JP2019078227A - Exhaust emission control system for internal combustion engine - Google Patents

Abstract

To suppress deterioration of an NOx elimination ratio in a NOx catalyst as much as possible.SOLUTION: An exhaust emission control system for an internal combustion engine applied to an exhaust emission control device for the internal combustion engine having a reducing agent supply device and a selective reduction type NOx catalyst includes: control means for executing supply treatment that is treatment for supplying urea water from the reducing agent supply device; and calculation means for calculating an evaporation time that is time from completion of supply of urea water through the supply treatment until evaporation of the urea water supplied through the supply treatment. The control means executes the supply treatment this time after elapse of the evaporation time after the supply of the urea water through the previous supply treatment is completed.SELECTED DRAWING: Figure 8

Description

  The present invention relates to an exhaust gas purification system for an internal combustion engine.

  A selective reduction type NOx catalyst (hereinafter simply referred to as “NOx catalyst”) is known which purifies NOx using ammonia as a reducing agent. And in the exhaust gas purification apparatus provided with the NOx catalyst, an addition valve etc. which add urea water which is a precursor of ammonia are installed in an exhaust passage on the upstream side of the NOx catalyst.

  Then, Patent Document 1 discloses a technology for storing urea in the NOx catalyst by adding urea water in a pulse form from the addition valve when the temperature of the NOx catalyst is low.

  Further, Patent Document 2 discloses a technology in which the spray particle diameter of urea water injected from the addition valve is changed by changing the supply pressure of urea water based on the temperature of the NOx catalyst.

JP, 2005-201283, A JP, 2009-293513, A

  In order to appropriately reduce NOx using the NOx catalyst, it is necessary to add a predetermined required amount of urea water from an addition valve or the like so that the amount of ammonia necessary for NOx reduction is supplied to the NOx catalyst There is.

  Here, the inventor of the present application has newly found that solid precipitates derived from urea are easily generated before the aqueous urea solution added from the addition valve or the like evaporates. And when the addition amount of urea water was the same, it turned out that the NOx purification rate in a NOx catalyst falls rather than the case where it does not produce | generate when a solid deposit is produced | generated.

  The present invention has been made in view of the above problems, and it is an object of the present invention to suppress the decrease in the NOx purification rate as much as possible in the NOx catalyst.

  In order to solve the above problems, an exhaust gas purification system for an internal combustion engine according to the present invention is a reducing agent supply device for supplying urea water, which is a precursor of ammonia, as a reducing agent into an exhaust passage of the internal combustion engine. A reducing agent supply device provided in the passage and having an addition valve for adding urea water, and a selective reduction type NOx catalyst provided in the exhaust passage downstream of the addition valve and reducing ammonia in the exhaust with ammonia In an exhaust gas purification system for an internal combustion engine applied to an exhaust gas purification apparatus for an internal combustion engine, the control means for executing a supply process that is a process for supplying urea water from the reducing agent supply apparatus, and urea water by the supply process Calculating means for calculating an evaporation time which is a time until the aqueous urea solution supplied by the supply process is evaporated after the supply of Wherein the supply of the urea water in the supply process after the evaporation time since the completion of executing the process for supplying the current.

  According to the present invention, it is possible to suppress the decrease in the NOx purification rate as much as possible in the NOx catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows schematic structure of the internal combustion engine which concerns on embodiment of this invention, and its intake / exhaust system. It is a figure which shows the correlation with the presumed adsorption amount of ammonia, and a NOx purification rate when urea water is added from an addition valve. It is a figure showing the decomposition | disassembly process of the urea water added from the addition valve. It is a first figure showing the correlation between the amount of adsorption of ammonia and the NOx purification rate. It is the 2nd figure which shows the correlation with the adsorption quantity of ammonia and the NOx purification rate. FIG. 2 represents by-products produced by the polymerization of urea and isocyanic acid. It is a figure for demonstrating the production | generation aspect of a by-product in the exhaust gas purification device of an internal combustion engine. It is a third diagram showing the correlation between the amount of adsorption of ammonia and the NOx purification rate. It is a flow chart which shows a control flow concerning a first embodiment. It is a figure which shows correlation with exhaust gas flow rate, injection particle size, the temperature of urea water, and the amount of injection, and an evaporation rate. It is a figure which shows the time chart of the supply process performed according to FIG. It is a figure which shows the time chart of the supply process performed in the modification of 1st embodiment. It is a flow chart which shows the control flow concerning the modification of a first embodiment. It is a flowchart which shows the control flow which concerns on 2nd embodiment. It is a figure showing the relationship between an evaporation rate and an attainment rate. This is an example in which the injection condition is changed such that the injection flow rate is increased. This is an example in which the injection condition is changed so that the injection amount for one injection decreases. It is a figure which shows the time transition of the injection flow volume which concerns on 3rd embodiment.

  Hereinafter, with reference to the drawings, modes for carrying out the present invention will be exemplarily described in detail. However, the dimensions, materials, shapes, relative positions, etc. of components described in this embodiment are not intended to limit the scope of the present invention to them unless otherwise specified.

(First embodiment)
FIG. 1 is a view showing a schematic configuration of an internal combustion engine and its intake and exhaust system according to the present embodiment. An internal combustion engine 1 shown in FIG. 1 is a compression ignition internal combustion engine (diesel engine). However, the present invention can also be applied to a spark ignition type internal combustion engine that uses gasoline or the like as a fuel.

  The internal combustion engine 1 includes a fuel injection valve 3 for injecting fuel into the cylinder 2. When the internal combustion engine 1 is a spark ignition type internal combustion engine, the fuel injection valve 3 may be configured to inject fuel to the intake port.

  The internal combustion engine 1 is connected to an intake passage 4. An air flow meter 40 and a throttle valve 41 are provided in the intake passage 4. The air flow meter 40 outputs an electrical signal corresponding to the amount (mass) of intake air (air) flowing in the intake passage 4. The throttle valve 41 is disposed downstream of the air flow meter 40 in the intake passage 4. The throttle valve 41 adjusts the intake air amount of the internal combustion engine 1 by changing the passage sectional area in the intake passage 4.

The internal combustion engine 1 is connected to an exhaust passage 5. In the exhaust passage 5, according to the flow of exhaust, in order,
A first NOx sensor 50, an addition valve 510, a selective reduction NOx catalyst 52 (hereinafter simply referred to as "NOx catalyst 52"), an exhaust temperature sensor 53, and a second NOx sensor 54 are provided. The NOx catalyst 52 has a function of reducing NOx in the exhaust gas using ammonia as a reducing agent. The first NOx sensor 50 outputs an electrical signal according to the NOx concentration of the exhaust discharged from the internal combustion engine 1, and the second NOx sensor 54 outputs an electrical signal according to the NOx concentration of the exhaust flowing out of the NOx catalyst 52. Output. Further, the exhaust temperature sensor 53 outputs an electrical signal according to the temperature of the exhaust flowing out of the NOx catalyst 52.

  Further, a reducing agent supply device 51 is provided in the exhaust gas purification device of the internal combustion engine according to the present embodiment. The reducing agent supply device 51 includes the addition valve 510, the pump 511, the pressure sensor 512, the temperature sensor 513, and the tank 514 described above.

  Urea water is stored in the tank 514, and the addition valve 510 adds the urea water in the tank 514 to the exhaust passage 5. Thus, the urea water injected from the addition valve 510 is hydrolyzed by the heat of the exhaust gas or the heat from the NOx catalyst 52 to be ammonia, and is adsorbed to the NOx catalyst 52. The ammonia is used as a reducing agent in the NOx catalyst 52. That is, the urea water supplied into the exhaust passage 5 by the reducing agent supply device 51 is decomposed, and the ammonia is supplied to the NOx catalyst 52. The exhaust passage 5 between the addition valve 510 and the NOx catalyst 52 may be provided with a mixer (not shown) for mixing the exhaust flowing through the exhaust passage 5.

  The pump 511 is provided in the tank 514 and discharges urea water. The pump 511 is an electric pump, and rotates when power is supplied from a connected power source. The pump 511 can change the discharge amount of urea water by changing the rotation speed. Thereby, the pressure of urea water can be adjusted.

  A pressure sensor 512 and a temperature sensor 513 are provided in the flow path of the urea water between the pump 511 and the addition valve 510. The pressure sensor 512 is a sensor that detects the pressure of urea water supplied to the addition valve 510, and the temperature sensor 513 is a sensor that detects the temperature of urea water supplied to the addition valve 510.

  Then, an electronic control unit (ECU) 10 is juxtaposed to the internal combustion engine 1. The ECU 10 is a unit that controls the operating state of the internal combustion engine 1 and the like. In addition to the air flow meter 40, the first NOx sensor 50, the exhaust temperature sensor 53, the second NOx sensor 54, the pressure sensor 512, and the temperature sensor 513, the ECU 10 includes the accelerator position sensor 7 and the crank position sensor 8 etc. The sensor is electrically connected. The accelerator position sensor 7 is a sensor that outputs an electric signal correlated with the operation amount (accelerator opening degree) of an accelerator pedal (not shown). The crank position sensor 8 is a sensor that outputs an electrical signal correlated to the rotational position of the engine output shaft (crankshaft) of the internal combustion engine 1. And the output signal of these sensors is inputted into ECU10. The ECU 10 derives the engine load of the internal combustion engine 1 based on the output signal of the accelerator position sensor 7 and derives the engine rotational speed of the internal combustion engine 1 based on the output signal of the crank position sensor 8. Further, the ECU 10 estimates the flow rate of exhaust flowing into the NOx catalyst 52 based on the output value of the air flow meter 40 (hereinafter sometimes referred to as "exhaust flow rate"), and based on the output value of the exhaust temperature sensor 53. Then, the temperature of the NOx catalyst 52 (hereinafter sometimes referred to as "catalyst temperature") is estimated. Further, the ECU 10 detects the NOx concentration of the exhaust flowing into the NOx catalyst 52 based on the output value of the first NOx sensor 50, and further, the amount of NOx flowing into the NOx catalyst 52 based on the NOx concentration and the exhaust flow rate. Estimate

Further, various devices such as the fuel injection valve 3, the throttle valve 41, the addition valve 510, and the pump 511 are electrically connected to the ECU 10. The ECU 10 controls these various devices. Here, the ECU 10 sends urea water from the reducing agent supply device 51 by transmitting a command signal to the reducing agent supply device 51. Hereinafter, the process in which the ECU 10 supplies urea water from the reducing agent supply device 51 will be referred to as “supply process”. When the ECU 10 executes the supply process, the ECU 10 first calculates the supply amount (requested supply amount) of ammonia to the NOx catalyst 52 necessary for appropriately reducing NOx in the NOx catalyst 52. Then, the ECU 10 sets the addition flow rate (required addition flow rate) of urea water from the addition valve 510 according to the required supply amount. Then, a command signal corresponding to the required addition flow rate is generated, and the command signal is transmitted to the reducing agent supply device 51. In the reducing agent supply device 51 having received the command signal, the applied voltage of the pump 511 or the valve opening degree of the addition valve 510 is controlled according to the command signal, and urea water is added from the addition valve 510. That is, the supply process is performed. In addition, when ECU10 performs a supply process, it functions as a control means which concerns on this invention.

  In such an exhaust gas purification apparatus, when the supply process is performed, the amount of adsorption of ammonia in the NOx catalyst 52 is increased. Then, the NOx purification rate of the NOx catalyst 52 tends to be high. FIG. 2 is a graph showing the correlation between the estimated amount of adsorption of ammonia in the NOx catalyst 52 and the NOx purification rate of the NOx catalyst 52 when urea water is added from the addition valve 510.

The estimated adsorption amount shown in FIG. 2 is an amount calculated based on the addition amount of urea water added from the addition valve 510. This will be described below. FIG. 3 is a diagram showing a decomposition process of urea water added from the addition valve 510. As shown in FIG. As shown in FIG. 3, the aqueous urea solution added from the addition valve 510 becomes urea as the water evaporates (see STEP 1). Then, the urea is thermally decomposed to generate ammonia (NH ₃ ) and isocyanate (HNCO) (see STEP 2). Ammonia (NH ₃ ) and CO ₂ are produced by hydrolysis of isocyanate (HNCO) (see STEP 3). The estimated adsorption amount shown in FIG. 2 is an adsorption amount calculated on the assumption that ammonia (NH ₃ ) is generated through such a decomposition process of urea water.

Then, as shown by line L1 in FIG. 2, when urea water is added from the addition valve 510, the estimated amount of adsorption of ammonia increases, and the ammonia is estimated until the amount of adsorption becomes a predetermined amount of adsorption. As the adsorption amount increases, the NOx purification rate tends to increase. Here, it is assumed that the estimated adsorption amount of ammonia becomes R1 shown in FIG. 2 by the addition of urea water from the addition valve 510. In this case, assuming that all of the aqueous urea solution added from the addition valve 510 is decomposed according to the decomposition step shown in FIG. 3 and the generated ammonia (NH ₃ ) is adsorbed to the NOx catalyst 52, the addition valve 510 Therefore, when the reduction of NOx is performed using the ammonia adsorbed on the NOx catalyst 52 without the addition of urea water, the NOx purification rate decreases along the line L1.

  Then, temporarily, the estimated amount of adsorption of ammonia described above and the amount of adsorption of ammonia actually adsorbed by the NOx catalyst 52 (hereinafter sometimes referred to as “the actual amount of adsorption of ammonia” may be substantially the same. In this case, the NOx purification rate can be uniquely determined according to the estimated adsorption amount of ammonia. However, the above-mentioned estimated adsorption amount of ammonia and the actual adsorption amount of ammonia may be different. For example, as shown in FIG. 4A, even if the estimated adsorption amount of ammonia is calculated as R1, the actual adsorption amount of ammonia may be R2, which is smaller than R1. In this case, when NOx is reduced using ammonia adsorbed on the NOx catalyst 52 without addition of urea water from the addition valve 510, the NOx purification rate will decrease along the line L2. . Such a situation is likely to occur under conditions where the exhaust gas temperature is relatively low and the exhaust gas flow rate is relatively large (low temperature / high flow condition).

  Since it is difficult to measure the actual amount of adsorption of ammonia, the NOx purification rate is actually estimated based on the estimated amount of adsorption of ammonia. Then, although the estimated adsorption amount of ammonia is calculated as R 1, when the actual adsorption amount of ammonia is R 2, urea water was not added from the addition valve 510 and was adsorbed to the NOx catalyst 52 without being added. When the reduction of NOx is performed using ammonia, as shown in FIG. 4B, the NOx purification rate will decrease along the line L3. If the estimated amount of adsorption of ammonia and the actual amount of adsorption of ammonia are substantially the same, the ammonia adsorbed on the NOx catalyst 52 without addition of urea water from the addition valve 510 is used to generate NOx. When the reduction is performed, the NOx purification rate will decrease along the line L1 of FIG. 4B.

  Here, as a result of intensive investigations, the inventor of the present application has found that by-products formed in the decomposition process of urea water are deposited as solid precipitates, whereby the estimated adsorption amount of ammonia and the actual adsorption amount of ammonia are I found that a difference occurred. This will be described in detail below.

  FIG. 5 is a diagram representing by-products produced by the polymerization of urea and isocyanic acid. As shown in FIG. 5, biuret is formed by the polymerization of urea and isocyanate, and triuret is formed by the polymerization of biuret and isocyanate. Then, triuret further produces a compound such as cyanuric acid, ammelide, ammeline and melamine. The compounds such as cyanuric acid, ammelide, ammeline and melamine have the property that they do not decompose into gaseous components (ammonia and isocyanic acid) at low temperatures. For example, melamine does not decompose into gaseous components unless it reaches 380 ° C. Have. Therefore, these compounds tend to be deposited as solid precipitates.

  Further, FIG. 6 is a view for explaining a generation aspect of the above-mentioned by-product in the exhaust gas purification apparatus for an internal combustion engine. Here, as shown in FIG. 6, ammonia and isocyanic acid generated by the thermal decomposition of urea flow into the NOx catalyst 52. Then, the NOx catalyst 52 adsorbs the inflowing ammonia. Further, by hydrolyzing isocyanate in the NOx catalyst 52, ammonia is generated, and the ammonia is adsorbed to the NOx catalyst 52. The hydrolysis of isocyanate in the NOx catalyst 52 tends to proceed relatively fast due to the catalytic reaction.

  On the other hand, it was newly found that when the droplets of urea water collide with isocyanate before flowing into the NOx catalyst 52, polymerization of urea and isocyanate becomes easy to occur. That is, when the droplets of urea water collide with isocyanate before flowing into the NOx catalyst 52, the above-described by-products are easily generated. And since the by-product generated in this way does not decompose into gas components unless it reaches a high temperature (for example, 380 ° C. or more), the by-product tends to be easily deposited as a solid deposit.

  Here, if the correlation between the estimated adsorption amount of ammonia and the NOx purification rate is obtained under the condition that the above-mentioned by-products are not generated, the hysteresis seen in FIG. 4B above is greatly reduced (this is shown in FIG. Represented by the lines L1 and L3 'of 7). In this case, the urea water (ammonia based on the urea water) added from the addition valve 510 can be efficiently used for the reduction of NOx, thereby reducing the NOx purification rate of the NOx catalyst 52 as much as possible. Can be suppressed. Then, in order to suppress the formation of the above-mentioned by-products, it was found that it is important to evaporate the aqueous urea solution added from the addition valve 510 sufficiently.

Here, when the ECU 10 executes the supply process, the urea water at the time of the previous supply process is not evaporated and remains in the exhaust passage 5, and the urea water at the time of the previous supply process and the current If interference with the aqueous urea solution added by the supply process interferes with the evaporation of the aqueous urea solution, the above-mentioned by-products may be easily generated. Therefore, in the exhaust gas purification system for an internal combustion engine according to the present embodiment, the evaporation time which is the time from the end of the supply of urea water by the supply process to the evaporation of the urea water supplied by the supply process after the ECU 10 Calculate Then, the ECU 10 executes the current supply process after the evaporation time has elapsed since the end of the supply of urea water in the previous supply process. Such processing executed by the ECU 10 will be described in detail below.

  FIG. 8 is a flowchart showing a control flow according to the present embodiment. In the present embodiment, the ECU 10 repeatedly executes the present flow at a predetermined operation cycle during operation of the internal combustion engine 1.

  In this flow, first, at S101, it is determined whether or not an execution condition for executing the supply process is satisfied. In S101, when the supply of ammonia to the NOx catalyst 52 is requested, it is determined that the execution condition is satisfied. Alternatively, in addition to the above, it is possible to determine that the execution condition is satisfied when, for example, the temperature of the aqueous urea solution belongs to the temperature range where the addition is possible, or for example, when the catalyst temperature belongs to the predetermined temperature range It can also be done. Whether or not the supply of ammonia to the NOx catalyst 52 is required can be determined by the ECU 10 executing a known flow different from the present flow. When the determination is affirmative in S101, the ECU 10 proceeds to the process of S102, and when the determination is negative in S101, the execution of the present flow is ended.

  When an affirmative determination is made in S101, next, in S102, parameters at the time of execution of the previous supply process are read. Here, the ECU 10 stores the execution parameter for each execution of the supply process, and in S102, the exhaust flow rate, the exhaust temperature, the temperature of the urea water, and the injection valve 510 at the time of the previous supply process are injected. The total amount of urea water (hereinafter sometimes referred to as “injection amount”), and the particle diameter of urea water injected from addition valve 510 (hereinafter sometimes referred to as “injection particle diameter”) are read. . The injection particle size is the flow rate of urea water injected from addition valve 510 (hereinafter sometimes referred to as “injection flow rate”), the pressure of urea water injected from addition valve 510 (hereinafter “injection pressure” And may be calculated based on the temperature of urea water and the like using a known method.

  Next, in S103, the evaporation time from the end of the addition of the aqueous urea solution by the previous supply process to the evaporation of the aqueous urea solution by the supply process is calculated. Here, the calculation method of the evaporation time in this embodiment is demonstrated using FIG. FIG. 9 is a view showing the correlation between the exhaust gas flow rate, the injected particle size, the temperature of urea water, and the injection amount, and the evaporation rate. As shown in FIG. 9, the evaporation rate becomes slower as the exhaust flow rate increases. In addition, the smaller the injected particle size, or the higher the temperature of the aqueous urea solution, the faster the evaporation rate. Also, the evaporation rate becomes slower as the injection amount increases. Such correlation is stored in advance in the ROM of the ECU 10 as a map or function. Then, in S103, the evaporation time is calculated based on such a correlation and the parameters read in S102. The ECU 10 functions as a calculation unit according to the present invention by calculating the evaporation time in this manner.

  Next, in S104, it is determined whether or not the evaporation time calculated in S103 has elapsed since the completion of the previous supply processing. When the determination is affirmative in S104, the ECU 10 proceeds to the process of S105, and when the determination is negative in S104, the ECU 10 repeats the process of S104.

  If an affirmative determination is made in S104, next, in S105, the supply of urea aqueous solution from the reducing agent supply device 51 is executed. And execution of this flow is ended after processing of S105.

  The supply process performed according to the control flow described above will be described using FIG. FIG. 10 is a diagram showing a time chart of the supply process performed according to FIG. 8 described above. The supply processing execution flag in FIG. 10 is a flag that is set to 1 when an execution condition for executing the supply processing is satisfied. As shown in FIG. 10, the execution of the previous supply process is ended at time t1. Then, at time t2, an execution condition of the current supply process is satisfied. Here, the evaporation time is represented by Δt in FIG. 10, and at time t2, the evaporation time Δt has not elapsed since the end of the previous supply process (from time t1). Therefore, at time t2, the current supply process is not executed yet. Then, at time t3 when the evaporation time Δt has elapsed from time t1, the current supply process is performed.

  As described above, by performing the supply process, the situation in which the evaporation of the urea aqueous solution is prevented is suppressed, whereby the above-described by-products are less likely to be generated. As a result, it is possible to suppress the decrease in the NOx purification rate of the NOx catalyst 52 as much as possible.

(Modification of the first embodiment)
Next, a modification of the first embodiment described above will be described. In the present modification, the detailed description of the configuration substantially the same as the first embodiment and the control processing substantially the same as the first embodiment will be omitted.

  FIG. 11 is a diagram showing a time chart of the supply process performed in the present modification. The supply process execution flag in FIG. 11 is as described in the description of FIG. In the first embodiment described above, as shown in FIG. 10 described above, urea solution is continuously injected from the addition valve 510 at a constant injection flow rate while the execution condition of the supply process is satisfied. On the other hand, in the present modification, as shown in FIG. 11, while the execution condition of the supply process is satisfied, urea water is injected in a pulse form from the addition valve 510. In such an embodiment, as shown in FIG. 11, while the execution condition of the supply process is satisfied, urea water is injected in a pulse form from the addition valve 510 at intervals of the evaporation time Δt 'or more.

  The control flow when such a supply process is performed will be described below. FIG. 12 is a flowchart showing a control flow according to the present modification. In the present modification, the ECU 10 repeatedly executes the present flow in a predetermined calculation cycle during operation of the internal combustion engine 1. In the processes shown in FIG. 12, processes substantially the same as the processes shown in FIG. 8 are given the same reference numerals, and the detailed description thereof is omitted.

  In the control flow shown in FIG. 12, when the positive determination is made in S101, next, in S201, the addition amount of urea water from the addition valve 510 necessary for appropriately reducing NOx in the NOx catalyst 52 (hereinafter referred to as “ It may be called "the required addition amount." Is calculated. In S201, based on the supply amount (request supply amount) of ammonia to the NOx catalyst 52 necessary for appropriately reducing NOx in the NOx catalyst 52, the required addition amount is calculated using a known method. Further, the required supply amount may be a known method in consideration of, for example, the inflow NOx amount to the NOx catalyst 52 estimated based on the NOx concentration detected by the first NOx sensor 50 and the exhaust flow rate, and the catalyst temperature. It can be calculated using. Then, after the process of S201, the ECU 10 proceeds to the process of S102.

  Further, in the control flow shown in FIG. 12, when the determination is affirmative in S104, next, the supply of urea water from the reducing agent supply device 51 is executed in S202. In this modification, as described above, while the execution condition of the supply process is satisfied, the urea water is injected in a pulse form from the addition valve 510. Therefore, in S202, the urea water from the reducing agent supply device 51 is After the supply is started, the supply of urea water from the reducing agent supply device 51 is temporarily stopped when a predetermined time determined in advance passes.

  Next, in S203, the total amount of urea water (hereinafter sometimes referred to as "current addition amount") that has been added from the addition valve 510 so far in the execution of the control flow shown in Fig. 12 this time is calculated. . Then, after the process of S203, it is determined in S204 whether or not the current addition amount calculated in S203 is equal to or more than the required addition amount calculated in S201. Then, when the determination is affirmative in S204, the execution of the present flow is ended, and when the determination is negative in S204, the ECU 10 returns to the process of S102.

  Even when the supply process is performed as described above, the situation in which the evaporation of the urea water is hindered is suppressed, and thus the above-described by-products are less likely to be generated. As a result, it is possible to suppress the decrease in the NOx purification rate of the NOx catalyst 52 as much as possible.

Second Embodiment
Next, a second embodiment of the present invention will be described. FIG. 13 is a flowchart showing a control flow according to the present embodiment. In the present embodiment, the ECU 10 repeatedly executes the present flow at a predetermined operation cycle during operation of the internal combustion engine 1. In the processes shown in FIG. 13, the processes substantially the same as the processes shown in FIG. 8 are denoted by the same reference numerals, and the detailed description thereof is omitted. In the present embodiment, the detailed description of the configuration substantially the same as the first embodiment will be omitted.

  In the control flow shown in FIG. 13, the required addition amount is calculated in S301 after the process of S104 shown in FIG. The process of S301 is substantially the same as the process of S201 shown in FIG. 12 described above. Next, in S302, the evaporation time associated with the execution of the current supply process is calculated. In S302, the evaporation time is calculated based on the correlation described in the description of the process of S103 shown in FIG. 8 and the parameters associated with the execution of the current supply process.

  Next, in S303, the arrival time associated with the execution of the current supply process is calculated. Here, the arrival time is the time until the droplets of urea water injected by the addition valve 510 reach the upstream end surface of the NOx catalyst 52. Alternatively, when a mixer is provided upstream of the NOx catalyst 52, the arrival time is the time until the droplets of urea water injected by the addition valve 510 reach the mixer. The arrival time is the distance from the injection hole of the addition valve 510 to the upstream end face (or mixer) of the NOx catalyst 52 and the velocity of urea water injected from the addition valve 510 (hereinafter referred to as “injection velocity” And the flow velocity of the exhaust. The injection speed can be calculated using a known method based on the injection flow rate and the like.

  Thus, in the present embodiment, the arrival time is calculated. This will be described below. As described above, by the droplets of urea water colliding with the isocyanate before flowing into the NOx catalyst 52, by-products are easily generated. Such a collision is likely to occur at the upstream end face (or mixer) of the NOx catalyst 52. Therefore, if the droplets of urea aqueous solution injected by the addition valve 510 are evaporated before they reach the upstream end face (or mixer) of the NOx catalyst 52, the above-described collision is less likely to occur. Thus, the formation of by-products is suppressed. Here, since the control flow according to the present embodiment is a flow that is executed after the evaporation time has elapsed since the previous completion of the supply processing, the urea water supplied by the previous supply interferes with the urea water supplied this time. However, by considering the above-described arrival time, it is possible to more preferably suppress the formation of by-products resulting from the aqueous urea solution currently supplied.

  Here, the relationship between the evaporation rate and the arrival rate is shown in FIG. As shown in FIG. 14, the ultimate velocity increases as the exhaust gas flow rate increases. Then, the generation of by-products is suppressed under the condition that the evaporation rate is equal to or higher than the ultimate velocity.

  Therefore, in the control flow shown in FIG. 13, after the process of S303, it is determined in S304 whether the arrival time calculated in S303 is longer than the evaporation time calculated in S302. When the determination is affirmative in S304, the ECU 10 proceeds to the process of S105, and when the determination is negative in S304, the ECU 10 proceeds to the process of S305.

  If an affirmative determination is made in S304, next, in S105, the supply of urea aqueous solution from the reducing agent supply device 51 is executed. If a positive determination is made in S304, it can be determined that the droplets of urea water injected by the addition valve 510 evaporate before reaching the upstream end surface (or mixer) of the NOx catalyst 52, in this case In the same manner as the process of S105 shown in FIG. 8 described above, the supply of urea aqueous solution can be performed. And execution of this flow is ended after processing of S105. The supply of urea water by the process of S105 is hereinafter referred to as supply of urea water at normal times.

  On the other hand, when the negative determination is made in S304, next, the injection condition of urea water is changed in S305. If a negative determination is made in S304, the droplets of urea aqueous solution injected by the addition valve 510, if the supply of urea aqueous solution at normal times is performed, the upstream end surface (or mixer) of the NOx catalyst 52 before evaporation. In this case, the injection condition of urea aqueous solution is changed. In S305, the injection conditions are changed so as to reduce the jetted particle size so as to increase the evaporation rate (to shorten the evaporation time). Alternatively, the injection condition is changed so that the injection amount per injection decreases. Then, in S306, the supply of urea water from the reducing agent supply device 51 is executed under the injection conditions determined in S305. And execution of this flow is ended after processing of S306.

  The aspect by which the injection conditions of urea water are changed as mentioned above is illustrated in FIG. 15 and FIG. FIG. 15 shows an example in which the injection condition is changed so as to increase the injection flow rate. As shown in FIG. 15, the urea aqueous solution is normally supplied at the injection flow rate FR1 while the injection flow rate is made to be FR2 higher than FR1 by changing the injection condition. By thus increasing the injection flow rate, the injection particle size can be reduced, and evaporation of urea water can be promoted. Further, FIG. 16 is an example in which the injection condition is changed so that the injection amount per injection is reduced. As shown in FIG. 16, while the injection amount is normally injected only once with the injection amount Q1, the injection amount is injected three times with Q2, which is smaller than Q1, by changing the injection condition. This also promotes the evaporation of the urea water. In the case where urea aqueous solution is injected in a pulse shape as shown in FIG. 16, as described in the modification of the first embodiment, urea aqueous solution is injected in a pulse shape at intervals equal to or longer than the evaporation time. .

  By changing the injection condition as described above, the above-described by-products are less likely to be generated, and it is possible to suppress the decrease in the NOx purification rate of the NOx catalyst 52 as much as possible.

Third Embodiment
Next, a third embodiment of the present invention will be described. The first embodiment and the second embodiment described above are examples in which urea water of the required addition amount is added from the addition valve 510 so that the supply amount of ammonia to the NOx catalyst 52 becomes the required supply amount. . On the other hand, in the present embodiment, the evaporation rate of the droplets of urea aqueous solution injected from the addition valve 510 is the rate-limiting, and it is an example of the case where urea aqueous solution of the required addition amount can not be added. In the present embodiment, the detailed description of the configuration substantially the same as that of the first embodiment described above and the control processing that is substantially the same will be omitted.

FIG. 17 is a view showing the time transition of the injection flow rate according to the present embodiment. The upper part of FIG.
Determined based on the required addition flow rate determined based on the supply amount of ammonia (required supply amount) to the NOx catalyst 52 necessary for appropriately reducing NOx in the Ox catalyst 52, and the evaporation rate of the urea aqueous solution droplets The lower portion of FIG. 17 shows the temporal transition of the injection flow rate (actual addition flow rate) actually injected from the addition valve 510.

  As shown in the upper part of FIG. 17, the addable flow rate is equal to or higher than the required addition flow rate from time t0 to time t11 and after time t12. On the other hand, the addable flow rate is smaller than the required addition flow rate from time t11 to time t12. Then, as shown in the lower part of FIG. 17, the actual addition flow rate is set to the required addition flow rate from time t0 to time t11, and the actual addition flow rate is set to the addition available flow rate from time t11 to time t12. That is, during the period from time t11 to time t12, the evaporation rate of the droplets of urea water becomes rate-limiting, and it is impossible to inject the urea water of the required addition flow rate. In this case, the amount of ammonia in the NOx catalyst 52 may be insufficient.

  Therefore, in the present embodiment, after the situation where the ammonia amount in the NOx catalyst 52 runs short, if the flow rate that can be added becomes equal to or higher than the required addition flow rate, as shown in the lower part of FIG. Is at the addable flow rate. That is, the amount of ammonia in the NOx catalyst 52 is increased by injecting more urea water than the required addition flow rate. This makes it possible to suppress the decrease in the NOx purification rate of the NOx catalyst 52 as much as possible.

1 ... internal combustion engine 3 ... fuel injection valve 4 ... intake passage 5 ... exhaust passage 10 ... ECU
50 ... first NOx sensor 51 ... reducing agent supply device 52 ... NOx catalyst 53 ... exhaust temperature sensor 54 ... second NOx sensor 510 ... addition valve 511 ... pump 512 ... pressure Sensor 513 · · Temperature sensor 514 · · Tank

Claims (1)

A reducing agent supply device for supplying urea water, which is a precursor of ammonia, as a reducing agent into an exhaust passage of an internal combustion engine, the reducing agent supply device being provided in the exhaust passage and having an addition valve for adding urea water.
A selective reduction type NOx catalyst provided in an exhaust passage downstream of the addition valve and reducing NOx in the exhaust gas with ammonia;
In an exhaust gas purification system for an internal combustion engine, the exhaust gas purification system for an internal combustion engine comprising:
A control unit that executes a supply process that is a process of supplying urea water from the reducing agent supply device;
Calculating means for calculating an evaporation time which is a time from the end of the supply of the aqueous urea solution by the supply process to the evaporation of the aqueous urea solution supplied by the supply process;
An exhaust gas purification system for an internal combustion engine, wherein the control means executes the current supply process after the evaporation time has elapsed since the last supply of urea water in the supply process is ended.

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