JP2009197741A - Exhaust emission control system for internal combustion engine - Google Patents

Abstract

In a urea SCR system, there is provided a technology capable of detecting deterioration of urea water while suppressing an increase in cost.
An SCR installed in an exhaust passage, an injection device that injects urea water into an exhaust passage upstream of the SCR, and that urea water for injection from the injection device is frozen or melted from a frozen state. Based on the detection means for detecting, the temperature acquisition means for acquiring the temperature of the urea water when the detection means detects the freezing of the urea water or the melting from the frozen state, and the temperature acquired by the temperature acquisition means Determination means for determining whether or not the water concentration is in a deteriorated state deviating from a predetermined reference concentration.
[Selection] Figure 4

Description

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

A selective reduction catalyst (SCR) that selectively reduces NOx in exhaust gas using ammonia (NH3) as a reducing agent is known as an exhaust purification device that purifies nitrogen oxide (NOx) discharged from an internal combustion engine by post-processing. It has been. As an exhaust purification system equipped with an SCR, a tank for storing urea water and an injection valve for injecting urea water in the tank into an exhaust passage upstream of the SCR and injecting urea water into the exhaust gas are added. There is known a urea SCR system in which NH 3 generated by a hydrolysis reaction of urea water by exhaust heat is supplied to the SCR as a reducing agent.
JP 2007-183244 A JP 2006-342771 A JP 2007-218146 A JP-A-10-33948

  By the way, the freezing point of urea water varies depending on the concentration of urea water. Specifically, when the concentration is 32.5%, the freezing point becomes the lowest temperature (−11 ° C.), and the freezing point becomes higher as the concentration shifts to a lower concentration side or a higher concentration side than 32.5%. In view of such properties of urea water, in the urea SCR system, the concentration of urea water to be stored in the tank is set to 32.5%, and in the feedback control of the urea water injection amount according to the NOx purification rate, etc. Some parameters such as a feedback gain are set on the assumption that the urea water concentration is 32.5%. Therefore, when urea water with a concentration different from the above concentration is replenished to the tank or when the urea water concentration in the tank is in a deteriorated state deviating from the above concentration, NOx purification by SCR is performed. May not be performed properly.

  On the other hand, it is possible to measure the concentration of urea water in the tank by newly installing a urea water concentration sensor and determine whether or not the urea water concentration in the tank is appropriate based on the measurement result. Conceivable. However, when such a new device is added, an increase in system cost becomes a problem. In addition, it becomes necessary to construct a system that detects a failure of the urea water concentration sensor and a system that provides an appropriate alternative means when the urea water concentration sensor fails, and this complicates the system and further increases costs.

  The present invention has been conceived in view of such circumstances, and it is an object of the present invention to provide a technology that enables detection of a deterioration state of urea water while suppressing an increase in cost in a urea SCR system. .

To achieve the above object, the present invention provides:
An SCR installed in the exhaust passage;
An injector for injecting urea water into an exhaust passage upstream of the SCR;
Detecting means for detecting that the urea water for injection from the injection device is frozen or melted from a frozen state;
Temperature acquisition means for acquiring the temperature of urea water when freezing of the urea water or melting from the frozen state is detected by the detection means;
Determination means for determining whether or not the concentration of urea water is in a deteriorated state deviating from a predetermined reference concentration based on the temperature acquired by the temperature acquisition means;
An exhaust gas purification system for an internal combustion engine is provided.

  Here, the “reference concentration” is a concentration determined in advance as a prescribed concentration of urea water to be used in the exhaust purification system. This is a concentration that is used as a reducing agent supply source and that is determined in advance as a prescribed concentration of urea water to be replenished as appropriate.

  The temperature of the urea water acquired by the temperature acquisition means when the detection means detects that the urea water is frozen corresponds to the freezing point of the urea water. Moreover, the temperature of the urea water acquired by the temperature acquisition means when the detection means detects that the urea water has melted from the frozen state corresponds to the melting point of the urea water.

  As described above, since there is a correspondence between the concentration of urea water and the freezing point of urea water (or melting point, hereinafter the same), the current freezing point of urea water acquired by the temperature acquisition means, and urea water By comparing the freezing point (preliminarily obtained) when the concentration is the reference concentration, it can be determined whether or not the current concentration of the urea water deviates from the reference concentration.

  In the present invention, the determination means may determine that the urea water is in a deteriorated state when the freezing point of the urea water acquired by the temperature acquisition means does not coincide with the freezing point corresponding to the reference concentration. Alternatively, when the freezing point of the urea water acquired by the temperature acquisition means does not belong to a certain temperature range including the freezing point corresponding to the reference concentration, it may be determined that the urea water is in a deteriorated state.

  As described above, the freezing point of urea water becomes the lowest temperature (hereinafter referred to as the lowest freezing point. For example, −11 ° C.) when the concentration of urea water is a predetermined concentration (for example, 32.5%). In both cases of shifting to a lower concentration side than the predetermined concentration and shifting to a higher concentration side, the freezing point of urea water has a property of becoming higher than the lowest freezing point.

  In view of such properties of urea water, an exhaust purification system that uses urea water as a supply source of the reducing agent is a system that can ensure the supply of the reducing agent in the widest possible temperature environment. As the standard concentration of urea water to be used, a concentration corresponding to the above minimum freezing point can be adopted.

  In this case, in the present invention, when the freezing point of the urea water acquired by the temperature acquisition means is higher than the lowest freezing point, it can be determined that the urea water is in a deteriorated state.

  Here, the determination unit may determine that the urea water is in a deteriorated state when the freezing point of the urea water acquired by the temperature acquisition unit is higher than the lowest freezing point by a certain temperature or more. This corresponds to determining that the urea aqueous solution is in a deteriorated state when the urea aqueous solution concentration does not belong to a certain concentration range including the concentration corresponding to the lowest freezing point.

  When the deterioration state of the urea water is determined by the configuration of the present invention as described above, it is desirable to eliminate the deterioration state by, for example, replenishing the urea water tank with an appropriate concentration of urea water according to the deterioration state. .

In order to appropriately perform such work, it is preferable to determine whether the concentration of urea water is shifted to a higher concentration side or a lower concentration side than the reference concentration. Therefore, in the configuration of the present invention,
A purification rate acquisition means for acquiring a NOx purification rate by the SCR;
Control means for feedback-controlling the injection amount of urea water by the injection device such that the NOx purification rate acquired by the purification rate acquisition means matches a predetermined target purification rate;
When the determination unit determines that the urea water is in the deteriorated state, the deteriorated state for determining whether the deteriorated state is a deteriorated state deviating to the higher concentration side or the lower concentration side from the reference concentration. Discrimination means;
Further comprising
The deterioration state determining means includes
When the feedback control amount of the urea water injection amount in the feedback control is greater than a predetermined reference control amount, it is determined that the urea water concentration is in a deteriorated state deviating to a lower concentration side than the reference concentration,
When the feedback control amount of the urea water injection amount in the feedback control is less than a predetermined reference control amount, it may be determined that the urea water concentration is in a deteriorated state deviating to a higher concentration side than the reference concentration. good.

  Here, the “NOx purification rate by SCR” is an index of the amount of NOx in the exhaust gas that is purified in the SCR when urea water injection is performed and NOx reduction processing by the SCR is performed. Is obtained based on the ratio of the NOx amount flowing into the SCR and the NOx amount flowing out from the SCR. In the feedback control by the control means, control parameters such as a feedback gain are set on the assumption that the urea water concentration is the reference concentration. The “predetermined reference control amount” is a feedback control amount of the urea water injection amount calculated on the assumption that the current urea water concentration is equal to the reference concentration.

  When the actual urea water concentration is lower than the reference concentration, the urea water injection amount required to make the NOx purification rate coincide with the target purification rate increases. Therefore, it is considered that the control amount of the urea water injection amount in actual feedback control is larger than the reference control amount. From this, when it is determined by the determination means that the urea water concentration is in a deteriorated state deviating from the reference concentration, if the control amount of the urea water injection amount in the actual feedback control is larger than the reference control amount, It can be determined that the urea water concentration is in a deteriorated state where the concentration is shifted to a lower concentration side than the reference concentration.

  On the other hand, when the actual urea water concentration is higher than the reference concentration, the urea water injection amount required to make the NOx purification rate coincide with the target purification rate is reduced. Therefore, it is considered that the control amount of the urea water injection amount in actual feedback control is smaller than the reference control amount. From this, when it is determined by the determination means that the urea water concentration is in a deteriorated state deviating from the reference concentration, if the control amount of the urea water injection amount in the actual feedback control is smaller than the reference control amount, It can be determined that the urea water concentration is in a deteriorated state where the concentration is shifted to a higher concentration side than the reference concentration.

  Next, specific means for detecting that the urea water is frozen or melted from the frozen state in the configuration of the present invention will be described.

  When urea water freezes, the urea water cannot be injected from the injection device. This is considered to appear as an abnormality occurring in the urea water injection control. For example, when the injection device is configured to include a pressurizing device that pressurizes urea water to inject urea water, freezing of urea water is said to cause excessive pressure applied by the pressurizing device. It appears to appear as an anomaly. Therefore, when it is detected that such an abnormality has occurred in the pressurizing device, it is possible to detect that the urea water is frozen.

  On the contrary, when the frozen urea water is melted, it is considered that the abnormality appearing in such a pressurizing apparatus is resolved. Therefore, when the abnormality that has occurred in the pressurizing device is resolved, it can be detected that the urea water has melted from the frozen state.

  Further, in a configuration including a tank for storing urea water to be injected from the injection device, and a pump that sucks up urea in the tank and supplies it to the injection device, freezing of urea water causes the urea water to be sucked up by the pump. This is considered to be an abnormality such that the normal operation cannot be performed normally or the urea estimation amount in the tank does not change despite the urea water injection request. Therefore, when it is detected that such an abnormality has occurred in the pump or the tank, it is possible to detect that the urea water is frozen.

  On the other hand, when the frozen urea water is melted, it is considered that such an abnormality appearing in the pump or tank is resolved. Therefore, it is possible to detect that the urea water has melted from the frozen state when the abnormality occurring in the pump or the tank is resolved.

  In addition, when urea water cannot be injected from the injector, the NOx reduction and purification reaction in the SCR does not proceed, so freezing of urea water is considered to appear as an abnormality that the NOx purification rate deteriorates rapidly. . Therefore, when it is detected that the NOx purification rate falls below a certain standard purification rate, it is possible to detect that the urea water has frozen. The “reference purification rate” is a constant determined based on the lower limit value of the NOx purification rate at which it can be determined that the NOx reduction purification reaction is normally progressing in the SCR.

  Conversely, when the frozen urea is melted, it is considered that such a state in which the NOx purification rate is excessively deteriorated is eliminated. Therefore, when the state where the NOx purification rate is excessively deteriorated is resolved, it is possible to detect that the urea water has melted from the frozen state.

  According to the present invention, in the urea SCR system, it is possible to detect deterioration of urea water while suppressing an increase in cost.

  The best mode for carrying out the present invention will be exemplarily described in detail below with reference to the drawings. The dimensions, materials, shapes, relative arrangements, and the like of the components described in the present embodiment are not intended to limit the technical scope of the invention to those unless otherwise specified.

  FIG. 1 is a diagram schematically illustrating a schematic configuration of an exhaust gas purification system for an internal combustion engine according to the present embodiment.

  In FIG. 1, the exhaust from the internal combustion engine 1 is discharged to the manifold 2 and flows into the exhaust passage 3. An oxidation catalyst 4, a particulate filter (DPF) 5, a selective reduction catalyst (SCR) 6, and an ammonia oxidation catalyst 7 are arranged in the exhaust passage 3 from the upstream side. Particulate matter (particulate matter) in the exhaust gas is collected and oxidized by the oxidation catalyst 4 and the DPF 5. The SCR 6 is a catalyst that selectively reduces and purifies NOx in the exhaust gas using ammonia (NH 3) as a reducing agent. NH 3 flowing out from the SCR 6 without contributing to the redox reaction in the SCR 6 is oxidized in the ammonia oxidation catalyst 7. Thereby, it is suppressed that NH3 is discharged into the atmosphere.

An injection valve 9 for injecting urea water into the exhaust passage 3 is attached to the exhaust passage 3 upstream of the SCR 6. The urea water 11 stored in the tank 10 is assembled to the injection valve 9 by the pump 12 and pressurized and supplied by the pressurizing device 15. The urea water injected from the injection valve 9 into the exhaust passage 3 is hydrolyzed by the heat of the exhaust to generate NH3. This NH3 flows into SCR6 and acts as a reducing agent. Therefore, the amount of NOx that can be reduced and purified in the SCR 6 is determined according to the urea water injection amount from the injection valve 9 and the urea water concentration in the tank 10. A temperature sensor 14 for measuring the temperature in the tank 10 is attached to the tank 10.

  A NOx sensor 8 for measuring the amount of NOx in the exhaust is attached to the exhaust passage 3 downstream of the ammonia oxidation catalyst 7. In the system of the present embodiment, the NOx amount in the exhaust gas from the internal combustion engine 1 is estimated based on the operating state of the internal combustion engine 1, and the estimated NOx amount is set as the NOx amount flowing into the SCR 6. Based on the ratio between the estimated SCR inflow NOx amount and the SCR outflow NOx amount measured by the NOx sensor 8, the NOx purification rate by the SCR 6 is calculated.

  In the present embodiment, the urea water injection amount from the injection valve 9 is adjusted by feedback control so that the NOx purification rate obtained in this way becomes a predetermined target purification rate. In this feedback control, control parameters such as feedback gain are set on the assumption that the concentration of urea water stored in the tank 10 and appropriately replenished is always constant at a predetermined reference concentration.

  The ECU 13 is an electronic control computer that controls the operating state of the internal combustion engine 1. The pressurization device 15, the injection valve 9, and the pump 12 are connected to the ECU 13, and operations of the pressurization device 15, the injection valve 9, and the pump 12 are controlled by a control signal from the ECU 13. Further, the NOx sensor 8 and the temperature sensor 14 are connected to the ECU 13, and the measured values by the NOx sensor 8 and the temperature sensor 14 are flowed into the ECU 13. In addition, information related to the operating state of the internal combustion engine 1 is input to the ECU 13. The ECU 13 performs the calculation of the NOx purification rate, the control of the urea water injection amount, the feedback control and the like based on the input information.

  Here, there is a correspondence between the freezing point of urea water and the concentration of urea water. Specifically, as shown in FIG. 2, the freezing point of urea water becomes the lowest temperature (−11 ° C.) at a concentration of 32.5%. Hereinafter, this lowest freezing point is referred to as “lowest freezing point”, and the concentration of urea water (32.5%) that gives the lowest freezing point is referred to as “standard concentration”. When the urea aqueous solution concentration is higher than the standard concentration or when the urea aqueous solution concentration is lower than the standard concentration, the freezing point of the urea aqueous solution is higher than the lowest freezing point. In the system of this embodiment, this standard concentration is adopted as the reference concentration of urea water to be stored in the tank 10. This is because by using the urea solution of the standard concentration, it is possible to suppress the urea solution from freezing as much as possible, and it is possible to perform NOx purification by the SCR 6 in a wide temperature environment.

  By the way, since replenishment of urea water to the tank 10 may be left to the user, there is a possibility that urea water having a concentration other than the reference concentration is replenished to the tank 10. In addition, there is a possibility that the urea water concentration may change from the reference concentration due to deterioration with time of the urea water. In such a case, there is a difference between the actual urea water concentration and the urea water concentration (that is, the reference concentration) assumed in the feedback control. Therefore, an appropriate NOx purification process is performed in the SCR 6. There is a risk that it will not be possible.

  Therefore, in order to appropriately execute the NOx purification process by the SCR 6, it is important to reliably detect when the urea water concentration in the tank 10 is in a deteriorated state deviating from the reference concentration. On the other hand, it is conceivable to attach a concentration sensor for measuring the urea water concentration to the tank 10 and determine the deterioration state of the urea water based on the measured value by the concentration sensor. Adding to this may lead to cost increase and system complexity.

Therefore, in the system of the present embodiment, the freezing point of the urea water in the tank 10 is acquired, and the tank 10 is based on the correspondence between the concentration of urea water and the freezing point of urea water shown in FIG. The deviation of the urea aqueous solution concentration from the reference concentration was estimated, and the deterioration state of the urea aqueous solution was determined based on the estimated deviation. Thereby, the deterioration state of the urea water in the tank 10 can be determined without using a new device such as a concentration sensor.

  Hereinafter, the deterioration state determination of urea water in the present embodiment will be described in detail.

  When the urea water in the tank 10 is frozen, the urea water cannot be injected from the injection valve 9. This appears as an abnormality occurring in the urea water injection system. For example, when the urea water freezes, the pump 12 cannot assemble the urea water in the tank 10. Therefore, in this case, although the urea water injection request is issued by the ECU 13, the urea water is not supplied to the injection valve 9, the amount of urea water in the tank 10 does not change, and the pressure in the pressurizing device 15 is excessive. It is thought that abnormalities such as a rise appear. In the present embodiment, when such an abnormality is detected, it is determined that the urea water in the tank 10 is frozen.

  Moreover, when urea water freezes and urea water injection from the injection valve 9 is no longer performed, the abnormality that the NOx purification rate by SCR6 deteriorates rapidly appears. Therefore, when the NOx purification rate calculated by the ECU 13 falls below a predetermined reference purification rate, it may be determined that the urea water in the tank 10 is frozen. This reference purification rate is a constant determined based on the lower limit value of the NOx purification rate at which it can be determined that the NOx reduction purification reaction is normally progressing in the SCR 6.

  On the other hand, when the frozen urea water is melted, it is considered that the abnormality appearing in the urea water injection system as described above is resolved. For example, the urea water supply to the injection valve 9 is performed again in a state where an abnormality occurs in which the urea water is not supplied to the injection valve 9 even though the urea water injection request is issued by the ECU 13. When it comes to it, it can be determined that the urea water has melted from the frozen state. Further, when the amount of urea water in the tank 10 changes again in a state where an abnormality such that the amount of urea water in the tank 10 does not change occurs, it can be determined that the urea water has melted from the frozen state. Moreover, when the excessive increase of the pressure in the pressurization apparatus 15 is eliminated, it can be determined that the urea water has melted from the frozen state.

  Further, when the NOx purification rate by the SCR 6 suddenly deteriorates and falls below the reference purification rate, the urea water in the tank 10 is frozen when the NOx purification rate by the SCR 6 returns to a value equal to or higher than the reference purification rate. It can be determined from the state that it has melted.

  In this way, when it is determined that the urea water is frozen, or when it is determined that the urea water is thawed from the frozen state, the temperature in the tank 10 is measured by the temperature sensor 14. At this time, the temperature measured by the temperature sensor 14 can be considered as a freezing point or a melting point of urea water.

  Then, the concentration of urea water in the tank 10 is estimated from the measured freezing point of the urea water based on the correspondence between the concentration of urea water and the freezing point shown in FIG. Here, if the freezing point of the urea water measured by the temperature sensor 14 is T (> -11 ° C.), as shown in FIG. 2, the urea water concentration corresponding to the freezing point T is the reference concentration (32 .5%), there are two possibilities: a density R1 on the lower density side and a density R2 on the higher density side than the reference density.

Here, when the estimated urea water concentration (R1 or R2) does not belong to the predetermined concentration range [R01, R02] including the reference concentration (32.5%), the urea water exceeds the allowable limit. It is determined that the battery is in a deteriorated state. Conversely, when the estimated urea water concentration belongs to this concentration range, it is determined that the urea water is not deteriorated. The concentration range allowed as the urea water concentration is a urea water concentration range in which it is possible to determine that the NOx purification process by the SCR 6 can be appropriately executed using the injection amount of urea water feedback-controlled by a predetermined control parameter as a reducing agent. As predetermined. In addition, when the estimated urea water density | concentration does not correspond with a reference | standard density | concentration, you may make it determine with urea water being in a degradation state.

  When it is determined that the urea water concentration is in a deteriorated state based on the urea water concentration estimated based on the freezing point of the urea water, for example, the tank 11 is replenished with urea water having an appropriate concentration according to the deteriorated state. It is preferable to eliminate the deterioration state.

  Here, when it is determined that the urea water is in a deteriorated state, as described above, the deviation of the urea water concentration from the reference concentration (32.5%) is a deviation toward the high concentration side or a deviation toward the low concentration side. Any of these possibilities is conceivable. In order to eliminate the deterioration state of the urea water, it is necessary to specify the tendency of deviation from the reference concentration.

  Therefore, in this embodiment, the tendency of deviation of the urea water concentration from the reference concentration is estimated based on the feedback control amount of the urea water injection amount in the feedback control of the NOx purification rate.

  Specifically, the urea water calculated based on the assumption that the urea water concentration is the reference concentration based on the deviation between the actual NOx purification rate calculated by the ECU 13 and the target purification rate in the feedback control. A feedback control amount (referred to as a reference control amount) of the injection amount is compared with a feedback control amount (referred to as an actual control amount) of the urea water injection amount actually performed by feedback control in order to eliminate the deviation.

  When the actual control amount is greater than the reference control amount, the urea water is determined to be in a deteriorated state in which the urea water concentration is shifted to a lower concentration side than the reference concentration. When the actual control amount is smaller than the reference control amount, the urea water is determined to be in a deteriorated state in which the urea water concentration is shifted to a higher concentration side than the reference concentration.

  When it is determined that the urea water is in a deteriorated state, referring to the comparison result between the actual control amount and the reference control amount in the above feedback control, the urea water concentration is finally lower than the reference concentration. It is determined whether the density R1 is shifted to the side or the density R2 is shifted to the higher density side with respect to the reference density.

  Thus, according to the present embodiment, it is possible to accurately determine the deterioration state of the urea water without using a new device such as a concentration sensor.

  The process for determining the deterioration state of the urea water according to the present embodiment described above will be described with reference to FIGS. FIG. 3 is a flowchart showing a processing routine for determining whether the urea water concentration is shifted to the high concentration side or the low concentration side with respect to the reference concentration.

  In FIG. 3, in step S101, the ECU 13 calculates the NOx purification rate, and calculates the instruction amount for urea water injection based on the deviation from the target purification rate in the feedback control. Here, the ECU 13 calculates the instruction amount for urea water injection on the assumption that the urea water concentration is the reference concentration.

  In step S102, urea water injection of the indicated amount calculated in step S101 is executed.

In step S103, the NOx purification rate as a result of the urea water injection in step S102 is calculated again and compared with the target purification rate in feedback control. Based on the comparison result, the excess or deficiency of the urea water injection instruction amount calculated in step S101 is evaluated.

  That is, if the deviation between the NOx purification rate calculated in step S103 and the target purification rate is not sufficiently reduced with respect to the deviation calculated in step S101, the urea water injection instruction amount calculated in step S101 is insufficient. It is determined that In this case, the process proceeds to step S104, and “−1” is set to the determination flag.

  If excessive NH3 is detected downstream of the ammonia oxidation catalyst 7 after the urea water injection in step S102, it is determined that the urea water injection command amount calculated in step S101 is excessive. In this case, the process proceeds to step S105, and the determination flag is set to “1”. The detection of NH3 flowing downstream from the ammonia oxidation catalyst 7 is performed, for example, by attaching an ammonia sensor to the exhaust passage 3 downstream of the ammonia oxidation catalyst 7 and measuring the amount of ammonia in the exhaust gas using the ammonia sensor. be able to. Other detection methods can also be used.

  The deviation between the NOx purification rate calculated in step S103 and the target purification rate is sufficiently reduced with respect to the deviation calculated in step S101, and a large amount of NH3 flows out downstream of the ammonia oxidation catalyst 7. If not, it is determined that the urea water injection instruction amount calculated in step S101 is not excessive or insufficient. In this case, this routine is terminated.

  Next, FIG. 4 is a flowchart showing a routine for determining the deterioration state of urea water.

  In FIG. 4, in step S201, the ECU 13 determines whether or not it has been detected that the urea water is frozen by the above-described method. When it is detected that it has been frozen, the process proceeds to step S203. If freezing is not detected, the process proceeds to step S202.

  In step S202, the ECU 13 determines whether or not it has been detected that the urea water has melted from the frozen state by the above-described method. When it is detected that the product has been thawed from the frozen state, the process proceeds to step S203. If it is not detected that the product has been thawed from the frozen state, the execution of this routine is terminated.

  In step S203, the ECU 13 measures the temperature in the tank 10 using the temperature sensor 14, and acquires the freezing point (or melting point, the same applies hereinafter) of urea water based on the measured value.

  In step S204, the ECU 13 determines the deterioration of the urea water based on the freezing point of the urea water acquired in step S203 and the correspondence between the urea water concentration and the freezing point shown in FIG. When the urea water concentration estimated from the freezing point belongs to the above-described predetermined concentration range including the reference concentration, the ECU 13 determines that the urea water is not in a deteriorated state, and ends the execution of this routine. If the urea water concentration does not belong to the predetermined concentration range, the ECU 13 determines that the urea water is in a deteriorated state, and proceeds to step S205.

  In step S205, the ECU 13 evaluates the determination flag set in the routine of FIG. If “1” is set in the determination flag, the ECU 13 proceeds to step S206 and determines that the urea water concentration is in a deteriorated state shifted to the low concentration side with respect to the reference concentration. On the other hand, when “−1” is set in the determination flag, the ECU 13 proceeds to step S207 and determines that the urea water concentration is in a deteriorated state shifted to the high concentration side with respect to the reference concentration.

  In the present embodiment, the ECU 13 that executes step S201 and step S202 corresponds to the detection means in the present invention. Moreover, ECU13 which performs step S203 is equivalent to the temperature acquisition means in this invention. ECU13 which performs step S204 is equivalent to the determination means in this invention. ECU13 which performs step S205, step S206, and step S207 is equivalent to the deterioration state determination means in this invention. ECU13 which performs step S101 and step S102 is equivalent to the purification rate acquisition means and control means in this invention.

  When the ECU 13 executes the routines shown in FIGS. 3 and 4 described above, the deterioration state of the urea water can be accurately determined. According to the deterioration determination method according to the present embodiment, it is possible to determine the deterioration state of the urea water without adding a new device such as a urea water concentration sensor. Therefore, it is possible to suppress an increase in cost and complexity of the system.

It is a figure which shows schematic structure of the exhaust gas purification system of the internal combustion engine in an Example. It is a figure which shows the correspondence of urea water density | concentration and the freezing point (melting | fusing point) of urea water. It is a flowchart showing the routine of the process which discriminate | determines whether the urea water density | concentration has shifted | deviated to the high concentration side or the low concentration side with respect to the reference concentration. It is a flowchart showing the routine of the process which determines the deterioration state of urea water.

Explanation of symbols

1 Internal combustion engine 2 Exhaust manifold 3 Exhaust passage 4 Oxidation catalyst 5 DPF
6 SCR
7 Ammonia oxidation catalyst 8 NOx sensor 9 Injection valve 10 Tank 11 Urea water 12 Pump 13 ECU
14 Temperature sensor 15 Pressurizing device

Claims (9)

An SCR installed in the exhaust passage of the internal combustion engine;
An injector for injecting urea water into an exhaust passage upstream of the SCR;
Detecting means for detecting that the urea water for injection from the injection device is frozen or melted from a frozen state;
Temperature acquisition means for acquiring the temperature of urea water when freezing of the urea water or melting from the frozen state is detected by the detection means;
Determination means for determining whether or not the concentration of urea water is in a deteriorated state deviating from a predetermined reference concentration based on the temperature acquired by the temperature acquisition means;
An exhaust gas purification system for an internal combustion engine, comprising: In claim 1,
The reference concentration is a concentration determined based on the concentration of urea water when the freezing point of urea water is lowest,
The determination means determines that the urea water is in the deteriorated state when the temperature acquired by the temperature acquisition means is higher than a predetermined reference temperature determined based on the lowest freezing point of the urea water. An exhaust purification system for an internal combustion engine. In claim 1 or 2,
A purification rate acquisition means for acquiring a NOx purification rate by the SCR;
Control means for feedback-controlling the injection amount of urea water by the injection device such that the NOx purification rate acquired by the purification rate acquisition means matches a predetermined target purification rate;
When the determination unit determines that the urea water is in the deteriorated state, the deteriorated state for determining whether the deteriorated state is a deteriorated state deviating to the higher concentration side or the lower concentration side from the reference concentration. Discrimination means;
Further comprising
The deterioration state determining means includes
When the feedback control amount of the urea water injection amount in the feedback control is greater than a predetermined reference control amount, it is determined that the urea water concentration is in a deteriorated state deviating to a lower concentration side than the reference concentration,
When the feedback control amount of the urea water injection amount in the feedback control is less than a predetermined reference control amount, it is determined that the urea water concentration is in a deteriorated state deviating to a higher concentration side than the reference concentration. An exhaust purification system for an internal combustion engine. In any one of Claims 1-3,
The injection device further includes a pressurizing device that pressurizes the urea water to inject the urea water,
The exhaust gas purification system for an internal combustion engine, wherein the detecting means detects that the urea water is frozen when an abnormality occurs in the pressurizing device. In claim 4,
The exhaust gas purification system for an internal combustion engine, wherein the detection means detects that urea water has melted from a frozen state when an abnormality occurring in the pressurizing device is resolved. In any one of Claims 1-3,
A tank for storing urea water for injection from the injection device;
A pump that sucks up urea water in the tank and supplies the urea water to the injection device;
Further comprising
The exhaust gas purification system for an internal combustion engine, wherein the detection means detects that the urea water is frozen when an abnormality occurs in the pump. In claim 6,
The exhaust gas purification system for an internal combustion engine, wherein the detection means detects that the urea water has melted from the frozen state when the abnormality that has occurred in the pump is resolved. In any one of Claims 1-3,
A purification rate acquisition means for acquiring a NOx purification rate by the SCR;
The exhaust gas purification system for an internal combustion engine, wherein the detection means detects that the urea water is frozen when the NOx purification rate acquired by the purification rate acquisition means falls below a predetermined reference purification rate. In claim 8,
The exhaust gas purification system for an internal combustion engine, wherein the detection means detects that the urea water has melted from the frozen state when the NOx purification rate that has been below the reference purification rate becomes equal to or higher than the reference purification rate. .

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