JP4297762B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

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

Conventionally, NO _X in the exhaust gas flowing in when the air-fuel ratio of the inflowing exhaust gas is lean is stored in the exhaust passage of the internal combustion engine in which combustion is performed under a lean air-fuel ratio, and the inflowing exhaust gas air-fuel ratio is arranged _the NO _X absorbent to reduce the amount of the NO _X are stored by reducing NO _X are stored as containing a reducing agent in the exhaust gas when lowered, NO _X absorption of stoichiometric air-fuel ratio of the exhaust gas flowing into the _the NO _X absorbent while maintaining the temperature of _the NO _X absorbent to sulfur loss required temperature such as about 600 ° C. the time to reduce the amount of sulfur that is stored in the agent There is known an internal combustion engine in which sulfur amount reduction control is performed to make the air-fuel ratio or rich. In this case, in order to make the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent to be the stoichiometric air-fuel ratio or rich, for example, fuel can be secondarily supplied into the exhaust passage upstream of the NO _X absorbent.

However, since a large amount of oxygen flows into the NO _X absorbent when the engine load is high, a large amount of oxygen is required to make the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent at this time the stoichiometric air-fuel ratio or rich. Requires fuel. The temperature of this large amount of fuel is oxidized in _the NO _X absorbent _the NO _X absorbent becomes excessively high, _the NO _X absorbent may also be melting.

  Accordingly, when the engine load becomes higher than a predetermined upper limit load when the sulfur amount reduction control is being performed, the normal operation is resumed, and when the engine load becomes lower than the upper limit load, the sulfur amount reduction control is resumed. Such an internal combustion engine is known.

However, once the normal operation is performed, the temperature of the NO _X absorbent decreases, and then the temperature of the NO _X absorbent needs to be increased to the required sulfur content reduction temperature in order to resume the sulfur reduction control. The sulfur reduction control cannot be resumed immediately.

In order to solve this problem, when the engine load becomes higher than the upper limit load when the sulfur amount reduction control is being performed, the temperature of the NO _X absorbent is maintained at a standby temperature higher than that during normal operation. While the standby control is performed to make the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent lean, the sulfur amount reduction control is resumed when the engine load becomes lower than the upper limit load during the standby control. An internal combustion engine is known. In this way, when the engine load becomes lower than the upper limit load, the sulfur amount reduction control can be started promptly.

  However, when the engine load becomes higher than the upper limit load due to the vehicle traveling on an expressway, for example, the engine load may be maintained higher than the upper limit load for a long time. In some cases, it may take a long time to be lower. Therefore, if the standby control is continued because the engine load is higher than the upper limit load, the standby control is performed for a long time, and energy cannot be used effectively.

  Therefore, an internal combustion engine is known in which standby control is stopped and returned to normal operation when standby control is continued for a certain time (see Patent Document 1). That is, in this internal combustion engine, when the standby control is continued for a certain time, the sulfur amount reduction control is stopped.

JP 2003-129830 A

However, in the internal combustion engine described in Patent Document 1, the control for reducing the amount of sulfur is stopped even if the amount of sulfur stored in the NO _X absorbent is large. Therefore, the internal combustion engine is stored in the NO _X absorbent. The amount of sulfur present cannot be reduced to a small amount, for example zero. As a result, the amount of sulfur stored in the NO _X absorbent increases in a short time, and there is a risk that sulfur amount reduction control must be frequently performed. This is not preferable from the viewpoint of energy consumption.

Therefore, an object of the present invention is to provide an exhaust gas purification apparatus for an internal combustion engine that can reduce the amount of sulfur stored in the NO _X absorbent while reducing the energy consumption.

In order to solve the above-described problem, according to the first invention, the exhaust gas flowing into the exhaust passage of the internal combustion engine in which the combustion is performed under the lean air-fuel ratio flows when the air-fuel ratio of the exhaust gas flowing in is lean. store up NO _X in the gas, decreasing the amount of the NO _X when the air-fuel ratio of the inflowing exhaust gas is stored by reducing NO _X are stored as containing a reducing agent in the exhaust gas when the reduced place make _the NO _X absorbent, it flows into _the NO _X absorbent while maintaining the temperature of _the NO _X absorbent to sulfur loss required temperature when to reduce the amount of sulfur that is accumulated in _the NO _X absorbent In the internal combustion engine in which the sulfur amount reduction control is performed to make the air-fuel ratio of the exhaust gas to be the stoichiometric air-fuel ratio or rich, when the sulfur amount reduction control is being performed, the engine load is higher than a predetermined upper limit load When Performs standby control for the air-fuel ratio of the exhaust gas flowing into the temperature of the NO _X absorbent to normal within _the NO _X absorbent while maintaining a high standby temperature than during operation in a lean, setting the standby control is predetermined When the amount of sulfur stored in the NO _X absorbent is determined at this time when the amount of sulfur is larger than a predetermined set amount, the standby control is continued. When the engine load becomes lower than the upper limit load, the sulfur amount reduction control is resumed. When the calculated sulfur amount is smaller than the set amount, the standby control is stopped and the normal operation is resumed.

Further, according to the second invention, in the first invention, in order to maintain the temperature of the NO _X absorbent at the standby temperature when the standby control is performed, the fuel is placed in the exhaust passage upstream of the NO _X absorbent. Is supplied secondarily.

Further, in the first aspect according to the third aspect, the _the NO _X absorbent on the particulate filter for trapping particulate in the exhaust gas and is carried and deposited on the particulate filter particles Particulate removal control is performed to raise the temperature of the particulate filter while keeping the air-fuel ratio of the exhaust gas flowing into the particulate filter lean so that the particulate oxidation control is completed. Sometimes the amount of sulfur stored in the NO _X absorbent is determined, and when the calculated amount of sulfur is greater than a predetermined allowable amount, the amount of sulfur stored in the NO _X absorbent is determined. When it is determined that the amount of sulfur to be reduced is less than the allowable amount, the sulfur stored in the NO _X absorbent is reduced. We try not to judge that the amount of should be reduced.

  In this specification, the ratio of the air supplied into the exhaust passage upstream of a certain position of the exhaust passage, the combustion chamber, and the intake passage and the reducing agent such as hydrocarbon HC and carbon monoxide CO This is called the air-fuel ratio of the exhaust gas at the position.

While reducing energy consumption, the amount of sulfur that is accumulated in _the NO _X absorbent can be reliably reduced.

  FIG. 1 shows a case where the present invention is applied to a compression ignition type internal combustion engine. However, the present invention can also be applied to a spark ignition type internal combustion engine.

  Referring to FIG. 1, the engine body 1 has, for example, four cylinders 1a. Each cylinder 1 a is connected to a common surge tank 3 via a corresponding intake branch pipe 2, and the surge tank 3 is connected to a compressor 6 of a variable nozzle exhaust turbocharger 5 via an intake duct 4. A throttle valve 8 driven by an electric control type or negative pressure control type actuator 7 is disposed in the intake duct 4, and a cooling device for cooling the intake air flowing in the intake duct 4 around the intake duct 4. 9 is arranged. An intake pipe 4 a is connected to the outlet of the compressor 6.

  Each cylinder 1 a is connected to an exhaust turbine 12 of an exhaust turbocharger 5 via an exhaust manifold 10 and an exhaust pipe 11, and an outlet of the exhaust turbine 12 is connected to a muffler 14 via an exhaust pipe 13. An exhaust throttle valve 16 driven by an electrically controlled or negative pressure controlled actuator 15 is disposed in the exhaust pipe 13. The exhaust throttle valve 16 is normally kept fully open.

A catalytic converter 14a is accommodated in the muffler 14, and the catalytic converter 14a carries a first NO _X absorbent 17 and a second NO _X absorbent 18 that are arranged in series while being separated from each other. 19 and an oxidation catalyst 20. Further, an exhaust pipe 21 is connected to the outlet of the muffler 14.

  Fuel injection valves 22 are arranged in the cylinders of the cylinders 1 a, and these fuel injection valves 22 are connected to an electrically controlled fuel pump 24 with variable discharge amount via a common rail 23. A fuel pressure sensor (not shown) for detecting the fuel pressure in the common rail 3 is attached to the common rail 3, and fuel is used so that the fuel pressure in the common rail 3 becomes the target fuel pressure based on the output signal of the fuel pressure sensor. The discharge amount of the pump 24 is controlled. Further, a fuel supply valve 25 for secondary supply of fuel, for example, light oil, is attached to the exhaust pipe 13 in the exhaust pipe 13, and fuel is also supplied to the fuel supply valve 25 from the fuel pump 24.

  Still referring to FIG. 1, the exhaust manifold 10 and the surge tank 3 are connected to each other via a recirculated exhaust gas (hereinafter referred to as EGR) passage 26, and an electrically controlled EGR control valve 27 is provided in the EGR passage 26. Be placed. A cooling device 28 for cooling the EGR gas flowing in the EGR passage 26 is disposed around the EGR passage 26, and an oxidation catalyst 29 is disposed in the EGR passage upstream of the cooling device 28.

The electronic control unit 30 is composed of a digital computer, and is connected to each other by a bidirectional bus 31. A ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, and a backup RAM (B-RAM). 35, an input port 36 and an output port 37. An air flow meter 40 for detecting the intake air amount Ga is attached to the intake pipe 4a. An exhaust pipe 13 and an exhaust pipe 21 upstream of the fuel supply valve 25 are provided with an air-fuel ratio sensor 41 for detecting the air-fuel ratio of the exhaust gas discharged from the engine and the air-fuel ratio of the exhaust gas flowing out from the catalytic converter 14a, respectively. , 42 are respectively attached. Further, the muffler 14 and the exhaust pipe 21 between the first NO _X absorbent 17 and the particulate filter 19 are attached with temperature sensors 43 and 44 for detecting the temperature of the exhaust gas flowing through the respective positions. Note that the temperatures of these exhaust gases represent the temperature of the catalytic converter 14a. Further, a differential pressure sensor 45 for detecting a differential pressure or pressure loss between the exhaust pipe 13 and the exhaust pipe 21 is provided. A depression amount sensor 46 that generates an output voltage proportional to the depression amount of the accelerator pedal is connected to an accelerator pedal (not shown). The output voltages of these sensors 40, 41, 42, 43, 44, 45, 46 are input to the input port 36 via corresponding AD converters 38. Further, a crank angle sensor 47 that generates an output pulse every time the crankshaft rotates, for example, 10 ° is connected to the input port 36. The CPU 34 calculates the engine speed based on this output pulse. On the other hand, the output port 37 is connected to the actuators 7 and 15, the fuel injection valve 22, the fuel pump 24, the fuel supply valve 25, and the EGR control valve 27 through corresponding drive circuits 39.

The particulate filter 19 has a honeycomb structure made of a porous material such as cordierite, and includes a plurality of exhaust gas passages extending in parallel with each other. These exhaust gas passages have a first exhaust gas passage whose one end is open and the other end is closed by a sealing material, and a second exhaust gas passage whose other end is open and one end is closed by a sealing material. These exhaust gas passages are alternately arranged through thin partition walls. The second NO _X absorbent 18 described above is supported on the partition wall of the particulate filter 19, that is, on both side surfaces of the partition wall and the inner wall surface of the pore. On the other hand, the first NO _X absorbent 17 is made of a porous material and supported on a base material having a honeycomb structure.

The NO _X absorbents 17 and 18 use, for example, alumina as a carrier, and on this carrier, for example, alkaline metals such as potassium K, sodium Na, lithium Li and cesium Cs, alkaline earths such as barium Ba and calcium Ca, lanthanum La , At least one selected from rare earths such as yttrium Y, and a noble metal such as platinum Pt, palladium Pd, rhodium Rh, and iridium Ir are supported.

The NO _X absorbent stores NO _X in the exhaust gas when the average air-fuel ratio of the inflowing exhaust gas is lean, and contains a reducing agent in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas decreases. The stored NO _X is reduced to reduce the amount of stored NO _X , and the NO _X accumulation and reduction action is performed.

The detailed mechanism of the NO _X accumulation and reduction action of the NO _X absorbent has not been fully clarified. However, the mechanism currently considered can be briefly described as follows, taking as an example the case where platinum Pt and barium Ba are supported on a support.

That is, when the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent becomes considerably leaner than the stoichiometric air-fuel ratio, the oxygen concentration in the flowing exhaust gas greatly increases, and oxygen O ₂ becomes O ₂ ⁻ or O ^2−. It adheres to the surface of platinum Pt. On the other hand, NO in the inflowing exhaust gas adheres to the surface of platinum Pt and reacts with O ₂ ⁻ or O ²⁻ on the surface of platinum Pt to become NO ₂ (NO + O ₂ → NO ₂ + O ^* , where O ^{* Indicates} active oxygen). Next, a part of the generated NO ₂ is further oxidized on platinum Pt while being absorbed into the NO _X absorbent and combined with barium oxide BaO, and diffuses into the NO _X absorbent in the form of nitrate ions NO ₃ ^−. To do. In this way, NO _X is stored in the NO _X absorbent.

On the other hand, when the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent becomes rich or the stoichiometric air-fuel ratio, the oxygen concentration in the exhaust gas decreases, the amount of NO ₂ generated decreases, and the reaction proceeds in the reverse direction (NO ₃ ^- → NO + 2O ^*) advances to, thus to _the NO _X absorbent in the nitrate ions NO ₃ ^- are released from _the NO _X absorbent in the form of NO. When the exhaust gas contains a reducing agent, that is, HC and CO, the released NO _X reacts with these HC and CO and is reduced. When NO _X no longer exists on the surface of platinum Pt in this way, NO _X is released from the NO _X absorbent to the next and reduced, and the amount of NO _X stored in the NO _X absorbent is reduced. Gradually decreases.

Incidentally, stored without any NO _X to form a nitrate, it believed it is possible to reduce without NO _X to release NO _X. Further, when attention is focused on the active oxygen O ^* , the NO _X absorbent can also be regarded as an active oxygen generating catalyst that generates active oxygen O ^* as NO _X is accumulated and released.

  On the other hand, the oxidation catalyst 20 is also supported on a base material made of a porous material and having a honeycomb structure. The oxidation catalyst 20 uses, for example, alumina as a carrier, and a noble metal such as platinum Pt, palladium Pd, rhodium Rh, and iridium Ir is supported on the carrier without supporting alkali metal, alkaline earth, and rare earth.

In the embodiment according to the present invention, the NO _X amount reduction control, the particulate removal control, the SO _X amount reduction control, and the standby control, which will be sequentially described, are executed. During normal operation in which these controls are not executed, the opening degree of the throttle valve 8 is controlled according to, for example, the amount of depression of the accelerator pedal, and the amount of fresh air detected by the air flow meter 40 is determined based on engine operating conditions such as fuel injection amount and engine rotation. The opening degree of the EGR control valve 27 is controlled so as to coincide with the target fresh air amount determined according to the number. Here, the fuel injection amount is determined according to, for example, the accelerator pedal depression amount and the engine speed. Further, during normal operation, only the main fuel injection FM is performed, for example, around the compression top dead center (TDC) as shown in FIG. 2 (A), or the main fuel injection FM as shown in FIG. 2 (B). At the same time, pilot injection FP is performed. In order to stabilize combustion, the pilot injection FP is performed, for example, at a crank angle of about 10 to 40 degrees before compression top dead center before the main fuel injection FM is performed.

Incidentally, the internal combustion engine shown in FIG. 1 are continuously performed combustion under a lean air-fuel ratio, therefore the air-fuel ratio of the exhaust gas flowing into the _the NO _X absorbent 17 is maintained lean ing. As a result, NO _X in the exhaust gas is stored in the NO _X absorbents 17 and 18.

The accumulated NO _X amount in the NO _X absorbents 17 and 18 gradually increases with time. In the embodiment according to the present invention, therefore, to reduce the accumulated amount of NO _X in _the NO _X absorbent 17 and reducing the NO _X that is stored in _the NO _X absorbent 17, _the NO _X absorbent 17 , and to perform the temporary amount of NO _X reduction control for switching to the rich air-fuel ratio of the exhaust gas flowing into the 18.

Specifically, in an embodiment according to the present invention, a fuel i.e. reducing agent from the fuel supply valve 25 so that the air-fuel ratio of the exhaust gas flowing into the _the NO _X absorbent 17 becomes rich is temporarily supplied The In this case, fuel is injected from the fuel feed valve 25 by an amount necessary to accumulate _the amount of NO _X in _the NO _X absorbent 17 becomes lower amount for example approximately zero. Such NO _X amount reduction control is repeatedly performed at predetermined time intervals during normal operation. This set time interval is set according to the engine operating state, for example, the fuel injection amount and the engine speed. Incidentally, obtains the accumulated amount of NO _X in _the NO _X absorbent 17, 18, to perform the amount of NO _X reduction control when the accumulated amount of NO _X in _the NO _X absorbent 17, 18 exceeds the upper limit amount of NO _X You can also

On the other hand, fine particles mainly composed of carbon contained in the exhaust gas are collected on the particulate filter 19. As described above, the internal combustion engine shown in FIG. 1 is continuously combusted under a lean air-fuel ratio, and the NO _x absorbents 17 and 18 have oxidizing ability. If the temperature of the curative filter 19 is maintained at or above the temperature at which the fine particles can be oxidized, the fine particles are oxidized on the particulate filter 19 and removed.

In this case, according to the NO _X accumulation and reduction mechanism of the NO _X absorbent described above, active oxygen O ^* is generated both when NO _X is stored in the NO _X absorbent and when NO _X is released. . This active oxygen O ^* has a higher activity than oxygen O ₂ , and therefore oxidizes fine particles deposited on the particulate filter 19 quickly. That is, when the NO _X absorbent 18 is carried on the particulate filter 19, the air-fuel ratio of the exhaust gas flowing into the particulate filter 19 is deposited on the particulate filter 19 regardless of whether it is lean or rich. Fine particles are oxidized. In this way, the fine particles are continuously oxidized.

  However, if the temperature of the particulate filter 19 is not maintained at a temperature that can oxidize the particulates, or if the amount of particulates flowing into the particulate filter 19 per unit time becomes considerably large, the particulates that accumulate on the particulate filter 19. Gradually increases and the pressure loss of the particulate filter 19 increases.

  Therefore, in the embodiment according to the present invention, the amount of accumulated particulates on the particulate filter 19 is obtained, and when the amount of accumulated particulates exceeds the upper limit particulate amount, the supply of EGR gas is stopped and the exhaust gas flowing into the particulate filter 19 The particulate filter 19 is controlled so that the temperature of the particulate filter 19 is raised to a required particulate removal temperature, for example, 600 ° C.

  More specifically, in the embodiment according to the present invention, the amount of deposited fine particles is calculated by sequentially integrating the amount of fine particles flowing into the particulate filter 19 per unit time. The amount of inflowing fine particles per unit time corresponds to the amount of fine particles discharged from the combustion chamber per unit time, and this discharged fine particle amount can be calculated according to the engine operating state, for example, the fuel injection amount and the engine speed. . Next, when the amount of deposited fine particles exceeds the upper limit fine particle amount, fine particle removal control is started. Until the temperature of the particulate filter 19 detected by the temperature sensor 43 reaches the particulate removal request temperature, first, pilot injection FP and after injection FA are performed together with the main fuel injection FM, as shown in FIG. . The after injection FA is performed, for example, at a crank angle of about 20 to 30 degrees after the compression top dead center in order to increase the temperature of the exhaust gas discharged from the combustion chamber. Further, at this time, the injection timings of the main fuel injection FM and the pilot injection FP are retarded as compared to the normal operation (see FIG. 2B). Next, when the temperature of the particulate filter 19 is raised to the particulate removal requirement temperature, as shown in FIG. 2D, pilot injection FP and after injection FA are performed together with the main fuel injection FM, and further, for example, compression Post injection FPO is performed from 90 to 120 degrees after the dead point. In this case, the injection amount of the post injection FPO is controlled so that the temperature of the particulate filter 19 is maintained at the particulate removal request temperature. The post-injection FPO is different from the after-injection FA in that it does not contribute to the engine output.

  Furthermore, when the idle operation is performed while the particulate removal control is being performed, the opening degree of the exhaust throttle valve 16 is reduced. In the internal combustion engine shown in FIG. 1, the fuel injection amount is controlled so that the engine speed is maintained at the target speed during idling. The target rotational speed is, for example, 650 rpm during normal operation, and is increased to, for example, 950 rpm when particulate removal control is performed.

  When the particulate removal control is started, the particulates deposited on the particulate filter 19 are ignited and burned and removed. In this case, the amount of fine particles removed per unit time can be calculated according to the temperature of the particulate filter 19, for example. In the embodiment according to the present invention, the amount of fine particles removed per unit time is successively subtracted from the amount of deposited fine particles, and the fine particle removal control is completed when the amount of deposited fine particles reaches a slight lower limit value QPML. That is, the normal operation is restored.

By the way, sulfur is contained in the exhaust gas in the form of SO _X , and not only NO _X but also SO _X is stored in the NO _X absorbents 17 and 18. It is considered that the storage mechanism of SO _{X in} the NO _X absorbents 17 and 18 is the same as the storage mechanism of NO _X. That is, when briefly described as an example the case of carrying platinum Pt and barium Ba on the carrier, the oxygen O ₂ as the air-fuel ratio of the exhaust gas is described above when the lean flowing into _the NO _X absorbent is O ₂ ^- Or, it is attached to the surface of platinum Pt in the form of O ²⁻ , and SO ₂ in the inflowing exhaust gas adheres to the surface of platinum Pt and reacts with O ₂ ⁻ or O ²⁻ on the surface of platinum Pt, the SO _3. Next, the generated SO ₃ is further oxidized on the platinum Pt and absorbed by the NO _X absorbent and combined with barium oxide BaO, and diffuses into the NO _X absorbent in the form of sulfate ions SO ₄ ⁻ . This sulfate ion SO ₄ ⁻ is then combined with barium ion Ba ⁺ to produce sulfate BaSO ₄ .

This sulfate BaSO ₄ is difficult to decompose, and even if the air-fuel ratio of the exhaust gas flowing into the NO _X absorbents 17 and 18 is simply made rich, the amount of sulfate BaSO ₄ in the NO _X absorbents 17 and 18 decreases. do not do. Therefore, increasing the amount of sulfate BaSO ₄ in _the NO _X absorbent 17, 18 over time, the amount of resulting _the NO _X absorbent 17 can stored NO _X is decreased.

However, if the average air-fuel ratio of the exhaust gas flowing into the NO _X absorbent 17, 18 is made the stoichiometric air-fuel ratio or rich while maintaining the temperature of the NO _X absorbent 17, 18 at, for example, 600 ° C. or higher, the NO _X absorbent 17 , sulfate BaSO ₄ in 18 is released from _the NO _X absorbent 17 in the form of SO ₃ is decomposed. The released SO ₃ reacts with HC and CO and is reduced to SO ₂ when a reducing agent, that is, HC and CO is contained in the exhaust gas. Thus gradually decreases the amount of SO _X, which is stored in the form of sulfate BaSO ₄ in _the NO _X absorbent 17 and 18, form from the time _the NO _X absorbent 17, 18 SO _X is SO ₃ It will not leak out.

Therefore, in the embodiment according to the present invention, the amount of accumulated SO _X in the NO _X absorbents 17 and 18 is obtained, and when this accumulated SO _X amount exceeds the predetermined upper limit SO _X amount, the NO _X absorbents 17 and 18 are obtained. In order to reduce the amount of accumulated SO _X in the exhaust gas, the exhaust gas flowing into the NO _X absorbent 17, 18 is maintained while maintaining the temperature of the NO _X absorbent 17, 18 at the required SO _X amount reduction temperature, for example, about 600 to 650 ° C. The SO _X amount reduction control is performed to switch the gas average air-fuel ratio to slightly rich.

More specifically, in the embodiment according to the present invention, the amount of accumulated SO _X is calculated by sequentially integrating the amount of SO _X flowing into the NO _X absorbents 17 and 18 per unit time. As in the case of NO _X and fine particles, the inflow SO _X amount per unit time corresponds to the amount of SO _X discharged from the combustion chamber per unit time, and this exhaust SO _X amount corresponds to the engine operating state, for example, fuel injection. It can be calculated according to the amount and the sulfur concentration in the fuel. Next, when the accumulated SO _X amount exceeds the upper limit SO _X amount, the above-described particulate removal control is started first. That is, when a large amount of fine particles are deposited on the particulate filter 19, a large amount of reducing agent is contained in the particulate filter 19 while maintaining the temperature of the NO _X absorbents 17, 18 and accordingly the temperature of the particulate filter 19 high. If a large amount of fine particles is supplied, the large amount of fine particles may cause abnormal combustion, and the particulate filter 19 may be melted. Therefore, in the embodiment according to the present invention, the particulate removal control is first performed when the SO _X amount reduction control is to be performed.

Next, when the particulate removal control is completed, the SO _X amount reduction control is started immediately. Since the temperature of the particulate removal control is performed particulate filter 19 therefore _the NO _X absorbent 17 is raised, the temperature of the NO _X absorbent 17, 18 for this way the SO _X amount reduction control It is not necessary to increase the temperature to the SO _X amount reduction required temperature, and thus energy can be used effectively.

The SO _X amount reduction control is performed as follows. That is, for example, low-temperature combustion is performed during low-load operation in which the fuel injection amount representing the engine load is less than a predetermined threshold amount, and high-EGR combustion is performed during high-load operation in which the fuel injection amount is greater than the threshold amount. Is called. These low temperature combustion and high EGR combustion will be briefly described with reference to FIG.

  FIG. 3 shows an example of simulation results showing the relationship between the EGR rate (= EGR gas amount / (EGR gas amount + intake air amount)) and the smoke discharge amount when the engine speed and the fuel injection amount are kept constant. Represents. As can be seen from FIG. 3, when the EGR rate is increased while maintaining the injection timing constant, the smoke discharge amount starts to increase. Next, when the EGR rate is further increased and the air-fuel ratio of the air-fuel mixture is decreased while making the injection timing constant, the amount of smoke generated increases rapidly and reaches a peak. Next, if the EGR rate is further increased while the injection timing is made constant and the air-fuel ratio of the air-fuel mixture is reduced, then the smoke is rapidly lowered. If the EGR rate is further increased while making the injection timing constant, the smoke becomes almost zero. That is, almost no wrinkles occur.

  Then, when the internal combustion engine shown in FIG. 1 increases the amount of EGR gas supplied into the combustion chamber while maintaining the injection timing constant, the generation amount of soot gradually increases and reaches a peak, and the injection timing is reduced. If the amount of EGR gas supplied into the combustion chamber is further increased while maintaining a constant level, the temperature of the fuel and the surrounding gas during combustion in the combustion chamber will be lower than the soot generation temperature, and soot will hardly be generated. It can also be seen that it consists of an internal combustion engine.

  As indicated by I in FIG. 3, combustion performed at an EGR rate that is higher than the EGR rate at which the amount of soot generation reaches a peak and hardly generates soot is low-temperature combustion. On the other hand, as shown by II in FIG. 3, high EGR combustion is combustion performed at a relatively high EGR rate among EGR rates having a lower EGR rate than the EGR rate at which soot generation peaks. It is. In this case, the air-fuel ratio of the air-fuel mixture burned in the combustion chamber is set to, for example, about 18 to 20 in the low temperature combustion, and is set to about 22 to 25 in the high EGR combustion. Note that the air-fuel ratio during normal operation is about 25 to 70. On the other hand, in low-temperature combustion, only main fuel injection is performed at a crank angle of about 20 degrees before compression top dead center, and in high EGR combustion, only main fuel injection is performed around compression top dead center. Note that hysteresis can be provided for the above-described threshold value.

  Regardless of which combustion is performed, the opening degree of the EGR control valve 27 coincides with the target opening degree determined according to the engine operating state, for example, the fuel injection amount and the engine speed. On the other hand, the opening degree of the throttle valve 8 is such that the output of the air-fuel ratio sensor 41 is such that the air-fuel ratio of the exhaust gas discharged from the engine matches the target air-fuel ratio determined according to the engine operating state, for example, the fuel injection amount and the engine speed. Controlled based on

Then, fuel is supplied from the fuel supply valve 25, for example, intermittently. In this case, the amount of fuel supplied from the fuel supply valve 25 is controlled based on the output of the air-fuel ratio sensor 42 so that the average air-fuel ratio of the exhaust gas flowing into the NO _X absorbents 17 and 18 becomes rich. The Note that when the SO _X amount reduction control is being performed, the target rotational speed during idle operation is increased to, for example, 1200 rpm.

When the sulfur amount reduction control is started, the amount of accumulated SO _X in the NO _X absorbents 17 and 18 gradually decreases. The amount of accumulated SO _X that decreases per unit time can be calculated according to the temperature of the NO _X absorbents 17 and 18, for example. In the embodiment according to the present invention, successively subtracts the accumulated SO _X amount which decreases per unit time from the accumulation SO _X amount, SO _X amount reduction control is completed when the amount of stored SO _X becomes the lower limit value for example zero. That is, the normal operation is restored.

Incidentally, as described above, when to perform the SO _X amount reduction control following the particle removal control, it is possible to effectively utilize the energy. Therefore, in the embodiment according to the present invention, when the amount of fine particles deposited on the particulate filter 19 becomes larger than the upper limit fine particle amount and then the fine particle removal control is performed, the accumulated SO _X amount at this time is smaller than the upper limit SO _X amount. However, the SO _X amount reduction control is performed following the particulate removal control.

However, when the accumulated SO _X amount at the time when the particulate removal control is completed is rather small, it is preferable not to perform the SO _X amount reduction control in terms of effective use of energy. Therefore, in the embodiment according to the present invention, an allowable SO _X amount smaller than the upper limit SO _X amount is set in advance, and when the accumulated SO _X amount is smaller than the allowable SO _X amount when the particulate removal control is completed, the SO _X amount decrease control is performed. Do not do. This allowable SO _X amount can be set to about ½ of the upper limit SO _X amount, for example.

By the way, as described at the beginning, if the SO _X amount reduction control is performed when the engine load is high, a large amount of fuel may be consumed or the temperature of the particulate filter 19 may become excessively high.

In the embodiment according to the present invention, therefore, it stops the SO _X amount reduction control when the fuel injection amount QF representing the engine load becomes greater than the upper-limit injection amount QFH predetermined when SO _X amount reduction control is being carried out Then, when the engine injection amount becomes smaller than the upper limit injection amount, the SO _X amount reduction control is resumed.

  FIG. 4 shows an example of the upper limit injection amount QFH. This upper limit injection amount QFH is stored in advance in the ROM 32 as a function of the engine speed N in the form of a map shown in FIG. In FIG. 4, the alternate long and short dash line represents the maximum injection amount corresponding to the full load. Moreover, a hysteresis can also be provided in the upper limit injection amount QFH.

In the embodiment according to the present invention, when the fuel injection amount QF is larger than the upper limit injection amount QFH and the SO _X amount reduction control is to be stopped, the temperature of the NO _X absorbents 17 and 18 is set higher than that in the normal operation. the air-fuel ratio of the exhaust gas flowing into the _the NO _X absorbent 17 and 18 while maintaining the temperature performs standby control back to lean, the fuel injection amount QF is an upper limit injection amount QFH when standby control is being performed _{If the} amount is less, the SO _X amount reduction control is resumed. In this way, when the fuel injection amount QF becomes smaller than the upper limit injection amount QFH, the SO _X amount reduction control can be started promptly.

In the standby control, as in the normal operation, for example, the opening degree of the throttle valve 8 is controlled in accordance with the depression amount of the accelerator pedal, and the fresh air amount detected by the air flow meter 40 is determined based on the engine operation state, for example, the fuel injection amount and the engine rotation. The opening degree of the EGR control valve 27 is controlled so as to coincide with the target fresh air amount determined according to the number. Further, only the main fuel injection FM or the pilot injection FP is performed together with the main fuel injection FM. On top of that, as the temperature of the NO _X absorbent 17 detected by the temperature sensor 43 is coincident with the standby temperature, the fuel from the fuel supply valve 25 is supplied secondarily.

The standby temperature may be determined in any way as long as it is higher than the temperature of the NO _X absorbents 17 and 18 during normal operation. However, in the embodiment according to the present invention, the standby temperature is set to the SO _X amount reduction required temperature, that is, about 600 to 630 ° C.

  However, it is not preferable to continue the standby control for a long time because the engine load is higher than the upper limit load. That is, when the standby control is continued for a certain time, it is preferable to stop the standby control and return to the normal operation.

However, considering that it is preferable to reduce the amount of accumulated SO _X in the NO _X absorbents 17 and 18 to zero as much as possible, the standby control should be continued as much as possible.

Therefore, in the embodiment according to the present invention, when the standby control is continued for a predetermined set time, the accumulated SO _X amount in the NO _X absorbents 17 and 18 at this time is obtained, and the accumulated SO _X amount is determined in advance. When the amount of SO _{X is} larger than the set SO _X amount, the standby control is continued, and when the amount of stored SO _X is smaller than the set SO _X amount, the standby control is stopped to return to the normal operation.

That is, as shown by the arrow X in FIG. 5, when the fuel injection amount QF becomes larger than the upper limit injection amount QFH, the SO _X amount reduction control is stopped and standby control is started. That is, the average air-fuel ratio AF of the exhaust gas flowing into the NO _X absorbents 17 and 18 is returned to lean while maintaining the temperature TCAT of the NO _X absorbents 17 and 18 at the standby temperature, that is, the SO _X amount reduction required temperature TS. . Next, when the standby control is continued for the set time tC as indicated by the arrow Y in FIG. 5, the standby control is continued when the accumulated SO _X amount QS is larger than the set SO _X amount QS1. Next, as shown by an arrow Z in FIG. 5, when the fuel injection amount QF becomes smaller than the upper limit injection amount QFH, the SO _X amount reduction control is resumed.

In contrast, when the standby control is continued for the set time tC as indicated by the arrow Y ′ in FIG. 6, when the stored SO _X amount QS is smaller than the set SO _X amount QS1, the standby control and the SO _X amount are Decrease control is stopped and normal operation is resumed.

In this way, it is possible to reliably reduce the accumulation SO _X amount while reducing energy consumption.

Therefore, generally speaking, when the engine load becomes higher than a predetermined upper limit load when the sulfur amount reduction control is being performed, the temperature of the NO _X absorbent is set higher than that during normal operation. While maintaining the temperature, standby control is performed to make the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent lean, and when the standby control is continued for a preset time, it is stored in the NO _X absorbent at this time. When the amount of sulfur that has been obtained is greater than a predetermined set amount, standby control is continued, and then when the engine load becomes lower than the upper limit load, sulfur amount reduction control is performed. Is resumed, and when the obtained amount of sulfur is smaller than the set amount, the standby control is stopped and the normal operation is resumed.

  7 to 9 show the exhaust gas purification routine of the embodiment according to the present invention described above. This routine is executed by interruption every predetermined time.

7 to 9, first, at step 100, it is judged if the SO _X amount reduction flag XS is set. The SO _X amount reduction flag XS is set when the SO _X amount reduction control is to be executed (XS = 1), and is otherwise reset (XS = 0). When the SO _X amount decrease flag XS is reset, the routine proceeds to step 101 where it is determined whether or not the particulate removal flag XPM is set. The particulate removal flag XPM is set when the particulate removal control is to be executed (XPM = 1), and otherwise is reset (XPM = 0). When the particulate removal flag XPM is reset, the routine proceeds to step 102 where it is determined whether or not the amount of particulate particulate QPM on the particulate filter 19 is larger than the upper limit particulate amount QPMH. Next, when QPM ≦ QPMH, the routine proceeds to step 103, where it is judged if the accumulated SO _X amount QS in the NO _X absorbents 17, 18 is larger than the upper limit SO _X amount QSH. When QS ≦ QSH, the routine proceeds to step 104, where it is determined whether or not an execution condition for the NO _x amount reduction control is satisfied. In the embodiment according to the present invention, when the temperature of the set time interval described above from the previous amount of NO _X reduction control is passed vital _the NO _X absorbent 17 is higher than the set temperature, for example 190 ° C. of the NO _X amount reduction control It is determined that the execution condition is satisfied, and otherwise, it is determined that the execution condition of the NO _x amount reduction control is not satisfied. When it is determined that the execution condition for the NO _X amount reduction control is not satisfied, the processing cycle is ended, that is, normal operation is performed. In contrast, the routine goes to step 105 when the execution condition of the NO _X amount reduction control is judged to be satisfied, _the amount of NO _X reduction control is performed.

  On the other hand, when QPM> QPMH at step 102 or when QS> QSH at step 103, the routine proceeds to step 106 where the particulate removal flag XPM is set.

When the particulate removal flag XPM is set, the routine proceeds from step 101 to step 107, where particulate removal control is executed. In the next step 108, it is determined whether or not the amount of accumulated particulates QPM on the particulate filter 19 is smaller than the lower limit amount QPML. When QPM ≧ QPML, the processing cycle is terminated. On the other hand, when QPM <QPML, the routine proceeds to step 109, where the particulate removal flag XPM is reset (XPM = 0). In the following step 110, the SO _X amount decrease flag XS is set (XS = 1). In the next step 111, it is determined whether or not the accumulated SO _X amount QS in the NO _X absorbents 17 and 18 is smaller than the allowable SO _X amount QSA. When QS ≧ QSA, the processing cycle is terminated while the SO _X amount decrease flag XS is set (XS = 1). On the other hand, when QS <QSA, the routine proceeds to step 112 where the SO _X amount decrease flag XS is reset (XS = 0), and the processing cycle is ended.

When the SO _X amount decrease flag XS is set, the routine proceeds from step 100 to step 113, where the upper limit injection amount QFH is calculated from the map of FIG. In the following step 114, it is determined whether or not the fuel injection amount QF is smaller than the upper limit injection amount QFH. When QF <QFH, the routine proceeds to step 115 where a counter C described later is cleared. In the following step 116, SO _X amount reduction control is executed. In the following step 117, it is determined whether or not the accumulated SO _X amount QS in the NO _X absorbents 17 and 18 has become zero. When QS> 0, the processing cycle is terminated. When QS = 0, the routine proceeds to step 118, where the SO _X amount decrease flag XS is reset (XS = 0). That is, the SO _X amount reduction control is completed.

When QF ≧ QFH at step 114, the routine proceeds to step 119, where standby control is executed. In the following step 120, the counter C indicating the time during which the standby control is continuously performed is incremented by one. In the following step 121, it is determined whether or not the counter C is larger than the set value C1 representing the set time tC described above. When C ≦ C1, the processing cycle is terminated, and when C> C1, the routine proceeds to step 122. Whether the accumulated SO _X amount QS in the NO _X absorbents 17 and 18 at this time is larger than the set SO _X amount QS1 or not. Is determined. When QS> QS1, the processing cycle is terminated, that is, the standby control and the SO _X amount reduction control are continued. On the other hand, when QS ≦ QS1, the routine proceeds to step 118, where the SO _X amount decrease flag XS is reset (XS = 0). That is, the standby control and the SO _X amount reduction control are stopped.

1 is an overall view of an internal combustion engine. It is a figure for demonstrating the form and timing of fuel injection. It is a figure for demonstrating low temperature combustion and high EGR combustion. It is a figure which shows the upper limit injection amount QFH. It is a time chart for demonstrating the Example by this invention. It is a time chart for demonstrating the Example by this invention. It is a time chart which shows the exhaust gas purification routine of the Example by this invention. It is a time chart which shows the exhaust gas purification routine of the Example by this invention. It is a time chart which shows the exhaust gas purification routine of the Example by this invention.

Explanation of symbols

1 ... engine body 17, 18 ... NO _X absorbent 19 ... particulate filter 25 ... fuel feed valve 43 ... Temperature sensor

Claims (3)

In the exhaust passage of an internal combustion engine in which combustion is performed under a lean air-fuel ratio, NO _X in the exhaust gas flowing in when the air-fuel ratio of the inflowing exhaust gas is lean is stored, and the air-fuel ratio of the inflowing exhaust gas There place _the NO _X absorbent to reduce the amount of the NO _X are stored by reducing NO _X are stored as containing a reducing agent in the exhaust gas when dropped, in the NO _X absorbent When the amount of stored sulfur should be reduced, the sulfur that makes the air-fuel ratio of the exhaust gas flowing into the NO _X absorbent into the stoichiometric air-fuel ratio or rich while maintaining the temperature of the NO _X absorbent at the sulfur amount reduction required temperature In an internal combustion engine configured to perform the amount reduction control, when the engine load becomes higher than a predetermined upper limit load when the sulfur amount reduction control is being performed, the temperature of the NO _X absorbent is set during normal operation. than There the air-fuel ratio of the exhaust gas flowing while being in _the NO _X absorbent held at a standby temperature performs standby control to lean, in this case _the NO _X absorbent when the standby control is continued by setting a predetermined time The amount of sulfur stored in the engine is obtained, and when the obtained amount of sulfur is larger than a predetermined set amount, the standby control is continued, and when the engine load becomes lower than the upper limit load, the sulfur is An exhaust emission control device for an internal combustion engine, wherein the amount reduction control is resumed, and when the calculated amount of sulfur is smaller than the set amount, the standby control is stopped to return to the normal operation. The fuel is secondarily supplied into an exhaust passage upstream of the NO _X absorbent in order to keep the temperature of the NO _X absorbent at the standby temperature when the standby control is being performed. Exhaust gas purification device for internal combustion engine. Wherein on the particulate filter for trapping particulate in the exhaust gas _the NO _X absorbent are carried is the exhaust gas flowing into the particulate filter in order to oxidize and remove the particulates deposited on the particulate filter Particulate removal control is performed to increase the temperature of the particulate filter while maintaining the air-fuel ratio of the exhaust gas lean, and the amount of sulfur stored in the NO _X absorbent when the particulate oxidation control is completed And determining that the amount of sulfur stored in the NO _X absorbent should be reduced when the amount of sulfur determined is greater than a predetermined allowable amount, the amount was prevented from being determined that it should reduce the amount of sulfur that is accumulated in _the nO _X absorbent when less than該許capacity The exhaust emission control device for an internal combustion engine according to claim 1.

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