Exhaust emission control system for internal combustion engines

Abstract

Exhaust emission control system for internal combustion engines, comprising: a catalyst (1) arranged in an exhaust path (12) leading to an internal combustion engine (50), wherein a reducing agent is supplied to the catalyst in order to purify a harmful product contained in exhaust emissions from the internal combustion engine (50); a supply device (2, 13, 7, 4, 5, 6) for supplying the reducing agent, which is stored in liquid form in a reservoir (8), upstream of the catalyst (1) within the exhaust gas path (12); and a collecting device (11, 7, 71, 72) for returning the reducing agent remaining in the supply device to the accumulator (8) in a reducing agent collecting mode, which is entered after the internal combustion engine (50) has stopped, wherein the collecting device comprises a pump (7, 71) which operates to suck the reducing agent remaining in the supply device back to the accumulator (8), wherein the collecting device carries out the reducing agent collecting mode such that the pump is operated for a predetermined time phase (T2) which is set to be shorter than a time phase (T1) required to continuously operate the pump to completely return the reducing agent from the supply device back to the accumulator, so that a certain amount of the reducing agent remains within the supply device.

Description

BACKGROUND OF THE INVENTION1. Technical Field of the InventionThe present invention relates generally to an exhaust emission control system of internal combustion engines such as vehicular diesel engines, and more particularly to an exhaust emission control device configured to use a catalyst installed in an exhaust path leading to the internal combustion engine and a reducing agent led to the catalyst to remove or reduce harmful components contained in the exhaust emissions.2. Prior ArtThere are known urea SCR (selective catalytic reduction) systems which are designed as exhaust emission control systems for vehicle internal combustion engines such as diesel engines to have catalysts (also referred to as an SCR catalyst or a catalyst for selective reduction of NOx) which are installed in an exhaust path for the internal combustion engine to reduce NOx contained in the exhaust gas discharged from the internal combustion engine. The urea SCR systems also include an additive injector that is installed upstream of the catalyst and sprays an aqueous urea solution (i.e., a reductant) into the exhaust gas. The catalyst functions to convert the urea aqueous solution supplied from the additive injector into ammonia and store it to decompose or reduce NOx in the exhaust gas into nitrogen and water.The aqueous urea solution supplied to the exhaust gas has a property of freezing at -11°C. When the urea aqueous solution freezes, it leads to an increase in the volume thereof so as to expand. The expansion of the aqueous urea solution remaining within a delivery system equipped with, for example, an additive injector, a pump, and piping, and serving to guide the aqueous urea solution to the exhaust path may therefore result in mechanical breakage of the delivery system. To address this problem, Japanese Patent No. 4730278 teaches operating the pump of the supply system in the rearward direction to collect the urea aqueous solution remaining in the supply system in a urea aqueous solution tank.However, it is physically impossible to completely remove the reducing agent from the delivery system, leaving a small amount of drops of the reducing agent in the delivery system. The water will evaporate from the drops of reductant over time, such that the reductant deposits, which may lead to conduit plugging or jamming of a displaceable portion of the additive injector.The collection or discharge of the reducing agent from the supply system to the tank requires that the supply system is completely filled with the reducing agent upon restart of the internal combustion engine, which leads to a delay of the operation of the entire supply system. Without the collection of reductant, this results in an increased likelihood of physical fracture damage to the delivery system, as described above.Furthermore, DE 10 2009 029 409 A1 discloses a method for operating an electromagnetically controllable metering valve which is arranged in an exhaust system of an internal combustion engine. For metering reducing agent in the exhaust system, the metering valve is actuated by a control and / or regulating device via an electromechanical drive unit having a first flow profile, which has a holding current phase having a first holding current level when the metering valve is open. The metering valve and the drive unit are part of a metering module. When the internal combustion engine is shut down, the metering valve is actuated via the drive unit with a second current profile which has a second holding current level which is increased compared to the first holding current level. Furthermore, a control device configured to carry out the method is disclosed.In addition, DE 10 2008 006 837 A1 discloses an assembly with an injection device and pump, in particular for an exhaust system, with an injection device and a pump, which can supply the injection device with a reactant via a reactant line, wherein a changeover device is provided, by means of which the flow direction in the reactant line can be reversed. A method for operating an injection device is also described, which includes the following steps: If a controller recognizes that reactant is to be introduced into the exhaust line, the pump is switched on, so that the injection device is supplied with reactant. If the controller recognizes that no further reactant is to be introduced into the exhaust line, the supply direction of the injection device is reversed, so that reactant is conveyed back by the injection device for a short time. Finally, the pump is turned off.Summary of the InventionIt is an object of the invention to provide an exhaust emission control system for an internal combustion engine, which is designed to eliminate the possibility of physical damage of a reducing agent supply system due to freezing of the reducing agent, avoid solidification of the reducing agent in the reducing agent supply system, and minimize the delay in completely starting the exhaust emission control system.The above object is achieved by the subject matter of claims 1 and 5. Advantageous further developments of the invention are the subject of the dependent claims that follow.According to an illustrative aspect of the present disclosure, an exhaust emission control system for an internal combustion engine is provided. The exhaust emission control system includes a catalyst, a supply device, and a collection device. The catalyst is disposed in an exhaust path leading to an internal combustion engine, and a reducing agent is supplied thereto to purify a harmful product contained in exhaust emissions from the internal combustion engine. The supply means operates to supply the reducing agent stored in liquid form in a reservoir upstream of the catalyst within the exhaust path. The collecting device operates to return the reducing agent remaining in the supply device to the storage device or receive it there in a reducing agent collecting mode which is entered after the internal combustion engine has stopped. The collecting means performs the reducing agent collecting mode so that a certain amount of the reducing agent remains within the supplying means.The exhaust gas emission control system is in particular designed such that it does not fully guide the reducing agent from the supply device back to the storage device, but leaves a small amount of the reducing agent within the supply device after the internal combustion engine has stopped. In other words, the reducing agent containing moisture is left in the supply device after the internal combustion engine stops, thereby minimizing the likelihood of the reducing agent solidifying inside the supply device. When the internal combustion engine is restarted, the supply device is already filled with the certain amount of reducing agent, thereby minimizing the time required to completely replenish the supply device with the reducing agent to achieve a quick start of the operation of the exhaust emission control system. The collection of the reducing agent minimizes the probability of physical damage to the supply device, which is caused by freezing and expansion of the reducing agent.In the preferred mode of the present disclosure, the particular amount of reductant remaining in the feeder may be selected to freeze and expand, but not cause damage to the feeder, and also have a moisture content that is not fully dried out.The predetermined amount of reducing agent remaining in the supply means may also be determined so as to provide in the supply means a space which is not occupied by the reducing agent and whose volume is equal to or larger than the volume of the freezing and expanding reducing agent within the supply means.The determined amount of reductant remaining in the supply device may be further determined to have a moisture content that is greater in amount than an amount of moisture that is likely to evaporate from the reductant depending on an amount of saturated water vapor in the supply device.BRIEF DESCRIPTION OF THE DRAWINGSThe present invention will be more fully understood from the detailed description given below and from the accompanying drawings of the preferred embodiments of the invention, which, however, are not to be taken to limit the invention to the specific embodiments, but are for the purpose of explanation and understanding only.In the figures, FIG. 1 is a schematic view showing an exhaust emission control device according to the first embodiment of the invention; FIG. 2 is a flowchart of a urea solution collection process to be executed by the exhaust emission control system of FIG. 1 ; FIG. 3 is a timing diagram illustrating operation of portions of the exhaust emission control system of FIG. 1 in connection with various examples of urea solution collection applications; FIG. 4 is a flowchart of a urea solution collection process to be executed by the exhaust emission control system of the second embodiment; FIG. 5 is a time chart illustrating the operation of parts of an exhaust emission control system of the third embodiment; FIG. 6 is a flowchart of a urea solution collection process to be executed by the exhaust emission control system of the third embodiment; FIG. 7 is a schematic view showing an exhaust emission control device according to the fourth embodiment of the invention; FIG. 8 is a time chart illustrating the operation of parts of the exhaust emission control system of FIG. 7; and FIG. 9 is a time chart showing the operation of parts of a modification of the exhaust emission control system of FIG. 7.DESCRIPTION OF THE PREFERRED EMBODIMENTSFirst EmbodimentReferring now to the drawings, wherein like reference numerals refer to like components throughout the several views, particularly to FIG. 1, there is shown an exhaust emission control system 100 for vehicular internal combustion engines according to the first embodiment of the invention. The exhaust emission control system 100 in this embodiment is configured as a urea SCR (selective catalytic reduction) system to purify NOx contained in exhaust gas output from a diesel engine 50 mounted on a vehicle. The exhaust emission control system 100 has an exhaust path (i.e., an exhaust pipe) 12 connected to the internal combustion engine 50. Specifically, the exhaust gas is discharged to the outside of the vehicle via the exhaust path 12.The exhaust path 12 includes an SCR catalyst 1 (i.e., a selective NOx reduction catalyst) mounted therein, which operates to selectively reduce NOx in the exhaust gas. Specifically, the urea SCR catalyst 1 hydrolyzes and stores an aqueous urea solution (i.e., a reducing agent) supplied from a urea solution injector 2 into ammonia (NH 3). The urea SCR catalyst 1 provides a reaction of ammonia stored therein with NOx contained in the exhaust gas according to Chemical Equations 1 and 2 below to convert NOx into nitrogen and water. 4N0+4NH 3+ O 2 →4N 2+ 6 H 2 O (1) 6NO 2+ 8 NH 3 →7N 2+ 3 H 2 O (2)The urea SCR catalyst 1 is not capable of storing an unlimited amount of ammonia therein, but has a maximum ammonia storage depending on the temperature of the urea SCR catalyst 1 (i.e., a catalytic temperature). A rapid drop in the temperature of the urea SCR catalyst 1 leads to the phenomenon known as ammonia slip, unreacted ammonia being discharged from the SCR catalyst 1. In order to clean the ammonia released by the ammonia slip from the SCR catalyst 1, an oxidation catalyst 3 is arranged in the exhaust gas path 12 downstream of the SCR catalyst 1. The oxidation catalyst 3 serves to carry out oxidation to convert the ammonia into water and nitrogen.The exhaust emission control system 100 is equipped with a urea solution supply system disposed in the exhaust path 12 upstream of the SCR catalyst 1 to supply the urea aqueous solution to the SCR catalyst 1. Specifically, the urea solution supply system has the urea solution injector 2 installed upstream of the SCR catalyst 1 to inject the urea aqueous solution into the exhaust path 12. The urea solution injector 2 is configured to have substantially the same structure as that of a fuel injector for use in injecting fuel into the internal combustion engine 50. The body has therein a movable member such as a needle reciprocally movable in a longitudinal direction thereof to open and close a urea solution flow path extending inside the body, and a spray hole formed in the head of the body. The urea solution injector 2 is opened by a drive signal output from an electronic control unit (ECU) 11. Specifically, the electromagnetic coil is energized by the drive signal from the ECU 11 to move the needle to open the injection hole, thereby injecting the urea aqueous solution into the exhaust path 12.The urea solution injector 2 is supplied with the aqueous urea solution from a urea solution tank 8 as needed. The urea solution tank 8 is made up of a hermetically sealed container having a filling cap and stores the urea aqueous solution therein at a specific concentration. The urea solution tank 8 is connected to the urea solution injector 2 via a line 13.The conduit 13 defines a urea solution path therein. The pipe 13 has, at one end thereof, an inlet which is disposed below the surface of the aqueous urea solution within the urea solution tank 8.The line 13 has a pump 7 installed therein, which is an electrically-driven series pump such as a three-phase AC motor, and is driven by a drive signal from the ECU 11. The pump 7 is capable of rotating in both a normal and a rearward direction. When it is rotated in the normal direction, the pump 7 operates to suck the urea aqueous solution from the urea solution tank 8 and supply it to the urea solution injector 2 via the pipe 13. Alternatively, when rotated in the rearward direction, the pump 7 operates to suck or collect the urea aqueous solution from the urea solution injector 2 and the conduit 13 back to the urea solution tank 8. FIG. 1 illustrates the pump 7 mounted inside the urea solution tank 8, but may alternatively be disposed outside the urea solution tank 8.The pipe 13 also has a porous urea solution filter 4 incorporated therein, which removes impurities from the aqueous urea solution to prevent penetration thereof into the urea solution injector 2 and the urea solution tank 8.The exhaust emission control system 100 also includes a urea solution pressure regulator 6 mounted downstream of the pump 7. The urea solution pressure regulator 6 operates to adjust the pressure of the urea aqueous solution to be supplied to the urea solution injector 2 to a selected level. An excess of the urea aqueous solution, which is generated by adjusting the pressure by the urea solution pressure regulator 6, is returned to the urea solution tank 8. The conduit 13 also has a urea solution pressure sensor 5 incorporated therein, which measures the pressure of the aqueous urea solution flowing inside the conduit 13. The regulation of the urea aqueous solution within the line 13 may alternatively be achieved by monitoring the output of the urea solution pressure sensor 5 and controlling the operation of the pump 7 by the ECU 11 without using the urea solution pressure regulator 6.The urea solution injector 2, the pipe 13, the pump 7, the urea solution pressure regulator 6, the urea solution filter 4, and the urea solution pressure sensor 5 constitute the urea solution supply system which operates to supply the urea aqueous solution to the exhaust path 12 (i.e., the SCR catalyst 1).Between the SCR catalyst 1 and the oxidation catalyst 3 there is arranged in the exhaust gas path 12 a NOx sensor 9 which measures the concentration of NOx in the exhaust gas which has passed through the SCR catalyst 1. The exhaust path 12 also has an exhaust temperature sensor 10 installed upstream of the SCR catalyst 1 for measuring the temperature of the exhaust gas. The outputs of the NOx sensor 9 and the exhaust temperature sensor 10 are used, for example, in controlling the amount of the urea aqueous solution to be injected from the urea solution injector 2 and the time at which the urea solution injector 2 is to be operated to inject the urea aqueous solution.The exhaust emission control system 100 has the ECU 11 equipped with a typical microcomputer to control, as described above, the amount of urea aqueous solution to be supplied through the urea solution injector 2 to the exhaust path 12 and the time at which the urea solution injector 2 is to start injecting the urea aqueous solution into the exhaust path 12 based on the outputs of the sensors as described above when the internal combustion engine 50 is operating. The ECU 11 is also equipped with a timer 111 for measuring the time, and connected to an ignition switch 14 which turns on or off the internal combustion engine 50.The ECU 11 is configured to perform a urea solution collecting task as described below to return the urea aqueous solution from the urea solution supply system back to the urea solution tank 8 after stopping the internal combustion engine 50 to avoid the physical damage of the urea solution supply system due to the urea aqueous solution remaining in the urea solution supply system (for example, the urea solution injector 2 or the piping 13).FIG. 2 is a flowchart of the urea solution collection process to be executed by the ECU 11. FIG. 3 is a timing diagram illustrating operation of portions of the exhaust emission control system 100 in connection with various examples of the urea solution collection applications. The top portion of FIG. 3 specifically illustrates an on-off operation of the ignition switch 14 for the internal combustion engine 50. The second upper most portion of FIG. 3 illustrates an on-off operation of the pump 7 and an open or closed state of the urea solution injector 2 in a comparative example of a full collection application to fully collect the aqueous urea solution. The third upper most portion of FIG. 3 illustrates the on-off operation of the pump 7 and the open or closed state of the urea solution injector 2 in a partial collection application to be executed by the exhaust emission control system 100 of this embodiment to partially collect the urea aqueous solution. The lowermost portion of FIG. 3 illustrates the on-off operation of the pump 7 and the open or closed state of the urea solution injector 2 in a replenishment application to be executed in another embodiment, as described in detail later.The urea solution collection routine of FIG. 2 is initiated simultaneously with the ignition switch 14 being turned on to start the internal combustion engine 50.After entering the routine, the routine proceeds to step S11, where it is determined whether the ignition switch 14 has been turned off to stop the internal combustion engine 50. If a NO answer is obtained, meaning that the internal combustion engine 50 is still in operation, the routine then repeats step S 11. Alternatively, if a YES answer is obtained, meaning that the internal combustion engine 50 has been stopped, the routine then proceeds to step S 12, where the ECU 11 starts injecting the urea aqueous solution into the exhaust path 12 as needed. Specifically, as illustrated in FIG. 3, the ECU 11 rotates the pump 7 in the normal direction and opens the urea solution injector 2.When the ignition switch 14 is in the off state, that is, the internal combustion engine 50 is stopped (i.e., YES in step S 11), the ECU 11 turns off the pump 7 in step S 12, and then performs the operations in step S 13 and the subsequent steps to return the urea aqueous solution collected in the urea solution injector 2 and the pipe 13 to the urea solution tank 8. Specifically, the ECU 11 opens the urea solution injector 2 at step S 13, and then operates the pump 7 in the rearward direction at step S 14, thereby starting to return the urea aqueous solution remaining in the urea solution supply system back to the urea solution tank 8. The ECU 11 also starts measuring the time using the timer 111 at the time when the pump 7 is driven in the rearward direction at step S 14.After step S14, the routine proceeds to step S15, where it is determined whether a predetermined time period T2 has elapsed since the pump 7 was driven in the rearward direction. The time period T 2 is, as seen in FIG. 3, set shorter than a time period T 1 required to operate the pump 7 for fully collecting the urea aqueous solution from the urea solution supply system back to the urea solution tank 8 in the rearward direction. In other words, the ECU 11 does not fully suck the urea aqueous solution back to the urea solution tank 8, but leaves a certain amount of the urea aqueous solution inside the urea solution supply system.The amount of urea aqueous solution to be left in the urea solution supply system is selected so as to avoid the breakage of the urea solution supply system due to freezing or expansion of the urea aqueous solution collected in the urea solution supply system. For example, the amount of the urea aqueous solution to be left in the urea solution supply system is determined so as to leave, in the urea solution supply system, a space in which no urea aqueous solution is accommodated and the volume of which is equal to or larger than the freezing and expanding urea aqueous solution after the urea aqueous solution is sucked back into the urea solution tank 8. When the aqueous urea solution freezes and expands, the volume thereof usually increases by about 7%. Therefore, the amount of the urea aqueous solution to be left in the urea solution supply system is determined such that a total volume of the urea aqueous solution after expanding by 7% inside the urea solution supply system is smaller than a total volume of the urea solution supply system that can be completely occupied by the urea aqueous solution. If it is difficult to know exactly the total volume of the urea solution supply system, the volume of the pipe 13 may be used as the total volume of the urea solution supply system.The amount of the urea aqueous solution to be left in the urea solution supply system is also selected such that a moisture content thereof is not completely dried and solidified. Such an amount is determined in particular in consideration of the amount of saturated water vapor of the urea aqueous solution within the urea solution supply system in order to prevent the amount of moisture in the urea aqueous solution remaining in the urea solution supply system from completely evaporating. Specifically, the moisture content in the aqueous urea solution that will evaporate from the aqueous urea solution can be calculated as a function of the amount of saturated water vapor and the degree of moisture in the urea solution supply system. Therefore, the amount of urea aqueous solution to be left in the urea solution supply system is calculated such that the amount of moisture contained in the urea aqueous solution remaining within the urea solution supply system is larger than the above calculated amount of water. This prevents the urea aqueous solution from completely evaporating to avoid the deposition thereof within the urea solution supply system.The amount of saturated water vapor and the degree of humidity in the urea solution supply system usually depend on the temperature in the urea solution supply system, but are fixed in this embodiment so as not to reduce the complexity of the structure or operation of the exhaust emission control system 100. However, in order to accurately calculate the amount of saturated water vapor in the urea solution supply system, a temperature sensor may be used to measure the temperature in the urea solution supply system and correct the set value of the amount of saturated water vapor as a function of the measured temperature to determine a target amount of the aqueous urea solution to be left in the urea solution supply system. In short, the amount of urea aqueous solution to be left in the urea solution supply system can be calculated as a function of the temperature in the urea solution supply system.As is apparent from the above explanation, the amount of the urea aqueous solution to be left in the urea solution supply system is selected such that the water content thereof is larger than the amount of water which will usually evaporate and disappear within the urea solution supply system, and the volume of the urea aqueous solution after freezing and expansion will be smaller than the entire inner volume of the urea solution supply system. In order to replenish the urea solution supply system with urea aqueous solution again as quickly as possible upon restarting of the internal combustion engine 50, it is preferable to leave a maximum possible amount of urea aqueous solution within the urea solution supply system as long as it is frozen but does not cause damage to the urea solution supply system and avoids deposition of urea aqueous solution within the urea solution supply system.The time period T 2 in step S 15 corresponds to a time obtained in consideration of freezing and expansion of urea aqueous solution, the amount of saturated water vapor, and the capacity of the pump 7. If a NO answer is obtained in step S 15, meaning that the time period T 2 has not yet elapsed, the routine repeats the process in step S 15, in other words, the ECU 11 continues to suck the urea aqueous solution from the urea solution supply system back to the urea solution tank 8. Alternatively, if a YES answer is obtained, meaning that the time period T2 has elapsed, the routine then proceeds to step S16 in which the pump 7 is turned off. The routine proceeds to step S17, where the urea solution injector 2 is closed to finish the injection of the aqueous urea solution into the urea solution supply system. When the internal combustion engine 50 is restarted, the ECU 11 first operates the pump 7 in the normal direction to replenish the urea solution supply system with the urea aqueous solution, and then supplies the urea aqueous solution to the exhaust path 12.The exhaust emission control system 100 of this embodiment is, as described above, configured to retain a certain amount of urea aqueous solution in the urea solution supply system in the urea solution collection mode after stopping the internal combustion engine 50, thereby enabling the urea solution supply system to be quickly replenished with the urea aqueous solution again upon restarting the internal combustion engine 50. This minimizes the delay in operation of the exhaust emission control system 100, which is objectionable to the prior art system as described in the introductory portion of this application. The exhaust emission control system 100 determines the amount of the aqueous urea solution to be left in the urea solution supply system in view of the amount by which the aqueous urea solution freezes and expands and the amount of saturated water vapor in the urea solution supply system, thereby minimizing the likelihood of mechanical damage to the urea solution supply system (for example, the urea solution injector 2) and avoiding the clogging of the pipe 13 or jamming of a displaceable part of the urea solution injector 2.Second EmbodimentThe exhaust emission control system 100 of the second embodiment is described below, which is configured to execute a urea solution collection application different in operation from the first embodiment. Other structural arrangements are identical to those in the first embodiment, and detailed explanation thereof is omitted here.FIG. 4 is a flowchart of a urea solution collection process to be executed by the ECU 11 of the exhaust emission control system 100 of the second embodiment. The lowermost portion of FIG. 3 illustrates the on-off operation of the pump 7 and the open or closed state of the urea solution injector 2 in a urea solution replenishment mode of this embodiment.Steps S 21 to S 24 are identical in operation to steps S 11 to S 14 in FIG. 2, in particular, after the ignition switch 14 is turned off (i.e., YES in step S 21) and the pump 7 is turned off (i.e., step S 22), the ECU 11 opens the urea solution injector 2 subsequently in the rearward direction to start sucking the urea aqueous solution from the urea solution supply system in the same manner as described in the first embodiment.After step S 24, the routine proceeds to step S 25 where it is determined whether the time period T 1 has elapsed from the time when the pump 7 was operated in the rearward direction to suck the urea aqueous solution back to the urea solution tank 8. As described above, the time period T 1 corresponds to a time period required to completely return the urea aqueous solution from the urea solution supply system back to the urea solution tank 8. Specifically, the ECU 11 operates the pump 7 to completely take out the urea aqueous solution from the urea solution supply system. If a NO answer is obtained at step S 25, meaning that the time period T 1 has not yet elapsed, the routine then repeats the process at step S 25. Specifically, the ECU 11 continues to take out the urea aqueous solution from the urea solution supply system. Alternatively, if a YES answer is obtained, meaning that the time period T 1 has elapsed, the routine then proceeds to step S 26, in which the ECU 11 drives the pump 7 in the normal direction to supply the urea aqueous solution from the urea solution tank 8 to the urea solution supply system. At the same time, the ECU 11 opens the urea solution injector 2 to inject the urea aqueous solution simultaneously with the replenishment of the urea solution supply system. The ECU 11 also starts counting the time using the timer 111 from the time when the pump 7 was operated to replenish the urea solution supply system with the urea aqueous solution.The routine proceeds to step S27, where it is determined whether a time period T4 has elapsed after the start of replenishment of the urea solution supply system. As can be seen from FIG. 3, the time period T 4 is set to be shorter than a time period T 3 required to drive the pump 7 in the normal direction to completely replenish the urea solution supply system with the aqueous urea solution. Specifically, the time period T 3 corresponds to a time period selected in consideration of the capacity of the pump 7 to bring the amount of the urea aqueous solution with which the urea solution supply system is to be replenished in accordance with that to be left in the urea solution supply system in the urea solution collection mode of the first embodiment.If a NO answer is obtained at step S 27, meaning that the time period T 4 has not yet elapsed, the routine then repeats the process at step S 27. Specifically, the ECU 11 continues to replenish the urea solution supply system with the aqueous urea solution. Alternatively, if a YES answer is obtained, the routine proceeds to step S28, wherein the pump 7 is turned off. The routine proceeds to step S29, wherein the urea solution injector 2 is closed to finish the injection of the aqueous urea solution into the urea solution supply system. The routine then ends.The exhaust emission control system 100 of the second embodiment is, as described above, configured to press air bubbles in the pipe 13 into the urea solution injector 2 in the urea solution replenishing mode at step S 26, thereby ejecting the urea aqueous solution adhering to the displaceable part of the urea solution injector 2 to the outside of the urea solution supply system. This further reduces the deposition of the aqueous urea solution in the urea solution injector 2.The exhaust emission control system 100 of the third embodiment is described below, which is configured to execute a urea solution collection application different in operation from that in the first and second embodiments. Other structural arrangements are identical to those in the first embodiment, and detailed explanation thereof is omitted here.Third EmbodimentThe exhaust emission control system 100 of this embodiment is a modification thereof in the second embodiment, and is configured to delay, as shown in a time chart of FIG. 5, the time when the urea solution injector 2 is to be opened from the start of replenishing the urea solution supply system, that is, the start of rotation of the pump 7 in the normal direction in the urea solution replenishment mode after the urea aqueous solution is accumulated by the urea solution supply system. In FIG. 5, the times T 1 and T 4 are identical to the times T 1 and T 4 in FIG. 3.FIG. 6 is a flowchart of a urea solution collection process to be executed by the ECU 11 of the exhaust emission control system 100 of the third embodiment. The same step numbers as in FIG. 4 denote the same operations, and detailed explanation thereof is omitted here. The flow of FIG. 6 is in particular only different from that of FIG. 4 in that steps S 261, S 262, and S 136 are performed between steps S 26 and S 28.Referring to the urea solution collection flow of FIG. 6, the routine proceeds to step S 261, wherein the urea solution injector 2 is closed when the pump 7 is operated in the normal direction to start replenishing the urea solution supply system with the urea aqueous solution at step S 26. In this embodiment, the urea solution injector 2 is closed substantially simultaneously with the start of replenishment of the urea solution supply system at step S 26, but this may alternatively be done before the operation at step S 26 to start replenishment of the urea solution supply system.After step S 261, the routine proceeds to step S 262 where it is determined whether the time period T 5 has elapsed from the start of replenishment of the urea solution supply system. As can be seen from FIG. 5, the time period T 5 is shorter than the time period T 4 at which the urea solution replenishment mode is to be ended, but may be identical in length to the time period T 4. It is expedient for the time phase T 4 to be as close as possible to the time phase T 5 in order to push the aqueous urea solution more strongly out of the urea solution injector 2 when the latter is opened. If the time phase T 5 is set to be identical to the time phase T 4, step S 27 may be omitted.If a NO answer is obtained, meaning that the time period T 5 has not yet elapsed, the routine repeats the process at step S 262. Specifically, the ECU 11 keeps the urea solution injector 2 closed. Alternatively, if a YES answer is obtained, meaning that the time period T 5 has elapsed, the routine then proceeds to step S 263, in which the ECU 11 opens the urea solution injector 2 to discharge the urea aqueous solution. The routine then proceeds to step 27, wherein it is determined whether a time period T4 has elapsed from the start of replenishment of the urea solution supply system. If a NO answer is obtained at step S 27, meaning that the time period T 4 has not yet elapsed, the routine then repeats the process at step S 27. Specifically, the ECU 11 continues to replenish the urea solution supply system with the aqueous urea solution. Alternatively, if a YES answer is obtained, the routine proceeds to step S28, wherein the pump 7 is turned off to end the urea solution replenishing mode. The routine then proceeds to step S29, where the urea solution injector 2 is closed. Then, the routine ends.As is apparent from the above discussion, the exhaust emission control system 100 of the third embodiment is configured to keep the urea solution injector 2 closed for a while after the urea solution replenishment mode starts, which results in a rise in pressure in the urea solution supply system (for example, the pipe 13 and the urea solution injector 2), which discharges the urea aqueous solution from the urea solution injector 2 at an increased pressure. This minimizes the deposition of the urea aqueous solution at the sliding part of the urea solution injector 2.Fourth EmbodimentFIG. 7 illustrates the exhaust emission control system 100 according to the fourth embodiment of the invention. The same reference numerals as used in the first embodiment refer to the same components, and detailed explanation thereof will be omitted here. Specifically, the exhaust emission control system 100 of this embodiment is only different from that of the first embodiment in that the pump 71 and the flow selection valve 72 are provided instead of the pump 7.The pump 71 is, as illustrated in FIG. 7, of a type such as a pinch pump configured to supply liquid only in a direction P 1. The pump 71 includes an inlet 711 through which liquid is sucked into the pump 71 and an outlet 712 from which the liquid is discharged to the outside of the pump 71. Specifically, the pump 71 operates to suck the urea aqueous solution through the inlet 711, discharge it in the direction P 1, and then discharge it from the outlet 712. The pump 71 does not suck the discharged aqueous urea solution back to it itself. The pump 71 is installed in the conduit 13.The flow selector valve 72 is also installed in the conduit 13. Specifically, the flow selection valve 72 is connected to the inlet 711 and the outlet 712 of the pump 71, a portion 133 of the pipe 13 leading to the urea solution tank 8, and a portion 134 of the pipe 13 leading to the urea solution injector 2 through the urea solution filter 4.The flow selection valve 72 responds to a switching signal output from the ECU 11 to switch between two directions in which the urea aqueous solution flows inside the flow selection valve 72. Specifically, the flow selection valve 72 selectively operates in two modes: a first flow mode that provides a first flow path in which the inlet 711 is connected to the portion 133 of the conduit 13 and the outlet 712 is connected to the portion 134 of the conduit 13, and a second flow mode that provides a second flow path in which the outlet 712 is connected to the portion 133 of the conduit 13 and the inlet 711 is connected to the portion 134 of the conduit 13. FIG. 7 indicates the first flow path by a solid line and the second flow path by a broken line inside the flow selection valve 72.When the flow selection valve 72 is in the first flow mode, the urea aqueous solution in the line 13 is supplied from the urea solution tank 8 to the urea solution injector 2 by the pump 71. Alternatively, when the flow selection valve 72 is located in the second flow path, the urea aqueous solution in the line 13 is supplied from the urea solution injector 2 to the urea solution tank 8 by the pump 71.As described above, the ECU 11 outputs the switching signal to the flow selection valve 72 to switch between the first flow mode and the second flow mode. Specifically, when the internal combustion engine 50 is operating, the ECU 11 provides the first flow mode to guide the urea aqueous solution through the urea solution injector 2 to the exhaust path 12. When the internal combustion engine 50 has stopped, the ECU 11 creates the second flow mode to return the urea aqueous solution back to the urea solution tank 8. FIG. 8 is a time chart illustrating the operation of parts of the exhaust emission control system 100 related to the urea solution collection application in the fourth embodiment. Specifically, the top portion of FIG. 8 illustrates the on-off operation of the ignition switch 14 for the internal combustion engine 50. The second upper most portion of FIG. 8 illustrates the on-off operation of the pump 7 and the open or closed state of the urea solution injector 2 in the full collection application to fully collect the urea aqueous solution, as already described with reference to FIG. 3 as a comparative example. The third upper most portion of FIG. 8 illustrates the on-off operation of the pump 7 and the open or closed state of the urea solution injector 2 in the partial collection application to partially collect the urea aqueous solution in the first embodiment. The lowermost portion of FIG. 8 illustrates the on-off operation of the pump 71, the first and second flow modes of the flow selection valve 72, and the open or closed state of the urea solution injector 2 in a replenishment application to be executed in this embodiment, as in the second embodiment. The time phases T1, T2 and T4 are identical to those in Fig. 3.In operation of the exhaust emission control system 100 of this embodiment, the ECU 11 maintains the pump 71 in the ON state, places the flow selection valve 72 in the first flow mode, and opens the urea solution injector 2 to inject the urea aqueous solution into the exhaust path 12 when the ignition switch 14 is in the ON state, as illustrated in the top portion of FIG. 8.As in the above embodiments, when the ignition switch 14 is turned off, the ECU 11 enters the urea solution collection mode (i.e., the partial collection mode) to return the remaining urea aqueous solution from the urea solution supply system (i.e., the urea solution injector 2 and the pipe 13) back to the urea solution tank 8. The ECU 11 leaves a certain amount of the urea aqueous solution within the urea solution supply system as in the above embodiments. Specifically, the ECU 11 operates in the partial collection mode to drive the pump 71 for the period T 2 shorter than the period T 1 for which the pump 7 is operated in the rearward direction in the full collection mode to fully return the urea aqueous solution from the urea solution supply system to the urea solution tank 8. The ECU 11 sets the flow selection valve 72 to the second flow mode for the time phase T 2. Referring to the flowchart of FIG. 2, instead of the process at step S 14, the ECU 11 switches the flow selection valve 72 to the second flow mode.When the urea solution replenishing mode is entered, the ECU 11 first drives the pump 71 for the period T 1 as shown in the lowermost portion of FIG. 8, and also sets the flow selection valve 72 for the period T 1 to the second flow mode to fully suck the urea aqueous solution from the urea solution supply system back to the urea solution tank 8. Thereafter, the ECU 11 drives the pump 71 for the period T 4 and also sets the flow selection valve 72 to the first flow mode for the period T 4 to replenish the urea solution supply system with a certain amount of the urea aqueous solution. The ECU 11 keeps the urea solution injector 2 opened after the start of the urea solution replenishment mode. Referring to the flowchart of FIG. 4, the ECU 11 switches the flow selection valve 72 to the second flow mode instead of the process at step S 24, and switches the flow selection valve 72 to the first flow mode instead of the process at step S 26. Other operations and useful advantages provided by the fourth embodiment are the same as those of the second embodiment.As illustrated in FIG. 9, the exhaust emission control system 100 may be configured to perform a urea solution replenishment mode similar to that in the third embodiment. Specifically, when it is required to replenish the urea solution supply system with the urea aqueous solution, the ECU 11 controls the operation of the pump 71 and the flow selection valve 72 in the same manner as this in the urea solution replenishment mode discussed in FIG. 8, but keeps the urea solution injector closed after the flow selection valve 72 starts to supply the urea aqueous solution to the urea solution supply system until the time period T 5 elapses. After the time period T 5 has elapsed, the ECU 11 opens the urea solution injector 2, referring to the flowchart of FIG. 6, the ECU 11 switches the flow selection valve 72 to the second flow mode instead of the process at step S 24, and switches to the first flow mode instead of the process at step S 26. Other operations and useful advantages provided by the fourth embodiment are the same as those in the third embodiment.While the present invention has been described in terms of the preferred embodiments in order to provide a better understanding thereof, it should be appreciated that the invention may be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications of the shown embodiments which may be practiced without departing from the principle of the invention as set forth in the appended claims. For example, the exhaust emission control system 100 of each of the third and fourth embodiments operates to open the urea solution injector 2 after a lapse of the time period T 5 in the urea solution replenishment mode, but may be configured to open the urea solution injector 2 when the pressure of the urea aqueous solution flowing inside the pipe 13 measured by the urea solution pressure sensor 5 has exceeded a certain level. This also leads to a pressure increase within the urea solution injector 2, which leads to the aqueous urea solution being ejected completely from the urea solution injector 2. The present invention may also be used with urea SCR systems for spark ignition engines, particularly lean burn engines. The exhaust emission control system 100 may be configured to use a reducing agent other than an aqueous urea solution, such as an ammonia-containing solution.

Claims

An exhaust emission control system for internal combustion engines, comprising: a catalyst (1) disposed in an exhaust path (12) leading to an internal combustion engine (50), wherein a reducing agent is supplied to the catalyst to purify a harmful product contained in exhaust emissions from the internal combustion engine (50); a supply device (2, 13, 7, 4, 5, 6) for supplying the reducing agent stored in liquid form in a storage (8) upstream of the catalyst (1) within the exhaust path (12); A collector (11, 7, 71, 72) for returning the reducing agent remaining in the feeder to the reservoir (8) in a reducing agent collecting mode entered after stopping the internal combustion engine (50), the collector comprising a pump (7, 71) that operates to suck the reducing agent remaining in the feeder back to the reservoir (8), wherein the collector performs the reducing agent collecting mode such that the pump is operated for a predetermined time period (T2) set shorter than a time period (T1) required to continuously operate the pump for fully returning the reducing agent from the feeder back to the reservoir so that a certain amount of the reducing agent remains within the feeder.The exhaust emission control system according to claim 1, wherein the determined amount of the reducing agent remaining in the feeder is selected to freeze and expand but not cause damage to the feeder, and also has a moisture content which does not fully dry out.An exhaust emission control system according to claim 2, wherein the determined amount of reducing agent remaining in the supply means is also determined so as to leave in the supply means a space which is not occupied by the reducing agent and whose volume is equal to or larger than the volume through which the reducing agent within the supply means freezes and expands.An exhaust emission control system according to claim 2 or 3, wherein the determined amount of the reducing agent remaining in the supply means is also determined to have a moisture content which is larger in amount than an amount of moisture which is expected to evaporate from the reducing agent depending on an amount of saturated water vapor in the supply means.An exhaust emission control system for internal combustion engines, comprising: a catalyst (1) disposed in an exhaust path (12) leading to an internal combustion engine (50), wherein a reducing agent is supplied to the catalyst to purify a harmful product contained in exhaust emissions from the internal combustion engine (50); a supply device (2, 13, 7, 4, 5, 6) for supplying the reducing agent stored in liquid form in a storage (8) upstream of the catalyst (1) within the exhaust path (12); and a collector (11, 7, 71, 72) for returning the reducing agent remaining in the supply device to the reservoir (8) in a reducing agent collecting mode entered after stopping the internal combustion engine (50), the collector performing the reducing agent collecting mode such that a certain amount of the reducing agent remains inside the supply device, the collector operating in the reducing agent collecting mode such that it fully returns the reducing agent from the supply device to the reservoir (8) and then supplies the certain amount of the reducing agent to the supply device.The exhaust emission control system according to claim 5, wherein the supply means includes an injector (2) that supplies the reducing agent into the exhaust path, and wherein the collection means includes an injector control device (11) that operates to open the injector to supply the determined amount of the reducing agent to the supply means, and performs a specific application (S261, S262, S263) to start opening the injector after a specific time period has elapsed following the supply of the determined amount of the reducing agent.The exhaust emission control system according to claim 5 or 6, wherein the supply means includes an injector (2) that operates to supply the reducing agent into the exhaust path, a pipe (13) that connects the reservoir and the injector, and a pump (7) that serves to supply the reducing agent stored in the reservoir to the injector via the pipe, the pump being configured to be switchable between a first flow mode and a second flow mode, the pump in the first flow mode supplies the reducing agent from the reservoir to the injector, the pump in the second flow mode supplies the reducing agent from the injector to the reservoir, and the collection means is equipped with a pump control device (11), which controls an operation of the pump and switches between the first flow mode and the second flow mode to return the reducing agent from the supply device back to the reservoir.The exhaust emission control system according to claim 5 or 6, wherein the supply means includes an injector (2) that operates to supply the reducing agent into the exhaust path, a pipe (13) that connects the reservoir and the injector, and a pump (7) that serves to supply the reducing agent stored in the reservoir to the injector via the pipe, the pump is configured to allow the reducing agent to flow only in one direction, and the collection means is equipped with a selector (72) that switches between a first flow path through which the reducing agent flows from the reservoir to the injector and a second flow path through which the reducing agent flows from the injector to the reservoir, and a selector control device (11), which controls the switching of the selection device in order to feed the reducing agent from the feed device back to the storage.The exhaust emission control system according to any one of claims 1 to 8, wherein the reducing agent is an aqueous urea solution, and wherein the catalyst (1) is a selective NOx reduction catalyst that operates to reduce NOx in the exhaust gas using ammonia generated from the aqueous urea solution supplied from the supply device.

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