JP6627840B2 - Engine exhaust purification control device - Google Patents

Description

  The present invention relates to an exhaust gas purification control device for an engine including a NOx catalyst and an SCR catalyst in an exhaust passage.

  Conventionally, it has been known to provide a catalyst or the like in an exhaust passage in order to purify NOx discharged from an engine.

For example, Patent Document 1 discloses that when the excess air ratio of the exhaust gas is greater than 1, that is, when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio, NOx in the exhaust gas is stored and the exhaust gas is exhausted. A NOx storage-reduction type NOx catalyst for reducing NOx stored in a state where the excess air ratio of the gas is 1 or less is provided in the exhaust passage, and NH ₃ (ammonia) is provided downstream of the NOx catalyst in the exhaust passage. An engine provided with an SCR catalyst for purifying NOx in exhaust gas by a reaction with the above is disclosed. In this engine, an ammonia compound injection device is provided upstream of the SCR catalyst in the exhaust passage to inject an ammonia compound serving as a source of NH ₃ , and NH ₃ is generated from the ammonia compound injected by the device. , NOx is purified in the SCR catalyst.

Japanese Patent No. 3518398

Here, in the NOx storage-reduction type NOx catalyst as described above, when NOx stored in contact with NOx is reduced, "N" in the stored NOx and H as a reducing agent introduced are introduced. And the like generate NH ₃ .

Accordingly, as in Patent Document 1, SCR catalyst disposed downstream of the NOx catalyst, the engine unit supplies provided to a substance which is a raw material of NH ₃ or NH ₃ in the exhaust passage, a simple constant weight If NH ₃ or a substance serving as a raw material of NH ₃ is configured to be supplied to the exhaust passage, when the NOx is reduced by the NOx catalyst, the amount of NH ₃ supplied to the SCR catalyst becomes excessive, and a large amount of NH ₃ is supplied. ₃ may pass downstream from the SCR catalyst.

On the other hand, when the NOx is reduced by the NOx catalyst or the like, it is conceivable to reduce the amount of the substance such as NH ₃ supplied to the exhaust passage from the device. However, if the amount of reduction is not properly controlled, there is still a problem that NH ₃ slips downstream of the SCR catalyst. Alternatively, the lack of NH ₃ supplied to the SCR catalyst, a problem that NOx can not be properly purified by the SCR catalyst occurs.

The present invention has been made in view of the above circumstances, and provides an exhaust gas purification control device for an engine that can suppress the passage of NH ₃ to the downstream side of the SCR catalyst while appropriately purifying NOx. Aim.

In order to solve the above problems, the present inventors have conducted intensive studies and found that when a NOx storage-reduction type NOx catalyst having an oxygen storage capacity was used, exhaust gas was reduced to reduce NOx stored in the NOx catalyst. excess air rate with 1 or less immediately be carried out playback control for playing the NOx catalyst without being released NH ₃ from NOx catalyst, the NH ₃ from the NOx catalyst since the start of the regeneration control start to be released In the meantime, it was found that there was a predetermined delay time. Furthermore, the inventors of the present application have found that the delay time is highly correlated with the stored oxygen reduction rate, which is the reduction rate of the oxygen stored in the NOx catalyst, and that the smaller the stored oxygen reduction rate, the longer the delay time. Ascertained. Here, the shorter the delay time is, the larger the amount of NH ₃ released from the NOx catalyst, supplied to the SCR catalyst, and stored (occluded) in the SCR catalyst is, compared to the longer delay time. . Therefore, if a certain amount of NH ₃ or its raw material is separately supplied to the exhaust passage regardless of the delay time during the regeneration control, there is a possibility that NH ₃ in the SCR catalyst becomes excessive or insufficient.

The present invention has been made based on this finding, and has an engine body in which a cylinder is formed, an exhaust passage through which exhaust gas discharged from the engine body flows, and a NOx catalyst provided in the exhaust passage. An exhaust purification control device for an engine, comprising: an SCR catalyst provided downstream of the NOx catalyst; an excess air ratio changing means for changing an excess air ratio of exhaust gas; between the catalyst and the SCR catalyst, the SCR for reducing agent supply means for supplying the SCR for reducing agent comprising raw material or NH ₃ in NH _3, and the excess air ratio changing means and said SCR for reducing agent supply means And control means for performing regeneration control for regenerating the NOx catalyst by reducing the excess air rate of the exhaust gas to 1 or less by the excess air rate changing means. Control means, as if the temperature of the NOx catalyst during the regeneration control is high, the amount of the SCR for reducing agent to be supplied to the exhaust passage from the SCR for reducing agent supply means than low is reduced, The SCR reducing agent supply means is controlled such that the lower the temperature of the NOx catalyst at the time of the regeneration control, the greater the rate of change of the supply amount of the SCR reducing agent with respect to the temperature of the NOx catalyst. (Claim 1).

  According to the findings of the present inventors, the stored oxygen reduction rate increases as the temperature of the NOx catalyst increases.

On the other hand, in this apparatus, the control is performed such that the higher the temperature of the NOx catalyst during the regeneration control, the smaller the amount of the SCR reducing agent supplied from the SCR reducing agent supply means to the exhaust passage. In other words, in this device, when the stored oxygen reduction rate increases with an increase in the temperature of the NOx catalyst and the delay time is short, further, the SCR catalyst is released from the NOx catalyst due to the short delay time. When the total amount of NH ₃ supplied to the exhaust passage increases, the amount of the SCR reducing agent supplied to the exhaust passage is reduced.

Therefore, during the regeneration control, the amount of NH ₃ supplied from the NOx catalyst to the SCR catalyst and the amount of NH ₃ supplied to the SCR catalyst by the SCR reducing agent supply means are combined and supplied to the SCR catalyst. It is possible to maintain the entire amount of NH _{3 at} an appropriate amount, and to suppress the passage of NH ₃ to the downstream side of the SCR catalyst while appropriately purifying NOx in the SCR catalyst.

Moreover , according to the findings of the present inventors, the lower the temperature of the NOx catalyst, the greater the rate of change of the stored oxygen reduction rate with respect to the temperature of the NOx catalyst.

On the other hand, the control means controls the SCR so that the lower the temperature of the NOx catalyst at the time of the regeneration control, the larger the rate of change of the supply amount of the SCR reducing agent with respect to the temperature of the NOx catalyst. that controls the use the reducing agent supply means.

Therefore , it is possible to more reliably maintain an appropriate amount of the combined amount of NH ₃ supplied from the NOx catalyst to the SCR catalyst and the amount of NH ₃ supplied to the SCR catalyst by the SCR reducing agent supply unit. it can.

In the above configuration, it is preferable that the control unit controls the excess air ratio changing unit such that the excess air ratio of the exhaust gas becomes larger than 0.9 during the regeneration control (claim 2 ).

In this way, by suppressing the variation width of the amount of reducing agent in the exhaust gas decreases, since it is possible to supply stably reducing agent to the NOx catalyst, thus the SCR catalyst 46 of NH ₃ released from the NOx catalyst 41 it is possible to maintain the NH ₃ in the more reliably appropriate amount.

In the above configuration, the excess air ratio changing means is configured to perform, during the regeneration control, a delay after the main injection, in addition to a main injection for injecting fuel for obtaining engine torque introduced into the cylinder into the cylinder. It is preferable that a post-injection for injecting fuel into the cylinder be performed at a timing on the corner side, and the excess air ratio of the exhaust be changed by changing an injection amount of the post-injection (Claim 3 ).

  This makes it possible to stabilize the excess air ratio of the exhaust gas as compared with a case where the amount of air introduced into the cylinder and the exhaust passage is changed to change the excess air ratio of the exhaust gas. Then, the excess air ratio of the exhaust gas can be more reliably maintained at a value larger than 0.9.

In the above configuration, the control means may control the reducing agent for the SCR before the predetermined period elapses after the end of the regeneration control than after the predetermined period elapses after the end of the regeneration control. as the amount of the SCR reducing agent for the supply means is supplied to the exhaust passage decreases, and controls the reducing agent supply means for the SCR, the NH ₃ from the NOx catalyst with the start of the regeneration control Estimating the NH ₃ release start timing, which is the release timing, and estimating the total amount of NH ₃ released from the NOx catalyst during the period from the NH ₃ release start timing to the end of the regeneration control. the person total emission amount of the NH ₃ is large, subjected to the exhaust passage from the SCR for reducing agent supply means during the period from the reproduction control is finished until the predetermined period elapses Wherein as the amount of SCR reducing agent for less is to control the SCR for reducing agent supply means, it is preferably (claim 4).

In this way, when the amount of NH ₃ in the SCR catalyst increases during the period from the end of the regeneration control to the lapse of the predetermined period and the execution of the regeneration control, Can be prevented from being supplied to the SCR catalyst, and a large amount of the SCR reducing agent can be prevented from slipping downstream of the SCR catalyst.

According to the engine control system according to the present invention, it is possible to suppress the slipping to the downstream side of the SCR catalyst NH ₃ while appropriately purify NOx.

1 is a schematic configuration diagram of an engine system to which an exhaust gas purification control device for an engine according to an embodiment of the present invention is applied. FIG. 2 is a block diagram illustrating a control system of the engine system. It is a figure showing a control area of passive DeNOx control and active DeNOx control. 4 is a flowchart illustrating a control procedure of fuel injection. 5 is a graph showing the relationship between the excess air ratio of exhaust gas and the concentration of each substance contained in the exhaust gas. 4 is a graph showing the relationship between the air shortage width of exhaust gas and the concentration of each substance contained in the exhaust gas. 5 is a graph showing a relationship between an excess air ratio and a first correction coefficient. 4 is a graph showing the relationship between the temperature of a NOx catalyst, the flow rate of exhaust gas, and the ratio of a oxidizing reducing agent. FIG. 4 is a diagram for explaining a procedure for estimating the amount of NH ₃ released from a NOx catalyst. 5 is a graph showing a relationship between the temperature of the NOx catalyst and a first temperature coefficient. 5 is a graph showing a relationship between an exhaust flow rate and a first flow coefficient. 5 is a graph showing a relationship between an air-fuel ratio of exhaust gas and a first A / F coefficient. 5 is a graph showing a relationship between an exhaust gas flow rate and a second flow coefficient. 5 is a graph showing a relationship between an air-fuel ratio of exhaust gas and a second A / F coefficient. 4 is a flowchart showing a control procedure of a urea injection amount.

  Hereinafter, an engine exhaust gas purification control device according to an embodiment of the present invention will be described with reference to the drawings.

(1) Overall Configuration FIG. 1 is a schematic configuration diagram of an engine system 100 to which an exhaust gas purification control device for an engine of the present embodiment is applied.

  The engine system 100 includes a four-stroke engine body 1, an intake passage 20 for introducing air (intake) into the engine body 1, an exhaust passage 40 for discharging exhaust gas from the engine body 1 to the outside, A first turbocharger 51 and a second turbocharger 52 are provided. The engine system 100 is provided in a vehicle, and the engine body 1 is used as a drive source of the vehicle. The engine main body 1 is, for example, a diesel engine, and has four cylinders 2 arranged in a direction perpendicular to the paper surface of FIG.

  The engine body 1 has a cylinder block 3 in which a cylinder 2 is formed, a cylinder head 4 provided on the upper surface of the cylinder block 3, and a piston 5 slidably inserted into the cylinder 2. I have. A combustion chamber 6 is formed above the piston 5.

  The piston 5 is connected to the crankshaft 7, and the crankshaft 7 rotates around its central axis in accordance with the reciprocating motion of the piston 5.

  An injector (excess air ratio changing means) 10 for injecting fuel into the combustion chamber 6 (inside the cylinder 2) and a glow plug for raising the temperature of a mixture of fuel and air in the combustion chamber 6 are provided in the cylinder head 4. 11 is provided for each cylinder 2. In the example shown in FIG. 1, the injector 10 is provided at the center of the ceiling surface of the combustion chamber 6 so as to face the combustion chamber 6 from above. In addition, the glow plug 11 has a heat generating portion at the front end that generates heat when energized, and the heat generating portion is attached to the ceiling surface of the combustion chamber 6 so as to be located near the front end portion of the injector 10. ing. For example, the injector 10 has a plurality of nozzles at the tip thereof, and the glow plug 11 has a heating portion located between the plurality of nozzles from the plurality of nozzles of the injector 10 so as not to come into direct contact with the fuel spray. , Is located.

  The injector 10 is an injection that is performed to obtain engine torque, and is a main injection that injects fuel that burns near the compression top dead center into the combustion chamber 6, and a fuel that is retarded from the main injection and burns. However, post-injection for injecting fuel into the combustion chamber 6 at a time when the combustion energy hardly contributes to the engine torque can be performed.

  The cylinder head 4 has an intake port for introducing air supplied from the intake passage 20 into the combustion chamber 6 of each cylinder 2, an intake valve 12 for opening and closing the intake port, and a gas generated by the combustion chamber 6 of each cylinder 2. An exhaust port for leading the exhaust gas to the exhaust passage 40 and an exhaust valve 13 for opening and closing the exhaust port are provided.

  In the intake passage 20, an air cleaner 21, a compressor 51a of the first turbocharger 51, a compressor 52a of the second turbocharger 52, an intercooler 22, a throttle valve 23, and a surge tank 24 are provided in this order from the upstream side. ing. In addition, the intake passage 20 is provided with an intake-side bypass passage 25 that bypasses the second compressor 52a, and an intake-side bypass valve 26 that opens and closes the passage. The intake side bypass valve 26 is switched between a fully closed state and a fully opened state by a driving device (not shown).

  In the exhaust passage 40, in order from the upstream side, the turbine 52b of the second turbocharger 52, the turbine 51b of the first turbocharger 51, the first catalyst 43, and particulate matter (PM) in the exhaust gas. ), A urea injector (reducing agent supply means for SCR) 45, an SCR (Selective Catalytic Reduction) catalyst 46, and a slip catalyst 47.

  The first catalyst 43 includes a NOx catalyst 41 that purifies NOx, and a DOC (diesel) that oxidizes hydrocarbons (HC), carbon monoxide (CO), and the like using oxygen in the exhaust gas to change them into water and carbon dioxide. Oxidation catalyst, Diesel Oxidation Catalyst 42.

The NOx catalyst 41 is a NOx storage reduction type catalyst (NSC: NOx Storage Catalyst). That is, the NOx catalyst 41 stores NOx in the exhaust gas when the excess air ratio λ of the exhaust gas is lean (state where the exhaust gas air-fuel ratio is higher than the stoichiometric air-fuel ratio). Then, the NOx catalyst 41 converts the stored NOx into a rich state in which the excess air ratio λ of the exhaust gas is close to 1 or smaller than 1 (when the air-fuel ratio of the exhaust gas is close to the stoichiometric air-fuel ratio or when the exhaust air has a stoichiometric air-fuel ratio). (Small state), that is, the exhaust gas passing through the NOx catalyst 41 is reduced under a reducing atmosphere containing a large amount of unburned HC.

Specifically, the NOx catalyst 41 is configured to be able to store oxygen contained in the exhaust gas in a lean state where the excess air ratio λ of the exhaust gas is larger than 1. For example, the NOx catalyst 41 includes ceria having oxygen storage ability. Then, the NOx catalyst 41 oxidizes NO in the exhaust gas by using oxygen contained in the exhaust gas and stored oxygen (referred to as NO ₂ ) and stores the NO.

The NOx catalyst 41 generates and releases NH ₃ (ammonia) when reducing the stored NOx. Specifically, at the time of NOx reduction, “N” in the NOx stored by the NOx catalyst 41 and NOx passing through the NOx catalyst 41 are combined with H, which is a reducing agent introduced into the NOx catalyst 41, and the like. Thus, NH ₃ is generated.

  The first catalyst 43 is formed, for example, by coating the surface of a DOC catalyst material layer with an NSC catalyst material.

  In this embodiment, no separate device for supplying air or fuel is provided in the exhaust passage, and the excess air ratio λ of the exhaust gas and the excess air ratio λ of the air-fuel mixture in the combustion chamber 6 correspond to each other. That is, when the excess air ratio λ of the air-fuel mixture in the combustion chamber 6 is larger than 1, the excess air ratio λ of the exhaust gas also becomes larger than 1, and when the excess air ratio λ of the air-fuel mixture in the combustion chamber 6 is 1 or less. Then, the excess air ratio λ of the exhaust gas also becomes 1 or less.

  The urea injector 45 injects urea into the exhaust passage 40 downstream of the DPF 44. The urea injector 45 is connected to a urea tank 45c via a urea supply path 45a and a urea delivery pump 45b, and injects urea pumped from the urea tank 45c by the urea delivery pump 45b into the exhaust passage 40. In the present embodiment, a heater 45d for preventing urea from freezing is provided. Urea injected from the urea injector 45 is introduced into the SCR catalyst 46.

The SCR catalyst 46 purifies by reacting (reducing) NH ₃ (ammonia) with NOx in the exhaust gas. The SCR catalyst 46 hydrolyzes urea injected from the urea injector 45 to generate NH ₃ (CO (NH ₂ ) ₂ + H ₂ O → CO ₂ + 2NH ₃ ), and converts the generated NH ₃ into exhaust gas. NOx is purified by reacting (reducing) with NOx.

As described above, in the present embodiment, the urea injected (supplied) into the exhaust passage 40 by the urea injector 45 functions as the NH ₃ raw material and the SCR reducing agent in the claims.

Specifically, in the SCR catalyst 46, the introduced NH ₃ is adsorbed, and the adsorbed NH ₃ reacts with NOx to reduce NOx. As described above, when NOx is reduced by the NOx catalyst 41, NH ₃ is also released from the NOx catalyst 41, and the SCR catalyst 46 exhausts the NH ₃ released from the NOx catalyst 41. NOx is also purified by reacting (reducing) with NOx in the inside.

For example, the SCR catalyst 46 has a catalyst metal (Fe, Ti, Ce, W, etc.) having a function of reducing NOx by NH ₃ carried on a zeolite having a function of trapping NH ₃ as a catalyst component. It is made by supporting the components on the cell walls of the honeycomb carrier.

  Although both the SCR catalyst 46 and the NOx catalyst 41 can purify NOx, they are different from each other in the temperature at which the purification rate increases, and the NOx purification rate of the SCR catalyst 46 is such that the exhaust gas temperature is relatively high. And the NOx purification rate of the NOx catalyst 41 increases when the temperature of the exhaust gas is relatively low.

  That is, in the present embodiment, NOx purification is performed using both the NOx catalyst 41 and the SCR catalyst 46. Specifically, when the temperature of the SCR catalyst 46 is lower than the first temperature and the NOx purification rate by the SCR catalyst 46 is low, NOx purification is performed only by the NOx catalyst 41, and the temperature of the SCR catalyst 46 is equal to or higher than the second temperature. When the second temperature is higher than the first temperature and the NOx purification rate by the SCR catalyst 46 is high, NOx purification is performed only by the SCR catalyst 46. When the temperature of the SCR catalyst 46 is between the first temperature and the second temperature, NOx purification is performed by both the NOx catalyst 41 and the SCR catalyst 46. Also, when the exhaust gas flow rate is large and the NOx purification rate by the SCR catalyst 46 is low, NOx purification is performed by both the NOx catalyst 41 and the SCR catalyst 46.

The slip catalyst 47 oxidizes and purifies unreacted NH ₃ discharged from the SCR catalyst 46.

In the exhaust passage 40, an exhaust-side bypass passage 48 for bypassing the second turbine 52b, an exhaust-side bypass valve 49 for opening and closing the same, a wastegate passage 53 for bypassing the first turbine 51b, and a wastegate for opening and closing the same A valve 54 is provided. The exhaust side bypass valve 49 and the wastegate valve 54 are respectively switched between a fully closed state and a fully opened state by a driving device (not shown), and are changed to an arbitrary opening degree therebetween. The openings of the exhaust-side bypass valve 49, the wastegate valve 54, and the intake-side bypass valve 26 are changed based on the engine speed and the engine load.

The engine system 100 further includes an EGR device 55 that recirculates a part of the exhaust gas to the intake air. The EGR device 55 includes an EGR passage 56 that connects a portion of the exhaust passage 40 upstream of the upstream end of the exhaust-side bypass passage 48 and a portion of the intake passage 20 between the throttle valve 23 and the surge tank 24. And an EGR cooler 58 that cools the exhaust gas passing through the EGR passage 56. Further, the EGR device 55 has an EGR cooler bypass passage 59 that bypasses the EGR cooler 58, and a second EGR valve 60 that opens and closes the EGR cooler bypass passage 59.

(2) Control System The control system of the engine system will be described with reference to FIG. The vehicle is mainly provided with a DCU (Dosing Control Unit) 300 for mainly controlling the urea injector 45 and a PCM (Power-train Control Module) 200 for controlling other components. The PCM 200 and the DCU 300 are microprocessors each including a CPU, a ROM, a RAM, an I / F, and the like. In the present embodiment, the PCM 200 and the DCU 300 constitute control means in the claims.

  Information from various sensors is input to PCM 200. For example, the PCM 200 detects a rotational speed of the crankshaft 7, that is, a rotational speed sensor SN1 for detecting an engine rotational speed, and an intake air amount which is provided near the air cleaner 21 and is an amount of fresh air (air) flowing through the intake passage 20. An air flow sensor SN2 provided in the surge tank 24, an intake pressure sensor SN3 for detecting the pressure of intake air in the surge tank 24 after being supercharged by the turbochargers 51 and 52, that is, a supercharging pressure, and an exhaust pressure sensor SN3. Among them, it is electrically connected to an exhaust O2 sensor SN4 or the like for detecting an oxygen concentration in a portion between the first turbocharger 51 and the first catalyst 43, and receives input signals from these sensors SN1 to SN4. . Further, the vehicle is provided with an accelerator opening sensor SN5 for detecting an accelerator opening which is an opening of an accelerator pedal (not shown) operated by a driver, a vehicle speed sensor SN6 for detecting a vehicle speed, and the like. Detection signals from these sensors SN5 and SN6 are also input to the PCM 200. The PCM 200 controls the injector 10 and the like by executing various calculations and the like based on input signals from the sensors (SN1 to SN6 and the like).

  The DCU 70 and the PCM 60 are communicably connected to each other. The DCU 70 controls the urea injector 45 by calculating the amount of urea to be injected into the exhaust passage 40 by the urea injector 45 using the calculation result of the PCM 200 and the like. The DCU 70 also controls the urea delivery pump 45b and the heater 45d.

(2-1) DeNOx control DeNOx which is a control for regenerating the NOx catalyst 41 by reducing NOx stored in the NOx catalyst 41 (hereinafter referred to as stored NOx as appropriate) and releasing (leaving off) the NOx catalyst 41. Control (reproduction control) will be described.

  In the present embodiment, DeNOx control, control for reducing SOx stored in the NOx catalyst 41 (so-called DeSOx control), and control for regenerating the DPF 44 (control for burning and removing particulate matter from the DPF 44). During the normal operation without performing the above operation, the excess air ratio λ of the air-fuel mixture in the combustion chamber 6 and thus the excess air ratio λ of the exhaust gas are set to λ> 1 (for example, approximately λ = 1.7) in order to improve fuel efficiency. You. Hereinafter, the excess air ratio λ of the air-fuel mixture in the combustion chamber 6 is simply referred to as the excess air ratio λ of the air-fuel mixture.

  On the other hand, as described above, in the NOx catalyst 41, the stored NOx is reduced and the NOx is released from the NOx catalyst 41 when the excess air ratio λ of the exhaust gas is close to 1 or in a rich state smaller than 1. ing. Therefore, in order to reduce the stored NOx, it is necessary to reduce the excess air ratio λ of the exhaust gas and the excess air ratio λ of the air-fuel mixture as compared with those in the normal operation.

  As one method of reducing the excess air ratio λ of the air-fuel mixture (excess air ratio λ of the exhaust gas), it is conceivable to reduce the amount of fresh air (air) introduced into the combustion chamber 6. However, if the amount of fresh air is simply reduced, the engine torque may not be properly obtained. In particular, if the amount of fresh air is reduced during acceleration, the accelerating property may be deteriorated. When adjusting the amount of fresh air, it is relatively difficult to control the excess air ratio λ of the air-fuel mixture with high accuracy.

  Therefore, in the present embodiment, post-injection is performed as DeNOx control, and thereby the excess air ratio of the air-fuel mixture is reduced while the amount of reduction in the amount of fresh air is kept small. That is, the PCM 200 performs control for causing the injector 10 to perform post-injection in addition to main injection as DeNOx control. During normal operation, post injection is stopped.

  In the present embodiment, the DeNOx control for performing the post-injection in order to reduce the stored NOx is performed only in the first region R1 and the second region R2 illustrated in FIG. The first region R1 is such that the engine speed is equal to or greater than a preset first reference speed N1 and equal to or less than a preset second reference speed N2, and the engine load is equal to or greater than a preset first reference load Tq1 and This is a region equal to or less than the set second reference load Tq2. The second region R2 is a region where the engine load is higher than the first region R1, and is a region where the engine load is equal to or more than a third reference load Tq3 set in advance.

  In the first region R1, the PCM 200 determines the timing at which the post-injected fuel burns in the combustion chamber 6 (the first half of the expansion stroke, specifically, from the compression top dead center to 90 ° CA after the compression top dead center). For example, active DeNOx control for performing post injection at 30 to 70 ° CA after compression top dead center) is performed. When the active DeNOx control is performed, the air-fuel mixture is heated by energizing the glow plug 11 in order to promote the combustion of the post-injected fuel.

  On the other hand, in the second region R2, the PCM 200 performs passive DeNOx control for performing post-injection at a timing when the post-injected fuel does not burn in the combustion chamber 6 (the latter half of the expansion stroke, for example, 110 ° CA after compression top dead center). Is carried out.

  This is for the following reason.

  In a region where the engine load is low or the engine speed is low, the temperature of the NOx catalyst 41 tends to become lower than the temperature at which the stored NOx can be reduced due to the low temperature of the exhaust gas. Therefore, in this embodiment, the DeNOx control is stopped in this region.

  Further, as described above, the post-injection is performed in the DeNOx control. However, when the post-injected fuel is directly discharged to the exhaust passage 40 without burning, the EGR cooler 58 and the like are caused by the deposit caused by the unburned fuel. There is a risk of blockage. Therefore, it is preferable to burn the post-injected fuel in the combustion chamber 6. However, in the region where the engine load is high or the engine speed is high, the gas in the combustion chamber 6 is exhausted due to the high temperature in the combustion chamber 6 or the short time per crank angle. It is difficult to sufficiently mix the post-injected fuel and air before the fuel injection is performed, and the post-injected fuel may not be sufficiently burned in the combustion chamber 6. In addition, soot may increase due to insufficient mixing. Therefore, in such a region, the DeNOx control is basically stopped.

  However, in the second region R2 where the engine load is extremely high, the excess air ratio of the air-fuel mixture is small even during normal operation due to the large injection amount of the main injection (hereinafter, appropriately referred to as the main injection amount). Can be suppressed. Therefore, in the second region R2, the post-injection injection amount (hereinafter, appropriately referred to as post-injection amount) required to reduce the stored NOx is reduced, and the unburned fuel is discharged to the exhaust passage 40. The influence can be suppressed small.

  Therefore, in the present embodiment, in the first region R1 in which neither the engine load nor the engine speed is too low and not too high, active DeNOx control in which the post-injected fuel burns in the combustion chamber 6 is performed. In the second region R2, passive DeNOx control for preventing post-injected fuel from burning in the combustion chamber 6 is performed. The second region R2 is a region where the exhaust gas temperature is sufficiently high and the DOC catalyst 42 is sufficiently activated. Therefore, the unburned fuel discharged into the exhaust passage 40 is purified by the DOC catalyst 42.

(2-2) Fuel Injection Control The control procedure of the fuel injection will be described with reference to the flowchart of FIG.

  First, in step S1, the PCM 200 acquires various types of vehicle information including the accelerator opening, the engine speed, the value of the active DeNOx control execution flag, and the passive DeNOx control execution flag.

  The active DeNOx control execution flag is a flag that becomes “1” when a basic condition for performing the active DeNOx control is satisfied, and becomes “0” at other times. In the present embodiment, the NOx occlusion amount, which is the amount of NOx occluded in the NOx catalyst 41, is equal to or greater than a predetermined first reference amount, and the temperature of the SCR catalyst 46 is set to a value close to the second temperature. When the temperature is lower than the determination temperature and the temperature of the NOx catalyst 41 is equal to or higher than the preset NOx reducible temperature, the active DeNOx control execution flag is set to “1”. When the active DeNOx control is performed for the first time after the engine is started, the first reference amount is set near the maximum amount of the NOx amount that can be stored by the NOx catalyst 41, and in other cases, the first reference amount is set to a value somewhat lower than the maximum amount. Is done. The NOx reduction possible temperature is the minimum temperature at which the NOx catalyst 41 can reduce NOx, and is set in advance.

  The passive DeNOx control execution flag is a flag that becomes “1” when a basic condition for performing the passive DeNOx control is satisfied, and becomes “0” otherwise. In the present embodiment, the case where the NOx storage amount is equal to or higher than the third reference amount set in advance, the temperature of the SCR catalyst 46 is lower than the SCR determination temperature, and the temperature of the NOx catalyst 41 is equal to or higher than the NOx reducible temperature Then, the passive DeNOx control execution flag is set to 1. The third reference amount is set to a value smaller than the first storage amount determination value. For example, the third storage amount determination value is set to a value that is about half of the maximum value of the amount of NOx that the NOx catalyst 41 can store.

  As described above, the NOx purification rate by the SCR catalyst 46 is relatively low, and although the NOx must be purified by the NOx catalyst 41, the amount of NOx stored in the NOx catalyst 41 is large. When the NOx catalyst 41 is capable of reducing NOx, the active DeNOx control execution flag and the passive DeNOx control execution flag become 1.

  The temperature of the NOx catalyst 41 is estimated based on, for example, a temperature detected by a temperature sensor provided immediately upstream of the NOx catalyst 41. The temperature of the SCR catalyst 46 is estimated based on, for example, a temperature detected by a temperature sensor provided immediately upstream of the SCR catalyst 46. The NOx storage amount is estimated by, for example, integrating the NOx amount in the exhaust gas estimated based on the operating state of the engine body 1, the flow rate and the temperature of the exhaust gas, and the like.

  After step S1, the process proceeds to step S2. In step S2, PCM 200 determines whether or not the active DeNOx control execution flag is “1”. If the determination in step S2 is NO, the process proceeds to step S10. On the other hand, if the determination in step S2 is YES, the process proceeds to step S3.

  In step S3, PCM 200 determines whether or not the engine is operating in first region R1. If the determination in step S3 is NO, the process proceeds to step S10.

  On the other hand, if the determination in step S3 is YES, the process proceeds to step S4. In step S4, the PCM 200 determines the post-injection injection amount and the injection timing in the active DeNOx control.

  Specifically, PCM 200 calculates, as a main injection amount, a fuel injection amount corresponding to a required torque calculated from an accelerator opening and the like. Next, the excess air ratio λ of the air-fuel mixture and the exhaust gas is determined based on the total injection amount obtained by combining the calculated main injection amount and the post injection amount and the amount of air introduced into the cylinder 2. The post-injection amount is determined based on the main injection and the air amount so that the target value of the excess ratio λ is obtained. The amount of air introduced into the cylinder 2 is estimated using a value detected by the air flow sensor SN2 and the like. As described above, in the present embodiment, the excess air ratio λ of the exhaust gas is changed by changing the injection amount of the post injection.

  Further, the PCM 200 sets the injection timing of the post injection for active DeNOx control to the first half of the expansion stroke as described above. Here, if the post-injection timing is advanced too much, this air-fuel mixture starts combustion in a state where the air and fuel are not sufficiently mixed, resulting in an increase in the amount of smoke generated. Therefore, in the present embodiment, the post-injection timing is adjusted based on the amount of air introduced into the cylinder 2 and the like so that the amount of smoke generation does not exceed a predetermined amount within the first half of the expansion stroke. In the present embodiment, at the time of active DeNOx control, an appropriate amount of EGR gas is introduced into the cylinder 2, thereby delaying the ignition of the post-injected fuel to suppress the generation of smoke.

  After step S4, the process proceeds to step S5. In step S5, the PCM 200 causes the injector 10 to main-inject the fuel of the main injection amount and post-inject the fuel of the post-injection amount for active DeNOx control determined in step S4 at the timing determined in step S4. . After step S5, the process ends (return to step S1). When the post-injection amount is small, the amount of air introduced into the cylinder 2 may be reduced by controlling the throttle valve 23 to be closed.

  On the other hand, in step S10 where the determination in step S2 is NO or the determination in step S3 is NO, PCM 200 determines whether the passive DeNOx control execution flag is "1". If the determination in step S10 is NO, the process proceeds to step S11.

  In step S11, the PCM 200 performs the normal control without executing the active DeNOx control and the passive DeNOx control. That is, the PCM 200 stops the post injection and performs the main injection of the fuel injection amount corresponding to the required torque calculated from the accelerator opening and the like. After step S11, the process ends (return to step S1). On the other hand, if the determination in step S10 is YES, the process proceeds to step S12.

In step S12, PCM200 determines whether the engine is operated in the second region R 2. If the determination in step S12 is NO, the process proceeds to step S11.

  On the other hand, if the determination in step S12 is YES, the process proceeds to step S13. In step S13, the PCM 200 determines the injection amount and the injection timing of the post injection in the passive DeNOx control.

  Specifically, PCM 200 calculates, as a main injection amount, a fuel injection amount corresponding to a required torque calculated from an accelerator opening and the like. Next, the excess air ratio λ of the air-fuel mixture and the exhaust gas is determined by the total injection amount obtained by adding the calculated main injection amount and post-injection amount and the amount of air introduced into the cylinder 2 to the passive DeNOx control air. The post-injection amount is determined based on the main injection and the air amount so that the target value of the excess ratio λ is obtained. In the present embodiment, the PCM 200 sets the injection timing of the passive DeNOx control post-injection to the first half of the expansion stroke as described above.

After step S13, the process proceeds to step S14. In step S14, PCM200 is the injector 10, the fuel of the main injection amount of the causes is the main injection, post-injection and post-injection quantity of fuel for passive DeNOx control determined in step S13 at the timing determined in step S1 3 Let it. After step S14, the process ends (return to step S1).

(3) Urea Water Injection Control Next, injection control of the urea injector 45 will be described. Hereinafter, the amount of urea injected from the urea injector 45 will be referred to as the urea injection amount as appropriate. As described above, the injection control of the urea injector 45 is performed by the DCU 300 while obtaining information from the PCM 200.

(3-1) Outline of Urea Injection Control During DeNOx Control As described above, when NOx is reduced, that is, when DeNOx control is performed, NH ₃ is released from the NOx catalyst 41 and introduced into the SCR catalyst 46. Therefore, if the urea injection amount when the DeNOx control is performed is the same as the urea injection amount when the DeNOx control is not performed (when the DeNOx control is not performed), the amount of NH ₃ supplied to the SCR catalyst 46 is reduced. Is excessive, and there is a possibility that NH ₃ may pass downstream of the SCR catalyst 46. Therefore, during the DeNOx control, the urea injection amount is made smaller than when the DeNOx control is not performed.

However, simply controlling the urea injection amount at the time of DeNOx control to a predetermined amount smaller than that at the time of non-DeNOx control irrespective of the operating conditions results in an excessive amount of NH ₃ supplied to the SCR catalyst 46. Alternatively, it was found that there was a possibility that a large amount of NH ₃ would pass downstream of the SCR catalyst 46 due to a shortage, and that the SCR catalyst 46 could not properly purify NOx.

The inventors of the present application have conducted intensive studies on this, and as a result, it has been found that the release of NH ₃ from the NOx catalyst 41 occurs only after a predetermined period has elapsed since the start of the DeNOx control. It was found that the above-mentioned problem arises because of the differences. Hereinafter, a period from the start of the DeNOx control to the start of the release of NH ₃ from the NOx catalyst 41 is appropriately referred to as a delay period.

That is, even if the execution period of the DeNOx control is the same, the shorter the delay period, the longer the supply of NH ₃ from the NOx catalyst 41 to the SCR catalyst 46 for a longer period. Therefore, assuming that the total amount of urea injected into the exhaust passage 40 during the execution period of the DeNOx control is constant, regardless of the delay period, the amount of NH ₃ supplied to the SCR catalyst 46 becomes excessive, and a large amount of NH ₃ There is a possibility that the NOx purification rate of the SCR catalyst 46 may be deteriorated due to slippage to the downstream side of the catalyst 46 or an insufficient amount of NH ₃ supplied to the SCR catalyst 46.

  Further, the inventors of the present application have found that there is a correlation between the delay period and the amount of decrease in the amount of oxygen stored in the NOx catalyst 41 per unit time (hereinafter referred to as the stored oxygen reduction rate). It was found that the larger the oxygen reduction rate, the shorter the delay period.

  It is considered that the longer the stored oxygen reduction rate is, the shorter the delay period is for the following reasons.

As described above, the reason why NH ₃ is generated in the NOx catalyst 41 is that “N” in the NOx stored in the NOx catalyst 41 is obtained by supplying the reducing agent to the NOx catalyst 41 in accordance with the DeNOx control. This is because NOx passing through the NOx catalyst 41 and H and the like as a reducing agent introduced into the NOx catalyst 41 are combined. However, as described above, the NOx catalyst 41 has an oxygen storage ability. The NOx catalyst 41 stores a large amount of oxygen as the excess air ratio λ of the exhaust gas is set to be larger than 1 during normal operation or the like. Therefore, immediately after the DeNOx control is started and the introduction of the reducing agent into the NOx catalyst 41 is started, the supplied reducing agent reacts not with “N” but with the oxygen stored in the NOx catalyst 41. , NH ₃ generation reaction is suppressed. Accordingly, it is considered that the release of NH ₃ is started only when the oxygen stored in the NOx catalyst 41 is exhausted. Here, the period from the start of the DeNOx control to the time when the oxygen stored in the NOx catalyst 41 is exhausted is shorter when the stored oxygen reduction rate is higher. Therefore, it is considered that the timing at which the release of NH ₃ starts is earlier and the delay period is shorter when the stored oxygen reduction rate is higher.

  Therefore, in the present embodiment, the stored oxygen reduction rate is estimated, and the delay period is estimated based on the estimated oxygen reduction rate.

Specifically, the amount of decrease in the amount of oxygen stored in the NOx catalyst 41 per unit time, which is the stored oxygen reduction rate, is integrated, and this integrated value is the maximum amount of stored oxygen and the NOx catalyst 41 The time when the amount of oxygen that can be stored becomes equal to or more than the maximum amount is estimated as the time when the release of NH ₃ from the NOx catalyst 41 is started (hereinafter, referred to as NH ₃ release start time). Then, a period from the start time of the DeNOx control to the NH ₃ release start time is estimated as the delay period. The procedure for estimating the stored oxygen reduction rate will be described later.

Until the delay period elapses after the start of the DeNOx control, the urea injection amount is maintained at the same amount as before the execution of the DeNOx control, that is, when the DeNOx control is not performed, and only after the delay period elapses (NH ₍₃ ) The urea injection amount is set to be smaller than when the DeNOx control is not performed (the first time the release start time comes).

  Therefore, in the present embodiment, even if the execution period of the DeNOx control is the same, the shorter the delay period is, the longer the period in which the urea injection amount is reduced than when the DeNOx control is not performed, and the implementation of the DeNOx control is shorter. The total amount of urea injection during the period, that is, the total amount of urea supplied to the exhaust passage 40 during the execution period of the DeNOx control decreases.

  Here, the longer the storage oxygen reduction rate, the shorter the delay period. Therefore, in the present embodiment, the larger the stored oxygen reduction rate, the smaller the total amount of the urea injection amount during the execution period of the DeNOx control.

More specifically, in the present embodiment, during the execution of the DeNOx control, when the delay period elapses (when the NH ₃ release start time comes), at each time, the amount of NH ₃ released from the NOx catalyst 41 (hereinafter, referred to as “NH ₃ release amount”). The amount of NH ₃ release is appropriately estimated). In addition, the estimated NH ₃ release amount is converted into urea (the minimum value of the amount of urea required to generate NH ₃ of the NH ₃ release amount is calculated). Then, the urea-converted value of the NH ₃ release amount is subtracted from the urea injection amount when the DeNOx control is not performed, and the obtained value is calculated as the urea injection amount.

Here, in the calculation procedure of the urea injection amount, the amount of the urea injection amount also changes depending on the NH ₃ release amount. However, the influence on the total amount of NH ₃ released from the NOx catalyst 41 during the execution period of the DeNOx control is greater in the oxygen reduction rate than in the NH ₃ release amount, as described above. In the case, the total amount of the urea injection amount during the execution period of the DeNOx control decreases as the stored oxygen reduction rate increases.

As will be described later, the NH ₃ emission amount is calculated to be a larger value when the exhaust gas is rich, and the stored oxygen reduction rate is smaller when the excess air ratio λ of the exhaust gas is smaller, that is, when the exhaust gas is rich. Is calculated to be a large value. Therefore, for the change in the excess air ratio λ of the exhaust gas, the effect of the NH ₃ release amount on the total amount of the urea injection amount and the effect of the stored oxygen reduction rate on the total amount of the urea injection amount are both on the same side ( (The side where the total amount increases or decreases). That is, when the excess air ratio λ of the exhaust gas is small, the total amount of the urea injection amount decreases as the occluded oxygen reduction rate increases, and also decreases as the NH ₃ release amount increases.

Similarly, the larger the flow rate of the exhaust gas (hereinafter, appropriately referred to as the exhaust flow rate), the larger the value of the NH ₃ release is calculated, and the larger the flow rate of the exhaust gas, the larger the value of the stored oxygen decreasing rate. The effect of the NH ₃ release amount on the total amount of the urea injection amount and the effect of the stored oxygen reduction rate on the total amount of the urea injection amount are both on the same side (for the change in the exhaust gas flow rate). The total amount increases or decreases).

(3-2) so that the summary of the urea injection control after DeNOx control end, during DeNOx control NH ₃ is introduced from the NOx catalyst 41 in SCR catalyst 46. Therefore, immediately after the DeNOx control ends, a large amount of NH ₃ is adsorbed on the SCR catalyst 46. Therefore, if the urea injection amount is greatly increased immediately after the end of the DeNOx control, the NH ₃ supplied to the SCR catalyst 46 becomes excessive, and the NH ₃ may pass through the downstream side of the SCR catalyst 46. There is.

Therefore, in the present embodiment, even when the DeNOx control is not performed, the urea injection amount is set to be smaller than the other periods during a predetermined period (hereinafter, appropriately referred to as a switching period) after the DeNOx control ends. . However, as described above, even in the switching period, because after the DeNOx control is finished as the NH ₃ emissions almost 0 is the emission of NH ₃ from NOx catalyst 41, even during the switching period, The urea injection amount is set to be larger than that during the DeNOx control. The switching period is set in advance.

Further, in the present embodiment, the total amount of NH ₃ discharged from the NOx catalyst 41 along with the execution of the DeNOx control is estimated, and during the switching period, the total amount is set so that the larger the total amount, the smaller the urea injection amount. decide.

Specifically, as described above, the amount of NH ₃ release at each time during the execution of the DeNOx control and after the elapse of the delay period is estimated, and the amount of released Nh3 is integrated until the DeNOx control is completed. The value is calculated as the total amount (total amount of NH ₃ discharged from the NOx catalyst 41 with the execution of the DeNOx control).

(3-3) Calculation procedure of stored oxygen reduction rate Next, a calculation procedure of the stored oxygen reduction rate will be described.

  The inventors of the present application have found out that it is possible to accurately estimate the stored oxygen reduction rate by calculating the stored oxygen reduction rate using MreO2 according to the following equation (2). Therefore, in the present embodiment, the stored oxygen reduction rate is calculated using the equation (2).

MreO2 = (1−λ) × K × Mex (2)
In the equation (2), λ is the excess air ratio of the exhaust gas. In the present embodiment, as described above, the target value of the excess air ratio λ of the exhaust gas at the time of the DeNOx control is set in advance, and the post injection is performed so as to realize this. Therefore, the excess air ratio λ in the equation (2) is substantially equal to the target value. In the present embodiment, the current excess air ratio λ of the exhaust gas is calculated from the fuel injection amount, the amount of air introduced into the cylinder 2, and the like. Then, this calculated value is used for the equation (2).

  In the equation (2), Mex is a flow rate of the exhaust gas. In the present embodiment, the exhaust flow rate Mex is calculated using a value detected by an air flow meter and the like.

  In the equation (2), K is a correction coefficient. The correction coefficient K is calculated by K = α1 × α2 × α3, where α1 is the first correction coefficient, α2 is the second correction coefficient, and α3 is the third correction coefficient.

  The value calculated in the term of (1−λ) × Mex included in the equation (2) is the value of the exhaust gas when all of the unburned fuel and the reducing agent flowing into the NOx catalyst 41 are oxidized. The amount of oxygen that is insufficient. Therefore, X1 obtained by the following equation (3) is considered to be a value close to the stored oxygen reduction rate. That is, at the start of the DeNOx control, it is considered that the NOx catalyst 41 consumes the amount of stored oxygen calculated in the term of (1−λ) × Mex.

X1 = (1−λ) × Mex (3)
However, there is a difference between the value X1 obtained by the equation (3) and the actual stored oxygen reduction rate, and the correction coefficients α1, α2, and α3 are coefficients for correcting the difference. Hereinafter, the parameter represented by “1-λ” will be described as an air shortage width.

(First correction coefficient)
As a result of intensive studies to reduce the deviation, the concentrations of CO, H ₂ , and THC, which are the reducing agents discharged from the engine body 1 when the excess air ratio λ of the exhaust gas is set to 1 or less, are shown in FIG. 5 was found. Further, as shown in FIG. 5, it was found that even when the excess air ratio λ of the exhaust gas was 1 or less, the exhaust gas discharged from the engine body 1 contained oxygen (O ₂ ).

Specifically, in FIG. 5, the horizontal axis represents the excess air ratio λ of the exhaust gas, and the vertical axis represents the concentration of each substance in the exhaust gas. As shown in FIG. 5, CO excess air ratio of the exhaust gas λ is contained as the exhaust gas is reduced, the concentration of H 2, _THC is increased respectively, contained in the air excess ratio λ is smaller the more the exhaust gas in the exhaust gas The concentration of O ₂ is reduced.

In addition, the present inventors have found that the amount of H ₂ in addition to CO mainly contained in the exhaust gas affects the stored oxygen reduction rate. This, H ₂ has a high reducing ability, oxygen stored in the NOx catalyst 41 (hereinafter referred to as the stored oxygen) is considered to be because the easily react with. Further, reduction of stored oxygen by O ₂ contained in the exhaust gas is suppressed, the amount of O ₂ contained in the exhaust gas was also discovered to affect the stored oxygen reduction rate.

Then, from these findings, the amount of reducing agent that reacts with actually stored oxygen in the NOx catalyst 41 from the total amount of CO and H ₂ contained in the exhaust gas, a quantity of O ₂ to react with these exhaust It was found that the value approximated to a value obtained by subtracting half the amount of O ₂ contained in the gas.

Here, FIG. 6 shows a value obtained by subtracting half the value of the concentration of O ₂ contained in the exhaust gas from the total value of the concentrations of CO and H ₂ contained in the exhaust gas. 5 is a graph in which the concentration of a reducing agent that is considered to react with stored oxygen in the NOx catalyst 41 (hereinafter, referred to as a net reducing agent concentration) is plotted for each air shortage width (“1-λ”) of exhaust gas. In FIG. 6, a line L1 indicated by a solid line is a line connecting representative points of these plot points.

  As shown in FIG. 6, the smaller the excess air ratio λ of the exhaust gas becomes (the richer the air becomes), the larger the net concentration of the reducing agent becomes.

  However, as the air shortage width of the exhaust gas increases (the excess air ratio λ of the exhaust gas decreases), the rate of change of the net concentration of the reducing agent with respect to the air shortage width of the exhaust gas and the excess air ratio λ of the exhaust gas increases. The net concentration of the reducing agent is not proportional to the width of the air shortage. That is, there is a difference between the line L2 which is a line connecting the points calculated by the equation (3) for each air deficiency width and the line L1 which connects the points proportional to the air deficiency width. Is considered to constitute at least a part of the difference between the value of X1 obtained by equation (3) and the actual stored oxygen reduction rate.

  The first correction coefficient α1 is a coefficient for correcting this shift, that is, a coefficient for correcting a point on the line L1 to a point on the line L2, and is set as shown in FIG. Specifically, the first correction coefficient α1 is set so that the value increases as the excess air ratio λ of the exhaust gas decreases. Further, the first correction coefficient α1 is set such that the change rate of the first correction coefficient α1 with respect to the excess air ratio λ of the exhaust gas increases as the excess air ratio λ of the exhaust gas increases.

(Second correction coefficient and third correction coefficient)
In addition, the inventors of the present application have determined that depending on the temperature of the NOx catalyst 41 and the exhaust flow rate, even during the DeNOx control, all the reducing agents introduced into the NOx catalyst 41 are not oxidized. It has been found that the ratio of the reducing agent oxidized by the stored oxygen in the NOx catalyst 41 during the DeNOx control differs depending on the exhaust gas flow rate.

  FIG. 8 is a graph in which the horizontal axis represents the temperature of the NOx catalyst 41 and the vertical axis represents the rate at which the reducing agent introduced into the NOx catalyst 41 is oxidized during the DeNOx control. The three lines L11, L12, L13 in the graph of FIG. 8 are lines when the exhaust flow rates are different from each other. The exhaust flow rate increases in the order of the lines L11, L12, and L13.

  As shown in FIG. 8, the lower the temperature of the NOx catalyst 41, the lower the ratio. More specifically, when the temperature of the NOx catalyst 41 is equal to or higher than a predetermined temperature (for example, about 350 ° C.), the ratio becomes 100%, and almost all the reducing agent introduced into the NOx catalyst 41 is oxidized. On the other hand, when the temperature of the NOx catalyst 41 is lower than the predetermined temperature, the above ratio decreases as the temperature of the NOx catalyst 41 decreases. In addition, the larger the exhaust flow rate, the smaller the ratio becomes.

Therefore, the amount of the reducing agent actually oxidized by the stored oxygen in the NOx catalyst 41, and thus the amount of the stored oxygen used for this oxidation, is determined by the value obtained by (1−λ) × Mex × α1 (the NOx catalyst 41). ing the value obtained by multiplying the percentage total amount of the introduced reducing agent and the value) estimated the amount of the corresponding stored oxygen to the total amount.

  Therefore, in the present embodiment, the ratio that changes according to the temperature of the NOx catalyst 41 is set as a second correction coefficient α2, and the ratio that changes according to the exhaust gas flow rate is set as a third correction coefficient α3. These α2 and α3 are multiplied by (1−λ) × Mex × α1, and the amount of the reducing agent oxidized by the stored oxygen in the NOx catalyst 41 and the amount of the stored oxygen used for the oxidation are estimated with higher accuracy. .

  As described above, the second correction coefficient α2, that is, the ratio that changes in accordance with the temperature of the NOx catalyst 41, becomes smaller as the temperature of the NOx catalyst 41 becomes lower.

  As described above, the third correction coefficient α3, that is, the ratio that changes according to the exhaust gas flow rate, is set to a smaller value when the exhaust gas flow rate is larger.

  The second correction coefficient α2 and the third correction coefficient α3 are constant values (for example, 1.0) regardless of the temperature of the NOx catalyst 41 and the exhaust flow rate when the temperature of the NOx catalyst 41 is equal to or higher than a predetermined temperature. Is done.

  It is considered that the reason why the higher the temperature of the NOx catalyst 41 is, the higher the ratio is because the higher the temperature of the NOx catalyst 41 is, the more the NOx catalyst 41 is in an active state and the oxidation reaction of the reducing agent is accelerated. Also, it is considered that the reason why the larger the exhaust flow rate is, the smaller the ratio is because the larger the exhaust flow rate is, the larger the amount of the substance other than the reducing agent is, and the chance of contact between the NOx catalyst 41 and the reducing agent is reduced.

  As described above, in the present embodiment, the stored oxygen reduction rate is accurately estimated by the equation (2).

In the present embodiment, the NH ₃ release start timing is estimated using the stored oxygen decrease rate estimated in this manner. Specifically, as described above, the time when the integrated value of the estimated stored oxygen decrease rate becomes equal to or more than the maximum amount of stored oxygen is estimated as the NH ₃ release start time.

(3-4) Target value of excess air ratio at the time of DeNOx control As described above, in the NOx catalyst 41, the excess air ratio λ of the introduced exhaust gas is set to be close to 1 or a rich state smaller than 1. , NOx is reduced. Therefore, if only NOx is reduced in the NOx catalyst 41, the excess air ratio λ of the exhaust gas may be near 1.

However, as described above, the NOx catalyst 41 has an oxygen storage capacity, and stores oxygen when the excess air ratio λ of the exhaust gas becomes 1 or more. When oxygen is stored in the NOx catalyst 41, the release of NH ₃ from the NOx catalyst 41 is stopped until the stored oxygen is exhausted.

Therefore, in the present embodiment, the excess air ratio λ of the exhaust gas is set to less than 1 in order to efficiently supply NH ₃ to the SCR catalyst 46. That is, in order to release NH ₃ from the NOx catalyst 41 and suppress the amount of urea injected from the urea injector 45 to reduce the amount of urea in the urea tank 45c, the excess air ratio λ of the exhaust gas is set to less than 1. To In particular, it has been found that when the excess air ratio λ of the exhaust gas becomes 1 or more, a large amount of oxygen is occluded in the NOx catalyst 41 in a short time and the release of NH ₃ is interrupted for a relatively long time. In order to release a large amount of NH ₃ from the NOx catalyst 41 inside, the excess air ratio λ of the exhaust gas needs to be less than 1.

However, the excess air ratio λ of the exhaust gas changes depending on the amount of air introduced into the cylinder 2 and the amount of fuel. Therefore, when the target value of the excess air ratio λ of the exhaust gas is set to a value very close to 1, the actual excess air ratio λ of the exhaust gas becomes 1 or more in accordance with the change in the air amount and the fuel amount. More opportunities. Further, as described above, when the excess air ratio λ of the exhaust gas becomes 1 or more, the release of NH ₃ is stopped. Therefore, when the excess air ratio λ of the exhaust gas fluctuates over 1, the supply to the SCR catalyst 46 is performed. The fluctuation of NH _{3 performed} becomes large. Thus, in order to efficiently and stably supply NH ₃ to the SCR catalyst 46, in the present embodiment, the target value of the excess air ratio λ of the exhaust gas is set to less than 0.98.

On the other hand, as shown by the arrow in FIG. 5, when the excess air ratio λ of the exhaust gas decreases, the variation width of the amount of the reducing agent in the exhaust gas increases as the amount of the reducing agent in the exhaust gas increases. Become. Therefore, even if the excess air ratio λ of the exhaust gas is set too small, the NH ₃ released from the NOx catalyst 41 and, consequently, the NH ₃ in the SCR catalyst 46 become unstable. In particular, when the excess air ratio λ of the exhaust gas is 0.9 or less , the variation width of the amount of the reducing agent in the exhaust gas becomes a predetermined value or more. Therefore, in the present embodiment, the target value of the excess air ratio λ of the exhaust gas is set to be larger than 0.9. That is, in the present embodiment, the target value of the excess air ratio λ of the exhaust gas at the time of the DeNOx control is set to a value larger than 0.9 and smaller than 0.98.

  For example, in the low-load first region R1_L in which the engine load is low in the first region R1, the target value of the excess air ratio λ of the air-fuel mixture and thus the exhaust gas is set to 0.98. In the first region R1, in regions other than the low-load-side first region R1_L, and in the second region R2, the target value of the excess air ratio λ of the air-fuel mixture and thus the exhaust gas is set to 0.96.

(3-5) Procedure for Estimating the Amount of NH ₃ Released from NOx Catalyst Next, the procedure for estimating the amount of NH ₃ released from the NOx catalyst 41 during DeNOx control will be described. FIG. 9 is a diagram for explaining this procedure.

  In the present embodiment, the DCU 300 is functionally provided with a first estimating unit 301 and a second estimating unit 302.

First estimation unit 301, at the time DeNOx control, the NOx stored in the NOx catalyst 41, the reducing agent NH ₃ to H and the like are generated by combining (hereinafter referred to as a 1N H ₃₎ of Estimate the amount.

During the DeNOx control, the second estimating unit 302 generates NOx (hereinafter, appropriately referred to as RawNOx) generated by the engine body 1 and flowing into the NOx catalyst 41 and H or the like as a reducing agent by the NOx catalyst 41. The amount of NH ₃ (hereinafter, appropriately referred to as second NH ₃ ) is estimated.

The first estimating unit 301 first estimates the current NOx storage amount of the NOx catalyst 41. Next, the first estimating unit 301 multiplies the estimated value of the NOx storage amount by the first temperature coefficient β1, the first flow coefficient β2, the first A / F coefficient β3, and the first thermal deterioration coefficient β4, respectively. Then, the amount of the first NH ₃ is calculated.

The first temperature coefficient β1 is a coefficient set based on the map shown in FIG. 10 and is set according to the temperature of the NOx catalyst 41. Specifically, the temperature coefficient β1 decreases as the temperature of the NOx catalyst 41 increases. That is, it is known that the higher the temperature of the NOx catalyst 41 is, the more the reaction of converting NOx stored in the NOx catalyst 41 to NH ₃ is promoted, and the higher the temperature of the NOx catalyst 41 is, the higher the temperature of the first NH _{3 is.} The first temperature coefficient β1 is set so that the amount is calculated to be large.

The first flow coefficient β2 is a coefficient set based on the map shown in FIG. 11, and is set according to the exhaust flow rate. Specifically, the first flow coefficient β2 is set to a larger value as the exhaust flow rate increases. That is, as the exhaust flow rate increases, the amount of the reducing agent flowing into the NOx catalyst 41 increases, and the amount of NH ₃ released from the NOx catalyst 41 increases. Accordingly, the higher the exhaust flow rate, the higher the first NH _3. The first flow coefficient β2 is set so that the amount is calculated to be large.

The first A / F coefficient β3 is a coefficient set based on the map shown in FIG. 12, and is set according to the air-fuel ratio (A / F) of the exhaust gas. Specifically, the A / F coefficient β3 is set to a larger value as the air-fuel ratio of the exhaust gas is smaller (richer). In other words, the richer the air-fuel ratio of the exhaust gas, the larger the amount of the reducing agent flowing into the NOx catalyst 41 and the larger the amount of NH ₃ released from the NOx catalyst 41, and accordingly, the larger the exhaust flow rate. The first A / F coefficient β3 is set so that the first NH ₃ amount is calculated to be larger.

The first thermal deterioration coefficient β4 is a coefficient set according to the degree of deterioration of the NOx catalyst 41. The PCM 200 estimates the degree of deterioration of the NOx catalyst 41 based on the running time of the vehicle, the number of executions of the DeNOx control, and the like, and the thermal deterioration coefficient β4 increases (the deterioration progresses as the estimated degree of deterioration increases). ), A large value. That is, it is known that the higher the degree of deterioration of the NOx catalyst 41 is, the more the reaction of converting NOx stored in the NOx catalyst 41 into NH ₃ is promoted, and the higher the degree of deterioration is, The thermal deterioration coefficient β4 is set so that the first NH ₃ amount is calculated to be large.

First, the second estimating unit 302 estimates the amount (flow rate) of RawNOx discharged from the engine body 1. In the present embodiment, the flow rate of RawNOx is estimated from the exhaust flow rate, the excess air ratio λ of the air-fuel mixture, and the like. Next, a second estimation unit 302, the estimated value of this, by multiplying the second flow coefficient β22 and the 2A / F factor β23 respectively, to calculate the amount of the 2NH _3.

The second flow coefficient β22 is a coefficient set based on the map shown in FIG. 13, and is set according to the exhaust flow rate. Specifically, similarly to the first flow coefficient β2, the second flow coefficient β22 is set to a larger value as the exhaust flow rate increases. However, unlike the NOx stored in the NOx catalyst 41, the influence of the exhaust flow rate on the RawNOx becomes equal when the exhaust flow rate becomes a predetermined value or more, and the second flow coefficient β22 becomes a constant value when the exhaust flow rate is a predetermined value or more. It is said.

The second A / F coefficient β23 is a coefficient set based on the map shown in FIG. 14, and is set according to the air-fuel ratio (A / F) of the exhaust gas. Specifically, like the second A / F coefficient β3, the second A / F coefficient β23 has a larger value as the air-fuel ratio of the exhaust gas is smaller (richer). However, unlike the NOx stored in the NOx catalyst 41, the influence of the air-fuel ratio of the exhaust gas on RawNOx becomes equivalent when the air-fuel ratio of the exhaust gas becomes equal to or less than a predetermined value, and the second A / F coefficient β23 is When the air-fuel ratio is equal to or lower than a predetermined value, the air-fuel ratio is set to a constant value.

In this manner, in the present embodiment, the amount and the amount of the 2NH ₃ of the 1N H ₃ is estimated. Then, DCU300 is the amount and the combined amount of the amount of the 2NH ₃ of the 1N H _3, during the DeNOx control (specifically, between the NH ₃ release start timing until the DeNOx control end) to, NOx It is calculated as NH ₃ released from the catalyst 41.

(3-6) Control Flow of Urea Injection Amount FIG. 15 is a flowchart summarizing the control procedure of the urea injection amount. The overall flow of the control of the urea injection amount will be described with reference to this flowchart. FIG. 15 is a control flow when NOx purification is performed by both the SCR catalyst 46 and the NOx catalyst 41.

  First, in step S21, the DCU 300 acquires various information of various vehicles including the engine speed, the engine load, the excess air ratio λ of the air-fuel mixture and the exhaust gas, the temperature of the NOx catalyst 41, and the exhaust flow rate.

Next, in step S22, the DCU 300 calculates a basic urea injection amount, which is a value of the urea injection amount during normal operation excluding the DeNOx control and during the switching period, as described later. In the present embodiment, the basic urea injection amount is determined based on engine operating conditions and the like so that the amount of NH ₃ adsorbed on the SCR catalyst 46 is maintained at a predetermined amount.

  Next, in step S23, DCU 300 determines whether or not DeNOx control is being performed. In the present embodiment, a flag is set to “1” during execution of the active DeNOx control or the passive DeNOx control, and is set to “0” in other cases. In step S23, whether or not this flag is 1 is set. Is determined.

  If the determination in step S23 is YES, the process proceeds to step S24. In step S24, the value of the correction coefficient K is determined. Specifically, the value of the correction coefficient K is calculated according to K = α1 × α2 × α3.

  Here, the first correction coefficient α1 is obtained from the map shown in FIG. 7 according to the excess air ratio λ of the exhaust gas. Specifically, the map shown in FIG. 7 is stored in DCU 300, and DCU 300 extracts a value corresponding to the current excess air ratio λ of the exhaust gas from this map.

  A map corresponding to FIG. 8 is stored in the DCU 300, and the DCU 300 extracts a value corresponding to the current exhaust gas flow rate and the temperature of the NOx catalyst 41 from this map, and calculates a value of α2 × α3. I do.

  After step S24, the process proceeds to step S25. In step S25, the DCU 300 calculates the stored oxygen reduction rate MreO2. Specifically, the DCU 300 uses the correction coefficient K determined in step S24 and the exhaust flow rate and the excess air ratio λ of the exhaust gas read in step S21 to calculate the stored oxygen reduction rate MreO2 according to equation (2). .

  After step S25, the process proceeds to step S26. In step S26, the stored oxygen reduction rate MreO2 calculated in step S25 is integrated. In other words, ΣMreO2 is calculated, and the stored oxygen amount of the NOx catalyst 41 that has been reduced to the present with the start of the DeNOx control is calculated.

After step S26, the process proceeds to step S27. In step S27, it is determined whether ΔMreO2 calculated in step S26 is equal to or greater than the maximum amount of stored oxygen, that is, whether or not the NH ₃ release start timing has been reached. As described above, the maximum amount of stored oxygen is set in advance and stored in the DCU 300.

If the determination in step S27 is NO, that is, if it is estimated that ΣMreO2 is less than the maximum amount of stored oxygen and it has not yet reached the NH ₃ release start timing, the process proceeds to step S28, where the NH ₃ release amount The release amount of NH ₃ from the NOx catalyst 41 is set to zero. After step S28, the process proceeds to step S29, where the final (actually injected) urea injection amount is set to the basic value of the urea injection amount calculated in step S22. After step S29, the process ends (return to step S21).

On the other hand, a YES determination at step S27, ShigumaMreO2 is not less occluded oxygen maximum amount or more, if it is estimated to have exceeded the or it reaches the NH ₃ release start timing, the process proceeds to step S30. At step S30, it is calculated by the procedure described amount of released NH ₃ from NH ₃ emissions clogging NOx catalyst 41 in (3-5).

After step S30, the process proceeds to step S31. In step S31, the DCU 300 calculates, as a reduction amount, a value obtained by converting the NH ₃ release amount calculated in step S30 into urea.

  After step S31, the process proceeds to step S32. In step S32, the DCU 300 subtracts the urea conversion value obtained in step S31 from the basic urea injection amount obtained in step S22, and sets the value as the final urea injection amount.

After step S32, the process proceeds to step S33. In step S33, DCU300 is by integrating the emission amount of NH ₃ calculated in step S 31, to calculate the total amount of NH ₃ released from the N Ox catalyst 41 to NH ₃ release start timing after. After step S32, the process ends (return to step S21).

As described above, in the present embodiment, even when the DeNOx control is being performed, the urea injection amount is reduced from the basic urea injection amount only after the determination in step S27 is YES and the NH ₃ release start timing is reached. Is performed. Therefore, the shorter the time from the start of the DeNOx control to the arrival of the NH ₃ release, the longer the decrease rate of the stored oxygen becomes, and the longer the decrease in the stored oxygen becomes, the longer the decrease time becomes. The total supplied urea injection amount is reduced.

As described above, the basic urea injection amount is appropriately adjusted so that the amount of NH ₃ adsorbed on the SCR catalyst 46 is maintained at a predetermined amount, but a reduction amount exceeding the basic urea injection amount is required. In this case, the excess reduction amount may be subtracted from the basic urea injection amount after the end of the DeNOx control.

  On the other hand, if the determination in step S23 is NO, the process proceeds to step S40.

In step S40, the DCU 300 resets the integrated value of the stored oxygen reduction rate (ΣMreO2) to 0 and resets the amount of released NH ₃ from the NOx catalyst 41 to 0. The total amount of NH ₃ release is stored.

  After step S40, the process proceeds to step S41. In step S41, the DCU 300 determines whether or not a switching period (preset as described above) has not elapsed since the end of the DeNOx control.

If the determination in step S41 is YES, the process proceeds to step S42. In step S42, a value calculated in step S3 3 during the practice of DeNOx control just before, in accordance with the embodiment of the DeNOx control based on the total amount of NH ₃ released from the NOx catalyst 41, the urea injection amount Determine the amount of reduction. As described above, in the present embodiment, this is determined such that the larger the total amount of NH ₃ , the larger the reduction amount. Also, as described above, reduction amount calculated in step S42 (reduction in switching period) is set to a value smaller than the reduction amount calculated in step S 31 (reduction in DeNOx control) You.

After step S42, the process proceeds to step S43. In step S43, the DCU 300 determines, as the final urea injection amount, a value obtained by subtracting the reduction amount calculated in step S42 from the basic urea injection amount calculated in step S22. After step S4 3 ends the processing (return to step S21).

On the other hand, if the determination in step S41 is NO, that is, if the reduction of the urea injection amount by the amount corresponding to the total amount of NH ₃ release generated by the DeNOx control is completed, the process proceeds to step S51. In step S51, the final urea injection amount is set to the basic value of the urea injection amount calculated in step S22. After step S51, the process ends (return to step S21).

(4) Operation As described above, in the present embodiment, the urea injection amount is reduced during the DeNOx control as compared to when the DeNOx control is not performed. Therefore, during the DeNOx control, it is possible to prevent the amount of NH ₃ supplied to the SCR catalyst 46 from becoming excessive and passing through the SCR catalyst 46 downstream.

Then, during the DeNOx control, the urea injection amount is changed according to the stored oxygen decreasing speed. Specifically, the time period from the start of the stored oxygen reduction rate increased DeNOx controlled to release NH ₃ from NOx catalyst 41 is started (delay time) shorter, the urea injection amount is small You.

Accordingly, the amount of NH ₃ supplied to the SCR catalyst 46 can be reliably set to an appropriate amount to prevent NH ₃ from passing through the downstream side of the SCR catalyst 46 and to appropriately purify NOx by the SCR catalyst 46. it can.

More specifically, when the delay time is short, a large amount of NH ₃ is released from the NOx catalyst 41 and a large amount of NH ₃ is accumulated (adsorbed) in the SCR catalyst 46. A large amount of NH ₃ can be prevented from being supplied to 46, and it is possible to prevent NH ₃ from passing downstream of the SCR catalyst 46 without being adsorbed by the SCR catalyst 46. In addition, a small amount of NH ₃ introduced from the NOx catalyst 41 to the SCR catalyst 46 due to the long delay time and a small amount of NH ₃ in the SCR catalyst 46 decrease, and a small amount of NH ₃ is supplied from the urea injector 45 to the SCR catalyst 46. The supply of only NH ₃ can be prevented, and the amount of NH ₃ in the SCR catalyst 46 can be secured to appropriately purify NOx.

  Here, as described above, the stored oxygen reduction rate is calculated by equation (2). Then, the second correction coefficient α2 constituting the correction coefficient K in the equation (2) is set so that the lower the temperature of the NOx catalyst 41 becomes, the smaller the temperature becomes (the higher the temperature of the NOx catalyst 41 becomes, the higher). ing. Therefore, the higher the temperature of the NOx catalyst 41, the higher the value of the stored oxygen reduction rate is calculated. During the DeNOx control, the higher the temperature of the NOx catalyst 41, the smaller the urea injection amount.

That is, in the present embodiment, during the DeNOx control, the urea injector 45 is controlled such that the higher the temperature of the NOx catalyst 41, the smaller the urea injection amount. Is controlled so as to be small, whereby the urea injection amount is appropriately changed in accordance with the occluded oxygen reduction rate, and an effect of efficiently purifying NOx while preventing the NH ₃ from slipping through is obtained. be able to.

  In the present embodiment, as shown in FIG. 8, the second correction coefficient α2 is set such that the lower the temperature of the NOx catalyst 41, the larger the rate of change of the second correction coefficient α2 with respect to the temperature of the NOx catalyst 41. Is set. Therefore, in the present embodiment, the urea injection amount is controlled such that the lower the temperature of the NOx catalyst 41, the larger the rate of change of the urea injection amount with respect to the temperature of the NOx catalyst 41.

  That is, in the present embodiment, during the DeNOx control, the urea injection amount is controlled such that the lower the temperature of the NOx catalyst 41, the greater the rate of change of the urea injection amount with respect to this temperature. The value is set to a more appropriate value according to the oxygen reduction rate, whereby the effect can be obtained more reliably.

  Further, in the present embodiment, the final (actually injected) urea injection amount is configured to be smaller than the basic urea injection amount until the switching period elapses after the DeNOx control ends. The urea injection amount is configured to be smaller than other periods when the DeNOx control is not performed.

Therefore, it is possible to prevent an excessive amount of NH _{3 from} being supplied from the urea injector 45 in a state where a large amount of NH ₃ is adsorbed on the SCR catalyst 46 in accordance with the execution of the DeNOx control, and the NH ₃ SCR catalyst 46 NOx can be efficiently purified while preventing slippage of the NOx to the downstream side.

Further, in the present embodiment, since the stored oxygen decreasing rate is calculated using the equation (2), it can be estimated with high accuracy. Then, by accurately estimating the stored oxygen reduction rate, the urea injection amount adjusted according to the stored oxygen reduction rate is set to a more appropriate value, that is, NH ₃ passes through the SCR catalyst 46 on the downstream side. It can be controlled to a value that can efficiently purify NOx while preventing NOx.

In the present embodiment, the target value of the excess air ratio λ of the exhaust gas at the time of the DeNOx control is set to a value larger than 0.9, and control is performed so that this value is realized. Therefore, the variation width of the amount of the reducing agent in the exhaust gas can be suppressed to be small, and the reducing agent can be stably supplied to the NOx catalyst 41, so that the NH ₃ released from the NOx catalyst 41 and the SCR catalyst 46 NH ₃ can be more reliably maintained at an appropriate amount.

  In the present embodiment, the injection amount of the post injection is adjusted so that the excess air ratio λ is realized. Therefore, the excess air ratio λ is controlled more accurately than when the excess air ratio λ is changed by adjusting the amount of air introduced into the cylinder 2 and the exhaust passage 40 by changing the opening of the throttle valve 23 or the like. can do.

(5) Modification In the above embodiment, the case where the excess air ratio λ of the exhaust gas is controlled to 0.96 or 0.98 at the time of DeNOx control has been described. Typical values are not limited to this.

However, as described above, if the excess air ratio λ of the exhaust gas is controlled to a value larger than 0.9 and smaller than 1.0 at the time of DeNOx control, NH ₃ can be efficiently and stably supplied to the SCR catalyst 46. Can be supplied.

  In the above-described embodiment, the case where the excess air ratio λ of the exhaust gas is adjusted by performing the post injection during the DeNOx control has been described. The excess air ratio λ of the gas may be adjusted. However, as described above, when the amount of air is reduced, the acceleration performance is deteriorated, and the control accuracy of the excess air ratio λ may be deteriorated. Therefore, as described above, at the time of DeNOx control, it is preferable to control the excess air ratio λ of the exhaust gas to the target value by changing the injection amount of the post injection.

FIG. 15 illustrates the case where the urea injection amount is reduced in a state where NOx is being purified by both the SCR catalyst 46 and the NOx catalyst 41. However, NOx is purified only by the NOx catalyst 41. In addition, when the purification of NOx by the SCR catalyst 46 is started after the execution of the DeNOx control, the urea injection amount after the start of the NOx purification by the SCR catalyst 46 is calculated from the basic urea injection amount by the NH calculated in step S33. _What is necessary is just to reduce the total amount of ₃ release amounts.

10 fuel injection valve (excess air ratio changing means)
40 exhaust passage 41 NOx catalyst 45 urea injector (reducing agent supply means for SCR)
46 SCR catalyst 200 PCM (control means)
300 DCU (control means)

Claims (4)

An engine body in which a cylinder is formed, an exhaust passage through which exhaust gas discharged from the engine body flows, a NOx catalyst provided in the exhaust passage, an SCR catalyst provided downstream of the NOx catalyst, An exhaust gas purification control device for an engine having:
Means for changing the excess air ratio of the exhaust gas,
SCR reducing agent supply means for supplying a source of NH ₃ or a reducing agent for SCR consisting of NH ₃ between the NOx catalyst and the SCR catalyst in the exhaust passage;
Control for controlling the excess air ratio changing means and the SCR reducing agent supply means, and performing regeneration control for reducing the excess air ratio of exhaust gas to 1 or less by the excess air ratio changing means to regenerate the NOx catalyst. Means,
The control means is configured such that, when the temperature of the NOx catalyst during the regeneration control is high, the amount of the SCR reducing agent supplied from the SCR reducing agent supply unit to the exhaust passage is smaller than when the temperature is low. The SCR reducing agent supply means is controlled such that the lower the temperature of the NOx catalyst at the time of the regeneration control, the larger the rate of change of the supply amount of the SCR reducing agent with respect to the temperature of the NOx catalyst. An exhaust gas purification control device for an engine. The exhaust gas purification control device for an engine according to claim 1 ,
The exhaust gas purification control device for an engine, wherein the control means controls the excess air ratio changing means such that the excess air ratio of the exhaust gas becomes larger than 0.9 during the regeneration control. The exhaust gas purification control device for an engine according to claim 1 or 2 ,
The excess air ratio changing means, during the regeneration control, in addition to the main injection for injecting fuel for obtaining engine torque introduced into the cylinder into the cylinder, a timing on the more retarded side than the main injection. A post-injection for injecting fuel into the cylinder, and changing an injection amount of the post-injection to change an excess air ratio of the exhaust gas. The exhaust gas purification control device for an engine according to any one of claims 1 to 3 ,
The control means may control the SCR reducing agent supply means from the SCR reducing agent supply means until the predetermined period elapses after the regeneration control ends and after the predetermined period elapses after the regeneration control ends. Controlling the SCR reducing agent supply means so that the amount of the SCR reducing agent supplied to the exhaust passage is reduced;
The NOx during the period from the NOx catalyst with the start of the regeneration control with the NH ₃ estimates the NH ₃ release start timing is a timing that is released, until the end timing of the playback control from the NH ₃ release start timing The total amount of NH ₃ released from the catalyst is estimated, and the larger the estimated total amount of released NH ₃ , the larger the estimated total amount of NH ₃ is between the end of the regeneration control and the elapse of the predetermined period. An exhaust gas purification control device for an engine, wherein the SCR reducing agent supply means is controlled so that the amount of the SCR reducing agent supplied from the reducing agent supply means to the exhaust passage is reduced.

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