas Aftertreatment
additional devices to regenerate the underfloor DPF without increasing oil dilution or SCR ageing (type 2). Consid-ering the pros and cons of each system as well as the potential to further reduce N2O-emissions, the EHC-SCR/DPF-SCR system was selected. The LNT-DPF-SCR technology also fulfilled the main require-ments in the virtual analysis and there-fore remains an interesting alternative for vehicles where an EHC may not be easily integrated. Robust system behav-iour over 150,000 mi lifetime is assumed.
SULEV30 CONCEPT
Based on the virtual concept study and according to the requirements of the Jag-uar XF vehicle selected, the inline four-cylinder 2.2-l diesel engine was equipped with an EHC-SCR/DPF-SCR exhaust aftertreatment system, ❶ [3]. The CAFE footprint of the car and the powertrain
specifications are very close to the Euro-pean Euro 5 baseline concept. In order to achieve the challenging SULEV30 and CAFE requirements, a robust high-per-formance aftertreatment system com-bined with a high efficiency drivetrain and power unit is needed. The exhaust aftertreatment system features a 1.8-l oxidation catalyst (Emitec EHC) with metallic substrate [4] in ultra-close-cou-pled position. The EHC coating was opti-mised to deliver excellent light-off per-formance, NO2 production and hydrocar-bon conversion efficiency. The catalysed DPF of the current production system was replaced by an integrated SCR/DPF system. The silicon carbide SCR/DPF substrate features an optimised pore size distribution enabling enhanced wash-coat loadings whilst maintaining a low pressure drop. A volume-optimised SCR catalyst is located in the underfloor po sition. Due to the close-coupled posi-
tion of the SCR/DPF, the reductant is injected downstream of the EHC into a very short mixing section. This close-coupled AdBlue dosing position increases thermal load on the dosing module, so a water cooled Delphi B3.25 dosing module was installed.
REDUCTANT AGENT MIXING
In order to enable maximum NOx con-version efficiency, the injected reductant needs to be homogeneously distributed at the SCR/DPF inlet surface area. Due to the close-coupled position of the SCR/DPF, the effective mixing section length is only 260 mm. The optimisation of the mixing performance has to be achieved with the lowest possible pressure drop across the mixing elements. ❷ illustrates the process of the mixing section optimi-sation [5].
The various complex fluid mechanics optimisation parameters for different dosing unit positions, spray types and mixer designs, require a CAE based pre-selection process to efficiently optimise the mixing performance at the lowest possible back pressure [5]. This process starts with high-quality optical spray characterisation to enable calibration of the CFD reductant injection subroutine. Thegeometry selected in the CFD process was constructed and tested.
SCR/DPF- AND SCR-CATALYST OPTIMISATION
The DeNOx functionality was achieved using a close-coupled SCR/DPF catalyst combined with an underbody SCR cata-lyst [3]. The latest SCR/DPF washcoat
eDOC (EHC)
SCR catalyst Integrated SCR/DPF
AdBlue doser, water cooled
V = 2.0 l,400 cpsi, Cu-zeolite
V = 2.5 l,300 cpsi, Cu-zeolite
V = 1.8 l,130/600 cpsi
2.2 l, 140 kW, CR,one TC, HD-AGR
I4 diesel engineAdBlue mixer
❶ SULEV30 demonstrator vehicle key concept features
Doser positions
Spray design
Mixer design
❷ CAE-based mixing section optimisation process (schematic)
COVER STORY EXHAUST GAS AFTERTREATMENT
12
technology [1], optimised for low temper-ature performance and high ammonia storage capacity, was used, and ❸ shows the very high DeNOx efficiency of both technologies for various NO2/NOx ratios in laboratory tests. The advanced SCR formulations provide sufficient perfor-mance during the cold part of the FTP75 test cycle between 200 and 250 °C, even at low NO2 concentrations, and the SCR/ DPF will operate at a higher temperature than the SCR and therefore can contri bute significantly to the overall NOx reduction. The combination of both technologies will provide the required potential for NOx reduction.
CONTROL AND DIAGNOSTICS
Accurate calculation and control of the ammonia storage levels of each indivi-dual SCR formulation is required, at vari-ous aging conditions, to meet the NOx reduction requirements for SULEV30. This requires estimation of the NOx and NH3 flux from the SCR/DPF to under-body SCR. Further, model-based sensors and their control strategies are required for plausibility checks of the physical sensors and system costs reduction. The feasibility of these sensors must consider complexity, real-time capability and accuracy. Requirements related to con-trollability, OBD feasibility, and the com-plexity of model-based sensors and sys-tem costs are essential for the sensor lay-out definition. IAV’s SCR dosing control
software was coupled with the virtual exhaust line [2] and the virtual vehicle (VeLoDyn) to assess system performance at the very start of the development pro-cess. This approach significantly acceler-ates algorithm development due to almost unlimited virtual test possibilities, even for difficult to measure signals (e.g. the specific NH3 storage levels of each cata-lyst). For the demonstrator vehicle, the advanced IAV SCR rapid prototyping control software has been adapted.
The exhaust aftertreatment configura-tion selection also affects the On-Board Diagnostics (OBDII) requirements. Both SCR components are monitored individu-ally with regard to their functionality [3].
Although the OBD threshold factors will not change for SULEV in 2013, the abso-lute values will decrease significantly due to the lower emission thresholds of the SULEV30 standards. The major OBD challenges for SULEV30 emissions com-pared to the current legislation are expected to be: : increased accuracy and durability
requirements : combined NMOG/NOx emission limit
(still under discussion) : increased FUL (full usefull life) stage
mileage threshold : new requirements for feed gas and
DPF-NMHC conversion monitors.The SULEV30 candidate system will affect the diagnostics of the DOC, DPF, and SCR efficiency, as well as OBD of the close-coupled doser, ❹.
TECHNOLOGY DEMONSTRATION
❺ illustrates the emission development status over the FTP75 test cycle. Without active heating of the oxidation catalyst (EHC) by post injection or applied elec-trical heating, the performance of the advanced exhaust aftertreatment concept is already in line with ULEV50 require-ments. By applying a combined active warm-up strategy, consisting of electrical heating and exothermic reactions over the EHC, the SULEV30 NOx+NMOG standards were achieved with a non-aged system. For the SULEV30 application of the Jaguar XF, the SCR/DPF component reaches its target temperature after just 100 s from engine start and the target temperature level for the underbody SCR
Temperature [°C]
NO
x con
vers
ion
effic
ienc
y [%
]
100
80
60
40
20
0150 200 250 300 350 400 450
NO2/NOx = 0 % SCR/DPFNO2/NOx = 0 % SCR
NO2/NOx = 50 % SCR/DPFNO2/NOx = 50 % SCR
Synthetic gas laboratory
❸ MinNOx performance of SCR/DPF- and SCR-brick
Requirement(§1968.2)
Feedgas monitor New requirement ≥ MY15
DOCNMHC monitor
Exothermal observer Exothermal observer➞ No PGM on SCR/DPF➞ Lower degradation of DOC tolerable
DPFNMHC monitor
New requirement≥ MY15
➞ No PGM on SCR/DPF➞ No NMHC-monitoring required
DPFFilter eff. monitor
pDiff based pDiff based (+PM-sensor)
SCRNOx eff. monitor
Via NOx sensors Via NOx sensors➞ SCR pinpointing expected
DosingSystem monitor
Underbody dosing➞ Air cooled doser
Close coupled dosing➞ Water cooled doser➞ Changed aging/ durability effects
❹ Selected of OBD aspects
01I2014 Volume 75 13
catalyst was reached after 220 s. In addi-tion, the engine-out NOx and NMOG lev-els need to be controlled by in-cylinder combustion measures for the first 200 s after the engine start.
The combustion strategy is affected by the engine load and speed. The combina-tion of an improved powertrain calibra-tion strategy, consisting of an improved gear shift schedule in combination with a modified combustion strategy, reduces the emission levels by an additional 10 %. As a result, the NOx + NMOG emission level was decreased by 90 % to meet the
SULEV30 standards. The optimised shift strategy also partially compensates for the fuel economy deterioration caused by the system warm-up. Over the FTP75 test cycle, the fuel penalty could be limited to approximately 1 % compared to the ULEV50 system without an active warm-up strategy, although the effects on drive-ability and NVH remain to be assessed. The NOx reduction efficiency achieved over the FTP75 test cycle is over 90 %, and due to the excellent DeNOx perfor-mance of the prototype system, the con-version efficiencies achieved over the
US06, HFET and SC03 test cycles exceed 97 %, ❻ [3]. The CAFE fuel consumption development target for 2020 of 41.5 mpg could also be potentially achieved by the demonstrator vehicle.
CONCLUSIONS
Using a simulation-supported develop-ment process, an advanced exhaust gas aftertreatment architecture has been selected from a large number of possible configurations. The chosen concept, employing an electrically heated oxi-
0
5
10
15
20
25
30
35
40
45
50
55
0 5 10 15 20 25 30 35 40 45 50 55
NO
x [m
g/m
i]
Legislation limit ULEV50
Legislation limit SULEV30
Status FTP75 w/o active heat-up
Status FTP75 – active heat-up andopt. ECU and TCU
Status FTP75 w/o active heat-up (aged)
Status FTP75 – active heat-up andopt. ECU and TCU (aged)
NMHC [mg/mi] (~ NMOG)
100 % 101 %
50 %
60 %
70 %
80 %
90 %
100 %
110 %
120 %
30 %
140 %
150 %
FTP75w/o active heat-up
FTP75w. active heat-up
w. ECU & TCU cal. opt.
Fuel
con
sum
ptio
n [%
]
❺ Development status in FTP75 test cycle (ECU: Engine Control Unit, TCU: Transmission Control Unit)
CAFE target XF
Fuel economy [mpg]
68
37
39
41
50
15 20 25 30 35 40 45 50 55 60 65 70
HFET
SC03
US06
FTP75
CAFE FE(FTP75 and HFET)
99 %
99 %
98 %
93 %
HFET
SC03
US06
FTP75
Target MinNOx efficiency
MinNOx efficiency [%]
50 60 70 80 90 100
CAFE 2020 final: 2025 augural: 41.5 mpg 51.5 mpg
❻ Development status summary for all test cycles (non-aged system)
COVER STORY EXHAUST GAS AFTERTREATMENT
14
dation catalyst (EHC), a close-coupled AdBlue dosing system and an SCR/DPF followed by an underbody SCR catalyst, was developed for application in a Jaguar XF demonstrator vehicle with a inline four-cylinder 2.2-l diesel engine. In this study, the system development and opti-misation included catalyst formulations, close coupled urea injection with a uni-form reductant distribution, dosing con-trol and warm-up strategies, as well as transmission and combustion calibration in order to provide the highest possible system performance. The potential to meet SULEV30 emission requirements was demonstrated with a fuel penalty of approximately 1 % over a ULEV50 sys-tem without an active warm-up strategy. It is clear that SULEV30 emission concepts do require a number of improvements over current production technologies and the requirements for on-board diagnos-tics would be challenging with the cur-rently available sensing technologies.
Although modular platform strategies and/or related sourcing strategies and increased production volumes of after-treatment components may reduce future system costs, different market requirements and emission standards in the US, Europe and Asia are limiting these savings, because worldwide emis-sion standards are not harmonised.
REFERENCES[1] Bunar, F.; Schrade, F.; Tourlonias, P.; Friedrichs, O.; Krämer, L.: SCReaming for Integration: Applying SCR/DPF to Passenger Cars. 4th MinNOx confer-ence, Berlin, 2012[2] Schrade, F.; Brammer, M.; Schäffner, J.; Langeheinecke, K.; Krämer, L.: Physico-Chemical Modeling of an Integrated SCR on DPF (SCR/DPF) System. In: SAE2012-01-1083, International Journal Engines 5 (3), pp. 958-974, 2012[3] Krämer, L.; Buschmann, G.; Stiegler, L.; Bunar, F.; Richardson, S.; Hansen, K.F.: SULEV: Diesel-Exhaust Gas Aftertreatment Heading to Super Ultra Low Emissions. 34th Vienna Motor Symposium, 2013[4] Schrade, F.; Spitta, J.; Krämer, L.; Mountstevens, E.; Swallow, D.; Wylie, J.: Minimizing NOx Emissions through Exhaust Aftertreatment. 3rd MinNOx confer-ence, Berlin, 2010
[5] Maass, J.; Eppler, A.; Scholz, J.; Gentgen, H.; Marohn, R.; Grumbrecht, F.: Influences of AdBlue spray targeting and mixing devices on UWS distri-bution upstream SCR catalyst. Institution of Mechanical Engineering: Fuel systems for IC engines, London, 2012
THANKS
The authors would kindly like to thank Johnson
Matthey Plc for the provision of advanced cata-
lyst technologies and for their highly valuable
contributions to this study. In addition, the
authors would like to thank the Delphi corpora-
tion for providing the dosing system and adapt-
ing it to the specific requirements.
01I2014 Volume 75 15