JP6686665B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust gas purification device for an internal combustion engine.

  2. Description of the Related Art There is known an exhaust gas purification device for an internal combustion engine in which a particulate filter for collecting particulate matter contained in exhaust gas is arranged in an engine exhaust passage. However, as the amount of the particulate matter deposited on the particulate filter increases, the pressure loss of the particulate filter increases. If the pressure loss of the particulate filter increases, the engine output may decrease. Therefore, in this exhaust emission control device, PM removal control is performed to raise and maintain the temperature of the particulate filter when the amount of particulate matter trapped in the particulate filter increases, thereby oxidizing and removing the particulate matter. I am trying.

  By the way, the exhaust gas also contains a non-combustible component called ash, and the ash is collected by the particulate filter together with the particulate matter. However, the ash does not burn or vaporize even if the PM removal control is performed. That is, the ash is not removed from the particulate filter but remains on the particulate filter. Therefore, even if the PM removal control is performed, the pressure loss of the particulate filter cannot be sufficiently recovered.

  Therefore, there is known an exhaust gas purification device for an internal combustion engine in which the ash collected in the particulate filter is atomized, whereby the ash passes through the particulate filter and is removed from the particulate filter (for example, patent Reference 1). In this exhaust emission control device, a solid acid is carried on the particulate filter, and the acid strength of this solid acid is higher than that of sulfurous acid and lower than that of sulfuric acid. Further, in order to atomize the ash, the oxygen concentration in the exhaust gas flowing into the particulate filter is temporarily lowered, and the temperature of the particulate filter is temporarily raised. If the ash passes through the particulate filter and is removed from the particulate filter, an increase in pressure loss of the particulate filter due to ash can be suppressed.

Japanese Patent Publication No. 2014-520226

  However, the solid acid is indispensable in the exhaust gas purification device of Patent Document 1. Therefore, there is a demand for a technique of suppressing an increase in pressure loss of the particulate filter due to ash with an inexpensive and simple structure.

Further, as will be described in detail later, it may be more difficult to release the ash in the form of calcium sulfate CaSO ₄ from the particulates. Therefore, an object of the present invention is to suppress the generation of ash in the form of calcium sulfate CaSO ₄ , while suppressing the increase in pressure loss of the particulate filter due to ash with an inexpensive and simple structure. To provide a purification device.

  According to the present invention, a particulate filter that is arranged in the engine exhaust passage and that collects particulate matter in the exhaust gas, and is arranged in the engine exhaust passage upstream of the particulate filter or in the particulate filter. Of the SOx trapping catalyst, which traps SOx in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean, the temperature of the SOx trapping catalyst is the SOx release temperature, and the air-fuel ratio of the inflowing exhaust gas is the theoretical air-fuel ratio. A SOx trapping catalyst that releases SOx trapped when the fuel ratio or rich, and raises the temperature of the particulate filter to a predetermined ash desorption temperature in order to reduce the amount of adhered ash of the particulate filter. Ash desorption means configured to perform ash desorption control for maintaining the ash desorption temperature. The ash desorption temperature is a temperature suitable for converting the ash to calcium oxide, and the ash desorption means and the temperature of the SOx trap catalyst for reducing the SOx trap amount of the SOx trap catalyst are set to SOx release configured to perform SOx release control for temporarily changing the air-fuel ratio of the exhaust gas flowing into the SOx trap catalyst to rich while raising the SOx release temperature to a predetermined value and maintaining the SOx release temperature Means and a control means for controlling the ash desorption means and the SOx release means so that the ash desorption control is performed first and then the SOx release control is performed when the SOx release control is to be performed. Further, an exhaust gas purification device for an internal combustion engine is provided.

  While suppressing the generation of ash in the form of calcium sulfate, it is possible to suppress an increase in pressure loss of the particulate filter due to ash with an inexpensive and simple structure.

1 is an overall view of an internal combustion engine. FIG. 3 is a cross-sectional view of a surface portion of a catalyst carrier of a NOx storage reduction catalyst. FIG. 3 is a cross-sectional view of a surface portion of a catalyst carrier of a NOx storage reduction catalyst. It is a schematic diagram which shows the state of the ash in a particulate filter. It is a schematic diagram which shows the state of the ash in a particulate filter. It is a time chart which shows the execution timing of PM removal control, ash desorption control, and SOx release control. It is a flow chart which shows a calculation routine of SOx trap quantity QS. It is a flow chart which shows a SOx release control routine.

  Referring to FIG. 1, 1 is a main body of a compression ignition type internal combustion engine, 2 is a combustion chamber of each cylinder, 3 is an electronically controlled fuel injection valve for injecting fuel into each combustion chamber 2, and 4 is an intake manifold. Reference numerals 5 and 5 respectively denote exhaust manifolds. The intake manifold 4 is connected to an outlet of a compressor 7c of an exhaust turbocharger 7 via an intake duct 6, and the inlet of the compressor 7c is connected to an air flow meter 9 and an air cleaner 10 via an intake introduction pipe 8 in order. An electrically controlled throttle valve 11 is arranged in the intake duct 6, and a cooling device 12 for cooling intake air flowing in the intake duct 6 is further arranged in the intake duct 6. On the other hand, the exhaust manifold 5 is connected to the inlet of the exhaust turbine 7t of the exhaust turbocharger 7, and the outlet of the exhaust turbine 7t is connected to the exhaust aftertreatment device 20.

  Each fuel injection valve 3 is connected to a common rail 14 via a fuel supply pipe 13, and this common rail 14 is connected to a fuel tank 16 via an electrically controlled variable discharge fuel pump 15. The fuel in the fuel tank 16 is supplied to the common rail 14 by the fuel pump 15, and the fuel supplied to the common rail 14 is supplied to the fuel injection valve 3 via each fuel supply pipe 13. A fuel pressure sensor (not shown) that detects the fuel pressure in the common rail 14 is attached to the common rail 14 so that the fuel pressure in the common rail 14 matches the target pressure based on the signal from the fuel pressure sensor. The fuel discharge amount of the fuel pump 15 is controlled. In the embodiment shown in FIG. 1, this fuel is composed of light oil. In another embodiment, not shown, the internal combustion engine comprises a spark ignition internal combustion engine. In this case, the fuel consists of gasoline.

  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 17, and an electric control type EGR control valve 18 is arranged in the EGR passage 17. A cooling device 19 for cooling the EGR gas flowing in the EGR passage 17 is arranged around the EGR passage 17.

  The exhaust aftertreatment device 20 includes an exhaust pipe 21 connected to an outlet of the exhaust turbine 7t, and the exhaust pipe 21 is connected to an inlet of a catalyst 22. The outlet of the catalyst 22 is connected to the inlet of a wall flow type particulate filter 24 via an exhaust pipe 23. The outlet of the particulate filter 24 is connected to the exhaust pipe 25. In another embodiment, not shown, the catalyst 22 is located within the particulate filter 24.

  The electronic control unit (ECU) or controller 30 is composed of a digital computer, and ROM (read only memory) 32, RAM (random access memory) 33, CPU (microprocessor) 34, input connected to each other by a bidirectional bus 31 It has a port 35 and an output port 36. A temperature sensor 26 for detecting the temperature of the exhaust gas flowing into the particulate filter 24 is attached to the exhaust pipe 23. The temperature of the exhaust gas flowing into the particulate filter 24 represents the temperature of the particulate filter 24. The output voltages of the air flow meter 9 and the temperature sensor 26 are input to the input port 35 via the corresponding AD converters 37. A load sensor 40 that generates an output voltage proportional to the depression amount of the accelerator pedal 39 is connected to the accelerator pedal 39, and the output voltage of the load sensor 40 is input to the input port 35 via the corresponding AD converter 37. It Further, a crank angle sensor 41 that generates an output pulse each time the crankshaft rotates, for example, 30 degrees is connected to the input port 35. The CPU 34 calculates the engine speed based on the output pulse from the crank angle sensor 41. On the other hand, the output port 36 is connected to the fuel injection valve 3, the drive device for the throttle valve 11, the fuel pump 15, and the EGR control valve 18 via a corresponding drive circuit 38, respectively. The electronic control unit constitutes ash desorption means and SOx release means.

  2A and 2B show the structure of the particulate filter 24. 2A shows a front view of the particulate filter 24, and FIG. 2B shows a side sectional view of the particulate filter 24. As shown in FIGS. 2A and 2B, the particulate filter 24 has a honeycomb structure, and a plurality of exhaust flow passages 71i and 71o extending in parallel with each other and partition walls separating the exhaust flow passages 71i and 71o from each other. And 72. In the embodiment shown in FIGS. 2A and 2B, the exhaust gas flow passages 71i and 71o have their upstream ends opened and their downstream ends closed by a plug 73d, and their upstream ends closed by a plug 73u. The exhaust gas outflow passage 71o is open at the downstream end. The hatched portion in FIG. 2A indicates the plug 73u. Therefore, the exhaust gas inflow passages 71i and the exhaust gas outflow passages 71o are alternately arranged via the thin partition walls 72. In other words, in the exhaust gas inflow passage 71i and the exhaust gas outflow passage 71o, each exhaust gas inflow passage 71i is surrounded by four exhaust gas outflow passages 71o, and each exhaust gas outflow passage 71o is surrounded by four exhaust gas inflow passages 71i. Are arranged as follows. In another embodiment not shown, the exhaust flow passage is composed of an exhaust gas inflow passage whose upstream and downstream ends are open, and an exhaust gas outflow passage whose upstream end is closed by a plug and whose downstream end is open. It

  The partition wall 72 is formed of a porous material, for example, a ceramic such as cordierite, silicon carbide, silicon nitride, zirconia, titania, alumina, silica, mullite, lithium aluminum silicate, zirconium phosphate. Therefore, as shown by the arrow in FIG. 2B, the exhaust gas first flows into the exhaust gas inflow passage 71i, then passes through the surrounding partition wall 72 and flows out into the adjacent exhaust gas outflow passage 71o. Thus, the partition wall 72 constitutes the inner peripheral surface of the exhaust gas inflow passage 71i.

  A catalyst having an oxidizing function is carried on both side surfaces of the partition wall 72 and the inner surfaces of the pores. The catalyst having an oxidizing function is composed of a noble metal such as platinum Pt, rhodium Rh and palladium Pd. In another embodiment not shown, the catalyst having an oxidizing function is composed of a complex oxide containing a base metal such as cerium Ce, praseodymium Pr, neodymium Nd, and lanthanum La. In yet another embodiment, not shown, the catalyst comprises a combination of noble metal and complex oxide.

  On the other hand, the catalyst 22 has a honeycomb structure and is provided with a plurality of exhaust flow passages that are separated from each other by a thin base material and extend in parallel with each other. A catalyst component is supported on this base material via a carrier made of alumina, for example. In the embodiment according to the present invention, the catalyst 22 is formed of a NOx storage reduction catalyst. The NOx occlusion reduction catalyst 22 includes a noble metal catalyst and a basic layer. In the examples according to the present invention, at least one selected from platinum Pt, rhodium Rh, and palladium Pd is used as the noble metal catalyst, and the components constituting the basic layer are, for example, potassium K, sodium Na, and cesium Cs. Alkali metals, alkaline earths such as barium Ba and calcium Ca, rare earths such as lanthanoids, and metals capable of donating electrons to NOx such as silver Ag, copper Cu, iron Fe and iridium Ir. At least one is used.

  The ratio of the air and the fuel supplied into the exhaust passage upstream of a certain position in the intake passage, the combustion chamber 2 and the exhaust passage is called the air-fuel ratio of the exhaust gas at that position, and includes both absorption and adsorption. When the term storage is used, the basic layer stores NOx when the air-fuel ratio of the inflowing exhaust gas is lean, and releases the stored NOx when the oxygen concentration in the inflowing exhaust gas decreases. To act.

That is, the case where platinum Pt is used as the noble metal catalyst and barium Ba is used as the component forming the basic layer will be described as an example. When the air-fuel ratio of the inflowing exhaust gas is lean, that is, the oxygen concentration in the inflowing exhaust gas is high. Sometimes, NO contained in the inflowing exhaust gas is oxidized on platinum Pt to become NO ₂ . The NO ₂ in the NO ₂ and the inflow exhaust gas generated as the then been donating electrons from platinum NO ₂ ^- and becomes. This NO ₂ ⁻ then diffuses into the basic layer in the form of nitrate ions NO ₃ ⁻ to form nitrates. In this way, NOx is absorbed in the basic layer in the form of nitrate. Note that NO and NO ₂ may be temporarily retained in the basic layer by adsorption.

On the other hand, when NOx is absorbed in the basic layer in the form of nitrate and the air-fuel ratio of the inflowing exhaust gas is made rich, the oxygen concentration in the inflowing exhaust gas is lowered, so that the reaction is performed in the reverse direction (NO ₃ ⁻ → NO ₂ ) and thus the nitrate ions NO ₃ ⁻ in the basic layer are released from the basic layer in the form of NO ₂ . Next, the released NO ₂ is reduced to N ₂ by a reducing agent such as HC, CO, H ₂ contained in the inflowing exhaust gas. Thus, NOx storage-reduction catalyst 22, the air-fuel ratio of the inflowing exhaust gas is occluded the NOx when the lean air-fuel ratio of the inflowing exhaust gas of NOx that has released with releasing NOx occluding becomes rich N ₂ It is configured to reduce to.

  In the engine body 1, combustion is performed under excess oxygen. Therefore, since the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 22 is lean, NOx in the exhaust gas is stored in the NOx storage reduction catalyst 22 at this time. However, the amount of NOx stored in the NOx storage reduction catalyst 22 increases with the passage of time. Therefore, in the embodiment according to the present invention, in order to release NOx from the NOx storage reduction catalyst 22, the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 22 is temporarily switched to rich.

The exhaust gas also contains SOx, and this SOx is stored in the NOx storage reduction catalyst 22 in the form of sulfate BaSO ₄ when the air-fuel ratio of the inflowing exhaust gas is lean. That is, the NOx storage reduction catalyst 22 has a function of trapping SOx in the exhaust gas. However, the sulfate BaSO ₄ is stable and difficult to decompose, and the sulfate BaSO ₄ remains without being decomposed by simply making the air-fuel ratio of the exhaust gas rich. On the other hand, when the temperature of the NOx storage reduction catalyst 22 is raised to the SOx release temperature and the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 22 is made rich, SOx is released from the NOx storage reduction catalyst 22. Therefore, in the embodiment according to the present invention, in order to release SOx from the NOx storage / reduction catalyst 22, the temperature of the NOx storage / reduction catalyst 22 is raised to the SOx release temperature and kept flowing at the SOx release temperature while flowing into the NOx storage / reduction catalyst 22. SOx release control is performed in which the air-fuel ratio of the exhaust gas is temporarily switched to rich. The SOx release temperature is 600 ° C., for example. That is, in the embodiment according to the present invention, the NOx storage reduction catalyst 22 is a SOx trapping catalyst arranged in the engine exhaust passage upstream of the particulate filter 24 or in the particulate filter 24, and the air-fuel ratio of the inflowing exhaust gas is An SOx trap catalyst that traps SOx in exhaust gas when lean, and releases SOx that is trapped when the temperature of the SOx trap catalyst is the SOx release temperature and the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio or rich Function as.

  In the embodiment according to the present invention, it is determined that the SOx release control should be performed when the SOx trapping amount QS of the particulate filter 24 exceeds the preset SOx amount. On the other hand, when the SOx trapping amount QS of the particulate filter 24 during SOx release control reaches a predetermined lower limit SOx amount, for example, zero, the SOx release control is completed. For the SOx trapping amount QS of the particulate filter 24, for example, an increase dQSi per unit time and a decrease dQSr per unit time are calculated based on the engine operating state, and the increase dQSi and the decrease dQSr are integrated. Is calculated by (QS = dQSi + dQSr). In another embodiment (not shown), when the differential pressure across the particulate filter 24 exceeds a predetermined threshold value, and when the differential pressure across the particulate filter 24 decreases below the predetermined threshold value. Even when the SOx trapping amount QS of the particulate filter 24 exceeds the preset SOx amount, it is determined that the SOx release control should be performed.

  In addition, in the embodiment according to the present invention, in order to switch the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 22 to rich, in addition to the main fuel for obtaining the engine output, additional fuel is used in the explosion stroke or the exhaust stroke. Is injected into. This additional fuel burns in the combustion chamber 2, the exhaust passage upstream of the NOx storage reduction catalyst 22, or the NOx storage reduction catalyst 22 with almost no engine output. In another embodiment (not shown), in order to switch the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 22 to rich, the fuel is added from the fuel addition valve arranged in the exhaust passage upstream of the NOx storage reduction catalyst 22 to the exhaust gas. Is added secondarily into the.

  Further, in the embodiment according to the present invention, in order to raise the temperature of the NOx storage reduction catalyst 22, the additional fuel is injected from the fuel injection valve 3 in the explosion stroke or the exhaust stroke. In another embodiment (not shown), in order to raise the temperature of the NOx storage reduction catalyst 22, the fuel is secondarily added to the exhaust gas from a fuel addition valve arranged in the exhaust passage upstream of the NOx storage reduction catalyst 22. To be done.

  Furthermore, the exhaust gas contains particulate matter formed mainly of solid carbon. The particulate matter is collected on the particulate filter 24. As described above, since the engine body 1 is burned under excess oxygen, the particulate filter 24 is in an oxidizing atmosphere. Further, the particulate filter 24 carries a noble metal catalyst having an oxidizing function. As a result, the particulate matter collected by the particulate filter 24 is sequentially oxidized. However, when the amount of the particulate matter collected per unit time becomes larger than the amount of the particulate matter oxidized per unit time, the amount of the particulate matter collected on the particulate filter 24 becomes large. It increases with the passage of engine operating time.

  Therefore, in the embodiment according to the present invention, in order to reduce the amount of particulate matter trapped by the particulate filter 24, the temperature of the particulate filter 24 is raised to a predetermined PM removal temperature and the PM removal temperature is maintained. Control is performed. As a result, the particulate matter on the particulate filter 24 is removed, and the pressure loss of the particulate filter 24 is reduced. The PM removal temperature is, for example, about 610 ° C.

  In the embodiment according to the present invention, the PM removal control is completed when the particulate matter trapping amount QPM of the particulate filter 24 reaches a predetermined lower limit PM amount, for example, zero during the PM removal control. As the particulate matter trapping amount QPM of the particulate filter 24, for example, the increment dQPMi and the decrement dQPMr per unit time are calculated based on the engine operating state, and the increment dQPMi and the decrement dQPMr are calculated. It is calculated by integrating (QPM = dQPMi + dQPMr).

  In the embodiment according to the present invention, in order to raise the temperature of the particulate filter 24, the additional fuel is injected from the fuel injection valve 3 in the expansion stroke or the exhaust stroke. In another embodiment (not shown), in order to raise the temperature of the particulate filter 24, fuel is secondarily added into the exhaust gas from a fuel addition valve arranged in the exhaust passage upstream of the particulate filter 24. .

By the way, the exhaust gas also contains ash, and this ash is also collected by the particulate filter 24 together with the particulate matter. It has been confirmed by the present inventor that the ash in this case is mainly formed of calcium carbonate CaCO ₃ . Calcium Ca is derived from engine lubricating oil, and carbon C is derived from fuel. That is, the engine lubricating oil flows into the combustion chamber 2 and burns, and calcium Ca in the lubricating oil combines with carbon C in the fuel to generate calcium carbonate CaCO ₃ . Alternatively, the calcium oxide CaO generated in the combustion chamber 2 is collected by the particulate filter 24, and the calcium oxide CaO then reacts with the carbon dioxide CO ₂ in the exhaust gas on the particulate filter 24 to thereby give calcium carbonate. CaCO ₃ is produced.

  However, even if PM removal control is performed, the ash does not burn or vaporize. That is, the ash is not removed from the particulate filter 24 but remains on the particulate filter 24. In this case, as shown by A in FIG. 3A, the ash adheres to the inner peripheral surface 71is of the exhaust gas inflow passage 71i so as to cover the inner peripheral surface 71is. As a result, the pressure loss of the particulate filter 24 increases due to the ash.

  In this case, the ash flows into the particulate filter 24 in the form of particles and accumulates on the inner peripheral surface 71is. As the amount of ash on the inner peripheral surface 71is increases, the granular ashes are bonded to each other to form a layer. In this process, the ash and the partition wall 72 are engaged with each other by, for example, anchoring, and as a result, the ash layer is firmly attached to the inner peripheral surface 71is. Therefore, even if the flow of exhaust gas acts on the ash, it is difficult for the ash to be detached from the inner peripheral surface 71is.

On the other hand, when the ash is kept at a high temperature, the ash is converted into calcium oxide CaO. That is, when holding the calcium carbonate CaCO ₃ is ash to a high temperature, calcium carbonate CaCO ₃ is decomposed into calcium oxide CaO and carbon dioxide CO _2, carbon dioxide CO ₂ is a gas is released from the layer of ash. As a result, the particle size of the ash is reduced and the density of the ash layer is reduced. As a result, the connection between the ash and the engagement between the ash and the partition wall 72 are weakened. Further, the binding energy of calcium oxide CaO is higher than the binding energy of calcium carbonate CaCO ₃ . In an ionic crystal, a material having a high binding energy is harder than a material having a low binding energy. Therefore, when calcium carbonate CaCO ₃ is converted to calcium oxide CaO, the ash becomes hard and the ash layer becomes brittle. Therefore, the ash can be easily detached from the inner peripheral surface 71is. This has been confirmed by experiments by the present inventors.

  Therefore, in the embodiment according to the present invention, in order to reduce the amount of adhered ash on the particulate filter 24, the temperature of the particulate filter 24 is raised to a predetermined ash desorption temperature and maintained at the ash desorption temperature. Control is performed. In this case, the ash desorption temperature is a temperature suitable for converting ash into calcium oxide CaO. As a result, it is possible to suppress an increase in pressure loss of the particulate filter 24 due to ash.

  As shown in FIG. 3B, the ash A desorbed from the inner peripheral surface 71is of the exhaust gas inflow passage 71i when the ash desorption control is performed is exhausted by the exhaust gas flowing in the exhaust gas inflow passage 71i. It is moved to the inner part 71ir of 71i. In this case, the ash A may adhere to the inner peripheral surface 71is in the inner portion 71ir. However, such an ash A hardly affects the pressure loss of the particulate filter 24.

  The ash desorption temperature, that is, the temperature suitable for converting the ash to calcium oxide CaO is, for example, 450 ° C. or higher. However, when the ash desorption temperature is low, the time required to complete the ash desorption control becomes long. On the other hand, when the ash desorption temperature is excessively high, the amount of fuel per unit time required to raise the temperature of the particulate filter 24 becomes excessively large, or the NOx storage reduction catalyst 22 or the particulate filter 24 becomes It may be damaged. Therefore, the ash desorption temperature is preferably about 620 ° C. or higher and about 800 ° C. or lower, and more preferably about 650 ° C. In the example according to the present invention, the ash desorption temperature is set to about 650 ° C. In the example according to the present invention, the PM removal temperature is about 610 ° C. as described above, so the ash desorption temperature is set higher than the PM removal temperature.

  In the embodiment according to the present invention, the ash desorption control is completed when the ash adhesion amount QA of the particulate filter 24 becomes a predetermined lower limit ash amount, for example, zero during the ash desorption control. For the ash adhesion amount QA of the particulate filter 24, for example, an increase dQAi per unit time and a decrease dQAr per unit time are calculated based on the engine operating state, and the increase dQAi and the decrease dQAr are integrated. Is calculated by (QA = dQAi + dQAr).

In the embodiment of the present invention, the time during which the filter temperature is maintained at the ash desorption temperature or higher in the ash desorption control is longer than the time during which the filter temperature is maintained at the PM removal temperature or higher in the PM removal control. . This is because the conversion of calcium carbonate CaCO ₃ into calcium oxide CaO does not proceed sufficiently in a short time. That is, in the embodiment according to the present invention, even if the filter temperature is maintained at a temperature higher than the PM removal temperature in the ash desorption control, if the retention time is the same as or shorter than the retention time of the PM removal control. In other words, it is difficult to obtain a sufficient scavenging action of ash. Considering that if the ash desorption temperature is set low, the holding time of the ash desorption control can be lengthened, and if the ash desorption temperature is set high, the holding time can be shortened. It can be also considered that the ash desorption temperature is set so that the holding time of is longer than the holding time of the PM removal control. In another embodiment not shown, the ash desorption temperature is set to about 800 ° C., and the ash desorption control holding time is set shorter than the PM removal control holding time. In other words, in this other embodiment, the ash desorption temperature is set such that the ash desorption control holding time is shorter than the PM removal control holding time.

By the way, in the embodiment according to the present invention, as described above, the ash is mainly formed of calcium carbonate CaCO ₃ . This is because the NOx storage reduction catalyst 22 that functions as an SOx trapping catalyst is arranged upstream of the particulate filter 24, and SOx does not substantially flow into the particulate filter 24. In other words, when SOx flows into the particulate filter 24, since the particulate filter 24 is in an oxidizing atmosphere at this time, part of the calcium carbonate CaCO ₃ is converted into calcium sulfate CaSO ₄ , that is, in the form of calcium sulfate CaSO ₄ . Ash is generated.

However, it may be more difficult to detach the ash in the form of calcium sulfate CaSO ₄ from the inner peripheral surface 71is, as compared with the ash in the form of calcium carbonate CaCO ₃ . That is, in order to desorb from the inner peripheral surface 71is the ash form of calcium sulfate CaSO _4, it is necessary to convert the calcium carbonate CaCO ₃ calcium oxide CaO converts the calcium sulfate CaSO ₄ in the calcium carbonate CaCO ₃ For this conversion, while maintaining the air-fuel ratio of the exhaust gas flowing into the particulate filter 24 substantially at the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio, that is, while maintaining the particulate filter 24 in the reducing atmosphere, The temperature of the curate filter 24 must be maintained at the ash desorption temperature.

On the other hand, when the SOx release control of the NOx storage reduction catalyst 22 is performed, a relatively high concentration of SOx flows into the particulate filter 24. At this time, if ash adheres to the particulate filter 24, calcium sulfate CaSO ₄ The form of ash may be generated.

Therefore, in the embodiment according to the present invention, when the SOx release control is to be performed, the ash desorption control is first performed, and then the SOx release control is performed. As a result, the SOx trapping ability of the NOx storage reduction catalyst 22 can be restored while suppressing the generation of ash in the form of calcium sulfate CaSO ₄ .

  On the other hand, the above-mentioned decomposition reaction of ash is promoted by the catalyst having an oxidizing function on the particulate filter 24. However, if a large amount of particulate matter is collected on the particulate filter 24, it becomes difficult for the ash to come into contact with the catalyst having an oxidizing function. As a result, the decomposition reaction of the ash becomes difficult, and thus it becomes difficult to detach the ash from the inner peripheral surface 71is.

  Therefore, in the embodiment according to the present invention, when ash desorption control should be performed, PM removal control is first performed, and then ash desorption control is performed. As a result, the ash is reliably detached from the inner peripheral surface 71is.

  That is, in the embodiment according to the present invention, when SOx release control should be performed, as shown in FIG. 4, PM removal control is first performed, then ash desorption control is performed, and then SOx release control is performed. In this case, since the ash desorption control and the SOx release control are sequentially performed when the temperature of the particulate filter 24 is high, it is possible to reduce the energy required to raise and maintain the temperature of the particulate filter 24.

  FIG. 5 shows a routine for calculating the estimated SOx trap amount QS according to the embodiment of the present invention. This routine is repeatedly executed by interruption every predetermined set time. Referring to FIG. 5, in step 100, the increase amount dQSi and the decrease amount dQSr of the SOx trapping amount are calculated using the maps of FIGS. 4A and 4B, respectively. In the following step 101, the SOx trapping amount estimated value QS is calculated (QS = QS + dQSi-dQSr).

  FIG. 6 shows an execution routine of PM removal control according to the embodiment of the present invention. This routine is repeatedly executed by interruption every predetermined set time. Referring to FIG. 6, at step 200, it is judged if the estimated SOx trap amount QS is larger than the SOx set value QS1. When QS ≦ QS1, the processing cycle is ended. When QS> QS1, the routine proceeds to step 201, where PM removal control is first started. When the PM removal control is completed, the routine then proceeds to step 202, where the ash desorption control is started. When the ash desorption control is completed, the routine proceeds to step 203, where the SOx release control is started. When the SOx release control is completed, the processing cycle is ended.

  In the embodiment according to the present invention, when the particulate matter trapping amount exceeds the preset PM amount, the PM removal control is performed even if the SOx trapping amount QS is smaller than the set SOx amount QS1. Be seen. Further, in the embodiment according to the present invention, when the ash adhesion amount exceeds a preset set ash amount, ash desorption control is performed even if the SOx trapping amount QS is smaller than the set SOx amount QS1 at this time.

  In another embodiment (not shown), after the SOx trap amount QS exceeds the set SOx amount QS1, the particulate matter trap amount exceeds the preset PM amount and the ash adhesion amount is the preset ash amount. When it exceeds, PM removal control, ash desorption control, and SOx release control are sequentially performed. In a further embodiment (not shown), when the SOx trapping amount QS exceeds the set SOx amount QS1, and the ash adhesion amount exceeds a preset set ash amount, the particulate matter trapping amount is set to a preset value. Even if the amount is less than the amount of PM, PM removal control, ash desorption control, and SOx release control are sequentially performed.

1 Engine Main Body 2 Combustion Chamber 3 Fuel Injection Valves 21 and 23 Exhaust Pipe 22 NOx Storage Reduction Catalyst 24 Particulate Filter 30 Electronic Control Unit

Claims (1)

A particulate filter arranged in the engine exhaust passage for collecting particulate matter in the exhaust gas,
A SOx trap catalyst arranged in an engine exhaust passage upstream of the particulate filter or in the particulate filter, for trapping SOx in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean, and trapping the SOx. An SOx trap catalyst that releases SOx trapped when the temperature of the catalyst is the SOx release temperature and the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio or rich;
In order to reduce the amount of adhered ash of the particulate filter, the temperature of the particulate filter is raised to a predetermined ash desorption temperature, and the ash desorption control of maintaining the ash desorption temperature is performed. An ash desorption means, wherein the ash desorption temperature is a temperature suitable for converting the ash to calcium oxide and is higher than the SOx release temperature ;
In order to reduce the SOx trapping amount of the SOx trapping catalyst, the temperature of the SOx trapping catalyst is raised to a predetermined SOx releasing temperature and kept at the SOx releasing temperature while the exhaust gas flowing into the SOx trapping catalyst is emptied. SOx release means configured to perform SOx release control for temporarily switching the fuel ratio to rich,
When the SOx release control is to be performed, first, the ash desorption control is performed, and subsequently, the ash desorption means and the SOx release means are controlled to control the SOx release.
An exhaust gas purification device for an internal combustion engine, comprising:

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