JP6158871B2 - Exhaust gas purification device for internal combustion engine - Google Patents

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, in Patent Document 2, immediately after the start of the internal combustion engine in which the three-way catalyst is not activated, the ignition timing is advanced from the MBT to reduce the HC emission amount from the internal combustion engine, and in the exhaust gas upstream of the three-way catalyst. A technique for promoting the oxidation of CO by introducing secondary air into the three-way catalyst in an oxidizing atmosphere is shown. According to the technique of Patent Document 2, it is possible to reduce HC and CO emissions immediately after the start of the internal combustion engine, and to activate the three-way catalyst at an early stage using the heat of oxidation reaction of CO.

Japanese Patent Application No. 2014-51628 International Publication No. 2008/105550

  By the way, it is thought that the NOx catalyst containing Pd and zeolite as shown in Patent Document 1 deteriorates with use in the same manner as other catalysts, and its NOx adsorption performance is also lowered. Conventionally, however, the degradation characteristics specific to the NOx catalyst, such as under which circumstances the degradation progresses, have not been clarified. For this reason, even if the technique for supplying secondary air to the catalyst as described in Patent Document 2 is applied to the NOx catalyst, deterioration of the NOx catalyst can be suppressed by supplying the secondary air at any timing. Is not clear.

  Moreover, it is thought that the magnitude | size of the function which a NOx catalyst exhibits according to the environment like other catalysts. Conventionally, however, the performance degradation characteristic unique to the NOx catalyst, such as under what circumstances the NOx adsorption performance becomes weak, has not been clarified. For this reason, it is not clear at what timing the secondary air can be supplied to the NOx catalyst to suppress a decrease in the NOx adsorption performance.

  An object of the present invention is to provide an exhaust emission control device for an internal combustion engine that supplies secondary air to the NOx catalyst at an appropriate timing according to the degradation characteristics and performance degradation characteristics unique to the NOx catalyst.

(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 a NOx catalyst (for example, an exhaust purification device 2 to be described later) having a support made of zeolite and Pd supported on the support. A downstream catalytic converter 62) to be described later is provided in an exhaust passage (for example, an exhaust pipe 13 to be described later) of the internal combustion engine, and NOx in the exhaust gas is purified by the NOx catalyst, and the NOx catalyst in the exhaust passage. Secondary air supply means (for example, a secondary air supply device 9 to be described later) for supplying air to the exhaust on the upstream side of the exhaust gas, and an exhaust gas whose combustion air-fuel ratio is less than stoichiometric and correlated with the temperature of the NOx catalyst temperature or temperature of the NOx catalyst (for example, the catalyst temperature Tcat below) a predetermined temperature (e.g., the second judgment temperature T_High below) the secondary air subjected to is equal to or greater than Control means for supplying air with a means (for example, the means related to execution of the processing of ECU5 and 7 below), characterized in that it comprises a, a.

  In the present invention, the “combustion air-fuel ratio” refers to the ratio of oxygen to the fuel component of the air-fuel mixture that contributes to combustion in the cylinder of the internal combustion engine. Further, the state in which the combustion air-fuel ratio is equal to or lower than the stoichiometric state is realized, for example, during high load operation in which the injection amount of main injection contributing to torque, the injection amount of after injection performed after the main injection, and the like increase.

(2) 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 a NOx catalyst (for example, an exhaust purification device 2 to be described later) having a support made of zeolite and Pd supported on the support. A downstream catalytic converter 62) to be described later is provided in an exhaust passage (for example, an exhaust pipe 13 to be described later) of the internal combustion engine, and NOx in the exhaust gas is purified by the NOx catalyst, and the NOx catalyst in the exhaust passage. Secondary air supply means (for example, a secondary air supply device 9 to be described later) for supplying air to the exhaust on the upstream side of the exhaust gas, and an exhaust gas whose combustion air-fuel ratio is less than stoichiometric and correlated with the temperature of the NOx catalyst temperature or temperature of the NOx catalyst (for example, the catalyst temperature Tcat below) within a temperature range of NOx adsorption function occurs in said NOx catalyst (e.g., the adsorption temperature zone below) Certain control means for supplying air by using the secondary air supply means when (e.g., the means related to execution of the processing of ECU5 and 7 below), characterized in that it comprises a, a.

  (3) In this case, a three-way catalyst (for example, an upstream catalytic converter 61 to be described later) provided upstream of an air supply section (for example, an air supply section 94 to be described later) of the secondary air supply means in the exhaust passage. ) And stoichiometric control means (for example, ECU 5 described later) for controlling the air-fuel ratio of the exhaust gas flowing into the three-way catalyst.

  In the present invention, the “air-fuel ratio of exhaust gas” refers to the ratio of oxygen to reducing components such as hydrocarbons and carbon monoxide in the exhaust gas. Specifically, when the exhaust air-fuel ratio is stoichiometric or rich, for example, after-injection or the like is performed, the combustion air-fuel ratio is stoichiometric or rich, post-injection is performed, or exhaust is performed by a fuel injector provided in the exhaust passage. This is realized by supplying unburned fuel to the exhaust passage by injecting fuel into the exhaust passage.

  (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, the NOx catalyst deteriorates when exposed to high-temperature exhaust gas, and its NOx adsorption performance also decreases. Further, the NOx catalyst as described above has a characteristic that deterioration is more likely to proceed faster as the air-fuel ratio of the exhaust gas becomes lower even under the same temperature environment (see FIG. 6 described later). In contrast, in the present invention, when the internal combustion engine is operated at a high load, more specifically, when the combustion air-fuel ratio of the internal combustion engine is not more than stoichiometric and the temperature of the exhaust or NOx catalyst is not less than a predetermined temperature, the secondary air supply is performed. The air is supplied to the exhaust gas upstream of the NOx catalyst. As a result, the air-fuel ratio of the exhaust gas in the NOx catalyst rises and the temperature rise is also suppressed, so that a reduction in NOx adsorption performance due to deterioration of the NOx catalyst can be suppressed.

(2) 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, so that the exhaust gas is discharged from the internal combustion engine immediately after the start for the same reason as in the invention of (1). NOx can be prevented from being discharged out of the exhaust purification device. Further, the NOx catalyst has a characteristic that the amount of NOx that can be adsorbed decreases as the CO concentration or H ₂ O concentration of the exhaust gas flowing into the NOx catalyst increases (see FIGS. 4 and 5 described later). In contrast, in the present invention, immediately after the start of the internal combustion engine, more specifically, the combustion air-fuel ratio of the internal combustion engine is not more than stoichiometric, and the temperature of the exhaust or NOx catalyst is within the temperature range where the NOx adsorption function occurs in the NOx catalyst. In this case, the secondary air supply means is used to supply air to the exhaust on the upstream side of the NOx catalyst. Thereby, the CO concentration and the H ₂ O concentration of the exhaust gas in the NOx catalyst are reduced, so that the reduction in the NOx adsorption performance due to the CO and H ₂ O of the NOx catalyst can be suppressed.

  (3) In the present invention, a three-way catalyst is provided upstream of the air supply unit of the secondary air supply means, and the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is stoichiometrically controlled. Thereby, for example, after the three-way catalyst is activated, NOx, HC, CO, etc. in the exhaust gas can be purified mainly using the three-way catalyst. According to the present invention, even if air is supplied from the secondary air supply means, the air-fuel ratio in the three-way catalyst is not affected. For this reason, the three-way catalyst can maintain the optimum air-fuel ratio for purifying the exhaust gas while supplying air from the secondary air supply means so as to reduce the performance degradation of the NOx catalyst as described above.

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 the relationship between adsorption amount and _{H 2} O concentration of NOx in the NOx catalyst. It is a figure which shows the relationship between the adsorption amount of NOx in a NOx catalyst, and CO density | concentration. It is a figure which compares the NOx adsorption | suction performance of a NOx catalyst after giving a thermal load under exhaust_gas | exhaustion from which an air fuel ratio differs. It is a flowchart which shows the procedure which determines whether the introduction of secondary air is possible. It is a figure which shows the relationship between the temperature which occurs in each temperature range, and the temperature of a NOx catalyst.

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, a catalyst purification device 6 that purifies exhaust gas using the function of a catalyst, and pressurizes intake air using kinetic energy of the exhaust gas. A supercharger 8 and an electronic control unit (hereinafter referred to as “ECU”) 5 for controlling the engine 1, the supercharger 8, and the secondary air supply device 9 are 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 secondary air supply device 9 includes a bypass pipe 91 that allows the intake pipe 12 and the exhaust pipe 13 to communicate with each other, a secondary air introduction valve 92 that can be opened and closed in the bypass pipe 91, and a command from the ECU 5. An actuator 93 that opens and closes the secondary air introduction valve 92.

  The bypass pipe 91 connects the air introduction part 95 downstream of the compressor wheel 82 in the intake pipe 12 and the air supply part 94 downstream of the turbine wheel 81 in the exhaust pipe 13. Since the air introduction unit 95 has a higher pressure than the air supply unit 94, air flows from the air introduction unit 95 to the air supply unit 94 when the secondary air introduction valve 92 is opened. Hereinafter, the air introduced into the exhaust pipe 13 via the bypass pipe 91 is also referred to as secondary air. The secondary air introduction valve 92 is opened and closed by the ECU 5 at a timing determined by the process of FIG.

  The catalyst purification device 6 includes an upstream catalytic converter 61, a downstream catalytic converter 62, an exhaust temperature sensor 63, and an air-fuel ratio sensor 64, which 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 in the exhaust pipe 13 between the turbine wheel 81 and the air supply unit 94 of the secondary air supply device 9. Therefore, even if air is supplied into the exhaust by the secondary air supply device 9, the air-fuel ratio, temperature, etc. of the exhaust in the three-way catalyst do not change greatly. Therefore, secondary air can be freely supplied using the secondary air supply device 9 while simultaneously purifying HC, CO, and NOx in the three-way catalyst.

  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 downstream from the turbine wheel 81, the upstream catalytic converter 61, and the air supply unit 94 of the secondary air supply device 9 in the exhaust pipe 13 in consideration of the characteristics of the NOx catalyst described in detail below. Provided on the side. Therefore, when the secondary air introduction valve 92 is opened, secondary air is supplied into the exhaust gas upstream of the NOx catalyst.

  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 amount [g / L] in the NOx catalyst and the H ₂ O concentration [%] of the exhaust gas flowing into the NOx catalyst. FIG. 5 is a graph showing the relationship between the NOx adsorption amount [g / L] in the NOx catalyst and the CO concentration [ppm] of the exhaust gas flowing into the NOx catalyst. 4 and 5 show the results of measuring the NOx adsorption amount using a model gas simulating exhaust discharged from the engine at the time of starting as shown below.
[Model gas composition]
NO ... 250ppm
O ₂ ... 10%
CO: 0 to 10,000 ppm
C ₃ H ₆ ... 600 ppmC
CO ₂ ... 15%
H ₂ O ... 0~15%

When changing the H ₂ O concentration, the CO concentration was fixed at 10000 ppm (see FIG. 4), and when changing the CO concentration, the H ₂ O concentration was fixed at 2%. As shown in FIGS. 4 and 5, the amount of NOx that can be adsorbed by the NOx catalyst decreases as the CO concentration or H ₂ O concentration in the exhaust gas increases.

  FIG. 6 is a diagram comparing the NOx adsorption performance of the NOx catalyst after applying a thermal load under exhaust having different air-fuel ratios. More specifically, in the order from the left side, the NOx adsorption amount of the NOx catalyst after applying a heat load equivalent to 800 ° C. × 5 hours with the lean / stoichi ratio of 100/0, 80/20, and 0/100 [g / L]. Here, the lean / stoichi ratio refers to the ratio of the total lean time and the total stoichiometric time. As shown in FIG. 6, the NOx adsorption performance lowering characteristic of the NOx catalyst greatly varies depending not only on the heat load but also on the air-fuel ratio of the exhaust. More specifically, the NOx adsorption performance of the NOx catalyst greatly decreases as the lean / stoichiometric ratio decreases, that is, the exhaust air-fuel ratio decreases.

  Returning to FIG. 1, the air-fuel ratio sensor 64 is provided upstream of the upstream catalytic converter 61 in the exhaust pipe 13. The air-fuel ratio sensor 64 detects the air-fuel ratio of the exhaust gas flowing into the upstream catalytic converter 61 (ratio of fuel components (HC, CO, etc.) to oxygen in the exhaust gas), and sends a signal substantially proportional to the detected value to the ECU 5. . As the air-fuel ratio sensor 64, for example, a sensor having a linear output characteristic from a rich region to a lean region is used. When the ECU 5 simultaneously purifies HC, CO, and NOx in the exhaust using the three-way purification reaction in the three-way catalyst of the upstream catalytic converter 61, the ECU 5 performs three-way feedback control using the output of the air-fuel ratio sensor 64. The air-fuel ratio of the exhaust gas flowing into the original catalyst is controlled stoichiometrically.

  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 NOx catalyst of the downstream catalytic converter 62 is estimated by calculation in the ECU 5 based on the output of the exhaust temperature sensor 63, for example.

  The ECU 5 determines the I / O interface for A / D converting the detection signal of the sensor, the CPU for executing the fuel injection control described below, the process according to the flowchart shown in FIG. It is the microcomputer comprised by the drive circuit which drives various devices in an aspect, RAM, ROM, etc. which memorize | store various data.

  FIG. 7 is a flowchart showing a procedure for determining whether or not secondary air can be introduced. The process of FIG. 7 is repeatedly executed in the ECU under a predetermined control cycle in response to an ignition switch (not shown) for starting the engine being turned on.

  In S1, the ECU determines whether or not the combustion air-fuel ratio is equal to or less than stoichiometric. If the determination in S1 is NO, the ECU determines that it is not necessary to introduce secondary air to suppress deterioration of the NOx catalyst, closes the secondary air introduction valve (see S2), and ends this process. .

  When the determination in S1 is YES and the combustion air-fuel ratio is equal to or lower than the stoichiometric ratio, the current NOx catalyst temperature Tcat is acquired, whether the catalyst temperature Tcat is lower than a predetermined first determination temperature T_Low, and the catalyst It is determined whether or not the temperature Tcat is equal to or higher than a predetermined second determination temperature T_High (see S3 and S4).

FIG. 8 is a diagram showing the relationship between the temperature of the NOx catalyst and the phenomenon occurring in each temperature range.
As described above, the NOx catalyst generates a function of adsorbing NOx in the exhaust gas at a temperature lower than the desorption temperature. On the higher temperature side than the desorption temperature, a function of desorbing NOx adsorbed at a low temperature is generated. Further, when the temperature of the NOx catalyst becomes excessively higher than the desorption temperature, the deterioration is promoted, and the NOx adsorption function that should be generated on the low temperature side is also lowered. For this reason, the temperature of the NOx catalyst is, as schematically shown in FIG. 8, in order from the low temperature side, the adsorption temperature zone where the NOx adsorption function occurs, the desorption temperature zone where the NOx desorption function occurs, and the deterioration of the NOx catalyst. It is divided into three types, that is, a deterioration temperature zone promoted by.

  The first determination temperature T_Low and the second determination temperature T_High in the determinations of S3 and S4 are set to determine the temperature zone to which the NOx catalyst belongs. That is, the first determination temperature T_Low is set to the boundary between the adsorption temperature zone and the desorption temperature zone, that is, the NOx catalyst desorption temperature (for example, about 200 ° C.). The second determination temperature T_High is set to a boundary between the desorption temperature zone and the deterioration temperature zone, more specifically, for example, 800 ° C. on the higher temperature side than the first determination temperature T_Low.

  Returning to FIG. 7, when the combustion air-fuel ratio is equal to or lower than the stoichiometric value and the catalyst temperature Tcat is equal to or higher than the second determination temperature T_High (when determination of S4 is YES), the ECU suppresses deterioration of the NOx catalyst. The secondary air introduction valve is opened (see S5), and this process is terminated. If the combustion air-fuel ratio is made lower than the stoichiometric state when the NOx catalyst is at a high temperature, the NOx catalyst is exposed to the stoichiometric or rich air-fuel ratio exhaust gas at a high temperature state. As described above, the deterioration of the NOx catalyst is accelerated when the temperature becomes high. However, as described with reference to FIG. 6, the progress of the deterioration becomes more remarkable when the air-fuel ratio becomes rich. Therefore, when the determination in S4 is YES, the secondary air introduction valve is opened and the secondary air is introduced into the exhaust. As a result, exhaust gas that has been leanly diluted with secondary air and reduced in temperature is introduced into the NOx catalyst, so that deterioration of the NOx catalyst is suppressed.

Further, the ECU is in a temperature range where the combustion air-fuel ratio is equal to or lower than the stoichiometric value and the catalyst temperature Tcat is lower than the first determination temperature T_Low (when the determination of S3 is YES), that is, within the temperature range where the NOx adsorption function occurs in the NOx catalyst. In this case, in order to improve this NOx adsorption function as much as possible, the secondary air introduction valve is opened (see S5), and this process is terminated. When the combustion air-fuel ratio is made less than stoichiometric, the CO concentration and H ₂ O concentration of exhaust exhausted from the engine increase, so as described with reference to FIGS. 4 and 5, the NOx adsorption function of the NOx catalyst is lowered. It becomes a factor to make. Therefore, when the determination in S3 is YES, CO and H ₂ O in the exhaust gas flowing into the NOx catalyst are diluted by introducing secondary air, and the NOx adsorption function in the NOx catalyst is maintained high. be able to.

  Further, the ECU closes the secondary air introduction valve (see S2) when the combustion air-fuel ratio is equal to or lower than the stoichiometric value and the catalyst temperature Tcat is within the NOx desorption temperature band (when the determination of S4 is NO). This process ends. When NOx is desorbed from the NOx catalyst as described above, if the stoichiometric or rich air-fuel ratio exhaust gas is introduced, the desorbed NOx is reduced and purified on the NOx catalyst. Therefore, when the determination of S4 is NO, it is preferable to stop the introduction of secondary air so as not to hinder the NOx reduction and purification function 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.

  For example, in the process of FIG. 7 in the above embodiment, the temperature Tcat of the NOx catalyst is acquired, and the catalyst temperature Tcat is compared with predetermined threshold values (T_Low and T_High) to determine whether secondary air can be introduced. The invention is not limited to this. Since the temperature of the NOx catalyst has a correlation with the temperature of the exhaust, the temperature of the exhaust may be compared with the threshold value, and whether or not the secondary air can be introduced may be determined according to the comparison result.

1. Engine (internal combustion engine)
13. Exhaust pipe (exhaust passage)
2 ... Exhaust gas purification device 5 ... ECU (control means)
61 ... Upstream catalytic converter (three-way catalyst)
62 ... Downstream catalytic converter (NOx catalyst)
9 ... Secondary air supply device (secondary air supply means)

Claims (3)

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,
Secondary air supply means for supplying air to the exhaust on the upstream side of the NOx catalyst in the exhaust passage;
When the combustion air-fuel ratio of the internal combustion engine is less than stoichiometric and the exhaust gas temperature correlated with the temperature of the NOx catalyst or the temperature of the NOx catalyst is higher than a predetermined temperature, the secondary air supply means is used to supply air. And an exhaust gas purifying device for an internal combustion engine. 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,
Secondary air supply means for supplying air to the exhaust on the upstream side of the NOx catalyst in the exhaust passage;
When the combustion air-fuel ratio of the internal combustion engine is less than stoichiometric and the temperature of the exhaust gas correlates with the temperature of the NOx catalyst or the temperature of the NOx catalyst is within the temperature range where the NOx adsorption function occurs in the NOx catalyst, the secondary And an exhaust gas purifying apparatus for an internal combustion engine, characterized by comprising: control means for supplying air using air supply means. A three-way catalyst provided upstream of the air supply part of the secondary air supply means in the exhaust passage;
The exhaust purification device for an internal combustion engine according to claim 1 or 2, further comprising stoichiometric control means for stoichiometrically controlling an air-fuel ratio of the exhaust gas flowing into the three-way catalyst.

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