WO2009008395A1

WO2009008395A1 - Exhaust purification device for internal combustion engine

Info

Publication number: WO2009008395A1
Application number: PCT/JP2008/062245
Authority: WO
Inventors: Takamitsu Asanuma; Shinya Hirota; Kohei Yoshida; Hiromasa Nishioka; Hiroshi Otsuki
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2007-07-11
Filing date: 2008-06-30
Publication date: 2009-01-15
Also published as: JP2009019553A

Abstract

In an exhaust purification device, a SOx trap catalyst (19) capable of capturing SOx contained in exhaust gas is placed at that portion of an engine exhaust path which is located on the upstream of a NOx occluding/reducing catalyst (21). The exhaust purification device has air/fuel ratio control means that, when the air/fuel ratio of exhaust gas flowing into the SOx trap catalyst (19) is rich and when it is predicted that the temperature of the SOx trap catalyst (19) becomes equal to or higher than a SOx release temperature, performs control such that the air/fuel ratio is lean. By doing so, the air/fuel ratio control means suppresses release of SOx captured by the SOx trap catalyst (19). In this process, a reducing agent supply valve (25) placed at that portion of the exhaust path which is located between the SOx trap catalyst (19) and the NOx occluding/reducing catalyst (21) injects a reducing agent so that the air/fuel ratio of the exhaust gas flowing into the NOx occluding/reducing catalyst (21) is a target air/fuel ratio at which NOx can be reduced and purified.

Description

Description Exhaust gas purification device for internal combustion engine Technical field

The present invention relates to an exhaust emission control device for an internal combustion engine. Background art

When the air-fuel ratio of the inflowing exhaust gas is lean, the NOx contained in the exhaust gas is occluded. When the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or the latch, the stored NOx is reduced and purified. NOx occlusion An internal combustion engine in which a reduction catalyst is arranged in an engine exhaust passage is known. In this internal combustion engine, NO generated when combustion is performed at a lean air-fuel ratio is stored in the NO X storage reduction catalyst. On the other hand, when the NO x storage capacity of the NO x storage reduction catalyst approaches saturation, the air-fuel ratio of the exhaust gas is temporarily turned on, so that NO x is reduced and purified from the NO x storage reduction catalyst.

By the way, sulfur is contained in the fuel and the lubricating oil, and therefore S O X is contained in the exhaust gas. This S O x is stored in the NO X storage reduction catalyst together with NO x. However, SOX is not released from the NO x storage reduction catalyst by simply switching the air-fuel ratio of the exhaust gas, and therefore the amount of SO x stored in the NO x storage reduction catalyst gradually increases. (Hereinafter referred to as sulfur poisoning). As a result, the amount of NO x that can be stored in the NO x storage reduction catalyst gradually decreases.

Therefore, a compression ignition type internal combustion engine in which an SO x trap catalyst is arranged in the engine exhaust passage upstream of the NO x storage reduction catalyst in order to prevent SOX from being stored in the NO X storage reduction catalyst is known. Open 2 0 0 5— 1 3 3 6 1 0)). This SO x trap catalyst captures SO x contained in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the SOX trap catalyst is lean, and when the air-fuel ratio of the exhaust gas is lean, S 0 X When the temperature of the wrap catalyst rises, the trapped SOX gradually diffuses inside the SOX trap catalyst. As a result, the Sx trap rate is recovered. Therefore, this internal combustion engine is provided with an estimation means for estimating the SO x trap rate by the SOX trap catalyst. When the SO x trap rate falls below a predetermined rate, the air-fuel ratio of the exhaust gas is lean. Increase the temperature of the SOX trap catalyst below. As a result, the SO x trap rate is recovered. Disclosure of the invention

When using an Sx trap catalyst such as that described above for a compression ignition internal combustion engine, the air-fuel ratio of the exhaust gas flowing into the SOX trap catalyst is made lean so that the Sx trap catalyst does not release SOX. It's not difficult to keep up. However, when the above-mentioned S O X trap catalyst is used in a spark ignition internal combustion engine, the air-fuel ratio of the exhaust gas flowing into the S O X trap catalyst during a high load or sudden acceleration tends to become a latch. Then, depending on the catalyst temperature of the SOx trap catalyst, SOx will be released from the SOx trap catalyst, and SOx will be occluded by the downstream NOx storage reduction catalyst. As a result, there is a problem that the amount of NOx that can be stored in the NOx storage reduction catalyst gradually decreases.

The above problem is that the air-fuel ratio of the exhaust gas flowing into the SOX trap catalyst is rich due to changes in the operating conditions of the internal combustion engine, and the temperature of the SO x trap catalyst is higher than the temperature at which SOX is released from the SOX trap catalyst. By controlling so that the air-fuel ratio of the exhaust gas flowing into the SOx trap catalyst is lean, the release of SOx trapped in the SOx trap catalyst is suppressed. It seems to be solved.

However, when the air-fuel ratio of the exhaust gas flowing into the SOX trap catalyst is controlled to be lean in this way, the amount of NO X stored in the downstream NO x storage reduction catalyst gradually increases, As a result, the amount of NO x that can be stored in the NO x storage reduction catalyst gradually decreases. If this happens, there will be a problem that the NOX that the NOX storage / reduction catalyst could not store is discharged to the outside air and the exhaust properties deteriorate.

Therefore, in view of the above problems, the present invention prevents NO X from being released to the outside air when the air-fuel ratio of the exhaust gas is controlled to be lean in order to suppress the release of SOX trapped by the S0 X trap catalyst. An object of the present invention is to provide an exhaust purification device for an internal combustion engine.

In order to solve the above-described problems, a first aspect of the present invention is a SO x trap catalyst that captures SO x contained in exhaust gas when the air-fuel ratio of the exhaust gas flowing into the engine exhaust passage is lean. Therefore, when the air-fuel ratio of the inflowing exhaust gas is lean and the temperature of the SOx trap catalyst rises, the trapped SOx gradually diffuses into the SOx trap catalyst and If the air-fuel ratio of the exhaust gas flowing into the SOX trap catalyst is under the latch and the temperature of the SOX trap catalyst is equal to or higher than the SO x release temperature, an SO x trap catalyst having the property of releasing the captured SO x is placed. When the air-fuel ratio of the exhaust gas flowing into the SO x trap catalyst downstream exhaust passage is lean, NOx contained in the exhaust gas is occluded and the air-fuel ratio of the exhaust gas flowing in is the theoretical air-fuel ratio or Rich NO x for reduction purification of occluded NO X and In an internal combustion engine equipped with an occlusion reduction catalyst, the air-fuel ratio of the exhaust gas flowing into the S0 x trap catalyst is rich and the SO x trap catalyst temperature is the SO x release temperature due to changes in operating conditions. An air-fuel ratio control means for suppressing the release of SOX trapped in the SO x trap catalyst by controlling the air-fuel ratio to be lean when expected to be above, the SOX trap catalyst and the NOX storage reduction catalyst A reducing agent supplying means for injecting the reducing agent so that the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst becomes a target air-fuel ratio capable of reducing and purifying NOx. An exhaust gas purification apparatus for an internal combustion engine is provided.

That is, in this aspect, by providing the air-fuel ratio control means, the air-fuel ratio of the exhaust gas flowing into the SOX trap catalyst due to a change in operating conditions such as a driver acceleration request is rich, and the SOX trap catalyst When the temperature is expected to be higher than the SO x release temperature, the air-fuel ratio is controlled to be lean, and the release of S 0 X trapped in the S 0 X trap catalyst is suppressed. As a result of making the air-fuel ratio lean, as described above, the NO x storage amount of the NO x storage reduction catalyst increases, so the amount of NO X that can be stored decreases, and there is concern about deterioration of exhaust properties. The Therefore, by injecting the reducing agent from the upstream side of the NO X storage reduction catalyst, the air / fuel ratio of the exhaust gas flowing into the NO X storage reduction catalyst is set to the target air / fuel ratio at which NO X can be reduced and purified. . As a result, there is an effect that it becomes possible to prevent the deterioration of exhaust properties. Here, S0 X release temperature is the temperature at which SO x trapped by S0 X trap catalyst is released when the air-fuel ratio of exhaust gas flowing into S0 x trap catalyst is a latch ( For example, 6 0 0).

In a second aspect of the present invention, the air-fuel ratio control means comprises the SO x An exhaust purification device for an internal combustion engine is provided that controls the air-fuel ratio of the exhaust gas flowing into the trap catalyst to a lean air-fuel ratio that is a limit that can ensure the required torque. That is, in this aspect, the air-fuel ratio control means controls the air-fuel ratio of the exhaust gas flowing into the S0 x trap catalyst so that the lean air-fuel ratio of the limit that can satisfy the required torque is obtained. A lean air-fuel ratio is ensured while satisfying the required torque. As a result, the release of SOX trapped in the SOX trap catalyst is suppressed.

In a third aspect of the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine, wherein the air-fuel ratio control means controls the air-fuel ratio of the exhaust gas flowing into the S0 trap catalyst to be lean near the stoichiometric air-fuel ratio. The That is, in this embodiment, the air-fuel ratio control means controls the SOx trap catalyst to capture the SOx trap catalyst by controlling the air-fuel ratio of the exhaust gas flowing into the S0 X trap catalyst so that it becomes lean near the stoichiometric air-fuel ratio. x release is suppressed. Therefore, since it is possible to operate at a leaner position closer to the stoichiometric air-fuel ratio than the invention according to claim 2, it is possible to satisfy a higher torque requirement.

In a fourth aspect of the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine, wherein the air-fuel ratio control means comprises means for supplying oxygen into an exhaust passage upstream of the S O x trap catalyst. In other words, in this embodiment, by supplying oxygen into the exhaust passage upstream of the SOX trap catalyst, the air-fuel ratio of the exhaust gas flowing into the S0 trap catalyst regardless of the air-fuel ratio of the air-fuel mixture combusted in the engine combustion chamber. Can always be lean.

In a fifth aspect of the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine, wherein the N O X storage reduction catalyst has a three-way catalyst function.

In a sixth aspect of the present invention, the target air-fuel ratio is a stoichiometric air-fuel ratio. There is provided an exhaust gas purification apparatus for an internal combustion engine in which the Nx storage / reduction catalyst functions as a three-way catalyst. That is, in the fifth and sixth modes, NOx in the exhaust gas is reduced and purified by using the function as a three-way catalyst, regardless of the amount of NO x stored in the NO x storage reduction catalyst. The effect of doing.

In a seventh aspect of the present invention, the target air-fuel ratio is a rich spike that temporarily changes from a lean air-fuel ratio to a rich air-fuel ratio, and NOx stored in the NO x storage-reduction catalyst is reduced and purified. An exhaust purification device for an internal combustion engine is provided. In other words, in this embodiment, NOX can be stored again by restoring the storage capacity of the NO x storage reduction catalyst by the Rich Spike process, and the storage reduction function of the NOX storage reduction catalyst can be utilized. Is possible.

According to an eighth aspect of the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine, further comprising a catalyst temperature raising means for setting the catalyst temperature of the NOx storage reduction catalyst to an activation temperature or higher. That is, in this embodiment, by providing the means for raising the temperature of the Nx storage reduction catalyst, it is possible to raise the catalyst temperature quickly when necessary.

Hereinafter, the present invention will be more fully understood from the accompanying drawings and the description of the preferred embodiments of the present invention. Brief Description of Drawings

FIG. 1 is a diagram showing a schematic configuration of an exhaust emission control device for an internal combustion engine. FIG. 2 is a diagram showing a schematic configuration of an exhaust gas purification apparatus for an internal combustion engine according to another embodiment.

Figure 3 shows the flow chart of the sulfur poisoning control operation.

Figure 4 shows the flow chart of the reducing agent supply operation.

FIG. 5 is a flowchart of a reducing agent supply operation according to another embodiment. is there.

FIG. 6 is a flow chart of a reducing agent supply operation according to still another embodiment.

FIG. 7 is a flowchart of a reducing agent supply operation according to still another embodiment.

FIG. 8 is a flowchart of sulfur poisoning suppression operation according to another embodiment.

FIG. 9 a and FIG. 9 b are diagrams showing a map of the amount of S O X trapped by the S0 x trap catalyst.

FIG. 10 is a diagram showing a map of the amount of NOx stored in the NOx storage reduction catalyst. BEST MODE FOR CARRYING OUT THE INVENTION

In the embodiment described below, the case where the present invention is applied to a spark ignition type internal combustion engine will be described, but it may be applied to a compression ignition type internal combustion engine. This spark ignition type internal combustion engine is operated by controlling so that the air-fuel ratio of the air-fuel mixture combusting in the engine combustion chamber is basically lean (for example, air-fuel ratio 30). Depending on the acceleration requirement, the air-fuel ratio of the air-fuel mixture combusted in the engine combustion chamber may fluctuate to the stoichiometric air-fuel ratio or the rich. The S O X trapped by the SOX trap catalyst is released because the air-fuel ratio of the exhaust gas flowing into the S O X trap catalyst is a latch and the temperature of the S O X trap catalyst is S O X This is the case above the discharge temperature. Therefore, in the present invention, when the temperature of the S 0 x trap catalyst is equal to or higher than the SO x release temperature, the control is performed so that the air-fuel ratio of the exhaust gas flowing into the SOX trap catalyst does not become rich. NOx occlusion / reduction that increases as a result of the following: The

Figure 1 shows an overall view of the internal combustion engine. Referring to FIG. 1, 1 is, for example, an engine body with four cylinders, 2 is a cylinder block, 3 is a cylinder head, 4 is a piston, 5 is a combustion chamber, 6 is an intake valve, and 7 is an intake port. 8 is an exhaust valve, 9 is an exhaust port, and 10 is a spark plug. The intake port 7 is connected to the surge tank 1 2 through the corresponding intake branch pipe 11, and the surge tank 12 is connected to the air cleaner 14 through the intake duct 1 3. In the intake duct 13, an air flow mechanism 15 for detecting the intake air flow rate and a throttle valve 16 are arranged. In addition, an electrically controlled fuel injection valve 17 for injecting fuel into the intake port 7 is disposed in the intake port 7. On the other hand, the exhaust port 9 is connected to the S0x trap catalyst 19 through the exhaust manifold 18, and the SOX trap catalyst 19 is connected to the NOX storage reduction catalyst 21 through the exhaust pipe 20 to store the NOX. The reduction catalyst 2 1 is connected to the exhaust pipe 2 2. An air-fuel ratio sensor 2 3 for detecting the air-fuel ratio AF is attached upstream of the SOX trap catalyst 1 9, and a catalyst temperature sensor 2 4 for detecting the catalyst temperature T s is attached to the S0 X trap catalyst 1 9. . Further, an electrically controlled reducing agent injection valve 25 for supplying, for example, a reducing agent made of hydrocarbons into the exhaust passage is disposed upstream of the NOX storage reduction catalyst 21, and the NOX storage reduction catalyst 21. 2 1 is provided with a catalyst temperature sensor 26 for detecting the catalyst temperature T n. Incidentally, placing 〇 ₂ sensor 2 7 abruptly output voltage changes in the vicinity of the stoichiometric air-fuel ratio N_〇 x storage-reduction catalyst 2 1 than the downstream of the exhaust passage, downstream in controlling the air-fuel ratio as described later The preferred electronic control unit (ECU) 30 is a digital computer and can be fed back to each other by means of a bidirectional bus 31. 3), RAM (random access memory) 3 3, CPU (microprocessor) 3 4., input port 3 5,. And output port 3 6. A load sensor 40 for detecting the amount of depression of the accelerator pedal 39 is connected to the accelerator pedal 39. Here, the amount of depression of the accelerator pedal 39 represents the required load. The output signals of the air flow meter 1 5, air-fuel ratio sensor 2 3, catalyst temperature sensor 2 4, 2 6, ₂ sensor 2 7, and load sensor 4 0 are input ports via the corresponding AD converter 3 7. 3 Entered in 5. Further, a crank angle sensor 41 that generates an output pulse every time the crankshaft rotates, for example, 30 ° is connected to the input port 35. The CPU 3 4 calculates the engine speed N based on the output pulse of the crank angle sensor 4 1. On the other hand, the output port 3 6 is connected to the spark plug 10, the throttle valve 16, the fuel injection valve 17, and the reducing agent supply valve 25 via the corresponding drive circuit 38, which are electronic Control unit (ECU) 30 Controlled based on output signal from 0.

First, an embodiment of the present invention will be described with reference to FIG. In this embodiment, the NO X storage reduction catalyst 21 also has a function as a three-way catalyst. That is, when the NO x storage reduction catalyst 21 is above a certain temperature and the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio, hydrocarbons, carbon monoxide, and NO x that are harmful components in the exhaust gas It plays a role in purifying the water with a high purification rate. The temperature at which purification is performed with such a high purification rate is called the activation temperature. The activation temperature is also used as the temperature at which the NOx occlusion reduction catalyst can fully exhibit the occlusion reduction function. The air-fuel ratio of the exhaust gas is defined as the intake port 7, the combustion chamber 5, and the air supplied to the exhaust passage upstream of the Sx trap catalyst 19 and the fuel and the reducing agent (hydrocarbon) described later. The ratio of

As the first method in this embodiment, for example, significant lean Assume that the operating state changes from an air-fuel ratio operating state to an operating state in an air-fuel ratio region (region from about 20 to the stoichiometric air-fuel ratio) where the degree of leanness is relatively low due to an acceleration request. According to the conventional air-fuel ratio control, when sudden acceleration is required, the air-fuel ratio region is relatively low after passing through the air-fuel ratio once. However, in the present invention, even if the torque required at that time is the maximum required torque assumed at the time of high load or sudden acceleration, the theoretical air-fuel ratio or the rich air-fuel ratio is not reached. The air-fuel ratio is controlled so that the degree of leanness is relatively low. By controlling in this way, it becomes possible to keep the air-fuel ratio of the exhaust gas flowing into the Sx trap catalyst 19 9 lean.

As a second method in the present embodiment, the air-fuel ratio of the exhaust gas flowing into the S0 x trap catalyst 19 is fed back using the air-fuel ratio sensor 2 3 to obtain the theoretical air-fuel ratio of the inflowing exhaust gas. In this method, the fuel ratio is controlled to be close to the fuel ratio (for example, the air-fuel ratio is 14.8). According to this, it is possible to operate at a leaner value closer to the stoichiometric air-fuel ratio than in the first method, so that higher torque requirements can be satisfied. The air-fuel ratio sensor 2 3 may be replaced with ○ ₂ sensor.

Other methods include, for example, maintaining the stoichiometric air-fuel ratio so that the air-fuel ratio of the exhaust gas flowing into the SOX trap catalyst 19 does not become a latch when there is a request for fuel increase during acceleration operation. It is also conceivable to control the fuel increase ratio so that it does not become a rich air-fuel ratio. In addition, even if there is a request for a spike spike treatment to temporarily switch the air-fuel ratio to reduce and purify the N0 x storage reduction catalyst 21, the temperature of the S0 x trap catalyst will be released from the S0 x If it is above the temperature, it may be prohibited. The first mentioned above, The second and other methods are collectively referred to below as the air-fuel ratio control method.

Furthermore, in these air-fuel ratio control methods, in order to satisfy the required torque, for example, the required torque is satisfied by supercharging with a mechanical supercharger or using an electric motor together. May be. By using such additional means, for example, in the first method, it is possible to reliably ensure a larger lean air-fuel ratio.

In the present embodiment, when the air-fuel ratio of the exhaust gas flowing into the SO trap catalyst 19 is controlled to be lean by the air-fuel ratio control method as described above, the NO x of the downstream NO X storage reduction catalyst 21 is reduced to NO x. The amount of stored N 0 x gradually increases, and as a result, the amount of NO x that can be stored in the NO x storage reduction catalyst 2 1 gradually decreases. As a result, NO x in the inflowing exhaust gas cannot be fully occluded by the NO x storage reduction catalyst 21 and is partially discharged to the outside air, resulting in deterioration of exhaust properties.

Therefore, the reducing agent is supplied into the exhaust passage upstream of the NO x storage reduction catalyst 21 using the reducing agent injection valve 25, and the air-fuel ratio of the exhaust gas flowing into the NO X storage reduction catalyst 21 is theoretically determined. Control to achieve an air-fuel ratio. By doing so, the function of the NO x storage-reduction catalyst 21 as a three-way catalyst allows simultaneous oxidation of hydrocarbons and carbon monoxide in the exhaust gas and reduction of N0 x. Purify these harmful components into harmless carbon dioxide, water and nitrogen. As a result, it is possible to prevent deterioration of exhaust properties.

Next, the flow chart of the sulfur poisoning suppression operation and the reducing agent supply operation is explained. These operations, which will be described below using some embodiments, are performed as routines that are executed by interruptions at predetermined time intervals set in advance by an electronic control unit (ECU) 30. FIG. 3 shows a flow chart of the sulfur poisoning suppression operation according to this embodiment. The sulfur poisoning suppression operation is an operation for controlling the air-fuel ratio of the exhaust gas flowing into the SOX trap catalyst 19 to be lean so that SO x is not released from the SO x trap catalyst 19. Referring to Fig. 3, first, the amount of S 〇 x trapped in S〇 X trap catalyst 1 9 is estimated in step 100, and the amount of SO x trapped in S 〇 trap catalyst 1 9 1 S 〇 X It is determined whether 1 is greater than a predetermined amount SOX 0. That is, if the trapped SO x amount 〇 S X 1 is less than the predetermined amount SOX 0, even if SOX is released from the S O trap catalyst 1 9 immediately, the SOX trap catalyst 1 again 9 It is rarely captured by itself and reaches the NO X storage reduction catalyst 2 1 almost. Accordingly, the subsequent operation is not performed, and the routine proceeds to step 104 to set 0 to the flag F used in the reducing agent supply operation described later, and the routine is terminated.

Here, a method for estimating the amount of S0 x trapped by the S O X trap catalyst 19 will be described. The fuel contains a certain amount of sulfur. Therefore, the amount of SOx contained in the exhaust gas, that is, the amount of SOx trapped in the SOx trap catalyst 19 is proportional to the fuel injection amount. The fuel injection amount is a function of the required torque and the engine speed, so the amount of S O X trapped in the S O X trap catalyst 19 is also a function of the required torque and the engine speed. In the embodiment according to the present invention, the SO x trap SO XA trapped per unit time in the SOx trap catalyst 19 in the form of a map as shown in FIG. 9 a as a function of the required torque TQ and the engine speed N. Stored in ROM 3 2 in advance.

The lubricating oil also contains a certain percentage of sulfur, and the amount of lubricating oil combusted in the combustion chamber 5, that is, contained in the exhaust gas, is captured by the S0 x trap catalyst 19. SOX amount is also required torque and engine speed Is a function of In an embodiment according to the present invention,

○ X trap catalyst 1 Amount of S x trapped per unit time in 9 SOXB is stored in advance in R OM 3 2 in the form of a map as shown in Fig. 9b as a function of required torque TQ and engine speed N And the sum of the SO x amount SO XA and the SO x amount SOXB is calculated to calculate the SO x amount ∑ SO X 1 captured by the SOX trap catalyst 19.

If the amount of SOx trapped by the SOX trap catalyst 1 9 calculated as described above is greater than or equal to the predetermined amount SOx0, proceed to step 1001 It is determined whether or not the catalyst temperature T s of the X trap catalyst 1 9 is equal to or higher than S0 X release temperature T s 0. That is, when the catalyst temperature T s is lower than the S O x release temperature T s O, the captured S O x is not released regardless of the air-fuel ratio. Therefore, no further operation is performed, and the process proceeds to step 104 to set flag F to 0, and the routine ends. In order to reliably suppress the release of the trapped SOx, it is desirable that the temperature determined in Step 1001 is slightly lower than the SOx release temperature Ts0.

If the catalyst temperature T s is equal to or higher than S0 x release temperature T s 0 in step 1 0 1, the process proceeds to step 1 0 2 and the air-fuel ratio AF of the exhaust gas flowing into the SOX trap catalyst 1 9 is the theoretical sky. It is determined whether or not the fuel ratio is AF 0 or rich (AF 0). Here, even when the air-fuel ratio is the stoichiometric air-fuel ratio, the object of determination is basically that SO x is hardly released when the stoichiometric air-fuel ratio AF 0. However, since the release starts when the air / fuel ratio becomes even slightly (G AF 0), the decrease in the air / fuel ratio is suppressed immediately before that. S ○ X Trap catalyst 1 The air-fuel ratio AF of the exhaust gas flowing into 9 is If the stoichiometric air-fuel ratio is not AFO or Litch (G AF 0), that is, if the air-fuel ratio (> AF 0), the S0 x trapped by the S0 x trap catalyst 19 is released. Never happen. Therefore, no further operation is performed, and the routine proceeds to step 104, where flag F is set to 0 and the routine is terminated.

On the other hand, if the air-fuel ratio AF of the exhaust gas flowing into the SO x trap catalyst 1 9 is the stoichiometric air-fuel ratio AF 0 or the rich (AF 0) in step 100, the SOX 卜 wrap catalyst 1 9 The trapped SOX will be released (or it may be the case if the stoichiometric air-fuel ratio is used). Accordingly, the process proceeds to step 103, and the air-fuel ratio control is performed by the air-fuel ratio control method as described above so that the air-fuel ratio of the exhaust gas flowing into the SOx trap catalyst 19 becomes lean. Then proceed to step 105, this time set flag F to 1 and end the routine.

Figure 4 shows the flow chart of the reducing agent supply operation. The reducing agent supply operation is an operation for controlling the air-fuel ratio of the exhaust gas flowing into the NO x storage reduction catalyst 21 to reduce and purify the NO X stored in the NO X storage reduction catalyst 21. Referring to FIG. 4, first, in step 200, it is determined whether or not the flag F set in the above-described sulfur poisoning suppression operation is 1. When the flag F is not 1, that is, when the air-fuel ratio of the exhaust gas flowing into the Sx trap catalyst 19 is not leanly controlled by the sulfur poisoning suppression operation shown in FIG. There is no need to supply an additional reducing agent according to the invention. Therefore, the routine is terminated without performing the subsequent operation.

On the other hand, when the flag F is 1 in step 200, that is, the state in which the air-fuel ratio of the exhaust gas flowing into the S0 trap catalyst 19 is leanly controlled by the sulfur poisoning suppression operation described above. If, go to step 2 0 1. Step 2 0 1 It is then determined whether or not the air-fuel ratio AF of the exhaust gas has already become a lean air-fuel ratio (> AF 0). This is a process that takes into account the case of the transition to the lean air-fuel ratio, etc., even though the air-fuel ratio control of the sulfur poisoning suppression operation is being executed. As a result, if it is not yet lean, the routine is terminated without performing further operations. The processing in step 210 may not be performed when the air-fuel ratio of the exhaust gas is reliably made lean by the sulfur poisoning suppression operation.

On the other hand, if the air-fuel ratio AF of the exhaust gas is already the lean air-fuel ratio (> AF 0) in step 2 0 1, the process proceeds to step 2 0 2. In step 2 0 2, the stoichiometric air-fuel ratio process is executed. This process is performed by introducing the reducing agent into the exhaust passage from the reducing agent supply valve 25 before the air-fuel ratio of the exhaust gas that has been made lean by the sulfur poisoning suppression operation flows into the NO X storage reduction catalyst 21. Spray. Accordingly, the control is performed so that the air-fuel ratio of the exhaust gas flowing into the N0 x storage reduction catalyst 2.1 becomes the stoichiometric air-fuel ratio. In that case, it is assumed that the air-fuel ratio of the exhaust gas flowing into the SOX trap catalyst 19 is equal to the air-fuel ratio of the exhaust gas flowing out. Then, based on the air-fuel ratio measured by the air-fuel ratio sensor 23, an electronic control unit (ECU) 30 is used so that the air-fuel ratio of the exhaust gas flowing into the NO x storage reduction catalyst 21 becomes the stoichiometric air-fuel ratio. The amount of reducing agent calculated by the above is injected from the reducing agent supply valve 25. Injection amount of the reducing agent, by 〇 ₂ sensor 2 7 downstream of NOX storage-reduction catalyst 2 1 is feedback so that more accurate stoichiometric air-fuel ratio.

Therefore, by controlling the air-fuel ratio of the exhaust gas flowing into the NO x storage reduction catalyst 2 1 to the stoichiometric air fuel ratio, the function of the NO x storage reduction catalyst 2 1 as a three-way catalyst is utilized, and the exhaust gas is contained in the exhaust gas. It is possible to reduce the NOx contained and prevent deterioration of exhaust properties. In addition, by utilizing the function as a three-way catalyst of N 0 x storage reduction catalyst 21, Regardless of the amount of occluded NOx, it is possible to prevent deterioration of exhaust properties. Note that the Nx storage reduction catalyst having a function as a three-way catalyst used in this embodiment may be a normal three-way catalyst.

By the way, since the downstream NO X storage reduction catalyst 21 is downstream in the exhaust passage, the catalyst temperature is less likely to rise than the upstream SOX trap catalyst 19. If the catalyst temperature is not sufficiently increased, the N O X storage reduction catalyst 21 cannot function sufficiently as a three-way catalyst or a NO X storage reduction catalyst. Therefore, as yet another embodiment, a means for quickly raising the temperature of the NO x occlusion reduction catalyst 2 1 to the activation temperature T n O is taken. Specifically, a heating element such as a heating coil is arranged in the exhaust passage under the floor or upstream of the NO X storage reduction catalyst 21 to heat the exhaust gas flowing into the N0 x storage reduction catalyst 21 or the catalyst itself. To raise the temperature. Alternatively, even if the activation temperature Tn has not been reached, if the temperature has risen to some extent, the reducing agent is added from the reducing agent supply valve 25 in a lean air-fuel ratio with sufficient oxygen in the exhaust gas. By doing so, it is possible to raise the temperature to the activation temperature T n using the heat of oxidation reaction between the reducing agent and oxygen on the χ χ occlusion reduction catalyst 21.

FIG. 5 shows a flow chart of the reducing agent supply operation when the above-described catalyst temperature raising means is provided in the reducing agent supply operation shown in FIG. Referring to FIG. 5, the operations of steps 3 0 0, 3 0 1 and 3 0 4 are the same as the corresponding steps 2 0 0, 2 0 1 and 2 0 2 of the operation shown in FIG. The difference is that step 3 0 2 for determining temperature and step 3 0 3 are added between step 3 0 1 and step 3 0 4. That is, when the air-fuel ratio AF of the exhaust gas is the lean air-fuel ratio (> AF 0) in step 3 0 1, the process proceeds to step 3 0 2. Then, in Step 3 0 2, it is determined whether or not the catalyst temperature T n of the NO X storage reduction catalyst 2 1 is equal to or higher than the activation temperature T n 0. If the catalyst temperature T n is lower than the activation temperature T n O, the process proceeds to Step 303, where the NO X storage reduction catalyst 21 is heated by the heating element or added with the reducing agent as described above. The temperature is raised by the heat of oxidation reaction. Then go to step 3 0 2 again to determine the catalyst temperature T n again. As a result, if the catalyst temperature T n is equal to or higher than the activation temperature T n O in step 3 0 2, the process proceeds to step 3 0 4 to execute the stoichiometric air-fuel ratio process, and the routine is terminated.

Until now, a reducing agent has been added to reduce the air-fuel ratio of the exhaust gas flowing into the NOX storage reduction catalyst 21 to a theoretical air-fuel ratio, and the function of the NO X storage reduction catalyst 21 as a three-way catalyst has been utilized. The embodiment for reducing and purifying NO X in the inflowing exhaust gas has been described. Next, the NO x storage reduction catalyst 2 1 stores and reduces the NO x stored in the NO x storage reduction catalyst 2 1 to restore the storage capacity of the NO x storage reduction catalyst 21 and then stores the inflowing NO x. A form is demonstrated. Specifically, a rich spike treatment is performed in which the air-fuel ratio of the exhaust gas flowing into the NO x storage reduction catalyst 21 is temporarily latched by injecting the reducing agent from the reducing agent supply valve 25. . By restoring the storage capacity, all NO X contained in the exhaust gas flowing into the NOx storage reduction catalyst 21 is stored, and deterioration of exhaust properties is prevented.

Fig. 6 shows the flow chart of the reducing agent supply operation that performs the above-described rich spike processing. Referring to FIG. 6, Step 4 0 0 and Step 4 0 1 are the same as Step 2 0 0 and Step 2 0 1 corresponding to the sulfur poisoning suppression operation shown in FIG. Accordingly, step 40 2 is executed when the air-fuel ratio AF of the exhaust gas flowing into the SO x trap catalyst 19 in step 4 0 1 is the stoichiometric air-fuel ratio AF 0 or the latch (g AF 0). . In Step 4 0 2, the NO x amount Σ 吸 stored in the NO x storage-reduction catalyst 2 1 is greater than or equal to the allowable value NX. It is determined whether or not. If the stored amount of N〇X∑N〇X is less than the permissible value NX, there is sufficient room to store the incoming NOX, so there is no need to restore the storage capacity. Therefore, in that case, the routine is terminated without performing the subsequent operation.

Here, a method for estimating the amount of N0 x Σ に Χ 吸 stored in the NO x storage reduction catalyst 21 will be described. The amount of NO x stored in the NO x storage-reduction catalyst 2 1 per unit time N0 XA is stored in advance in the ROM 3 2 as a function of the required torque TQ and the engine speed N in the form of the map shown in Fig. 10. The stored NO x amount NO XA is added to calculate the NO x amount Σ 吸 stored in the NO x storage reduction catalyst 2 1. On the other hand, in step 4 0 2, the N 0 x amount If Σ ΝΟ 以上 is greater than or equal to the permissible value NX, the process proceeds to step 4 0 3, where the reducing agent is injected from the reducing agent supply valve 25 and the rich spike process is executed. By doing so, the stored NO X is reduced and purified, and the storage capacity of the NO x storage reduction catalyst 21 is restored. Accordingly, even if the exhaust gas flowing into the NO X storage reduction catalyst 21 is lean thereafter, it becomes possible to store N 0 x until the N 0 X amount ∑ N 0 X reaches the allowable value NX again. Then the routine ends. When the air-fuel ratio is controlled by sulfur poisoning suppression operation at high load or sudden acceleration, a large amount of NO X is contained in the exhaust gas. Therefore, it is more preferable to perform the Rich Spike process in advance before the Rich Spike process is performed by the reducing agent supply operation to restore the NOx storage reduction catalyst 21 storage capacity.

FIG. 7 shows a flow chart of the reducing agent supply operation when the catalyst temperature raising means is provided in the reducing agent supply operation shown in FIG. 6, similarly to the reducing agent supply operation shown in FIG. Step 4 of reducing agent supply operation in Fig. 6 0 FIG. 6 is the same as FIG. 6 except that steps 6 0 2 and 6 0 3 corresponding to steps 3 0 2 and 3 0 3 of the operation shown in FIG. 5 are inserted between 1 and 4 0 2.

Next, FIG. 2 shows an embodiment having a configuration different from that of FIG. In this embodiment, an electrically controlled air (oxygen) supply valve 28 is further installed upstream of the S0 x trap catalyst 19, and is based on an output signal from the electronic control unit (ECU) 30. The air / fuel ratio of the exhaust gas can be adjusted by supplying air (oxygen) into the exhaust passage. In other words, the air (oxygen) supply valve 2 5 upstream of the S0 x trap catalyst 1 9 is supplied with air (oxygen), so that the air-fuel ratio of the exhaust gas flowing into the SO x trap catalyst 1 9 is reduced to lean air. The fuel ratio is adjusted to become the fuel ratio. The advantage of this is that it is possible to adjust the air / fuel ratio of the exhaust gas flowing into the Sx trap catalyst 19 regardless of the air / fuel ratio of the air-fuel mixture combusting in the engine combustion chamber.

As a further effect, even when the catalyst temperature of the S0 x trap catalyst 19 rises excessively and the catalyst may deteriorate due to its heat, the temperature of the air (oxygen), which is lower than the exhaust gas, It is also possible to cool the catalyst by increasing the feed rate. As a result, the NO x occlusion reduction catalyst 21 downstream of the S O x trap catalyst 19 has a fuel (HC) or a reducing agent (HC, CO) and air not yet burned.

(Oxygen) will be supplied. There is a risk that the NO x storage reduction catalyst 2 1 will also deteriorate due to the oxidation reaction and the generation of heat. However, even in such a case, the NOx storage reduction catalyst 21 can be cooled by further increasing the amount of supplied air (oxygen).

FIG. 8 shows a flow chart of the sulfur poisoning suppression operation according to this embodiment. Referring to Figure 8, from step 6 0 0 to step 6 0 The operations up to 2 are the same as the corresponding steps 100 to 100 in the sulfur poisoning suppression operation according to the above-described embodiment shown in FIG. Therefore, step 6 0 3 is executed when the air-fuel ratio AF of the exhaust gas flowing into the S0 trap catalyst 1 9 in step 6 0 2 is the stoichiometric air-fuel ratio AF 0 or the rich (<AF 0). Is done. In step 60, the air (oxygen) supply valve 2 is adjusted so that the air-fuel ratio of the exhaust gas flowing into the S0 x trap catalyst 19 becomes lean so as to suppress the release of the trapped SOX as described above. Supply air (oxygen) from 5. Then go to step 6 0 5 to set the flag to 1 and end the routine.

The sulfur poisoning suppression operation shown in Fig. 8 can be used in combination with the reducing agent supply operation shown in Figs. 4 to 7, just like the sulfur poisoning suppression operation shown in Fig. 3 above.

Although the present invention has been described based on specific embodiments, those skilled in the art can make various changes and modifications without departing from the scope and spirit of the present invention.

Claims

The scope of the claims

1. An SOX trap catalyst that captures SOX contained in exhaust gas when the air-fuel ratio of the exhaust gas flowing into the engine exhaust passage is lean, and the air-fuel ratio of the inflowing exhaust gas is below the lean Therefore, the trapped SOX gradually diffuses into the SOX trap catalyst when the temperature of the S0x 卜 wrap catalyst rises, and the air-fuel ratio of the exhaust gas flowing into the SOx trap catalyst is If the temperature of the SOx trap catalyst is equal to or higher than the SOx release temperature, an SOx trap catalyst that releases the trapped SOx is placed and flows into the SOx trap catalyst downstream exhaust passage. When the air-fuel ratio of the exhaust gas is lean, NO X contained in the exhaust gas is occluded, and when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or the latch, the stored NOx is reduced and purified NO X Occlusion reduction In the internal combustion engine, the air-fuel ratio of the exhaust gas flowing into the S.O.X trap catalyst due to changes in operating conditions is rich, and the temperature of the S.O.X trap catalyst is higher than the S.O.X release temperature. An air-fuel ratio control means for controlling the release of SO x trapped in the SO x trap catalyst by controlling the air-fuel ratio to be lean when expected to be, the S0 x trap catalyst and the NO X occlusion Reducing agent supply means for injecting the reducing agent so that the air-fuel ratio of the exhaust gas flowing into the NO X storage reduction catalyst becomes a target air-fuel ratio capable of reducing and purifying NO X, disposed in the exhaust passage between the reduction catalysts; An exhaust purification device for an internal combustion engine comprising:

2. The exhaust gas purification of the internal combustion engine according to claim 1, wherein the air-fuel ratio control means controls the air-fuel ratio of the exhaust gas flowing into the SO trap catalyst to a lean air-fuel ratio that is a limit that can ensure the required torque. apparatus.

3. The air-fuel ratio control means flows into the SOX trap catalyst The exhaust emission control device for an internal combustion engine according to claim 1, wherein the air-fuel ratio of the exhaust gas is controlled to be lean near the stoichiometric air-fuel ratio.

4. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the air-fuel ratio control means includes means for supplying oxygen into an exhaust passage upstream of the S O X trap catalyst.

5. The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 4, wherein the NO X storage reduction catalyst has a three-way catalyst function.

6. The exhaust gas purification apparatus for an internal combustion engine according to claim 5, wherein the target air-fuel ratio is a stoichiometric air-fuel ratio, and the NO x storage-reduction catalyst exhibits a function as a three-way catalyst.

7. The target spike according to claim 1, wherein the target air-fuel ratio is a rich spike that temporarily changes from a lean air-fuel ratio to a rich air-fuel ratio, and NOx stored in the NOX storage-reduction catalyst is reduced and purified. The exhaust gas purification apparatus for an internal combustion engine according to one.

8. The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 7, further comprising a catalyst temperature raising means for setting a catalyst temperature of the NO x storage reduction catalyst to an activation temperature or higher.