JP2019157739A - Exhaust purifying device for internal combustion engine - Google Patents

Abstract

To execute purification of harmful material as much as possible under unfavorable conditions for catalyst activation.SOLUTION: An exhaust purifying device for an internal combustion engine 3A has a selective-reduction type first NOx catalyst 11, a filter 13 disposed downstream from the first NOx catalyst in exhaust flow direction, and an NOx adsorbent 14 disposed downstream from the filter.SELECTED DRAWING: Figure 2

Description

  The present disclosure relates to an exhaust emission control device for purifying exhaust gas of an internal combustion engine.

  As this type of exhaust emission control device, a configuration in which a plurality of catalysts or the like for removing or reducing harmful substances in exhaust gas are arranged in series is generally used (see, for example, Patent Documents 1 to 3).

JP 2009-13932 A JP 2011-94482 A JP-A-8-270440

  By the way, an exhaust emission control device for a vehicle internal combustion engine has a general problem of dealing with exhaust gas regulations in each country that are becoming stricter year by year. In particular, recently, there has been a demand for an exhaust emission control device capable of purifying harmful substances as much as possible under conditions unfavorable for catalyst activation, such as after a cold start of an internal combustion engine or when the exhaust gas temperature is low.

  Accordingly, the present disclosure has been created in view of such circumstances, and an object thereof is to provide an exhaust purification device capable of performing as much of the purification of harmful substances as possible under conditions unfavorable for catalyst activation.

According to one aspect of the present disclosure,
A selective reduction type first NOx catalyst;
A filter disposed downstream of the first NOx catalyst in the exhaust flow direction;
A NOx adsorbent disposed downstream of the filter;
An exhaust emission control device for an internal combustion engine is provided.

  Preferably, the exhaust emission control device further includes an oxidation catalyst disposed on the downstream side of the first NOx catalyst and on the upstream side of the filter.

  Preferably, the exhaust emission control device further includes a first ammonia slip catalyst disposed downstream of the first NOx catalyst and upstream of the oxidation catalyst.

  Preferably, the exhaust emission control device further includes a selective reduction type second NOx catalyst disposed on the downstream side of the NOx adsorbent.

  Preferably, the exhaust purification device further includes a second ammonia slip catalyst disposed on the downstream side of the second NOx catalyst.

  Preferably, the NOx adsorbent has an electric heater.

  According to the present disclosure, purification of harmful substances can be performed as much as possible under conditions unfavorable for catalyst activation.

1 is a schematic view of an internal combustion engine. It is a figure which shows the exhaust gas purification apparatus of 1st Embodiment. It is a figure which shows the exhaust gas purification apparatus of 2nd Embodiment. It is a figure which shows the exhaust gas purification apparatus of 3rd Embodiment. It is a figure which shows the exhaust gas purification apparatus of 4th Embodiment. It is a figure which shows the exhaust gas purification apparatus of 5th Embodiment.

  Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings.

[First Embodiment]
FIG. 1 schematically shows an internal combustion engine according to this embodiment. The internal combustion engine (engine) 1 is for a vehicle and is a multi-cylinder compression ignition internal combustion engine, that is, a diesel engine. However, this embodiment is intended for an engine that is operated in an excessive oxygen state in which the excess air ratio λ, which is the mixing ratio of fuel and air, is greater than 1. Therefore, the engine may be a so-called lean burn type spark ignition internal combustion engine, that is, a gasoline engine. The use of the engine, the cylinder arrangement type, the number of cylinders, etc. are arbitrary.

  The engine 1 has an exhaust passage 2 that is defined by an exhaust pipe or the like and through which exhaust gas G flows. The exhaust passage 2 in the illustrated example is simply a straight line for simplification, but may be appropriately bent or meandered. An exhaust gas purification device 3 for purifying exhaust gas is provided in the middle of the exhaust passage 2. In FIG. 1, the exhaust purification device 3 is simply indicated by a broken-line square for simplification.

  As will be described in detail later, the exhaust purification device 3 includes a plurality of various catalysts and filters that remove or reduce various harmful substances in the exhaust. These catalysts and filters are collectively referred to as post-processing members. The post-processing members used in the embodiments of the present disclosure mainly include the following.

(1) Selective reduction type NOx catalyst A selective reduction type NOx catalyst (hereinafter referred to as SCR (Selective Catalytic Reduction)) is a catalyst for reducing and removing nitrogen oxides NOx in exhaust gas by a chemical reaction. The SCR can continuously reduce NOx when a reducing agent is added. Therefore, an addition valve for adding urea water as a reducing agent into the exhaust passage 2 is provided upstream of the SCR in the exhaust flow direction.

SCR is a material in which a noble metal such as Pt is supported on the surface of a substrate such as zeolite or alumina, a material in which a transition metal such as Cu is ion-exchanged on the surface of the substrate, and titania / vanadium on the surface of the substrate. Examples include those carrying a catalyst (V ₂ O ₅ / WO ₃ / TiO ₂ ). The SCR is activated when the catalyst temperature (catalyst bed temperature) is equal to or higher than the activation start temperature, and NOx is reduced and purified when urea water is added. When urea water is added, urea water is hydrolyzed and ammonia is generated. On the SCR, ammonia reacts with NOx to reduce NOx.

(2) NOx adsorbent NOx adsorbent (hereinafter referred to as PNA (Passive NOx Adsorbers)) is for removing nitrogen oxide NOx in the exhaust gas by physically adsorbing it. PNA is a composite oxide of transition element and rare earth element, or a mixture of transition element oxide and rare earth element coated on a cordierite honeycomb, or a flow-through type formed of porous zeolite Examples include carriers.

  The minimum value of the PNA activation start temperature, that is, the temperature at which the PNA becomes active is lower than the SCR activation start temperature. Therefore, after the cold start of the engine and before the activation of the SCR, the PNA starts to activate and can adsorb and remove NOx. When the temperature of the PNA rises above the activation end temperature, that is, the maximum value of the temperature at which the PNA enters the active state, the PNA stops the NOx adsorption and desorbs and releases the NOx that has been adsorbed until then. The activation end temperature of this PNA is generally near the activation start temperature of the SCR.

  As its name suggests, PNA is a passive NOx adsorbent that adsorbs and desorbs NOx depending on its own temperature. In this respect, PNA is an active NOx adsorbent that desorbs and releases adsorbed NOx by forcing exhaust gas into a rich atmosphere, that is, an NOx storage reduction catalyst (LNT: Lean NOx Trap or NSR: NOx Storage Reduction). ) Is different.

(3) The filter filter (hereinafter referred to as DPF (Diesel Particulate Filter)) is for collecting and removing particulate matter (PM) contained in the exhaust gas. DPF is any type that physically collects PM, such as a so-called wall flow type in which both ends of a honeycomb-shaped heat-resistant substrate are alternately closed in a checkered pattern, or a mesh-shaped foam shape. These filters can be used. In this embodiment, a wall flow type is used.

  The DPF is a so-called continuous regeneration type DPF with a catalyst in which a noble metal such as Pt is supported on the inner wall of the base material. In this case, HC in the exhaust gas supplied to the DPF is oxidized and burned by the catalytic action, and at this time, PM accumulated in the DPF is burned and removed.

(4) Oxidation catalyst An oxidation catalyst (hereinafter referred to as DOC (Diesel Oxidation Catalyst)) is for oxidizing and purifying unburned components (hydrocarbon HC and carbon monoxide CO) in exhaust gas. The DOC is formed by dispersing a large number of noble metals such as Pt in a coating material on a substrate surface. The DOC has a function of heating and raising the temperature of the exhaust gas with heat generated during the oxidation reaction of HC and CO in the exhaust gas. The DOC also has a function of oxidizing NO in the exhaust to NO _2. When DPF exists on the downstream side of the DOC, PM collected in the DPF can be efficiently oxidized and removed by the high oxidation ability of NO ₂ .

(5) Ammonia slip catalyst An ammonia slip catalyst (hereinafter referred to as ASC (Ammonia Slip Catalyst)) is used to oxidize and purify excess ammonia that is not used for NOx reduction in the SCR but discharged downstream of the SCR. It is a catalyst. When ammonia is released into the atmosphere, it causes a strange odor and the like, so that excess ammonia is treated by ASC to prevent this.

  FIG. 2 shows the exhaust purification device 3 of the first embodiment. For convenience, the exhaust purification device 3 of the first embodiment is denoted by reference numeral 3A.

  The exhaust purification device 3A includes a selective reduction type first NOx catalyst, that is, the first SCR 11, a DPF 13 disposed on the downstream side of the first SCR 11 in the exhaust flow direction, and a PNA 14 disposed on the downstream side of the DPF 13. The exhaust purification device 3 </ b> A further includes a DOC 12 disposed on the downstream side of the first SCR 11 and the upstream side of the DPF 13. The exhaust purification device 3A further includes a selective reduction type second NOx catalyst, that is, a second SCR 15 disposed on the downstream side of the PNA 14.

  Thereby, in the exhaust passage 2, the first SCR 11, the DOC 12, the DPF 13, the PNA 14, and the second SCR 15 are arranged in series from the upstream side.

  A first addition valve 16 for adding urea water into the exhaust passage 2 is disposed upstream of the first SCR 11. The first addition valve 16 adds urea water used for the NOx reduction reaction in the first SCR 11. Accordingly, the first addition valve 16 and the first SCR 11 are paired, and no post-processing member or member for changing the exhaust gas component is disposed between them. The arrangement of both in such a relationship is called a pair arrangement. For example, the first addition valve 16 is disposed in a pair with respect to the first SCR 11.

  A pair of second addition valves 17 that add urea water into the exhaust passage 2 are also arranged upstream of the second SCR 15.

  The features of this exhaust purification device 3A are as follows. First, since the first SCR 11, the PNA 14 and the second SCR 15 that purify NOx and the DPF 13 that collects PM are provided, NOx and PM that are main harmful substances of the diesel engine can be removed.

  Further, since the first SCR 11, the DPF 13, and the PNA 14 are arranged as described above, the following advantages are brought about.

  In the present embodiment, by providing the PNA 14 that starts activation at a lower temperature than the first SCR 11, NOx can be adsorbed and removed by the PNA 14 even in a low temperature region where the first SCR 11 is not activated. In this embodiment, the activation start temperature Ts1 of the first SCR 11, that is, the minimum value of the temperature at which the first SCR 11 becomes active is, for example, about 150 ° C. On the other hand, the activation start temperature Tps of the PNA 14 is lower than the activation start temperature Ts1 of the first SCR 11, and is, for example, about 120 to 130 ° C. Therefore, even when the first SCR 11 is lower than the activation start temperature Ts1, if the PNA 14 is equal to or higher than the activation start temperature Tps, NOx can be adsorbed and removed by the PNA 14. Therefore, purification of NOx, which is a harmful substance, can be performed as much as possible even under conditions unfavorable for catalyst activation.

  Further, the activation end temperature Tpe of the PNA 14, that is, the maximum value of the temperature at which the PNA 14 becomes active is, for example, about 200 ° C., and the PNA 14 desorbs and releases adsorbed NOx when the temperature is higher than the activation end temperature Tpe. Therefore, the PNA 14 can perform NOx adsorption only within the activation temperature range of the activation start temperature Tps or more and the activation end temperature Tpe or less.

  By the way, although the exhaust gas temperature tends to decrease before and after passing through the DPF 13 because the heat capacity of the DPF 13 is relatively large, the exhaust temperature at the position immediately after the DPF 13 is relatively stable regardless of changes in the engine operating state and the like. is doing. In the present embodiment, the PNA 14 is provided on the downstream side of the DPF 13, specifically at a position immediately after the DPF 13. For this reason, the temperature of the PNA 14 can be stably maintained within the active temperature range as much as possible, and NOx adsorption by the PNA 14 can be continued as much as possible regardless of changes in the engine operating state. This also makes it possible to carry out NOx purification as much as possible under conditions unfavorable for catalyst activation.

  On the other hand, since the PNA 14 is provided on the downstream side of the DPF 13, there is a drawback that the temperature of the PNA 14 does not easily rise above the activation start temperature Tps as a contradiction to stabilize the temperature of the PNA 14. In the case of an engine specification where the temperature of the original exhaust gas flowing into the exhaust purification device 3A is high, it may not be a problem so much, but in the case of an engine specification where the original exhaust gas temperature is low, it may become a problem. There is sex.

  Therefore, in the present embodiment, the first SCR 11 is provided on the upstream side of the DPF 13 in order to overcome such drawbacks. If it carries out like this, high temperature exhaust gas can be supplied to 1st SCR11, and 1st SCR11 can be easily heated up to activation start temperature Ts1 or more. Thereby, NOx can be removed by the first SCR 11, and even if the PNA 14 is deactivated, NOx can be surely removed. That is, the configuration of the present embodiment is effective in the case where the exhaust gas temperature drop larger than (Ts1−Tps) occurs immediately before the first SCR 11 and immediately before the PNA 14. In some cases, the NOx removal can be started by activating the first SCR 11 earlier than the PNA 14 after the engine cold start.

  In the present embodiment, among the five post-processing members included in the exhaust purification device 3A, the first SCR 11 is disposed on the most upstream side of the exhaust purification device 3A, so that the first SCR 11 can receive the hottest exhaust gas. This is very advantageous for promoting the activation of the first SCR11.

  Further, the first SCR 11 of the present embodiment may be operable at a lower temperature than a commonly used normal SCR. In other words, the activation start temperature Ts1 of the first SCR 11 is higher than the activation start temperature Tsn of the normal SCR. It may be low. In this case, for example, the activation start temperature Tsn of the normal SCR is about 200 ° C., whereas the activation start temperature Ts 1 of the first SCR 11 is about 150 ° C. Therefore, the first SCR 11 can be activated earlier than the normal SCR after the cold start of the engine 1, and NOx can be removed at an earlier timing. Further, even when the operating state of the engine 1 becomes an idle or low load operating state and the exhaust gas temperature becomes low, the active state of the first SCR 11 can be maintained for a longer time than the normal SCR, in other words, the first SCR 11 is in the active state. Can be delayed from the normal SCR. Therefore, NOx purification can be performed as much as possible under conditions unfavorable for catalyst activation.

  When such a low temperature operation type first SCR 11 is provided, it is possible to remove NOx at a lower temperature side and earlier than a normal SCR. However, below the activation start temperature Ts1, the first SCR 11 still cannot remove NOx. Therefore, in order to cover the temperature range, it is preferable to provide PNA 12 as an auxiliary.

  Of course, when both of the first SCR 11 and the PNA 14 are within the active temperature range, NOx can be removed using the two, so that the NOx removal efficiency can be increased.

  In the present embodiment, since the DOC 12 is provided on the downstream side of the first SCR 11 and the upstream side of the DPF 13, the exhaust gas heated by the DOC 12 can be sent to the downstream side, and the downstream post-processing member, particularly the activation of the PNA 14 can be performed. Is advantageous. As is well known, if additional HC is supplied to the DOC 12 by post injection or exhaust pipe injection, the exhaust gas is heated to a significantly high temperature (about 500 ° C.) by the DOC 12 and the PM accumulated in the DPF 13 is forced. Filter regeneration can be performed to remove the combustion. At this time, the high-temperature exhaust gas that has passed through the DPF 13 can be supplied to the PNA 14 so that sulfur (S) adsorbed on the PNA 14 can be desorbed (purged). Therefore, the S purge of the PNA 14 can be executed at the same time using the filter regeneration opportunity, and the convenience can be improved and the fuel consumption can be reduced as compared with the case where both are performed individually.

  In the present embodiment, since the second SCR 15 is disposed on the downstream side of the PNA 14, NOx desorbed and released from the PNA 14 can be reduced by the second SCR 15, and the release of the NOx into the atmosphere can be suppressed.

  Here, preferably, the capacity of the first SCR 11 is smaller than the capacity of the second SCR 15. For example, the capacity of the first SCR 11 is about the same as the displacement of the engine 1, whereas the capacity of the second SCR 15 is about twice the displacement of the engine 1. If the capacity of the first SCR 11 on the upstream side is reduced in this way, the heat capacity is reduced and the first SCR 11 is easily heated, which is particularly advantageous for early activation of the first SCR 11 after engine cold start. On the other hand, since the capacity of the second SCR 15 is large, even when a relatively large amount of NOx is generated that cannot be processed by the first SCR 11 alone, for example, during high-load operation of the engine 1, the NOx is reduced by the second SCR 15. It can be purified reliably.

  For the sake of convenience, the configuration of the first embodiment is surrounded by a frame in the figure and denoted by reference numeral 100.

  Note that at least one of the DOC 12 and the second SCR 15 can be omitted as necessary.

  Hereinafter, other embodiments will be described based on the first embodiment. However, the description of the same parts as in the first embodiment will be omitted, and differences from the first embodiment will be mainly described.

[Second Embodiment]
FIG. 3 shows an exhaust emission control device 3 according to the second embodiment. For convenience, the exhaust purification device 3 of the second embodiment is denoted by reference numeral 3B.

  The second embodiment is different from the first embodiment in that a first ASC (first ammonia slip catalyst) 18 is disposed on the downstream side of the first SCR 11 and the upstream side of the DOC 12. When the first ASC 18 is provided in this way, surplus ammonia that has not been used in the first SCR 11 and has passed through the first ASC 18 can be removed by the first ASC 18, and the release of ammonia into the atmosphere can be suppressed.

  The first SCR 11 and the first ASC 18 may be integrated. For example, the first SCR 11 may be formed in the upstream (front) portion of the common carrier and the first ASC 18 may be formed in the downstream (rear) portion by zone coating.

  In the third embodiment (FIG. 4), the fourth embodiment (FIG. 5), and the fifth embodiment (FIG. 6), which will be described later, the first ASC 18 is arranged downstream of the first SCR 11 and upstream of the DOC 12. Is possible.

[Third Embodiment]
FIG. 4 shows an exhaust emission control device 3 according to the third embodiment. For convenience, the exhaust purification device 3 of the third embodiment is denoted by reference numeral 3C.

  The third embodiment is different from the first embodiment in that a second ASC (second ammonia slip catalyst) 19 is disposed on the downstream side of the second SCR 15. When the second ASC 19 is provided in this way, surplus ammonia that has passed through the second SCR 15 and possibly surplus ammonia that has passed through the first SCR 11 can also be removed by the second ASC 19, and the release of ammonia into the atmosphere can be suppressed. .

  In the second embodiment (FIG. 3), the fourth embodiment (FIG. 5), and the fifth embodiment (FIG. 6), it is possible to arrange the second ASC 19 on the downstream side of the second SCR 15. The second SCR 15 and the second ASC 19 may be integrated.

[Fourth Embodiment]
FIG. 5 shows an exhaust purification device 3 of the fourth embodiment. For convenience, the exhaust purification device 3 of the fourth embodiment is denoted by reference numeral 3D.

  The fourth embodiment differs from the first embodiment in that the DPF 13 of the first embodiment is replaced with a selective reduction type NOx catalyst with a filter in which DPF and SCR are integrated, that is, SCRF (SCR-Filter) 20.

  The SCRF 20 is obtained by, for example, forming an SCR layer on the surface of a base material of a wall flow type DPF by a wash coat or the like. SCRF 20 is substantially the same as a separate DPF and SCR arranged in series. A pair of third addition valves 21 for adding urea water into the exhaust passage 2 is disposed upstream of the SCRF 20.

  In the second embodiment (FIG. 3), the third embodiment (FIG. 4), and the fifth embodiment (FIG. 6), it is possible to replace the DPF 13 with the SCRF 20.

[Fifth Embodiment]
FIG. 6 shows an exhaust emission control device 3 according to the fifth embodiment. For convenience, the exhaust emission control device 3 of the fifth embodiment is denoted by reference numeral 3E.

  The fifth embodiment is different from the first embodiment in that a PNA (PNA + EH) 22 having an electric heater (EH: Electric Heater) is used instead of the simple PNA 14. Providing this PNA 22 provides the following advantages.

  That is, by turning on the electric heater of the PNA 22, it is possible to promote the warming-up of the PNA 22 itself and the activation thereof. Therefore, by turning on the electric heater of the PNA 22 after the cold start of the engine, even if the first SCR 11 is not activated, the PNA 22 can be activated early and NOx removal can be started quickly. Even when the operating state of the engine 1 is in an idling state or a low load operating state and the temperature of the exhaust gas becomes low, the PNA 22 is kept active as much as possible by turning on the electric heater of the PNA 22, or once lost. The activated PNA 22 can be activated again. Thereby, purification of NOx can be performed as much as possible even under conditions unfavorable for catalyst activation.

  In addition, the temperature of the PNA 22 can be positively controlled, specifically, the temperature can be raised when necessary, and convenience can be improved.

  Further, it is possible to raise the temperature of the exhaust gas by turning on the electric heater of the PNA 22, which is advantageous for the activation of the downstream post-processing member, specifically, the second SCR 15. In particular, if additional HC is supplied to the PNA 22 by post injection, exhaust pipe injection or the like with the electric heater of the PNA 22 turned on, the HC can be directly ignited and burned by the electric heater, and the PNA 22 can function as a burner. it can. As a result, an extremely high exhaust temperature raising effect can be obtained.

  In the second embodiment (FIG. 3), the third embodiment (FIG. 4), and the fourth embodiment (FIG. 5), it is possible to replace the PNA 14 with the PNA 22 with an electric heater.

  In addition, the configurations of the respective embodiments described above can be combined partially or wholly unless there is a particular contradiction. The embodiment of the present disclosure is not limited to the above-described embodiment, and includes all modifications, applications, and equivalents included in the concept of the present disclosure defined by the claims. Therefore, the present disclosure should not be construed as being limited, and can be applied to any other technique belonging to the scope of the idea of the present disclosure.

1 Internal combustion engine
2 Exhaust passages 3, 3A, 3B, 3C, 3D, 3E Exhaust gas purification device 11 Selective reduction type first NOx catalyst (first SCR)
12 Oxidation catalyst (DOC)
13 Filter (DPF)
14 NOx adsorbent (PNA)
15 selective reduction type second NOx catalyst (second SCR)
18 First ammonia slip catalyst (first ASC)
19 Second ammonia slip catalyst (2nd ASC)
20 Selective reduction type NOx catalyst with filter (SCRF)
22 NOx adsorbent with heater (PNA + EH)

Claims (6)

A selective reduction type first NOx catalyst;
A filter disposed downstream of the first NOx catalyst in the exhaust flow direction;
A NOx adsorbent disposed downstream of the filter;
An exhaust emission control device for an internal combustion engine, comprising: The exhaust emission control device for an internal combustion engine according to claim 1, further comprising an oxidation catalyst disposed downstream of the first NOx catalyst and upstream of the filter. The exhaust emission control device for an internal combustion engine according to claim 2, further comprising a first ammonia slip catalyst disposed downstream of the first NOx catalyst and upstream of the oxidation catalyst. The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 3, further comprising a selective reduction type second NOx catalyst disposed downstream of the NOx adsorbent. The exhaust emission control device for an internal combustion engine according to claim 4, further comprising a second ammonia slip catalyst disposed downstream of the second NOx catalyst. The exhaust purification device for an internal combustion engine according to any one of claims 1 to 5, wherein the NOx adsorbent has an electric heater.

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