JP4748664B2 - Exhaust purification device - Google Patents

Description

  The present invention relates to an exhaust purification device for purifying engine exhaust, and more particularly to an exhaust purification device including a NOx catalyst that reduces and purifies NOx in exhaust using ammonia as a reducing agent.

As an exhaust purification device for purifying NOx (nitrogen oxide), which is one of the pollutants contained in the exhaust discharged from the engine, a selective reduction type NOx catalyst is disposed in the exhaust passage of the engine and reduced. An exhaust gas purification apparatus is used that purifies NOx in exhaust gas by supplying ammonia to the NOx catalyst as an agent.
In such an exhaust purification device, it is known that urea water, which is easier to handle than ammonia, is added to the exhaust gas in order to supply ammonia to the NOx catalyst. The urea water added to the exhaust gas is hydrolyzed by the heat of the exhaust gas, and the resulting ammonia is supplied to the NOx catalyst. Ammonia supplied to the NOx catalyst is once adsorbed by the NOx catalyst, and NOx purification is performed by promoting the denitration reaction between this ammonia and NOx in the exhaust gas by the NOx catalyst.

At this time, since the urea water is atomized so as to be easily hydrolyzed and supplied into the exhaust gas, the urea water is mixed in the pressurized air in advance by the urea water injection device and is discharged into the exhaust gas from the urea water addition nozzle together with the pressurized air. It comes to be sprayed.
The amount of urea water added varies depending on the amount of NOx to be purified by the NOx catalyst. However, when the amount of urea water added is relatively small, the urea water is taken away by the pressurized air, and urea crystals are precipitated. There is a possibility that clogging may occur in the urea water supply path such as the water injection device, the urea water addition nozzle, or the passage therebetween.

Further, even when the flow rate of the pressurized air is decreased in accordance with the decrease in the urea water supply amount, the urea water supply is reduced when the urea water supply amount decreases and the pressurized air flow rate decreases. There is a possibility that the urea water adhered in the path is not carried away by the pressurized air, and clogging may occur due to urea crystals precipitated from the adhered urea water.
In order to prevent the occurrence of such clogging, a method for maintaining the supply amount of urea water to be equal to or higher than the lower limit value when the supply amount of urea water is less than a predetermined lower limit value, and a supply amount of urea water are A method of stopping the supply amount of urea water when it becomes less than a predetermined lower limit value can be considered.

An exhaust purification device that prevents the supply amount of urea water from falling below the lower limit when the target supply amount of urea water is less than a predetermined lower limit is proposed by Patent Document 1 or the like, although the purpose is different. ing.
The exhaust gas purification apparatus disclosed in Patent Document 1 has a problem in that the temperature of the urea water addition nozzle rises due to a decrease in the urea water cooling effect accompanying a decrease in the urea water supply amount, and the urea water crystallizes and causes clogging. The purpose is to prevent. In order to achieve such an object, a urea water supply amount capable of cooling the urea water addition nozzle below the temperature at which the urea water crystallizes in the urea water addition nozzle is obtained, and the urea water supply amount is not reduced below this urea water supply amount. Water supply control is performed.
JP 2005-113688 A

  As in the exhaust gas purification device disclosed in Patent Document 1, when the urea water exceeding the lower limit value is supplied even when the target supply amount of urea water is lower than the predetermined lower limit value, NOx An amount of ammonia exceeding the supply amount necessary for purification of NOx is supplied to the NOx catalyst. For this reason, not only is the urea water wasted more than necessary, but also the problem is that excess ammonia that did not contribute to the purification of NOx by the NOx catalyst flows out from the NOx catalyst and is released into the atmosphere. .

In addition, when the supply of urea water is stopped when the target supply amount of urea water falls below a predetermined lower limit value, ammonia necessary for NOx purification is not supplied to the NOx catalyst. The efficiency is lowered, and there arises a problem that NOx that has not been purified by the NOx catalyst is released into the atmosphere.
The present invention has been made in view of such problems, and the object of the present invention is to add urea water into the exhaust gas without causing clogging and supply an appropriate amount of ammonia to the NOx catalyst. An object of the present invention is to provide an exhaust emission control device that can perform the above-described process.

In order to achieve the above object, an exhaust emission control device according to the present invention is provided in an exhaust passage of an engine, reduces NOx in exhaust using ammonia as a reducing agent, and urea water in the exhaust of the engine. A urea water supply means for supplying ammonia to the NOx catalyst by adding; a urea water supply amount adjusting means for adjusting the amount of the urea water added to the exhaust gas from the urea water supply means; and the NOx catalyst. The target supply amount of the urea water necessary for NOx reduction purification by the above is obtained, and when the target supply amount is less than a predetermined lower limit, the moment when the urea water is actually supplied from the urea water supply means on the specific feed amount was the lower limit value or more, control for controlling the urea water supply amount adjusting means so as to add intermittently the urea water in correspondence to the target supply amount in the exhaust gas Characterized in that a means (claim 1).

According to the exhaust purification apparatus configured as described above, when the target supply amount of urea water necessary for NOx reduction purification by the NOx catalyst is less than a predetermined lower limit value, the urea water is actually supplied from the urea water supply means. The urea water supply amount adjusting means is configured to intermittently add urea water into the exhaust gas corresponding to the target supply amount after setting the instantaneous supply amount during supply to the lower limit value or more. It is controlled by the control means.

Specifically, the lower limit value is clogged due to crystallization of the urea water in the urea water supply means when the urea water is continuously added to the exhaust gas by the urea water supply means. The value is larger than the upper limit value of the supply amount (Claim 2).
In the exhaust emission control device, the control means adjusts the urea water supply amount so as to continuously add the urea water corresponding to the target supply amount when the target supply amount is not less than the lower limit value. The means is controlled (claim 3).

  According to the exhaust gas purification apparatus configured as described above, when the target supply amount is less than a predetermined lower limit value, the supply amount per unit time when urea water is supplied from the urea water supply means is set to the lower limit value. In addition to the above, the urea water is intermittently added into the exhaust gas corresponding to the target supply amount, while when the target supply amount is equal to or greater than the lower limit value, the urea water is continuously corresponding to the target supply amount. The urea water is added to the exhaust gas.

In the exhaust purification apparatus, more specifically, the control means performs the intermittent addition of the urea water by performing PWM control of the urea water supply amount adjusting means in accordance with the target supply amount. It is characterized (claim 4).
Alternatively, in the exhaust emission control device, specifically, the control means performs intermittent addition of the urea water by performing PFM control of the urea water supply amount adjusting means according to the target supply amount. It is characterized (claim 5).

According to the exhaust gas purification apparatus of the present invention, when the target supply amount of urea water necessary for NOx reduction purification by the NOx catalyst is less than a predetermined lower limit value, urea water is intermittently exhausted corresponding to the target supply amount. To be added. In that case, since the instantaneous supply amount when urea water is actually supplied from the urea water supply means is set to the lower limit value or more, the urea when urea water flows in the urea water supply means The amount of ammonia flowing to the NOx catalyst can be maintained at an amount that prevents the clogging due to crystallization of urea water from occurring in the urea water supply means. An appropriate amount required for NOx purification can be obtained.

Therefore, while preventing clogging due to a decrease in the flow rate of urea water, an appropriate amount of ammonia necessary for NOx reduction and purification is supplied to the NOx catalyst to reduce NOx purification efficiency and to the atmosphere of ammonia. Can be prevented from being released.
Further, as in the exhaust purification device of claim 4, when the urea water supply amount adjusting means is PWM controlled according to the target supply amount, intermittent addition of the reducing agent is performed, or claim 5 When the urea water is intermittently added by performing PFM control of the urea water supply amount adjusting means according to the target supply amount as in the exhaust gas purification apparatus of the above, the ammonia supplied to the NOx catalyst This amount can be controlled to an appropriate amount with higher accuracy.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a system configuration diagram of a four-cylinder diesel engine (hereinafter referred to as an engine) to which an exhaust emission control device according to an embodiment of the present invention is applied. The exhaust emission control device according to the present invention is based on FIG. The structure of will be described.
The engine 1 includes a high-pressure accumulator chamber (hereinafter referred to as a common rail) 2 common to each cylinder, and supplies light oil, which is high-pressure fuel stored in the common rail 2, to an injector 4 provided in each cylinder. From this, light oil is injected into each cylinder.

  The intake passage 6 is equipped with a turbocharger 8. The intake air drawn from an air cleaner (not shown) flows into the compressor 8a of the turbocharger 8 from the intake passage 6, and the intake air supercharged by the compressor 8a is intercooler. 10 and the intake control valve 12 are introduced into the intake manifold 14. An intake flow rate sensor 16 for detecting the intake air flow rate to the engine 1 is provided upstream of the compressor 8 a in the intake passage 6.

On the other hand, an exhaust port (not shown) through which exhaust is discharged from each cylinder of the engine 1 is connected to an exhaust pipe (exhaust passage) 20 via an exhaust manifold 18. An EGR passage 24 that communicates the exhaust manifold 18 and the intake manifold 14 via the EGR valve 22 is provided between the exhaust manifold 18 and the intake manifold 14.
The exhaust pipe 20 passes through the turbine 8 b of the turbocharger 8 and is connected to an exhaust aftertreatment device 28 via an exhaust throttle valve 26. The rotating shaft of the turbine 8b is connected to the rotating shaft of the compressor 8a, and the turbine 8b receives the exhaust flowing in the exhaust pipe 20 and drives the compressor 8a.

The exhaust aftertreatment device 28 accommodates a NOx catalyst 30 that adsorbs ammonia and selectively reduces NOx in the exhaust gas using the adsorbed ammonia as a reducing agent. The NOx catalyst 30 is provided downstream of the NOx catalyst 30. An oxidation catalyst 32 for oxidizing the discharged ammonia is accommodated.
An inlet temperature sensor 34 for detecting the inlet side exhaust temperature of the NOx catalyst 30 is provided on the inlet side of the NOx catalyst 30, and an outlet temperature for detecting the outlet side exhaust temperature of the NOx catalyst 30 is provided on the outlet side of the NOx catalyst 30. A sensor 36 is provided.

A urea water addition nozzle 38 for injecting and adding urea water into the exhaust gas in the exhaust pipe 20 is provided on the upstream side of the exhaust aftertreatment device 28 in the exhaust pipe 20. The urea water addition nozzle 38 is urea. It is connected to a urea water injection device 42 via a water injection pipe 40.
The urea water tank 44 stores urea water, and the urea water in the urea water tank 44 is supplied to the urea water injection device 42 via the urea water supply pipe 46. The urea water supply pipe 46 is provided with a urea water control valve 48 that is an electromagnetic valve, which is opened when the urea water in the urea water tank 44 is supplied to the urea water injection device 42. When the supply of urea water to 42 is stopped, the valve is closed.

The air tank 50 stores pressurized air compressed by an air pump (not shown), and the pressurized air in the air tank 50 is supplied to the urea water injection device 42 via the air supply pipe 52. The air supply pipe 52 is provided with an air control valve 54 that is an electromagnetic metering valve for controlling the amount of pressurized air supplied to the urea water injector 42.
The urea water injection device 42 mixes the urea water supplied from the urea water tank 44 into the pressurized air supplied from the air tank 50 as described above, and the urea water is fed to the urea water addition nozzle 38 by the pressurized air. The amount of urea water added to the exhaust gas from the urea water addition nozzle 38 is adjusted by controlling the amount of pressurized air supplied to the urea water injection device 42 by the air control valve 54. It has become so.

Therefore, in this embodiment, the urea water addition nozzle 38, the urea water injection pipe 40, and the urea water injection device 42 correspond to the urea water supply means of the present invention, and the air control valve 54 corresponds to the urea water supply amount of the present invention. It corresponds to the adjusting means.
The mist-like urea water mixed into the pressurized air by the urea water injection device 42 and supplied to the urea water addition nozzle 38 is injected into the exhaust gas in the exhaust pipe 20 together with the pressurized air, and is hydrolyzed by the heat of the exhaust gas. To ammonia. The ammonia thus generated is supplied to the NOx catalyst 30 and once adsorbed on the NOx catalyst 30. The denitration reaction with NOx in the exhaust and the adsorbed ammonia in the NOx catalyst 30 is promoted by the NOx catalyst 30, exhaust gas is purified by the NOx is converted to harmless N _2.

The ECU (control means) 56 is a control device for performing comprehensive control including the operation control of the engine 1, and includes a CPU, a memory, a timer counter, and the like, and performs various control amount calculations. Various devices are controlled based on the control amount.
On the input side of the ECU 56, in order to collect information necessary for various controls, in addition to the intake flow rate sensor 16, the inlet temperature sensor 34, and the outlet temperature sensor 36 described above, a rotation speed sensor 58 that detects the engine speed, and Various sensors such as an accelerator opening sensor 60 for detecting the amount of depression of the accelerator pedal are connected, and on the output side, the injector 4 of each cylinder that is controlled based on the calculated control amount, the intake control valve 12, EGR Various devices such as the valve 22, the exhaust throttle valve 26, the urea water control valve 48, and the air control valve 54 are connected.

  The ECU 56 also performs calculation of the fuel supply amount to each cylinder of the engine 1 and fuel supply control from the injector 4 based on the calculated fuel supply amount. The fuel supply amount (main injection amount) necessary for the operation of the engine 1 is stored in advance based on the engine speed detected by the speed sensor 58 and the accelerator opening detected by the accelerator opening sensor 60. Read from the map and decide. The amount of fuel supplied to each cylinder is adjusted by the valve opening time of the injector 4, and each injector 4 is driven to open in a driving time corresponding to the determined fuel amount, and main injection is performed in each cylinder. Thus, the amount of fuel necessary for the operation of the engine 1 is supplied.

  Further, the ECU 56 selects urea water necessary for selectively reducing NOx discharged from the engine 1 by the NOx catalyst 30 based on the engine operating state such as the engine speed and the main injection amount of fuel detected by the speed sensor 58. Is obtained from map data stored in advance. Then, the ECU 56 opens the urea water control valve 48 and controls the air control valve 54 to adjust the amount of pressurized air supplied to the urea water injection device 42, so that urea corresponding to the target supply amount is obtained. Water is added into the exhaust gas from the urea water addition nozzle 38. The urea water added to the exhaust gas in this way is hydrolyzed by the heat of the exhaust gas to become ammonia. Using this ammonia as a reducing agent, the NOx catalyst 30 selectively reduces NOx in the exhaust gas as described above.

Such supply control of urea water by the ECU 56 is performed in a predetermined control cycle according to the flowchart of FIG.
First, it is determined in step S2 whether urea water can be supplied.
That is, for example, when the NOx catalyst 30 has not reached the activation temperature immediately after the engine 1 is started, or when the exhaust temperature has not reached a temperature enabling hydrolysis of the urea water, the urea water is being exhausted. However, NOx cannot be purified by the NOx catalyst 30. Therefore, in step S2, is it possible to supply urea water based on the engine operating state such as the exhaust temperature on the inlet side and outlet side of the NOx catalyst 30 detected by the inlet temperature sensor 34 and the outlet temperature sensor 36? Determine whether or not.

If it is determined in step S2 that urea water cannot be supplied, the process proceeds to step S4 and the urea water control valve 48 is closed, and in the next step S6, the air control valve 54 is closed and the current control is performed. End the cycle. Accordingly, in this case, urea water is not added from the urea water addition nozzle 38 into the exhaust gas.
On the other hand, if it is determined in step S2 that urea water can be supplied, the process proceeds to step S8. In the following description, it is assumed that urea water can be supplied.

In step S8, a target supply amount Qu of urea water necessary for purifying NOx in the exhaust with the NOx catalyst 30 is obtained. This target supply amount Qu is determined as follows.
First, the NOx emission amount from the engine 1 is estimated based on the engine operation state such as the engine rotation number detected by the rotation number sensor 58 and the fuel main injection amount calculated by the ECU 56. Further, the NOx purification rate of the NOx catalyst 30 is obtained from the map data stored in advance based on the exhaust temperature at the inlet side of the NOx catalyst 30 detected by the inlet temperature sensor 34. Next, the NOx purification amount by the NOx catalyst 30 is obtained from the estimated NOx emission amount and the NOx purification rate, and the ammonia amount corresponding to the NOx purification amount is obtained. The target supply amount Qu of urea water is obtained from the ammonia amount thus obtained.

Next, in step S10, it is determined whether or not the target supply amount Qu of urea water obtained in step S8 is smaller than a preset lower limit value Qmin.
In the state where urea water is continuously injected into the exhaust gas from the urea water addition nozzle 38, when the supply amount of urea water decreases, the amount of pressurized air also decreases as the urea water supply amount decreases. Therefore, the urea water attached in the urea water supply path such as the urea water injection device 42, the urea water injection pipe 40, and the urea water addition nozzle 38 is difficult to be carried away by the pressurized air. For this reason, when the amount is less than a certain supply amount, the attached urea water is crystallized and clogs the urea water supply path. Therefore, in this embodiment, the supply amount of urea water in which such clogging occurs is obtained in advance by experiments or the like, and an amount slightly larger than the upper limit value of the urea water supply amount in which clogging occurs is set as the lower limit value Qmin. ing.

Therefore, if it is determined in step S10 that the target supply amount Qu of the urea water is equal to or greater than the lower limit value Qmin, the urea water supply path is clogged even if the urea water is continuously supplied. The process proceeds to step S12 on the assumption that no occurs.
In step S12, the urea water control valve 48 is opened so that the urea water in the urea water tank 44 can be supplied to the urea water injector 42, and the process proceeds to the next step S14.

In step S14, an opening corresponding to the target supply amount Qu of urea water obtained in step S8 in the current control cycle is determined from a map indicating the relationship between the target supply amount of urea water stored in advance and the opening degree of the air control valve 54. The degree is read, and the air control valve 54 is controlled to reach the opening degree, and the current control cycle is completed.
In the next control cycle, after obtaining the target supply amount Qu of urea water again in step S8, it is determined in step S10 whether or not the target supply amount Qu is smaller than the lower limit value Qmin, and the target supply amount Qu is lower limit value Qmin. If it is above, it will progress to step S12.

In step S12, the urea water control valve 48 is opened, but if the urea water control valve 48 is already opened in the previous control cycle, the valve opened state is maintained.
In the next step S14, the urea water corresponding to the target supply amount Qu is added into the exhaust gas from the urea water addition nozzle 38 by controlling the opening degree of the air control valve 54 as described above.

Therefore, as long as the target supply amount Qu of urea water obtained in step S8 for each control cycle is equal to or higher than the lower limit value Qmin, urea water corresponding to this target supply amount is continuously exhausted from the urea water addition nozzle 38. To be added.
On the other hand, if it is determined in step S10 that the target supply amount Qu of the urea water is smaller than the lower limit value Qmin, if continuous addition is performed from the urea water addition nozzle 38, urea is crystallized because the urea water supply amount is small. The process proceeds to step S16 on the assumption that clogging may occur.

In step S16, as in step S12, the urea water control valve 48 is opened to enable the urea water in the urea water tank 44 to be supplied to the urea water injector 42, and the process proceeds to the next step S18.
In step S18, intermittent control is performed by alternately switching the air control valve 54 between a predetermined opening position and a fully closed position. At this time, the amount of urea water continuously added from the urea water addition nozzle 38 when the air control valve 54 is set to the predetermined opening is the lower limit value Qmin used in step S10. Such an opening is used. That is, when the air control valve 54 is intermittently controlled, the instantaneous urea water supply amount when urea water is added from the urea water addition nozzle 38 when the air control valve 54 is at a predetermined opening position. (Supply amount per unit time) is controlled to be the lower limit Qmin.

Further, the intermittent control of the air control valve 54 at this time is performed by PWM (Pulse Width Modulation) control corresponding to the target supply amount Qu set in step S8, and urea water supplied from the urea water addition nozzle 38 is used. The valve opening time of the air control valve 54 is controlled so that the supply amount becomes equal to the target supply amount Qu.
Also in the next control cycle, if the target supply amount Qu of urea water obtained in step S8 is smaller than the lower limit value Qmin, the process proceeds to step S16, and the urea water control valve 48 is opened. In addition, when the urea water control valve 48 has already been opened in the previous control cycle, the opened state is maintained.

  In the next step S18, the unit when the air control valve 54 is intermittently controlled by the PWM control corresponding to the target supply amount Qu as described above and the urea water is added from the urea water addition nozzle 38. The urea water is urea so that the supply amount per time is an amount corresponding to the lower limit value Qmin used in step S10, and the supply amount of urea water intermittently supplied by PWM control becomes the target supply amount Qu. The water is added intermittently from the water addition nozzle 38 into the exhaust.

Accordingly, while the target supply amount Qu of urea water obtained in step S8 for each control cycle is smaller than the lower limit value Qmin, the urea water is converted into urea water by PWM control of the air control valve 54 corresponding to the target supply amount Qu. The gas is intermittently added to the exhaust gas from the addition nozzle 38.
FIG. 3 shows a state of continuous addition of urea water and a state of intermittent addition when urea water supply control is performed as described above.

  The upper part of FIG. 3 shows the amount of NOx in the exhaust gas flowing into the NOx catalyst 30 over time. The middle part of FIG. 3 shows the supply amount of urea water necessary for purifying the NOx flowing in the NOx catalyst 30 in this way, That is, the target supply amount Qu is indicated. The lower part of FIG. 3 shows the urea addition amount actually added to the exhaust gas from the urea water addition nozzle 38 in correspondence with such a target supply amount Qu.

Note that Qmin in FIG. 3 is a lower limit value used in the above-described urea water supply control. If a smaller amount of urea water is supplied, urea will crystallize and clog even if it is continuously added. there's a possibility that.
As the amount of NOx flowing into the NOx catalyst 30 decreases, the target supply amount Qu of the urea water also decreases. As long as the target supply amount Qu is not less than the lower limit value Qmin, as described above, step S14 in FIG. Thus, the air control valve 54 is continuously opened to an opening corresponding to the target supply amount Qu, and urea water is continuously added into the exhaust gas from the urea water addition nozzle 38.

Then, at the point a in the figure, when the target supply amount Qu becomes equal to the lower limit value Qmin and then becomes lower than the lower limit value Qmin, the operation is switched to intermittent urea water addition by PWM control of the air control valve 54. In this intermittent addition, the air control valve 54 is set to a predetermined opening position during a part of the period ts in step S18 of FIG. 2 and is fully closed during other periods.
At this time, the predetermined opening is an instantaneous supply amount of urea water when the air control valve 54 is in the predetermined opening position and urea water is added from the urea water addition nozzle 38, that is, in the drawing. The amplitude of the urea water supply waveform at the time of intermittent addition is set to be equal to the lower limit value Qmin. Further, the period during which the air control valve 54 is set to a predetermined opening is set according to the target supply amount Qu by PWM control and gradually shortens as the target supply amount Qu decreases as shown in the figure (tp1>tp2>tp3> tp4). Thus, the supply amount of urea water added into the exhaust gas from the urea water addition nozzle 38 is controlled to be equal to the target supply amount Qu.

  Thus, when the target supply amount Qu of urea water necessary for NOx purification by the NOx catalyst 30 is smaller than the lower limit value Qmin, urea water when urea water is added from the urea water addition nozzle 38 is used. Since the urea water is intermittently supplied from the urea water addition nozzle 38 so that the supply amount per unit time becomes the lower limit value Qmin, when the urea water flows, the amount becomes larger than the amount of urea crystallizing. The occurrence of clogging due to urea crystallization can be prevented.

  Further, the supply amount of urea water supplied into the exhaust gas from the urea water addition nozzle 38 is controlled to be equal to the target supply amount Qu by PWM control of the air control valve 54 corresponding to the target supply amount Qu. For this reason, ammonia required for NOx purification of the NOx catalyst 30 is supplied to the NOx catalyst 30 without excess or deficiency, and it is possible to satisfactorily prevent NOx purification efficiency from being lowered and release of ammonia into the atmosphere. Extra consumption of water can be eliminated.

Although the description of the exhaust emission control device according to one embodiment of the present invention is finished above, the present invention is not limited to the above embodiment.
For example, in the above-described embodiment, the air control valve 54 is controlled by PWM control and urea water is intermittently added, but PFM (Pulse Frequency Modulation) control may be used instead of PWM control.

  FIG. 4 is a diagram showing a state of addition of urea water when urea water is intermittently added by PFM control. The upper part of FIG. 4 and the middle part of FIG. 4 show the amount of NOx in the exhaust gas flowing into the NOx catalyst 30 and the supply of urea water necessary for purifying the NOx flowing in this way with the NOx catalyst 30 as in FIG. The amount, that is, the target supply amount Qu is shown. The lower part of FIG. 4 shows the urea addition amount actually added to the exhaust gas from the urea water addition nozzle 38 in correspondence with the target supply amount Qu.

As the amount of NOx flowing into the NOx catalyst 30 decreases, the target supply amount Qu of the urea water also decreases. As long as the target supply amount Qu is equal to or higher than the lower limit value Qmin, the air control valve 54 keeps the target supply amount Qu. And the urea water is continuously added into the exhaust gas from the urea water addition nozzle 38.
Then, at the point a in the figure, when the target supply amount Qu becomes equal to the lower limit value Qmin and then becomes smaller than the lower limit value Qmin, switching to intermittent urea water addition by PFM control of the air control valve 54 is performed. In this intermittent addition, the air control valve 54 is set to the predetermined opening position for a certain period tp, and the repetition period changes according to the target supply amount Qu.

  At this time, the predetermined opening is the supply amount of urea water per unit time when the air control valve 54 is in the predetermined opening position and the urea water is being added from the urea water addition nozzle 38, that is, in the drawing. The amplitude of the urea water supply waveform at the time of intermittent addition is set to be equal to the lower limit value Qmin. Further, the period for setting the air control valve 54 to a predetermined opening is set according to the target supply amount Qu by PFM control, and gradually increases as the target supply amount Qu decreases as shown in the figure (ts1 <ts2). <Ts3 <ts4). Thus, the supply amount of urea water added into the exhaust gas from the urea water addition nozzle 38 is controlled to be equal to the target supply amount Qu.

In this way, even when the air control valve 54 is PFM-controlled, the same effect as in the above embodiment can be obtained.
The intermittent control of the air control valve is not limited to the PWM control and the PFM control as in the above embodiment, and both the valve opening period and the valve opening interval of the air control valve 54 are changed according to the target supply amount Qu. In any method, the supply amount of urea water per unit time when urea water is added from the urea water addition nozzle 38 by intermittent control becomes the lower limit value Qmin. The supply amount of urea water from the urea water addition nozzle 38 only needs to increase or decrease corresponding to the increase or decrease of the target supply amount Qu.

  In the above embodiment, the air control valve 54 is the reducing agent supply amount adjusting means of the present invention. However, the air control valve 54 is only switched between fully open and fully closed like the urea water control valve 48 in the above embodiment. The urea water control valve 48 may be an electromagnetic metering valve like the air control valve 54 in the above embodiment. In this case, the urea water control valve 48 corresponds to the reducing agent supply amount adjusting means of the present invention.

  In this case, the air control valve 54 is opened, and the urea water control valve 48 is intermittently controlled to open and close between a predetermined opening position and a fully closed opening position, so that the urea water addition nozzle 38 is opened. The urea water is intermittently added to the exhaust gas. At this time, the supply amount per unit time of the urea water when the urea water control valve 48 is in the predetermined opening position and the urea water is being added from the urea water addition nozzle 38 is the lower limit. It is set to be equal to the value Qmin.

  Finally, in the above embodiment, the present invention is applied to an exhaust emission control device for a diesel engine, but the engine type is not limited to this.

1 is an overall configuration diagram of an exhaust emission control device according to an embodiment of the present invention. It is a flowchart of the urea water supply control performed with the exhaust gas purification device of FIG. It is a figure which shows the supply state of the urea water by the urea water supply control of FIG. It is a figure which shows the supply state of the urea water by the modification of the urea water supply control of FIG.

Explanation of symbols

1 Engine 20 Exhaust pipe (exhaust passage)
30 NOx catalyst 38 Urea water addition nozzle (urea water supply means)
40 Urea water injection pipe (urea water supply means)
42 Urea water injection device (urea water supply means)
48 Urea water control valve 54 Air control valve (Urea water supply amount adjusting means)
56 ECU (control means)

Claims (5)

A NOx catalyst that is disposed in the exhaust passage of the engine and reduces and purifies NOx in the exhaust gas using ammonia as a reducing agent;
Urea water supply means for supplying ammonia to the NOx catalyst by adding urea water into the exhaust of the engine;
Urea water supply amount adjusting means for adjusting the amount of urea water added to the exhaust gas from the urea water supply means;
A target supply amount of the urea water necessary for NOx reduction purification by the NOx catalyst is obtained, and when the target supply amount is less than a predetermined lower limit value, the urea water is actually supplied from the urea water supply means. The urea water supply amount adjusting means is controlled so that the urea water is intermittently added to the exhaust gas in correspondence with the target supply amount, with the instantaneous supply amount at the time being equal to or higher than the lower limit value. An exhaust emission control device comprising: a control means.   The lower limit value is an upper limit value of the supply amount that causes clogging due to crystallization of the urea water in the urea water supply means when the urea water is continuously added to the exhaust gas by the urea water supply means. The exhaust emission control device according to claim 1, wherein the exhaust purification device has a larger value.   The control means controls the urea water supply amount adjusting means so as to continuously add the urea water corresponding to the target supply amount when the target supply amount is not less than the lower limit value. The exhaust emission control device according to claim 1.   2. The exhaust emission control device according to claim 1, wherein the control unit performs intermittent addition of the urea water by performing PWM control of the urea water supply amount adjusting unit according to the target supply amount. .   2. The exhaust emission control device according to claim 1, wherein the control unit performs intermittent addition of the urea water by performing PFM control of the urea water supply amount adjusting unit according to the target supply amount. .

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