HK1092514A - Exhaust system for lean burn ic engine including particulate filter and nox absorbent - Google Patents

Description

Exhaust system for lean burn internal combustion engine comprising particulate filter and NOx absorbent

The present invention relates to an exhaust system for a lean burn internal combustion engine, and in particular to an exhaust system comprising a particulate filter and a NOx absorbent.

Generally, acceptable emission standards from vehicle internal combustion engines are legislated. In recent years, such standards have become more demanding, and it has therefore been a challenge for vehicle manufacturers (original equipment manufacturers or OEMs) how to meet these requirements. The exhaust gas constituents required by these legislations include Particulate Matter (PM), nitrogen oxides (NOx), carbon monoxide (CO) and Hydrocarbons (HC). A widely adopted measure to meet legislative standard requirements for particulate matter is a particulate or soot filter. Generally, such filters extend the residence time of particulate matter in the exhaust system to enable destruction thereof, and can include ceramic wall-flow filters or wire mesh filters.

Typically, the wall-flow filter is honeycomb shaped. The honeycomb has an inlet end and an outlet end and a plurality of openings extending from the inlet end to the outlet end, the openings having porous walls, wherein a portion of the total number of openings at the inlet end is plugged along a portion of their length, for example to a depth of about 5-20mm, and the remainder of the openings are open at the inlet end and plugged along a portion of their length at the outlet end, whereby a flowing exhaust gas stream flows from the inlet through the openings of the honeycomb into the open openings, through the walls of the openings, and out of the filter through the open openings at the outlet end. A combination of plugging these screens is described in U.S. patent No.4,329,162 (incorporated herein by reference). A typical arrangement is to block every other mesh on a given surface, as in a checkerboard pattern.

One problem associated with the use of particulate filters is how to destroy the particulate matter collected from the exhaust throughout the engine cycle of lean combustion. Generally, the particulate matter of diesel fuel is in oxygen (O)₂) Combustion above about 550 ℃. However, during certain phases of the drive cycle, for example, diesel exhaust gas temperatures (particularly in light-duty diesel engines) can be as low as 150 ℃, due to the increasingly severe use of Exhaust Gas Recirculation (EGR) to reduce NOx emissions. If particulate matter is allowed to accumulate, the back pressure may increase, thereby increasing the load on the engine. Increased engine load can lead to increased fuel consumption and, in the worst case, engine wear and filter destruction due to uncontrolled combustion of large amounts of particulate matter. Although increasing engine load (e.g., by increasing backpressure due to particulate matter accumulation) may also increase exhaust temperature sufficiently to combust particulate matterHowever, this temperature increase is not sufficient to keep the filter reliably clean.

Light duty diesel engines are subject to European legislation by European regulation 70/220/EEC (post-modification 93/59EC and 98/69/EC). In the united states passenger cars, light trucks (LLDT) less than 6000 pounds Gross Vehicle Weight Rating (GVWR) and heavy light trucks (HLDT) greater than 6000 pounds are included in the light diesel vehicle category. The temperature of the gas emitted from a light-duty diesel engine is generally lower than the exhaust temperature of a heavy-duty diesel engine (subject to regulations).

Catalyzed particulate filters are known to reduce soot combustion temperatures, thereby facilitating passive regeneration of the filter by oxidation of particulate matter at exhaust temperatures encountered during normal engine/vehicle operation (typically in the range of 300-. Without the catalyst, the particulate matter can be oxidized at appreciable rates at temperatures in excess of 500 ℃, which are rarely seen during operation over the actual life of a diesel engine. Such catalyzed filters are often referred to as catalyzed soot filters (or CSF).

A common problem with passive filter regeneration is that the driving regime can prevent the exhaust gas temperature from reaching lower temperatures that are conducive to adequate and reliable prevention of particulate matter accumulation on the filter through the often catalyzed filter. Such driving conditions include extended periods of engine idling or slow drive in urban areas, which is a problem particularly acute with gases emitted from light-duty diesel engines. One solution to this problem that has been employed by OEMs is to regenerate the filter using active techniques, either at normal intervals or when a predetermined filter backpressure is detected in addition to passive filter regeneration. A typical configuration in light-duty diesel vehicles is to place a Diesel Oxidation Catalyst (DOC) on a separate monolith upstream of the CSF and to regulate in-cylinder fuel combustion by various engine management techniques in order to introduce increased amounts of unburned fuel into the exhaust. The increased fuel is combusted over the DOC, thereby raising the temperature in the downstream CSF sufficiently to promote combustion of particulate matter thereon.

Processing microAn important advance in particulate matter was the discovery that particulate matter in diesel fuel can be made lower than nitrogen dioxide (NO) at temperatures below 400 deg.C₂) In a combustion chamber (see EP-B-0341832, incorporated herein by reference). NO₂May be obtained by oxidizing Nitric Oxide (NO) in the exhaust over a suitable oxidation catalyst and reacting with particulate matter on a downstream filter. This advance can destroy particulate matter in the normal exhaust temperature window of many diesel engines. We use CRT^The name of (a) sells the apparatus comprising the process. However, while this process has been successfully used for heavy duty diesel applications, difficulties remain in its use with certain lean-burn internal combustion engines, particularly light duty diesel engines. A recurring problem is low exhaust gas temperature, e.g. for NO₂Thermodynamic limitation of combustion of medium particulate matter and NO to NO₂Balancing of (1).

NOx from lean exhaust gas is absorbed on a NOx absorber such as barium to "store" it as nitrates and release NOx and reduce it to dinitrogen (N) in the exhaust gas containing less oxygen₂) Is known, for example from EP 0560991 (incorporated herein by reference). Generally, when the process is used in practice, various techniques are utilized to assess the remaining capacity of the NOx absorber and to control the engine to switch on and off momentarily and intermittently to operate with lower O production relative to normal lean operating conditions₂The state of the exhaust gas of concentration (i.e., enriched exhaust gas) so as to be dinitrogen (N)₂) And removes the stored NOx, thereby regenerating the NOx absorbent.

As used herein, "absorbent" includes both "adsorbent," i.e., a substance that takes up and combines a solid, a vapor, or a gaseous species on its surface with another solid, a vapor, or a gaseous species with which it comes into contact.

As used herein, "rich" refers to a lower O relative to normal lean operating conditions₂Concentrations, including both values of λ > 1 and λ < 1.

From a NO comprising a catalyst such as platinum and rhodiumDevices composed of an x-absorber are referred to as lean NOx traps or simply NOx traps, in which a catalyst platinum promotes the oxidation of NO to NO in the lean exhaust state₂And the catalyst rhodium can catalytically reduce the NOx released from the NOx absorber to N during the periodic enrichment state₂。

We have now discovered a method of improving particulate matter and NOx emissions using a NOx absorbent during the motoring cycle of a lean burn internal combustion engine such as a light duty diesel engine.

According to one aspect, the present invention provides an exhaust system for a lean burn internal combustion engine comprising a particulate filter, a first NOx absorbent disposed upstream of the filter and a second NOx absorbent disposed downstream of the filter.

The term "particulate filter" refers to any device that increases the residence time of particulate matter in the device relative to the airflow through a single sheet of the same material, wall thickness, open frontal area, and mesh density comprising an array of parallel straight channels disposed parallel to the exhaust flow direction. Examples of such devices are wall flow filters made of cordierite or silicon carbide, metal filters made for example of wire mesh and devices comprising channels which present tortuous paths to exhaust gas flowing therethrough, such as EP 1057519 (incorporated herein by reference).

EP 0758713 (incorporated herein by reference) describes the reaction with NO₂The use of NOx sorbers in connection with processes for burning particulate matter. However, in this configuration, a separate NOx absorber is provided downstream of the filter.

In one embodiment, the first NOx absorbent is adapted to release stored NOx at about 300 ℃ and above at λ > 1. In this embodiment, the first NOx absorbent may comprise a material capable of absorbing NOx under lean exhaust conditions below about 300 ℃. Such materials may include cerium, lanthanum, alumina (Al)₂O₃) At least one of iron, zinc, calcium, sodium and magnesium and mixtures of any two or more thereof. It is believed that under fuel lean conditions the elements will be in the state of their oxide compounds, although they may also be present in the carbonate and/or hydroxide states. It is understood that these compounds form nitrates when contacted with NOx. However, it is believed that these nitrates are thermodynamically unstable above about 300 ℃ even in lean exhaust gases and can decompose to release as NO or NO₂NOx of (c) is added. NO and NO released in the presence of a reducing catalyst such as rhodium at a lower oxygen concentration₂Can be reduced to N₂。

As described above, according to the process described in EP 0341832, one aspect of the present invention is that NOx in the exhaust gas may be absorbed by the first NOx absorbent when the exhaust gas temperature is low (e.g., after a cold start, or during a drive cycle when the exhaust gas temperature is decreasing), while NOx may be treated as NO during fuel-lean operating conditions for burning particulate matter held downstream of the filter when the temperature is thermodynamically more favorable for the combustion of the particulate matter₂And released.

In general, NOx stored on the first NOx absorbent may be desorbed in a fuel-rich exhaust gas component at a lower temperature. In this case, if the first NOx absorbent includes a NOx reduction catalyst such as rhodium, NOx can be reduced to N₂. However, rhodium is likely not capable of forming a net reduction of NOx released under lean operating conditions.

According to another embodiment, a second NOx absorbent disposed downstream of the filter is capable of storing NOx during lambda > 1 conditions at about 300 ℃ to about 550 ℃. Suitable materials for use as the second NOx absorbent include at least one alkali metal such as potassium or cesium, at least one alkaline earth metal such as strontium or barium, or at least one rare earth metal, or mixtures of one or more thereof. The at least one rare earth metal may be yttrium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, or a mixture of any two or more thereof.

One advantage of using the above materials in the first NOx absorbent is that NOx can be treated in the system during extended periods of low exhaust temperatures, such as after cold start or idling or slow driving. The NOx released from the first absorbent may be treated with the second NOx absorbent when it has reached a desired operating temperature.

According to another embodiment, at least one of the first and second NOx absorbers comprises at least one Platinum Group Metal (PGM). The at least one platinum group metal may be, for example, platinum, palladium or rhodium. Although both the first and second NOx absorbents may comprise platinum and rhodium or palladium, in one embodiment the first NOx absorbent comprises platinum as the sole platinum group metal. This is for at least two reasons. First, in embodiments where the first NOx absorbent is designed to release stored NOx in a lean exhaust at temperatures of about 300 ℃ and above, to reduce the released NOx to N₂Rhodium need not be present in the enriched exhaust. Second, if the rich engine-driven exhaust is intended to regenerate the second NOx absorbent, the presence of rhodium on the first NOx absorbent may undesirably remove some of the HC upstream of the second NOx absorbent.

In another embodiment, a filter in an exhaust system is catalyzed. The catalyst may comprise at least one platinum group metal which may be supported directly from the material forming the filter or on a large surface area particulate refractory oxide and coated onto a filter substrate. Methods of making directly supported substrates are known and include soaking filter materials (e.g., cordierite) in an aqueous solution of a platinum group metal, followed by drying and firing the formed article.

If the platinum group metal is supported on a particulate refractory oxide, it may be fixed to the refractory oxide by calcination prior to application to the substrate, or a washcoat of the refractory oxide may be applied to the substrate and subsequently impregnated with an aqueous solution of the platinum group metal using known techniques. However, it is important that the size of the particulate support is selected such that the refractory compound does not block the pores of the filter substrate, so that the backpressure of the filter is significantly increased relative to an uncoated filter, or filtration efficiency may be compromised. We have found that pores smaller than 25 μm (e.g. 15-25 μm) are generally useful for filtering particulate matter of diesel fuel and therefore we prefer that the refractory oxide particles should be smaller than this size. This means that particles of the repair coating can remain in the pores without completely clogging.

Alternatively, or in addition, the catalyst may comprise a soot combustion catalyst consisting of a molten salt selected from the group consisting of: alkali metal salts of vanadium, tungsten or molybdenum, alkaline earth metals of vanadium, tungsten or molybdenum or lanthanum salts of vanadium, tungsten or molybdenum, vanadium pentoxide, silver vanadate and copper vanadate. Suitable alkali metals include one or both of potassium or cesium. The alkaline earth metal may be selected from magnesium, calcium, strontium, barium and any two or more thereof.

Another aspect of the present invention is to utilize the components of the exhaust system of the present invention to more effectively manage heat in the system and thereby improve conversion of targeted exhaust constituents. As mentioned above, one problem with treating exhaust gas from lean-burn internal combustion engines, particularly light-duty diesel engines, is that the temperature of the exhaust gas can be undesirably low during certain phases of a drive cycle. This can make it difficult to catalytically treat the exhaust to meet legislative limits. These problems may be reduced or avoided by more effectively managing the retention or generation of heat within the system.

Although the present invention is capable of enhancing the NO reduction of particulate matter at moderate temperatures₂The extent of combustion, but it is anticipated that the particulate matter may contain readily available O₂The composition of the combustion. Operation of the invention may include higher temperatures (relative to NO) under lean conditions₂Middle combustion) in O₂While burning particulate matter on the filter, regeneration of the NOx trap typically requires higher temperatures and/or an enriched state to remove NOx, and even higher temperatures and a more enriched state to remove sulfur oxides (SOx).

To promote lean combustion of particulate matter at higher temperatures on the filter, the particulate matter may be driedThe HC in the exhaust gas, either post-injected HC or engine-derived HC, is combusted with the catalyst in the first NOx absorber by modulating the air/fuel ratio of the engine, thereby increasing the temperature of the filter. The provision of exotherm O upon combustion of HC and/or CO by injecting secondary air or lean exhaust air (e.g. from a parallel exhaust line) between the first NOx absorber and the filter₂. Alternatively, or in addition, an Oxygen Storage Component (OSC) such as ceria or a ceria-zirconia mixed oxide may be provided downstream of the first NOx absorbent (optionally downstream of any HC injector) or between the first NOx absorbent and the particulate filter. The first NOx absorbent can also be partially or fully regenerated by the action of HC to generate an exotherm. Where a catalyst is present, an additional exotherm may be generated on the filter catalyst.

The resulting increase in temperature and temperature in the filter can result in an increase in temperature of the second NOx absorbent, but typically the exhaust gas contacting the second NOx absorbent will be lean. Accordingly, a mechanism for introducing HC between the filter and the second NOx absorbent may be provided to change the composition of the exhaust gas to enrich it, thereby releasing NOx and/or SOx as needed. Of course, in some embodiments (e.g. where the filter is uncatalyzed), the system may be arranged such that sufficient HC slips through the filter to regenerate the second NOx absorbent, thereby avoiding the need for an injector between the filter and the second NOx absorbent, or reducing the amount of HC required to be injected. Additional O to combust HC to create an exotherm on the second NOx absorber can also be provided by injecting secondary air or a lean exhaust gas between the filter and the second NOx absorber, or by providing an oxygen storage component downstream of the HC injection point, if desired₂(while maintaining a fuel rich exhaust gas composition).

In use, control of the enrichment of exhaust gases with a reductant such as hydrocarbons used as fuel for engine power and the introduction of secondary or lean exhaust air may be controlled by an Engine Control Unit (ECU) comprising, for example, a suitably programmed processor or computer "chip".

In another embodiment, the system includes a method for oxidizing NO in an exhaust gas to NO₂The oxidation catalyst of (3), which may be disposed between the first NOx absorbent and the filter. This embodiment employs the configuration described in EP 341832 above. One advantage of this arrangement and/or the embodiment in which the filter is catalysed is that HC slip is reduced as much as possible during NOx absorber regeneration of the first NOx absorber. According to another embodiment, the oxidation catalyst may be arranged between the exhaust manifold and the first NOx absorbent, preferably upstream of any HC injector.

In a particular embodiment, the oxidation catalyst and the oxygen storage component are combined. In one such configuration, the catalyst comprises ceria, such as a ceria-zirconia mixed oxide, and optionally at least one platinum group metal supported thereon.

The or each NOx absorber and any filter catalyst or NO oxidation catalyst used in the invention may comprise a catalyst consisting of alumina, silica-alumina, zirconia, titania, ceria-zirconia or a mixture of any two or more thereof or a mixed or composite oxide of any two or more thereof.

The "composite oxide" herein refers to a mostly amorphous oxide material composed of an oxide of at least two elements, which is not a true mixed oxide composed of at least two metals.

The support may be stabilized with at least one rare earth metal, as is known in the art. The at least one rare earth metal may be lanthanum, yttrium, cerium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, or a mixture of any two or more thereof.

According to another aspect, the present invention provides a lean burn internal combustion engine comprising an exhaust system according to the present invention. In one embodiment, the engine is a diesel engine, preferably a light-duty diesel engine.

According to another aspect, the present invention provides a method of controlling NOx in an exhaust system of a lean burn internal combustion engine, the method comprising: the method includes the steps of collecting particulate matter from the exhaust gas downstream of the first NOx absorbent, absorbing NOx in the first NOx absorbent when the temperature of the first NOx absorbent is below 300 ℃, desorbing the absorbed NOx to add to pre-existing NOx in the exhaust gas when the temperature of the first NOx absorbent is above 300 ℃, combusting soot within the collected NOx in the exhaust gas, and absorbing NOx derived from combustion of the soot within the NOx.

For a more complete understanding of the present invention, an illustrative embodiment and example are provided below by way of example only with reference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of an exhaust system according to the present disclosure;

FIG. 2 is a schematic diagram illustrating operation of the exhaust system when in a cold state (e.g., after a cold start or during a drive cycle that produces cooler exhaust gas);

FIG. 3 is a schematic diagram illustrating the operation of the exhaust system when the temperature is 300 ℃ and above;

FIG. 4 is a schematic illustrating operation of the exhaust system in regenerating the first NOx absorbent;

FIG. 5 is a schematic illustrating operation of the exhaust system in regenerating the catalyzed soot filter and the second NOx absorber;

FIG. 6 is a trace of gas concentration versus time showing the specificity of NOx downstream of the NOx trap (1);

FIG. 7 is a graph showing exhaust temperature versus time during a fuel rich pulse causing a temperature rise on the NOx trap (1) with reductive combustion of residual oxygen;

FIG. 8 is a trace of exhaust lambda values taken upstream and downstream of the NOx trap (1) with air injection downstream of the NOx trap (1);

FIG. 9 is a trace of exhaust gas temperature taken upstream and downstream of a catalyzed soot filter after air injection between the NOx trap (1) and the filter;

FIG. 10 is a trace graph showing exhaust temperature versus time resulting from a lean fuel temperature rise across the NOx trap (1); and

fig. 11 is a trace graph of NOx concentration versus time showing how the NOx trap (2) stores NOx that escapes from the NOx trap (1) in both the normal lean mode and during the fuel rich pulse in which air is injected between the CSF and the NOx trap (1).

It is believed that the annotations of fig. 1-5 are self explanatory. "NOx (1)" in the figure is a first NOx absorbent; "NOx (2)" is a second NOx absorbent; and "CSF" is an abbreviation for catalyzed soot filter.

Examples of the present invention

A light duty diesel engine with in-cylinder calibration of rich fuel is fitted with an exhaust system comprising the arrangement shown in figure 1, except that engine management is used to provide hydrocarbon enrichment of the exhaust gases occurring from the engine, i.e. no fuel is injected downstream of the exhaust manifold, and an air injector is provided between the NOx trap (1) and the catalysed soot filter. The catalyzed soot filter is a cordierite-walled gas flow filter catalyzed with a washcoat comprised of platinum supported on both the alumina-based particulate refractory oxide and the filter material itself. The preparation process of the filter comprises the following steps: coating an uncoated filter with a washcoat comprising a refractory oxide, drying and calcining the formed article, and then applying a load of 100g ft^-3The aqueous platinum salt solution of (a) impregnates the repair-coated filter.

The NOx trap (1) is a low temperature trap comprising a monolithic ceramic substrate coated with a repair coating by gas flow, the coating consisting of an aluminium based particulate refractory oxide and platinum, barium, cerium, rhodium supporting an oxygen storage member. The high temperature NOx trap (2) has a similar construction except that cesium is included in the formulation.

Operating at an exhaust temperature of 350 ℃, fig. 6 shows the specificity of NOx slip after NOx trap (1). NO upstream of NOx trap₂The concentration was 14ppm (6% of the total NOx). It can be seen that the high ratio of NOx is NO₂(up to 30% of the total NOx slip), and thus NO₂Can be used to react with soot in an upstream catalyzed soot filter according to the process disclosed in EP 0341832.

The system is configured to cycle between lean and rich operating conditions at a temperature of 450 ℃ of the engine exhaust gas. The fuel-lean time was adjusted to 300 seconds long and each fuel-rich period was 8 seconds long. It can be seen from fig. 7 that introducing a fuel rich pulse on the NOx trap (1) by reducing the oxygen concentration in the exhaust gas upstream of the NOx trap (1) creates an increase in exhaust gas temperature as the reductant is combusted in the remaining oxygen. This additional heat can be used to regenerate the NOx trap (1) for NOx or SOx in a fuel rich state.

As can be seen in fig. 8, the introduction of air after the NOx trap (1) during a fuel rich pulse creates a constant fuel lean condition downstream of the Catalyzed Soot Filter (CSF). The results of fig. 7 and 8 show that the exhaust gas lambda is fuel rich before the NOx trap (1), which allows the NOx trap to be regenerated (see fig. 7), while the injection of air after the NOx trap (1) can provide a constant lean state in the CSF downstream of the NOx trap (1). The elevated temperature resulting from the combustion of the reductant above NOx (1) (fig. 7) can be high enough to allow soot regeneration of CSF to occur in a fuel lean state. Alternatively, excess reductant can escape through the NOx trap (1) and burn over CSF in a fuel lean condition resulting from air/fuel lean exhaust gas injection, again resulting in high CSF temperatures, which can allow soot regeneration to occur, see fig. 9. Fig. 9 shows the rise in CSF temperature due to fuel-rich pulsed reductant combustion in a fuel-lean state resulting from air injection after the NOx trap (1) but before the CSF.

Additional fuel is introduced into the exhaust upstream of the NOx trap (1), but only enough to maintain a lean overall composition. This creates a temperature rise which can be used to regenerate soot in the CSF (see figure 10). NOx can also be released from the NOx trap (1) heat in the same way, causing NO upstream of the CSF₂The increase in concentration, which can be used to react with soot on CSF according to the process described in EP 0341832.

The NOx trap (2) can store NOx that escapes from the NOx trap (1) and CSF both during lean fuel and (when there is air injection between the NOx trap (1) and CSF) during the fuel rich pulse (see fig. 11). Regeneration of the NOx trap (2) can be easily accomplished by fuel injection between the CSF and the NOx trap (2) operating as each normal NOx trap (see EP 0758713).

Claims (39)

1. An exhaust system for a lean burn internal combustion engine, the system comprising a particulate filter, a first NOx absorbent disposed upstream of the filter and a second NOx absorbent disposed downstream of the filter.

2. A system according to claim 1, wherein the first NOx absorbent is adapted to release stored NOx at temperatures of about 300 ℃ and above during conditions where λ > 1.

3. A system according to claim 2, wherein the first NOx absorbent comprises cerium, lanthanum, alumina (Al)₂O₃) At least one of iron, zinc, calcium, sodium, magnesium and mixtures of any two or more thereof.

4. A system according to claim 1, 2 or 3, wherein the second NOx absorbent is capable of storing NOx at a temperature of from about 300 ℃ to about 550 ℃ during a lambda > 1 regime.

5. A system according to claim 4, wherein the second NOx absorbent comprises at least one alkali metal, at least one alkaline earth metal or at least one rare earth metal or a mixture of any two or more thereof.

6. A system according to claim 5, wherein the at least one alkali metal is potassium or caesium or a mixture thereof.

7. A system according to claim 5 or 6, wherein the at least one alkaline earth metal is strontium or barium or a mixture thereof.

8. A system according to claim 5, 6 or 7, wherein the at least one rare earth metal is yttrium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium or a mixture of any two or more thereof.

9. A system according to any preceding claim, wherein the first and/or second NOx absorber comprises at least one Platinum Group Metal (PGM).

10. A system according to claim 9, wherein at least one platinum group metal of the first NOx absorbent consists of platinum.

11. A system according to claim 9, wherein the at least one platinum group metal comprises platinum and rhodium.

12. A system according to any preceding claim, wherein the filter is catalysed.

13. A system according to claim 12, wherein the filter catalyst comprises at least one platinum group metal.

14. A system according to claim 13, wherein the at least one platinum group metal is supported by the material from which the filter is formed.

15. A system according to claim 12, 13 or 14, wherein the at least one additional catalyst comprises a soot-burning catalyst comprising a molten salt selected from the group consisting of: alkali metal salts of vanadium, tungsten or molybdenum, alkaline earth metal salts of vanadium, tungsten or molybdenum, or lanthanum salts of vanadium, tungsten or molybdenum, vanadium pentoxide, silver vanadate and copper vanadate.

16. A system according to any preceding claim, comprising a means for oxidising NO in the exhaust gas to NO₂The catalyst is disposed between the first NOx absorbent and the filter and/or between the exhaust manifold and the first NOx absorbent.

17. A system according to claim 16, wherein the NO oxidation catalyst comprises at least one platinum group metal.

18. A system according to any preceding claim, comprising an Oxygen Storage Component (OSC) arranged between the first NOx absorbent and the filter and/or between the filter and the second NOx absorbent.

19. A system according to claim 18, wherein the oxygen storage means comprises ceria, optionally a ceria-zirconia mixed oxide.

20. A system according to any preceding claim, wherein each NOx absorber and, where present, the filter catalyst and/or the NO oxidation catalyst comprises a support of alumina, silica-alumina, zirconia, titania, ceria-zirconia or a mixture of any two or more thereof or a mixed oxide or composite oxide of any two or more thereof.

21. A system according to claim 20, wherein the oxidation catalyst comprises platinum supported on alumina.

22. A system according to claim 20 or 21, wherein the support is stabilised with at least one rare earth metal.

23. A system according to claim 22, wherein the at least one rare earth metal is lanthanum, yttrium, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium or a mixture of any two or more thereof.

24. A system according to any preceding claim, comprising first means for enriching exhaust gas with reductant upstream of the first NOx absorbent.

25. A system according to claim 24, wherein the first enrichment means comprises a first port for injecting the reductant, the first port being disposed between the exhaust manifold and the first NOx absorbent.

26. A system according to claim 24 or 25, comprising second means for enriching exhaust gas with a reductant between the first NOx absorbent and the filter, the second enriching means comprising a second port for injecting the reductant, the second port being disposed between the first NOx absorbent and the filter.

27. A system according to claim 24, 25 or 26, comprising first means for introducing secondary air or a lean exhaust gas into the exhaust gas between the first NOx absorbent and the filter.

28. A system according to any of claims 24 to 27, comprising third means for enriching an exhaust gas with a reductant between the filter and the second NOx absorbent, the third enriching means comprising a third port for injecting the reductant, the third port being disposed between the filter and the second NOx absorbent.

29. A system according to any of claims 24 to 28, comprising second means for introducing secondary air or a lean exhaust gas into the exhaust gas between the filter and the second NOx absorbent.

30. A system according to any of claims 24 to 29, comprising means, in use, for controlling enrichment of exhaust gas with a reductant between the exhaust manifold and the first NOx absorbent.

31. A system according to any of claims 26 to 30, comprising means, in use, for controlling enrichment of exhaust gas with a reductant between the first NOx absorbent and the filter.

32. A system according to any of claims 28 to 31, comprising means, in use, for controlling enrichment of exhaust gas with a reductant between the filter and the second NOx absorbent.

33. A system according to claim 27 and any of claims 28 to 32 when dependent on claim 26, comprising means for controlling the introduction of secondary air or a lean exhaust gas into the exhaust gas between the first NOx absorbent and the filter, in use.

34. A system according to claim 29 and any of claims 30 to 33 when dependent on claim 29, comprising means, in use, for controlling the introduction of secondary air or a lean exhaust gas into the exhaust gas between the first NOx absorbent and the filter.

35. A system according to any of claims 30 to 34, wherein the control means comprises an Engine Control Unit (ECU).

36. A system according to any of claims 24 to 35, wherein the reductant is a hydrocarbon, optionally a hydrocarbon that is fuel for the engine.

37. A lean burn internal combustion engine comprising an exhaust system according to any preceding claim.

38. An engine according to claim 37, characterized in that it is a diesel engine, preferably a light-duty diesel engine.

39. A method of controlling nitrogen oxides (NOx) and Particulate Matter (PM) in an exhaust system of a lean burn internal combustion engine, the method comprising: collecting particulate matter from the exhaust gas downstream of the first NOx absorbent; absorbing NOx in the first NOx absorbent when the temperature of the first NOx absorbent is up to 300 ℃; desorbing the absorbed NOx when the temperature of the first NOx absorbent is above 300 ℃ while adding pre-existing NOx to the exhaust gas, NO in the exhaust gas₂Combusting the collected soot, and absorbing the NO therefrom₂NO obtained by combustion of soot in₂。