CN1330212A

CN1330212A - Device for purifying waste gas of diesel motor

Info

Publication number: CN1330212A
Application number: CN 01118752
Authority: CN
Inventors: 南利贵; 目时尚仁
Original assignee: Isuzu Motors Ltd
Current assignee: Isuzu Motors Ltd
Priority date: 2000-06-16
Filing date: 2001-06-07
Publication date: 2002-01-09
Anticipated expiration: 2021-06-07
Also published as: JP2001355431A; CN1195940C

Abstract

The invention provides an exhaust emission control device for a diesel engine prevented from the burning or damage of a filter when particulate matters collected by a DPF(diesel particulate filter) are burnt. A first exhaust emission control unit 21 and a second exhaust emission control unit 22 are disposed in an exhaust passage 13. The first exhaust emission control unit 21 is provided with an oxidation catalyst 51 and the first DP composed of a fibrous material. The second exhaust emission control unit 22 is disposed downstream of the first exhaust emission control unit 21 and provided with an oxidation catalyst 52 and the second DPF 40 composed of honeycomb structure wherein a plurality of cells are formed parallel of a porous material and the inlets and outlets of the cells are alternately blocked.

Description

Exhaust gas purification device for diesel engine

The present invention relates to an exhaust gas purification device for a diesel engine, and more particularly to an exhaust gas purification device for a diesel engine that removes particulates in exhaust gas.

The restriction on the exhaust gas of diesel engine vehicles has been intensified year by year, and in particular, it is urgent to reduce the particulate matter (hereinafter, referred to as PM) mainly containing carbon. As a device for removing such PM, a diesel particulate filter (hereinafter, referred to as DPF) is known, and it is now under the obligation to install the DPF in a diesel engine vehicle.

Various studies have been made on the material and structure of the DPF, and for example, there is known a fiber filter in which a metal mesh is overlapped on both sides of a felted ceramic fiber to form a layered body, and the layered body is wound in a bellows shape around a porous pipe; or a wall-flow type (ゥォ - ルフロ I type) honeycomb filter having a honeycomb structure in which a plurality of parallel cells are formed by porous cordierite and the inlet and outlet of the cells are alternately plugged. In general, a fibrous DPF can collect PM having a small particle size when the fiber density is high, but if it is configured to have a fiber density higher than a predetermined value, the pressure loss increases, the back pressure increases, and the engine performance is impaired. Therefore, for a fiber type DPF having a fiber density set within a range not affecting engine performance, it is limited to collect PM having a very small particle diameter. Further, the wall-flow type DPF has a small pressure loss, and can collect small-particle-size PM that cannot be collected by the fiber type DPF within a range that does not affect the engine performance. The technology of the fiber type DPF is disclosed in japanese patent application laid-open No. 9-137712, and the technology of the wall flow type DPF is disclosed in japanese patent application laid-open No. 9-94434.

In addition, various studies have been made on a method for regenerating a DPF by burning PM collected by the DPF, and for example, a method for burning PM by heating with an electric heater, a combustion furnace or the like, or a method for oxidizing NO in exhaust gas to NO via an oxidation catalyst is known₂In combination with NO₂A method of burning PM (hereinafter, referred to as a continuous regeneration type). Wherein the continuous regeneration type DPF is at 400 DEG CSince PM burns at a low temperature as described below, it is not necessary to provide a special heating device such as an electric heater, which is very advantageous in simplifying and compacting the entire device. Further, a technique related to a continuous regeneration type DPF is disclosed in japanese patent No. 3012249 and the like.

In the DPF that burns PM by the continuous regeneration method, combustion of PM is excited at an exhaust gas temperature in the range of 250 to 400 ℃. Therefore, in the operating range where the exhaust gas temperature is outside the above range, the collected PM is not burned, and the PM is accumulated in the DPF. In addition, when the PM is at a temperature of about 600 ℃ or higher, if a proper amount of oxygen is present, the PM will burn naturally due to the spontaneous combustion property, for example, when exhaust gas of 600 ℃ or higher flows. Therefore, in a state where a large amount of PM is accumulated in the DPF, the PM spontaneously ignites, and the temperature of the DPF itself rises to 2000 ℃.

In the fiber type DPF, even if such spontaneous combustion occurs, if ceramic fibers having high heat resistance are used, the fibers themselves are not burned out, and even if the ceramic fibers are thermally expanded, the fibers themselves are soft, and the expanded portions are absorbed to some extent by spaces between the fibers, and the expanded ceramic fibers receive stress to the wire mesh or the porous tube constituting the filter, so that the filter is not broken. However, when the same phenomenon occurs in the wall-flow type DPF, the porous material is cordierite, and the heat resistant temperature is 1000 ℃. In addition, when the porous material is silicon carbide, although it has heat resistance of 2000 ℃ or higher, it has a high thermal expansion rate, and this expansion cannot be absorbed by the honeycomb structure, resulting in problems such as cracking and breakage of the filter.

In view of the above problems, it is a primary object of the present invention to provide an exhaust gas purification device for a diesel engine, in which a high-temperature resistant fiber type DPF and a wall-flow type DPF capable of collecting small-particle-size PM are combined to collect PM separately, thereby reducing the load of collecting PM in the wall-flow type DPF, suppressing PM accumulation, preventing rapid combustion due to self-ignition, and preventing burning out or breakage of a filter without fail.

According to the present invention, in order to solve the above-described main technical object, there is provided an exhaust gas purification apparatus for a diesel engine, characterized in that a 1 st exhaust gas purification module and a 2 nd exhaust gas purification module located on a downstream side of the 1 st exhaust gas purification module are provided in an exhaust passage of the diesel engine, wherein the 1 st exhaust gas purification module has an oxidation catalyst and a 1 st DPF composed of a fibrous material and collecting PM in exhaust gas, and the 2 nd exhaust gas purification module has an oxidation catalyst and a 2 nd DPF composed of a honeycomb structure in which a plurality of parallel cells are formed by a porous material and an inlet and an outlet of the cells are alternately blocked and collecting PM in exhaust gas.

The minimum particle size of the particulates collected by the 1 st diesel particulate filter is set to 2 to 4 μm; the minimum particle diameter of the particulates collected by the 2 nd diesel particulate filter was set to 0.5 μm.

Embodiments of the present invention are described below with reference to the drawings.

FIG. 1 is a schematic configuration diagram of an embodiment of an exhaust gas purifying apparatus for a diesel engine according to the present invention,

fig 2 is a radial sectional view of a 1 st diesel particulate filter constituting the exhaust gas purifying apparatus shown in fig 1,

figure 3 is a cross-sectional view in the exhaust gas flow direction of the 1 st diesel particulate filter shown in figure 2,

fig 4 is a radial sectional view of a 2 nd diesel particulate filter constituting the exhaust gas purifying apparatus shown in fig 1,

figure 5 is a cross-sectional view in the exhaust gas flow direction of the 2 nd diesel particulate filter shown in figure 4,

fig. 6(a) is a graph showing the particle size distribution of PM at a rotation speed of 1000rpm and an exhaust gas temperature of 250 ℃, and (b) is a graph showing the particle size distribution of PM at a rotation speed of 3200rpm and an exhaust gas temperature of 700 ℃.

An embodiment of the present invention is described below with reference to the drawings.

Fig. 1 is a schematic configuration diagram of an embodiment of an exhaust gas purifying apparatus for a diesel engine according to the present invention, fig. 2 is a radial sectional view of a 1 st diesel particulate filter (1 st DPF) configuring the exhaust gas purifying apparatus shown in fig. 1, fig. 3 is a sectional view of the 1 st DPF shown in fig. 2 in an exhaust gas flowing direction, fig. 4 is a radial sectional view of a 2 nd diesel particulate filter (2 nd DPF) configuring the exhaust gas purifying apparatus shown in fig. 1, and fig. 5 is a sectional view of the 2 nd DPF shown in fig. 4 in an exhaustgas flowing direction.

The diesel engine 10 shown in fig. 1 includes an engine body 11 including a cylinder block, a cylinder head, and the like, an intake passage 12 for introducing air into a cylinder formed in the engine body 11, and an exhaust passage 13 for discharging exhaust gas from the cylinder of the engine body 11. The exhaust passage 13 is constituted by an exhaust pipe 14 and an exhaust manifold 15, and the exhaust manifold 15 is connected to the engine body 11.

The exhaust pipe 14 is provided with a 1 st exhaust gas purification unit 21 and a 2 nd exhaust gas purification unit 22 from the upstream side thereof. First, the 1 st exhaust gas purification module 21 will be explained.

The 1 st exhaust purification module 21 is constituted by the oxidation catalyst 51 and the 1 st DPF 30. The oxidation catalyst 51 and the 1 st DPF30 are disposed in series, with the oxidation catalyst located upstream. For the oxidation catalyst 51, for example, cordierite in a honeycomb form or a carrier made of heat-resistant steel is coated with activated alumina or the like on the surface thereof to form a light coating layer (ゥォッシュコ - ト) in which a catalytic active component made of a noble metal such as platinum, palladium or rhodium is supported. The oxidation catalyst 51 oxidizes NO in the exhaust gas to generate NO₂While HC in the exhaust gas is oxidized with CO to generate H₂O and CO₂。

Next, the 1 st DPF30 will be described with reference to fig. 2 and 3.

The 1 st DPF30 has a cylindrical cover body 31, and a hollow cylindrical filter 32 is provided coaxially with the cover body 31 inside the cover body 31. The filter 32 has a structure in which a heat-resistant wire mesh 32b is laminated on both surfaces of a felt-type ceramic fiber 32a toform a laminate body in which a cylindrical porous tube 33 bent in a wave shape is surrounded. For example, silicon carbide fiber or the like can be used as the ceramic fiber 32a, and the fiber diameter or the like of the felted ceramic fiber 32a is set so that the minimum particle diameter of the collected PM in the 1 st DPF30 is 2 to 4 μm. In order to collect the PM having the above particle diameter, the fiber diameter of the ceramic fiber 32a to be felted is preferably 14 to 15 μm. The porous tube 33 is closed at the inlet side (left side in fig. 3) and open at the outlet side (right side in fig. 3), and has a peripheral wall formed with a plurality of small holes 33 a. The band 34 is wound around both ends of the filter 32, and the filter 32 is fastened in the axial direction by the band 34. The filter 32 is provided at both ends with

annular plates

35a, 35 b. The inner diameters of the

annular plates

35a, 35b are substantially the same as the outer diameter of the porous pipe 33, and the porous pipe 33 is inserted through and welded to the

annular plates

35a, 35 b. Thus, the 1 st DPF30 constitutes a fiber type filter. Once the exhaust gas flows into the 1 st DPF30, the exhaust gas passes through the filter 32 via the outer periphery of the annular plate 35a as shown by the arrows in fig. 3. Once the exhaust gas passes through the filter 32, PM in the exhaust gas is collected by the ceramic fibers 32 a. Thereafter, the exhaust gas flows into the porous pipe 33 through the small holes 33a, and flows out from the outlet of the porous pipe 33.

The 2 nd exhaust gas purifying module 22 will be explained below.

The 2 nd exhaust purification unit 22 is constituted by the oxidation catalyst 52 and the 2 nd DPF 40. The oxidation catalyst 52 and the 2 nd DPF40 are disposed in series, with the oxidation catalyst 52 being located on the upstream side. The oxidation catalyst 52 can oxidize NO in the exhaust gas to produce NO using an oxidation catalysthaving the same composition or components as the oxidation catalyst 51 constituting the above-described 1 st exhaust gas purification device 21₂。

Next, the 2 nd DPF is described with reference to fig. 4 and 5.

The 2 nd DPF40 is formed of a porous material in a columnar honeycomb structure in which a plurality of cells 41 are formed in parallel to the exhaust gas flow direction. Examples of the porous material include cordierite and silicon carbide. The cells 41 are formed to have a substantially square cross section in the radial direction, and fine holes for preventing PM in the exhaust gas from passing therethrough are formed in the partition walls 45 that partition the cells 41. The pore diameter of the pores is set so that even small-particle-size PM that cannot be collected by the 1 st DPF30 can be collected by the 2 nd DPF40, and specifically, the minimum particle size of the collected PM is set to 0.5 μm. In order to collect the particle size PM, the pore diameter of the fine pores is preferably 0.5 to 1.0 μm. Both ends of the cells 41 are alternately plugged with plugging materials 44 made of ceramics or the like so that the inlet portions 42 (left side in fig. 5) and the outlet portions 43 of the cells 41 are alternately closed. Thus, the 2 nd DPF40 constitutes a wall-flow honeycomb filter. Once the exhaust gas flows into the 2 nd DPF40, as shown by the arrows in fig. 5, the exhaust gas flows into the cells 41 from the open inlet portion 42 without the plugging material 44 and passes through the partition wall 45. Once the exhaust gas passes through the partition wall 45, PM in the exhaust gas is collected by the partition wall 45, and the exhaust gas flows out from the open outlet portion 43 without the plugging material 44.

The exhaust gas purification apparatus of the diesel engine 10 of the embodiment shown in fig. 1 is configured as described above, and exhaust gas discharged from the cylinders of the engine body 11 is purified by the 1 st exhaust gas purification unit 21 and the 2 nd exhaust gas purification unit 22 provided in the exhaust passage 13. Next, a series of processes for purifying PM in the exhaust gas by the 1 st exhaust gas purification unit 21 and the 2 nd exhaust gas purification unit 22 will be described.

First, the exhaust gas flows into the 1 st exhaust purification assembly 21, and once the exhaust gas passes through the oxidation catalyst 51, the NO in the exhaust gas is oxidized to generate NO₂. Then, when the exhaust gas passes through the 1 st DPF30, PM having a particle size of 2 to 4 μm or more is collected by the 1 st DPF 30. Here, the PM collected by the 1 st DPF30 and NO generated by the oxidation catalyst 51₂Generating 、 The chemical reaction of (1). That is, the PM collected by the 1 st DPF30 passes through NO₂Oxidation (combustion) to CO or CO₂. In this manner, once the exhaust gas is purified by the 1 st exhaust purification unit 21, the exhaust gas flows into the 2 nd exhaust purification unit 22. In the 2 nd exhaust gas purification module 22, the collection and combustion of PM are performed through the same process as in the 1 st exhaust gas purification module 21, but in the 2 nd DPF40, it is possible to collect PM having a small particle size that cannot be collected by the 1 st DPF30, that is, PM having a particle size of 0.5 μm or more. By the above processing, PM in the exhaust gas can be purified.

Here, the relationship between the PM collected by each DPF and the collection efficiency will be described with reference to fig. 6.

Fig. 6(a) shows a PM particle size distribution when the rotation speed is 1000rpm and the exhaust gas temperature is 250 ℃, and fig. 6(b) shows a PM particle size distribution when the rotation speed is 3200rpm and the exhaust gas temperature is 700 ℃. In the diesel engine 10, when the exhaust gas is passed through the exhaust passage 13 under the conditions shown in fig. 6(a), that is, when the engine speed is 1000rpm and the exhaust gas temperature is 250 ℃, the 1 st DPF30 can collect PM having a particle size of 2 to 4 μm or more, and 60 to 70% of the total amount of PM can be collected. Also, almost all of the remaining PM that cannot be collected by the 1 st DPF30 can be collected with the 2 nd DPF 40. In the diesel engine 10, when the exhaust gas is caused to pass through the exhaust passage 13 under the conditions shown in fig. 6, that is, when the engine speed is 3200rpm and the exhaust gas temperature is 700 ℃, 90% or more of the total amount of PM can be collected by the 1 st DPF 30. While the 2 nd DPF40 may collect nearly all of the remaining PM that the 1 st DPF30 was unable to collect.

As described above, in the exhaust gas purifying apparatus of the diesel engine 10 of the illustrated embodiment, most of the PM is collected by the 1 st DPF30, and only the PM having a small particle size is collected by the 2 nd DPF40, so that only a small amount of PM is accumulated. Therefore, even if high-temperature exhaust gas of 600 ℃ or higher flows through the exhaust passage 13, the 1 st DPF30 having a large amount of PM deposited thereon is formed of a fiber filter, and therefore, even if PM is rapidly burned, the PM is not burned. In addition, since only a small amount of PM is accumulated in the 2 nd DPF40 as described above, the PM does not rapidly burn, and burnout or breakage does not occur.

In the illustrated embodiment, since the minimum particle size of the PM collected by the 1 st DPF30 is set to 2 to 4 μm and the minimum particle size of the PM collected by the 2 nd DPF40 is set to 0.5 μm, the 2 nd DPF40collects a small amount of 10% or less of the total amount of PM in the high speed rotation region shown in fig. 6(b) of the diesel engine 10, and the 2 nd DPF40 collects 30 to 40% of the total amount of PM in the low and medium speed rotation region shown in fig. 6 (a). That is, the 2 nd DPF40 collects a little more PM in the low/middle rotation region than in the high rotation region, but the exhaust gas temperature at this time is 250 ℃ to 400 ℃ because PM passes through NO₂Combustion does not cause a large amount of PM to be accumulated in the 2 nd DPF40, and rapid combustion due to self-ignition does not occur.

In the illustrated embodiment, the oxidation catalyst and the DPF constituting each exhaust gas purification unit are provided separately, but the exhaust gas purification unit may be constituted by placing a catalytically active component on the DPF and forming the oxidation catalyst and the DPF as one device. That is, in the 1 st exhaust gas purification module, a catalytically active component made of a noble metal such as platinum is supported on a fibrous material constituting the 1 st DPF, and in the 2 nd exhaust gas purification module, a catalytically active component is supported on a surface of a porous material constituting the 2 nd DPF. With this configuration, NO can be generated in the DPF₂And collecting and burning the PM, it is not necessary to provide an oxidation catalyst separately, and the entire device can be made compact.

The present invention has been described above with reference to the illustrated embodiments, but the present invention is not limited thereto. For example, the 1 st DPF may have a structure in which a ceramic long fiber or a ceramic woven fabric is directly wound around a porous tube. Further, a Nox catalyst may be disposed in the exhaust passage downstream of the 2 nd exhaust gas purification unit to reduce nitrogen oxides such as NO in the exhaust gas to harmless N₂Or H₂And O. That is, any embodiment may be used as long as it has the respective elements constituting the present invention and obtains the same operational effects.

In the exhaust gas purification apparatus for a diesel engine according to the present invention, since the 1 st exhaust gas purification module and the 2 nd exhaust gas purification module DPF located on the downstream side of the 1 st exhaust gas purification module are provided in the exhaust passage, the 1 st exhaust gas purification module has the 1 st DPF composed of a fibrous material and an oxidation catalyst, and the 2 nd exhaust gas purification module has the 2 nd DPF composed of a honeycomb structure in which a plurality of parallel cells are formed by a porous material and the inlets and outlets of the cells are alternately blocked, the load of collecting PM in the wall-flow type DPF can be reduced. Therefore, accumulation of PM in the wall-flow DPF can be suppressed, and rapid combustion due to self-ignition does not occur, and thus burning out or breakage of the filter can be prevented without fail.

Claims

1. An exhaust gas purification apparatus of a diesel engine, characterized in that a 1 st exhaust gas purification unit and a 2 nd exhaust gas purification unit located on a downstream side of the 1 st exhaust gas purification unit are provided in an exhaust passage of the diesel engine, wherein the 1 st exhaust gas purification unit has an oxidation catalyst and a 1 st diesel particulate filter composed of a fibrous material that collects particulates in exhaust gas; the 2 nd exhaust gas purification module has an oxidation catalyst, and a 2 nd diesel particulate filter which is constituted by a honeycomb structure in which a plurality of parallel cells are formed by a porous material, and an inlet and an outlet of the cells are alternately plugged, and which collects particulates in exhaust gas.

2. The exhaust gas purifying apparatus for a diesel engine according to claim 1, wherein the minimum particle diameter of the particulates collected by the 1 st diesel particulate filter is set to 2 to 4 μm; the minimum particle diameter of the particulates collected by the 2 nd diesel particulate filter was set to 0.5 μm.