JP4986973B2 - Exhaust purification device - Google Patents

Description

  The present invention relates to an exhaust purification device for purifying engine exhaust, and more particularly, to an ammonia selective reduction type NOx catalyst that reduces NOx in exhaust using ammonia generated from urea water supplied in exhaust as a reducing agent. The present invention relates to an exhaust purification device.

As an exhaust purification device for purifying NOx (nitrogen oxide), which is one of the pollutants contained in engine exhaust, an ammonia selective reduction type NOx catalyst is disposed in the exhaust passage of the engine, and ammonia as a reducing agent. There is known an exhaust gas purification device that purifies exhaust gas by reducing NOx by supplying NOx to an ammonia selective reduction type NOx catalyst.
In such an exhaust purification device, in order to supply ammonia to the ammonia selective reduction type NOx catalyst, it is common to supply urea water, which is easier to handle than ammonia, into the exhaust gas. Used to inject urea water into the exhaust. The atomized urea water supplied into the exhaust gas from the urea water injector is hydrolyzed by the heat of the exhaust gas, and the resulting ammonia is supplied to the ammonia selective reduction type NOx catalyst. Thus, the NOx catalyst promotes the denitration reaction between the ammonia supplied to the NOx catalyst and the NOx in the exhaust gas, whereby NOx is reduced and the exhaust gas is purified.

  At this time, part of the mist-like urea water injected from the urea water injector is liquefied by colliding with the wall surface in the exhaust passage and adheres to the exhaust passage and the urea water injector. The urea water adhering in this way becomes solid urea crystals or the like when the water vaporizes, and accumulates on the wall surface in the exhaust passage or the urea water injector, and becomes a urea-derived deposit. In addition, since a cold spot is generated due to the latent heat of vaporization when the water of the attached urea water evaporates, the area around the area where the urea water adheres becomes more liable to adhere due to liquefied mist of urea water. The production of things will be promoted.

  If the generation of urea-derived deposits continues in this way, there is a risk of causing problems such as an increase in exhaust flow resistance in the exhaust passage, blockage of the exhaust passage, and malfunction of the urea water injector. In addition, the generation of urea-derived deposits causes a problem that the amount of ammonia that is originally required for NOx reduction is insufficient and the exhaust purification rate decreases. Furthermore, when the exhaust gas temperature becomes high, a large amount of urea-derived deposits are converted into ammonia all at once, and more ammonia than necessary is supplied to the ammonia selective catalytic reduction catalyst. There is a problem that release into the atmosphere, that is, ammonia slip occurs.

Therefore, in order to remove urea-derived deposits accumulated in the exhaust passage, when the urea-derived deposits have accumulated to some extent, the temperature of the exhaust flowing in the exhaust passage is temporarily raised, and the urea-derived deposits are gasified. It is conceivable to convert to ammonia.
An exhaust emission control device that removes urea-derived deposits by such an exhaust temperature rise is proposed in Patent Document 1. In the exhaust gas purification apparatus of Patent Document 1, when the exhaust gas temperature is low, NOx and ammonia in the exhaust gas are reacted to generate ammonium nitrate, and this ammonium nitrate is positively deposited on the ammonia selective reduction catalyst. By doing so, the release of NOx into the exhaust gas is prevented even at a low exhaust temperature at which the reduction effect of the ammonia selective reduction catalyst is not sufficiently obtained.

  If the amount of ammonium nitrate deposited positively on the ammonia selective reduction catalyst in this way becomes excessive, the exhaust flow resistance in the ammonia selective reduction catalyst may increase and the ammonia selective reduction catalyst may be clogged. is there. Therefore, in the exhaust emission control device of Patent Document 1 described above, when the particulate filter disposed in the upstream exhaust passage of the ammonia selective reduction type catalyst is forcibly regenerated, it accumulates on the ammonia selective reduction type catalyst by raising the temperature of the exhaust gas. The ammonium nitrate is burned and decomposed, and removed from the ammonia selective reduction catalyst.

As described above, the exhaust purification device of Patent Document 1 actively deposits ammonium nitrate, but when the amount of deposition becomes excessive, ammonium nitrate is removed by raising the temperature of the exhaust.
JP 2006-320854 A

  However, when removing the urea-derived deposits by increasing the temperature of the exhaust, if the temperature of the exhaust is immediately raised to a high temperature when the removal starts, that is, in a state where a relatively large amount of the urea-derived deposits are deposited, Origin deposits are gasified at once and converted to ammonia. For this reason, there is a problem that ammonia more than necessary is supplied to the ammonia selective reduction catalyst, and ammonia slips and ammonia slip occurs.

  The present invention has been made in view of such a problem, and an object thereof is to satisfactorily suppress the deposition of urea-derived deposits in the exhaust passage and to prevent ammonia slip accompanying the removal of urea-derived deposits. An object of the present invention is to provide an exhaust emission control device that can prevent the above.

  In order to achieve the above object, an exhaust emission control device of the present invention is provided in an exhaust passage of an engine, an ammonia selective reduction type NOx catalyst that selectively reduces NOx in exhaust using ammonia as a reducing agent, and the ammonia selective reduction type. A urea water supply means for supplying urea water into the exhaust gas upstream of the NOx catalyst, and an operation region of the engine, wherein the urea-derived deposit produced by the urea water supplied from the urea water supply means is the exhaust passage. A stacking operation region determining means for determining whether or not the engine is in a deposition operation region, and a timing operation when the operation region of the engine is determined to be in the deposition operation region by the deposition operation region determining unit. The time measuring means to be executed and the time measured by the time measuring means reach the deposition limit judgment time set based on the deposition limit of the urea-derived deposit in the exhaust passage. Control means for performing exhaust gas temperature raising control so that the exhaust temperature in the exhaust passage becomes a target temperature when it is determined that the target temperature is reached. The temperature is raised (claim 1).

  According to the exhaust gas purification apparatus configured as described above, it is determined by the deposition operation region determination unit whether or not the engine operation region is in the deposition operation region, and when it is determined that the engine operation region is in the deposition operation region, Timekeeping operation is executed. The deposition state of urea-derived deposits varies depending on the engine operating range, and the amount of urea-derived deposits increases in one engine operating region, and the amount of urea-derived deposits decreases in another engine operating region. . As the deposition operation region, an engine operation region is set in advance when urea-derived deposits generated by urea water accumulate in the exhaust passage, that is, when the amount of urea-derived volume deposits changes in an increasing direction. Based on this accumulation operation area, the determination process of the accumulation operation area determination means is executed.

  Therefore, when the time counted when the engine operating region is in the deposition operation region reaches the deposition limit judgment time, it can be determined that the amount of urea-derived deposits in the exhaust passage has increased and approached the limit. Exhaust temperature raising control is performed by the means so that the exhaust temperature in the exhaust passage becomes the target temperature, and urea-derived deposits are gradually gasified. Since the target temperature at this time is set so as to increase as the deposition amount of urea-derived deposits decreases, the exhaust temperature rises to a relatively low target temperature at the beginning of the exhaust gas temperature increase control, and thereafter The exhaust gas temperature gradually increases due to the increase in the target temperature accompanying the decrease in the amount of urea-derived deposits.

On the other hand, whether or not the exhaust gas temperature raising control is necessary is determined by a simple process of measuring the time when the engine operation region is in the accumulation operation region and comparing the time measured with the accumulation limit determination time.
Further, in the exhaust purification apparatus, the control means sets a target temperature increase rate that increases as the amount of deposits of urea-derived deposits decreases, and the rate of change of the exhaust temperature when raising the exhaust temperature is The exhaust gas temperature raising control is executed so as to achieve a target temperature raising rate (Claim 2).

  According to the exhaust gas purification apparatus configured as described above, when the exhaust gas temperature raising control is performed to raise the exhaust gas temperature to the target temperature, the rate of increase in the exhaust gas temperature is controlled to the target temperature raising rate. At this time, the target temperature increase rate is set so as to increase as the amount of urea-derived deposits decreases, so at the beginning of the exhaust gas temperature increase control, the exhaust temperature reaches the target temperature at a relatively small rate of change. As the amount of urea-derived deposits decreases, the rate of change in exhaust temperature gradually increases.

In the exhaust gas purification apparatus, the control means stops supplying urea water from the urea water supply means when executing the exhaust gas temperature raising control (claim 3).
According to the exhaust gas purification apparatus configured as described above, urea water is not supplied from the urea water supply means when the exhaust gas temperature raising control is being executed.

The exhaust gas purification apparatus further includes an oxidation catalyst interposed in the exhaust passage upstream of the urea water supply means, and the control means supplies fuel into the exhaust gas flowing into the oxidation catalyst. Thus, the exhaust gas temperature raising control is performed (claim 4).
According to the exhaust gas purification apparatus configured as described above, the temperature of the exhaust gas is increased by oxidizing the fuel component supplied in the exhaust gas in the exhaust gas temperature increase control by the oxidation catalyst, and the exhaust gas whose temperature has increased is supplied with urea water. It flows into the exhaust passage downstream of the means.

Further, in the exhaust purification apparatus, the accumulation operation region determination means is based on at least one of the supply amount of urea water into the exhaust gas, the exhaust amount of the engine, and the exhaust temperature of the engine. Is determined to be in the deposition operation region (Claim 5).
According to the exhaust gas purification apparatus configured as described above, the determination of the deposition operation region determination unit is performed based on at least one of the supply amount of urea water into the exhaust gas, the exhaust amount of the engine, and the exhaust temperature of the engine. . For example, urea-derived deposits are more likely to be generated as the amount of urea water supplied into the exhaust gas is increased, are generated more easily as the exhaust emission amount from the engine is smaller, and are generated more easily as the exhaust gas temperature is lower. The amount of urea-derived deposits on the soil also changes. Therefore, in view of this characteristic, an operation region where there is a large amount of urea water supplied into the exhaust gas, an operation region where the exhaust emission amount is small, and an operation region where the exhaust gas temperature is low are set as the accumulation operation region. When it is in the operating range, it can be assumed that the amount of urea-derived deposits in the exhaust passage changes in an increasing direction, and the urea-related deposits in the exhaust passage are compared by comparing the measured time with the deposition limit judgment time. It is possible to accurately determine that the amount of deposits has increased and approached the limit.

  Further, in the exhaust gas purification apparatus, the control means is based on a deviation between an exhaust temperature in the vicinity of the urea water supply means and an exhaust temperature in the vicinity of the inlet of the ammonia selective reduction type NOx catalyst. The accumulation limit determination time is corrected, and the necessity of the exhaust gas temperature raising control is determined based on the corrected time keeping time or the accumulation limit determination time.

  According to the exhaust gas purification apparatus configured as described above, the accumulation limit determination time is corrected by the deviation between the exhaust gas temperature in the vicinity of the urea water supply means and the exhaust gas temperature in the vicinity of the inlet of the ammonia selective reduction type NOx catalyst. The determination is performed by the control means based on the limit determination time. One of the factors affecting the deposition state of urea-derived deposits in the exhaust passage is a decrease in exhaust temperature when exhaust gas flows in the exhaust passage from the urea water supply means to the inlet of the ammonia selective reduction type NOx catalyst. As the exhaust temperature decreases, the amount of urea-derived deposits in the exhaust passage tends to increase. The influence of the decrease in the exhaust temperature is compensated by correcting the time measurement or the accumulation limit determination time based on the deviation corresponding to the temperature decrease at this time.

According to the exhaust purification device of the present invention, when performing exhaust gas temperature increase control for removing urea-derived deposits accumulated in the exhaust passage, the exhaust temperature is raised to a relatively low target temperature at the beginning of the control, It is possible to prevent the urea-derived deposit from being gasified at a stretch and converting to ammonia, thereby preventing the occurrence of ammonia slip.
If the exhaust temperature is constant, ammonia slip is less likely to occur as the amount of urea-derived deposits decreases, but with exhaust temperature increase control, the amount of urea-derived deposits decreases, Since the exhaust temperature rises as the target temperature rises, it is possible to quickly remove urea-derived deposits from the exhaust passage while preventing the occurrence of ammonia slip.

In addition, because it is determined whether or not the exhaust gas temperature raising control is necessary by a simple process of measuring the time when the engine operating region is in the accumulation operation region and comparing the time measured with the accumulation limit determination time. The entire control can be simplified.
According to the exhaust gas purification apparatus of the second aspect, when the exhaust gas temperature raising control is performed to raise the exhaust gas temperature to the target temperature, the rate of increase of the exhaust gas temperature is controlled to the target temperature raising rate. Since the target temperature increase rate is set so as to increase with a decrease in the amount of urea-derived deposits, the exhaust temperature is initially directed to the target temperature at a relatively small rate of change. Rise. Urea-derived deposits tend to gasify and become ammonia earlier as the rate of change in exhaust temperature during temperature rise is larger, but when exhaust temperature rise control is started in this way, the exhaust temperature changes relatively little. Therefore, even when the exhaust gas temperature is raised toward the target temperature, it is possible to reliably prevent the occurrence of ammonia slip.

  In addition, as described above, if the exhaust temperature is constant, ammonia slip is less likely to occur as the amount of urea-derived deposits decreases. As the target temperature increase rate increases, the rate of change in the exhaust temperature increases, so even when the exhaust temperature is raised toward the target temperature, ammonia slip can be prevented reliably and quickly derived from urea. Deposits can be removed from the exhaust passage.

According to the exhaust gas purification apparatus of claim 3, when the exhaust gas temperature raising control is being executed, urea water is not supplied from the urea water supply means, so that ammonia slip can be prevented more reliably. Can do.
According to the exhaust gas purification apparatus of claim 4, the temperature of the exhaust gas is raised by oxidizing the fuel component supplied in the exhaust gas with the oxidation catalyst, so that the exhaust gas temperature raising control can be executed relatively easily. It becomes possible. That is, for example, when fuel is supplied to the engine by fuel injection into the cylinder, it is possible to supply fuel into the exhaust by additionally injecting fuel at a timing that does not contribute to power generation. Therefore, it is not necessary to provide a new device for supplying fuel into the exhaust gas. In the case of a diesel engine, a particulate filter for collecting particulates in the exhaust is generally installed in the exhaust passage, and fuel is exhausted into the exhaust by forced regeneration of the particulate filter. Since a supply mechanism is already provided, it is not necessary to provide a new device for supplying fuel into the exhaust gas.

  According to the exhaust gas purification apparatus of the fifth aspect of the present invention, the determination is made based on the supply amount of urea water into the exhaust gas, the exhaust gas discharge amount of the engine, and the exhaust gas temperature of the engine that correlate with the generation state of the urea-derived deposit. Thus, it is possible to accurately determine whether or not the engine operation region is in a deposition operation region in which urea-derived deposits are accumulated in the exhaust passage, and accordingly, whether or not the exhaust gas temperature raising control is necessary can be appropriately determined and executed.

  According to the exhaust gas purification apparatus of claim 6, the time measuring means or the accumulation limit judgment time is corrected by the deviation between the exhaust temperature near the urea water supply means and the exhaust temperature near the inlet of the ammonia selective reduction type NOx catalyst. In order to determine whether or not the exhaust gas temperature raising control is necessary based on the corrected timing or accumulation limit determination time, the exhaust gas when the exhaust gas flows in the exhaust passage from the urea water supply means to the inlet of the ammonia selective reduction type NOx catalyst It is possible to compensate for the influence of the temperature decrease and appropriately determine whether or not the exhaust gas temperature raising control is necessary.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 shows an overall configuration diagram of a four-cylinder diesel engine (hereinafter referred to as an engine) to which an exhaust emission control device according to an embodiment of the present invention is applied, and the exhaust emission control device according to the present invention is based on FIG. The structure of will be described.
The engine 1 includes a high-pressure accumulator chamber (hereinafter referred to as a common rail) 2 common to each cylinder, and high-pressure fuel supplied from a fuel injection pump (not shown) and stored in the common rail 2 is supplied to an injector 4 provided in each cylinder. Then, fuel is injected from each injector 4 into each cylinder.

  The intake passage 6 is equipped with a turbocharger 8. The intake air drawn from an air cleaner (not shown) flows into the compressor 8a of the turbocharger 8 from the intake passage 6, and the intake air supercharged by the compressor 8a is intercooler. 10 and the intake control valve 12 are introduced into the intake manifold 14. An intake air amount sensor 16 for detecting an intake air flow rate to the engine 1 is provided upstream of the compressor 8a in the intake passage 6.

On the other hand, an exhaust port (not shown) through which exhaust is discharged from each cylinder of the engine 1 is connected to an exhaust pipe 20 via an exhaust manifold 18. An EGR passage 24 that communicates the exhaust manifold 18 and the intake manifold 14 via the EGR valve 22 is provided between the exhaust manifold 18 and the intake manifold 14.
The exhaust pipe 20 passes through the turbine 8 b of the turbocharger 8 and is connected to an exhaust aftertreatment device 28 via an exhaust throttle valve 26. The rotating shaft of the turbine 8b is connected to the rotating shaft of the compressor 8a so that the turbine 8b receives the exhaust flowing in the exhaust pipe 20 and drives the compressor 8a.

  The exhaust aftertreatment device 28 includes an upstream casing 30 and a downstream casing 34 that is communicated with the downstream side of the upstream casing 30 via a communication path 32. The upstream casing 30, the communication path 32, and the downstream casing 34 are configured. Thus, the exhaust passage of the present invention is configured. A pre-stage oxidation catalyst 36 is accommodated in the upstream casing 30, and a particulate filter (hereinafter referred to as a filter) 38 is accommodated on the downstream side of the pre-stage oxidation catalyst 36. The filter 38 is provided to purify the exhaust of the engine 1 by collecting particulates in the exhaust.

Since the pre-stage oxidation catalyst 36 oxidizes NO (nitrogen monoxide) in the exhaust gas to generate NO ₂ (nitrogen dioxide), the pre-stage oxidation catalyst 36 and the filter 38 are arranged in this manner, so that the filter 38 captures them. The collected particulates react with NO ₂ supplied from the pre-stage oxidation catalyst 36 to be oxidized, and the filter 38 is continuously regenerated.
On the other hand, in the downstream casing 34, an ammonia selective reduction type NOx catalyst (hereinafter referred to as an SCR catalyst) 40 that purifies exhaust by selectively reducing NOx (nitrogen oxide) in exhaust using ammonia as a reducing agent is housed. At the same time, a downstream oxidation catalyst 42 for removing ammonia flowing out from the SCR catalyst 40 is accommodated downstream of the SCR catalyst 40.

The post-stage oxidation catalyst 42 also has a function of oxidizing CO (carbon monoxide) generated when particulates are incinerated by forced regeneration of the filter 38 and discharging it into the atmosphere as CO ₂ (carbon dioxide). Yes.
Further, the communication passage 32 is provided with a urea water injector (urea water supply means) 44 for injecting and supplying urea water into the exhaust gas in the communication passage 32, and is not shown from the urea water tank 46 storing the urea water. The urea water is supplied to the urea water injector 44 via the urea water supply pump, and the urea water is injected into the exhaust gas in the communication passage 32 from the urea water injector 44 by opening and closing the urea water injector 44. Yes.

The atomized urea water injected from the urea water injector 44 is hydrolyzed by the heat of the exhaust to become ammonia, and is supplied to the SCR catalyst 40. The SCR catalyst 40 reduces NOx to harmless N ₂ by promoting a denitration reaction between the supplied ammonia and NOx in the exhaust. At this time, if ammonia flows out of the SCR catalyst 40 without reacting with NOx, this ammonia is removed by the post-stage oxidation catalyst 42.

Further, an exhaust gas temperature sensor 48 for detecting the temperature of the exhaust gas flowing in the exhaust gas aftertreatment device 28 is provided downstream of the upstream oxidation catalyst 36 in the upstream casing 30.
The ECU (control means) 50 is a control device for performing comprehensive control including operation control of the engine 1, and includes a CPU, a memory, a timer counter, and the like, and calculates various control amounts. Various devices are controlled based on the control amount.

  On the input side of the ECU 50, in order to collect information necessary for various controls, in addition to the intake air amount sensor 16 and the exhaust gas temperature sensor 48 described above, a rotational speed sensor 52 for detecting the rotational speed of the engine 1, and an accelerator pedal (not shown) Various sensors such as an accelerator opening sensor 54 for detecting the depression amount of the vehicle are connected. Further, various devices such as the injector 4, the intake control valve 12, the EGR valve 22, the exhaust throttle valve 26, and the urea water injector 44 that are controlled based on the calculated control amount are connected to the output side of the ECU 50. Has been.

  The ECU 50 also performs calculation of the fuel supply amount to each cylinder of the engine 1 and control of fuel supply from the injector 4 based on the calculated fuel supply amount. The fuel supply amount (main injection amount) necessary for the operation of the engine 1 is preliminarily determined based on the engine speed detected by the engine speed sensor 52 and the accelerator pedal depression amount detected by the accelerator opening sensor 54. It is determined by reading from the stored map. The amount of fuel supplied to each cylinder is adjusted by the valve opening time of the injector 4, and each injector 4 is driven to open in a driving time corresponding to the determined fuel amount, and main injection is performed in each cylinder. As a result, an amount of fuel necessary for the operation of the engine 1 is supplied.

  In addition to such fuel supply control to each cylinder, the ECU 50 also performs forced regeneration of the filter 38 and urea water supply control for supplying ammonia to the SCR catalyst 40. The forced regeneration of the filter 38 is already widely known and will not be described in detail here, but based on the detection value of the exhaust temperature sensor 48, fuel is supplied from the injector 4 to each cylinder separately from the main injection. By injecting, fuel is supplied into the exhaust gas, and the filter 36 is forcibly regenerated by raising the temperature of the exhaust gas by the oxidation reaction of the fuel in the exhaust gas at the pre-stage oxidation catalyst 36. Below, urea water supply control is demonstrated in detail.

  The ECU 50 determines the unit time of the engine 1 based on the main injection amount from the injector 4, the rotational speed of the engine 1 detected by the rotational speed sensor 52, the intake air flow rate to the engine 1 detected by the intake air amount sensor 16, and the like. Per unit exhaust emission amount and NOx emission amount are obtained, and a target supply amount of urea water is obtained from the amount of ammonia necessary for the selective reduction of NOx by the SCR catalyst 40 with respect to this NOx emission amount. Then, by controlling the urea water injector 44 based on this target supply amount, urea water is supplied from the urea water injector 44 into the exhaust gas upstream of the SCR catalyst 40.

As described above, the mist-like urea water injected from the urea water injector 44 is hydrolyzed to ammonia by the heat of the exhaust gas, and this ammonia is supplied to the SCR catalyst 40. The SCR catalyst 40 reduces NOx to harmless N ₂ by promoting a denitration reaction between the supplied ammonia and NOx in the exhaust.
In order to properly supply the urea water by the urea water injector 44, the ECU 50 performs urea water supply control at a predetermined control period according to the flowchart shown in FIG. The urea water supply control is started when the engine 1 is started, and is ended when the engine 1 is stopped.

  When the control is started, the ECU 50 determines in step S101 whether or not a condition for enabling the supply of the urea water is established and the state in which the supply of ammonia to the SCR catalyst 40 is required. Specifically, based on the exhaust temperature detected by the exhaust temperature sensor 48, it is determined whether or not the SCR catalyst 40 is activated. When it is determined that the SCR catalyst 40 is activated, urea water can be supplied. It is determined that the condition is satisfied and the state where the supply of ammonia to the SCR catalyst 40 is required has been reached.

  When it is determined in step S101 that the condition for enabling urea water supply is satisfied, the ECU 50 proceeds to step S102, while in step S101, the ECU 50 determines that the condition for enabling urea water supply is not satisfied. Ends the current control cycle, and determines whether or not the condition for enabling the urea water supply is satisfied again in step S101 of the next control cycle. Accordingly, the ECU 50 advances the process to step S102 only when it is determined in step S101 that the condition for enabling urea water supply is satisfied. In the following description, it is assumed that the condition for enabling the supply of urea water is satisfied, and the process proceeds from step S101 to step S102.

  In step S102, the ECU 50 determines whether or not the value of the flag F is 1. This flag F is based on the timing and determination described later, whether or not the amount of urea-derived deposits in the exhaust aftertreatment device 28 has reached its limit, in other words, the exhaust gas temperature rise for removing urea-derived deposits. This indicates whether or not control is necessary, and a value of 1 indicates that the amount of urea-derived deposits has reached its limit.

  The initial value of the flag F is 0, and when the process proceeds to step S103 after determining that the value of the flag F is not 1 in step S102, the ECU 50 exhausts urea-derived deposits in advance in the current engine operating region. It is determined whether or not the engine is in the accumulation operation region set as the engine operation region for accumulation in the post-processing device 28. The deposition operation region is set based on the deposition characteristics of the urea-derived deposit shown in FIG. Urea-derived deposits are more likely to be generated as the amount of urea water supplied into the exhaust gas is larger, more easily generated as the exhaust emission amount from the engine 1 is smaller, and more likely to be generated as the exhaust temperature is lower. The generation state of urea-derived deposits correlates with the amount of urea-derived deposits deposited in the exhaust aftertreatment device 28 per unit time, and as shown in FIG. The ratio of the urea water supply amount per unit time from the injector 44, that is, the urea water deposit amount increases as the urea water supply amount / exhaust exhaust amount increases, and the urea origin deposit increases as the exhaust temperature decreases. To increase. The accumulation operation region is set as all regions except a partial region where the amount of deposits of urea-derived deposits is 0 or extremely small (region outside the upper left hatching in FIG. 3), and the operation region of the engine 1 is this region. When it is inside, it can be considered that the deposition amount of the urea-derived deposit in the exhaust aftertreatment device 28 is changing in the increasing direction.

  In step S103, it is determined whether or not the engine 1 is operating in the deposition operation region set in this way (deposition operation region determination means). When it is determined that the engine 1 is in the deposition operation region, the process proceeds to step S104. After the deposition determination timer A is started in step S105, the process proceeds to step S105. The initial value of the accumulation determination timer A is 0. The accumulation determination timer A performs a timing operation over the control cycle of the ECU 50 this time. As a result, the accumulation determination timer A performs a timing operation only when the operation region of the engine 1 is in the accumulation operation region, and corresponds to the accumulation operation region. When it stops, the timing operation is stopped, and the value of the accumulation determination timer A represents the duration when the engine 1 is operated in the accumulation operation area (time measuring means).

  Further, when it is determined that the operation region of the engine 1 does not correspond to the region of the accumulation operation region, the process directly proceeds from step S103 to step S105. In step S105, it is determined whether or not the value of the deposition determination timer A has reached the deposition limit determination time A0. The deposition limit determination time A0 is a threshold set based on the deposition limit of the urea-derived deposit in the exhaust aftertreatment device 28. Specifically, as the operation of the engine 1 in the accumulation operation region is continued, the amount of urea-derived deposits in the exhaust aftertreatment device 28 gradually increases, and at any timing, the urea-derived deposits are deposited. The function in the exhaust aftertreatment device 28 is impaired due to problems such as an increase in exhaust flow resistance caused by deposition. The deposition limit determination time A0 is set as a value slightly shorter than the elapsed time when such a defect becomes apparent.

  When it is determined in step S105 that the value of the accumulation determination timer A has not reached the accumulation limit determination time A0, the process proceeds to step S106, after urea water injection is permitted, the routine is once terminated. That is, in this case, even if urea-derived deposits are further generated by the supply of urea water, there is no problem caused by deposition of urea-derived deposits such as an increase in exhaust flow resistance. Allowed. In this way, by permitting urea water injection in step S106, the ECU 50 controls the urea water injector 44 to perform injection supply of urea water into the exhaust.

  Each time the engine 1 is operated in the accumulation operation region, the value of the accumulation determination timer A is sequentially increased in step S104, and at any point in time, the ECU 50 sets the value of the accumulation determination timer A to the accumulation limit determination time A0 in step S105. When it is determined that it has reached, the process proceeds to step S107, and the injection of urea water from the urea injector 44 is stopped. That is, the amount of urea-derived deposits is approaching its limit, and if the supply of urea water is continued further, the amount of urea-derived deposits becomes excessive, causing problems such as an increase in exhaust flow resistance. As a result, the supply of urea water is stopped.

The ECU 50 sets the value of the flag F to 1 in the next step S108, and then proceeds to step S109. In the subsequent step S109, the ECU 50 resets the accumulation determination timer A in preparation for the next time measurement process, and then ends the routine.
On the other hand, when the value of the flag F is set to 1, in step S102 of the next control cycle, the ECU 50 proceeds to step S110, starts the completion determination timer B, and then proceeds to step S111. Note that the initial value of the completion determination timer B is 0. As with the accumulation determination timer A, the completion determination timer B performs a time measuring operation over the current control cycle of the ECU 50, and as a result, the value of the completion determination timer B has elapsed after the value of the flag F becomes 1. It represents time. In step S111, it is determined whether or not the value of the completion determination timer B has reached the completion determination time B0. The completion determination time B0 is used to determine the completion of the exhaust gas temperature raising control for removing the urea-derived deposit described below, and the exhaust rise from the state in which the urea-derived deposit has been deposited near the limit in advance in an experiment. It is set as a value obtained by measuring the required time until temperature control is performed and erasing all accumulated urea-derived deposits, and adding a slight margin to the measured required time.

  When it is determined that the value of the completion determination timer B has not reached the completion determination time B0, the process proceeds to step S112, and the urea water injection from the urea water injector 44 is stopped by stopping the urea water injection. Canceling continues. When it is determined that the value of the completion determination timer B has gradually increased and reached the completion determination time B0 in step S111, the ECU 50 determines that all the urea-derived deposits in the exhaust aftertreatment device 28 have disappeared, and the process is stepped. After proceeding to S113, the urea water injection from the urea injector 44 is permitted. Next, after the value of the flag F is set to 0 in step S114, the completion determination timer B is reset in preparation for the next timing process in step S115. End the routine. As described above, when the value of the completion determination timer B reaches the completion determination time B0, the urea water injection from the urea injector 44 that has been continuously stopped is restarted.

In parallel with such urea water supply control, the ECU 50 performs exhaust gas temperature raising control for appropriately removing urea-derived deposits according to the deposition state of urea-derived deposits in the exhaust aftertreatment device 28 as shown in FIG. It performs according to the flowchart shown in FIG. The exhaust gas temperature raising control is also started when the engine 1 is started, and is ended when the engine 1 is stopped.
When the control is started, in step S201, the ECU 50 is in an operating state in which the engine 1 can raise the exhaust temperature based on the main injection amount from the injector 4, the rotational speed of the engine 1 detected by the rotational speed sensor 52, and the like. It is determined whether or not. When it is determined that the engine 1 is not in an operation state in which the exhaust gas temperature can be raised, the ECU 50 advances the process to step S202, stops the exhaust gas temperature raising, and ends the control cycle. Also in the next control cycle, if it is determined in step S201 that the engine 1 is not in an operation state in which the exhaust gas temperature can be increased, the ECU 50 proceeds to step S202 and stops the exhaust gas temperature increase. The exhaust gas temperature raising by the exhaust gas temperature raising control is not performed unless the operation state becomes possible. Hereinafter, the description will be made assuming that the engine 1 is in an operation state in which the exhaust gas temperature can be raised.

  If it is determined in step S201 that the engine 1 is in an operation state in which the exhaust gas temperature can be raised, the ECU 50 advances the process to step S203 and determines whether or not the value of the flag F is 1. The value of the flag F is set to 1 when the value of the accumulation determination timer A reaches the accumulation limit determination time A0 in the urea water supply control described above, that is, when the accumulation amount of urea-derived deposits is approaching the limit. When the value of the accumulation determination timer A has not reached the accumulation limit determination time A0, it is set to 0. Accordingly, when the value of the accumulation determination timer A has not reached the accumulation limit determination time A0 and the value of the flag F is 0, the ECU 50 advances the process to step S202, stops the exhaust gas temperature increase, and sets its control cycle. finish. Also in the next control cycle, when the process proceeds from step S201 to step S203 and it is determined that the value of the flag F is not 1, the ECU 50 proceeds to the process to step S202 and stops the exhaust gas temperature raising. As long as the value does not become 1, that is, unless the value of the accumulation determination timer A reaches the accumulation limit determination time A0, the exhaust gas temperature increase by the exhaust gas temperature increase control is not performed. At this time, as described above, in the urea water supply control, as long as the urea water supply condition is satisfied, urea water is supplied into the exhaust gas from the urea water injector 44, and ammonia generated from the urea water is used as a reducing agent. Reduction of NOx by the catalyst 40 is performed.

  On the other hand, when the urea-derived deposit accumulates due to the supply of urea water into the exhaust gas, and it is determined in the urea water supply control that the value of the deposition determination timer A has reached the deposition limit determination time A0, the value of the flag F is 1. Therefore, the ECU 50 advances the process from step S203 to step S204, and raises the exhaust gas temperature by the process after step S204. At this time, since the value of the flag F is 1, the urea water injection from the urea injector 44 is stopped in the urea supply control, and the exhaust gas temperature raising is executed by the exhaust gas temperature raising control without supplying the urea water. .

  First, in step S204, the ECU 50 reads and sets a target temperature for raising the temperature of the exhaust gas from a preset target temperature map. The amount of ammonia produced by the gasification of the urea-derived deposit due to the temperature rise of the exhaust gas varies depending on the exhaust temperature when the amount of urea-derived deposit is the same, and the higher the exhaust temperature, the higher the ammonia per unit time. The production amount of increases. Further, in a state where the exhaust gas temperature is kept constant, the amount of ammonia generated from the urea-derived deposit per unit time increases as the amount of urea-derived deposit increases. Accordingly, the supply amount of ammonia per unit time that can be supplied to the SCR catalyst 40 without causing ammonia slip in various operating states of the engine 1 is obtained in advance through experiments or the like. Then, the exhaust temperature capable of maintaining the ammonia production amount per unit time from the urea-derived deposit corresponding to the supply amount of ammonia is obtained using the deposition amount of the urea-derived deposit as a parameter, and this exhaust temperature is the target. The temperature is stored in the target temperature map.

  Therefore, the relationship between the deposition amount of the urea-derived deposit and the target temperature in the target temperature map is such that the target temperature increases as the deposition amount of the urea-derived deposit decreases, as shown in FIG. It has become. In the example of the target temperature map shown in FIG. 5, when the amount of urea-derived deposits reaches the upper limit value Qr when the amount reaches the limit, the target temperature is set to T1, and urea-derived deposits are deposited. If not, the target temperature is set to T2, which is higher than T1, and as the amount of urea-derived deposits decreases from the upper limit value Qr, the target temperature increases from T1 to T2. It is set to rise. Note that the method of changing the target temperature according to the change in the deposition amount of the urea-derived deposit is not limited to the example in FIG. 5, and changes in various forms according to the characteristics of the SCR catalyst 40 and the engine 1. Yes.

In step S204, the ECU 50 reads and sets the target temperature corresponding to the deposition amount of the urea-derived deposit at that time from the target temperature map. The deposition amount of the urea-derived deposit used at this time is, for example, the following procedure: It can be calculated by
First, in the urea water supply control, until the value of the accumulation determination timer A reaches the accumulation limit determination time A0 and the flag F becomes 1, the exhaust gas temperature rise is continued in the processing after step S204, and the value of the completion determination timer B is When the completion determination time B0 is reached and the flag F becomes 0, the temperature rise of the exhaust gas is stopped in step S202, and at this time, it can be considered that all the urea-derived deposits in the exhaust aftertreatment device 28 have disappeared. In response to the flag F becoming 0, the urea water supply control restarts the injection of urea water from the urea injector 44 in the process of step S113. Therefore, from this point of time, urea-derived deposits in the exhaust aftertreatment device 28 are removed. The amount of deposition increases gradually. As shown in FIG. 3, the amount of urea-derived deposits deposited in the exhaust aftertreatment device 28 increases as the urea water supply amount / exhaust discharge amount increases, and increases as the exhaust temperature decreases. Therefore, the characteristics shown in FIG. 3 are used as a map for calculating the deposition amount per unit time of the urea-derived deposit, and the deposition when the flag F becomes 0 and all the urea-derived deposit disappears due to the temperature rise of the exhaust gas. If the amount is regarded as 0 and the amount of urea-derived deposits deposited per unit time is calculated for each control cycle of the ECU 50 and added sequentially, the urea-derived deposits increasing due to the injection of urea water from the urea injector 44 Can be obtained.

  On the other hand, in the exhaust gas temperature raising performed by stopping the injection of urea water from the urea injector 44, the urea-derived deposits accumulated in the exhaust aftertreatment device 28 are gradually converted into ammonia and disappear. The urea-derived deposits disappear in accordance with the characteristics shown in FIG. That is, urea-derived deposits are more easily converted to ammonia and disappeared as the exhaust emission amount from the engine 1 is larger, and urea is more easily converted and disappeared as the exhaust temperature is higher. The amount of disappearance of the deposits increases, and the amount of disappearance increases as the exhaust temperature increases. Therefore, the characteristic shown in FIG. 6 is used as a map for calculating the disappearance amount of urea-derived deposits per unit time, and the amount of urea-derived deposits gradually increases to reach the limit and flag F becomes 1. When the exhaust gas temperature rise is started and the disappearance amount per unit time of the urea-derived deposit is calculated and sequentially subtracted every control cycle of the ECU 50, the urea-derived deposit that is decreasing due to the exhaust gas temperature increase Can be obtained.

In order to calculate the target temperature from the target temperature map of FIG. 5, the deposition amount of the urea-derived volume gradually decreasing with the exhaust gas temperature increase is applied, and the deposition amount of the urea-derived deposit is 0 from the upper limit value Qr. The target temperature is set to increase from T1 to T2 in response to the decrease.
When the target temperature is thus set in step S204, the ECU 50 proceeds to the next step S205, and sets the target temperature increase rate as a target value for the rate of change of the exhaust temperature at the time of exhaust gas temperature increase.

  The amount of ammonia produced from urea-derived deposits per unit time due to the temperature rise of the exhaust gas also varies depending on the exhaust gas temperature rise rate when the exhaust temperature is changed with the same amount of urea-derived deposits deposited. As the rate of increase increases, it increases. Therefore, based on the supply amount per unit time of ammonia that can be supplied to the SCR catalyst 40 without causing the ammonia slip that has been obtained in advance by experiments or the like as described above, the unit time from the urea-derived deposit corresponding to this supply amount The change rate of the exhaust temperature capable of maintaining the pertinent ammonia generation amount is obtained using the deposition amount of urea-derived deposit as a parameter, and this increase rate is stored in the target temperature increase rate map as the target temperature increase rate. Therefore, the relationship between the deposition amount of the urea-derived deposit and the target temperature increase rate in the target temperature increase rate map is, as shown in FIG. 7, an example of the relationship between the target temperature increase as the urea-derived deposit accumulation amount decreases. The speed is increasing.

  In the example of the target temperature increase rate map shown in FIG. 7, when the deposition amount of the urea-derived deposit is the upper limit value Qr, the target temperature increase rate is set to ΔT1, and the urea-derived deposit is not deposited. The target temperature increase rate is set to ΔT2 which is larger than ΔT1, and as the deposition amount of urea-derived deposits decreases from the upper limit value Qr, the target temperature increase rate increases from ΔT1 to ΔT2. It is set to increase. Note that the method of changing the target temperature increase rate is not limited to the example of FIG. 7, and can be changed in various forms according to the characteristics of the SCR catalyst 40 and the engine 1.

In step S205, the ECU 50 reads out and sets the target temperature increase rate corresponding to the deposition amount of the urea-derived deposit at that time from the target temperature increase rate map. This is the same as that used in step S204.
When the target temperature increase rate is thus set in step S205, the ECU 50 advances the process to step S206, and the exhaust temperature detected by the exhaust temperature sensor 48 is detected using the target temperature and the target temperature increase rate set in steps S204 and S205, respectively. The temperature of the exhaust is increased so as to increase to the target temperature at the target temperature increase rate, and the control cycle ends.

  Also in the next control cycle, if the value of the flag F is 1 when the engine 1 is in an operating state in which the temperature can be raised, the ECU 50 advances the process from step S201 to step S203. Then, as described above, in step S204, a target temperature corresponding to the deposition amount of the urea-derived deposit at that time is set, and in step S205, the target temperature increase corresponding to the deposition amount of the urea-derived deposit at that time is set. The speed is set, and the temperature of the exhaust is raised in step S206.

Therefore, as long as the engine 1 is in an operation state in which the temperature can be raised and the value of the flag F is 1, the target temperature and the target temperature raising rate corresponding to the amount of urea-derived deposits at that time are maintained in each control cycle. It is set and the temperature of the exhaust is raised.
Here, the temperature increase of the exhaust gas in step S206 is performed by injecting additional fuel from the injector 4 to each cylinder in the exhaust stroke separately from the main injection. When the additional fuel is injected into each cylinder at such an injection timing, the additional fuel reaches the pre-stage oxidation catalyst 36 without burning in the cylinder or the exhaust manifold 18, and the HC of the fuel is oxidized by the pre-stage oxidation catalyst 36. By causing the reaction, the temperature of the exhaust gas flowing out from the pre-stage oxidation catalyst 36 increases.

  In step S206, if the exhaust temperature detected by the exhaust temperature sensor 48 has not reached the target temperature set in step S204, the ECU 50 sets the change rate of the exhaust temperature detected by the exhaust temperature sensor 48 in step S205. The amount of additional fuel supplied is adjusted so that the target temperature increase rate is achieved. Since the exhaust gas temperature does not reach the target temperature at the beginning of the exhaust gas temperature increase in the exhaust gas temperature increase control, the exhaust gas temperature increase rate increases until the exhaust gas temperature reaches the target temperature due to the exhaust gas temperature increase. Additional fuel will be supplied so that it may become a temperature rate.

On the other hand, when the exhaust temperature detected by the exhaust temperature sensor 48 reaches the target temperature set in step S204, the ECU 50 causes the exhaust temperature detected by the exhaust temperature sensor 48 to become the target temperature set in step S204. In addition, the supply amount of the additional fuel is adjusted.
By supplying additional fuel in each control cycle in this way, the exhaust temperature rises to a temperature corresponding to the target temperature at a rate of change corresponding to the target temperature increase rate, and is substantially maintained at the target temperature.

  In this way, the temperature of the exhaust gas is raised by the exhaust gas temperature raising control, so that the urea-derived deposits accumulated in the exhaust aftertreatment device 28 are gradually gasified and converted into ammonia. At this time, the target temperature and the target temperature increase rate used in the exhaust gas temperature increase control are the exhaust gas temperature and the exhaust gas that realize the ammonia supply amount per unit time that can be supplied to the SCR catalyst 40 without causing ammonia slip as described above. The temperature change rate is set, and the ECU 50 increases the target temperature and increases the target temperature increase rate as the amount of urea-derived deposits decreases. Therefore, in a state where a relatively large amount of the urea-derived deposits at the beginning of the exhaust gas temperature increase is deposited, the target temperature is set to be relatively low and the exhaust temperature rises relatively slowly toward the target temperature. To go. As a result, the generated ammonia is supplied to the SCR catalyst 40 both during the exhaust temperature rising toward the target temperature and after reaching the target temperature. Is used for selective reduction of NOx to remove urea-derived deposits while preventing the occurrence of ammonia slip.

  In addition, as described above, the amount of ammonia produced per unit time from urea-derived deposits decreases as the amount of urea-derived deposits decreases in a state where the exhaust temperature is kept constant, and urea-derived deposits. When the amount of deposits is the same, the higher the exhaust gas temperature or the higher the exhaust gas temperature increase rate, the higher the exhaust gas temperature. For this reason, when the exhaust temperature is constant, as the amount of urea-derived deposits decreases, ammonia slip hardly occurs due to a decrease in the amount of ammonia produced per unit time from urea-derived deposits. However, after the exhaust temperature reaches the target temperature, the exhaust temperature rises due to the increase in the target temperature with the decrease in the deposition amount of the urea-derived deposit as described above. The decrease in the amount of ammonia produced from the urea-derived deposit is offset by an increase in the exhaust temperature, and the urea-derived deposit can be quickly removed from the exhaust passage while preventing the occurrence of ammonia slip.

  On the other hand, while the exhaust gas temperature is rising toward the target temperature by the exhaust gas temperature increase control, the rate of change in the exhaust gas temperature is increased by the increase in the target temperature increase rate as the amount of urea-derived deposits decreases as described above. Therefore, the decrease in the amount of ammonia generated from the urea-derived deposit accompanying the decrease in the amount of deposit is offset by the increase in the rate of change in the exhaust temperature, and the urea-derived deposit is promptly prevented while preventing ammonia slip. Can be removed from the exhaust passage.

  Further, as described above, while the value of the flag F is 1, the supply of urea water is stopped in the urea water supply control. Therefore, the exhaust gas temperature increase control is performed to remove urea-derived products by the exhaust gas temperature increase control. During the operation, the supply of urea water is stopped. As a result, when removing urea-derived deposits as quickly as possible while preventing ammonia slip in the exhaust gas temperature raising control, a situation in which ammonia slip occurs due to the supply of excess urea water. It can be surely prevented.

In the present embodiment, the temperature of the exhaust gas is raised by using the pre-stage oxidation catalyst 36 provided for forced regeneration of the filter 36, the exhaust temperature sensor 48, and the injector 4 provided for injecting fuel into each cylinder. Since the exhaust gas temperature is raised in the control, it is not necessary to add a new device for removing the urea-derived deposit, and the urea-derived deposit can be easily removed.
Further, during the injection of urea water from the urea injector 44 by the urea water supply control, when the operation region of the engine 1 is in the accumulation operation region, that is, into the exhaust aftertreatment device 28 by the processing in steps S103 to S105. A simple process in which the duration time when the engine 1 is operated in an operation region in which the amount of deposits of urea-derived deposits increases is counted by the deposition determination timer A, and the measured time is compared with the deposition limit determination time A0. Therefore, the necessity of the exhaust gas temperature raising control is determined, so that the entire control executed by the ECU 50 can be simplified.

  In view of the fact that the amount of urea-derived deposits in the exhaust aftertreatment device 28 correlates with the urea water supplied into the exhaust, the amount of exhaust discharged from the engine 1, and the exhaust temperature, as shown in FIG. Since the determination process of step S103 related to the deposition operation region is executed based on the urea water supply amount / exhaust discharge amount and the exhaust temperature according to the characteristic diagram, it is determined whether or not the current operation region of the engine 1 is in the deposition operation region. Therefore, it is possible to accurately determine, and thus to appropriately determine whether or not the exhaust gas temperature raising control is necessary.

  By the way, in this embodiment, as shown in FIG. 1, since the SCR catalyst 40 is arranged immediately downstream of the urea water injector 44, a decrease in the exhaust temperature from the urea water injector 44 to the SCR catalyst 40 is so small that it can be ignored. However, for example, the communication path 32 may have to be extended due to the routing of the exhaust pipe 20 at the lower part of the vehicle body. In this case, the distance from the urea water injector 44 to the SCR catalyst 40 becomes long. For this reason, the exhaust gas temperature significantly decreases between the urea water injector 44 and the SCR catalyst 40. Further, when the number of bends of the communication passage 32 is increased, the exhaust temperature is significantly reduced. In step S103, when the determination relating to the deposition operation region is executed according to the characteristic diagram of FIG. 3, the detected value of the exhaust temperature sensor 48 is used as the exhaust temperature. The detection value of the exhaust temperature sensor 48 is close to the exhaust temperature near the urea water injector 44, but is far from the exhaust temperature near the downstream SCR catalyst inlet. This factor leads to a determination error related to the deposition operation region in step S103, which is executed based on the detection value of the exhaust temperature sensor 48, and thus a determination error regarding whether or not the exhaust gas temperature raising control is necessary in step S105.

  Therefore, for example, exhaust temperature sensors are provided in the vicinity of the urea water injector 44 and in the vicinity of the inlet of the SCR catalyst 40, respectively, and the value of the accumulation determination timer A or the accumulation is determined based on the deviation of the exhaust temperature detected by both exhaust temperature sensors. The limit determination time A0 may be corrected, and the determination in step S105 may be executed based on the corrected value of the deposition determination timer A or the deposition limit determination time A0. Specifically, the larger the temperature deviation and the more the exhaust temperature decreases, the more the amount of urea-derived deposits in the exhaust aftertreatment device 28, more specifically, from the vicinity of the urea water injector 44 to the inlet of the SCR catalyst 40. As a result, the amount of urea-derived deposits in the region up to this point tends to increase, and the need to perform exhaust gas temperature raising control earlier increases. Therefore, as the temperature deviation is larger, the exhaust gas temperature increase control is started at an earlier time point by correcting the value of the accumulation determination timer A to the increase side or correcting the accumulation limit determination time A0 to the decrease side. It may be. If comprised in this way, the influence by the fall of the exhaust gas temperature from the urea water injector 44 to the inlet_port | entrance of the SCR catalyst 40 can be compensated, and the necessity of exhaust gas temperature raising control can be determined more appropriately.

Alternatively, as the exhaust deviation becomes larger, the accumulation operation region in the characteristic diagram of FIG. 3 may be corrected to be enlarged in the upper left direction. In this case as well, it is determined that the operation region of the engine 1 is in the accumulation operation region. Since the frequency of being increased increases, the same effect as described above can be obtained.
Although the description of the exhaust emission control device according to one embodiment of the present invention is finished above, the present invention is not limited to the above embodiment.

  For example, in the above embodiment, the supply of urea water is stopped when the urea-derived deposit is removed by the temperature rise of the exhaust. However, ammonia generation from the urea-derived deposit is based on the exhaust temperature, the exhaust flow rate, etc. If the amount is estimated and the amount of ammonia that can be supplied to the SCR catalyst 40 is not reached, urea water corresponding to the shortage of ammonia may be supplied from the urea water injector 44.

In the above-described embodiment, the temperature of the exhaust gas in the exhaust gas temperature increase control is increased by supplying additional fuel from the injector 4 and the oxidation reaction of the fuel by the pre-stage oxidation catalyst 36. The method is not limited to this, and various known methods can be used.
Further, in the above embodiment, the target temperature and the target temperature increase rate are continuously changed according to the change in the deposition amount of the urea-derived deposit. In each section, it may be changed stepwise while maintaining at least one of the target temperature and the target temperature rising rate constant.

In the above embodiment, the target temperature and the target temperature increase rate are set, but only the target temperature is used, and in addition to the control of the exhaust temperature when the exhaust gas temperature is raised, the target temperature change rate is adjusted. Thus, the change rate of the exhaust temperature may be controlled.
Further, in the above embodiment, the exhaust post-treatment device 28 is provided with the front-stage oxidation catalyst 36, the filter 38, the SCR catalyst 40, and the rear-stage oxidation catalyst 42, but the parts other than the SCR catalyst 40 may be changed as necessary. Is possible.

1 is an overall configuration diagram of an engine to which an exhaust emission control device according to an embodiment of the present invention is applied. It is a flowchart of the urea water supply control which ECU performs. It is a characteristic view which shows the deposition characteristic and deposition operation area | region of a urea origin deposit. It is a flowchart of exhaust gas temperature raising control which ECU performs. It is a graph which shows the relationship between the deposition amount and target temperature in a target temperature map. It is a characteristic view which shows the extinction characteristic of a urea origin deposit. It is a graph which shows the relationship between the deposition amount in a target temperature increase rate map, and a target temperature increase rate.

Explanation of symbols

1 Engine 30 Upstream casing (exhaust passage)
32 Communication passage (exhaust passage)
34 Downstream casing (exhaust passage)
40 Ammonia selective reduction type NOx catalyst 44 Urea water injector (urea water supply means)
50 ECU (deposition operation region determination means, timing means, control means)

Claims (6)

An ammonia selective reduction type NOx catalyst that is disposed in the exhaust passage of the engine and selectively reduces NOx in the exhaust gas using ammonia as a reducing agent;
Urea water supply means for supplying urea water into the exhaust gas upstream of the ammonia selective reduction type NOx catalyst;
Deposition operation region determination means for determining whether or not the engine operation region is in a deposition operation region in which urea-derived deposits generated by urea water supplied from the urea water supply means are deposited in the exhaust passage. When,
Clocking means for performing a timekeeping operation when it is determined by the accumulation operation area determination means that the engine operation area is in the accumulation operation area;
When it is determined that the time counted by the time measuring means has reached the deposition limit determination time set based on the deposition limit of urea-derived deposits in the exhaust passage, the exhaust temperature in the exhaust passage is set to the target temperature. And a control means for performing exhaust gas temperature raising control so that
The exhaust gas purification apparatus, wherein the control means increases the target temperature in accordance with a decrease in the accumulation amount of the urea-derived deposit.   The control means sets a target temperature increase rate that increases as the amount of urea-derived deposits decreases, so that the rate of change of the exhaust temperature when increasing the exhaust temperature becomes the target temperature increase rate. The exhaust gas purification apparatus according to claim 1, wherein the exhaust gas temperature raising control is executed.   2. The exhaust emission control device according to claim 1, wherein the control unit stops supply of urea water from the urea water supply unit when executing the exhaust gas temperature raising control. An oxidation catalyst interposed in the exhaust passage on the upstream side of the urea water supply means;
The exhaust emission control device according to claim 1, wherein the control means performs the exhaust gas temperature raising control by supplying fuel into the exhaust gas flowing into the oxidation catalyst.   Whether the engine operation area is in the accumulation operation area based on at least one of the supply amount of urea water into the exhaust, the exhaust emission amount of the engine, and the exhaust temperature of the engine. The exhaust emission control device according to claim 1, wherein it is determined whether or not.   The control means corrects the time measured by the time measuring means or the accumulation limit determination time based on the deviation between the exhaust temperature near the urea water supply means and the exhaust temperature near the inlet of the ammonia selective reduction type NOx catalyst, 2. The exhaust emission control device according to claim 1, wherein whether or not the exhaust gas temperature raising control is necessary is determined based on a time count after correction or a deposition limit determination time.

Images