Patent Publication Number: US-2010122525-A1

Title: Exhaust purification control device and exhaust purification system of internal combustion engine

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on and incorporates herein by reference Japanese Patent Application No. 2008-294029 filed on Nov. 18, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an exhaust purification control device of an internal combustion engine and to an exhaust purification system having the exhaust purification control device, wherein the exhaust purification control device is applied to an exhaust purification device having a purification device provided in an exhaust passage of the internal combustion engine for purifying nitrogen oxides in exhaust gas and an addition device for adding a reducing agent into the exhaust gas upstream of the purification device and performs purification control of the nitrogen oxides with the purification device while adjusting addition quantity of the reducing agent based on operation of the addition device. 
     2. Description of Related Art 
     In recent years, development of an exhaust purification system (urea SCR system) using a selective reduction catalyst (SCR: selective catalytic reduction), which selectively purifies NOx (nitrogen oxides) in exhaust gas by using urea solution as a reducing agent in an in-vehicle internal combustion engine (specifically, diesel engine), has been advanced, and such the exhaust purification system (urea SCR system) has been partly put into practical use. In the urea SCR system, a selective reduction NOx catalyst is provided in an exhaust pipe connected to an engine main body, and a urea solution addition valve for adding the urea solution (urea aqueous solution) as a NOx reducing agent into the exhaust pipe is provided upstream of the NOx catalyst. 
     In the above-described system, the urea solution is added by the urea solution addition valve into the exhaust pipe and thus NOx in the exhaust gas is selectively reduced and removed on the NOx catalyst. More specifically, ammonia (NH3) is generated when the urea solution is hydrolyzed with exhaust heat and is adsorbed to the NOx catalyst, thereby causing a reduction reaction on the NOx catalyst using the ammonia. Thus, NOx is reduced and purified. 
     When exhaust gas temperature of the internal combustion engine is low, there is a possibility that an efficiency of the hydrolysis from the urea solution to the ammonia lowers and urea pyrolysates such as a cyanuric acid deposit in an exhaust passage. The deposit turns into the ammonia if the exhaust gas temperature increases. Therefore, when the deposit has accumulated in the exhaust passage, there is a possibility that the ammonia supplied to the SCR becomes excessive with the increase of the exhaust gas temperature and controllability of the ammonia supply quantity to the SCR lowers. 
     Therefore, conventionally, there has been proposed a scheme that provides a bypass passage to the exhaust passage for passing small quantity of exhaust gas and provides a hydrolysis catalyst of urea solution and a heater in the bypass passage, e.g., as described in Patent document 1 (JPA-2007-327377). Thus, when the exhaust gas temperature is low, the ammonia is extracted from the urea solution through the bypass passage and is supplied to the NOx catalyst, thereby suitably inhibiting or avoiding the deposition of the urea pyrolysate in the exhaust passage. 
     Patent document 2 (JP-A-2007-239500) describes another conventional exhaust purification control device. 
     The above-described conventional technology employs the additional hardware such as the bypass passage, the hydrolysis catalyst and the heater for the NOx purification control in the low exhaust gas temperature range. Accordingly, lowering of cost performance is not ignorable. Furthermore, there is also a problem of increase in energy consumption since the heater is used. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an exhaust purification control device of an internal combustion engine and an exhaust purification system having the exhaust purification control device, wherein the exhaust purification control device performs purification control of nitrogen oxides with a purification device by operating an addition device adding a reducing agent into exhaust gas upstream of the purification device and is capable of suitably inhibiting accumulation of deposit in an exhaust passage due to the addition of the reducing agent, while inhibiting increase in the number of parts. 
     According to a first example aspect of the present invention, an exhaust purification control device of an internal combustion engine is applied to an exhaust purification device having a purification device provided in an exhaust passage of the internal combustion engine for purifying nitrogen oxides in exhaust gas and an addition device for adding a reducing agent into the exhaust gas upstream of the purification device and performs purification control of the nitrogen oxides with the purification device while adjusting addition quantity of the reducing agent based on operation of the addition device. The exhaust purification control device has an estimating section for estimating accumulation quantity of deposit on an inner wall of the exhaust passage resulting from the addition of the reducing agent and a decreasing section for compulsorily decreasing the addition quantity of the reducing agent when at least one of a condition that the estimated accumulation quantity is equal to or larger than a specified value and a condition that increase speed of the accumulation quantity is equal to or higher than specified speed is established. 
     The addition of the reducing agent by the addition device is performed to purify the nitrogen oxides. Therefore, if the addition quantity of the reducing agent is decreased, there is a possibility that a purification rate of the nitrogen oxides lowers. However, the inventors of the present invention found that a degree of the lowering of the purification rate of the nitrogen oxides is small or negligible even if the addition quantity is decreased under a situation where the accumulation quantity of the deposit onto the inner wall of the exhaust passage is large or under a situation where the increase speed of the accumulation quantity is high. That is, under the situation where the accumulation quantity of the deposit is large, adsorption quantity of the reducing substance to the purification device is large. Therefore, even if the addition quantity of the reducing agent is decreased, the shortfall of the addition quantity for the purification of the nitrogen oxides is compensated by the reducing substance having been adsorbed to the purification device. Under the situation where the increase speed of the accumulation of the deposit is high, the exhaust gas temperature is low and therefore the quantity of the nitrogen oxides in the exhaust gas is small. 
     In view of this point, according to the above-described first example aspect of the present invention, the addition quantity of the reducing agent is compulsorily decreased when at least one of the above-described conditions is established. Accordingly, the accumulation of the deposit on the inner waif of the exhaust passage can be suitably inhibited. 
     According to a second example aspect of the present invention, the exhaust purification control device further has an exhaust gas temperature increasing section for increasing exhaust gas temperature of the internal combustion engine to temperature capable of removing the deposit when the estimated accumulation quantity is equal to or larger than a predetermined value. 
     For example, when request torque of the internal combustion engine increases, the deposit on the inner wall of the exhaust passage decomposes and is supplied to the purification device since the exhaust gas temperature increases. If the quantity of the deposit on the inner wall of the exhaust passage is excessively large, excessive quantity of the reducing substance is supplied to the purification device, e.g., when the request torque increases. In such the case, there is a possibility that the large quantity of the reducing substance flows out to a downstream side of the purification device. 
     In view of this point, according to the above-described second example aspect of the present invention, the exhaust gas temperature is increased when the accumulation quantity on the inner wall of the exhaust passage is equal to or larger than the predetermined value. Thus, the excessive increase of the accumulation quantity of the deposit on the inner wall of the exhaust passage can be inhibited suitably, Moreover, since the temperature is increased to the temperature capable of removing the deposit, the supply quantity of the reducing substance to the purification device resulting from the decomposition of the deposit can be determined relatively easily. Therefore, the supply control of the reducing substance to the purification device can be performed appropriately. 
     According to a third example aspect of the present invention, the increase processing of the exhaust gas temperature is stopped when it is determined that the accumulation quantity has become equal to or smaller than a predetermined value. 
     When the processing for increasing the exhaust gas temperature is performed, there is a possibility that the fuel consumption of the internal combustion engine increases. Regarding this point, according to the above-described third example aspect of the present invention, the increase of the fuel consumption can be inhibited to the minimum by stopping the increase processing of the exhaust gas temperature when the accumulation quantity becomes equal to or smaller than the predetermined value. 
     According to a fourth example aspect of the present invention, the exhaust purification control device further has a torque increase timing temperature increasing section for increasing the exhaust gas temperature of the internal combustion engine to temperature capable of removing the deposit at higher speed than speed of exhaust gas temperature increase accompanying increase of torque of the internal combustion engine when request torque of the internal combustion engine increases. 
     When the request torque of the internal combustion engine increases, the exhaust gas temperature of the internal combustion engine normally increases. Therefore, the deposit on the inner wall of the exhaust passage decomposes and is supplied to the purification device. However, since components of the deposit start to decompose at different temperatures respectively, it is difficult to determine the quantity of the reducing substance supplied to the purification device with the increase of the exhaust gas temperature. 
     In view of this point, according to the above-described fourth example aspect of the present invention, the exhaust gas temperature is increased rapidly when the request torque of the internal combustion engine increases. Accordingly, the lowering of the determination accuracy of the quantity of the reducing substance due to the difference in the temperatures, at which the respective components of the deposit start to decompose, can be suitably inhibited. 
     According to a fifth example aspect of the present invention, the addition quantity of the reducing agent added by the addition device is decreased when the exhaust gas temperature increasing section performs the increase processing of the exhaust gas temperature. 
     If the increase processing of the exhaust gas temperature is performed, the deposit on the inner wall of the exhaust passage decomposes and eventually the reducing substance is supplied to the purification device. Therefore, in this case, the reducing substance supplied to the purification device contains both of the reducing substance supplied by the addition device and the reducing substance resulting from the decomposition. Therefore, if the addition quantity of the addition device is set without taking the quantity resulting from the decomposition into account, there is a possibility that the quantity of the reducing substance actually supplied to the purification device becomes excessive. 
     In view of this point, according to the above-described fifth example aspect of the present invention, the addition quantity of the reducing agent added by the addition device is decreased when the increase processing of the exhaust gas temperature is performed. Thus, the increase of the supply quantity of the reducing substance to the purification device resulting from the decomposition can be compensated suitably. Eventually, desired quantity of the reducing substance can be supplied to the purification device. 
     According to a sixth example aspect of the present invention, the exhaust gas temperature increasing section performs at least one of delaying processing of fuel injection timing of the internal combustion engine, fuel supplying processing to the exhaust passage of the internal combustion engine and increasing processing of exhaust gas recirculation quantity of the internal combustion engine. 
     According to a seventh example aspect of the present invention, the internal combustion engine is an in-vehicle internal combustion engine, an output shaft of which is connected to a drive wheel through a transmission. The exhaust gas temperature increasing section operates a change gear ratio of the transmission to decrease rotation speed of the output shaft of the internal combustion engine, while inhibiting fall of running speed of a vehicle. 
     If the rotation speed of the internal combustion engine is lowered, the air quantity charged into a combustion chamber of the internal combustion engine decreases. Therefore, the exhaust gas temperature can be increased. 
     According to an eighth example aspect of the present invention, the estimating section estimates the accumulation quantity based on a parameter correlated with temperature of an exhaust system of the internal combustion engine and the addition quantity. 
     With the above construction, the parameter correlated with the temperature of the exhaust system, which is a parameter correlated with the accumulation quantity, and the addition quantity are used. Thus, the accumulation quantity can be estimated appropriately. 
     According to a ninth example aspect of the present invention, the estimating section estimates the accumulation quantity based on a time, in which idling of the internal combustion engine is performed, during the idling. 
     Since the exhaust gas temperature is low during the idling of the internal combustion engine, the deposit tends to accumulate on the inner wall of the exhaust passage during the idling. The accumulation quantity of the deposit increases as an engine idling time (idling time) lengthens. In view of this point, according to the above-described ninth example aspect of the present invention, the accumulation quantity can be suitably estimated by using the idling time as the parameter correlated with the accumulation quantity. 
     According to a tenth example aspect of the present invention, the reducing agent is urea solution. 
     In the case where the urea solution is used as the reducing agent, the urea pyrolysate deposits on the inner wall of the exhaust passage under a situation where heating is insufficient. Moreover, the deposit contains various components and decomposition start temperatures differ among the components. Therefore, the increase of the supply quantity of the reducing substance to the purification device due to the decomposition of the deposit causes lowering of controllability of the supply control of the reducing substance. Therefore, according to the tenth example aspect of the present invention, utility values of the decreasing section and the exhaust gas temperature increasing section are specifically high. 
     According to an eleventh example aspect of the present invention, an exhaust purification system of the internal combustion engine has the exhaust purification control device and the purification device. 
     The above-described eleventh example aspect of the present invention has the decreasing device and the exhaust gas temperature increasing device, thereby realizing a system with high reliability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features and advantages of embodiments will be appreciated, as well as methods of operation and the function of the related parts, from a study of the following detailed description, the appended claims, and the drawings, all of which form a part of this application. In the drawings: 
         FIG. 1  is a system configuration diagram according to a first embodiment of the present invention; 
         FIG. 2  is a diagram showing melting points of urea pyrolysates according to the first embodiment; 
         FIG. 3  is a diagram showing a measurement result of temporal changes of the urea pyrolysate according to the first embodiment; 
         FIG. 4  is a time chart showing a decrease control mode of urea solution addition quantity according to the first embodiment; 
         FIG. 5  is a time chart showing an increasing processing mode of exhaust gas temperature according to the first embodiment; 
         FIG. 6  is a time chart showing an increasing processing mode of the exhaust gas temperature at request torque increasing timing according to the first embodiment; 
         FIG. 7  is a flowchart showing a processing procedure of exhaust purification control according to the first embodiment; and 
         FIG. 8  is a flowchart showing a processing procedure of exhaust purification control according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     First Embodiment 
     Hereafter, an exhaust purification control device of an internal combustion engine according to a first embodiment of the present invention will be explained with reference to the drawings. 
     A diesel engine  10  is an internal combustion engine having a reciprocating engine structure. An air cleaner  14  is provided upstream of an intake passage  12  of the diesel engine  10 . An intake temperature sensor  16  for sensing intake air temperature and an airflow meter  18  for sensing an intake air flow rate are provided to the air cleaner  14 . A turbocharger  20  is provided downstream of the air cleaner  14 . An air supercharged by the turbocharger  20  is cooled by an intercooler  22  and then supplied to a downstream side of the intake passage  12 . The air is supplied to a combustion chamber  28  of the diesel engine  10  through a throttle valve  24 , which adjusts a flow passage area of the intake passage  12 , and an intake valve  26 , which opens and closes communication between the combustion chamber  28  and the intake passage  12 . 
     The air thus supplied to the combustion chamber  28  is compressed with high-pressure fuel (e.g., fuel at tens to 200 MPa) such as light oil injected by an injector  30 , whose tip portion protrudes into the combustion chamber  28 , and is used for combustion. An energy generated by the combustion is converted into a rotational energy of a crankshaft  34  via a piston  32 . The rotational energy of the crankshaft  34  is transmitted to drive wheels via a continuously variable transmission  35  (CVT). A crank angle sensor  36  that senses a rotation angle of the crankshaft  34  is provided near the crankshaft  34 . 
     The air and fuel used for the combustion in the combustion chamber  28  are discharged to an exhaust passage  40  as an exhaust gas in connection with an opening action of an exhaust valve  38 . A part of the exhaust passage  40  upstream of the turbocharger  20  is connected to the intake passage  12  through an exhaust gas recirculation passage  42 . A part of the exhaust gas discharged into the exhaust passage  40  is cooled by an EGR cooler  44  and then supplied to the intake passage  12  according to an opening degree of an exhaust gas recirculation valve  46  (EGR valve) that adjusts a flow passage area of the exhaust gas recirculation passage  42 . 
     An after treatment device is provided downstream of the turbocharger  20  in the exhaust passage  40 . The after treatment device includes an oxidation catalyst  50 , a urea selective reduction catalyst  52  (referred to as urea SCR, hereafter) and an ammonia slip catalyst  54  in this order from the upstream side of the exhaust passage  40 . The ammonia slip catalyst  54  removes surplus ammonia, which is not consumed in a reaction with NOx in the urea SCR  52  and is discharged downstream of the urea SCR  52 . The ammonia slip catalyst  54  is constituted by an oxidation catalyst, for example. 
     An upstream NOx sensor  56  that senses a NOx concentration in the exhaust gas and an exhaust temperature sensor  58  that senses exhaust gas temperature are provided between the oxidation catalyst  50  and the urea SCR  52 . A downstream NOx sensor  60  that senses the NOx concentration is provided between the urea SCR  52  and the ammonia slip catalyst  54 . The after treatment device further includes a diesel particulate filter (DPF) that collects particulate matters in the exhaust gas. The DPF may be provided to be integrated with the oxidation catalyst  50  or may be provided downstream of the oxidation catalyst  50 . 
     A urea solution addition valve  62  is further provided between the oxidation catalyst  50  and the urea SCR  52 . An injection hole of the urea solution addition valve  62  is directed to a downstream side of the exhaust passage  40 . The urea solution addition valve  62  is an electronically-controlled valve member that injects a urea solution, which is supplied from a urea solution tank  64 , into the exhaust passage  40 , thereby adding the urea solution to the exhaust gas. The urea solution tank  64  is constituted by a hermetic container having a supplying cap. The urea solution tank  64  stores a urea solution of a prescribed concentration (for example, 32.5%) therein. The urea solution tank  64  is connected to the urea solution addition valve  62  through a urea solution pipe  66 . An electronically-controlled urea solution pump  68  is provided in the urea solution pipe  66 . The urea solution pump  68  draws the urea solution in the urea solution tank  64  and pressure-feeds (pumps) the urea solution to the urea solution addition valve  62 . A pressure sensor  70  that senses pressure of the pumped urea solution is provided downstream of the urea solution pump  68 . 
     A swirl flow generating member  72  is provided upstream of the urea solution addition valve  62 . The swirl flow generating member  72  generates a swirl flow in the exhaust gas flowing through the exhaust passage  40  by changing a cross-sectional structure of a flow passage inside the exhaust passage  40 . 
     In the urea SCR system constituted by the urea SCR  52 , the urea solution addition valve  62  and the like, the urea solution is added and supplied into the exhaust passage  40  by the urea solution addition valve  62 , thereby supplying the urea solution to the urea SCR  52  together with the exhaust gas in the exhaust passage  40 . Thus, in the urea SCR  52 , the exhaust gas is purified through a reduction reaction of NOx. 
     More specifically, the urea solution injected from the urea solution addition valve  62  is hydrolyzed by exhaust heat. At that time, the ammonia (NH3) as a reducing substance is generated by a chemical reaction expressed by a following expression (c1). 
       (NH2)2CO+H2O→2NH3+CO2  (c1) 
     NOx in the exhaust gas is selectively reduced and purified by the ammonia when the exhaust gas passes through the urea SCR  52 . More specifically, NOx is reduced and purified through reduction reactions shown by following expressions (c2) to (c4). 
       4NO+4NH3+O2→4N2+6H2O  (c2) 
       6NO2+8NH3→7N2+12H2O  (c3) 
       NO+NO2+2NH3→2N2+3H2O  (c4) 
     An electronic control unit  80  (ECU) controls the diesel engine  10  and operates various actuators such as the injector  30 . Sensing signals of the above-described various sensors that sense the operation states of the diesel engine  10 , a sensing signal of an accelerator sensor  82  that senses accelerator operation amount ACCP by a user, a sensing signal of a vehicle speed sensor  84  that sensing running speed Vc of a vehicle and the like are successively inputted to the ECU  80 , and the ECU  80  controls control amounts of the diesel engine  10  (torque, exhaust characteristic and the like) based on the sensing signals. 
     In order to control the characteristic of the exhaust gas discharged from the exhaust passage  40  as the above-described control amount, the ECU  80  operates the urea solution addition valve  62  and the urea solution pump  68  to perform NOx purification control using the urea SCR  52 . First, urea solution addition quantity is calculated based on the NOx concentration in the exhaust gas sensed with the upstream NOx sensor  56 . Then, a valve opening command pulse having a predetermined cycle is outputted to the urea solution addition valve  62  based on the calculated urea solution addition quantity. When a driving current flows to a drive section (solenoid section) of the urea solution addition valve  62  in connection with the output of the pulse, valve opening of the urea solution addition valve  62  is performed and the urea solution is added (injected). 
     When temperature of an exhaust system of the diesel engine  10  is low, there is a possibility that an efficiency of the hydrolysis from the urea solution to the ammonia lowers and urea pyrolysates such as a cyanuric acid deposit and accumulate on an inner wall surface of the exhaust passage  40 . The deposit having accumulated on the inner wall of the exhaust passage  40  decomposes with increase of the exhaust temperature, thereby generating the ammonia. Thus, the deposit having accumulated on the inner wall of the exhaust passage  40  decomposes and the ammonia is supplied to the urea SCR  52  irrespective of the operation of the urea solution addition valve  62 . The deposit contains various components having different decomposition start temperatures.  FIG. 2  shows decomposition start temperatures (melting points) of biuret and the cyanuric acid among the components constituting the above deposit.  FIG. 2  shows measurement results of the decomposition start temperatures of the biuret and the cyanuric acid under oxygen environment. The decomposition start temperatures of the biuret and the cyanuric acid are different from each other by 100 degrees C. as shown in  FIG. 2 . Therefore, it is quite difficult to anticipate how much deposit on the inner wall of the exhaust passage  40  decomposes with the increase of the exhaust gas temperature and how much ammonia is supplied to the urea SCR  52  as the result of the decomposition. 
     Furthermore, component concentrations of the above-described deposit can change with elapse of time.  FIG. 3  shows temporal changes of the composition ratios of the biuret, ammelide and the cyanuric acid among the components constituting the above deposit.  FIG. 3  shows a measurement result of a relationship between a heating time of heating treatment applied to a solid urea at 180 degrees C. and the composition ratios of the components remaining as solids. As shown in  FIG. 3 , the composition ratio of the cyanuric acid increases as the heating time lengthens. That is, the composition ratio of the component having high decomposition start temperature increases as the heating time lengthens. Such the phenomenon makes it more difficult to anticipate how much deposit on the inner wall of the exhaust passage  40  decomposes with the increase of the exhaust gas temperature and how much ammonia is supplied to the urea SCR  52  as the result of the decomposition. 
     Therefore, in the present embodiment, excessive increase of the ammonia supply quantity to the urea SCR  52  is suitably avoided by processing shown in  FIGS. 4 to 6 . 
       FIG. 4  shows a first processing mode according to the present embodiment. Parts (a), (b), (c), (d) and (e) of  FIG. 4  respectively show transitions of the vehicle running speed Vc, the exhaust gas temperature Tex, the urea solution addition quantity Qur, the urea deposit accumulation quantity Dur on the inner wall of the exhaust passage  40  and the NOx purification rate Rnox. The NOx purification rate Rnox is quantified with a value calculated by dividing a difference between the NOx concentration upstream of the urea SCR  52  and the NOx concentration downstream of the urea SCR  52  by the NOx concentration upstream of the urea SCR  52 . 
     The first processing shown in  FIG. 4  is to decrease the urea addition quantity when the urea deposit accumulation quantity on the inner wall of the exhaust passage  40  increases. More specifically, in the present embodiment, the urea addition quantity Qur is decreased when the urea deposit accumulation quantity Dur (explained later) becomes equal to or larger than a threshold value β and the exhaust gas temperature Tex is equal to or lower than threshold temperature γ. The condition that the exhaust gas temperature Tex is equal to or lower than the threshold temperature γ is used in order to accurately determine the situation where the urea deposit accumulation quantity Dur increases. In  FIG. 4 , the decrease control of the urea solution addition quantity Qur is performed at time t 3  when the exhaust gas temperature Tex becomes equal to or lower than the threshold temperature y. By performing the decrease control of the urea solution addition quantity Qur in this way, the increase of the accumulation quantity Dur of the urea deposit onto the inner wall of the exhaust passage  40  can be inhibited suitably. 
     Originally, the urea solution addition quantity Qur is set at quantity necessary to purify NOx. Therefore, if the urea solution addition quantity Qur is decreased unnecessarily, it can cause the increase of the NOx concentration in the exhaust gas discharged to the downstream side of the after treatment device. 
     However, the inventors of the present invention found that the ammonia adsorbed to the urea SCR  52  also increases under the situation where the deposit accumulation quantity Dur onto the inner wall of the exhaust passage  40  increases and that the decrease of the ammonia supply quantity to the urea SCR  52  due to the decrease of the urea solution addition quantity Qur can be compensated with the ammonia having been adsorbed to the urea SCR  52 . 
     The quantity of the adsorbed ammonia increases because the NOx purification rate Rnox decreases due to the decrease of the exhaust gas temperature Tex after time t 2  shown in  FIG. 4 . In the present embodiment, the decrease control of the urea solution addition quantity Qur is performed based on these findings. In fact, in the example shown in  FIG. 4 , the NOx purification rate Rnox does not fall even when the decrease control of the urea solution addition quantity Qur is performed. 
       FIG. 5  shows a second processing mode according to the present embodiment. Parts (a) to (e) of  FIG. 5  correspond to parts (a) to (e) of  FIG. 4 , respectively. 
     The second processing shown in  FIG. 5  is to perform processing for increasing the exhaust gas temperature Tex (temperature increase processing) when the urea deposit accumulation quantity Dur on the inner wall of the exhaust passage  40  becomes equal to or larger than a threshold value a. In  FIG. 5 , the temperature increase control is performed at time t 3  when the accumulation quantity Dur becomes equal to or larger than the threshold value α. Then, the temperature increase control is stopped when the accumulation quantity Dur becomes equal to or smaller than the threshold value β. Similarly, the temperature increase control is performed also in a period t 5  to t 6  and a period t 7  to t 8 . Marks A in  FIG. 5  indicate the periods for performing the temperature increase control. 
     The temperature increase control is processing for rapidly increasing the exhaust gas temperature Tex to or over the highest value of the decomposition start temperature of the deposit accumulating on the inner wall of the exhaust passage  40 . More specifically, the temperature increase control is processing for increasing the exhaust gas temperature Tex stepwise to 300 degrees C. The stepwise increase is defined as increase at speed higher than average increase speed of the exhaust gas temperature Tex caused by normal increase of request torque of the diesel engine  10  or the like. Thus, the quantity of the ammonia supplied to the urea SCR  52  due to the decomposition of the deposit can be estimated easily. Accordingly, the decrease quantity of the urea solution addition quantity Qur from the urea solution addition valve  62  can be set based on the estimated ammonia supply quantity resulting from the decomposition of the deposit, In  FIG. 5 , as an example compulsorily decreasing the urea solution addition quantity Qur, the urea solution addition quantity Qur is decreased gradually with the start of the temperature increase control and fixed when the urea solution addition quantity Qur becomes predetermined quantity. 
     The above-described temperature increase control may be performed by at least one of post-injection, increase processing of EGR quantity and increase processing of a change gear ratio of the CVT  35 . The post-injection is to inject the fuel to the combustion chamber  28  at timing largely delayed from a compression top dead center of the diesel engine  10 . Thus, the injected fuel is combusted not in the combustion chamber  28  but in the exhaust passage  40 . The increase processing of the EGR quantity can be performed by increase operation of the opening degree of the EGR valve  46 . If the EGR quantity increases, temperature of the gas supplied from the intake passage  12  to the combustion chamber  28  increases. Therefore, the exhaust gas temperature Tex can be increased. The increase processing of the change gear ratio of the CVT  35  is performed to decrease the rotation speed of the diesel engine  10  without decreasing the running speed Vc of the vehicle. If the rotation speed of the diesel engine  10  decreases, charging quantity of the gas supplied to the combustion chamber  28  decreases. Accordingly, the air quantity per unit quantity of the fuel decreases and eventually the exhaust gas temperature Tex increases. 
       FIG. 6  shows a third processing mode according to the present embodiment. Parts (a) to (d) of  FIG. 6  correspond to parts (a) to (d) of  FIG. 4  respectively. 
     The third processing shown in  FIG. 6  is to increase the exhaust gas temperature Tex to or over the maximum value of the decomposition start temperature of the above-described deposit at higher speed than the increase speed of the exhaust gas temperature Tex accompanying the acceleration request of the diesel engine  10  when the acceleration of the diesel engine  10  is requested. More specifically, the processing is to increase the exhaust gas temperature Tex stepwise to 300 degrees C. Since the exhaust gas temperature Tex increases when the acceleration request occurs, the deposit having accumulated on the inner wall of the exhaust passage  40  decomposes. However, since the decomposition start temperature differs among the components of the deposit, it is difficult to determine timing and amount of emergence of the ammonia. Therefore, the exhaust gas temperature Tex is increased stepwise (as shown by mark B in  FIG. 6 ) to facilitate anticipation of the amount of emergence of the ammonia resulting from the decomposition of the deposit. Thus, it can be facilitated to adjust the urea solution addition quantity Qur, which is added from the urea solution addition valve  62 , to suitable quantity. 
     The temperature increase control according to the present embodiment may be performed by means of the post-injection. 
       FIG. 7  shows a procedure of purification processing of the nitrogen oxides according to the present embodiment. The ECU  80  repeatedly performs the processing, for example, in a predetermined cycle. 
     In a series of the processing, first in S 10  (S means “Step”), it is determined whether a present operation range of the diesel engine  10  is a range for performing the urea solution addition processing. For example, the range for performing the urea solution addition processing may be a temperature range equal to or higher than activation temperature of the urea SCR  52 . When the present operation range is the range for performing the addition processing of the urea solution, the accumulation quantity Dur of the urea deposit is estimated based on the temperature of the inner wall of the exhaust passage  40  and the urea solution addition quantity Qur in S 12 . It is estimated that the accumulation quantity Dur increases as the inner wall temperature decreases and that the accumulation quantity Dur increases as the urea solution addition quantity Qur increases. 
     The temperature of the inner wall of the exhaust passage  40  is estimated based on the vehicle speed Vc sensed with the vehicle speed sensor  84 , the exhaust gas temperature Tex and ambient temperature. It is thought that a wall surface of the exhaust passage  40  receives more heat from the exhaust gas as the exhaust gas temperature Tex increases. Therefore, the inner wall temperature is estimated to be higher as the exhaust gas temperature Tex increases. It is thought that more heat amount flows out of a wall surface of the exhaust passage  40  to an exterior as the ambient temperature decreases. Therefore, the inner wall temperature is estimated to be lower as the ambient temperature decreases. Furthermore, it is thought that a flow rate of an ambient air blowing against the wall surface of the exhaust passage  40  increases as the vehicle speed Vc increases. Therefore, the inner wall temperature is estimated to be lower as the vehicle speed Vc increases. For example, the estimation may be performed using a model of heat transfer based on a specific heat of the wall surface of the exhaust passage  40  and the like. In the present embodiment, the intake air temperature sensed with the intake temperature sensor  16  is used as the ambient temperature. 
     In following S 14 , it is determined whether the acceleration is being performed. More specifically, in S 14 , it is determined whether the request torque of the diesel engine  10  is increased based on the sensing value of the accelerator sensor  82  and the like. When it is determined that the request torque is increased, the temperature increase control is performed in S 16  in the mode shown in  FIG. 6 . 
     When the determination result in S 14  is negative, it is determined in S 18  whether an idling state is present. This processing is provided because the urea solution addition quantity decrease control and the temperature increase control are performed based on the urea deposit accumulation quantity Dur estimated by a method different from the processing of S 12  during the idling. 
     When it is determined in S 18  that the idling is not performed, it is determined in S 20  whether the temperature increase control of the exhaust gas temperature Tex shown in  FIG. 5  is in execution. When the determination result in S 20  is negative, the process proceeds to  322 . It is determined in S 22  whether the accumulation quantity Dur estimated by the processing of S 12  is “equal to or larger than” the threshold value α. This processing is to determine whether to perform the temperature increase control shown in  FIG. 5 . When the determination result of S 22  is negative, it is determined in S 24  whether the accumulation quantity Dur estimated by the processing of S 12  is “equal to or larger than” the threshold value β. The threshold value β is set as a value smaller than the above-described threshold value α. This processing is to determine whether to perform the decrease control of the urea solution addition quantity Qur shown in  FIG. 4 . When the determination result of S 24  is affirmative, it is determined in S 26  whether the exhaust gas temperature Tex is “equal to or lower than” the threshold temperature v. This processing is also for determining whether to perform the decrease control of the urea solution addition quantity Qur shown in  FIG. 4 . When the determination result of S 26  is affirmative, the decrease control of the urea solution addition quantity Qur is performed in S 28  in the mode shown in  FIG. 4 . 
     When the determination result in S 22  is affirmative, the temperature increase control shown in  FIG. 5  is performed in S 30 . When the processing of S 30  completes or the determination result in S 20  is affirmative, the process proceeds to S 32 . In S 32 , as shown in  FIG. 5 , the processing for decreasing the urea solution addition quantity Qur based on the temperature increase control is performed. More specifically, the quantity Qur of the urea solution added from the urea solution addition valve  62  is decreased according to the amount of emergence of the ammonia resulting from the decomposition of the deposit due to the temperature increase control based on the accumulation quantity Dur estimated in S 12 . In order to perform such the processing easily, it is desirable to quantify the deposition quantity as the estimation object of S 12  as the amount of emergence of the ammonia in the case where the deposit is decomposed. 
     When the processing of S 32  completes, the process proceeds to S 34 . In S 34 , it is determined whether the accumulation quantity Dur estimated in S 12  is “equal to or smaller than” a threshold value c. This processing is to determine whether to stop the temperature increase control. The threshold value ε is set as a value larger than the threshold value β. When the determination result of S 34  is affirmative, the temperature increase control and the processing of S 32  are stopped in S 36 , and the usual urea solution addition control is resumed. 
     When the determination result of S 18  is affirmative, an engine idling time (idling time) is counted in S 38 . The idling time is a parameter for quantifying the accumulation quantity Our of the deposit on the inner wall of the exhaust passage  40 . In following S 40 , it is determined whether the idling time is longer than a threshold time T 1 . This processing is to determine whether to perform the temperature increase control shown in  FIG. 5 . The threshold time T 1  is set to a certain time, during which the accumulation quantity Dur of the deposit on the inner wall of the exhaust passage  40  is assumed to reach approximately the threshold value a. 
     When the determination result of S 40  is negative, it is determined that the accumulation quantity Dur is not large to such an extent that the temperature increase control should be performed. In this case, the process proceeds to S 42 . In S 42 , it is determined whether the idling time is longer than a threshold time T 0 . This processing is to determine whether to perform the decrease control of the urea solution addition quantity Qur. The threshold time T 0  is set to a certain time, during which the accumulation quantity Dur of the deposit on the inner wall of the exhaust passage  40  is assumed to reach approximately the threshold value β. When the determination result of S 42  is affirmative, the decrease control of the urea solution addition quantity Qur is performed in S 44 . 
     When the determination result in S 40  is affirmative, the temperature increase control is performed in S 46 . In following S 48 , processing similar to S 32  is performed. In S 50 , it is determined whether a temperature increase control time is “equal to or longer than” a threshold time T 2 . This processing is to determine whether to stop the temperature increase control. The threshold time T 2  is set to a certain time, during which the accumulation quantity Dur of the deposit on the inner wall of the exhaust passage  40  is assumed to decrease approximately to the threshold value e due to the temperature increase control. When the determination result of S 50  is affirmative, the temperature increase control and the processing of S 48  are stopped in S 52 , and the usual urea solution addition control is resumed. Further, the idling time is initialized. 
     The series of the processing is once ended when the determination result is negative in S 10 , S 24 , S 26 , S 34 , S 42  or  550  or when the processing of S 16 , S 28 , S 36 , S 44  or S 52  completes. 
     The present embodiment described above exerts following effects. 
     (1) When the estimated accumulation quantity is equal to or larger than the specified value, the urea solution addition quantity is decreased compulsorily. Thus, the accumulation of the deposit onto the inner wall of the exhaust passage  40  can be inhibited suitably. 
     (2) When the estimated accumulation quantity is equal to or larger than the predetermined value, the exhaust gas temperature of the diesel engine  10  is increased stepwise to the temperature capable of removing the deposit. Thus, the excessive increase of the accumulation quantity of the deposit on the inner wall of the exhaust passage  40  can be inhibited suitably. 
     (3) When it is determined that the accumulation quantity has become equal to or smaller than the predetermined value, the increase processing of the exhaust gas temperature is stopped. Thus, the increase of fuel consumption can be inhibited to the minimum. 
     (4) When the request torque of the diesel engine  10  increases, the exhaust gas temperature of the diesel engine  10  is increased stepwise to the temperature capable of removing the deposit. Thus, lowering of the accuracy of the determination of the ammonia supply quantity to the urea SCR  52  due to the difference in the decomposition start temperatures of the components of the deposit can be inhibited suitably. 
     (5) When the temperature increase control is performed, the addition quantity of the urea solution added by the urea solution addition valve  62  is decreased. Thus, the increase of the ammonia supply quantity to the urea SCR  52  resulting from the decomposition of the deposit can be compensated suitably. Eventually, desired quantity of the ammonia can be supplied to the urea SCR  52 . 
     (6) The accumulation quantity is estimated based on the parameter (exhaust gas temperature) correlated with the temperature of the exhaust system of the diesel engine  10  and the addition quantity. Thus, the accumulation quantity can be estimated suitably. 
     (7) The vehicle running speed and the ambient temperature are also used when estimating the accumulation quantity. Thus, a diffusion mode of the heat from the exhaust passage  40  to the exterior can be grasped with high accuracy. Accordingly, the inner wall temperature of the exhaust passage  40  can be determined with high accuracy and eventually the accumulation quantity can be estimated with high accuracy. 
     (8) The accumulation quantity is estimated based on the time, in which the idling is performed, when the idling of the diesel engine  10  is performed. Thus, the accumulation quantity can be suitably estimated by using the idling time as the parameter correlated with the accumulation quantity. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described with reference to the drawings, focusing on the differences from the first embodiment. 
     In the present embodiment, the addition quantity of the urea solution added by the urea solution addition valve  62  is calculated based on the NOx purification rate Rnox. The NOx purification rate Rnox is calculated based on the both sensing values of the upstream NOx sensor  56  and the downstream NOx sensor  60 . 
       FIG. 8  shows a procedure of purification processing of the nitrogen oxides according to the present embodiment. The ECU  80  repeatedly performs the processing, for example, in a predetermined cycle. Processing in  FIG. 8  corresponding to the processing in  FIG. 7  is indicated with the same step number as in  FIG. 7 . 
     As shown in  FIG. 8 , in the present embodiment, as the execution condition of the decrease control of the urea solution addition quantity Qur in the case where the idling is not performed presently, a condition that increase speed of the accumulation quantity Dur is equal to or higher than threshold speed Sth is used (refer to S 24   a ) in place of the condition that the accumulation quantity Dur is equal to or larger than the threshold value β. Thus, the excessive increase of the accumulation quantity of the deposit on the inner wall of the exhaust passage  40  can be surely avoided. When the increase speed of the accumulation quantity is very high, the NOx concentration in the exhaust gas lowers. Therefore, even if the decrease control of the urea solution addition quantity Qur is performed, the NOx purification rate Rnox does not fall. 
     In the present embodiment, the urea solution addition quantity Qur is set based on the NOx purification rate Rnox. Therefore, even if there is a situation where the NOx concentration in the exhaust gas falls, it does not necessarily lead directly to decrease of the set urea solution addition quantity Qur. Therefore, as in the present embodiment, it is specifically effective to perform the processing for decreasing the urea solution addition quantity Qur when the increase speed of the accumulation quantity Dur is high. 
     Modified Embodiments 
     The above described embodiments may be modified and implemented as follows, for example. 
     In the above embodiments, the exhaust temperature sensor  58  is provided to sense the exhaust gas temperature. Alternatively, the exhaust gas temperature may be estimated by using a parameter indicating an operation state of the diesel engine  10  as an input. Such the parameter may be fuel injection quantity, the rotation speed or the like, for example. 
     In the above-described second embodiment, a condition that the accumulation quantity is equal to or larger than the threshold value β may be used as the start condition of the decrease control of the urea solution addition quantity in addition to the condition that the increase speed of the accumulation quantity is equal to or higher than the threshold speed Sth. 
     In the above-described first embodiment, the urea solution addition quantity is set based on the NOx concentration in the exhaust gas. Alternatively, the urea solution addition quantity may be set based on the NOx purification rate in the urea SCR  52  as in the second embodiment, for example. Alternatively, a device or a section for estimating ammonia adsorption quantity in the urea SCR  52  may be provided, and the urea solution addition quantity may be set based on the estimated adsorption quantity. 
     In the above-described second embodiment, the urea solution addition quantity is set based on the NOx purification rate in the urea SCR  52 . Alternatively, for example, a device or a section for estimating the ammonia adsorption quantity in the urea SCR  52  may be provided, and the urea solution addition quantity may be set based on the estimated adsorption quantity. Alternatively, for example, as in the above-described first embodiment, the urea solution addition quantity may be set based on the NOx concentration in the exhaust gas. 
     As mentioned above, it is thought that the case where the increase speed of the accumulation quantity is equal to or higher than the threshold speed Sth is a situation where the exhaust gas temperature is low and the NOx concentration in the exhaust gas is low. However, in the case where the urea solution addition quantity is set based on the NOx purification rate or the ammonia adsorption quantity in the urea SCR  52 , there is a possibility that decrease of the urea solution addition quantity delays as compared to the case where the urea solution addition quantity is set based on the NOx concentration in the exhaust gas. Therefore, in the case where the NOx concentration of the exhaust gas discharged from the combustion chamber  28  of the diesel engine  10  is not used as the direct input parameter of the estimation of the urea solution addition quantity, it is specifically effective to perform the decrease control of the urea addition quantity when the increase speed of the accumulation quantity is equal to or higher than the threshold speed Sth in order to promptly perform the decrease control of the urea addition quantity. 
     In the above-described embodiments, the decrease control of the urea solution addition quantity is performed when a condition of the conjunction between the condition that the accumulation quantity is equal to or larger than the threshold value β and the condition that the exhaust gas temperature is equal to or lower than the threshold temperature γ is established in the range of the operation other than the idling. Alternatively, for example, the decrease control of the urea solution addition quantity may be performed when the condition that the accumulation quantity is equal to or larger than the threshold value β is established, irrespective of the exhaust gas temperature. 
     In the above-described embodiments, the threshold value ε for the determination of the stop of the temperature increase control in the range of the operation other than the idling is set larger than the threshold value β for the determination of the start of the decrease control of the urea solution addition quantity. Alternatively, for example, the threshold value ε may be set equal to or smaller than the threshold value β. With such the configuration, the deposit accumulation quantity on the inner wall surface of the exhaust passage  40  can be sufficiently decreased even before the request torque of the diesel engine  10  increases. 
     In the above embodiments, the idling time is used as the estimate of the accumulation quantity of the urea pyrolysate during the idling. Alternatively, for example, accumulation quantity estimated based on a parameter correlated with the temperature of the exhaust system and the urea solution addition quantity may be used also during the idling. In the case where the accumulation quantity is estimated based on the parameter correlated with the temperature of the exhaust system and the urea solution addition quantity, the accumulation quantity may be estimated in accordance with the idling time during the idling such that the accumulation quantity increases with the idling time instead of using the above parameter and the urea solution addition quantity. With such the modification, the execution conditions of the temperature increase control of the exhaust gas temperature and the addition quantity decrease control can be equalized between the case where the idling is performed and the case where the idling is not performed. 
     The scheme of the estimation of the accumulation quantity of the deposit is not limited to those illustrated in the above-described embodiments and the modifications thereof. For example, estimation processing of the accumulation quantity in each estimation processing cycle may be performed based on the idling time and at least one of a parameter correlated with the temperature of the exhaust system and the urea solution addition quantity during the idling. 
     In the above-described embodiments, the temperature increase control of the exhaust gas temperature is stopped when the estimate of the accumulation quantity of the deposit becomes equal to or smaller than the threshold value β in the range of the operation other than the idling. Alternatively, for example, the temperature increase control may be stopped on a condition that the temperature increase control time reaches a predetermined time. In this case, the temperature increase control time serves as the parameter indicating the accumulation quantity of the deposit. That is, it is meant that the accumulation quantity decreases as the temperature increase control time lengthens. 
     in the above-described embodiments, the temperature increase control of the exhaust gas temperature is stopped when the temperature increase control time reaches the threshold time T 2  in the idling range. Alternatively, for example, the temperature increase control may be stopped when the estimate of the accumulation quantity of the deposit becomes equal to or smaller than the threshold value β. 
     The fuel supply processing to the exhaust passage  40  performed to increase the exhaust gas temperature is not limited to the processing for performing the post-injection. For example, in the case where another injector for injecting the fuel into the exhaust passage  40  is provided separately, processing for injecting the fuel into the exhaust passage  40  with the another injector may be performed. 
     The transmission used for increasing the exhaust gas temperature is not limited to the above-described CVT  35 . For example, a transmission with discrete gear ratios may be used. 
     The control for increasing the exhaust gas temperature is not limited to the control that increases the exhaust gas temperature to approximately 300 degrees C. For example, control that increases the exhaust gas temperature over 300 degrees C. may be used. In this case, it is thought that the deposit in the exhaust passage  40  decomposes at once into the ammonia. Therefore, it is thought that the estimation of the ammonia supply quantity to the urea SCR  52  is made much easier. 
     The purification device for purifying the nitrogen oxides in the exhaust gas is not limited to the above-described urea SCR  52 . For example, a selective reduction catalyst that uses a reducing agent, which is different from the urea solution and is added to the exhaust gas upstream of the catalyst, may be used. The present invention can be effectively applied to such the case if there is a possibility that deposit containing multiple components having different decomposition start temperatures accumulates because of the reducing agent when the inner wall surface temperature of the exhaust passage  40  is low. In this case, it is desirable to set the target temperature of the temperature increase processing of the exhaust gas temperature to or over the maximum value among the decomposition start temperatures of the components of the deposit. 
     The internal combustion engine is not limited to the compression ignition internal combustion engine such as the diesel engine. Alternatively, for example, even if the internal combustion engine is a spark ignition internal combustion engine such as a direct injection gasoline engine, the present invention can be effectively applied to the engine if a selective reduction catalyst is used for the purification of NOx. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.