JP5812952B2 - Exhaust gas purification system for internal combustion engine - Google Patents

Description

The present invention relates to an exhaust gas purification system for an internal combustion engine. More specifically, the present invention relates to an exhaust gas purification system for an internal combustion engine provided with a selective reduction catalyst and an NH ₃ sensor provided downstream thereof.

Conventionally, as one of exhaust purification systems for purifying NOx in exhaust, a system in which a selective reduction catalyst for selectively reducing NOx in exhaust with a reducing agent such as ammonia (NH ₃ ) is provided in the exhaust passage has been proposed. (For example, refer to Patent Document 1). For example, in the exhaust purification system of urea addition type, the NH ₃ by supplying urea water which is a precursor of NH ₃ from the upstream side of the selective reduction catalyst, thermal decomposition or hydrolysis in the exhaust heat from the urea water The NOx in the exhaust gas is selectively reduced by this NH ₃ . In addition to such a urea addition type system, for example, a system in which NH ₃ is generated by heating a NH ₃ compound such as ammonia carbide and this NH ₃ is directly added has also been proposed.

Selective reduction catalysts have a lean air-fuel ratio that is leaner than stoichiometric and exhibits high NOx purification performance under lean air-fuel ratio exhaust gas that contains a large amount of oxygen. Therefore, lean combustion such as lean-burn gasoline engines and diesel engines Often used in engine exhaust purification systems based on Further, in this case, by controlling the supply amount of urea water or NH ₃ supplied to the selective reduction catalyst by feedback control based on the detection signal of the NH ₃ sensor provided on the downstream side of the selective reduction catalyst, It is also known that the NOx purification rate can be maintained high while minimizing the NH ₃ slip amount (see, for example, Patent Document 1).

  However, during acceleration operation in which the amount of NOx emissions increases, NOx may not be sufficiently purified with only the selective reduction catalyst. Therefore, as in the system disclosed in Patent Document 2, it is conceivable to purify NOx using a three-way purification reaction in a three-way catalyst provided upstream of the selective reduction catalyst during acceleration operation. In the exhaust purification system of Patent Document 2, in a system in which a three-way catalyst is provided on the upstream side of the NOx storage reduction catalyst, the air-fuel ratio of the air-fuel mixture is made lean to use the three-way purification reaction in the three-way catalyst during acceleration operation. Switch from side to stoichiometric.

JP 2011-163195 A JP 2009-293585 A

  However, if the three-way purification reaction in the three-way catalyst is to be performed with high efficiency, the air-fuel ratio of the air-fuel mixture must be accurately maintained at or near the stoichiometry. Further feedback from an oxygen concentration sensor that directly detects the oxygen concentration of the gas is required. Even in a system that does not include such a three-way catalyst, the air-fuel ratio of the air-fuel mixture can also be used during regeneration of an exhaust purification filter that collects soot in exhaust gas or during SOx poisoning regeneration of a selective reduction catalyst. In order to enrich the stoichiometry or the richer side than the stoichiometry, an oxygen concentration sensor is required.

  The present invention has been made in consideration of the above points, and in an exhaust purification system including a selective reduction catalyst and an ammonia sensor provided downstream thereof, a new oxygen concentration sensor is not provided. An object of the present invention is to provide an exhaust purification system that can determine the state of the air-fuel ratio of exhaust.

(1) In order to solve the above problems, the present invention provides a nitrogen oxide provided in an exhaust passage (for example, an exhaust pipe 11 to be described later) of an internal combustion engine (for example, an engine 1 to be described later) in the exhaust in the presence of ammonia. A selective reduction catalyst (for example, an SCR catalyst of a downstream catalytic converter 33 described later) and a reducing agent supply device (for example, an ammonia or a precursor thereof as a reducing agent upstream of the selective reduction catalyst) A urea water supply device 4 (to be described later), an ammonia sensor (for example, an NH ₃ sensor 5 to be described later), and a reducing agent supply amount control means (for example, a control unit for controlling a supply amount of the reducing agent according to a detection signal of the ammonia sensor An exhaust gas purification system for an internal combustion engine (for example, an exhaust gas purification system 2 described later) is provided. The ammonia sensor includes a reference electrode having activity against oxygen (for example, a reference electrode 53 to be described later), a detection electrode having activity to oxygen and ammonia (for example, a detection electrode 52 to be described later), and the detection electrode and the reference electrode. And an oxygen ion conductive solid electrolyte (for example, a solid electrolyte layer 51 described later). The reference electrode and the detection electrode are provided in the exhaust passage so as to be exposed to the same exhaust gas downstream of the selective reduction catalyst. In the exhaust purification system, when the value of the detection signal of the ammonia sensor changes beyond a predetermined stoichiometric determination threshold, the air-fuel ratio of the exhaust on the downstream side of the selective reduction catalyst is stoichiometric or stoichiometric from the lean side to the stoichiometric. Further, stoichiometric determination means (for example, a downstream stoichiometric determination unit 73 described later) for determining that the rich side has been reached is further provided.

(1) During the lean operation in which the air-fuel ratio of the air-fuel mixture is leaner than the stoichiometry to reduce NOx mainly in the selective reduction catalyst, both the detection electrode and the reference electrode of the ammonia sensor are sufficient. It is exposed to exhaust gas containing a large amount of oxygen. In this case, a reduction reaction that ionizes oxygen (O ₂ ) proceeds on the reference electrode side, oxygen ions (O ²⁻ ) move through the electrolyte, and ammonia (NH ₃ ) and oxygen ions in the exhaust gas are moved on the detection electrode side. Oxidation proceeds, and an electromotive force (detection signal) that varies at least according to the ammonia concentration in the exhaust gas is generated between the detection electrode and the reference electrode. The reducing agent supply amount control means controls the reducing agent supply amount based on the detection signal of such an ammonia sensor, for example, while suppressing the amount of ammonia discharged from the selective reduction catalyst to a minimum. An appropriate amount of reducing agent (ammonia or a precursor thereof) can be supplied to the selective reduction catalyst so that the purification rate is maintained high.

  If the air-fuel ratio stoichiometric control that makes the air-fuel ratio of the air-fuel mixture stoichiometric or the air-fuel ratio rich control that makes the air-fuel ratio richer than the stoichiometric condition from the state of lean operation as described above, both electrodes of the ammonia sensor downstream of the selective reduction catalyst After a short time, the atmosphere is switched to a low oxygen concentration atmosphere in which the oxygen concentration is almost zero. At this time, since the electron transfer by oxygen ions does not proceed at the interface between the electrode and the solid electrolyte, the potential difference between the two electrodes increases, so the value of the detection signal greatly changes to the positive or negative side regardless of the ammonia concentration. The maximum value or the minimum value is indicated. In other words, a larger inherent spontaneous electromotive force is generated at the detection electrode and the reference electrode than when the oxygen concentration is high. In the present invention, a predetermined stoichiometric determination threshold value is set for the value of the detection signal of the ammonia sensor, and when the value of the detection signal changes beyond the stoichiometric determination threshold value, the selective reduction catalyst is changed accordingly. It is determined that the air-fuel ratio of the exhaust on the downstream side of the engine has changed from the lean side to the stoichiometric or richer side than the stoichiometric. Therefore, according to the present invention, it is possible to determine the actual air-fuel ratio state of the exhaust gas by using the ammonia sensor without providing a new sensor for detecting the oxygen concentration in the exhaust passage.

(2) In this case, the reducing agent supply amount control means has a normal output range in which the value of the detection signal of the ammonia sensor is set inside the stoichiometric determination threshold (for example, a normal NH ₃ output range described later). It is preferable to control the supply amount of the reducing agent based on the detection signal of the ammonia sensor.

  (2) Under a stoichiometric or rich air-fuel ratio, NOx cannot basically be reduced by the selective reduction catalyst. During this time, it is not necessary to control the reducing agent supply amount by the reducing agent supply amount control means. Therefore, the normal output range is set inside the stoichiometric determination threshold for determining the state of the air-fuel ratio of the exhaust based on the detection signal of the ammonia sensor, and the value of the detection signal of the ammonia sensor is within this normal output range. Sometimes, by controlling the supply amount of the reducing agent based on the detection signal of the ammonia sensor, various controls performed based on the supply control of the reducing agent by the reducing agent supply amount control means and the determination of the stoichiometry by the stoichiometric determination means Can be prevented from interfering with each other.

(3) In this case, the exhaust gas purification system is based on the detection signal of the ammonia sensor when the value of the detection signal of the ammonia sensor is within a normal output range set inside the stoichiometric determination threshold value. And an ammonia concentration calculating means (for example, NH ₃ concentration calculating section 72 described later) for calculating the ammonia concentration in the exhaust downstream of the selective reduction catalyst, wherein the reducing agent supply amount control means It is preferable to control the supply amount of the reducing agent based on the concentration.

  (3) When the oxygen concentration in the exhaust gas is reduced to 0 or in the vicinity thereof, the detection signal of the ammonia sensor largely changes to the positive or negative side regardless of the ammonia concentration in the exhaust gas as described above. The ammonia concentration cannot be calculated from the detection signal of the sensor. In the present invention, the normal output range is set inside the stoichiometric threshold for determining the state of the air-fuel ratio of the exhaust based on the detection signal of the ammonia sensor, and the value of the detection signal of the ammonia sensor falls within this normal output range. The ammonia concentration can be calculated with high accuracy by calculating the ammonia concentration of the exhaust gas based on the detection signal of the ammonia sensor. In addition, by controlling the supply amount of the reducing agent based on the ammonia concentration calculated accurately in this way, for example, the NOx purification rate is kept high while minimizing the amount of ammonia discharged from the selective reduction catalyst. As such, an appropriate amount of reducing agent (ammonia or precursor thereof) can be fed to the selective reduction catalyst.

  (4) In this case, the exhaust purification system is provided on the upstream side of the selective reduction catalyst in the exhaust passage and has an upstream catalyst having a three-way purification function (for example, an oxidation catalyst or three of the upstream catalytic converter 31 described later). Parameters of the original catalyst and the oxidation catalyst or the three-way catalyst of the exhaust purification filter 32) and the NOx emission amount of the engine (for example, an estimated value of NOx emission amount and a required torque value, which will be described later) When the air-fuel ratio of the air-fuel mixture is less than the predetermined threshold when the air-fuel ratio is less than the predetermined threshold, and when the parameter value is larger than the threshold, the air-fuel ratio of the air-fuel mixture is determined based on the determination by the stoichiometric determination means. It is preferable to further include air-fuel ratio control means (for example, an air-fuel ratio control unit 76 described later) for controlling the fuel ratio to stoichiometric.

  (4) According to the present invention, when the value of the parameter correlated with the NOx emission amount is equal to or less than the predetermined threshold value, the air-fuel ratio of the air-fuel mixture is controlled to be leaner than the stoichiometric, and NOx is reduced by the selective reduction catalyst. In the case where the value of the parameter correlated with the NOx emission amount is larger than the stoichiometric determination threshold value, the NOx is reduced using the three-way purification reaction in the upstream catalyst by controlling the air-fuel ratio of the air-fuel mixture to stoichiometric. Can be reduced. At this time, in the present invention, the oxygen concentration sensor is not newly provided by controlling the air-fuel ratio of the air-fuel mixture to stoichiometric based on the determination by the stoichiometric determination means using the ammonia sensor provided downstream of the selective reduction catalyst. In both cases, the existing ammonia sensor can efficiently advance the three-way purification reaction to purify the exhaust.

  (5) In this case, the exhaust purification system controls an EGR device (for example, an EGR device 13 described later) that recirculates a part of the exhaust gas flowing through the exhaust passage to the intake passage, and an exhaust gas recirculation amount by the EGR device. EGR control means (e.g., a high EGR control unit 77 to be described later), and the EGR control means in response to the determination that the air-fuel ratio has become richer than stoichiometric or stoichiometric by the stoichiometric determining means. Thus, it is preferable to correct the exhaust gas recirculation amount to the reduction side.

  (5) In the present invention, when the exhaust gas recirculation amount is controlled by the EGR control means, the stoichiometric determination means determines that the exhaust air-fuel ratio has become richer than stoichiometric or stoichiometric. By correcting the exhaust gas recirculation amount to the decreasing side, it is possible to prevent the air-fuel ratio from exceeding the stoichiometric condition and causing excessive fuel. Also in this case, since it is not necessary to newly provide an oxygen concentration sensor, the cost can be reduced accordingly.

  (6) In this case, the selective reduction catalyst includes an oxygen storage / release material, and the stoichiometric determination means starts from a predetermined time after the start of the control to make the air-fuel ratio of the air-fuel mixture richer than stoichiometric or stoichiometric. To measure the time (for example, rich determination time described later) required until it is determined that the air-fuel ratio downstream of the selective reduction catalyst has become richer than stoichiometric or stoichiometric, and the selection is performed based on the measured time. It is preferable to further include catalyst deterioration determination means (for example, an SCR catalyst deterioration determination control unit 78 described later) for determining deterioration of the reduction catalyst.

  (6) By including an oxygen storage / release material in the selective reduction catalyst, even after switching the air-fuel ratio of the air-fuel mixture from the lean side to the stoichiometric or rich side, the stored oxygen is used to make a transient transition with the selective reduction catalyst. NOx can be purified. Further, by including the oxygen storage / release material in the selective reduction catalyst, it is possible to estimate the deterioration of the selective reduction catalyst through detection of a decrease in the oxygen storage / release function of the oxygen storage / release material. In the present invention, the air-fuel ratio of the exhaust gas downstream of the selective reduction catalyst is richer than stoichiometric or stoichiometric by a stoichiometric determination means from a predetermined timing after starting the control to make the air-fuel ratio of the air-fuel mixture stoichiometric or richer than stoichiometric. Measure the time taken until it is determined that the oxygen storage / release material has been determined, that is, the time taken until the oxygen occluded / released material has completely released the oxygen, and determine the degradation of the selective reduction catalyst based on this measurement time. . Thereby, it is possible to determine the deterioration of the selective reduction catalyst without newly providing an oxygen concentration sensor on the downstream side of the selective reduction catalyst.

(7) In order to solve the above-mentioned problem, the present invention is provided in an exhaust passage (for example, an exhaust pipe 11 to be described later) of an internal combustion engine (for example, an engine 1 to be described later), and nitrogen oxide in the exhaust in the presence of ammonia. A selective reduction catalyst (for example, an SCR catalyst of a downstream catalytic converter 33 described later) and a reducing agent supply device (for example, an ammonia or a precursor thereof as a reducing agent upstream of the selective reduction catalyst) A urea water supply device 4 described later) and an ammonia sensor (for example, described later) that is provided downstream of the selective reduction catalyst and that generates an electromotive force according to at least ammonia and oxygen concentrations in the exhaust as a detection signal. NH ₃ sensor 5) and a reducing agent supply amount control means (for example, urea water injection control unit 7 described later) for controlling the supply amount of the reducing agent based on the detection signal of the ammonia sensor. 4) and an exhaust gas purification system for an internal combustion engine (for example, an exhaust gas purification system 2 described later). When the oxygen concentration of the surrounding exhaust gas becomes 0 or almost 0 between the electrodes of the ammonia sensor, an electromotive force exceeding a predetermined stoichiometric threshold is generated regardless of the ammonia concentration. In the exhaust purification system, when the value of the detection signal of the ammonia sensor changes beyond the stoichiometric determination threshold value, the air-fuel ratio of the exhaust on the downstream side of the selective reduction catalyst is from the stoichiometric or stoichiometric side from the lean side. A stoichiometric determination unit (for example, a downstream stoichiometric determination unit 73 described later) that determines that the rich side is reached is provided.

  (7) According to the present invention, it is possible to determine the actual air-fuel ratio state of exhaust using an existing ammonia sensor without providing a sensor for newly detecting the oxygen concentration in the exhaust passage.

(8) In this case, in the exhaust purification system, the value of the detection signal of the ammonia sensor is within a normal output range (for example, a normal NH ₃ output range described later) set inside the stoichiometric determination threshold value. In some cases, the apparatus further comprises ammonia concentration calculating means (for example, NH ₃ concentration calculating unit 72 described later) for calculating the ammonia concentration of the exhaust downstream of the selective reduction catalyst based on the detection signal of the ammonia sensor, The supply amount control means preferably controls the supply amount of the reducing agent based on the calculated ammonia concentration.

  (8) According to the present invention, the normal output range is set inside the stoichiometric determination threshold value for determining the state of the air-fuel ratio of the exhaust based on the detection signal of the ammonia sensor, and the value of the detection signal of the ammonia sensor is By calculating the ammonia concentration of the exhaust gas based on the detection signal of the ammonia sensor when within the normal output range, the ammonia concentration can be calculated with high accuracy. In addition, by controlling the supply amount of the reducing agent based on the ammonia concentration calculated accurately in this way, for example, the NOx purification rate is kept high while minimizing the amount of ammonia discharged from the selective reduction catalyst. As such, an appropriate amount of reducing agent (ammonia or precursor thereof) can be fed to the selective reduction catalyst. Further, it is possible to prevent interference between the reducing agent supply control by the reducing agent supply amount control means and various controls performed based on the stoichiometric determination by the stoichiometric determination means.

  According to the present invention, by utilizing a peculiar change in the low oxygen concentration atmosphere of the detection signal of the ammonia sensor, the existing ammonia sensor can control the air-fuel ratio of the exhaust gas without providing a new oxygen concentration sensor in the exhaust passage. The state can be determined.

It is a figure which shows the structure of the exhaust gas purification system which concerns on one Embodiment of this invention. The structure of the O ₂ sensor is a diagram schematically showing the detection principle of the O ₂ concentration. NH ₃ and the structure of the sensor is a diagram schematically showing the detection principle of the NH ₃ concentration. The change in the definitive O ₂ sensor and NH ₃ detection signals of the sensor when the air-fuel ratio of the mixture was controlled from the temporarily stoichiometric from the lean side stoichiometric is a diagram schematically illustrating. It is a figure which shows the control block comprised by ECU. It is a flowchart which shows the procedure of the temperature rising process of filter reproduction | regeneration processing. It is a flowchart which shows the procedure of stoichi air fuel ratio control. It is a flowchart which shows the procedure of high EGR control. It is a flowchart which shows the procedure of SCR catalyst deterioration determination control.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram showing a configuration of an internal combustion engine (hereinafter referred to as “engine”) 1 and an exhaust purification system 2 thereof according to the present embodiment. The engine 1 is based on so-called lean combustion in which the air-fuel ratio of the air-fuel mixture is leaner than the stoichiometric state during steady operation, more specifically, a diesel engine, a lean burn gasoline engine, or the like.

  The exhaust purification system 2 recirculates part of the exhaust gas flowing through the exhaust pipe 11 to the intake pipe 12 reaching the intake port of the engine 1 and the catalyst purification device 3 provided in the exhaust pipe 11 extending from the exhaust port of the engine 1. An EGR device 13 and an electronic control unit (hereinafter referred to as “ECU”) 7 that controls the engine 1, the catalyst purification device 3, and the EGR device 13 are provided.

  The engine 1 is provided with a fuel injection valve 17 that injects fuel into each cylinder. The actuator that drives the fuel injection valve 17 is electromagnetically connected to the ECU 7. The air-fuel ratio of the air-fuel mixture of the engine 1 depends on the amount of fresh air introduced into the cylinder by the intake control valve 16, the amount of exhaust gas introduced into the cylinder by the EGR device 13, and the fuel provided in each cylinder of the engine It is controlled by adjusting the fuel injection amount from the injection valve 17 and the like.

  The catalyst purification device 3 includes an upstream catalytic converter 31, an exhaust purification filter 32, a downstream catalytic converter 33, and a urea water supply device 4. The upstream catalytic converter 31 is provided in the exhaust pipe 11 immediately below the engine 1. The downstream catalytic converter 33 is provided downstream of the upstream catalytic converter 31 in the exhaust pipe 11. The exhaust purification filter 32 is provided between the upstream catalytic converter 31 and the downstream catalytic converter 33 in the exhaust pipe 11. These upstream catalytic converter 31 and downstream catalytic converter 33 are provided with a catalyst for promoting a reaction for purifying components such as CO, HC and NOx contained in the exhaust of the engine 1.

  As the upstream catalyst included in the upstream catalytic converter 31, a catalyst having at least a three-way purification function is used. The three-way purification function is a three-way purification reaction, that is, the oxidation of HC and CO and the reduction of NOx at the same time in the exhaust air atmosphere of the stoichiometric air-fuel ratio generated when the air-fuel ratio of the mixture is controlled stoichiometrically. This refers to the function of the reaction that takes place. Examples of the catalyst having such a three-way purification function include an oxidation catalyst and a three-way catalyst. As the upstream catalyst, an oxidation catalyst or a three-way catalyst is preferably used.

The oxidation catalyst (DOC) purifies HC, CO, and NOx with high efficiency by the above three-way purification reaction under a stoichiometric atmosphere. The oxidation catalyst purifies by oxidizing HC and CO under the lean air-fuel ratio exhaust atmosphere generated when the air-fuel ratio of the air-fuel mixture is controlled to be leaner than the stoichiometry, and a part of NO in the exhaust gas Is oxidized to NO ₂ to improve the NOx purification rate in the downstream catalytic converter 33 described later. A three-way catalyst (TWC) corresponds to an oxidation catalyst added with an oxygen storage / release material. The three-way catalyst and the oxidation catalyst have the same basic purification function. However, the three-way catalyst is superior to the oxidation catalyst in that it has an oxygen storage / release material and has a wide three-way purification window.

  The exhaust purification filter 32 collects particulate matter (Particulate Matter, hereinafter simply referred to as “PM”) in the exhaust from the engine 1. The exhaust purification filter 32 carries the above-described oxidation catalyst and three-way catalyst so that the accumulated PM can be burned and removed from a lower temperature and the components such as CO, HC, NOx in the exhaust are purified. Preferably it is. Hereinafter, the abbreviation “CSF” is used for the exhaust purification filter 32 on which the catalyst is thus supported.

Downstream catalytic converter 33, NH ₃ selective reduction catalyst (hereinafter, referred to as "SCR catalyst") that reduces NOx in the exhaust gas in the presence of NH ₃ comprises a. In the downstream catalytic converter 33, in the presence of NH ₃ as a reducing agent, a Fast-SCR reaction (see the following formula (1)), a Standard-SCR reaction (see the following formula (2)), and a Slow-SCR reaction ( The following formula (3)) proceeds.
NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O (1)
4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O (2)
6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O (3)

Of the above three reactions, the reaction of formula (2) that reduces NO proceeds only under a lean air-fuel ratio in which oxygen is present. The reaction shown in Equation (1) reducing the formula (3) or NO and NO ₂ for reducing the NO _2, although the oxygen concentration of the exhaust gas can proceed even in the stoichiometric or under a rich air-fuel ratio becomes substantially 0, in this case In the upstream catalytic converter 31 and the CSF 32, the reaction of oxidizing NO in the exhaust to NO ₂ does not proceed, and the amount of NO ₂ flowing into the SCR catalyst is also reduced. Therefore, the reactions of these equations (1) and (3) are also performed. It will not progress. Therefore, the SCR catalyst can basically purify NOx only during lean operation and when the oxygen concentration in the exhaust is sufficient.

The SCR catalyst has a function of reducing NOx in the exhaust with NH ₃ generated from urea water, and also has a function of storing the generated NH ₃ in a predetermined amount. Hereinafter, the NH ₃ amount stored in the SCR catalyst and NH ₃ storage amount, the maximum NH ₃ storage _capacity, NH ₃ amount that can be stored in the SCR catalyst. The NH ₃ stored in the SCR catalyst in this way is also consumed as appropriate in the reduction of NOx in the exhaust. For this reason, the SCR catalyst has a characteristic that the NOx purification rate increases as the NH ₃ storage amount increases.

The urea water supply device 4 includes a urea water tank 41 and a urea water injector 42. The urea water tank 41 stores urea water that is a precursor of NH ₃ . The urea water tank 41 is connected to a urea water injector 42 via a urea water supply path 43 and a urea water pump (not shown). The urea water injector 42 opens and closes when driven by an actuator (not shown), and injects urea water supplied from the urea water tank 41 to the upstream side of the downstream catalytic converter 33 in the exhaust pipe 11 as a reducing agent in the SCR catalyst. . The urea water injected from the injector 42 is hydrolyzed into NH ₃ in the exhaust gas or in the downstream catalytic converter 33 and consumed for the reduction of NOx. The actuator of the urea water injector 42 is electromagnetically connected to the ECU 7. As will be described in detail later, the ECU 7 determines the urea water injection amount by the urea water injection control based on the detection signal of the NH ₃ sensor 5, and drives the urea water injector 42 so that this amount of urea water is injected.

  The EGR device 13 includes an EGR pipe 14, an EGR control valve 15, an EGR cooler (not shown), and the like. The EGR pipe 14 connects the intake pipe 12 to the upstream side of the upstream catalytic converter 31 in the exhaust pipe 11. The EGR control valve 15 is provided in the EGR pipe 14 and controls the amount of exhaust gas recirculated into the cylinder of the engine 1 via the EGR pipe 14 (hereinafter referred to as “EGR amount”). The actuator that drives the EGR control valve 15 is electromagnetically connected to the ECU 7. The ECU 7 calculates an estimated value and a target value of the EGR amount (or EGR rate) by a process (not shown), and drives the EGR control valve 15 so that the estimated value becomes the target value.

  The ECU 7 shapes input signal waveforms from various sensors, corrects the voltage level to a predetermined level, converts an analog signal value into a digital signal value, and a central processing unit (hereinafter “ CPU ”), a storage circuit for storing various calculation programs executed by the CPU, calculation results, and the like, a fuel injection valve 17, an EGR control valve 15 and an intake control valve 16 for controlling the air-fuel ratio, and urea water And an output circuit that outputs a control signal to the urea water injector 42 for injection control.

The ECU 7 is connected with an O ₂ sensor 6 and an NH ₃ sensor 5 as sensors for detecting the states of the exhaust purification system 2 and the engine 1.

The O ₂ sensor 6 is provided between the CSF 32 and the downstream catalytic converter 33 in the exhaust pipe 11. The O ₂ sensor 6 acts as will be described in detail later with reference to FIG. 2, and outputs an electromotive force that varies according to the O ₂ concentration of the exhaust gas between the CSF 32 and the downstream catalytic converter 33 as a detection signal.
The NH ₃ sensor 5 is provided on the downstream side of the downstream catalytic converter 33 including the SCR catalyst in the exhaust pipe 11. The NH ₃ sensor 5 acts as will be described in detail later with reference to FIG. 3, and outputs an electromotive force that fluctuates according to the NH ₃ concentration in the exhaust downstream of the downstream catalytic converter 33 as a detection signal.

Next, the principle of detection of the O ₂ concentration by the O ₂ sensor 6 will be described with reference to FIG.
FIG. 2 is a diagram schematically showing the configuration of the O ₂ sensor 6 and the principle of detection of the O ₂ concentration.
The O ₂ sensor 6 includes a solid electrolyte layer 61, a porous detection electrode 62 and a reference electrode 63 provided on the solid electrolyte layer 61, and the solid electrolyte layer 61 and the electrodes 62, 63 at a certain temperature (for example, And a heater (not shown) maintained at about 600 ° C.). The detection electrode 62 and the reference electrode 63 may be provided so as to sandwich the solid electrolyte layer 61 from both sides as shown in FIG. 2, or may be provided apart from each other in the same plane of the solid electrolyte layer 61.

For the solid electrolyte layer 61, an oxygen ion (O ²⁻ ) conductive material is used. More specifically, as the material of the solid electrolyte layer 61, for example, a material obtained by adding yttrium oxide (Y ₂ O ₃ ) to zirconia (ZrO ₂ ) is used.
The reference electrode 63 and the detection electrode 62 are made of the same material having activity with respect to oxygen (O ₂ ). More specifically, for example, platinum (Pt) is used for the reference electrode 63 and the detection electrode 62 as a material having activity against oxygen.

Further, since an electromotive force according to the oxygen concentration in the exhaust pipe 11 is generated between the electrodes 62 and 63 as a detection signal, the detection electrode 62 is exposed to the exhaust in the exhaust pipe 11 downstream of the CSF. The reference electrode 63 is provided so as to be exposed to the atmosphere outside the exhaust pipe 11. The oxygen (O ₂ ) concentration in the atmosphere is about 21% and constant, whereas the oxygen concentration in the exhaust gas flowing through the exhaust pipe 11 is lower than the oxygen concentration in the atmosphere and the engine is in an operating state. Fluctuate accordingly.

As shown in FIG. 2, on the reference electrode 63 side having a relatively high oxygen concentration, a reduction reaction for ionizing oxygen in the atmosphere proceeds. In addition, since conduction electrons are transferred by this oxygen reduction reaction, the interface between the solid electrolyte layer 61 and the reference electrode 63 is filled with holes (h ⁺ ), so that the reference electrode 63 side becomes the positive electrode. Oxygen ions generated by the reference electrode 63 move through the oxygen ion conductive solid electrolyte layer 61 to the detection electrode 62 side. On the detection electrode 62 side, an oxidation reaction in which oxygen ions are converted into oxygen molecules proceeds. Further, surplus electrons are transferred by oxidation of oxygen ions, so that the detection electrode 62 side becomes a negative electrode. As described above, when the detection electrode 62 is provided on the exhaust side and the reference electrode 63 is provided on the atmosphere side, the detection electrode 62 is a negative electrode, the reference electrode 63 is a positive electrode, and an electromotive force E _O2 as shown in the following formula (4) is obtained. Occur. Here, Pa is the oxygen partial pressure on the reference electrode 63 side, and Pb is the oxygen partial pressure on the detection electrode 62 side. k is a Boltzmann constant, T is a sensor temperature (absolute temperature), and e is an elementary electric quantity.
E _O2 = (kT / 4e) · ln (Pa / Pb) (4)

Next, the detection principle of NH ₃ by the NH ₃ sensor 5 will be described with reference to FIG.
FIG. 3 is a diagram schematically showing the configuration of the NH ₃ sensor 5 and the detection principle of its NH ₃ concentration.
The NH ₃ sensor 5 includes a solid electrolyte layer 51, a porous detection electrode 52 and a reference electrode 53 provided on the solid electrolyte layer 51, and a sensor element including the solid electrolyte layer 51 and the electrodes 52 and 53. A heater (not shown) that maintains a temperature (for example, about 600 ° C.) is provided. The detection electrode 52 and the reference electrode 53 may be provided so as to sandwich the solid electrolyte layer 51 from both sides as shown in FIG. 3, or may be provided apart from each other in the same plane of the solid electrolyte layer 51.

For the solid electrolyte layer 51, an oxygen ion conductive material is used as in the O ₂ sensor. More specifically, as the material of the solid electrolyte layer 51, a material obtained by adding yttrium oxide to zirconia is used.

In order for the NH ₃ sensor 5 to exhibit a function of generating an electromotive force that varies according to the NH ₃ concentration in the exhaust gas as a detection signal, a material having activity against oxygen is used for the reference electrode 53. A material different from that of the reference electrode 53 having activity against ammonia and oxygen is used. Unlike the O ₂ sensor, the reference electrode 53 and the detection electrode 52 are provided on the exhaust pipe 11 downstream of the downstream catalytic converter 33 so as to be exposed to the same exhaust gas. In order to satisfy these conditions, for example, platinum (Pt) is used for the reference electrode 53, and for example, BiVO ₄ is used for the detection electrode 52. FIG. 3 shows an example of a reaction that proceeds when the detection electrode 52 and the reference electrode 53 of the NH ₃ sensor 5 as described above are exposed to exhaust gas containing NH ₃ and O ₂ .

An L acid point is formed on the reference electrode 53 side, and a reduction reaction for oxygen ionization (O ²⁻ ) of oxygen (O ₂ ) in the exhaust proceeds as shown in the following formula (5).
O ₂ + 4e ⁻ → 2O ²⁻ (5)

On the other hand, on the detection electrode 52 side, O ⁻ and H ⁺ are polarized to form strong B acid spots. And, as shown in the following formula (6), a reaction for selectively oxidizing NH ₃ in the exhaust proceeds by this B acid point.
3O ^2- + 2NH ₃ → N ₂ + 3H ₂ O + 6e ⁻ (6)

As described above, by using different materials for both the electrodes 52 and 53 and exposing them to the same exhaust gas, between the reference electrode 53 and the detection electrode 52, the reference electrode 53 is a positive electrode, the detection electrode 52 is a negative electrode, An electromotive force E _NH3 as shown in the following formula (7) corresponding to the NH ₃ concentration C _NH3 , O ₂ concentration C _O2 , and H ₂ O concentration C _H2O in the exhaust gas is generated. Note that the NH ₃ sensor configured as described above generates an electromotive force in the opposite direction to the NH ₃ concentration C _NH3 with respect to the O ₂ concentration C _O2 and the H ₂ O concentration C _H2O .
E _NH3 = (kT / 3e) · ln (C _NH3 )
-(KT / 4e) · ln (C _O2 )-(kT / 2e) · ln (C _H2O ) (7)

  Further, when the oxygen concentration in the exhaust gas decreases to 0 or in the vicinity thereof, the electron transfer by oxygen ions does not proceed at the interface between the electrodes 52 and 53 and the solid electrolyte layer 51, and the potential difference between the two electrodes becomes large. The value of changes greatly to the positive side and shows the maximum value. The magnitude of the electromotive force generated between the electrodes 52 and 53 at this time depends on the electrochemical potential at the interface between the electrodes 52 and 53 and the solid electrolyte layer 51, and is specific to the material of the electrodes 52 and 53 and the solid electrolyte layer 51. It is the value of.

FIG. 4 shows changes in the detection signal E _O2 [mV] of the O ₂ sensor and the detection signal E _NH3 [mV] of the NH ₃ sensor when the air-fuel ratio of the air-fuel mixture is temporarily controlled from the lean side to the stoichiometric side. It is a figure shown typically. In FIG. 4, the air-fuel ratio stoichiometric control is performed in which the air-fuel ratio of the air-fuel mixture is leaner than the stoichiometric time from time t0 to t1, and the air-fuel ratio of the air-fuel mixture is stoichiometric from time t1 to t2. It was carried out, after time t2 is a diagram showing a change of the O ₂ sensor and NH ₃ detection signals of the sensor in the case of performing a lean operation again.

During the lean operation from time t0 to time t1, the oxygen concentration of the exhaust is relatively high and is close to the oxygen concentration of the outside air. Therefore, the value of the detection signal of the O ₂ sensor is a small value near 0 as shown in the above equation (4).

Further, the value of the detection signal of the NH ₃ sensor varies within a predetermined range according to the NH ₃ concentration, the O ₂ concentration, and the H ₂ O concentration at that time according to the above equation (7). Hereinafter, the range in which the value of the detection signal of the NH ₃ sensor during the lean operation (that is, when the oxygen concentration of the exhaust gas is 0 or not substantially 0) can be defined as the normal NH ₃ output range. The upper limit value (NH ₃ usage upper limit threshold value in FIG. 4) and the lower limit value (NH ₃ usage lower limit threshold value in FIG. 4) of the normal NH ₃ output range are the NH ₃ concentration of exhaust gas downstream of the downstream catalytic converter, O It is predetermined according to the fluctuation range of the ₂ concentration and the H ₂ O concentration. The fluctuation range of the NH ₃ concentration is determined from, for example, the supply amount of urea water that can be supplied from the urea water supply device, the amount of NH ₃ that can slip from the SCR catalyst of the downstream catalytic converter, and the like. In addition, regarding the O ₂ concentration and the H ₂ O concentration, the respective fluctuation ranges can be determined from the normal usage mode during the lean operation.

When the stoichiometric control is started at the time t1, the oxygen concentration in the exhaust gas in the vicinity of the O ₂ sensor and the NH ₃ sensor is rapidly decreased until near zero. Then, the value of the detection signal of the O ₂ sensor rapidly increases from the state in the vicinity of 0 according to the above equation (4) and shows a substantially maximum value. Further, when the oxygen concentration in the exhaust gas near the NH ₃ sensor suddenly decreases to near 0, the value of the detection signal of the NH ₃ sensor changes beyond the normal NH ₃ output range described above, and the positive side maximum Indicates the value.

When the stoichiometric operation is switched to the lean operation at time t2, the oxygen concentration of the exhaust gas in the vicinity of the O ₂ sensor and the NH ₃ sensor increases soon. Therefore, the value of the detection signal of the O ₂ sensor decreases to a small value near 0 as in the period from time t0 to t1. The value of the detection signal of the NH ₃ sensor also varies within the normal NH ₃ output range according to the NH ₃ concentration, O ₂ concentration, and H ₂ O concentration at that time, as in the period from time t0 to t1.

In FIG. 4, the change in the detection signals of the O ₂ sensor and the NH ₃ sensor when the air-fuel ratio of the air-fuel mixture is switched from the lean side to the stoichiometric is described. The change in the detection signal of these sensors is qualitatively the same even when the lean side is switched from the stoichiometric side to the rich side, but the maximum value of each detection signal also increases as the oxygen concentration is closer to 0. .

FIG. 5 is a diagram showing a control block configured in the ECU 7 in order to execute various controls based on the detection signals of the O ₂ sensor 6 and the NH ₃ sensor 5 as described above. The ECU 7 includes an upstream stoichiometric determination unit 71, a downstream NH ₃ concentration calculation unit 72, and a downstream stoichiometric determination unit 73 in order to process detection signals from the O ₂ sensor 6 and the NH ₃ sensor 5. Further, the ECU 7 determines various types of exhaust purification systems such as the urea water injector 42, the catalyst deterioration warning lamp 45, the fuel injection valve 17, the intake control valve 16, and the EGR control valve 15 based on the processing results in these blocks 71-73. In order to control the apparatus, a urea water injection control unit 74, a filter regeneration control unit 75, an air-fuel ratio control unit 76, a high EGR control unit 77, and an SCR catalyst deterioration determination control unit 78 are configured. The catalyst deterioration warning light 45 indicates that the SCR catalyst of the downstream catalytic converter has deteriorated, and is provided at a position that can be seen from the driver seat of the vehicle (not shown).
Hereinafter, processing in these blocks 71 to 78 will be described.

[Upstream stoichiometric determination unit 71]
As shown in FIG. 4, the detection signal of the O ₂ sensor 6 varies in a substantially binary manner with respect to the air-fuel ratio variation from the lean side to the stoichiometric side (or the rich side from the stoichiometric side). Upstream stoichiometric determination unit 71 to which, O ₂ is set within the output range of the sensor 6 (0 from a given until the maximum value) in stoichiometric determination threshold (see the dashed line in FIG. 4), the O ₂ sensor 6 air-fuel ratio of the exhaust gas upstream side of the downstream catalytic converter O ₂ sensor 6 is provided when the value of the detection signal is equal to or greater than the stoichiometric determination threshold determines that is richer than stoichiometric or stoichiometric, the O ₂ sensor When the value of the detection signal 6 is smaller than the stoichiometric determination threshold value, it is determined that it is leaner than the stoichiometric value. As described above, the upstream stoichiometric determination unit 71 determines whether the air-fuel ratio of the exhaust on the upstream side of the downstream catalytic converter is richer than stoichiometric or richer than stoichiometric, based on the detection signal of the O ₂ sensor 6. Whether or not is binary.

[NH ₃ concentration calculator 72]
NH ₃ detection signals of the sensor 5, when the NH ₃ concentration and H _{2 O} concentration in the exhaust constant, with respect to the air-fuel ratio variation between the lean side from the stoichiometric to stoichiometric (or richer than the stoichiometric), O ₂ Like the sensor 6, it fluctuates in a binary manner. Further, the value of the detection signal of the NH ₃ sensor 5 during the lean operation varies within the above-described normal NH ₃ output range according to the NH ₃ concentration of the exhaust gas, etc., so that the NH ₃ concentration can be calculated (detected) by calculation. However, when the air-fuel ratio of the exhaust gas is changed from the lean side to the stoichiometric side or the rich side from the stoichiometric side, the detection signal of the NH ₃ sensor 5 is low oxygen outside the normal NH ₃ output range regardless of the NH ₃ concentration of the exhaust gas. Since it is a unique value at the time of concentration, the NH ₃ concentration cannot be calculated in principle.

NH ₃ concentration calculator 72, in order to accurately calculate the NH ₃ concentration by excluding the period during which can not be calculated NH ₃ concentration on the basis of a detection signal of such NH ₃ sensor 5, the detection signal of the NH ₃ sensor 5 When the value is within the normal NH ₃ output range, the NH ₃ concentration of the exhaust on the downstream side of the downstream catalytic converter is calculated based on the detection signal of the NH ₃ sensor 5. More specifically, the NH ₃ concentration calculation unit 72 finishes the stoichiometric control during the period in which the stoichiometric control is performed by an air-fuel ratio control unit 76 (to be described later) (for example, times t1 to t2 in FIG. 4) and leans. The period from when the operation is switched to when a predetermined NH ₃ redetection determination time elapses (for example, times t2 to t3 in FIG. 4), the value of the detection signal of the NH ₃ sensor 5 is substantially normal NH ₃ output. It is considered that the period is not within the range, and during this period, NOx cannot be reduced by the SCR catalyst, and it is not necessary to perform supply control of the reducing agent. Therefore, these periods are defined as NH ₃ non-detectable periods. The NH ₃ concentration calculator 72, a period other than this is defined as NH ₃ detection period, only during the NH ₃ detection period, calculates the NH ₃ concentration on the basis of the value of the detection signal of the NH ₃ sensor 5 .

As shown in the above equation (7), the detection signal (electromotive force E _NH3 ) of the NH ₃ sensor has a term dependent on the oxygen concentration (second term on the right side) and a term dependent on the H ₂ O concentration (third on the right side). Item). Therefore, even when the detection signal of the NH ₃ sensor is in the normal NH ₃ within the output range, the value of the detection signal of the NH ₃ sensor is associated with NH ₃ concentration and monovalent downstream of the downstream catalytic converter exhaust Not. Therefore, the NH ₃ concentration calculation unit 72 estimates the estimated value of the term depending on the oxygen concentration and the H ₂ O concentration by a not-shown process by a known method based on the not-shown sensor and engine operating state. Then, the NH ₃ concentration of the exhaust downstream of the downstream catalytic converter is calculated from the value of the detection signal of the NH ₃ sensor. The urea water injection control unit 74 described later basically controls the injection amount of urea water based on the NH ₃ concentration calculated by the NH ₃ concentration calculation unit 72, and therefore performs urea water injection control. The period and the period for calculating the NH ₃ concentration are substantially the same.

[Downstream stoichiometric determination unit 73]
As shown in FIG. 4, when the stoichiometric control (or rich control) is performed during the lean operation, and the oxygen concentration of the exhaust gas is reduced to near zero, the value of the detection signal of the NH ₃ sensor is the normal NH ₃ output. It changes from the range to a specific value at a low oxygen concentration outside the normal NH ₃ output range. Further, when the stoichiometric control (or rich control) is finished and the operation is switched to the lean operation, and the exhaust gas oxygen concentration rises from around 0, the value of the detection signal of the NH ₃ sensor is usually at a low oxygen concentration outside the NH ₃ output range. After a short time, the value changes to a predetermined value within the normal NH ₃ output range.

Downstream stoichiometric determination unit 73, when detecting a specific spontaneous electromotive force of a low oxygen concentration at the detection signals of the NH ₃ sensor 5, the air-fuel ratio of the exhaust gas downstream side of the downstream catalytic converter NH ₃ sensor 5 is provided It is determined that the stoichiometric or stoichiometric side is richer than the stoichiometric side. More specifically, the downstream stoichiometric determination unit 73 sets the stoichiometric determination threshold outside the normal NH ₃ output range, and the value of the detection signal of the NH ₃ sensor changes from the inside to the outside exceeding the stoichiometric determination threshold. In this case, it is determined that the air-fuel ratio of the exhaust gas downstream of the downstream catalytic converter has become richer than stoichiometric or stoichiometric, and the value of the detection signal of the NH ₃ sensor changes from the outside to the inside exceeding the stoichiometric threshold It is determined that the air-fuel ratio of the exhaust gas from the downstream catalytic converter has changed from stoichiometric or richer than stoichiometric to leaner than stoichiometric.

Hereinafter, the air-fuel ratio of the determination result in the upstream stoichiometric determination unit 71 and the downstream stoichiometric determination unit 73, as well as use of NH ₃ concentrations calculated in NH ₃ concentration calculator 72, the urea solution injection control unit 74, the filter regeneration control unit 75 The processing procedure in the air-fuel ratio control unit 76, the high EGR control unit 77, and the SCR catalyst deterioration determination control unit 78 will be described.

[Urea water injection control unit 74]
As described above, the SCR catalyst can basically purify NOx only when the lean operation is being performed and the oxygen concentration of the exhaust gas is sufficient. The period during which NOx can be purified by the SCR catalyst in the presence of the reducing agent substantially coincides with the period during which the NH ₃ concentration of exhaust gas can be calculated based on the detection signal of the NH ₃ sensor. Therefore, the urea water injection control unit 74 is in the NH ₃ detection period determined by the NH ₃ concentration calculation unit 72, that is, the value of the detection signal of the NH ₃ sensor is normally within the NH ₃ output range, and the NH ₃ concentration is relatively low. Only during the period considered to be accurately calculated, the injection amount of the reducing agent is controlled based on the NH ₃ concentration calculated by the NH ₃ concentration calculation unit 72.

More specifically, the urea solution injection control unit 74, than 0 for NH ₃ concentration of the exhaust sets a target value slightly greater, the NH ₃ concentration calculated by the NH ₃ concentration calculator 72, The injection amount of urea water is controlled so that the state matching the target value is maintained. In other words, the urea water injection control unit 74 controls the injection amount of urea water so that a very small amount of NH ₃ slips from the downstream catalytic converter 33. The SCR catalyst of the downstream catalytic converter 33 has the ability to store NH ₃ by the maximum NH ₃ storage capacity that changes according to the temperature, and the NO ₃ reduction performance increases as the amount of NH ₃ stored increases. Accordingly, the urea water injection control unit 74 performs urea water injection control so as to maintain a state in which a slight amount of NH ₃ slips, thereby minimizing the slip amount of NH ₃ and reducing the downstream catalytic converter. 33 NOx purification performance can be maintained high. The NH ₃ slipping from the downstream catalytic converter 33 is provided with an SCR catalyst, an oxidation catalyst, or the like further downstream of the NH ₃ sensor 5 and appropriately treated with these catalysts, whereby NH ₃ is discharged outside the system. Can be prevented. Further, a specific algorithm for realizing such urea water injection control based on the NH ₃ concentration calculated from the detection signal of the NH ₃ sensor is, for example, International Publication No. 2009/128169 by the present applicant. Therefore, detailed description thereof is omitted here.

[Filter regeneration control unit 75]
PM discharged from the engine accumulates on the CSF. Therefore, the filter regeneration control unit 75 estimates or directly detects, for example, the amount of PM accumulated in the CSF, and determines that it is time to regenerate the CSF when the amount of accumulation exceeds a predetermined threshold. A filter regeneration process is performed to burn and remove the. In this filter regeneration process, a rich control is performed to raise the temperature of the CSF to the combustion temperature of PM in a low oxygen concentration atmosphere, and then the lean operation is switched to supply a large amount of oxygen to the CSF. Can be broadly divided into two processes: a combustion process for burning PM. Hereinafter, an example in which the air-fuel ratio determination based on the output signal of the NH ₃ sensor is applied to the temperature raising process of the filter regeneration process will be described with reference to FIG. The PM accumulation amount is calculated, for example, by integrating the PM emission amount calculated based on the operating state of the engine. In addition, the accumulation amount of PM can also be estimated based on the output of the differential pressure sensor that detects the differential pressure before and after the CSF.

FIG. 6 is a flowchart showing the procedure of the temperature raising process of the filter regeneration process. The process shown in FIG. 6 is executed by the filter regeneration control unit 75 in response to reaching the filter regeneration time.
In S1, air-fuel ratio rich control for controlling the air-fuel ratio of the air-fuel mixture to be richer than stoichiometric is executed in order to raise the CSF, and in subsequent S2, the time elapsed since the start of air-fuel ratio rich control in S1 is executed. The corresponding rich determination time is integrated. In the air-fuel ratio rich control in S1, target values of engine operating parameters such as the intake air amount, the fuel injection amount, and the EGR amount are set so that the air-fuel ratio of the air-fuel mixture becomes a predetermined target value set to the rich side from the stoichiometry. The intake control valve, the fuel injection valve, the EGR control valve and the like are driven and controlled based on the target values of these operating parameters. Here, as the target value of each operation parameter, a map value determined by searching a predetermined map or a value obtained by correcting these map values in S4 described later is used.

In S3, it is determined whether or not the value of the detection signal of the NH ₃ sensor is greater than or equal to the stoichiometric determination threshold value. If the determination in S3 is NO, it is determined that the enrichment of the air-fuel ratio is not yet sufficient, and the target value of the operation parameter determined in S1 is corrected so that the air-fuel ratio is enriched (S4), Move to S1. If the determination in S3 is YES, it is determined that the enrichment is sufficiently performed, and the process proceeds to S5. As described above, in S1 to S4, the air-fuel ratio of the air-fuel mixture is corrected to the rich side until it is determined that the actual exhaust air-fuel ratio has become richer than stoichiometric or stoichiometric based on the detection signal of the NH ₃ sensor. To do.

  In S5, it is determined whether or not the rich determination time at which integration is started in S2 has passed a predetermined time. This specified time corresponds to the time required to raise the temperature of the CSF to the combustion temperature of PM. If the determination in S5 is NO, air-fuel ratio rich control is executed again (S6), and the rich determination time is integrated (S7). If the determination in S5 is YES, it is determined that the CSF has risen to the combustion temperature of PM, and this process is terminated.

As described above, by executing the air-fuel ratio rich control using the NH ₃ sensor in the CSF temperature raising step, for example, even when there is no O ₂ sensor, the air-fuel ratio can be accurately controlled. The time required for temperature rise can be shortened.

[Air-fuel ratio control unit 76]
For example, when the engine is at a low and medium load, the urea water injection control is performed under lean operation, so that CO and HC in the exhaust gas are purified by an oxidation reaction in DOC and CSF, and NOx is in the presence of NH ₃ . It can be purified by a reduction reaction in the SCR catalyst. However, if the amount of NOx discharged from the engine increases due to a high load, NOx may not be sufficiently purified with only the SCR catalyst. Therefore, the air-fuel ratio control unit 76 performs lean operation so that the air-fuel ratio of the air-fuel mixture is leaner than the stoichiometric engine when the engine is at low load and medium load, for example, NOx emission from the engine such as during high load operation When the amount increases, the air-fuel ratio of the air-fuel mixture is controlled at or near the stoichiometric range in order to purify CO, HC and NOx using a three-way purification reaction in DOC and CSF. However, in order to advance the three-way purification reaction efficiently in the DOC and CSF, it is necessary to control the air-fuel ratio to or near the stoichiometry with high accuracy based on the output of the sensor provided in the exhaust pipe. Hereinafter, an example in which air-fuel ratio determination based on the output signal of the NH ₃ sensor is applied to such stoichiometric air-fuel ratio control will be described with reference to FIG.

FIG. 7 is a flowchart showing a procedure of stoichiometric air-fuel ratio control. The process shown in FIG. 7 is executed by the air-fuel ratio control unit 76 at a predetermined timing during the lean operation.
In S11, it is determined whether or not the value of the parameter correlated with the NOx emission amount from the engine is larger than a predetermined threshold value. The parameter value correlated with the NOx emission amount from the engine includes an estimated value of NOx emission amount, a required torque value of the engine, or an indicated mean effective pressure value. The estimated value of the NOx emission amount is calculated from a known arithmetic expression or control map using a plurality of parameters indicating the engine operating state such as the fuel injection amount and the rotational speed as arguments. The required torque value of the engine is calculated based on the output of an accelerator opening sensor (not shown), for example. The indicated mean effective pressure is calculated based on, for example, the output of a cylinder pressure sensor (not shown). If the determination in S11 is NO, this process is terminated to continue the lean operation.

  If the determination in S11 is YES, the process moves to S12, and air-fuel ratio stoichiometric control is executed to purify the exhaust gas by the three-way purification reaction in the DOC and CSF. In this air-fuel ratio stoichiometric control, target values of engine operating parameters such as the intake air amount, fuel injection amount, and EGR amount are determined so that the air-fuel ratio of the air-fuel mixture becomes stoichiometric, and based on the target values of these operating parameters. The intake control valve, the fuel injection valve, the EGR control valve, and the like are driven and controlled. Here, the target value of the operation parameter is a map value determined by searching a predetermined map, or a value obtained by correcting these map values in S14 or S15 described later.

In S13, it is determined whether or not the value of the detection signal of the NH ₃ sensor is greater than or equal to the stoichiometric determination threshold value. If the determination in S13 is NO, the target value of the operation parameter is corrected so that the air-fuel ratio becomes richer (S14), and the process proceeds to S11. If the determination in S13 is YES, the target value of the operation parameter is corrected so that the air-fuel ratio becomes leaner (S15), and the process proceeds to S11.

As described above, by or lean or rich air-fuel ratio in accordance with a detection signal of the NH ₃ sensor, the NH ₃ value of the detection signal of the sensor is equal to or greater than stoichiometric determination threshold state (richer than stoichiometric), The state in which the value of the detection signal of the NH ₃ sensor falls below the stoichiometric determination threshold value (lean side from stoichiometric) is alternately realized, and as a result, the air-fuel ratio of the air-fuel mixture is approximately controlled to stoichiometric.

[High EGR control unit 77]
As one of the combustion techniques for diesel engines, there is known a technique for reducing the NOx emission amount and the PM emission amount by controlling an EGR control valve so that the EGR gas becomes larger than that during normal combustion. If the amount of EGR gas is increased, the air-fuel ratio tends to change in the rich direction. Therefore, when performing such high EGR control, the amount of EGR gas is controlled so that the air-fuel ratio does not change too much in the rich direction. There is a need. Hereinafter, an example in which air-fuel ratio determination based on the detection signal of the NH ₃ sensor is applied to such high EGR control will be described with reference to FIG.

FIG. 8 is a flowchart showing a procedure of high EGR control. The process shown in FIG. 8 is executed by the high EGR control unit 77 at a predetermined timing.
In S21, it is determined whether or not a predetermined high EGR control execution condition is satisfied. If the determination in S21 is YES, the process moves to S22, and if the determination is NO, this process is immediately terminated. In S22, high EGR control is executed, and the process proceeds to S23. In the high EGR control of S22, a target value of the EGR amount is determined, and the EGR control valve is driven and controlled based on the target value. Here, as the target value of the EGR amount, a map value determined by searching a map predetermined for high EGR control, or a value obtained by correcting this map value in S3 described later is used. According to this map, the target value of the EGR amount is determined so that the air-fuel ratio of the air-fuel mixture becomes slightly lean (slightly leaner than stoichiometric).

In S23, it is determined whether or not the value of the detection signal of the NH ₃ sensor is greater than or equal to the stoichiometric determination threshold value. If the determination in S23 is NO and the air-fuel ratio of the exhaust is leaner than the stoichiometry, the process proceeds to S21. If the determination in S23 is YES and the air-fuel ratio of the exhaust gas is stoichiometric or richer than stoichiometric, it is determined that the air-fuel ratio has changed too much in the rich direction, and the target value of the EGR amount is corrected to the decreasing side. Then (S24), the process proceeds to S21.
As described above, by correcting the EGR amount to the decreasing side according to the detection signal of the NH ₃ sensor, high EGR control can be executed while maintaining the air-fuel ratio at a weak lean level.

[SCR catalyst deterioration determination control unit 78]
As described above, the SCR catalyst can basically purify NOx only in the lean operation in which the exhaust gas containing a large amount of oxygen flows. For this reason, for example, when the lean operation is switched to the stoichiometric or rich operation, the reaction of the above equation (2) is transiently advanced. Therefore, the downstream catalytic converter 33 stores or releases oxygen in the exhaust gas. An oxygen storage / release material (OSC material) may be provided. For example, cerium oxide (CeO ₂ ) is used as the OSC material. When the OSC material is included in the SCR catalyst as described above, the thermal load applied to the SCR catalyst is equal to the thermal load applied to the OSC material, and therefore, the degree of deterioration of the OSC material and the degree of deterioration of the SCR catalyst can be correlated.

  The SCR catalyst deterioration determination control unit 78 detects a decrease in the oxygen storage / release function of the OSC material using the determination result of the air-fuel ratio by the downstream stoichiometric determination unit 73, thereby indirectly reducing the purification performance of the SCR catalyst. The catalyst deterioration warning lamp 45 is turned on according to the determination result. Hereinafter, with reference to FIG. 9, the procedure of the process in the SCR catalyst deterioration determination control unit 78 will be described.

FIG. 9 is a flowchart showing a procedure of SCR catalyst deterioration determination control. The process shown in FIG. 9 is executed by the SCR catalyst deterioration determination control unit 78 at a predetermined timing during the lean operation.
In S31, air-fuel ratio rich control for controlling the air-fuel ratio of the air-fuel mixture to be richer than stoichiometric is performed in order to reduce the oxygen concentration of the exhaust gas over a certain period. In this air-fuel ratio rich control, target values of engine operating parameters such as the intake air amount, fuel injection amount, and EGR amount are determined so that the air-fuel ratio of the air-fuel mixture becomes a predetermined target value set to a richer side than stoichiometric. The intake control valve, the fuel injection valve, the EGR control valve, and the like are driven and controlled based on the target values of these operating parameters. Here, as the target value of each operation parameter, for example, a map value determined by searching a predetermined map is used.

In S32, whether or not the upstream stoichiometric determination unit determines that the air-fuel ratio of the exhaust gas upstream of the downstream catalytic converter has become richer than stoichiometric, that is, the upstream stoichiometric determination unit determines the value of the detection signal of the O ₂ sensor. It is determined whether or not it is determined that the stoichiometric determination threshold is exceeded. When the determination in S32 is NO, the process proceeds to S31, and the air-fuel ratio rich control is continuously executed. If the determination in S32 is yes, the process proceeds to S33. In S33, the rich determination time corresponding to the time elapsed since the air-fuel ratio of the exhaust on the upstream side of the downstream catalytic converter has become richer than the stoichiometric value is integrated, and the process proceeds to S34. In S34, whether or not the downstream stoichiometric determination unit determines that the air-fuel ratio of the downstream exhaust of the downstream catalytic converter has become richer than stoichiometric or stoichiometric, that is, the value of the detection signal of the NH ₃ sensor is determined by the downstream stoichiometric determination unit. It is determined whether or not it is determined that the stoichiometric determination threshold is exceeded. When the determination in S34 is NO, the process proceeds to S31, and the air-fuel ratio rich control is continuously executed. If the determination in S34 is YES, it is determined that the air-fuel ratio of the exhaust on the downstream side is also stoichiometric or richer than stoichiometric after the upstream side of the downstream catalytic converter, and the process proceeds to S35.

In S35, it is determined whether or not the rich determination time accumulated in S32 is equal to or more than a deterioration determination threshold set for determining the deterioration degree of the SCR catalyst. This rich determination time is determined after the air-fuel ratio rich control is started in S31, and based on the detection signal of the O ₂ sensor, it is determined that the air-fuel ratio of the exhaust on the upstream side of the downstream catalytic converter has become richer than stoichiometric or stoichiometric. From this, it corresponds to the time taken until it is determined that the air-fuel ratio of the exhaust gas downstream of the downstream catalytic converter becomes richer than stoichiometric based on the detection signal of the NH ₃ sensor. The rich determination time becomes shorter as the OSC material provided in the downstream catalytic converter deteriorates and its oxygen storage / release function decreases, so this rich determination time is a measure for determining the degree of deterioration of the SCR catalyst. . If the determination in S35 is YES, it is determined that the SCR catalyst is normal (S36), and this process ends. If the determination in S35 is NO, it is determined that the SCR catalyst is in a deteriorated state, the catalyst deterioration warning lamp is turned on (S37), and this process is terminated.

Although one embodiment of the present invention has been described above, the present invention is not limited to this.
For example, as shown in the above formula (7), the electromotive force generated between the electrodes of the NH ₃ sensor is proportional to the sensor temperature T. For this reason, the normal NH ₃ output range and the magnitude of the electromotive force generated when the oxygen concentration is reduced to 0 or in the vicinity thereof also vary depending on the sensor temperature T. Therefore, the stoichiometric determination threshold value for determining the air / fuel ratio of the exhaust based on the normal NH ₃ output range or the detection signal of the NH ₃ sensor may be changed according to the sensor temperature. The sensor temperature can be estimated based on the heater current of the sensor element, the exhaust temperature, and the like.

In the example shown in FIG. 9, the rich determination time is determined in response to the determination that the air-fuel ratio of the exhaust on the upstream side of the downstream catalytic converter is richer than stoichiometric based on the detection signal of the O ₂ sensor. However, the start of accumulation of the rich determination time is not limited to this. The time required from the start of the air-fuel ratio rich control until the air-fuel ratio of the exhaust on the upstream side of the downstream catalytic converter becomes richer than the stoichiometric value can also be estimated based on experiments conducted in advance. Therefore, in a system that does not include the O ₂ sensor, the timing at which the air-fuel ratio of the exhaust on the upstream side of the downstream catalytic converter becomes richer than stoichiometric is estimated, and the accumulation of the rich determination time is started accordingly. The same effect as the process shown in FIG. 8 is obtained.

1. Engine (internal combustion engine)
11 ... Exhaust pipe (exhaust passage)
13 ... EGR device (EGR device)
2 ... Exhaust purification system 33 ... Downstream catalytic converter (SCR catalyst)
4 ... Urea water supply device (reducing agent supply device)
31 ... Upstream catalytic converter (upstream catalyst)
32. Exhaust gas purification filter (upstream catalyst)
5 ... NH ₃ sensor (ammonia sensor)
53 ... reference electrode 52 ... detection electrode 51 ... solid electrolyte layer 72 ... NH ₃ concentration calculator (ammonia concentration calculating means)
73: Downstream stoichiometric determination unit (stoichiometric determining means)
74: urea water injection control unit (reducing agent supply amount control means)
76: Air-fuel ratio control section (air-fuel ratio control means)
77 ... High EGR control section (EGR control means)
78 ... SCR catalyst deterioration determination control unit (catalyst deterioration determination means)

Claims (8)

A selective reduction catalyst that is provided in an exhaust passage of the internal combustion engine and selectively reduces nitrogen oxides in the exhaust in the presence of ammonia;
A reducing agent supply device for supplying ammonia or a precursor thereof as a reducing agent upstream of the selective reduction catalyst;
An ammonia sensor comprising a reference electrode having activity against oxygen, a sensing electrode having activity against oxygen and ammonia, and an oxygen ion conductive solid electrolyte provided with these sensing electrode and reference electrode;
An exhaust gas purification system for an internal combustion engine, comprising a reducing agent supply amount control means for controlling a supply amount of the reducing agent according to a detection signal of the ammonia sensor,
The reference electrode and the detection electrode are provided in the exhaust passage so as to be exposed to the same exhaust gas downstream of the selective reduction catalyst,
When the value of the detection signal of the ammonia sensor has changed beyond a predetermined stoichiometric determination threshold, the air-fuel ratio of the exhaust downstream of the selective reduction catalyst has changed from lean to lean or stoichiometric from the stoichiometric side. An exhaust purification system for an internal combustion engine, further comprising stoichiometric determination means for determining.   When the value of the detection signal of the ammonia sensor is within a normal output range set inside the stoichiometric determination threshold, the reducing agent supply amount control means is configured to reduce the reducing agent based on the detection signal of the ammonia sensor. 2. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the supply amount of the engine is controlled. When the value of the detection signal of the ammonia sensor is within a normal output range set inside the stoichiometric determination threshold, the ammonia in the exhaust gas downstream of the selective reduction catalyst based on the detection signal of the ammonia sensor An ammonia concentration calculation means for calculating the concentration;
2. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the reducing agent supply amount control means controls the supply amount of the reducing agent based on the calculated ammonia concentration. An upstream catalyst provided on the upstream side of the selective reduction catalyst in the exhaust passage and having a three-way purification function;
When the value of the parameter correlated with the NOx emission amount of the engine is equal to or less than a predetermined threshold, the air-fuel ratio of the air-fuel mixture is controlled to be leaner than the stoichiometric, and when the value of the parameter is larger than the threshold The exhaust gas purification of an internal combustion engine according to any one of claims 1 to 3, further comprising air-fuel ratio control means for controlling the air-fuel ratio of the air-fuel mixture to stoichiometric based on determination by the stoichiometric determination means. system. An EGR device that recirculates a portion of the exhaust gas flowing through the exhaust passage to the intake passage;
EGR control means for controlling the exhaust gas recirculation amount by the EGR device,
5. The EGR control means corrects the exhaust gas recirculation amount to a decrease side when the stoichiometric determination means determines that the air-fuel ratio has become richer than stoichiometric or stoichiometric. 5. An exhaust purification system for an internal combustion engine according to any one of the above. The selective reduction catalyst includes an oxygen storage / release material,
When the air-fuel ratio on the downstream side of the selective reduction catalyst has become richer than stoichiometric or stoichiometric by the stoichiometric determination means from a predetermined time after starting the control to make the air-fuel ratio of the air-fuel mixture stoichiometric or richer than stoichiometric 4. The catalyst deterioration determination unit according to claim 1, further comprising a catalyst deterioration determination unit that measures a time taken until the determination, and determines deterioration of the selective reduction catalyst based on the measurement time. An exhaust purification system for an internal combustion engine. A selective reduction catalyst that is provided in an exhaust passage of the internal combustion engine and selectively reduces nitrogen oxides in the exhaust in the presence of ammonia;
A reducing agent supply device for supplying ammonia or a precursor thereof as a reducing agent upstream of the selective reduction catalyst;
An ammonia sensor that is provided on the downstream side of the selective reduction catalyst, and that generates an electromotive force according to the concentration of at least ammonia and oxygen in the exhaust as a detection signal between the electrodes;
An exhaust gas purification system for an internal combustion engine comprising a reducing agent supply amount control means for controlling a supply amount of the reducing agent based on a detection signal of the ammonia sensor,
When the oxygen concentration of the surrounding exhaust gas becomes 0 or almost 0 between the electrodes of the ammonia sensor, an electromotive force exceeding a predetermined stoichiometric threshold is generated regardless of the ammonia concentration,
When the value of the detection signal of the ammonia sensor changes beyond the stoichiometric determination threshold value, it is determined that the air-fuel ratio of the exhaust gas downstream of the selective reduction catalyst has changed from lean to lean or stoichiometric. An exhaust purification system for an internal combustion engine, further comprising: When the value of the detection signal of the ammonia sensor is within a normal output range set inside the stoichiometric determination threshold, the ammonia in the exhaust gas downstream of the selective reduction catalyst based on the detection signal of the ammonia sensor An ammonia concentration calculation means for calculating the concentration;
The exhaust gas purification system for an internal combustion engine according to claim 7, wherein the reducing agent supply amount control means controls the supply amount of the reducing agent based on the calculated ammonia concentration.

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