SAE TECHNICAL
`PAPER SERIES
`
`2003-01-3753
`E
`
`Developments In Diesel Emission
`Aftertreatment Technology
`
`Philip G. Blakeman
`Johnson Matthey
`
`Andrew F. Chiffey
`Senior Catalyst Development Scientist
`
`Paul R. Phillips
`Johnson Matthey
`
`Martyn V. Twigg
`Johnson Matthey
`
`Andrew P. Walker
`Johnson Matthey
`
`AV. PAULISTA, 2073 - HORSA II - CJ. 2001 - CEP 01311-940 - SÃO PAULO – SP
`
`12º Congresso e Exposição Internacionais
`de Tecnologia da Mobilidade
`São Paulo, Brasil
`18 a 20 de novembro de 2003
`
`BASF-2009.001
`
`

`
`Developments In Diesel Emission Aftertreatment Technology
`
`2003-01-3753
`
`Copyright © 2003 Society of Automotive Engineers, Inc.
`
`ABSTRACT
`
`The modern Diesel engine is one of the most versatile power
`sources available for mobile applications. The high fuel
`economy and torque of the Diesel engine has long resulted in
`global application for heavy-duty applications. Moreover, the
`high power and excellent driveability of today’s turbo-charged
`small high-speed Diesel engines, coupled with their low CO2
`emissions, has resulted in an increasing demand for Diesel
`powered light-duty vehicles.
`
`However, the demand for Diesel vehicles can only be
`realised if their exhaust emissions meet the increasingly
`stringent emissions legislation being introduced around the
`world. In the USA, light-duty Diesel (LDD) vehicles will
`have to meet the same emissions legislation as gasoline
`vehicles from 2004 onwards, while in Europe a similar target
`is expected when European Stage 5 legislation is introduced.
`In practice, such targets mean very high reductions (up to
`90%) of nitrogen oxides (NOx) and particulate matter (PM)
`emissions may be required from today’s levels. Drastically
`reduced NOx and PM emissions from heavy-duty Diesel
`(HDD) engines are also required in Europe and the USA in a
`similar time frame.
`
`This paper reviews the developments in Diesel exhaust
`emissions control devices. The application of Diesel
`oxidation catalysts, particulate filters, and NOx control
`catalysts (NOx adsorber catalysts and Selective Catalytic
`Reduction systems) to help meet both light- and heavy-duty
`legislation is discussed. An overview of likely catalyst
`system designs to achieve high levels of PM and NOx
`conversion is given.
`
`INTRODUCTION
`
`Future legislation around the world for Diesel vehicles is
`becoming increasingly stringent, with the emphasis placed on
`lower PM and NOx emissions. Advances in engine design and
`control has shown, and continues
`to show, significant
`reductions in emissions. At the same time further advances in
`the high torque and excellent driveability of the Diesel engine
`has fuelled expansion of the Diesel sector.
`
`Philip G. Blakeman
`Andrew F. Chiffey
`Paul R. Phillips
`Martyn V. Twigg
`Andrew P. Walker
`Johnson Matthey
`
`To help achieve the tighter emissions requirements, advanced
`aftertreatment technology from oxidation catalysts to systems
`capable of high conversion of all four pollutants is being
`developed.
`
`For passenger cars in Europe, Stage 4 legislation will be
`introduced in 2005, with significantly lower emissions limits
`than the currently applied Stage 3 legislation (Table 1). For
`Stage 4, NOx and PM emissions have been halved from the
`Stage 3 limits, while CO and HC emissions require more
`moderate reductions. In addition, the durability requirement
`over which the emission standards have to be met has increased
`from 80,000 km for Stage 3 to 100,000 km for Stage 4.
`
`While LDD Stage 5 has yet to be set, it is widely expected that
`a further reduction of NOx and PM limits will occur, to give
`emissions levels comparable to those required from gasoline
`cars (0.08 g/km NOx in Stage 4), and similar to the standards
`already set in the USA.
`
`Future USA legislation for LDD vehicles also focuses on NOx
`and PM emissions. For passenger vehicles, Tier 2 limits phase
`in between 2004 and 2007 with the limits for Diesel vehicles
`being the same as gasoline-powered vehicles. By 2007, the
`fleet average NOx emissions must meet 0.07 g/mile for all
`passenger vehicles up to 8500 lb. Within Tier 2, there are a
`number of emissions levels (bins) which vehicles can be
`certified to – the 0.07 g/mile fleet average NOx value occurs
`in bin 5 (Table 2). The emissions must be durable over
`120,000 miles.
`
`Emissions legislation for HDD engines in Europe is also
`forcing lower NOx and PM emissions. Table 3 shows the
`emissions limits for the European steady-state cycle (ESC) test
`procedure. Stage 4 (commencing in 2005) has a significant
`drop in PM and a more moderate NOx decrease. Stage 5
`(commencing in 2008) introduces a further drop in NOx.
`
`The steady decrease of NOx emissions for HDD engine
`legislation in Europe is in contrast to that in the USA. In the
`2007 – 2010 timeframe, the NOx and PM emissions for HDD
`engines in the US are reduced ten-fold from the 2004 limits
`
`BASF-2009.002
`
`

`
`(Table 4). The PM legislation is effective immediately in
`2007, while the NOx and NMHC limits will be gradually
`phased in between 2007 and 2010. In addition, not-to-exceed
`limits of 150% of the legislation have been set, and these
`apply to a large proportion of the engine map in an effort to
`ensure high levels of emission control occur under a whole
`range of driving conditions. The emissions have to be durable
`for up to 435,000 miles.
`
`In Brazil, Diesel engines are used in trucks and busses,
`however, and legislation for these has been based on the
`European standards. Currently,
`legislation equivalent
`to
`European Stage 2 is in force (ECE R49 test, 4.0 g/kW-
`hr CO, 1.1 g/kW-hr HC, 7.0 g/kW-hr NOx and 0.15 g/kW-hr
`PM). In 2006 the legislation will become equivalent to
`European Stage 3, and in 2009 it will be equivalent to
`European Stage 4 (Table 3).
`
`Table 1: European LDD emissions legislation (g/km).
`
`Table 4: USA HDD emissions legislation (g/bhp-hr).
`
`Stage
`(Year)
`
`CO
`
`HC+NOx
`
`NOx
`
`PM
`
`Year
`
`CO
`
`HC /
`NMHCa
`
`NOx
`
`PM
`
`3 (2000)
`
`0.64
`
`4 (2005)
`
`0.5
`
`0.56
`
`0.3
`
`0.5
`
`0.05
`
`0.25
`
`0.025
`
`Table 2: USA Tier 2 LDD emissions legislation (g/mile) to be
`phased in from 2004.
`
`Bin
`
`CO
`
`NMOG
`
`NOx
`
`8
`
`7
`
`6
`
`4.2
`
`4.2
`
`4.2
`
`0.125
`
`0.09
`
`0.09
`
`0.2
`
`0.15
`
`0.1
`
`PM
`
`0.02
`
`0.02
`
`0.01
`
`1998
`
`2004b
`
`2007 –
`2010
`
`15.5
`
`15.5
`
`15.5
`
`1.3
`
`0.5
`
`0.14
`
`4.0
`
`2.0
`
`0.2
`
`0.1
`
`0.1
`
`0.01
`
`a) HC applies to 1998 legislation
`
` b)
`
`An alternative limit of NOx + NMHC = 2.5 g/bhp-hr also exisits
`
`Legislation has been set for light vehicles (< 1700 kg) to be
`effective in 2007 and 2009. The test cycle is to be the FTP-75
`cycle, but the legislated values are not equivalent to those in
`the USA legislation (Table 5). As in the rest of the world,
`there is a significant tightening of the NOx legislation by
`2009.
`
`5
`
`4
`
`3
`
`2
`
`1
`
`4.2
`
`2.1
`
`2.1
`
`2.1
`
`0.0
`
`0.09
`
`0.07
`
`0.055
`
`0.01
`
`0.0
`
`0.07
`
`0.04
`
`0.03
`
`0.02
`
`0.0
`
`0.01
`
`0.01
`
`0.01
`
`0.01
`
`0.0
`
`Table 5: Brazilian LDD emissions legislation (g/km).
`
`Level
`(Year)
`
`4 (2007)
`
`5 (2009)
`
`CO
`
`NMHC
`
`NOx
`
`PM
`
`2.0
`
`2.0
`
`0.16
`
`0.05
`
`0.6
`
`0.25
`
`0.05
`
`0.05
`
`Table 3: European HDD emissions legislation (g/kW-hr) for
`the ESC test.
`
`Stage
`(Year)
`
`3 (2000)
`
`4 (2005)
`
`5 (2008)
`
`CO
`
`2.1
`
`1.5
`
`1.5
`
`HC
`
`NOx
`
`PM
`
`0.66
`
`0.46
`
`0.46
`
`5.0
`
`3.5
`
`2.0
`
`0.1
`
`0.02
`
`0.02
`
`Such legislation is forcing the use of exhaust aftertreatment
`systems of greater complexity. To help ensure prolonged
`activity and durability of these systems, Diesel fuel sulfur
`levels are being reduced. For Stage 4 in Europe, 50 ppm sulfur
`fuel is required (down from 350 ppm sulfur for Stage 3), while
`in the US 15 ppm sulfur fuel is required in 2006 (down from
`500 ppm today). In Brazil, 500 ppm sulfur fuel is the target for
`2009 (although 50 ppm is proposed for metropolitan regions),
`and this could limit the types of aftertreatment technology
`applicable in this time frame.
`
`The main aftertreatment devices for use in Diesel exhaust are
`discussed below.
`
`BASF-2009.003
`
`

`
`in Figure 2. The improved low temperature activity of the
`Stage 4 DOC results in a faster light-off on the European test
`cycle. The improved activity enables lower emissions to be
`achieved with almost half of the platinum content than that
`used in the Stage 3 catalyst (50 g/ft3 on the Stage 4 DOC and
`90 g/ft3 on the Stage 3 DOC). Thus the improved activity and
`durability of the Stage 4 DOC demonstrated on the engine
`bench translates to an emissions improvement at significantly
`lower platinum cost on a vehicle.
`
`CO
`HC
`
`0.25
`
`0.2
`
`0.15
`
`0.1
`
`0.05
`
`0
`
`CO, HC emissions (g/km)
`
`EU3 DOC technology Pt @ 90
`
`EU4 DOC technology Pt @ 50
`
`Figure 2: CO and HC emissions over the European Stage 3
`test cycle (on a Stage 3 vehicle) for a Stage 3 DOC with a Pt
`loading of 90 g/ft3 and a Stage 4 DOC with a Pt loading of 50
`g/ft3 after extended low temperature ageing.
`
`DEVICES FOR CONTROL OF PM EMISSIONS
`
`A number of methods of achieving high levels of removal of
`PM emissions in the exhaust of Diesel vehicles have been
`developed that are mainly based on a filtration technique for
`removal of soot from the exhaust gas.
`
`The most common and well-known filtration devices are wall-
`flow filters. These filters typically consist of a honeycomb
`substrate and may be made from cordierite or silicon carbide
`(although other materials have also been employed) [3, 4, 5].
`Typically, half of the cells of the honeycomb substrate are
`sealed at the inlet face in a checkerboard pattern, and the
`remaining cells are sealed at the outlet face of the substrate.
`When placed in the exhaust, the gas cannot pass straight
`through the device, but instead is forced by the seals to pass
`through the walls of the cells. This process results in the
`removal of soot (and other particles) suspended in the gas. The
`filtration efficiency of the wall-flow filters can be extremely
`high, with greater than 95% removal of particulate mass
`possible. This type of wall-flow filter is already fitted to a
`number of passenger cars in Europe [6].
`
`Other filtration devices have also been developed. Sintered
`metal, ceramic foams and compacted fibres have also been
`demonstrated to remove PM [7, 8]. In addition, filter designs
`have been developed with partial flow through the filter
`material to give partial filtration [9].
`
`Whatever the type of PM control device employed, the
`common system control issue is that soot removal by the
`device cannot continue indefinitely without the soot itself
`being removed from the system, typically by combustion. The
`
`DIESEL OXIDATION CATALYSTS (DOC)
`
`Today, the main market for LDD vehicles is in Europe, and
`the majority of these vehicles currently meet Stage 3
`legislation with a DOC as the only aftertreatment device. The
`DOC will remain a principal aftertreatment device for future
`vehicles, either used alone or, as discussed later, as part of
`more advanced emissions control systems.
`
`The DOC principally controls HC, CO and the volatile organic
`fraction (VOF) of PM emissions. The legislation for these
`pollutants is tighter for Stage 4 than Stage 3, and there is a
`general trend towards lower exhaust gas temperatures and
`higher CO and HC emissions from LDD engines, mainly as
`the result of advanced engine control methods that are used to
`minimise NOx emissions. These factors place greater
`emphasis on the need for high catalyst activity at low
`temperatures [1, 2].
`
`High temperature durability is important in order to withstand
`maximum exhaust temperatures and exotherm events that may
`occur on the catalyst surface (the latter usually caused by HC
`that can be adsorbed on
`the catalyst surface at
`low
`temperatures). High durability of the catalyst with respect to
`extended low temperature ageing is also required. Sulfur,
`heavy HC, and carbonaceous deposits can build-up on the
`catalyst surface at low temperatures and poison the catalytic
`activity. This ageing mechanism is particularly important
`because of the general trend towards lower exhaust gas
`temperatures on modern LDD aplications.
`
`Figure 1 illustrates the progress that has been made in
`improving low temperature performance. The engine bench
`CO light-off activity of various catalysts is shown after
`extended low temperature ageing (< 320°C). A typical Stage 3
`DOC demonstrates 50% CO conversion at 180°C (the T50
`value) on this test. By contrast, a typical DOC suitable for
`Stage 4 applications has a T50 of 164°C, clearly a significant
`improvement. The latest developments in DOC technology
`demonstrate an even lower T50 value of 154°C – a 26°C
`improvement in light-off activity compared to a current Stage
`3 DOC technology.
`
`EU3 DOC
`
`EU4 DOC
`
`Latest DOC
`
`26 ° C improvement in light - off
`
`140
`
`200
`180
`160
`Temperature (°C)
`
`220
`
`240
`
`100
`90
`80
`70
`60
`50
`40
`30
`20
`10
`0
`120
`
`CO conversion (%)
`
`Figure 1: Engine bench CO light-off of various DOC
`technologies after extended low temperature ageing.
`
`The improvement in vehicle emissions between a Stage 3 and
`Stage 4 DOC after extended low temperature ageing is shown
`
`BASF-2009.004
`
`

`
`80
`70
`60
`50
`40
`30
`20
`10
`0
`
`NO
`Conversion
`(%)
`
`C a ta ly st In let T em p e ra tu re (°C )
`
`150
`
`250
`
`350
`
`450
`
`550
`
`Figure 4: Temperature window for oxidation of NO to NO2
`over an oxidation catalyst.
`
`
`
`NO2NO2
`
`
`
`O2O2
`
`1
`
`0.8
`
`0.6
`
`0.4
`
`0.2
`
`
`
`00
`
`(arbitrary units)
`(arbitrary units)
`Soot burn rate
`Soot burn rate
`
`
`
`00
`
`
`
`100100
`
`
`
`
`
`200200
`300300
`400400
`500500
`
`Temperature (C)Temperature (C)
`Figure 5: Combustion of Diesel soot in oxygen and nitrogen
`dioxide gas feeds.
`
`
`
`600600
`
`
`
`700700
`
`In practice, the minimum temperature required to ensure a
`significant soot combustion rate with NO2 is around 270°C.
`The soot combustion rate will also depend on the ratio of PM
`and NO2 available for a given engine condition. In general, a
`weight ratio of around 16:1 NOx:PM is required to ensure
`continuous removal of soot. Under operating conditions that
`have sufficient temperature, combustion of the filtered soot
`with NO2 can either provide continuous, or at least partial
`removal of soot (depending on the NOx:PM ratio), and the
`optimised design of the catalyst and overall system layout can
`maximise
`the efficiency of
`this
`low
`temperature soot
`combustion.
`
`Whilst NO2 combustion of soot can be utilised in certain
`driving conditions, modern LDD vehicles are characterised by
`low exhaust gas temperatures and low NOx concentrations
`emitted from the engine. The use of high levels of EGR mean
`that engine NOx emissions are low relative to PM emissions,
`and therefore the concentration of NO2 that can be made by
`the oxidation catalyst is also low. Low concentrations of NO2
`result in low rates of soot burn, even if sufficient temperature
`is achieved in the exhaust gas. Figure 6 illustrates the effect of
`NOx concentration on soot combustion (monitored by the
`backpressure of the filter, and initial and final soot mass
`within the filter) on a LDD engine with and without EGR
`enabled at 350°C. Without EGR, there is 4.2 times higher
`mass flow of NOx than with EGR enabled, and a high rate of
`soot combustion occurred. With a high EGR rate enabled (i.e.
`as found on advanced engine applications), there is only half
`the amount of soot removed in the same time. Whilst complete
`
`implications of soot regeneration for LDD and HDD
`applications are now discussed.
`
`CONTROL OF LDD PARTICULATE EMISSIONS
`
`In Europe, advanced engine design and combustion control
`has been successfully employed in combination with low
`temperature light-off oxidation catalysts to reduce soot and
`heavy HC emissions from LDD vehicles. The contribution of
`sulfate emissions to total PM emissions has also been lowered
`by the reduction of the sulfur level in the fuel.
`
`The further lowering of PM and NOx limits however has
`increased the need for PM filtration devices to meet European
`Stage 4 limits particularly for heavier vehicles, and for LDD
`vehicles to meet USA ’07 limits. Improvements in engine
`technology can further reduce PM and NOx emissions,
`however, the general trade-off between NOx and PM engine-
`derived
`emissions
`necessitates
`the
`requirement
`for
`aftertreatment devices.
`
`The effect on PM emissions of fitting a wall-flow filter device
`to a Stage 3 passenger car is shown in Figure 3. As expected,
`highly efficient removal of particulate is demonstrated.
`
`0.08
`
`0.07
`
`0.06
`
`0.05
`
`0.04
`
`0.03
`
`0.02
`
`0.01
`
`0
`
`PM (g/km)
`
`Engine out
`
`Post DOC + wall-flow filter
`
`Figure 3: Effect on PM emissions over the European Stage 3
`test cycle of fitting a DOC + wall-flow filter device to a Stage
`3 passenger car.
`
`In order to maintain functionality of the filter system, the filter
`device has to be regenerated. Combustion of soot within the
`filter can be achieved either by reaction with oxygen (Reaction
`1), or by reaction with nitrogen dioxide (NO2, Reaction 2). In
`the case of Reaction 2, a catalyst can be used to oxidise
`engine-out NO to NO2 (Reaction 3), as shown in Figure 4, and
`the NO2 can then react with the collected soot.
`
` PM + O2 → CO2
`
`PM + NO2 → CO2 + NO
`
`NO + O2 → NO2
`
`(1)
`
`(2)
`
`(3)
`
`Figure 5 shows the temperatures at which soot combustion is
`achieved with either O2 or NO2 gas. The combustion of soot
`with NO2 clearly occurs at significantly lower temperatures
`than with O2.
`
`BASF-2009.005
`
`

`
`therefore a significant rise in filter bed temperature can occur.
`The effects of this type of transient gas flow conditions during
`soot regeneration of a SiC filter has been modelled to show the
`influence of different oxygen concentrations under the low
`flow condition, as shown in Figure 7. The simulation is based
`on a 650°C inlet gas condition to initiate the soot combustion
`of a filter at an exhaust flow equivalent to idle conditions. At
`high oxygen concentrations, a maximum bed temperature of
`1800°C is obtained. Lowering the oxygen concentration in the
`exhaust
`(for example, by
`increasing
`the EGR
`rate)
`significantly slows the rate of soot combustion and the peak
`temperatures within the filter are much lower.
`
`2000
`
`1800
`
`1600
`
`1400
`
`1200
`
`1000
`
`20% O2
`15% O2
`12% O2
`10% O2
`
`800
`
`600
`
`400
`
`200
`
`0
`
`High carbon loading at 0.003kg/s mass flow
`
`0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
`Time (s)
`
`Maximum Bed Temperature (°C)
`
`Figure 7: Modelling results showing maximum predicted
`filter bed temperatures for a SiC wall-flow filter under idle
`conditions.
`
`There are three different system architectures possible for a
`filter system, shown schematically in Figure 8.
`
`Exhaust Flow
`
`
`
`
`
`
`
`
`
`
`
`
`
`DPF
`
`
`
`
`
`
`
`
`
`
`
`
`CDPF
`
`
`
`
`
`
`
`
`
`
`
`
`
`DOC
`
`
`
`
`
`
`
`
`
`
`
`
`DOC
`
`
`
`
`
`
`
`
`
`
`
`
`
`CDPF
`
`(A)
`
`(B)
`
`(C)
`
`Figure 8: Three possible DPF systems (DOC = Diesel
`Oxidation Catalyst, DPF = Diesel Particulate Filter, CDPF =
`Catalysed Diesel Particulate Filter).
`
`System A
`
`This system utilises the Diesel oxidation catalyst (DOC)
`placed upstream of the DPF to create the exotherm needed for
`soot combustion. The temperature required at the rear of the
`DOC during an active regeneration may be significantly
`higher than the temperature at the inlet to the DPF because of
`heat losses between the DOC and DPF. Therefore the
`
`regeneration of the filter would take a relatively long time
`under the high EGR rate condition, it can be seen that the
`balance point is exceeded such that the filter loses soot mass.
`This NO2-based passive regeneration can be used to minimise
`the build-up of soot, even when the engine-out NOx/PM ratio
`is too low to allow totally passive regeneration. This soot
`combustion via passive regeneration can significantly reduce
`the frequency of active regeneration.
`
`Gasflow: 180 kg/h
`
`DOC+CDPF - low NOx (160ppm)
`
`DOC+CDPF - high NOx (410ppm)
`
`soot: 8.1g/litre - 0.9g/litre (89% regeneration)
`
`Gasflow: 110 kg/h
`
`soot: 6.9g/litre - 3.2g/litre (54% regeneration)
`
`2000
`
`4000
`
`8000
`6000
`Time (sec.)
`
`10000
`
`12000
`
`14000
`
`0.1
`
`0.09
`
`0.08
`
`0.07
`
`0.06
`
`0.05
`
`0.04
`
`0.03
`
`0.02
`
`0.01
`
`Back pressure over CDPF (bar)
`
`0
`
`0
`
`Figure 6: Effect of NOx mass flow (EGR on or off) at 350°C
`on soot combustion with NO2.
`
`In addition to the low temperature NO2 soot combustion
`mechanism, in order to ensure the filter can be regenerated in
`all operating conditions, an active strategy is required to
`periodically combust soot from the filter. For example,
`combustion of the soot can be achieved by raising the
`temperature of the exhaust gas to that required for combustion
`with oxygen. The excess oxygen conditions present in the
`Diesel exhaust enables a fast rate of soot combustion to take
`place. Thus, the higher temperature conditions for oxygen
`regeneration only need to be maintained for a relatively short
`time.
`
`The two key steps involved in an oxygen-based combustion of
`soot are raising the exhaust gas temperature in the filter to
`initiate soot combustion, and then controlling the combustion
`under transient operation once soot burning has been initiated.
`
`Raising the exhaust gas temperature within the filter can be
`done through modifications to the engine calibration to create
`higher temperatures. Fuel can be injected into the cylinder
`after the main fuel combustion event, and this late injected
`fuel can be combusted in-cylinder to increase the gas
`temperatures. Alternatively, excess fuel can be injected very
`late after the main injection into the cylinder, or directly into
`the exhaust gas. This fuel can then be combusted over an
`oxidation catalyst to create a certain exotherm. A combination
`of both techniques can also be employed.
`
`Once soot combustion is initiated, it is important that the
`regeneration is maintained and controlled under transient
`operating conditions. Following
`the
`initiation of soot
`combustion, the gas flow rate may substantially change to a
`lower flow, for example, as expected under engine idling
`conditions. The resulting lower gas flow will be less effective
`at removing the heat generated by soot combustion, and
`
`BASF-2009.006
`
`

`
`post-DPF CO
`post-CDPF CO
`
`100
`
`200
`
`300
`
`500
`400
`Time (sec)
`
`600
`
`700
`
`800
`
`900
`
`300
`
`250
`
`200
`
`150
`
`100
`
`50
`
`Concentration (ppm)
`
`0
`
`0
`
`Figure 10: Comparison of tailpipe CO emissions from System
`A and System B during an oxygen-based soot regeneration at
`620°C.
`
`System C
`
`System C is the most thermally efficient of the systems
`because there is only one substrate to heat-up, and the
`exotherm created during the CDPF regeneration is created at
`the same location as the soot. As there is no separate flow-
`through DOC, all CO and HC emissions must be controlled by
`the CDPF, and therefore this arrangement places very high
`demands on the catalyst coating. The catalyst must have very
`high thermal durability to keep high activity over repeated
`soot regenerations where severe temperature conditions may
`be encountered. In addition, the catalyst has a high exposure to
`inorganic ash deposits, such as calcium, zinc and phosphorus,
`to which it must also be very tolerant.
`
`The common element to all three systems is the extreme
`conditions to which the DOC and/or CDPF are exposed. New
`specialised catalyst formulations have been developed for
`these applications.
`
`CONTROL OF HDD PARTICULATE EMISSIONS
`
`In Europe, the five-fold reduction in PM emissions required
`by the incoming Stage 4 legislation is not accompanied by a
`large reduction in the NOx emission limit with respect to the
`Stage 3 level. In principle there are two ways to meet the
`Stage 4 limits. EGR may be applied to reduce the NOx level,
`but this will lead to an increase in PM emissions, so it is
`expected that a filter will need to be added to control PM
`emissions when using this strategy. The alternative approach
`is to advance the injection timing, which leads to low PM
`emissions (within the Stage 4 limits) but high NOx emissions
`(outside the Stage 4 limits). Selective Catalytic Reduction
`(SCR) will then be applied to control the NOx (see below).
`The latter strategy is expected to be the principal one used
`since advancing the timing leads to a significant fuel economy
`benefit.
`
`A similar argument applies to European Stage 5 limits, as the
`PM limit is currently expected to be unchanged from the Stage
`4 level. The NOx limit for European Stage 5 is lower than
`Stage 4. However, the possibility still remains that European
`Stage 5 could be achieved by a combination of engine-out PM
`control and SCR.
`
`to high
`oxidation catalyst must have high durability
`temperature conditions. The upstream oxidation catalyst can
`also be used to maximise the passive soot combustion via the
`oxidation of NO to NO2, and subsequent reaction of NO2 with
`soot. A variation on system A is already in production in
`Europe, where a fuel additive is used to lower the temperature
`required to combust soot with oxygen to around 450°C [6].
`
`System B
`
`System B also utilises the upstream DOC to create exotherms
`to aid the active regeneration strategy, and to generate NO2. In
`addition, the filter is catalysed to improve soot combustion.
`This system provides improved NO2-based regeneration, since
`NO formed via reaction 2 can be reoxidised on the CDPF and
`re-used to react further with soot in the filter. In this way, the
`system makes better use of the available NOx.
`
`Figure 9 compares the low temperature performance of
`System A and System B over a low temperature cycle, in
`which the inlet temperature is always below 270oC. The
`backpressure was used to provide an indication of the extent
`of regeneration. The backpressure of a bare filter is also
`shown, to demonstrate the rate of backpressure increase from
`soot accumulation in the absence of any regeneration. The
`backpressure of System B is significantly below that of
`System A, revealing that System B has better low temperature
`passive regeneration [10].
`
`Bare Filter Alone
`
`System A
`
`System B
`
`5
`
`10
`
`15
`
`25
`20
`Time (Hours)
`
`30
`
`35
`
`40
`
`45
`
`0.16
`
`0.14
`
`0.12
`
`0.1
`
`0.08
`
`0.06
`
`0.04
`
`0.02
`
`Back Pressure (Bar)
`
`0
`
`0
`
`Figure 9: Passive regeneration of Systems A and B over a low
`temperature cycle.
`
`In addition, CO can be formed by incomplete oxidation of soot
`or the HC used for the exotherm generation, and the CDPF is
`very efficient at removing this (Figure 10). In addition, the
`exotherm does not need to be created 100% by the DOC. The
`catalyst on the filter can also be utilised, thus creating extra
`heat in the same location as the soot.
`
`BASF-2009.007
`
`

`
`NOX ADSORBER CATALYSTS (NAC)
`
`The chemical principles behind NOx adsorber catalysts have
`been well documented [14, 15]. Under lean conditions (the
`normal operating mode of a Diesel engine), the catalyst
`promotes NOx adsorption as illustrated by reactions 4 and 5,
`where M is the NOx adsorbing element, MO is the stable
`oxidized form of the element, and MNO3 is the stable NOx
`containing compound formed by element M.
`
` 2NO + O2 → 2NO2
`
` (4)
`
` NO2 + MO → MNO3
`
` (5)
`
`Under fuel-rich conditions, the catalyst promotes the reverse
`reaction i.e. decomposition of the nitrate phase to release the
`stored NOx (reaction 6). Finally, the catalyst reduces the NOx
`to form N2 (reaction 7).
`
`MNO3 → MO + ½O2 + NO
`
`(6)
`
`NO + CO / HC → ½N2 + CO2
`
` (7)
`
`Fuel rich conditions on Diesel engines can be achieved in-
`cylinder via engine calibration techniques [16, 17], or with
`fuel injection directly into the exhaust gas [18 - 21].
`
`For most vehicles requiring the use of a NAC, the use of PM
`control devices is also expected to be required. Additionally, if
`the operation of NAC systems is through the use of in-cylinder
`techniques, engine-out PM emissions during rich operation
`may increase such that a PM control device is necessary [22].
`
`A possible exhaust arrangement for a NAC + CDPF system is
`illustrated in Figure 12. The NAC is shown upstream of the
`CDPF because of thermal considerations – the high thermal
`mass of filter devices (especially silicon carbide wall-flow
`filters) would significantly delay the light-off of a catalyst
`placed behind it. This thermal lag is especially important in
`cold-start situations.
`
`Also illustrated in Figure 12 is a DOC upstream of the NAC.
`This catalyst is optional, but can bring some benefits to the
`system, such as better oxidation activity for CO and HC, and
`enhancing the NAC activity. Figure 13 shows the NOx
`conversion from engine bench evaluation of a NAC versus a
`DOC + NAC system. An improvement of 10 – 30 % in NOx
`conversion is seen across a wide temperature range.
`
`Exhaust Flow
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`DOC
`(optional)
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`NOx
`Adsorber
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`CDPF
`
`Figure 12: Schematic of a possible exhaust architecture for a
`NAC system.
`
`In the USA, many HDD engines are already being fitted with
`EGR to meet the USA 2004 legislation. This strategy,
`combined with the large reduction in both NOx and PM
`emissions required to meet the US 2007 legislation, means
`that one possible strategy to meet these limits is to combine
`EGR with a particulate filter [11].
`
`The same operational considerations discussed above for
`LDD also apply to HDD particulate control devices.
`However, due to the higher temperatures generally prevalent
`in HDD applications, the extent of NO2-based regeneration is
`likely to be higher in HDD systems. Indeed, there is
`substantial experience with retrofitting such devices to
`existing vehicles and relying entirely upon NO2 to regenerate
`the filter [12, 13].
`
`Nevertheless an oxygen-based regeneration must still be used
`to ensure the system works in all situations. Figure 11 shows
`an oxygen regeneration of System A (with a cordierite wall-
`flow filter) conducted on a HDD engine. Diesel fuel was
`injected directly into the exhaust to create a temperature of
`600°C after the DOC, with an engine-out temperature of
`250°C. The filter regeneration was monitored by measuring
`the backpressure of the system. Once regeneration starts, it is
`rapid and controlled, with the backpressure dropping from 0.1
`bar to 0.03 bar in about 4 minutes.
`
`Back Pressure (Bar)
`
`0.12
`
`0.1
`
`0.08
`
`0.06
`
`0.04
`
`0.02
`
`0
`1200
`
`T before Cat
`T after Cat
`T after DPF
`Delta P
`
`Inject Fuel
`
`0
`
`200
`
`400
`
`600
`time (s)
`
`800
`
`1000
`
`650
`
`600
`
`550
`
`500
`
`450
`
`400
`
`350
`
`Temperature (C)
`
`300
`
`250
`
`200
`
`Figure 11: System backpressure and temperatures during an
`oxygen-based soot regeneration with System A on a HDD
`engine.
`
`DIESEL NOX CONTROL AFTERTREATMENT
`
`Both LD and HD Diesels in the USA will have to meet
`extremely stringent NOx emissions legislation in the 2007 –
`2010 time frame, almost certainly necessitating the use of
`aftertreatment for NOx control.
`
`In Europe, catalytic treatment of NOx emissions is also
`assumed to be required on LDD vehicles for Stage 5 limits
`(when they are set). In addition, HDD engines are likely to use
`NOx control for Stage 4 onwards.
`
`There are two main aftertreatment technologies for high levels
`of NOx conversion – NOx Adsorber Catalysts (NACs) and
`Selective Catalytic Reduction (SCR).
`
`BASF-2009.008
`
`

`
`700C 16hr conditioned
`72h 750°°°°C l/r aged
`
`200
`
`300
`T