JP2020169640A - Exhaust emission control system - Google Patents

Abstract

To secure a capacity for reducing a nitrogen oxide amount, in an exhaust emission control system having an SCR catalyst and a three-dimensional catalyst at a catalyst layer.SOLUTION: An exhaust emission control system 10 for purifying a prescribed component in exhaust emission from an internal combustion engine 20 comprises: a catalyst layer 40 formed at an exhaust pipe 12 of the internal combustion engine 20, an ozone supply device 30 for supplying ozone to the catalyst layer 40, and a control device 50 for controlling the ozone supply device 30. The catalyst layer 40 includes at least either of a selective reduction catalyst for selectively reducing a nitrogen oxide by using a reductant supplied to the exhaust pipe as a catalyst for purifying the nitrogen oxide in the exhaust emission, and a three-dimensional catalyst for reducing the nitrogen oxide with a carbon monoxide or hydrocarbon in the exhaust emission as a reductant. The control device comprises a supply ratio control part 53 for controlling a supply ratio of an ozone amount supplied to the catalyst layer 40 with respect to a nitrogen oxide amount discharged from the internal combustion engine 20.SELECTED DRAWING: Figure 1

Description

The present invention relates to an exhaust gas purification system that purifies the exhaust gas of an internal combustion engine.

Lean NOx Trap (LNT) catalyst, Selective Catalytic Reduction (SCR) catalyst, and three-way catalyst are typical catalysts used to purify nitrogen oxides (NOx) contained in the exhaust of internal combustion engines. Catalysts (Three Way Catalyst: TWC) and the like are known. The LNT catalyst is a NOx occlusal reduction type purification catalyst that temporarily occludes NOx in the exhaust gas and then reduces it. As described in Patent Document 1, in an exhaust gas purification system provided with an LNT catalyst in a catalyst layer, a technique of supplying ozone to the upstream side of the catalyst layer is known. By supplying ozone, nitric oxide (NO) in the exhaust is oxidized to nitrogen dioxide (NO2), which is more easily adsorbed by the LNT catalyst, so that the amount of NOx adsorbed by the LNT catalyst increases. This promotes purification by the NOx reduction reaction in the LNT catalyst.

JP-A-2015-108350

In the NOx purification catalyst, the catalytic activity related to NOx purification may not be sufficiently obtained due to the low temperature of the catalyst layer or the like. A NOx storage reduction type purification catalyst called an LNT catalyst or the like needs to store NOx when reducing NOx, and is therefore prepared so that the amount of NOx adsorbed is increased by introducing a NOx adsorbed species or the like. .. Therefore, when a NOx storage reduction type NOx purification catalyst is used, the amount of NOx in the exhaust can be reduced to some extent by adsorbing NOx even when the catalytic activity of the NOx reduction reaction is not sufficiently obtained. Can be done. On the other hand, NOx purification catalysts such as SCR catalysts and three-way catalysts, which are not NOx storage reduction type, do not necessarily need to store NOx when reducing NOx, and are not necessarily prepared so that the amount of NOx adsorbed is high. Therefore, when an SCR catalyst or a three-way catalyst is used, NOx can hardly be adsorbed when the catalytic activity of the NOx reduction reaction cannot be sufficiently obtained, and the amount of NOx in the exhaust gas can hardly be reduced. There is.

In view of the above, it is an object of the present invention to provide a technique for ensuring the ability to reduce the amount of nitrogen oxides in an exhaust gas purification system provided with an SCR catalyst or a three-way catalyst in the catalyst layer.

The present invention provides an exhaust gas purification system that purifies the exhaust gas from an internal combustion engine. This exhaust purification system includes a catalyst layer arranged in an exhaust pipe of the internal combustion engine, an ozone supply device that supplies ozone to the catalyst layer, and a control device that controls the ozone supply device. The catalyst layer includes a selective reduction catalyst that selectively reduces nitrogen oxides with a reducing agent supplied to the exhaust pipe as a catalyst for purifying nitrogen oxides in the exhaust, and carbon monoxide in the exhaust. Alternatively, it includes at least one of a three-way catalyst that reduces nitrogen oxides using a hydrocarbon as a reducing agent. The control device includes a supply ratio control unit that controls the supply ratio of the amount of ozone supplied to the catalyst layer to the amount of nitrogen oxides discharged from the internal combustion engine.

As a result of diligent research, the present inventor has found that the amount of NOx adsorbed in each catalyst can be increased by supplying ozone to the selective reduction catalyst (SCR catalyst) and the three-way catalyst (TWC).

In the exhaust purification system according to the present invention, the catalyst layer uses a selective reducing catalyst that selectively reduces NOx by a reducing agent supplied to the exhaust pipe, and NOx using carbon monoxide or hydrocarbon in the exhaust as a reducing agent. It contains at least one of the reducing ternary catalysts as a catalyst. Then, ozone can be supplied to this catalyst layer by the ozone supply device and the control device. Further, the control device includes a supply ratio control unit that controls the supply ratio of the amount of ozone supplied to the catalyst layer to the amount of nitrogen oxides discharged from the internal combustion engine. This makes it possible to increase the amount of NOx adsorbed in the selective reduction catalyst or the three-way catalyst. Even if the state of the selective reduction catalyst or the three-way catalyst contained in the catalyst layer is such that the catalytic activity related to the NOx reduction reaction cannot be sufficiently obtained, by supplying ozone from the ozone supply device to the catalyst layer, The ability to increase the amount of NOx adsorbed in each catalyst and reduce the amount of NOx can be ensured.

The schematic diagram of the exhaust gas purification system which concerns on 1st Embodiment. A table showing catalyst names, their alternative names, and specific examples. The figure which shows the relationship between the catalyst temperature and NOx purification rate. The figure which shows the relationship between the amount of NOx adsorbed by the SCR catalyst and the supply ratio. The figure which shows the relationship between the amount of NOx adsorbed by DOC and the supply ratio. The figure which shows the relationship between the amount of NOx adsorbed by alumina and the supply ratio. FIG. 7A is a microscopic FT-IR imaging diagram of the surface of the LNT catalyst without ozone supply. FIG. 7B is a microscopic FT-IR imaging diagram of the surface of the LNT catalyst with ozone supply. The figure which shows the relationship between the amount of NOx adsorbed by the SCR catalyst and the supply ratio. The figure explaining the supply ratio control. The flowchart of the exhaust gas purification control which concerns on 1st Embodiment. The schematic diagram which shows the mode of the catalyst layer which concerns on other embodiment. The figure which illustrates the arrangement of the catalyst layer and the supply position of ozone and a reducing agent. The figure which illustrates the arrangement of the catalyst layer and the supply position of ozone and a reducing agent. The schematic diagram which shows the mode of the catalyst layer which concerns on other embodiment. The figure which illustrates the arrangement of the catalyst layer and the supply position of ozone and a reducing agent. The figure which illustrates the arrangement of the catalyst layer and the supply position of ozone and a reducing agent.

(First Embodiment)
As shown in FIG. 1, the exhaust gas purification system 10 is configured as a system capable of purifying a predetermined component contained in the exhaust gas discharged from the internal combustion engine 20 by the catalyst layer 40. The exhaust gas purification system 10 further includes an ozone supply device 30 and a reducing agent supply device 60.

The internal combustion engine 20 is a diesel engine, and the air sucked from the intake pipe 11 is compressed by the supercharging device 13 and sucked into the combustion chamber of the internal combustion engine 20, and the fuel injected from the fuel injection valve in this combustion chamber. It is used for combustion together.

The supercharging device 13 includes an intake compressor 14 arranged in the intake pipe 11, an exhaust turbine 15 arranged in the exhaust pipe 12, and a rotating shaft 16 connecting the intake compressor 14 and the exhaust turbine 15. When the exhaust turbine 15 is rotated by the exhaust from the internal combustion engine 20, the intake compressor 14 is rotated along with the rotation, and the intake air is supercharged.

The exhaust pipe 12 is provided with an exhaust NOx sensor 22 near the outlet of the exhaust turbine 15. The emission NOx sensor 22 detects the amount of nitrogen oxides (NOx amount) generated in the internal combustion engine 20 as the concentration. In the exhaust pipe 12, the catalyst layer 40 is arranged on the downstream side of the exhaust NOx sensor 22. The exhaust gas from the internal combustion engine 20 passes through the exhaust pipe 12 and is purified in the catalyst layer 40. The exhaust pipe 12 is provided with a post-catalyst NOx sensor 24 near the outlet of the catalyst layer 40. The post-catalyst NOx sensor 24 detects the amount of NOx in the exhaust gas after being purified by the catalyst layer 40 as a concentration.

The catalyst layer 40 includes a first catalyst layer 41 and a second catalyst layer 42 in this order from the upstream side. A catalyst temperature sensor 23 for detecting the temperature of the catalyst layer is installed in the first catalyst layer 41. FIG. 2 shows the catalyst names in the present specification, their aliases, and specific examples. In the present specification, the catalyst name shown in FIG. 2 is treated as including each catalyst referred to by the alias.

The first catalyst layer 41 includes at least a catalyst layer containing a NOx purification catalyst that purifies nitrogen oxides (NOx). Specifically, the NOx purification catalyst includes one or both of a selective reduction catalyst (SCR catalyst) and a three-way catalyst (TWC).

The SCR catalyst is a catalyst that selectively reduces NOx by a reducing agent supplied to the exhaust pipe 12. Typical examples of substances used as reducing agents in SCR catalysts include ammonia (NH3), hydrocarbons (HC), hydrogen (H2), and aldehydes (RCHO). In the SCR catalyst, NOx (NO, NO2) is reduced to N2 by a reducing agent. The reducing agent is oxidized to produce water (H2O), carbon dioxide (CO2), N2 and the like depending on the type of reducing agent. When ammonia is used as a reducing agent, it is supplied to the exhaust pipe 12 as urea water or the like, and ammonia is generated in the exhaust pipe 12.

As the SCR catalyst, for example, a catalyst containing a carrier such as a metal oxide and an active species of a noble metal or a base metal is known. More specifically, as shown in FIG. 2, as a specific example of the SCR catalyst using ammonia as a reducing agent, a zeolite catalyst such as ZSM5 containing copper (Cu), iron (Fe) or the like as a cation species can be exemplified. Further, as a specific example of the SCR catalyst using HC, H2 and RCHO as a reducing agent, a catalyst in which silver (Ag) or platinum (Pt) is supported on alumina (Al2O3) can be exemplified.

The three-way catalyst is a catalyst that reduces NOx using carbon monoxide (CO) or hydrocarbon (HC) in the exhaust as a reducing agent. According to the three-way catalyst, for example, by the catalytic reaction represented by the following formulas (1) to (3), CO is oxidized to carbon dioxide (CO2), HC is oxidized to water (H2O) or CO2, and NOx is nitrogen. It is reduced to (N2).

2CO + 2NO → 2CO2 + N2 ... (1)
2H2 + 2H2 + 2NO → 2H2 + 2NO → 2H2O + N2 ... (2)
[HC] + NO → N2 + CO2 + H2O ... (3)

As the three-way catalyst, for example, a catalyst containing a carrier such as a metal oxide and an active species of a noble metal is known. More specifically, as shown in FIG. 2, as a specific example of the three-way catalyst, a noble metal catalyst such as platinum (Pt), palladium (Pd), rhodium (Rh) is used as alumina, ceria / zirconia (CeO2 / ZrO2). Etc. can be exemplified as a catalyst carried in the above. The three-way catalyst may further contain, as a co-catalyst, ceria (CeO2), zirconia (ZrO2), lanthania (La2O3), etc., which have an oxygen storage capacity.

The SCR catalyst and the three-way catalyst are NOx purification catalysts that are not NOx storage reduction type, and are generally not prepared so as to have high NOx adsorption property. Therefore, the amount of NOx adsorbed is generally lower than that of a NOx storage reduction type catalyst called an LNT catalyst or the like.

The LNT catalyst occludes NOx on the catalyst during normal operation of the internal combustion engine 20, and occasionally reduces oxygen (O2) in the exhaust gas by rich spikes (injecting a large amount of fuel). Increases carbon oxide (CO), hydrocarbons (HC), etc. Then, it reacts with the occluded NOx and reduces it to nitrogen. As the LNT catalyst, for example, a catalyst containing a carrier such as a metal oxide, an active species of a noble metal or a base metal, and a NOx adsorbed species is used. More specifically, as shown in FIG. 2, specific examples of the LNT catalyst include a noble metal species such as Pt, Pd, and Rh, and a catalyst containing barium (Ba) and cerium (Ce) as NOx adsorbing species. it can.

Examples of the LNT catalyst include NSR (NOx Strage Reduction) catalyst, NAC (NOx Absorption Catalyst), DPNR (Diesel Particulate-NOx Reduction) catalyst and the like. In the present specification, the term LNT catalyst is used to include each catalyst referred to by the above alternative names.

As the second catalyst layer 42, a catalyst layer provided with a catalyst for removing components not purified by the first catalyst layer 41 is arranged. For example, when an SCR catalyst using ammonia as a reducing agent is used as the first catalyst layer 41, it is preferable to use an ammonia slip catalyst (ASC) or the like as a catalyst for the second catalyst layer 42. The ASC is an oxidation catalyst and is mainly installed for the purpose of suppressing ammonia from slipping (passing through) the SCR catalyst and being released into the outside air when the exhaust gas temperature is low (for example, less than 200 ° C.). Will be done. In ASC, ammonia is converted to nitrogen, water and NOx.

Further, the catalyst layer 40 may further contain a catalyst other than the NOx purification catalyst. For example, a catalyst layer containing a diesel oxidation catalyst (DOC) or a diesel particulate filter (DPF) may be included.

DOC is a catalyst that oxidizes soluble organic components (SOF) in particulate matter (PM) contained in exhaust gas, and CO and HC in exhaust gas. As the DOC, for example, a catalyst containing a carrier such as a metal oxide and an active species of a noble metal or a base metal is used. As shown in FIG. 2, as a typical example of DOC, a catalyst in which a noble metal catalyst such as Pt or Pd is supported on alumina or zeolite can be exemplified. When arranged in the exhaust pipe 12, it is often used in the form of supporting Pt and Pd on an alumina carrier having a honeycomb structure.

The DPF is a particulate filter equipped with a catalyst for purifying particulate components in exhaust gas. As the DPF, for example, a catalyst containing a carrier such as a metal oxide and an active species of a noble metal or a base metal is used. As shown in FIG. 2, as a typical example of the DPF, a catalyst in which a noble metal catalyst such as Pt or Pd is supported on alumina having a honeycomb structure can be exemplified. The DPF filters and collects particulate matter (PM) in the exhaust.

As another name for DPF, DPR (Diesel Particulate Active System System), DPD (Diesel Particulate Defecter), CRT (Continuous Particulate Filter), CRT (Continuous Particulate Filter, Generic Filter, etc.) In the present specification, the term DPF is used to include each catalyst referred to by the above alternative names.

Instead of DOC or DPF, a NOx adsorption (PASSA) catalyst may be placed. As the PNA catalyst, for example, a catalyst containing a carrier such as a metal oxide and a NOx-adsorbed species is used. As shown in FIG. 2, as a specific example of the PNA catalyst, a catalyst having a structure that does not contain Rh, which is a reducing agent, can be exemplified with respect to the LNT catalyst.

In the present embodiment, a case where an SCR catalyst using ammonia as a reducing agent is used as the first catalyst layer 41 and ASC is used as the second catalyst layer 42 will be described below.

The ozone supply device 30 includes an air pump 32 and an ozone generator 33. The air pump 32 is, for example, an electric pump, and can pressurize the air sucked from the outside and blow it to the ozone generator 33. An air amount sensor 35 is provided at the outlet of the air pump 32, and can detect the flow rate of air blown from the air pump 32 to the ozone generator 33.

The ozone generator 33 is connected via an ozone supply pipe 31. The ozone supply pipe 31 is connected to the exhaust pipe 12 at a position on the upstream side of the catalyst layer 40 and on the downstream side of the exhaust NOx sensor 22. An ozone supply port 37 is provided at the tip of the ozone supply pipe 31, and the ozone supply port 37 is inserted inside the exhaust pipe 12. The ozone supply pipe 31 is provided with an on-off valve 34 for the purpose of suppressing the backflow of exhaust gas from the exhaust pipe 12. The on-off valve 34 is opened when ozone is supplied to the exhaust pipe 12, and is closed when ozone supply is stopped.

The ozone generator 33 includes a plurality of electrode plates in the housing. When the air supplied from the air pump 32 to the ozone generator 33 passes between the plurality of electrode plates, a high voltage is applied between the plurality of electrode plates and an electric discharge occurs, so that ozone (O3) can be generated. it can. The ozone generated by the ozone generator 33 is supplied from the ozone supply port 37 into the exhaust pipe 12 via the ozone supply pipe 31. The ozone supply port 37 is inserted into the exhaust pipe 12 on the upstream side of the catalyst layer 40, and ozone is supplied to the upstream side of the catalyst layer 40.

The reducing agent supply device 60 includes an injector 61, a tank 62, a reducing agent supply pipe 63, and a pump 64. The injector 61 is connected to the exhaust pipe 12 at a position on the upstream side of the catalyst layer 40 and the ozone supply pipe 31 and on the downstream side of the exhaust NOx sensor 22. Urea water (urea aqueous solution) is stored in the tank 62 as a liquid containing a reducing agent. The reducing agent supply pipe 63 connects the injector 61 and the tank 62, and the pump 64 is provided in the reducing agent supply pipe 63. By driving the pump 64, the urea water stored in the tank 62 passes through the reducing agent supply pipe 63 and is supplied to the injector 61, and the urea water can be injected from the injector 61 into the exhaust pipe 12.

The urea water droplets ejected from the injector 61 collide with the wall surface to become finer, or adhere to the wall surface and evaporate, and then the thermal decomposition reaction represented by the following formula (4) and the hydrolysis represented by the following formula (5). As a result, urea (CO (NH2) 2) is changed to ammonia and supplied to the first catalyst layer 41.

CO (NH2) 2 → NH3 + HNCO… (4)
HNCO + H2O → NH3 + CO2 ... (5)

In the first catalyst layer 41, the catalytic reaction represented by the following formulas (6) to (8) occurs due to the SCR catalyst. As a result, NOx is reduced using NH3 as a reducing agent and converted into nitrogen (N2) and water (H2O). The following formula (6) is called the Fast reaction, the formula (7) is called the Standard reaction, and the formula (8) is called the Slow reaction.

NO + NO2 + 2NH3 → 2N2 + 3H2O ... (6)
4NO + 4NH3 + O2 → 4N2 + 6H2O ... (7)
6NO2 + 8NH3 → 7N2 + 12H2O ... (8)

The detected values of the discharge NOx sensor 22, the catalyst temperature sensor 23, the post-catalyst NOx sensor 24, and the air amount sensor 35 are output to the ECU 50. The ECU 50 is an electronic control unit mainly composed of a microcomputer including a CPU, ROM, RAM, etc., and by executing various control programs stored in the ROM, it is based on the detection signals of the various sensors described above. , Performs various controls of the internal combustion engine 20 and the exhaust purification system 10. The ECU 50 has a function of executing combustion control of the internal combustion engine 20, and also functions as a control device for executing ozone supply amount control from the ozone supply device 30 and reduction agent supply amount control from the reducing agent supply device 60. Has.

The ECU 50 includes a temperature information acquisition unit 51, a component amount acquisition unit 52, a supply ratio control unit 53, an estimation unit 54, an ozone amount control unit 55, and a combustion control unit 56.

The temperature information acquisition unit 51 acquires temperature information of each catalyst layer constituting the catalyst layer 40. The temperature information acquired by the temperature information acquisition unit 51 may be stored in the ECU 50. The temperature information of each catalyst layer may be a detected value of the temperature detected by a temperature sensor (for example, the catalyst temperature sensor 23) installed in each catalyst layer, or may be an estimated value.

The temperature information acquisition unit 51 may acquire other parameters related to the temperature of the first catalyst layer 41 as temperature information. The temperature information includes other parameters related to the temperature of the first catalyst layer 41, for example, the elapsed time from the start of temperature rise of the first catalyst layer 41, the completion of warming up the internal combustion engine 20, and the internal combustion engine 20. Examples include the temperature of the cooling return water, the fuel consumption from the start of the internal combustion engine 20, the elapsed time, the mileage, and the like.

The component amount acquisition unit 52 acquires a detected value or an estimated value of the component amount of each component contained in the exhaust gas. For example, as the amount of NOx components, the amount of NOx in the exhaust gas detected by the exhaust NOx sensor 22 and the post-catalyst NOx sensor 24 is acquired. That is, the component amount acquisition unit 52 functions as a discharge NOx amount acquisition unit that acquires the NOx amount discharged from the internal combustion engine 20, and the post-catalyst NOx amount that acquires the NOx amount in the exhaust gas after passing through the catalyst layer 40. It has a function as an acquisition unit. The data acquired by the component amount acquisition unit 52 may be stored in the ECU 50.

The supply ratio control unit 53 controls the supply ratio of the amount of ozone supplied to the catalyst layer 40 to the amount of NOx acquired by the component amount acquisition unit 52. For example, assuming that the amount of substance of NOx supplied to the catalyst layer 40 is N1 and the amount of substance of ozone is N2, the supply ratio K calculated as the substance amount ratio can be expressed by the following formula (9).

K = N2 / N1 ... (9)

The supply ratio control unit 53 controls the amount of ozone supplied to the catalyst layer 40 based on the temperature information of the first catalyst layer 41. More specifically, "ozone" that supplies ozone to the catalyst layer 40 when the temperature information of the first catalyst layer 41 indicates that the temperature of the first catalyst layer 41 is less than a predetermined temperature threshold value. Select "Supply Mode". When the temperature information of the first catalyst layer 41 is in a state indicating that it is equal to or higher than a predetermined temperature threshold value, the "normal mode" in which ozone is not supplied to the catalyst layer 40 is selected. The temperature threshold can be set to, for example, the active temperature TA of a predetermined catalyst contained in the first catalyst layer 41. More specifically, the activity temperature TA of the SCR catalyst or the three-way catalyst arranged in the first catalyst layer 41 may be set.

The active temperature TA is a temperature at which the NOx purification capacity of the catalyst (an index in which the maximum purification capacity of the catalyst is 100%) has a high probability (for example, about 90%). As shown in FIG. 3, the SCR catalyst and the three-way catalyst have almost no catalytic activity in the low temperature range where the catalyst temperature is up to about T1. Then, when the catalyst temperature rises from the low temperature range, the catalytic activity increases remarkably in the medium temperature range shown by the catalyst temperatures T1 to T2 to reach a high probability. Then, in the high temperature region after the catalyst temperature reaches T2, the catalytic activity becomes substantially constant. For example, the catalyst temperature T2 at which the significant increase in catalytic activity is completed can be used as the active temperature TA.

The supply ratio control unit 53 controls the amount of ozone supplied to the catalyst layer 40 for the purpose of adsorbing NOx on the catalyst layer 40. The supply ratio control unit 53 is used in a low temperature range where the catalyst temperature at which the NOx reduction reaction hardly occurs is up to about T1 and a medium temperature range where the catalyst temperature in the NOx reduction reaction is not sufficiently high is about T1 to T2. , It is configured to supply ozone to the catalyst layer 40.

For example, as in JP-A-2016-70181, for the purpose of promoting the NOx reduction reaction in the SCR catalyst, exhaust gas is exhausted on the upstream side of the SCR catalyst so that the Fast reaction represented by the above formula (6) becomes the main reaction. The technology to supply ozone inside is known. In this technique, the amount of ozone supplied is controlled so that the component ratio (NO2 / NO) in NOx in the exhaust gas supplied to the SCR catalyst is 1. Specifically, by supplying ozone, NO is changed to NO2 by the reaction represented by the following formula (10), and the component ratio (NO2 / NO) is controlled to 1. Therefore, the ozone supply amount is controlled so as to be equal to the amount of substance of NO that is desired to be changed to NO2. For example, the amounts of NO2 and NO substances in the exhaust gas are individually detected or estimated, and the amount of ozone supplied is calculated and controlled so that the component ratio (NO2 / NO) becomes 1.

NO + O3 → NO2 + O2… (10)

Further, ozone is supplied in the medium temperature range because it is supplied for the purpose of promoting the reduction reaction of NOx in the SCR catalyst or the like. In the low temperature region, the NOx reduction reaction hardly occurs in the catalyst layer 40, so even if the component ratio (NO2 / NO) in the exhaust gas supplied to the SCR catalyst is controlled to 1, the NOx reduction reaction in the SCR catalyst is promoted. Little effect is obtained. Therefore, when ozone is supplied for the purpose of promoting the reduction reaction of NOx in the SCR catalyst or the like as in the conventional case, ozone is not supplied in the low temperature region.

On the other hand, in the present embodiment, the amount of ozone supplied to the catalyst layer 40 is controlled not for the purpose of promoting the NOx reduction reaction in the catalyst layer 40 but for the purpose of adsorbing NOx on the catalyst layer 40. Therefore, the supply ratio control unit 53 is configured to calculate the amount of ozone to be supplied based on the total amount of NOx to be adsorbed in the catalyst layer 40, and the breakdown thereof is the amount of individual substances of NO2 and NO. There is no need to get a detected or estimated value. Further, in order to control the ozone supply amount for the purpose of adsorbing NOx on the catalyst layer 40, the supply ratio control unit 53 uses the catalyst not only in the medium temperature range as in the conventional case but also in the low temperature range where the catalytic activity is hardly obtained. It is configured to supply ozone to layer 40.

The control of the ozone supply amount by the supply ratio control unit 53 will be further described with reference to the experimental data. FIG. 4 is a diagram showing the relationship between the adsorption rate (%) of NOx supplied to the SCR catalyst and the amount of NOx accumulated in the SCR catalyst, which was obtained by the NOx adsorption / thermal desorption (TPD) test. is there. As the sample, an SCR catalyst in which the base material was cordierite, the coating material was zeolite, and the active species were Cu and Fe was used. When NOx is adsorbed, the sample temperature is set to 100 ° C., and an N2 balanced supply gas containing NO; 100 ppm, H2O; 3 vol%, O2; 10 vol%, CO2; 3 vol% as components is used as a space velocity SV; 47000 (1 /). It was supplied for 20 minutes in h). At the time of temperature rise desorption of NOx, an N2 balanced supply gas containing 10 vol% of O2 as a component was supplied with a space velocity SV; 47,000 (1 / h), and the temperature was raised from 100 ° C. to 550 ° C. in 20 minutes.

In FIG. 4, the reference number 1 indicates the case where K = 1 in the above equation (9), the reference number 3 indicates the case where K = 3 in the above equation (9), and the reference number 5 is The case where ozone is not supplied (that is, K = 0) is shown. The adsorption rate Y (%) shown on the vertical axis was calculated based on the following formula (11). Cni indicates the NOx concentration in the gas upstream of the catalyst, and Cno indicates the NOx concentration in the gas downstream of the catalyst.

Y = 100 × (Cni-Cno) / Cni… (11)

As shown in Reference No. 5, the adsorption rate of NOx was almost zero when ozone was not supplied. On the other hand, as shown in Reference Nos. 1 and 3, when ozone is supplied, the NOx adsorption rate in the SCR catalyst is 40 to 50% by supplying ozone to the SCR catalyst at a supply ratio of K = 1. Was able to be improved. Furthermore, by supplying ozone to the SCR catalyst at a supply ratio of K = 3, the NOx adsorption rate in the SCR catalyst could be improved to 90% or more. That is, as shown in FIG. 4, when ozone is not supplied, the NOx purification rate can be improved by supplying ozone to the SCR catalyst having almost zero NOx purification rate. It was found as a new finding by the person.

Further, FIG. 5 was obtained by similarly performing a NOx adsorption / temperature desorption test using DOC in which the base material is cordierite, the coating material is alumina, and the active species are Pt and Pd as samples. It is a figure which shows the test result. In FIG. 5, reference number 4 shows a case where K = 3 in the above formula (9), and reference number 2 shows a case where ozone is not supplied (that is, K = 0).

As shown in Reference No. 2, the adsorption rate of NOx was almost zero when ozone was not supplied. On the other hand, as shown in Reference No. 4, by supplying ozone to the DOC at a supply ratio of K = 3, the NOx adsorption rate in the DOC could be improved to 80% or more. That is, as shown in FIG. 5, when ozone is not supplied, the NOx purification rate can be improved by supplying ozone to the DOC whose NOx purification rate is almost zero. Was found as a new finding by.

When the supply ratio K = 1, ozone is supplied at the same magnification as the total amount of NOx in the exhaust gas. Therefore, the reaction represented by the above formula (10) causes ozone to be used in the NOx in the exhaust gas. It is presumed that the supply amount is sufficient to oxidize carbon monoxide (NO) to carbon dioxide (NO2). However, as shown in FIG. 4, in the SCR catalyst, the NOx adsorption rate is 40 to 50% when the supply ratio K = 1, and the NOx adsorption rate is close to 100% when the supply ratio K = 3. It was found as a new finding that cannot be predicted from the above equation (10) that it can be increased to. Further, as shown in FIG. 5, it was found as a new finding that the NOx adsorption rate can be increased to 80% or more even in DOC when the supply ratio is K = 3.

As shown in FIGS. 4 and 5, in order to secure the NOx adsorption rate in the ozone supply mode, the supply ratio when supplying ozone to the catalyst layer 40 is set to the same value or more by calculating the substance amount ratio. It is preferably set to 1 to 3 times, or 3 times or more. That is, in order to secure the NOx adsorption rate, it is preferable to set the supply ratio K represented by the above formula (9) in the range of K ≧ 1, and the range of 1 ≦ K ≦ 3 or K ≧ 3. It may be set in a range. When the supply ratio K is in the range of about 1 to 3 times, the NOx adsorption rate improves as the supply ratio K increases. Therefore, the NOx adsorption rate is efficiently improved with respect to the amount of ozone supplied. Therefore, the supply ratio K is preferably set in the range of about 1 to 3 times. That is, in order to efficiently improve the NOx adsorption rate with respect to the amount of ozone to be supplied, it is preferable to set the supply ratio K represented by the above formula (9) in the range of 1 ≦ K ≦ 3. The supply ratio K may be stored in the ECU 50.

The reason why the NOx adsorption rate improves as the supply ratio K increases in the range where the supply ratio K is about 1 to 3 times is that nitrogen trioxide (NO3) is generated by the reaction represented by the following formula (12). It is presumed that this is because it was adsorbed on metal oxides such as alumina used as a carrier for SCR catalyst and DOC. By supplying ozone at a supply ratio K exceeding the same magnification, the reaction represented by the following formula (12) is promoted, and the production of NO3 is promoted. Since NO3 has a higher acidity than NO2, it is easily adsorbed by metal oxides. Therefore, by promoting the production of NO3, it is adsorbed by a metal oxide having a low basicity (for example, alumina) as compared with a metal oxide having a high basicity (for example, barium oxide; BaO) used as a NOx adsorbing species. It is presumed that the NOx purification rate could be improved.

NO2 + O3 → NO3 + O2… (12)

The effect of ozone supply on the NOx adsorption capacity of metal oxides such as alumina was further verified by experiments. FIG. 6 is a diagram showing test results obtained by similarly performing a NOx adsorption / temperature rise desorption test using alumina contained as a composition in a coating material as a sample. FIG. 6 shows a case where K = 3 in the above equation (9). Even when ozone was supplied to alumina at a supply ratio of K = 3, the NOx adsorption rate could be improved to 80% or more. From the results of FIG. 6, it was clarified that the NOx purification rate can be improved by supplying ozone to alumina having a NOx purification rate of almost zero.

FIG. 7 shows a microscopic FT-IR (Fourier transform infrared spectroscopy) imaging method in which the substrate is cordierite, the coating material is a noble metal species such as alumina, Pt, Pd, Rh, and the surface of an LNT catalyst which is Ba as a NOx adsorption species. It is a figure which shows the result of the analysis by. The microscopic FT-IR imaging method is a method of visualizing the distribution of functional groups characteristic of a compound by performing two-dimensional measurement using a two-dimensional array (FPA) detector. FIG. 7A shows a case where ozone is not supplied, and FIG. 7B shows a case where ozone is supplied. In FIG. 7, for the sake of clarity, the region where Ba exists (the region circled) and the region where alumina exists are shown.

As shown in FIG. 7A, when ozone was not supplied, NOx was adsorbed in the region where Ba was present, while NOx was hardly adsorbed in the region where alumina was present. .. On the other hand, as shown in FIG. 7B, when ozone was supplied, NOx was adsorbed even in the region where alumina was present. From the results shown in FIG. 7, it was clarified that alumina does not adsorb NOx so much when ozone is not supplied, while the amount of NOx adsorbed increases by supplying ozone.

From the experimental results shown in FIGS. 6 and 7, the effect of improving the NOx adsorption capacity of the SCR catalyst and DOC by supplying ozone is that the NOx adsorption capacity of metal oxides such as alumina contained in the carrier is improved. It became clear that. Further, as shown in FIG. 7, it was clarified that even in BaO used as a NOx adsorbing species, the NOx adsorption ability is further improved by supplying ozone. From this result, in the metal oxide generally used as a carrier or the like in the SCR catalyst, the three-way catalyst, the DOC, DPF, the LNT catalyst, and the PNA catalyst, the amount of ozone sufficient to promote the reaction represented by the above formula (12) is increased. It was clarified that the supply has the effect of improving the purification rate of NOx in the exhaust gas.

In particular, in order to promote the reaction represented by the above formula (12) in a metal oxide having a low basicity and not a very high NO2 adsorption capacity as compared with a highly basic metal oxide used as a NOx adsorbing species. It has been clarified that the NOx adsorption capacity can be improved more remarkably by supplying a sufficient amount of ozone. Specific examples of such metal oxides having relatively low basicity include alumina, titania, and zirconia. Metal oxides with relatively low basicity such as alumina, titania, and zirconia, and SCR catalysts containing composite metal oxides and ceramics containing this oxide as a composition (for example, cordierite, zeolite, ceria / zirconia, etc.). In the three-way catalyst, DOC, DPF, LNT catalyst, and PNA catalyst, the effect of improving the NOx adsorption amount by the above-mentioned ozone supply and the effect of improving the NOx purification rate obtained as a result can be remarkably obtained.

FIG. 8 is a diagram showing the results of further studies on the relationship between the adsorption rate (%) of NOx supplied to the SCR catalyst and the amount of NOx accumulated in the SCR catalyst by changing the supply ratio K. .. In the examination, the same sample was used under the same conditions as the NOx adsorption / thermal desorption (TPD) test shown in FIG. Furthermore, the ozone concentration in the gas downstream of the catalyst was also detected.

In FIG. 8, reference numerals 6 to 9 indicate the case where K = 1,1.25,1.5,3 in the above formula (9), respectively. The supply ratio K = 1.25 shown in the reference number 7 is an intermediate supply ratio between the reference number 6 and the reference number 7, and the NOx adsorption rate is also a value intermediate between the reference number 6 and the reference number 7. It was. That is, when the supply ratio K was between 1 and 1.5, the rate of increase in the NOx adsorption rate almost corresponded to the rate of increase in the supply ratio K. Further, as shown in Reference No. 8, when the supply ratio K = 1.5, the NOx adsorption rate increased to the same extent as in the case of the supply ratio K = 3 shown in Reference No. 9. That is, the NOx adsorption rate increases slightly between the supply ratios of 1.5 to 3, but the NOx adsorption rate increases with respect to the increase rate of the supply ratio K as compared with the supply ratio K = 1 to 1.5. The rate was significantly reduced. Further, for each of the supply ratios K = 1,1.25,1.5,3, the ozone concentrations in the gas downstream of the catalyst as a sample were 0 ppm, 0 ppm, 30 ppm, and 300 ppm, respectively.

From the experimental results shown in FIG. 8, it was clarified that by controlling the supply ratio K to 1 to 3, the higher the supply ratio K, the higher the NOx adsorption rate. Further, by controlling the supply ratio K to 1 to 1.5 (1 to 1.5 times), the NOx adsorption rate is improved in a state where the increase rate of the NOx adsorption rate is high with respect to the increase rate of the supply ratio K. It became clear that it could be done. Furthermore, it was clarified that the detected value of the ozone concentration in the gas downstream of the catalyst, which is a sample, can be reduced to 0 ppm by controlling the supply ratio K to 1 to 1.5. That is, when the supply ratio K is controlled between 1 and 1.5, the NOx adsorption rate can be efficiently improved by increasing the amount of ozone supplied, and ozone leaking to the downstream side can be improved. It became clear that the amount could be reduced.

When ozone is supplied to a NOx purification catalyst such as an SCR catalyst, the NOx purification reaction proceeds by the reactions represented by the following formulas (13) to (15). [M] in the following formula (16) indicates a NOx purification catalyst.

NO + O3 → NO2 + O2… (13)
NO2 + O3 → NO3 + O2 ... (14)
NO2 + NO3 → N2O5… (15)
[M] + H2O + N2O5 → [M] + 2HNO3 ... (16)

When ozone is supplied to the first catalyst layer 41, ozone reacts with NOx in the exhaust gas by the reactions represented by the above formulas (13) to (15), and dinitrogen pentoxide (dinitrogen pentoxide), which is a precursor of nitric acid, is produced. N2O5) is generated. The produced N2O5 further reacts with H2O on the SCR catalyst as shown in the above formula (16) to produce nitric acid (HNO3). The generated HNO3 is adsorbed on the SCR catalyst as nitrate ions. The following formula (17) can be obtained by summarizing the reactions represented by the above formulas (13) to (16).

NO + 1.5O3 + 0.5H2O → HNO3 ... (17)

From the general reaction formula shown in the above formula (17), it can be understood that 1 mol of HNO3 is produced from 1 mol of NO and 1.5 mol of O3. According to the above formula (17), when the substance amount ratio of NO and O3 in the first catalyst layer 41 is NO: O3 = 1: 1.5, O3 can be efficiently consumed to generate HNO3. As shown in FIG. 8, by controlling the supply ratio K to 1 to 1.5, the NOx adsorption rate can be determined in a state where the increase rate of the NOx adsorption rate is high with respect to the increase rate of the supply ratio K. This is in agreement with the experimental results that it can be improved and the amount of ozone leaking to the downstream side can be significantly reduced.

Further, from the experimental results shown in FIG. 8, it was clarified that the NOx adsorption rate of about 80 to 90% or more can be secured by setting the supply ratio K to 1.5 or more. As shown in FIG. 8, when the supply ratio K = 3, the NOx adsorption rate could be further improved as compared with the case where the supply ratio K = 1.5. That is, it was clarified that when the supply ratio K = 1.5, which corresponds to the stoichiometric ratio shown in the above formula (17), is exceeded and ozone is excessively supplied, the NOx adsorption rate can be further increased.

From this finding, when the supply ratio K = 1.5 to 3 (1.5 times to 3 times) is controlled, the amount of ozone leaking to the downstream side is relatively small, compared to the case where the supply ratio K = 1.5. It was found that the NOx adsorption rate can be further improved and the NOx adsorption rate of about 80 to 90% or more can be secured. On the other hand, if the supply ratio K is controlled to 1.5 to 3, the amount of ozone leaking to the downstream side increases. Further, when the supply ratio K is controlled to 3 or more, it is estimated that the amount of ozone leaking to the downstream side is further increased. Therefore, when the supply ratio K is controlled to 1.5 to 3 or 3 or more with respect to the amount of ozone supplied to the first catalyst layer 41, ozone is used or is used on the downstream side of the first catalyst layer 41. It is preferable to configure the exhaust purification system so as to include a recovery mechanism.

Further, the supply ratio control unit 53 can control the amount of NOx and the amount of ozone supplied to the catalyst layer 40 so as to obtain the calculated supply ratio. For example, the supply ratio control unit 53 executes a control command to the ozone amount control unit 55 or the combustion control unit 56, which will be described later, so that the supply ratio K is at least the substance amount N1 of NOx and the substance amount N2 of ozone. Adjust one or the other. Further, in the normal mode, since ozone is not supplied to the catalyst layer 40, the supply ratio control unit 53 executes a control command to the ozone amount control unit 55 to stop the supply of ozone.

For example, the supply ratio control unit 53 controls the amount of NOx and the amount of ozone supplied to the catalyst layer 40 based on the relationship between the NOx concentration and the ozone concentration stored in the ECU 50 as shown in FIG. It may be configured. In FIG. 9, the straight line L1 shows a straight line having a supply ratio K = 1.5, and the straight line L2 shows a straight line having a supply ratio K = 1. The supply ratio control unit 53 can control the supply ratio K = 1 to 1.5 by adjusting the amount of NOx and the amount of ozone within the range of the region AR surrounded by the straight line L1 and the straight line L2. it can. As a result, ozone can be efficiently used to increase the NOx adsorption rate, and the amount of ozone leaking to the downstream side can be reduced.

The estimation unit 54 calculates estimated values of parameters related to the state of the catalyst layer 40 based on various data acquired by the temperature information acquisition unit 51, the component amount acquisition unit 52, and the like. For example, the amount of NOx adsorbed on the catalyst is estimated based on the amount of NOx in the exhaust gas and the amount of NOx after the catalyst.

As shown in Reference No. 3 of FIG. 4, in the region where the amount of NOx accumulated in the SCR catalyst shown on the horizontal axis is small, a substantially constant high NOx adsorption rate with respect to the amount of NOx can be obtained. However, when the amount of NOx increases to some extent, the adsorption rate decreases as the amount of NOx increases. Therefore, the estimation unit 54 determines to supply ozone to the catalyst layer 40 when the integrated adsorption amount is equal to or less than a predetermined adsorption amount threshold value. Specifically, for example, Y1 shown on the vertical axis of FIG. 4 is set as the target value of the NOx adsorption rate, and the maximum value of the NOx amount at which the NOx adsorption rate is Y1 or more is set as the adsorption amount threshold value X1. Then, when the integrated adsorption amount is equal to or less than the adsorption amount threshold value X1, the supply of ozone is permitted. Thereby, it is possible to control the supply of ozone in the range where the NOx adsorption rate is Y1 or more.

The ozone amount control unit 55 controls the amount of ozone supplied from the ozone supply device 30 to the catalyst layer 40. When ozone is supplied to the exhaust pipe 12, a voltage is applied to the discharge plate in the ozone generator 33 to generate ozone. When the air pump 32 is driven and the on-off valve 34 is opened under the state where ozone is generated, ozone flows into the exhaust pipe 12 together with the air passing through the ozone generator 33.

The ozone amount control unit 55 controls the ozone amount by controlling the amount of air blown by the air pump 32 and the voltage applied to the discharge plate. The amount of air blown by the air pump 32 can be adjusted by controlling the air pump 32 based on the detected value of the air amount sensor 35. Further, the supply of ozone to the catalyst layer 40 can be stopped by controlling the ozone generator 33 to stop the production of ozone and the on-off valve 34 to be closed.

The combustion control unit 56 controls the combustion state of the internal combustion engine 20. The combustion control unit 56 can control the amount of NOx in the exhaust gas by controlling the amount and timing of the fuel and air supplied to the internal combustion engine 20 based on the detected value of the exhaust NOx sensor 22.

FIG. 10 shows a flowchart of the exhaust gas purification process executed by the ECU 50. The process according to FIG. 10 is repeatedly executed at regular intervals.

In step S101, the amount of NOx contained in the exhaust gas from the internal combustion engine 20 is acquired. For example, the exhaust NOx amount Cnb, which is the NOx concentration in the exhaust detected by the exhaust NOx sensor 22, is acquired as the NOx amount.

In step S102, the catalyst temperature Tcat of the catalyst layer 40 is acquired. For example, the temperature Tcat1 of the first catalyst layer 41 detected by the catalyst temperature sensor 23 is acquired.

In step S103, it is determined whether or not the catalyst temperature Tcat is equal to or higher than the active temperature TA used as the temperature threshold. In this case, the activity temperature TA is the activity temperature of the SCR catalyst used for the first catalyst layer 41. If Tcat <TA, the process proceeds to step S104 to select the ozone supply mode. In the ozone supply mode, the processes shown in steps S105 to S111 are executed. On the other hand, when Tact ≧ TA, the process proceeds to step S120 and the normal mode is selected. In the normal mode, as shown in step S121, it is determined not to supply ozone, and the process ends.

In the ozone supply mode, first, in step S105, the amount of NOx detected on the outlet side of the catalyst layer 40 is acquired. For example, the post-catalyst NOx amount Cna, which is the NOx concentration in the exhaust gas detected by the post-catalyst NOx sensor 24, is acquired as the NOx amount. After that, the process proceeds to step S106.

In step S106, the amount of NOx adsorbed on the first catalyst layer 41, that is, the integrated adsorption amount Acc, is calculated based on the discharged NOx amount Cnb and the post-catalyst NOx amount Cna acquired in steps S101 and S105.

In step S107, it is determined whether or not the integrated adsorption amount Acc is equal to or less than a predetermined adsorption amount threshold value X1. The adsorption amount threshold value X1 is a threshold value set as a NOx adsorption amount that can make the NOx adsorption rate of the catalyst layer 40 equal to or higher than the target value Y1. If Acc ≦ X1, the process proceeds to step S109. If Acc> X1, the process proceeds to step S108, it is determined not to supply ozone, and the process ends. That is, when ozone is supplied, the supply is stopped, and when ozone is not supplied, ozone is not supplied.

In step S109, the supply ratio K represented by the above equation (9) is acquired. The supply ratio K is stored in the ECU 50 in advance and is read out from the ECU 50 in step S109. After that, the process proceeds to step S110.

In step S110, the control target value of the amount of ozone supplied to the catalyst layer 40 is calculated. Specifically, the supply ratio K acquired in step S109 and the emission NOx amount Cnb are applied to the above formula (9), and the ozone amount Co to be supplied to the catalyst layer 40 is calculated as a control target value. When the supply ratio K is 3, the amount of substance of ozone supplied to the catalyst layer 40 is calculated to be three times the amount of substance of NOx discharged from the internal combustion engine 20 and supplied to the catalyst layer 40. .. After that, the process proceeds to step S111.

In step S111, the amount of ozone supplied to the catalyst layer 40 is controlled to the amount of ozone Co, which is the control target value calculated in step S110. For example, the ozone supply device 30 is controlled so that the amount of ozone supplied from the ozone supply pipe 31 is Co. As shown in FIG. 4, by supplying ozone corresponding to three times the amount of the substance of NOx in the exhaust gas flowing into the catalyst layer 40 to the catalyst layer 40, a reduction reaction of NOx almost occurs in the first catalyst layer 41. Even if it is not present, Y1% or more of NOx in the exhaust can be removed.

As described above, according to the exhaust gas purification system 10, when the temperature of the first catalyst layer 41 is lower than the active temperature TA of the SCR catalyst or the three-way catalyst, the ozone supply mode can be executed. When the temperature of the first catalyst layer 41 is lower than the activity temperature TA and the state of the SCR catalyst or the three-way catalyst contained in the first catalyst layer 41 does not sufficiently obtain the catalytic activity related to the NOx reduction reaction, In the ozone supply mode, ozone is supplied from the ozone supply device 30 to the catalyst layer 40. Furthermore, the amount of ozone to be supplied is calculated using the supply ratio K set based on the NOx purification rate in the SCR catalyst. Therefore, the amount of NOx adsorbed in each catalyst can be increased, and a desired NOx purification rate can be achieved. As a result, even when the temperature of the first catalyst layer 41 is lower than the active temperature TA, the ability to reduce the amount of NOx can be ensured.

In FIG. 10, as shown in step S103, switching between the ozone supply mode and the normal mode was executed according to the temperature of the first catalyst layer 41 detected by the catalyst temperature sensor 23, but other than that related to the catalyst temperature. The ozone supply mode and the normal mode may be switched based on the parameters of. Specifically, for example, the ozone supply mode is selected when the elapsed time S from the start of temperature rise of the first catalyst layer 41 is less than the predetermined time threshold value Xs, and the normal mode is selected when Xs is equal to or more than the predetermined time threshold value Xs. You may choose. In this case, it is preferable that the time threshold value Xs is set, for example, the time when the first catalyst layer 41 reaches the active temperature TA.

(Modification example)
Each catalyst layer constituting the catalyst layer 40 may be divided into a plurality of catalyst layers. Specifically, for example, as shown in FIG. 11, the first catalyst layer 41 may be divided into a plurality of divided catalyst layers 41a and 41b separated by a pipe 12m. In this case, even if the split catalyst layers 41a and 41b are provided with catalyst temperature sensors 23a and 23b, injectors 61a and 61b for injecting a reducing agent upstream thereof, and ozone supply pipes 31a and 31b for supplying ozone. Good. Further, an intermediate NOx sensor 25 for detecting the amount of NOx in the exhaust gas in the pipe 12 m may be provided. The amount of NOx detected by the intermediate NOx sensor 25 is the amount of NOx after the catalyst in the split catalyst layer 41a and the amount of NOx supplied to the split catalyst layer 41b.

FIG. 12 illustrates the catalysts arranged in the split catalyst layers 41a and 42b and the second catalyst layer 42, and the supply positions of ozone and the reducing agent. In FIGS. 12A to 12C, a catalyst layer containing an SCR catalyst is arranged on the split catalyst layers 41a and 41b, and a catalyst layer containing an ASC is arranged on the second catalyst layer 42.

As shown in FIGS. 12A to 12C, it is preferable that the reducing agent is supplied to the upstream side of both the split catalyst layers 41a and 41b. The reducing agent can be appropriately supplied to the SCR catalysts arranged in both the split catalyst layers 41a and 41b. As a result, the amount of NOx can be reduced by appropriately proceeding the selective reduction reaction in the SCR catalyst.

As shown in FIG. 12A, ozone may be supplied only upstream of the split catalyst layer 41a. Alternatively, as shown in FIG. 12B, it may be supplied only upstream of the split catalyst layer 41b. Alternatively, as shown in FIG. 12 (c), both of the split catalyst layers 41a and 41b may be supplied to the upstream side thereof. The amount of ozone supplied to the split catalyst layers 41a and 41b is preferably adjusted based on the catalyst temperature in the catalyst layers arranged in the split catalyst layers 41a and 41b, the calculated cumulative adsorption amount Acc and the like.

For example, as shown in FIG. 12C, when ozone supply pipes 31a and 31b are provided upstream of the split catalyst layers 41a and 41b, the temperature information of the split catalyst layers 41a and 41b, respectively, , The amount of ozone supplied from the ozone supply pipe 31a and the amount of ozone supplied from the ozone supply pipe 31b are calculated based on the detected values of the emission NOx sensor 22, the intermediate NOx sensor 25, and the post-catalyst NOx sensor 24, respectively. Is preferable.

The first catalyst layer 41 may contain a NOx storage reduction type catalyst called an LNT catalyst or the like in addition to the SCR catalyst or the three-way catalyst. For example, the split catalyst layer 41a shown in FIG. 11 may be provided with an LNT catalyst. In this case, the catalyst layer 40 has, for example, the configuration shown in FIG. In FIG. 13, as the first catalyst layer 41, a catalyst layer having an LNT catalyst is arranged on the upstream split catalyst layer 41a, and a catalyst layer having an SCR catalyst is arranged on the downstream split catalyst layer 41b. A catalyst layer having an ASC is arranged on the layer 42.

In this case, it is preferable to arrange the injector 61 and the ozone supply pipe 31 downstream of the split catalyst layer 41a and upstream of the split catalyst layer 41b. By supplying ozone and a reducing agent to the SCR catalyst at the inlet of the split catalyst layer 41b, the amount of NOx adsorbed in the SCR catalyst can be improved, and the selective reduction reaction in the SCR catalyst can be promoted to reduce the amount of NOx. .. Compared with SCR catalysts and the like, LNT catalysts have a higher ability to adsorb NOx. Therefore, when a catalyst layer containing an LNT catalyst is arranged in the split catalyst layer 41a, ozone is not supplied to the split catalyst layer 41a. , NOx is adsorbed by the LNT catalyst, and NOx is purified to some extent. Therefore, in the configuration of FIG. 13, it is not necessary to supply ozone upstream of the split catalyst layer 41a, but ozone may be supplied.

For example, as shown in FIG. 14, a third catalyst layer 43 is provided on the upstream side of the first catalyst layer 41, and the third catalyst layer 43 is provided with a catalyst layer having a catalyst having no NOx purification function. It may be arranged. For example, it is preferable to arrange a catalyst layer containing DOC or a DPF in the third catalyst layer 43.

According to DOC, hydrocarbons (HC), CO, and NO in the exhaust gas are oxidized by the catalytic reaction represented by the following formulas (18) to (20).

HC + O2 → H2O + CO2 ... (18)
2CO + O2 → 2CO2 ... (19)
2NO + O2 → 2NO2 ... (20)

According to the DPF, the carbon (C) in the exhaust gas is oxidized by the catalytic reaction represented by the following formula (21). Further, as the DPF, a catalyst having activity in the catalytic reaction represented by the above formula (10) may be used. By promoting the reaction of the above formula (20) to generate NO2, the catalytic reaction represented by the following formula (21) can be promoted.

C + 2NO2 → CO2 + 2NO ... (21)

Like the SCR catalyst and the three-way catalyst, the DOC and DPF do not need to occlude NOx, so they are not necessarily prepared so that the amount of NOx adsorbed is high. Therefore, the DOC and DPF generally have a lower NOx adsorption amount than the NOx storage reduction type NOx purification catalyst such as the LNT catalyst. Further, similarly to the SCR catalyst and the three-way catalyst, the NOx purification rate can be improved by supplying ozone to the DOC and DPF having a NOx purification rate of almost zero.

When the catalyst layer 40 includes the third catalyst layer 43, the catalyst layer 40 has, for example, the configuration shown in FIG. In FIG. 14, a catalyst layer containing a DOC or DPF is arranged as the third catalyst layer 43, a catalyst layer having an SCR catalyst is arranged as the first catalyst layer 41, and a catalyst layer having an ASC is arranged as the second catalyst layer 42. Has been done. An ozone supply pipe 31 is provided in a pipe 12 m upstream of the first catalyst layer 41, and an ozone supply pipe 31c is provided upstream of the third catalyst layer 43.

In the configuration of the catalyst layer 40 shown in FIG. 14, it is preferable to supply ozone from the ozone supply pipe 31c upstream of the third catalyst layer 43, as shown in FIG. 15A. By supplying ozone to the DOC from the upstream of the third catalyst layer 43, the amount of NOx adsorbed in the DOC can be improved. When the amount of ozone is sufficient, ozone is also supplied to the first catalyst layer 41 on the downstream side thereof, and the amount of NOx adsorbed in the SCR catalyst can also be improved. Further, as shown in FIG. 15B, ozone may be supplied from both the ozone supply pipe 31c and the ozone supply pipe 31. In this case, the amount of ozone supplied from the ozone supply pipe 31c is controlled by the temperature information of the third catalyst layer 43 and the like, and the amount of ozone supplied from the ozone supply pipe 31 is controlled by the temperature information of the first catalyst layer 41 and the like. You may do so.

When supplying ozone to the third catalyst layer 43, the supply ratio control unit 53 may control the ozone supply amount based on the temperature information of the first catalyst layer 41 as shown in FIG. The ozone supply amount may be controlled based on the temperature information of the third catalyst layer 43. For example, when the temperature information of the third catalyst layer 43 indicates that the temperature of the third catalyst layer 43 is less than a predetermined temperature threshold TB, “ozone supply” that supplies ozone to the third catalyst layer 43 You may choose "mode". Further, when the temperature information of the third catalyst layer 43 indicates that the temperature of the third catalyst layer 43 is equal to or higher than a predetermined temperature threshold TB, ozone is not supplied to the catalyst layer 40 in the "normal mode". May be selected.

The temperature threshold TB can be set, for example, based on the temperature dependence of the amount of NOx adsorbed on the third catalyst layer 43. When the temperature Tdoc of the third catalyst layer 43 becomes high, the NOx adsorption / desorption equilibrium in the third catalyst layer 43 shifts to the desorption side, and the amount of NOx adsorbed in the third catalyst layer 43 decreases. For example, the target value Y2 of the NOx adsorption amount in the third catalyst layer 43 is set, and the upper limit value of the catalyst temperature of the third catalyst layer 43 that can secure the target value Y2 or more is set in the temperature threshold TB. As a result, the ozone supply mode can be executed when the NOx adsorption amount of the target value Y2 or more can be secured.

Further, the injector 61 is preferably arranged upstream of the first catalyst layer 41. By supplying the reducing agent to the SCR catalyst from the inlet of the first catalyst layer 41, the selective reduction reaction in the SCR catalyst can proceed and the amount of NOx can be reduced.

Further, as shown in FIG. 16, a PNA catalyst may be arranged in the third catalyst layer 43 instead of the DOC or DPF. Compared to DOC or DPF, PNA catalysts have a higher ability to adsorb NOx. Therefore, when a catalyst layer containing a PNA catalyst is arranged in the third catalyst layer 43, NOx is adsorbed by the third catalyst layer 43 even if ozone is not supplied to the third catalyst layer 43, and NOx is adsorbed to some extent. NOx is purified. Therefore, when the PNA catalyst is arranged in the third catalyst layer 43, it is not necessary to supply ozone from the upstream side of the third catalyst layer 43, but ozone may be supplied.

According to each of the above embodiments, the following effects can be obtained.

The exhaust gas purification system 10 can purify NOx and the like contained in the exhaust gas from the internal combustion engine 20. The exhaust purification system 10 includes a catalyst layer 40 arranged in the exhaust pipe 12 of the internal combustion engine 20, an ozone supply device 30 that supplies ozone to the catalyst layer 40, and an ECU 50 that controls the ozone supply device 30. The catalyst layer 40 contains at least one of an SCR catalyst and a three-way catalyst as a catalyst for purifying NOx in the exhaust gas.

According to the exhaust purification system 10, even if the state of the SCR catalyst or the three-way catalyst provided in the catalyst layer 40 is such that the catalytic activity related to the NOx reduction reaction cannot be sufficiently obtained, the catalyst is supplied from the ozone supply device 30. By supplying ozone to the layer 40, the amount of NOx adsorbed in each catalyst can be increased. Therefore, NOx can be adsorbed on the SCR catalyst or the three-way catalyst to reduce the amount. That is, even if the catalytic activity related to the NOx reduction reaction cannot be sufficiently obtained, by supplying ozone, the SCR catalyst or the three-way catalyst is imparted with NOx adsorption ability and the ability to reduce the amount of NOx is secured. can do.

The catalyst layer 40 may further contain at least one of a DOC that oxidizes an unburned component in the exhaust and a DPF that purifies the particulate component in the exhaust. By supplying ozone, the amount of NOx adsorbed can be increased also for DOC and DPF. Therefore, when the SCR catalyst or the three-way catalyst is in a state where the catalytic activity related to the NOx reduction reaction cannot be sufficiently obtained, ozone is supplied to the DOC or DPF contained in the catalyst layer 40 to reduce the amount of NOx adsorption. By increasing the amount, the ability to reduce the amount of NOx can be ensured.

The ECU 50 is preferably configured to control the amount of ozone supplied to the catalyst layer 40 based on the temperature information of the catalyst layer 40. Further, the ECU 50 is configured to supply ozone to the catalyst layer 40 when the temperature information of the catalyst layer 40 indicates that the temperature of the catalyst layer 40 is less than a predetermined temperature threshold value. Is more preferable. The catalytic activity of the SCR catalyst or the three-way catalyst for the NOx reduction reaction depends on the catalyst temperature, and when the catalyst temperature is equal to or higher than the active temperature TA, the NOx reduction ability can be sufficiently exhibited. Based on the temperature information, which is a parameter related to the temperature that affects the catalytic activity, it is determined whether or not ozone is supplied, and the amount of ozone supplied is determined according to the NOx reduction reaction activity of the SCR catalyst or the three-way catalyst. Ozone supply can be carried out appropriately.

The ECU 50 preferably includes a supply ratio control unit 53 that controls the supply ratio of the amount of ozone supplied to the catalyst layer 40 with respect to the amount of NOx discharged from the internal combustion engine 20. Regarding the supply ratio, it is preferable to control the supply ratio K calculated by the substance amount ratio to 1 or more. As a result, 40 to 50% of NOx can be adsorbed and purified. Further, the supply ratio K calculated by the substance amount ratio may be controlled to 1 to 3 (1 ≦ K ≦ 3). The NOx adsorption rate can be efficiently improved with respect to the amount of ozone supplied. Alternatively, the supply ratio K calculated by the substance amount ratio may be controlled to 3 or more. There is a possibility that the NOx adsorption rate can be further improved. Further, the supply ratio K calculated by the substance amount ratio may be controlled to 1 to 1.5 (1 ≦ K ≦ 1.5). The NOx adsorption rate can be efficiently improved with respect to the amount of ozone supplied, and the ozone leaking to the downstream side can be significantly reduced. Alternatively, the supply ratio K calculated by the substance amount ratio may be controlled to 1.5 to 3 (1.5 ≦ K ≦ 3). In a state where the amount of ozone leaking to the downstream side is relatively small, the NOx adsorption rate can be further improved as compared with the case where the supply ratio K = 1.5, and the NOx adsorption rate of about 80 to 90% or more can be secured.

The ECU 50 is configured so that the internal combustion engine 20 and the ozone supply device 30 can also be controlled. Therefore, the supply ratio can be controlled by controlling the internal combustion engine 20 and adjusting the amount of NOx discharged from the internal combustion engine 20. Further, the ozone supply device 30 can be controlled to adjust the amount of ozone supplied to the catalyst layer 40, and the supply ratio can be controlled.

The ECU 50 sets the ozone supply device 30 so as to supply ozone to the catalyst layer 40 when the integrated adsorption amount Acc, which is the integrated value of the NOx amount adsorbed on the catalyst layer 40, is equal to or less than a predetermined adsorption amount threshold value X1. Control. In the SCR catalyst, the three-way catalyst, the DOC, and the DPF, if the integrated adsorption amount Acc becomes too high, the NOx adsorption property cannot be sufficiently obtained even if ozone is supplied. Therefore, it is possible to avoid unnecessary supply of ozone by supplying ozone to the catalyst layer 40 when Acc ≦ X1 and not supplying ozone when Acc> X1. Can be done.

In the above, as a general tendency, it is explained that the NOx adsorption amount in the SCR catalyst, the three-way catalyst, the DOC and the DPF is lower than the NOx adsorption amount of the NOx storage reduction type such as the LNT catalyst. , SCR catalysts, three-way catalysts, DOCs and DPFs do not exclude SCR catalysts, three-way catalysts, DOCs and DPFs prepared so that the amount of NOx adsorbed is increased by introducing NOx adsorbed species and the like. Even in the SCR catalyst, the three-way catalyst, the DOC, and the DPF prepared so that the NOx adsorption amount is high, the NOx adsorption amount is improved by supplying ozone, so that the exhaust purification system 10 according to each of the above embodiments is used. For example, the amount of NOx adsorbed below the catalyst active temperature can be further improved, and the amount of NOx discharged from the catalyst layer 40 can be reduced.

Further, in each of the above embodiments, the case where ozone is not supplied in the normal mode has been described as an example, but the present invention is not limited to this. For example, in the normal mode, ozone supply may be executed not for the purpose of adsorbing NOx on the catalyst layer 40 but for the purpose of adjusting the component ratio of NO and NO2 in the exhaust gas. When ozone is supplied in the normal mode, the amount of ozone is controlled by using a parameter different from the supply ratio K, which is a parameter suitable for adsorbing NOx on the catalyst layer 40.

The controls and methods thereof described in the present disclosure are realized by a dedicated computer provided by configuring a processor and memory programmed to perform one or more functions embodied by a computer program. May be done. Alternatively, the controls and methods thereof described in the present disclosure may be implemented by a dedicated computer provided by configuring the processor with one or more dedicated hardware logic circuits. Alternatively, the control unit and method thereof described in the present disclosure may be a combination of a processor and memory programmed to perform one or more functions and a processor composed of one or more hardware logic circuits. It may be realized by one or more dedicated computers configured. Further, the computer program may be stored in a computer-readable non-transitional tangible recording medium as an instruction executed by the computer.

10 ... Exhaust purification system, 12 ... Exhaust pipe, 20 ... Internal combustion engine, 30 ... Ozone supply device, 40 ... Catalyst layer, 50 ... ECU, 53 ... Supply ratio control unit

Claims (11)

An exhaust gas purification system (10) that purifies a predetermined component in exhaust gas from an internal combustion engine (20).
The catalyst layer (40) arranged in the exhaust pipe of the internal combustion engine and
An ozone supply device (30) that supplies ozone to the catalyst layer, and
A control device (50) for controlling the ozone supply device is provided.
The catalyst layer includes a selective reduction catalyst that selectively reduces nitrogen oxides with a reducing agent supplied to the exhaust pipe as a catalyst for purifying nitrogen oxides in the exhaust, and carbon monoxide in the exhaust. Alternatively, it contains at least one of a three-way catalyst that reduces nitrogen oxides using a hydrocarbon as a reducing agent.
The control device is an exhaust gas purification system including a supply ratio control unit (53) that controls the supply ratio of the amount of ozone supplied to the catalyst layer to the amount of nitrogen oxides discharged from the internal combustion engine. The exhaust according to claim 1, wherein the catalyst layer further includes at least one of an oxidation catalyst that oxidizes an unburned component in the exhaust and a particle collection filter that purifies the particulate component in the exhaust. Purification system. Claim 1 or 2 in which the supply ratio control unit supplies ozone to the catalyst layer when the temperature information of the catalyst layer indicates that the temperature of the catalyst layer is less than a predetermined temperature threshold value. Exhaust purification system described in. The exhaust gas purification system according to any one of claims 1 to 3, wherein the supply ratio control unit controls the supply ratio calculated by the substance amount ratio to the same magnification or more. The exhaust gas purification system according to any one of claims 1 to 4, wherein the supply ratio control unit controls the supply ratio calculated by the substance amount ratio to be 1 to 3 times. The exhaust gas purification system according to any one of claims 1 to 5, wherein the supply ratio control unit controls the supply ratio calculated by the substance amount ratio to be 1 to 1.5 times. The exhaust gas purification system according to any one of claims 1 to 5, wherein the supply ratio control unit controls the supply ratio calculated by the substance amount ratio to 1.5 times to 3 times. The exhaust gas purification system according to any one of claims 1 to 4, wherein the supply ratio control unit controls the supply ratio calculated by the substance amount ratio to be three times or more. The exhaust gas purification system according to any one of claims 1 to 8, wherein the supply ratio control unit adjusts the amount of nitrogen oxides discharged from the internal combustion engine to control the supply ratio. The exhaust gas purification system according to any one of claims 1 to 9, wherein the supply ratio control unit adjusts the amount of ozone supplied to the catalyst layer to control the supply ratio. The control device includes an estimation unit (54) that estimates an integrated adsorption amount, which is an integrated value of the amount of nitrogen oxides adsorbed on the catalyst layer, and the integrated adsorption amount estimated by the estimation unit is a predetermined adsorption amount. The exhaust purification system according to any one of claims 1 to 10, wherein ozone is supplied to the catalyst layer when it is equal to or less than a threshold value.