JP2017025757A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine in which a three-way catalyst and an NOx catalyst are arranged at positions appropriate to sufficiently exhibit functions of the catalysts.SOLUTION: An exhaust emission control device 2 for an engine 1 includes: an upstream catalytic converter 61 having a three-way catalyst; a downstream catalytic converter 62 that has an NOx catalyst including a carrier comprising zeolite and Pd supported on the carrier; an exhaust gas recirculation device 3 for taking out part of exhaust gas flowing in an exhaust pipe 13 to recirculate the exhaust gas in an intake pope 12 of the engine 1; and a supercharger 8 for pressurizing intake air by using exhaust gas energy. The upstream catalytic converter 61 is provided upstream of a turbine wheel 81. The downstream catalytic converter 62 is provided downstream of the upstream catalytic converter 61, the turbine wheel 81 and an exhaust taking-out section of the exhaust gas recirculation device 3.SELECTED DRAWING: Figure 1

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 temperature of the NOx catalyst rises due to warm-up. NOx can be reduced and purified.

Japanese Patent Application No. 2014-51628

  The NOx catalyst shown in Patent Document 1 is used in particular to suppress NOx emission under low-temperature conditions immediately after start-up, and is often used in combination with a three-way catalyst responsible for exhaust purification after a while from start-up. . However, in such an exhaust purification device that combines a NOx catalyst and a three-way catalyst, the position where these two catalysts should be provided has been sufficiently studied so that the respective functions can be fully exhibited. Not. In addition, when devices using exhaust gas such as an exhaust gas recirculation device and a supercharger are provided, it has not been sufficiently studied as to where the two catalysts should be provided for these devices.

  An object of the present invention is to provide an exhaust gas purification apparatus for an internal combustion engine in which a three-way catalyst and a NOx catalyst are arranged at suitable positions so that each function can be sufficiently exhibited.

  (1) An exhaust purification device (for example, an exhaust purification device 2 to be described later) of an internal combustion engine (for example, an engine 1 to be described later) of the present invention is provided in an exhaust passage (for example, an exhaust pipe 13 to be described later) of the internal combustion engine. A three-way catalyst (for example, an upstream catalytic converter 61 described later), a NOx catalyst (for example, a downstream catalytic converter 62 described later) provided in the exhaust passage and having a support made of zeolite and Pd supported on the support; An exhaust gas recirculation device (for example, an exhaust gas recirculation device 3 to be described later) that extracts a part of the exhaust gas flowing through the exhaust passage and recirculates it to the intake air passage of the internal combustion engine, and the NOx catalyst is downstream of the three-way catalyst. And provided on the downstream side of the exhaust extraction portion of the exhaust gas recirculation device.

  (2) In this case, the exhaust passage is provided with a supercharger (for example, a supercharger 8 to be described later) that pressurizes intake air using the energy of the exhaust, and the exhaust take-out part and the NOx catalyst are It is preferable that the exhaust passage is provided downstream of the turbocharger turbine (for example, a turbine wheel 81 described later).

  (3) In this case, it is preferable that the three-way catalyst is provided upstream of the turbine in the exhaust passage.

  (1) In the present invention, a support made of zeolite and a NOx catalyst having Pd supported on the support are provided downstream of the three-way catalyst 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. Therefore, according to the present invention, NOx before the activation of the three-way catalyst such as immediately after startup can be adsorbed to the NOx catalyst, so that this NOx is prevented from being discharged out of the exhaust purification device. it can.

  By the way, the NOx adsorption performance of the NOx catalyst varies depending on the temperature. More specifically, the NOx catalyst exhibits a high NOx adsorption performance at a low temperature immediately after the start of the internal combustion engine or the like, but this NOx adsorption performance has a characteristic of gradually decreasing as the temperature rises (for example, FIG. 2 and FIG. 4 etc.). In the present invention, by providing the NOx catalyst on the downstream side of the three-way catalyst, the temperature increase of the NOx catalyst after starting the internal combustion engine can be moderated more than that of the upstream three-way catalyst. Further, in the present invention, by providing the NOx catalyst downstream from the exhaust extraction portion of the exhaust gas recirculation device, the amount of exhaust gas flowing into the NOx catalyst can be reduced, so that the temperature rise of the NOx catalyst due to exhaust gas flow can be further moderated. . In addition, by moderately increasing the temperature of the NOx catalyst in this way, for example, after starting the internal combustion engine, before the three-way catalyst is activated, until the temperature of the NOx catalyst has lost its NOx adsorption performance. It is possible to prevent the NOx purification performance of the entire exhaust gas purification apparatus from being temporarily lowered.

  Further, the NOx adsorption performance of the NOx catalyst also changes depending on the amount of NOx adsorbed on the NOx catalyst. More specifically, the NOx adsorption efficiency of the NOx catalyst has a characteristic that it decreases as the amount of NOx adsorbed on the NOx catalyst increases (see, for example, FIG. 2 and FIG. 5 described later). In the present invention, by providing the NOx catalyst on the downstream side of the exhaust extraction portion of the exhaust gas recirculation device, the amount of exhaust flowing into the NOx catalyst can be reduced, so that the increase in the NOx adsorption amount in the NOx catalyst can be moderated. Thus, by gradually increasing the NOx adsorption amount of the NOx catalyst, for example, after starting the internal combustion engine, the NOx adsorption amount loses its NOx adsorption performance before the three-way catalyst is activated. It is possible to prevent the NOx purification performance of the entire exhaust gas purification apparatus from temporarily decreasing.

  (2) In this invention, the exhaust extraction part and NOx catalyst of an exhaust gas recirculation apparatus are provided in the downstream rather than the turbine of a supercharger. That is, a so-called low-pressure exhaust gas recirculation device is used as the exhaust gas recirculation device. As a result, the exhaust temperature on the downstream side of the turbine further decreases, so that the temperature increase of the NOx catalyst can be further moderated.

  (3) In the present invention, the three-way catalyst is provided upstream of the turbine. Thereby, after starting the internal combustion engine, the time taken to activate the three-way catalyst can be shortened. Therefore, it is possible to prevent the NOx adsorption performance of the NOx catalyst from being lowered before the three-way catalyst is activated, and the NOx purification performance in the entire exhaust gas purification apparatus to be temporarily lowered.

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 figure which shows the relationship between the NOx adsorption rate in a NOx catalyst, the temperature of a NOx catalyst, and NOx adsorption amount. It is a figure which shows the relationship between the NOx adsorption amount of the NOx adsorption rate when the temperature of the NOx catalyst is fixed at a predetermined temperature. It is a flowchart which shows the specific procedure of a NOx purification process at the time of starting. It is a figure for demonstrating the effect of the exhaust gas purification apparatus of the said embodiment.

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 includes an intake pipe 12 through which intake air flows, an exhaust pipe 13 through which exhaust gas flows, and an exhaust gas recirculation device 3 that extracts a part of the exhaust gas in the exhaust pipe 13 and recirculates the exhaust gas as EGR gas to the intake pipe 12; Controls a catalyst purification device 6 that purifies exhaust using the function of the catalyst, a supercharger 8 that pressurizes intake air using the kinetic energy of the exhaust, an engine 1, an exhaust gas recirculation device 3, and a supercharger 8. An electronic control unit (hereinafter referred to as “ECU”) 5 is provided.

  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 5. The ECU 5 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 supercharger 8 includes a turbine wheel 81 provided in the exhaust pipe 13, a compressor wheel 82 provided in the intake pipe 12, and a shaft 83 that connects the turbine wheel 81 and the compressor wheel 82. The turbine wheel 81 is rotationally driven by blowing exhaust discharged from the engine 1. The compressor wheel 82 is rotationally driven by the turbine wheel 81 to pressurize the intake air of the engine 1 and pump it into the intake pipe 12.

  The exhaust gas recirculation device 3 includes an EGR pipe 31 that connects the exhaust pipe 13 and the intake pipe 12, and an EGR valve 32 that adjusts the amount of exhaust gas flowing through the EGR pipe 31. The exhaust gas recirculation device 3 includes a portion where the EGR pipe 31 and the exhaust pipe 13 are connected, that is, a portion where the exhaust extraction portion is located downstream of the turbine wheel 81, and a portion where the EGR pipe 31 and the intake pipe 12 are connected, ie, the exhaust gas. This is a so-called low-pressure type exhaust gas recirculation device in which the return portion is upstream of the compressor wheel 82.

  The EGR valve 32 is an electromagnetic valve that can be opened and closed in the EGR pipe 31 and is connected to the ECU 5 via an actuator 33. The opening degree of the EGR valve 32 is controlled by adjusting the drive current supplied from the battery (not shown) to the actuator 33 by the ECU 5. The ECU 5 determines the target EGR rate and the opening of the EGR valve 32 according to the target EGR rate in accordance with the operating state of the engine 1, and determines the duty ratio of the drive current so that the opening is realized. The details of the control of the EGR valve 32 will be omitted.

  The catalyst purification device 6 includes an upstream catalytic converter 61, a downstream catalytic converter 62, and an exhaust temperature sensor 63 that are provided in the exhaust pipe 13, respectively.

  The upstream catalytic converter 61 is configured by supporting a three-way catalyst on a base material of a flow-through type honeycomb structure. 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 upstream catalytic converter 61 is provided on the upstream side of the turbine wheel 81 in the exhaust pipe 13 so that the three-way catalyst is activated as soon as possible after the start of the engine 1.

  The downstream catalytic converter 62 is provided downstream of the upstream catalytic converter 61 in the exhaust pipe 11. The downstream catalytic converter 62 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. The 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 61 reaches the activation temperature). It has the function of adsorbing and reducing and purifying NOx that was not present. The downstream catalytic converter 62 is provided downstream of the exhaust catalytic converter 61, the turbine wheel 81, and the exhaust gas extraction portion of the exhaust gas recirculation device 3 in the exhaust pipe 13 in consideration of the characteristics of the NOx catalyst described in detail below. It is done.

  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 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 graph showing the relationship between the NOx adsorption rate in the NOx catalyst, the temperature of the NOx catalyst, and the NOx adsorption amount. The NOx adsorption rate is defined as the ratio of the amount of NOx adsorbed to the NOx catalyst out of the total amount of NOx flowing into the NOx catalyst. The NOx adsorption amount is defined by the amount of NOx adsorbed by NOx. As shown in FIG. 4, the NOx adsorption rate has a characteristic of decreasing as the temperature of the NOx catalyst increases, and becomes 0 when exceeding a predetermined NOx desorption temperature.

  FIG. 5 is a diagram showing the relationship between the NOx adsorption rate and the NOx adsorption amount when the temperature of the NOx catalyst is fixed at a predetermined temperature. As shown in FIG. 5, as the amount of NOx adsorbed on the NOx catalyst increases, it becomes more difficult for new NOx to be adsorbed. Therefore, the NOx adsorption rate decreases as the NOx adsorption amount increases. For this reason, in order to keep the NOx adsorption rate in the NOx catalyst high, it is preferable to keep the NOx adsorption amount as small as possible. Further, there is a limit to the amount of NOx that can be adsorbed by the NOx catalyst, and when the NOx adsorption amount exceeds this maximum value, new NOx cannot be adsorbed.

  Returning to FIG. 1, the exhaust temperature sensor 63 is provided on the downstream side of the downstream catalytic converter 62 in the exhaust pipe 13. The exhaust temperature sensor 63 detects the temperature of the exhaust gas flowing out from the downstream catalytic converter 62 and transmits a signal substantially proportional to the detected value to the ECU 5. The temperature of the three-way catalyst of the upstream catalytic converter 61 and the temperature of the NOx catalyst of the downstream catalytic converter 62 are estimated by calculation in the ECU 5 based on the output of the exhaust temperature sensor 63, for example.

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

  FIG. 6 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. 6, 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 in FIG. 6 is repeatedly executed in the ECU under a predetermined control cycle in response to turning on an ignition switch (not shown) that starts and stops 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 in 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. 6 from the previous value of the purge timer.

  FIG. 7 is a diagram for explaining the effect of the exhaust purification apparatus of the present embodiment as described above. FIG. 7 schematically shows, in order from the top, changes in the temperature of the three-way catalyst, the temperature of the NOx catalyst, the NOx adsorption amount of the NOx catalyst, and the NOx adsorption rate of the NOx catalyst in a predetermined period immediately after the start of the engine. FIG. In FIG. 7, the solid line indicates the result of the exhaust purification device of the present embodiment of FIG. 1, and the broken line indicates the result of the exhaust purification device of the comparative example in which the position of the NOx catalyst is changed from the exhaust purification device of the present embodiment of FIG. 1. Indicates. In this comparative example, the NOx catalyst was provided between the three-way catalyst and the turbine.

  First, the position where the three-way catalyst is provided is the same in the comparative example and the exhaust purification device of the present embodiment. Therefore, as shown in the uppermost stage of FIG. 7, the temperature of the three-way catalyst immediately after the start of the engine rises at substantially the same speed in both the comparative example and the exhaust purification device of the present embodiment, and the three-way catalyst is predetermined at time tact. The activation temperature of is reached.

  Further, the position where the NOx catalyst is provided is different between the comparative example and the exhaust purification apparatus of the present embodiment. More specifically, in the comparative example, the NOx catalyst is provided on the upstream side of the exhaust extraction portion of the turbine and the low pressure EGR device, whereas in the exhaust purification device of the present embodiment, the NOx catalyst is provided on the downstream side. For this reason, in the exhaust gas purification apparatus of the present embodiment, the amount of exhaust gas flowing into the NOx catalyst is smaller than that in the comparative example, so that the temperature rise of the NOx catalyst can be moderated accordingly (see the second stage from the top in FIG. 7). ). As described with reference to FIGS. 2 to 5 and the like, the adsorption rate of the NOx catalyst gradually decreases as the temperature rises with a peak at about 50 ° C. For this reason, as shown in the lowermost stage of FIG. 7, in the comparative example, the temperature of the NOx catalyst quickly rises, and as a result, the NOx adsorption rate is greatly reduced before the three-way catalyst is activated. There is. On the other hand, in the exhaust gas purification apparatus of the present embodiment, a sufficient NOx adsorption rate can be maintained even when the three-way catalyst is activated by slowing the temperature rise of the NOx catalyst.

  Further, in the exhaust purification apparatus of the present embodiment, the amount of NOx flowing into the NOx catalyst can be reduced by reducing the amount of exhaust flowing into the NOx catalyst as described above. The amount of adsorption can also be moderately increased (see the second stage from the bottom in FIG. 7). As described with reference to FIGS. 2 to 5 and the like, the adsorption rate of the NOx catalyst decreases as the NOx adsorption amount increases. For this reason, as shown in the lowermost stage of FIG. 7, in the comparative example, the NOx adsorption amount quickly increases, and as a result, the NOx adsorption rate may be greatly reduced before the three-way catalyst is activated. is there. On the other hand, in the exhaust purification apparatus of this embodiment, by sufficiently increasing the NOx adsorption amount, a sufficient NOx adsorption rate can be maintained even when the three-way catalyst is activated.

  In summary, in the exhaust purification system of the present embodiment, the NOx catalyst is provided further downstream than the exhaust extraction portion of the low pressure EGR device, so that the temperature of the NOx catalyst and the NOx adsorption amount of the NOx catalyst immediately after the start of the engine is started. Since both rises can be moderated, a sufficient NOx adsorption rate can be maintained even when the three-way catalyst is activated.

  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)
13. Exhaust pipe (exhaust passage)
3 ... Exhaust gas recirculation device 6 ... Exhaust gas purification device 61 ... Upstream catalytic converter 62 ... Downstream catalytic converter 8 ... Supercharger 81 ... Turbine wheel

Claims (3)

A three-way catalyst provided in the exhaust passage of the internal combustion engine;
A NOx catalyst provided in the exhaust passage and made of zeolite and having Pd supported on the carrier;
An exhaust gas recirculation device that takes out part of the exhaust gas flowing through the exhaust passage and recirculates it to the intake passage of the internal combustion engine,
The exhaust gas purification apparatus for an internal combustion engine, wherein the NOx catalyst is provided downstream from the three-way catalyst and downstream from an exhaust extraction portion of the exhaust gas recirculation device. The exhaust passage is provided with a supercharger that pressurizes intake air using the energy of the exhaust,
2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the exhaust extraction unit and the NOx catalyst are provided in a downstream side of a turbine of the supercharger in the exhaust passage.   The exhaust purification device for an internal combustion engine according to claim 2, wherein the three-way catalyst is provided upstream of the turbine in the exhaust passage.

Images