JP2016205351A - Exhaust emission control system of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To avoid as much as possible that a selection reduction type catalyst is fallen into a sulfur-poisoned state, and NOx purification performance is lowered, in an exhaust system of an internal combustion engine having the selection reduction type NOx catalyst in an exhaust passage, and a front stage catalyst having a characteristic for occluding or adsorbing a sulfur component at its upstream side.SOLUTION: In the case that an ammonia amount which is estimated to be adsorbed to a selection reduction type NOx catalyst is smaller than a prescribed amount when it is determined that an execution condition of restoration processing for eliminating a sulfur oxide which is occluded or adsorbed to a front stage catalyst by raising a temperature of the front stage catalyst is established, an exhaust system of an internal combustion engine increases an ammonia adsorption amount to at least a prescribed amount by supplying ammonia to the selection reduction type NOx catalyst, and after that, executes the restoration processing.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an exhaust gas purification control system for an internal combustion engine.

  The exhaust passage of the internal combustion engine may be provided with a selective reduction type NOx catalyst for reducing NOx in the exhaust. When ammonia as a reducing agent is supplied to the selective reduction type NOx catalyst, ammonia is adsorbed on the active point of the catalyst, and NOx purification is performed by showing a selective reduction reaction with respect to NOx in the exhaust gas. Here, in Patent Document 1, the NOx storage reduction catalyst and the selective reduction NOx catalyst are arranged in the exhaust passage from the upstream side, so that the reaction with the fuel component in the exhaust gas in the NOx storage reduction catalyst is performed. An exhaust purification device that generates ammonia and uses the generated ammonia for a NOx reduction reaction with a selective reduction type NOx catalyst is disclosed. Here, the NOx storage reduction catalyst has a characteristic that its performance as a catalyst deteriorates by storing an oxide of a sulfur component contained in exhaust gas and poisoning with sulfur. Therefore, in order to remove the stored sulfur component, a sulfur poisoning recovery process is performed in which the NOx storage reduction catalyst is heated to recover its catalytic performance.

  Specifically, in the technique disclosed in Patent Document 1, when the sulfur poisoning recovery process is performed to recover the performance of the NOx storage reduction catalyst, urea is injected from the upstream side of the NOx storage reduction catalyst by an injection valve. Water is jetted. With this configuration, the ammonia produced by the urea water supplied by injection acts on the upstream storage-reduction NOx catalyst, so that sulfur poisoning caused by the stored sulfur oxide can be eliminated. Further, the generated ammonia leaks from the NOx storage reduction catalyst and reaches the downstream selective reduction NOx catalyst, so that NOx purification in the exhaust gas is also promoted.

JP 2009-85178 A

  According to the prior art, the sulfur poisoning elimination process in the NOx storage reduction catalyst is disclosed. However, the catalytic performance of the selective reduction NOx catalyst is also reduced by the adsorption of the sulfur component contained in the exhaust gas. obtain. Therefore, even in the selective reduction type NOx catalyst, the sulfur poisoning state should be avoided as much as possible. Here, in the example shown in the above prior art, when the sulfur poisoning recovery process of the NOx storage reduction catalyst is performed, the sulfur component stored therein is released out of the catalyst by the action of ammonia. However, the selective reduction type NOx catalyst arranged on the downstream side thereof is exposed to the released sulfur component, and thus falls into a sulfur poisoning state, and its NOx purification performance tends to be lowered.

  The present invention has been made in view of the above problems, and an exhaust system of an internal combustion engine having a selective reduction type NOx catalyst in an exhaust passage and a pre-stage catalyst having a property of storing or adsorbing a sulfur component upstream thereof. The purpose of this invention is to avoid, as much as possible, that the selective reduction type NOx catalyst falls into a sulfur poisoning state and the NOx purification performance is lowered when the sulfur component of the upstream catalyst is occluded or adsorbed.

In order to solve the above-mentioned problems, the present applicant paid attention to the ammonia adsorption amount in the selective reduction type NOx catalyst when the sulfur component is occluded and the adsorption is eliminated in the preceding catalyst. This is because the sulfur poisoning form by the sulfur component in the selective reduction type NOx catalyst is easily affected by the ammonia adsorption amount in the selective reduction type NOx catalyst, and even if the selective reduction type NOx catalyst is sulfur poisoned, ammonia This is because as the amount of adsorption increases, the reduction in NOx reduction performance by the selective reduction type NOx catalyst is suppressed, and the sulfur poisoning state in the selective reduction type NOx catalyst is easily eliminated.

  More specifically, the present invention relates to an exhaust gas purification control system for an internal combustion engine, which is a catalyst provided in an exhaust passage of the internal combustion engine and has a property of storing or adsorbing sulfur components in the exhaust gas. A selective reduction type NOx catalyst that is provided in the exhaust passage on the downstream side of the pre-stage catalyst and selectively reduces NOx using ammonia as a reducing agent, an ammonia supply means for supplying ammonia to the selective reduction type NOx catalyst, and the selection Estimating means for estimating the amount of ammonia adsorbed on the reduced NOx catalyst, and recovery means for executing a recovery process for raising the temperature of the preceding catalyst and removing sulfur oxides occluded or adsorbed on the preceding catalyst Determining means for determining whether the execution condition for the recovery process is satisfied; and when the determination means determines that the execution condition for the recovery process is satisfied, the estimating means When the estimated ammonia amount is less than a predetermined amount, the ammonia supply amount is increased to at least the predetermined amount by supplying ammonia by the ammonia supply means, and then the poisoning recovery is performed. Control means for executing the recovery processing by the means.

  According to the present invention, in an exhaust system of an internal combustion engine having a selective reduction type NOx catalyst in an exhaust passage and a upstream catalyst having a property of storing or adsorbing a sulfur component upstream thereof, storage and adsorption of the sulfur component of the upstream catalyst. When eliminating the above, it is possible to avoid as much as possible that the selective reduction type NOx catalyst falls into a sulfur poisoning state and the NOx purification performance is lowered.

It is a figure which shows schematic structure of the exhaust gas purification control system of the internal combustion engine which concerns on this invention. It is a figure which shows the correlation with the ammonia adsorption amount and the ratio of sulfur poisoning in a selective reduction type NOx catalyst. It is a figure which shows the correlation with the sulfur poisoning amount and NOx purification rate in a selective reduction type NOx catalyst. It is a figure which shows the correlation with the catalyst temperature in the selective reduction type | mold NOx catalyst, and the removal | desorption amount of sulfur oxide. 2 is a flowchart relating to sulfur poisoning recovery control executed in the exhaust purification control system shown in FIG. 1.

  Hereinafter, specific embodiments of the present invention will be described with reference to the drawings. The dimensions, materials, shapes, relative arrangements, and the like of the components described in the present embodiment are not intended to limit the technical scope of the invention to those unless otherwise specified.

  An embodiment of an exhaust gas purification control system for an internal combustion engine according to the present invention will be described based on the drawings attached to the present specification. FIG. 1 is a diagram showing a schematic configuration of an exhaust gas purification system for an internal combustion engine according to the present embodiment. The internal combustion engine 1 is a gasoline engine for driving a vehicle.

An exhaust passage 2 is connected to the internal combustion engine 1. The exhaust passage 2 includes a three-way catalyst 41, an NOx storage reduction catalyst (hereinafter referred to as “NSR catalyst”) 42, and a selective reduction NOx catalyst (hereinafter referred to as “SCR catalyst”) 43 in order from the upstream side. ing. The three-way catalyst 41 has an oxidation function, and purifies NOx, HC and CO with maximum efficiency when the catalyst atmosphere is at the stoichiometric air-fuel ratio. Further, the NSR catalyst 42 has a storage agent that stores NOx. When the oxygen concentration of the inflowing exhaust gas is high, the NSR catalyst 42 stores NOx in the exhaust gas, the oxygen concentration of the inflowing exhaust gas decreases, and there is a reducing agent. Sometimes the stored NOx is released and reduced. As the reducing agent supplied to the NSR catalyst 42, the fuel discharged from the internal combustion engine 1 can be used. The amount of the fuel is controlled through adjustment of fuel injection conditions and combustion conditions in the internal combustion engine 1.

  Here, when the exhaust gas passes through the three-way catalyst 41 or the NSR catalyst 42, NOx in the exhaust gas may react with HC or hydrogen to generate ammonia. For example, when hydrogen is generated from CO or water in the exhaust gas by a water gas shift reaction or a steam reforming reaction, the hydrogen reacts with NO in the three-way catalyst 41 or the NSR catalyst 42 to generate ammonia. Ammonia is generated when the air-fuel ratio of the exhaust gas passing through the three-way catalyst 41 or the NSR catalyst 42 is equal to or lower than the stoichiometric air-fuel ratio. The SCR catalyst 43 adsorbs the ammonia generated by the three-way catalyst 41 or the NSR catalyst 42, and selectively reduces NOx by the adsorbed ammonia when NOx passes.

  Further, a temperature sensor 44 that detects an exhaust gas temperature flowing out from the NSR catalyst 42 is provided on the downstream side of the NSR catalyst 42. In addition, a crank position sensor 21, an accelerator opening sensor 22, an air flow meter 26 provided in the exhaust passage 25, and the like are also arranged. These sensors are electrically connected to the ECU 20, and the detected values are sent to the ECU 20. Passed. And ECU20 can grasp | ascertain the driving | running state of the internal combustion engine 1 based on these detection values.

  In the exhaust purification control system configured as described above, the exhaust gas discharged from the internal combustion engine 1 is purified by the exhaust gas purification capabilities of the three-way catalyst 41, the NSR catalyst 42, and the SCR catalyst 43. At that time, sulfur oxide (SOx) contained in the exhaust gas may be stored in the storage agent of the NSR catalyst 42 together with NOx and accumulated. As SOx accumulates in the occlusion material, the capacity to occlude NOx decreases, and as a result, it becomes difficult to exhibit the NOx purification function and ammonia generation capability that the NSR catalyst 42 should originally perform. Therefore, in order to release SOx accumulated in the storage agent of the NSR catalyst 42, a sulfur poisoning recovery process is performed in which the temperature of the NSR catalyst 42 is raised and the NSR catalyst 42 is placed in a rich atmosphere. Specifically, the fuel is sent to the three-way catalyst 41 having an oxidation function via exhaust, and the temperature of the NSR catalyst 42 is increased so that the temperature of the NSR catalyst 42 becomes a predetermined temperature by the oxidation reaction heat of the fuel there. It is done. Note that the temperature of the NSR catalyst 42 is detected based on the detection value of the temperature sensor 44. In addition, since it is necessary to place the NSR catalyst 42 in a rich atmosphere in the sulfur poisoning recovery process, the fuel supplied to the exhaust gas flowing into the three-way catalyst 41 considers the formation of the rich atmosphere as the temperature of the NSR catalyst 42 rises. It becomes the amount.

  When such sulfur poisoning recovery processing is performed, the temperature of the SCR catalyst 43 rises as the temperature of the NSR catalyst 42 rises, and SOx released from the NSR catalyst 42 flows into the SCR catalyst 43. Therefore, when the SCR catalyst 43 is exposed to the separated SOx, the SCR catalyst 43 is poisoned with sulfur, and the NOx purification performance (NOx purification rate) may be reduced.

Here, sulfur poisoning in the SCR catalyst 43 will be described with reference to FIGS. 2A to 2C. FIG. 2A is a graph showing the correlation between the ammonia adsorption amount in the SCR catalyst 43 and the sulfur poisoning rate (poisoning rate). The sulfur poisoning in the SCR catalyst 43 can be roughly classified into two poisoning forms. The first poisoning form is poisoning caused by the combination of the active component contained in the SCR catalyst 43, for example, Cu or the like and SOx, and CuSO ₄ or the like is generated in the SCR catalyst 43. The second poisoning form is poisoning caused by the combination of ammonia adsorbed on the SCR catalyst 43 and SOx, and (NH ₄ ) ₂ SO ₄ or the like is generated in the SCR catalyst 43. . Therefore, the poisoning ratio is the ratio of the sulfur poisoning caused by the first poisoning form in the sulfur poisoning generated in the SCR catalyst 43 (the transition indicated by the line L1 in FIG. 2A), or the second poisoning. It is defined as the ratio of sulfur poisoning due to poisoning form (transition indicated by line L2 in FIG. 2A). As can be understood from the characteristic of the poisoning ratio shown in FIG. 2A, the poisoning ratio due to the first poisoning form decreases as the ammonia adsorption amount in the SCR catalyst 43 increases, while the second poisoning form. The poisoning rate due to increases.

  Next, FIG. 2B is a graph showing the correlation between the sulfur poisoning amount of the SCR catalyst 43 and the NOx purification performance of the SCR catalyst 43. As the NOx purification performance, the NOx purification rate by the SCR catalyst 43 (= 1−NOx concentration in the exhaust gas flowing out from the SCR catalyst 43 / NOx concentration in the exhaust gas flowing into the SCR catalyst 43) can be used. Further, the sulfur poisoning amount of the SCR catalyst 43 is the amount of SOx contained in the SCR catalyst 43 in the first poisoning form or the second poisoning form. In FIG. 2B, the line L3 indicates the transition of the NOx purification performance according to the first poisoning form, and the line L4 indicates the transition of the NOx purification performance according to the second poisoning form. is there. As can be understood from the characteristics of the NOx purification performance shown in FIG. 2B, the NOx purification performance of the SCR catalyst 43 according to the first poisoning mode increases as the sulfur poisoning amount in the SCR catalyst 43 increases. There is a tendency that the NOx purification performance of the SCR catalyst 43 is greatly reduced. As described above, in the case of the second poisoning form, the degree of decrease in the NOx purification performance is smaller. In the second poisoning form, SOx is lower than the active component that determines the reduction performance of the SCR catalyst 43. This is probably because they are not connected.

  Next, FIG. 2C is a graph showing the transition of the amount of sulfur component desorbed from the SCR catalyst 43 with respect to the temperature of the SCR catalyst 43. When the temperature of the SCR catalyst 43 is about 450 ° C., the SOx bonded to the SCR catalyst 43 is mainly detached due to the second poisoning mode. Further, when the temperature of the SCR catalyst 43 is about 650 degrees on the higher temperature side, the SOx bonded to the SCR catalyst 43 is mainly detached due to the first poisoning form. That is, the SOx under the second poisoning form is separated from the SCR catalyst 43 at a lower temperature side than the SOx under the first poisoning form, in other words, it is relatively easy. It can be said that it can be detached from.

  As described above, when the first poisoning form and the second poisoning form are compared, even if the SCR catalyst 43 is sulfur poisoned, the sulfur poisoning by the second poisoning form is more sulfur. It can be understood that the drop in NOx purification performance due to poisoning is small and that recovery from the sulfur poisoning state is easy. Therefore, in the present invention, by performing the sulfur poisoning recovery control shown in FIG. 3, the sulfur poisoning recovery process is performed in the NSR catalyst 42, so that the SCR catalyst 43 disposed on the downstream side thereof is sulfur poisoned. Even if there is a possibility of this, the ammonia adsorption amount of the SCR catalyst 43 is controlled so that the poisoning form becomes the second poisoning form as much as possible. The sulfur poisoning recovery control is repeatedly executed at a predetermined timing by a control program stored in the ECU 20.

  First, in S101, the sulfur poisoning state of the NSR catalyst 42 is estimated. That is, it is estimated how much SOx is occluded in the NSR catalyst 42 which is the target of the sulfur poisoning recovery process in S106 described later. Specifically, the estimation is performed based on the operating state of the internal combustion engine 1 related to the amount of sulfur contained in the exhaust gas from the completion of the previous sulfur poisoning recovery process to the present time. Since the estimation is based on the prior art, its detailed description is omitted. When the process of S101 ends, the process proceeds to S102.

In S102, the ammonia adsorption amount X1 in the SCR catalyst 43 is calculated. As for the adsorption amount of ammonia in the SCR catalyst 43, as an element for increasing the adsorption amount,
A new adsorption of the amount of ammonia produced by the NSR catalyst 42 according to the fuel component supplied from the internal combustion engine 1 to the SCR catalyst 43 can be mentioned. This new ammonia adsorption amount is the amount of fuel component supplied from the internal combustion engine 1, the rate of the ammonia generation reaction at the NSR catalyst 42, the rate at which the supplied ammonia is adsorbed there without passing through the SCR catalyst 43, etc. It can be calculated in consideration of the following factors. On the other hand, factors that reduce the amount of ammonia adsorbed on the SCR catalyst 43 include consumption due to a reduction reaction for NOx purification by the SCR catalyst 43 and detachment of ammonia adsorbed on the SCR catalyst 43. The former can be calculated based on the catalyst temperature related to the NOx reduction reaction in the SCR catalyst 43, the NOx concentration in the exhaust gas, etc., and the latter can be calculated based on the catalyst temperature related to the desorption of ammonia. Can do. Since the calculation is based on the prior art, a detailed description thereof is omitted. When the process of S102 ends, the process proceeds to S103.

  In S103, it is determined whether the execution condition of the sulfur poisoning recovery process of the NSR catalyst 42 is satisfied. In the sulfur poisoning state of the NSR catalyst 42 estimated in S101, if the sulfur poisoning amount exceeds a predetermined threshold amount, the catalyst performance of the NSR catalyst 42 is reduced due to sulfur poisoning, so sulfur poisoning. It can be considered that the recovery process is to be executed. Therefore, the first execution condition can include that the sulfur poisoning amount exceeds a predetermined threshold amount. Further, as described above, the sulfur poisoning recovery process needs to raise the temperature of the NSR catalyst 42 and place the NSR catalyst 42 in a rich atmosphere by sending fuel from the internal combustion engine 1 to the three-way catalyst 41. The operating state of the internal combustion engine 1 needs to be in a state where such fuel supply is possible. One example is an idle operation state. Therefore, the second execution condition can include that the operation state of the internal combustion engine is an idle operation state. As described above, whether or not the first and second execution conditions are satisfied is determined in S103. If both the execution conditions are satisfied, an affirmative determination is made, and otherwise, a negative determination is made. If an affirmative determination is made in S103, the process proceeds to S104. If a negative determination is made, the present control is terminated.

  In S104, it is determined whether or not the ammonia adsorption amount X1 calculated in S102 is smaller than a predetermined amount X0. As described based on FIGS. 2B and 2C, even if the SCR catalyst 43 is sulfur poisoned due to the sulfur poisoning recovery process of the NSR catalyst 42, the sulfur poisoning by the second poisoning mode is performed. The decrease in NOx purification performance due to sulfur poisoning is smaller, and recovery from the sulfur poisoning state is easier. As shown in FIG. 2A, the ratio between the first poisoning form and the second poisoning form that occurs in the SCR catalyst 43 varies depending on the ammonia adsorption amount in the SCR catalyst 43. Specifically, As the ammonia adsorption amount increases, sulfur poisoning due to the second poisoning form is likely to occur. Therefore, in consideration of this point, when the ammonia adsorption amount is smaller than the predetermined amount X0, the SCR catalyst 43 is liable to cause sulfur poisoning due to the first poisoning form, and the NOx purification performance is deteriorated or sulfur poisoning is caused. It can be considered that the difficulty for elimination becomes remarkable. In accordance with such an idea, the predetermined amount X0 can be set based on the minimum NOx purification performance required for the SCR catalyst 43, the energy amount required for the sulfur poisoning recovery of the SCR catalyst 43, and the like. If a positive determination is made in S104, the process proceeds to S105, and if a negative determination is made, the process proceeds to S106.

  In S105, the process for increasing the ammonia adsorption amount of the SCR catalyst 43 is executed through the fuel supply from the internal combustion engine 1. Even if the SCR catalyst 43 is sulfur poisoned due to the sulfur poisoning recovery process of the NSR catalyst 42, the increase process is performed as the amount of ammonia adsorption increases as described above. This is because the poisoning is likely to occur. Therefore, in S105, the amount increasing process is performed until the ammonia adsorption amount of the SCR catalyst 43 reaches at least the predetermined amount X0. When the process of S105 ends, the process proceeds to S106. In S106, the sulfur poisoning recovery process for the NSR catalyst 42 described above is executed.

  As described above, according to this control, the execution condition of the sulfur poisoning recovery process of the NSR catalyst 42 is established, and before the execution, the ammonia adsorption amount of the SCR catalyst 43 disposed downstream thereof is greater than the predetermined amount X0. When the amount is small, the sulfur poisoning recovery process is executed after the increase process. Thereby, at the time of execution of the sulfur poisoning recovery process, it is ensured that the ammonia adsorption amount in the SCR catalyst 43 is equal to or greater than the predetermined amount X0. When sulfur poisoning by the first poisoning form occurs in the catalyst 43, it is possible to avoid inconveniences such as a significant decrease in the NOx purification performance of the SCR catalyst 43.

<Modification>
As a modification of the exhaust system of the internal combustion engine 1, an S trap for storing SOx in the exhaust gas may be arranged instead of the NSR catalyst 42 shown in FIG. Here, the S trap absorbs SOx in exhaust flowing through the S trap and removes SOx from the exhaust, and is composed of a catalyst supporting layer, a noble metal catalyst supported on the catalyst supporting layer, and a SOx absorbent. The The catalyst support layer is composed of a generally used porous oxide material, and for example, alumina, silica, zirconia, silica-alumina, zeolite, or the like is used. Examples of the noble metal catalyst include platinum (Pt), rhodium (Rh), palladium (Pd), iridium (Ir), ruthenium (Ru), and the like, and one or more of these can be used. As the SOx absorbent, a substance capable of forming a sulfate can be used, and at least one selected from alkali metals, alkaline earth metals, rare earth metals, and transition metals can be exemplified.

  Since the S trap is for storing SOx in the exhaust as described above, in this modification, in order to supply ammonia as a reducing agent to the SCR catalyst 43, urea water as a precursor thereof is exhausted. It is necessary to arrange a configuration to be added therein (for example, a urea water supply valve and a tank for storing the urea water).

  As described above, even when the exhaust system of the internal combustion engine 1 is constituted by the three-way catalyst 41, the S trap, and the SCR catalyst 43 from the upstream side, the processing for releasing the SOx stored in the S trap, that is, by the S trap. When the recovery process for recovering the storage capacity of SOx is executed, the SOx released along with it flows into the SCR catalyst 43 located downstream, so that the sulfur of the SCR catalyst 43 is the same as in the case shown in FIG. Poisoning can occur. Therefore, even in such a case, the control equivalent to the sulfur poisoning recovery control described above is executed, and the increase according to the ammonia adsorption amount in the SCR catalyst 43 is performed before the recovery process for the S trap. Processing may be performed. Thereby, the fall of the NOx purification performance resulting from the sulfur poisoning of the SCR catalyst 43 can be avoided.

DESCRIPTION OF SYMBOLS 1 Internal combustion engine 2 Exhaust passage 41 Three-way catalyst 42 NSR catalyst (Occlusion reduction type NOx catalyst)
43 SCR catalyst 44 Temperature sensor 20 ECU

Claims (1)

A pre-stage catalyst provided in an exhaust passage of an internal combustion engine, having a property of storing or adsorbing a sulfur component in exhaust;
A selective reduction type NOx catalyst that is provided in the exhaust passage on the downstream side of the preceding catalyst and selectively reduces NOx using ammonia as a reducing agent;
Ammonia supply means for supplying ammonia to the selective reduction type NOx catalyst;
Estimating means for estimating the amount of ammonia adsorbed on the selective reduction type NOx catalyst;
A recovery means for performing a recovery process for raising the temperature of the preceding catalyst and removing sulfur oxides occluded or adsorbed by the preceding catalyst;
Determining means for determining whether the execution condition of the recovery process is satisfied;
When it is determined by the determination means that the conditions for executing the recovery process are satisfied, if the ammonia amount estimated by the estimation means is less than a predetermined amount, ammonia is supplied by the ammonia supply means. And a control means for executing the recovery process by the poisoning recovery means after increasing the ammonia adsorption amount in the selective reduction type NOx catalyst to at least the predetermined amount;
An exhaust gas purification control system for an internal combustion engine.

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