Exhaust gas control apparatus for internal combustion engine

Abstract

An exhaust gas control apparatus for an internal combustion engine includes an addition valve, a tank, a urea water passage, a pump, a nitrogen oxides catalyst, and an electronic control unit. The electronic control unit is configured to: control the pump, as a first control, such that a specified amount of the urea water stored in the addition valve and the urea water passage is returned to the tank after the internal combustion engine is stopped; control the pump, as a second control, such that the specified amount or more of the urea water in the tank is discharged from the tank to the urea water passage after the first control is executed; and control the pump, as a third control, such that all the urea water stored in the addition valve and the urea water passage is returned to the tank after the second control is executed.

What is claimed is 
     
         1 . An exhaust gas control apparatus for an internal combustion engine, the exhaust gas control apparatus comprising:
 an addition valve configured to add urea water to an exhaust passage of the internal combustion engine;   a tank configured to store the urea water;   a urea water passage that connects the addition valve and the tank so as to circulate the urea water;   a pump configured to circulate the urea water to the urea water passage;   a selective reduction-type nitrogen oxides catalyst disposed in a portion of the exhaust passage on a downstream side of the addition valve, the selective reduction-type nitrogen oxides catalyst being configured to purify nitrogen oxides in the exhaust passage using the urea water; and   an electronic control unit configured to:   (i) control the pump, as a first control, such that a specified amount of the urea water stored in the addition valve and the urea water passage is returned to the tank after the internal combustion engine is stopped;   (ii) control the pump, as a second control, such that the specified amount or more of the urea water in the tank is discharged from the tank to the urea water passage after the first control is executed; and   (iii) control the pump as a third control, such that all the urea water stored in the addition valve and the urea water passage is returned to the tank after the second control is executed.   
     
     
         2 . The exhaust gas control apparatus according to  claim 1 , wherein the specified amount in the first control is smaller than a capacity of the addition valve and the urea water passage. 
     
     
         3 . The exhaust gas control apparatus according to  claim 1 , wherein:
 the addition valve or the urea water passage includes a radiator configured to radiate heat from the addition valve or the urea water passage; and   the specified amount in the first control is equal to or more than a capacity of a portion extending from a front end of the addition valve to an end portion of the radiator on a tank side.   
     
     
         4 . The exhaust gas control apparatus according to  claim 1 , wherein:
 the urea water passage is higher in resistance against corrosion by ammonia than the addition valve; and   the specified amount in the first control is the capacity of the addition valve.   
     
     
         5 . The exhaust gas control apparatus according to  claim 1 , wherein:
 the electronic control unit is configured to execute the first control, the second control, and the third control when a temperature of exhaust gas at a time of stopping the internal combustion engine is equal to or more than a predetermined temperature; and   the electronic control unit is configured to control the pump, as fourth control, such that all the urea water stored in the addition valve and the urea water passage returns to the tank without executing the first control, the second control, and the third control, when the temperature of the exhaust; gas at the time of stopping the internal combustion engine is less than the predetermined temperature.   
     
     
         6 . The exhaust gas control apparatus according to  claim 1 , wherein the electronic control unit is configured to make a period from time of ending the first control to time of starting the second control shorter and make a period from time of ending the second control to time of starting the third control longer as the temperature of the exhaust gas at the time of stopping the internal combustion engine is higher. 
     
     
         7 . The exhaust gas control apparatus according to  claim 1 , wherein the electronic control unit is configured to control the pump, as the second control, such that the specified amount or more of the urea water in the tank is discharged to the urea water passage and that the temperature of the exhaust gas becomes lower than an allowable temperature.