JP4285193B2 - 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 NOx contained in exhaust gas when combustion is performed under a lean air-fuel ratio, a layer of NOx absorbent made of alkali metal or alkaline earth on the surface of a carrier made of alumina. And a catalyst in which a noble metal catalyst such as platinum is supported on the surface of the support is known (see, for example, Patent Document 1). In this catalyst, when the catalyst is activated, NOx contained in the exhaust gas is stored in the NOx absorbent when the air-fuel ratio of the exhaust gas is lean, and is stored in the NOx absorbent when the air-fuel ratio of the exhaust gas is made rich. The NOx that has been released is released and reduced.

  By the way, it is considered that such NOx absorption / release action is not performed unless the catalyst is activated. Therefore, in the internal combustion engine described in Patent Document 1, when the catalyst is not activated, the catalyst is activated by an electric heater. I try to heat it.

JP-A-6-108826 JP 2002-89246

However, as a result of repeated studies by the present inventors on the catalyst that performs the NOx absorption / release action as described above, the nitric oxide NO contained in the exhaust gas is not stored in the NOx absorbent unless the catalyst is activated. It turns out that nitrogen dioxide NO ₂ contained in the exhaust gas is stored in the NOx absorbent (stored at low temperature) by a mechanism different from that in which NO is stored when the catalyst is activated even if the catalyst is not activated. It was.

And by utilizing such facts found by the present inventors, that is, that NO ₂ is stored in the NOx absorbent at a low temperature even when the catalyst is not activated, the exhaust gas can be exhausted more efficiently than before. It becomes possible to remove NOx in the gas. However, on the other hand, in such a case, the NOx purification capacity of the catalyst (including NOx purification capacity based on low-temperature occlusion when the catalyst is inactive) varies greatly depending on the catalyst state, that is, the degree of catalyst deterioration and sulfur poisoning. Therefore, it has been found that there is a case where sufficient NOx removal cannot be performed as intended. Further, it has been found that such a problem is particularly remarkable when the catalyst is inactive, since the NOx purification ability when the catalyst is inactive is greatly influenced by the catalyst state.

Therefore, in the present invention, in the exhaust purification device that purifies exhaust gas by utilizing the low temperature storage of nitrogen dioxide NO ₂ in the NOx absorbent even when the catalyst is not activated, the catalyst state is considered. An object of the present invention is to provide an exhaust emission control device capable of purifying exhaust gas.

The invention according to claim 1 is a NOx storage catalyst comprising a noble metal catalyst and a NOx absorbent disposed in the engine exhaust passage, and is activated when the air-fuel ratio of the inflowing exhaust gas is lean. when non will cold occlude nitrogen dioxide nO ₂ contained in the exhaust gas in the NOx absorbent, a NOx storage catalyst to a high temperature storage of nitrogen oxides NOx in the exhaust gas in the NOx absorbent when being activated Means for estimating the NOx purification capacity of the NOx storage catalyst based on the active state of the NOx storage catalyst, wherein the NOx purification capacity estimation means is the oxidation performance of the NOx storage catalyst. comprise means for estimating the reduction degree of, as reduction degree of oxidation performance estimated by the oxidation degradation degree estimating means is high above NOx storage catalyst Serial to provide an exhaust purification device of the NOx purification capacity based on cold storage large estimates.

In the process of shifting from a state in which the NOx storage catalyst, that is, the precious metal catalyst is not activated to a fully activated state, during the oxidation reaction of hydrocarbon HC in the precious metal catalyst, nitrogen dioxide NO ₂ is converted to nitric oxide NO. A reduction reaction occurs. Nitrogen dioxide NO ₂ contained in the exhaust gas is occluded in the NOx absorbent at a low temperature even if the NOx occlusion catalyst, that is, the noble metal catalyst is not activated, but nitrogen monoxide NO contained in the exhaust gas is occluded in the NOx occlusion. Unless the catalyst is activated and oxidized to nitrogen dioxide NO ₂ , it is not stored in the NOx absorbent at high temperature.

As described above, when the reduction reaction of nitrogen dioxide NO ₂ to nitric oxide NO occurs as described above, the NOx storage capacity when the NOx storage catalyst is not activated (that is, NOx based on the low temperature storage). (Purification capacity) will decrease. On the other hand, as described above, the reduction reaction of nitrogen dioxide NO ₂ to nitric oxide NO occurs with the oxidation reaction of hydrocarbon HC in the noble metal catalyst, so that the NOx storage catalyst deteriorates and the noble metal catalyst is sintered, etc. If the oxidation performance is lowered, it is difficult to occur. If the reduction reaction of the nitrogen dioxide NO ₂ to nitric oxide NO decreases, as a result, the decrease in the NOx purification capacity when the NOx storage catalyst is not activated is suppressed. In other words, reduction of the NOx purifying ability at the time when the oxidation performance of the NOx storage catalyst decreases Then the NOx storage catalyst is not activated can be suppressed.

According to the first aspect of the present invention, the means for estimating the NOx purification capacity of the NOx storage catalyst based on the active state of the NOx storage catalyst, for example, the temperature, estimates the degree of decrease in the oxidation performance of the NOx storage catalyst. The NOx purification ability based on the said low temperature storage of the said NOx storage catalyst is estimated largely, so that the fall degree of the estimated oxidation performance is high. As a result, as a result, the higher the estimated degree of decrease in the oxidation performance, the lower the NOx purification capacity in the state where the activity of the NOx storage catalyst is relatively low (low activity side), that is, for example, the lower the temperature of the NOx storage catalyst. In this case, the NOx purification capacity is estimated to be larger. More specifically, for example, the NOx purification capacity is estimated to be larger in a range where the temperature of the NOx storage catalyst is equal to or lower than a predetermined temperature determined by the degree of reduction in oxidation performance .

By doing so, the NOx purification ability of the NOx storage catalyst, in particular, the NOx purification ability in a relatively low activity state of the NOx storage catalyst that is easily affected by the degree of decrease in the oxidation performance of the NOx storage catalyst is further improved. It can be estimated accurately. Therefore, by performing exhaust gas purification based on the NOx purification capacity estimated in this way, it is possible to efficiently remove sufficient NOx as intended.
In this specification, the NOx purification capacity means, for example, the NOx occlusion speed (the amount of NOx occluded in the NOx absorbent per unit time).

The invention according to claim 2 is an NOx storage catalyst comprising a noble metal catalyst and a NOx absorbent disposed in the engine exhaust passage, and activated when the air-fuel ratio of the inflowing exhaust gas is lean. NOx storage catalyst that stores nitrogen dioxide NO ₂ contained in the exhaust gas in the NOx absorbent at a low temperature when it is not activated and stores NOx absorbent in the NOx absorbent at a high temperature when the NOx absorbent is activated And an NOx purification capacity of the NOx storage catalyst based on an active state of the NOx storage catalyst, wherein the NOx purification capacity estimation means is a sulfur of the NOx storage catalyst. Means for estimating the degree of poisoning, and the higher the degree of sulfur poisoning estimated by the sulfur poisoning degree estimating means, the lower the temperature of the NOx storage catalyst. To provide an exhaust purifying apparatus that estimates reduce the NOx purifying capability-based built.

  When the NOx occlusion catalyst is poisoned with sulfur, the NOx occlusion capacity of the NOx occlusion catalyst is reduced. In particular, the NOx occlusion capacity when the NOx occlusion catalyst is not activated (that is, the low temperature occlusion catalyst). The reduction in the NOx purification capacity based on the above is remarkable.

  According to the second aspect of the present invention, the means for estimating the NOx purification capacity of the NOx storage catalyst based on the active state of the NOx storage catalyst, for example, the temperature, estimates the degree of sulfur poisoning of the NOx storage catalyst. The NOx purification ability based on the said low temperature occlusion of the said NOx occlusion catalyst is estimated to be smaller as the estimated degree of sulfur poisoning is higher. Consequently, as a result, the higher the estimated degree of sulfur poisoning, the lower the NOx purification capacity in the state where the activity of the NOx storage catalyst is relatively low (low activity side), that is, for example, the temperature of the NOx storage catalyst is lower. In this case, the NOx purification capacity is estimated to be smaller. More specifically, for example, the NOx purification capacity is estimated to be smaller in a range where the temperature of the NOx storage catalyst is equal to or lower than a predetermined temperature determined by the degree of sulfur poisoning.

  By doing so, the NOx purification capacity of the NOx storage catalyst, particularly the NOx purification capacity in a relatively low state of activity of the NOx storage catalyst that is easily affected by the degree of sulfur poisoning of the NOx storage catalyst is further improved. It can be estimated accurately. Therefore, by performing exhaust gas purification based on the NOx purification capacity estimated in this way, it is possible to efficiently remove sufficient NOx as intended.

  According to a third aspect of the present invention, in the first aspect of the invention, the NOx purification capacity estimating means further includes means for estimating the degree of sulfur poisoning of the NOx storage catalyst, and the estimated sulfur poisoning. The higher the degree is, the smaller the NOx purification capacity of the NOx storage catalyst based on the low temperature storage is estimated.

  According to the third aspect of the present invention, it is possible to accurately estimate the NOx purification capacity at a relatively low activity of the NOx storage catalyst, that is, for example, at a low temperature, and exhaust gas based on the estimated NOx purification capacity. By purifying the gas, it is possible to efficiently remove sufficient NOx as intended.

The invention according to claim 4 is a NOx occlusion catalyst comprising a noble metal catalyst and a NOx absorbent disposed in the engine exhaust passage, and is activated when the air-fuel ratio of the inflowing exhaust gas is lean. A NOx storage catalyst that stores nitrogen dioxide NO ₂ contained in the exhaust gas in the NOx absorbent at a low temperature when it is not activated, and stores nitrogen oxide NOx contained in the exhaust gas in the NOx absorbent at a high temperature when activated. In an exhaust gas purification apparatus for an internal combustion engine, in which temperature rise control is performed to increase the temperature of the NOx storage catalyst to a predetermined target temperature or higher after the engine is started, the degree of reduction in the oxidation performance of the NOx storage catalyst is further reduced . comprise means for estimating, as the degree of reduction in oxidation performance estimated by the oxidation degradation degree estimating means is high, is to start time of the temperature increase control is delayed Or the temperature of the NOx storage catalyst from the start of the engine to provide an exhaust purification device to be so that the time until the target temperature becomes longer.

According to the fourth aspect of the present invention, when the oxidation performance of the NOx occlusion catalyst is lowered, the temperature rise of the NOx occlusion catalyst is delayed, and the low temperature state of the NOx occlusion catalyst is lengthened. And, as described above, when the oxidation performance of the NOx storage catalyst is lowered , the NOx purification ability on the low temperature side increases, so by increasing the low temperature state of the NOx storage catalyst as in the present invention, Efficient and sufficient removal of NOx becomes possible.

The invention according to claim 5 is a NOx storage catalyst comprising a noble metal catalyst and a NOx absorbent disposed in the engine exhaust passage, and is activated when the air-fuel ratio of the inflowing exhaust gas is lean. A NOx storage catalyst that stores nitrogen dioxide NO ₂ contained in the exhaust gas in the NOx absorbent at a low temperature when it is not activated, and stores nitrogen oxide NOx contained in the exhaust gas in the NOx absorbent at a high temperature when activated. In an exhaust gas purification apparatus for an internal combustion engine, in which temperature rise control is performed to increase the temperature of the NOx storage catalyst to a predetermined target temperature or higher after the engine is started, the degree of sulfur poisoning of the NOx storage catalyst is further increased. Means for estimating, and the higher the degree of sulfur poisoning estimated by the sulfur poisoning degree estimating means, the earlier the start timing of the temperature increase control, or Temperature from the start of the NOx storage catalyst related to provide an exhaust purification device which is as time until the target temperature is shortened.

  According to the fifth aspect of the present invention, when the NOx occlusion catalyst is poisoned with sulfur, the temperature rise of the NOx occlusion catalyst is accelerated and the low temperature state of the NOx occlusion catalyst is shortened. As described above, when the NOx storage catalyst is poisoned with sulfur, the NOx purification ability on the low temperature side is particularly lowered. Therefore, it is sufficient to shorten the low temperature state of the NOx storage catalyst as in the present invention. It is possible to realize the removal of NOx.

The invention according to claim 6 is a NOx occlusion catalyst comprising a noble metal catalyst and a NOx absorbent disposed in the engine exhaust passage, and is activated when the air-fuel ratio of the inflowing exhaust gas is lean. when non will cold occlude nitrogen dioxide nO ₂ contained in the exhaust gas in the NOx absorbent, a NOx storage catalyst to a high temperature storage of nitrogen oxides NOx in the exhaust gas in the NOx absorbent when being activated , and means for estimating the reduction degree of the oxidation performance of the NOx storage catalyst, the long deterioration degree of oxidation performance estimated after the start of the engine by the oxidation degradation degree estimating means is equal to or greater than a predetermined reduction degree in the exhaust purification apparatus for an internal combustion engine cryostat control is performed for maintaining a low temperature of the NOx storage catalyst, which is estimated by the oxidation degradation degree estimating means The higher the degree of reduction of performance, to increase the execution time of the low temperature maintenance control, or to provide an exhaust gas purification apparatus adapted to lower the temperature to maintain in the cryostat control.

According to the sixth aspect of the present invention, when the oxidation performance of the NOx storage catalyst is lowered , the NOx storage catalyst is maintained at a low temperature for a long time. As described above, when the oxidation performance of the NOx storage catalyst is lowered , the NOx purification capacity on the low temperature side is increased, so that the NOx storage catalyst can be maintained at a low temperature for a long time as in the present invention. Thus, efficient and sufficient removal of NOx becomes possible.

The invention according to claim 7 is a NOx storage catalyst comprising a noble metal catalyst and a NOx absorbent disposed in the engine exhaust passage, and is activated when the air-fuel ratio of the inflowing exhaust gas is lean. A NOx storage catalyst that stores nitrogen dioxide NO ₂ contained in the exhaust gas in the NOx absorbent at a low temperature when it is not activated, and stores nitrogen oxide NOx contained in the exhaust gas in the NOx absorbent at a high temperature when activated. And when the temperature of the NOx storage catalyst is lower than a predetermined temperature, the ratio of nitrogen dioxide NO _{2 to} nitrogen monoxide NO generated when combustion is performed under a lean air-fuel ratio is expressed as the temperature of the NOx storage catalyst. In the exhaust emission control device that is increased as compared with the case of the same engine operating state when the temperature is equal to or higher than the predetermined temperature, the degree of reduction in the oxidation performance of the NOx storage catalyst Comprise means for estimating the case, the higher the degree of reduction in oxidation performance estimated by the oxidation degradation degree estimating means, to provide an exhaust purification device that is adapted to the predetermined temperature is set high.

As described above, nitrogen dioxide NO ₂ contained in the exhaust gas is occluded at a low temperature by the NOx absorbent even if the NOx occlusion catalyst is not activated. However, nitrogen monoxide NO contained in the exhaust gas is activated by the NOx occlusion catalyst. If it is not oxidized to nitrogen dioxide NO _2, it will not be stored in the NOx absorbent at high temperature. For this reason, when the NOx storage catalyst is not activated or has a low activity, the amount of nitrogen monoxide NO in the exhaust gas is reduced and the amount of nitrogen dioxide NO ₂ in the exhaust gas is increased when the NOx storage catalyst is low in temperature. It is preferable. Therefore, for example, if the predetermined temperature is set to a temperature at which the amount of nitrogen dioxide NO ₂ stored in the NOx absorbent at a low temperature per unit time is equal to the amount of nitrogen oxide NOx stored at a high temperature per unit time. In addition, the amount of nitrogen dioxide NO ₂ in the exhaust gas is increased in a temperature range where low-temperature storage of nitrogen dioxide NO ₂ can be expected, and the NOx purification rate can be improved.

Since NOx purification performance based on the low-temperature storage as described above is consequently increased when the oxidation performance of the NOx storage catalyst is reduced, a higher degree of decrease of the oxidation performance of the NOx storage catalyst as in the present invention As the predetermined temperature is increased and the temperature range expected for low-temperature storage is increased, efficient and sufficient removal of NOx becomes possible.

The invention according to claim 8 is a NOx occlusion catalyst comprising a noble metal catalyst and a NOx absorbent disposed in the engine exhaust passage, and is activated when the air-fuel ratio of the inflowing exhaust gas is lean. A NOx storage catalyst that stores nitrogen dioxide NO ₂ contained in the exhaust gas in the NOx absorbent at a low temperature when it is not activated, and stores nitrogen oxide NOx contained in the exhaust gas in the NOx absorbent at a high temperature when activated. And when the temperature of the NOx storage catalyst is lower than a predetermined temperature, the ratio of nitrogen dioxide NO _{2 to} nitrogen monoxide NO generated when combustion is performed under a lean air-fuel ratio is expressed as the temperature of the NOx storage catalyst. In the exhaust emission control device that is increased as compared with the same engine operating state when the temperature is equal to or higher than the predetermined temperature, the degree of sulfur poisoning of the NOx storage catalyst is further increased. Provided is an exhaust emission control device provided with a means for estimating, wherein the predetermined temperature is set lower as the degree of sulfur poisoning estimated by the sulfur poisoning degree estimating means is higher.

  As described above, the NOx purification capacity based on low-temperature storage is greatly affected when the NOx storage catalyst is poisoned with sulfur. That is, when the NOx storage catalyst is poisoned with sulfur, the NOx purification ability based on the low temperature storage becomes extremely low. Therefore, as the degree of sulfur poisoning of the NOx storage catalyst is higher as in the present invention, the predetermined temperature is lowered, and the temperature range expected for low temperature storage is reduced, thereby realizing sufficient removal of NOx. Can do.

The invention described in each claim is an exhaust purification apparatus that purifies exhaust gas by utilizing low-temperature storage of nitrogen dioxide NO ₂ in the NOx absorbent even in a state where the NOx storage catalyst is not activated. There is a common effect that it is possible to remove sufficient NOx as intended.

  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 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 (intercooler) 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 intercooler 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 NOx 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. A cooling device (EGR cooler) 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 EGR cooler 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. A temperature sensor 20 for detecting the temperature of the NOx storage catalyst 11 is attached to the NOx storage catalyst 11, and an output signal of the temperature sensor 20 is input to the input port 35 via the corresponding AD converter 37. A load sensor 41 that generates an output voltage proportional to the depression amount L 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. Is done. Further, the input port 35 is connected to a crank angle sensor 42 that generates an output pulse every time the crankshaft rotates, for example, 15 °. On the other hand, the output port 36 is connected to the fuel injection valve 3, the throttle valve 9 driving step motor, the reducing agent supply valve 13, the EGR control valve 15, and the fuel pump 19 through corresponding drive circuits 38.

  The NOx storage catalyst 11 shown in FIG. 1 is composed of a monolith catalyst, and a catalyst carrier made of alumina, for example, is supported on the base of the NOx storage catalyst 11. 2A to 2C schematically show a cross section of the surface portion of the catalyst carrier 45. FIG. As shown in FIGS. 2A to 2C, a noble metal catalyst 46 is dispersedly supported on the surface of the catalyst carrier 45, and a layer of NOx absorbent 47 is further formed on the surface of the catalyst carrier 45. Is formed.

  In the embodiment of the present invention, for example, platinum Pt is used as the noble metal catalyst 46, and the components constituting the NOx absorbent 47 are, for example, alkali metals such as potassium K, sodium Na, cesium Cs, barium Ba, calcium. At least one selected from alkaline earth such as Ca, rare earth such as lanthanum La and yttrium Y is used.

  When the ratio of air and fuel (hydrocarbon) supplied to the engine intake passage, the combustion chamber 2 and the exhaust passage upstream of the NOx storage catalyst 11 is called the air-fuel ratio of the exhaust gas, the NOx absorbent 47 is activated by the noble metal catalyst 46. If the NOx occlusion catalyst 11 is activated, NOx is occluded at a high temperature when the air-fuel ratio of the exhaust gas is lean, and NOx occlusion that releases the NOx occluded at a high temperature when the oxygen concentration in the exhaust gas decreases. Performs release action. When fuel (hydrocarbon) or air is not supplied into the exhaust passage upstream of the NOx storage catalyst 11, the air-fuel ratio of the exhaust gas matches the air-fuel ratio of the air-fuel mixture in the combustion chamber 2. Therefore, in this case The NOx absorbent 47 stores NOx at a high temperature when the air-fuel ratio of the mixture in the combustion chamber 2 is lean, and releases the NOx stored at a high temperature when the oxygen concentration in the mixture in the combustion chamber 2 decreases.

That is, the case where barium Ba is used as a component constituting the NOx absorbent 47 will be described as an example. When the air-fuel ratio of the exhaust gas is lean, that is, when the oxygen concentration in the exhaust gas is high, the noble metal catalyst 46 is activated. Then, the NO contained in the exhaust gas is oxidized on the noble metal catalyst 46 to become NO ₂ as shown in FIG. 2 (A), and then stored in the NOx absorbent 47 at a high temperature and combined with the barium oxide BaO. It diffuses into the NOx absorbent 47 in the form of nitrate ions NO ₃ ⁻ . In this way, NOx is occluded in the NOx absorbent 47 at a high temperature. That is, such occlusion in the form of nitrate ions NO ₃ ⁻ is referred to as high-temperature occlusion. NO ₂ on the surface as long as the precious metal catalyst 46 a high oxygen concentration in the exhaust gas is generated, as long as NO ₂ to a high temperature storage capacity is not saturated in the NOx absorbent 47 is a high temperature occluded in the NOx absorbent 47 nitrate ions NO ₃ ^- is generated.

On the other hand, if the air-fuel ratio in the combustion chamber 2 is made rich or stoichiometric, or the reducing agent is supplied from the reducing agent supply valve 13 to make the air-fuel ratio rich or stoichiometric, the exhaust gas is exhausted. Since the oxygen concentration in the gas decreases, the reaction proceeds in the reverse direction, and thus nitrate ions NO ₃ ⁻ in the NOx absorbent 47 are released from the NOx absorbent 47 in the form of NO ₂ or NO. In this case, since the reducing agent (unburned HC, CO, etc.) is present in the exhaust gas, the released NOx is then contained in the exhaust gas (unburned HC, CO, etc.). Reduced by

Now, the nitrogen oxides NOx in the exhaust gas is not hot stored in the NOx absorbent 47 in the form of nitrogen monoxide NO, are not hot stored in the NOx absorbent 47 unless not in the form of nitrogen dioxide NO _2. That is, nitrogen monoxide NO contained in the exhaust gas does not become nitrogen dioxide NO ₂ , that is, is not stored in the NOx absorbent 47 unless it is oxidized. The noble metal catalyst 46, for example, platinum Pt inherently has activity at a low temperature, but the basicity of the NOx absorbent 47 is quite strong, and therefore the activity at a low temperature of the noble metal catalyst 46, that is, the oxidizing property is weakened. It will be. As a result, when the temperature TC of the NOx storage catalyst 11 decreases, the action of oxidizing nitric oxide NO to nitrogen dioxide NO ₂ is weakened. For this reason, in order to oxidize nitric oxide NO, it is necessary that the temperature of the noble metal catalyst 46 is high and activated, that is, that the NOx storage catalyst 11 is activated. Has been considered to require that the noble metal catalyst 46 is activated, that is, that the NOx storage catalyst 11 is activated.

However, as a result of the inventor's repeated research on the NOx storage catalyst 11, the NOx absorbent contained in the exhaust gas does not activate the noble metal catalyst 46, that is, the NOx storage catalyst 11 does not activate. Nitrogen dioxide NO _{2 that} is not stored in the exhaust gas but is contained in the exhaust gas is low in the NOx absorbent 47 in the form of nitrous acid NO ₂ ⁻ as shown in FIG. 2B even if the NOx storage catalyst 11 is not activated. It was found that it was occluded. In the present specification, such occlusion in the form of nitrous acid NO ₂ ⁻ is referred to as low-temperature occlusion to distinguish it from the above-described high-temperature occlusion. Further, when it is not necessary to distinguish between high temperature storage and low temperature storage, these are collectively referred to simply as storage.

Thus, even if the NOx storage catalyst 11 is not activated, nitrogen dioxide NO ₂ is stored at a low temperature. Therefore, in the embodiment of the present invention, for example, when combustion is performed under a lean air-fuel ratio, the inflow In the case where the air-fuel ratio of the exhaust gas being lean is lean, nitrogen dioxide NO ₂ contained in the exhaust gas is stored in the NOx absorbent at a low temperature while the NOx storage catalyst 11 is not activated. That is, in the compression ignition type internal combustion engine as shown in FIG. 1, the exhaust gas air-fuel ratio during normal operation is lean. For example, the NOx storage catalyst 11 is not activated for a while after the cold start of the engine. For this reason, for example, for a while after the engine is started, the NOx absorbent 47 of the NOx storage catalyst 11 stores nitrogen dioxide NO ₂ contained in the exhaust gas at a low temperature.

  By the way, the transition from the state where the NOx storage catalyst 11 as shown in FIG. 2 (B) is not activated to the state where the NOx storage catalyst 11 as shown in FIG. Actually, this gradually occurs as the temperature TC of the NOx storage catalyst 11 increases. An intermediate state (hereinafter referred to as “intermediate state”) as shown in FIG. 2C exists between these two states.

In this intermediate state, as shown in FIG. 2C, a reaction occurs in which nitrogen dioxide NO ₂ reacts with hydrocarbon HC in the exhaust gas on the noble metal catalyst 46 and is reduced to nitric oxide NO. . That is, when hydrocarbon HC is present when the NOx storage catalyst 11 is fully activated and the noble metal catalyst 46 is sufficiently oxidized, nitrogen dioxide NO ₂ and hydrocarbon HC in the exhaust gas on the noble metal catalyst 46. It reacts and is reduced to nitrogen N ₂ (in other words, hydrocarbon HC is oxidized by oxygen of nitrogen dioxide NO ₂ ). However, in the intermediate state, the NOx storage catalyst 11 is in the process of activation and the noble metal catalyst 46. Nitrogen dioxide NO ₂ can only be reduced up to nitrogen monoxide NO.

In this intermediate state, nitrogen dioxide NO _{2 in} the exhaust gas is reduced to nitrogen monoxide NO, so that the proportion of nitrogen dioxide NO ₂ in the exhaust gas decreases and the proportion of nitrogen monoxide NO increases. Become. In the intermediate state, nitrogen dioxide NO ₂ can be stored in the NOx absorbent 47 at a low temperature, but as described above, the ratio of nitrogen dioxide NO ₂ in the exhaust gas decreases and the ratio of nitrogen monoxide NO increases. It is considered that the NOx purification capacity in this intermediate state, for example, the NOx occlusion speed is lowered.

In more detail, each reaction in the NOx storage catalyst 11 described with reference to FIGS. 2A to 2C is changed according to the change in the active state of the NOx storage catalyst 11, that is, the temperature change. It happens while changing the ratio. In other words, the active state of the NOx storage catalyst 11 is determined according to the temperature TC of the NOx storage catalyst 11, and each of the reactions described above occurs at different rates according to the active state. That is, taking the case where the temperature TC of the NOx storage catalyst 11 gradually increases as an example, the NOx storage catalyst 11 described with reference to FIG. 2B is active because the NOx storage catalyst 11 is initially at a low temperature. In the intermediate state described with reference to FIG. 2 (C) when the temperature TC of the NOx storage catalyst 11 rises, the ratio of the reaction (ie, the low temperature storage of nitrogen dioxide NO ₂ ) is large. The ratio of the reaction (ie, the reduction reaction of nitrogen dioxide NO ₂ to nitric oxide NO) increases. When the temperature TC further increases, the reaction rate in the intermediate state decreases, and the reaction when the NOx storage catalyst 11 described with reference to FIG. 2A is activated (that is, the high temperature of the nitrogen oxide NOx). (Occlusion) ratio increases.

  Then, from the relationship between the temperature change of the NOx storage catalyst 11 and the active state of the NOx storage catalyst 11 (that is, each of the reactions according to the active state) as described above, the NOx storage capability Nc of the NOx storage catalyst 11 and the NOx storage are stored. It is considered that the relationship with the temperature TC of the catalyst 11 is as shown in FIG. Each of Tm, Ta, and Tk in the figure has a large proportion of each reaction of the reaction when the above-described NOx storage catalyst 11 is not activated, the reaction when it is in an intermediate state, and the reaction when it is activated. The example of temperature TC of the NOx storage catalyst 11 corresponding to an active state is represented.

Hereinafter, the case where the temperature TC of the NOx storage catalyst 11 gradually increases from the temperature Tm to the temperature Tk will be described with reference to FIG. When the temperature TC of the NOx storage catalyst 11 is the temperature Tm, the NOx storage catalyst 11 is not activated, so the NOx purification capacity Nc in this case is mostly based on the low temperature storage of nitrogen dioxide NO _2. It is. Therefore, as the temperature TC of the NOx storage catalyst 11 increases from the temperature Tm, the NOx purification capability Nc gradually decreases. That is, as the temperature TC of the NOx storage catalyst 11 rises from the temperature Tm, the reduction reaction from nitrogen dioxide NO ₂ to nitrogen monoxide NO in the intermediate state described above gradually increases, so nitrogen dioxide NO _{2 that} can be stored at a low temperature. Decrease. As a result, the NOx purification capacity Nc gradually decreases.

  On the other hand, when the temperature TC of the NOx occlusion catalyst 11 further increases and approaches the temperature Ta, the NOx occlusion catalyst gradually starts to be activated and high temperature occlusion of NOx becomes possible. When the temperature TC of the NOx storage catalyst 11 exceeds the temperature Ta, the degree of increase in the NOx purification capacity Nc based on the low temperature storage exceeds the degree of decrease in the NOx purification capacity Nc based on the high temperature storage, and as the temperature of the NOx storage catalyst 11 increases. The overall NOx purification capacity Nc increases. That is, when the temperature TC of the NOx storage catalyst 11 reaches the temperature Ta, the NOx purification capacity Nc takes a minimum value.

When the temperature TC of the NOx storage catalyst 11 exceeds the temperature Ta, as the temperature TC of the NOx storage catalyst 11 increases, the oxidation reaction of oxidizing nitric oxide NO to nitrogen dioxide NO ₂ is activated, and as a result, gradually. A large amount of NOx is occluded and the NOx purification capacity Nc increases. When the temperature TC of the NOx storage catalyst 11 reaches the temperature Tk, the NOx storage catalyst 11 is completely activated, and in this case, most of the NOx purification capacity Nc is based on the high temperature storage of the nitrogen oxide NOx. .

  By the way, ascertaining the relationship between the NOx purification capacity Nc of the NOx storage catalyst 11 and the temperature TC of the NOx storage catalyst 11 as shown in FIG. 3 is controlled by controlling the temperature TC of the NOx storage catalyst 11 in the exhaust gas. This is important when purifying NOx. That is, if the relationship between the NOx purification capacity Nc of the NOx storage catalyst 11 and the temperature TC of the NOx storage catalyst 11 is accurately grasped, the temperature TC of the NOx storage catalyst 11 is controlled to efficiently reduce NOx in the exhaust gas. And it becomes possible to remove sufficiently.

  On the other hand, as described above, the relationship between the NOx purification capacity Nc of the NOx storage catalyst 11 and the temperature TC of the NOx storage catalyst 11 is such that each reaction occurring in the NOx storage catalyst 11 when the NOx storage catalyst 11 is at each temperature TC (that is, Each reaction described with reference to FIGS. 2 (A) to 2 (C) is determined by the balance. Therefore, the relationship between the NOx purification capacity Nc of the NOx storage catalyst 11 and the temperature TC of the NOx storage catalyst 11 depends on factors affecting the above reactions in the NOx storage catalyst 11, that is, for example, the catalyst state of the NOx storage catalyst 11. to be influenced. Here, examples of the catalyst state include the degree of deterioration of the NOx storage catalyst 11 and the degree of sulfur poisoning.

That is, when the NOx occlusion catalyst 11 deteriorates, the noble metal catalyst 46 is sintered and its surface area is reduced. Therefore, the reduction reaction from nitrogen dioxide NO ₂ to nitrogen monoxide NO in the intermediate state described above is suppressed, and oxidation from nitrogen monoxide NO to nitrogen dioxide NO ₂ in a state where the NOx storage catalyst 11 is activated. The reaction is suppressed. As a result, when the NOx storage catalyst 11 is deteriorated, the relationship between the NOx purification capability Nc of the NOx storage catalyst 11 and the temperature TC of the NOx storage catalyst 11 is as shown in FIG.

  That is, in FIG. 4, the solid line represents the relationship between the NOx purification capacity Nc of the NOx storage catalyst 11 and the temperature TC of the NOx storage catalyst 11 when the NOx storage catalyst 11 has deteriorated, and the dotted line represents the NOx storage catalyst. The relationship between the NOx purification capability Nc of the NOx storage catalyst 11 and the temperature TC of the NOx storage catalyst 11 when the catalyst 11 is not deteriorated is shown. That is, when the NOx storage catalyst 11 is deteriorated, the NOx purification capability Nc on the low temperature side is increased and the NOx purification capability Nc on the high temperature side is decreased compared to the case where the NOx storage catalyst 11 is not deteriorated. As a result, the temperature Tb at which the NOx purification capacity Nc is minimized when the NOx storage catalyst 11 is deteriorated is higher than that when the NOx storage catalyst 11 is not deteriorated (temperature Ta). Such a tendency increases as the degree of deterioration of the NOx storage catalyst 11 increases.

Here, the NOx purification capacity Nc increases on the low temperature side because the reduction reaction from nitrogen dioxide NO ₂ to nitrogen monoxide NO in the intermediate state is suppressed, so that the NOx purification capacity based on low temperature storage is reduced. This is because the decrease is suppressed.

  The deterioration of the NOx storage catalyst 11 is caused by, for example, the higher the temperature of the NOx storage catalyst 11, the longer the time when the temperature is higher than the upper limit temperature determined by the catalyst type, or the oxygen concentration of the inflowing exhaust gas. It has been found that the higher the value, the faster the progression. Therefore, the degree of deterioration of the NOx storage catalyst 11 can be estimated using this fact. That is, for example, the degree of deterioration of the NOx storage catalyst 11 can be estimated based on the temperature history of the NOx storage catalyst 11 since the new NOx storage catalyst 11 is attached.

On the other hand, sulfur poisoning of the NOx storage catalyst 11 is stored in the NOx absorbent 47 by the same mechanism as the sulfur oxide SOx in the exhaust gas stores the nitrogen oxide NOx at a high temperature, and sulfate is contained in the NOx absorbent 47. For example, by producing barium sulfate BaSO ₄ . Since such a sulfate has a low basicity, when the NOx storage catalyst 11 is poisoned with sulfur, the base point on the NOx absorbent 47 is reduced, and low temperature storage of nitrogen dioxide NO ₂ is difficult to occur. Further, as described above, since sulfate is stored by storing sulfur oxide SOx by the same mechanism as storing nitrogen oxide NOx at a high temperature, NOx can be stored at a high temperature by the amount of sulfate in the NOx absorbent 47. The amount will decrease. Therefore, when the NOx storage catalyst 11 is poisoned with sulfur, high temperature storage of nitrogen oxides NOx hardly occurs. From the above, when the NOx storage catalyst 11 is poisoned with sulfur, the relationship between the NOx purification capability Nc of the NOx storage catalyst 11 and the temperature TC of the NOx storage catalyst 11 is as shown in FIG.

  That is, in FIG. 5, the solid line represents the relationship between the NOx purification capacity Nc of the NOx storage catalyst 11 and the temperature TC of the NOx storage catalyst 11 when the NOx storage catalyst 11 is poisoned with sulfur, and the dotted line represents The relationship between the NOx purification capability Nc of the NOx storage catalyst 11 and the temperature TC of the NOx storage catalyst 11 when the NOx storage catalyst 11 is not sulfur-poisoned is shown. That is, when the NOx storage catalyst 11 is sulfur-poisoned, the NOx purification capacity Nc is reduced in the entire temperature range as compared to the case where the NOx-occlusion catalyst 11 is not sulfur-poisoned. However, the effect is large on the low temperature side, and as a result, the temperature Ts at which the NOx purification capacity Nc is minimized when the NOx storage catalyst 11 is sulfur poisoned is the case where the NOx storage catalyst 11 is not sulfur poisoned. It is lower than (temperature Ta). Such a tendency increases as the degree of sulfur poisoning of the NOx storage catalyst 11 increases.

  The degree of sulfur poisoning of the NOx storage catalyst 11 can be estimated from, for example, the fuel consumption after the previous sulfur poisoning elimination control. That is, the sulfur poisoning of the NOx storage catalyst 11 is caused by raising the temperature of the NOx storage catalyst 11 to about 600 ° C. and making the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst 11 the stoichiometric air-fuel ratio or rich. Has been found to be released from the NOx absorbent 47 and eliminated. Therefore, in the exhaust gas purification apparatus provided with the NOx storage catalyst as described above, the NOx storage catalyst 11 is usually heated to about 600 ° C. and the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst 11 is made the stoichiometric air-fuel ratio or rich. Sulfur poisoning elimination control is regularly implemented. If such sulfur poisoning elimination control is carried out, sulfur poisoning is basically eliminated. Therefore, if the sulfur content of the fuel is obtained in advance, the fuel consumption after the sulfur poisoning elimination control is implemented. The current degree of sulfur poisoning can be estimated. Further, the sulfur content of the oil may be obtained, and the degree of sulfur poisoning may be obtained in consideration of the oil consumption.

  Alternatively, a NOx sensor that detects the NOx concentration in the exhaust gas may be provided on the downstream side of the NOx storage catalyst 11, and the degree of sulfur poisoning may be estimated based on the output value of the NOx sensor. That is, in this case, for example, a reference output when sulfur poisoning is not performed is obtained in advance, and the degree of sulfur poisoning is estimated by comparing the reference output with the actual NOx sensor output value. .

  As described above, the NOx purification ability Nc of the NOx storage catalyst 11 is not limited to the temperature TC (that is, the active state) of the NOx storage catalyst 11, but the catalyst state such as the degree of deterioration of the NOx storage catalyst 11 and the degree of sulfur poisoning. Also affected. Therefore, in the embodiment of the present invention, the exhaust gas is purified in consideration of the catalyst state, that is, in consideration of the influence of the catalyst state on the NOx purification ability Nc as described above.

  That is, for example, in one embodiment of the present invention, if the temperature TC of the NOx storage catalyst 11 is less than the predetermined target temperature To after a predetermined time ts has elapsed since the start of the engine, the NOx storage catalyst 11 is activated for the purpose of activating the NOx storage catalyst 11. In the exhaust gas purification apparatus that performs the temperature rise control for raising the temperature TC of the catalyst 11 to the target temperature To, the higher the estimated deterioration degree of the NOx storage catalyst 11, the higher the temperature of the NOx storage catalyst 11 from the start. The time until TC reaches the target temperature To is increased. More specifically, for example, the predetermined time ts is lengthened, or the rate of temperature rise is slowed (temperature rise per unit time is made small). Further, for example, in the temperature increase control, when the target temperature is set stepwise until the final target temperature To and the time ti for maintaining each target temperature is determined, the rate of temperature increase is determined. In other words, the higher the degree of deterioration of the NOx storage catalyst 11, the longer the maintenance time ti at each target temperature.

  In this way, when the NOx storage catalyst 11 is deteriorated, the temperature rise of the NOx storage catalyst 11 is delayed, and the state where the temperature TC of the NOx storage catalyst 11 is low is lengthened. As described above, when the NOx storage catalyst 11 is deteriorated, the NOx purification ability Nc on the low temperature side increases, so by increasing the low temperature TC state of the NOx storage catalyst 11 as described above, More efficient removal of NOx becomes possible.

  It should be noted that how long the time from the start until the temperature TC of the NOx storage catalyst 11 reaches the target temperature To is increased (more specifically, for example, how long the predetermined time ts is increased, or the rate of temperature increase) For example, how much late is to be made), a map is prepared in advance, and the map is obtained based on the map. That is, the relationship between the degree of deterioration of the NOx storage catalyst 11 and the increase in the NOx purification capacity Nc on the low temperature side is obtained in advance, and based on this, control parameters necessary for the degree of deterioration of the NOx storage catalyst 11 (for example, the above-mentioned) A map for which an appropriate value for a predetermined time ts or the like is obtained is created and obtained based on the map. In this map, a plurality of values may be defined corresponding to a plurality of degrees of deterioration, or only two values, that is, a case where deterioration is caused and a case where deterioration is not caused may be prescribed. .

  In another embodiment of the present invention, if the temperature TC of the NOx storage catalyst 11 is lower than a predetermined target temperature To after a predetermined time ts has elapsed from the start of the engine, the NOx storage catalyst 11 is activated for the purpose of activating the NOx storage catalyst 11. In the exhaust gas purification apparatus that performs the temperature raising control for raising the temperature TC of the catalyst 11 to the target temperature To, the higher the estimated degree of sulfur poisoning of the NOx storage catalyst 11, the more the NOx storage catalyst from the start. The time until the temperature TC 11 becomes the target temperature To is shortened. More specifically, for example, the predetermined time ts is shortened, or the rate of temperature increase is increased (the temperature increase per unit time is increased). Further, for example, in the temperature increase control, when the target temperature is set stepwise until the final target temperature To and the time ti for maintaining each target temperature is determined, the rate of temperature increase is determined. First, the higher the degree of sulfur poisoning of the NOx storage catalyst 11, the shorter the maintenance time ti at each target temperature.

  In this way, when the NOx storage catalyst 11 is poisoned with sulfur, the temperature rise of the NOx storage catalyst 11 is accelerated, and the state where the temperature TC of the NOx storage catalyst 11 is low is shortened. As described above, when the NOx storage catalyst 11 is poisoned with sulfur, the NOx purification ability Nc on the low temperature side is particularly lowered, so that the low temperature TC state of the NOx storage catalyst 11 is shortened as described above. Thus, it is possible to achieve sufficient removal of NOx.

  It should be noted that how much time is required from the start until the temperature TC of the NOx storage catalyst 11 reaches the target temperature To (more specifically, how much the predetermined time ts is reduced, for example, or the rate of temperature increase) As to the degree of deterioration, the degree of sulfur poisoning of the NOx storage catalyst 11 and the appropriate control parameters (for example, the predetermined time ts) are appropriately set in advance. A map in which values are associated with each other is created and obtained based on the map.

  Further, the temperature increase control here can be performed by controlling the fuel injection pattern as follows, for example. That is, FIG. 6 is a schematic diagram showing four examples of fuel injection patterns that can be implemented in the internal combustion engine shown in FIG. 1, but the main fuel qm is normally indicated by (I) in FIG. Is injected near the compression top dead center. On the other hand, when the temperature raising control is started, a fuel injection pattern as shown in FIG. That is, the injection timing of the main fuel qm is retarded until after the compression top dead center. Thus, if the injection timing of the main fuel qm is retarded until after the compression top dead center, the afterburning period becomes longer, and thus the exhaust gas temperature rises. As the exhaust gas temperature increases, the temperature TC of the NOx storage catalyst 11 increases accordingly.

  Further, in order to increase the temperature TC of the NOx storage catalyst 11, in addition to the main fuel qm as shown in FIG. 6 (III), the auxiliary fuel qv can be injected near the intake top dead center. When the auxiliary fuel qv is additionally injected in this manner, the amount of fuel combusted by the amount corresponding to the auxiliary fuel qv increases, so that the exhaust gas temperature rises, and thus the temperature TC of the NOx storage catalyst 11 rises.

  On the other hand, when the auxiliary fuel qv is injected in the vicinity of the intake top dead center, intermediate products such as aldehyde, ketone, peroxide, and carbon monoxide are generated from the auxiliary fuel qv by the compression heat during the compression stroke. The product accelerates the reaction of the main fuel qm. Therefore, in this case, as shown in FIG. 6 (III), even if the injection timing of the main fuel qm is greatly delayed, good combustion can be obtained without causing misfire. That is, since the injection timing of the main fuel qm can be greatly delayed in this way, the exhaust gas temperature becomes considerably high, and thus the temperature TC of the NOx storage catalyst 11 can be quickly raised.

  Further, in order to increase the temperature TC of the NOx storage catalyst 11, in addition to the main fuel qm as shown in FIG. 6 (IV), the auxiliary fuel qp can be injected during the expansion stroke or the exhaust stroke. That is, in this case, most of the auxiliary fuel qp is discharged into the exhaust passage in the form of unburned HC without burning. The unburned HC is oxidized on the NOx storage catalyst 11 by excess oxygen, and the temperature TC of the NOx storage catalyst 11 is raised by the oxidation reaction heat generated at this time.

  In the fuel injection pattern as described above, the rate of temperature increase of the NOx storage catalyst 11 can be controlled by adjusting the retard amount of the main injection qm, the injection timing and the injection amount of the auxiliary fuels qv and qp, and the like. it can.

  In still another embodiment of the present invention, for example, after the cold start of the engine, the NOx occlusion catalyst 11 is activated for the purpose of activating the NOx occlusion catalyst 11 when the temperature TC of the NOx occlusion catalyst 11 rises to a predetermined temperature Td. In the exhaust gas purification apparatus that performs the temperature raising control for raising the temperature TC of the catalyst 11 to a predetermined target temperature To, the higher the estimated degree of deterioration of the NOx storage catalyst 11, the higher the predetermined temperature Td. It is set up.

  In this way, when the NOx storage catalyst 11 is deteriorated, the NOx storage catalyst 11 is maintained for a long time at a low temperature TC. As described above, when the NOx storage catalyst 11 is deteriorated, the NOx purification ability Nc on the low temperature side increases, so that the NOx storage catalyst 11 is maintained at a low temperature TC for a long time as described above. Thus, more efficient removal of NOx becomes possible.

Further, in this case, the start temperature (that is, the predetermined temperature Td) and the end temperature (that is, the target temperature To) of the temperature increase control are set in consideration of the influence of the deterioration degree of the NOx storage catalyst 11 on the NOx purification ability Nc. By doing so, the time when the NOx purification capacity Nc is insufficient can be efficiently shortened. This will be described with reference to FIG. That is, in FIG. 7, the solid line represents the relationship between the NOx purification capacity Nc of the NOx storage catalyst 11 and the temperature TC of the NOx storage catalyst 11 when the NOx storage catalyst 11 has deteriorated, and the dotted line represents the NOx storage catalyst 11. The relationship between the NOx purification capacity Nc of the NOx storage catalyst 11 and the temperature TC of the NOx storage catalyst 11 when NO is not deteriorated represents the NOx purification capacity that requires Nr. In the example shown in FIG. 7, when the NOx storage catalyst 11 is not deteriorated, the temperature increase control is started when the temperature TC of the NOx storage catalyst 11 reaches the temperature T ₁ and reaches the temperature T ₂ . Sometimes it ends. When the NOx storage catalyst 11 is deteriorated, the temperature rise control is started when the temperature TC of the NOx storing catalyst 11 becomes the temperature T _3, is caused to exit when it becomes the temperature T _4.

In this way, the temperature rise control start temperature (ie, the predetermined temperature Td) and end temperature (ie, the target temperature To) are set in consideration of the influence of the deterioration degree of the NOx storage catalyst 11 on the NOx purification capacity Nc. Thus, the temperature rise control is performed over an appropriate temperature range according to the degree of deterioration, and the time during which the NOx purification capability Nc is equal to or less than the required NOx purification capability Nr can be efficiently shortened. That is, for example, even when the NOx storage catalyst 11 is deteriorated, the temperature increase control is started when the temperature TC of the NOx storage catalyst 11 reaches the temperature T ₁ and is ended when the temperature T ₂ reaches the temperature T ₂ . temperature TC of the NOx storing catalyst 11 will continue to be a state where the NOx purifying ability Nc is insufficient to increase the temperature T ₂ to the temperature T ₄ by the normal operation of the engine.

  In order to set the start temperature and the end temperature of the temperature increase control according to the deterioration degree of the NOx storage catalyst 11 as described above, the deterioration degree of the NOx storage catalyst 11 and an appropriate temperature increase control corresponding thereto are performed. The relationship between the start temperature and the end temperature may be obtained in advance and mapped.

  Furthermore, in another embodiment of the present invention, for example, for the purpose of activating the NOx storage catalyst 11 when the temperature TC of the NOx storage catalyst 11 rises to a predetermined temperature Td after a cold start of the engine or the like. In the exhaust gas purification apparatus in which the temperature raising control for raising the temperature TC of the storage catalyst 11 to the predetermined target temperature To is performed, the higher the estimated degree of sulfur poisoning of the NOx storage catalyst 11, the higher the predetermined value. The temperature Td is set low.

  In this way, when the NOx storage catalyst 11 is poisoned with sulfur, the temperature increase control is started when the temperature TC of the NOx storage catalyst 11 is lower, and as a result, the temperature TC of the NOx storage catalyst 11 is increased. The low state is shortened. As described above, when the NOx storage catalyst 11 is poisoned with sulfur, the NOx purification ability Nc on the low temperature side is particularly reduced. Therefore, the temperature increase control is performed by the temperature TC of the NOx storage catalyst 11 as described above. By starting from a low time, it is possible to realize sufficient removal of NOx.

In this case also, the temperature rise control start temperature (that is, the predetermined temperature Td) and end temperature (that is, the target temperature To) are considered in consideration of the influence of the sulfur poisoning degree of the NOx storage catalyst 11 on the NOx purification capability Nc. ) Can be set to efficiently reduce the time for which the NOx purification capacity Nc is insufficient. This will be described with reference to FIG. That is, in FIG. 8, the solid line represents the relationship between the NOx purification capacity Nc of the NOx storage catalyst 11 and the temperature TC of the NOx storage catalyst 11 when the NOx storage catalyst 11 is poisoned with sulfur, and the dotted line represents the NOx storage catalyst. The relationship between the NOx purification capacity Nc of the NOx storage catalyst 11 and the temperature TC of the NOx storage catalyst 11 when the catalyst 11 is not poisoned with sulfur is shown, and the NOx purification capacity that requires Nr is shown. In the example shown in FIG. 8, when the NOx storage catalyst 11 is not poisoned with sulfur, the temperature increase control is started when the temperature TC of the NOx storage catalyst 11 reaches the temperature T _1, and reaches the temperature T ₂ . It is finished when it becomes. When the NOx storage catalyst 11 is sulfur poisoning, the temperature increase control is always performed when the temperature TC of the NOx storing catalyst 11 is the temperature T ₅ less, allowed ends when it becomes the temperature T ₅ It is done.

As described above, the start temperature (that is, the predetermined temperature Td) and the end temperature (that is, the target temperature To) of the temperature increase control are set in consideration of the influence of the degree of sulfur poisoning of the NOx storage catalyst 11 on the NOx purification ability Nc. As a result, the temperature increase control is performed over an appropriate temperature range corresponding to the degree of sulfur poisoning, and the time during which the NOx purification capability Nc is equal to or less than the required NOx purification capability Nr can be efficiently shortened. it can. That is, for example, when the NOx storage catalyst 11 is poisoned with sulfur, the temperature increase control is started when the temperature TC of the NOx storage catalyst 11 reaches the temperature T ₁ and is ended when the temperature TC reaches the temperature T _2. then, the state between the between and temperature T ₂ to the temperature TC of the NOx storage catalyst 11 is raised to the temperature T ₁ of the normal operation of the engine until the rise in the temperature T ₅ is the NOx purifying ability Nc is insufficient It will continue.

  In order to set the start temperature and the end temperature of the temperature increase control according to the sulfur poisoning degree of the NOx storage catalyst 11 as described above, the sulfur poisoning of the NOx storage catalyst 11 is the same as in the case of the deterioration degree. The relationship between the degree and the start temperature and end temperature of appropriate temperature increase control corresponding to the degree is preferably obtained in advance and mapped.

  In yet another embodiment of the present invention, low temperature maintenance control for keeping the temperature TC of the NOx occlusion catalyst 11 low according to the estimated degree of deterioration of the NOx occlusion catalyst 11 after engine startup or the like is performed. To be implemented. That is, the low temperature maintenance control is performed when the deterioration degree of the NOx storage catalyst 11 is high (for example, when the deterioration degree is higher than a predetermined reference deterioration degree). In this case, the higher the degree of deterioration, the longer the time for low temperature maintenance control, or the lower the temperature to be maintained. At this time, the specific value of the control parameter (for example, the time for maintaining the low temperature) is stored in advance based on the relationship between the degree of deterioration of the NOx storage catalyst 11 and the increase in the NOx purification capacity Nc on the low temperature side. A map in which the degree of deterioration of the catalyst 11 is associated with an appropriate value of a necessary control parameter can be created and obtained based on the map. Note that the low temperature maintenance control here includes, for example, at least one of advancing the fuel injection timing, reducing the EGR amount, and introducing air upstream of the NOx storage catalyst 11. Can be realized.

  According to such an embodiment, when the NOx storage catalyst 11 is deteriorated, the NOx storage catalyst 11 is maintained at a low temperature TC for a long time. As described above, when the NOx storage catalyst 11 is deteriorated, the NOx purification ability Nc on the low temperature side increases, so that the NOx storage catalyst 11 is maintained at a low temperature TC for a long time as described above. Thus, more efficient removal of NOx becomes possible.

Furthermore, in another embodiment of the present invention, when the temperature TC of the NOx storage catalyst 11 is less than the predetermined temperature Tf, nitrogen dioxide with respect to nitrogen monoxide NO generated when combustion is performed under a lean air-fuel ratio. In the exhaust gas purification apparatus in which the ratio of NO ₂ is increased compared to the same engine operating state when the temperature TC of the NOx storage catalyst 11 is equal to or higher than the predetermined temperature Tf, that is, the same rotational speed and the same torque. The predetermined temperature Tf is set higher as the estimated degree of deterioration of the NOx storage catalyst 11 is higher.

As described above, nitrogen dioxide NO ₂ contained in the exhaust gas is stored in the NOx absorbent 47 at a low temperature even if the NOx storage catalyst 11 is not activated, but nitrogen monoxide NO contained in the exhaust gas is stored in the NOx storage catalyst. The NOx absorbent 47 is not occluded at high temperature unless 11 is activated and oxidized to nitrogen dioxide NO ₂ . Therefore, when the NOx storage catalyst 11 is not activated or has a low activity, the NOx storage catalyst 11 has a low temperature, the amount of nitrogen monoxide NO in the exhaust gas is reduced, and the amount of nitrogen dioxide NO ₂ in the exhaust gas is reduced. It is preferable to increase. Therefore, for example, the predetermined temperature Tf is set to a temperature at which the amount of nitrogen dioxide NO ₂ stored in the NOx absorbent 47 at a low temperature per unit time is equal to the amount of nitrogen oxide NOx stored at a high temperature per unit time. In this case, the amount of nitrogen dioxide NO ₂ in the exhaust gas is increased in a temperature range where low-temperature storage of nitrogen dioxide NO ₂ can be expected, and the NOx purification rate can be improved.

  As described above, the NOx purification capacity Nc based on the low-temperature storage increases as a result when the NOx storage catalyst 11 is deteriorated. Therefore, the higher the degree of deterioration of the NOx storage catalyst 11 as described above, the higher the predetermined temperature. By increasing Tf and expanding the temperature range expected for low-temperature storage, more efficient removal of NOx becomes possible.

In another embodiment of the present invention, when the temperature TC of the NOx storage catalyst 11 is lower than the predetermined temperature Tf, nitrogen dioxide with respect to nitrogen monoxide NO generated when combustion is performed under a lean air-fuel ratio. In the exhaust gas purification apparatus in which the ratio of NO ₂ is increased compared to the same engine operating state when the temperature TC of the NOx storage catalyst 11 is equal to or higher than the predetermined temperature Tf, that is, the same rotational speed and the same torque. The predetermined temperature Tf is set lower as the estimated degree of sulfur poisoning of the NOx storage catalyst 11 is higher.

  As described above, the NOx purification capacity Nc based on low-temperature storage is greatly affected when the NOx storage catalyst 11 is poisoned with sulfur. That is, when the NOx storage catalyst 11 is poisoned with sulfur, the NOx purification ability Nc based on the low temperature storage becomes extremely low. Therefore, the higher the degree of sulfur poisoning of the NOx storage catalyst 11 as described above, the lower the predetermined temperature Tf and the lower the temperature range expected for low temperature storage, thereby realizing sufficient removal of NOx. be able to.

It has been found that the ratio of NO ₂ (= NO ₂ amount / NO amount) increases when slow combustion is performed. For example, the fuel injection timing is retarded or the EGR gas amount is increased. When at least one of the above is increased, pilot injection is performed, or premixed gas combustion is performed, the combustion becomes slow. Therefore, in the embodiment according to the present invention, in order to increase the amount of nitrogen dioxide NO ₂ in the exhaust gas as described above, at least the method of slowing down the combustion when the amount of nitrogen dioxide NO ₂ needs to be increased is required. Either one is implemented to cause slow combustion.

  In the above description, the temperature raising control is performed by controlling the fuel injection pattern. However, the present invention is not limited to this, and the NOx occlusion catalyst is used by other means such as using an electric heater. 11 may be heated.

  Moreover, in the above description, in order to facilitate understanding, the control performed according to the deterioration degree of the NOx storage catalyst 11 and the control performed according to the sulfur poisoning degree of the NOx storage catalyst 11 are separately described. The control to be performed may be determined in consideration of both of these degrees. In this case, for example, if the NOx storage catalyst 11 is deteriorated and sulfur-poisoned, the NOx purification capacity Nc on the low temperature side increases by the amount corresponding to the degree of deterioration and decreases by the amount corresponding to the degree of sulfur poisoning. The control to be implemented is determined. Therefore, taking as an example the predetermined time ts until the temperature raising control is started after the cold start of the engine described above, it is increased by an amount corresponding to the degree of deterioration and then reduced by an amount corresponding to the degree of sulfur poisoning. Thus, a final predetermined time ts used for actual control is determined.

FIG. 1 shows a case where the present invention is applied to a compression ignition type internal combustion engine. FIG. 2 is a diagram schematically showing a cross-section of the carrier surface portion of the NOx storage catalyst. FIG. 3 is a diagram showing the relationship between the NOx purification capacity Nc of the NOx storage catalyst and the temperature TC of the NOx storage catalyst. FIG. 4 is a view similar to FIG. 3 and shows a comparison between a case where the NOx storage catalyst is deteriorated and a case where the NOx storage catalyst is not deteriorated. FIG. 5 is a view similar to FIG. 3 and shows a comparison between a case where the NOx storage catalyst is sulfur-poisoned and a case where it is not sulfur-poisoned. FIG. 6 is a diagram showing various fuel injection patterns. FIG. 7 is a diagram for explaining a method for determining the start temperature and the end temperature of the temperature increase control of the NOx storage catalyst in an embodiment of the present invention. FIG. 8 is a diagram for explaining a method for determining the start temperature and the end temperature of the temperature rise control of the NOx storage catalyst in another embodiment of the present invention.

Explanation of symbols

3 ... Fuel injection valve 4 ... Intake manifold 5 ... Exhaust manifold 7 ... Exhaust turbocharger 11 ... NOx storage catalyst 13 ... Reductant supply valve

Claims (8)

A NOx occlusion catalyst comprising a noble metal catalyst and a NOx absorbent disposed in the engine exhaust passage, and when the air-fuel ratio of the inflowing exhaust gas is lean, it is included in the exhaust gas when it is not activated NOx storage catalyst that stores nitrogen dioxide NO ₂ in the NOx absorbent at a low temperature and, when activated, stores nitrogen oxide NOx contained in the exhaust gas in the NOx absorbent at a high temperature;
Means for estimating the NOx purification capacity of the NOx storage catalyst based on the active state of the NOx storage catalyst,
The above NOx purification ability estimation means comprise means for estimating the reduction degree of the oxidation performance of the NOx storage catalyst, as degree of reduction in oxidation performance estimated by the oxidation degradation degree estimating means is high, the NOx storage catalyst An exhaust gas purification apparatus that largely estimates the NOx purification capacity based on the low temperature storage. A NOx occlusion catalyst comprising a noble metal catalyst and a NOx absorbent disposed in the engine exhaust passage, and when the air-fuel ratio of the inflowing exhaust gas is lean, it is included in the exhaust gas when it is not activated NOx storage catalyst that stores nitrogen dioxide NO ₂ in the NOx absorbent at a low temperature and, when activated, stores nitrogen oxide NOx contained in the exhaust gas in the NOx absorbent at a high temperature;
Means for estimating the NOx purification capacity of the NOx storage catalyst based on the active state of the NOx storage catalyst,
The NOx purification capacity estimation means includes means for estimating the degree of sulfur poisoning of the NOx storage catalyst, and the higher the degree of sulfur poisoning estimated by the sulfur poisoning degree estimation means, the higher the NOx storage catalyst. An exhaust gas purification apparatus in which the NOx purification capacity based on the low-temperature storage is estimated to be small.   The NOx purification capacity estimating means further includes means for estimating the degree of sulfur poisoning of the NOx storage catalyst, and the higher the estimated degree of sulfur poisoning, the more the NOx purification based on the low temperature storage of the NOx storage catalyst. The exhaust emission control device according to claim 1, wherein the capacity is estimated to be small. A NOx occlusion catalyst comprising a noble metal catalyst and a NOx absorbent disposed in the engine exhaust passage, and when the air-fuel ratio of the inflowing exhaust gas is lean, it is included in the exhaust gas when it is not activated A NOx storage catalyst that stores nitrogen dioxide NO ₂ in the NOx absorbent at a low temperature and, when activated, stores nitrogen oxide NOx contained in the exhaust gas in the NOx absorbent at a high temperature, is provided after the engine is started. In the exhaust gas purification apparatus for an internal combustion engine in which the temperature raising control for raising the temperature of the storage catalyst to a predetermined target temperature or higher is performed
Further comprise means for estimating the reduction degree of the oxidation performance of the NOx storage catalyst, as degree of reduction in oxidation performance estimated by the oxidation degradation degree estimating means is high, so that the starting time of the temperature increase control is delayed Or an exhaust purification device in which the time from the start of the engine until the temperature of the NOx storage catalyst reaches the target temperature is lengthened. A NOx occlusion catalyst comprising a noble metal catalyst and a NOx absorbent disposed in the engine exhaust passage, and when the air-fuel ratio of the inflowing exhaust gas is lean, it is included in the exhaust gas when it is not activated A NOx storage catalyst that stores nitrogen dioxide NO ₂ in the NOx absorbent at a low temperature and, when activated, stores nitrogen oxide NOx contained in the exhaust gas in the NOx absorbent at a high temperature, is provided after the engine is started. In the exhaust gas purification apparatus for an internal combustion engine in which the temperature raising control for raising the temperature of the storage catalyst to a predetermined target temperature or higher is performed
Furthermore, a means for estimating the degree of sulfur poisoning of the NOx storage catalyst is provided, so that the higher the degree of sulfur poisoning estimated by the sulfur poisoning degree estimating means, the earlier the start timing of the temperature increase control. Or an exhaust purification device in which the time from the start of the engine until the temperature of the NOx storage catalyst reaches the target temperature is shortened. A NOx occlusion catalyst comprising a noble metal catalyst and a NOx absorbent disposed in the engine exhaust passage, and when the air-fuel ratio of the inflowing exhaust gas is lean, it is included in the exhaust gas when it is not activated Nitrogen dioxide NO ₂ is stored in the NOx absorbent at a low temperature, and when activated, the NOx storage catalyst that stores NOx absorbent in the exhaust gas at a high temperature is stored in the NOx absorbent, and the oxidation performance of the NOx storage catalyst And a means for estimating the degree of decrease, and the temperature of the NOx occlusion catalyst is kept low if the degree of decrease in oxidation performance estimated by the degree of decrease in oxidation performance estimated after starting the engine is equal to or greater than a predetermined degree of decrease. In an exhaust gas purification apparatus for an internal combustion engine in which low temperature maintenance control is performed for
The exhaust gas purification apparatus in which the lower the oxidation performance estimated by the oxidation performance degradation degree estimation means, the longer the duration of the low temperature maintenance control, or the lower the temperature maintained in the low temperature maintenance control. A NOx occlusion catalyst comprising a noble metal catalyst and a NOx absorbent disposed in the engine exhaust passage, and when the air-fuel ratio of the inflowing exhaust gas is lean, it is included in the exhaust gas when it is not activated nitrogen dioxide NO ₂ is cold stored in the NOx absorbent, when being activated comprising the NOx storage catalyst to a high temperature storage of nitrogen oxides NOx in the exhaust gas in the NOx absorbent, the temperature of the NOx storage catalyst Is lower than the predetermined temperature, the ratio of nitrogen dioxide NO _{2 to} nitrogen monoxide NO generated when combustion is performed at a lean air-fuel ratio is determined when the temperature of the NOx storage catalyst is equal to or higher than the predetermined temperature. In the exhaust emission control device that is increased compared to the case of the same engine operating state,
Further comprise means for estimating the reduction degree of the oxidation performance of the NOx storage catalyst, the higher the degree of reduction in oxidation performance estimated by the oxidation degradation degree estimating means, so that the predetermined temperature is set high Exhaust gas purification device. A NOx occlusion catalyst comprising a noble metal catalyst and a NOx absorbent disposed in the engine exhaust passage, and when the air-fuel ratio of the inflowing exhaust gas is lean, it is included in the exhaust gas when it is not activated nitrogen dioxide NO ₂ is cold stored in the NOx absorbent, when being activated comprising the NOx storage catalyst to a high temperature storage of nitrogen oxides NOx in the exhaust gas in the NOx absorbent, the temperature of the NOx storage catalyst Is lower than the predetermined temperature, the ratio of nitrogen dioxide NO _{2 to} nitrogen monoxide NO generated when combustion is performed at a lean air-fuel ratio is determined when the temperature of the NOx storage catalyst is equal to or higher than the predetermined temperature. In the exhaust emission control device that is increased compared to the case of the same engine operating state,
Further, a means for estimating the degree of sulfur poisoning of the NOx storage catalyst is provided, and the predetermined temperature is set lower as the degree of sulfur poisoning estimated by the sulfur poisoning degree estimating means is higher. Exhaust gas purification device.

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