JP2010270616A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine, suppressing a decrease in a NOx purifying rate and occurrence of an ammonia slip. <P>SOLUTION: This exhaust emission control device for the internal combustion engine includes: a NOx adsorption catalyst 25; a NOx catalyst 35; a urea water adding valve 70 as a reducing agent supply means; and a burner 60 as a heating means. When a temperature raising process is performed to raise the bed temperature of the NOx catalyst 35 to a predetermined temperature range for relatively increasing the NOx conversion rate by the burner 60, a reducing agent required for converting the maximum emission of NOx to be emitted from the NOx adsorption catalyst 25 with temperature raised from a first temperature range toward a second temperature range with the temperature increasing process is set as the maximum supply, and the reducing agent within a range not exceeding the maximum supply is supplied before the temperature raising process for the NOx catalyst 35 is finished. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine.

  As an exhaust purification device that purifies nitrogen oxide (NOx) contained in exhaust gas discharged from an internal combustion engine, Patent Document 1 discloses a NOx adsorption catalyst (oxidation catalyst) that adsorbs NOx provided in an exhaust passage, and this oxidation. A catalyst having a NOx catalyst that selectively reduces and purifies NOx using ammonia as a reducing agent provided downstream of the catalyst is disclosed. In this exhaust purification device, in order to suppress the release of NOx and ammonia to the atmosphere, the supply amount of the reducing agent is adjusted according to the NOx adsorption amount of the oxidation catalyst. In particular, when the NOx adsorption amount of the oxidation catalyst approaches the saturated adsorption amount, in order to prevent the release of NOx into the atmosphere, the ammonia adsorption amount of the NOx catalyst is maintained high in advance, and the NOx release amount due to the temperature increase of the oxidation catalyst Discloses a technique for preparing for a temporary increase in

JP 2008-261253 A

  By the way, as the bed temperature of the oxidation catalyst rises, the bed temperature of the NOx catalyst also rises. When the bed temperature of the NOx catalyst rises, the saturated adsorption amount, which is the maximum amount that can adsorb ammonia, decreases. For this reason, if the ammonia adsorption amount of the NOx catalyst is set to be high in advance, ammonia may be released into the atmosphere due to an increase in the bed temperature.

  An object of the present invention is to provide an exhaust purification device for an internal combustion engine that can suppress a decrease in the NOx purification rate and the occurrence of ammonia slip.

An exhaust gas purification apparatus for an internal combustion engine according to the present invention is provided in an exhaust passage of the internal combustion engine, capable of adsorbing and holding NOx contained in exhaust gas up to a saturated NOx adsorption amount corresponding to the bed temperature, and the NOx A NOx catalyst which is provided downstream of the adsorption catalyst and has a function of adsorbing the supplied reducing agent, and selectively reduces and purifies NOx from the NOx adsorption catalyst based on the reducing agent; and the NOx The reducing agent supply means for supplying a reducing agent to the catalyst, and heating means for heating the NOx adsorption catalyst and the exhaust gas introduced into the NOx catalyst to raise the temperature of both the NOx adsorption catalyst and the NOx catalyst. When the temperature raising process is performed to raise the bed temperature of the NOx catalyst to a predetermined temperature range for relatively increasing the NOx purification rate by the heating means, The maximum supply is defined as the maximum supply amount of the reducing agent necessary for purifying the maximum release amount of NOx that can be released from the NOx adsorption catalyst that is heated from the first temperature range toward the second temperature range. The reducing agent in a range not exceeding the amount is supplied to the NOx catalyst before the temperature raising process is completed.
According to this configuration, since the adsorption characteristics according to the bed temperature of the NOx adsorption catalyst are known, the temperature of the NOx adsorption catalyst was raised from the first temperature range to the second temperature range during the temperature raising process. The maximum amount of NOx released from the NOx adsorption catalyst can be determined. By supplying the reducing agent within a range that does not exceed the maximum supply amount necessary for purification of this maximum amount of NOx, slip of the reducing agent can be suppressed.

In the above configuration, the predetermined temperature range may be a temperature range where the NOx catalyst is activated.
According to this configuration, NOx released from the NOx adsorption catalyst can be purified efficiently.

  In the above configuration, a reducing agent in an amount necessary for purifying NOx that can be released from the NOx adsorption catalyst by the temperature raising process according to an estimated value of the NOx adsorption amount of the NOx adsorption catalyst in the first temperature range. The structure which supplies can be adopted.

  The said structure WHEREIN: The structure which supplies the said reducing agent before the said temperature rising process and makes it adsorb | suck to the said NOx catalyst is employable.

In the above configuration, an amount of reducing agent that does not exceed the saturated reducing agent adsorption amount of the NOx catalyst in the predetermined temperature range is supplied before the temperature raising process, and a deficient amount of the reducing agent for NOx purification is raised in the temperature raising. It is possible to adopt a configuration that supplies after the start of processing.
According to this configuration, since the amount of the reducing agent adsorbed on the NOx catalyst before the temperature rise does not exceed the saturated reducing agent adsorption amount in the predetermined temperature range, the reducing agent slip can be reliably prevented. In addition, since the reducing agent adsorbed on the NOx catalyst is consumed after the temperature raising process is started, even if a deficient amount of reducing agent is supplied, occurrence of reducing agent slip can be prevented.

  According to the present invention, it is possible to suppress a decrease in the NOx purification rate and the occurrence of ammonia slip.

1 is a schematic diagram illustrating a configuration of an exhaust gas purification apparatus for an internal combustion engine according to an embodiment of the present invention. It is a flowchart which shows an example of the process by ECU. It is a timing chart which shows the change of various state quantities. It is a graph which shows an example of the relationship between the bed temperature of a NOx adsorption catalyst, and NOx adsorption amount. It is a graph which shows the relationship between NOx catalyst bed temperature, ammonia adsorption amount, and NOx purification rate. It is a graph which shows the relationship between the bed temperature of the NOx adsorption catalyst in other embodiment of this invention, and NOx adsorption amount. It is a timing chart which shows the change of the various state quantities in other embodiments of the present invention. It is a graph which shows the relationship between NOx catalyst bed temperature and ammonia adsorption amount. It is a flowchart which shows an example of the process by ECU in further another embodiment of this invention. It is a timing chart which shows the change of the various state quantity in other embodiments of the present invention.

Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a configuration diagram of an exhaust emission control device for an internal combustion engine according to an embodiment of the present invention.
The internal combustion engine 10 is, for example, a diesel engine, and a burner 60 as a heating unit is provided on the upstream side of the exhaust passage 15 of the internal combustion engine 10. An air supply path 61 for supplying air and a fuel supply path 62 for supplying fuel are connected to the burner 60 from the internal combustion engine 10 side. The burner 60 combusts the fuel supplied from the fuel supply path 62 and supplies the combustion gas to the exhaust passage 15. Further, by controlling the amount of air from the air supply path 61 and the amount of fuel from the fuel supply path 62, the air-fuel ratio of the combustion gas is controlled. The burner 60 is used to raise the bed temperature of the NOx adsorption catalyst 25, DPF (diesel particulate filter) 30, and NOx catalyst 35, which will be described later.

  The combustion gas exhausted from the burner 60 may be exhausted to the exhaust passage 15 in a completely burned state, or may be exhausted to the exhaust passage 15 in a state containing unburned fuel.

  On the downstream side of the burner 60 in the exhaust passage 15, a NOx adsorption catalyst 25, a DPF 30 and a NOx catalyst 35 are provided in this order.

  In the exhaust passage 15, NOx sensors 95 </ b> A, 95 </ b> B, and 95 </ b> C that detect the concentration of nitrogen oxide are provided at the inlet and outlet of the NOx adsorption catalyst 25 and the inlet of the NOx catalyst 35, respectively. Further, exhaust temperature sensors 90A and 90B are provided at the inlet and the outlet of the DPF 30, respectively. Detection signals from these sensors are input to the ECU 100.

  Further, in the exhaust passage 15, between the DPF 30 and the NOx catalyst 35, a urea water addition valve 70 as a reducing agent supply means for adding an aqueous urea solution to the exhaust passage 15, and downstream of the urea water addition valve 70. And an addition valve downstream mixer 80 for mixing the exhaust gas EG and the urea aqueous solution.

  The NOx adsorption catalyst 25 is composed of an adsorption catalyst such as zeolite and adsorbs NOx contained in the exhaust gas EG. The saturated NOx adsorption amount, which is the maximum NOx adsorption amount that can be adsorbed by the NOx adsorption catalyst 25, varies according to the bed temperature, as will be described later. Utilizing this characteristic, the NOx adsorbed catalyst 25 is purged by changing the bed temperature of the NOx adsorption catalyst 25 to change the saturated NOx adsorption amount.

  The DPF 30 is a filter that collects particulate matter (PM) contained in the exhaust gas EG. As is well known, the structure of the DPF 30 is composed of, for example, a honeycomb body made of metal or ceramics. The DPF 30 needs to be regenerated when a predetermined amount of PM is deposited. Specifically, exhaust gas EG and unburned fuel heated by the burner 60 are supplied to the DPF 30. As a result, the collected PM is burned and the filter function is regenerated. The temperature of the DPF 30 in this regeneration process is, for example, about 600 to 700 ° C. Note that the determination of whether a predetermined amount of PM has accumulated on the DPF 30 is a well-known technique, and a description thereof will be omitted. Further, the DPF 30 may be configured to carry an oxidation catalyst made of a noble metal.

  The urea water addition valve 70 is supplied with urea water from a tank 75 that stores a urea aqueous solution, and adds an amount of urea water according to a control signal from the ECU 100 to the exhaust passage 15.

  The NOx catalyst (hereinafter also referred to as SCR catalyst) 35 selectively reduces NOx contained in the exhaust gas EG into nitrogen gas and water by using a urea aqueous solution added from the urea addition valve 70 as a reducing agent. To do. Specifically, the urea aqueous solution added to the exhaust gas EG is hydrolyzed by the heat of the exhaust gas EG, changed to ammonia as a reducing agent, and adsorbed and held on the NOx catalyst 35. The ammonia adsorbed and held by the NOx catalyst 35 reacts with NOx and is reduced to water and harmless nitrogen. If the ammonia adsorption amount of the NOx catalyst 35 exceeds the saturated adsorption amount, ammonia slip may occur, and if it is too small, NOx may not be sufficiently purified. In addition, it is also possible to supply ammonia directly instead of urea water.

  The NOx catalyst 35 has a well-known structure. For example, the NOx catalyst 35 is composed of zeolite containing Si, O, and Al as main components and containing Fe ions, for example, a vanadium catalyst on the surface of a base material made of aluminum oxide alumina. A material carrying (V2O5) or the like can be used, and is not particularly limited thereto.

  The ECU 100 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), a backup memory such as an EEPROM (Electronically Erasable and Programmable Read Only Memory), an A / D converter, a buffer, and the like. It comprises hardware including an output interface circuit including an input interface circuit, a drive circuit, etc., and necessary software. The ECU 100 controls the amount of urea water added from the urea water addition valve 70 based on signals from the exhaust temperature sensors 90A and 90B, NOx sensors 95A to 95c, and the bed of the NO adsorption catalyst 25 by a burner 60 described later. Control of the temperature and the bed temperature of the NOx catalyst 35 is executed.

Next, an example of the temperature raising process by the ECU 100 will be described with reference to FIGS.
First, the burner 60 is activated to raise the bed temperature of the NOx adsorption catalyst 25 until it reaches the activation temperature range (first temperature range) Ta (° C.) (step S1). Here, as shown in FIG. 4, in the NOx adsorption catalyst 25, the saturated NOx adsorption amount, which is the maximum NOx adsorption amount, changes according to the bed temperature. Specifically, when the temperature of the NOx adsorption catalyst 25 rises from a low temperature state such as during cold start, the saturated NOx adsorption amount becomes maximum in the temperature range Ta, and the adsorption amount decreases as the temperature rises thereafter. To go. For this reason, in order to maximize the NOx adsorption amount, the bed temperature of the NOx adsorption catalyst 25 is controlled to the temperature range Ta.

  As the bed temperature of the NOx adsorption catalyst 25 increases toward the temperature range Ta, the NOx adsorption amount adsorbed by the NOx adsorption catalyst 25 also increases as shown in FIG. Further, as the NOx adsorption catalyst 25 rises in temperature, the NOx catalyst bed temperature (SCR bed temperature) also rises.

  Here, in the NOx catalyst 35, as shown in FIG. 5, the saturated ammonia adsorption amount, which is the maximum ammonia adsorption amount, changes according to the bed temperature. Specifically, as the temperature of the NOx catalyst 35 rises from a low temperature state such as during cold start, the saturated ammonia adsorption amount becomes maximum in the temperature range T1, and the saturated ammonia adsorption amount decreases as the temperature rises thereafter. I will do it. On the other hand, the NOx catalyst 35 increases in the NOx purification rate as the bed temperature rises from the low temperature state, and is activated in the temperature range T2 that is higher than the temperature range T1 and has a relatively high purification rate. . For this reason, when purifying NOx, it is necessary to control the bed temperature of the NOx catalyst 35 to the temperature range T2.

  In order to control the temperature increase of the NOx catalyst 35, it is first determined whether or not the SCR bed temperature has reached a predetermined temperature range T0 (step S2). The predetermined temperature range T0 is a temperature range in which the NOx catalyst 35 can adsorb a sufficient amount of ammonia, as shown in FIG.

  In step S2, urea water (ammonia) is supplied when the SCR bed temperature reaches a predetermined temperature range T0 (step S3). Here, the amount of ammonia to be supplied is the maximum amount of ammonia Amax that is required to purify the maximum amount of NOx that can be released from the NOx adsorption catalyst 25 that is heated in accordance with the temperature raising process of the NOx catalyst 35. The amount is in a range not exceeding the maximum supply amount Amax. Specifically, as shown in FIG. 4, when the bed temperature of the NOx adsorption catalyst 25 is γ and β when the saturated NOx adsorption amounts in the temperature ranges Ta and Tb are γ and β, the NOx catalyst 35 is heated to perform the NOx adsorption catalyst 25. When the bed temperature of 25 rises from the temperature range Ta to the temperature range Tb, the maximum NOx release amount Emax that may be released is β-γ. The maximum supply amount Amax of ammonia is an amount capable of purifying NOx of the maximum release amount Emax.

  In the present embodiment, as shown in FIG. 3, ammonia (urea water) having a maximum supply amount Amax is set at a constant amount (per unit time) before starting the temperature raising process of the NOx catalyst 35 so that ammonia slip does not occur. ) Is added.

  Next, it is determined whether the bed temperature of the NOx catalyst 35 has reached a predetermined temperature range T1 (step S4). As shown in FIG. 5, the temperature range T1 is a temperature range where the ammonia adsorption amount becomes maximum. From the viewpoint of maximizing the amount of adsorption of ammonia, the temperature raising process of the NOx catalyst 35 is started when this temperature range is reached (step S6). The predetermined temperature range T1 is not limited to this, and can be set as appropriate. In step S4, when the bed temperature of the NOx catalyst 35 has not reached the predetermined temperature range T1, it is determined whether the NOx adsorption amount estimated by the NOx catalyst 35 has reached the threshold value N (step S5). The threshold value N is set to, for example, the saturated NOx adsorption amount β. Even when the bed temperature of the NOx catalyst 35 does not reach the predetermined temperature range T1, if the ammonia adsorption amount of the NOx catalyst 35 is saturated, the temperature raising process of the NOx catalyst 35 is started (step S6). ).

  When the temperature of the NOx catalyst 35 is increased toward the predetermined temperature range T2 for activating the NOx catalyst 35, as shown in FIG. 3, the bed temperature of the NOx adsorption catalyst 25 also increases, and the temperature range as the second temperature range. Tb is reached. As the bed temperature of the NOx adsorption catalyst 25 increases, the saturated NOx adsorption amount of the NOx adsorption catalyst 25 decreases as shown in FIG. For this reason, NOx is released from the NOx adsorption catalyst 25, and the NOx adsorption amount of the NOx adsorption catalyst 25 also decreases. Further, as shown in FIG. 5, the ammonia adsorption amount of the NOx catalyst 35 decreases simultaneously with the start of the temperature raising process of the NOx catalyst 35, and the ammonia adsorption amount does not increase during the temperature raising process. For this reason, ammonia slip hardly occurs.

  In the above embodiment, the amount of ammonia supplied to the NOx catalyst 35 before the start of the temperature raising process of the NOx catalyst 35 is the maximum supply amount Amax that can purify the maximum NOx release amount Emax. However, as shown in FIG. 6, for example, when the bed temperature of the NOx adsorption catalyst 25 is raised to the temperature range Ta, the actual NOx adsorption amount X may not reach the saturated NOx adsorption amount β. In such a case, if ammonia with the maximum supply amount Amax is supplied, ammonia may become excessive.

  For this reason, as shown in FIG. 7, it is also possible to adjust the ammonia supply amount to Am in accordance with the estimated NOx adsorption amount X adsorbed on the NOx adsorption catalyst 25 in the temperature range Ta. The ammonia supply amount Am is the amount of ammonia necessary to purify the NOx amount (X−γ) released when the NOx adsorption catalyst 25 is heated from the temperature range Ta to the temperature range Tb.

Next, still another embodiment of the present invention will be described with reference to FIGS.
As shown in FIG. 8, the NOx catalyst 35 has an ammonia adsorption amount that changes according to the bed temperature. For example, assuming that the ammonia adsorption amounts in the predetermined temperature range T1 and T2 are A1 and A2, respectively, when the bed temperature rises from the predetermined temperature range T1 to T2, ammonia corresponding to A1-A2 is released from the NOx catalyst 35. The When all of the ammonia corresponding to A1-A2 is not consumed by NOx purification, the rest slips and is discharged to the outside. In the present embodiment, in order to reliably prevent ammonia slip due to an increase in the bed temperature of the NOx catalyst 35, an amount of ammonia that does not exceed the saturated ammonia adsorption amount A2 of the NOx catalyst 25 in the predetermined temperature region T2 is increased in temperature of the NOx catalyst 35. It is supplied before the treatment, and a shortage of ammonia for NOx purification is supplied after the temperature raising treatment is started.

FIG. 9 is a flowchart illustrating an example of processing by the ECU 100.
First, steps S11 and S12 are the same as steps S1 and S2 described in FIG.

  When the bed temperature of the NOx catalyst 35 exceeds the predetermined temperature range T0, addition of ammonia (urea water) to the NOx catalyst 35 is started (step S13). Then, it is determined whether the ammonia addition amount A has reached the saturated ammonia adsorption amount A2 in the predetermined temperature range T2 (step S14). When the ammonia addition amount A reaches the saturated ammonia adsorption amount A2, the ammonia addition is stopped (step S15). As a result, as shown in FIG. 10, the saturated ammonia adsorption amount A2 of ammonia is adsorbed to the NOx catalyst 35.

  Next, it is determined whether the bed temperature of the NOx catalyst 35 has reached a predetermined temperature range T1 (step S16). The temperature range T1 is the temperature range shown in FIG. When reaching this temperature range, the temperature raising process of the NOx catalyst 35 is started (step S18). In step S16, if the bed temperature of the NOx catalyst 35 has not reached the predetermined temperature range T1, it is determined whether the NOx adsorption amount estimated by the NOx catalyst 35 has reached the threshold value N (step S17). The threshold value N is set to, for example, the saturated NOx adsorption amount in the temperature range T1. Even when the bed temperature of the NOx catalyst 35 does not reach the predetermined temperature range T1, if the ammonia adsorption amount of the NOx catalyst 35 is saturated, the temperature raising process of the NOx catalyst 35 is started (step S18). ).

  When starting toward the temperature range Tb in which the bed temperature of the NOx catalyst 35 is activated, the ammonia adsorption amount of the NOx catalyst 35 is consumed for NOx purification and decreases, as shown in FIG. At the same time, a shortage of ammonia (A1-A2) necessary for NOx purification is added (step S19). Note that the short amount of ammonia (A1-A2) is all added until the bed temperature of the NOx catalyst 35 reaches the predetermined temperature range Tb.

  Before the NOx catalyst 35 is heated, an amount of ammonia that does not exceed the saturated ammonia adsorption amount A2 in the predetermined temperature range T2 is added in advance, and after the NOx catalyst 35 is activated and NOx purification is started, a shortage of ammonia ( By adding A1-A2), it is possible to reliably suppress ammonia slip caused by the temperature rise of the NOx catalyst 35.

  In the above embodiment, the burner 60 is used as the heating means. However, for example, other heating means such as a heater for heating the exhaust gas itself may be used.

DESCRIPTION OF SYMBOLS 10 ... Internal combustion engine 15 ... Exhaust passage 25 ... NOx adsorption catalyst 30 ... DPF (filter)
35 ... NOx catalyst 60 ... Burner 70 ... Urea water addition valve 100 ... ECU
90A, 90B ... exhaust temperature sensors 95A to 95c ... NOx sensors

Claims (5)

A NOx adsorption catalyst provided in the exhaust passage of the internal combustion engine, capable of adsorbing and holding NOx contained in the exhaust gas up to a saturated NOx adsorption amount corresponding to the bed temperature;
A NOx catalyst provided on the downstream side of the NOx adsorption catalyst, having a function of adsorbing the supplied reducing agent, and selectively reducing and purifying NOx from the NOx adsorption catalyst based on the reducing agent;
The reducing agent supply means for supplying a reducing agent to the NOx catalyst;
Heating means capable of heating the NOx adsorption catalyst and the exhaust gas introduced into the NOx catalyst to raise the temperature of both the NOx adsorption catalyst and the NOx catalyst,
When performing the temperature raising process in which the heating means raises the bed temperature of the NOx catalyst to a predetermined temperature range for relatively increasing the NOx purification rate, the heating temperature increases from the first temperature range to the first temperature range. The reduction within a range not exceeding the maximum supply amount, with the maximum supply amount being a reducing agent necessary to purify the maximum release amount of NOx that can be released from the NOx adsorption catalyst heated to the temperature range of 2. An agent is supplied to the NOx catalyst before the temperature raising process is completed.
An exhaust emission control device for an internal combustion engine.   The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the predetermined temperature range is a temperature range where the NOx catalyst is activated.   Supplying a reducing agent in an amount necessary for purifying NOx that can be released from the NOx adsorption catalyst by the temperature raising process according to an estimated value of the NOx adsorption amount of the NOx adsorption catalyst in the first temperature range; The exhaust emission control device for an internal combustion engine according to claim 1 or 2.   The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 3, wherein the reducing agent is supplied and adsorbed to the NOx catalyst before the temperature raising process.   An amount of reducing agent that does not exceed the saturated reducing agent adsorption amount of the NOx catalyst in the predetermined temperature range is supplied before the temperature raising process, and a deficient amount of the reducing agent for NOx purification is supplied after the temperature raising process is started The exhaust emission control device for an internal combustion engine according to claim 1, wherein the exhaust gas purification device is supplied.

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