JP2005105828A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device of an internal combustion engine, capable of effectively removing a sulfur component held in a NOx holding agent, while restraining the deterioration in fuel consumption. <P>SOLUTION: This exhaust emission control device has the NOx holding agent arranged on an engine exhaust passage, and a sulfur holding quantity estimating means for estimating a sulfur component holding quantity held in the NOx holding agent. Sulfur poisoning recovery control is performed for controlling so as to substantially set the air-fuel ratio of inflow exhaust gas to the NOx holding agent to the stoichiometric air-fuel ratio or rich, and to maintain the temperature of the NOx holding agent in the sulfur separation starting temperature or more by raising the temperature, when the sulfur component holding quantity estimated by the sulfur holding quantity estimating means becomes a predetermined holding quantity or more. The sulfur poisoning recovery control is stopped when idle operation is performed when performing the sulfur poisoning recovery control. The sulfur poisoning recovery control is stopped after a predetermined delay time (t<SB>0</SB>to t<SB>1</SB>) passes after starting the idle operation. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

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

  As a technology for purifying nitrogen oxides (NOx) in exhaust gases of automobiles, etc., when the air-fuel ratio of the exhaust gas flowing in is lean (when the oxygen concentration of the exhaust gas is high), NOx in the exhaust gas is reduced. When the air-fuel ratio of the exhaust gas that is retained and flows in is approximately the stoichiometric air-fuel ratio or rich (when the oxygen concentration is low; hereinafter referred to as “stoichiometric rich”), the retained NOx is released, and at this time the exhaust gas is exhausted. A NOx holding agent that reduces NOx with a reducing agent contained in gas is known.

  On the other hand, the exhaust gas also contains sulfur components such as SOx, and the NOx retention agent is a mechanism similar to the retention mechanism of NOx. When the air-fuel ratio of the inflowing exhaust gas is lean, the SOx in the exhaust gas When the air-fuel ratio of the inflowing exhaust gas is stoichiometric and rich and the temperature of the NOx holding agent becomes equal to or higher than the sulfur release start temperature, the held SOx is released.

  The SOx retention amount and the NOx retention amount of the NOx retention agent are related to each other, and when the SOx retention amount increases, the amount of NOx that can be retained by the NOx retention agent decreases, and the exhaust purification performance of the NOx retention agent decreases ( Sulfur poisoning). Therefore, when the SOx retention amount increases, the air-fuel ratio of the exhaust gas flowing into the NOx retention agent is stoichiometric rich, and the temperature of the NOx retention agent is set to the sulfur desorption start temperature or higher, and is retained by the NOx retention agent. Sulfur poisoning recovery control is performed to release the SOx.

  However, generally, in a diesel internal combustion engine, the air-fuel ratio of the exhaust gas discharged from the internal combustion engine is extremely high. Therefore, in order to make the air-fuel ratio of the exhaust gas flowing into the NOx retention agent stoichiometric rich, the NOx retention agent A large amount of fuel must be added to the exhaust gas by means of a fuel addition device arranged upstream. For this reason, conventionally, by performing the sulfur poisoning recovery control when the engine operating state is at a low rotation speed, from the fuel addition device for making the air-fuel ratio of the exhaust gas flowing into the NOx holding agent stoichiometric rich The amount of fuel added is reduced.

  An exhaust emission control device in which sulfur poisoning recovery control is performed in this way is described in Patent Document 1. In the exhaust emission control device described in Patent Document 1, sulfur poisoning recovery control is executed when the engine operating state is an idle operation or a deceleration operation. Further, in the exhaust gas purification apparatus described in Patent Document 1, a large amount of hydrocarbon (HC) is present in the exhaust pipe upstream of the NOx retention agent because the idling operation state continues for a long time during execution of the sulfur poisoning recovery control. In order to prevent the adhesion, the fuel addition from the fuel addition device to the exhaust pipe is prohibited when the duration of the idle operation exceeds a predetermined time during the sulfur poisoning recovery control.

JP 2002-38932 A JP-A-6-88518 JP 2003-129830 A

  By the way, in the sulfur poisoning recovery control, the temperature of the exhaust gas discharged from the internal combustion engine is increased, and the fuel is added from the fuel addition device provided on the upstream side of the NOx retention agent and burned with the NOx retention agent. Is done by. However, when the sulfur poisoning regeneration control is executed during the idling operation as in the exhaust gas purification device described in Patent Document 1, it is difficult to maintain the temperature of the NOx retention agent at the sulfur desorption start temperature or higher. That is, it is difficult to maintain a high temperature of the exhaust gas discharged from the internal combustion engine because it is in a no-load state during idle operation. Even when fuel is added from the fuel addition device, most of the fuel adheres to the exhaust pipe and remains without burning, making it difficult to raise the temperature of the NOx retention agent. For example, the added fuel retains NOx. Even if it reaches the agent, most of it burns in the downstream portion of the NOx retention agent, so that the temperature distribution is generated in the NOx retention agent and the sulfur component is only partially removed. Therefore, even if the sulfur poisoning recovery control is executed during the idling operation, the sulfur component held in the NOx holding agent cannot be effectively removed.

  On the other hand, if the sulfur poisoning recovery control is not executed during the idling operation, the fuel consumption worsens due to the sulfur poisoning recovery control. That is, since a large amount of fuel is required to raise the temperature of the low-temperature NOx retention agent during normal operation to the sulfur desorption start temperature or higher, it is preferable that the number of times of temperature rise is small. However, if the sulfur poisoning recovery control is not executed during the idle operation, if the idle operation is performed during the sulfur poisoning recovery control, a large amount of sulfur component is still held in the NOx retention agent. Since the sulfur poisoning recovery control is canceled even in the state of being present, the period until the next sulfur poisoning recovery control is shortened, and thus the number of times of raising the temperature of the NOx retention agent as described above is increased. Therefore, the fuel consumption is greatly deteriorated.

  Accordingly, an object of the present invention is to provide an exhaust purification device for an internal combustion engine that can effectively remove the sulfur component held in the NOx holding agent while suppressing deterioration in fuel consumption.

In order to solve the above-mentioned problem, according to the first aspect of the present invention, there is provided an NOx holding agent that is disposed on the engine exhaust passage and purifies NOx in the exhaust gas, and the air-fuel ratio of the inflowing exhaust gas is substantially the stoichiometric air-fuel ratio. Alternatively, when the temperature of the NOx retention agent is higher than the sulfur desorption start temperature, the NOx retention agent that desorbs the retained sulfur component, and the sulfur component retention amount retained in the NOx retention agent are estimated. And a sulfur retention amount estimation means that performs an approximately theoretical calculation of the air-fuel ratio of the exhaust gas flowing into the NOx retention agent when the sulfur component retention amount estimated by the sulfur retention amount estimation means exceeds a predetermined retention amount. Sulfur poisoning recovery control is performed to control the air-fuel ratio or rich so that the temperature of the NOx holding agent is maintained at a temperature higher than the sulfur desorption starting temperature, and idle operation is performed during execution of the sulfur poisoning recovery control. In the exhaust gas purification apparatus for an internal combustion engine in which the sulfur poisoning recovery control is stopped when the operation is performed, the sulfur poisoning recovery control is stopped after a predetermined delay time has elapsed since the start of idle operation. .
As described above, when the idling operation is performed, the temperature of the NOx holding agent is lowered to a temperature lower than the sulfur desorption start temperature. However, due to the residual heat stored in the NOx retention agent, the temperature of the NOx retention agent is maintained at the sulfur desorption start temperature or higher for a certain period after the idling operation is started. According to the first invention, even if the idling operation is started during the execution of the sulfur poisoning recovery control, the sulfur poisoning is continued until the predetermined delay time corresponding to the certain period of time has elapsed after the idling operation is started. Since recovery control is continued, sulfur poisoning recovery control is performed as long as possible, and more sulfur components can be released. That is, when the idling operation is started after the temperature of the NOx retaining agent is once raised to the sulfur desorption starting temperature or more, the NOx retaining agent is stopped until the sulfur poisoning recovery control is stopped due to the start of the idling operation. It is possible to increase the amount of sulfur component released from the. Therefore, the number of executions of sulfur poisoning recovery control, that is, the number of times of temperature increase to the sulfur desorption start temperature can be reduced, and the desorption of sulfur components from the NOx holding agent can be performed efficiently.
The above-mentioned “predetermined delay time” is, for example, the time it takes for the temperature of the NOx retention agent to fall below the sulfur desorption start temperature after the start of idle operation during execution of sulfur poisoning recovery control. It may be a predetermined value, or a value that varies depending on the temperature of the NOx retention agent when the idle operation is started during execution of the sulfur poisoning recovery control or the temperature of the NOx retention agent during the idle operation. Also good. Further, in the present specification, the term “idle operation” is used when the engine load is substantially zero and the driving force generated by the internal combustion engine is not used to drive a vehicle equipped with the internal combustion engine. It means the driving state.

According to a second invention, in the first invention, after the sulfur poisoning recovery control is stopped, the temperature of the NOx holding agent is started to be released from sulfur while the air-fuel ratio of the exhaust gas flowing into the NOx holding agent is made lean. The temperature maintenance control for maintaining the temperature near the temperature is executed, and when the operation is changed from the idle operation to the operation other than the idle operation during the temperature maintenance control, the sulfur poisoning recovery control is executed again.
According to the second invention, when the engine operation is changed from idle operation to operation other than idle operation (hereinafter referred to as “non-idle operation”), the temperature of the NOx retention agent is already in the vicinity of the sulfur desorption start temperature. Since the temperature is higher than the temperature, the sulfur component can be quickly released from the NOx retention agent.
By the way, unlike the second invention, the sulfur poisoning recovery control is executed again when the non-idle operation is started without maintaining the temperature of the NOx holding agent at a high temperature in the state of performing the idle operation. In this case, for example, when the idle operation and the non-idle operation are repeated at a short time interval because the vehicle equipped with the internal combustion engine is traveling in the city, the temperature of the NOx retention agent even if the non-idle operation is started. It takes a long time to raise the temperature of the NOx retention agent above the sulfur desorption start temperature. Furthermore, immediately after the temperature of the NOx holding agent is raised to the sulfur desorption starting temperature or higher, that is, when the idle operation is started again before the sulfur component is actually almost released from the NOx holding agent, the sulfur component is consequently reduced. In some cases, the fuel may be consumed in order to raise the temperature of the NOx retention agent while it cannot be almost separated. On the other hand, according to the second invention, when the engine operation is changed from the idle operation to the non-idle operation, the separation of the sulfur component from the NOx holding agent is started immediately. Even in this case, the sulfur component can be sufficiently released from the NOx retention agent.
For example, when the engine load is high and it is difficult to make the air-fuel ratio of the exhaust gas flowing into the NOx holding agent nearly stoichiometric or rich, it is difficult to remove the sulfur component from the NOx holding agent. Therefore, in such a case, as the sulfur poisoning recovery control, only the NOx retention agent is maintained above the sulfur desorption start temperature.

According to a third invention, in the second invention, when the temperature maintenance control is not changed from an idle operation to an operation other than the idle operation during the temperature maintenance control, the sulfur poisoning recovery control is stopped. After a predetermined waiting time has elapsed.
As described above, a large amount of fuel is required to raise the low-temperature NOx retention agent to the sulfur desorption start temperature when the sulfur poisoning recovery control and the temperature maintenance control are not executed. If the sulfur poisoning recovery control is stopped in a state where the sulfur component remains retained in the NOx retention agent, the interval until the next temperature increase of the NOx retention agent as described above is shortened. As a result, fuel consumption is deteriorated. Therefore, as in the second aspect of the invention, even after the sulfur poisoning recovery control is stopped, the temperature of the NOx retention agent is maintained at a temperature close to the sulfur desorption start temperature, and the engine operation is changed from the idle operation to the non-idle operation. By executing the sulfur poisoning recovery control again when changed, it is possible to reduce the number of times of temperature rise of the NOx retention agent, and thus suppress deterioration in fuel consumption.
However, since the fuel is also consumed in order to maintain the temperature of the NOx retention agent at a temperature near the sulfur desorption start temperature or higher, if the execution time of the temperature maintenance control is long, that is, the sulfur component does not desorb from the NOx retention agent. In addition, if the time for maintaining the temperature of the NOx holding agent at a temperature close to the sulfur desorption start temperature or longer is long, fuel consumption deteriorates.
Here, according to the third aspect of the invention, since the temperature maintenance control execution time is limited to a predetermined standby time, the amount of fuel consumed to maintain the temperature of the NOx retention agent at or above the temperature near the sulfur desorption start temperature. The number of times the temperature of the NOx retention agent is raised can be reduced as much as possible.
Note that the “predetermined standby time” is determined in consideration of a balance between suppression of deterioration in fuel consumption due to reduction in the number of times the temperature of the NOx retention agent is increased and fuel consumption deterioration due to maintenance of the NOx retention agent at a high temperature. This period is set so that the deterioration of the most is suppressed.

According to a fourth invention, in the invention according to any one of the first to third aspects, the sulfur poisoning recovery control includes sucking a part of the exhaust gas discharged from the combustion chamber into the combustion chamber again, This is performed by adding a reducing agent into the exhaust gas from a reducing agent addition device disposed on the exhaust passage upstream of the NOx retention agent.
According to the fourth aspect of the invention, since the reducing agent (for example, fuel) is added into the exhaust gas from the reducing agent addition device, the hydrocarbon (HC) is removed from the exhaust passage when the temperature of the exhaust gas discharged from the internal combustion engine is low. Adhere inside.
However, by combining the fourth invention and the first invention, the period during which the temperature of the exhaust gas discharged from the internal combustion engine is low due to the idling operation ends with a predetermined delay time. The adhesion of HC is also reduced.

According to a fifth aspect of the invention, in the first to fourth aspects of the invention, the temperature maintenance control is slower than when the temperature maintenance control is not performed for the main injection performed near the compression top dead center. Angling and performing a small amount of auxiliary injection before the main injection, performing a small amount of auxiliary injection after the main injection, and exhausting upstream of the NOx retention agent This is performed by at least one of adding a reducing agent into the exhaust gas from a reducing agent addition device disposed on the passage.
According to the fifth aspect of the invention, the temperature of the exhaust gas discharged from the internal combustion engine is raised by performing the retard of the main injection and the auxiliary injection. For this reason, the reducing agent adhering to the exhaust passage due to the temperature of the exhaust gas exhausted from the internal combustion engine when the idling operation is performed can be burned and removed by executing the temperature maintenance control. .

  According to the present invention, since the sulfur poisoning recovery control is continued for a predetermined delay time after the idling operation is started, the sulfur poisoning recovery control is performed as long as possible. As a result, it is possible to reduce the number of executions of sulfur poisoning recovery control, that is, the number of times of temperature increase to the sulfur desorption start temperature, thereby effectively removing the sulfur component held in the NOx holding agent while suppressing fuel consumption deterioration. can do.

  Hereinafter, the particulate matter removing method of the present invention will be described with reference to the drawings. FIG. 1 shows a diesel-type compression self-ignition internal combustion engine in which the particulate matter removing method of the present invention is used. The present invention is also applicable to a spark ignition type internal combustion engine.

  Referring to FIG. 1, 1 is an engine body, 2 is a cylinder block, 3 is a cylinder head, 4 is a piston, 5 is a combustion chamber, 6 is an electrically controlled fuel injection valve, 7 is an intake valve, 8 is an intake port, 9 Is an exhaust valve, and 10 is an exhaust port. The intake port 8 is connected to a surge tank 12 via a corresponding intake branch pipe 11, and the surge tank 12 is connected to a compressor 15 of an exhaust turbocharger 14 via an intake duct 13.

  A throttle valve 17 driven by a step motor 16 is arranged in the intake duct 13, and a cooling device 18 for cooling intake air flowing in the intake duct 13 is arranged around the intake duct 13. In the internal combustion engine shown in FIG. 1, engine cooling water is guided into the cooling device 18 and the intake air is cooled by the engine cooling water. On the other hand, the exhaust port 10 is connected to an exhaust turbine 21 of an exhaust turbocharger 14 via an exhaust manifold 19 and an exhaust pipe 20, and an outlet of the exhaust turbine 21 is a casing containing a NOx holding catalyst 23 described later via an exhaust pipe 22. 24.

  The exhaust manifold 19 and the surge tank 12 are connected to each other via an exhaust gas recirculation (hereinafter referred to as EGR) passage 25, and an electrically controlled EGR control valve 26 is disposed in the EGR passage 25. Around the EGR passage 25, an EGR cooling device 27 for cooling the EGR gas flowing in the EGR passage 25 is disposed. In the internal combustion engine shown in FIG. 1, engine cooling water is guided into the EGR cooling device 27, and the EGR gas is cooled by the engine cooling water.

  On the other hand, each fuel injection valve 6 is connected to a fuel reservoir, so-called common rail 28, through a fuel supply pipe 6a. Fuel is supplied into the common rail 28 from an electrically controlled fuel pump 29 with variable discharge amount, and the fuel supplied into the common rail 28 is supplied to the fuel injection valve 6 through each fuel supply pipe 6a. A fuel pressure sensor 30 for detecting the fuel pressure in the common rail 28 is attached to the common rail 28, and a fuel pump 29 is used so that the fuel pressure in the common rail 28 becomes the target fuel pressure based on the output signal of the fuel pressure sensor 30. The discharge amount is controlled.

  The electronic control unit 40 is composed of a digital computer, and is connected to each other by a bidirectional bus 41. A ROM (read only memory) 42, a RAM (random access memory) 43, a CPU (microprocessor) 44, an input port 45 and an output port 46 are connected. It comprises. The output signal of the fuel pressure sensor 30 is input to the input port 45 via the corresponding AD converter 47.

  A load sensor 50 that generates an output voltage proportional to the amount of depression of the accelerator pedal 49 is connected to the accelerator pedal 49, and the output voltage of the load sensor 50 is input to the input port 45 via the corresponding AD converter 47. Further, the input port 45 is connected to a crank angle sensor 51 that generates an output pulse every time the crankshaft rotates, for example, 30 °. The output port 46 is connected to the fuel injection valve 6, the throttle valve driving step motor 16, the EGR control valve 26, and the fuel pump 29 via a corresponding drive circuit 48. The exhaust pipe 22 is provided with a reducing agent addition device 52 for adding a reducing agent to the exhaust gas flowing into the NOx holding catalyst 23, and the output port 46 is connected to the reducing agent addition device 52 via a corresponding drive circuit 48. Also connected to. In the present embodiment, the reducing agent addition device 52 is hereinafter referred to as a fuel addition device because it adds fuel as a reducing agent to the exhaust gas.

  The NOx retention catalyst 23 described above carries a NOx retention agent described later. The NOx retention agent is an air-fuel ratio (exhaust passage on the upstream side of the NOx retention catalyst 23) of exhaust gas flowing into the NOx retention catalyst 23 (hereinafter referred to as “inflow exhaust gas to the NOx retention agent”) by a mechanism described later. When the ratio of the air and the fuel supplied to the combustion chamber 5 and the intake passage is lean, the NOx in the inflowing exhaust gas is held, and the inflowing exhaust gas to the NOx holding agent is almost stoichiometric or rich (hereinafter, referred to as “ratio”). In the case of "Stoichi Rich"), the held NOx is released. The desorbed NOx reacts with a reducing agent such as HC or CO in the exhaust gas and is reduced and purified.

  In the diesel internal combustion engine used in the present embodiment, the air-fuel ratio of the exhaust gas discharged from the engine body 1 (ratio of air and fuel supplied to the combustion chamber 5 and the intake passage) is basically lean. Normally, NOx in the exhaust gas is held in the NOx holding agent. However, the amount of NOx that can be retained by the NOx retention agent is determined, and if the NOx retention amount of the NOx retention agent increases, the NOx retention agent cannot retain NOx any more. When the amount exceeds a certain amount, rich control is performed to make the air-fuel ratio of the exhaust gas flowing into the NOx holding agent stoichiometrically rich.

  Further, the NOx retaining agent retains a sulfur component (for example, SOx) in the inflowing exhaust gas when the air-fuel ratio of the inflowing exhaust gas flowing into the NOx retaining agent is lean, and the inflowing exhaust gas is stoichiometric rich. When the temperature of the NOx retention agent is equal to or higher than the sulfur desorption start temperature (for example, 600 ° C.), the retained sulfur component is desorbed. When the amount of sulfur component retained in the NOx retention agent (hereinafter referred to as “sulfur component retention amount”) increases, the amount of NOx that can be retained by the NOx retention agent decreases, and the NOx purification performance of the NOx retention agent. Is reduced (sulfur poisoning).

  Therefore, many of the exhaust gas purification apparatuses for internal combustion engines recover sulfur poisoning by removing the sulfur component held in the NOx holding agent when the sulfur component holding amount of the NOx holding agent exceeds a predetermined limit amount. Control is executed. In the sulfur poisoning recovery control, control is performed so that the temperature of the NOx retention agent becomes equal to or higher than the sulfur desorption start temperature and the air-fuel ratio of the exhaust gas flowing into the NOx retention agent becomes stoichiometric rich. The sulfur poisoning recovery control is basically continued until the sulfur component retention amount of the NOx retention agent becomes substantially zero, and thereby the NOx purification ability of the NOx retention agent is maintained in a relatively high state.

  In the present embodiment, as the sulfur poisoning recovery control, a temperature raising process for raising the temperature of the NOx retention agent to the sulfur desorption start temperature or higher, and the NOx retention agent that has already been heated to the sulfur desorption start temperature or higher. A high-temperature rich process is performed in which the air-fuel ratio of the exhaust gas flowing into the NOx holding agent is stoichiometrically rich while maintaining the temperature as it is above the sulfur desorption start temperature. Therefore, at the start of the sulfur poisoning recovery control, the temperature raising process is first performed. After the temperature of the NOx retention agent is raised to the sulfur desorption start temperature or higher by the temperature raising process, the high temperature rich process is performed. During processing, the sulfur component held in the NOx holding agent is released.

  Specifically, in the temperature raising process, the combustion of the engine is so-called retarded combustion. The retarded combustion is a normal control of the main injection from the fuel injection valve 6 performed in the vicinity of the compression top dead center (that is, a control performed at a normal time when control such as sulfur poisoning recovery control and PM removal control described later is not performed). ) Combustion performed at a later angle than the time. Thereby, since the energy converted into the work to a piston is reduced among the thermal energy obtained by combustion, temperature rise of the exhaust gas discharged | emitted from a combustion chamber is achieved. Further, in this case, in order to prevent the combustion from becoming unstable by retarding the main injection, a small amount of auxiliary injection (hereinafter referred to as “pilot injection”) is performed before the main injection. . Further, an injection (hereinafter referred to as “after-injection”) that is started immediately after the end of the main injection or when a relatively short crank angle interval (for example, within 15 ° CA after the end of the main injection) has elapsed is also performed. Also in the combustion by the after injection, the temperature of the exhaust gas discharged from the combustion chamber is raised for the same reason as the retard combustion. Therefore, in the present embodiment, retard combustion, pilot injection, and after injection are performed as the temperature raising process.

  On the other hand, in the high temperature rich process, the combustion of the engine is so-called EGR combustion, and the fuel is added to the exhaust gas flowing into the NOx holding agent from the fuel addition device 52. EGR combustion is combustion performed by causing EGR gas, which is part of exhaust gas discharged from the combustion chamber, to be again sucked into the combustion chamber, and its EGR rate (EGR gas amount / (EGR gas amount + intake air) In general, in a diesel-type internal combustion engine, the air-fuel ratio of the exhaust gas discharged from the engine body 1 is almost lean, and the degree of leanness is high (hereinafter referred to as “strong lean”). On the other hand, the air-fuel ratio of the exhaust gas is lean even during EGR combustion as described above, but since the amount of intake air into the combustion chamber 5 is suppressed in EGR combustion, The degree of leanness of the air-fuel ratio of the exhaust gas to be discharged is kept low (for example, the air-fuel ratio of the exhaust gas is 18 to 25. Hereinafter, referred to as “weak lean”), and a lot of exhaust gas returns to the combustion chamber 5 again. Therefore, the amount of exhaust gas that reaches the NOx retention agent is small, so even if the amount of fuel added from the fuel addition device to the exhaust gas is small, the air-fuel ratio of the exhaust gas flowing into the NOx retention agent is stoichiometric. Can be rich.

  Further, in EGR combustion, a large amount of EGR gas having a temperature higher than that of the intake air into the combustion chamber 5 is sucked, so that the temperature of the exhaust gas discharged from the engine body 1 is higher than when EGR combustion is not performed. Further, since the fuel is added from the fuel addition device 52 to the exhaust gas having such a high temperature, the added fuel is combusted until the exhaust gas flows into the NOx retention agent and in the NOx retention agent, and the exhaust gas is exhausted. And the temperature of the NOx retention agent are increased. Therefore, by setting the engine combustion to EGR combustion and adding fuel from the fuel addition device 52, the temperature of the NOx retention agent can be maintained at or above the sulfur desorption start temperature.

  In this way, in the high temperature rich process, the combustion of the engine is set to EGR combustion and the fuel is added from the fuel addition device, so that the air-fuel ratio of the exhaust gas flowing into the NOx holding agent is stoichiometrically rich and the NOx holding agent is The temperature is maintained above the sulfur desorption start temperature.

  EGR combustion includes low temperature combustion and high EGR combustion. Here, the low temperature combustion and the high EGR combustion will be briefly described. In general, in a diesel-type internal combustion engine, when the ratio of EGR gas in the air-fuel mixture flowing into the combustion chamber 5 at a constant fuel injection timing (hereinafter referred to as “EGR rate”) increases, the amount of smoke generated accordingly However, if the peak of a certain amount of smoke generation is exceeded, the amount of smoke generation decreases conversely. Low temperature combustion refers to combustion performed by inhaling EGR gas into the combustion chamber 5 at a high EGR rate that exceeds the peak of the amount of smoke generated, and high EGR combustion is a range that does not exceed the peak of the amount of smoke generated The combustion is performed by sucking EGR gas into the combustion chamber at a relatively high EGR rate.

  In this specification, these low-temperature combustion and high EGR combustion are simply described as EGR combustion, but in this embodiment, these low-temperature combustion and high EGR combustion are selectively used depending on the situation during EGR combustion. Yes.

  Further, EGR combustion including low temperature combustion and high EGR combustion becomes difficult to execute when the engine load is high, the fuel injection amount is large, or the required torque for the internal combustion engine is large. This is because the temperature of the fuel and surrounding gas during combustion increases. Therefore, in such a case, it is difficult to make the air-fuel ratio of the exhaust gas flowing into the NOx holding agent stoichiometric rich. Therefore, in the sulfur poisoning recovery control of the present embodiment, the high temperature rich process is not performed in such a case, the combustion is performed with a considerably low or almost zero EGR rate as in the normal control, and the fuel is added from the fuel addition device. Thus, the temperature of the NOx retention agent is maintained at the sulfur desorption start temperature or higher. That is, in the sulfur poisoning recovery control, the air-fuel ratio of the exhaust gas flowing into the NOx retention agent is basically made stoichiometric and rich, and the temperature of the NOx retention agent is raised to the sulfur release start temperature or higher and maintained. However, since it is difficult to make the air-fuel ratio stoichiometric rich when the engine load is high, only the temperature rise and maintenance of the NOx holding agent is executed. However, in the following description, it is assumed that the high-temperature rich process is performed even at the time described above.

  However, the internal combustion engine performs an idle operation (an operation state in which the engine load is substantially zero and the driving force generated by the internal combustion engine is not used to drive a vehicle equipped with the internal combustion engine). In this case, since the amount of fuel injected into each combustion chamber 5 is small in each cycle, the temperature of the exhaust gas discharged from the engine body 1 does not increase so much even if the above-described EGR combustion is performed. Furthermore, since the temperature of the exhaust gas discharged from the engine body 1 is low, the fuel added from the fuel addition device 52 is also difficult to burn, and therefore the temperature of the exhaust gas to which fuel is added is also difficult to increase. As described above, when the idling operation is performed, it is difficult to maintain the temperature of the NOx retention agent at the sulfur desorption start temperature or higher even if the high temperature rich process as described above is performed.

  If the temperature of the NOx retention agent cannot be maintained above the sulfur desorption start temperature, the sulfur component retained in the NOx retention agent cannot be desorbed. For this reason, if the high temperature rich process is continuously performed while the idling operation is being performed, the fuel is added from the fuel addition device 52 even though the sulfur component cannot be removed, This leads to deterioration of fuel consumption.

  Therefore, in the present invention, when the idle operation is started during the sulfur poisoning recovery control (particularly, the high temperature rich process), the high temperature rich process is stopped after a predetermined delay time from the start of the idle operation. As a result, when the temperature of the NOx retention agent becomes lower than the sulfur desorption start temperature, the addition of fuel from the fuel addition device 52 to the exhaust gas is stopped, and deterioration of fuel consumption is suppressed.

  Here, due to the residual heat stored in the NOx holding agent, the temperature of the NOx holding agent is maintained at the sulfur desorption start temperature or higher for a certain period even when the idling operation is started. Accordingly, during this period, even if the internal combustion engine is in an idling state, the sulfur component can be released from the NOx holding agent as long as the air-fuel ratio of the exhaust gas flowing into the NOx holding agent is made stoichiometric rich. As described above, in the present embodiment, the high temperature rich process is continued for a predetermined delay time from the start of the idle operation. This is shown in FIG. FIG. 2 shows the control mode, the fuel addition amount Qf from the fuel addition device 52, the temperature of exhaust gas discharged from the engine body 1 (hereinafter referred to as “exhaust temperature”) Tgas, the temperature Tcat of the NOx holding agent, the fuel addition Fuel adhering amount Qhc in the portion of the exhaust pipe (hereinafter referred to as “exhaust pipe portion”) between the apparatus and the NOx holding agent, the air-fuel ratio AFeg of the exhaust gas discharged from the engine body 1, and the NOx holding agent (NOx It is a time chart of the air-fuel ratio AFat of the exhaust gas flowing into the holding catalyst 23).

In FIG. 2, at time t ₀ , the engine operation is changed from an operation other than the idle operation (hereinafter referred to as “non-idle operation”) to the idle operation. Prior to time t ₀ , a high temperature rich treatment is performed. Therefore, since EGR combustion is performed in the engine body 1, the air-fuel ratio AFeg of the exhaust gas discharged from the engine body 1 is weakly lean. Further, before the time t ₀ , the exhaust gas temperature Tgas is high because no idle operation is performed. Further, fuel is added from the fuel addition device 52, the air-fuel ratio AFat of the exhaust gas flowing into the NOx holding agent is made stoichiometric rich, the exhaust gas temperature Tgas is high, and the added fuel burns. The temperature Tcat of the NOx retention agent is also maintained at a high temperature. However, as can be seen from FIG. 2, the fuel addition amount Qf from the fuel addition device 52 is not constant with respect to time, and a period in which fuel is added and a period in which fuel is not added are repeated. The period when the air-fuel ratio AFeg of the exhaust gas flowing into the agent is also stoichiometric rich is repeated and the period when it is lean. This is because when the fuel is continuously added so that the air-fuel ratio of the exhaust gas flowing into the NOx retention agent becomes continuously stoichiometric and rich, the NOx retention agent becomes excessively hot and the NOx retention agent (NOx retention catalyst) Therefore, the air-fuel ratio of the inflowing exhaust gas is not continuously made rich and stoichiometric.

Since the idling operation is performed after the time t ₀ , the temperature Tgas of the exhaust gas discharged from the engine body 1 is lowered when EGR combustion is performed. Along with this, the temperature Tcat of the NOx retention agent gradually decreases. However, due to the heat accumulated in the NOx retention agent at time t ₀ , the temperature of the NOx retention agent is maintained above the NOx release start temperature until time t ₁ is reached.

Therefore, in the present embodiment, the high temperature rich process is continued until time t ₁ even when the internal combustion engine starts idling. Therefore, EGR combustion continues until time t _1, and thus the air-fuel ratio AFeg of the exhaust gas exhausted from the engine body 1 is also maintained weakly lean. Further, the fuel is added from the fuel addition device so that the air-fuel ratio of the exhaust gas flowing into the NOx holding agent becomes stoichiometric and rich. Therefore, the sulfur component held in the NOx holding agent is released between time t ₀ and time t ₁ .

The time t ₁ may be when the temperature detected by the temperature sensor that detects the temperature of the NOx retention catalyst 23 becomes lower than the sulfur desorption start temperature, or a predetermined time from the time t _0. It may be when (for example, 30 seconds) has elapsed, or when time calculated from a map or the like based on the temperature of the NOx retention agent from time t ₀ to time t ₀ has elapsed.

Thus, by continuing the high temperature rich process for a predetermined delay time (time from time t ₀ to time t ₁ ) after the start of the idling operation, the temperature of the exhaust gas discharged from the engine body 1 decreases. Even after that, the sulfur component held in the NOx holding agent can be released. That is, the sulfur component is released after the sulfur poisoning recovery control is started and the temperature of the NOx retention agent is raised to the release start temperature or higher and the sulfur poisoning recovery control is stopped by the idle operation. The amount can be increased. Therefore, according to the present invention, it is possible to efficiently remove the sulfur component and suppress deterioration in fuel consumption.

  By the way, when the exhaust gas temperature Tgas is low, the fuel added to the exhaust gas from the fuel addition device 52 is almost oxidized and burned in the exhaust pipe portion (the portion of the exhaust pipe between the fuel addition device and the NOx holding agent). It will not adhere to the inner surface of the exhaust pipe part. In this way, the fuel adhering to the inner surface of the exhaust pipe part is desorbed all at once due to some factor (for example, high-temperature exhaust gas flowing in the exhaust pipe part due to an increase in engine load) and released into the atmosphere without being purified. If it is done, white smoke will be discharged.

Here, in the present embodiment, the period during which fuel is added from the fuel addition device 52 to the exhaust gas while the exhaust temperature Tgas is low is the predetermined delay time (the time between time t ₀ and time t ₁ ). Therefore, it is possible to suppress the fuel from adhering to the inner surface of the exhaust pipe portion. That is, as shown in FIG. 2, the high temperature rich process is stopped in a state where the fuel adhesion amount Qhc to the exhaust pipe portion is small. For this reason, even if high-temperature exhaust gas subsequently flows through the exhaust pipe portion, etc., these fuels are purified by the NOx retention agent, so that they are suppressed from being released into the atmosphere as white smoke.

In the present embodiment, when the engine operation is changed from the idle operation to the non-idle operation before the predetermined delay time has elapsed from time t ₀ , the high temperature rich process is continued as it is. If the engine operation returns to the non-idle operation, the exhaust temperature Tgas is raised again by continuing the high temperature rich process, and accordingly, the temperature of the NOx retention agent is also maintained above the sulfur desorption start temperature. The sulfur component can be released from the NOx retention agent.

On the other hand, if the idle operation is continued even when the predetermined delay time has elapsed and the time t ₁ has elapsed since the start of the idle operation, the temperature maintenance control is executed from the time t ₁ . In the temperature maintenance control, the temperature of the NOx holding agent that is near the sulfur desorption start temperature at time t ₁ is referred to as a temperature near the sulfur desorption start temperature or higher (hereinafter, “near sulfur desorption start temperature”). ) To maintain. Specifically, retard combustion, pilot injection, and after injection are performed in the same manner as the temperature increase process of the sulfur poisoning recovery control described above. However, unlike the temperature increase process of the sulfur poisoning recovery control, the temperature maintenance control is performed during idle operation, so the temperature of the NOx holding agent may not be maintained above the sulfur desorption start temperature only by the combustion and injection. In addition to the combustion and injection, fuel is added from the fuel addition device so that the temperature of the NOx retention agent is maintained at or above the sulfur desorption start temperature. This will be described with reference to FIG. The broken lines after time t _{1 in} FIG. 2 indicate the transition of each parameter when the high temperature rich process is performed without performing the temperature maintenance control after time t ₁ .

At time t ₁ , EGR combustion is stopped, retard combustion, pilot injection, and after injection are performed. Therefore, the air-fuel ratio AFeg of the exhaust gas discharged from the engine body 1 becomes strong lean, and the exhaust temperature Tgas also rises. To do. Fuel is added from the fuel addition device 52 to the exhaust gas at a time interval different from that of the high temperature rich process, and this state is shown in the fuel addition amount Qf. Thus, during the temperature maintenance control, the temperature Tcat of the NOx retention agent is maintained at or above the sulfur desorption start temperature by the high exhaust temperature Tgas and the addition of fuel from the fuel addition device 52.

  If the engine operation is changed from idle operation to non-idle operation during this temperature maintenance control, the control immediately changes from temperature maintenance control to sulfur poisoning recovery control (especially high-temperature rich processing). Is done. At this time, the temperature of the NOx retention agent is equal to or higher than the sulfur desorption start temperature, so that the sulfur component can be desorbed from the NOx retention agent almost simultaneously with the start of the sulfur poisoning recovery control.

  Further, during the temperature maintenance control, the exhaust gas temperature Tgas becomes high, and the temperature of the exhaust gas passing through the exhaust pipe portion also becomes high. For this reason, a small amount of fuel that adheres to the inner surface of the exhaust pipe portion in the high temperature rich process during the idling operation that is executed immediately before the temperature maintenance control can be desorbed from the inner surface of the exhaust pipe portion. That is, as shown in FIG. 2, by performing the temperature maintenance control, the fuel adhesion amount Qhc in the exhaust pipe portion can be made substantially zero. The desorbed fuel flows into the high-temperature NOx holding agent, and is oxidized and removed by this NOx holding agent.

Here, for example, when the high temperature rich process is continued without stopping at time t ₁ , the exhaust gas temperature Tgas remains low as shown by the broken line in FIG. 2, and therefore the fuel adhesion amount Qhc to the exhaust pipe portion is Increase with time. When the amount Qhc of fuel adhering to the exhaust pipe portion increases in this way, the fuel adhering to the inner surface of the exhaust pipe portion will be desorbed all at once when the high-temperature exhaust gas flows through the exhaust pipe portion. The fuel cannot be purified, and white smoke is emitted into the atmosphere. However, in the present embodiment, the fuel adhesion amount Qhc to the exhaust pipe portion does not become so large, and thus the fuel attached to the exhaust pipe portion becomes white smoke and is discharged into the atmosphere. It is suppressed.

  By the way, as described above, a large amount of fuel is required to raise the low-temperature NOx retention agent to the sulfur desorption start temperature when normal control is performed. If the sulfur poisoning recovery control is stopped in a state where the sulfur component remains retained in the NOx retention agent, the interval until the next temperature increase of the NOx retention agent as described above is shortened. As a result, fuel consumption is deteriorated. On the other hand, if the temperature maintenance control is executed during the idling operation to maintain the temperature of the NOx retention agent, and the sulfur poisoning recovery control is performed again when the operation is changed to the non-idle operation, the number of times the temperature of the NOx retention agent is increased Therefore, the deterioration of fuel consumption can be suppressed.

  However, it is also conceivable that the engine operation state is maintained for a long time in the idle operation state. In such a case, if the temperature maintenance control is performed for a long time as it is, the NOx retention agent is kept above the sulfur desorption start temperature for a long time even though the sulfur component cannot be separated from the NOx retention agent. Will be maintained. However, in such temperature maintenance control, pilot injection and after injection are performed, and fuel is added into the exhaust gas from the fuel addition device 52, so that compared to the case where these injections and fuel addition are not performed. High fuel consumption. Therefore, if the temperature maintenance control is executed for a long time, the fuel consumption deteriorates.

Therefore, in this embodiment, if the idle operation is continued until time t _2, the that is, when the idle operation over from the start of the temperature maintaining control at a predetermined standby time is continued, the temperature maintaining control Discontinue. Here, the predetermined standby time is the time when the balance between the suppression of fuel consumption deterioration due to the temperature maintenance control and the deterioration of fuel consumption due to the temperature maintenance control being executed for a long time is optimal, and the fuel consumption deterioration can be most suppressed. The The predetermined waiting time may be a predetermined value (for example, 4 minutes), or may be a value that varies depending on the atmospheric temperature or the sulfur component retention amount of the NOx retention agent.

  In addition, normal control is performed after temperature maintenance control is stopped. Here, the normal control means that the above-described retarded combustion, pilot injection, and after-injection are not performed, and even if EGR gas is sucked into the combustion chamber, normal combustion is performed with an EGR rate smaller than the EGR rate in EGR combustion. Further, it means control in which fuel is not added to the exhaust gas from the fuel addition device.

  FIG. 3 shows a routine related to recovery from sulfur poisoning in the above-described embodiment. This routine is executed by interruption every predetermined time.

  Referring to FIG. 3, first, at step 101, it is judged if the sulfur poisoning recovery flag XS is set. The sulfur poisoning recovery flag XS is a flag that is set when the sulfur poisoning recovery control is to be executed (XS = 1), and is not set otherwise (XS = 0). When it is determined that the sulfur poisoning recovery flag XS is not set, the routine proceeds to step 102, where it is determined whether or not the sulfur component retention amount QS of the NOx retention agent is greater than the upper limit value QSH. Here, the sulfur component retention amount QS of the NOx retention agent is determined based on the total amount of fuel injected and added by the fuel injection valve 6 and the fuel addition device 52 since the previous sulfur poisoning recovery control was completed. (Retention amount estimating means) 40. When it is determined that the sulfur component retention amount QS of the NOx retention agent is equal to or lower than the upper limit value QSH, the routine proceeds to step 103 where normal control is executed. On the other hand, when it is determined in step 102 that the sulfur component retention amount QS of the NOx retention agent is larger than the upper limit value QSH, the routine proceeds to step 104, where the sulfur poisoning recovery flag is set (XS = 1).

  When it is determined at step 101 that the sulfur poisoning recovery flag XS is set, the routine proceeds to step 105. In step 105, it is determined whether or not the engine operation state is an idle operation state. If it is determined in step 105 that the engine operating state is not in the idle operating state, the routine proceeds to step 106 where the counter C is set to zero. The counter C is a time counter that represents the time during which the idle operation state continues. Next, at step 107, sulfur poisoning recovery control is executed. In step 108, it is determined whether or not the sulfur component retention amount QS of the NOx retention agent has become substantially zero. If it is determined in step 108 that the sulfur component retention amount QS is not substantially zero, the routine is terminated. On the other hand, if it is determined in step 108 that the sulfur component retention amount QS of the NOx retention agent is substantially zero, the sulfur poisoning recovery flag XS is reset.

If it is determined in step 105 that the engine operating state is the idle operating state, the routine proceeds to step 110. In step 110, the counter C indicating the time during which the idle operation state continues is incremented by 1 (C = C + 1). Then, in step 111, the counter C is less than or whether than the set value C1 representing a predetermined delay time described above (time corresponding from time t ₀ to time t ₁₎ is determined. If it is determined that the counter C1 is smaller than the set value C1, the routine proceeds to step 112, where sulfur poisoning recovery control is executed.

On the other hand, if it is determined in step 111 that the counter C is greater than or equal to the set value C1, the process proceeds to step 113. In step 113, the counter C is whether smaller than the set value C2 representing a predetermined delay time and a predetermined waiting time and the added time described above (time corresponding from time t ₀ to time t ₂₎ is determined. When it is determined that the counter C is smaller than the setting C2, the process proceeds to step 114, and the temperature maintenance control described above is executed. On the other hand, if it is determined in step 113 that the counter C is greater than or equal to the set value C2, the process proceeds to step 115, where the counter C is reset to zero. Next, at step 116, the sulfur poisoning recovery flag XS is reset (XS = 0).

  Next, another embodiment of the present invention will be described with reference to FIG. The configuration shown in FIG. 4 is basically the same as the configuration shown in FIG. 1, and the description of the parts common to the configuration shown in FIG. 1 is omitted. In FIG. 4, the same or similar components as those in FIG. 1 are denoted by the same reference numerals.

  Referring to FIG. 4, in the present embodiment, an intake air temperature sensor 61 and an intake pressure sensor 62 for detecting the temperature and pressure of the intake air flowing in the intake duct 13 are provided on the downstream side of the cooling device 18. Yes. An air-fuel ratio sensor 63 for detecting the air-fuel ratio of the exhaust gas flowing in the exhaust pipe 22 is provided between the outlet of the exhaust turbine 21 and the fuel addition device 52. Further, in the present embodiment, a NOx holding catalyst 64, a particulate filter (hereinafter referred to as “filter”) 65 carrying an NOx holding agent, and an oxidation catalyst 66 are built in the casing 24 from the upstream side. An exhaust purifier 67 is configured. A temperature sensor 68 for detecting the temperature of the exhaust purifier 67 is provided between the NOx holding catalyst 64 and the filter 65 in the casing 24.

  An exhaust throttle valve 69 used for an exhaust brake or the like is further provided between the outlet of the exhaust turbine 21 and the casing 24. In the configuration shown in FIG. 4, the exhaust throttle valve 69 is disposed on the downstream side of the fuel addition device 52, but the exhaust throttle valve 69 is disposed anywhere between the outlet of the exhaust turbine 21 and the casing 24. Also good.

  Further, a differential pressure sensor 70 for detecting a differential pressure upstream and downstream of the casing 24, that is, upstream and downstream of the exhaust gas purification device 67 is also provided. By this differential pressure sensor 70, the pressure loss in the exhaust purifier can be obtained. Further, a temperature sensor 71 and an air-fuel ratio sensor 72 for detecting the temperature and the air-fuel ratio of the exhaust gas flowing out from the exhaust purifier are provided downstream of the casing 24, respectively.

  The output signal of each sensor is input to the input port 45 via the corresponding AD converter 47. The exhaust throttle valve 69 is connected to the output port 46 via a corresponding drive circuit 48.

  Here, the filter 65 will be described. The filter 65 has a partition wall made of a porous material such as cordierite. By passing the partition wall through the exhaust gas flowing into the filter 65, the particulate matter contained in the exhaust gas is collected. To do. Thereby, release of particulate matter to the atmosphere is prevented. Further, the above-mentioned NOx holding agent is carried on the partition wall of the filter 65, and thus has both NOx holding / releasing and reducing / purifying actions.

  In such a filter 65, the particulate matter collected by the filter 65 is normally burned and removed continuously. However, when the amount of particulate matter in the exhaust gas flowing into the filter 65 is so large that the continuous removal capability of the filter 65 is exceeded, particulate matter is deposited on the surface of the filter 65. Thus, if particulate matter accumulates, it will become difficult for exhaust gas to pass the filter 65, and the pressure loss of the exhaust gas resulting from the filter 65 will become large. For this reason, the temperature of the filter 65 is raised to a temperature higher than the temperature at which the particulate matter starts to burn (hereinafter referred to as “PM combustion start temperature”), and the particulate matter deposited on the surface of the filter 65 is forcibly burned. Therefore, it is necessary to execute removal control of particulate matter to be removed (hereinafter referred to as “PM removal control”).

  In the PM removal control, the temperature of the filter 65 is increased in the same manner as the temperature increase process of the sulfur poisoning recovery control described above. That is, the combustion of the engine is set to retarded combustion, and pilot injection and after injection are performed before and after the main injection. Further, in PM removal control, fuel may be added to the exhaust gas from the fuel addition device 52 in addition to these.

  In this embodiment, PM removal control and sulfur poisoning recovery control must be executed for the exhaust gas purification device 67 having the NOx retention catalyst 64 and the filter 65. Both of these controls are performed by the exhaust gas purification device 67. The temperature needs to be raised, and the temperature to be raised is almost the same in any control (that is, the sulfur desorption start temperature and the PM combustion start temperature are the same). Therefore, if these controls are performed continuously when any control is required, the number of times these controls can be executed can be reduced compared to when each control is required, thus reducing fuel consumption. Can be made.

  Therefore, in the present embodiment, when the amount of sulfur component retained in the exhaust gas purification device 67 (NOx retention catalyst 64 and filter 65) increases or the amount of particulate matter deposited on the exhaust gas purification device 67 (filter 65) increases. In this case, PM removal control and sulfur poisoning recovery control are continuously performed. When the sulfur poisoning recovery control is executed while particulate matter is accumulated, the air-fuel ratio of the exhaust gas flowing into the exhaust purifier 67 is stoichiometric rich while adjusting the temperature of the exhaust purifier 67 optimally. Therefore, the sulfur poisoning recovery control is executed after the PM removal control is executed.

  FIG. 5 and FIG. 6 show a routine related to recovery from sulfur poisoning in the above-described embodiment. This routine is executed by interruption every predetermined time. In FIG. 5, Steps 143 to 154 are the same as Steps 105 to 116 in FIG. However, in the present embodiment, when the sulfur poisoning recovery control is executed, the temperature of the exhaust gas purification device 67 is already high due to the PM removal control, so only the high temperature rich process is performed as the sulfur poisoning recovery control. Is called.

  Referring to FIGS. 5 and 6, first, at step 131, it is determined whether or not the sulfur poisoning recovery flag XS is set. When it is determined that the sulfur poisoning recovery flag XS is not set, the routine proceeds to step 132. In step 132, it is determined whether the PM removal flag XPM is set. This PM removal flag XPM is a flag that is set when PM removal control is to be executed (XPM = 1), and is not set otherwise (XPM = 0). When it is determined that the PM removal flag XPM is not set (XPM = 0), the routine proceeds to step 133, and whether or not the particulate matter accumulation amount QPM on the filter 65 of the exhaust purifier 67 is larger than the upper limit amount QPMH. Is determined. The amount of particulate matter deposited on the filter 65 is estimated based on the output of the differential pressure sensor 70. When it is determined that the particulate matter deposition amount QPM is equal to or less than the upper limit value QPMH (QPM ≦ QPMH), the routine proceeds to step 134, where the sulfur component retention amount QS is larger than the upper limit amount QSH as in step 102 of FIG. It is determined whether or not. When it is determined that the sulfur component retention amount QS is less than or equal to the upper limit value QSH (QS ≦ QSH), the routine proceeds to step 135 where normal control that is performed when the sulfur poisoning recovery control and PM removal control are not being performed is performed. Is called.

  On the other hand, when it is determined in step 133 that the particulate matter deposition amount QPM is larger than the upper limit value QPMH (QPM> QPMH) or in step 134, the sulfur component retention amount QS is larger than the upper limit amount QSH (QS> QSH). When it is determined, the routine proceeds to step 136 where the PM removal flag XPM is set.

  When the PM removal flag XPM is set, the routine proceeds from step 132 to step 137, where PM removal control is executed. In the following step 138, it is determined whether or not the particulate matter deposition amount QPM has become substantially zero. When it is determined that the particulate matter accumulation amount QPM is not zero (QPM> 0), the routine is terminated. On the other hand, the routine proceeds to step 139 where it is determined that the particulate matter deposition amount QPM has become substantially zero (QPM≈0), and the PM removal flag XPM is reset (XPM = 0). In the following step 140, the sulfur poisoning recovery flag XS is set (XS = 1). In the following step 141, it is determined whether or not the sulfur component retention amount QS is smaller than the allowable amount QSA. When it is determined that the sulfur component retention amount QS is equal to or greater than the permissible amount QSA (QS ≧ QSA), the routine is terminated while the sulfur poisoning recovery flag XS is set (XS = 1). On the other hand, when it is determined that the sulfur component retention amount QS is less than the allowable amount QSA (QS <QSA), the routine proceeds to step 142, and after the sulfur poisoning recovery flag XS is reset (XS = 0), the routine ends. I'm damned.

  Finally, the exhaust gas purification mechanism by the NOx retention agent of the present invention, in particular, the retention / desorption of NOx in the exhaust gas and the reduction purification action will be described with reference to FIG. The mechanism of the NOx retention / desorption and reduction / purification action will be described with reference to the case where platinum (Pt) and barium (Ba) are supported, but the same applies to the case where other noble metals, alkali metals, alkaline earths, and rare earths are used. Mechanism. 7A and 7B schematically show enlarged views of the surface of the carrier layer formed on the surface of the partition wall of the NOx retention agent and the pore surface of the partition wall.

When the air-fuel ratio of the inflowing exhaust gas becomes considerably leaner, the oxygen concentration in the exhaust gas greatly increases. As shown in FIG. 7A, these oxygens are in the form of O ₂ ⁻ or O ²⁻ on the surface of platinum. Adhere to. On the other hand, NO in the inflowing exhaust gas reacts with O ₂ ⁻ or O ²⁻ on the surface of platinum to become NO ₂ (2NO + O ₂ → 2NO ₂ ). Next, a part of the produced NO ₂ is further oxidized on platinum while being absorbed in the NO _x retention agent and combined with barium oxide BaO, and as shown in FIG. 7A, nitrate ions (NO ₃ ⁻ ) Diffuses into the NOx retention agent. Thus NO _x is held on the NOx retention agent.

Oxygen concentration in the inflowing exhaust gas is generated NO ₂ in as high as the surface of the platinum, as long as NO ₂ to NOx holding capacity of the NOx holding agent is not saturated is held in the NOx keeping agent nitrate ion (NO ₃ ^-) is Generated. In contrast, when the oxygen concentration in the exhaust gas decreases and the amount of NO ₂ produced decreases, the reaction proceeds in the reverse direction (NO ₃ ⁻ → NO ₂ ), and thus nitrate ions (NO ₃ in the NOx retention agent). ^-) is released from the NOx keeping agent in the form of NO _2. That is, when the oxygen concentration in the inflowing exhaust gas decreases, NO _x is released from the NO _x holding agent. Becomes lower lean degree of the inflowing exhaust gas is the oxygen concentration in exhaust gas decreases, therefore NO _x will be induced to leave from the NOx retention agent if the lower degree of lean inflowing exhaust gas.

On the other hand, when the air-fuel ratio of the exhaust gas flowing in at this time is reduced, HC and CO react with O ₂ ⁻ or O ²⁻ on the platinum and are oxidized. Further, when the air-fuel ratio of the inflowing exhaust gas is reduced, the oxygen concentration in the exhaust gas extremely decreases, so that NO ₂ is desorbed from the NOx holding agent, and this NO ₂ is not as shown in FIG. 7B. It reacts with fuel HC and CO to be reduced and purified. Such is NO ₂ on the surface of the platinum in the NO ₂ is detached from comes to NOx retention agent absent Next the following. Therefore to reduce the air-fuel ratio of the exhaust gas flowing into, and NO _x from the NOx retention material in a short time when a state in which the reducing agent is present is to be reduced and purified is disengaged.

Next, the mechanism of sulfur poisoning of the NOx retention agent will be described. When the SOx component is contained in the exhaust gas, the NOx retention agent retains SOx in the exhaust gas by the same mechanism as the above-described retention of NOx. That is, when the air-fuel ratio of the exhaust gas is lean, SOx (for example, SO ₂ ) in the exhaust gas is oxidized on platinum (Pt) to become SO ₃ ⁻ , SO ₄ ⁻ and combined with barium oxide (BaO). BaSO ₄ is formed. BaSO ₄ is relatively stable, and since crystals are likely to be coarse, once they are produced, they are not easily decomposed and released. For this reason, when the production amount of BaSO ₄ in the NOx retention agent increases, the amount of BaO that can participate in the retention of NOx decreases, and the NOx retention capability decreases.

In order to eliminate this sulfur poisoning, BaSO ₄ produced in the NOx retention agent is decomposed at a high temperature, and SO ₃ ⁻ and SO ₄ ⁻ sulfate ions produced thereby are decomposed in a stoichiometric rich atmosphere. It must be reduced, converted to gaseous SO ₂ and released from the NOx retention agent. Therefore, in order to perform sulfur poisoning regeneration, it is necessary to bring the NOx retention agent to a high temperature and rich atmosphere.

  In the present specification, it has been described that when the sulfur component is released from the NOx retention agent, the air-fuel ratio of the inflowing exhaust gas is substantially the stoichiometric air-fuel ratio or rich (stoichiometric rich). When the oxygen concentration of the exhaust gas is lower than the predetermined oxygen concentration, the sulfur component is easily released. Therefore, the description “making the air-fuel ratio of the exhaust gas substantially the stoichiometric air-fuel ratio or rich” in the above-described embodiment also means “making the oxygen concentration of the inflowing exhaust gas equal to or lower than a predetermined oxygen concentration”. Therefore, any reducing agent may be added to the exhaust gas from the fuel addition device as long as the oxygen concentration can be reduced and NOx released from the NOx holding agent can be reduced.

In this specification, the term “retaining” is used to include both “absorption” and “adsorption”. Therefore, the “NOx retention agent” includes both the “NOx absorbent” and the “NOx adsorbent”, and the former accumulates NOx in the form of nitrate or the like, and the latter adsorbs in the form of NO ₂ or the like. Further, the term “detachment” from the NOx holding agent is also used to include the meaning of “desorption” corresponding to “adsorption” in addition to “release” corresponding to “absorption”.

It is a figure which shows the whole internal combustion engine which has an exhaust gas purification apparatus of this invention. It is a time chart of various parameters about an exhaust emission control device. It is a figure which shows the routine regarding sulfur poisoning recovery | restoration of 2nd embodiment of this invention. It is a figure which shows the whole internal combustion engine which has the exhaust emission purification device of 2nd embodiment of this invention. It is a figure which shows a part of routine regarding sulfur poisoning recovery | restoration of 2nd embodiment of this invention. It is a figure which shows a part of routine regarding sulfur poisoning recovery | restoration of 2nd embodiment of this invention. It is a figure for demonstrating the holding | maintenance / detachment | desorption and reduction | restoration / purification | cleaning action of NOx.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Engine main body 5 ... Combustion chamber 6 ... Fuel injection valve 23 ... NOx holding catalyst 25 ... EGR passage 40 ... ECU
52 ... Reducing agent addition device

Claims (5)

An NOx holding agent that is disposed on the engine exhaust passage and purifies NOx in the exhaust gas, and the air-fuel ratio of the inflowing exhaust gas is substantially stoichiometric or rich, and the temperature of the NOx holding agent is sulfur desorption A NOx holding agent for releasing the held sulfur component when the temperature is equal to or higher than the start temperature; and a sulfur holding amount estimating means for estimating the sulfur component holding amount held in the NOx holding agent. When the sulfur component retention amount estimated by the amount estimation means exceeds a predetermined retention amount, the air-fuel ratio of the exhaust gas flowing into the NOx retention agent is made substantially stoichiometric or rich, and the temperature of the NOx retention agent is increased. Sulfur poisoning recovery control is performed to control the temperature so that the temperature rises above the sulfur desorption start temperature and is maintained. When idle operation is performed during execution of the sulfur poisoning recovery control, the sulfur poisoning recovery control is performed. Cancel In the exhaust gas purification device for an internal combustion engine,
Stopping the sulfur poisoning recovery control is an internal combustion engine exhaust gas purification device that is executed after a predetermined delay time has elapsed since the start of idle operation.   Temperature maintenance control for maintaining the temperature of the NOx retention agent at a temperature near the sulfur desorption start temperature or more while making the air-fuel ratio of the exhaust gas flowing into the NOx retention agent lean after executing the cancellation of the sulfur poisoning recovery control. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the sulfur poisoning recovery control is executed again when the control is changed to an operation other than the idle operation during the temperature maintenance control.   The temperature maintenance control is stopped after a predetermined waiting time has elapsed since execution of the sulfur poisoning recovery control is canceled when the temperature maintenance control is not changed from idle operation to operation other than idle operation. 2. An exhaust emission control device for an internal combustion engine according to 2.   In the sulfur poisoning recovery control, a part of the exhaust gas discharged from the combustion chamber is again sucked into the combustion chamber and exhausted from the reducing agent addition device disposed on the exhaust passage upstream of the NOx retention agent. The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 3, which is performed by adding a reducing agent to the gas.   In the temperature maintenance control, the main injection performed near the compression top dead center is delayed more than when the temperature maintenance control is not executed, and a small amount of auxiliary injection is performed before the main injection. Performing a small amount of auxiliary injection after the main injection, and adding the reducing agent into the exhaust gas from the reducing agent addition device disposed on the exhaust passage upstream of the NOx retention agent. The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 4, which is performed by at least one of the above.

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