Exhaust system for a lean burn internal combustion engine

[0001] This application is a continuation-in-part application of U.S. patent application Ser. No. 10/494,406, filed May 3, 2004, which is the U.S. National Phase application of PCT International Application No. PCT/GB02/04750, filed Oct. 21, 2002, and claims priority of British Patent Application No. 0126346.6, filed Nov. 2, 2001.

[0002] The present invention relates to an exhaust system for a lean burn internal combustion engine, and in one illustrative embodiment, to an exhaust system for a diesel engine.

[0003] Lean burn internal combustion engines, such as diesel engines and gasoline engines, produce a number of pollutants including carbon monoxide (CO), unburnt hydrocarbon (HC), particulate matter (PM) and nitrogen oxides (NO_x). Whilst not as visible to the naked eye as PM emitted by diesel engines, gasoline engines produce PM of the size-order of <1 μm such as 10-100 nm. Interest in gasoline PM is growing as particles of this size can penetrate deep into the human lungs and can be detrimental to health.

[0004] The amount of these pollutants that can be emitted by vehicular internal combustion engines is prescribed by legislation in various countries and regions of the world, such as the USA and Europe, and these amounts are set to decrease as the legislation tightens step-wise over the next ten years or so. Similarly, International agreements between countries have led to moves toward vehicular internal combustion engines that use fuel more efficiently. The legislation acts as a stimulus to vehicle manufacturers and to their suppliers to devise new engines that are more fuel-efficient and that emit fewer pollutants and to exhaust systems that can clean up the exhaust gas before it passes to atmosphere.

[0005] One such exhaust system component primarily for treating diesel exhaust comprises an oxidation catalyst for oxidising NO in the exhaust gas to NO₂ and a downstream filter for trapping PM. A process for treating diesel PM that uses this arrangement is described in EP-B-0341382 or U.S. Pat. No. 4,902,487, both of which are incorporated herein by reference. The process claimed in EP-B-0341382 comprises passing an exhaust gas, such as a diesel exhaust gas, including PM and NO unfiltered over an oxidation catalyst to convert the NO to NO₂, collecting soot on the filter and combusting the collected soot by reaction with the NO₂. This technology is commercially available as Johnson Matthey's Continuously Regenerating Trap or CRT®. Further steps may be added, for example downstream NO_x removal by injection of reductant e.g. HC or NO_x-specific reactant e.g. NH₃ or urea (see for example our WO-A-00/21647, incorporated herein by reference). An advantage of this process is that it is possible to combust diesel soot at temperatures of up to 400° C., whereas combustion of diesel soot in oxygen occurs at about 500° C. This is significant since diesel exhaust gas is generally cooler than exhaust gas from gasoline engines and soot would accumulate on the filter causing back-pressure problems in the system if the process relied on combustion of soot in oxygen.

[0006] One form of gasoline engine is a gasoline direct injection engine, which is designed to operate under stoichiometric and lean conditions. When running lean, relatively low levels of NO_x are formed that cannot be reduced (removed) in the presence of the relatively high levels of oxygen in the exhaust gas. Reducing species, e.g. HC, can reduce NO_x to N₂ during stoichiometric- or rich-running conditions, as comparatively less oxygen is present than during lean-running conditions.

[0007] In order to control NO_x in lean-bum engines, there has been devised a NO_x absorber/catalyst which can store NO_x, e.g. as nitrate, when an engine is running lean. In a stoichiometric or rich environment, the nitrate is understood to be thermodynamically unstable, and the stored NO_x is released and is reduced by the reducing species present in the exhaust gas. This NO_x absorber/catalyst is commonly called a NO_x-trap and is described in EP-A-0560991 (the entire contents of which is incorporated herein by reference). By periodically controlling the engine to run stoichiometrically or rich, stored NO_x is reduced and the NO_x-trap regenerated.

[0008] A typical NO_x-trap formulation comprises a washcoat comprising a catalytic oxidation component, such as platinum, a NO_x-storage component, such as barium, and a reduction catalyst e.g. rhodium on a surface area-enhancing support material, e.g. a metal oxide such as alumina, or a mixed or composite (i.e. not a mixed phase pure) metal oxide containing one or more of aluminium, zirconium, titanium, cerium, silicon and chromium. One mechanism commonly given for NO_x-storage during lean engine operation for this formulation is:
NO+½O₂→NO₂; and  (i)
BaO+NO₂+½O₂→Ba(NO₃)₂ .  (ii)

[0009] In the first step, the nitric oxide reacts with oxygen on active oxidation sites on the platinum to form NO₂. The second step involves adsorption of the NO₂ by the storage material in the form of an inorganic nitrate.

[0010] When the engine runs under rich conditions or at elevated temperatures, the nitrate species become thermodynamically unstable and decompose, producing NO or NO₂ according to equation (iii) below. Under rich conditions, these nitrogen oxides are subsequently reduced by carbon monoxide, hydrogen and hydrocarbons to N₂, which can take place over the reduction catalyst.
Ba(NO₃)₂→BaO+2NO+ 3/2O₂ or Ba(NO₃)₂→BaO+2NO₂+½O₂; and   (iii)
NO+CO→½N₂+CO₂ (and other reactions).  (iv)

[0011] In the reactions of (i)-(iv) above the reactive barium species is given as the oxide. However, it is understood that in the presence of air most of the barium is in the form of the carbonate or possibly the hydroxide. The above reaction schemes can be adapted accordingly for species of barium other than the oxide.

[0012] NO_x-traps are typically coated on a flow through honeycomb monolith substrates. One form of such substrate is made from a ceramic material such as cordierite. Typical commercially available flow-through monolith substrates have a mean pore size of 3 μm and a porosity of about 35%. NO_x-trap washcoat formulations for coating on flow through honeycomb monolith substrates have a D50, of about 15 μm, i.e. at least 50% of the particle sizes in the washcoat are 15 μm in diameter or less. Similarly a D70, would mean that at least 70% of the particle sizes in the washcoat are 15 μm in diameter or less.

[0013] Using sophisticated engine management techniques to provide for rich/lean cycling and common rail fuel injection, vehicle manufacturers are now adopting NO_x-trap technology into diesel exhaust systems. One such system is described in EP-A-0758713 (the entire contents of which is incorporated herein by reference). Means for reducing the redox composition of the exhaust gas, as defined by lambda, for the purpose of regenerating a NO_x-trap include injecting HC into the exhaust gas downstream of the engine, adjusting the ignition timing of at least one engine cylinder or adjusting the engine air-to-fuel ratio. Since the NO_x trap disclosed in EP-A-0758713 comprises a flow through monolith substrate, it can be assumed that the substrate porosity and mean pore size of the substrate and the D50 of the washcoat composition will be in the order of that mentioned above.

[0014] Where a NO_x-trap formulation comprises a reduction catalyst such as rhodium, it is known to locate the NO_x-trap components where they can maximise their activity, i.e. of the oxidation catalyst component, e.g. platinum, to oxidise NO to NO₂ and HC and CO during lambda >1 conditions; and for the rhodium to catalyse the reduction of NO_x to N₂ with HC during 1 ≧lambda conditions. One such arrangement is disclosed in U.S. Pat. No. 6,413,483 (the entire contents of which is incorporated herein by reference), wherein the rhodium component is in an overlayer on a layer containing the Pt component.

[0015] Another technique that can be used to control emissions is exhaust gas recirculation (EGR). In this, a portion of the exhaust gas is taken returned to the engine air intake so that the engine is fed a mixture of air and exhaust gas. Because the resulting mixture is lower in oxygen than in air, the temperature of the combustion event is reduced so that there is less NO_x in the exhaust gas. This technique does cause an increase in PM, so there is a pay-off between NO_x and PM, but by managing the rate of EGR to the load on the engine, it is possible to obtain an overall reduction in pollutant emissions.

[0016] In Japanese Patent No. 2722987 and EP-A-1079084 (the entire contents of which documents are incorporated herein by reference), Toyota describes an exhaust system including a component including a combination of certain of the catalyst features described above. Essentially it describes a catalysed particulate trap including a NO_x-trap. In particular, the component comprises a particulate trapping device comprising a NO_x absorbent capable of absorbing NO_x contained in exhaust gas when the air-fuel ratio of the exhaust gas is lean, and capable of releasing the NO_x absorbed in the NO_x component when the air-fuel ratio of the exhaust gas is substantially equal to the stoichiometric air-fuel ratio or rich. One method of coating a particulate filter with a catalyst washcoat is disclosed in EP-A-0766993 (the entire contents of which is incorporated herein by reference). The method of this patent provides a coating layer which is uniformly formed on the surfaces of the pores in the porous walls of the wall-flow filter and the catalyst is carried thereon.

[0017] In JP-B-2722987, the mechanism suggested for the combustion of soot trapped on the particulate trap is that during lean running, a high concentration of oxygen O₂ is deposited in the form of O₂⁻ or O²⁻on the surface of platinum (Pt). NO contained in the flowing exhaust gas reacts with O₂⁻ or O²⁻on the surface of the Pt to form NO₂ (2NO+O₂→2NO₂). Then, part of the NO₂ thus formed is absorbed into the NO_x absorbent while being oxidised on Pt, and diffused in the form of nitrate ion NO₃⁻ while combining with BaO.

[0018] If the air-fuel ratio is adjusted rich, the oxygen concentration in the exhaust gas is reduced, and consequently the amount of NO₂ formed on the surface of the Pt is reduced. If the amount of NO₂ produced is reduced, the reaction proceeds in the reverse direction (NO₃⁻→NO₂) and thus the nitrate ion NO₃⁻ is released in the form of NO₂ from the absorbent.

[0019] The suggestion is that “activated oxygen” species such as O₂⁻ and O²⁻are responsible for combusting particulate during rich and lean running, but also that NO₂ could also be responsible for combustion of particulate, particularly during rich running.

[0020] We have investigated Toyota's combined particulate filter-NO_x trap and have found, very surprisingly, that by introducing an oxidation catalyst active for oxidation of NO to NO₂ upstream of the filter/trap in a similar arrangement to that described in EP-B-0341382 or U.S. Pat. No. 4,902,487 that filter regeneration is improved compared with filter regeneration employing the particulate filter-NO_x trap alone. We have been able to show this by measuring the back-pressure in the system on a bench mounted engine. Increased back-pressure is an indication of increased particulate build up, i.e. that particulate deposition and particulate combustion are not in balance. It is also believed that the system represents an improvement over the system described in EP-A-758713 in that NO_x released from the NO_x absorbent can combust trapped particulate, but also oxidise HC to carbon dioxide (CO₂) and water (H₂O) and oxidise carbon monoxide CO to CO₂. Accordingly, the system provides an improved management of pollutant species in the exhaust gas.

[0021] According to the invention, there is provided an exhaust system for an internal combustion engine, which system comprising a catalysed ceramic wall flow particulate filter coated with a washcoat composition comprising an oxidation catalyst comprising at least one platinum group metal selected from the group consisting of platinum and palladium and a NO_x absorbent comprising at least one metal selected from the group consisting of alkali metals, alkaline earth metals and rare earth metals for absorbing NO_x contained in an exhaust gas when the composition of the exhaust gas is lambda >1, and for releasing the NO_x absorbed in the NO_x absorbent when the exhaust gas composition is 1≧lambda, and a platinum group metal catalyst upstream of the filter for oxidising NO to NO₂ at least when the composition of the exhaust gas is lambda >1, wherein the uncoated ceramic wall flow particulate filter has a porosity of >40% and a mean pore size of 8-25 μm and the D50 of the washcoat composition is less than or equal to 8 μm.

[0023] One theory that an exhaust system including the particulate filter-NO_x trap alone is less active for particulate combustion is because the combustion of trapped particulate occurs only where it is in contact with the Pt or other washcoat components. Accordingly, particulate further from the surface of the filter-trap is combusted later than that which is nearer the surface. In the present invention particulate in contact with the Pt on the trap can be combusted at the same time as particulate not in contact with the Pt, because the particulate not in contact with the Pt is combusted in exhaust gas including increased levels of NO₂ downstream of the oxidation catalyst.

[0024] The invention is advantageous in that by reducing back-pressure in the system, fuel economy is improved and wear on the engine is reduced or eliminated.

[0025] In one embodiment the porosity of the uncoated filter is from 42-60%.

[0026] In another embodiment, the mean pore size of the uncoated filter is from 11-22 μm.

[0027] Known catalysts for producing NO₂ from NO and O₂ may be used to generate the NO₂ oxidant for the purpose of combusting particulate. Such catalysts are extensively used in the catalytic conversion of automotive exhaust gases. This includes, for example, Pt, rhodium (Rh), ruthenium (Ru), palladium (Pd) or combinations thereof, platinum group metal oxides such as RhO₃ and the like. Conveniently, the catalyst is coated onto a monolith substrate e.g. a ceramic or metal honeycomb.

[0028] The filter may be in conventional form and structure. Typically this comprises a ceramic wall-flow filter of appropriate pore size, but one or more wire meshes of appropriate metal e.g. stainless steel or the like can also be used.

[0029] The NO_x absorbent includes alumina, for example as a support, and at least one selected from, for example, alkali metals, such as potassium (K), sodium (Na), lithium (Li) and caesium (Cs), alkaline earth metals, such as barium (Ba) and calcium (Ca), and rare earth metals, such as lanthanum (La) and yttrium (Y), and a noble metal such as Pt carried on the support. A reduction catalyst such as rhodium can also be included. In a particular embodiment, the rhodium is coated on the exit channels of the filter.

[0030] According to a further aspect, the invention provides an internal combustion engine including an exhaust system according to the invention. The engine can be a diesel engine, such as a heavy duty diesel engine (as defined by the relevant European or US Federal or California State legislation) or a diesel engine for a light duty diesel engine, such as for a passenger vehicle or van. The engine can also be a gasoline engine, such as a lean-burn gasoline engine including a gasoline direct injection engine. However, the engine can be powered by alternative fuel means such as CNG, LPG or methanol, and engines powered by these alternative fuels are within the scope of the present invention.

[0031] In a further aspect, the invention comprises a vehicle including an internal combustion engine according to the invention. However, the exhaust system can be also be used in connection with stationary power plants.

[0032] According to a further aspect, the invention provides a method of treating an exhaust gas of an internal combustion engine, which method comprising oxidising NO in the exhaust gas to NO₂ at least when the composition of the exhaust gas is lambda >1, trapping particulate on a catalysed ceramic wall flow filter coated with a washcoat including a NO_x absorbent, oxidising NO to NO₂ on the filter when the composition of the exhaust gas is lambda >1, absorbing the NO₂ in the NO_x absorbent when the composition of the exhaust gas is lambda >1, releasing the absorbed NO_x as NO₂ when the exhaust gas composition is 1≧lambda and combusting particulate trapped on the filter in NO₂ wherein the step of oxidising NO to NO₂ is performed upstream of the filter and wherein the uncoated ceramic wall flow particulate filter has a porosity of >40% and a mean pore size of 8-25 μm and wherein the D50 of the washcoat composition is less than or equal to 8 μm.

[0033] In order that the invention may be more fully understood, the following Example is provided by way of illustration only and with reference to the accompanying Figure which shows a graph depicting the effect of pre-NO oxidation catalyst and combined NO_x and particulate trap.

[0034] A diesel particulate wall-flow filter (5.66 inches (14.38 cm) diameter by 6 inches (15.24 cm) long, 200 cells per square inch (31 cells cm⁻²)) was coated with a conventional NO_x trap composition comprising supported platinum and barium prepared using known incipient wetness solution impregnation and conventional coating techniques. The coated filter was dried in an airflow and calcined at 500° C.

[0035] The resulting piece, now termed a NO_x particulate trap (NPT), was mounted in a stainless steel can using standard procedures, and fitted to the exhaust gas system of a bench-mounted 1.9 liter common rail diesel engine. The engine was coupled to a dynamometer in the conventional manner, with both engine and dynamometer being controlled by computer. Exhaust emissions at pre- and post-NPT positions were measured at 10 second intervals. Gas pressures and temperatures at pre- and post-NPT positions were measured over the same time interval.

[0036] The engine was operated to give cycles of lean-running and rich-running conditions. The engine was run at 2300 rpm and the torque was adjusted to give a NPT gas inlet temperature of 350° C. After 60 seconds of lean-running the engine conditions were changed to rich conditions for 2 seconds by means of fuel post-injection, air intake throttling, and increased exhaust gas recirculation (EGR) rate. After two hours of cycling 60 seconds lean and 2 seconds rich the engine was kept at lean-running conditions and the torque was increased to give a NPT gas inlet temperature of 450° C. These lean-running conditions were maintained for 1 hour. The reaction between soot and NO₂ during this period was monitored by the reduction in back pressure of the system.

[0037] The above test conditions were repeated on a combined system comprising of diesel oxidation catalyst (DOC) followed by a NPT filter.

[0038] The DOC was prepared by coating a cordierite monolith (5.66 inches (14.38 cm) diameter by 3 inches (7.62 cm) long, 400 cells per square inch (62 cells cm⁻²)) with platinum supported on alumina using conventional coating techniques. The DOC was mounted in a stainless steel can and fitted to the exhaust gas system of the diesel engine. The NPT filter was then fitted 1 inch (2.54 cm) behind the DOC. Emissions and back pressure measurements were carried out over the lean-rich cycling and lean only conditions detailed above.

[0039] As can be seen from FIG. 1, during the rich-lean cycling, the back-pressure in the system including the DOC upstream of the NPT is consistently lower than the back-pressure in the system without the DOC. Furthermore, it can be seen that following the switch to constant lean running, NO₂ increases downstream of the NPT in both systems. This is because the NO_x absorbent is “full” or substantially all the NO_x absorbent is in the nitrate form. With no rich regeneration events to reduce the nitrate and regenerate the NO_x absorbent, the system including the DOC+NPT essentially becomes a CRT as described in EP-B-341832. NO₂ generated over the Pt of the NO_x trap on the NPT appears to be responsible for the combustion of particulate on the NPT only system. In both cases, increased NO₂ is detected downstream of the NPT.