JP2009293513A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve a NOx purification rate adding a suitable reducing agent while suppressing the influence on an internal combustion engine. <P>SOLUTION: An SCR catalyst 42 is provided in an exhaust pipe 22 of an engine, and a urea water adding valve 44 is provided upstream. Urea water is supplied by pressurization by a urea water pump 53 to the urea water adding valve 44. In an ECU 60, a temperature of the SCR catalyst 42 is detected, and a urea water pressure pressurized by the urea water pump 53 based on the SCR catalyst is changed based on the SCR catalyst temperature. At this time, if the SCR catalyst temperature is in a predetermined low temperature region, the urea water pressure is raised, and an atomization particle size of the urea water to be added from the urea water adding valve 44 is changed. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification apparatus for an internal combustion engine, and more particularly, an exhaust gas that employs a selective catalytic reduction (SCR) that selectively purifies NOx (nitrogen oxide) in exhaust gas by ammonia as a reducing agent. The present invention is suitably applied to a purification system. This system is generally known as a urea SCR system because an aqueous urea solution is used as a reducing agent.

  In recent years, a urea SCR system has been developed as an exhaust purification system for purifying NOx in exhaust gas at a high purification rate in engines (particularly diesel engines) applied to automobiles and the like, and some have been put into practical use. . The following configuration is known as a urea SCR system. That is, in the urea SCR system, a selective reduction type NOx catalyst is provided in an exhaust pipe connected to the engine body, and urea water (urea aqueous solution) as a NOx reducing agent is added into the exhaust pipe upstream thereof. A water addition valve is provided.

  In the above system, urea water is added into the exhaust pipe by the urea water addition valve, so that NOx in the exhaust gas is selectively reduced and removed on the NOx catalyst. When NOx is reduced, urea water is hydrolyzed by exhaust heat to produce ammonia (NH3), which is adsorbed on the NOx catalyst and undergoes a reduction reaction based on ammonia on the NOx catalyst. As a result, NOx is reduced and purified.

In addition, in order to improve the NOx purification performance of the NOx catalyst, a technique for promoting evaporation and diffusion in the exhaust pipe of the reducing agent added from the urea water addition valve has been proposed (for example, see Patent Document 1). That is, in the invention of Patent Document 1, a shielding member is provided in the exhaust passage on the exhaust upstream side of the NOx catalyst, and a purification agent is injected downstream of the shielding member to promote atomization of the purification agent. . In addition, the purifier sprayed into the exhaust passage is collided with the dispersion member to promote atomization of the purifier.
JP 2007-255343 A

  However, in the invention of Patent Document 1, the pressure in the exhaust passage increases with the provision of the shielding member and the dispersion member in the exhaust passage. Since this pressure increase in the exhaust passage always occurs during the operation of the internal combustion engine, there is a risk of adverse effects on the operation of the internal combustion engine such as deterioration of fuel consumption. That is, the exhaust flow rate varies depending on the engine operating state. At this time, for example, in a high-load operation state in which the intake air amount (exhaust flow rate) is large, it is considered that the loss in the exhaust increases due to the shielding member or the dispersion member provided in the exhaust passage, and the fuel consumption decreases.

  The main object of the present invention is to provide an exhaust purification device for an internal combustion engine that can add a suitable reducing agent while suppressing the influence on the internal combustion engine, thereby improving the NOx purification rate. .

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

  An exhaust emission control device according to the present invention includes a NOx catalyst (SCR catalyst) that is provided in an exhaust passage of an internal combustion engine and selectively purifies NOx in exhaust gas by a reducing agent, and a liquid reducing agent on the exhaust upstream side of the NOx catalyst. The present invention is applied to an exhaust gas purification system including a reducing agent adding unit to be added and a pressurizing unit to pressurize the reducing agent supplied to the reducing agent adding unit. And in invention of Claim 1, the temperature information of NOx catalyst or the temperature information correlated with it is acquired by measurement or estimation, and the reducing agent pressure pressurized by the pressurizing means based on the catalyst temperature or temperature information Is changed to change the spray particle diameter of the reducing agent added by the reducing agent addition means. For example, a pressurizing pump is used as the pressurizing means.

  Here, as described in claim 2, when the temperature of the NOx catalyst is in a predetermined low temperature range, the reducing agent pressure may be increased by the pressurizing means. By increasing the reducing agent pressure, the spray particle size of the reducing agent can be made fine. More specifically, a temperature threshold value for switching the spray particle size is determined, and the reducing agent pressure is increased when the catalyst temperature becomes lower than the temperature threshold value.

  In short, the inventors of the present application have confirmed that when the particle size of the reducing agent added and supplied from the reducing agent adding means is different, the NOx purification performance of the NOx catalyst is different. Specifically, as shown in FIG. 2, when the NOx catalyst becomes higher than a certain temperature (purification rate saturation temperature), the NOx purification rate is saturated to a predetermined high purification rate level. It was confirmed that the saturation temperature was lower as the particle size was smaller, depending on the diameter. In the present invention, focusing on such characteristics, the spray particle size of the reducing agent is changed in accordance with the catalyst temperature. Therefore, when the NOx purification rate of the NOx catalyst tends to be low, the NOx catalyst is made finer by reducing the spray particle size. The purification rate can be improved. Further, in the present invention, the pressure of the reducing agent is changed as necessary, and thereby the spray particle size of the reducing agent is changed. Therefore, unlike the conventional configuration in which a shielding member or a dispersion member is provided in the exhaust passage. The influence on the internal combustion engine can be suppressed. As a result, it is possible to add a suitable reducing agent while suppressing the influence on the internal combustion engine, thereby improving the NOx purification rate.

  Further, when the particle size of the reducing agent is changed, the NOx catalyst purification rate saturation temperature when the particle size is large is set as a temperature threshold, and a temperature lower than the temperature threshold is set in a predetermined low temperature range. (Claim 3). And when a catalyst temperature exists in a predetermined low temperature range, it is good to make the spray particle diameter of a reducing agent small. Thereby, when the NOx purification rate of the NOx catalyst does not reach the saturation value with the particle size of the reducing agent kept large, the NOx purification rate can be increased by atomizing the reducing agent.

  Immediately after the internal combustion engine is started, the NOx catalyst is in a low temperature state, and after the internal combustion engine is started, the temperature is gradually raised by exhaust heat. Therefore, as described in claim 4, it is preferable to reduce the spray particle size of the reducing agent when starting the internal combustion engine and increase the spray particle size after a predetermined time has elapsed. Thereby, a suitable reducing agent addition can be implemented according to the change of the catalyst temperature accompanying the start of the internal combustion engine.

  The spray particle size of the reducing agent is considered to change depending on the temperature of the reducing agent. Therefore, as described in claim 5, in addition to the pressurization of the reducing agent by the pressurizing means, the spraying particle diameter of the reducing agent may be changed by heating the reducing agent by the heating means. In this case, atomization of the reducing agent can be further promoted by pressurizing and heating the reducing agent.

  In the configuration in which the reducing agent pressure is changed as described above, the reducing agent addition amount per addition time (reducing agent addition amount per valve opening time when the reducing agent addition valve is used, “addition rate”) The amount of reducing agent introduced into the NOx catalyst varies. In this regard, as described in claim 6, the pressure of the reducing agent is detected, and at least the period of addition of the reducing agent by the reducing agent addition means according to the detected pressure of the reducing agent, and the addition time per one time Either of them may be set to be variable. Thereby, even when the reducing agent pressure is variably set, a desired amount of reducing agent can be added.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, an embodiment of the invention will be described with reference to the drawings. In this embodiment, an engine control system is constructed with a multi-cylinder diesel engine for a vehicle as a control target. In the control system, various controls of the engine are performed with an electronic control unit (hereinafter referred to as ECU) as a center. The In this embodiment, a common rail fuel injection system is adopted as the fuel injection system, and a urea SCR system is adopted as the exhaust purification system. First, an overall outline of the present system will be described with reference to FIG.

  The engine 10 has an engine body 11 having a reciprocating engine structure. As a basic structure, the engine 10 is provided in a piston 12 reciprocating in a cylinder, and each port on the intake side and the exhaust side, and is individually opened and closed. An intake valve 13 and an exhaust valve 14 are provided. As the piston 12 reciprocates, the crankshaft 15 rotates. The cylinder head is provided with a fuel injection valve 16 for each cylinder. Fuel is directly injected into the combustion chamber 17 by the fuel injection valve 16, and the injected fuel is used for combustion in the combustion chamber 17.

  The crankshaft 15 is provided with a crank angle sensor 18 for detecting the rotation of the crankshaft 15. Further, the cylinder block is provided with a water temperature sensor 19 for detecting the temperature of the engine cooling water.

  The configuration of the fuel supply system will be briefly described (however, since it is a well-known configuration, description by illustration is omitted). A high-pressure pump and a common rail (pressure accumulation pipe) are provided as a configuration of the fuel supply system, and the fuel in the fuel tank is increased in pressure by the high-pressure pump and is pumped to the common rail. A high pressure fuel of about several tens to 200 MPa is stored in the common rail, and this high pressure fuel is supplied to the fuel injection valve 16 of each cylinder. Note that the fuel pressure in the common rail is appropriately adjusted according to the engine operating condition and the like.

  An intake pipe (including a manifold portion) 21 is connected to the intake port of the engine body 11, and an exhaust pipe (including a manifold portion) 22 is connected to the exhaust port. The intake pipe 21 is provided with a throttle actuator 23 having an electrically driven throttle valve. The intake passage in the intake pipe 21 and the exhaust passage in the exhaust pipe 22 are connected by an EGR passage 24, and an EGR valve 25 and an EGR cooler 26 are provided in the EGR passage 24. An air cleaner 27 is provided at the most upstream portion of the intake pipe 21, and an air flow meter (intake air amount sensor) 28 is provided in the air cleaner 27.

  Further, in this system, a turbocharger 30 is provided as a supercharging device. The turbocharger 30 includes an intake compressor 31 provided in the intake pipe 21 and an exhaust turbine 32 provided in the exhaust pipe 22, and the exhaust turbine 32 rotates by the exhaust gas flowing through the exhaust pipe 22. The force is transmitted to the intake compressor 31 via the shaft 33. Then, the intake air flowing through the intake pipe 21 is compressed by the intake compressor 31 and supercharging is performed. The air supercharged by the turbocharger 30 is cooled by the intercooler 34 and then fed to the downstream side of the intake pipe 21.

  In addition, the intake pipe 21 is provided with sensors such as an intake pressure sensor and an intake air temperature sensor, but description thereof is omitted for convenience.

  Next, an exhaust purification system provided in the exhaust system will be described. In the exhaust pipe 22, an oxidation catalyst 41, an SCR catalyst (ammonia selective reduction catalyst) 42, and an ammonia slip catalyst 43 are disposed in order from the upstream side. The SCR catalyst 42 corresponds to a “NOx catalyst”. Further, a urea water addition valve 44 for adding and supplying urea water (urea aqueous solution) as a reducing agent into the exhaust pipe 22 is provided between the oxidation catalyst 41 and the SCR catalyst 42 in the exhaust pipe 22. . The urea water addition valve 44 has substantially the same configuration as an existing fuel injection valve (electromagnetically driven injector), and the tip injection hole of the urea water addition valve 44 is opened by a valve opening operation associated with an electrical control command. The urea water is injected (added) from the part.

  To the urea water addition valve 44, urea water is sequentially supplied from the urea water tank 51. Next, the configuration of the urea water supply system will be described.

  The urea water tank 51 is configured by a sealed container with a liquid supply cap, and urea water having a predetermined concentration (32.5%) is stored therein. One end of a urea water pipe 52 is connected to the urea water tank 51, and a urea water pump 53 as a pressurizing means is provided in the middle of the urea water pipe 52. The other end of the urea water pipe 52 is connected to the urea water addition valve 44. Similarly, the urea water pipe 52 is provided with a pressure sensor 54 for detecting the pressure of the urea water in the pipe. The urea water pump 53 is an electric pump that is rotationally driven by a drive signal from the ECU 60. When the urea water pump 53 is rotationally driven, the urea water in the urea water tank 51 is pumped up and passed through the urea water pipe 52. It is discharged to the urea water addition valve 44 side. In the present embodiment, in particular, the urea water pumping amount (pump pumping amount) by the urea water pump 53 can be variably adjusted, and the urea water pressure in the urea water pipe 52 can be changed by changing the pumping amount. It has become. The urea water pump 53 may be an in-tank pump that is installed in the urea water tank 51 while being immersed in the urea water.

  Further, a mixer 55 is provided between the oxidation catalyst 41 and the SCR catalyst 42 in the exhaust pipe 22 for generating a swirling flow in the exhaust gas flowing through the exhaust pipe 22. The mixer 55 is, for example, an exhaust stirrer composed of a rotating body having a plurality of blade pieces. The mixer 55 rotates with the passage of the exhaust gas, and the exhaust gas flows into the SCR catalyst 42 while rotating.

  In the exhaust purification system having the above configuration, when urea water is added and supplied into the exhaust pipe 22 by the urea water addition valve 44 during engine operation, urea water is supplied to the SCR catalyst 42 together with the exhaust gas in the exhaust pipe 22. The exhaust gas is purified by performing a reduction reaction of NOx in the SCR catalyst 42.

Specifically, the urea water injected from the urea water addition valve 44 is hydrolyzed by exhaust heat, and at that time, (NH2) 2CO + H2O → 2NH3 + CO2 (Formula 1)
Ammonia (NH3) is generated by the reaction as described above. When the exhaust gas passes through the SCR catalyst 42, NOx in the exhaust gas is selectively reduced and purified by ammonia. At that time, NOx is reduced and purified by the following reduction reaction.
4NO + 4NH3 + O2 → 4N2 + 6H2O (Formula 2)
6NO2 + 8NH3 → 7N2 + 12H2O (Formula 3)
NO + NO2 + 2NH3 → 2N2 + 3H2O (Formula 4)
In this way, when the reduction and purification of NOx by ammonia is performed, if ammonia does not react with NOx and becomes surplus, the surplus ammonia is mixed with the exhaust and released downstream of the exhaust. In such a case, surplus ammonia is removed by an ammonia slip catalyst 43 (for example, an oxidation catalyst) on the downstream side of the SCR catalyst.

  In addition, an oxygen concentration sensor 45 and a temperature sensor 46 are provided between the oxidation catalyst 41 and the SCR catalyst 42 in the exhaust pipe 22, and the oxygen concentration in the exhaust gas and the catalyst are based on the outputs of these sensors. The temperature is detected. Further, on the downstream side of the SCR catalyst 42, there is provided a NOx sensor 47 for detecting the NOx amount (NOx concentration) in the exhaust with the exhaust after passing through the SCR catalyst as a detection target. Based on this, the NOx purification rate of the SCR catalyst 42 is detected.

  Although omitted in FIG. 1, a DPF (diesel particulate filter) is installed in the exhaust pipe 22, and PM (particulate matter) in the exhaust is collected by the DPF. Yes.

  The ECU 60 includes a known microcomputer (not shown) including a CPU, a ROM, a RAM, and the like. The ECU 60 includes detection signals from the various sensors described above and other fuel pressures (rail pressure) in the common rail. Detection signals are sequentially input from a rail pressure sensor for detecting), an accelerator sensor for detecting an accelerator operation amount (accelerator opening) by a driver, and the like. Then, the ECU 60 performs fuel injection control, fuel pressure control (rail pressure control), and the like based on engine operation information such as engine rotation speed and accelerator operation amount. As a result, the fuel injection operation of the fuel injection valve 16 and the fuel pumping operation by the high-pressure pump are controlled. In addition, the ECU 60 appropriately executes control of the throttle actuator 23, the EGR valve 25, and the like based on each engine operating state.

  Based on the output of the NOx sensor 47, the ECU 60 calculates the NOx amount on the downstream side of the SCR catalyst 42 or calculates the NOx purification rate. Further, the urea water addition amount is controlled based on the NOx purification rate. Incidentally, the NOx purification rate X1 is calculated based on the NOx emission amount Y1 from the engine and the NOx amount Y2 on the downstream side of the SCR catalyst 42 (X1 = (Y1-Y2) / Y1). At this time, the NOx emission amount Y1 is calculated by a map or a mathematical formula based on the respective engine operating state (engine speed, fuel injection amount). Further, the NOx amount Y2 on the downstream side of the SCR catalyst 42 is calculated from the NOx sensor output.

  In addition, regarding the control of the urea water addition amount, the ammonia adsorption amount of the SCR catalyst 42 may be calculated, and the urea water addition amount may be controlled based on the ammonia adsorption amount. Specifically, the actual ammonia adsorption amount (actual adsorption amount) is calculated based on the balance between the ammonia supply amount and the ammonia consumption amount (ammonia reaction amount) in the SCR catalyst 42, and the actual adsorption amount and the target adsorption amount are calculated. The amount of urea solution added is feedback-controlled based on the deviation.

  Specifically, regarding the drive of the urea water addition valve 44, the ECU 60 outputs a valve opening command pulse with a predetermined cycle to the urea water addition valve 44, and the drive unit (solenoid) of the urea water addition valve 44 according to the pulse output. Drive current flows through Then, with the energization, the urea water addition valve 44 is opened and urea water is added (injected). At this time, the urea water addition amount is increased or decreased by variably adjusting the output period (or output frequency) of the valve opening command pulse. When reducing the urea water addition amount, the output cycle of the valve opening command pulse is increased. Further, when increasing the urea water addition amount, the output period of the valve opening command pulse is reduced. In addition, when decreasing the urea water addition amount, the valve opening drive of the urea water addition valve 44 may be stopped for a certain period.

  By the way, the NOx purification rate in the SCR catalyst 42, the SCR catalyst temperature, and the spray particle diameter of urea water have a correlation, and for example, it has been confirmed by the experiment of the present inventor that the relationship shown in FIG. FIG. 2 is a diagram showing the relationship between the SCR catalyst temperature and the NOx purification rate when the spray particle size of urea water is changed. FIG. 2 shows a case where the spray particle size is 100 μm (solid line) and a case where the spray particle size is 20 μm (two-dot chain line). Among these, the former is a spray particle size when a normal pump is driven. The latter corresponds to the spray particle size in the case of miniaturization with respect to normal driving.

  In FIG. 2, if the SCR catalyst temperature is higher than about 220 ° C., there is no difference in the NOx purification performance depending on the spray particle diameter, and the NOx purification rate is saturated to a predetermined high purification rate level (about 80% in the figure). On the other hand, when the temperature is lower than about 220 ° C., it can be confirmed that the smaller the spray particle size, the better the NOx purification performance. This is considered to be because if the SCR catalyst 42 is at a low temperature, the smaller the spray particle size, the easier it is to obtain the amount of heat from the atmospheric gas (exhaust gas), and the generation of ammonia is promoted. The saturation temperature of the NOx purification rate is about 220 ° C. when the particle size = 100 μm, and about 200 ° C. when the particle size = 20 μm. That is, the NOx purification rate can be improved by using a fine particle size urea spray in a low temperature range of about 220 ° C. or less (the purification rate saturation temperature or less when the particle size is 100 μm).

  Incidentally, the catalyst temperature at which the NOx purification rate is 50% in the case of performing normal urea water addition (spray particle size = 100 μm) is the activation temperature of the SCR catalyst 42, and in this embodiment about 180 ° C. It is.

  In addition, there is a correlation between the pressure of urea water (urea water pressure) supplied to the urea water addition valve 44 and the spray particle diameter of urea water, for example, the relationship shown in FIG. That is, the larger the urea water pressure, the smaller the spray particle size of the urea water. Therefore, in this embodiment, the urea water pressure is increased or decreased by adjusting the urea water pumping amount by the urea water pump 53, and the spray particle diameter of the urea water is changed accordingly.

  In the present embodiment, when the SCR catalyst 42 is at a relatively low temperature (in other words, when the degree of catalytic activity is small), urea water addition is performed with the urea water pressure being increased (high pressure state). When the SCR catalyst 42 is at a relatively high temperature (in other words, when the degree of catalytic activity is high), the urea water addition is performed in a state where the urea water pressure is reduced (a state where the pressure is not increased). By changing the urea water pressure as described above, atomization of the urea water spray is performed as necessary, and the NOx purification performance is maintained at a high level.

  When the SCR catalyst 42 is in a predetermined high temperature state, the NOx purification performance can be secured without depending on the spray particle diameter of the urea water (that is, regardless of the increase in the pressure of the urea water), while the urea water pressure is reduced. By doing so, the pump load can be reduced. Thereby, the effect of reducing the power consumption of a vehicle-mounted battery is acquired.

  FIG. 4 is a flowchart showing the procedure of urea water addition control, and this process is repeatedly executed by the ECU 60 at a predetermined time period.

  In FIG. 4, in step S11, it is determined whether or not the current engine speed NE is greater than a predetermined value K1. The predetermined value K1 is a threshold value for determining that the engine 10 is in an operating state, and for example, K1 = 800 rpm. Next, in step S12, it is determined whether or not the SCR catalyst temperature Tscr is higher than a predetermined value K2. The SCR catalyst temperature Tscr is calculated based on the output of the temperature sensor 46 provided on the upstream side of the SCR catalyst 42. The predetermined value K2 is a temperature threshold value for determining whether the SCR catalyst 42 is in an active completion state or an inactive state, and is, for example, K2 = 180 ° C. This temperature value (180 ° C.) corresponds to the catalyst temperature at which the NOx purification rate is 50% in the SCR catalyst 42 of the present embodiment.

  If at least one of the above steps S11 and S12 is NO, this process is terminated as it is, and if both of steps S11 and S12 are YES, the process proceeds to the subsequent step S13.

  In step S13, it is determined whether or not the current urea water pressure Pn is greater than a predetermined value K3. The urea water pressure Pn is a value calculated based on the detection result of the pressure sensor 54 provided in the urea water pipe 52. The predetermined value K3 is a pressure threshold value (minimum pressure that can be added) for determining that urea water can be added by the urea water addition valve 44, and is, for example, K3 = 0.4 MPa.

  If Pn ≦ K3, the process proceeds to step S14, and if Pn> K3, the process proceeds to step S15. In step S14, the urea water pump 53 is driven at a predetermined rotational speed. That is, the pump is driven so that the urea water pressure Pn is equal to or higher than the minimum pressure that can be added by the urea water addition valve 44.

  In step S15, it is determined whether or not the SCR catalyst temperature Tscr is higher than a predetermined value K4. The predetermined value K4 is a temperature threshold value for determining whether or not to atomize the urea water spray. For example, K4 = 220 ° C. The predetermined value K4 is the purification rate saturation temperature when the spray particle size = 100 μm (see FIG. 2).

  If Tscr> K4, the process proceeds to step S16, and if Tscr ≦ K4, the process proceeds to step S17. In step S16, the first target pressure PT1 is set, and the driving of the urea water pump 53 is controlled based on the first target pressure PT1. In step S17, the second target pressure PT2 is set, and the driving of the urea water pump 53 is controlled based on the second target pressure PT2. The first target pressure PT1 is a normal urea water pressure, for example, PT1 = 0.5 MPa. On the other hand, the second target pressure PT2 is a pressure value higher than the first target pressure PT1, for example, PT2 = 5 MPa. At this time, if the urea water pressure is in the relationship shown in FIG. 3, the spray particle diameter becomes 100 μm by setting the urea water pressure to the first target pressure PT1 (0.5 MPa), and the urea water pressure is the second pressure. By setting the target pressure PT2 (5 MPa), the spray particle size becomes 20 μm. The first target pressure PT1 is larger than a predetermined value K3 (0.4 MPa) which is the minimum pressure that can be added.

  Thereafter, in step S18, the urea water pressure is detected. In step S19, a control parameter for the urea water addition valve 44 is determined. Specifically, for example, the relationship shown in FIG. 5 is used, and the valve opening time (addition time per time) of the urea water addition valve 44 is determined based on the urea water addition amount and the urea water pressure at that time. The urea water addition amount is calculated based on the NOx purification rate or ammonia adsorption amount at each time. As a control parameter of the urea water addition valve 44, it is possible to determine the urea water addition cycle based on the urea water pressure at that time. Instead of the configuration in which the control parameter is calculated using the detected value of the urea water pressure, the configuration in which the control parameter is calculated using the target pressure (PT1, PT2) for each time may be used.

  Finally, in step S20, urea addition by the urea water addition valve 44 is executed based on the control parameter determined in step S19.

  FIG. 6 is a time chart for more specifically explaining the urea water control of the present embodiment. In FIG. 6, (a) shows the change in engine speed, (b) shows the change in SCR catalyst temperature, and (c) shows the change in urea water pressure.

  In FIG. 6, the engine 10 is started at the timing t1, and the idling operation is performed during the period from t1 to t2. After the engine is started, the SCR catalyst temperature gradually rises due to exhaust heat.

  Thereafter, when the engine speed increases due to an acceleration operation or the like at timing t2, the SCR catalyst temperature rapidly rises, and at time t3 when the SCR catalyst temperature reaches a predetermined value K2 (180 ° C.), driving of the urea water pump 53 is started. As a result, the urea water pressure rises. And when urea water pressure becomes larger than predetermined value K3 (0.4 MPa), control of urea water pressure based on 2nd target pressure PT2 (5 MPa) will be started. That is, the urea water pressure is increased, the urea water spray is atomized, and urea water is added (period t3 to t4).

  Thereafter, when the SCR catalyst temperature reaches a predetermined value K4 (220 ° C.) at timing t4, the target value of the urea water pressure is changed from the second target pressure PT2 (5 MPa) to the first target pressure PT1 (0.5 MPa). . That is, the urea water pressure is set to the normal pressure, and the urea water addition by the normal spray is performed without atomizing the urea water spray (period t4 to t5).

  Thereafter, when the SCR catalyst temperature decreases with deceleration and becomes equal to or lower than the predetermined value K4 (220 ° C.) at timing t5, the target value of the urea water pressure is changed again to the second target pressure PT2 (5 MPa), and the urea water spray is performed. Atomization is performed (period from t5 to t8). However, during the period from t6 to t7, since the SCR catalyst temperature is equal to or lower than the predetermined value K2 (180 ° C.) that is the catalyst activation temperature, the urea water addition is temporarily stopped and the urea water pump 53 during the same period. The pumping amount is changed to a minute amount.

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

  Based on each SCR catalyst temperature, the urea water pressure pressurized by the urea water pump 53 is changed, thereby changing the spray particle diameter of the urea water injected from the urea water addition valve 44. In particular, when the SCR catalyst temperature is in a predetermined low temperature range, the urea water pressure was increased to reduce the spray particle size of the urea water. Thereby, in a predetermined low temperature range where the NOx purification rate in the SCR catalyst 42 tends to be low, the NOx purification rate can be improved by making the spray particle size finer. In addition, the urea water pressure is changed as necessary to change the spray particle size of the urea water, so unlike the conventional configuration in which a shielding member or dispersion member is provided in the exhaust passage, adverse effects on the engine are suppressed. it can. As a result, it is possible to add a suitable urea water while suppressing adverse effects on the engine, thereby improving the NOx purification rate.

  Further, in the case where the spray particle size is not miniaturized (when the urea water pressure is not increased), the amount of current supplied from the power source (battery) to the urea water pump 53 is increased by the amount that the urea water pressure need not be increased. The energy can be reduced.

  Immediately after engine start (immediately after cold start), the SCR catalyst 42 is in a low temperature state, and the increase in the NOx purification rate tends to be delayed, but by reducing the spray particle size of urea water as described above, It is possible to speed up the NOx purification rate immediately after engine startup. Therefore, the exhaust purification performance immediately after the engine is started can be improved.

  Further, as described in the prior art, when a shielding member or a dispersion member is provided in the exhaust passage, these members are attached to the passage wall (exhaust pipe wall) by welding or the like. There are concerns about the problem of reliability and the reliability problem that the shielding member and the dispersion member may be damaged by long-term use. In this respect, in this embodiment, since the shielding member and the dispersion member are not required, the configuration can be simplified and the installation work can be prevented from becoming complicated. Further, it is possible to eliminate the occurrence of inconvenience due to breakage of the shielding member and the dispersion member in the exhaust pipe.

  Further, when changing the spray particle size of urea water, the saturation temperature of the NOx purification rate when the particle size is large is set as a temperature threshold value (predetermined value K4 = 220 ° C.), which is higher than the temperature threshold value. It was set as the structure which refines | miniaturizes urea water spray considering the low temperature side as a predetermined low temperature range. As a result, when the NOx purification rate of the SCR catalyst 42 does not reach the saturation value while the particle size of the urea water remains large, the NOx purification rate can be increased by atomizing the urea water.

  Since at least one of the urea water addition cycle by the urea water addition valve 44 or the addition time per time is variably set according to the urea water pressure, it is desirable even when the urea water pressure is changed in size. An amount of urea water can be added.

(Other embodiments)
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, the urea water pressure is changed based on the comparison result between the SCR catalyst temperature and the temperature threshold value (predetermined value K4), thereby reducing the spray particle size of the urea water. A configuration in which this is changed and the spray particle diameter is changed according to the passage of time in a predetermined period immediately after the engine is started may be employed. That is, at the time of engine start (particularly during cold start), the elapsed time from the start of the start is measured, the spray particle diameter of urea water is reduced until the elapsed time reaches a predetermined time, and the timing at which the predetermined time has elapsed. Increase spray particle size. The time threshold for switching from the large spray particle size to the small spray particle size is determined in advance based on experiments or can be varied according to the warm-up state at the time of engine start (for example, engine water temperature). It is good to set. Also in this structure, the spray particle diameter of urea water can be changed according to the temperature rise of the SCR catalyst 42, and suitable urea water addition can be implemented.

  In the above embodiment, the urea water spray particle size is switched in two stages (100 μm, 20 μm), but this may be changed and switched in three or more stages.

  -When changing the spray particle diameter of urea water, it is good also as a structure which changes a spray particle diameter in consideration of the amount of urea water addition in addition to SCR catalyst temperature. For example, when the urea water addition amount is large, the urea water pressure is increased to reduce the spray particle size of the urea water.

  The spray particle diameter of urea water is considered to change depending on the urea water temperature in addition to the urea water pressure. Thus, in addition to the urea water pressurization by the urea water pump 53, the urea water spray particle size may be reduced by heating the urea water by the heating means. In this case, it is preferable to provide a heater as a heating means in the urea water pipe 52 and to heat the urea water by the heater when reducing the spray particle diameter of the urea water. In this case, atomization of the urea water spray can be further promoted by pressurizing and heating the urea water.

  In the above embodiment, the SCR catalyst temperature Tscr is detected based on the output of the temperature sensor 46 installed on the upstream side of the SCR catalyst 42, and the spray particle size of urea water is changed based on the SCR catalyst temperature Tscr. Instead of this, the engine exhaust temperature may be measured by a sensor or the like, or may be estimated by calculation based on the engine operating state, and the spray particle diameter of urea water may be changed based on the exhaust temperature. In this case, the exhaust gas temperature corresponds to “NOx catalyst temperature information”.

  Moreover, it is good also as a structure which changes the spray particle diameter of urea water based on engine load of every time using a correlation with an engine driving | running load and SCR catalyst temperature. That is, when the engine load is low, the SCR catalyst temperature decreases as the exhaust gas temperature decreases. Therefore, when the engine load is less than the predetermined value, the spray particle diameter of urea water is reduced. The engine load can be estimated from the engine speed, fuel injection amount, accelerator operation amount, intake air flow rate, NOx emission amount, exhaust temperature, and the like.

  -It is also possible to embody the present invention other than the urea SCR system described above. For example, a system that uses solid urea as an ammonia generation source and generates urea water or ammonia water as a reducing agent from the urea, a system that uses fuel such as light oil as an ammonia generation source, and a system that directly adds ammonia water to the exhaust passage It can also be embodied in a system using a reducing agent (HC or the like) other than ammonia.

The block diagram which shows the outline of the engine control system in embodiment of invention. The figure which shows the relationship between the NOx purification rate in an SCR catalyst, SCR catalyst temperature, and the spray particle diameter of urea water. The figure which shows the relationship between urea water pressure and the spray particle diameter of urea water. The flowchart which shows the process sequence of urea water addition control. The figure which shows the relationship between the urea water addition amount, urea water pressure, and the valve opening time of a urea water addition valve. The time chart for demonstrating urea water control more concretely.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Engine (internal combustion engine), 22 ... Exhaust pipe, 42 ... SCR catalyst (NOx catalyst), 44 ... Urea water addition valve (reducing agent addition means), 53 ... Urea water pump (pressurization means), 60 ... ECU ( Acquisition means and spray particle size control means).

Claims (6)

NOx catalyst provided in an exhaust passage of an internal combustion engine for selectively purifying NOx in exhaust gas by a reducing agent, reducing agent adding means for adding a liquid reducing agent to the exhaust upstream side of the NOx catalyst, and adding the reducing agent Applied to an exhaust purification system comprising a pressurizing means for pressurizing a reducing agent supplied to the means,
Acquisition means for acquiring the temperature of the NOx catalyst or temperature information correlated therewith by measurement or estimation;
Based on the catalyst temperature or temperature information acquired by the acquisition unit, the spray particle size of the reducing agent added from the reducing agent addition unit is changed by changing the reducing agent pressure pressurized by the pressurization unit. Spraying particle size control means,
An exhaust emission control device for an internal combustion engine, comprising:   2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the spray particle size control means increases the reducing agent pressure by the pressurizing means when the temperature of the NOx catalyst is in a predetermined low temperature range.   In the case where the particle size of the reducing agent is changed, the NOx catalyst purification rate saturation temperature when the particle size is large is set as a temperature threshold, and the temperature lower than the temperature threshold is the predetermined low temperature range. The exhaust emission control device for an internal combustion engine according to claim 2, wherein   The internal combustion engine according to any one of claims 1 to 3, wherein the spray particle size control means reduces the spray particle size of the reducing agent when starting the internal combustion engine, and increases the spray particle size after a predetermined time has elapsed. Exhaust purification device. A heating means for heating the reducing agent added by the reducing agent addition means;
5. The internal combustion engine according to claim 4, wherein the spray particle size control unit changes the spray particle size of the reducing agent by performing heating of the reducing agent by the heating unit in addition to pressurization of the reducing agent by the pressurizing unit. Engine exhaust purification system. Means for detecting the pressure of the reducing agent;
Means for variably setting at least one of a reducing agent addition period by the reducing agent addition means and an addition time per one time according to the detected pressure of the reducing agent;
An exhaust emission control device for an internal combustion engine according to any one of claims 1 to 5, further comprising:

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