KR20050062530A - Exhaust system for a lean-burn ic engine - Google Patents

Abstract

An exhaust system for a lean-burn internal combustion engine comprises a filter (30, 36) for particulate matter and means (24) for generating an oxidant more active than molecular oxygen for combusting particulate matter disposed on the filter (30, 36), wherein the filter (30, 36) comprises a mass of elongate flat, narrow strip metal.

Description

Exhaust system for lean burn IC engine {EXHAUST SYSTEM FOR A LEAN-BURN IC ENGINE}

The present invention relates to an exhaust system for a lean-burn internal combustion engine, and more particularly to a system for treating soot-containing gas.

Filtering soot from engine exhaust by a ceramic wall-flow filter is well established while the use of metal-based filters falls short of that. Metal-based filters are disclosed in particular in US-A-4,270,936 and US-A-4,902,487 and in the SAE newspapers 820184 (Enga et al.) And 890404 (Cooper, Thoss). They have reached an advanced stage of research at Johnson Mattsay's 'Catalytic Trap Oxidiser' ('CTO') but do not appear to compete successfully with wall-flow filters in the commercial market. We have recently identified a system in which metal-based filters can be advantageously used.

In order that the present invention may be more fully understood, embodiments thereof will be described in detail with reference to the accompanying drawings and examples:

1 shows in schematic cross section a diesel engine with an exhaust system.

2 is a diagram showing the exhaust aftertreatment component inlet temperature and outlet temperature in the vehicle exhaust system over time and also showing the vehicle speed.

3 is a schematic cross-sectional view through components of an exhaust gas treatment system used in the present invention.

4 is a bar graph showing the amount of particulates measured over the drive cycles for the exhaust treatment systems 1, 2 and 3.

5 shows a model analysis of exhaust pipe NO ₂ from an exhaust system comprising systems 1, 2 and 3 (continuously every second).

With reference to FIG. 1, item 10 is a fuel inlet 12 and air inlet 12 fed with a hydrocarbon content of 5 ppm sulfur content, which is varied as typically practiced, but at an air / fuel weight ratio of about 30 during stable operation. Represents a four-cylinder diesel engine with Engine exhaust 16 is fed to a cylindrical processing reactor generally indicated by reference numeral 18 having an insulated inner wall 20. The filter cartridge 22 is provided in the wall 20.

At the inlet end of the cartridge 22 is a catalyst bed 24 which occupies the entire diameter of the cartridge 22, which is a 310 stainless steel knitted 0.33 mm wide and 0.2 mm thick of 6 volume percent solids. Filled with steel flattened wire, containing alumina washcoat and Pt up to 70-100 (1.98-2.83 gm ^-3 ), possibly up to 300 (8.50 gm-3) g / ft ³ of bed volume And provides low temperature ignition. The next downstream section of the cartridge 22 is a circular feed containing washcoat and soot oxidation catalyst surrounding the filled primary filter 30 with the same flattened wire as in bed 24 but in an amount of 12% by volume. Occupied by channel 28. Filter 30 provides an in-axis gas flow to outlet 32. Feed channel 28 terminates longitudinally in bypass 34, the function of which will be described below. The next downstream zone is the secondary filter 36 which provides longitudinal gas flow. Filter 36 is filled with the same flattened wire as used in bed 24 but in an amount of 16% by volume and contains a soot oxidation catalyst such as, for example, La / Cs / V ₂ O ₅ . Filters 30 and / or 36 may be hardened by electrical internal spot welding using an axial outlet or axial rod 38 as one electrode, respectively, and an outer interface member as the other electrode, respectively, in construction. Surrounding filter 36 is bypass channel 40. Bypasses 34 and 40 are darkened to show the possible inclusion of flow-blocking material to provide a pressure-drop balance with that of the filter when the filter is partially sooted. In lieu of or in addition to the soot oxidation catalyst on filter 36, there may be a secondary oxidation catalyst, similar to reference numeral 24, between the filters.

In operation, the exhaust gas entering the bed NO 24 NO ₂ in most gas passing through the bed (24) is oxidized as it is held on the soot filter 30, which is oxidized to CO by NO _2. If the filter 30 is insufficient in design or engine faults produce excessive soot, the soot accumulates in the filter 30 and blocks gas flow through the filter 30. By design, i.e. at a predetermined, level of pressure drop due to such a shutoff, the bypass 34 allows gas to pass through the exhaust system, so that engine operation is performed until the soot-oxidation conditions are regenerated or remedial action is taken. Can be continued. Likewise, if soot accumulates in filter 36 at a design level, bypass channel 40 allows gas to pass through.

Possibly serially ordered Nos. 30 and 36 filters sequentially increase the geometric surface per unit volume and capture finer or bypassed particles. If the concentration of soot entering is smaller than in the first filtering stage, such continuous filters need not include a bypass. Such additional step (s) may comprise an oxidation catalyst as mentioned above to regenerate the component of NO ₂ after reduction by soot on the preceding filter.

According to the present invention there is provided an exhaust system for a lean burn internal combustion engine, which is more active than molecular oxygen (O ₂ ) for burning soot collected in a long, narrow strip form of metal and soot collected on a filter. And an exhaust gas treatment system comprising a soot filter filled with means for producing a.

The term "wait" here means the width of the plane of the flat strip metal. The term "narrow" is interpreted appropriately.

The term "filled" here means "dense" and "compressed".

Typically, exhaust gases from such engines are unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NO _x ), carbon dioxide (CO ₂ ), water vapor, which are gaseous components of soot (or particulate matter (PM)). (H ₂ O), O ₂ , and nitrogen (N ₂ ). The means for removing the soot collected in the filter by combustion is preferably operated continuously. Oxidizers that are more active than O ₂ are, for example, ozone and / or plasma, the most convenient being NO ₂ . Preferably such NO ₂ is provided at least in part by catalytic oxidation of the NO component of NO _x , for example on a platinum supported on an alumina particulate upstream of the filter, for example. Such a catalyst may conveniently be supported on a metal substrate in the form of a flat, narrow strip filled, but at the low packing density allows the passage of soot particles.

Alternatively or additionally, the filter charge may contain a catalyst layer for soot oxidation, possibly by a mechanism involving the oxidation of NO to NO ₂ . Such soot oxidation catalysts include platinum on supported platinum group metals, for example alumina and / or base metals such as La / Cs / V ₂ O ₅ . If the engine-out NO _x available in gaseous state is insufficient to burn soot continuously, it may be introduced more as gas produced by the introduction of plasma or NO _x or nitric acid, possibly by ammonia oxidation during riding. Can be.

Ozone and / or plasma may be generated by suitable means such as UV light sources and / or corona discharge devices. It is understood that plasma and / or ozone may oxidize NO to NO ₂ . In one embodiment, the exhaust system comprises both ozone and / or plasma generating means and a NO oxidation catalyst.

The external structure of the filter may have features that provide operational advantages. For example, it can be formed as a monolith that is easily inserted into the reactor shell or easily recovered. Whether monolithic or not, it may be disposed as a cartridge in the shell to be easily insertable or recoverable. It can conduct electricity as a whole, thereby allowing heat transfer at the onset of cooling. If the filter is placed in the shell, such electrical conduction can be used to construct the unit by local welding between adjacent strips. It may contain an axial metal rod so that it can act as one electrode during such welding, and the shell serves as the other metal. Further external features are mentioned below.

The metal of the filter must be able to withstand the exhaust treatment process conditions. Since the filter is more easily replaceable than the ceramic filter and the material can be recovered and reused, the service life of the filter does not have to be as long as the ceramic filter. The filter may be replaced during the normal service period of the vehicle.

Typically, the metal is a corrosion resistant iron alloy. Typical alloys contain secondary components such as nickel and chromium and type 300 or type 400 stainless steel. The use depends on whether the exhaust treatment system is required to temporarily operate in rich conditions, where some stainless steel is unstable. In a wide range of exhaust compositions, preferred iron alloys contain at least 11.5% Cr, 4% Al and secondary components, such as, for example, rare earths, zirconium or hafnium. The metal in the filter may be a mixture of different components and may include components that provide an electrically conductive bridge or welding function.

The filter, as a whole or in a domain, may have a regular structure, for example a coiled, woven or knitted structure. The width of the metal strip of the filter may for example be in the range of up to 2 mm, in particular from 0.1 to 0.5 mm. The metal strip of the filter should be thick enough to provide mechanical strength under the conditions in which it will be used. Typically, the thickness of the metal strip is in the range of 0.2 to 0.8 of the width. Suitably, the geometric surface area per unit length of the metal strip is in the range of 1.2 to 1.5 times the geometric surface area of the same weight of metal of circular cross section. It is suitably the result of flattening the circular cross-section wire. The metal in the filter unit may be a mixture of strips of various sizes and may comprise circular cross-sectional wires as mutually elongated wires that are not flattened or as added sub-units. In one embodiment, the metal in the form of a flat narrow strip is a wire made flat.

The fill level may be selected to provide the desired level of filtration and / or back pressure in the system and may depend on the width and depth of the flat metal strip. However, the inventors generally believe that a range of packing densities of 2.5 to 30% v / v, such as for example 5 to 15% v / v, may provide desirable results. We advantageously used a filling concentration of 10%.

Typically, the catalyst coating on the filter comprises a washcoat of oxides, for example a rare earth and an active material, in particular a washcoat of alumina with oxides of Pt or Pd or Cs and V. The coating may contain perovskite. If catalytic oxidation of NO is used, the catalyst typically comprises Pt and / or Pd on such washcoat. If ozone is used, its generator may be, for example, a corona discharge tube passing between two electrodes where air is maintained at a large potential difference, or may comprise a high energy lamp. If plasma is used, the plasma generator may be operated by, for example, corona discharge, surface plasma discharge or dielectric barrier discharge, or may comprise a dielectric packed layer electron beam reactor. It can be enhanced by electromagnetic radiation, for example microwave radiation. The generator can process all or part of the air or exhaust gas before and after treatment.

The size of the filter relative to the engine and certain devices for introducing additional oxidants more active than O ₂ may be the subject of the intended feature. In the simplest case the filter capacity is large enough to allow soot to be burned continuously by the oxidant. That is, any accumulation during low speed operation is quickly removed during high speed operation, and the overall trend is continuous combustion. Cheaper filter capacities are sized to accommodate the accumulation of more soot, which is sufficient to significantly increase the pressure-drop before the next high speed operation period. Such filter (s) preferably comprise a bypass, wherein the pressure-drop through the bypass is equal to the maximum allowable pressure-drop in the design. Bypass prevents engine stall or low power due to excessive pressure drop, but allows some soot release into the atmosphere. In order to overcome such soot emission, a secondary stage, for example a filter or an impingement collector and / or an oxidation catalyst, may be provided downstream of the bypass. The bypass with the second stage filter and / or oxidation catalyst or the bypass without them may be part of the filter cartridge.

The direction of gas flow through the filter and / or oxidation catalyst (if used) may be or have components that are parallel or transverse to the general flow direction. The transverse flow can, for example, be symmetric, in particular symmetric inward with respect to the outflow head axis in the cylindrical filter or with the plenum in the elliptical or rectangular filter. Alternatively, unidirectional cross-flow can be provided.

To further refine the process and system, continuous filter elements present different soot-treatment capacities in the gas, for example to collect smaller and smaller particles, and / or to provide graded catalysis environments. Preferably, the gas flow in the filter element (s) at the inlet of the series is or has a component that traverses the general flow direction. If the process and system provide an oxidant that is more active than molecular oxygen in soot, successive filter elements may alternate with means for providing an oxidation catalyst and / or plasma or ozone. Downstream of such a filter element (s) and the oxidation catalyst (s) (if used) may be ceramic.

Particularly useful systems include a plurality of metal-based filters for continuously capturing smaller and smaller particles arranged downstream, and optionally at least one wall-flow filter for capturing smaller particles. In this system, the pores of the wall-flow filter are single-stage wall-flow because the metal-based filters in front of them remove larger particles that can block or block them, that is, reduce the flow-through of the gas. May be smaller than in capture. Any or all filters can be catalyzed.

Alternatively or additionally, a separate NO-oxidation catalyst may be disposed at least upstream of the primary filter. Such a catalyst on and / or between the filters may have the effect of restoring the NO ₂ component which may be reduced by reacting with soot in the preceding filter. The filter and, if present, the catalyst can be assembled as a single unit inside the cartridge. Such NO-oxidation catalyst may be supported on a flow-through substrate, for example a ceramic or metal substrate.

According to a further aspect, the present invention provides a system according to the invention, wherein the oxidant more active than O ₂ is at least one of ozone, plasma or NO ₂ .

The exhaust treatment system may comprise other completes, as used or proposed, for example three-dimensional catalyst (TWC), nitrogen oxide (NOx) traps + regeneration means, for example hydrocarbon or ammonia as reducing agent. Is a selective catalytic reduction (SCR), lean-NOx catalysis, renewable or disposable sulfur oxide (SOx) trap. The engine and system may include a removal gear for reducing emissions by controlling the operation of the exhaust system in use and a built-in diagnostic gear suitable for conventional or novel features of the invention.

The lean burn engine can be any engine that produces current or potentially soot-containing engines. For example, the engine may be a compression ignition engine such as a diesel engine, or a spark ignition engine such as lean burn gasoline, for example a gasoline direct injection (GDI ^™ ) engine. It may have an exhaust gas recirculation (EGR). It may be for light or heavy loads. The S content of the fuel used to provide a low SO ₂ exhaust gas should be below 500 ppm, in particular below 50 ppm w / w, as S. Low sulfur fuels and lubricants that provide 20 ppm SO ₂ or less of exhaust gases are preferred.

A 2.5-liter Audi TDI vehicle fueled by 50 ppm sulfur containing diesel fuel, certified to European Tier 2 legislation, is available in square inches (cpsi) (62 cells cm ^-2 ) and 6 mils (1 inch). A one-hundred thousandth (0.15 mm) of ceramic monolith with a cell density of 400 cells per wall thickness was flown through a 5.66 in (144 mm) diameter and 9 in (225 mm) length. The vehicle was placed on a standard chassis rolling rod dynamometer, accelerated to 120 kph within 100 seconds after 20 seconds idling and held at this speed for the remainder of the test. After an additional 300 to 400 seconds the inlet temperature in the catalyst system achieved a stable temperature of 330 to 350 ° C. The vehicle was then operated at this temperature (FIG. 2) for a period of about 20 minutes. During the 20 minute period, the particles were collected on two sets of filter paper by standard measurements (one set for each 10 minute period) to enable calculating the average particle weight during the test. Nitrogen oxides (NO and NO ₂ ) were measured in the feed gas to the monolith during the same 20 minute period and the exhaust pipes after the monolith were measured by chemiluminescence analysis and Fourier transform infrared (FTIR) respectively. This is labeled System 1.

The flow through the monolith was removed from the vehicle and replaced with a 5.66 in (144 mm) diameter and 4 in (100 mm) length flow through the same cell density and wall thickness monolith coated with 75 gft ^-3 (2.6 / liter) platinum catalyst. This was followed by exposure flow through the monolith 5.66 in (144 mm) diameter and 4 in (100 mm) length. The same test cycle was performed and the measurement was repeated. This is labeled System 2.

The oxidation catalyst was replaced by a single 5.66 (144 mm) diameter and 3 in (76 mm) length flow through a monolith coated with 75 gft ^-3 (2.6 g / liter ^-1 ) of platinum, and the exposed monolith was placed in a new design particle trap. Replaced by It consists of a bed filled with 0.10 mm wide and 0.05 mm thick wire, which is a flattened wire made of stainless steel, knitted, which occupies and abuts the entire diameter of the backside of the catalyst. The wire bed is 4 inches (100 mm) long and has a packing density of 10% v / v. This is followed by a 5.66 in. (144 mm) diameter and 1 in. (25.4 mm) length flow through a 75 gft ^-3 (2.6 g / liter) ("catalyst piece") platinum-coated monolith facing the back of the knitted wire substrate. Followed. This catalyst was followed by an alternate monolithic wire substrate and exposed monolith, alternately of the same size as in the preceding primary knit wire substrate and catalyst piece. The apparatus is shown in FIG. Thus, the total volume and appearance ratio of the catalyzed flow through the monolith was the same as that of System 2. This last system was labeled System 3. The same drive cycles and measurements were repeated as for systems 1 and 2.

4 outlines the particle mass measured over the drive cycles for the three systems. Thus, lowering the particle mass from the "baseline" system 1 by the addition of an oxidation catalyst is achieved in system 2 and is further improved by the addition of a filled wire bed that retains some of the soot, thereby leaving NO ₂ exiting the primary catalyst. React with it. It can be seen that the conversion efficiency of system 2 over system 1 is 13%, while the conversion efficiency of system 3 over system 1 is 38%.

FIG. 5 shows an analysis (continuous every _second) of NO ₂ in the exhaust pipe downstream of three systems. System 1 with an exposed monolith has very low NO ₂ emissions similar to those from the engine. Higher NO ₂ concentrations are measured after System 2 because NO is oxidized on the catalyst, but only a small amount reacts with soot. NO ₂ formed on the primary oxidation catalyst in system 3 reacts with the soot collected in the primary filter bed. The unoxidized NO on the primary catalyst passes through the primary filter bed and is oxidized to NO ₂ on the secondary catalyst. In addition, there may be some re-oxidation of NO formed from the reaction between NO ₂ + C → NO + CO. The NO ₂ NO ₂ with some unreacted in the first-order filter is reacted with the collected soot in the secondary filter bed as compared to system 2, will have a lower soot and NO ₂ levels of the exhaust pipe as a result. Turbulence initiated in the gas stream by its passage through the filter bed increases the reaction of soot and NO ₂ and the oxidation of NO on the secondary catalyst.

Claims (21)

For lean burn internal combustion engines comprising a soot filter filled with a number of elongated flat narrow strips of metal and a means for producing oxidants that are more active than molecular oxygen (O ₂ ) for burning soot collected on the filter Exhaust system. The system of claim 1, wherein the oxidant more active than O ₂ is at least one of ozone, plasma or NO ₂ . 3. The system of claim 2, wherein the NO ₂ is provided at least in part by catalytic oxidation of the NO component of NO x contained in the exhaust gas. 4. The system of claim 3, wherein the system comprises a NO-oxidation catalyst upstream of the filter, which catalyst is supported on a metal substrate of the type used in the filter but allows passage of soot particles at low packing densities. system. The system of any one of the preceding claims, wherein the filter is of regular coiled, woven, or knitted construction, in whole or in domain. The system of any one of the preceding claims, wherein the metal of the filter is type 300 or type 400 stainless steel. The method of any one of the preceding claims, wherein the metal of the filter comprises an iron alloy containing at least 11.5% Cr, 4% Al and minor components such as 0.02 to 0.25% rare earth, zirconium or hafnium. System. The system according to claim 1, wherein the width of the metal strip of the filter is in the range of at most 2 mm, in particular in the range of 0.1 to 0.5 mm, the thickness of which is 0.2 to 0.8 of the width. 10. The system of claim 9, wherein the metal in the form of a flat narrow strip is a flattened wire. The system according to claim 1, wherein the filter charge has a catalyst layer for soot oxidation. The system of claim 10, wherein the filter comprises a catalyst coating comprising a washcoat comprising oxides of Pt or Cs and V. 12. The system of any one of the preceding claims, comprising an exhaust gas treatment system comprising a plurality of metal-based filters in sequence from upstream to downstream suitable for continuously capturing smaller and smaller particles. 13. The system of claim 12, comprising at least one wall-flow filter to capture smaller particles. 14. A system according to claim 12 or 13, comprising a flow-through monolith between metal-based filters or between each pair of metal-based filters. 15. The system of claim 14, wherein the flow through monolith or each flow through monolith comprises a NO-oxidation catalyst, thereby recovering the reduced NO ₂ component by reacting with soot in the preceding filter. The system according to any one of the preceding claims, wherein the filter capacity is sufficient to allow soot to be continuously burned by the oxidant. 17. The filter capacity according to any of claims 1 to 16, wherein the filter capacity is sized to be suitable for the accumulation of soot sufficient to significantly increase the pressure-drop before the next high speed operation period, and the system includes a bypass and a bypass The system is characterized in that the pressure drop through is equal to the maximum allowable pressure drop in the design through the filter (s), thereby avoiding engine stall. 18. The system of claim 17, comprising means for limiting soot emissions to the atmosphere, including secondary stages such as filters or impact collectors and / or oxidation catalysts downstream of the bypass. An internal combustion engine comprising an exhaust system according to any of the preceding claims. Diesel engine according to claim 19. A method of treating the exhaust gas of an internal combustion engine, the method comprising: trapping soot in exhaust gas on a filter, oxidation more active than molecular oxygen (O ₂ ) for burning soot collected on the filter from exhaust gas components Producing a sieve, and combusting the trapped soot in the oxidant, wherein the filter comprises a metal in the form of a plurality of elongated flat narrow strips.