Abstract:
Embodiments of the invention provide an engine aftertreatment apparatus comprising first and second diesel particulate filters (DPFs), the first DPF being provided upstream of the second DPF, the second DPF being provided with a coating of a second DPF catalyst, the second DPF catalyst being arranged to promote reduction of NOx.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus and a method for filtering exhaust gas generated by a diesel engine. Aspects of the invention relate to an apparatus, to a vehicle and to a method. 
     BACKGROUND 
     Engine exhaust gases contain a number of gases and particulates that it is undesirable to release to the atmosphere. The gases typically include hydrocarbons, carbon monoxide (CO), carbon dioxide (CO 2 ) and nitrogen oxides (NOx) whilst the particulates include carbon particles and other solid and liquid phase components. 
     It is known to provide engine aftertreatment apparatus employing a diesel oxidation catalyst (DOC) module, a diesel particulate filter (DPF) module and a selective catalytic reduction (SCR) catalyst module.  FIG. 1(   a ) shows an example of such an apparatus  100 . 
     The apparatus  100  has a DOC module  120  coupled to an exhaust outlet  111  of an engine  110 . The DOC module  120  is provided upstream of a DPF module  130  which is in turn provided upstream of an SCR module  140 . An ammonia source  142  is arranged to inject ammonia gas or an ammonia precursor compound into a flowstream of exhaust gases from the DPF module  130  to the SCR module  140 . In some systems, the order of the SCR catalyst and DPF can be reversed. 
     The DOC module  120  is arranged to oxidise hydrocarbons and CO to form CO 2  and H 2 O. Some NO 2  may also be formed by oxidation of NO in the exhaust gas stream. The generation of NO 2  by the DOC  120  can be helpful in promoting formation of N 2  in the SCR module, and in oxidising particulate matter trapped on the DPF. The DOC module  120  may comprise a ceramic monolith having an oxidation catalyst provided thereover. The catalyst may be applied by means of a wash coat, and may comprise one or more types of metal such as platinum, rhodium, palladium or an alloy of two or more metals. 
     The DPF module  130  is a wall-flow filter module and contains a porous DPF filter material arranged to trap soot particles entrained in the exhaust gases. The filter material is coated with an oxidation catalyst that oxidises the trapped particles to form CO 2 . DPF modules  130  are typically arranged to operate in a passive regeneration role in which oxidation of the trapped particles takes place to a sufficient extent without a requirement to provide external heat energy to the DPF module  130  other than that provided by the exhaust gases as exhausted from the engine  110  at the exhaust outlet  111 . 
       FIG. 2(   a ) is a cross-sectional schematic illustration of a wall-flow filter  30  in which exhaust gases flow through parallel channels  31 C defined by walls  31  of the filter  30 . The channels  31 C are blocked alternately at a first end  30 A or a second end  30 B such that gases entering the filter  30  through one channel  31 C at the first end  30 A are forced to flow through a wall  31  of the filter  30  in order to exit the filter through an adjacent channel  31 C at the second end  30 B. 
       FIG. 2(   b ) is a cross-sectional schematic illustration of a filter  40  of flow-through or deep bed type. The filter has a porous matrix  41  having a plurality of pores or channels  41 C therethrough. Gases entering the filter  40  at a first end  40 A may pass through the filter  40  by passing along the channels  41 C and emerge from a second end  40 B opposite the first end  40 A. 
     The problem exists that the amount of soot trapped in the DPF module  130  may become too high, for example due to the DPF module  130  not attaining a sufficiently high temperature for a sufficiently long period during a given drivecycle. An active regeneration operation may therefore be required. A number of technologies exist for performing active regeneration operations. One common technology is an operation in which the engine is operated under conditions in which additional fuel is injected into the cylinder late in the engine operating cycle. Excess unburned fuel becomes entrained in the exhaust gas and enters the DOC module  120 . The DOC module  120  oxidises the unburned fuel generating heat which causes an increase in the temperature of catalyst and soot in the DPF module  130 , promoting oxidation of soot particles trapped in the DPF module  130 . 
     It is to be understood that it is undesirable to operate the engine  110  under active regeneration conditions for at least the following reasons: (1) the process causes an increase in fuel consumption and CO 2  and NOx emissions; and (2) some of the fuel mixes with engine oil causing a reduction in viscosity and lubricity of the oil. 
     Thus it is desirable to keep a flow path of exhaust gases from the engine  110  to the DOC module  120  and DPF module  130  as short as possible, so that active regenerations may be performed in as short a time period as possible and passive regeneration can be maximised. 
     The SCR module  140  is arranged to reduce NO x  gases passing therethrough to N 2 . A reduction catalyst such as Cu, Fe, V or a mixture thereof is provided in the SCR module  140  and ammonia (NH 3 ) is injected into the flow of exhaust gases into the SCR module  140 , either directly or as an ammonia precursor, in order to effect reduction of the NO x  gases. 
     Exhaust gases flowing through the SCR module  140  are exhausted from the apparatus  100  by means of an exhaust outlet  150  downstream of the SCR module  140 . 
     In some known arrangements, the total volume and weight of catalyst in the aftertreatment apparatus is reduced by combining the DPF and SCR modules in a single wall-flow filter. This is achieved by coating a DPF filter material with SCR catalyst (a reducing catalyst) rather than the usual DPF catalyst (an oxidising catalyst) to form a so-called SCR coated DPF (or SCRDPF) module. 
       FIG. 1(   b ) shows an example of an apparatus  200  having such an arrangement. Like features of the arrangement of  FIG. 1(   b ) to that of  FIG. 1(   a ) are provided with like reference numerals prefixed numeral 2 instead of numeral 1. 
     A conventional DOC module  220  is provided immediately downstream of an exhaust gas outlet  211  of an engine  210 . An SCR coated DPF (SCRDPF) module  240  is provided downstream of the DOC module  220 . An ammonia source  242  is arranged to inject ammonia gas or an ammonia precursor compound into a flowstream of exhaust gases from the DOC module  220  to the SCRDPF module  240 . Exhaust gases flowing through the SCRDPF module  240  are exhausted from the apparatus  200  by means of an exhaust outlet  250  downstream of the module  240 . 
     Because the DPF of the module  240  is coated with SCR catalyst and not the normal oxidation catalyst, the only way to oxidise trapped soot particles is by operating the SCRDPF module  240  at a higher temperature than would otherwise be desirable. However, operation of the module  240  at a higher temperature has the disadvantage that the NH 3  that is provided in order to promote reduction of NO and NO 2  to N 2  in the presence of the SCR catalyst becomes oxidised and pre-stored NH 3  is released and therefore does not reduce NO and NO 2  to N 2 . 
     A further disadvantage of the SCRDPF module  240  is that it requires a relatively large package size and may need to be mounted underneath the vehicle. This results in an increase in a length of the flowpath of exhaust gases from the engine  210 , and therefore a greater drop in temperature of the gases by the time they reach the module  240 . Consequently active regeneration of the SCRDPF module  240  is required to be performed regularly in order to oxidise the soot particles trapped by it. 
     In addition to the disadvantage stated above that SCRDPF filters require frequent active regeneration events, such filters also suffer the disadvantage that they may provide an increased backpressure on exhaust gases flowing through the aftertreatment apparatus. This reduces an efficiency of the engine. 
     It is desirable to provide an improved engine aftertreatment apparatus that does not suffer the disadvantages of known SCRDPF modules  240 . 
     STATEMENT OF THE INVENTION 
     In one aspect of the invention there is provided an engine aftertreatment apparatus comprising first and second diesel particulate filter (DPF) modules, the first DPF being provided upstream of the second DPF, the second DPF comprising a second DPF catalyst, the second DPF catalyst being arranged to promote reduction of one or more nitrogen oxides. 
     This has the advantage that the first DPF may be arranged to trap particles entrained in engine exhaust gases passing through the apparatus thereby reducing a particle loading on the second DPF. The first DPF may therefore perform a pre-filter function, acting as an exhaust gas pre-filter for the second DPF. The first DPF may also be made of reduced size compared with known SCR coated DPFs thereby enabling the first DPF to be mounted closer to the engine. The first DPF may therefore be exposed to exhaust gases of a higher temperature, allowing oxidisation of particulate matter and a reduction in the number of active regeneration events required. Furthermore, because the particle loading on the second DPF is reduced compared with known SCR coated DPF systems, the number of active regeneration events required by the second DPF filter may be reduced. In some embodiments of the invention the requirement to perform active regeneration of one or both of the first and second DPFs may be eliminated. 
     Advantageously the first DPF may be arranged to trap particles having a size in a first size range from a first DPF lower particle size value to a first DPF upper particle size value and to allow particles having a size above the first DPF upper particle size value to flow therethrough. 
     The first DPF lower particle size value may be in the range of from around 1 nm to around 10 nm. 
     The first DPF lower particle size value may be around 1 nm or around 10 nm. 
     Optionally the first DPF upper particle size value is one selected from amongst around 100 nm and around 200 nm. 
     The first DPF may be more effective at trapping smaller particles of up to 100 nm in size. In some embodiments the first DPF is arranged to trap particles in the size range from around 1 nm to around 100 nm. In some embodiments the first DPF is arranged to trap particles in the size range from around 10 nm to around 100 nm. 
     Optionally the first DPF is arranged to cause agglomeration of particles having a size in the first size range. 
     Advantageously the first DPF may comprise a filter of flow-through or deep bed type. 
     Advantageously the second DPF may be arranged to trap particles having a size range from a second DPF lower particle size value to a second DPF higher particle size value. 
     Further advantageously the second DPF lower particle size value may be higher than the first DPF lower particle size value. 
     This feature has the advantage that a backpressure created by the second DPF may be made lower than the backpressure created by a known SCRDPF device. This is because the second DPF has a higher particle size trapping threshold than known SCRDPF devices. 
     Optionally the second DPF lower particle size is less than the first DPF upper particle size. 
     The second DPF may be arranged to trap particles over substantially the whole particle size spectrum. 
     The second DPF may be arranged to trap particles in the range from around 10 nm to around 100 microns, optionally from around 100 nm to around 100 microns. 
     Advantageously the second DPF may be of the wall-flow type. 
     Further advantageously the first DPF may be pr with a coating of a first DPF catalyst arranged to oxidise carbon particles trapped by the first DPF. 
     Advantageously the first DPF catalyst may comprise a platinum and/or palladium catalyst. The catalyst may be supported by a metallic foam, metal fleece or corrugated foil. Other supports are also useful. 
     Further advantageously the second DPF catalyst may comprise a selective catalytic reduction (SCR) catalyst. 
     Optionally the second DPF catalyst may comprise one selected from amongst Cu, Fe and V. 
     Advantageously the first DPF may be provided in a first DPF module and the second DPF may be provided in a second DPF module different from the first DPF module. 
     This feature has the advantage that the first DPF may be packaged in the form of a relatively small module (compared with known SCRDPF modules  240 ) and therefore be provided at a location having a shorter flowpath from an exhaust gas outlet of the engine. 
     Optionally the first DPF may be provided within or in the vicinity of an exhaust manifold. 
     In a further aspect of the invention there is provided a method of treatment of engine exhaust gases comprising: passing the exhaust gases through a first diesel particulate filter (DPF); and subsequently passing the exhaust gases through a second DPF downstream of the first DPF, the second DPF comprising a second DPF catalyst, the second DPF catalyst being arranged to promote reduction of one or more nitrogen oxides. 
     In a still further aspect of the invention there is provided an engine aftertreatment system comprising: a first diesel particulate filter (DPF) of flow-through or deep bed type arranged to cause agglomeration and trapping of smaller particles; and a second DPF downstream of the first, the second DPF also being of the wall-flow type and arranged to trap larger particles than the first DPF, the second DPF being provided with a coating of a second DPF catalyst, the second DPF catalyst being arranged to reduce one or more nitrogen oxides. 
     Advantageously the second DPF may be arranged to and trap larger particles than the first DPF and not the smaller particles. 
     This feature has the advantage that the second DPF may be formed to exert a reduced backpressure on engine exhaust gases passing therethrough. 
     Within the scope of this application it is expressly envisaged that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. Features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures in which: 
         FIG. 1  is a schematic illustration of a known aftertreatment apparatus; 
         FIG. 2  is a schematic illustration of filters of (a) wall-flow type and (b) flow-through or deep bed type; 
         FIG. 3  is a schematic illustration of an aftertreatment apparatus according to an embodiment of the present invention; 
         FIG. 4  is a schematic illustration of three particulate filtration trapping mechanisms; and 
         FIG. 5  is a schematic cross-sectional illustration of a filter module in use. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  is a schematic illustration of an engine aftertreatment apparatus  300  according to an embodiment of the present invention. The apparatus  300  has a diesel oxidation catalyst (DOC) module  320  coupled to an exhaust gas outlet  311  of an engine  310 . The DOC module  320  is coupled upstream of a partial diesel particulate filter (PDPF) module  330  which is in turn provided upstream of a selective catalytic reduction diesel particulate filter (SCRDPF) module  340  in which the DPF is coated with SCR catalyst. 
     The PDPF module  330  is of the flow-through or deep bed type whilst the DPF module  340  is of the wall-flow type. It is to be understood that the PDPF module  330  is a partial filter module because it is arranged to allow certain particles entrained in engine exhaust gases to pass directly through it whilst other entrained particles are trapped by it. The PDPF module  330  is arranged to trap particles having a size below a prescribed size range whilst particles having a size above this range are allowed to pass therethrough. In some embodiments the PDPF module  330  is arranged to trap particles having a size below around 200 nm. In some embodiments the PDPF module  330  is arranged to trap particles having a size in the range from around 1 nm to around 100 nm, from 1 nm to around 200 nm, from around 10 nm to around 100 nm or from 10 nm to around 200 nm. Other size ranges may also be useful. 
     The deployment of a particle filter module  330  upstream of the SCRDPF module  340  has the advantage that a particle loading on the module  340  may be reduced as described in more detail below. This in turn has the advantage that a backpressure on exhaust gases due to the module  340  may be reduced, enabling an increase in efficiency of operation of the apparatus  300 . 
     The PDPF module  330  is arranged to trap particulates by deep bed filtration mechanisms such as inertial deposition, flow line interception or diffusional deposition. Electrophoresis or thermophoresis phenomena may also play a part in trapping particulates. As such the PDPF module  330  is arranged to trap particles of relatively small size as described above. Thus particles of relatively large size typically pass through the PDPF module  330  without becoming trapped. In some embodiments a width of pores or channels through the PDPF module  300  is from around 0.5 mm (millimetres) to around 5 mm, optionally around 1 to 3 mm. In some embodiments the pores or channels have a breadth that is of a similar size. In some embodiments the pores or channels have a breadth that is larger than the width, i.e. the pores or channels are elongate in cross-section. In other words, the pores or channels may have two orthogonal dimensions normal to a flowpath of gas that are of sizes that are substantially the same or which are different to one another. 
     The three filtration mechanisms noted above are illustrated schematically in  FIG. 4 , in which particles P are trapped at a wall W of a filter. 
     Trapping by inertial deposition is a trapping mechanism in which an entrained particle P 1  ‘collides’ directly with a wall W of a filter because it is unable to follow directly a flowpath Fg of gases through pores or channels of the filter due to inertia. Its actual flowpath F 1  therefore results in a direct collision with the wall W. 
     Trapping by flowline interception is a mechanism in which a flowpath F 2  of gases in which a particle P 2  is entrained brings the particle into contact with the wall W, trapping the particle P 2 . 
     Trapping by diffusional deposition is a mechanism in which a flowpath F 3  of a particle P 3  entrained in gas flow causes the particle P 3  to become trapped at the wall W by diffusion of the particle P 3  to the wall W. 
       FIG. 5  is a schematic cross-sectional illustration of the SCRDPF module  340  in use. The DPF module  340  contains a particulate filter material  341  ( FIG. 5 ) of wall-flow type that is coated with a selective catalytic reduction (SCR) catalyst and may be referred to as an SCR coated DPF or SCRDPF module  340 . In some embodiments the filter material  341  has a channel density of around 300 channels per square inch. In some arrangements, walls defining the channels have a thickness of around 0.3 mm. In some arrangements a size of the channels or pores in the filter  341  is in the size range from around 1 micron (μm) to around 50 microns, optionally from around 10 microns to around 50 microns. 
     The filter material  431  defines an array of substantially parallel gas flow passageways  340 P, adjacent passageways being blocked at opposite respective ends with respect to a direction of flow of fluid through the module  340 . Thus gas flowing into the module is forced to flow through one of the alternate passageways, through the portion of the filter material  341  separating adjacent passageways and out from the module  340  via a passageway adjacent the one through which it entered the module  340 . Thus the gas is forced to flow through a wall of filer material  341  separating adjacent passageways  340 P. 
     The SCRDPF module  340  is arranged initially to filter particles P by one or more of the deep bed filtration mechanisms described above. 
     However, as the amount of particles P trapped by the filter material  341  increases, the filter material  341  becomes clogged and a layer  341  L of particles builds up at an inlet surface  341 S of the filter material  341  being a surface through which gases pass into the material  341 . As the layer  341 L increase in thickness the module  340  begins to filter by a sieve filtration (or cake filtration) mechanism. 
     It is to be understood that during the early stages of particle loading of a fresh or newly regenerated SCRDPF module  340 , before the cake filtration mechanism has begun to operate, an efficiency of the module  340  may be lower, especially for smaller particles. Advantageously, the additional filtration provided by the DPF module  330  reduces the number of particulates that pass through the apparatus  300  during this period. 
     Furthermore, it is found that the PDPF module  330  may be made smaller than a conventional SCRDPF module  240  (and of a size more comparable to a conventional DPF module  130 ) due to the presence of the PDPF module  330  upstream of the SCRDPF module  340  without compromising performance relative to prior art aftertreatment apparatus. The smaller size of the PDPF module  330  allows the module  330  to be provided closer to the engine exhaust outlet  311 , allowing agglomerates of particles trapped in the module  330  to be oxidised in the module  330  without a requirement for active regeneration operations to be performed. 
     It is to be understood that in some embodiments the filter material of the PDPF module  330  is coated with an oxidation catalyst to promote oxidation of particles trapped within the PDPF module  330  in a similar manner to known DPF modules  130 . 
     As noted above the PDPF module  330  has a bed filter ( FIG. 2(   a )) such as a metal foam, metal fleece or perforated foil. In contrast, the SCR coated DPF module  340  is formed of a high porosity wall flow ceramic filter, as shown schematically in  FIG. 2(   b ). 
     As noted above, embodiments of the invention have the advantage that because a PDPF module  330  is employed upstream of the SCRDPF module  340 , a particle loading on the SCRDPF module  340  may be reduced. The PDPF module  330  may be made sufficiently small that it may be located sufficiently close to the engine  310  that active regeneration events may not be required to be performed in order to oxidise particles trapped by the module  330 . 
     Furthermore, the PDPF module  330  may be arranged to trap smaller particles allowing the SCRDPF module  340  to be formed to have a reduced backpressure on a flow of exhaust gases therethrough. 
     In some embodiments, because the SCRDPF module  340  has a reduced particle loading relative to SCR coated DPF devices provided in known aftertreatment systems, a time period between regenerations (if required) may be increased. This has the advantage of reducing an amount by which engine oils are diluted by diesel fuel and an amount of undesirable emissions emitted by the aftertreatment apparatus  300 . 
     Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other moieties, additives, components, integers or steps. 
     Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
     Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.