JP2010242602A - Exhaust gas purifying system and control method of exhaust gas purifying system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas purifying system and a control method of an exhaust gas purifying system capable of efficiently purifying NOx, even in an operating condition of an internal combustion engine such that temperature of an exhaust gas flowing into a carbon monoxide oxidation catalyst (NO oxidation catalyst) is in a low-temperature zone of 150 to 200°C, utilizing carbon monoxide (CO) contained in the exhaust gas, in an exhaust gas purifying system provided with an exhaust gas purifying apparatus having a NOx occlusion reduction type catalyst in an exhaust passage of the internal combustion engine. <P>SOLUTION: In an exhaust gas purifying system 1 provided with a NOx occlusion reduction type catalyst 6 in an exhaust passage 3 of an internal combustion engine 2, a carbon monoxide oxidation catalyst 5 is provided on the upstream side of the NOx occlusion reduction type catalyst 6 of the exhaust passage 3, and a carbon monoxide amount increase control for increasing the carbon monoxide contained in the exhaust gas flowing into the carbon monoxide oxidation catalyst 5, if an operating condition of the internal combustion engine 2 turns into a preset operating condition. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification system having an NOx occlusion reduction type catalyst in an exhaust passage of an internal combustion engine and a control method for the exhaust gas purification system, and more specifically, effectively utilizes NOx discharged from the internal combustion engine from a low temperature. The present invention relates to an exhaust gas purification system and an exhaust gas purification system control method that can efficiently purify NOx from a low temperature range.

  In an internal combustion engine mounted on a vehicle such as a diesel engine, a NOx occlusion reduction type catalyst that purifies NOx has been put into practical use as one method for purifying nitrogen oxide (NOx) in an atmosphere containing excess oxygen. ing. This NOx occlusion reduction type catalyst stores NOx in the NOx occlusion reduction type catalyst when the air-fuel ratio is lean to treat NOx in an oxygen-excess atmosphere such as exhaust gas of a diesel engine, and the NOx occlusion amount is reduced. Before reaching saturation, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio or rich, and the stored NOx is released and reduced.

  However, the NOx occlusion reduction type catalyst has a problem that the NOx occlusion performance in the lean air-fuel ratio state is low in the low temperature range and NOx cannot be sufficiently purified. Therefore, there is an attempt to improve the storage performance in the lean air-fuel ratio state by disposing a nitric oxide oxidation catalyst (NO oxidation catalyst) upstream of the NOx storage reduction catalyst and generating nitrogen dioxide with the nitric oxide oxidation catalyst. Has been made.

  For example, an exhaust gas purification method and an exhaust gas purification device for an internal combustion engine in which an NO oxidation device and a NOx trapping reduction catalyst are installed in the exhaust gas flow path of the internal combustion engine to purify NOx in the exhaust gas during the internal combustion engine start period have been proposed (for example, , See Patent Document 1).

  However, in the low temperature range of 150 ° C. to 200 ° C., the nitrogen dioxide generating activity and the adsorption retention of nitrogen dioxide in the normal nitric oxide oxidation catalyst are not sufficient, and the NOx storage performance of the NOx storage reduction catalyst is also 150 ° C. to 200 ° C. There is a problem that the temperature is low in a low temperature region of ° C., and NOx cannot be sufficiently purified in this low temperature region.

JP 2002-89246 A

  The present invention has been made in view of the above situation, and an object of the present invention is to provide carbon monoxide (CO) in exhaust gas in an exhaust gas purification system including an NOx occlusion reduction type catalyst in an exhaust passage of an internal combustion engine. Is used to efficiently purify NOx even in an operating state of an internal combustion engine in which the temperature of exhaust gas flowing into the nitric oxide oxidation catalyst (NO oxidation catalyst) is in a low temperature range of 150 ° C. to 200 ° C. It is an object of the present invention to provide an exhaust gas purification system and a control method for the exhaust gas purification system.

  An exhaust gas purification system for achieving the above object is an exhaust gas purification system comprising an NOx occlusion reduction type catalyst in an exhaust passage of an internal combustion engine, upstream of the NOx occlusion reduction type catalyst in the exhaust passage. Provided with a nitric oxide oxidation catalyst, and increases the carbon monoxide in the exhaust gas flowing into the nitric oxide oxidation catalyst when the operation state of the internal combustion engine becomes a preset operation state. It is configured with a control device that performs carbon increase control.

According to this configuration, when carbon monoxide (CO) is present, the nitric oxide oxidation catalyst (NO oxidation catalyst) oxidizes nitrogen monoxide (NO) to nitrogen dioxide (NO ₂ ) to generate nitrogen dioxide. Since the rate of generation increases significantly, in combination with carbon monoxide increase control, nitrogen monoxide is oxidized and adsorbed to nitrogen dioxide with a nitric oxide oxidation catalyst (NO oxidation catalyst) in the low temperature range of 150 ° C to 200 ° C. Since it can be retained, NOx can be purified even in this low temperature range.

  Further, when the temperature of the exhaust gas at which the NOx occlusion performance of the NOx occlusion reduction type catalyst becomes higher than the activation temperature is 200 ° C. or more, the nitrogen dioxide held in the nitric oxide oxidation catalyst is desorbed, but the downstream side The NOx occlusion reduction catalyst can be used for purification.

  That is, NOx is oxidized and held in the nitric oxide oxidation catalyst using carbon monoxide in a low temperature range of 150 ° C. to 200 ° C., and NOx held in a high temperature range of 200 ° C. or higher is released. As a result, NOx is retained by the nitric oxide oxidation catalyst in the low temperature range where the purification performance of the NOx selective reduction type catalyst is low, and this retained NOx is supplied to the NOx occlusion reduction type catalyst in the high temperature range for purification. To do. This makes it possible to purify NOx in the exhaust gas from a low temperature range that could not be purified by the prior art.

  The carbon monoxide increase control is performed by increasing the amount of fuel injected into the cylinder (inside the cylinder) while checking the oxygen concentration detected by the excess air sensor and the nitrogen oxide concentration detected by the nitrogen oxide sensor. Main injection without pre-injection or direct fuel injection in the exhaust pipe, throttle of the intake air amount, increase of the EGR amount, etc. can be performed.

  In the internal combustion engine, the preset operating state includes a case where the temperature of the exhaust gas flowing into the nitric oxide oxidation catalyst is in a low temperature range of 150 ° C to 200 ° C. With this configuration, it is possible to generate nitrogen dioxide having strong adsorptive power and hold it in the nitric oxide oxidation catalyst even in the low temperature range of 150 ° C to 200 ° C, which was difficult with the prior art, and purify NOx from a lower temperature range. become able to.

  Further, in the internal combustion engine, in the carbon monoxide increase control, the carbon monoxide is increased so that the volume concentration of carbon monoxide in the exhaust gas is not less than 5 times and not more than 20 times the volume concentration of nitrogen oxides. Configured as follows. In other words, in the carbon monoxide increase control, the carbon monoxide is increased so that the molar ratio (CO / NOx) of carbon monoxide (CO) to nitrogen oxide (NOx) in the exhaust gas is 5 or more and 20 or less. . With this configuration, nitrogen dioxide can be efficiently generated and held in the nitric oxide oxidation catalyst.

  An exhaust gas purification system control method for achieving the above object includes a NOx occlusion reduction type catalyst in an exhaust passage of an internal combustion engine, and is provided upstream of the NOx occlusion reduction type catalyst in the exhaust passage. In a control method of an exhaust gas purification system provided with a nitrogen oxide oxidation catalyst, when the operation state of the internal combustion engine becomes a preset operation state, monoxide in the exhaust gas flowing into the nitrogen monoxide oxidation catalyst It is a method characterized by performing carbon monoxide increase control for increasing carbon.

According to this method, when carbon monoxide (CO) is present, in the nitric oxide oxidation catalyst (NO oxidation catalyst), nitrogen monoxide (NO) is oxidized to nitrogen dioxide (NO ₂ ) to generate nitrogen dioxide. Since the rate of generation increases significantly, in combination with carbon monoxide increase control, nitrogen monoxide is oxidized and adsorbed to nitrogen dioxide with a nitric oxide oxidation catalyst (NO oxidation catalyst) in the low temperature range of 150 ° C to 200 ° C. Since it can be retained, NOx can be purified even in this low temperature range.

  Further, the nitrogen dioxide held in the nitric oxide oxidation catalyst is desorbed and released when the temperature of the exhaust gas is 200 ° C. or higher. At this exhaust gas temperature, the NOx occlusion reduction catalyst on the downstream side is released. Since the temperature is equal to or higher than the temperature at which the NOx occlusion performance is activated, the NOx occlusion reduction catalyst can be used for purification.

  Further, in the above control method of the exhaust gas purification system, if the temperature of the exhaust gas flowing into the nitric oxide oxidation catalyst is in a low temperature range of 150 ° C. to 200 ° C., this method makes it difficult for the prior art. NO2 can be purified from a low temperature range of 150 ° C. to 200 ° C. because nitrogen dioxide having strong adsorptive power can be generated and held in the nitric oxide oxidation catalyst even in a low temperature range of 150 ° C. to 200 ° C. .

  Further, in the control method for an internal combustion engine, in the carbon monoxide increase control, the carbon monoxide so that the volume concentration of carbon monoxide in the exhaust gas is not less than 5 times and not more than 20 times the volume concentration of nitrogen oxides. Increasing the amount effectively oxidizes nitric oxide to form nitrogen dioxide and retains NOx.

  According to the exhaust gas purification system and the exhaust gas purification system control method of the present invention, the temperature of the exhaust gas flowing into the nitrogen monoxide oxidation catalyst using carbon monoxide (CO) in the exhaust gas is 150. In the operation state of the internal combustion engine that is set in a low temperature range from ℃ to 200 ℃, the carbon monoxide increase control is performed to efficiently generate nitrogen dioxide with the nitric oxide oxidation catalyst. Nitrogen dioxide is held in the nitric oxide oxidation catalyst to purify NOx, and nitrogen dioxide held in the nitric oxide oxidation catalyst is desorbed. Since the NOx occlusion performance of the NOx occlusion reduction catalyst is higher than the temperature at which it is activated, it can be purified by the NOx occlusion reduction catalyst. Therefore, NOx purification performance can be improved from a low temperature range.

It is the figure which showed the structure of the exhaust-gas purification system of embodiment of this invention. It is the figure which showed the relationship between the nitrogen oxide (NOx) adsorption rate in a nitric oxide oxidation catalyst (NO oxidation catalyst), and the temperature of a nitric oxide oxidation catalyst. It is the figure which showed the relationship between a NOx purification rate and a catalyst inlet_port | entrance temperature. It is a schematic diagram for demonstrating the role of carbon monoxide (CO) in a nitric oxide oxidation catalyst (NO oxidation catalyst). It is a schematic diagram for demonstrating the oxidation of nitric oxide (NO) in a nitric oxide oxidation catalyst (NO oxidation catalyst). It is the figure which showed the relationship between the ratio (CO / NOx) of carbon monoxide (CO) and nitrogen oxide (NOx) in a nitric oxide oxidation catalyst (NO oxidation catalyst), and nitrogen oxide (NOx) adsorption amount.

  Hereinafter, an exhaust gas purification system and an exhaust gas purification system control method according to embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a configuration of an exhaust gas purification system 1 according to an embodiment of the present invention.

  In this exhaust gas purification system 1, an exhaust pipe direct fuel injection device 4, a nitric oxide oxidation catalyst 5, and a NOx occlusion reduction type catalyst 6 are provided in an exhaust passage 3 of an engine (internal combustion engine) 2 in order from the upstream side. . A control device (not shown) of the exhaust gas purification system 1 is provided, and this control device is incorporated in a control device (not shown) called an ECU (engine control unit).

  This nitric oxide oxidation catalyst 5 is composed of platinum (Pt), palladium (Pd) on a metal oxide carrier in which the metal oxide as a carrier contains at least one oxide of aluminum (Al) and cerium (Ce). It is formed by supporting a metal catalyst containing at least one of the following. In addition to rhodium (Pt), rhodium (Ph), platinum-palladium (Pt-Pd), palladium (Pd), and the like exhibit the same effect as the noble metal, and these can be used.

Carriers used in this nitric oxidation catalyst 5 may be a typical aluminum oxide (Al ₂ O _3), but the oxygen absorbing function of cerium oxide with (OSC) (CeO _2), cerium oxide dioxide zirconia (CeO ₂ -It is more preferable to form it with a material containing ZrO ₂ ) because the effect of generating nitrogen dioxide is even greater.

  In the exhaust pipe fuel direct injection device 4, in the regeneration process for recovering the NOx occlusion capacity of the NOx occlusion reduction type catalyst 6, in order to bring the air-fuel ratio of the exhaust gas into the stoichiometric air-fuel ratio or rich state, the fuel is supplied to the exhaust passage 3. It is a device for injecting.

  NOx occlusion reduction catalyst (LNT) 6 is formed by supporting alkali metal or alkaline earth metal together with noble metal, and oxidizes nitrogen monoxide (NO) in exhaust gas containing excess oxygen and adsorbs as nitrate on the catalyst. To purify NOx. The NOx occlusion reduction catalyst 6 occludes NOx when the exhaust gas is lean, and releases the occluded NOx when the exhaust gas is rich, and reduces the released NOx in a reducing atmosphere. Reduce.

  Further, the control device performs carbon monoxide increase control for increasing carbon monoxide in the exhaust gas flowing into the nitrogen monoxide oxidation catalyst 5 when the operation state of the internal combustion engine 1 is set to a preset operation state. Configured to do. This preset operation state includes a case where the temperature of the exhaust gas flowing into the nitric oxide oxidation catalyst 5 is in a low temperature range of 150 ° C to 200 ° C. That is, unless the temperature is higher than the oxidation activation temperature of carbon monoxide, oxygen on the catalyst surface cannot be removed, and the nitrogen dioxide generation activity is low. When the carbon monoxide purification rate is 20% or more, the generation of nitrogen dioxide is greatly increased. In addition, since the activity is constant at 200 ° C. or higher and the effect of increasing carbon monoxide is lost, it is desirable that the carbon monoxide increase control be in the range of the temperature at which the carbon monoxide purification rate is 20% or higher to 200 ° C. or lower.

  This carbon monoxide increase control is performed in the cylinder (inside the cylinder) while checking the oxygen concentration detected by the excess air sensor (not shown) and the nitrogen oxide (NOx) concentration detected by the nitrogen oxide sensor (not shown). ), Or by direct injection of fuel in the exhaust pipe, restriction of the intake air amount, increase of the EGR amount, and the like. Speaking of the concentration of carbon monoxide, it is usually about 10 ppm to 300 ppm, but increases to 1000 ppm to 2000 ppm.

Next, regarding the increase in the amount of nitrogen dioxide produced by carbon monoxide, nitrogen oxide (NOx) adsorption on the catalyst surface of the nitrogen monoxide oxidation catalyst (NO oxidation catalyst) when carbon monoxide is used, and nitrogen dioxide (NO) ₂ ) A model of generation and desorption will be described with reference to FIGS. In an exhaust gas purification system 1 such as a diesel engine, carbon monoxide (CO) reacts with oxygen (O ₂ ) on platinum (Pt) as shown in FIG. Nitrogen oxide (NOx) is oxidized with carbon monoxide (CO) by the nitric oxide oxidation catalyst 5, so that oxygen of noble metal active sites such as platinum (Pt) of the nitric oxide oxidation catalyst 5 is consumed. The

On the other hand, as shown in FIG. 5 in which nitrogen monoxide (NO) reacts with oxygen (O ₂ ) on platinum (Pt) to generate nitrogen dioxide (NO ₂ ), as shown in FIG. Oxidation results in an oxygen-free (O ₂ ) foot state, and therefore, a site that stably retains nitric oxide oxidation products such as nitrogen dioxide (NO ₂ ) by attracting the carrier oxide lattice oxygen in the vicinity of the noble metal to the noble metal. It is formed. At this stable holding site, compounds such as nitrogen dioxide (NO ₂ ) generated by oxidation of nitric oxide (NO) on the noble metal quickly move and be held. Therefore, oxidation of nitric oxide (NO) and adsorption of oxidized nitrogen dioxide (NO ₂ ) on the nitric oxide oxidation catalyst 5 occur continuously.

The adsorbed nitrogen dioxide (NO ₂ ) is desorbed when the temperature rises. The desorption temperature is about 200 ° C. to 300 ° C., and is a temperature range in which nitrogen dioxide is hardly generated at the outlet of the nitric oxide oxidation catalyst 5 in the conventional control method.

That is, as shown in FIG. 4, carbon monoxide (CO) cleans platinum (Pt), thereby promoting adsorption of nitrogen oxide (NOx) on the platinum (Pt) surface. Further, as shown in FIG. 5, the oxygen (O ₂ ) on the surface of platinum (Pt), the surface of the carrier (Al ₂ O ₃ , CeO ₂ , ZrO _2, etc.) and the oxygen released from the carrier, NOx + (2 -X) / 2 × O ₂ → NO ₂ (x ≦ 2) reaction is promoted. The generated nitrogen dioxide (NO ₂ ) is adsorbed on platinum (Pt) and the surface of the carrier, and desorbed into the exhaust gas due to a temperature rise (200 ° C. to 300 ° C.).

FIG. 6 shows the adsorption rate (apparent NOx purification rate) of nitrogen oxide (NOx) at 160 ° C. in the experiment with the model gas before and after the nitric oxide oxidation catalyst (NO oxidation catalyst) 5. Nitrogen oxide (NOx) adsorption rate varies depending on the carbon monoxide (CO) concentration, and NOx adsorption performance varies depending on the molar ratio (CO / NOx) of carbon monoxide (CO) and nitrogen oxide (NOx). The molar ratio is 5 to 20, and the NOx retention performance in the low temperature range is maximized. The retained NOx (mainly as NO ₂ ) is desorbed in a temperature range of 200 ° C. or higher.

  As shown in FIG. 6, the amount of carbon monoxide (CO) used in this reaction is preferably in the range of CO / NOx of 5 to 20, and the upper limit varies depending on the amount of catalyst, the concentration of nitrogen oxide (NOx), etc. In general, CO / NOx is about 10 to 20.

  By increasing the amount of carbon monoxide flowing into the nitric oxide oxidation catalyst 5 by controlling the increase in the amount of carbon monoxide in this low-temperature region, the nitrogen monoxide generated by the effect of this carbon monoxide is retained in the nitric oxide oxidation catalyst 5. NOx can be purified.

  Therefore, according to the exhaust gas purification system 1 and the exhaust gas purification system control method configured as described above, the NOx occlusion reduction performance of the NOx occlusion reduction catalyst 6 is low, and the NOx purification is not sufficiently performed. In the low temperature range, NOx is oxidized and held by the nitric oxide oxidation catalyst 5, and the NOx in the exhaust gas is purified. The NOx oxidized and held by the nitric oxide oxidation catalyst 5 is released in a high temperature range of 200 ° C. or higher, and this released NOx is stored in the downstream side of the NOx occluded reduction that has been heated to the activation temperature or higher. As a result, the NOx in the exhaust gas can be purified from a low temperature range that could not be purified by the prior art.

Next, examples of the present invention will be described. As shown in FIG. 1, a nitrogen monoxide oxidation catalyst 5 is arranged on the upstream side of the exhaust passage 3, and a NOx occlusion reduction type catalyst 6 is arranged on the downstream side, and a simulated gas test was conducted to confirm the effect of the present invention. . The volume ratio of carbon monoxide (CO) to nitrogen oxide (NOx) (same as the molar ratio) (CO / NOx) with platinum (Pt) -supported aluminum oxide (Al ₂ O ₃ ) as the nitric oxide oxidation catalyst 5 The case of 20 was designated as A in the figure. Further, the case where the volume ratio (CO / NOx) is 20 with cerium oxide (CeO ₂ ) supported by platinum (Pt) / aluminum oxide (Al ₂ O ₃ ) supported by platinum (Pt) is set as B in the figure. Moreover, the comparative example which made the volume ratio (CO / NOx) 0 using the catalyst of Example 1 was set to C in the figure.

  As a result of the experiment, the NOx purification rate and the catalyst inlet temperature (nitrogen monoxide oxidation catalyst as shown in FIG. 3) are obtained using each nitric oxide oxidation catalyst that can obtain the nitrogen oxide (NOx) adsorption rate as shown in FIG. Inlet gas temperature) relationship was obtained.

  FIG. 2 is a graph of the NOx adsorption rate of the nitric oxide oxidation catalyst in Examples A and B and Comparative Example C. When the NOx adsorption rate shows a positive value, the progress of NOx adsorption and retention to the nitric oxide oxidation catalyst is shown. When the NOx adsorption rate shows a negative value, the adsorbed NOx is released.

  As shown in FIG. 3, in Examples A and B, by adjusting the volume ratio (= molar ratio) (CO / NOx), nitrogen monoxide is converted into nitric oxide oxidation catalyst 5 in a low temperature range of 200 ° C. or lower. It was found that the NOx purification performance of NOx discharged at a low temperature could be improved by maintaining the oxidation at a low temperature and releasing it in the activation temperature range of the NOx selective reduction catalyst 6 at 200 ° C. or higher.

  From this result, the NOx occlusion capacity of the NOx occlusion reduction type catalyst 6 is low, and it is oxidized and held in the nitric oxide oxidation catalyst 5 by adjusting the volume ratio (CO / NOx) in the low temperature region in the low temperature region where NOx purification is difficult. It was confirmed that the NOx that was oxidized and held at a high temperature could be purified by the NOx storage reduction catalyst 6.

  According to the exhaust gas purification system and the exhaust gas purification system control method of the present invention, a NOx occlusion reduction type catalyst is provided in an exhaust passage of an internal combustion engine such as a diesel engine, and nitric oxide oxidation is performed upstream of the NOx occlusion reduction type catalyst. A carbon monoxide increase control that increases the carbon monoxide in the exhaust gas flowing into the nitric oxide oxidation catalyst when a catalyst is provided and the internal combustion engine is in a preset operating state such as at low temperatures. In this operation state, NOx is oxidized and held in the nitric oxide oxidation catalyst using carbon monoxide in the exhaust gas, and the temperature of the exhaust gas is changed to that of the NOx occlusion reduction type catalyst. When the temperature rises above the activation temperature, it is desorbed and released and purified by the downstream NOx storage reduction catalyst to reduce NOx.

  Therefore, the exhaust gas purification system and the exhaust gas purification system control method of the present invention can be used as an exhaust gas purification system for an internal combustion engine mounted on an automobile or a control method for an exhaust gas purification system.

1 Exhaust gas purification system 2 Engine (internal combustion engine)
3 Exhaust passage 4 Fuel direct injection device in exhaust pipe 5 Nitric oxide oxidation catalyst (NO oxidation catalyst)
6 NOx storage reduction catalyst

Claims (6)

  In an exhaust gas purification system provided with a NOx storage reduction catalyst in an exhaust passage of an internal combustion engine, a nitrogen monoxide oxidation catalyst is provided upstream of the NOx storage reduction catalyst in the exhaust passage, and the operating state of the internal combustion engine Exhaust gas, comprising: a control device that performs carbon monoxide increase control for increasing carbon monoxide in the exhaust gas flowing into the nitric oxide oxidation catalyst when the engine enters a preset operating state. Gas purification system.   2. The exhaust gas purification system according to claim 1, wherein the preset operation state includes a case where the temperature of the exhaust gas flowing into the nitric oxide oxidation catalyst is in a low temperature range of 150 ° C. to 200 ° C. 3. .   2. The carbon monoxide increase control includes increasing the amount of carbon monoxide so that the volume concentration of carbon monoxide in the exhaust gas is 5 to 20 times the volume concentration of nitrogen oxides. Or the exhaust gas purification system of 2.   In the control method of an exhaust gas purification system, wherein the exhaust passage of the internal combustion engine is provided with a NOx occlusion reduction type catalyst and a nitrogen monoxide oxidation catalyst is provided upstream of the NOx occlusion reduction type catalyst in the exhaust passage. Exhaust gas purification characterized by performing carbon monoxide increase control for increasing carbon monoxide in the exhaust gas flowing into the nitric oxide oxidation catalyst when the operating state of the engine becomes a preset operating state How to control the system.   The control method of the exhaust gas purification system according to claim 4, including a case where the temperature of the exhaust gas flowing into the nitric oxide oxidation catalyst is in a low temperature range of 150 ° C to 200 ° C.   5. The carbon monoxide is increased in the carbon monoxide increase control so that the volume concentration of carbon monoxide in the exhaust gas is not less than 5 times and not more than 20 times the volume concentration of nitrogen oxides. Or the control method of the exhaust gas purification system of 5.

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