JP2012057567A - Method for controlling exhaust cleaning apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maintain a high NOreducing rate without degrading performance while improving fuel efficiency than before.SOLUTION: In the method for controlling an exhaust cleaning apparatus, fuel 8 is added to an entry side of an NOoccluding reducing catalyst 5, the added fuel 8 is decomposed to Hand CO with a reforming catalyst 9 and is introduced to the NOoccluding reducing catalyst 5. An exhaust temperature to gain a prescribed level NOreducing rate or more with a reducer HC in the NOoccluding reducing catalyst 5 is made a reference temperature. When an exhaust temperature is lower than the reference temperature, a first fuel adding is executed in the range not reaching fuel rich atmosphere to heighten floor temperature of the reforming catalyst 9 to a fuel reforming temperature. Then, following the first fuel adding, a second fuel adding is executed to form fuel rich atmosphere. Further, when the exhaust temperature is not lower than the reference temperature, the fuel adding is controlled so as to discontinue the first fuel adding and to execute only the second fuel adding.

Description

  The present invention relates to a method for controlling an exhaust emission control device.

Conventionally, in a diesel engine, when the exhaust air-fuel ratio is lean, NOx in the exhaust gas is oxidized and temporarily stored in the form of nitrate, and when the O ₂ concentration in the exhaust gas decreases, unburned HC and CO For example, a NOx storage reduction catalyst having a property of decomposing and releasing NOx by the intervention of the above and the like to reduce and purify is installed in the middle of the exhaust pipe, and the NOx emission concentration is reduced by this NOx storage reduction catalyst.

However, in the NOx occlusion reduction catalyst, if the NOx occlusion amount increases and reaches the saturation amount, no more NOx can be occluded, and therefore the NOx occlusion reduction catalyst before the NOx occlusion amount reaches the saturation amount. It is necessary to decompose and release NOx by reducing the O ₂ concentration of the exhaust gas flowing into the exhaust gas with a reducing agent such as HC.

For example, when used in a gasoline engine, the operating air-fuel ratio of the engine is reduced (the engine is operated at a rich air-fuel ratio), thereby reducing the O ₂ concentration in the exhaust gas and unburned HC in the exhaust gas. It is possible to promote the decomposition and release of NOx by increasing reducing components such as CO and CO. However, when the NOx storage reduction catalyst is used as an exhaust gas purification device for a diesel engine, it is difficult to operate the engine at a rich air-fuel ratio. is there.

For this reason, when the NOx storage reduction catalyst is used as an exhaust gas purification device for a diesel engine, by adding fuel to the exhaust gas upstream of the NOx storage reduction catalyst, the HC generated from the added fuel is reduced. It is necessary to reduce the O ₂ concentration in the exhaust gas by reacting with O ₂ on the NOx storage reduction catalyst.

However, in such a system in which fuel is added upstream of the NOx storage reduction catalyst, a part of HC generated by evaporation of the added fuel reacts with O ₂ in the exhaust gas on the surface of the NOx storage reduction catalyst. (Combustion), and NOx decomposition and release starts after the O ₂ concentration in the atmosphere around the NOx storage reduction catalyst becomes zero, so that HC is converted to O ₂ on the surface of the NOx storage reduction catalyst. Under operating conditions where the combustion temperature (about 300 ° C) required for reaction (combustion) cannot be obtained (for example, slow driving in a city with heavy traffic), NOx is efficiently decomposed and released from the NOx storage reduction catalyst. As a result, the regeneration of the NOx storage reduction catalyst does not proceed efficiently, so that there is a concern that the recovery rate of the NOx storage site in the volume of the catalyst is reduced and the storage capacity is lowered.

For this reason, a reforming catalyst capable of decomposing the added fuel into H ₂ and CO is arranged in front of the NO x storage reduction catalyst, and a high NO x reduction rate is obtained from a relatively low temperature range by the highly reactive H ₂ and CO. In fact, as shown in the graph of FIG. 5, in particular, when H ₂ is used as the reducing agent, the performance is significantly improved as compared with the case where HC is used as the reducing agent. It has been confirmed that

  As prior art document information related to this type of exhaust purification device, there is the following Patent Document 1 and the like.

Japanese Patent Laid-Open No. 2007-9718

However, in order to decompose the added fuel into H ₂ and CO by the reforming catalyst, the bed temperature of the reforming catalyst is about 600 ° C. (fuel reforming temperature: this temperature is exemplified in the case of a general reforming catalyst) The temperature of the reforming catalyst varies depending on the type of reforming catalyst), and it is necessary to form a fuel-rich atmosphere with zero O ₂ concentration. It is necessary to add extra fuel to raise the temperature.

As shown in the graph of FIG. 5, if the exhaust temperature is about 350 ° C. or higher, a high NOx reduction rate of about 80% can be obtained even when HC is used as the reducing agent. are known, in the high temperature range of such exhaust gas temperature, since the performance difference between the case of of H ₂ and the reducing agent is small, the disadvantage of also fuel efficiency by subtracting the benefits of of H ₂ with a reducing agent There was a problem of being big.

  In addition, as shown in the graph of FIG. 6, the saturated storage amount of a general NOx storage reduction catalyst has a peak temperature in the vicinity of about 300 ° C., but increases as the catalyst bed temperature increases until this peak temperature is reached. The catalyst bed temperature rises when the catalyst bed temperature further rises beyond the peak temperature. Therefore, the bed temperature of the reforming catalyst is raised to about 600 ° C. in an operation state where the exhaust temperature is relatively high. When fuel is added, the bed temperature of the NOx occlusion reduction catalyst also rises, leading to a decrease in the NOx saturation occlusion amount, and the NOx occlusion reduction catalyst occludes significantly.

That is, if the saturated storage amount of the NOx occlusion reduction catalyst decreases due to the addition of fuel to raise the bed temperature of the reforming catalyst to about 600 ° C. and the occlusion capacity decreases, the exhaust air / fuel ratio becomes lean and NOx is reduced. The interval between occlusions has to be shortened, so that fuel addition must be performed frequently, resulting in a vicious circle in which the temperature of the NOx occlusion reduction catalyst is further increased, and H ₂ having high reactivity. There is a problem that the generation of NO does not lead to an improvement in the NOx reduction rate.

  The present invention has been made in view of the above-described circumstances, and provides a control method for an exhaust gas purification device that can maintain a high NOx reduction rate without causing performance degradation while improving fuel consumption deterioration as compared with the prior art. For the purpose.

The present invention is equipped with a NOx occlusion reduction catalyst in the middle of an exhaust pipe, fuel is added to the inlet side of the NOx occlusion reduction catalyst, and the added fuel is decomposed into H ₂ and CO by a reforming catalyst, and the NOx occlusion is performed. A control method for an exhaust gas purification device that can be led to a reduction catalyst, wherein an exhaust temperature at which a NOx reduction rate equal to or higher than a predetermined value is obtained using HC as a reducing agent in a NOx storage reduction catalyst is a reference temperature, and the exhaust temperature is a reference temperature The first fuel addition is performed within a range that does not reach a fuel-rich atmosphere when the temperature falls below the temperature, and the bed temperature of the reforming catalyst is raised to the fuel reforming temperature. While the second fuel addition for forming a rich atmosphere is performed, the fuel addition control is performed so that when the exhaust temperature is equal to or higher than the reference temperature, the first fuel addition is stopped and only the second fuel addition is performed. What is characterized by doing It is.

Thus, in this way, when the exhaust temperature falls below the reference temperature, the first fuel addition is performed, and the added fuel reacts with O ₂ coexisting in the surroundings in the reforming catalyst. The bed temperature of the reforming catalyst is raised to the fuel reforming temperature, and the second fuel addition is carried out following the first fuel addition, and a fuel-rich atmosphere in which the O ₂ concentration is zero around the reforming catalyst. Therefore, the fuel remaining in the atmosphere is decomposed into H ₂ and CO by the reforming catalyst and supplied to the NOx occlusion reduction catalyst, and immediately after the supply, the O ₂ concentration in the atmosphere becomes zero and the NOx is reduced. the onset of decomposition release immediately, NOx from a low temperature is reduced processed efficiently N ₂ than when the highly reactive H ₂ and CO, and the HC as a reducing agent directly on the surface of the NOx storage reduction catalyst It will be.

  On the other hand, when the exhaust temperature is higher than the reference temperature, the first fuel addition is stopped and only the second fuel addition is performed. However, if the exhaust temperature is higher than the reference temperature, HC is used as a reducing agent. However, it is possible to obtain a sufficiently effective NOx reduction rate that is greater than or equal to a predetermined value, and it is not necessary to cause a performance degradation due to the stop of the first fuel addition.

On the contrary, a fuel-rich atmosphere is formed by the addition of the second fuel in a temperature range where the exhaust temperature is higher than the reference temperature, so that a small amount of H ₂ , CO, and highly active HC can be obtained without raising the temperature to the fuel reforming temperature. Can be produced by a reforming catalyst, and a higher NOx reduction rate can be obtained than when HC is used as a reducing agent without a reforming catalyst.

  In addition, since the amount of the first fuel addition that has been used to raise the temperature of the reforming catalyst has been reduced so far, the fuel consumption can be greatly improved, and the temperature of the reforming catalyst can be raised by the first fuel addition. As a result, the increase in the bed temperature of the NOx occlusion reduction catalyst is suppressed, the decrease in the saturated occlusion amount is suppressed, and the NOx occlusion reduction catalyst's occlusion capacity is largely prevented from being lowered.

  Further, in the present invention, it is preferable to perform gradual change control so as to gradually increase the amount of fuel added at the start of the first fuel addition, and in this way, the bed temperature of the reforming catalyst is low and the catalyst activity is reduced. It is possible to prevent an excessive amount of fuel addition exceeding the processing capacity in the state where the gas is falling, and to avoid a decrease in the bed temperature (fuel cooling) due to the fuel addition exceeding the processing capacity. Generation of good reaction heat is promoted, and the bed temperature of the reforming catalyst can be raised quickly from the start of fuel addition.

  In the present invention, when setting the reference temperature of the exhaust temperature for switching the fuel addition control, if the exhaust temperature at which an NOx reduction rate of at least 80% is obtained when HC is used as the reducing agent is set as the reference temperature. good.

  According to the control method of the exhaust purification apparatus of the present invention described above, various excellent effects as described below can be obtained.

(I) When the exhaust temperature falls below the reference temperature, the bed temperature of the reforming catalyst is raised to the fuel reforming temperature by the first fuel addition, and the second fuel addition is continued to add O around the reforming catalyst. ₂ A fuel-rich atmosphere with a zero concentration is formed, and the added fuel is decomposed into H ₂ and CO and supplied to the NOx storage reduction catalyst, so that NOx is efficiently removed from a lower temperature than when HC is used as the reducing agent. _In addition, when the exhaust temperature is equal to or higher than the reference temperature, the first fuel addition is stopped and only the second fuel addition is performed, and in a temperature range where the exhaust temperature is higher than the reference temperature. By forming a fuel-rich atmosphere, a small amount of H ₂ , CO, and highly active HC can be generated in the reforming catalyst without raising the temperature to the fuel reforming temperature, and HC is used as a reducing agent without the reforming catalyst. Higher NOx reduction rate than the case can be obtained. By not increasing the temperature of the reforming catalyst due to the addition of fuel, the increase in the bed temperature of the NOx storage reduction catalyst is suppressed, and the decrease in the saturated storage amount is suppressed to prevent the NOx storage reduction catalyst from significantly decreasing the storage capacity. Therefore, reducing the amount of the first fuel added, which has been used to raise the temperature of the reforming catalyst until now, when the exhaust temperature is higher than the reference temperature, improves the fuel efficiency and lowers the performance. A high NOx reduction rate can be maintained without any problems.

  (II) If the gradual change control is performed so that the fuel addition amount is gradually increased at the start of the first fuel addition, the bed temperature is lowered due to an excessive amount of fuel addition exceeding the processing capacity (fuel cooling). Can be avoided, and the generation of extremely efficient reaction heat can be promoted. As a result, the bed temperature of the reforming catalyst can be raised at an early stage from the start of fuel addition, thereby avoiding unnecessary fuel addition. As a result, further improvement in fuel consumption can be achieved.

It is the schematic which shows an example of the form which implements this invention. It is a flowchart which shows the control procedure regarding the fuel addition of the control apparatus of FIG. It is a graph explaining addition fuel control when exhaust gas temperature falls below reference temperature. It is a graph explaining addition fuel control when exhaust gas temperature is more than reference temperature. The reducing agent is a graph showing the performance difference between the case of the and H ₂ For HC. It is a graph which shows the relationship between the saturated occlusion amount of NOx occlusion reduction catalyst, and temperature.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 shows an example of an embodiment for carrying out the present invention. In the exhaust purification apparatus of this embodiment, the exhaust pipe 4 through which the exhaust gas 3 discharged from the diesel engine 1 through the exhaust manifold 2 flows is shown. In addition, a NOx occlusion reduction catalyst 5 having a flow-through type honeycomb structure is mounted and mounted on the catalyst case 6, and the occlusion site of the NOx occlusion reduction catalyst 5 is recovered on the entrance side of the catalyst case 6 A fuel addition valve 7 for injecting fuel 8 (light oil) as a reducing agent is provided.

Further, in this embodiment, a reforming catalyst 9 for decomposing the fuel 8 added by the fuel addition valve 7 into H ₂ and CO is provided in the preceding stage of the NOx storage reduction catalyst 5 in the catalyst case 6. As the reforming catalyst 9, for example, a catalyst in which Pd, Pt, Rh, Rb or the like is supported as an active metal using an oxide such as alumina or silica or a composite oxide such as zeolite as a carrier is used. .

  Further, a temperature sensor 10 for detecting the exhaust temperature (the temperature of the exhaust gas 3) is disposed on the inlet side of the reforming catalyst 9, and a detection signal 10a from the temperature sensor 10 is sent to an engine control computer (ECU). : An electronic control unit).

  Here, in the control device 11, the exhaust temperature on the inlet side of the reforming catalyst 9 is monitored based on the detection signal 10 a from the temperature sensor 10 as shown in the flowchart in FIG. The first fuel addition is performed within a range that does not reach a fuel rich atmosphere when the exhaust temperature falls below a predetermined reference temperature, and the bed temperature of the reforming catalyst is raised to the fuel reforming temperature. Following the first fuel addition, the second fuel addition is performed to form a fuel-rich atmosphere. When the exhaust temperature is equal to or higher than the reference temperature, the first fuel addition is stopped and the second fuel addition is performed. Only the fuel addition control is performed.

More specifically, when the exhaust gas temperature falls below a predetermined reference temperature, a control signal 11a for instructing the first fuel addition and the second fuel addition as shown in FIG. In the first fuel addition for the purpose of raising the bed temperature of the reforming catalyst 9, the upper limit addition amount is appropriately set so that the exhaust air-fuel ratio does not become the stoichiometric state. Then, the fuel 8 is added while leaving the residual O _2, and the oxidation reaction of the fuel 8 is continued.

  In addition, particularly in this embodiment, the gradual change control is applied so that the fuel addition amount is gradually increased at the start of the first fuel addition, and the exhaust temperature is in a low temperature range below the reference temperature. An excessive amount of fuel exceeding the processing capacity of the reforming catalyst 9 is prevented from being added.

Note that in the second fuel addition for the purpose of creating a fuel-rich atmosphere, the fuel addition is performed in a sufficient amount so that the exhaust air-fuel ratio greatly exceeds the stoichiometric state. The fuel that exceeds the Ichiometric state will be decomposed into H ₂ and CO.

Further, when the exhaust gas temperature is equal to or higher than the reference temperature, a control signal 11a for commanding only the second fuel addition as shown in FIG. 4 is outputted to the fuel addition valve 7, and is added at this time. Most of the fuel 8 is supplied to the NOx occlusion reduction catalyst 5 in the form of HC. At this time, the exhaust gas temperature is already in a high temperature range higher than the reference temperature, so a very small amount of H ₂ , CO Highly active HC is produced by the reforming catalyst 9.

  Note that the reference temperature of the exhaust temperature for switching the fuel addition control is preferably set, for example, as an exhaust temperature at which a NOx reduction rate of at least 80% is obtained when HC is used as a reducing agent. Based on the previous graph of FIG. 5, 350 ° C. at which an NOx reduction rate of 80% is obtained when HC is used as the reducing agent is set as the reference temperature.

Thus, when the exhaust gas purification apparatus is operated in such a control system, the first fuel addition is performed when the exhaust gas temperature detected by the temperature sensor 10 falls below the reference temperature, and the added fuel 8 The reforming catalyst 9 reacts with surrounding O ₂ to raise the temperature of the reforming catalyst 9 to the fuel reforming temperature, and the second fuel addition is carried out following the first fuel addition. As a result, a fuel-rich atmosphere having a zero O ₂ concentration is formed around the reforming catalyst 9, so that the fuel 8 remaining in the atmosphere is decomposed into H ₂ and CO by the reforming catalyst 9, and the NOx storage reduction catalyst. 5 immediately after the supply, the decomposition and release of NOx is started immediately after the O ₂ concentration in the atmosphere becomes zero, and as it is, H ₂ and CO having a high reactivity on the surface of the NO x storage reduction catalyst 5 NOx is more efficient at lower temperatures than when used as a reducing agent It will be reduction treatment N _2.

  At this time, in this embodiment, since the gradual change control is applied so as to gradually increase the fuel addition amount at the start of the first fuel addition, the bed temperature of the reforming catalyst 9 is low and the catalytic activity is lowered. In this state, it is possible to prevent an excessive amount of fuel addition exceeding the processing capacity from being performed, and to avoid a decrease in the bed temperature (fuel cooling) due to the fuel addition exceeding the processing capacity. Heat generation is promoted, and the bed temperature of the reforming catalyst 9 can be raised at an early stage from the start of fuel addition.

  On the other hand, when the exhaust temperature detected by the temperature sensor 10 is equal to or higher than the reference temperature, the first fuel addition is stopped and only the second fuel addition is performed, but the exhaust temperature is increased above the reference temperature. For example, even if HC is used as the reducing agent, it is possible to obtain a sufficiently effective NOx reduction rate of a predetermined value or more, and it is not necessary to cause a performance deterioration due to the stop of the first fuel addition.

On the contrary, a fuel-rich atmosphere is formed by the addition of the second fuel in a temperature range where the exhaust temperature is higher than the reference temperature, so that a small amount of H ₂ , CO, and highly active HC can be obtained without raising the temperature to the fuel reforming temperature. Can be produced by the reforming catalyst 9, and a higher NOx reduction rate than that obtained when HC is used as the reducing agent without the reforming catalyst 9 can be obtained.

  In addition, since the amount of the first fuel addition that has been used to raise the temperature of the reforming catalyst 9 is reduced so far, the fuel consumption can be greatly improved, and the reforming catalyst 9 by the first fuel addition can be improved. By not raising the temperature, the rise in the bed temperature of the NOx storage reduction catalyst 5 is suppressed, the decrease in the saturated storage amount is suppressed, and the significant reduction in the storage capacity of the NOx storage reduction catalyst 5 is prevented.

Therefore, according to the above embodiment, when the exhaust gas temperature falls below the reference temperature, the bed temperature of the reforming catalyst 9 is raised to the fuel reforming temperature by the first fuel addition, and the second fuel addition is performed subsequently. Compared to the case where HC is used as a reducing agent, a fuel-rich atmosphere having a zero O ₂ concentration is formed around the reforming catalyst 9 and the added fuel is decomposed into H ₂ and CO and supplied to the NOx storage reduction catalyst 5. NOx can be efficiently reduced to N ₂ from a lower temperature, and when the exhaust temperature is equal to or higher than the reference temperature, the first fuel addition is stopped and only the second fuel addition is performed. By forming a fuel-rich atmosphere in the high exhaust gas temperature range, a small amount of H ₂ , CO, and highly active HC are generated by the reforming catalyst 9 without raising the temperature to the fuel reforming temperature. NOx lower than when HC is used as the reducing agent without the catalyst 9 Further, by not raising the temperature of the reforming catalyst 9 due to the addition of the first fuel, the rise in the bed temperature of the NOx occlusion reduction catalyst 5 is suppressed, and the decrease in the saturated occlusion amount is suppressed. Since a significant decrease in the storage capacity of the NOx storage reduction catalyst 5 can be prevented, the amount of the first fuel added that has been used to increase the temperature of the reforming catalyst 9 is reduced when the exhaust temperature is equal to or higher than the reference temperature. Thus, while improving the deterioration of fuel consumption as compared with the prior art, it is possible to maintain a high NOx reduction rate without causing performance degradation.

  In addition, since the gradual change control is performed so that the fuel addition amount is gradually increased at the start of the first fuel addition, the bed temperature is lowered due to an excessive amount of fuel addition exceeding the processing capacity (fuel cooling). Can be avoided and the generation of extremely efficient reaction heat can be promoted. As a result, the bed temperature of the reforming catalyst 9 can be raised at an early stage from the start of fuel addition. By avoiding this, it is possible to further improve fuel consumption.

  It should be noted that the method for controlling the exhaust purification apparatus of the present invention is not limited to the above-described embodiment, and it is needless to say that various changes can be made without departing from the scope of the present invention.

Reference Signs List 3 exhaust gas 4 exhaust pipe 5 NOx storage reduction catalyst 8 fuel 9 reforming catalyst 10 temperature sensor 10a detection signal 11 control device 11a control signal

Claims (3)

A NOx occlusion reduction catalyst is provided in the middle of the exhaust pipe, fuel is added to the inlet side of the NOx occlusion reduction catalyst, and the added fuel is decomposed into H ₂ and CO by the reforming catalyst, leading to the NOx occlusion reduction catalyst. A control method for an exhaust emission control device, wherein the exhaust gas temperature at which a NOx reduction rate equal to or higher than a predetermined value is obtained using HC as a reducing agent in a NOx occlusion reduction catalyst is a reference temperature, and the exhaust gas temperature falls below the reference temperature. The first fuel addition is performed within a range not reaching the fuel rich atmosphere to raise the reforming catalyst bed temperature to the fuel reforming temperature, and the fuel rich atmosphere is formed following the first fuel addition. The fuel addition control is performed so that when the exhaust temperature is equal to or higher than the reference temperature, the first fuel addition is stopped and only the second fuel addition is performed. Of exhaust purification equipment Control method.   The method of controlling an exhaust emission control device according to claim 1, wherein gradual change control is performed so that the fuel addition amount is gradually increased at the start of the first fuel addition.   The exhaust gas purification apparatus control method according to claim 1 or 2, wherein an exhaust gas temperature at which an NOx reduction rate of at least 80% is obtained when HC is used as a reducing agent is set as a reference temperature.

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