JP2018062882A - Exhaust purification system - Google Patents

Abstract

An exhaust purification system capable of sufficiently reducing NOx emission at a low exhaust temperature until reaching a predetermined activation temperature of a selective reduction catalyst is provided. An exhaust gas purification system comprising a selective reduction catalyst for detoxifying NOx in exhaust gas exhausted into an exhaust pipe of an internal combustion engine by reacting with a reducing agent in the presence of oxygen in the presence of oxygen. A porous heat-resistant carrier layer is supported on the inner wall surface portion of the upstream side exhaust pipe located upstream from the selective reduction catalyst, and the NOx purification rate by the selective reduction catalyst is within an allowable range. A NOx adsorption catalyst that adsorbs NOx in the exhaust gas in a low temperature range with respect to a predetermined active temperature that can be reached, and desorbs the NOx in a high temperature range that is relatively high with respect to the low temperature range And an exhaust gas purification system. [Selection] Figure 1

Description

  The present invention relates to an exhaust purification system, and in particular, when nitrogen oxide (NOx), which is an air pollutant, is contained in exhaust from an internal combustion engine mounted on a vehicle, the NOx is selectively reacted with a reducing agent. The present invention relates to a detoxifying exhaust purification system.

  In vehicles equipped with internal combustion engines, especially trucks and buses equipped with diesel engines, urea water is added to the exhaust discharged from the engine to generate ammonia, and NOx in the exhaust is selectively reduced. Exhaust gas purification systems that are rendered harmless by reacting with ammonia in the presence of oxygen with a catalyst (selective reduction type NOx catalyst) are frequently used.

As an exhaust purification system of this type, for example, exhaust (exhaust gas) from an engine flows into an oxidation catalyst to oxidize a specific air pollutant, while the exhaust flowing into the selective reduction catalyst downstream thereof falls within a predetermined temperature range. In some cases, an appropriate amount of urea-based liquid is supplied into the inflow gas to generate ammonia (NH ₃ ), and NOx in the exhaust gas is converted into harmless nitrogen (N ₂ ) and water (H ₂ O) using the ammonia as a reducing agent. What is changed is known (for example, refer to Patent Document 1).

In this apparatus, the composition ratio of NO and NO ₂ is improved by the oxidation catalyst to increase the efficiency of exhaust gas purification by the selective reduction catalyst, and the selective reduction catalyst is connected to the first catalyst layer having a relatively low active temperature region. By configuring with the second catalyst layer having a relatively high activation temperature region, it is possible to employ a processing pattern in which the exhaust purification processing is executed in a relatively wide exhaust temperature range.

  On the other hand, as an exhaust purification system mounted on a motorcycle equipped with a small internal combustion engine, an oxidation catalyst or a three-way catalyst is supported on a metal carrier made of punching metal in an exhaust pipe upstream of the exhaust port of the internal combustion engine. A thin-walled catalyst is also known to improve the purification efficiency while suppressing the exhaust resistance (see, for example, Patent Document 2).

JP 2009-106913 A JP-A-8-93461

  However, in the conventional exhaust purification system as described above, it cannot be said that the NOx emission amount can be sufficiently reduced at a low exhaust temperature until the selective reduction catalyst reaches its activation temperature.

  Further, it is conceivable to add a NOx adsorption catalyst to the exhaust purification device having a selective reduction catalyst to reduce the NOx emission amount at the low exhaust temperature, but in this case, the adsorption site of the NOx adsorption catalyst is limited. Not only can NOx be adsorbed sufficiently, but the NOx adsorbed on the NOx adsorbing catalyst is desorbed in one piece when the catalyst temperature rises and released to the selective reduction catalyst side, causing the NOx concentration in the exhaust to fluctuate. There was concern.

  On the other hand, for heavy vehicles equipped with diesel engines, new emission regulations and test methods (for example, the transient test cycle WHTC based on the international unified test method), such as the introduction of exhaust gas regulations at cold start, are introduced. (World Harmonized Transient Cycle)), and there is a tendency for higher demands to reduce NOx emissions. In commercial vehicles such as trucks and buses, NOx emissions at low exhaust temperatures are more Strict and sufficient reduction has become an important issue.

  The present invention has been made to solve such an unsolved problem, and is an exhaust purification capable of sufficiently reducing the NOx emission amount at a low exhaust temperature until reaching a predetermined activation temperature of the selective reduction catalyst. The purpose is to provide a system.

  In order to achieve the above object, an exhaust purification system according to the present invention is an exhaust purification system including a selective reduction catalyst for purifying NOx in exhaust discharged into an exhaust pipe of an internal combustion engine by reacting with a reducing agent. A heat-resistant carrier layer having a porosity on an inner wall surface of an upstream side exhaust pipe located upstream of the selective reduction catalyst in the exhaust pipe, and NOx purification by the selective reduction catalyst supported on the heat-resistant carrier layer The NOx in the exhaust is adsorbed in the low temperature region with respect to a predetermined activation temperature at which the rate can reach an allowable range, while the NOx is absorbed in a high temperature region that is relatively high with respect to the low temperature region. And a NOx adsorption catalyst layer to be desorbed.

  With this configuration, at a low exhaust temperature until the selective reduction catalyst reaches the activation temperature, NOx in the exhaust is adsorbed in the upstream exhaust pipe by the NOx adsorption catalyst layer in the low temperature region, and the selective reduction catalyst becomes the activation temperature. In the high exhaust temperature region after reaching the NOx, NOx is released from the NOx adsorption catalyst layer with respect to the exhaust gas flowing into the selective reduction catalyst. When the internal combustion engine is cold started, the timing at which the NOx adsorption catalyst layer reaches the high temperature side temperature region is relatively early on the upstream side close to the internal combustion engine, and relatively late on the downstream side. Therefore, NOx is effectively adsorbed by NOx adsorption by the NOx adsorption catalyst layer in the low temperature region at low exhaust temperatures, while NOx emissions are effectively suppressed, while gradually increasing from the upstream side to the downstream side when the exhaust temperature rises. NOx is gradually released into the exhaust gas by the NOx adsorption catalyst layer that reaches the side temperature region, so that NOx is not released uniformly into the exhaust gas, and effective NOx purification processing by the selective reduction catalyst is executed. It becomes possible.

  In the exhaust purification system of the present invention, the heat-resistant carrier layer may have a cordierite layer that supports the NOx adsorption catalyst layer.

  The heat-resistant carrier layer may support the NOx adsorption catalyst layer in at least one cylindrical shape.

  A PM collection filter that collects PM in the exhaust gas may be provided downstream of the heat-resistant carrier layer and the NOx adsorption catalyst layer and upstream of the selective reduction catalyst.

  The heat-resistant carrier layer may have a metal support layer supported by the exhaust pipe.

  ADVANTAGE OF THE INVENTION According to this invention, the exhaust gas purification system which can fully reduce the NOx discharge | emission amount at the time of low exhaust gas temperature until it reaches the predetermined activation temperature of a selective reduction catalyst can be provided.

1 is a schematic system configuration diagram of an exhaust purification system according to a first embodiment of the present invention. FIG. 2 (a) is a partial longitudinal sectional view of the inner wall surface portion of the upstream side exhaust pipe provided with the heat-resistant carrier layer and the NOx adsorption catalyst layer in the exhaust purification system according to the first embodiment of the present invention, FIG. 2B is a cross-sectional view taken along arrow B2-B2 in FIG. 3 (a), FIG. 3 (b), FIG. 3 (c), and FIG. 3 (d) are partial longitudinal sectional views of the III portion of the inner wall surface portion of the upstream side exhaust pipe in FIG. Four different types of layer structures are schematically shown. FIG. 5 is a graph showing a change in the maximum NOx adsorption amount according to the temperature of the NOx adsorption catalyst provided on the inner wall surface portion of the upstream side exhaust pipe in the exhaust purification system according to the first embodiment of the present invention, and the vertical axis indicates NOx adsorption. The maximum NOx adsorption amount per unit volume of the catalyst is shown, and the horizontal axis shows the catalyst temperature. FIG. 5 (a) is a partial longitudinal sectional view of the inner wall surface portion of the upstream side exhaust pipe in the exhaust purification system according to the second embodiment of the present invention, and FIG. 5 (b) is a view in FIG. 5 (a). It is B5-B5 arrow sectional drawing of. FIG. 6A is a partial longitudinal sectional view of the inner wall surface portion of the upstream side exhaust pipe in the exhaust purification system according to the third embodiment of the present invention, and FIG. 6B is a view in FIG. It is B6-B6 arrow sectional drawing of. FIG. 7A is a partial longitudinal sectional view of the inner wall surface portion of the upstream side exhaust pipe in the exhaust purification system according to the fourth embodiment of the present invention, and FIG. 7B is a view in FIG. It is B7-B7 arrow sectional drawing of.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings.

(First embodiment)
1 to 4 show an exhaust purification system according to a first embodiment of the present invention. The exhaust purification system of this embodiment is mounted on a commercial vehicle, for example, a large truck.

  First, the configuration of the exhaust purification system will be described.

  The exhaust gas purification system for an internal combustion engine of the present embodiment shown in FIG. 1 is installed in an exhaust system of an engine 10 that is an internal combustion engine. The engine 10 is a multi-cylinder diesel that drives a large truck (vehicle) (not shown). It consists of an engine.

  The engine 10 is provided with an injector 12 for each cylinder 11 and a common rail 13 and a high-pressure fuel pump 14 for supplying high-pressure fuel to the plurality of injectors 12.

  The engine 10 is also equipped with an intake device 20 and an exhaust device 30 and an exhaust turbocharger 40, and an exhaust purification device that forms a main part of the exhaust purification system as a part of the exhaust device 30. 50 is provided. The engine 10 may constitute a hybrid drive type power unit that drives the vehicle in cooperation with the motor generator.

  The intake device 20 includes a first intake pipe 21 connected to the inlet side of the compressor section 41 of the exhaust turbocharger 40, an air cleaner 22 provided on the upstream side of the first intake pipe 21, and an outlet of the compressor section 41. A second intake pipe 24 connected to the second intake pipe 24, an intercooler 25 that is mounted on the second intake pipe 24 and is cooled by running air, and a downstream end of the second intake pipe 24. And an intake manifold 27 connected to the intake port portion.

  The exhaust device 30 is connected to the inlet side of the exhaust turbine section 42 of the exhaust turbocharger 40 and connected to a plurality of exhaust ports of the engine 10, and the exhaust turbine section of the exhaust turbocharger 40. 42, an EGR pipe 33 and an EGR valve that can recirculate a part of the exhaust gas to the downstream end side of the exhaust pipe 32 connected to the outlet side 42 and the second intake pipe 24 connected to the intake manifold 27 from the exhaust manifold 31. 34 and an EGR cooler 35 that cools the recirculated exhaust gas that passes through the EGR pipe 33. An exhaust purification device 50 is arranged in the middle of the exhaust pipe 32 of the exhaust device 30.

  The compressor section 41 of the exhaust turbocharger 40 can pressurize the air taken in from the first intake pipe 21 to supercharge the engine 10, and the exhaust gas flowing into the exhaust turbine section 42 from the exhaust manifold 31 side. A VGT (variable geometry turbo) actuator 43 for controlling the supercharging pressure by adjusting the gas inflow amount is mounted.

  The exhaust purification device 50 is an integrated catalyst device combined with a urea SCR (Selective Catalytic Reduction) method and a DPF (Diesel Particulate Filter) method, and is installed in the middle of the exhaust pipe 32. ing. A known fuel addition valve may be installed in the upstream side exhaust pipe 36 located on the upstream side of the exhaust purification device 50. An exhaust silencer (not shown) is attached to the exhaust pipe 32 further downstream from the exhaust purification device 50.

  The exhaust purification device 50 includes a case 51 connected to the exhaust pipe 32 and forming a part thereof, a front diesel oxidation catalyst (hereinafter referred to as a front DOC) 52 housed in the case 51, and a downstream side of the front DOC 52. The DPF 53 is located, and the urea SCR catalyst 54 and the ammonia strip prevention catalyst 55 are located further downstream than the DPF 53.

  The case 51 is formed so that a part of the exhaust passage 32a in the exhaust pipe 32 has a substantially S-shaped folded shape, and the upstream DOC 52 and the upstream DOC 52 and The DPF 53 is accommodated.

  The pre-stage DOC 52 oxidizes hydrogen (HC) and carbon monoxide (CO) in the exhaust gas flowing into the case 51 to make them harmless, and also oxidizes nitrogen monoxide (NO) to oxidize nitrogen dioxide ( The concentration of NO2) can be increased to the same level as NO, and further, the exhaust can be heated to a temperature higher than the self-combustion temperature of PM, which is a particulate matter floating in the exhaust, by reaction heat.

  The DPF 53 is a wall flow type monolithic PM collection filter having a high PM collection rate. The DPF 53 is a porous carrier made of ceramic such as cordierite, and the flow paths in the cells extending in the direction of the exhaust passage 32a and parallel to each other are alternately sealed at the upstream end side of the DPF 53. As for the flow path whose upstream end side is not sealed, the downstream end side of the DPF 53 is sealed. And it permeate | transmits the porous cell wall which divides the upstream flow path opened on the upstream end side of DPF53, and the downstream flow path sealed by the downstream end side of DPF53 among several flow paths of DPF53. Only the exhausted gas flows from the upstream channel to the adjacent downstream channel, and at that time, PM is collected on the porous cell wall of the DPF 53.

  The urea SCR catalyst 54 and the ammonia strip prevention catalyst 55 are formed in the downstream case portion 51b after the two turn-ups in the case 51 in which a part of the downstream exhaust passage 32a is turned twice. It is stored in.

  Further, a mixing pipe 57 with a urea water injector 56 attached is interposed between the upstream case portion 51a and the downstream case portion 51b, and the urea water injector 56 passes through the DPF 53 and collects in the collected exhaust gas. From the above, urea water, which is a diesel exhaust fluid, is injected.

  The urea water is pumped from the urea water tank 59 by the urea water supply pump 58 and pressurized, and is added to the exhaust gas when the urea water injector 56 is opened.

  The urea water mixed with the exhaust gas in the mixing pipe 57 is decomposed into ammonia and carbon dioxide gas in the high temperature exhaust gas, and the exhaust passage 32a is expanded and folded when entering the downstream case portion 51b. To be distributed.

The urea SCR catalyst 54 is a selective reduction catalyst having the property of selectively reacting NOx with ammonia even in the presence of oxygen, and the exhaust gas in which ammonia (NH ₃ ) and carbon dioxide (CO ₂ ) are dispersed is urea. When flowing into the SCR catalyst 54, NOx (nitrogen oxide) in the exhaust gas is reduced by ammonia to nitrogen (N ₂ ) and water (H ₂ O) is generated, and the exhaust gas is purified. Yes.

  The ammonia strip prevention catalyst 55 can oxidize surplus ammonia to make it harmless with nitrogen and water, and can effectively prevent ammonia from remaining in the exhaust discharged into the atmosphere.

  As shown in FIGS. 2 and 3, among the exhaust pipes 32 (members forming the exhaust passage 32 a), the upstream exhaust pipe 36 located upstream from the exhaust purification device 50 is adsorbed in the main length region 60. The inner cylinder wall 61 a (inner wall surface portion) of the pipe portion 61 has a heat-resistant carrier layer 62 having a large number of fine holes and a NOx adsorption catalyst layer 63 supported on the heat-resistant carrier layer 62.

  The upstream side exhaust pipe 36 extends in the longitudinal direction of the vehicle over a long section on the upstream side of the exhaust passage 32a in the exhaust pipe 32, for example, a long section that is 1/4 or more of the separation distance between the front and rear wheel axles of a large truck. . The main length region 60 covers, for example, half or more of the entire length of the upstream exhaust pipe 36 from the downstream end of the exhaust manifold 31 to the inlet of the case 51 of the exhaust purification device 50.

  The main length region 60 is shown in a straight shape in FIG. 2, but may have a bent shape similar to that of an exhaust pipe of a general vehicle internal combustion engine. The outer cylindrical wall 61b (metal support layer) of the adsorption pipe portion 61 in the main length region 60 is integrally coupled to the upstream end side portion 36a and the downstream end side portion 36b of the upstream side exhaust pipe 36 by welding or the like. Yes. The adsorption pipe portion 61 has a pipe shape in which the ratio of the length to the diameter (outer diameter) exceeds, for example, five times.

The heat-resistant carrier layer 62 is a heat-resistant material having a small thermal expansion coefficient with respect to the material of the steel pipe forming the upstream side exhaust pipe 36, light weight and excellent mechanical strength at high temperatures, such as magnesium oxide (MgO). ), Cordierite containing three components of aluminum oxide (Al ₂ O ₃ ) and silicon dioxide (SiO ₂ : silica), and is formed into a porous layer. That is, the heat-resistant carrier layer 62 has a cordierite layer that supports the NOx adsorption catalyst layer 63.

  The cordierite forming the heat-resistant carrier layer 62 may be a combination of many cordierite grains within a predetermined particle size range, or may be a combination containing a large number of cordierite fibers. It may have a cylindrical porous core member. However, the heat-resistant carrier layer 62 has at least one cylindrical shape extending in the axial direction of the upstream side exhaust pipe 36, and has a plurality of cylindrical shapes arranged in parallel or in series or coaxially. You may have.

  Specifically, as shown in FIG. 3A, for example, the heat-resistant carrier layer 62 is blown toward the inner peripheral surface 61c side of the outer cylindrical wall 61b that has been subjected to roughening treatment, grooving, or other uneven processing. It is formed of a cordierite layer fixed by attaching or the like. For example, the cordierite layer is coated with a slurry obtained by mixing a cordierite granulated into a granule and a resin binder on the inner peripheral surface side of the outer cylindrical wall 61b by spraying, and then the temperature of the adsorption pipe portion 61 is increased. The cordierite layer is fired and the resin binder is degreased.

  As shown in FIG. 3 (b), the heat-resistant carrier layer 62 is coated with slurry mixed with cordierite and a resin binder on the inner and outer peripheral surfaces and the porous portion of a cylindrical core member 62h made of punching metal. The tube 61 may be heated to fire the cordierite layer 62c and the resin binder may be degreased.

  The heat-resistant carrier layer 62 is formed in a porous portion of a cylindrical core member 62i that is an aggregate of heat-resistant fibers (for example, metal fibers) as shown in FIG. 3C, or as shown in FIG. After coating a slurry of cordierite and a resin binder on the porous portion of the heat-resistant net-like cylindrical core member 62j, the adsorption pipe portion 61 is heated to fire the cordierite layer, and the resin binder May be degreased.

  As schematically shown in the partially enlarged view in FIG. 3A, the NOx adsorption catalyst layer 63 is not only supported on the surface of the heat-resistant carrier layer 62 but also enters the pores of the heat-resistant carrier layer 62. It is supported.

  The NOx adsorption catalyst layer 63 includes, for example, a transition element whose metal valence is easy to change with low energy and a rare earth element having an oxygen absorption function and a release function and can adsorb NOx by their interaction.

The NOx adsorption catalyst layer 63 adsorbs NOx in the exhaust gas in a low temperature side temperature region that is a low temperature side with respect to a predetermined activation temperature t ₂ (for example, 200 ° C.) set in advance. Here, the predetermined activation temperature t ₂ is equal to or higher than the activation temperature t ₀ of the urea SCR catalyst 54 and the urea addition start temperature t ₁ (for example, 180 ° C.), and the NOx of the downstream urea SCR catalyst 54 is reduced. It is set as a catalyst temperature corresponding to a state in which the purification rate can fall within a preset allowable range.

The low temperature side temperature range is a temperature range including a temperature range lower than the predetermined activation temperature t ₂ as a main range, but including the predetermined activation temperature t ₂ or a temperature range near the predetermined activation temperature t ₂ . Then, the adsorption activity of the NOx adsorption catalyst layer 63 becomes sufficiently high in the low temperature side specific adsorption temperature region R ₁ (see FIG. 4) corresponding to the state where NOx can be discharged in the low temperature side temperature region, and the urea SCR catalyst. The NOx in the exhaust gas is set so as to be adsorbed by the NOx adsorption catalyst layer 63 until the NOx purification rate by 54 reliably reaches the preset allowable range.

The NOx adsorption catalyst layer 63 only needs to be capable of NOx adsorption at the catalyst temperature when NOx is exhausted when the engine 10 is cold started, and the adsorption start temperature t ₀ is a temperature lower than 150 ° C., for example, 100 ° C. Can be set to a degree. Also, if the adsorption start temperature is relatively high and NOx adsorption catalyst layer 63 may not be able to purify NOx with a sufficient purification rate at a low exhaust temperature, the EGR valve 34 is opened to recirculate part of the exhaust. The required purification rate can be ensured by suppressing the generation of NOx due to the decrease in the combustion temperature.

  FIG. 4 is a graph showing the temperature characteristics of the NOx adsorption catalyst layer 63. In FIG. 4, the vertical axis represents the NOx maximum adsorption amount per unit volume (mass) of the NOx adsorption catalyst layer 63 [mol / g. The horizontal axis represents the temperature [° C.] of the NOx adsorption catalyst layer 63.

As shown in the figure, the NOx adsorption catalyst layer 63 is based on the NOx maximum adsorption amount A ₀ [mol / g] when the predetermined activation temperature t ₂ is reached, and the specific adsorption temperature region R ₁ on the low temperature side described above. In the low temperature side temperature region containing NO, the maximum NOx adsorption amount is large, while in the high temperature side temperature region that is relatively high with respect to the low temperature side temperature region, at least in the specific desorption temperature region R2, the unit volume (unit mass) ), The maximum NOx adsorption amount [mol / g] per unit area decreases with increasing temperature. Higher side temperature range described above, a temperature range exceeding the predetermined activation temperature t _2.

The NOx adsorption catalyst layer 63 has a high temperature side temperature range, for example, specific desorption, which is a high temperature side with respect to a predetermined activation temperature t ₂ (eg, 200 ° C.) at which the NOx purification rate by the urea SCR catalyst 54 can fall within an allowable range. When the temperature region R2 is reached, NOx is desorbed as the temperature rises, and NOx can be released into the exhaust gas flowing into the exhaust purification device 50.

The specific desorption temperature region R2 described above is a temperature region below a predetermined temperature t ₃ (for example, 250 ° C.) exceeding the predetermined activation temperature t _2, for example. The NOx adsorption catalyst layer 63 reaches the predetermined activation temperature t ₂ under a state where the NOx purification rate by the urea SCR catalyst 54 can reach the allowable range, and starts to desorb NOx when the catalyst temperature exceeds the predetermined activation temperature t _2. .

On the other hand, in the stage where the temperature of the NOx trap catalyst layer 63 becomes the exhaust gas temperature is a medium high temperature exceeds a predetermined temperature t _3, in addition to the NOx exhaust concentration with increasing combustion temperature becomes higher, depending on operating conditions NOx The emission concentration can vary. Therefore, although the urea SCR catalyst 54 has reached a predetermined activation temperature t ₂ at which a required purification rate can be expected, a temperature region R ₃ (hereinafter referred to as a high temperature) below a temperature t ₄ (for example, 350 ° C.) exceeding the predetermined temperature t _3. It adsorbs NOx at the side of that certain adsorption temperature region R _3), another of the NOx adsorption catalyst (NOx storage catalyst and hydrocarbon selective reduction which can be the NOx in a temperature range desorbed above its temperature t ₄ Catalyst may be added) as a part of the NOx adsorption catalyst layer 63 on the heat resistant carrier layer 62, or plural types of NOx adsorption catalyst layers 63 having different adsorption temperature regions are arranged in different sections on the exhaust passage 32a. It can be considered.

  In the main length region 60 of the upstream side exhaust pipe 36 in which the NOx adsorption catalyst layer 63 performs the above-described NOx adsorption function and desorption function on the upstream side of the exhaust purification device 50, when the engine 10 is cold started, There is a time difference in the temperature rise of the exhaust pipe 32 between the upstream side close to the engine 10 and the downstream side far from the engine 10.

  The NOx adsorption catalyst layer 63 is supported on the exhaust pipe 32 and is supported on the heat-resistant carrier layer 62 formed of cordierite having a low thermal conductivity with respect to the exhaust pipe 32. Therefore, the NOx adsorption catalyst layer 63 is supported. The timing at which the temperature reaches the high temperature side temperature range also tends to be relatively early on the upstream side near the engine 10 and relatively late on the downstream side away from the engine 10.

  Therefore, when the engine 10 is cold-started, NOx is adsorbed by the NOx adsorption catalyst layer 63 in the low temperature side temperature region at a low exhaust temperature until the urea SCR catalyst 54 reaches a predetermined activation temperature. While effectively suppressed, when the exhaust gas temperature rises, the exhaust gas in the upstream exhaust pipe 36 (flows into the exhaust gas purification device 50) by the NOx adsorption catalyst layer 63 that gradually reaches the high temperature side temperature region from the upstream side to the downstream side. NOx is gradually released during (exhaust).

  Next, the operation will be described.

  In the present embodiment configured as described above, the upstream side exhaust pipe 36 and its main length region 60 extend in the vehicle front-rear direction over a long section on the upstream side of the exhaust passage 32a in the exhaust pipe 32. Therefore, the NOx adsorption site is sufficiently secured on the inner cylinder wall 61a side of the main length region 60 located on the upstream side of the exhaust purification device 50.

  When the engine 10 is cold-started, at a low exhaust temperature where the exhaust temperature of the engine 10 is equal to or lower than a predetermined temperature, the low temperature side in the upstream exhaust pipe 36 of the exhaust passage 32a in the exhaust pipe 32 The NOx adsorption catalyst layer 63 in the temperature region is sequentially heated from the upstream side and gradually reaches the adsorption start temperature, and NOx in the exhaust is adsorbed by the NOx adsorption catalyst layer 63 in the low temperature region.

On the other hand, in the high exhaust temperature region after reaching the activation temperature of the urea SCR catalyst 54 and the urea addition start temperature t ₁ by the urea water injector 56, the NOx adsorption catalyst layer 63 produces exhaust gas flowing into the urea SCR catalyst 54. NOx is released.

During this time, the timing at which the NOx adsorption catalyst layer 63 reaches the high temperature side temperature region as the partial temperature of the exhaust pipe 32 rises is relatively early on the upstream side near the engine 10 and relatively on the downstream side away from the engine 10. Become slow. Therefore, after the NOx adsorbing catalyst layer 63 is gradually reached the adsorption start temperature sequentially heated from an upstream side, to the urea SCR catalyst 54 reaches a predetermined activation temperature t _2, NOx is continued upstream within the exhaust pipe 36 As a result, the exhaust of NOx to the outside is effectively suppressed.

  When the exhaust temperature of the engine 10 rises, NOx is gradually introduced into the exhaust gas flowing into the exhaust purification device 50 by the NOx adsorption catalyst layer 63 having a temperature gradient so as to gradually reach the high temperature region from the upstream side to the downstream side. Thus, NOx is not released uniformly (abruptly and in large quantities) into the exhaust gas, and an effective NOx purification process by the urea SCR catalyst 54 can be executed.

As a result, it is possible to urea SCR catalyst 54 of the exhaust purification device 50 can be sufficiently reduced NOx emissions at low exhaust gas temperature reaches a predetermined activation temperature t _2, e.g. FTP (Federal Test Procedure; Federal Test Procedure) Even in tests such as the cold start mode of WHTC (World Harmonized Transient Cycle), a required NOx purification rate can be achieved.

  In the present embodiment, the main length region 60 of the upstream side exhaust pipe 36 extends in the vehicle front-rear direction over a long section on the upstream side in the exhaust passage 32a. Since the porous cordierite layer supporting the adsorption catalyst layer 63 is provided, the NOx adsorption catalyst layer 63 forms a sufficient NOx adsorption site, and a required NOx purification rate at a low exhaust temperature is achieved. be able to.

  In addition, the movement of heat in the pipe radial direction of the exhaust pipe 32 is suppressed by the heat shielding (heat insulation) effect of the heat-resistant carrier layer 62 having a porous cordierite layer, and noise from the exhaust pipe 32 to the outside. The sound insulation effect is also obtained, and the effect that the exterior material for heat insulation and sound insulation is unnecessary or can be thinned is also obtained.

  Furthermore, in the present embodiment, the heat-resistant carrier layer 62 has at least one cylindrical shape (flow-through shape) extending in the axial direction of the upstream exhaust pipe 36, and therefore can be easily disposed in the upstream exhaust pipe 36. At the same time, the pressure loss of the exhaust can be suppressed.

  In addition, in the present embodiment, the DPF 53 that collects PM in the exhaust gas is provided downstream of the heat-resistant carrier layer 62 and the NOx adsorption catalyst layer 63 and upstream of the urea SCR catalyst 54. In the emission reduction control by controlling the operating conditions of the engine 10, both NOx and PM, which are in a trade-off relationship, can be effectively reduced.

(Second Embodiment)
FIG. 5 shows a cross section of the main part of an exhaust purification system according to the second embodiment of the present invention. Each embodiment described below has the same overall configuration as the exhaust purification system of the first embodiment described above, and only the internal configuration of the upstream side exhaust pipe is different from the first embodiment. It is different. Therefore, regarding the same or similar configuration as the first embodiment, the difference from the first embodiment will be described below using the reference numerals of the corresponding components shown in FIGS. 1 to 4. .

  As shown in FIG. 5, in the present embodiment, an inner cylinder wall 71a is provided inside an outer cylinder wall 61b in the main length region 60 of the upstream side exhaust pipe 36, and the inner cylinder wall 71a (inner wall surface portion) is provided. ) Has a heat-resistant carrier layer 72 having a large number of micropores (porous), and a NOx adsorption catalyst layer 63 supported on the heat-resistant carrier layer 72.

  The heat-resistant carrier layer 72 has a cordierite layer, and includes a cylindrical portion 72a supported by the outer cylindrical wall 61b and a plurality of rib portions 72b protruding radially inward from the cylindrical portion 72a. .

  The heat-resistant carrier layer 72 can be produced by extrusion molding, and the method itself for supporting the NOx adsorption catalyst layer 63 on the heat-resistant carrier layer 72 is substantially the same as in the first embodiment.

  Also in this embodiment, the same effect as the above-described embodiment can be obtained.

(Third embodiment)
FIG. 6 shows a cross section of the main part of an exhaust purification system according to the third embodiment of the present invention.

  As shown in FIG. 6, in the present embodiment, a heat-resistant carrier layer 82 having a rough flow-through carrier structure is formed inside the outer cylindrical wall 61b in the main length region 60 of the upstream side exhaust pipe 36, And a NOx adsorption catalyst layer 63 supported on the heat-resistant carrier layer 82.

  The heat-resistant carrier layer 82 includes a cordierite layer, and a cylindrical portion 82a (inner wall surface portion) that is an inner cylindrical wall supported by the outer cylindrical wall 61b, and an orthogonal wall that protrudes radially inward from the cylindrical portion 82a. Part 82b, and inner cylinder part 82c supported by orthogonal wall part 82b coaxially with cylindrical part 82a.

  The heat-resistant carrier layer 82 can be produced by extrusion molding, and the method itself for supporting the NOx adsorption catalyst layer 63 on the heat-resistant carrier layer 82 is substantially the same as in the first embodiment.

  Also in this embodiment, the same effect as the above-described embodiment can be obtained.

(Fourth embodiment)
FIG. 7 shows a cross section of the main part of an exhaust purification system according to the fourth embodiment of the present invention.

  As shown in FIG. 7, in the present embodiment, an inner cylinder wall 91a is provided inside the outer cylinder wall 61b in the main length region 60 of the upstream side exhaust pipe 36, and both end portions of the inner cylinder wall 91a are provided. A cordierite layer 92 including bottomed tubular members 91b and 91c made of punched metal, which are integrally coupled to each other, is provided. Here, the inner cylinder wall 91a may be a cordierite layer using a punching metal core member in combination as shown in FIG. 3B as one aspect of the inner cylinder wall 61a of the first embodiment. Alternatively, the bottomed tubular members 91b and 91c may be coated with cordierite.

  Alternatively, only the both end portions of the inner cylindrical wall 91a use a core member made of a punching metal as shown in FIG. 3B, and the intermediate portion of the inner cylindrical wall 91a is shown in FIGS. It may have a layer structure as shown in c) or FIG.

  Also in this embodiment, the same effect as the above-described embodiment can be obtained.

  In each of the above-described embodiments, the metal support in which the main length region 60 of the upstream side exhaust pipe 36 is integrally supported by the upstream end side part 36a and the downstream end side part 36b of the upstream side exhaust pipe 36 is provided. The outer cylinder wall 61b of the adsorption pipe part 61 is formed as a layer, and the inner cylinder wall 61a of the adsorption pipe part 61 is set to a value close to the diameter of the exhaust passage 32a in the upstream and downstream exhaust pipes 32. Was. However, the diameter of the outer cylinder wall 61b of the main length region 60 may be enlarged, or the exhaust passage 32a may be divided into a plurality of small-diameter parallel passages in the main length region 60. Further, the outer cylindrical wall 61b may be configured as a reinforcing pipe that supports the inner cylindrical wall 61a having a cordierite layer as a main part, and may be fitted into the exhaust pipe 32. Furthermore, it goes without saying that the cross-sectional shape of the inner peripheral surface of the exhaust passage 32a and the inner cylinder wall 61a may be circular or non-circular (for example, substantially elliptical).

  In each of the above-described embodiments, the selective reduction catalyst is a urea SCR catalyst. However, the selective reduction catalyst referred to in the present invention is not limited to this, and a hydrocarbon-based selective reduction catalyst or NOx occlusion is used. A reduced catalyst may be used.

  The heat resistant carrier layer 62 may be divided into a plurality of cylindrical bodies adjacent in the axial direction of the exhaust passage 32a, or may be divided into a plurality of divided cylindrical bodies adjacent in the circumferential direction. Further, when a metal core member for supporting the cordierite layer is provided, the core member has a spiral cross section, or is formed in a spiral band shape along the inner peripheral surface of the outer cylindrical wall 61b of the adsorption pipe portion 61. It can be considered.

  As described above, the exhaust purification system of the present invention can sufficiently reduce the NOx emission amount at a low exhaust temperature until reaching the predetermined activation temperature of the selective reduction catalyst, and is an internal combustion engine mounted on a vehicle. This is useful for exhaust gas purification systems that detoxify NOx contained in exhaust gas.

10 engines (internal combustion engines, diesel engines)
20 Intake device 30 Exhaust device 31 Exhaust manifold 32 Exhaust pipe 32a Exhaust passage 36 Upstream exhaust pipe 36a Upstream end portion 36b Downstream end portion 40 Exhaust turbocharger 50 Exhaust purification device 51 Case 51a Upstream case portion 51b Downstream side Case part 52 Previous DOC
53 DPF (PM collection filter)
54 Urea SCR catalyst (selective reduction catalyst)
56 Urea water injector 60 Main length region 61 Adsorption pipe part 61a, 71a, 91a Inner cylinder wall (inner wall surface part)
61b Outer cylinder wall (metal support layer)
61c Inner peripheral surface 62, 72, 82 Heat-resistant carrier layer 62c Cordierite layer 62h, 62i, 62j Cylindrical core member 63 NOx adsorption catalyst layer 82a Cylindrical portion (inner cylindrical wall)
91b, 91c Bottomed cylindrical member A ₀ NOx maximum adsorption amount at a predetermined activation temperature R ₁ Specific adsorption temperature region (low temperature side temperature region) on the low temperature side
R ₂ specific desorption temperature region (higher side temperature range)
R ₃ Specific adsorption temperature region on the high temperature side t ₀ Activation temperature of the urea SCR catalyst t ₁ Urea addition start temperature t ₂ Predetermined activation temperature

Claims (5)

An exhaust purification system comprising a selective reduction catalyst that renders NOx in exhaust exhausted into an exhaust pipe of an internal combustion engine to be harmless by reacting with a reducing agent in the presence of oxygen,
A heat-resistant carrier layer having a porosity on an inner wall surface portion of the upstream side exhaust pipe located upstream of the selective reduction catalyst in the exhaust pipe, and a NOx purification rate by the selective reduction catalyst supported on the heat-resistant carrier layer NOx in the exhaust gas is adsorbed in a low temperature region with respect to a predetermined activation temperature that can reach an allowable range, while the NOx is desorbed in a high temperature region that is relatively high with respect to the low temperature region. And an NOx adsorption catalyst layer to be separated.   The exhaust purification system according to claim 1, wherein the heat-resistant carrier layer includes a cordierite layer that supports the NOx adsorption catalyst layer.   The exhaust heat purification system according to claim 1 or 2, wherein the heat-resistant carrier layer has at least one cylindrical shape extending in an axial direction of the upstream exhaust pipe.   A PM collection filter for collecting PM in the exhaust gas is provided downstream of the heat-resistant carrier layer and the NOx adsorption catalyst layer and upstream of the selective reduction catalyst. Item 4. The exhaust gas purification system according to any one of Items 1 to 3.   The exhaust heat purification system according to any one of claims 1 to 4, wherein the heat-resistant carrier layer has a metal support layer supported by the exhaust pipe.

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