JP4254505B2 - 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.

As a catalyst for purifying nitrogen oxide (NO _x ) contained in exhaust gas when combustion is performed under a lean air-fuel ratio, an alkali metal or alkaline earth is provided on the surface of a support made of alumina. the layers of the NO _X absorbent is formed consisting of a further noble metal known catalyst that is supported on a support surface, such as platinum (for example, see Patent Document 1). In this catalyst, when the catalyst is not activated, nitrogen dioxide (NO ₂ ) contained in the exhaust gas is occluded in the NO _x storage agent when the air-fuel ratio of the inflowing exhaust gas is lean, and when the catalyst is activated, the inflow exhaust gas when the air-fuel ratio of the gas is lean NO _X contained in the inflowing exhaust gas is occluded in _the NO _X storage agent, the air-fuel ratio of the inflowing exhaust gas has been occluded in when it is made rich _the NO _X storage agent NO _X Is released and reduced.

In such a catalyst, nitrogen monoxide contained in the inflowing exhaust gas (NO), the catalyst is not stored in the NO _X storage agent unless activated, NO ₂ contained in the inflowing exhaust gas catalyst is not activated Even if not, it is stored in the NO _X storage agent. Therefore, in order to increase the purification rate of NO _x in the exhaust gas when the catalyst is not activated, the NO contained in the exhaust gas is oxidized to NO ₂ before the exhaust gas flows into the catalyst. It is necessary to make it easy to occlude.

In the exhaust emission control device disclosed in Patent Document 1, for example, an oxidation catalyst is provided on the exhaust upstream side of the catalyst, and the oxidation catalyst oxidizes NO contained in the exhaust gas to NO ₂ before the exhaust gas flows into the catalyst. ing. In particular, when the temperature of the oxidation catalyst is low, the oxidation capability of the oxidation catalyst is maintained by forcibly raising the temperature of the oxidation catalyst with an electric heater.

JP 2002-89246 A Japanese Patent Laid-Open No. 10-26014

However, _the NO _X storage storage capacity of the NO ₂ by the agent (i.e., _the NO _X storage agent abundance of absorbing possible NO ₂ amount), the temperature of the NO _X absorbent, i.e. varies with the temperature of the catalyst. Since NO ₂ storage ability of the NO _X absorbent is not constant with respect to temperature, in order to maintain a high NO ₂ adsorption capacity by _the NO _X storage agent, NO in temperature, such as NO ₂ storage capability is maintained high It is necessary to maintain the temperature of the _X storage catalyst.

Accordingly, an object of the present invention is to provide an exhaust gas purification apparatus for an internal combustion engine that maintains a high NO ₂ storage capacity.

In order to solve the above problems, the first aspect of the present invention, in the exhaust gas when the air-fuel ratio of the exhaust gas is lean flowing into the NO _X storage catalyst when the temperature of the NO _X storage catalyst is lower than the activation temperature NO ₂ that contains occluded in the NO _X absorbent, if the temperature of the NO _X storage catalyst is not less than the activation temperature, the air-fuel ratio of the exhaust gas flowing into the NO _X storage catalyst when the lean contained in the exhaust gas NO _X is stored in the NO _X storage agent, NO _X when the air-fuel ratio of the exhaust gas flowing into the NO _X storage catalyst is occluded when almost the stoichiometric air-fuel ratio or rich is released from _the NO _X storage agent, the NO _X An exhaust gas purification apparatus for an internal combustion engine is provided in which the storage catalyst is positioned at a position where the temperature is at least lower than the activation temperature in most engine operating regions.
According to the first invention, the temperature of the NO _x storage catalyst is maintained below its activation temperature during most of the operation period. When the temperature of the NO _x storage catalyst is lower than the activation temperature, low temperature storage is performed in which NO ₂ in the inflowing exhaust gas is stored in the NO _x storage agent as nitrite. The NO ₂ storage capacity of the NO _X storage agent in this low-temperature storage is the storage performed when the temperature of the NO _X storage catalyst is higher than the activation temperature, that is, the high temperature at which NO ₂ in the exhaust gas is stored in the NO _X storage agent as nitrate. It is higher than the NO ₂ storage capacity of the NO _X storage agent in storage. Therefore, the NO ₂ storage capacity can be maintained high by maintaining the temperature of the NO _X storage catalyst lower than its activation temperature.

In a second invention, in the first invention, the NO _x storage catalyst has a temperature range in which the temperature is lower than the activation temperature and the NO ₂ storage capacity of the NO _x storage agent is not less than a certain level in most engine operating ranges. It is positioned at such a position as to be inside.

According to a third aspect, in the second aspect, the NO _x storage catalyst is positioned on the most upstream side of positions where the temperature is within the temperature range in most engine operating regions.
According to the third aspect of the invention, the temperature of the NO _x storage catalyst is within the temperature range where the temperature is lower than the activation temperature and the NO ₂ storage capacity of the NO _x storage agent is greater than or equal to a certain level in most engine operating ranges. Maintained in the highest temperature range. For example, the more the amount of the NO _X which is stored in _the NO _X storage agent, when to emit NO _X which is stored in _the NO _X storage catalyst, the temperature of the NO _X storage catalyst above its activation temperature However, according to the present invention, since the temperature of the NO _x storage catalyst is maintained at a relatively high temperature, the temperature can be easily raised to the activation temperature or higher.

According to a fourth invention, in any one of the first to second inventions, an oxidation catalyst having a noble metal is disposed in the engine exhaust passage and upstream of the NO _x storage catalyst, and the oxidation catalyst is mostly It is positioned at a position that falls within a temperature range that exhibits a certain level of oxidation ability in the engine operation region.
According to the fourth aspect of the invention, the oxidation catalyst is disposed upstream of the NO _x storage catalyst, and NO in the exhaust gas discharged from the engine main body in most engine operation regions can be oxidized to NO ₂ . Therefore, most of the NO _{x in} the exhaust gas flowing into the NO _x storage catalyst can be made NO ₂ . Since the temperature of the NO _X storage catalyst is lower than its activation temperature in most engine operating region, it can not be occluded only NO ₂ only in the inflowing exhaust gas, NO _X in the inflowing exhaust gas is almost NO ₂ Therefore, most of the NO _x in the inflowing exhaust gas can be occluded.

In a fifth aspect, in the first to fourth any one invention of comprises means for estimating _the NO _X storage amount occluded in the _the NO _X storage agent, _the NO _X storage amount estimated is When a predetermined allowable value is exceeded, temperature increase control is performed so that the temperature of the NO _x storage catalyst becomes equal to or higher than the activation temperature, and the air-fuel ratio of the exhaust gas is made substantially stoichiometric or rich.

In a sixth invention, in any one of the first to fifth inventions, the NO _x storage catalyst has a noble metal, and the activation temperature is an activation temperature of the noble metal.

In the seventh invention, in any one invention of the first to fifth, the active temperature, said activation temperature, the temperature range and NO ₂ that _the NO _X storage agent occludes NO ₂ as nitrite as nitrate It is a boundary temperature with the temperature range to occlude.

NO NO ₂ storage capacity of the _X storing catalyst was higher when the temperature of the NO _X storage catalyst is lower than its activation temperature, according to the present invention, the temperature of the NO _X storage catalyst in most engine operating region of its By maintaining the temperature lower than the activation temperature, the NO ₂ storage capacity can be maintained high.

  FIG. 1 shows a case where the present invention is applied to a compression ignition type internal combustion engine. The present invention can also be applied to a spark ignition type internal combustion engine.

Referring to FIG. 1, 1 is an engine body, 2 is a combustion chamber of each cylinder, 3 is an electronically controlled fuel injection valve for injecting fuel into each combustion chamber 2, 4 is an intake manifold, and 5 is an exhaust manifold. Respectively. The intake manifold 4 is connected to the outlet of the compressor 7 a of the exhaust turbocharger 7 through the intake duct 6, and the inlet of the compressor 7 a is connected to the air cleaner 8. A throttle valve 9 driven by a step motor is arranged in the intake duct 6, and a cooling device 10 for cooling intake air flowing in the intake duct 6 is arranged around the intake duct 6. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 10 and the intake air is cooled by the engine cooling water. On the other hand, the exhaust manifold 5 is connected to the inlet of the exhaust turbine 7 b of the exhaust turbocharger 7, and the outlet of the exhaust turbine 7 b is connected to the casing 12 containing the NO _x storage catalyst 11. A reducing agent supply valve 13 for supplying a reducing agent made of, for example, hydrocarbons into the exhaust gas flowing through the exhaust manifold 5 is disposed at the outlet of the collecting portion of the exhaust manifold 5.

  The exhaust manifold 5 and the intake manifold 4 are connected to each other via an exhaust gas recirculation (hereinafter referred to as EGR) passage 14, and an electronically controlled EGR control valve 15 is disposed in the EGR passage 14. An EGR cooling device 16 for cooling the EGR gas flowing in the EGR passage 14 is disposed around the EGR passage 14. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 16, and the EGR gas is cooled by the engine cooling water. On the other hand, each fuel injection valve 3 is connected to a fuel reservoir, so-called common rail 18 via a fuel supply pipe 17. Fuel is supplied into the common rail 18 from an electronically controlled fuel pump 19 with variable discharge amount, and the fuel supplied into the common rail 18 is supplied to the fuel injection valve 3 via each fuel supply pipe 17.

The electronic control unit 30 is composed of a digital computer, and is connected to each other by a bidirectional bus 31. A ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, an input port 35 and an output port 36. It comprises. The the NO _X storing catalyst 11 is attached a temperature sensor 20 for detecting the temperature of the NO _X storage catalyst 11, the output signal of the temperature sensor 20 is input to the input port 35 via a corresponding AD converter 37 . A load sensor 41 that generates an output voltage proportional to the amount of depression of the accelerator pedal 40 is connected to the accelerator pedal 40, and the output voltage of the load sensor 41 is input to the input port 35 via the corresponding AD converter 37. The Further, a crank angle sensor 42 that generates an output pulse every time the crankshaft rotates, for example, 15 ° is connected to the input port 35. On the other hand, the output port 36 is connected to the fuel injection valve 3, the step motor for driving the throttle valve 9, the reducing agent supply valve 13, the EGR control valve 15, and the fuel pump 19 through corresponding drive circuits 38.

The NO _x storage catalyst 11 shown in FIG. 1 is composed of a monolith catalyst, and a carrier made of alumina, for example, is supported on the base of the NO _x storage catalyst 11. 2A and 2B schematically show a cross section of the surface portion of the carrier 50. As shown in FIGS. 2A and 2B, catalyst noble metal 51 is dispersed and supported on the surface of the carrier 50, and a layer of NO _x storage agent 52 is formed on the surface of the carrier 50. Has been.

In the embodiment of the present invention, platinum (Pt) is used as the catalyst noble metal 51, and the component constituting the NO _x storage agent 52 is, for example, an alkali such as potassium (K), sodium (Na), or cesium (Cs). At least one selected from metals, alkaline earths such as barium (Ba) and calcium (Ca), and rare earths such as lanthanum (La) and yttrium (Y) is used.

When the ratio of the air and fuel (hydrocarbon) supplied into the engine intake passage, the combustion chamber 2 and the exhaust passage upstream of the NO _x storage catalyst 11 is referred to as the air-fuel ratio of the exhaust gas, the NO _x storage agent 52 is a catalyst noble metal. If the temperature of 51 is equal to or higher than the activation temperature, that is, if the temperature of the NO _x storage catalyst 11 is equal to or higher than the activation temperature, the air-fuel ratio of the exhaust gas flowing into the NO _x storage catalyst (hereinafter referred to as “inflow exhaust gas”) absorbs NO _X when the lean performs absorbing and releasing action of the NO _X that releases NO _X concentration of oxygen absorbed to decrease in the inflowing exhaust gas.

The case where barium (Ba) is used as a component constituting the NO _X storage agent 52 will be described as an example. When the air-fuel ratio of the inflowing exhaust gas is lean, that is, when the oxygen concentration in the inflowing exhaust gas is high, the platinum 51 is activated. As shown in FIG. 2 (A), the NO contained in the inflowing exhaust gas is oxidized on the platinum 51 to become NO ₂ and then absorbed into the NO _x storage agent 52 to be barium oxide (BaO). And diffused into the NO _x storage agent 52 in the form of nitrate ions (NO ₃ ⁻ ). In this way, NO _X is absorbed in the NO _X storage agent 52. To flow in the oxygen concentration in the exhaust gas is NO ₂ with high long as the surface of the platinum 51 generation, NO _X unless NO ₂ to NO ₂ absorption capacity is not saturated the absorbent 52 is absorbed in the NO _X absorbent 52 nitrate ions (NO ₃ ⁻ ) is generated.

On the other hand, the inflow exhaust gas is obtained by making the air-fuel ratio in the combustion chamber 2 substantially the stoichiometric air-fuel ratio or rich (hereinafter referred to as “stoichiometric rich”) or by supplying the reducing agent from the reducing agent supply valve 13. When the air-fuel ratio of the gas is stoichiometrically rich, the oxygen concentration in the inflowing exhaust gas decreases, so that the reaction proceeds in the reverse direction (NO ₃ ⁻ → NO ₂ ), whereby nitrate ions (NO ₃ ) in the NO _X storage agent 52 ₃ ⁻ ) is released from the NO _X storage agent 52 in the form of NO ₂ . Next, the released NO _x is reduced by unburned HC and CO contained in the inflowing exhaust gas. In the above description, NO _X is described as being absorbed by the NO _X storage agent 52 (that is, accumulated in the form of nitrate or the like). However, whether NO _X is actually absorbed or adsorbed ( That is, it is not always clear whether NO _x is adsorbed in the form of NO ₂ or the like, and the term occlusion including both the concepts of absorption and adsorption is used. In the present specification, in particular, the occlusion performed when the temperature of the NO _x occlusion catalyst 11 is equal to or higher than the activation temperature is referred to as “high temperature occlusion”. The term “release” from the NO _X storage agent 52 is also used to include the meaning of “desorption” corresponding to “adsorption” in addition to “release” corresponding to “absorption”.

By the way, the oxidation ability of the platinum 51 varies depending on the temperature. If the temperature of the platinum 51 is low, the oxidation ability is low. For this reason, as shown in FIG. 3, when the temperature of the NO _x storage catalyst 11 decreases, the oxidation rate from NO to NO ₂ by platinum 51 (hereinafter referred to as “NO ₂ conversion rate”) decreases. In this embodiment, as can be seen from FIG. 3, when the temperature of the NO _x storage catalyst 11 becomes lower than about 250 ° C., the NO ₂ conversion rate decreases rapidly, and when the temperature of the NO _x storage catalyst 11 becomes about 200 ° C. ₂ conversion rate is almost 50% of the maximum NO ₂ conversion (e.g. NO ₂ conversion at about 250 ° C.). In this embodiment, the temperature of the NO _X storage catalyst 11 when the NO ₂ conversion rate becomes approximately 50% of the maximum NO ₂ conversion rate is set to the activation temperature (approximately 200 ° C. (= Ts)), and the NO _X storage catalyst 11 When the temperature is equal to or higher than the activation temperature, it is determined that the NO _x storage catalyst 11 is in an active state.

Now, NO _X in the inflowing exhaust gas is not stored in the NO _X storage agent 52 in the form of nitric oxide (NO), and is not stored in the NO _X storage agent 52 unless it is in the form of nitrogen dioxide (NO ₂ ). The NO _x contained in the inflowing exhaust gas usually contains not only NO ₂ but also a lot of NO, and this NO is not absorbed by the NO _x storage agent 52 unless it is oxidized to NO ₂ . In order to oxidize NO to NO ₂ with the NO _x storage catalyst 11, it is necessary that the catalytic noble metal 51 is activated. Therefore, in order to purify NO _x so far, the catalytic noble metal 51 must be activated. It has been considered necessary.

However results the present inventors for the NO _X storing catalyst 11 is repeated research, the NO contained in the exhaust gas flowing platinum 51 is not activated, i.e. the NO _X storing catalyst 11 is not activated when _the NO _X storage agent The NO ₂ contained in the inflowing exhaust gas is, for example, in the form of nitrous acid (NO ₂ ⁻ ) as shown in FIG. 2B even if the NO _x storage catalyst 11 is not activated. It was found that the _X occlusion agent 52 occludes. Also in this case, as in the case of the NO _X storing catalyst 11 is activated, whether the nitrogen dioxide NO ₂ is absorbed in or or _the NO _X storage agent 52 adsorbs the NO _X absorbent 52 is not always It is not clear, and the term “occlusion” that combines these adsorption and absorption is used. In the present specification, in particular, the storage performed when the NO _X storage catalyst 11 is not activated, that is, when the temperature of the NO _X storage catalyst is lower than the activation temperature is referred to as “low temperature storage”.

Furthermore, the inventors of the present inventors have repeated research through both occluding the form of cold storage and high temperature storage, abundance NO ₂ storage ability _(the NO _X storage agent 52 of the NO _X absorbent 52 is an amount of storage available NO ₂ ) Is not always constant and changes according to the temperature as shown in FIG. As can be seen from FIG. 4, the NO ₂ storage capacity of the NO _X storage agent 52 is mainly the low temperature region in which low temperature storage is performed (region A in the figure) and the high temperature region in which high temperature storage is performed (region in the figure). It is high in B), while it is low in the temperature region (region C in the figure) between these low temperature regions and high temperature regions. Therefore, from the viewpoint of the NO ₂ storage capacity of the NO _X storage agent 52, it is preferable to always maintain the temperature of the NO _X storage catalyst 11 in a low temperature region or a high temperature region.

As described above, when considering that the temperature of the NO _x storage catalyst 11 is always maintained in the low temperature region or the high temperature region, if the cooling or temperature increase is actively performed in order to maintain the temperature in the low temperature region or the high temperature region, the energy consumption amount is increased. Therefore, from the viewpoint of reducing energy consumption, the NO _X storage catalyst 11 may be disposed at a position where the temperature of the NO _X storage catalyst 11 is in the low temperature region or the high temperature region in most engine operating states. Is optimal.

Here, in the compression self-ignition internal combustion engine as in the present embodiment, the temperature of the exhaust gas discharged from the engine body is relatively low, for example, during idling, etc., so the NO _x storage catalyst is optimally disposed in the engine exhaust passage. Even if it is disposed at the position, the temperature of the NO _x storage catalyst cannot often be maintained in a high temperature region. Further, when trying to maintain the temperature of the NO _x storage catalyst in a high temperature region, it is necessary to suppress the high temperature exhaust gas discharged from the engine body from being cooled before reaching the NO _x storage catalyst, The NO _x storage catalyst must be disposed in the vicinity of the engine body, which increases design restrictions on a vehicle or the like equipped with the NO _x storage catalyst.

Therefore, in the embodiment of the present invention, the NO _x storage catalyst 11 is arranged at a position in the engine exhaust passage so that the temperature of the NO _x storage catalyst 11 can be maintained in the low temperature region in most engine operation regions. Is done. Here, the low temperature region refers to a temperature that is lower than the activation temperature of the NO _x storage catalyst 11 and at which the NO ₂ storage capacity of the NO _x storage agent 52 becomes equal to or greater than 200 ° C., for example. In addition, the most operating range refers to an operating range in which steady operation is performed, excluding, for example, during a temperature increasing process described later, when high load operation continues, during cold start of an internal combustion engine, during transient conditions, etc. Refers to the operating state.

As described above, the NO _x storage catalyst 11 is arranged in such a manner that the exhaust passage from the engine body to the NO _x storage catalyst 11 is lengthened, the exhaust gas is cooled and reached before reaching the NO _x storage catalyst 11. It can be considered that the temperature of the exhaust gas is low in most engine operating regions. Further, when the NO _X storage catalyst 11 is provided in a vehicle or the like, the NO _X storage catalyst 11 is arranged at a position where the traveling wind is positively hit and the NO _X storage catalyst 11 is cooled by the traveling wind. The temperature of the NO _x storage catalyst 11 may be maintained in the low temperature region in the partial engine operation region.

When the NO _x storage catalyst 11 is arranged in this manner, the NO _x storage catalyst 11 is actually used in most engine operation ranges, unlike when it is attempted to maintain the NO _x storage catalyst 11 in a high temperature range that cannot actually be maintained in the target temperature range. Can be maintained in a low temperature region. For this reason, the NO ₂ storage capacity of the NO _X storage catalyst 11 can be maintained high during most of the operation period. Further, NO ₂ storage ability of the NO _X absorbent 52 in the low temperature region, as seen from Figure 4, higher than the NO ₂ storage capacity in the high temperature region, and capable of absorbing an effective NO _X.

However, even if kept in this way increase the NO ₂ storage ability of the NO _X absorbent 52, the NO ₂ stored amount of the NO _X absorbent 52 is limited, continued combustion under a lean air-fuel ratio If this is done, the NO ₂ storage capacity of the NO _X storage agent 52 becomes saturated during that time, and therefore NO ₂ cannot be stored by the NO _X storage agent 52. Therefore, in this embodiment, before the NO ₂ storage capacity of the NO _X storage agent 52 saturates, the internal combustion engine becomes in the special operating state described above, so that the temperature of the NO _X storage catalyst 11 is equal to or higher than its activation temperature. when became, the air-fuel ratio of the inflowing exhaust gas by supplying a reducing agent from the reducing agent feed valve 13 to temporarily stoichiometric-rich and thereby so as to release NO _X from _the NO _X storage agent 52 (NO _x release treatment).

More specifically, in the present embodiment, the stored NO _X amount ΣNOX stored in the NO _X storage agent 52 of the NO _X storage catalyst 11 is calculated, and the calculated stored NO _X amount ΣNOX is a predetermined allowable value. When the temperature exceeds the NX and the temperature of the NO _x storage catalyst 11 is equal to or higher than the activation temperature, the air-fuel ratio of the inflowing exhaust gas is switched from lean to stoichiometric rich, thereby the NO _x storage agent 52. NO _x is released from the gas. In the present embodiment, since the NO _X releasing process is executed when the temperature of the NO _X storage catalyst 11 becomes equal to or higher than the activation temperature, that is, the NO _X releasing process cannot always be executed. The predetermined allowable value NX in the embodiment is a value that is considerably smaller than the limit value of the stored NO _X amount at which the NO ₂ storage capacity of the NO _X storage catalyst 11 is saturated. Further, in the present embodiment, the allowable value NX is a predetermined value, but may be a value that changes according to the temperature of the NO _x storage catalyst 11, for example.

When the temperature of the NO _x storage catalyst 11 is lower than its activation temperature, the NO _x storage amount per unit time varies depending on the amount of NO ₂ in the exhaust gas discharged from the engine body 1 as described above. The amount of NO ₂ in the exhaust gas discharged from the engine body 1 is a function of the fuel injection amount Q and the engine rotational speed N, as well as the later-described EGR rate and the retard amount of the injection timing, and therefore NO _X per unit time. The NO ₂ storage amount NO2A stored in the storage agent 52 is a function of the fuel injection amount Q, the engine speed N, the EGR rate, the retard amount of the injection timing, and the like. In the present embodiment, the NO ₂ occlusion amount NO2A per unit time corresponding to these parameters is obtained in advance by experiments and stored in advance in the ROM 32 in the form of a map. The integrated value of the NO ₂ storage amount per unit time represents the integrated storage NO _X amount stored in the NO _X storage agent 52.

In the NO _x releasing process, it has been described that the air-fuel ratio of the inflowing exhaust gas is temporarily stoichiometrically rich by supplying the reducing agent from the reducing agent supply valve 13, but compression auto-ignition is performed as in the present embodiment. When the internal combustion engine is used, the lean degree of the air-fuel ratio of the exhaust gas discharged from the engine body 1 is usually very large. Therefore, in order to make the air-fuel ratio of the inflowing exhaust gas stoichiometric and rich, a large amount of reducing agent is required. Must be supplied from the reducing agent supply valve 13. Therefore, in the present embodiment, for example, by introducing a large amount of EGR gas into the combustion chamber 2, that is, by increasing the EGR rate (amount of EGR gas / amount of total intake gas), the exhaust gas is discharged from the engine body 1. The reducing agent is supplied from the reducing agent supply valve 13 after the lean degree of the air-fuel ratio of the exhaust gas is relatively small.

Further, from the viewpoint of execution of the NO _X emission process of the NO _X storage catalyst 11, the temperature of the NO _X storage catalyst 11 even temporarily in the there by the internal combustion engine becomes a special operating state of some of the above-described It is necessary to be higher than the activation temperature. Therefore, for example, if the NO _x storage catalyst 11 is arranged far away from the engine body and its temperature is always in the low temperature region, it is difficult to execute the NO _x releasing process.

Therefore, the NO _x storage catalyst 11 is preferably arranged so that its temperature can be maintained in a relatively high temperature region in a low temperature region in most engine operation regions. Therefore, for example, when the temperature of the NO _x storage catalyst 11 is to be maintained in the low temperature region by disposing the NO _x storage catalyst 11 away from the engine body, the temperature of the NO _x storage catalyst 11 is low in most engine operation regions. It is preferable to be arranged at the most upstream position among positions that can be maintained in the region.

Figure 5 shows a control routine to maintain the NO _X purifying ability of the NO _X storage catalyst 11, this routine is executed by interruption every predetermined time.

Referring to FIG. 6, first, at step 101, it is judged if the NO _x release flag XN is set. The NO _X release flag XN is a flag that is set when the NO _X release process is to be executed (XN = 1) and is not set otherwise (XN = 0). When it is determined that the NO _X release flag XN is not set, the routine proceeds to step 102, where the NO ₂ storage amount NOXA per unit time is calculated from the map described above. Next, at step 103, the NO ₂ storage amount NO2A per unit time calculated at step 102 is added to ΣNOX, and an integrated storage NO _X amount ΣNOX is calculated.

Next, at step 104 and step 105, whether or not the cumulative stored NO _X amount ΣNOX calculated at step 103 exceeds the allowable value NX, and the temperature TC of the NO _X storage catalyst 11 is a set temperature Tsc, for example, 200 ° C. or more. Whether or not there is is determined. If the integrated storage NO _X amount ΣNOX exceeds the allowable value NX (ΣNOX> NX) and the temperature TC of the NO _X storage catalyst 11 detected by the temperature sensor 20 is equal to or higher than the set temperature Tsc, the routine proceeds to step 106. Then, the NO _X release flag XN is set (XN = 1), otherwise the control routine is terminated. When the NO _X release flag XN is set, the routine proceeds from step 101 to step 107 when the next routine is executed, and the NO _X release process is executed.

FIG. 6 shows a processing routine of the NO _x releasing process started at step 107 in FIG.

Referring to FIG. 6, first, at step 121, the supply amount of the reducing agent necessary for setting the air-fuel ratio of the inflowing exhaust gas to a rich air-fuel ratio of about 13, for example, is calculated. Next, at step 122, the supply time of the reducing agent is calculated. The supply time of this reducing agent is usually 10 seconds or less. Next, at step 123, supply of the reducing agent from the reducing agent supply valve 13 is started. Next, at step 124, it is judged if the reducing agent supply time calculated at step 122 has elapsed. When the supply time of the reducing agent has not elapsed, step 124 is repeated. At this time, the supply of the reducing agent is continued, and the air-fuel ratio of the inflowing exhaust gas is maintained at a rich air-fuel ratio of about 13. On the other hand, when the supply time of the reducing agent has elapsed, that is, when the NO _X releasing action from the NO _X storage agent 52 is completed, the routine proceeds to step 125 where the supply of the reducing agent is stopped, and then proceeds to step 126 and ΣNOX. Is cleared, and in step 127, the NO _X release flag XN is reset.

By the way, as in the above-described embodiment, when executing the NO _X releasing process, when the internal combustion engine is in a special operating state, it waits for the temperature of the NO _X storage catalyst 11 to exceed its activation temperature. sometimes NO ₂ storage ability of the NO _X absorbent 52 from _the NO _X storage agent 52 over a long period can not release the NO _X is saturated.

Therefore, in the second embodiment of the present invention, as the NO _X releasing process, the NO _X storage catalyst 52 waits for the temperature of the NO _X storage catalyst 11 to be equal to or higher than its activation temperature before the NO ₂ storage capacity of the NO _X storage agent 52 is saturated. Instead, the temperature raising process is actively executed to raise the temperature of the NO _x storage catalyst 11 to the activation temperature or higher, and then the air-fuel ratio of the inflowing exhaust gas is temporarily stoichiometric rich, thereby NO. NO _X is released from the _X storage agent 52.

More specifically, in the present embodiment, when the calculated stored NO _X amount ΣNOX exceeds a predetermined allowable value NX, the temperature increasing process of the NO _X storage catalyst 11 is executed to perform the NO _X storage catalyst. the temperature of 11 and the activation temperature or more, then the air-fuel ratio of the inflowing exhaust gas from a lean switched to stoichiometric-rich and thereby release the NO _X from _the NO _X storage agent 52. In the present embodiment, there is no need to wait for the NO _x storage catalyst 11 to reach the activation temperature or higher, and the NO _x releasing process can be executed at any time. Therefore, the predetermined allowable value NX in the present embodiment is The value may be larger than the permissible value NX in the first embodiment. For example, the value is substantially the same as or slightly smaller than the limit value of the stored NO _X amount at which the NO ₂ storage capacity of the NO _X storage catalyst 11 is saturated. Can be a value.

In the present embodiment, the temperature raising process is performed by raising the temperature of the inflowing exhaust gas (hereinafter referred to as the exhaust gas temperature raising process). Specifically, in this exhaust gas temperature raising process, a timing for injecting fuel into the combustion chamber 2 of the internal combustion engine is delayed, or a small amount of fuel is injected after injecting fuel for driving the engine into the combustion chamber 2 of the internal combustion engine. By burning a large amount of EGR gas into the combustion chamber 2 of the internal combustion engine, or by providing an electric heater or glow plug upstream of the NO _x storage catalyst 11 and operating these electric heaters or glow plugs, The temperature of the inflowing exhaust gas is raised, and as a result, the NO _x storage catalyst 11 is heated. In addition, when an ignition plug for igniting fuel is provided in the combustion chamber 2, the temperature of the inflowing exhaust gas can be raised also by delaying the ignition timing of the fuel by the ignition plug.

FIG. 7 is a diagram similar to FIG. 5, and shows a control routine for maintaining the NO _X purification ability of the NO _X storage catalyst 11, and this routine is executed by interruption at regular intervals. Steps 131 to 134 and step 138 are the same as steps 101 to 104 and step 107 in FIG.

In step 135, it is determined whether or not the temperature TC of the NO _x storage catalyst 11 detected by the temperature sensor 20 is a set temperature Tsc, for example, 200 ° C. or higher. When the temperature TC of the NO _x storage catalyst 11 is lower than the set temperature Tsc, the routine proceeds to step 136 where the temperature raising process is performed. When the temperature raising process is performed and the temperature TC of the NO _x storage catalyst 11 becomes equal to or higher than the set temperature Tsc, the routine proceeds from step 135 to step 137 when the routine is executed, and the NO _x release flag XN is set (XN = 1). ).

Incidentally, the exhaust gas discharged from the engine body 1 contains not only nitrogen dioxide (NO ₂ ) but also nitrogen monoxide (NO). If the temperature of the NOx storage catalyst 11 is lower than the activation temperature, The NO _X storage agent 52 can store only NO ₂ . That is, when the temperature of the NO _x storage catalyst 11 is equal to or higher than the activation temperature, the NO _x storage catalyst 11 can be stored after oxidizing NO to NO ₂ by the oxidizing action of the supported platinum. When it is lower than that, NO cannot be stored. Accordingly, the temperature of the NO _X storage catalyst is not occluded NO is the the NO _X absorbent 52 when in the low temperature region, NO _X purification rate by the NO _X storage catalyst 11 is reduced.

Therefore, in the third embodiment of the present invention, the exhaust gas upstream of the NO _x storage catalyst 11 is further added to the exhaust gas purification apparatus as shown in the first embodiment or the second embodiment as shown in FIG. A casing 56 containing an oxidation catalyst 55 is provided. The oxidation catalyst 55 is a catalyst having an oxidation ability supporting a catalytic noble metal such as platinum, and oxidizes NO in the exhaust gas to NO ₂ before flowing into the NO _x storage catalyst 11 so that the NO _x is stored in the NO _x. The catalyst 11 is easily occluded.

Here, the oxidation ability of the oxidation catalyst 55, that is, the oxidation rate of NO to NO ₂ (NO ₂ conversion rate) in the oxidation catalyst 55 also changes according to the temperature of the oxidation catalyst 55. That is, as shown in FIG. 9, when the temperature of the oxidation catalyst 55 is low (for example, 200 ° C. or less), the noble metal supported on the oxidation catalyst 55 is not activated, and therefore the NO ₂ conversion in the oxidation catalyst 55 is not performed. The rate is low. Further, even when the temperature of the oxidation catalyst 55 is very high, the oxidation ability of the noble metal is lowered, and as a result, the NO ₂ conversion rate in the oxidation catalyst 55 is lowered. For this reason, in order to maintain the NO ₂ conversion rate in the oxidation catalyst 55 high, the temperature range of the oxidation catalyst 55 excluding the temperature range where the NO ₂ conversion rate in the oxidation catalyst 55 is low, that is, the oxidation catalyst 55 is It is necessary to maintain in a temperature region (region D in the figure, hereinafter referred to as “high oxidation ability temperature region”) that exhibits a certain or higher NO ₂ conversion rate. The high oxidation ability temperature region is, for example, 200 ° C to 300 ° C. Note that the NO ₂ conversion in the temperature of the lowest temperature side of the high oxidation ability temperature region as can be seen from Figure 9, may be different from the NO ₂ conversion in the temperature of the highest temperature side.

Therefore, in this embodiment, the oxidation catalyst 55 is disposed at a position where the temperature is maintained in the high oxidation ability temperature region in most engine operation regions. Thus, most of the NO _X flowing into the NO _X storing catalyst 11 in most engine operating region of the can to NO _2. As described above, since the NO _X storage catalyst 11 of the present invention can store NO ₂ in the inflowing exhaust gas in most engine operation regions, the NO _{X in} the exhaust gas discharged from the engine body 1 can be stored. Most of the NO _X storage catalyst 11 can be stored, and the NO _X purification rate by the NO _X storage catalyst 11 is increased.

Note that most of the engine operation region in which the temperature of the oxidation catalyst 55 is maintained in the high oxidation capacity temperature region is the same operation region as the most engine operation region in which the NO _x storage catalyst 11 is maintained in the low temperature region. It may be a different operating region.

In the present embodiment, the NO _x releasing process is performed in the same manner as in the first embodiment or the second embodiment, but the method for estimating the stored NO _x amount of the NO _x storage catalyst 11 in the present embodiment is described above. Unlike the embodiment, the stored NO _x amount is not estimated based on the amount of NO ₂ discharged from the engine body 1, but is estimated based on the amount of NO _x discharged from the engine body 1. This is because, in this embodiment, because all NO _X not only NO ₂ is discharged from the engine body 1 is occluded in the NO _X storing catalyst 11.

More specifically, NO NO _X amount exhausted from the engine body 1 per unit time is a function of the fuel injection amount Q and the engine speed N, is thus occluded in the NO _X absorbent 52 per unit time _X The storage amount NOXA is a function of the fuel injection amount Q and the engine speed N. In this embodiment, the NO _X storage amount NOXA per unit time corresponding to the fuel injection amount Q and the engine speed N is obtained in advance by experiments, and this NO _X storage amount NOXA is determined based on the fuel injection amount Q and the engine speed N. Is stored in advance in the ROM 32 in the form of a map. The integrated value ΣNOX of the NO _X storage amount NOXA per unit time represents the integrated storage NO _X amount stored in the NO _X storage agent 52.

In the exhaust purification system of this embodiment, in addition to the exhaust manifold 5, that is, the reducing agent addition valve 13 provided in the engine exhaust passage upstream of the oxidation catalyst 55, the oxidation catalyst 55 and the NO _x storage catalyst 11 A reducing agent addition valve 57 is provided in the engine exhaust passage. In the temperature raising process for raising the temperature of the NO _x storage catalyst 11, a reducing agent is added from the reducing agent addition valve 13 provided upstream of the oxidation catalyst 55, and NO is generated by the combustion heat of the reducing agent in the oxidation catalyst 55. _The temperature of the _X storage catalyst 11 is increased. In the NO _x releasing process, a reducing agent is added from a reducing agent addition valve 57 provided between the oxidation catalyst 55 and the NO _x storage catalyst 11, and the air-fuel ratio of the inflowing exhaust gas is made stoichiometric rich. Alternatively, the reducing agent addition valve 13 may be provided only upstream of the exhaust of the oxidation catalyst 55 and used for both the temperature raising process and the NO _x releasing process.

When the oxidation catalyst 55 is provided upstream of the exhaust of the NO _x storage catalyst 11, the temperature raising process is not the above-described temperature raising method, but generates a chemical reaction in the oxidation catalyst 55 to generate heat. The heat may be transmitted to the NO _x storage catalyst 11 through the exhaust gas (hereinafter referred to as exothermic temperature raising process). Specifically, in the heat generation and temperature raising process, a small amount of fuel is injected after injecting fuel for driving the engine into the combustion chamber 2 of the internal combustion engine, and the fuel is discharged from the combustion chamber 2 as it is without being burned. A device for adding fuel to the exhaust gas is provided upstream of the oxidation catalyst 55, and the fuel is added to the exhaust gas from this device, and the fuel is supplied to the oxidation catalyst 55. exothermic reaction at the oxidation catalyst 55 by burned occur, _the NO _X storage catalyst 11 is raised this heat is transmitted to the NO _X storing catalyst 11 via an exhaust gas Te.

Next, the case where the NO _X storage catalyst 11 shown in FIG. 1 is composed of a particulate filter (hereinafter referred to as “filter”) will be described.

  FIGS. 10A and 10B show the structure of the filter 11. 10A shows a front view of the filter 11, and FIG. 10B shows a side sectional view of the filter 11. As shown in FIG. As shown in FIGS. 10A and 10B, the filter 11 has a honeycomb structure and includes a plurality of exhaust flow passages 60 and 61 extending in parallel with each other. These exhaust flow passages include an exhaust gas inflow passage 60 whose downstream end is closed by a plug 62 and an exhaust gas outflow passage 61 whose upstream end is closed by a plug 63. In addition, the hatched part in FIG. Therefore, the exhaust gas inflow passages 60 and the exhaust gas outflow passages 61 are alternately arranged via the thin partition walls 64. In other words, in the exhaust gas inflow passage 60 and the exhaust gas outflow passage 61, each exhaust gas inflow passage 60 is surrounded by four exhaust gas outflow passages 61, and each exhaust gas outflow passage 61 is surrounded by four exhaust gas inflow passages 60. To be arranged.

  The filter 11 is made of, for example, a porous material such as cordierite. Therefore, the exhaust gas flowing into the exhaust gas inflow passage 60 passes through the surrounding partition wall 64 as indicated by an arrow in FIG. And flows into the adjacent exhaust gas outflow passage 61.

When the NO _x storage catalyst 11 is constituted by a particulate filter in this way, the peripheral wall surfaces of the exhaust gas inflow passages 60 and the exhaust gas outflow passages 61, that is, on both side surfaces of the partition walls 64 and in the partition walls 64. A catalyst carrier layer made of alumina is formed on the inner wall surface of the hole. As shown in FIGS. 2A and 2B, the catalyst noble metal 51, the NO _x storage agent 52, and the catalyst carrier 50 are formed on the catalyst carrier 50. Is carried. In this case, platinum Pt is also used as the catalyst noble metal. Thus the temperature of the NO _X storage catalyst 11 even if you configure the NO _X storing catalyst 11 from the particulate filter is NO ₂ in the inflowing exhaust gas when less than its activation temperature is occluded in the NO _X storage catalyst 11. The same of the NO _X emission process and the NO _X release processing for the NO _X storing catalyst 11 shown in FIGS. 5 and 6 or 7 in this case is carried out.

Further, when the NO _x storage catalyst 11 is composed of a particulate filter, particulate matter contained in the inflowing exhaust gas is captured in the filter 11 and the captured particulate matter is sequentially burned by the exhaust gas heat. It is done. When a large amount of particulate matter accumulates on the filter 11, the temperature of the exhaust gas discharged from the engine body 1 is increased, or the reducing agent is supplied from the reducing agent supply valve 13 to supply the oxidation catalyst 55 and the filter. 11 is burned, and the temperature of the filter 11 is increased, whereby the accumulated particulates are ignited and burned.

In each of the above embodiments, the NO _X storage catalyst 11 carries the NO _X storage agent 52 and the catalyst noble metal (for example, platinum) 51. However, the NO _X storage agent 52 in the low temperature region has NO _2. From the viewpoint of storage capacity, it is preferable that the catalyst noble metal 51 is supported in a very small amount or not at all.

In other words, in a temperature range where the NO _x storage agent 52 performs low temperature storage, the catalyst noble metal 51 supported on the NO _x storage catalyst 11 cannot only oxidize NO in the inflowing exhaust gas to NO ₂ . , resulting in a reduction of the NO ₂ to NO. As described above, the NO _X storage agent 52 can store only NO _2, and cannot store NO. Therefore, when the many supporting the catalyst noble metal 51 in the NO _X storage catalyst 11, NO _X purification rate by the NO _X storage catalyst 11 at the temperature region is lowered. If no catalyst precious metal 51 is supported on the NO _x storage catalyst 11, NO ₂ is not reduced, and the NO _x purification rate by the NO _x storage catalyst 11 can be increased.

However, the catalytic noble metal 51 is not at all carried on the NO _X storing catalyst 11, when releasing NO _X from the NO _X storing catalyst 11, in the the NO _X storing catalyst 11 alone released from the NO _X storing catalyst 11 it is not possible to reduce the nO _X to N _2, exhaust downstream oxidation catalyst of the nO _X storage catalyst 11 to reduce the released nO _X from the nO _X storing catalyst 11 is required. Therefore, a very small amount of the catalytic noble metal 51 may be supported on the NO _x storage catalyst 11. The amount of the catalyst noble metal 52 is, for example, an amount smaller than the amount of the catalyst noble metal 51 set so as to be optimal when considering the reduction ability and the oxidation ability at least above the activation temperature. When platinum is used, it is preferably 2 g / l or less, for example.

It is a figure which shows the whole internal combustion engine. It is a diagram for explaining the absorbing action of the NO _X storage catalyst. It is a diagram illustrating a NO ₂ conversion with platinum. It is a diagram illustrating a NO ₂ storage ability of the NO _X absorbent. It is a flowchart of the NO _X purifying ability maintain control of the NO _X storage catalyst. Is a flow chart for performing the NO _X release process. It is a flowchart of NO _X purification ability maintenance control of the NO _X storage catalyst in the second embodiment. It is a figure which shows the whole internal combustion engine in 3rd embodiment. It is a diagram illustrating a NO ₂ conversion rate of the oxidation catalyst. It is a figure which shows a particulate filter.

Explanation of symbols

3 ... fuel injection valve 4 ... intake manifold 5 ... exhaust manifold 7 ... exhaust turbocharger 11 ... NO _X storing catalyst 13 ... reducing agent feed valve

Claims (7)

Place _the NO _X storage catalyst with _the NO _X storage agent engine exhaust passage, when the temperature of the NO _X storage catalyst is lower than the activation temperature when the air-fuel ratio of the exhaust gas flowing into the NO _X storage catalyst is lean is NO ₂ contained in the exhaust gas is occluded in the NO _X absorbent to, when the temperature of the NO _X storage catalyst is not less than the activation temperature, the exhaust when the air-fuel ratio of the exhaust gas flowing into the NO _X storage catalyst is lean NO _X contained in the gas is occluded in the NO _X absorbent, NO _X when the air-fuel ratio of the exhaust gas flowing into the NO _X storage catalyst is occluded when almost the stoichiometric air-fuel ratio or rich is released from _the NO _X storage agent An exhaust purification device for an internal combustion engine in which the NO _X storage catalyst is positioned at a position where the temperature of the NO _X storage catalyst is at least lower than its activation temperature in most engine operating ranges. The NO _x storage catalyst is positioned in a temperature range where the temperature is lower than the activation temperature and the NO ₂ storage capacity of the NO _x storage agent is at a certain level or more in most engine operation regions. 2. An exhaust emission control device for an internal combustion engine according to 1. The exhaust purification device for an internal combustion engine according to claim 2, wherein the NO _x storage catalyst is positioned on the most upstream side in a position where the temperature is within the temperature range in most engine operation regions. An oxidation catalyst having a noble metal is disposed in the engine exhaust passage and upstream of the NO _x storage catalyst, and the oxidation catalyst is located within a temperature range that exhibits a certain level of oxidation capability in most engine operation regions. The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 3, wherein the exhaust gas purification apparatus is positioned at the position. Means for estimating the NO _X storage amount stored in the NO _X storage agent are provided, and when the estimated NO _X storage amount exceeds a predetermined allowable value, the temperature of the NO _X storage catalyst becomes the active temperature. The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 4, wherein the temperature rise control is performed so as to achieve the above, and the air-fuel ratio of the exhaust gas is substantially stoichiometric or rich. The exhaust purification device for an internal combustion engine according to any one of claims 1 to 5, wherein the NO _x storage catalyst has a noble metal, and the activation temperature is an activation temperature of the noble metal. The activation temperature, an internal combustion according to any one of claims 1 to 5 which is a boundary temperature between a temperature range of occluding temperature range and NO ₂ that _the NO _X storage agent occludes NO ₂ as nitrite as nitrate Engine exhaust purification system.

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