JP2009174443A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine, certainly raising the catalyst temperature of a NOx storage reduction catalyst to desorption temperature of a sulfur compound when performing sulfur poisoning recovery control. <P>SOLUTION: In this exhaust emission control device, as the sulfur poisoning recovery control over the NOx storage reduction catalyst 10, temperature rise control is performed so that exhaust gas of a rich air-fuel ratio is discharged from first cylinder groups #1, #4, exhaust gas of a lean air-fuel ratio is discharged from second cylinder groups #2, #3, exhaust gas of the rich air-fuel ratio and exhaust gas of the lean air-fuel ratio are reacted in the NOx storage reduction catalyst after being mixed in one common exhaust pipe, and the temperature of the Nox storage reduction catalyst is raised to the desorption temperature of the sulfur compound with the sulfur compound desorbed from the NOx storage reduction catalyst, by heat of reaction obtained according to a difference between the air fuel ratios of exhaust gas. When the temperature of the Nox storage reduction catalyst is determined not be raised to the desorption temperature of the sulfur compound within a predetermined time, air-fuel ratio difference increasing control is performed for increasing the difference between the air-fuel ratio. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

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

  An NOx storage reduction catalyst that stores NOx contained in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and reduces and purifies the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich An internal combustion engine arranged in an exhaust passage is known. In this internal combustion engine, NOx generated when combustion is performed under a lean air-fuel ratio is stored in the NOx storage reduction catalyst. On the other hand, when the NOx occlusion capacity of the NOx occlusion reduction catalyst approaches saturation, the air-fuel ratio of the exhaust gas is temporarily made rich, whereby NOx is reduced and purified from the NOx occlusion reduction catalyst.

By the way, sulfur is contained in the fuel and the lubricating oil. Therefore, the exhaust gas contains sulfur compounds (for example, SOx, H ₂ S, etc.). This sulfur compound is stored in the NOx storage reduction catalyst together with NOx. However, this sulfur compound is not released from the NOx occlusion reduction catalyst simply by making the air-fuel ratio of the exhaust gas rich, and therefore the amount of the sulfur compound occluded in the NOx occlusion reduction catalyst gradually increases (hereinafter, “ Called sulfur poisoning). As a result, the amount of NOx that can be stored in the NOx storage reduction catalyst gradually decreases.

  In order to release the sulfur compound from the NOx occlusion reduction catalyst (that is, recover from sulfur poisoning), the catalyst temperature of the NOx occlusion reduction catalyst is set to the temperature at which the sulfur compound is released, that is, the sulfur compound emission temperature (for example, 600 ° C. And the sulfur poisoning recovery control to make the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst the stoichiometric air-fuel ratio or rich air-fuel ratio.

  Therefore, an exhaust purification device for an internal combustion engine is known in which the catalyst temperature of the NOx storage reduction catalyst is raised to the sulfur compound release temperature and the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst is made the stoichiometric air-fuel ratio (Patent Document). 1).

  The internal combustion engine disclosed in Patent Document 1 includes a plurality of cylinders, and the cylinders are divided into two cylinder groups, and exhaust pipes (hereinafter referred to as “exhaust branch pipes”) are connected to the respective cylinder groups, and these exhaust gases are connected. The branch pipes are joined downstream and connected to one common exhaust pipe (hereinafter referred to as “common exhaust pipe”), and the NOx storage reduction catalyst is disposed in the common exhaust pipe. In this configuration, when the sulfur poisoning recovery of the NOx occlusion reduction catalyst is to be performed, exhaust gas with a rich air-fuel ratio (hereinafter referred to as “rich exhaust gas”) is discharged from one cylinder group and from the other cylinder group. Lean air-fuel ratio exhaust gas (hereinafter referred to as “lean exhaust gas”) is discharged. Then, when the rich exhaust gas and the lean exhaust gas are mixed upstream of the NOx storage reduction catalyst, the rich degree of the rich exhaust gas and the lean degree of the lean exhaust gas are set so that the total air-fuel ratio of the exhaust gas becomes the stoichiometric air-fuel ratio. Is adjusted. Further, at this time, by mixing the rich exhaust gas and the lean exhaust gas, the fuel in the rich exhaust gas reacts with the oxygen in the lean exhaust gas in the NOx storage reduction catalyst, and the reaction heat causes the NOx storage reduction catalyst to react. The catalyst temperature rises.

  Thus, in the internal combustion engine disclosed in Patent Document 1, the catalyst temperature of the NOx storage reduction catalyst is raised to the sulfur compound release temperature, and the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst is set to the stoichiometric air-fuel ratio or rich. The air-fuel ratio is set, and the sulfur compound is released from the NOx storage reduction catalyst.

JP 2007-2784 A

  By the way, according to the sulfur poisoning recovery control disclosed in Patent Document 1, the reaction heat in the NOx storage reduction catalyst depends on the air-fuel ratio difference between the rich exhaust gas and the lean exhaust gas mixed in the upstream common exhaust pipe. To do. That is, the total air-fuel ratio of the exhaust gas when mixed upstream of the NOx storage-reduction catalyst is controlled to the stoichiometric air-fuel ratio. Therefore, the fuel with which the difference in the air-fuel ratio between the rich exhaust gas and the lean exhaust gas is larger can be reacted. The amount and the amount of oxygen increase. Therefore, a large reaction heat can be obtained. Therefore, in the sulfur poisoning recovery control, depending on the difference between the target sulfur compound release temperature and the current catalyst temperature of the NOx storage reduction catalyst, the air-fuel ratio difference between the rich exhaust gas and the lean exhaust gas, that is, The total amount of fuel and oxygen are adjusted.

  However, the fuel amount in the rich exhaust gas is less than the target due to the manufacturing error of the fuel injection valve or the variation or error in the fuel injection amount due to deformation due to aging, etc. The fuel amount in the lean exhaust gas is less than the target It may increase. In this case, there is a problem that the target air-fuel ratio difference cannot be obtained, and the catalyst temperature of the NOx storage reduction catalyst cannot be raised to the sulfur compound release temperature, or it takes time to raise the temperature.

  Accordingly, in view of the above problems, the present invention provides an exhaust purification device for an internal combustion engine that can reliably raise the catalyst temperature of the NOx storage reduction catalyst to the sulfur compound release temperature when the sulfur poisoning recovery control is executed. The purpose is to do.

In order to solve the above-mentioned problem, according to the first aspect of the present invention, a plurality of cylinders are provided, the cylinders are divided into two cylinder groups, and exhaust branch pipes are connected to the respective cylinder groups, and the exhaust branch pipes are connected. Are exhaust gas purification apparatuses for an internal combustion engine in which the NOx occlusion reduction catalyst is disposed in the common exhaust pipe, and the sulfur coverage of the NOx occlusion reduction catalyst is When poison recovery is to be performed, exhaust gas having a rich air-fuel ratio is exhausted from the first cylinder group, and exhaust gas having a lean air-fuel ratio is exhausted from the second cylinder group. After mixing with the air-fuel ratio exhaust gas in the common exhaust pipe, it reacts in the NOx occlusion reduction catalyst, and the sulfur compound is released from the NOx occlusion reduction catalyst by the reaction heat obtained according to the air-fuel ratio difference of these exhaust gases. In the exhaust purification device that controls the temperature rise to raise the sulfur compound release temperature, when the NOx storage reduction catalyst is determined not to raise the temperature to the sulfur compound release temperature within a predetermined time, the air-fuel ratio difference is increased. An exhaust emission control device for an internal combustion engine that performs an increase control to increase an air-fuel ratio difference is provided. Here, in the present invention, the sulfur compound includes, for example, SOx and H ₂ S.

  According to a second aspect of the present invention, in the first aspect of the present invention, the temperature increase control includes a rich-side guard value that limits a rich-side lower limit value of the air-fuel ratio, and the air-fuel ratio difference An exhaust purification device for an internal combustion engine is provided in which the increase control is a control for changing the rich-side guard value to be richer and making the air-fuel ratio of exhaust gas discharged from the first cylinder group richer.

  According to a third aspect of the present invention, in the first or second aspect of the present invention, the temperature increase control includes a lean-side guard value that limits a lean-side upper limit value of the air-fuel ratio, and There is provided an exhaust gas purification apparatus for an internal combustion engine in which the fuel ratio difference increasing control is a control for changing the lean side guard value to be leaner and making the air-fuel ratio of the exhaust gas discharged from the second cylinder group leaner. The

  According to a fourth aspect of the present invention, in the first aspect of the present invention, the temperature increase control includes a rich-side guard value and a lean-side upper limit value that limit the rich-side lower limit value of the air-fuel ratio. A lean side guard value to be limited, and when the air-fuel ratio difference increase control does not change the guard value and does not perform the air-fuel ratio difference increase control from the first cylinder group, the second cylinder group The exhaust gas having a leaner air-fuel ratio than the exhaust gas discharged from the exhaust gas is discharged, and the second cylinder group is richer than the exhaust gas discharged from the first cylinder group when the air-fuel ratio difference increase control is not performed. An exhaust gas purification apparatus for an internal combustion engine, which is control for exhausting air-fuel ratio exhaust gas, is provided.

  Further, according to the invention described in claim 5, in the invention described in any one of claims 1 to 4, the difference between the estimated catalyst temperature estimated from the air-fuel ratio difference and the sulfur compound release temperature is determined in advance. An internal combustion engine exhaust gas purification apparatus that determines that the NOx occlusion reduction catalyst does not rise to a sulfur compound release temperature within the predetermined time when a state smaller than the predetermined value continues for a predetermined time or longer. Provided.

  According to the invention described in each claim, when the sulfur poisoning recovery control is executed, there is a common effect that the catalyst temperature of the NOx occlusion reduction catalyst can be more reliably raised to the sulfur compound release temperature. .

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an internal combustion engine equipped with the exhaust emission control device of the present invention. In FIG. 1, 1 indicates a main body of an internal combustion engine, and # 1 to # 4 indicate a first cylinder, a second cylinder, a third cylinder, and a fourth cylinder, respectively. Each cylinder is provided with fuel injection valves 21, 22, 23, 24 correspondingly. Each cylinder is connected to an intake pipe 4 via a corresponding intake branch pipe 3. Further, a first exhaust branch pipe 5 is connected to the first cylinder and the fourth cylinder, and a second exhaust branch pipe 6 is connected to the second cylinder and the third cylinder. That is, when the first cylinder and the fourth cylinder are collectively referred to as a first cylinder group, and the second cylinder and the third cylinder are collectively referred to as a second cylinder group, the first cylinder group includes: A first exhaust branch pipe 5 is connected, and a second exhaust branch pipe 6 is connected to the second cylinder group. And these exhaust branch pipes 5 and 6 merge in the downstream, and are connected to the common exhaust pipe 7 which is one common exhaust pipe.

  The first exhaust branch pipe 5 is one exhaust branch pipe on the downstream side, but is branched into two on the upstream side, and the exhaust branch pipes branched into these two are the first cylinder and the first cylinder, respectively. It is connected to 4 cylinders. Similarly, the second exhaust branch pipe 6 is also one exhaust branch pipe on the downstream side, but is branched into two on the upstream side, and the two exhaust branch pipes branched into the second cylinder and the second branch branch pipe, respectively. Connected to the third cylinder.

  Three-way catalysts 8 and 9 are disposed in the gathering portions of the exhaust branch pipes 5 and 6, respectively, and a NOx storage reduction catalyst 10 is disposed in the exhaust pipe 7. In addition, oxygen sensors 11 and 12 are arranged at the collection portions of the exhaust branch pipes 5 and 6 upstream of the three-way catalysts 5 and 6, respectively. An air-fuel ratio sensor 13 and an oxygen sensor 14 are disposed in the exhaust pipe upstream and downstream of the NOx storage reduction catalyst 10, respectively. Furthermore, the NOx storage reduction catalyst 10 is provided with a temperature sensor 15 for detecting the catalyst temperature. An air flow meter 16 and a throttle valve 17 for detecting the intake air flow rate are arranged upstream of the intake pipe 4.

  The electronic control unit (ECU) 30 is a digital computer and includes a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, a CPU (Microprocessor) 34, an input port 35, and the like connected to each other by a bidirectional bus 31. An output port 36 is provided. As shown in FIG. 2, the air-fuel ratio sensor 13 generates an output voltage substantially proportional to the air-fuel ratio of the exhaust gas based on the oxygen concentration in the exhaust gas passing through the exhaust branch pipe or the common exhaust pipe. On the other hand, as shown in FIG. 3, the oxygen sensors 11, 12, and 14 are exhaust gas passing through the exhaust pipe downstream of the NOx storage reduction catalyst 10, that is, oxygen in the exhaust gas after passing through the NOx storage reduction catalyst 10. Based on the concentration, an output voltage that varies greatly depending on whether the air-fuel ratio of the exhaust gas is richer or leaner than the stoichiometric air-fuel ratio (about 14.7). These output voltages are input to the input port 35 via the corresponding AD converter 37. The output signals of the temperature sensor 15 and the air flow meter 16 are input to the input port 35 via the corresponding AD converters 37, respectively.

  A load sensor 40 that generates an output voltage proportional to the amount of depression 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. Further, the input port 35 is connected to a crank angle sensor 41 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 valves 21, 22, 23, and 24 and a step motor (not shown) for driving the throttle valve 17 through a corresponding drive circuit 38.

  The three-way catalysts 8 and 9 have a temperature equal to or higher than a certain temperature (so-called activation temperature) and the air-fuel ratio of the exhaust gas flowing into the three-way catalysts 8 and 9 is close to the theoretical air-fuel ratio. Nitrogen oxide (NOx), carbon monoxide (CO), and hydrocarbon (HC) are simultaneously purified at a high purification rate. On the other hand, when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is leaner than the stoichiometric air-fuel ratio, the three-way catalyst absorbs oxygen in the exhaust gas, and the air-fuel ratio of the exhaust gas flowing into the exhaust gas is greater than the stoichiometric air-fuel ratio. When it is also rich, it has an oxygen absorption / release capability of releasing absorbed oxygen. As long as this oxygen absorption / release capability functions normally, the air / fuel ratio of the exhaust gas flowing in is leaner or richer than the stoichiometric air / fuel ratio. Therefore, NOx, CO, and HC in the exhaust gas are simultaneously purified with a high purification rate.

  The NOx occlusion reduction catalyst 10 is an exhaust gas that occludes and flows in NOx contained in the exhaust gas when the temperature is equal to or higher than a certain temperature (so-called activation temperature) and the air-fuel ratio of the inflowing exhaust gas is lean. When the air-fuel ratio of the gas becomes the stoichiometric air-fuel ratio or rich, the stored NOx is reduced and purified.

By the way, as described above, if the exhaust gas contains a sulfur compound (for example, SOx, H ₂ S, etc.) under the condition in which NOx is retained in the NOx storage reduction catalyst 10, this sulfur compound is also stored in the NOx. It is occluded by the reduction catalyst 10. When a sulfur compound is occluded in the NOx occlusion reduction catalyst 10, the amount of NOx that can be occluded by the NOx occlusion reduction catalyst 10 decreases accordingly. For this reason, in order to maintain the NOx storage capacity of the NOx storage reduction catalyst 10 as high as possible, it is necessary to release the sulfur compound from the NOx storage reduction catalyst 10. Here, in the state where the temperature of the NOx occlusion reduction catalyst 10 is set to the minimum necessary temperature for releasing the sulfur compound from the NOx occlusion reduction catalyst 10, ie, the sulfur compound release temperature, the NOx occlusion reduction catalyst 10 is theoretically empty. If exhaust gas having a fuel ratio or rich (preferably, very close to the stoichiometric air-fuel ratio) is supplied, the sulfur compound can be released from the NOx storage reduction catalyst 10.

  Therefore, when the sulfur compound is to be released from the NOx storage reduction catalyst 10, in this embodiment, the sulfur poisoning recovery control described below is executed, thereby changing the catalyst temperature of the NOx storage reduction catalyst 10 to the sulfur compound release temperature. At the same time, exhaust gas having a stoichiometric air-fuel ratio or rich air-fuel ratio is supplied to the NOx storage reduction catalyst 10. Here, the time when the sulfur compound should be released from the NOx storage reduction catalyst 10 refers to the time when the estimated sulfur compound amount ΣS stored in the NOx storage reduction catalyst 10 exceeds a predetermined value MAX.

  Sulfur is contained in the fuel at a certain ratio. Therefore, the amount of sulfur compound contained in the exhaust gas, that is, the amount of sulfur compound stored in the NOx storage reduction catalyst 10 is proportional to the fuel injection amount. The fuel injection amount is a function of the required torque and the engine speed. Therefore, the amount of sulfur compound stored in the NOx storage reduction catalyst 10 is also a function of the required torque and the engine speed. Therefore, the amount of sulfur compounds stored in the NOx storage reduction catalyst 10 per unit time is obtained in advance by experiments as a function of the required torque and engine speed, and stored in the ROM 32 as a map or calculation formula. By integrating the amount of sulfur compound per unit time calculated based on this map or calculation formula, the estimated amount of sulfur compound ΣS stored in the NOx storage reduction catalyst 10 can be calculated.

  During execution of the sulfur poisoning recovery control according to the present embodiment, rich air-fuel ratio exhaust gas (hereinafter referred to as “rich exhaust gas”) from the first cylinder # 1 and the fourth cylinder # 4 (that is, the first cylinder group). Each cylinder is discharged so that exhaust gas having a lean air-fuel ratio (hereinafter referred to as “lean exhaust gas”) is discharged from the second cylinder # 2 and the third cylinder # 3 (that is, the second cylinder group). The air-fuel ratio of the air-fuel mixture to be filled is controlled.

  That is, during execution of the sulfur poisoning recovery control, the air-fuel ratio of the exhaust gas discharged in the first cylinder group is set to the rich air-fuel ratio, and the exhaust gas discharged in the second cylinder group is set to the lean air-fuel ratio. (Hereinafter referred to as “sulfur poisoning recovery air-fuel ratio control”). At this time, the rich degree of the rich exhaust gas from the first cylinder group and the lean degree of the lean exhaust gas from the second cylinder group are determined based on whether the rich exhaust gas and the lean exhaust gas discharged from each cylinder are NOx. Based on the output of the air-fuel ratio sensor 13 so that the air-fuel ratio of the total exhaust gas becomes the stoichiometric air-fuel ratio or the desired rich air-fuel ratio when mixed upstream of the storage-reduction catalyst 10 and flows into the NOx storage-reduction catalyst 10. Controlled. As a result, the exhaust gas having a stoichiometric air-fuel ratio or a rich air-fuel ratio is supplied to the NOx storage reduction catalyst 10.

  When the rich exhaust gas and the lean exhaust gas are mixed, the fuel in the rich exhaust gas reacts with the oxygen in the lean exhaust gas in the NOx storage reduction catalyst 10 to generate reaction heat. As described above, this reaction heat depends on the air-fuel ratio difference between the rich exhaust gas and the lean exhaust gas. That is, when the total air-fuel ratio of the exhaust gas when mixed upstream of the NOx storage reduction catalyst 10 is controlled to the stoichiometric air-fuel ratio, the larger the air-fuel ratio difference between the rich exhaust gas and the lean exhaust gas, the more fuel that can be reacted And the amount of oxygen increases. Therefore, a large reaction heat can be obtained. Hereinafter, the air-fuel ratio difference between the rich exhaust gas and the lean exhaust gas discharged from each cylinder group when the total air-fuel ratio is controlled to the stoichiometric air-fuel ratio is simply referred to as “air-fuel ratio difference”.

  Therefore, in the sulfur poisoning recovery control, the air-fuel ratio difference, that is, the reactable fuel amount and oxygen amount is adjusted according to the target sulfur compound release temperature. Specifically, the relationship between the air-fuel ratio difference and the catalyst temperature of the NOx storage reduction catalyst 10 is obtained in advance as a map or a calculation formula by experiment or calculation, and is stored in the ROM 32. Based on the map or calculation formula, the magnitude of the air-fuel ratio difference is controlled so that the catalyst temperature of the NOx storage reduction catalyst 10 becomes the sulfur compound release temperature. When the temperature of the NOx storage reduction catalyst 10 becomes the sulfur compound release temperature and the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 10 becomes the stoichiometric air-fuel ratio or the rich air-fuel ratio, the NOx storage reduction catalyst 10 releases the sulfur compound. Will be released.

  When the sulfur poisoning recovery control is not executed, the air-fuel ratio of the exhaust gas discharged from each cylinder group is controlled by the normal air-fuel ratio control such that the air-fuel ratio becomes the stoichiometric air-fuel ratio. Accordingly, the exhaust gas mixed in the common exhaust pipe and flowing into the NOx storage reduction catalyst 10 has almost no reactive fuel and oxygen, and no reaction heat is generated in the NOx storage reduction catalyst 10. Therefore, since the catalyst temperature of the NOx storage reduction catalyst 10 does not rise to the sulfur compound release temperature, the sulfur compound is not released.

First, the normal air-fuel ratio control of this embodiment will be described more specifically. In the present embodiment, during normal operation other than during execution of sulfur poisoning recovery control described later, control is performed so that the air-fuel ratio of the exhaust gas discharged from each cylinder group becomes the stoichiometric air-fuel ratio. The reference valve opening time (hereinafter referred to as “reference valve opening time”) for determining the air-fuel ratio of the exhaust gas as the stoichiometric air-fuel ratio is determined by equation (1). Here, α is a constant, Ga is the intake air amount (the amount of air taken into the cylinder), and Ne is the engine speed. That is, according to the present embodiment, the reference valve opening time becomes longer as the intake air amount per unit engine speed is larger.
TAUB = α × Ga / Ne (1)

Then, the actual valve opening time (hereinafter referred to as “actual valve opening time”) TAU of the fuel injection valve is calculated according to the equation (2). β is a constant determined according to the engine operating state.
TAU = TAUB × β (2)

  Thus, according to the present embodiment, during normal operation, the air-fuel ratio of the exhaust gas discharged from each cylinder group is maintained at the stoichiometric air-fuel ratio.

Next, during execution of the sulfur poisoning recovery control of the present embodiment, the air-fuel ratio of the exhaust gas discharged from one cylinder group is set to the rich air-fuel ratio, and the air-fuel ratio of the exhaust gas discharged from the other cylinder group is set to the lean air-fuel ratio. The sulfur poisoning recovery air-fuel ratio control for making the air-fuel ratio will be described more specifically. In the present embodiment, the reference valve opening time is determined by equation (3). This equation (3) is the same as the above equation (1), α is a constant, Ga is the intake air amount, and Ne is the engine speed.
TAUB = α × Ga / Ne (3)

Then, the actual valve opening time (hereinafter referred to as “lean actual valve opening time”) TAUL in the cylinder that burns so as to discharge lean exhaust gas is calculated according to the equation (4), and the cylinder that burns so as to discharge rich exhaust gas The actual valve opening time (hereinafter referred to as “rich actual valve opening time”) TAUR is calculated according to the equation (5).
TAUR = TAUB × β × KR (4)
TAUL = TAUB × β × KL (5)

Here, KR is a rich correction coefficient (KR ≧ 1) for extending the reference valve opening time so that the fuel injection amount increases, and KL shortens the reference valve opening time so that the fuel injection amount decreases. Therefore, the lean correction coefficient (0 <KL ≦ 1). Since the rich correction coefficient KR and the lean correction coefficient KL are controlled so that the total air-fuel ratio of the exhaust gas becomes the stoichiometric air-fuel ratio, Expressions (6) and (7) are obtained using the variable s. β is a constant determined according to the engine operating state.
KR = 1 + s (6)
KL = 1-s (7)

Here, the expressions (4) and (5) are expressed by the rich correction coefficient KR in the actual valve opening time TAU for setting the air-fuel ratio of the exhaust gas discharged from each cylinder group shown in the expression (2) to the stoichiometric air-fuel ratio. Or it is multiplied by the lean correction coefficient KL. Accordingly, the air-fuel ratio AFR of the rich exhaust gas and the air-fuel ratio AFL of the lean exhaust gas are calculated according to the equations (8) and (9) using the theoretical air-fuel ratio AF0. Therefore, the air-fuel ratio difference ΔAF is calculated according to the equation (10) from the equations (8) and (9).
AFR = AF0 / KR (8)
AFL = AF0 / KL (9)
ΔAF = AF0 × (1 / KL−1 / KR) (10)

  Therefore, the map or calculation formula showing the relationship between the air-fuel ratio difference ΔAF and the catalyst temperature of the NOx storage reduction catalyst 10 can be expressed as a function of the rich correction coefficient KR and the lean correction coefficient KL. That is, based on the target catalyst temperature, the corresponding air-fuel ratio difference ΔAF is calculated from the map or the calculation formula, and the rich correction coefficient KR and the lean correction coefficient KL are calculated from the formulas (6), (7), and (10). It is possible to calculate. Accordingly, the rich actual valve opening time TAUR and the lean actual valve opening time TAUL are determined by the equations (4) and (5).

  In summary, when the target catalyst temperature (for example, the sulfur compound release temperature) of the NOx storage reduction catalyst 10 is determined, the air-fuel ratio difference corresponding to the target catalyst temperature is calculated, and should finally be controlled based on the air-fuel ratio difference. The actual valve opening time can be calculated.

  By the way, as described above, the fuel amount in the rich exhaust gas is less than the target due to the variation or error in the fuel injection amount due to the manufacturing error of the fuel injection valve or the deformation due to aging, etc. The amount of fuel may be higher than the target. In this case, there is a problem that the target air-fuel ratio difference cannot be obtained, and the catalyst temperature of the NOx storage reduction catalyst 10 cannot be raised to the sulfur compound release temperature, or it takes time to raise the temperature. .

  In this regard, in the configuration of the internal combustion engine according to the present invention as shown in FIG. 1, the actual air-fuel ratio difference and the actual air-fuel ratio of the exhaust gas discharged from each cylinder group cannot be directly measured. That is, the oxygen sensors 11 and 12 are disposed upstream of the three-way catalysts 8 and 9, but the oxygen sensor is richer than the stoichiometric air-fuel ratio as is apparent from the output characteristics shown in FIG. Whether it is lean or not is only determined accurately, and it is difficult to measure the richness or the leanness in detail. Therefore, in the configuration of the internal combustion engine according to the present invention shown in FIG. 1, the air-fuel ratio of the cylinder group having variations or errors in the fuel injection amount is determined from the outputs of the oxygen sensors 11 and 12, and the fuel injection amount is corrected. It is difficult to do.

  Further, if the air-fuel ratio sensor 13 having the characteristics shown in FIG. 2 is used, the air-fuel ratio can be measured accurately. However, since the air-fuel ratio sensor 13 is measured after the exhaust gas from each cylinder group is mixed, the air-fuel ratio of the exhaust gas from each cylinder group cannot be determined. Even if the air-fuel ratio sensor having the output characteristics shown in FIG. 2 is arranged on the upstream side of the three-way catalysts 8 and 9 instead of the oxygen sensors 11 and 12, if the air-fuel ratio sensor fails, it is still an accurate air-fuel ratio. Can not be measured.

  Therefore, in the present invention, the actual air-fuel ratio difference is estimated from the map or the calculation formula based on the actual catalyst temperature detected by the temperature sensor 15.

  This will be described in detail with reference to FIG. FIG. 4 shows the air-fuel ratio A / F, the catalyst temperature T of the NOx storage reduction catalyst 10, the valve opening time of the first cylinder group (hereinafter referred to as “first cylinder group valve opening time”) TAU1, and the second cylinder group. 4 is a time chart of a sulfur poisoning recovery control operation showing a relationship with a valve opening time (hereinafter referred to as “second cylinder group valve opening time”) TAU2. The sulfur poisoning recovery control is executed from time t0 to time t1. Regarding the air-fuel ratio A / F, the solid line indicates the air-fuel ratio AF1 of the exhaust gas discharged from the first cylinder group, and the broken line indicates the air exhaust gas exhausted from the second cylinder group. Represents the fuel ratio AF2. Note that the air-fuel ratios AF1 and AF2 cannot actually be measured as described above.

  A broken line indicated by GL on the lean side indicates a lean side guard value for restricting the setting of the target value of the air-fuel ratio so as not to further lean to prevent misfire, and a broken line indicated by GR on the rich side Indicates a rich-side guard value for limiting the setting of the rich-side target value so as not to become rich any more in order to prevent exhaust emission from deteriorating or misfire. Therefore, the target value cannot be set beyond these. That is, it is impossible to set the rich actual valve opening time TAUR and the lean actual valve opening time TAUL that are assumed to be the air-fuel ratio exceeding the guard value.

  First, assuming that the sulfur compound release temperature for releasing the sulfur compound is the target catalyst temperature TGT, the target air-fuel ratio difference ΔAFT is calculated from the target catalyst temperature TGT by a map or a calculation formula. As described above, the rich correction coefficient KR and the lean correction coefficient KL are calculated based on the target air-fuel ratio difference ΔAFT, and the rich actual valve opening time TAUR and the lean actual valve opening time TAUL to be controlled are calculated. In the present embodiment, the rich actual valve opening time TAU1 is set as the first cylinder group valve opening time TAU1, and the lean actual valve opening time TAUL is set as the second cylinder group valve opening time TAU2.

  As described above, the fuel amount exactly as determined in advance is accurately determined based on the rich actual valve opening time TAUR of the first cylinder group valve opening time TAU1 and the lean actual valve opening time TAUL of the second cylinder group valve opening time TAU2. In the case of injection, that is, when the air-fuel ratio difference ΔAF is accurately controlled to the target air-fuel ratio difference ΔAFT, the estimated catalyst temperature TMT of the catalyst temperature T estimated from the air-fuel ratio difference ΔAF by a map or a calculation formula is the target catalyst temperature. It becomes TGT.

  However, if the fuel amount in the rich exhaust gas is less than the target due to variations or errors in the fuel injection amount of the fuel injection valve, and the fuel amount in the lean exhaust gas becomes larger than the target, FIG. 4 shows. Thus, the actual air-fuel ratio difference (hereinafter referred to as “actual air-fuel ratio difference”) ΔAFR is smaller than the target air-fuel ratio difference ΔAFT. This is because the amount of fuel injected from the fuel injection valve of the first cylinder group that discharges the rich exhaust gas is insufficient, or is discharged from the fuel injection valve of the second cylinder group that discharges the lean exhaust gas. Either the fuel amount is high or both are conceivable. Then, as shown by a solid line in FIG. 4, the actual catalyst temperature (hereinafter referred to as “actual catalyst temperature”) TMR does not rise to the target temperature TGT, and therefore, the sulfur compound cannot be released.

  Here, if the cylinder group with variations in the fuel injection amount is not known, the exhaust gas discharged from the first cylinder group is made richer and the exhaust gas discharged from the second cylinder group. It is conceivable to perform control so that the air-fuel ratio becomes a leaner air-fuel ratio and the target air-fuel ratio difference ΔAFT itself becomes larger. However, due to the rich side guard value GR and the lean side guard value GL, the rich actual valve opening time TAUR and the lean actual valve opening time TAUL may not be changed so as to increase the target air-fuel ratio difference ΔAFT. .

  In addition, it is considered that variations and errors in the fuel injection amount from the fuel injection valve always show the same tendency. That is, a fuel injector that tends to inject more fuel than the target always injects more fuel than the target, and a fuel injector that tends to inject fuel less than the target always However, it is thought that fuel will be injected to a lesser extent.

  Therefore, according to the first embodiment of the present invention, if the catalyst temperature T cannot be raised to the sulfur compound release temperature even after performing the sulfur poisoning recovery control, the guard value is changed, In order to further increase the target air-fuel ratio difference ΔAFT, the rich actual valve opening time TAUR and the lean actual valve opening time TAUL can be changed.

  The change of the guard value will be described with reference to FIG. FIG. 5, like FIG. 4, shows the relationship between the air-fuel ratio A / F, the catalyst temperature T of the NOx storage reduction catalyst 10 and the first cylinder group valve opening time TAU1 and the second cylinder group valve opening time TAU2. The time chart of poison recovery control operation is shown.

  According to the present embodiment, first, the rich side guard value GR is changed to the rich side, and the lean side guard value GL is changed to the lean side. By doing so, it is possible to change the rich actual valve opening time TAUR to be longer and become a richer air-fuel ratio, and the lean actual valve-opening time TAUL to be shorter and become a leaner air-fuel ratio. Thus, the air-fuel ratio difference ΔAF can be further increased.

  More specifically, although the air-fuel ratio difference ΔAF is currently controlled to become the target air-fuel ratio difference ΔAFT, the target air-fuel ratio difference ΔAFT has not been reached due to variations in fuel injection valves and the like. Therefore, the actual catalyst temperature TMR does not reach the target catalyst temperature TGT. Therefore, it is necessary to change the target air-fuel ratio ΔAFT to a larger value, and for that purpose, it is necessary to change the rich side guard value GR and the lean side guard value GL.

Here, the rich-side guard value GR and the lean-side guard value GL before the change are expressed by Expression (11) and Expression (12), respectively, using the variable g. For example, when the relationship between the above-described air-fuel ratio difference and the catalyst temperature is a proportional relationship in the map or calculation formula, the changed target air-fuel ratio ΔAFT, rich-side guard value GRR, and lean-side guard value GLL are the target catalyst temperature. The calculation is performed according to the equations (13) to (15) with the ratio of the TGT and the actual catalyst temperature TMR as a coefficient.
GR = AF0 × (1-g) (11)
GL = AF0 × (1 + g) (12)
ΔAFT = TMT / TMR × ΔAFT (13)
GRR = AF0 × (1-TMT / TMR × g) (14)
GLL = AF0 × (1 + TMT / TMR × g) (15)

  The rich actual valve opening time TAURR and the lean actual valve opening time TAULL after the change are based on the rich correction coefficient KR or the lean correction based on the above formulas (6), (7), (10), and (13). After obtaining the coefficient KL, the coefficient KL is calculated according to the equations (4) and (5).

  If the target air-fuel ratio difference ΔAFT is set to the maximum within the limit of the current guard value and the catalyst temperature T can be raised to the sulfur compound release temperature, the change of the guard value according to the present embodiment is as follows. There is no need to do it. Further, although the common coefficient TMT / TMR is used for calculation of the guard value after the change, it may be calculated by multiplying each value by a common or individual predetermined constant. It may be calculated based on the following formula.

  FIG. 6 is a flowchart of a sulfur poisoning recovery control operation for releasing sulfur compounds from the NOx storage reduction catalyst 10 according to the first embodiment and performing sulfur poisoning recovery. This operation is performed as a routine executed by interruption every predetermined time set in advance by the electronic control unit (ECU) 30.

  Referring to FIG. 6, first, at step 101, it is determined whether or not the estimated sulfur compound amount ΣS stored in the NOx storage reduction catalyst 10 is greater than or equal to the allowable amount MAX. Here, if the estimated sulfur compound amount ΣS is less than the allowable amount MAX, the routine is terminated without performing sulfur poisoning recovery, assuming that the sulfur compound amount is still within the allowable range. In step 101, if the estimated sulfur compound amount ΣS is equal to or greater than the allowable amount MAX, the routine proceeds to step 102 in order to recover sulfur poisoning.

  Next, at step 102, in order to raise the catalyst temperature T for sulfur poisoning recovery, the target air-fuel ratio difference ΔAFT obtained from the map or the calculation formula according to the target catalyst temperature TGT, formulas (4) and (5). Accordingly, the rich actual valve opening time TAUR is set to the first cylinder group valve opening time TAU1, and the rich actual valve opening time TAUL is set to the second cylinder group valve opening time TAU2. Thereafter, the process proceeds to step 103.

  Next, at step 103, the estimated catalyst temperature TMT is calculated from a map or a calculation formula, and the routine proceeds to step 104. Next, at step 104, a temperature difference Δt that is the difference between the target catalyst temperature TGT and the estimated catalyst temperature TMT calculated at step 103 is calculated, and the routine proceeds to step 105.

  Next, at step 105, it is determined whether or not the temperature difference Δt calculated at step 104 is less than a predetermined allowable temperature tsh. That is, in step 105, when the temperature difference Δt is equal to or greater than the allowable temperature tsh, it is considered that the catalyst temperature T has almost reached the target catalyst temperature TGT or soon. In this case, the routine proceeds to step 110 where a sulfur compound releasing process for releasing the sulfur compound is performed. On the other hand, when the temperature difference Δt is less than the allowable temperature tsh, the temperature difference Δt is set in step 102 until the temperature difference Δt becomes equal to or higher than the allowable temperature tsh or until a predetermined time elapses as described in the next step 106. The temperature is raised based on the actual valve opening time. Therefore, the routine proceeds to step 106.

  In step 106, it is determined whether or not the time counter CNT is greater than or equal to the allowable value CNTsh. Here, when step 106 is less than the allowable value CNTsh, it is determined that the predetermined time has not yet passed in a state where the temperature difference Δt is less than the allowable temperature tsh, and the process proceeds to step 107 to maintain the current state. Next, at step 107, the time counter is incremented and the routine proceeds to step 103. Thereafter, the above process is repeated until the temperature difference Δt becomes equal to or higher than the allowable temperature tsh or until a predetermined time elapses in Step 106.

  On the other hand, if the time counter CNT is greater than or equal to the allowable value CNTsh in step 106, it means that a predetermined time has elapsed with the temperature difference Δt being less than the allowable temperature tsh, and the fuel injection amount of the fuel injection valve varies. Thus, it is estimated that the catalyst temperature T cannot be raised to the target catalyst temperature TGT. Accordingly, in this case, the process proceeds to step 108.

  Next, at step 108, the rich-side guard value GRR and the lean-side guard value GLL after the change are calculated using, for example, the above-described equations (14) and (15) or the other methods. Then, the rich side guard value GRR after the change is set to the rich side guard value GR and the lean side guard value GLL after the change is set to the lean side guard value GL. As a result, the target air-fuel ratio difference ΔAFT can be set more widely without being limited to the guard value.

  Next, at step 109, the changed rich actual valve opening time TAURR and the lean actual valve opening time TAULL are calculated based on the changed target air-fuel ratio difference ΔAFT calculated by, for example, the equation (13) or other methods. Then, the changed rich actual valve opening time TAURR is set as the first cylinder group valve opening time TAU1, the changed rich actual valve opening time TAURL is set as the second cylinder group valve opening time TAU2, and the routine proceeds to step 110. move on. As a result, the catalyst temperature T can be reliably raised to the target catalyst temperature TGT.

  Next, in step 110, the catalyst temperature T is raised to the target catalyst temperature TGT, that is, the sulfur compound release temperature by the processing in step 109, and therefore the estimated amount of stored sulfur compound ΣS is estimated to be zero, The sulfur compound releasing process is executed, and then the process proceeds to step 111.

  Next, at step 111, the estimated sulfur compound amount ΣS and the time counter CNT are reset to zero, the guard value is returned to the original value, a reset process is performed to return the air-fuel ratio control to the normal air-fuel ratio control, and the routine ends. To do.

  In the present embodiment, both the rich side guard value GRR and the lean side guard value GLL are changed, but the amount of fuel injected from the fuel injection valve of the first cylinder group that discharges the rich exhaust gas is sufficient. If it is known that the amount of fuel discharged from the fuel injection valve of the second cylinder group that discharges the lean exhaust gas is large, only one of the rich side guard value GRR or the lean side guard value GLL May be changed.

  According to the second embodiment of the present invention, even if the sulfur poisoning recovery control is executed, if the catalyst temperature T cannot be raised to the sulfur compound release temperature, it is discharged from each cylinder group. Control is performed to change the air-fuel ratio of the exhaust gas. That is, at present, the control is performed so that the rich exhaust gas is discharged from the first cylinder group and the lean exhaust gas is discharged from the second cylinder group. From the second cylinder group, control is performed so as to discharge the rich exhaust gas so that the lean exhaust gas is discharged from the second cylinder group.

  As described above, the temperature cannot be raised to the sulfur compound release temperature, that is, the target air-fuel ratio difference ΔAFT cannot be obtained, the amount of fuel in the rich exhaust gas is less than the target, and the lean exhaust gas The amount of fuel inside has exceeded the target. The variations and errors in the fuel injection amount from the fuel injection valve that cause it are considered to always show the same tendency. Therefore, by changing the air-fuel ratio of the exhaust gas discharged from each cylinder group, the fuel injection amount The influence of variations and errors can be eliminated.

  That is, the fuel injection valve of the first cylinder group that discharges the rich exhaust gas currently has a tendency to inject a fuel amount smaller than the target, and the second cylinder group that discharges the lean exhaust gas. This is a problem because there is a variation in the tendency of the fuel injection valves to inject a fuel amount larger than the target. Therefore, if the lean exhaust gas is discharged from the first cylinder group by switching the air-fuel ratio, the fuel amount smaller than the target is injected from the tendency, and the rich amount is discharged from the second cylinder group. If the exhaust gas is discharged, a fuel amount larger than the target is injected from the tendency. As a result, since the air-fuel ratio difference ΔAF becomes large as a whole, the purpose of raising the temperature to the sulfur compound release temperature is achieved.

  FIG. 7, like FIG. 4, shows the relationship between the air-fuel ratio A / F, the catalyst temperature T of the NOx storage reduction catalyst 10, the first cylinder group valve opening time TAU1, and the second cylinder group valve opening time TAU2. The time chart of poison recovery control operation is shown. According to FIG. 7, the tendency of the first cylinder group valve opening time TAU1 and the second cylinder group valve opening time TAU2 is reversed as compared with FIG. Lean exhaust gas is discharged from the cylinder group, and rich exhaust gas is discharged from the second cylinder group. Thus, the catalyst temperature T is raised to the target catalyst temperature TGT by reversing the air-fuel ratio of the exhaust gas to be discharged.

  FIG. 8 is a flowchart of a sulfur poisoning recovery control operation for releasing sulfur compounds from the NOx storage reduction catalyst 10 according to the second embodiment and performing sulfur poisoning recovery. This operation is performed as a routine executed by interruption every predetermined time set in advance by the electronic control unit (ECU) 30.

  Referring to FIG. 8, steps 201 to 207 are the same as corresponding steps 101 to 107 in FIG. Therefore, the description is omitted, and the processing after step 208 will be described.

  The case of proceeding from step 206 to step 208 means that the temperature difference Δt is less than the predetermined allowable temperature tsh determined in step 205 and the time counter CNT is equal to or greater than the allowable value CNTsh in step 206. This is the case when it is considered to have elapsed. In step 208, contrary to the actual valve opening time set in step 202, the lean actual valve opening time TAUL is set at the first cylinder group opening time TAU1, and the rich actual valve opening time is set at the second cylinder group opening time TAU2. TAUR is set and the process proceeds to step 209. As a result, the influence of variations in the fuel injection valves and the like is eliminated as described above, the target air-fuel ratio difference ΔAFT is obtained, and the catalyst temperature T can be raised to the target catalyst temperature TGT.

  Next, at step 209, the catalyst temperature T is raised to the target catalyst temperature TGT, that is, the sulfur compound release temperature by the processing at step 208, and therefore the estimated amount of stored sulfur compound ΣS is estimated to be zero, The sulfur compound releasing process is executed, and then the process proceeds to step 210.

  Next, at step 210, the estimated sulfur compound amount ΣS and the time counter CNT are reset to zero, and further, a reset process for returning the air-fuel ratio control to the normal air-fuel ratio control is performed, and the routine ends.

  In the above-described embodiment, whether or not the catalyst temperature T has substantially reached the target catalyst temperature TGT or soon will be determined using the temperature difference Δt that is the difference between the target catalyst temperature TGT and the estimated catalyst temperature TMT. Although it has been determined, it is also possible to make a determination based on, for example, the rate of temperature rise instead of the temperature difference Δt.

It is a figure of the whole internal combustion engine in which the exhaust emission control device of the present invention is used. It is the figure which showed the relationship between an air fuel ratio and the output voltage of an air fuel ratio sensor. It is the figure which showed the relationship between an air fuel ratio and the output voltage of an oxygen sensor. It is a time chart of sulfur poisoning recovery control operation. It is a time chart of sulfur poisoning recovery control operation. It is a flowchart which shows sulfur poisoning recovery control operation. It is a time chart of sulfur poisoning recovery control operation. It is a flowchart which shows sulfur poisoning recovery control operation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Engine body 4 Intake pipe 5,6 Exhaust branch pipe 8,9 Three way catalyst 10 NOx storage reduction catalyst 21-24 Fuel injection valve # 1- # 4 Cylinder

Claims (5)

  An internal combustion engine having a plurality of cylinders, divided into two cylinder groups, each having an exhaust branch pipe connected to each cylinder group, and joining these exhaust branch pipes on the downstream side to one common exhaust pipe When the NOx occlusion reduction catalyst is disposed in the common exhaust pipe, and the sulfur poisoning recovery of the NOx occlusion reduction catalyst is to be performed, the rich air-fuel ratio is removed from the first cylinder group. The exhaust gas having the lean air-fuel ratio is discharged from the second cylinder group, and the rich air-fuel ratio exhaust gas and the lean air-fuel ratio exhaust gas are mixed in the common exhaust pipe. Exhaust gas is controlled to raise the temperature to the sulfur compound release temperature at which the sulfur compound is released from the NOx storage reduction catalyst by the reaction heat obtained by reacting in the NOx storage reduction catalyst and according to the air-fuel ratio difference of these exhaust gases. In the purifying apparatus, when it is determined that the NOx storage reduction catalyst does not rise to the sulfur compound release temperature within a predetermined time, the exhaust gas purifying apparatus for the internal combustion engine that performs the air-fuel ratio difference increasing control for increasing the air-fuel ratio difference. .   The temperature increase control includes a rich guard value that limits a lower limit value on the rich side of the air-fuel ratio, and the air-fuel ratio difference increase control changes the rich guard value to be richer, and the first cylinder The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the control is performed to make the air-fuel ratio of the exhaust gas discharged from the group richer.   The temperature increase control includes a lean-side guard value that limits the lean-side upper limit value of the air-fuel ratio, and the air-fuel ratio difference increase control changes the lean-side guard value to be leaner, and the second cylinder The exhaust gas purification apparatus for an internal combustion engine according to claim 1 or 2, wherein the control is performed to make the air-fuel ratio of the exhaust gas discharged from the group leaner.   The temperature increase control includes a rich-side guard value that limits a rich-side lower limit value of an air-fuel ratio and a lean-side guard value that limits a lean-side upper limit value, and the air-fuel ratio difference increase control includes these guard values. Without changing the air-fuel ratio, the exhaust gas having a leaner air-fuel ratio than the exhaust gas discharged from the second cylinder group when the air-fuel ratio difference increase control is not performed is discharged from the first cylinder group. The exhaust of the internal combustion engine according to claim 1, wherein the exhaust of the rich air-fuel ratio is exhausted from the cylinder group when exhaust control of the air-fuel ratio is not performed from the cylinder group. Purification equipment.   When the state where the difference between the estimated catalyst temperature estimated from the air-fuel ratio difference and the sulfur compound release temperature is smaller than a predetermined value continues for a predetermined time or more, the NOx is within the predetermined time. The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 4, wherein the storage reduction catalyst determines that the temperature does not rise to the sulfur compound release temperature.

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