JP2006204969A - Exhaust purifying apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust purifying apparatus capable of preventing choking of an oxidation catalyst arranged in the exhaust pipe. <P>SOLUTION: The apparatus has a flow-through type oxidation catalyst 14 arranged in the exhaust pipe 11 guiding exhaust gas 9 from a diesel engine 1, and the amount of the noble metal per unit volume in the specified area A from the front to the rear edge is increased partially at least twice compared with the amount of the noble metal per unit volume in the rear common area B. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust emission control device.

  Particulate matter (particulate matter) discharged from diesel engines is mainly composed of carbonaceous soot and SOF (Soluble Organic Fraction) consisting of high-boiling hydrocarbon components. Furthermore, the composition contains a small amount of sulfate (mist-like sulfuric acid component). As a measure to reduce this type of particulates, a particulate filter is installed in the middle of the exhaust pipe through which the exhaust gas flows. Has been performed conventionally.

  This type of particulate filter has a porous honeycomb structure made of a ceramic such as cordierite, and the inlets of the flow paths partitioned in a lattice pattern are alternately sealed, and the inlets are not sealed. About the flow path, the exit is sealed, and only the exhaust gas which permeate | transmitted the porous thin wall which divides each flow path is discharged | emitted downstream.

  Then, the particulates in the exhaust gas are collected and deposited on the inner surface of the porous thin wall, so that the particulates are appropriately burned and removed before the exhaust resistance increases due to clogging. It is necessary to regenerate, but in normal diesel engine operating conditions, there are few opportunities to obtain exhaust temperatures that are high enough for the particulates to self-combust. For example, an appropriate amount for platinum-supported alumina Adoption of a catalyst regeneration type particulate filter in which an oxidation catalyst formed by adding a rare earth element such as cerium is integrally supported has been studied.

  That is, if such a catalyst regeneration type particulate filter is employed, the oxidation reaction of the collected particulates is promoted to lower the ignition temperature, and the particulates can be burned and removed even at an exhaust temperature lower than the conventional one. It becomes possible.

  However, even when such a catalyst regeneration type particulate filter is used, the trapped amount exceeds the particulate processing amount in the operation region where the exhaust temperature is low, so such a low exhaust gas. If the operation state at the temperature continues, there is a possibility that the particulate filter will fall into an over trapped state without the regeneration of the particulate filter proceeding well.

  Therefore, a flow-through type oxidation catalyst is separately arranged in front of the particulate filter, and fuel is added to the exhaust gas upstream of the oxidation catalyst at the stage where the amount of particulate accumulation has increased. It is considered to perform forced regeneration.

  In other words, the fuel (HC) added upstream from the particulate filter undergoes an oxidation reaction while passing through each cell of the preceding oxidation catalyst, and the particulates immediately after the inflow of exhaust gas heated by the reaction heat. The catalyst bed temperature of the filter is raised, the particulates are burned out, and the particulate filter is regenerated.

  As a specific means for performing this type of fuel addition, exhaust gas is produced by performing post-injection at a non-ignition timing later than compression top dead center following main injection of fuel performed near compression top dead center. It is common to add fuel to the inside of the cylinder. However, the fuel may be added to the exhaust gas by delaying the timing of main injection into the cylinder from the usual timing. As well as a means to add fuel by controlling the fuel injection to the exhaust gas so that a large amount of unburned fuel remains in the exhaust gas, an injector is installed as a fuel addition means at an appropriate position in the exhaust pipe (or an exhaust manifold) However, it is also possible to add fuel by directly injecting it into the exhaust gas by this injector.

Incidentally, as prior art document information related to the forced regeneration of such a particulate filter, there is the following Patent Document 1 by the same applicant as the present invention.
JP 2003-193824 A

  However, when performing forced regeneration of the particulate filter by adding this type of fuel, a light load with a low exhaust temperature is required for vehicles operating in a congested road such as a city bus. Although it is assumed that the operation is continued continuously, if the fuel addition is performed under the condition that the catalyst activity lower limit temperature (about 190 ° C.) of the preceding stage oxidation catalyst is just below, the front end of the oxidation catalyst that does not have a very high catalyst activity The mist of the added fuel that cannot be processed on the surface accumulates as SOF, and the front end surface of the oxidation catalyst becomes sticky in a wet state, and the particulates of the particulates easily adhere to the surface. The mixture with the SOF component is agglomerated after incomplete combustion to form a bridge at the opening of each cell, and the agglomeration proceeds from this bridge as oxidation starts. There has been clogging occurs fear the front end surface of the medium.

  The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide an exhaust purification device capable of preventing clogging of an oxidation catalyst provided in the middle of an exhaust pipe.

  The present invention relates to an exhaust emission control apparatus having a flow-through type oxidation catalyst in the middle of an exhaust pipe that guides exhaust gas from an engine, the amount of noble metal per unit volume in a required range from the front end of the oxidation catalyst to the rear. Further, the amount is partially increased so as to be at least twice as much as the amount of noble metal per unit volume in the general portion behind this.

  Thus, if the amount of noble metal per unit volume in the required range from the front end to the rear of the oxidation catalyst is increased in this way, the catalytic activity performance in this range is partially enhanced, and the front end surface of the oxidation catalyst is increased. Since the oxidation treatment of the attached SOF component starts from a relatively low temperature, and the reaction heat generated by this oxidation treatment raises the catalyst bed temperature near the front end face of the oxidation catalyst, and synergistically improves the activity. Thus, the SOF component adhering to the front end surface of the oxidation catalyst is oxidized without delay, and the front end surface of the oxidation catalyst is held in a dry state.

  As a result, even when light load operation with a low exhaust temperature is continued continuously, apportioning becomes difficult to adhere to the front end surface of the oxidation catalyst, and apportioning (apportioning apportioning) The bridge of the mixture with the SOF component) is not formed, and the problem of agglomeration starting from this bridge is avoided in advance, and clogging at the front end face of the oxidation catalyst is prevented.

  Also, in order to prevent clogging of the oxidation catalyst, the amount of noble metal per unit volume is not increased evenly over the entire oxidation catalyst, but noble metal per unit volume in the required range from the front end of the oxidation catalyst to the rear. Since the amount is partially increased, the cost increase of the raw material for the oxidation catalyst is negligible.

  It should be noted that clogging can be prevented only by partially increasing the amount of noble metal per unit volume in the required range from the front end to the rear of the oxidation catalyst, and has already been confirmed through extensive research by the present inventors. In this case, an effective clogging prevention effect cannot be obtained unless the amount is increased by at least twice as much as the amount of precious metal per unit volume in the general portion behind this range. This is also obtained as a result of the present inventors' verification experiment.

  Furthermore, in the present invention, the range in which the amount of the precious metal per unit volume is partially increased is preferably limited to a range corresponding to about 1/10 of the total length of the oxidation catalyst. This is because, in order to prevent clogging of the front end face of the oxidation catalyst, it is sufficient to increase the amount of noble metal in this partial range, and even if it is expanded beyond this range, the effect of preventing clogging is markedly changed. This is because it has been confirmed by the present inventors that the raw material cost only increases unnecessarily.

  Further, in the present invention, it is preferable that the present invention is applied to a device provided with a fuel addition means for adding fuel into the exhaust gas upstream of the oxidation catalyst. A fuel injection device that injects fuel may be adopted, and fuel addition may be performed by controlling fuel injection into the cylinder and leaving a large amount of unburned fuel in the exhaust gas.

  That is, in the case of a usage mode in which fuel is added to the exhaust gas upstream of the oxidation catalyst and the added fuel is oxidized on the oxidation catalyst and the exhaust gas is heated by reaction heat, Since the mist of the added fuel tends to accumulate as SOF on the front end surface, it becomes a clogging prevention means that is particularly effective in such usage.

  According to the above-described exhaust purification device of the present invention, even when a light load operation with a low exhaust temperature is continuously continued, it is possible to reliably prevent clogging of the oxidation catalyst provided in the middle of the exhaust pipe. In addition, the amount of noble metal per unit volume required to prevent the clogging can be increased to the minimum necessary amount, and an excellent effect that the cost of the raw material for the oxidation catalyst can be remarkably suppressed can be obtained.

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

  1 to 4 show an example of an embodiment for carrying out the present invention. Reference numeral 1 in FIG. 1 denotes a diesel engine equipped with a turbocharger 2, and intake air 4 guided from an air cleaner 3 is an intake pipe 5. And the intake air 4 pressurized by the compressor 2a is sent to the intercooler 6 to be cooled, and the intake air 4 further flows from the intercooler 6 to the intake manifold 7. It is guided and distributed to each cylinder 8 of the diesel engine 1 (the case of in-line 6 cylinders is illustrated in FIG. 1).

  Further, the exhaust gas 9 discharged from each cylinder 8 of the diesel engine 1 is sent to the turbine 2b of the turbocharger 2 through the exhaust manifold 10, and the exhaust gas 9 that has driven the turbine 2b passes through the exhaust pipe 11. It is designed to be discharged outside the vehicle.

  A filter case 12 is interposed in the middle of the exhaust pipe 11, and a catalyst regeneration type particulate filter 13 that integrally carries an oxidation catalyst is provided in the rear stage in the filter case 12. As shown schematically in FIG. 2, the particulate filter 13 has a porous honeycomb structure made of ceramic, and each flow path 13 a (cell) partitioned in a lattice shape. The flow passages 13a in which the inlets are alternately sealed and the entrances are not sealed are sealed in the outlets, and permeate through the porous thin walls 13b partitioning the flow passages 13a. Only the exhaust gas 9 is discharged downstream.

  A flow-through type oxidation catalyst 14 having a honeycomb structure as shown in an enlarged view in FIG. 3 is accommodated in the filter case 12 immediately before the particulate filter 13. A large number of flow paths 14a (cells) opened in the direction are defined by the cell walls 14b.

Then, as shown in FIG. 4, in the required range A from the front end of the oxidation catalyst 14 to the rear corresponding to the length L ₂ of about 1/10 of the total length L ₁ of the oxidation catalyst 14, The amount of noble metal per unit volume is partially increased so as to be at least twice as much as the amount of noble metal per unit volume (Pt, Pd, Rh, etc.) in the rear general portion B.

  In the example shown here, a control device 15 constituting an engine control computer (ECU: Electronic Control Unit) is connected to an accelerator sensor 16 (load sensor) that detects the accelerator opening as a load of the diesel engine 1. An accelerator opening signal 16a and a rotation speed signal 17a from a rotation sensor 17 that detects the engine rotation speed of the diesel engine 1 are input. The accelerator opening signal 16a and the rotation speed signal 17a are input to the accelerator opening signal 16a. Based on this, a fuel injection signal 18 a is output to a fuel injection device 18 that injects fuel into each cylinder 8 of the diesel engine 1.

  Here, the fuel injection device 18 is composed of a plurality of injectors 19 provided for each cylinder 8, and the electromagnetic valves of these injectors 19 are appropriately controlled to open by the fuel injection signal 18a. The injection timing and the injection amount are appropriately controlled.

  On the other hand, in the control device 15, the fuel injection signal 18a in the normal mode is determined based on the accelerator opening signal 16a and the rotation speed signal 17a. On the other hand, it is necessary to perform forced regeneration of the particulate filter 13. In this case, the mode is switched from the normal mode to the regeneration mode, and the non-ignition timing (start timing is 90 ° crank angle) after the main injection of fuel performed near the compression top dead center (crank angle 0 °). The fuel injection signal 18a is determined so as to perform the post-injection in the range of .degree.

  That is, when post-injection is performed at a non-ignition timing later than the compression top dead center following main injection, unburned fuel (mainly HC: hydrocarbon) is added to the exhaust gas 9 by this post-injection. As a result, the HC gas generated by the unburned fuel undergoes an oxidation reaction on the oxidation catalyst on the surface of the particulate filter 13, and the catalyst bed temperature rises due to the reaction heat, thereby causing the particulate matter in the particulate filter 13. Will be burned off.

  The filter case 12 in FIG. 1 also serves as a muffler outer cylinder for the purpose of improving the mountability of the particulate filter 13 and the preceding oxidation catalyst 14 to the vehicle. 20 and 21 are attached.

  Thus, when it becomes necessary to perform forced regeneration of the particulate filter 13, the regeneration mode is selected by the control device 15, and the fuel injection pattern is switched from the normal mode to the regeneration mode by the control device 15. As a result of adopting an injection pattern in which post-injection is performed at the timing of non-ignition later than the compression top dead center following main injection, fuel added unburned into exhaust gas 9 by the post-injection is highly concentrated by thermal decomposition As a result, the exhaust gas 9 passing through the oxidation catalyst 14 is significantly heated by the oxidation reaction by the oxidation catalyst 14 and is heated by the oxidation catalyst 14. When the gas 9 is introduced into the particulate filter 13, the entire area of the particulate filter 13 is heated to a high temperature. It is well burned and removed so that the regeneration of the particulate filter 13 is achieved.

  At this time, especially in a light load operation where the exhaust gas temperature is low, the mist of the added fuel tends to adhere to the front end surface of the oxidation catalyst 14 as the SOF component. However, in the oxidation catalyst 14, the front end is directed rearward. Since the amount of noble metal per unit volume in the required range A is partially increased, the catalytic activity performance in this range A is partially enhanced, and the oxidation treatment of SOF adhering to the front end surface of the oxidation catalyst 14 is performed. Since the catalyst bed temperature in the vicinity of the front end face of the oxidation catalyst 14 is raised by the heat of reaction generated by this oxidation treatment, and the synergistic improvement in activity is achieved, the oxidation catalyst during forced regeneration is started. The heat-up performance in the vicinity of the front end surface of 14 is greatly enhanced, the SOF component adhering to the front end surface of the oxidation catalyst 14 is oxidized without delay, and the front end surface of the oxidation catalyst 14 is brought into a dry state. It will be lifting.

  As a result, even when light load operation with a low exhaust temperature is continuously continued, apportioning is difficult to adhere to the front end surface of the oxidation catalyst 14, and apportioning (approaching to the opening of each cell 14a) The mixture of the component and the SOF component is not formed, and the problem of agglomeration starting from this bridge is avoided in advance, and clogging of the front end surface of the oxidation catalyst 14 is prevented.

  In order to prevent clogging of the oxidation catalyst 14, the amount of noble metal per unit volume is not increased uniformly with respect to the entire oxidation catalyst 14, but the unit in the required range A from the front end to the rear of the oxidation catalyst 14. Since the amount of noble metal per volume is partially increased, the raw material cost of the oxidation catalyst 14 is slightly increased.

  Note that clogging can be prevented only by partially increasing the amount of noble metal per unit volume in the required range A from the front end to the rear side of the oxidation catalyst 14 after extensive research by the present inventors. In this case, effective clogging prevention effect is required unless the amount is increased by at least twice as much as the amount of precious metal per unit volume in the general portion B behind this range. The fact that it cannot be obtained is also obtained as a result of the verification experiments of the present inventors.

Further, the range A in which the amount of the precious metal per unit volume is partially increased is a range corresponding to a length L ₂ of about 1/10 of the total length L ₁ of the oxidation catalyst 14. In order to prevent clogging at the front end surface of the slab, it is sufficient to increase the amount of the noble metal in this partial range A, and even if it is expanded beyond this range, there is no particular change in the clogging prevention effect, This is because it has been confirmed by the present inventors that the raw material cost only increases unnecessarily.

  Therefore, according to the above embodiment, even when the light load operation at a low exhaust temperature is continuously continued, clogging of the oxidation catalyst 14 provided in the preceding stage of the particulate filter 13 is surely prevented. In addition, the amount of noble metal per unit volume required to prevent the clogging can be kept to a minimum increase, and an increase in the raw material cost of the oxidation catalyst 14 can be remarkably suppressed.

  The exhaust emission control device of the present invention is not limited to the above-described embodiment. In the above embodiment, the fuel injection device is used as the fuel addition means, and the fuel is performed near the compression top dead center. Fuel is added to the exhaust gas by performing post-injection at a timing of non-ignition later than the compression top dead center following the main injection of the engine, but the timing of main injection into the cylinder is delayed from normal The fuel may be added to the exhaust gas, and only the means for adding fuel by controlling the fuel injection into the cylinder and leaving a large amount of unburned fuel in the exhaust gas. In addition, an injector may be provided as a fuel addition means at an appropriate position of the exhaust pipe (or an exhaust manifold), and fuel may be directly injected into the exhaust gas by this injector, and The downstream stage of the oxidation catalyst may not necessarily be equipped with a particulate filter, and may be equipped with a post-treatment device other than the particulate filter, and the scope of the present invention is not deviated. Of course, various changes can be made.

It is the schematic which shows an example of the form which implements this invention. It is sectional drawing which shows the structure of a particulate filter typically. It is a perspective view which cuts off one part and shows the structure of an oxidation catalyst. It is a perspective view which shows the range which increased the amount of noble metals per unit volume of an oxidation catalyst.

Explanation of symbols

1 Diesel engine (engine)
8 cylinder 9 exhaust gas 11 exhaust pipe 14 oxidation catalyst 18 fuel injection device (fuel addition means)
A Required range B General part L ₁ Total length of oxidation catalyst L ₂ About 1/10 of the length

Claims (4)

  In an exhaust emission control device having a flow-through type oxidation catalyst in the middle of an exhaust pipe for leading exhaust gas from an engine, the amount of noble metal per unit volume in a required range from the front end to the rear side of the oxidation catalyst is An exhaust purification device characterized in that it is partially increased so as to be at least twice as much as the amount of noble metal per unit volume in the general portion of   2. The exhaust gas purification according to claim 1, wherein a range in which the amount of the precious metal per unit volume is partially increased is limited to a range corresponding to about 1/10 of the total length of the oxidation catalyst. apparatus.   The exhaust emission control device according to claim 1 or 2, further comprising fuel addition means for adding fuel to the exhaust gas upstream of the oxidation catalyst.   Fuel injection device that injects fuel into each cylinder of the engine is adopted as fuel addition means, and fuel addition is executed by controlling fuel injection into the cylinder and leaving a large amount of unburned fuel in the exhaust gas The exhaust emission control device according to claim 1 or 2, wherein the exhaust purification device is configured.

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