JP2014118945A - Exhaust purification device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust purification device, for an internal combustion engine, capable of diagnosing a concentration of urea water.SOLUTION: An exhaust purification device for an internal combustion engine: injects urea water through an urea water injector after a lapse of a predetermined time since a regeneration process of a diesel particulate filter is started and an exhaust gas temperature increases to not less than a predetermined temperature (S10 to S16); detects a NOx concentration in exhaust gas with a plurality of NOx sensors (S18); calculates NOx purification efficiency on the basis of detection results (S20); calculates a concentration of the urea water from the NOx purification efficiency (S22); and warns a driver that the concentration of the urea water is less than a predetermined value when the concentration of the urea water is less than the predetermined value, for example 32.5% (S24 to S26).

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine, and more particularly to quality diagnosis of urea water.

In diesel engines and lean-burn gasoline engines, the amount of nitrogen oxide (NOx) discharged into the exhaust gas increases because the mixture of fuel and air contains a large amount of oxygen.
Therefore, conventionally, in a diesel engine or a lean burn gasoline engine, a selective reduction catalyst, a NOx trap catalyst, or the like is provided in the exhaust passage. Particularly in a diesel engine, the amount of NOx emission increases during high rotation speed and high load operation. Therefore, ammonia water (NH3) generated by adding urea water to the exhaust gas in the exhaust passage and hydrolyzing the urea water A selective catalytic reduction device that reduces and purifies NOx is used.

  In such a selective catalytic reduction apparatus, a NOx sensor is disposed upstream and downstream of the selective catalytic reduction catalyst, and a technology for detecting abnormalities in the selective catalytic reduction catalyst or the NOx sensor based on the detected value of the NOx sensor is known. It has been developed (Patent Document 1).

Japanese Patent No. 4874364

In the exhaust gas purification apparatus for an internal combustion engine disclosed in Patent Document 1, abnormality is detected in the selective reduction catalyst and the NOx sensor, and NOx is purified by the selective reduction catalyst.
By the way, since the exhaust gas purification apparatus for an internal combustion engine of Patent Document 1 does not have detection means for detecting the urea concentration of urea water, when urea water whose urea concentration is not appropriate is used for the selective catalytic reduction catalyst, Even if the selective reduction catalyst and NOx sensor function normally, the amount of ammonia is insufficient relative to the NOx emission amount, and NOx is not properly reduced and purified by the selective reduction catalyst, but NOx is discharged. In addition, there is a possibility that the amount of ammonia increases with respect to the NOx emission amount, and ammonia is discharged without using all ammonia for NOx reduction purification in the selective reduction catalyst.

Thus, it is not preferable that NOx or ammonia is discharged into the atmosphere.
The present invention has been made to solve such problems, and an object of the present invention is to provide an exhaust purification device for an internal combustion engine that can diagnose the concentration of urea water.

  In order to achieve the above object, in the exhaust gas purification apparatus for an internal combustion engine according to claim 1, a selective reduction type that reduces and purifies nitrogen oxides contained in the exhaust gas of the internal combustion engine with urea water supplied by urea water supply means. A catalyst, first concentration detecting means for detecting the concentration of the nitrogen oxide flowing into the selective catalytic reduction catalyst, and second concentration detecting means for detecting the concentration of the nitrogen oxide flowing out of the selective catalytic reduction catalyst. , Hydrocarbon desorption means for desorbing hydrocarbons adsorbed on the selective catalytic reduction catalyst, and after completion of the hydrocarbon desorption processing by the hydrocarbon desorption means, the first concentration detection means and the Concentration determining means for determining the concentration of the urea water based on the detection result of the second concentration detecting means.

Further, in the exhaust gas purification apparatus for an internal combustion engine according to claim 2, the hydrocarbon desorption means according to claim 1, wherein the hydrocarbon desorption means is based on an adsorption amount of the hydrocarbon adsorbed on the selective reduction catalyst. The desorption treatment is performed.
According to a third aspect of the present invention, there is provided an exhaust purification apparatus for an internal combustion engine according to the first or second aspect, further comprising a diesel particulate filter for removing particulate matter in the exhaust gas upstream of the urea water supply means. The filter regeneration process also serves as the hydrocarbon desorption process.

  According to a fourth aspect of the present invention, there is provided the exhaust gas purification apparatus for an internal combustion engine according to the third aspect, wherein the concentration determining means is in the regeneration process of the diesel particulate filter and the urea is filtered after a lapse of a predetermined time from the start of the regeneration process. It is characterized by carrying out water concentration determination.

According to the first aspect of the present invention, after the desorption treatment of the hydrocarbons adsorbed on the selective catalytic reduction catalyst is performed, the urea water supply means is used based on the detection results of the first concentration detection means and the second concentration detection means. The concentration of the supplied urea water is determined.
Thus, by desorbing the hydrocarbons adsorbed on the selective catalytic reduction catalyst prior to the determination of the urea water concentration, it is possible to prevent a reduction in the purification efficiency of nitrogen oxides due to the adsorption of hydrocarbons.

Therefore, when the concentration of urea water is determined, the nitrogen oxide purification efficiency of the selective catalytic reduction catalyst can be stabilized, so that the concentration of urea water can be accurately determined.
According to the second aspect of the invention, the hydrocarbon desorption means performs the hydrocarbon desorption process based on the hydrocarbon adsorption amount of the selective catalytic reduction catalyst.
Therefore, for example, when the adsorption amount of the hydrocarbon adsorbed on the selective catalytic reduction catalyst is large, the hydrocarbon can be surely desorbed by lengthening the hydrocarbon desorption treatment time.

Therefore, since the hydrocarbon adsorbed on the selective catalytic reduction catalyst can be desorbed before the urea water concentration is determined, the urea water concentration can be determined with high accuracy.
According to the invention of claim 3, the regeneration process of the diesel particulate filter also serves as the desorption process of the hydrocarbon adsorbed on the selective catalytic reduction catalyst.
That is, the exhaust gas temperature in the regeneration process of the diesel particulate filter is equal to or higher than the temperature at which the hydrocarbons are desorbed from the selective reduction catalyst. Therefore, the regeneration process of the diesel particulate filter is performed by the carbonization adsorbed on the selective reduction catalyst. It can also serve as a hydrogen desorption process.

Therefore, by performing the regeneration process of the diesel particulate filter based on the adsorption amount of the hydrocarbon adsorbed on the selective catalytic reduction catalyst, the hydrocarbon can be desorbed reliably and the urea water can be accurately obtained. The concentration can be determined.
According to the invention of claim 4, during the regeneration process of the diesel particulate filter, the concentration of urea water is determined after a lapse of a predetermined time from the start of the regeneration process.

During the regeneration process of the diesel particulate filter, the temperature of the exhaust gas is high in order to burn the particulate matter in the diesel particulate filter. The temperature of the exhaust gas is generally higher than the temperature at which urea water is hydrolyzed and ammonia production starts.
Therefore, the amount of ammonia produced is greater than that of urea water, that is, the ammonia production rate is increased.

Therefore, the ammonia production rate from the urea water is high, and it is possible to suppress the urea water from being discharged from the selective catalytic reduction catalyst without being hydrolyzed, so that the second concentration detection means discharges from the selective catalytic reduction catalyst. Only the amount of nitrogen oxide discharged can be accurately detected.
Further, during the regeneration process of the diesel particulate filter, the exhaust gas temperature is kept high, and thus the amount of nitrogen oxide discharged increases.

Therefore, since the amount of nitrogen oxide flowing into the selective reduction catalyst is large, it is possible to easily determine the change in the purification efficiency of nitrogen oxide in the selective reduction catalyst.
Therefore, the nitrogen oxide purification efficiency of the selective reduction catalyst is calculated based on the detection results of the first concentration detection means and the second concentration detection means, and the concentration of urea water is determined from the purification efficiency. The concentration of urea water can be accurately determined.

  In addition, by determining the urea water concentration after a predetermined time from the start of the regeneration process of the diesel particulate filter, the desorption of hydrocarbons adsorbed on the selective catalytic reduction catalyst is ensured before the urea water concentration determination. It can be carried out.

1 is a schematic configuration diagram of an engine to which an exhaust gas purification apparatus for an internal combustion engine according to the present invention is applied. It is a control flowchart of the urea concentration determination which the engine control unit of the exhaust gas purification apparatus of the internal combustion engine which concerns on this invention performs.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram of an engine 1 to which an exhaust gas purification apparatus for an internal combustion engine is applied.
As shown in FIG. 1, an engine (internal combustion engine) 1 mounted on a vehicle (not shown) is a multi-cylinder direct injection internal combustion engine (for example, a common rail type diesel engine), and more specifically, pressure is accumulated on the common rail. High pressure fuel is supplied to the fuel injection nozzle 2 of each cylinder, and can be injected from the fuel injection nozzle 2 into the combustion chamber 3 of each cylinder at an arbitrary injection timing and injection amount.

Each cylinder of the engine 1 is provided with a piston 4 that can slide up and down. The piston 4 is connected to the crankshaft 6 via a connecting rod 5. A flywheel is provided at one end of the crankshaft 6.
An intake port 7 and an exhaust port 8 are communicated with the combustion chamber 3.
The intake port 7 is provided with an intake valve 9 that communicates and shuts off the combustion chamber 3 and the intake port 7. Further, the exhaust port 8 is provided with an exhaust valve 10 for performing communication between the combustion chamber 3 and the exhaust port 8 and shutting off.

An intake manifold 11 that distributes the sucked air to each cylinder is provided upstream of the intake port 7 so as to communicate therewith. Further, an exhaust manifold 12 that collects exhaust gas discharged from each cylinder is provided downstream of the exhaust port 8.
The intake manifold 11 upstream of the branch for distributing the intake air to each cylinder of the intake manifold 11 is provided with an oxygen concentration sensor 13 for detecting the oxygen concentration so that the sensor portion projects into the intake manifold 11. . Further, an intake air temperature sensor 14 for detecting the temperature of intake air taken into the combustion chamber 3 is provided downstream of the air-fuel ratio sensor 13 so as to protrude into the intake manifold 11.

  The intake manifold 11 and the exhaust manifold 12 are provided with a high-pressure exhaust gas recirculation passage 15 for returning a part of the high-temperature and high-pressure exhaust gas to the intake air, that is, for introducing the high-temperature and high-pressure exhaust gas recirculation gas into the intake air so as to communicate with each other. ing. The high-pressure exhaust gas recirculation passage 15 is connected to the intake manifold 11 upstream of the oxygen concentration sensor 13 via an exhaust gas recirculation valve 16 that adjusts the amount of high-temperature, high-pressure exhaust gas returning to the intake air, that is, the flow rate of the exhaust gas recirculation gas. ing. Further, the high-pressure exhaust gas recirculation passage 15 is provided with an exhaust gas recirculation cooler 17 that cools the exhaust gas introduced into the intake manifold 11.

  Upstream of the intake manifold 11, an air cleaner 18 that removes fresh dust sucked from the uppermost stream, an intercooler 20 that cools the compressed fresh air, and a flow rate of the fresh air are adjusted. An electronic control throttle valve 39 for adjusting the flow rate of low-pressure exhaust gas recirculation gas introduced from a low-pressure exhaust gas recirculation passage 41, which will be described later, and a turbocharger 19 that compresses fresh air sucked in using exhaust energy. The compressor housing that is not connected is connected to the intake manifold 11 via the intake pipe 21. An electronically controlled throttle valve 22 for adjusting the flow rate of the exhaust gas recirculation gas introduced from the high-pressure exhaust gas recirculation passage 15 is disposed between the intake manifold 11 and the intake pipe 21. The electronically controlled throttle valves 22 and 39 are provided with throttle position sensors 23 and 40 for detecting the degree of opening of the throttle valve.

The intake pipe 21 between the air cleaner 18 of the intake pipe 21 and the compressor housing of the turbocharger 19 is provided with an intake temperature sensor 38 that detects the temperature of the intake air so as to protrude into the intake pipe 21. Yes.
Downstream of the exhaust manifold 12, a turbine housing (not shown) for introducing exhaust gas into the turbocharger 19 and an exhaust pipe 24 are provided so as to communicate with each other.

  The exhaust pipe 24 includes an oxidation catalyst 25 that sequentially oxidizes components to be oxidized in the exhaust gas from the upstream side, a diesel particulate filter 26 that collects and burns particulate matter mainly composed of graphite in the exhaust gas, and exhaust gas. A selective reduction catalyst 28 for reducing and purifying nitrogen oxide (hereinafter referred to as NOx) therein using ammonia is provided so as to communicate therewith. Note that the oxidation catalyst 25 and the diesel particulate filter 26 are disposed in a casing 29. The selective reduction catalyst 28 is disposed in the casing 30.

The selective catalytic reduction catalyst 28 is supplied with ammonia generated by hydrolysis of urea water injected from the urea water injector 36 to reduce and purify NOx in the exhaust gas.
Between the turbocharger 19 of the exhaust pipe 24 and the casing 29, an oxygen concentration sensor 31 for detecting the oxygen concentration in the exhaust gas is provided so as to protrude into the exhaust pipe 24.
An exhaust gas temperature sensor 32 that detects the temperature of the exhaust gas flowing into the oxidation catalyst 25 is provided upstream of the oxidation catalyst 25 in the casing 29 so as to protrude into the casing 29. An exhaust temperature sensor 33 that detects the temperature of the exhaust gas flowing out from the oxidation catalyst 25 is provided between the oxidation catalyst 25 of the casing 29 and the diesel particulate filter 26 so as to protrude into the casing 29. An exhaust gas temperature sensor 34 that detects the temperature of the exhaust gas flowing out from the diesel particulate filter 26 is provided downstream of the diesel particulate filter 26 in the casing 29 so as to project into the casing 29.

  Between the diesel particulate filter 26 and the selective reduction catalyst 28 in the exhaust pipe 24, that is, between the casing 29 and the casing 30 of the exhaust pipe 24, a NOx sensor (first concentration) that detects the concentration of NOx in the exhaust gas. Detection means) 35 is provided so as to protrude into the exhaust pipe 24. A urea water injector (urea water supply means) 36 is provided between the NOx sensor 35 of the exhaust pipe 24 and the casing 30 so as to protrude into the exhaust pipe 24. Note that the detection value of the NOx sensor 35 is influenced by ammonia due to the characteristics of the sensor. That is, the detected value of the NOx sensor 35 outputs a high detected value (NOx concentration) when the concentration of ammonia becomes high even if the concentration of NOx in the exhaust gas is constant.

  The urea water injector 36 injects urea water for supplying ammonia to the selective reduction catalyst 28 into the exhaust pipe 24. The urea water is supplied from a urea water tank (not shown) provided outside. The distance from the urea water injector 36 to the selective catalytic reduction catalyst 28 is set to be longer than the distance necessary for the urea water injected from the urea water injector 36 to be hydrolyzed to generate ammonia. Therefore, the urea water injected from the urea water injector 36 undergoes hydrolysis and generates ammonia before reaching the selective reduction catalyst 28.

  A NOx sensor (second concentration detection means) 37 that detects the concentration of NOx in the exhaust gas flowing out from the selective reduction catalyst 28 is disposed downstream of the selective reduction catalyst 28 in the exhaust pipe 24, that is, downstream of the casing 30. It is provided so as to protrude into 24. Similar to the NOx sensor 35, the NOx sensor 37 is affected by ammonia due to the characteristics of the sensor. That is, the detection value of the NOx sensor 37 outputs a high detection value (NOx concentration) when the concentration of ammonia becomes high even if the concentration of NOx in the exhaust gas is constant.

  A part of the low-temperature and low-pressure exhaust gas is returned to the intake air so that the intake pipe 21 between the electronically controlled throttle valve 39 and the turbocharger 19 and the exhaust pipe 24 downstream of the diesel particulate filter 29 communicate with each other. That is, a low-pressure exhaust gas recirculation passage 41 is provided for introducing low-temperature and low-pressure exhaust gas recirculation gas into the intake air. The low-pressure exhaust gas recirculation passage 41 includes an exhaust gas recirculation valve 42 that adjusts the amount of exhaust gas that returns to the intake air, that is, the flow rate of the exhaust gas recirculation gas, an exhaust gas recirculation cooler 43 that cools the exhaust gas that is returned to the intake air, And an exhaust gas recirculation filter 44 for removing water.

The warning device 50 is arranged in the passenger compartment and urges the driver to warn that the concentration of the urea water stored in the urea water tank is abnormal by uttering a voice or lighting a warning light. is there.
The fuel injection nozzle 2, oxygen concentration sensors 13, 31, intake air temperature sensors 14, 38, exhaust gas recirculation valves 16, 42, electronically controlled throttle valves 22, 39, throttle position sensors 23, 40, exhaust gas temperature sensors 32, 33, 34, NOx sensors 35 and 37, urea water injector 36, alarm device 50, various sensors for detecting the operating state of the engine 1, and an accelerator for detecting the degree of operation of an accelerator pedal operated by a driver of the vehicle on which the engine 1 is mounted. Various devices such as a position sensor are control devices for performing overall control of the engine 1, and are input / output devices, storage devices (ROM, RAM, nonvolatile RAM, etc.), timers, and a central processing unit (CPU). Engine control unit (hydrocarbon desorption means, concentration determination means) 6 It is electrically connected to the. The engine control unit 60 controls the operation of the engine 1 by controlling the operation of various devices based on information from various sensors.

On the input side of the engine control unit 60, oxygen concentration sensors 13, 31, intake temperature sensors 14, 38, throttle position sensors 23, 40, exhaust temperature sensors 32, 33, 34, NOx sensors 35, 37, and accelerator position sensor. Sensors such as these are electrically connected, and detection information from these various devices and various sensors is input.
On the other hand, on the output side of the engine control unit 60, the fuel injection nozzle 2, the exhaust gas recirculation valves 16, 42, the electronic control throttle valves 22, 39, the urea water injector 36, and the alarm device 50 are electrically connected.

The engine control unit 60 then determines the fuel injection amounts and injection timings of the pre-injection, main injection, and after-injection from the fuel injection nozzle 2 and the exhaust recirculation valves 16 and 42 and the electronically controlled throttle valve based on the detection values of the sensors. The opening degree of 22, 39 and the injection amount of urea water from the urea water injector 36 are optimally controlled, and the engine 1 is controlled with high accuracy.
The engine control unit 60 calculates a pressure difference from the pressure of exhaust gas upstream and downstream of the diesel particulate filter 26 detected by a pressure sensor (not shown), and when the pressure difference becomes a predetermined pressure difference, When a forced regeneration button (not shown) or the like is operated by the control, the diesel particulates which control the fuel injection amount and the injection timing from the fuel injection nozzle 2 and burn the particulate matter deposited in the diesel particulate filter 26 Perform filter regeneration processing.

  Further, the engine control unit 60 injects urea water from the urea water injector 36 during regeneration processing of the diesel particulate filter, and based on the detection values of the NOx sensor 35 and the NOx sensor 37, the NOx of the selective catalytic reduction catalyst 28 is reduced. The NOx purification efficiency which is the purification efficiency is calculated, and the urea concentration of the urea water, that is, the urea water concentration is calculated based on the NOx purification efficiency, the urea water injection amount and the exhaust gas temperature. Then, the engine control unit 60 performs urea concentration determination for determining whether or not the urea aqueous solution concentration being used is a prescribed urea aqueous solution concentration based on the urea aqueous solution concentration. The urea water concentration is calculated from the map by storing the relationship between the NOx purification efficiency, the urea water injection amount, the exhaust gas temperature, and the urea water concentration in the engine control unit 60 based on experiments and analysis in advance. Is done.

Next, urea concentration determination in the engine control unit 60 of the exhaust gas purification apparatus for an internal combustion engine according to the present invention will be described.
FIG. 2 is a control flowchart of urea concentration determination in the engine control unit 60 of the exhaust gas purification apparatus for an internal combustion engine according to the present invention.
As shown in FIG. 2, in step S10, it is determined whether or not the reproduction process is started. Specifically, it is determined whether or not the regeneration process of the diesel particulate filter 26 is started. If the determination result is true (Yes) and the regeneration process of the diesel particulate filter 26 is started, the process proceeds to step S12. If the determination result is NO (No) and the regeneration process of the diesel particulate filter 26 has not been started, the routine returns.

  In step S12, it is determined whether or not the exhaust gas temperature is equal to or higher than a predetermined temperature (for example, 400 ° C.). Specifically, the exhaust gas temperature between the urea water injector 36 and the selective reduction catalyst 28 is estimated and estimated based on the temperature of the exhaust gas discharged from the diesel particulate filter 26 detected by the exhaust gas temperature sensor 34. Whether the exhaust gas temperature is equal to or higher than a predetermined temperature is determined. If the exhaust gas temperature estimated with the determination result being true (Yes) is equal to or higher than the predetermined temperature, the process proceeds to step S14. If the exhaust gas temperature estimated as a result of the determination is NO (No) is not equal to or higher than a predetermined temperature, this routine is returned. The predetermined temperature is equal to or higher than the temperature at which the hydrocarbon (HC) adsorbed on the selective reduction catalyst 28 can be desorbed, and the urea water injected from the urea water injector 36 is hydrolyzed to become ammonia. Set as above.

  In step S14, it is determined whether or not a predetermined time has elapsed. Specifically, it is determined whether or not the regeneration process of the diesel particulate filter 26 has been started and a predetermined time has elapsed after the exhaust gas temperature has become equal to or higher than the predetermined temperature. If the determination result is true (Yes) and a predetermined time has elapsed, the process proceeds to step S16. If the determination result is NO (No) and the predetermined time has not elapsed, this routine is returned. The predetermined time is set to be equal to or longer than the time during which HC adsorbed on the selective reduction catalyst 28 can be completely desorbed based on the amount of HC adsorbed on the selective reduction catalyst 28. That is, HC adsorbed on the selective catalytic reduction catalyst 28 can be completely desorbed when the exhaust gas temperature becomes equal to or higher than a predetermined temperature and a predetermined time elapses. Further, the amount of HC adsorbed on the selective catalytic reduction catalyst 28 is determined in advance through experiments, analyzes, etc., when the exhaust gas temperature in the regeneration processing interval of the diesel particulate filter 26 is below a predetermined temperature (for example, 200 ° C. or 300 ° C.). The amount of adsorption of HC adsorbed by the selective catalytic reduction catalyst 28 with respect to the accumulated value of time is calculated and stored as a map, and the exhaust gas temperature in the interval between the map and the actual regeneration process of the diesel particulate filter 26 is stored. The actual amount of HC adsorbed on the selective catalytic reduction catalyst 28 is calculated based on the accumulated value of the time during which the temperature is equal to or lower than the predetermined temperature.

In step S16, urea water injection from the urea water injector 36 is started. That is, urea water is injected from the urea water injector 36, ammonia generated by hydrolysis of the urea water is supplied to the selective reduction catalyst 28, and NOx is purified by the selective reduction catalyst 28. Then, the process proceeds to step S18.
In step S18, the NOx sensor 35 and the NOx sensor 37 detect the NOx concentration in the exhaust gas. Then, the process proceeds to step S20.

  In step S20, NOx purification efficiency is calculated. More specifically, the NOx concentration difference is calculated by subtracting the NOx concentration detected by the NOx sensor 37 from the NOx concentration detected by the NOx sensor 35. Then, the calculated NOx concentration difference is divided by the NOx concentration detected by the NOx sensor 35 and further multiplied by 100 to calculate the NOx purification efficiency. Then, the process proceeds to step S22.

  In step S22, the urea water concentration is calculated. Specifically, the NOx purification efficiency detected in step S20, the total urea water injection amount from the urea water injector 36, and the urea water concentration that is the urea concentration of urea water injected from the urea water injector 36 from the exhaust gas temperature. Is calculated. Then, the process proceeds to step S24. Note that the urea water concentration here is preliminarily stored as a map of the relationship between the urea water concentration and the NOx purification efficiency with the total urea water injection amount and the exhaust gas temperature as variables, through experiments and analysis, and the like. To calculate. The total urea water injection amount is an integrated value of the urea water injected when calculating the NOx purification efficiency.

  In step S24, it is determined whether or not the urea water is abnormal. Specifically, it is determined whether or not the urea water concentration calculated in step S22 is less than a specified value (for example, 32.5%). If the determination result is true (Yes) and the urea water concentration is less than a specified value (for example, 32.5%), the process proceeds to step S26. If the determination result is NO (No) and the urea aqueous solution concentration is not less than a specified value (for example, 32.5%), this routine is returned.

In step S26, the warning device 50 warns the driver that the urea water concentration is less than the specified value. Then, this routine is returned.
As described above, in the exhaust gas purification apparatus for an internal combustion engine according to the present invention, when the regeneration process of the diesel particulate filter 26 is started and the exhaust gas temperature becomes equal to or higher than a predetermined temperature, the urea water is supplied from the urea water injector 36 when a predetermined time elapses. The ammonia generated by the injection and hydrolysis of the urea water is supplied to the selective reduction catalyst 28, and the selective reduction catalyst 28 purifies NOx. The NOx sensor 35 and the NOx sensor 37 respectively detect the NOx concentration in the exhaust gas, and calculate the NOx purification efficiency based on the detection result. Then, the urea water concentration is calculated from the NOx purification efficiency, and if the urea water concentration is less than a specified value (for example, 32.5%), the warning device 50 informs the driver that the urea water concentration is less than the specified value. I am trying to warn you.

Before determining the concentration of urea water, the regeneration process of the diesel particulate filter 26 is started, and the exhaust gas temperature not lower than a predetermined temperature is maintained for a predetermined time, so that the hydrocarbon (HC) adsorbed on the selective catalytic reduction catalyst 28 is removed. It can be detached.
Therefore, it is possible to prevent a reduction in nitrogen oxide (NOx) purification efficiency due to adsorption of HC to the selective reduction catalyst 28.

Therefore, when the concentration of urea water is determined, the NOx purification efficiency of the selective catalytic reduction catalyst 28 can be stabilized, so that the concentration of urea water can be accurately determined.
Further, since the predetermined time is set based on the adsorption amount of HC adsorbed on the selective reduction purification catalyst 28, for example, when the adsorption amount of HC is large, the regeneration processing time of the diesel particulate filter 26 is set. By increasing the length, HC can be reliably desorbed from the selective reduction purification catalyst 28.

Further, during the regeneration process of the diesel particulate filter 26, in order to burn the particulate matter (PM) in the diesel particulate filter 26, the exhaust gas temperature is higher than the temperature at which the urea water is hydrolyzed and generation of ammonia is started. It is high.
Therefore, the amount of ammonia produced is greater than that of urea water, that is, the ammonia production rate is increased.

Therefore, the ammonia production rate from the urea water is high, and it is possible to suppress the urea water from being discharged from the selective reduction catalyst 28 without being hydrolyzed, so that the NOx sensor 37 discharges from the selective reduction catalyst 28. Only the amount of NOx emitted can be detected accurately.
Further, during the regeneration process of the diesel particulate filter, the exhaust gas temperature is high, and thus the amount of NOx emission increases.

Therefore, since the inflow amount of NOx flowing into the selective reduction catalyst 28 is large, it is possible to easily determine the change in the NOx purification efficiency in the selective reduction catalyst 28.
Therefore, by calculating the NOx purification efficiency of the selective reduction catalyst 28 based on the detection results of the NOx sensor 35 and the NOx sensor 37, and determining the concentration of the urea water from the purification efficiency, the urea water can be accurately obtained. Can be determined.

Although the description of the embodiment of the invention is finished as above, the embodiment of the present invention is not limited to the above embodiment.
In the above embodiment, the engine 1 is provided with the exhaust gas recirculation device that recirculates the exhaust of the high pressure exhaust gas recirculation passage 15 and the low pressure exhaust gas recirculation passage 41 to the intake air. Needless to say, the present invention is also applicable to an internal combustion engine having only one of the exhaust gas recirculation devices, or an internal combustion engine not equipped with any exhaust gas recirculation device.

1 engine (internal combustion engine)
26 Diesel particulate filter 28 Selective reduction catalyst 35 NOx sensor (first concentration detection means)
36 Urea water injector (urea water supply means)
37 NOx sensor (second concentration detection means)
60 Engine control unit (hydrocarbon desorption means, concentration determination means)

Claims (4)

A selective reduction catalyst for reducing and purifying nitrogen oxides contained in exhaust gas of an internal combustion engine with urea water supplied by urea water supply means;
First concentration detecting means for detecting the concentration of the nitrogen oxide flowing into the selective catalytic reduction catalyst;
Second concentration detecting means for detecting the concentration of the nitrogen oxide flowing out from the selective catalytic reduction catalyst;
Hydrocarbon desorption means for desorbing hydrocarbons adsorbed on the selective catalytic reduction catalyst;
After completion of the hydrocarbon desorption process by the hydrocarbon desorption means, based on the purification efficiency of the nitrogen oxides calculated based on the detection results of the first concentration detection means and the second concentration detection means, An exhaust gas purification apparatus for an internal combustion engine, comprising: concentration determination means for determining the concentration of the urea water.   2. The internal combustion engine according to claim 1, wherein the hydrocarbon desorption unit performs the hydrocarbon desorption process based on the adsorption amount of the hydrocarbon adsorbed on the selective catalytic reduction catalyst. Engine exhaust purification system. An upstream of the urea water supply means, comprising a diesel particulate filter that removes particulate matter in the exhaust gas,
The exhaust gas purification apparatus for an internal combustion engine according to claim 1 or 2, wherein the regeneration process of the diesel particulate filter also serves as a desorption process of the hydrocarbon.   4. The concentration determination unit according to claim 3, wherein the concentration determination unit performs the concentration determination of the urea water during a regeneration process of the diesel particulate filter and after a predetermined time has elapsed since the start of the regeneration process. An exhaust purification device for an internal combustion engine.

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