JP2009167852A - Exhaust emission control system of hydrogen engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To properly control exhaust emission by restraining the emission of NOx when starting a hydrogen engine in a cold state. <P>SOLUTION: In this hydrogen engine using hydrogen as fuel, while arranging a water adsorbent 24 in an exhaust pipe 14, a bypass pipe 26 is formed on the downstream side of the water adsorbent 24, and an NOx adsorbent 28 is arranged in the bypass pipe 26. A switching valve 30 for switching a flow passage between the bypass pipe 26 and the exhaust pipe 14 is arranged in a branch 14A of the bypass pipe 26. When starring the engine in the cold state, the switching valve 30 is controlled so that exhaust gas flows in the bypass pipe 26. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification system for a hydrogen engine, and more particularly to an exhaust gas purification system that suppresses NOx emission during cold start.

  In a gasoline engine, a three-way catalyst that purifies exhaust gas containing harmful components such as HC, NOx, and CO is provided in the exhaust pipe. However, the catalyst is not activated at low temperatures, and the exhaust gas is sufficiently purified at cold start. Can not. Therefore, an exhaust gas purification device provided with an HC adsorbent and a NOx adsorbent has been proposed (see Patent Document 1).

  There, a bypass pipe branching from the main exhaust pipe is formed, and an HC adsorbent and a NOx adsorbent for adsorbing HC and NOx, respectively, are provided on the bypass pipe. At the time of cold start, the exhaust gas is caused to flow into the bypass pipe by the switching valve, and HC and NOx are respectively adsorbed by the corresponding adsorbents. After the warm-up is completed, NOx, HC and moisture adsorbed on each adsorbent are purged by circulating the exhaust gas to the intake pipe.

  Further, since moisture in the exhaust gas has stronger adsorption performance than NOx and HC, a water adsorbent is provided on the bypass pipe upstream of the HC adsorbent and NOx adsorbent. As a result, moisture in the exhaust gas is removed in advance at the time of cold start, and moisture is prevented from adsorbing to the HC and NOx adsorbent.

By the way, in the case of a hydrogen engine using hydrogen as fuel, HC and CO are not generated by combustion, but NOx is generated as in the gasoline engine. Therefore, a catalyst for purifying NOx is provided (see, for example, Patent Document 2). However, during cold start, since the catalyst is not activated, NOx is not sufficiently purified, and harmful exhaust gas is discharged to the outside.
JP 2006-342700 A Japanese Patent Laid-Open No. 6-200749

  Also for a hydrogen engine, it is desirable to adsorb NOx to the NOx adsorbent before activating the catalyst in order to suppress NOx emission during cold start. In order to prevent moisture from adsorbing to the NOx adsorbent, it is conceivable to provide a water adsorbent. However, in the configuration described in Patent Document 1, that is, the configuration in which the water adsorbent and the NOx adsorbent are provided in the bypass pipe, the adsorption and desorption of NOx and moisture cannot be appropriately performed.

  In the configuration of Patent Document 1, when the switching valve shuts off the bypass pipe due to catalyst activation, the exhaust gas does not flow through the water adsorbent, and moisture desorption from the water adsorbent does not proceed sufficiently. For this reason, when moisture remaining in the water adsorbent is purged, NOx is easily purged from the NOx adsorbent, and harmful exhaust gas may be discharged. In Patent Document 1, since a purification system based on the characteristics of exhaust gas components in a hydrogen engine and the adsorption and desorption characteristics of each adsorbent based on such exhaust gas components is not configured, NOx purification is appropriately performed. I can't do it.

  The present invention relates to an exhaust gas purification system for an internal combustion engine using hydrogen as fuel, and an exhaust gas purification catalyst for purifying exhaust gas and a water adsorbent for adsorbing moisture in the exhaust gas in an exhaust passage of the internal combustion engine Are provided. Further, a bypass passage that branches off from the exhaust passage and bypasses is provided downstream of the water adsorbent. However, the side closer to the internal combustion engine in the exhaust passage is the upstream side, and the opposite is the downstream side. The bypass passage is provided with a NOx adsorbing material that adsorbs NOx, and is further provided with a switching valve that switches the exhaust gas flow path between the exhaust passage and the bypass passage. The exhaust gas purification catalyst is constituted by a catalyst (for example, a three-way catalyst) having a NOx purification action, and the NOx adsorbent is constituted by, for example, zeolite, metal ion exchange zeolite, or the like. The water adsorbent may also be composed of zeolite or the like.

  The exhaust gas purification system of the present invention includes exhaust gas purification control means for controlling the switching valve, and during cold start, the exhaust gas purification control means controls the switching valve so that the exhaust gas flows into the bypass passage. . Here, the cold start is an engine start in a state where the engine is not warmed up, and represents a start in a state where the catalyst is not activated and the exhaust gas purification performance cannot be normally exhibited. Whether or not the engine is cold-started may be detected based on the temperature of the engine cooling water, and the exhaust gas purification control means may detect the exhaust gas if the temperature of the engine cooling water does not reach a predetermined temperature when the engine is started. The switching valve is controlled so as to flow through the bypass passage.

  At the time of cold start, exhaust gas flows into the bypass passage, and NOx that has not been purified is adsorbed by the NOx adsorbent. As a result, exhaust gas containing NOx is suppressed. In addition, since the water adsorbent is provided upstream of the bypass passage of the exhaust passage, moisture contained in the exhaust gas is adsorbed by the water adsorbent, and moisture adsorption on the NOx adsorbent is suppressed. As a result, the NOx adsorption performance of the NOx adsorbent can be enhanced, NOx that is difficult to adsorb can be adsorbed reliably, and NOx emission can be suppressed. When the catalyst is activated and the exhaust gas purification action is in a normal functioning state, the exhaust gas purification control means may switch the switching valve to flow the exhaust gas to the exhaust passage.

  In the present invention, the water adsorbent is provided in the exhaust passage (not the bypass passage) in consideration of the exhaust gas characteristics generated by the hydrogen fuel. Therefore, the exhaust gas always passes through the water adsorbent regardless of the state of the switching valve. As a result, the frequency of desorption of the adsorbed water is improved, and the adsorbed water can be desorbed early and discharged into the atmosphere. As a result, the influence of moisture adsorption on the NOx adsorbent can be suppressed, and the NOx adsorbent can sufficiently exhibit the NOx adsorption performance. The water adsorbent usually has HC adsorption performance. Therefore, in the case of an internal combustion engine using gasoline fuel, if a water adsorbent is provided in the exhaust passage, HC is adsorbed and desorbed, and HC is discharged to the outside. However, since no HC is generated in an internal combustion engine of hydrogen fuel, such a problem does not occur.

  In order to purge the adsorbed NOx, it is desirable to provide a purge passage for circulating exhaust gas upstream of the catalyst in the engine intake system or exhaust system. The exhaust gas purification catalyst is preferably provided upstream of the bypass passage in the exhaust passage. In this case, the water adsorbent is preferably provided on the downstream side of the exhaust gas purification catalyst in order to absorb moisture as much as possible by the exhaust passage itself and not to adsorb moisture more than necessary. As the configuration of the purge passage, for example, a purge passage branched from the bypass passage and communicating with the intake system passage of the internal combustion engine, and a purge valve for opening and closing the purge passage are provided. The exhaust gas purification control means controls the switching valve and the purge valve so that the exhaust gas flows back to the intake system passage through the NOx adsorbent after the temperature of the exhaust gas purification catalyst (catalyst temperature) reaches a predetermined temperature. That's fine.

  Here, the predetermined temperature indicates a lower limit temperature at which the purification action of the catalyst normally works, and the value is determined according to the catalyst characteristics and the like. The catalyst temperature may be detected based on the exhaust gas temperature passing through the catalyst, the gas flow rate, and the like.

  In the present invention, since the water adsorbent is provided on the exhaust passage, when the exhaust gas is recirculated to purge NOx, the exhaust gas passing through the water adsorbent passes through the NOx adsorbent and flows into the purge passage. . Therefore, the exhaust gas having a relatively high moisture concentration passes through the NOx adsorbent, and the purge of NOx adsorbed on the NOx adsorbent becomes easy.

  The NOx adsorbent adsorbs moisture as well as NOx. In order to exhibit NO adsorption performance even at the next cold start, it is desirable to purge the remaining moisture. For this reason, the exhaust gas purification control means sets the switching valve so that the exhaust gas flows into the bypass passage when the engine is operating at an air-fuel ratio leaner than the stoichiometric air-fuel ratio after the catalyst temperature reaches a predetermined temperature. It is good to control. For example, after purging NOx by exhaust gas recirculation, the switching valve is switched so that the exhaust gas flows through the bypass passage, and the operation is switched to lean operation. Alternatively, the switching valve may be controlled so that the exhaust gas flows through the bypass passage in accordance with the timing when the lean operation is performed depending on the operating conditions.

  The adsorption performance of NOx adsorbents decreases with the course of use, and can be divided into permanent performance degradation due to deterioration of the material and the like, and temporary performance degradation due to oxidation degradation (recoverable performance degradation). In the case of temporary performance degradation, the adsorption performance is recovered by exposing the reducing atmosphere to the NOx adsorbent. Therefore, it is desirable to diagnose and recover from the temporary performance degradation.

  Therefore, based on the correlation between the amount of moisture adsorbed on the NOx adsorbent and the amount of NOx adsorbed, and the correlation between the amount of moisture adsorbed on the NOx adsorbent and the amount of NOx adsorbed, It is preferable to provide an adsorption performance determining means for comparing and determining a reference NOx amount according to the amount of moisture that has been made. When the detected NOx amount does not satisfy the reference NOx amount, the exhaust gas purification control means causes the engine to operate at an air / fuel ratio richer than the stoichiometric air / fuel ratio. By performing the rich operation, ammonia generated in the exhaust gas purification catalyst together with hydrogen is supplied to the NOx adsorbent. The exhaust gas having a reducing action of hydrogen and ammonia passes through the NOx adsorbent, so that the oxidized and deteriorated NOx adsorbent is reduced.

  Ammonia has higher adsorption performance than NOx. Therefore, it is preferable that the exhaust gas purification control means operates the engine at an air-fuel ratio richer than the stoichiometric air-fuel ratio when the catalyst temperature does not reach a predetermined temperature when the engine is started. As a result, ammonia produced in the catalyst is reliably adsorbed on the NOx adsorbent, and emission of harmful gas components is suppressed.

  According to the present invention, NOx emission can be suppressed during the cold start of the hydrogen engine, and the exhaust gas can be appropriately purified.

  Below, with reference to drawings, the exhaust gas purification system of the hydrogen engine which is an embodiment of the present invention is explained.

  FIG. 1 is a schematic configuration diagram of an exhaust gas purification system of a hydrogen engine.

  The engine 10 is a spark ignition engine using hydrogen as fuel, and an intake pipe 12 and an exhaust pipe 14 are connected to each other. The air that has entered the intake pipe 12 is mixed with the hydrogen fuel injected from the injector 15 and is taken into the cylinder 16 of the engine 10. The hydrogen fuel is stored, for example, in a tank (not shown) having a hydrogen storage alloy, and is supplied to the injector 15 by delivery (not shown). The hydrogen fuel may be stored as a gas or a liquid.

  The air-fuel mixture sucked into the cylinder 16 is ignited by the spark plug 18 and burned. The combustion gas generated in the cylinder 16 is discharged from the exhaust port 12 </ b> B as exhaust gas and flows downstream of the exhaust pipe 14. The crankshaft (not shown) rotates in conjunction with the vertical movement of the piston 20, and the intake valve 19A and the exhaust valve 19B are respectively connected to the intake port 12A and the exhaust port 12B in accordance with the rotation of the camshaft (not shown). Open and close at a predetermined timing.

  A catalyst 22 and a water adsorbent 24 are provided in the exhaust pipe 14. The catalyst 22 purifies harmful components in the exhaust gas such as NOx, and here is constituted by a three-way catalyst. The water adsorbent 24 is made of a zeolite material having small diameter holes, and adsorbs moisture contained in the exhaust gas.

  On the downstream side of the water adsorbent 24, a bypass pipe 26 is formed which branches from the branch portion 14A of the exhaust pipe 14 and then joins the exhaust pipe 14 again. The bypass pipe 26 is provided with a NOx adsorbent 28 that adsorbs NOx in the exhaust gas, and is composed of, for example, Fe ion exchange zeolite.

  A switching valve 30 for switching the flow path between the exhaust pipe 14 and the bypass pipe 26 is provided at the branch portion 14A of the exhaust pipe 14. Assuming that the portion of the exhaust pipe 14 parallel to the bypass pipe 26 is the main pipe line 14T, the switching valve 30 is operated by an actuator 31 such as a diaphragm, and shuts off the bypass pipe 26 so that the exhaust gas flows through the main pipe line 14T as it is. The exhaust gas is caused to flow into the bypass pipe 26 by blocking the main pipe line 14T.

  A purge pipe 32 that circulates the exhaust gas is branched upstream of the NOx adsorbent 28 in the bypass pipe 26 and communicates with the intake pipe 12. A purge valve 34 that opens and closes the purge pipe 32 is provided in the middle of the purge pipe 32. The purge valve 34 is opened / closed by an actuator (not shown) to allow exhaust gas to pass through or to be shut off.

The ECU 60 including a CPU, RAM, ROM and the like controls the engine 10 and detects signals from the sensors such as the O ₂ sensor 42, the cooling water temperature sensor 43, and the catalyst temperature sensor 44. Then, a control signal is output to the injector 15, the spark plug 18, the throttle valve 46, etc. according to the rotational position of the crankshaft. Further, a control signal for controlling switching of the switching valve 30 and opening / closing of the purge valve 34 is output.

  In order to measure the moisture adsorption amount and the NOx adsorption amount in the NOx adsorbent 28, moisture sensors 52A and 54A and NOx sensors 52B and 54B for detecting the moisture amount and the NOx amount are provided before and after the NOx adsorbent 28, respectively. It has been. An exhaust gas temperature sensor 57 disposed in the bypass pipe 26 detects the temperature of the exhaust gas flowing through the bypass pipe 26.

  FIG. 2 is a view showing a flowchart of the exhaust gas purification control process executed by the ECU 60. FIG. 3 is a view showing an exhaust gas flow path after the operation is started. Exhaust gas purification control processing will be described with reference to FIGS. When the engine 10 is started, the process is started.

  In step S101, it is determined whether or not the engine coolant temperature is equal to or higher than a predetermined temperature T0. When the engine is cold-started, that is, when the engine is started in a state where the engine is not warmed up, the original exhaust gas purification performance is not exhibited because the catalyst is not activated. Whether or not this is a cold start is determined based on the temperature of the engine coolant. Here, when the cooling water temperature detected by the cooling water temperature sensor 43 is not equal to or higher than the predetermined temperature T0, it is regarded as a cold start time.

  If it is determined that the engine coolant temperature is equal to or higher than the set temperature T0, the process proceeds to step S102, and the switching valve 30 is controlled to shut off the bypass pipe 26. As a result, the exhaust gas that has passed through the catalyst 22 and the water adsorbent 24 flows through the main pipeline 14T as it is. Further, the fuel injection amount is controlled so that the stoichiometric operation based on the stoichiometric air-fuel ratio is performed. When step S102 is executed, the process proceeds to step S109.

  On the other hand, when it is determined in step S101 that the coolant temperature is not equal to or higher than the set temperature T0, the process proceeds to step S103, and the switching valve 30 is switched so as to shield the main pipeline 14T. As a result, the exhaust gas that has passed through the catalyst 22 and the water adsorbent 24 flows through the bypass pipe 26 (see FIG. 3A). In step S104, the fuel injection amount is controlled so that the engine is operated at an air / fuel ratio richer than the stoichiometric air / fuel ratio.

  Since the catalyst 22 is not activated at the time of cold start, the exhaust gas flows without purifying NOx, but the NOx component in the exhaust gas is adsorbed when the exhaust gas passes through the bypass pipe 26. The NOx adsorbent 28 adsorbs NOx even when the exhaust gas is at a relatively low temperature. Further, at the time of cold start, a relatively large amount of moisture is contained in the exhaust gas, but the moisture is adsorbed by the water adsorbent 24 provided on the upstream side of the bypass pipe 26.

  Moisture has a strong adsorption action and has a property of being more easily adsorbed to the NOx adsorbent 28 than NOx. However, since water is removed in advance by the water adsorbent 24, NOx adsorption is not hindered. The water adsorbent 24 also adsorbs the HC component in the exhaust gas, but it is not necessary to consider the HC component because HC is not discharged by the hydrogen fuel.

  On the other hand, when the rich operation is performed at the cold start, the amount of exhausted NOx is reduced. Furthermore, in a state where the catalyst 22 has not been sufficiently activated, the exhausted NOx reacts in the catalyst 22 to generate hydrogen and ammonia. Since ammonia has a higher adsorption rate than NOx, the produced ammonia is reliably adsorbed by the NOx adsorbent 28. This further suppresses NOx emissions.

  In step S105, in order to detect the deterioration of the NOx adsorption performance, the amount of moisture and the amount of NOx adsorbed on the NOx adsorbent are detected. The diagnosis of NOx adsorption performance deterioration will be described later.

  In step S106, it is determined whether or not the catalyst temperature is equal to or higher than a predetermined temperature T1. Here, the predetermined temperature T1 indicates a lower limit temperature at which the catalyst is activated and can normally purify harmful components such as NOx. The catalyst temperature sensor 44 detects the temperature of exhaust gas passing through the catalyst 22 and obtains the catalyst temperature (bed temperature) based on the exhaust gas temperature. The exhaust gas continues to flow through the bypass pipe 26 until the predetermined temperature T1 is reached. If it is determined that the catalyst temperature is equal to or higher than the predetermined temperature T1, the process proceeds to step S106.

  In step S107, the rich operation is switched to the stoichiometric operation, and the switching valve 30 is switched so that the exhaust gas flows into the main pipeline 14T. Since the catalyst is activated, the NOx component in the exhaust gas is purified by the catalyst 22. Further, since the exhaust gas passes through the water adsorbent 24 even after the switching valve 30 is switched, the desorption of moisture adsorbed on the water adsorbent 24 proceeds as the temperature of the exhaust gas rises.

  In step S108, the purge valve 34 is opened so that the exhaust gas circulates to the intake pipe 12. As a result, a part of the exhaust gas that has passed through the main pipeline 14T of the exhaust pipe 14 flows into the bypass pipe 26, and then flows into the purge pipe 32 through the NOx adsorbent 28 (see FIG. 3B). As the temperature of the exhaust gas rises, NOx adsorbed on the NOx adsorbent 28 is desorbed, and NOx recirculated to the intake pipe 12 is purified by the active catalyst 22. Further, the EGR functions by opening the purge pipe 34, and the generation of NOx in the cylinder 20 is suppressed. The moisture passes through the exhaust pipe 14 and is discharged into the atmosphere.

  Exhaust gas passing through the NOx adsorbent 28 passes through the water adsorbent 24. Since the moisture concentration in the exhaust gas is relatively high, NOx adsorbed on the NOx adsorbent 28 is easily desorbed due to the effect of moisture adsorption, and NOx can be effectively purged from the NOx adsorbent 28. Regarding the purge period, for example, the purge amount is estimated from the exhaust gas temperature detected by the temperature sensor 57 and the elapsed time after the purge valve is released, and the elapsed time until the purge amount reaches the set amount is defined as the purge period. When the NOx purge period elapses, the purge valve 34 is closed.

  In step S109, based on the detected moisture content and NOx content of the NOx adsorbent, it is determined whether or not the adsorption performance of the NOx adsorbent 28 has deteriorated. In the following, the NOx adsorption performance diagnosis will be described with reference to FIGS.

  First, the NOx adsorption performance will be described. It is known that the NOx adsorption performance decreases with use, and a non-recoverable decrease due to material performance deterioration (permanent performance decrease) and a recoverable decrease (temporary) Performance degradation). Temporary performance degradation is presumed to be oxidative degradation, and the adsorption performance is restored by exposing the NOx adsorbent to a reducing atmosphere.

  FIG. 4 is a diagram showing a correlation with the moisture adsorption amount of the NOx adsorbent 28. When there is no temporary performance deterioration, the NOx adsorption amount and the moisture adsorption amount follow a curve (hereinafter referred to as a reference line) L shown in FIG. This reference line L represents the change in the NOx adsorption amount and the moisture adsorption amount due to permanent deterioration.

  On the other hand, when temporary performance deterioration due to oxidation occurs, the NOx adsorption amount decreases, while the moisture adsorption amount does not change. For example, when a temporary deterioration occurs from a predetermined position P1 on the reference line L, the relationship between the moisture adsorption amount and the NOx adsorption amount shifts to a position P2 that deviates from the reference line L. Therefore, by detecting the moisture amount and NOx amount adsorbed by the NOx adsorbent 28 and determining whether the amount is on the reference line L or deviated, whether or not a temporary performance degradation has occurred. Can be judged.

  In order to perform the adsorption performance diagnosis, as described above, the NOx adsorption amount and the moisture adsorption amount adsorbed on the NOx adsorbent 28 at the time of cold start are detected (S105). The diagnostic condition that the temperature of the exhaust gas is relatively low is satisfied, and the adsorbed NOx is not discharged to the outside. The moisture adsorption amount and NOx adsorption amount are measured as follows.

  FIG. 5 is a graph showing a moisture adsorption amount and a NOx adsorption amount. The vertical axis represents the moisture adsorption amount or NOx adsorption amount per unit time, and the horizontal axis represents time. First, the measurement of the moisture adsorption amount will be described. The moisture sensor 52B on the upstream side of the NOx adsorbent 28 measures the amount of moisture flowing into the NOx adsorbent 28, and the moisture sensor 54B on the downstream side measures the amount of moisture flowing out of the NOx adsorbent 28. Then, the difference value is detected as a moisture adsorption amount per unit time. When the moisture adsorption amount becomes saturated, the difference value becomes zero, and by integrating the difference value until it becomes zero (by calculating the area of the hatched portion A), the moisture adsorption amount x1 is Desired.

  Regarding the NOx adsorption amount, the difference between the NOx inflow amount detected by the NOx sensor 52A upstream of the NOx adsorbent 28 and the NOx outflow amount detected by the downstream NOx sensor 54A is obtained, and the difference value is integrated. As a result, the NOx adsorption amount y1 is obtained.

  By the way, the NOx adsorbed by the NOx adsorbent 28 and saturated is desorbed by the action of moisture adsorbed later. For this reason, the output difference (adsorption amount per unit time) between the NOx sensor 52A and the NOx sensor 54 becomes a positive value for a while after the start of adsorption, but when NOx begins to desorb, the output difference becomes a negative value. . Considering such NOx adsorption characteristics, the NOx adsorption amount until the output difference is reversed from positive to negative is integrated (the area of the hatched portion B is calculated), and this integrated value is determined as the NOx adsorption amount y1.

  Data relating to the reference line L shown in FIG. 5 is stored in the ROM of the ECU 60 in advance. When the NOx adsorption amount y1 and the moisture adsorption amount x1 are measured, a NOx adsorption amount (hereinafter referred to as a reference NOx adsorption amount) y0 corresponding to the moisture adsorption amount x1 is detected based on the reference line L. Then, a difference Δy (= y0−y1) between the detected NOx adsorption amount y1 and the reference NOx adsorption amount y0 is obtained.

  After the difference Δy is thus obtained, it is determined whether or not the difference Δy is zero (step S109 in FIG. 2). When the difference Δy is zero, it is determined that there is no temporary (recoverable) adsorption performance deterioration. On the other hand, when the difference Δy is not zero, it is determined that there is a temporary performance decrease, and it is considered that the NOx adsorption performance decrease progresses as the difference Δy increases.

  If it is determined in step S109 in FIG. 2 that a temporary performance deterioration has occurred, the process proceeds to step S110, and the fuel injection amount is controlled so that the engine is operated with an air-fuel ratio richer than the stoichiometric air-fuel ratio. The rich operation is continued for a predetermined period, and the predetermined period is controlled to be longer as the difference Δy is larger. Data indicating the relationship between the difference Δy and the rich operation period is stored in the ROM in advance.

  When the air-fuel ratio is made rich, the catalyst 22 generates a large amount of hydrogen and ammonia having a relatively strong reducing action. When the exhaust gas containing a large amount of hydrogen and ammonia components passes through the NOx adsorbent, the NOx adsorbent 28 is exposed to a reducing atmosphere, and the adsorption performance of the NOx adsorbent 28 that has temporarily deteriorated is recovered. On the other hand, if the temporary performance has not deteriorated, the process proceeds from step S109 to step S111 as it is.

  In step S111, the switching valve 30 is controlled so that the exhaust gas flows into the bypass pipe 26 in order to desorb the moisture adsorbed on the NOx adsorbent, and at the same time, the engine is controlled by the air-fuel ratio leaner than the stoichiometric air-fuel ratio. The fuel injection amount is controlled so as to operate (see FIG. 3C).

  As described above, since water has higher adsorption performance than NOx, the NOx adsorbent 28 adsorbs moisture together with NOx. In order to purge the adsorbed moisture, exhaust gas is passed through the bypass pipe 26 after the NOx purge. When the lean operation is performed, the moisture concentration in the exhaust gas becomes relatively low, so that the moisture in the NOx adsorbent 28 is easily desorbed. The moisture purge process is performed only for a predetermined period. When the moisture purge period has passed, the lean valve is changed to the stoichiometric operation, and the switching valve 30 is controlled so that the exhaust gas flows through the main line 14T.

  In this example, the lean operation is actively switched to desorb the moisture from the NOx adsorbent, but the moisture purge is performed in accordance with the timing when the lean operation is performed for other purposes such as fuel cut performed during engine deceleration. May be. In this case, the switching valve 30 may be switched so that the exhaust gas flows to the bypass side in accordance with the lean operation, and the exhaust gas may be switched to flow to the main pipe line 14T after the moisture purge period.

  When the switching valve 30 is controlled so that the exhaust gas flows through the main pipe line 14T after the moisture purge is completed, the above-described series of control processing at the time of starting the engine is completed.

  Thus, according to the present embodiment, the water adsorbent 24 is provided in the exhaust pipe 14, while the bypass pipe 26 formed on the downstream side of the water adsorbent 24 is provided with the NOx adsorbent 28, A switching valve 30 for switching the flow path between the bypass pipe 26 and the exhaust pipe 14 is provided. At the time of cold start, the switching valve 30 is controlled so that the exhaust gas flows into the bypass pipe 26. Thereby, at the time of cold start, moisture is adsorbed by the water adsorbent 24, and NOx not purified by the catalyst 22 is adsorbed by the NOx adsorbent.

  Since the water adsorbent 24 is provided in the exhaust pipe 14, the exhaust gas passes through the water adsorbent 24 regardless of the state of the switching valve 30. As a result, the desorption of the moisture adsorbed with the temperature rise of the exhaust gas proceeds quickly, and the water purge of the water adsorbent 24 can be completed early. Therefore, the water adsorbent 24 can take up moisture adsorption in the exhaust gas from cold start to warm-up, and the NOx adsorbent 28 can sufficiently exhibit the NOx adsorption performance.

  Further, by circulating the exhaust gas to the intake pipe 12, NOx adsorbed on the NOx adsorbent 28 is surely purged, and after the NOx purge, the exhaust gas is flowed to the bypass pipe 26 by lean operation, so that moisture is surely obtained. Purged. Further, when the adsorption performance of the NOx adsorbent 28 temporarily decreases, the adsorption performance is recovered by the rich operation. As a result, the water adsorbent 24 and the NOx adsorbent 28 function normally even at the next cold start, and the exhaust gas is always sufficiently purified at the cold start.

  Note that it may be determined whether or not it is a cold start other than the cooling water temperature, for example, it may be determined based on the catalyst temperature. Further, regarding the deterioration of the adsorption performance of the NOx adsorbent, any configuration that detects the NOx adsorption amount and the moisture amount may be used so that NOx is not discharged.

  The exhaust gas recirculation may be configured to return to the catalyst upstream side of the exhaust pipe. Moreover, you may comprise so that a catalyst may be arrange | positioned downstream from a bypass pipe.

  The rich operation at the cold start, the lean operation at the time of moisture purging, and the rich operation for recovering the adsorption performance of the NOx adsorbent may be selectively executed in consideration of the operation condition such as the operation load.

1 is a schematic configuration diagram of an engine exhaust gas purification system. It is the figure which showed the flowchart of the exhaust-gas purification | cleaning control process performed by ECU. It is the figure which showed the exhaust gas flow path after starting a driving | operation. It is the figure which showed the correlation with the moisture adsorption amount of NOx adsorbent. It is the figure which showed the graph of the moisture adsorption amount and the NOx adsorption amount.

Explanation of symbols

10 Engine 12 Intake pipe (intake system passage)
14 Exhaust pipe (exhaust passage)
14T Main pipeline (exhaust passage)
22 Catalyst (Exhaust gas purification catalyst)
24 Water adsorbent
26 Bypass pipe (bypass passage)
28 NOx adsorbent 30 Switching valve 32 Purge pipe (purge passage)
34 Purge valve

Claims (8)

An exhaust gas purification system for an internal combustion engine using hydrogen as a fuel,
An exhaust gas purification catalyst provided in the exhaust passage of the internal combustion engine;
A water adsorbent provided in the exhaust passage to adsorb water;
A bypass passage that branches off from the exhaust passage on the downstream side of the water adsorbent and detours;
A NOx adsorbent provided in the bypass passage and adsorbing NOx;
A switching valve for switching the flow path of the exhaust gas between the exhaust passage and the bypass passage;
An exhaust gas purification system comprising exhaust gas purification control means for controlling the switching valve so that exhaust gas flows into the bypass passage at the time of cold start.   2. The exhaust gas purification system according to claim 1, wherein the exhaust gas purification catalyst is provided upstream of the bypass passage in the exhaust passage. A purge passage branched from the bypass passage and communicating with the intake system passage of the internal combustion engine;
A purge valve for opening and closing the purge passage;
The exhaust gas purification control means controls the switching valve and the purge valve so that the exhaust gas flows back to the intake system passage through the NOx adsorbent after the temperature of the exhaust gas purification catalyst reaches a predetermined temperature. The exhaust gas purification system according to claim 1, wherein the exhaust gas purification system is controlled.   The exhaust gas purification control means causes the exhaust gas to flow into the bypass passage when the engine is operating at an air-fuel ratio leaner than the stoichiometric air-fuel ratio after the temperature of the exhaust gas purification catalyst reaches a predetermined temperature. The exhaust gas purification system according to any one of claims 1 to 3, wherein the switching valve is controlled. Adsorption amount detection means for detecting the amount of moisture adsorbed on the NOx adsorbent and the amount of NOx;
Based on the correlation between the moisture adsorption amount and the NOx adsorption amount in the NOx adsorbent, the adsorption performance judgment means for judging whether or not the detected NOx amount satisfies a reference NOx amount corresponding to the detected moisture amount. And
5. The exhaust gas purification control means, when the detected NOx amount does not satisfy the reference NOx amount, causes the engine to operate at an air / fuel ratio richer than the stoichiometric air / fuel ratio. The exhaust gas purification system described.   The exhaust gas purification system according to any one of claims 2 to 5, wherein the exhaust gas purification control means operates the engine at an air-fuel ratio richer than a stoichiometric air-fuel ratio at the time of cold start.   The exhaust gas purification system according to any one of claims 2 to 6, wherein the water adsorbent is provided downstream of the exhaust gas purification catalyst. The exhaust gas purification control means controls the switching valve so that exhaust gas flows through the bypass passage when the temperature of engine cooling water does not reach a predetermined temperature when the engine is started. The exhaust gas purification system according to any one of 7.

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