JP2018150834A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device capable of suppressing a reduction in the controllability of an addition amount while suppressing the accumulation amount of deposits on a mixer.SOLUTION: On condition that deposits are produced on a mixer 28 by using an addition valve 26 for adding urea water and the accumulation amount is not smaller than a threshold value, a CPU 42 changes a target value for a pressure Ps of the urea water and controls the pressure Ps of the urea water to be a new target value with the operation of a pump 25. Then, the CPU 42 changes an injection pulse width as an injection time for the addition valve 26 per injection on the basis of the new target value.SELECTED DRAWING: Figure 1

Description

  The present invention is applied to an internal combustion engine including an addition valve for adding a reducing agent into exhaust gas, a mixer for diffusing the reducing agent added by the addition valve, and a selective reduction catalyst in order from the upstream side in the exhaust passage. The present invention relates to an exhaust gas purification control device for an internal combustion engine.

  For example, Patent Document 1 discloses that when the exhaust gas flow rate is smaller than a threshold value and the fin (mixer) temperature is lower than the threshold value, the injection pressure of the injector (addition valve) is lowered and urea water (reducing agent) is discharged into the exhaust gas. An apparatus for adding is described. Here, the threshold value of the fin temperature is set to a temperature at which urea water colliding with the fin is deposited on the fin.

Japanese Patent Laying-Open No. 2006-151241

  By the way, in order for the addition amount per predetermined time from the addition valve to be the required amount, it is necessary to change the addition time per one time and the frequency of addition in accordance with the injection pressure. Here, when the target value is used as the injection pressure, in the period from when the target value is changed until the actual injection pressure follows the target value, the addition time and the frequency of addition are determined by adding the addition amount as the required amount. Deviate from the amount necessary to do. For this reason, when the target value is frequently changed, the controllability of the addition amount is lowered.

  In order to solve the above problems, in an exhaust gas purification control apparatus for an internal combustion engine, an addition valve for adding a reducing agent into exhaust gas in order from an upstream side in an exhaust passage, a mixer for diffusing the reducing agent added by the addition valve, and An injection pressure control process, which is applied to an internal combustion engine including a selective reduction catalyst, and controls an injection pressure of the reducing agent by the addition valve to a target value by operating a pump that supplies the reducing agent to the addition valve; Based on the addition amount setting process for setting the required addition amount of the reducing agent and the target value and the required addition amount, the addition valve is operated to inject the required addition amount of the reducing agent from the addition valve. An addition process, an acquisition process for acquiring the accumulation amount of the deposit on the mixer, and a target value changing process for changing the target value on condition that the accumulation amount exceeds a threshold value are executed.

  When deposits are generated in the mixer, the deposits have a fine structure, so that the reducing agent is easily absorbed. For this reason, deposits are generated in the mixer, and when the reducing agent collides with a portion of the mixer where the deposits are generated, the deposition rate may rapidly increase. Here, in the above configuration, by changing the target value on the condition that the accumulation amount exceeds the threshold value, the position where the reducing agent collides with the mixer changes, so the collision position of the reducing agent in the mixer can be changed. . In that case, since the additive is prevented from colliding with the surface of the deposit already generated in the mixer, the already generated deposit is reduced by thermal decomposition.

  In addition, in the above configuration, the target value is changed on condition that the accumulation amount exceeds the threshold value, so that the frequency of changing the target value can be suppressed from increasing. For this reason, it is possible to suppress a decrease in the controllability of the addition amount while suppressing the accumulation amount of the deposit on the mixer.

The figure which shows one Embodiment concerning an exhaust-gas-purification control apparatus, and an internal combustion engine. The flowchart which shows the procedure of the process regarding addition of the urea water concerning the embodiment. The figure explaining the effect | action of the embodiment.

  Hereinafter, an embodiment of an exhaust gas purification control device for an internal combustion engine will be described with reference to the drawings.

  In the internal combustion engine 10 shown in FIG. 1, the air sucked from the intake passage 12 is sucked into the combustion chamber 14 of each cylinder. The combustion chamber 14 is provided with a fuel injection valve 16 that injects fuel (for example, light oil) into the combustion chamber 14. When the mixture of fuel and air is used for combustion in the combustion chamber 14, the mixture supplied for combustion is discharged to the exhaust passage 18 as exhaust. An oxidation catalyst 20 and a diesel particulate filter (DPF 22) are provided in the exhaust passage. An addition valve 26 for injecting urea water is provided downstream of the DPF 22 in the exhaust passage 18 in order to add urea water in the urea tank 24 supplied from the pump 25 into the exhaust gas. A mixer 28, which is a plate-like member for diffusing the urea water injected from the addition valve 26, is provided in the exhaust passage 18 downstream of the addition valve 26. A selective reduction catalyst (SCR 30) is provided downstream of the mixer 28.

  The control device 40 controls the internal combustion engine 10 and operates actuators such as the fuel injection valve 16, the pump 25, and the addition valve 26 in order to control the control amounts (torque and exhaust components) of the internal combustion engine 10. At this time, the control device 40 detects the intake air amount Ga detected by the air flow meter 50, the NOx concentration in the exhaust upstream of the SCR 30 detected by the NOx sensor 52, and the upstream of the addition valve 26 detected by the exhaust temperature sensor 54. Refer to the exhaust temperature Tex. The CPU 42 refers to the urea water pressure Ps detected by the urea water pressure sensor 56 and the urea water temperature Ts detected by the urea water temperature sensor 58. The control device 40 includes a CPU 42, a ROM 44, and a RAM 46, and the control amount is controlled by the CPU 42 executing a program stored in the ROM 44.

  FIG. 2 shows a processing procedure regarding the addition of urea water. The processing shown in FIG. 2 is realized by the CPU 42 repeatedly executing a program stored in the ROM 44 at a predetermined cycle.

  In the series of processing shown in FIG. 2, the CPU 42 first sets a required addition amount Da, which is a required amount of urea water added by the addition valve 26 per predetermined time (S10). That is, the CPU 42 grasps the exhaust amount from the intake air amount Ga, calculates the NOx flow rate in the exhaust based on the exhaust amount and the NOx concentration, and calculates the amount of urea water that can react with the NOx of the NOx flow rate without excess or deficiency. The required addition amount Da is calculated by calculating as a reaction equivalent, and adding a correction amount for setting the ammonia adsorption amount in the SCR 30 as a target value to the reaction equivalent.

  Next, the CPU 42 acquires the deposit amount Mm of the deposit on the mixer 28 (S12). The accumulation amount Mm is calculated by the CPU 42 in a process different from the process of FIG. That is, when the surface temperature of the mixer 28 is in a predetermined temperature range in which urea water is likely to adhere to the mixer 28, the CPU 42 updates the accumulation amount Mm according to the urea water addition amount, the intake air amount Ga, and the like. To do.

  The predetermined temperature region is set as follows. The urea water injected from the addition valve 26 can be transformed into biuret, triuret, cyanuric acid, ammelide, ammelin, and melamine over time. Here, since cyanuric acid is insoluble, it tends to be a deposit. However, when the temperature is low, it is difficult to change the quality after cyanuric acid. On the other hand, when the temperature is high, thermal decomposition is likely to occur, and deposits are not easily generated. For this reason, the predetermined temperature region is set to a temperature region that is easily altered after cyanuric acid and is difficult to be thermally decomposed.

  The surface temperature of the mixer 28 is calculated by the CPU 42 executing a process different from the process shown in FIG. The CPU 42 calculates the surface temperature of the mixer 28 to a value between the exhaust gas temperature Tex and the urea water temperature Ts. Specifically, when the exhaust gas temperature Tex is the same, the CPU 42 calculates the temperature closer to the urea water temperature Ts than when the required addition amount Da is large and smaller. In addition, when the addition interval is long, a value closer to the exhaust temperature Tex is calculated than when the addition interval is short.

  Then, when the intake air amount Ga is the same, the CPU 42 sets the accumulation amount Mm to a value larger when the urea water addition amount is large than when the urea water addition amount is small. Further, when the amount of urea water added is the same, the CPU 42 sets the accumulation amount Mm to a smaller value when the intake air amount Ga is large than when the intake air amount Ga is large.

  Next, the CPU 42 determines whether or not the accumulation amount Mm exceeds the threshold value Mth (S14). Here, the threshold value Mth is set based on the lower limit value of the amount by which the deposition rate of the deposit on the mixer 28 increases rapidly. This is in view of the fact that when the surface of the mixer 28 is covered with a deposit, the deposition rate of the deposit becomes higher than when the surface is not covered. This is because urea water is more easily adsorbed than the base material of the mixer 28 because the deposit has a fine structure.

  When determining that the accumulation amount Mm exceeds the threshold value Mth (S14: YES), the CPU 42 changes the target value of the urea water pressure Ps (S16). The pressure Ps of urea water is normally set to a maximum value that suppresses the acceleration of deterioration even if the pump 25 is driven for a long time, and the process of S16 lowers the target value below the maximum value. It is processing.

  Next, the CPU 42 feedback-controls the urea water pressure Ps to the target value by operating the pump 25 (S18). Then, the CPU 42 resets the accumulation amount Mm (S20). This is because the site of collision between the mixer 28 and urea water changes due to the change in the injection pressure of the urea water by the addition valve 26, so that the deposit already deposited on the mixer 28 is easily decomposed and reduced. It is in view of what we do.

Further, the CPU 42 sets the injection time (injection pulse width) per one time by the addition valve 26 based on the target value (S22). That is, in the present embodiment, the required addition amount Da is an addition amount per predetermined time, and the addition amount per predetermined time is set as the required addition amount Da by opening and closing the addition valve 26 at a predetermined frequency. On the other hand, even if the injection pulse width is the same, when the pressure Ps of the urea water is high, the injection amount per time is larger than when it is low. For this reason, the CPU 42 adjusts the injection pulse width according to the target value.
And CPU42 performs the process which adds urea water in exhaust_gas | exhaustion by operating the addition valve 26, setting the injection pulse width as the set value (S24).
Note that the CPU 42 once ends the series of processes shown in FIG. 2 when the process of S24 is completed or when a negative determination is made in S14.
Here, the operation of the present embodiment will be described.

  When the CPU 42 determines that the deposit amount Mm exceeds the threshold value Mth, the CPU 42 changes the injection pressure of the urea water from the addition valve 26. Thereby, as shown in FIG. 3, the collision site | part of the mixer 28 and urea water changes.

  In FIG. 3, the solid line schematically shows the flow of exhaust gas, the alternate long and short dash line schematically shows the scattering state of urea water when the target value is high, and the broken line shows the scattering state of urea water when the target value is low Is shown schematically. As shown in the figure, when the pressure is low, the scattering path of the urea water is more susceptible to the flow of the exhaust gas than when the pressure is high, so the collision site with the mixer 28 changes depending on the urea water injection pressure. To do.

  The threshold value Mth is set to be smaller than the deposition amount at which the deposition rate of the deposit on the mixer 28 increases rapidly. For this reason, when the injection pressure is changed, the amount of accumulation on the mixer 28 is not so large. Therefore, if the urea water is further prevented from colliding with the deposit, the deposit is reduced by the thermal decomposition reaction. I will do it.

Moreover, since the target value is changed on condition that the deposition amount Mm is equal to or greater than the threshold value Mth, it is possible to suppress an increase in the frequency of changing the target value. When the target value is changed, during the transition until the pressure Ps follows the target value, the addition amount per predetermined time may be set as the required addition amount Da depending on the injection pulse width set by the process of S22. Can not. On the other hand, in this embodiment, since it can suppress that the change frequency of target value becomes high, the fall of controllability of addition amount can be suppressed.
<Correspondence>

The correspondence relationship between the items in the above embodiment and the items described in the column “Means for Solving the Problems” is as follows. The injection pressure control process corresponds to the process of S18, the addition amount setting process corresponds to the process of S10, the addition process corresponds to the processes of S22 and S24, and the acquisition process corresponds to the process of S12. The target value changing process corresponds to the process of S16, and the exhaust purification control device corresponds to the control device 40.
<Other embodiments>
In addition, you may change at least 1 of each matter of the said embodiment as follows.
-Instead of resetting the deposition amount Mm in the process of S20 of FIG. 2, it may be gradually decreased according to time.

  In the above embodiment, as a method for calculating the deposit amount Mm of the deposit on the mixer 28, a change in the exhaust flow rate or the like is not particularly considered, but this may be considered. That is, as in the above-described embodiment, when the target value is fixed in principle, the collision site of the urea water to the mixer 28 can change according to the exhaust flow rate. For this reason, when it is determined that the collision site changes due to a change in the exhaust flow rate, the deposition amount Mm may be reduced or reset. Further, for example, the deposition amount Mm may be calculated based on an output value of an image sensor that detects an image of the mixer 28 portion.

  In the above embodiment, the target value is set to the maximum value in principle, but is not limited to this. For example, the urea water may collide with the optimum position of the mixer 28 by variably setting according to the intake air amount Ga.

  In the above embodiment, when changing the target value, the addition amount per predetermined time is set to the required addition amount Da by changing the injection pulse width, but this is not restrictive. For example, the addition frequency that is the frequency for opening and closing the addition valve 26 while fixing the injection pulse width may be changed, or both the injection pulse width and the addition frequency may be changed.

  The exhaust purification control device is not limited to one that includes the CPU 42 and the ROM 44 and executes software processing. For example, a dedicated hardware circuit (for example, an ASIC) that performs hardware processing on at least a part of the software processed in the above embodiment may be provided.

  DESCRIPTION OF SYMBOLS 10 ... Internal combustion engine, 12 ... Intake passage, 14 ... Combustion chamber, 16 ... Fuel injection valve, 18 ... Exhaust passage, 20 ... Oxidation catalyst, 22 ... DPF, 24 ... Urea tank, 25 ... Pump, 26 ... Addition valve, 28 ... Mixer, 30 ... SCR, 40 ... Control device, 42 ... CPU, 44 ... ROM, 46 ... RAM, 50 ... Air flow meter, 52 ... NOx sensor, 54 ... Exhaust temperature sensor, 56 ... Urea water pressure sensor, 58 ... Urea water temperature Sensor.

Claims (1)

In order from the upstream side in the exhaust passage, it is applied to an internal combustion engine having an addition valve for adding a reducing agent into the exhaust, a mixer for diffusing the reducing agent added by the addition valve, and a selective reduction catalyst,
An injection pressure control process for controlling the injection pressure of the reducing agent by the addition valve to a target value by operating a pump that supplies the reducing agent to the addition valve;
An addition amount setting process for setting a required addition amount of the reducing agent;
Based on the target value and the required addition amount, an addition process for operating the addition valve to inject the required addition amount of the reducing agent from the addition valve;
An acquisition process for acquiring the amount of deposit deposited on the mixer;
An exhaust purification control apparatus for an internal combustion engine, which executes a target value change process for changing the target value on condition that the accumulation amount exceeds a threshold value.

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