JP2008138583A - Exhaust emission control device of engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To execute suitable exhaust emission purification by addition of a reducing agent while protecting a reducing agent addition valve and a reducing agent supply part. <P>SOLUTION: A DPF 12 and an SCR catalyst 13 are arranged on an exhaust pipe 11, and a urea aqueous solution addition valve 15 for adding and supplying a urea aqueous solution in the exhaust pipe 11 is arranged between the DPF 12 and the SCR catalyst 13. As a structure of a urea aqueous solution supply part, a urea aqueous solution having predetermined concentration is stored in a urea aqueous solution tank 21, and a urea aqueous solution pump 22 is arranged in the tank 21. An ECU 30 opens the urea aqueous solution addition valve 15 in response to congelation change of the urea aqueous solution remaining in the urea aqueous solution addition valve 15 and the like after stopping an engine. Thereby, the remaining urea aqueous solution in the urea aqueous solution addition valve 15 and the like is discharged. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an engine exhaust gas purification device, and more particularly, to an exhaust gas purification device suitably employed in a urea SCR (Selective Catalytic Reduction) system.

  In recent years, the urea SCR system has been developed as an exhaust purification device for purifying NOx (nitrogen oxide) in exhaust gas with a high purification rate in engines (particularly diesel engines) applied to automobiles and the like. It has been put to practical use. The following configuration is known as a urea SCR system.

  That is, in the urea SCR system, an SCR catalyst is provided in an exhaust pipe connected to the engine body, and a urea water addition valve for adding urea water (urea aqueous solution) as a reducing agent into the exhaust pipe is provided upstream of the SCR catalyst. It has been. A urea water tank is connected to the urea water addition valve via a urea water supply pipe. For example, when the pump disposed in the urea water tank is driven to discharge, the urea water is removed from the urea water tank. The urea water supply valve is supplied through a urea water supply pipe.

  In such a system, the urea water is added to the exhaust pipe by the urea water addition valve, whereby the urea water is supplied to the SCR catalyst together with the exhaust gas, and the exhaust gas is purified by the NOx reduction reaction on the SCR catalyst. When NOx is reduced, urea water is hydrolyzed with exhaust heat to generate ammonia (NH3), and ammonia is added to NOx in the exhaust gas selectively adsorbed by the SCR catalyst. Then, NOx is reduced and purified by performing a reduction reaction based on ammonia on the SCR catalyst.

By the way, in the urea SCR system described above, urea water used as a reducing agent is frozen in a low temperature range (for example, −11 ° C.) within the use temperature range, and the use of urea water is hindered by the freezing. Therefore, as a countermeasure against freezing of urea water, a cooling water circulation pipe that leads part of the engine cooling water to the urea water tank is provided, and a cooling water shut-off valve is provided in the middle of the cooling water circulation pipe. A technique has been proposed in which the cooling water is circulated through the cooling water circulation pipe (see, for example, Patent Document 1).
JP 2006-125331 A

  In the case of the prior art such as Patent Document 1 described above, urea water can be used (NOx reduction or purification by adding urea water) by thawing the urea water after the engine is started. This is a measure that can be effective after freezing, and no measures are taken to prevent freezing. For this reason, it is considered that inconvenience caused by freezing cannot be solved. In other words, if it is noticed that the volume increases by about 7% when the urea water freezes, it is considered that the urea water supply pipe or the like may be damaged due to the volume increase.

  The main object of the present invention is to provide an engine exhaust purification device that can perform suitable exhaust purification by adding a reducing agent while protecting the reducing agent addition valve and the reducing agent supply unit. is there.

  Hereinafter, means for solving the above-described problems and the effects thereof will be described.

  In the engine exhaust gas purification apparatus according to the present invention, the liquid reducing agent is supplied from the reducing agent supply unit to the reducing agent addition valve during engine operation, and the reducing agent is added and supplied from the outlet of the reducing agent addition valve into the exhaust passage. Is done. Thereby, a specific exhaust purification reaction based on the addition of the reducing agent is promoted in the exhaust purification catalyst (reduction catalyst). In such an exhaust purification device, particularly in the present invention (Claim 1), after the engine is stopped, the reducing agent addition valve is set according to the freezing change of the reducing agent remaining in the reducing agent addition valve or the reducing agent supply unit. The valve is opened to release the residual reducing agent from the jet port into the exhaust passage.

  In short, when the engine is running, the reducing agent addition valve and other reducing agent supply section (reducing agent passage, etc.) are filled with the reducing agent, and after the engine is stopped, they are also filled with the reducing agent. It becomes. In this case, when the reducing agent (residual reducing agent) freezes at night or the like in a cold region, the volume increases, which may cause inconvenience such as breakage in the reducing agent addition valve and the reducing agent supply unit. In this respect, in the present invention, after the engine is stopped, the reducing agent addition valve is opened according to the freezing change of the residual reducing agent and the residual reducing agent is released. Damage to the agent supply unit can be suppressed. As a result, it is possible to perform suitable exhaust purification by adding the reducing agent while protecting the reducing agent addition valve and the reducing agent supply unit.

  In the invention according to claim 2, after the engine is stopped, it is determined that the residual reducing agent such as the reducing agent addition valve is in a freezing transition process in which the reductant transitions to a frozen state, and it is determined that it is in the freezing transition process In this case, the reducing agent addition valve is opened. That is, in the freezing transition process, the reducing agent moves from the liquid phase state to the solid phase state, and the volume of the reducing agent increases during the state transition. In such a case, by determining that the process is in the freezing transition process as described above, the residual reducing agent can be released in accordance with the volume increase in the freezing transition process. Therefore, the volume increase of the reducing agent due to freezing can be appropriately discharged from the reducing agent addition valve or the like.

  Specifically, as a method for determining freezing, as described in claim 3, the freezing determination process is based on the reducing agent temperature after the engine is stopped (the temperature of the remaining reducing agent in the reducing agent addition valve or the like). It is good to judge. That is, when the temperature of the reducing agent decreases to the freezing point temperature, the reducing agent gradually starts to be frozen. In this case, it is possible to determine that the reducing agent is in the freezing transition process by sequentially detecting the reducing agent temperature after the engine is stopped.

  Note that the reducing agent temperature is directly measured by a sensor or the like provided in a reducing agent addition valve or a reducing agent supply unit (piping, etc.), or the ambient temperature of the reducing agent addition valve (exhaust pipe temperature, outside temperature, engine water temperature, etc. ) To be detected based on estimation.

  Alternatively, as described in claim 4, it is preferable to determine that the process is in the freezing transition process based on the reducing agent pressure after stopping the engine (pressure of the remaining reducing agent in the reducing agent addition valve or the like). That is, in the freezing transition process, a pressure change occurs with an increase in the volume of the reducing agent. In this case, it is possible to determine that the reducing agent is in the freezing transition process by sequentially detecting the reducing agent pressure after the engine is stopped.

  In the invention according to claim 5, the reducing agent addition valve is opened a plurality of times in the freezing transition process. Thereby, even when freezing of the reducing agent gradually proceeds during the freezing transition process, it is possible to reliably release the reducing agent corresponding to the volume increase.

  As described above, the freeze transition process can be determined by the reducing agent temperature and the reducing agent pressure, but it is considered difficult to determine the process without error. In this regard, as described in claim 6, it is determined that it is a state transition period including the immediately preceding liquid phase state and the immediately following solid phase state with respect to the time corresponding to the freezing transition process, and the state The reducing agent addition valve may be opened during the transition period. Thereby, even if there is some error in the determination of the freezing transition process, the reducing agent corresponding to the volume increase can be discharged without omission.

  The reducing agent release process described above is performed after the engine is stopped. For example, assuming application to an automobile, each time the freezing determination or the driving of the reducing agent addition valve is performed, the on-vehicle controller (ECU) is started, It is necessary to supply power to sensors and actuators. Regarding freezing determination, for example, it is repeatedly determined whether or not the process is in the freezing transition process every time a predetermined time elapses after the engine stops. In such a case, it is desirable to reduce power consumption as much as possible while the engine is stopped. Therefore, as described in claim 7, after the reducing agent is released by opening the reducing agent addition valve, the freezing determination by the freezing determining means may be stopped. As a result, it is possible to prevent the freezing determination and the like from being performed unnecessarily after the release of the reducing agent, thereby reducing power consumption.

  Further, as described in claim 8, based on temperature information such as the outside air temperature when the engine is stopped, the temperature of the residual reducing agent is a reference temperature (for example, a freezing point) set based on the freezing point temperature of the reducing agent. (Temperature + 10 ° C.) or less is predicted, and it is good to determine whether or not it is in the freezing transition process at the predicted time. In this case, since the freezing determination is performed only when the residual reducing agent is likely to shift to the frozen state, it is possible to reduce the power consumption.

  According to the ninth aspect of the present invention, the freeze transition timing when the residual reducing agent such as the reducing agent addition valve shifts to the frozen state is predicted when the engine is stopped or immediately after the engine is stopped. Then, the reducing agent addition valve is opened at the predicted freeze transition time. That is, it is considered that the temperature transition of the reducing agent after the engine is stopped can be predicted to some extent, and therefore it is considered possible to predict the freezing transition time. Even in such a case, the residual reducing agent can be released in accordance with the increase in volume during the freezing transition process, and the volume increase of the reducing agent due to freezing can be appropriately discharged from the reducing agent addition valve or the like.

  It is considered that the temperature transition of the reducing agent after the engine stops changes according to the outside air temperature or the like. Therefore, as described in claim 10, the freeze transition time may be predicted based on temperature information such as the outside air temperature when the engine is stopped or immediately after the engine is stopped.

  In the invention according to claim 11, the reducing agent addition valve is opened a plurality of times at the freeze transition time (predicted time). Thereby, even when freezing of the reducing agent gradually proceeds during the freezing transition time, the reducing agent corresponding to the volume increase can be reliably released.

  In particular, when predicting the freezing transition time when the engine is stopped or immediately after the stopping, it is difficult to accurately estimate the actual freezing transition process. Therefore, it is necessary to set the freezing transition time to a relatively long period. In such a case, the reducing agent addition valve is opened (reducing agent release) a plurality of times at the time of freezing transition, so that the reducing agent can be reliably released in the actual freezing transition process.

  In general, the reducing agent supply unit is provided with a pumping pump for pumping the reducing agent, and the reducing agent is pumped to the reducing agent addition valve by driving the pumping pump. In such a configuration, immediately after the engine is stopped, pressure remains inside the reducing agent addition valve or the like, and the internal pressure becomes higher than the atmospheric pressure. When pressure remains inside the reducing agent addition valve or the like in this way, the internal pressure becomes significantly higher when the volume increases during freezing change, and the reduction of the reducing agent addition valve or the like is further concerned. Therefore, as described in claim 12, immediately after the engine is stopped, the reducing agent addition valve may be temporarily opened regardless of the freezing change of the residual reducing agent. Thereby, reduction of internal pressures, such as a reducing agent addition valve, can be aimed at.

  In the present exhaust purification apparatus, the reducing agent is a urea aqueous solution, and the exhaust purification catalyst uses the NOx reduction reaction for reducing NOx by ammonia generated from the urea aqueous solution as the exhaust purification reaction, and promotes the NOx reduction reaction. It is good if it is.

  As represented by the urea SCR system, a NOx purification device using an aqueous urea solution as a reducing agent is expected as an exhaust purification device that purifies NOx in exhaust gas at a high purification rate. Thus, the present invention is particularly beneficial when applied to a urea SCR system. In addition, for example, when this exhaust purification device is adopted in the field of automobiles and this device is mounted on a vehicle equipped with a diesel engine, the generation of NOx is allowed in the combustion process to improve fuel consumption and PM. As a result, it becomes possible to greatly contribute to improving the performance of automobiles and purifying exhaust gas.

  Hereinafter, an embodiment of an exhaust emission control device according to the present invention will be described with reference to the drawings. The exhaust purification device of this embodiment purifies NOx in exhaust using a selective reduction catalyst, and is constructed as a urea SCR system. First, the configuration of this system will be described in detail with reference to FIG. FIG. 1 is a configuration diagram showing an outline of a urea SCR system according to the present embodiment.

  As shown in FIG. 1, the present system uses exhaust discharged from a diesel engine (not shown) mounted on an automobile as a purification target, and generally includes various actuators and various sensors for purifying exhaust, and an ECU ( (Electronic control unit) 30 and the like.

  Specifically, an exhaust pipe 11 connected to an engine body (not shown) is provided as a configuration of the engine exhaust system. The exhaust pipe 11 includes a DPF (Diesel Particulate Filter) 12 and a selective reduction catalyst (hereinafter referred to as an SCR catalyst). 13) is provided. Further, a urea water addition valve 15 for adding and supplying urea water (urea aqueous solution) as a reducing agent into the exhaust pipe 11 is provided between the DPF 12 and the SCR catalyst 13 in the exhaust pipe 11. As shown in the figure, the exhaust pipe 11 is configured by connecting a plurality of pipe materials, but here, these are collectively referred to as the exhaust pipe 11.

  In the exhaust pipe 11, an exhaust sensor 16 having both a NOx detection unit (NOx sensor) and an exhaust temperature detection unit (exhaust temperature sensor) is provided on the downstream side of the SCR catalyst 13. On the downstream side, the amount of NOx in the exhaust (and thus the NOx purification rate by the SCR catalyst 13) and the temperature of the exhaust are detected. Further downstream of the exhaust pipe 11, an ammonia removal device (for example, an oxidation catalyst) for removing excess ammonia (NH3), an ammonia sensor for detecting the amount of ammonia in the exhaust, and the like are provided as necessary. .

  The DPF 12 is a continuous regeneration PM removal filter that collects PM (particulate matter) in exhaust gas. The DPF 12 carries a platinum-based oxidation catalyst and can remove HC and CO together with a soluble organic component (SOF) which is one of the PM components. Incidentally, the PM collected by the DPF 12 can be removed by combustion by post-injection after the main fuel injection in the diesel engine or the like (corresponding to the regeneration process), whereby the DPF 12 can be used continuously.

The SCR catalyst 13 promotes a NOx reduction reaction (exhaust gas purification reaction).
4NO + 4NH3 + O2 → 4N2 + 6H2O (Formula 1)
6NO2 + 8NH3 → 7N2 + 12H2O (Formula 2)
NO + NO2 + 2NH3 → 2N2 + 3H2O (Formula 3)
Such a reaction is promoted to reduce NOx in the exhaust gas. The urea water addition valve 15 provided on the upstream side of the SCR catalyst 13 is additionally supplied with ammonia (NH 3) as a NOx reducing agent in these reactions.

  The urea water addition valve 15 has substantially the same configuration as an existing fuel injection valve (injector), and since a known configuration can be adopted, the configuration will be briefly described here. The urea water addition valve 15 is an electromagnetic on-off valve provided with a drive unit composed of an electromagnetic solenoid or the like, a urea water passage through which urea water flows, and a valve body unit having a needle for opening and closing the tip jet port 15a. It is comprised and opens or closes based on the drive signal from ECU30. That is, when the electromagnetic solenoid is energized based on the drive signal, the needle moves in the valve opening direction along with the energization, and urea water is added (injected) from the tip outlet 15a as the needle moves.

  The urea water is sequentially supplied from the urea water tank 21 to the urea water addition valve 15. Next, the configuration of the urea water supply system will be described.

  The urea water tank 21 is configured by a sealed container with a liquid supply cap, and urea water having a predetermined concentration is stored therein. As a countermeasure against freezing of urea water in the tank, it is possible to attach a heater to the urea water tank 21 or arrange a heat insulating material such as a heat insulating sheet around the tank.

  A urea water pump 22 is provided in the urea water tank 21 while being immersed in the urea water. The urea water pump 22 is an electric pump that is rotationally driven by a drive signal from the ECU 30. One end of a urea water supply pipe 23 is connected to the urea water pump 22, and the other end of the urea water supply pipe 23 is connected to the urea water addition valve 15. The urea water supply pipe 23 is a urea water passage. When the urea water pump 22 is driven to rotate, the urea water is pumped up and discharged (pressure-fed) to the urea water addition valve 15 side through the urea water supply pipe 23. The urea water pump 22 may be configured to be provided in the middle part of the urea water supply pipe 23 in addition to being provided in the urea water tank 21 while being immersed in the urea water.

  The urea water supply pipe 23 is provided with a filter 24 for filtering the urea water and a pressure adjustment valve 25 for adjusting the pressure of the urea water. Accordingly, the urea water discharged (pressure-fed) from the urea water pump 22 is removed of foreign matters by the filter 24, and then adjusted to a predetermined supply pressure by the pressure adjustment valve 25. As a result of the pressure adjustment, the excess urea water is returned to the urea water tank 21 through the return pipe 26. The urea water supply pipe 23 is provided with a pressure sensor 27 for detecting the pressure of the urea water in the urea water supply pipe 23 and a temperature sensor 28 for detecting the temperature of the urea water. . The urea water tank 21, the urea water pump 22, the urea water supply pipe 23, and the like correspond to the “reducing agent supply unit”.

  The ECU 30 is a part of the system that mainly performs control relating to exhaust gas purification as an electronic control unit. The ECU 30 includes a well-known microcomputer (not shown), and operates various actuators including the urea water addition valve 15 in a desired mode based on detection values of various sensors, thereby performing various types of exhaust purification. The control is performed. Specifically, for example, by controlling the energization time of the urea water addition valve 15 and the driving amount of the urea water pump 22, an appropriate amount of urea water is added and supplied into the exhaust pipe 11 at an appropriate time. In addition to the exhaust sensor 16, the pressure sensor 27, and the temperature sensor 28 described above, detection signals from the water temperature sensor 31 and the outside air temperature sensor 32 are sequentially input to the ECU 30.

In the system according to this embodiment, during operation of the engine, the urea water in the urea water tank 21 is pumped to the urea water addition valve 15 through the urea water supply pipe 23 by driving the urea water pump 22, and the urea water addition valve 15. Thus, urea water is added and supplied into the exhaust pipe 11. Then, urea water is supplied to the SCR catalyst 13 together with the exhaust gas in the exhaust pipe 11, and the exhaust gas is purified by the NOx reduction reaction in the SCR catalyst 13. When reducing NOx, for example,
(NH2) 2CO + H2O → 2NH3 + CO2 (Formula 4)
As a result of the above reaction, urea water is hydrolyzed with exhaust heat to produce ammonia (NH3), and this ammonia is added to NOx in the exhaust selectively adsorbed by the SCR catalyst 13. . Then, the reduction reaction based on the ammonia (the above reaction formulas (Formula 1) to (Formula 3)) is performed on the SCR catalyst 13, whereby NOx is reduced and purified.

  By the way, urea water used as a reducing agent freezes at −11 ° C., and the volume increases by about 7% with the freezing. In this case, when urea water remains in each component (urea water addition valve 15 and urea water supply pipe 23) of the urea water supply system after the engine is stopped, and the volume of the residual urea water increases due to freezing, The water addition valve 15 and the urea water supply pipe 23 may be damaged. Therefore, in this embodiment, as a countermeasure against freezing of urea water, after the engine is stopped, the freezing change of urea water is monitored, and when the volume increases with the freezing change, the urea water addition valve 15 is temporarily opened (ON). The residual urea water from the urea water addition valve 15 and the urea water supply pipe 23 is discharged into the exhaust pipe 11 by opening the valve. As a result, inconveniences such as component damage due to an increase in volume during freezing of urea water are suppressed.

  FIG. 2 is a flowchart showing a process for releasing residual urea water. This process is executed by the ECU 30 after the engine is stopped. Specifically, the ECU 30 is provided with a timer (soak timer) that measures an elapsed time after the engine stops, and each time a predetermined time (for example, 10 to several tens of minutes) elapses after the engine stops, The computer is automatically activated temporarily and the arithmetic processing of FIG. 2 is executed.

  In FIG. 2, in step S11, it is determined whether or not the engine is currently stopped. In subsequent step S12, it is determined whether or not the urea water release execution condition is satisfied. The execution condition is for determining whether there is a possibility that the residual urea water in the urea water addition valve 15 or the urea water supply pipe 23 is frozen after the engine is stopped. For example, the outside air temperature when the engine is stopped Is stored, and when the outside air temperature is equal to or lower than a predetermined threshold temperature (for example, 0 ° C.), it is assumed that the execution condition is satisfied that the residual urea water may be frozen.

  In step S13, it is determined whether or not the release of residual urea water after the engine is stopped is incomplete. For the determination, for example, a urea water discharge completion flag is used, and when the flag is set, it is determined that the urea water discharge is completed. If any of steps S11 to S13 is NO, this process is terminated as it is.

  When steps S11 to S13 are all YES, the process proceeds to step S14. In step S14, the temperature of the residual urea water at that time is estimated. The temperature estimation method of the residual urea water may be arbitrary, but in this embodiment, the temperature information of the urea water is acquired by the temperature sensor 28 installed in the urea water supply pipe 23, and the temperature of the residual urea water is based on the temperature information. Is estimated. In addition, the temperature sensor (not shown) installed in the urea water addition valve 15, the temperature sensor installed in the exhaust pipe 11 (for example, the exhaust temperature detection unit of the exhaust sensor 16), the water temperature sensor 31, and the outside air temperature sensor 32 detection results. It is also possible to estimate the temperature of the residual urea water by at least one of the following.

  Thereafter, in step S15, based on the estimation result of the urea water temperature in step S14, the current phase transition period including the freezing transition process of urea water (the liquid phase immediately before the time corresponding to the freezing transition process) Whether or not it is necessary to open the urea water addition valve 15 at a time including the state and the immediately following solid phase state). Specifically, if the urea water temperature does not fall to a predetermined freezing temperature range, it is determined that the urea water addition valve 15 does not need to be opened in the state transition period. Moreover, if the urea water temperature has fallen to the predetermined freezing temperature range, it is determined that the urea water addition valve 15 needs to be opened in the state transition period. In the former case, the process proceeds to step S18, and in the latter case, the process proceeds to step S16. The freezing temperature range is set based on the freezing temperature (freezing point temperature) of the urea water, and is, for example, a temperature range of freezing temperature −5 ° C. to freezing temperature + 5 ° C. (temperature range of −11 ° C. ± 5 ° C.).

  In step S16, the urea water addition valve 15 is temporarily opened. Then, after waiting for a predetermined time in step S17, the process returns to step S14, and the temperature of the remaining urea water is estimated again. Further, similarly to the above, it is determined whether or not the urea water addition valve 15 needs to be opened, and the urea water addition valve 15 is opened (steps S15 to S17).

  If the temperature of the residual urea water has not decreased to the freezing temperature range, or if the temperature of the residual urea water has further decreased to the freezing temperature range and then changed to a lower temperature side, step S15 becomes NO and the process proceeds to step S18. . In step S18, it is determined whether or not the urea water addition valve 18 is driven to open to release the residual urea water (step S16) due to the current ECU activation. And if discharge | release of residual urea water is performed, it will progress to step S19 and will set a urea water discharge completion flag. The urea water discharge completion flag is information stored in a backup RAM or the like, and the flag information is continuously held and cleared at the next engine start (when the ignition is turned on).

  After the urea water discharge completion flag is set, even if the ECU 30 is started thereafter, step S13 becomes NO, so that the temperature estimation of the residual urea water and the opening process of the urea water addition valve 15 are not performed unnecessarily. It has become.

  Next, the temperature change, pressure change, etc. of the residual urea water after the engine is stopped will be described with reference to FIGS. Here, FIG. 3 shows changes in the urea water temperature and the urea water pressure when the above-described residual urea water discharge process is not performed, and FIG. 4 shows the case where the above-described residual urea water discharge process is performed. The change of urea water temperature and urea water pressure is shown for (this embodiment). 3 and 4, it is assumed that the outside air temperature after the engine is stopped is extremely low (about −15 ° C.) for the vehicle used in the cold region. Is supposed to decline.

  In FIG. 3, after the engine is stopped, the urea water temperature (the temperature of the residual urea water such as the urea water addition valve 15) decreases according to the ambient temperature or the like. When the urea water temperature reaches the freezing point temperature (−11 ° C.) at timing t1, the urea water temperature is held at the freezing point temperature until timing t2. At this time, during the period from the timing t1 to the time t2, the urea water is in the freezing transition process, and the urea water pressure increases with the volume increase. Before the timing t1, the urea water is in a liquid phase state, and the period from t1 to t2 is a transition period in which the urea water moves from the liquid phase to the solid phase. Thereafter, when all of the urea water is frozen, the urea water becomes a solid phase and the temperature further decreases.

  As described above, when the urea water changes state, the urea water addition valve 15 and the urea water supply pipe 23 may be damaged due to the pressure increase (volume increase) during the liquid phase → solid phase transfer.

  In contrast to the above-described inconvenience, in the present embodiment, as shown in FIG. 4, a period of time t11 to t14 (state transition period) in which the urea water temperature falls within a predetermined freezing temperature range (temperature range of −11 ° C. ± 5 ° C.). , The urea water addition valve 15 is opened a plurality of times, and the urea water pressure is reduced as the valve is opened. That is, the urea water addition valve 15 is opened a plurality of times during the freezing transition process. Timing t12 to t13 is a period during which the urea water temperature is maintained at the freezing point temperature (corresponding to the period from t1 to t2 in FIG. 3). In FIG. 4, the pressure change when the residual urea water release process is performed is indicated by a solid line, and the pressure change when the release process is not performed is indicated by a two-dot chain line.

  Specifically, at timing t11, it is determined that the urea water temperature has decreased to a predetermined freezing temperature range, and accordingly, the urea water addition valve 15 is temporarily opened. At this time, the residual urea water in the urea water addition valve 15 and the urea water supply pipe 23 is discharged into the exhaust pipe 11 from the front end jet port 15a of the urea water addition valve 15, and the urea water pressure is at the atmospheric pressure level by opening the valve. Reduced to. After the timing t11, the urea water addition valve 15 is temporarily opened every time the predetermined time TA elapses, and each time the urea water pressure is reduced to the atmospheric pressure level.

  As described above, the urea water addition valve 15 is opened in the process of freezing transition of urea water, so that residual urea water is released in accordance with the increase in volume of the urea water accompanying freezing. At this time, the volume increase in urea water due to freezing is reliably discharged from the urea water addition valve 15.

  When the engine is stopped, the ECU 30 is activated every predetermined time, and the temperature of the residual urea water is monitored in accordance with the activation. Therefore, actually, as shown in FIG. 4, the urea water addition valve 15 does not open immediately after the urea water temperature reaches the freezing temperature range, and if the urea water temperature is within a predetermined temperature range when the ECU is started. The urea water addition valve 15 is opened.

  According to the embodiment described in detail above, the following excellent effects can be obtained.

  After the engine is stopped, it is determined that the residual urea water is in the freezing transition process, and when it is determined that the freezing transition process is in progress, the urea water addition valve 15 is temporarily opened. Damage to the urea water addition valve 15 and the urea water supply pipe 23 due to the increase can be suppressed. As a result, it is possible to perform suitable exhaust purification by adding urea water while protecting the urea water addition valve 15, the urea water supply pipe 23, and the like.

  Since the urea water addition valve 15 is opened a plurality of times at predetermined time intervals in the freezing transition process after the engine is stopped, the urea water can be further frozen after the urea water addition valve 15 is opened. The urea water can be released. Therefore, it is possible to reliably release urea water corresponding to the volume increase due to freezing.

  A freezing temperature range including a freezing temperature (−11 ° C. ± 5 ° C., corresponding to the temperature range in the state transition period) is set, and the urea water addition valve 15 is opened when the urea water temperature is in the freezing temperature range. Therefore, the urea water addition valve 15 can be opened to release the residual urea water even in the liquid phase state just before the freezing transition process and the solid phase state just after the freezing transition process. Thereby, the urea water for the volume increase by freezing can be discharged | emitted more reliably. In addition, it may be difficult to determine that there is no error in determining whether the urea water is in the freezing transition process, but even if there is some error in the determination in the freezing transition process, leakage of urea water for the volume increase will occur. It can discharge without.

  After completion of the urea water release by opening the urea water addition valve 15, the temperature estimation of the residual urea water and the opening process of the urea water addition valve 15 are not performed. Therefore, these processes are carried out wastefully. Power consumption can be prevented. As a result, power consumption while the engine is stopped can be reduced as much as possible, and problems such as battery exhaustion can be suppressed.

  Urea water is used as a reducing agent for the SCR catalyst 13, and the SCR catalyst 13 promotes a NOx reduction reaction (the above reaction formulas (Formula 1) to (Formula 3)) in which NOx is reduced by ammonia generated from the urea water. The system was installed in a vehicle equipped with a diesel engine. As a result, it is possible to improve the fuel consumption and PM by allowing the generation of NOx in the combustion process, and as a result, it can greatly contribute to the improvement of the performance of the automobile and the cleaner exhaust.

  The present invention is not limited to the description of the above embodiment, and may be implemented as follows, for example.

  In the above embodiment, it is determined that the process is in the freezing transition process based on the urea water temperature (temperature of residual urea water) after the engine is stopped, but this is changed to change the urea water pressure (residual urea water after the engine is stopped). It may be determined that the process is in the freezing transition process based on the pressure). That is, in the freezing transfer process, a pressure change accompanying an increase in the volume of urea water occurs. Therefore, by monitoring the pressure change, it is determined that the freezing transfer process is in progress. Specifically, the ECU 30 detects the urea water pressure based on the detection signal of the pressure sensor 27 installed in the urea water supply pipe 23, and determines that the process is in the freezing transition process based on the change in the urea water pressure. .

  Based on temperature information such as the outside air temperature when the engine is stopped, the time when the temperature of the residual urea water falls below the reference temperature set based on the freezing point temperature of the urea water (for example, the freezing point temperature + 10 ° C.) is predicted. It is also possible to determine whether or not the process is in the process of freezing transition at the predicted time. For example, the urea water temperature falls below the reference temperature (freezing point temperature + 10 ° C.) based on the outside air temperature and engine water temperature (engine warm-up state) when the engine is stopped, or after 4 hours from the engine stop, or the engine is stopped. 5 hours later. Then, the ECU 30 is activated at the predicted time, and the temperature of the residual urea water is estimated and the urea water addition valve 15 is opened. In this case, the ECU 30 is activated only at a time when the residual urea water is likely to shift to the frozen state, and the freezing determination or the like is performed. Therefore, power consumption can be reduced.

  In the above embodiment, it is determined that the urea water is in the freezing transition process by monitoring the temperature of the residual urea water as needed after the engine is stopped, and the urea water addition valve 15 is opened in the freezing transition process. This is changed as follows. That is, when the engine is stopped or immediately after the stop, the freezing transition time at which the residual urea water such as the urea water addition valve 15 shifts to the frozen state is predicted. Then, the urea water addition valve 15 is opened at the predicted freeze transition timing (in this case, monitoring of the urea water temperature or the like is not performed after the engine is stopped). For example, based on the outside air temperature when the engine is stopped and the engine water temperature (engine warm-up state), it is predicted when the freezing transition time after the engine stops will be. Then, the ECU 30 is activated at the predicted freezing transition timing, and the urea water addition valve 15 is opened. Also in this configuration, as described above, the residual urea water can be discharged in accordance with the volume increase in the freezing transition process, and the volume increase of urea water due to freezing is appropriately discharged from the urea water addition valve 15 or the like. Can do. Even in such a case, it is preferable to open the urea water addition valve 15 a plurality of times during the freezing transition time (predicted time). Thereby, even when freezing of urea water gradually progresses at the time of freezing transition, urea water corresponding to the volume increase can be reliably released.

  Immediately after the engine is stopped, pressure remains inside the urea water addition valve 15 and the like, and the internal pressure becomes higher than the atmospheric pressure. If the pressure remains inside the urea water addition valve 15 or the like in this way, the internal pressure becomes higher when the volume is increased at the time of freezing change, and the urea water addition valve 15 or the like is further concerned about damage. Therefore, immediately after the engine is stopped, the urea water addition valve 15 is temporarily opened regardless of the freezing change of the residual urea water. Thereby, the internal pressure of the urea water addition valve 15 etc. can be reduced.

  The control mode of the urea water addition valve 15 after the engine is stopped may be opened a plurality of times by gradually shortening the time interval in addition to opening the valve a plurality of times at regular intervals as described above. It is also possible to open the urea water addition valve 15 only once after the engine is stopped.

  When the outside air temperature when the engine is stopped is equal to or higher than a threshold temperature (for example, 0 ° C.) or in the summer, the ECU 30 is not started after the engine is stopped (ie, the ECU is started for releasing urea water). It is also possible.

  It is also possible to use an air assist type addition valve as the urea water addition valve. Specifically, the compressed air compressed by the compressor (on-vehicle compressor) is guided to the urea water supply system, and the urea water is atomized by the compressed air. Incidentally, some large trucks and the like are equipped with an air supply source for adjusting the brake pressure, and it is preferable to use this as an air supply source for air assist.

  At present, the practical application of the urea SCR system for in-vehicle diesel engines is being studied. However, the practical application of the urea SCR system for other engines such as gasoline engines (spark ignition engines) Is possible. Further, the present invention can be similarly applied to an exhaust purification system using a reducing agent other than urea water.

The block diagram which shows the outline of the urea SCR system in embodiment of invention. The flowchart which shows discharge | release process of residual urea water. The time chart which shows the change of urea water temperature and urea water pressure after an engine stop. The time chart which shows the change of urea water temperature and urea water pressure after an engine stop.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Exhaust pipe (exhaust passage), 12 ... DPF, 13 ... SCR catalyst (exhaust purification catalyst), 15 ... Urea water addition valve (reducing agent addition valve), 15a ... Front end jet port, 21 ... Urea water tank, 22 ... urea water pump, 23 ... urea water supply pipe (reducing agent supply unit), 30 ... ECU.

Claims (13)

An exhaust passage of the engine, provided with a reducing agent addition valve provided upstream of the exhaust purification catalyst, and supplying the liquid reducing agent supplied from the reducing agent supply section from the outlet of the reducing agent addition valve to the exhaust passage An exhaust purification device for an engine that promotes a specific exhaust purification reaction based on the addition of a reducing agent in the exhaust purification catalyst by being additionally supplied into the exhaust purification catalyst,
After the engine is stopped, the reducing agent addition valve is opened in accordance with the freezing change of the reducing agent remaining in the reducing agent addition valve or the reducing agent supply unit, and the residual reducing agent is discharged from the outlet through the exhaust passage. An exhaust emission control device for an engine, comprising a reducing agent release control means for releasing the inside. Freezing determination means for determining that a residual reducing agent such as the reducing agent addition valve is in a freezing transition process in which it is frozen after the engine is stopped,
2. The engine exhaust gas purification apparatus according to claim 1, wherein the reducing agent release control means opens the reducing agent addition valve when the freezing determination means determines that the freezing transition process is in progress. A temperature detecting means for detecting the temperature of the reducing agent remaining in the reducing agent addition valve or the like;
The engine exhaust gas purification apparatus according to claim 2, wherein the freezing determination means determines that the process is in the freezing transition process based on a detection result of the temperature detection means after the engine is stopped. Pressure detecting means for detecting the pressure of the reducing agent remaining in the reducing agent addition valve or the like,
The engine exhaust gas purification apparatus according to claim 2, wherein the freezing determination unit determines that the process is in the freezing transition process based on a detection result of the pressure detection unit after the engine is stopped.   The engine exhaust purification device according to any one of claims 2 to 4, wherein the reducing agent release control means opens the reducing agent addition valve a plurality of times during the freezing transition process based on a determination result of the freezing determination means. . The freezing determination means determines that it is in a state transition period including the immediately preceding liquid phase state and the immediately following solid phase state with respect to the time corresponding to the freezing transition process,
The engine exhaust gas purification apparatus according to any one of claims 2 to 5, wherein the reducing agent release control means opens the reducing agent addition valve in the state transition period. The freezing determination means repeatedly determines whether or not it is in the freezing transition process every time a predetermined time elapses after the engine is stopped.
The engine exhaust gas purification apparatus according to any one of claims 2 to 6, wherein the freezing determination by the freezing determination means is stopped after the reducing agent is released by the reducing agent release control means.   The freezing determination means predicts a time when the temperature of the residual reducing agent is equal to or lower than a reference temperature set based on the freezing point temperature of the reducing agent, based on temperature information such as an outside air temperature when the engine is stopped. The engine exhaust gas purification apparatus according to any one of claims 2 to 7, wherein it is determined whether or not the freezing transition process is in progress at the predicted time. Freezing prediction means for predicting the freezing transition time when the residual reducing agent such as the reducing agent addition valve or the like shifts to a frozen state immediately after the engine stops or immediately after the stop,
The engine exhaust gas purification apparatus according to claim 1, wherein the reducing agent release control means opens the reducing agent addition valve at a freezing transition time predicted by the freezing prediction means.   The engine exhaust gas purification apparatus according to claim 9, wherein the freezing prediction means predicts the freezing transition time based on temperature information such as an outside air temperature when the engine is stopped or immediately after the engine is stopped.   The engine exhaust emission control device according to claim 9 or 10, wherein the reducing agent release control means opens the reducing agent addition valve a plurality of times at the freezing transition timing based on a prediction result of the freezing prediction means.   The engine exhaust gas purification apparatus according to any one of claims 1 to 11, wherein the reducing agent addition valve is temporarily opened immediately after the engine is stopped regardless of a freezing change of the residual reducing agent.   The reducing agent is an aqueous urea solution, and the exhaust purification catalyst promotes the NOx reduction reaction by setting the NOx reduction reaction in which NOx is reduced by ammonia generated from the aqueous urea solution as the exhaust purification reaction. The exhaust emission control device for an engine according to any one of 1 to 12.

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