JP2009162158A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine capable of improving NOx conversion performance without depending on an operating state. <P>SOLUTION: This exhaust emission control device includes: a fuel reformer 50 arranged separately from an exhaust pipe 4, reforming fuel to generate reducing gas, and supplying the reducing gas from the upstream side of a NOx conversion catalyst 33 in the exhaust pipe 4 into the exhaust pipe 4; and an enriching means enriching an exhaust gas air-fuel ratio. The enriching means has a first enriching control means enriching the exhaust gas air-fuel ratio while supplying the reducing gas into the exhaust pipe 4 by the fuel reformer 50, and a second enriching control means enriching the exhaust gas air-fuel ratio without supplying the reducing gas into the exhaust pipe 4, and selects according to a predetermined condition whether or not the exhaust air-fuel ratio is enriched by the first enriching control means or the second enriching control means. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine. Specifically, the present invention relates to an exhaust gas purification apparatus for an internal combustion engine that includes a NOx purification catalyst that adsorbs or occludes NOx in exhaust gas and reduces the adsorbed or occluded NOx.

  In the present invention, the term “rich” indicates that the air / fuel ratio of the fuel in question (hereinafter referred to as “air-fuel ratio”) is smaller than the stoichiometric air-fuel ratio, and is referred to as “lean”. The term indicates that the air / fuel ratio of the fuel in question is greater than the stoichiometric air / fuel ratio described above. In the following description, the weight ratio of air to fuel in the air-fuel mixture flowing into the engine is referred to as engine air-fuel ratio, and the weight ratio of air in the exhaust pipe to combustible gas is referred to as exhaust air-fuel ratio.

Conventionally, a technique for purifying nitrogen oxide (hereinafter referred to as “NOx”) contained in exhaust gas is known.
For example, in Patent Documents 1 and 2 and Non-Patent Document 1, a NOx occlusion reduction catalyst (hereinafter referred to as “LNT”) is provided in the exhaust passage, and the exhaust gas is passed through an oxidation catalyst during a lean operation in which the exhaust gas has excess oxygen. There is shown an exhaust emission control device that stores NOx by reacting with alkali metal or alkaline earth metal and stores the reduced NOx during a rich operation in which the oxygen concentration of the exhaust gas is reduced. In this exhaust purification apparatus, NOx occlusion and NOx reduction can be performed periodically by repeating the lean operation and the rich operation.

  Further, for example, in Non-Patent Document 2, NOx is adsorbed on the catalyst during a lean operation in which the exhaust gas is excessive in oxygen, and then a lean operation is performed to periodically create a state where the oxygen concentration in the exhaust gas is low. A method is shown in which NOx adsorbed during lean operation is periodically reduced by periodically synthesizing and supplying carbon.

More specifically, in the method shown in Non-Patent Document 2, first, during the lean operation in which the exhaust gas is excessive in oxygen, the following formulas (1) to (3) indicate that nitrogen monoxide present in the exhaust gas and Nitrogen dioxide is adsorbed on the catalyst.
NO → NO (adsorption) (1)
2NO + O ₂ → 2NO ₂ (2)
NO ₂ → NO ₂ (Adsorption) (3)
Next, rich operation is performed and carbon monoxide is synthesized. The carbon monoxide synthesized here produces hydrogen by an aqueous gas shift reaction represented by the following formula (4) in an environment where the oxygen partial pressure is low.
CO + H ₂ O → H ₂ + CO ₂ (4)
Further, this hydrogen reacts with nitric oxide in a reducing atmosphere to generate ammonia, and this ammonia is adsorbed on the catalyst by the following formula (5).
5H ₂ + 2NO → 2NH ₃ (adsorption) + 2H ₂ O (5)
As described above, NOx in the exhaust gas or NOx adsorbed on the catalyst is reduced by the following formulas (6) to (8) using ammonia generated from carbon monoxide as a final reducing agent.
4NH ₃ + 4NO + O ₂ → 4N ₂ + 6H ₂ O (6)
2NH ₃ + NO ₂ + NO → 2N ₂ + 3H ₂ O (7)
8NH ₃ + 6NO ₂ → 7N ₂ + 12H ₂ O (8)

  In addition, for example, in Patent Documents 3 and 4, a fuel reformer in which an LNT is provided in an exhaust passage and further, a hydrocarbon gas is reformed upstream of the LNT to produce a reducing gas containing hydrogen and carbon monoxide. An exhaust purification system is provided with a vacuum vessel. In particular, this exhaust purification system uses a fuel reformer that produces a reducing gas such that hydrogen is larger than carbon monoxide by volume ratio. According to this system, it is possible to selectively reduce NOx in the exhaust gas by adding a reducing gas containing hydrogen from the upstream side of the LNT into the exhaust gas.

Here, for example, as a method for producing a reducing gas from a hydrocarbon fuel, a partial oxidation reaction using oxygen as an oxidant is known, for example, as shown in the following formula (9).
_{C n H m + 1 / 2nO} 2 → nCO + 1 / 2mH 2 (9)
This partial oxidation reaction is an exothermic reaction using fuel and oxygen, and the reaction proceeds spontaneously. For this reason, once reaction starts, it can continue producing hydrogen, without supplying heat from the outside. Further, in such a partial oxidation reaction, when a fuel and oxygen coexist at a high temperature, a combustion reaction as shown in the following formula (10) also proceeds.
_{C n H m + (n +} 1 / 4m) O 2 → nCO 2 + 1 / 2mH 2 O (10)
Further, a steam reforming reaction represented by the following formula (11) using steam as an oxidizing agent is known.
_{C n H m + nH 2 O} → nCO + (n + 1 / 2m) H 2 (11)
This steam reforming reaction is an endothermic reaction using fuel and steam, and is not a reaction that proceeds spontaneously. For this reason, the steam reforming reaction is easily controlled with respect to the partial oxidation reaction described above.
Japanese Patent No. 2,586,738 Japanese Patent No. 2600492 Japanese Patent No. 3642273 JP 2002-89240 A "Development of NOx storage-reduction three-way catalyst system" Proceedings of the Society of Automotive Engineers of Japan Vol.26, No.4, October 1995 “A NOx Reduction System Using Ammonia Storage- Selective Catalytic Reduction in Rich and Lean Poerations”, 15 Aachener Kolloquium Fahrzeug-und Motorentechnik 2006 p. 259-270

However, there are the following problems when the engine is repeatedly operated in a lean operation and a rich operation, for example, as shown in the above-mentioned Patent Documents 1 and 2 and Non-Patent Documents 1 and 2.
That is, when rich operation is performed and the exhaust air-fuel ratio is made rich, NOx stored in the LNT is desorbed, but if the stored amount of NOx in the LNT is large, the NOx is not reduced and flows out downstream of the LNT. As a result, the NOx purification performance may be reduced.
Therefore, it is conceivable to increase the frequency of rich operation and reduce NOx frequently so that the NOx occlusion amount in the LNT does not increase. However, the region where the rich operation can be performed is limited depending on the operation state of the engine such as the rotation speed and the torque. For this reason, it is difficult to reduce NOx frequently regardless of the operating state of the engine.

In addition, the exhaust purification systems of Patent Documents 3 and 4 are different from those shown in Patent Documents 1 and 2 described above, basically, hydrogen in the exhaust gas with excess oxygen regardless of lean operation and rich operation. Carbon monoxide and hydrocarbons are added.
However, in the case where LNT is purified by adding a reducing agent such as hydrogen or carbon monoxide into the exhaust gas containing excess oxygen, NOx can be purified only at about 200 ° C. For example, when the temperature of LNT is 200 ° C. or higher, the added hydrogen and carbon monoxide are combusted. For this reason, at such a temperature, the amount of the additive is insufficient, and the reduction reaction of NOx does not proceed sufficiently.

In addition, in the case where a fuel reformer is provided in an exhaust passage in which the exhaust amount constantly varies as in the exhaust gas purification systems of Patent Documents 3 and 4, in order to effectively produce hydrogen with this fuel reformer Therefore, it is necessary to increase the reaction time between the reforming catalyst of the fuel reformer and the exhaust. However, in order to increase the reaction time in this way, it is necessary to enlarge the reforming catalyst, which may be costly.
In order to operate the fuel reformer in a stable state, it is necessary to keep the reaction temperature in the reforming catalyst of the fuel reformer constant. However, when the fuel reformer is provided in the exhaust passage in which the oxygen amount, the water vapor amount, and the temperature constantly change as in the above-described exhaust gas purification systems of Patent Documents 3 and 4, the fuel reformer is operated in a stable state. It becomes difficult to do.

  The present invention has been made in consideration of the above-described points, and an object of the present invention is to provide an exhaust purification device for an internal combustion engine that can improve NOx purification performance regardless of the operating state.

  In order to achieve the above object, an invention according to claim 1 is provided in an exhaust passage (4, 5) of an internal combustion engine (1), and an exhaust air / fuel ratio of exhaust gas flowing through the exhaust passage is used as an exhaust air / fuel ratio. An internal combustion engine comprising a NOx purification catalyst (33) that adsorbs or occludes NOx in exhaust when the air-fuel ratio is made lean, and reduces the adsorbed or occluded NOx when the exhaust air-fuel ratio is made rich. In the exhaust purification apparatus, provided separately from the exhaust passage, the fuel is reformed to produce a reducing gas containing hydrogen and carbon monoxide, and the reducing gas is supplied to the NOx purification catalyst in the exhaust passage. A fuel reformer (50) for supplying the exhaust passage from the upstream side; and a rich means (40) for enriching the exhaust air-fuel ratio, the rich means comprising the fuel reformer Reducing First enrichment control means (40) for enriching the exhaust air / fuel ratio while supplying the exhaust gas into the exhaust passage, and the exhaust air without supplying reducing gas into the exhaust passage by the fuel reformer. Second enrichment control means (40) for enriching the fuel ratio, and the exhaust air / fuel ratio is enriched by the first enrichment control means or the second enrichment according to a predetermined condition. It is characterized by selecting whether to enrich by the control means.

According to a second aspect of the present invention, the exhaust gas purification apparatus for an internal combustion engine according to the first aspect further comprises a purification rate estimating means (40) for estimating a NOx purification rate (CE) of the NOx purification catalyst, wherein the rich When the NOx purification rate (CE) estimated by the purification rate estimation unit is smaller than a predetermined purification rate determination value (CEATH), the purification unit enriches the exhaust air-fuel ratio by the first enrichment control unit. It is characterized by doing.
According to a third aspect of the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine according to the first or second aspect, wherein the flow rate of exhaust gas flowing through the exhaust passage is used as an exhaust flow rate to estimate or detect the exhaust flow rate. Means (40, 21), and the enrichment means is configured to perform the operation when the exhaust flow rate (GE) estimated or detected by the exhaust flow rate estimation means is greater than or equal to a predetermined flow rate determination value (GETH). 1 The enrichment control means enriches the exhaust air-fuel ratio.
According to a fourth aspect of the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine according to the third aspect, wherein the temperature of the NOx purification catalyst is used as a catalyst temperature to estimate or detect the catalyst temperature. And a flow rate determination value determining means (40) for determining the predetermined flow rate determination value based on the catalyst temperature (TLNC) estimated or detected by the catalyst temperature estimation means. .

According to a fifth aspect of the present invention, in the exhaust gas purification apparatus for an internal combustion engine according to the first aspect, an exhaust flow rate estimating means for estimating or detecting the exhaust flow rate with the flow rate of the exhaust gas flowing through the exhaust passage as an exhaust flow rate. 40, 21), the catalyst temperature estimating means (40, 22) for estimating or detecting the catalyst temperature using the temperature of the NOx purification catalyst as the catalyst temperature, and the exhaust flow rate estimated or detected by the exhaust flow rate estimating means ( GE) or when the exhaust air-fuel ratio is enriched by the first enrichment control means and the second enrichment control means based on the catalyst temperature (TLNC) estimated or detected by the catalyst temperature estimation means. Exhaust air-fuel ratio target value determining means (40) for determining exhaust air-fuel ratio target values (HAFTV, NAFTV) in the engine, and further comprising the first enrichment control means and Said second enrichment control means, respectively, the exhaust air-fuel ratio, characterized in that enriching to match the target value determined by said exhaust air-fuel ratio target value determination means.
According to a sixth aspect of the present invention, in the exhaust gas purification apparatus for an internal combustion engine according to the fifth aspect, the second enrichment control means supplies a reducing gas supplied from the fuel reformer (GRG). Is adjusted so that the exhaust air-fuel ratio (AF) becomes rich so as to coincide with the target value (HAFTV) determined by the exhaust air-fuel ratio target value determining means.

According to the first aspect of the present invention, when the exhaust air / fuel ratio is enriched, the exhaust air / fuel ratio is enriched while supplying a reducing gas containing hydrogen and carbon monoxide into the exhaust passage, Whether to enrich the exhaust air-fuel ratio without supplying reducing gas is selected according to a predetermined condition. Thereby, for example, even when the reduction reaction in the NOx purification catalyst is difficult to occur, the exhaust air-fuel ratio is made rich while supplying the reducing gas containing hydrogen whose reaction rate is higher than that of the hydrocarbon. Further, it is possible to prevent the NOx from flowing out without being reduced by promoting the reduction reaction in the NOx purification catalyst.
Further, by supplying the reducing gas into the exhaust passage, the exhaust air / fuel ratio of the exhaust gas flowing into the NOx purification catalyst can be enriched while the engine air / fuel ratio is made lean and combustion in the internal combustion engine is kept in an optimal state. . Therefore, the NOx purification performance can be improved regardless of the operating state of the internal combustion engine. Also, since the engine air-fuel ratio can be kept lean, oil dilution occurs or the combustion of the internal combustion engine does not occur as in the case of changing the exhaust air-fuel ratio by supplying fuel by post injection or the like. There is no instability.
In addition, by providing a fuel reformer that produces reducing gas separately from the exhaust passage, the reducing gas is always supplied with optimum efficiency regardless of the operating state of the internal combustion engine, the oxygen concentration and the water vapor concentration of the exhaust gas, etc. While being able to be manufactured, this reducing gas can be supplied into the exhaust passage. Further, by providing the fuel reformer separately from the exhaust passage, it is possible to supply reducing gas as required, such as the operating state of the internal combustion engine.
On the other hand, when the fuel reformer is provided in the exhaust passage, it is necessary to enlarge the fuel reformer so that the fuel reformer can be operated without affecting the exhaust gas component, temperature, and flow velocity. According to this invention, by providing the fuel reformer separately from the exhaust passage, stable operation can be performed without increasing the size of the apparatus. Further, by providing the fuel reformer separately from the exhaust passage, it is possible to activate the catalyst included in the fuel reformer at an early stage by performing control of a system different from the control of the internal combustion engine.
Moreover, the reduction reaction rate in the NOx purification catalyst can be improved by supplying a reducing gas containing hydrogen. Thereby, when the exhaust air-fuel ratio is made rich, NOx adsorbed or stored in the NOx purification catalyst can be prevented from being desorbed without being reduced.

  According to the second aspect of the present invention, the NOx purification rate of the NOx purification catalyst is estimated, and when the estimated NOx purification rate is smaller than a predetermined purification rate judgment value, the reducing gas is supplied. Enrich exhaust air-fuel ratio. As a result, the amount of NOx adsorbed or occluded by the NOx purification catalyst increases, and the amount of NOx flowing out without being reduced when enriched can be reduced.

  According to the third aspect of the invention, the exhaust flow rate is estimated, and when the exhaust flow rate is equal to or higher than a predetermined flow rate determination value, the reducing air is supplied to enrich the exhaust air-fuel ratio. That is, when the exhaust gas flow rate is large, NOx occluded or adsorbed by the NOx purification catalyst is easily desorbed. Even in such a case, a reducing gas is supplied to promote the reduction reaction in the NOx purification catalyst. Thus, NOx can be prevented from flowing downstream without being reduced.

  According to the fourth aspect of the present invention, the temperature of the NOx purification catalyst is estimated, and the above-described flow rate determination value is determined according to this temperature. By the way, the reduction reaction in the NOx purification catalyst is less likely to occur as the temperature of the NOx purification catalyst becomes lower, so that NOx tends to flow out without being reduced. According to the invention of claim 4, the NOx purification performance can be improved more efficiently by determining the flow rate determination value in consideration of the temperature of the NOx purification catalyst.

According to the fifth aspect of the present invention, the target value of the exhaust air-fuel ratio is determined based on the estimated or detected exhaust flow rate or the estimated or detected temperature of the NOx purification catalyst, and the exhaust air-fuel ratio is the exhaust air-fuel ratio. Enrich to match the target value. In this way, by adjusting the exhaust air-fuel ratio at the time of enrichment according to the exhaust flow rate and the temperature of the NOx purification catalyst, it is possible to efficiently prevent NOx from flowing out from the NOx purification catalyst without being reduced.
According to the sixth aspect of the present invention, when the exhaust air / fuel ratio is enriched by supplying the reducing gas, the exhaust air / fuel ratio matches the target value determined by the exhaust air / fuel ratio target value determining means. As described above, the supply amount of the reducing gas is adjusted. As a result, the exhaust gas having the optimum exhaust air / fuel ratio can always be caused to flow into the NOx purification catalyst.

  FIG. 1 is a diagram showing a configuration of an internal combustion engine and an exhaust purification device thereof according to an embodiment of the present invention. An internal combustion engine (hereinafter referred to as “engine”) 1 is a diesel engine that directly injects fuel into the combustion chamber of each cylinder 7, and each cylinder 7 is provided with a fuel injection valve (not shown). These fuel injection valves are electrically connected by an electronic control unit (hereinafter referred to as “ECU”) 40, and the valve opening time and valve closing time of the fuel injection valve are controlled by the ECU 40.

  The engine 1 includes an intake pipe 2 through which intake air circulates, an exhaust pipe 4 through which exhaust gas circulates, an exhaust gas recirculation passage 6 that recirculates part of the exhaust gas in the exhaust pipe 4 to the intake pipe 2, and intake air into the intake pipe 2. And a supercharger 8 for pressure-feeding.

  The intake pipe 2 is connected to the intake port of each cylinder 7 of the engine 1 through a plurality of branch portions of the intake manifold 3. The exhaust pipe 4 is connected to the exhaust port of each cylinder 7 of the engine 1 through a plurality of branch portions of the exhaust manifold 5. The exhaust gas recirculation passage 6 branches from the exhaust manifold 5 and reaches the intake manifold 3.

  The supercharger 8 includes a turbine (not shown) provided in the exhaust pipe 4 and a compressor (not shown) provided in the intake pipe 2. The turbine is driven by the kinetic energy of the exhaust flowing through the exhaust pipe 4. The compressor is rotationally driven by the turbine, pressurizes the intake air, and pumps it into the intake pipe 2. The turbine includes a plurality of variable vanes (not shown), and is configured to change the turbine rotation speed (rotational speed) by changing the opening of the variable vanes. The vane opening degree of the turbine is electromagnetically controlled by the ECU 40.

  A throttle valve 9 for controlling the intake air amount GA of the engine 1 is provided upstream of the supercharger 8 in the intake pipe 2. The throttle valve 9 is connected to the ECU 40 via an actuator, and the opening degree is electromagnetically controlled by the ECU 40. In addition, an intercooler 11 for cooling the intake air pressurized by the supercharger 8 is provided on the downstream side of the supercharger 8 in the intake pipe 2.

  The exhaust gas recirculation passage 6 connects the exhaust manifold 5 and the intake manifold 3 and recirculates part of the exhaust discharged from the engine 1. The exhaust gas recirculation passage 6 is provided with an EGR cooler 12 that cools the exhaust gas that is recirculated, and an EGR valve 13 that controls the flow rate of the recirculated exhaust gas. The EGR valve 13 is connected to the ECU 40 via an actuator (not shown), and the valve opening degree is electromagnetically controlled by the ECU 40.

A NOx purification catalyst 33 that purifies the exhaust gas is provided downstream of the supercharger 8 in the exhaust pipe 4.
The NOx purification catalyst 33 acts as a catalyst supported on a support of alumina (Al ₂ O ₃ ), ceria (CeO ₂ ), and a composite oxide of cerium and rare earth (hereinafter referred to as “ceria-based composite oxide”). Platinum (Pt), ceria or ceria-based composite oxide having NOx adsorption capability, and zeolite having a function of holding ammonia (NH ₃ ) generated in the catalyst as ammonium ions (NH ₄ ⁺ ) .

In the present embodiment, the NOx purification catalyst 33 is formed by supporting a NOx reduction catalyst having two layers on a catalyst carrier.
The lower layer of the NOx reduction catalyst is platinum (4.5 g / L), ceria 60 (g / L), alumina 30 (g / L), and Ce—Pr—La—Ox 60 (g). / L) and 20 (g / L) of Zr-Ox together with an aqueous medium are put into a ball mill, stirred and mixed to produce a slurry, and this slurry is coated on a catalyst carrier. Formed.
Moreover, the upper layer of the NOx reduction catalyst is 75 (g / L) obtained by ion-exchange of β-type zeolite with iron (Fe) and cerium (Ce), 7 (g / L) alumina, and 8 binders. A material composed of (g / L) is put into a ball mill together with an aqueous medium, stirred and mixed to produce a slurry, and this slurry is coated on the lower layer.

  When the amount of adsorbed ammonia in the NOx purification catalyst 33 decreases, the NOx purification capacity decreases, so that a reducing agent is supplied to the NOx purification catalyst 33 (hereinafter referred to as “reduction”) in order to reduce NOx as appropriate. . In this reduction, for example, the air-fuel ratio (engine air-fuel ratio) of the air-fuel mixture in the combustion chamber of the engine 1 is changed to a chemical amount by increasing the amount of fuel injected from the fuel injection valve and decreasing the intake air amount GA by the throttle valve 9. By making it richer than the theoretical ratio, the reducing agent is supplied to the NOx purification catalyst 33. That is, by enriching the air-fuel ratio (exhaust air-fuel ratio) of the exhaust discharged from the engine 1, the concentration of the reducing agent in the exhaust flowing into the NOx purification catalyst 33 becomes higher than the oxygen concentration, and reduction is performed. The

The NOx purification in the NOx purification catalyst 33 will be described.
First, when the engine air-fuel ratio is set leaner than the stoichiometric ratio and so-called lean burn operation is performed, the concentration of the reducing agent in the exhaust gas flowing into the NOx purification catalyst 33 becomes lower than the oxygen concentration. As a result, nitrogen monoxide (NO) and oxygen (O ₂ ) in the exhaust gas react with each other by the action of the catalyst, and are adsorbed as NO ₂ on the ceria or ceria-based composite oxide. In addition, carbon monoxide (CO) that has not reacted with oxygen is also adsorbed to ceria or ceria-based composite oxide.

Next, so-called rich operation is performed in which the engine air-fuel ratio is set to be richer than the stoichiometric ratio, and the exhaust air-fuel ratio is enriched. That is, when reduction is performed to make the reducing agent concentration in the exhaust gas higher than the oxygen concentration, carbon monoxide (CO) in the exhaust gas reacts with water (H ₂ O), and carbon dioxide (CO ₂ ) and hydrogen ( H ₂ ) is generated, and hydrocarbons (HC) in the exhaust gas react with water to generate hydrogen together with carbon monoxide (CO) and carbon dioxide (CO ₂ ). Furthermore, NOx contained in the exhaust gas, NOx (NO, NO ₂ ) adsorbed on ceria or ceria-based complex oxide (and platinum), and the produced hydrogen react with each other by the action of the catalyst, and ammonia (NH ₃ ) and water are produced. Further, the ammonia generated here is adsorbed on the zeolite in the form of ammonium ions (NH ₄ ⁺ ).

Next, when lean burn operation is performed in which the engine air-fuel ratio is set leaner than the stoichiometric ratio, and the reducing agent concentration in the exhaust gas flowing into the NOx purification catalyst 33 is set lower than the oxygen concentration, ceria or ceria NOx is adsorbed on the system complex oxide. Further, when ammonium ions are adsorbed on the zeolite, NOx and oxygen in the exhaust gas react with ammonia to generate nitrogen (N ₂ ) and water.

  Thus, according to the NOx purification catalyst 33, ammonia generated during the supply of the reducing agent is adsorbed by the zeolite, and the ammonia adsorbed during the lean burn operation reacts with NOx, so that the NOx purification can be performed efficiently. Can do.

Further, on the upstream side of the NOx purification catalyst 33 in the exhaust pipe 4, a fuel reformer 50 that reforms the fuel gas to produce a reformed gas containing hydrogen (H ₂ ) and carbon monoxide (CO). Is connected. The fuel reformer 50 supplies the produced reformed gas as a reducing gas into the exhaust pipe 4 from the inlet 14 formed on the upstream side of the NOx purification catalyst 33 in the exhaust pipe 4.

  The fuel reformer 50 is provided in the gas passage 51, a gas passage 51 whose one end is connected to the exhaust pipe 4, a fuel gas supply device 52 that supplies fuel gas from the other end of the gas passage 51, and the gas passage 51. And a reforming catalyst 53 as a reforming catalyst.

  The fuel gas supply device 52 mixes fuel stored in the fuel tank and air supplied by the compressor at a predetermined ratio to produce fuel gas, and supplies the fuel gas to the gas passage 51. The fuel gas supply device 52 is connected to the ECU 40, and the fuel gas supply amount and the mixing ratio thereof are controlled by the ECU 40. Further, by controlling the supply amount of the fuel gas, the supply amount GRG of the reducing gas supplied to the exhaust pipe 4 (the amount of reducing gas supplied into the exhaust pipe 4 per unit time) can be controlled. It is possible.

  The reforming catalyst 53 contains rhodium and ceria. The reforming catalyst 53 is a catalyst that reforms the fuel gas supplied from the fuel gas supply device 52 to produce a reformed gas containing hydrogen, carbon monoxide, and hydrocarbons. More specifically, the reforming catalyst 53 has a pressure higher than atmospheric pressure due to a partial oxidation reaction between the hydrocarbon fuel constituting the fuel gas and air, and has a volume ratio of carbon monoxide higher than hydrogen. A reformed gas containing a large amount of is produced. Further, as described above, the partial oxidation reaction is an exothermic reaction. Thus, the fuel reformer 50 can supply reducing gas having a temperature higher than that of the exhaust gas in the vicinity of the inlet 14 in the exhaust pipe 4 into the exhaust pipe 4.

  The reforming catalyst 53 is connected to a heater (not shown) including a glow plug, a spark plug, and the like, so that the reforming catalyst 53 can be heated when the fuel reformer 50 is started. It is possible. The fuel reformer 50 is provided separately from the exhaust pipe 4. That is, the fuel gas supply device 52 and the reforming catalyst 53 of the fuel reformer 50 are not provided in the exhaust pipe 4.

The ECU 40 includes a crank angle position sensor (not shown) for detecting the rotation angle of the crankshaft of the engine 1, an accelerator sensor (not shown) for detecting an accelerator pedal depression amount AP of a vehicle driven by the engine 1, An air flow meter 21 for detecting the intake air amount GA of the engine 1 (the amount of air newly sucked into the engine 1 per unit time), an exhaust temperature sensor 22 for detecting the temperature TE of the exhaust gas flowing into the NOx purification catalyst 33, and The UEGO sensor 23 detects the oxygen concentration of the exhaust gas flowing into the NOx purification catalyst 33, that is, the exhaust air-fuel ratio AF, and the NOx concentrations DNU and DND of the exhaust on the upstream side and the downstream side of the NOx purification catalyst 33 in the exhaust pipe 4 are detected. An upstream NOx sensor 27 and a downstream NOx sensor 28 are connected, and a detection signal of these sensors is an ECU 40. It is supplied.
Here, the rotational speed NE of the engine 1 is calculated by the ECU 40 based on the output of the crank angle position sensor. Further, the generated torque TRQ of the engine 1 is calculated by the ECU 40 based on the fuel injection amount of the fuel injection valve determined according to the depression amount AP of an accelerator pedal (not shown).

  The ECU 40 shapes an input signal waveform from various sensors, corrects a voltage level to a predetermined level, converts an analog signal value into a digital signal value, and a central processing unit (hereinafter, “ CPU ”). In addition, the ECU 40 is a storage circuit that stores various calculation programs executed by the CPU, calculation results, and the like, a fuel reformer 50, a throttle valve 9, an EGR valve 13, a supercharger 8, and a fuel injection of the engine 1. An output circuit for outputting a control signal to a valve or the like.

  The engine 1 is normally operated with the engine air-fuel ratio set to a lean side with respect to the stoichiometric ratio, and rich control for setting the engine air-fuel ratio to the rich side with respect to the stoichiometric ratio is periodically performed.

FIG. 2 is a flowchart showing a procedure of enrichment control by the ECU. As shown in FIG. 2, the enrichment control of the present embodiment includes an H ₂ enrichment control that enriches the exhaust air-fuel ratio by supplying a reducing gas produced by the fuel reformer to the exhaust pipe, and a reducing gas. The normal enrichment control for enriching the exhaust air / fuel ratio by enriching the engine air / fuel ratio without supplying the engine can be selectively executed according to a predetermined condition.

In step S1, the NOx purification rate CE in the NOx purification catalyst is estimated, and it is determined whether or not the NOx purification rate CE is smaller than a predetermined first purification rate determination value CEATH. If this determination is YES, the process proceeds to step S6, and if NO, the process proceeds to step S2. Here, the NOx purification rate CE is calculated based on the NOx concentrations DNU and DND of the exhaust on the upstream and downstream sides of the NOx purification catalyst detected by the upstream NOx sensor and the downstream NOx sensor.
In step S2, it is determined whether or not the NOx purification rate CE is smaller than a predetermined second purification rate determination value CEBTH. If this determination is YES, the process proceeds to step S3, and if this determination is NO, the process ends. Here, the second purification rate determination value CEBTH is set to a value larger than the first purification rate determination value CEATH.

In step S3, it is determined whether or not the exhaust air-fuel ratio can be enriched based on the operating state of the engine. If this determination is YES, the process proceeds to step S4, and if NO, the enrichment control is terminated.
FIG. 3 is a diagram showing an operating state of the engine that can be enriched. The operating state of the engine is indicated by the rotational speed on the horizontal axis and the fuel injection amount on the vertical axis. As shown in FIG. 3, the operating state in which enrichment is possible is limited. In step S3, it is determined by such a control map whether or not the engine operating state is in a state where enrichment is possible.

Returning to FIG. 2, in step S4, the temperature TLNC of the NOx purification catalyst is estimated based on the exhaust temperature TE detected by the exhaust temperature sensor, and the flow rate determination value GETH is determined based on the catalyst temperature TLNC. Move on to S5.
In step S5, the flow rate GE of the exhaust gas flowing through the exhaust pipe is estimated based on the intake air amount GA detected by the air flow meter, and it is determined whether or not the exhaust flow rate GE is equal to or higher than the flow rate determination value GETH. If this determination is YES, the process proceeds to step S6 and the H ₂ enrichment control is executed. If NO, the process proceeds to step S8 and the normal enrichment control is executed.

FIG. 4 is a diagram showing the relationship between the catalyst temperature TLNC and the flow rate determination value GETH. In FIG. 4, the horizontal axis represents the catalyst temperature TLNC, and the vertical axis represents the exhaust gas flow rate GE.
Flow rate determination value GETH, depending on the exhaust flow rate GE, is intended to determine whether to perform a normal enrichment control whether to perform and H ₂ enrichment control, and H ₂ enrichment control in the case of GE ≧ GETH And when the GE <GETH, the normal enrichment control is executed. Further, the flow rate determination value GETH increases substantially in proportion to the catalyst temperature TLNC.

Returning to FIG. 2, in step S6, the target value HAFTV of the exhaust air-fuel ratio at the time of H ₂ enrichment control is determined based on the exhaust flow rate GE, and the process proceeds to step S7.
In step S7, H ₂ enrichment control is executed in accordance with the exhaust air / fuel ratio target value HAFTV. More specifically, the reducing gas is supplied into the exhaust pipe by the fuel reformer and the reducing gas supply amount GRG is adjusted so that the exhaust air-fuel ratio AF detected by the UEGO sensor becomes the exhaust air-fuel ratio target value HAFTV. The exhaust air / fuel ratio is enriched so that

In step S8, the target value NAFTV of the exhaust air-fuel ratio at the time of normal enrichment control is determined based on the exhaust flow rate GE, and the process proceeds to step S9. Here, the exhaust air / fuel ratio target value NAFTV during normal enrichment control is set to a value different from the exhaust air / fuel ratio target value HAFTV during H2 enrichment control.
In step S9, normal enrichment control is executed in accordance with the exhaust air / fuel ratio target value NAFTV. More specifically, by making the engine air-fuel ratio rich, the exhaust air-fuel ratio is made rich so that the exhaust air-fuel ratio AF detected by the UEGO sensor matches the exhaust air-fuel ratio target value HAFTV.

As described above in detail, according to the present embodiment, when the exhaust air-fuel ratio is enriched, the exhaust air-fuel ratio is enriched while reducing gas containing hydrogen and carbon monoxide is supplied into the exhaust pipe 4. Whether to enrich the exhaust air-fuel ratio without supplying this reducing gas is selected according to a predetermined condition. Thereby, for example, even when the reduction reaction in the NOx purification catalyst 33 is unlikely to occur, the exhaust air-fuel ratio is made rich while supplying a reducing gas containing hydrogen that has a higher reaction rate than hydrocarbons. Thus, the reduction reaction in the NOx purification catalyst 33 can be promoted to prevent NOx from flowing out without being reduced.
Further, by supplying the reducing gas into the exhaust pipe 4, the exhaust air / fuel ratio of the exhaust gas flowing into the NOx purification catalyst can be made rich while the engine air / fuel ratio is made lean and combustion in the internal combustion engine is kept in an optimal state. it can. Therefore, the NOx purification performance can be improved regardless of the operating state of the internal combustion engine. Also, since the engine air-fuel ratio can be kept lean, oil dilution occurs or the combustion of the internal combustion engine does not occur as in the case of changing the exhaust air-fuel ratio by supplying fuel by post injection or the like. There is no instability.
Further, by providing the fuel reformer 50 for producing reducing gas separately from the exhaust pipe 4, the reducing performance can be always reduced with optimum efficiency regardless of the operating state of the engine 1 and the oxygen concentration and water vapor concentration of the exhaust gas. Gas can be produced and this reducing gas can be supplied into the exhaust pipe 4. Further, by providing the fuel reformer 50 separately from the exhaust pipe 4, it is possible to supply reducing gas as required, such as the operating state of the engine 1.
On the other hand, when the fuel reformer 50 is provided in the exhaust pipe 4, it is necessary to make the fuel reformer 50 large so that the fuel reformer 50 can be operated without affecting the exhaust components, temperature, and flow velocity. According to the present embodiment, by providing the fuel reformer 50 separately from the exhaust pipe 4, a stable operation can be performed without increasing the size of the apparatus. Further, by providing the fuel reformer 50 separately from the exhaust pipe 4, the reforming catalyst 53 provided in the fuel reformer 50 is activated at an early stage by controlling the system different from the control of the engine 1. It is also possible.
In addition, the reduction reaction rate in the NOx purification catalyst 33 can be improved by supplying a reducing gas containing hydrogen. Thereby, when the exhaust air-fuel ratio is made rich, NOx adsorbed or stored in the NOx purification catalyst can be prevented from being desorbed without being reduced.

Further, according to the present embodiment, the NOx purification rate CE of the NOx purification catalyst 33 is estimated, and when the NOx purification rate CE is smaller than the first purification rate determination value CEATH, exhaust gas is exhausted by supplying reducing gas. Enrich the fuel ratio. As a result, the amount of NOx adsorbed or stored in the NOx purification catalyst 33 increases, and the amount of NOx that flows out without being reduced when enriched can be reduced.
Further, according to the present embodiment, the exhaust flow rate GE is estimated, and when the exhaust flow rate GE is equal to or higher than the flow rate determination value GETH, the exhaust air-fuel ratio is enriched by supplying the reducing gas. That is, when the exhaust gas flow rate is large, NOx occluded or adsorbed by the NOx purification catalyst 33 is likely to be desorbed. Even in such a case, a reducing gas is supplied to cause a reduction reaction in the NOx purification catalyst 33. By promoting, it is possible to prevent NOx from flowing out downstream without being reduced.

Further, according to this embodiment, the temperature TLNC of the NOx purification catalyst 33 is estimated, and the above-described flow rate determination value GETH is determined according to this temperature TLNC. By the way, the reduction reaction in the NOx purification catalyst 33 is less likely to occur as the temperature of the NOx purification catalyst 33 becomes lower, so that NOx is likely to flow out without being reduced. According to the present embodiment, the NOx purification performance can be improved more efficiently by determining the flow rate determination value GETH in consideration of the temperature TLNC of the NOx purification catalyst 33.
Further, according to the present embodiment, the exhaust air / fuel ratio target values HAFTV and NAFTV are determined based on the estimated exhaust gas flow rate GE, and the exhaust air / fuel ratio AF is enriched so as to coincide with the exhaust air / fuel ratio target values HAFTV and NAFTV. To do. In this way, by adjusting the exhaust air-fuel ratio at the time of enrichment according to the exhaust flow rate, it is possible to efficiently prevent NOx from flowing out from the NOx purification catalyst 33 without being reduced.
Further, according to the present embodiment, when the exhaust air-fuel ratio is enriched by supplying the reducing gas, the reducing air-fuel ratio AF is adjusted so that the exhaust air-fuel ratio AF matches the determined target values HAFTV and NAFTV. The supply amount GRG is adjusted. As a result, the exhaust gas having the optimum exhaust air / fuel ratio can always flow into the NOx purification catalyst 33.

  In the present embodiment, the ECU 40 performs enrichment means, first enrichment control means, second enrichment control means, part of the purification rate estimation means, part of exhaust flow rate estimation means, part of catalyst temperature estimation means, flow rate The determination value determining means and the exhaust air / fuel ratio target value determining means are configured. Specifically, the means relating to steps S1 to S9 in FIG. 2 corresponds to the enrichment means, the means relating to step S7 corresponds to the first enrichment control means, and the means relating to step S9 is the second enrichment control. The means according to step S1, the upstream NOx sensor 27, and the downstream NOx sensor 28 correspond to the purification rate estimation means, the means according to step S5 and the air flow meter 21 correspond to the exhaust flow rate estimation means, and step S4. And the exhaust temperature sensor 22 correspond to the catalyst temperature estimating means, the means related to step S4 corresponds to the flow rate determination value determining means, and the means related to steps S6 and S8 correspond to the exhaust air / fuel ratio target value determining means. .

  Next, a NOx purification evaluation test for verifying the effect of supplying the reducing gas to the exhaust pipe as in the above embodiment will be described with reference to FIGS.

[NOx purification performance evaluation test method]
FIG. 5 is a schematic diagram showing the configuration of the test apparatus 80 for the NOx purification performance evaluation test.
The test apparatus 80 includes a supply device 81 for supplying a model gas with a predetermined composition, a heater 83 in which an adsorbent 84 is provided, a gas analyzer 85 for analyzing the model gas, and a data acquisition computer 86. , Including.

The supply device 81 supplies a model gas composed of N ₂ , CO ₂ , O ₂ , H ₂ O, CO, HC, NOx, and H ₂ to the heater 83. The supply device 81 can adjust the flow rate of each component of the model gas.
The heater 83 includes a reactor 82 and an adsorbent 84 therein, and heats the reactor 82 and the adsorbent 84. The reactor 82 mixes the model gas supplied from the supply device 81 and supplies it to the adsorbent 84.

The adsorbent 84 is composed of a three-way catalyst and a NOx purification catalyst.
More specifically, in the three-way catalyst, a slurry having a composition described later is stirred and mixed with a water-based medium in a ball mill, and the slurry is coated on a support made of Fe—Cr—Al alloy. Use the one prepared by drying and baking at 2 ° C. for 2 hours. The composition of the three-way catalyst is Pt = 2.4, Rh = 1.2, Pd = 6.0, CeO ₂ = 50, Al ₂ O ₃ = 150, and binder = 10.
Since the same NOx purification catalyst 33 (see FIG. 1 described above) described in the above embodiment is used as the NOx purification catalyst, the description thereof is omitted.

In the heater 83, the gas analyzer 85 is supplied from the reactor 82 to the adsorbent 84 and measures the NOx concentration of the model gas that has passed through the adsorbent 84. The data acquisition computer 86 processes the data related to the NOx concentration and calculates the NOx purification rate for each temperature. The NOx purification rate is calculated based on the following equation.
NOx purification rate [%] = (Cin−Cout) / Cin × 100
Here, Cin is the NOx concentration of the model gas at the inlet of the adsorbent 84, and Cout is the NOx concentration of the model gas at the outlet of the adsorbent 84. The NOx concentration of the model gas was measured by the chemical luminescence method.

In this evaluation test, in the test apparatus 80 configured as described above, while supplying the model gas, the model gas passed through the adsorbent 84 while being heated from 50 ° C. to 450 ° C. at 20 ° C./min. The NOx purification rate of gas was measured.
As the model gas, a model gas having a lean atmosphere composition and a model gas having a rich atmosphere composition were alternately supplied over 55 seconds and 5 seconds, respectively.

[Example]
In the model gas of the example, the following gas is modeled on the one obtained by adding the reducing gas (including carbon monoxide, hydrogen, and hydrocarbon) produced by the fuel reformer according to the above embodiment to the exhaust gas. A model gas having a composition was used.
Lean atmosphere NO: 90ppm
CO: 6000 ppm
HC (propylene): 500 ppmC (C ₃ H ₆ )
O ₂ : 6%
CO ₂ : 6%
H ₂ O: 7%
N ₂ : Balance gas H ₂ : 5000 ppm
SV = 50000h-1
Rich atmosphere NO: 90ppm
CO: 2.1%
HC (propylene): 500 ppmC (C ₃ H ₆ )
O ₂ : 0%
CO ₂ : 6%
H ₂ O: 7%
N ₂ : Balance gas H ₂ : 6000 ppm
SV = 50000h-1

[Comparative example]
As a model gas of the comparative example, a model gas having the following composition was used as a model of exhaust gas to which the above-described reducing gas was not added.
Lean atmosphere NO: 90ppm
CO: 1000 ppm
HC (propylene): 500 ppmC (C ₃ H ₆ )
O ₂ : 6%
CO ₂ : 6%
H ₂ O: 7%
N ₂ : Balance gas H ₂ : 0 ppm
SV = 50000h-1
Rich atmosphere NO: 90ppm
CO 2%
HC (propylene): 500 ppmC (C ₃ H ₆ )
O ₂ : 0%
CO ₂ : 6%
H ₂ O: 7%
H ₂ : 0 ppm
N ₂ : Balance gas H ₂ : 0 ppm
SV = 50000h-1

[Test results]
FIG. 6 is a diagram showing test results of Examples and Comparative Examples. In FIG. 6, the horizontal axis indicates the temperature of the model gas, and the vertical axis indicates the NOx purification rate. Moreover, a black circle shows the relationship between the gas temperature and the NOx purification rate in the example, and a white circle shows the relationship between the gas temperature and the NOx purification rate in the comparative example.
Comparing the NOx purification rate of the example and the NOx purification rate of the comparative example, the NOx purification rate of the example is substantially constant over the entire temperature range, whereas the comparative example has a particularly small NOx purification rate in the low temperature range. It has become. Therefore, it was verified that the NOx purification performance in the low temperature range can be improved by adding the reducing gas containing carbon monoxide and hydrogen as in the embodiment.

The present invention is not limited to the embodiment described above, and various modifications can be made.
In the above embodiment, the temperature TLNC of the NOx purification catalyst 33 is estimated based on the exhaust gas temperature TE detected by the exhaust gas temperature sensor 22, but the present invention is not limited to this and may be detected directly. Moreover, in the said embodiment, although the flow volume GE of the exhaust_gas | exhaustion which distribute | circulates the exhaust pipe 4 was estimated based on the intake air amount GA detected by the airflow meter 21, you may detect not only this but directly.

  In the above embodiment, the target values HAFTV and NAFTV of the exhaust air / fuel ratio are determined based on the exhaust flow rate GE in steps S6 and S8, but the present invention is not limited to this. For example, these target values may be determined based on the temperature of the NOx purification catalyst. Even in such a case, the same effects as in the above embodiment can be obtained.

  Moreover, although the example which applied this invention to the diesel internal combustion engine was shown in the said embodiment, this invention is applicable also to a gasoline internal combustion engine. The present invention can also be applied to an exhaust emission control device such as a marine vessel propulsion engine such as an outboard motor having a vertical crankshaft.

1 is a diagram illustrating a configuration of an internal combustion engine and an exhaust purification device thereof according to an embodiment of the present invention. It is a flowchart which shows the procedure of the enrichment control by ECU which concerns on the said embodiment. It is a figure which shows the driving | running state of the engine which can be enriched based on the said embodiment. It is a figure which shows the relationship between the catalyst temperature and flow volume determination value which concern on the said embodiment. It is the schematic which shows the structure of the test apparatus of a NOx purification performance evaluation test. It is a figure which shows the test result of an Example and a comparative example.

Explanation of symbols

1. Engine (internal combustion engine)
4 ... Exhaust pipe (exhaust passage)
5. Exhaust manifold (exhaust passage)
14 ... Inlet 21 ... Air flow meter (exhaust flow rate estimating means)
22 ... Exhaust temperature sensor (catalyst temperature estimation means)
27 ... Upstream NOx sensor (purification rate estimating means)
28. Downstream NOx sensor (purification rate estimating means)
33 ... NOx purification catalyst 40 ... Electronic control unit (riching means, first enrichment control means, second enrichment control means, purification rate estimation means, exhaust flow rate estimation means, catalyst temperature estimation means, flow rate judgment value determination means, Exhaust air / fuel ratio target value determining means)
50 ... Fuel reformer (fuel reformer)

Claims (6)

The exhaust air-fuel ratio is provided in the exhaust passage of the internal combustion engine and adsorbs or occludes NOx in the exhaust when the exhaust air-fuel ratio is made lean, with the air-fuel ratio of the exhaust gas flowing through the exhaust passage being made lean. In an exhaust gas purification apparatus for an internal combustion engine comprising a NOx purification catalyst that reduces the adsorbed or occluded NOx when the gas is made rich.
Provided separately from the exhaust passage, reforming the fuel to produce a reducing gas containing hydrogen and carbon monoxide, the reducing gas from the upstream side of the NOx purification catalyst in the exhaust passage, A fuel reformer supplied into the exhaust passage;
Enriching means for enriching the exhaust air-fuel ratio,
The enrichment means is:
First enrichment control means for enriching the exhaust air-fuel ratio while supplying reducing gas into the exhaust passage by the fuel reformer;
A second enrichment control means for enriching the exhaust air-fuel ratio without supplying reducing gas into the exhaust passage by the fuel reformer, and according to a predetermined condition, the exhaust air-fuel ratio The exhaust gas purifying apparatus for an internal combustion engine, wherein the first enrichment control means or the second enrichment control means is selected. A purification rate estimating means for estimating a NOx purification rate of the NOx purification catalyst;
The enrichment means enriches the exhaust air / fuel ratio by the first enrichment control means when the NOx purification rate estimated by the purification rate estimation means is smaller than a predetermined purification rate determination value. The exhaust emission control device for an internal combustion engine according to claim 1. Exhaust flow rate estimation means for estimating or detecting the exhaust flow rate, using the flow rate of exhaust gas flowing through the exhaust passage as an exhaust flow rate,
The enrichment means enriches the exhaust air / fuel ratio by the first enrichment control means when the exhaust flow rate estimated or detected by the exhaust flow rate estimation means is greater than or equal to a predetermined flow rate determination value. The exhaust emission control device for an internal combustion engine according to claim 1 or 2, characterized in that Catalyst temperature estimating means for estimating or detecting the temperature of the NOx purification catalyst as a catalyst temperature;
The internal combustion engine according to claim 3, further comprising a flow rate determination value determining unit that determines the predetermined flow rate determination value based on the catalyst temperature estimated or detected by the catalyst temperature estimation unit. Exhaust purification device. An exhaust flow rate estimating means for estimating or detecting the exhaust flow rate, using the flow rate of the exhaust gas flowing through the exhaust passage as an exhaust flow rate;
Catalyst temperature estimating means for estimating or detecting the temperature of the NOx purification catalyst as a catalyst temperature;
Based on the exhaust flow rate estimated or detected by the exhaust flow rate estimation means or the catalyst temperature estimated or detected by the catalyst temperature estimation means, the first enrichment control means and the second enrichment control means An exhaust air / fuel ratio target value determining means for determining a target value of the exhaust air / fuel ratio when enriching the exhaust air / fuel ratio,
The first enrichment control unit and the second enrichment control unit respectively enrich the exhaust air / fuel ratio so as to coincide with the target value determined by the exhaust air / fuel ratio target value determination unit. The exhaust emission control device for an internal combustion engine according to claim 1.   The second enrichment control means adjusts the supply amount of the reducing gas supplied from the fuel reformer so that the exhaust air-fuel ratio becomes the target value determined by the exhaust air-fuel ratio target value determination means. 6. The exhaust emission control device for an internal combustion engine according to claim 5, wherein enrichment is performed so as to match.

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