JP2019167823A - Exhaust emission control device for internal combustion engine for getting driving force through combustion of ammonia and method - Google Patents

Abstract

To restrict discharge of ammonia and also simplify a device and make a small-sized formation of the device in an exhaust emission control device for internal combustion engine for getting driving force through combustion of ammonia.SOLUTION: An exhaust emission control device for an internal combustion engine for getting driving force under combustion of ammonia, comprises: an oxidation-reduction catalyst with oxidation and reduction actions arranged at a main flow passage in which exhaust gas from the internal combustion engine flows; a selective reduction catalyst arranged at the main flow passage; a temperature acquisition unit for getting temperature of the selective reduction catalyst; and a control unit for changing mixture ratio of exhaust emission at upstream sides of the oxidation-reduction catalyst and the selective reduction catalyst from stoichiometric to lean when the temperature of the selective reduction catalyst acquired by the temperature acquisition unit exceeds activation temperature of the selective reduction catalyst.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a technology for purifying exhaust gas from an internal combustion engine that obtains a driving force by burning ammonia.

A technique for purifying harmful substances contained in exhaust gas from an internal combustion engine such as an engine using a catalyst is known. For example, in Non-Patent Document 1, in a catalyst system equipped with a three-way catalyst, the concentration of oxygen (O ₂ ) in exhaust gas is monitored, reducing agent components (HC, CO) in exhaust gas, and oxidizing agent components (NO And the air-fuel ratio are controlled so that they are always balanced. For example, in Patent Document 1, in an exhaust purification system including zeolite and a main converter equipped with a catalyst, exhaust from an internal combustion engine is guided to zeolite during a cold start, and the zeolite is heated. It is described that after reaching the desorption temperature, it leads to the main converter. For example, in Patent Document 2, exhaust gas from an internal combustion engine is guided in the order of a first catalyst zone, an adsorption zone containing zeolite, and a second catalyst zone during the cold start operation period, and the first catalyst zone and the second catalyst zone. It describes the transfer of heat to and from the zone.

Soji Tsuchiya and Atsuko Hirakawa, “Science of Materials, Reactions and Properties”, Chapter 15 Chemistry of Catalysts, 4. Application of catalyst, Internet <URL: http://www.campus.ouj.ac.jp/~hamada/TextLib/rm/chap15/Text/Cr991504.html>

JP-A-7-166852 Japanese Patent No. 3435652

Incidentally, an internal combustion engine (hereinafter also referred to as “ammonia engine”) that obtains driving force by burning ammonia (NH ₃ ) gas is known. Since ammonia does not contain carbon (C), the ammonia engine has an advantage that the exhaust gas does not contain carbon dioxide (CO ₂ ). In this regard, the technologies described in Non-Patent Document 1 and Patent Documents 1 and 2 consider only exhaust gas purification in gasoline engines and diesel engines, and do not describe exhaust gas purification in ammonia engines.

  For example, when the technology of Non-Patent Document 1 is applied to an ammonia engine, if the three-way catalyst has reached the activation temperature, ammonia that is a harmful substance in the exhaust is oxidized and nitrogen oxide (NOx) By reducing the toxic substances, it can be purified. However, when the three-way catalyst has not reached the activation temperature, there has been a problem that ammonia and nitrogen oxides are unreacted and discharged through the three-way catalyst. For example, when the techniques of Patent Document 1 and Patent Document 2 are applied to an ammonia engine, ammonia emission can be suppressed by adsorbing ammonia to the zeolite before the catalyst reaches the activation temperature. However, since zeolite has the property that desorption of adsorbed ammonia occurs at high temperatures, a catalyst for purifying the desorbed ammonia is further required on the downstream side of the zeolite, which complicates equipment and systems. There was a problem of increasing the size.

  The present invention has been made to solve at least a part of the above-described problems, and in an exhaust purification device that purifies exhaust gas of an internal combustion engine that obtains driving force by combustion of ammonia, suppresses ammonia emission, The object is to simplify and miniaturize the apparatus.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms.

(1) According to one aspect of the present invention, an exhaust emission control device for an internal combustion engine that obtains a driving force by combustion of ammonia is provided. The exhaust gas purification apparatus includes an oxidation-reduction catalyst having an oxidation action and a reduction action provided in a main flow path through which exhaust gas from the internal combustion engine flows, a selective reduction catalyst provided in the main flow path, and the selective reduction catalyst When the temperature of the selective reduction catalyst acquired by the temperature acquisition unit exceeds the activation temperature of the selective reduction catalyst, the oxidation reduction catalyst and the selective reduction catalyst A control unit that changes the mixing ratio of the exhaust gas on the upstream side from stoichiometry to lean.

  According to this configuration, the exhaust purification device includes the oxidation-reduction catalyst and the selective reduction catalyst having an ammonia adsorption function, so that the oxidation-reduction catalyst and the selective reduction catalyst reach the activation temperature, for example, immediately after the start of the internal combustion engine. In the method, ammonia can be prevented from being discharged by adsorbing ammonia in the exhaust gas to the selective reduction catalyst. In addition, after the oxidation-reduction catalyst reaches the activation temperature, the mixing ratio is stoichiometric. Therefore, the oxidation-reduction catalyst causes the ammonia in the exhaust to oxidize and the nitrogen oxide in the exhaust to reduce. It is possible to suppress ammonia emission. Furthermore, after the selective reduction catalyst reaches the activation temperature, the mixing ratio is changed from stoichiometric to lean, so that the ammonia adsorbed by the selective reduction catalyst and the nitrogen oxides in the exhaust are reduced. Thus, ammonia emission can be suppressed and ammonia adsorbed on the selective reduction catalyst can be removed. Further, according to this configuration, the configuration of the exhaust purification device can be simplified and the exhaust purification device can be downsized by the combination of the oxidation-reduction catalyst and the selective reduction catalyst.

(2) In the exhaust emission control device of the above aspect, the oxidation-reduction catalyst may be disposed upstream of the selective reduction catalyst in the main flow path. According to this configuration, since the redox catalyst is arranged upstream of the main flow path in the exhaust purification apparatus, the temperature of the redox catalyst can be easily increased, and the time until the redox catalyst reaches the activation temperature. Can be shortened. Further, the saturated adsorption amount of ammonia in the selective reduction catalyst has a characteristic that it decreases as the temperature of the catalyst increases. According to this configuration, since the selective reduction catalyst is disposed on the downstream side of the main flow path, it is difficult to increase the temperature of the selective reduction catalyst, and a decrease in the saturated adsorption amount of ammonia in the selective reduction catalyst can be suppressed.

(3) In the exhaust emission control device according to the above aspect, the control unit may change the exhaust gas mixture ratio to stoichiometric after a lapse of a fixed time after changing the exhaust gas mixture ratio to lean. . According to this configuration, the control unit changes the exhaust gas mixing ratio on the upstream side of the oxidation-reduction catalyst and the selective reduction catalyst to be lean, and advances the reduction reaction between ammonia and nitrogen oxides by the selective reduction catalyst, By changing the mixing ratio to stoichiometric after a certain period of time, the oxidation reaction of ammonia and the reduction reaction of nitrogen oxides by the oxidation-reduction catalyst can be continued.

(4) The exhaust emission control device of the above aspect further includes a second temperature acquisition unit that acquires the temperature of the oxidation-reduction catalyst, and a mixture ratio acquisition unit that acquires the mixture ratio of the exhaust gas, and the control unit includes Further, the selective reduction catalyst is adsorbed using the temperature of the oxidation-reduction catalyst acquired by the second temperature acquisition unit and the mixture ratio of the exhaust gas acquired by the mixture ratio acquisition unit. The amount of ammonia is estimated, and the predetermined time may be set as the time until the estimated amount of ammonia is reduced in the selective reduction catalyst. According to this configuration, the control unit can estimate the amount of ammonia adsorbed on the selective reduction catalyst using the temperature of the redox catalyst and the exhaust gas mixture ratio. In addition, the control unit sets a predetermined time for maintaining a lean exhaust gas mixture ratio upstream of the redox catalyst and the selective reduction catalyst until the estimated amount of ammonia is reduced, thereby reducing the selective reduction catalyst. The removal of ammonia adsorbed on the catalyst can be completed.

  The present invention can be realized in various modes. For example, an exhaust purification device for an internal combustion engine that obtains a driving force by combustion of ammonia, a system including the internal combustion engine and the exhaust purification device, these devices, and a system And a computer program executed in these devices and systems, a server device for distributing the computer program, a non-temporary storage medium storing the computer program, and the like.

1 is a schematic view of an engine system as an embodiment of the present invention. It is a figure showing the ammonia adsorption | suction characteristic of a zeolite. It is a flowchart which shows an example of the control procedure in a control part. It is a figure shown about the change of the state of each part after the start of an internal combustion engine. It is a figure which shows the outline | summary of reaction in a three-way catalyst and an SCR catalyst.

<Embodiment>
FIG. 1 is a schematic diagram of an engine system 1 as an embodiment of the present invention. The engine system 1 is mounted on a vehicle, for example, and generates a driving force for driving the vehicle. The engine system 1 includes a combustion device 20 that generates driving force, and an exhaust purification device 10 that purifies harmful substances such as ammonia (NH ₃ ) and nitrogen oxide (NOx) in exhaust gas from the combustion device 20. .

  The combustion device 20 includes a combustion state control unit 21 and an internal combustion engine 22. The internal combustion engine 22 is an ammonia engine that obtains driving force by burning ammonia gas. Hereinafter, the ammonia gas is simply referred to as “ammonia”. The combustion state control unit 21 adjusts the throttle (not shown) to change the air flow rate to the internal combustion engine 22 and adjusts the fuel supply valve (not shown) to change the fuel supply amount to the internal combustion engine 22. . Thereby, the combustion state control unit 21 changes the mass ratio (mixing ratio) of the air and the fuel in the internal combustion engine 22 and in the exhaust discharged from the internal combustion engine 22 to each of the rich state, the stoichiometric state, and the lean state. And control. When “Φ = theoretical air / fuel ratio / the actual air / fuel ratio of the air / fuel mixture”, the overconcentrated state means a mixture ratio where Φ> 1, and the stoichiometric state means a mixture ratio where Φ = 1. This means that the lean state means a mixing ratio where Φ <1. Φ is also called “equivalent ratio”. The combustion state control unit 21 is mounted by, for example, an electronic control unit (ECU, Electronic Control Unit).

  The exhaust purification device 10 of this embodiment purifies harmful substances in exhaust using a redox catalyst and a selective reduction catalyst. In the following explanation, a three-way catalyst (Three-Way Catalyst) is exemplified as the oxidation-reduction catalyst, and an SCR catalyst (Selective Catalytic Reduction catalyst) is exemplified as the selective reduction catalyst. As the oxidation-reduction catalyst, a catalyst other than the three-way catalyst can be used as long as the catalyst has an oxidation action and a reduction action. For example, as the oxidation-reduction catalyst, a catalyst using ceramics, titanium oxide or the like as a carrier and supporting a noble metal such as platinum, rhodium or palladium as an active catalyst component can be used. Similarly, as the selective reduction catalyst, a catalyst other than the SCR catalyst can be used as long as the catalyst has an ammonia adsorption action. For example, as the selective reduction catalyst, a catalyst using ceramics, titanium oxide or the like as a carrier and supporting zeolite as an active catalyst component can be used.

  Further, in the following description, in the exhaust purification device 10, the side closer to the internal combustion engine 22 is referred to as “upstream side”, and the side farther from the internal combustion engine 22 is referred to as “downstream side”. In the case of FIG. 1, the left side corresponds to the upstream side, and the right side corresponds to the downstream side.

  The exhaust purification device 10 includes a control unit 11 that controls each part of the exhaust purification device 10, an exhaust pipe 19 extending from the internal combustion engine 22, a three-way catalyst 12 provided on the exhaust pipe 19, and an SCR catalyst 13, respectively. A mixing ratio acquisition unit 14, a second temperature acquisition unit 15, and a first temperature acquisition unit 16 are provided.

  The control unit 11 receives signals representing the acquired values acquired from the mixture ratio acquisition unit 14, the second temperature acquisition unit 15, and the first temperature acquisition unit 16. The control unit 11 performs control (FIG. 3), which will be described later, using each received acquired value and switches the mixing ratio of the internal combustion engine 22, thereby mixing the exhaust gas upstream of the three-way catalyst 12 and the SCR catalyst 13. Switch ratio. The control part 11 is mounted by ECU, for example. The exhaust pipe 19 forms a main flow path through which exhaust from the internal combustion engine 22 flows. Exhaust gas from the internal combustion engine 22 passes through the main flow path in the exhaust pipe 19, passes through the three-way catalyst 12 and the SCR catalyst 13, and is discharged to the outside air.

The mixture ratio acquisition unit 14 acquires the oxygen (O ₂ ) concentration (that is, the mixture ratio) of the exhaust (exhaust gas) from the internal combustion engine 22 on the upstream side of the three-way catalyst 12. The mixing ratio acquisition unit 14 can be realized, for example, by acquiring a measurement signal measured by an oxygen sensor or an ammonia sensor provided in the exhaust pipe 19. The mixing ratio acquisition unit 14 corresponds to a “mixing ratio acquisition unit”.

The three-way catalyst 12 is arranged on the most upstream side in the main flow path, in other words, on the upstream side of the SCR catalyst 13. Although the three-way catalyst 12 can purify ammonia, NOx, and hydrogen (H ₂ ) in the exhaust gas, when the mixing ratio deviates from a predetermined range in the vicinity of the stoichiometry, the purification performance is reduced. Has characteristics. The 2nd temperature acquisition part 15 is a sensor which measures the temperature of the three way catalyst 12, and measures the temperature (what is called bed temperature) in the catalyst of the three way catalyst 12 in this embodiment. The second temperature acquisition unit 15 may measure the temperature in the vicinity of the inlet / outlet of the three-way catalyst 12 instead of the bed temperature of the three-way catalyst 12. The second temperature acquisition unit 15 corresponds to a “second temperature acquisition unit”.

  The SCR catalyst 13 is disposed on the most downstream side in the main flow path, in other words, on the downstream side of the three-way catalyst 12. The SCR catalyst 13 can purify NOx in the exhaust gas using ammonia as a reducing agent. The 1st temperature acquisition part 16 is a sensor which measures the temperature of the SCR catalyst 13, and measures the temperature (what is called bed temperature) in the catalyst of the SCR catalyst 13 in this embodiment. The first temperature acquisition unit 16 may measure the temperature in the vicinity of the inlet / outlet of the SCR catalyst 13 instead of the bed temperature of the SCR catalyst 13. The first temperature acquisition unit 16 corresponds to a “temperature acquisition unit”.

  FIG. 2 is a diagram showing the ammonia adsorption characteristics of zeolite. The zeolite contained in the SCR catalyst 13 has a property of adsorbing ammonia. The vertical axis in FIG. 2 represents the ammonia concentration on the downstream side of the SCR catalyst 13 (zeolite), and the horizontal axis represents the amount of ammonia flowing into the SCR catalyst 13 (zeolite). As shown in the drawing, the SCR catalyst 13 can adsorb the entire amount of supplied ammonia in the range L1 until the inflow amount of ammonia exceeds a predetermined value. On the other hand, in the range L2 after the inflow amount of ammonia exceeds a predetermined value, the amount of ammonia adsorbed on the SCR catalyst 13 gradually decreases. In the SCR catalyst 13 of the present embodiment, an amount of zeolite that can adsorb the entire amount of ammonia discharged after the internal combustion engine 22 is started and before the temperature of the SCR catalyst 13 reaches about 200 ° C. It is supported. Note that the saturated adsorption amount of ammonia in the zeolite has a characteristic of decreasing as the temperature increases.

  FIG. 3 is a flowchart illustrating an example of a control procedure in the control unit 11. The control in FIG. 3 is a control for removing the ammonia adsorbed on the SCR catalyst 13 while monitoring the state of the exhaust purification device 10.

  In step S1, the control unit 11 determines whether a start condition is satisfied. Various conditions can be used as the start condition. For example, at least one of the following conditions a1 to a3 can be used. In the following description, a case where the condition a1 is employed will be described as an example.

(A1) When the internal combustion engine 22 is started: When the control unit 11 detects the start of the internal combustion engine 22, it determines that the start condition is satisfied. This condition is useful when it is desired to remove ammonia adsorbed on the SCR catalyst 13 immediately after the internal combustion engine 22 is started.
(A2) When a certain period of time has elapsed since the previous control: For example, the control unit 11 determines that the start condition is satisfied when an arbitrarily determined period of time has elapsed since the previous execution of step S6. This condition is useful when it is desired to periodically remove ammonia adsorbed on the SCR catalyst 13.
(A3) When the discharge of ammonia from the exhaust purification device 10 is detected: the control unit 11 acquires or estimates the ammonia concentration on the downstream side of the SCR catalyst 13, and when this ammonia concentration exceeds a certain value, the start condition Is determined to have been established. This condition is useful because ammonia adsorbed on the SCR catalyst 13 can be detected and removed. In this case, the exhaust purification device 10 may include an ammonia sensor (not shown) on the downstream side of the SCR catalyst 13.

  When the start condition of step S1 is not satisfied (step S1: NO), the control unit 11 shifts the process to step S1 and monitors the satisfaction of the start condition. On the other hand, when the start condition is satisfied (step S1: YES), the control unit 11 shifts the process to step S2.

  In step S <b> 2, the control unit 11 acquires the temperature of the SCR catalyst 13 from the first temperature acquisition unit 16. In step S <b> 3, the control unit 11 determines whether or not the acquired temperature of the SCR catalyst 13 exceeds the activation temperature of the SCR catalyst 13. “Activation temperature” in step S3 means a lower limit temperature at which the reduction reaction of NOx by ammonia is activated in the SCR catalyst 13, and is predetermined and stored in a storage unit (not shown) in the control unit 11. . When the temperature of the SCR catalyst 13 does not exceed the activation temperature (step S3: NO), the control unit 11 shifts the process to step S2 and monitors the temperature of the SCR catalyst 13. On the other hand, when the temperature of the SCR catalyst 13 exceeds the activation temperature (step S3: YES), the controller 11 shifts the process to step S4.

  In step S4, the control unit 11 changes the mixing ratio of the internal combustion engine 22 from stoichiometry to lean. Specifically, the control unit 11 transmits an instruction signal for changing the mixing ratio of the internal combustion engine 22 to lean to the combustion state control unit 21. The combustion state control unit 21 that has received the instruction signal changes the mixing ratio of the internal combustion engine 22 from stoichiometry to lean by changing the amount of air and fuel supplied to the internal combustion engine 22. Accordingly, the exhaust gas mixing ratio on the upstream side of the three-way catalyst 12 and the SCR catalyst 13 is also changed from stoichiometry to lean.

  In step S5, the control unit 11 determines whether an end condition is satisfied. Various conditions can be used as the termination condition. For example, one of the following conditions b1 and b1-1 can be used. In the following description, a case where the condition b1-1 is employed will be described as an example.

(B1) When a certain time has elapsed since the air-fuel mixture is lean: The control unit 11 determines that the end condition is satisfied when a certain time has elapsed since the air-fuel mixture of the internal combustion engine 22 is made lean in step S4. To do. This fixed time can be arbitrarily determined, and can be, for example, the time until the total amount of ammonia that can be adsorbed in the SCR catalyst 13 is consumed by the reduction action of NOx.
(B1-1) When the removal of the ammonia adsorbed on the SCR catalyst 13 is completed: The control unit 11 obtains the “certain time” of the condition b1 as follows, thereby setting the end condition to the SCR catalyst 13. The removal of the adsorbed ammonia may be completed. First, the control unit 11 calculates a calculation formula obtained from an experiment or the like in advance from the temperature of the three-way catalyst 12 acquired by the second temperature acquisition unit 15 and the exhaust gas mixture ratio acquired by the mixture ratio acquisition unit 14. The amount of ammonia adsorbed on the SCR catalyst 13 is estimated using a map or the like. The control unit 11 obtains a time (consumption time) until the estimated amount of ammonia is consumed by the reduction action of NOx, and sets the obtained consumption time as “a certain time”.

  When the termination condition of step S5 is not satisfied (step S5: NO), the control unit 11 shifts the process to step S5 and monitors the satisfaction of the termination condition. During this time, the mixture ratio of the internal combustion engine 22, that is, the exhaust gas mixture ratio upstream of the three-way catalyst 12 and the SCR catalyst 13, is maintained lean. On the other hand, when the termination condition is satisfied (step S5: YES), the control unit 11 shifts the process to step S6.

  In step S6, the control unit 11 changes the mixing ratio of the internal combustion engine 22 from lean to stoichiometric. Specifically, the control unit 11 transmits an instruction signal for changing the mixing ratio of the internal combustion engine 22 to the stoichiometry to the combustion state control unit 21. The combustion state control unit 21 that has received the instruction signal changes the mixing ratio of the internal combustion engine 22 from lean to quantitative by changing the air and fuel supply amounts to the internal combustion engine 22. Thereby, the exhaust gas mixing ratio upstream of the three-way catalyst 12 and the SCR catalyst 13 is also changed from lean to stoichiometric. After the end of step S6, the control unit 11 shifts the process to step S1 and waits until the start condition is satisfied again. As a result, until the start condition is satisfied again, the mixing ratio of the internal combustion engine 22, that is, the mixing ratio of the exhaust gas upstream of the three-way catalyst 12 and the SCR catalyst 13, is maintained in a stoichiometric manner.

  FIG. 4 is a diagram showing a change in the state of each part after the internal combustion engine 22 is started. FIG. 4A shows the change over time in the ammonia concentration C4 at the outlet of the SCR catalyst 13, in other words, the ammonia concentration C4 discharged from the exhaust purification device 10. FIG. 4B shows a change with time of the amount AA (ammonia adsorption amount) of ammonia adsorbed on the SCR catalyst 13. FIG. 4C shows changes over time in the ammonia concentration C2 and the NOx concentration C3 at the outlet of the three-way catalyst 12, in other words, the ammonia concentration C2 and the NOx concentration C3 flowing into the SCR catalyst 13. FIG. 4D shows the change over time in the exhaust gas mixture ratio MR on the upstream side of the three-way catalyst 12 and the SCR catalyst 13. FIG. 4E shows the change over time of the ammonia concentration C1 at the outlet of the internal combustion engine 22. FIG. 4F shows the change over time of the temperature ORt of the three-way catalyst 12 and the temperature SCRt of the SCR catalyst 13.

FIG. 5 is a diagram showing an outline of the reaction in the three-way catalyst 12 and the SCR catalyst 13. FIG. 5 also shows oxygen, nitrogen (N ₂ ), water (H ₂ O), hydrogen, and the like other than harmful substances in addition to ammonia and NOx that are harmful substances in exhaust gas. Hereinafter, the change of each part through the control of FIG. 3 will be described with reference to FIGS. 4 and 5.

  Section r1 from time t0 when the internal combustion engine 22 is started to time t2 when the temperature ORt of the three-way catalyst 12 exceeds the activation temperature Th1 (FIG. 3, step S1):

  As shown in FIG. 4 (E), from the time t0 when the internal combustion engine 22 is started to the time t1 immediately after the start, the internal combustion engine 22 has a large amount of unburned ammonia. The outlet ammonia concentration C1 increased rapidly. Thereafter, the combustion in the internal combustion engine 22 is stabilized from time t1 to time t2, and therefore the outlet ammonia concentration C1 of the internal combustion engine 22 gradually decreases. The mixing ratio MR is maintained at a stoichiometry as shown in FIG.

  At this time, as shown in FIG. 4F, the temperature ORt of the three-way catalyst 12 is lower than the activation temperature Th1 (the lower limit temperature at which the oxidation-reduction reaction is activated in the three-way catalyst 12). Therefore, the oxidation of ammonia and the reduction of NOx by the three-way catalyst 12 do not proceed sufficiently, and the ammonia and NOx in the exhaust gas flowing into the three-way catalyst 12 are shown in FIGS. 4 (C) and 5 (A). As shown, it is discharged from the outlet of the three-way catalyst 12. In FIG. 4C, the outlet ammonia concentration C2 and the outlet NOx concentration C3 of the three-way catalyst 12 are gradually decreased as the temperature ORt of the three-way catalyst 12 increases. This is because the oxidation of ammonia and the reduction of NOx proceed gradually.

  Almost all of the ammonia discharged from the outlet of the three-way catalyst 12 is adsorbed by the SCR catalyst 13 as shown in FIG. 5A, and the ammonia adsorption amount AA in the SCR catalyst 13 is as shown in FIG. 4B. It gradually increases. Note that NOx discharged from the outlet of the three-way catalyst 12 passes through the SCR catalyst 13 and is discharged from the exhaust purification device 10.

  As described above, in the interval r1 before the three-way catalyst 12 (oxidation-reduction catalyst) or the SCR catalyst 13 (selective reduction catalyst) reaches the activation temperatures Th1, Th2, such as immediately after the start of the internal combustion engine, the exhaust gas is exhausted to the SCR catalyst 13. By adsorbing the ammonia therein, the discharge of ammonia can be suppressed (FIGS. 4A and 5A).

  Section r2 from time t2 when the temperature ORt of the three-way catalyst 12 exceeds the activation temperature Th1 to time t3 when the temperature SCRt of the SCR catalyst 13 exceeds the activation temperature Th2 (FIG. 3, steps S2 and S3: NO):

  As shown in FIG. 4E, since the combustion in the internal combustion engine 22 is stable from time t2 to time t3, the outlet ammonia concentration C1 of the internal combustion engine 22 is substantially constant. The mixing ratio MR is maintained at a stoichiometry as shown in FIG.

  At this time, as shown in FIG. 4F, the temperature ORt of the three-way catalyst 12 is equal to or higher than the activation temperature Th1. For this reason, the oxidation reaction of ammonia in the three-way catalyst 12 and the reduction reaction of NOx proceed, and the ammonia and NOx in the exhaust gas flowing into the three-way catalyst 12 are shown in FIGS. 4 (C) and 5 (B). As shown in FIG. 2, the three-way catalyst 12 is purified and not discharged. As a result, since ammonia does not flow into the SCR catalyst 13, the ammonia adsorption amount AA in the SCR catalyst 13 does not substantially change as shown in FIG.

  Thus, in the section r2 after the three-way catalyst 12 (redox catalyst) reaches the activation temperature Th1 and before the SCR catalyst 13 (selective reduction catalyst) reaches the activation temperature Th2, the three-way catalyst Since the exhaust gas mixing ratio MR on the upstream side of the catalyst 12 and the SCR catalyst 13 is stoichiometric, the three-way catalyst 12 can oxidize ammonia in the exhaust gas and reduce NOx in the exhaust gas. Can be suppressed (FIGS. 4A and 5B).

  In a section r3 after time t3 when the temperature SCRt of the SCR catalyst 13 exceeds the activation temperature Th2, from time t3 to time t4 when the end condition is satisfied (FIG. 3, steps S3: YES, S4, S5: NO). :

  As shown in FIG. 4E, since the combustion in the internal combustion engine 22 is stable from time t3 to time t4, the outlet ammonia concentration C1 of the internal combustion engine 22 is substantially constant. Further, by step S4 in FIG. 3, the mixture ratio MR is changed from stoichiometry to lean as shown in FIG. Since the mixing ratio MR is no longer stoichiometric, the reduction of NOx by the three-way catalyst 12 does not proceed sufficiently, and the NOx in the exhaust gas flowing into the three-way catalyst 12 is shown in FIGS. 4 (C) and 5 (C). As shown, it is discharged from the outlet of the three-way catalyst 12.

  At this time, as shown in FIG. 4F, the temperature SCRt of the SCR catalyst 13 is equal to or higher than the activation temperature Th2 (the lower limit temperature at which the NOx reduction reaction is activated in the SCR catalyst 13). For this reason, in the SCR catalyst 13, as shown in FIG. 5C, the reducing action of reducing NOx in the exhaust gas proceeds using the accumulated ammonia as a reducing agent. As ammonia is consumed by the reducing action at the SCR catalyst 13, the ammonia adsorption amount AA at the SCR catalyst 13 gradually decreases as shown in FIG. 4B.

  As described above, in the interval r3 after the SCR catalyst 13 (selective reduction catalyst) reaches the activation temperature Th2, the exhaust gas mixing ratio MR on the upstream side of the three-way catalyst 12 and the SCR catalyst 13 decreases from stoichiometry. Therefore, the ammonia adsorbed by the SCR catalyst 13 and the NOx in the exhaust gas can be reduced, the ammonia emission can be suppressed, and the ammonia adsorbed by the SCR catalyst 13 can be removed. (FIG. 4 (A), FIG. 5 (C) and (D)).

  Of the section r3 after time t3 when the temperature SCRt of the SCR catalyst 13 exceeds the activation temperature Th2, after time t4 when the end condition is satisfied (FIG. 3, Steps S5: YES, S6):

  As shown in FIG. 4E, since the combustion in the internal combustion engine 22 is stable from time t4 to time t5, the outlet ammonia concentration C1 of the internal combustion engine 22 is substantially constant. Further, by step S6 in FIG. 3, the mixture ratio MR is changed from lean to stoichiometric as shown in FIG.

  Since the mixing ratio MR returns to the stoichiometry, ammonia oxidation reaction and NOx reduction reaction in the three-way catalyst 12 proceed, and the ammonia and NOx in the exhaust gas flowing into the three-way catalyst 12 are shown in FIG. ) And FIG. 5B, the three-way catalyst 12 is purified and not discharged. As a result, since ammonia does not flow into the SCR catalyst 13, the ammonia adsorption amount AA in the SCR catalyst 13 does not substantially change as shown in FIG.

  In this way, in the interval r3 after the SCR catalyst 13 (selective reduction catalyst) reaches the activation temperature Th2, the exhaust mixture ratio MR on the upstream side of the three-way catalyst 12 and the SCR catalyst 13 becomes stoichiometric. After the change, the three-way catalyst 12 (oxidation-reduction catalyst) can oxidize ammonia in the exhaust gas, and the three-way catalyst 12 can reduce the NOx in the exhaust gas, thereby suppressing ammonia discharge. (FIGS. 4A and 5B).

  Even after the internal combustion engine 22 is stopped and the internal combustion engine 22 is started again after the three-way catalyst 12 and the SCR catalyst 13 return to a temperature lower than the activation temperature, for example, at room temperature, the SCR catalyst 13 is also restarted. Since the ammonia accumulated in is removed, the processing described with reference to FIGS. 3 to 5 can be repeatedly performed.

  As described above, in the exhaust purification device 10 of the above-described embodiment, the exhaust of ammonia from the exhaust purification device 10 can be suppressed from the start immediately after the internal combustion engine 22 to the stop. Further, after the SCR catalyst 13 (selective reduction catalyst) reaches the activation temperature Th2, the ammonia adsorbed on the SCR catalyst 13 can be removed, so that the repeated operation shown in FIG. 5 is possible. Furthermore, according to the exhaust purification device 10 of the present embodiment, the combination of the three-way catalyst 12 (oxidation-reduction catalyst) and the SCR catalyst 13 simplifies the configuration of the exhaust purification device 10 and reduces the size of the exhaust purification device 10. Can be

  Further, in the exhaust purification device 10 of the above embodiment, the three-way catalyst 12 (oxidation-reduction catalyst) is disposed on the upstream side of the main flow path, so that the temperature of the three-way catalyst 12 is easily increased, The time until the activation temperature Th1 is reached can be shortened. Further, the saturated adsorption amount of ammonia in the SCR catalyst 13 (selective reduction catalyst) has a characteristic that it decreases as the temperature of the catalyst increases. According to the exhaust purification device 10 of the present embodiment, since the SCR catalyst 13 is disposed on the downstream side of the main flow path, it is difficult to increase the temperature of the SCR catalyst 13, and the amount of saturated adsorption of ammonia in the SCR catalyst 13 is reduced. Can be suppressed.

  Further, the control unit 11 changes the mixture ratio MR to lean in step S4 of FIG. 3 and advances the reduction reaction of ammonia and NOx by the SCR catalyst 13 (selective reduction catalyst), and then in step S5, the control unit 11 performs a certain period of time. By changing the mixing ratio to stoichiometric after the lapse of time, it is possible to continue the ammonia oxidation reaction by the three-way catalyst 12 (redox catalyst) and the NOx reduction reaction by the three-way catalyst 12. If the condition b1-1 is adopted as the end condition of step S5, the control unit 11 estimates the amount of ammonia adsorbed on the SCR catalyst 13 using the temperature of the three-way catalyst 12 and the exhaust gas mixture ratio. The predetermined time for maintaining the exhaust gas mixing ratio MR on the upstream side of the three-way catalyst 12 and the SCR catalyst 13 is made to be a period until the estimated amount of ammonia is reduced. The removal of the ammonia adsorbed on 13 can be completed.

<Modification of this embodiment>
The present invention is not limited to the above-described embodiment, and can be implemented in various modes without departing from the gist thereof. For example, the following modifications are possible.

[Modification 1]
In the said embodiment, an example of the structure of the engine system was shown. However, the engine system configuration can be variously modified. For example, the engine system may include another device (not shown) (for example, a device that monitors the state of the three-way catalyst or the SCR catalyst).

[Modification 2]
In the said embodiment, an example of the structure of the exhaust gas purification apparatus was shown. However, the configuration of the exhaust emission control device can be variously modified. For example, in the above-described embodiment, the three-way catalyst is disposed on the upstream side of the main flow path, and the SCR catalyst is disposed on the downstream side, but these may be reversed. That is, the SCR catalyst may be disposed on the upstream side of the main flow path, and the three-way catalyst may be disposed on the downstream side. Even in this case, before the three-way catalyst reaches the activation temperature, ammonia is suppressed by adsorbing ammonia to the SCR catalyst, and after the three-way catalyst reaches the activation temperature, the three-way catalyst reaches the activation temperature. By purifying ammonia and NOx with the original catalyst, the emission of ammonia is suppressed, and after the SCR catalyst reaches the activation temperature, the ammonia adsorbed on the SCR catalyst and the NOx in the exhaust are reduced and reacted. Since the ammonia adsorbed on the SCR catalyst can be removed, the same effects as in the above embodiment can be obtained. However, from the viewpoint of quickly moving the three-way catalyst to the catalytically active region and increasing the saturated adsorption amount of ammonia in the SCR catalyst, the three-way catalyst is arranged on the upstream side as in the above embodiment, It is preferable to arrange an SCR catalyst on the downstream side.

  For example, the exhaust purification device further includes various catalysts with different functions such as NOx storage reduction catalyst (NSR catalyst), oxidation catalyst (DOC catalyst: Diesel Oxidation Catalyst), DPF (Diesel particulate filter), etc. May be. The arrangement of these various catalysts can be arbitrarily determined.

[Modification 3]
In the said embodiment, an example of control by a control part was shown. However, the control by the control unit can be variously modified. For example, some procedures in steps S1 to S6 may be omitted, and other steps may be added and executed. For example, the start conditions a1 to a3 illustrated in step S1 are merely examples, and various start conditions can be employed. For example, the end conditions b1 and b1-1 exemplified in step S5 are merely examples, and various end conditions can be adopted.

  As mentioned above, although this aspect was demonstrated based on embodiment and a modification, embodiment of an above-described aspect is for making an understanding of this aspect easy, and does not limit this aspect. This aspect can be changed and improved without departing from the spirit and scope of the claims, and equivalents are included in this aspect. Further, if the technical feature is not described as essential in the present specification, it can be deleted as appropriate.

DESCRIPTION OF SYMBOLS 1 ... Engine system 10 ... Exhaust purification device 11 ... Control part 12 ... Three-way catalyst 13 ... SCR catalyst 14 ... Mixing ratio acquisition part 15 ... 2nd temperature acquisition part 16 ... 1st temperature acquisition part 19 ... Exhaust pipe 20 ... Combustion apparatus 21 ... Combustion state control unit 22 ... Internal combustion engine

Claims (5)

An exhaust purification device for an internal combustion engine that obtains driving force by burning ammonia,
A redox catalyst having an oxidizing action and a reducing action provided in a main flow path through which exhaust from the internal combustion engine flows;
A selective reduction catalyst provided in the main flow path;
A temperature acquisition unit for acquiring the temperature of the selective reduction catalyst;
When the temperature of the selective reduction catalyst acquired by the temperature acquisition unit exceeds the activation temperature of the selective reduction catalyst, the mixing ratio of the exhaust gas upstream of the redox catalyst and the selective reduction catalyst is stoichiometrically determined. A control unit that changes from sparse to lean,
An exhaust emission control device. The exhaust emission control device according to claim 1,
The exhaust purification apparatus, wherein the oxidation-reduction catalyst is disposed upstream of the selective reduction catalyst in the main flow path. The exhaust emission control device according to claim 1 or 2,
The controller is
An exhaust emission control device that changes the exhaust gas mixture ratio to stoichiometric after a predetermined time has elapsed after changing the exhaust gas mixture ratio to lean. The exhaust emission control device according to claim 3, further comprising:
A second temperature acquisition unit for acquiring the temperature of the oxidation-reduction catalyst;
A mixing ratio acquisition unit for acquiring a mixing ratio of the exhaust,
The control unit further includes:
The amount of ammonia adsorbed on the selective reduction catalyst using the temperature of the oxidation-reduction catalyst acquired by the second temperature acquisition unit and the mixture ratio of the exhaust gas acquired by the mixture ratio acquisition unit Estimate
The exhaust emission control device, wherein the predetermined time is a time until the estimated amount of ammonia is reduced in the selective reduction catalyst. A method for purifying exhaust gas from an internal combustion engine that obtains driving force by combustion of ammonia using a redox catalyst and a selective reduction catalyst,
Obtaining the temperature of the selective reduction catalyst;
When the temperature of the selective reduction catalyst exceeds the activation temperature of the selective reduction catalyst, changing the mixing ratio of the exhaust gas upstream of the redox catalyst and the selective reduction catalyst from stoichiometric to lean;
A method comprising: