JP2017025862A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device being for an internal combustion engine, capable of purifying NOin an appropriate mode corresponding to an HC adsorption amount in an NOcatalyst, and configured to purify NOusing the NOcatalyst that adsorbs NOexhausted from the internal combustion engine just after start-up.SOLUTION: In an exhaust emission control device, an exhaust pipe comprises a NOcatalyst containing a carrier consisting of zeolite and Pd carried by the carrier, and NOin an exhaust gas is purified by the NOcatalyst. The exhaust emission control device comprises: HC adsorption amount estimation means which calculates an estimate ΣHC of the HC adsorption amount in the NOcatalyst; and HC desorption mens for desorbing HC adsorbed to the NOcatalyst by causing a temperature of the NOcatalyst to rise in response to the estimate ΣHC of the HC adsorption amount exceeding a predetermined upper limit value.SELECTED DRAWING: Figure 6

Description

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

  An exhaust purification device that purifies exhaust gas from an internal combustion engine is mounted on a vehicle that travels using power generated by the internal combustion engine. As the exhaust gas purification apparatus, an apparatus for purifying exhaust gas using a catalyst provided in an exhaust pipe has become the mainstream. The catalyst cannot exhibit sufficient exhaust purification performance while the temperature does not reach an appropriate temperature. Therefore, in recent years, various techniques for improving the purification performance of exhaust gas immediately after starting the internal combustion engine have been proposed.

  For example, Patent Document 1 discloses a NOx catalyst having characteristics suitable for purification of exhaust gas from an internal combustion engine at the time of starting. The NOx catalyst is characterized by having zeolite and palladium supported on the zeolite. According to the NOx catalyst disclosed in Patent Document 1, NOx in the exhaust is adsorbed under a low temperature condition immediately after the start of the internal combustion engine, and then adsorbed as the NOx catalyst is warmed up. NOx can be reduced and purified.

  Further, for example, in Patent Document 2, when a NOx catalyst having a function of adsorbing NOx in exhaust gas is provided in the exhaust passage, the NOx adsorption amount in the NOx catalyst is estimated, and when the NOx adsorption amount becomes a predetermined value or more. Shows a technique for desorbing NOx adsorbed so far by executing temperature rise control.

  Since there is a limit to the amount of NOx that can be adsorbed by the NOx catalyst, the temperature rise control is executed at an appropriate timing as shown in Patent Document 2 to maintain the adsorption performance of the NOx catalyst by desorbing NOx. be able to.

Japanese Patent Application No. 2014-51628 JP 2009-257231 A

  By the way, the NOx catalyst having zeolite and palladium shown in Patent Document 1 has a characteristic of adsorbing not only NOx in exhaust gas but also HC. Adsorption of HC is thought to affect the NOx adsorption performance of the NOx catalyst, but this point has not been sufficiently studied in the past. For this reason, it is difficult to say that the NOx purification performance by the NOx catalyst is sufficiently utilized only by determining the timing for executing the temperature increase control according to the NOx adsorption amount as in Patent Document 2.

  The present invention purifies NOx using a NOx catalyst that adsorbs NOx discharged from an internal combustion engine immediately after starting, and can purify NOx in an appropriate manner according to the amount of HC adsorbed in the NOx catalyst. An object of the present invention is to provide an exhaust purification device.

  (1) An exhaust purification device (for example, an exhaust purification device 2 described later) of an internal combustion engine according to the present invention is a NOx catalyst (for example, a downstream catalytic converter 32 described later) having a support made of zeolite and Pd supported on the support. Is provided in an exhaust passage (for example, an exhaust pipe 11 described later) of an internal combustion engine (for example, an engine 1 described later), and NOx in the exhaust gas is purified by the NOx catalyst. HC adsorption amount estimation means for estimation (for example, means relating to execution of the processing of S15 in FIG. 6 described later, exhaust temperature sensor 33, air-fuel ratio sensor 34, ECU 6, etc.), and the estimated HC adsorption amount is a predetermined value. The HC desorbing means for desorbing the HC adsorbed on the NOx catalyst by increasing the temperature of the NOx catalyst in response to exceeding (for example, S1 in FIG. 6 described later) Comprising means related to the execution of the processing, and the ECU6 etc.), the.

  (2) In this case, it is preferable that the HC adsorption amount estimation means estimates the HC adsorption amount by using at least one of an air-fuel ratio of exhaust gas flowing into the NOx catalyst and a temperature of the NOx catalyst.

  (1) In the present invention, a NOx catalyst having a support made of zeolite and Pd supported on the support is provided in the exhaust passage. This NOx catalyst has a characteristic of adsorbing NOx at a low temperature and desorbing the adsorbed NOx at a high temperature. For this reason, according to the present invention, for example, NOx discharged from the internal combustion engine immediately after the start can be adsorbed to the NOx catalyst, so that this NOx can be prevented from being discharged out of the exhaust purification device.

  By the way, this NOx catalyst has a function of adsorbing HC in the exhaust gas in addition to the function of adsorbing NOx as described above. For this reason, it is suppressed that unpurified HC is discharged out of the exhaust purification device. However, the NOx adsorption performance of the NOx catalyst has a characteristic that it decreases as the HC adsorption amount increases (for example, see FIG. 4 described later). Therefore, in the present invention, the amount of HC adsorbed on the NOx catalyst is estimated, and the HC adsorbed on the NOx catalyst is desorbed by increasing the temperature of the NOx catalyst in response to the fact that this exceeds a predetermined value. Thereby, it is possible to prevent the amount of HC adsorption in the NOx catalyst from exceeding a predetermined value, so that the NOx adsorption performance of the NOx catalyst can be maintained high. When the temperature of the NOx catalyst is raised, NOx is desorbed together with HC, but NOx desorbed from the NOx catalyst is reduced and purified using HC as a reducing agent. Therefore, it is possible to suppress the HC desorbed from the NOx catalyst from being discharged out of the exhaust purification device as it is.

  (2) When the air-fuel ratio of the exhaust gas flowing into the NOx catalyst becomes low, the amount of HC flowing into the NOx catalyst and adsorbed increases. Further, as the temperature of the NOx catalyst increases, the amount of HC that directly oxidizes in the NOx catalyst increases. As described above, in the present invention, the HC adsorption amount can be accurately estimated by using at least one of the exhaust air-fuel ratio and the NOx catalyst temperature correlated with the HC adsorption amount in the NOx catalyst as described above. As a result, HC can be desorbed from the NOx catalyst at an appropriate timing. Further, by desorbing HC at an appropriate timing, it is possible to suppress a decrease in NOx adsorption performance of the NOx catalyst and thermal deterioration of the NOx catalyst.

1 is a diagram illustrating a configuration of an engine and an exhaust purification device thereof according to an embodiment of the present invention. It is a figure which shows the adsorption | suction and desorption behavior of NOx in a NOx catalyst. It is a figure which compares the adsorption amount and desorption amount of NOx in a NOx catalyst. It is a diagram showing a relationship between the amount of adsorption of the adsorption amount and _C 3 _{H 6} of NOx in the NOx catalyst. It is a flowchart which shows the specific procedure of the NOx purification process at the time of starting which purifies NOx discharged | emitted from the engine immediately after starting. 3 is a flowchart showing a specific procedure of HC desorption control for desorbing HC adsorbed on a NOx catalyst at a predetermined timing. It is an example of the map which determines HC inflow amount according to an exhaust air fuel ratio. It is an example of the map which determines HC purification | cleaning rate according to a catalyst temperature. It is an example of the map which determines HC adsorption | suction efficiency according to a catalyst temperature and HC adsorption amount.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram showing a configuration of an internal combustion engine (hereinafter referred to as “engine”) 1 and an exhaust purification device 2 for purifying exhaust thereof according to the present embodiment.

  The engine 1 is based on so-called lean combustion in which the combustion air-fuel ratio is leaner than stoichiometric, more specifically, a diesel engine, a lean burn gasoline engine, or the like. The engine 1 is provided with a fuel injection valve 17 that injects fuel into each cylinder. The actuator that drives the fuel injection valve 17 is electromagnetically connected to the ECU 6. The ECU 6 determines the fuel injection amount and fuel injection timing from the fuel injection valve 17 under fuel injection control (not shown), and drives the fuel injection valve 17 so that this is realized.

  The catalyst purification device 3 includes an upstream catalytic converter 31, a downstream catalytic converter 32, an exhaust temperature sensor 33, and an air / fuel ratio sensor 34 provided in the exhaust pipe 11.

  The upstream catalytic converter 31 has a flow-through type honeycomb structure as a base material and a three-way catalyst supported on the base material. In a three-way catalyst, under a stoichiometric exhaust ratio, a three-way purification reaction, that is, an oxidation reaction of HC and CO and a reduction reaction of NOx proceed simultaneously. In the three-way catalyst, the oxidation reaction of HC and CO proceeds under a lean air-fuel ratio exhaust.

  The downstream catalytic converter 32 is provided downstream of the upstream catalytic converter 31 in the exhaust pipe 11. The downstream catalytic converter 32 has a flow-through honeycomb structure as a base material, and a NOx catalyst is supported on the base material. The NOx catalyst includes a support made of zeolite and Pd supported on the support. This NOx catalyst can be completely purified by the three-way catalyst, for example, under relatively low temperature conditions immediately after the engine 1 is started (more specifically, for example, before the three-way catalyst of the upstream catalytic converter 31 reaches the activation temperature). It has the function of adsorbing and reducing and purifying NOx that was not present.

  The zeolite of the NOx catalyst, under stoichiometric or rich air-fuel ratio exhaust, takes and adsorbs HC contained in the exhaust into the pores in the skeleton under low temperature conditions, and adsorbs the adsorbed HC under high temperature conditions. Has the property of desorption. The HC desorption temperature at which HC desorption starts is substantially equal to the NOx desorption temperature at which NOx begins to desorb from Pd described later.

  Zeolite includes ZSM-5, ferrierite, mordenite, Y-type zeolite, beta-type zeolite, and CHA-type zeolite. In the present embodiment, any of these may be used alone, or a plurality of them may be used in combination. By supporting Pd on such a zeolite, excellent NOx adsorption performance is exhibited.

Here, usually, zeolite has a characteristic of adsorbing NOx supplied as NO ₂ in its pores. Therefore, in order to convert NO, which mainly constitutes NOx in the exhaust gas, to NO ₂ , the exhaust gas is made lean, has a high oxygen concentration and a high temperature atmosphere, and further requires active species such as Pt. On the other hand, the NOx catalyst of this embodiment exhibits excellent NOx adsorption performance even when the air-fuel ratio of the exhaust is stoichiometric or rich under low temperature conditions by supporting Pd on the support zeolite. The reason is as follows.

That is, in the NOx catalyst, Pd is arranged in the vicinity of Al which is an acid point among Al, Si and O constituting the zeolite. Therefore, Pd is present as divalent Pd ²⁺ because its electronic state is changed by the interaction with Al. Unlike the conventional NOx adsorption of zeolite, this divalent Pd ²⁺ has a characteristic of adsorbing NO as it is without oxidizing NO to NO ₂ . As a result, the NOx catalyst can obtain excellent NOx adsorption performance even when the air-fuel ratio of the exhaust is stoichiometric or rich under low temperature conditions.

  The content of Pd with respect to the entire NOx catalyst is preferably 0.01 to 10% by mass. If the content of Pd is within this range, excellent NOx adsorption performance can be obtained. A more preferable content is 0.1 to 3% by mass.

Further, the NOx catalyst is not limited to a catalyst in which Pd is supported on a support made of zeolite as described above. In addition to the above Pd, at least one additive element selected from the group consisting of Fe, Ce, Pr, Sr, Ba, La, Ga, In and Mn may be co-supported on the zeolite. That is, at least one additive element selected from the group consisting of Ce, Pr, Sr, Ba, La, Ga, In, and Mn is interposed between Pd, so that divalent Pd ²⁺ is zero-valent. Reduction to Pd ⁰ is suppressed, and movement and aggregation of Pd are suppressed, so that deterioration of dispersibility of Pd is suppressed. Therefore, according to such a NOx catalyst, excellent NOx adsorption performance is maintained, and heat resistance in a low oxygen concentration atmosphere is improved.

  FIG. 2 is a diagram showing NOx adsorption and desorption behavior in the NOx catalyst. This FIG. 2 shows that the model gas having the composition shown below is supplied to the NOx catalyst, and the NOx concentration of the gas flowing into the NOx catalyst and the NOx catalyst are maintained in an oxygen-excess atmosphere (oxygen excess ratio λ = 2). The NOx concentration of the gas flowing out from the NOx catalyst when the temperature of the NOx catalyst is changed is shown. In FIG. 2, the horizontal axis represents time (seconds), the right vertical axis represents the temperature of the NOx catalyst [° C.], and the left vertical axis represents the NOx concentration [ppm].

In the model gas, CO is constant at 1000 ppm, O ₂ is constant at 0.1%, NO is changed in a predetermined manner, and N ₂ is used as a balance gas, so that the total oxygen excess ratio λ = 2. did. Here, the NOx concentration of the model gas flowing into the NOx catalyst is set to a predetermined value larger than 0 until about 200 seconds have elapsed from the start of supply of the model gas, as indicated by a broken line in FIG. 0. Further, the temperature of the NOx catalyst is constant at about 50 ° C. until about 1200 seconds from the start of supply of the model gas, as shown by a one-dot chain line in FIG. 2, and about 500 ° C. after about 1200 seconds. Gradually increased until it reached. In addition, since there is almost no change in the NOx concentration, the temperature of the NOx catalyst, etc. between about 400 and 1000 seconds, the illustration between these is omitted in FIG.

  As shown in FIG. 2, the NOx concentration of the gas flowing out from the NOx catalyst is about 200 seconds after the NOx catalyst is in a low temperature state of 50 ° C. and the model gas containing NOx starts to be supplied to the NOx catalyst. (Solid line) is lower than the NOx concentration (broken line) of the gas flowing into the NOx catalyst. In particular, immediately after the supply of the model gas is started (about 0 to 100 seconds), the NOx concentration in the exhaust gas flowing out from the NOx catalyst is approximately 0 ppm. This means that almost all NOx (NO) contained in the gas flowing into the NOx catalyst is adsorbed by the NOx catalyst. From this result, it is understood that the NOx catalyst of the downstream catalytic converter can efficiently adsorb NOx (NO) under a low temperature condition of 50 ° C. before the three-way catalyst of the upstream catalytic converter reaches the activity.

  The NOx concentration in the gas flowing out from the NOx catalyst gradually increases until approximately 200 seconds elapse after the supply of the model gas starts, and becomes substantially equal to the NOx concentration of the gas flowing into the NOx catalyst. (See around 200 seconds in FIG. 2). This means that there is a limit to the amount of NOx that can be adsorbed by the NOx catalyst, and as the amount of NOx adsorbed to the NOx catalyst approaches this limit amount, NOx becomes difficult to adsorb (decrease in the NOx adsorption rate). To do. That is, when about 200 seconds have elapsed, the NOx catalyst has adsorbed an amount of NOx that is almost close to the limit amount. Further, the area of the region Tad in FIG. 2 represents the total amount of NOx adsorbed by the NOx catalyst (that is, the NOx adsorption amount).

  When the NOx concentration of the model gas is reduced to 0 ppm when about 200 seconds have elapsed, the NOx concentration of the gas flowing out from the NOx catalyst is immediately reduced to 0 ppm accordingly. Thereafter, as shown in FIG. 2, the NOx concentration of the gas flowing out from the NOx catalyst is approximately 0 ppm. That is, the NOx catalyst has a function of continuously holding NOx adsorbed on the NOx catalyst in 0 to 200 seconds in an oxygen-excess atmosphere.

Further, after the NOx concentration is reduced to 0 ppm, the temperature of the NOx catalyst is increased at a substantially constant rate between about 1200 and 2500 seconds. At this time, the NOx concentration of the gas flowing out from the NOx catalyst starts to increase from 0 ppm in about 1800 seconds as shown in FIG. 2, and returns to 0 ppm again in about 2300 seconds. Note that the temperature of the NOx catalyst is about 250 ° C. for about 1800 seconds and about 450 ° C. for about 2300 seconds. This means that the NOx adsorbed on the NOx catalyst was desorbed between the time when the temperature of the NOx catalyst exceeded about 250 ° C. and the time when it exceeded 450 ° C. Hereinafter, the temperature at which the desorption of NOx adsorbed on the NOx catalyst starts (about 250 ° C. in the example of FIG. 2) is referred to as the NOx desorption temperature. At this time, almost all NOx desorbed from the NOx catalyst was NO, and almost no NO ₂ or N ₂ O was observed. The area of the region Tdes in FIG. 2 represents the total amount of NOx desorbed from the NOx catalyst (that is, the NOx desorption amount).

  FIG. 3 is a diagram comparing the NOx adsorption amount and desorption amount in the NOx catalyst. In FIG. 3, the NOx adsorption amount and the NOx desorption amount were obtained by performing tests for changing the NOx concentration and temperature in accordance with the same procedure as in FIG. 2 using a model gas having a predetermined excess air ratio. The left side of FIG. 3 is a result obtained using a gas having an oxygen excess ratio λ = 2, and the right side of FIG. 3 is a result obtained using a gas having an oxygen excess ratio λ = 0.9.

  As shown on the left side of FIG. 3, under the model gas in an oxygen-excess atmosphere (λ = 2), the NOx desorption amount is almost equal to the NOx adsorption amount. That is, in an oxygen-excess atmosphere, almost all NOx adsorbed on the NOx catalyst at a low temperature is desorbed as it is when the temperature is raised to about 500 ° C.

On the other hand, as shown on the right side of FIG. 3, under the model gas of λ = 0.9, the NOx adsorption amount is almost the same as that of λ = 2, but the NOx desorption amount is the NOx adsorption amount. Significantly less than the amount. That is, in the reducing atmosphere, almost all of the NOx adsorbed on the NOx catalyst at a low temperature is desorbed and reduced to N _{2 while} being heated to about 500 ° C. In other words, the NOx catalyst desorbs HC and CO contained in the exhaust and HC adsorbed on the zeolite as described above as a reducing agent under an appropriate air-fuel ratio exhaust with λ of 1 or less. It means that there is a function to reduce and purify the separated NOx.

FIG. 4 is a diagram showing the relationship between the NOx adsorption amount [g / L] in the NOx catalyst and the adsorption amount [g / L] of C ₃ H ₆ (hereinafter simply referred to as “HC”). That is, when HC is adsorbed on the NOx catalyst, it is a diagram showing the magnitude of the influence of this HC on NOx adsorption. As shown in FIG. 4, the total amount of NOx that can be adsorbed by the NOx catalyst decreases as the HC adsorption amount increases. This is thought to be because if HC is adsorbed on the NOx catalyst, this interferes with NOx adsorption. Therefore, in order to maintain the NOx adsorption performance of the NOx catalyst as high as possible, it is preferable that the HC adsorption amount in the NOx catalyst is appropriately managed so as not to increase excessively.

  Returning to FIG. 1, the exhaust gas temperature sensor 33 is provided on the downstream side of the downstream catalytic converter 32 in the exhaust pipe 11. The exhaust temperature sensor 33 detects the temperature of the exhaust gas flowing out from the downstream catalytic converter 32 and transmits a signal substantially proportional to the detected value to the ECU 6. The temperature of the three-way catalyst of the upstream catalytic converter 31, the temperature of the NOx catalyst of the downstream catalytic converter 32, and the like are estimated by calculation in the ECU 6 based on the output of the exhaust temperature sensor 33, for example.

  The air-fuel ratio sensor 34 is provided between the upstream catalytic converter 31 and the downstream catalytic converter 32 in the exhaust pipe 11. The air-fuel ratio sensor 34 detects the air-fuel ratio of the exhaust gas flowing into the downstream catalytic converter 32 (ratio of fuel components (HC, CO, etc.) to oxygen in the exhaust gas), and transmits a signal substantially proportional to the detected value to the ECU 6. . As the air-fuel ratio sensor 34, for example, a sensor having linear output characteristics from a rich region to a lean region is used.

  The ECU 6 includes an I / O interface for A / D converting sensor detection signals, a CPU for executing processing in accordance with flowcharts shown in FIGS. 5 and 6 to be described later, and various devices in a mode determined under this processing. The microcomputer includes a driving circuit for driving and a RAM, a ROM, and the like for storing various data.

  FIG. 5 is a flowchart showing a specific procedure of start-up NOx purification processing for purifying NOx discharged from the engine immediately after start-up. In the start-up NOx purification process of FIG. 5, NOx contained in the exhaust of the engine immediately after start-up is once adsorbed to the NOx catalyst and then adsorbed by executing NOx purge control at a predetermined timing. NOx is reduced and purified on the NOx catalyst. The process of FIG. 5 is repeatedly executed in the ECU under a predetermined control cycle in response to turning on an ignition switch (not shown) for starting and stopping the engine.

  In S1, the ECU determines whether or not a purge control completion flag is 1. This purge control completion flag is a flag indicating that the first NOx purge control has been completed since the engine was started. The completion flag is set to 0 immediately after the engine is started, and is set to 1 in S8 described later in response to the execution of the NOx purge control over a predetermined time. If the determination in S1 is NO, the ECU proceeds to S3, and if the determination is YES, the ECU immediately ends the process of FIG.

  In S3, the ECU calculates the temperature of the NOx catalyst (hereinafter simply referred to as “catalyst temperature”) based on the output of the exhaust temperature sensor, and proceeds to S4. In S4, the ECU determines whether or not the catalyst temperature acquired in S3 is higher than the desorption temperature of the NOx catalyst, that is, whether or not it is time for NOx adsorbed immediately after the engine starts to desorb from the NOx catalyst. .

  If the determination in S4 is YES, the ECU proceeds to S5, and performs NOx purge control for controlling the air-fuel ratio of the exhaust gas flowing into the NOx catalyst to be stoichiometric or rich in accordance with the timing at which NOx adsorbed at the start is desorbed. Execute and move to S7. Here, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is, for example, making the air-fuel ratio in the combustion chamber of the engine stoichiometric or rich by performing after injection, or performing post injection to supply unburned fuel into the exhaust pipe. Is controlled to be stoichiometric or rich. Thereby, NOx desorbed from the NOx catalyst is reduced and purified on the NOx catalyst.

  In S7, the ECU determines whether or not the value of the purge timer that measures the time during which the purge control in S5 is executed is zero. The value of the purge timer is set to a predetermined initial value greater than 0 immediately after the engine is started, and is set to 0 as the minimum value, and is subtracted by the elapsed time for each control cycle in the process of S9 described later. Therefore, the state where the value of the purge timer is 0 means that the purge control has been executed for a time corresponding to the initial value after starting the engine. If the determination in S7 is YES, the ECU moves to S8, sets the value of the purge control completion flag to 1, and ends the process of FIG.

  If the determination in S7 is NO, the ECU moves to S9, updates the value of the purge timer, and ends the process of FIG. More specifically, the value of the purge timer is updated by subtracting the time corresponding to the control period of FIG. 5 from the previous value of the purge timer.

  FIG. 6 is a flowchart showing a specific procedure of HC desorption control for desorbing HC adsorbed on the NOx catalyst at a predetermined timing. The HC desorption control in FIG. 6 is repeatedly executed in the ECU under a predetermined control cycle in response to turning on an ignition switch (not shown) for starting and stopping the engine.

  In the start-up NOx purification process of FIG. 5 described above, the NOx exhausted immediately after engine startup is purified by making use of the low temperature characteristics of the NOx catalyst described with reference to FIGS. In order to efficiently purify NOx immediately after engine startup by the start-up NOx purification process of FIG. 5, the NOx catalyst needs to maintain its NOx purification performance high. Further, as described with reference to FIG. 4, if HC is adsorbed on the NOx catalyst, the NOx adsorption performance is also lowered accordingly. In the HC desorption control of FIG. 6, an estimated value of the HC adsorption amount in the NOx catalyst is calculated so that the NOx purification performance of the NOx catalyst is maintained high, and the NOx catalyst is adsorbed to the NOx catalyst at an appropriate timing according to this estimated value. The released HC is removed. Hereinafter, a specific procedure of HC desorption control will be described.

  First, in S11, the ECU acquires the HC inflow amount HCin in the NOx catalyst, and proceeds to S12. Here, the HC inflow amount HCin specifically corresponds to the amount of HC flowing into the NOx catalyst during the control cycle from the previous time to the current time. The HC inflow amount HCin is calculated, for example, by searching a map as illustrated in FIG. 7 using an output signal of an air-fuel ratio sensor that changes in accordance with the air-fuel ratio of exhaust flowing into the NOx catalyst. As shown in FIG. 7, the HC inflow amount HCin increases as the exhaust air-fuel ratio changes to the rich side.

  In S12, the ECU calculates the HC purification rate ηHC in the NOx catalyst, and proceeds to S13. Here, the HC purification rate ηHC specifically refers to the proportion of HC flowing into the NOx catalyst that is purified by being oxidized in the NOx catalyst. That is, the amount purified by adsorbing on the NOx catalyst is not included. The HC purification rate ηHC is calculated, for example, by searching a map as illustrated in FIG. 8 using the temperature of the NOx catalyst. As shown in FIG. 8, the higher the temperature of the NOx catalyst, the higher the HC purification rate ηHC.

  In S13, the ECU calculates the HC adsorption efficiency ηHCad in the NOx catalyst, and proceeds to S14. Here, the HC adsorption efficiency ηHCad specifically refers to the ratio of the HC that has not been oxidized and purified by the NOx catalyst and that is adsorbed to the NOx catalyst without being discharged to the downstream side of the NOx catalyst. The HC adsorption efficiency ηHCad is calculated, for example, by searching a map as illustrated in FIG. 9 using the NOx catalyst temperature and the estimated value ΣHC of the HC adsorption amount. As shown in FIG. 9, the NOx adsorption efficiency increases as the temperature of the NOx catalyst decreases, and decreases as the HC adsorption amount increases.

In S14, the ECU calculates a new adsorption amount ΔHC of HC in the NOx catalyst, and proceeds to S15. The new adsorption amount ΔHC corresponds to the amount of HC newly adsorbed on the NOx catalyst during the control cycle from the previous time to the current time. The new adsorption amount ΔHC is calculated by the following equation using, for example, the HC inflow amount ΔHC, the HC purification rate ηHC, and the HC adsorption efficiency ηHCad obtained in the above step.
ΔHC = HCin × (1−ηHC) × ηHCad

  In S15, the ECU updates the estimated value ΣHC by adding the new adsorption amount ΔHC to the previous value of the estimated value ΣHC of the HC adsorption amount, and proceeds to S16. In S16, the ECU determines whether or not the estimated value ΣHC is smaller than a predetermined upper limit value. If the determination in S16 is YES, the ECU determines that sufficient NOx purification performance is secured at this time without removing the HC adsorbed on the NOx catalyst, and immediately ends this processing. If the determination in S16 is NO, it is determined that the time suitable for removing the HC adsorbed on the NOx catalyst has been reached, and the process proceeds to S17.

  In S17, the ECU executes temperature increase control for forcibly increasing the temperature of the NOx catalyst to the HC desorption temperature over a predetermined time, desorbs and removes HC adsorbed on the NOx catalyst, and proceeds to S18. This temperature increase control is realized, for example, by retarding the injection timing of post injection, after injection, main injection, and the like with respect to the combustion parameters of the engine in a normal operating state, and increasing the temperature of the exhaust gas. . As described above, the temperature at which HC is desorbed from the NOx catalyst is substantially equal to the temperature at which NOx is desorbed. Therefore, when the temperature of the NOx catalyst is raised to a temperature at which HC is desorbed in S17, the desorbed HC is consumed for NOx reduction in the NOx catalyst, and thus is not directly discharged to the downstream side of the NOx catalyst. . In S18, the ECU resets the estimated value ΣHC of the HC adsorption amount to 0 in accordance with desorption and removal of the HC adsorbed on the NOx catalyst, and ends this process.

According to the exhaust purification device 2 of the present embodiment as described above, the following effects are obtained.
(1) In the exhaust purification apparatus 2 of the present embodiment, a NOx catalyst having a support made of zeolite and Pd supported on the support is provided in the exhaust pipe 11. Thereby, for example, NOx discharged from the engine immediately after start-up can be adsorbed to the NOx catalyst, so that this NOx can be prevented from being discharged out of the exhaust purification device 2. Further, the NOx catalyst has a function of adsorbing HC in the exhaust gas in addition to the function of adsorbing NOx as described above. For this reason, it is suppressed that unpurified HC is discharged out of the exhaust purification device. However, as described with reference to FIG. 4 and the like, the NOx adsorption performance of the NOx catalyst has a characteristic that it decreases as the HC adsorption amount increases. Therefore, the exhaust purification device 2 estimates the amount of HC adsorbed on the NOx catalyst, and desorbs the HC adsorbed on the NOx catalyst by increasing the temperature of the NOx catalyst in response to the fact that this exceeds a predetermined upper limit. As a result, it is possible to prevent the HC adsorption amount in the NOx catalyst from exceeding the upper limit value, so that the NOx adsorption performance of the NOx catalyst can be maintained high. When the temperature of the NOx catalyst is raised, NOx is desorbed together with HC, but NOx desorbed from the NOx catalyst is reduced and purified using HC as a reducing agent. Therefore, it is possible to suppress the HC desorbed from the NOx catalyst from being discharged out of the exhaust purification device as it is.

  (2) In the exhaust purification device 2 of the present embodiment, the HC adsorption amount can be accurately estimated by using at least one of the exhaust air-fuel ratio and the NOx catalyst temperature correlated with the HC adsorption amount in the NOx catalyst. As a result, HC can be desorbed from the NOx catalyst at an appropriate timing. Further, by desorbing HC at an appropriate timing, it is possible to suppress a decrease in NOx adsorption performance of the NOx catalyst and thermal deterioration of the NOx catalyst.

  As mentioned above, although embodiment of this invention was described, this invention is not restricted to this. Within the scope of the gist of the present invention, the detailed configuration may be changed as appropriate.

1. Engine (internal combustion engine)
11 ... Exhaust passage (exhaust pipe)
2 ... Exhaust purification device 32 ... Downstream catalytic converter (NOx catalyst)
33 ... Exhaust temperature sensor (HC adsorption amount estimation means)
34 ... Air-fuel ratio sensor (HC adsorption amount estimating means)
6 ... ECU (HC adsorption amount estimation means, HC desorption means)

Claims (2)

An exhaust purification device for an internal combustion engine, wherein a NOx catalyst having a support made of zeolite and a Pd supported on the support is provided in an exhaust passage of the internal combustion engine, and the NOx catalyst purifies NOx in the exhaust,
HC adsorption amount estimation means for estimating the HC adsorption amount in the NOx catalyst;
HC desorption means for desorbing HC adsorbed on the NOx catalyst by raising the temperature of the NOx catalyst in response to the estimated amount of HC adsorption exceeding a predetermined value. An exhaust purification device for an internal combustion engine.   The HC adsorption amount estimation means estimates the HC adsorption amount by using at least one of an air-fuel ratio of exhaust gas flowing into the NOx catalyst and a temperature of the NOx catalyst. Exhaust gas purification device for internal combustion engine.

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