JP2006328996A - Exhaust emission control device and exhaust emission control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device and an exhaust emission control method of properly purifying nitrogen oxides included in exhaust gas exhausted from an automobile and a factory. <P>SOLUTION: This exhaust emission control device is characterized by having a reactor 20 arranged in an exhaust passage 70 and discharging to the exhaust gas, a detecting means 40m for detecting the concentration of the nitrogen oxides in the exhaust gas in the exhaust passage 70 on the downstream side of the reactor 20, and a control means 50 for controlling output of the reactor 20 on the basis of a comparing result between the concentration of the nitrogen oxides detected by the detecting means 40m and a predetermined reference value; and controls the output of the reactor 20 on the basis of its comparing result by comparing the detected concentration of the nitrogen oxides with the predetermined reference value by detecting the concentration of the nitrogen oxides in the exhaust gas in the exhaust passage 70 on the downstream side of the reactor 20. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification apparatus and an exhaust gas purification method for treating exhaust gas.

  In general, exhaust gas discharged from automobiles, factories and the like contains harmful substances such as nitrogen oxides, and it is desired to purify them by using a catalyst or the like. However, when the oxygen concentration in the exhaust gas is high, there is a problem that it is difficult to purify nitrogen oxides with a catalyst such as a three-way catalyst.

  Therefore, for example, the nitrogen oxide is oxidized by discharge, the oxidized nitrogen oxide is adsorbed by the adsorption part, and then an electric field is applied to the adsorption part to concentrate the adsorbed and ionized nitrate ions to the positive polarity side, Finally, it has been proposed to purify nitrogen oxides (NOx) by reducing concentrated nitrate ions (see Patent Document 1).

JP 2004-68797 A

However, when the exhaust gas is discharged as described in Patent Document 1, a reaction suitable for the purification of nitrogen oxides, for example, a reaction in which NO is oxidized to NO ₂ may occur, but depending on the discharge conditions, the ambient temperature May rise and oxidation may occur in N ₂ in the exhaust gas, for example, NO may be generated. In this case, instead of promoting the purification of nitrogen oxides, it is increased, and there is a possibility that the purification of nitrogen oxides may not be performed properly.

  Then, an object of this invention is to provide the exhaust gas purification apparatus and exhaust gas purification method which purify | clean appropriately the nitrogen oxide contained in the exhaust gas discharged | emitted from a motor vehicle, a factory, etc.

  In order to solve the above problems, an exhaust gas purifying apparatus according to the present invention is provided in an exhaust passage and discharges into the exhaust gas, and the concentration of nitrogen oxides in the exhaust gas in the exhaust passage downstream of the reactor. And a control means for controlling the output of the reactor based on a comparison result between the concentration of nitrogen oxides detected by the detection means and a predetermined reference value. To do.

  With the above configuration, the reactor is controlled based on the comparison result between the concentration of nitrogen oxides in the exhaust gas downstream of the reactor and a predetermined reference value, so that the nitrogen oxidation using the reactor is performed. It becomes possible to purify the object appropriately.

  In order to solve the above-mentioned problem, the exhaust gas purification method according to the present invention discharges exhaust gas flowing through an exhaust passage by a reactor provided in the exhaust passage and promotes purification of nitrogen oxides in the exhaust gas. A purification method, the step of detecting the concentration of nitrogen oxides in the exhaust gas in the exhaust passage on the downstream side of the reactor, the concentration of nitrogen oxides detected in the detecting step and a predetermined reference value And a step of controlling the output of the reactor based on the result of the comparison.

  As a result, the reactor is controlled based on the comparison result between the concentration of nitrogen oxides in the exhaust gas downstream of the reactor and the predetermined reference value, so that the purification of nitrogen oxides using the reactor can be performed. It can be done appropriately.

  Preferably, the reference value is a concentration of nitrogen oxides in the exhaust gas upstream of the reactor. Thereby, the output of the reactor is controlled on the basis of the concentration of nitrogen oxides in the exhaust gas before entering the reactor.

  Further, the controlling step includes a step of suppressing the operation of the reactor when the concentration of nitrogen oxides in the exhaust passage on the downstream side of the reactor exceeds the reference value. To do. As a result, an increase in nitrogen oxides by the reactor is suppressed, and as a result, the operating power of the reactor can be saved.

  According to the present invention, nitrogen oxides contained in exhaust gas discharged from automobiles, factories and the like are appropriately purified.

Before describing the embodiment of the present invention, one experimental result regarding how the concentration of NO and NOx fluctuates by discharging to exhaust gas with a reactor comprising a pair of electrodes will be described with reference to FIG. explain. In this experiment, exhaust gas having a constant composition is caused to flow through an exhaust passage, and a discharge is generated by applying various values of voltage toward the exhaust gas by a reactor provided in the exhaust passage. NO gas concentration (curve t1), NO ₂ concentration (curve t2), and NOx concentration (curve T), which is the sum of NO and NO ₂ , were measured. When the applied voltage is gradually increased from the applied voltage “0 kV” in FIG. 1, that is, from the state in which the reactor is not discharged toward the exhaust gas, the NOx concentration does not substantially change to around 30 kV, but a higher level than that. When a voltage is applied, the NOx concentration tends to increase (see curve T). As apparent from the curves t1 and t2, this is mainly due to the change in the NO concentration that has been in a decreasing trend to an increasing trend. That is, first, when the applied voltage is gradually increased from 0 kV, the NO concentration starts to decrease as NO is oxidized to NO ₂ from about 20 kV, and the NO ₂ concentration increases accordingly. This trend continues to approximately 30 kV. However, when the voltage reaches approximately 30 kV, NO (so-called thermal NOx) is generated because a high voltage is applied to N ₂ in the exhaust gas and reacts with O ₂ to generate NO (so-called thermal NOx), so that the concentration of NO tends to increase. On the other hand, with decreasing oxygen concentration, NO is hardly changed into NO _2, NO ₂ turns to decrease. However, since the increase amount of the NO concentration exceeds the decrease amount of the NO ₂ concentration, the total NOx concentration starts to increase. From this, it is understood that if the voltage applied to the reactor is not properly controlled, it is difficult to purify NOx through the reactor. Therefore, in this embodiment, the applied voltage to the reactor is controlled to an appropriate applied voltage, for example, from 30 kV as shown in FIG. 1 within a predetermined range according to the NOx purification level.

  Hereinafter, an exhaust gas purification apparatus and an exhaust gas purification method according to embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 2 is a schematic configuration diagram showing that the exhaust gas purification apparatus 10 according to the embodiment of the present invention is applied to an exhaust system of a vehicle on which the engine 60 is mounted. The exhaust gas purification apparatus 10 is installed in an exhaust system of an engine 60 mounted on a vehicle, that is, in an exhaust passage 70 downstream of an exhaust port (not shown) of the engine 60, and the exhaust passage 70 is partitioned by an exhaust pipe 72. Has been. In addition, the engine 60 of this embodiment is a diesel engine. However, the exhaust gas purifying apparatus according to the present invention can be applied to various internal combustion engines, that is, an exhaust system of a spark ignition engine or a compression ignition engine, and is particularly applied to a lean burn gasoline engine having a high oxygen concentration in exhaust gas. Also good.

  The exhaust gas purification apparatus 10 of the present embodiment includes a reactor 20, a catalyst 30, a NOx concentration meter (NOx sensor) 40 (40u and 40m), and a control device (ECU) 50 that controls them. ing. The reactor 20 is provided in the exhaust passage 70, and the catalyst 30 is provided in the exhaust passage 70 downstream from the reactor 20. As the NOx sensor 40 of this embodiment, the NOx sensor 40u is disposed in the exhaust passage 70 upstream of the reactor 20, and the NOx sensor 40 is disposed in the exhaust passage 70 downstream of the reactor 20 and upstream of the catalyst 30. 40m is provided.

  The reactor 20 of this embodiment is installed in the exhaust pipe 72 so as to discharge toward the exhaust gas flowing through the exhaust passage 70. The reactor 20 is of a coaxial cylindrical type, and the reactor 20 is incorporated in the exhaust pipe 72 so that its own axis substantially coincides with the axis of the exhaust passage 70. The reactor 20 includes a case 22, a center electrode 24 that extends through the case 22, a ground electrode 26 that passes through the center of the case 24 and encloses the outer peripheral surface of the case 22, and a pulse voltage applied to the center electrode 24. And a high voltage power supply 28 to be applied. The ground electrode 26 is electrically grounded 26a.

  The center electrode 24 of the reactor 20 extends in the same direction as the flow direction of the exhaust gas, and a high voltage power source 28 is connected to the upstream end of the center electrode 24 via a metal connection 28a. The center electrode 24 can be made of chromium steel having excellent corrosion resistance (for example, 10Cr5Al), but is not limited thereto, and other metals having excellent corrosion resistance and other conductors may be used. it can. The case 22 is preferably made of a material having excellent heat resistance such as ceramics. However, since the case 22 is a diesel engine and the engine 60 has a low exhaust system temperature, the case 22 is relatively heat resistant such as acrylic resin. It may be made from a low material. On the other hand, the ground electrode 26 is formed of a heat-resistant and corrosion-resistant conductor such as stainless steel. The high voltage power supply 28 includes a DC-DC converter and a switching circuit so that a DC pulse wave having an arbitrary voltage value and waveform can be output by boosting the output of a DC power supply (not shown) (for example, a vehicle-mounted battery). Has been. The high voltage power supply 28 is controlled by a control output from an ECU 50 described later so as to apply a desired high voltage to the center electrode 24.

  The catalyst 30 of the present embodiment is a NOx storage reduction catalyst (NSR catalyst), and is provided to remove at least nitrogen oxides (NOx) from the exhaust gas discharged from the engine 60. Specifically, the catalyst 30 is a platinum / rhodium catalyst having a honeycomb structure, on which barium (Ba) is supported in order to enhance the NOx occlusion function. In addition to these, palladium, copper, manganese, or the like may be used as the catalyst component. The catalyst 30 is not limited to an NSR catalyst as long as at least NOx can be purified, and may be a three-way catalyst. For example, as the three-way catalyst, there is a platinum catalyst on which cerium (Ce) is supported in order to enhance the oxygen storage function.

  The NOx sensors 40u and 40m detect the concentration of NOx in the exhaust gas flowing through the exhaust passage 70.

  The ECU 50 is configured as a microcomputer including a CPU, ROM, RAM, A / D converter, input interface, output interface, and the like. NOx sensors 40u and 40m are connected to the input interface of the ECU 50 via electric wiring. A high voltage power supply 28 is connected to the output interface of the ECU 50. The applied voltage applied from the high voltage power supply 28 to the center electrode 24 in the reactor 20 is appropriately promoted to oxidize NOx based on the comparison result between the detected NOx concentration and a predetermined reference value. Be controlled. The voltage applied to the center electrode 24 is adjusted by varying the voltage command signal to the high voltage power supply 28.

In such a configuration, when a discharge is generated by the reactor 20 toward the exhaust gas flowing through the exhaust passage 70, plasma is generated under certain conditions, and electrons collide with oxygen molecules O ₂ in the air and oxygen atoms are generated. Oxygen atoms are combined with oxygen molecules to generate ozone O ₃ . Since this ozone has a strong oxidizing power, NOx contained in the exhaust gas, particularly NO, is oxidized and NO ₂ is generated and introduced into the catalyst 30. Note that NO ₂ is also generated when oxygen atoms generated from oxygen molecules react with NO, for example.

On the other hand, on the catalyst 30 which is an NSR catalyst, NOx is generally purified by being reduced to N ₂ or O ₂ , but it is preferable that oxidation of NOx is performed in the previous stage. That is, for example, NO is preferably oxidized to NO ₂ before being introduced into the catalyst 30 and then converted to N ₂ or O ₂ . Before exhaust gas is introduced into the catalyst 30, NOx purification is efficiently performed by promoting oxidation of NOx in the reactor 20 as described above.

  However, as described above, depending on the degree of discharge to the exhaust gas by the reactor 20, there may be a case where NOx is not appropriately purified. Therefore, in the present embodiment, the discharge to the exhaust gas by the reactor 20 is controlled according to the NOx purification level.

  Hereinafter, the control of the power supplied to the reactor 20, that is, the voltage applied to the center electrode 24 will be described based on the flowchart of FIG.

  First, in step S301, the ECU 50 determines whether a control stop flag described later is “0”. Since the flag is reset in the initial state, it is affirmed here. Next, in step S303, the NOx concentration upstream of the reactor 20 by the NOx sensor 40u (hereinafter referred to as the first NOx concentration) Cu is also reduced by the NOx concentration downstream of the reactor 20 by the NOx sensor 40m (hereinafter referred to as the NOx concentration 40m). , Referred to as the second NOx concentration.) Cm is detected and stored in the RAM. Next, in step S305, it is determined whether or not the second NOx concentration Cm stored in the RAM is higher than the first reference value Co. The first reference value Co is a reference value for determining that the concentration of NOx in the exhaust gas flowing through the exhaust passage is so low that it is not necessary to promote NOx purification by the reactor 20, and is obtained in advance through experiments. Stored in ROM.

  When it is determined that the second NOx concentration Cm is equal to or less than the first reference value Co, the process proceeds to step S307, a timer counter value n described later is reset, and in step S309, the reactor 20 is not discharged toward the exhaust gas. The applied voltage sp from the high voltage power supply 28 in the reactor 20 is set to “0”, that is, turned OFF, and the routine is terminated.

On the other hand, if it is determined in step S305 that the second NOx concentration Cm is higher than the predetermined first reference value Co, the process proceeds to step S311 and the second NOx concentration Cm stored in the RAM is similarly stored in the RAM. It is determined whether or not the first NOx concentration Cu is higher. That is, in step S311, the first NOx concentration Cu is used as the predetermined second reference value. When it is determined that the second NOx concentration Cm is equal to or lower than the first NOx concentration Cu, the process proceeds to step S313, the timer counter value n is reset, and the applied voltage sp is set to sp1 in step S315. With this setting, an output signal is issued from the ECU 50 so that the applied voltage sp in the reactor 20 becomes sp1. The term sp1 is a value that the reactor 20 for example NO ₂ oxidation is suitably carried out in the NOx in the exhaust gas by generated. For example, 30 kV in FIG. 1 corresponds to this sp1.

On the other hand, if it is determined in step S311 that the second NOx concentration Cm is higher than the first NOx concentration Cu, the process proceeds to step S317. Here, the fact that the second NOx concentration Cm is higher than the first NOx concentration Cu means that the NOx concentration of the exhaust gas is increased by the discharge in the reactor 20. This phenomenon can be considered to be caused by, for example, the reaction of N ₂ and O ₂ in the exhaust gas to induce undesirable so-called thermal NOx production. Therefore, it is necessary to suppress the operation of the reactor 20, that is, the applied voltage sp in the reactor 20 in this embodiment so that such thermal NOx is not generated. Therefore, the applied voltage sp is suppressed at step S317 and subsequent steps.

  In step S317, the applied voltage sp is suppressed, that is, reduced. Specifically, the applied voltage sp is updated by a value obtained by subtracting a predetermined decrease amount Δsp from the applied voltage sp1 during normal operation. Then, a voltage instruction signal is issued from the ECU 50 to the high voltage power supply 28 so that the applied voltage sp becomes the updated one. That is, the operation of the reactor 20 is suppressed when the second NOx concentration Cm in the exhaust passage on the downstream side of the reactor 20 exceeds the first NOx concentration Cu that is a predetermined second reference value. Also, the timer counter value n is incremented (step S319). The timer counter value n is used to measure the application time with the suppressed applied voltage sp.

  The application of the applied voltage sp suppressed in this way is repeated until the second NOx concentration Cm becomes equal to or lower than the first NOx concentration Cu (step S311).

  However, even if the operation of the reactor 20 is suppressed for a predetermined time, the second NOx concentration Cm may not be equal to or lower than the first NOx concentration Cu. In such a case, the NOx purification is not properly promoted by the reactor 20, and thus the failure of the reactor 20 is suspected. Therefore, in step S321, the timer counter value n is compared with a threshold value x stored in advance in the ROM, and if the timer counter value n is equal to or greater than the threshold value x, the determination is negative and the process proceeds to step S323, and an abnormal alarm is generated, for example, And a predetermined control stop flag is set to “1” so that the control of the reactor 20 is stopped. Since the flag is set to “1” as a result of referring to step S301 described above, the process of step S309 is executed and the power supply to the reactor 20 is turned off (applied voltage sp = 0).

  As described above, experiments were conducted to confirm the effect of the present embodiment. In the experiment, the apparatus was applied to a vehicle equipped with a diesel engine, and the NOx purification rate measured by the NOx sensor 40 in the 10-15 mode and the input power to the reactor 20 were measured. In order to measure the NOx purification rate, the NOx concentration upstream of the reactor 20 is measured by the NOx sensor 40u, and an additional NOx sensor 40d (not shown) is disposed in the exhaust passage 70 downstream of the catalyst 30. The NOx concentration there was also measured. And it compared with the experimental result which continued charging 500W and 50kV which are the maximum input energy of a power supply into the reactor 20. FIG. As a result, in the comparative example in which the power to the reactor 20, that is, the applied voltage was not adjusted, the purification rate was 70% and the input energy was 500W. In contrast, in the example in which the applied voltage was adjusted based on the control of the present embodiment, the purification rate was 85% and the input energy was 300W. As a result, it has been clarified that the control of this embodiment promotes the purification of NOx while suppressing the generation of thermal NOx and saves the power supplied to the reactor 20.

  Although the present invention has been described based on the preferred embodiments, the present invention is applicable not only to an exhaust system of an internal combustion engine but also to various exhaust systems such as an exhaust system from a factory. The reactor 20 may be of any other type, such as a parallel plate type as well as a coaxial cylindrical type, as long as it can purify exhaust gas by the action of electric power. Also good. In the above embodiment, the catalyst 30 is disposed in the exhaust passage 70 on the downstream side of the reactor 20. However, the catalyst 30 is not limited to the catalyst, and other devices such as an apparatus that absorbs and stores gas, electricity, etc. An apparatus that directly decomposes NOx in exhaust gas by force may be used. Furthermore, in the above embodiment, the NOx sensor is provided upstream of the reactor 20, that is, between the exhaust gas generation source and the reactor 20, and between the reactor 20 and the catalyst 30. It suffices that at least one exhaust passage 70 is disposed. Of course, any number of exhaust passages 70 may be provided as long as at least one of them is provided there.

  In the above embodiment, the voltage value sp1 is a fixed value. However, since the optimum applied voltage also varies depending on the NOx concentration contained in the exhaust gas, the predetermined map is referred to according to the first NOx concentration Cu, etc. The applied voltage sp1 may be changed. Further, the voltage value sp1 may be determined according to not only the first NOx concentration Cu upstream from the reactor 20, but also the NOx concentration downstream from the reactor 20, for example, the second NOx concentration Cm. Furthermore, since it is preferably also fluctuated depending on the operating condition of the engine, it can be one of two or more of exhaust gas temperature, catalyst temperature, exhaust gas amount, engine speed, and engine load. It may be determined according to the combination. In such a case, corresponding sensors, for example, a temperature sensor, a rotation speed sensor, and a load sensor are appropriately provided, or detection values of these sensors already provided in the engine 60 are used.

  In the above embodiment, the NOx concentration Cm on the downstream side of the reactor 20 detected by the NOx sensor 40m is compared with the NOx concentration Cu on the upstream side of the reactor 20 detected by the NOx sensor 40u. (Step S311) As a comparison target, a fixed value may be used instead of the actually measured upstream NOx concentration Cu.

  In the above embodiment, the present invention has been described with a certain degree of concreteness, but various modifications and changes can be made to the present invention without departing from the spirit and scope of the invention described in the claims. It must be understood that. That is, the present invention includes modifications and changes that fall within the scope and spirit of the appended claims and their equivalents.

NO with respect to the applied voltage in the reactor is a graph showing an example of changes in the concentration of NO 2, _NOx. It is a schematic block diagram which shows the place which the exhaust gas purification apparatus in embodiment of this invention was applied to the exhaust system of the vehicle carrying an engine. It is an example of the control flowchart of the exhaust gas purification apparatus in the embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Exhaust gas purification apparatus 20 Reactor 22 Case 24 Center electrode 26 Ground electrode 26a Ground 28 High voltage power supply 30 Catalyst 40u, 40m NOx sensor 60 Engine 70 Exhaust passage 72 Exhaust pipe

Claims (4)

A reactor provided in the exhaust passage for discharging into exhaust gas;
Detecting means for detecting the concentration of nitrogen oxides in the exhaust gas in the exhaust passage downstream of the reactor;
Control means for controlling the output of the reactor based on a comparison result between the concentration of nitrogen oxides detected by the detection means and a predetermined reference value;
An exhaust gas purification apparatus comprising: The exhaust gas flowing through the exhaust passage is discharged by a reactor provided in the exhaust passage, and the exhaust gas purification method for promoting the purification of nitrogen oxides in the exhaust gas,
Detecting the concentration of nitrogen oxides in the exhaust gas in the exhaust passage downstream of the reactor;
Comparing the concentration of nitrogen oxides detected in the detecting step with a predetermined reference value;
Controlling the output of the reactor based on the result of the comparison;
An exhaust gas purification method comprising:   The exhaust gas purification method according to claim 2, wherein the reference value is a concentration of nitrogen oxides in the exhaust gas upstream of the reactor. The step of controlling includes the step of suppressing the operation of the reactor when the concentration of nitrogen oxide in the exhaust passage on the downstream side of the reactor exceeds the reference value. Item 4. The exhaust gas purification method according to Item 2 or 3.

Images