JP5258426B2 - Engine exhaust purification system - Google Patents

Description

  The present invention relates to an exhaust emission control device for an engine, and more particularly, to a technique for suppressing emission of nitrogen oxides (hereinafter referred to as “NOx”) and particulates into the atmosphere with a compact configuration as a whole system.

  As a device for purifying engine exhaust by post-processing, a selective catalytic reduction (SCR) type exhaust purification device that reduces NOx in the exhaust with ammonia is known. In this exhaust purification device, a NOx reduction catalyst is installed in the exhaust passage, a urea water supply device is installed upstream of the NOx reduction catalyst, and an oxidation catalyst for ammonia purification is installed downstream. Ammonia is generated by hydrolysis of urea added to the exhaust gas by the supply device, and this ammonia is supplied as a reducing agent to the NOx reduction catalyst. On the other hand, ammonia (slip ammonia) that has passed through the NOx reduction catalyst without contributing to the reduction of NOx is oxidized and purified by the ammonia purification catalyst before being released into the atmosphere.

  In order to cope with exhaust gas regulations that will become stricter in the future, it is conceivable to add a particulate filter to the SCR type exhaust gas purification device. This particulate filter mainly collects and reduces diesel particulates discharged from a diesel engine by filtration of exhaust gas. However, simply adding a particulate filter to the NOx reduction catalyst (and the ammonia purification catalyst downstream of it) inevitably increases the overall size of the system, making it difficult to mount the vehicle or increasing the weight excessively. Problems occur.

In relation to this problem, it has been studied that a particulate filter is loaded with a NOx reduction catalyst and the particulate collection function and the NOx reduction function are combined with the particulate filter. As such a catalyst-carrying particulate filter, in Patent Document 1, in the wall flow type, a hydrolysis catalyst of urea is supported on the inner wall of a filter carrier that forms a cell passage on the inlet side, while on the outlet side In which a NOx reduction catalyst is supported on the inner wall of the carrier forming the cell passage.
JP 2006-183507 A (FIG. 2)

  However, in the exhaust gas purification device described in Patent Document 1, in order to ensure the particulate collection ability as a particulate filter, the filter carrier itself needs a large capacity, and in addition to this catalyst-carrying particulate filter, Since an ammonia purifying catalyst is required separately, the entire system is still large and the weight is increased.

  The present invention provides an engine exhaust gas purification device that takes the above problems into consideration, and enables a series of chemical reactions related to selective catalytic reduction with urea to be efficiently performed in a filter carrier. The purpose is to suppress the release of curate into the atmosphere with a compact configuration as a whole system.

An exhaust emission control device for an engine according to the present invention has a plurality of cell passages along a flow of exhaust gas defined by a partition wall capable of collecting particulates in exhaust gas by filtration, and seals adjacent cell passages. A filter carrier configured to be alternately closed on the inlet side or the outlet side by a material is provided. Here, in the present invention, in the filter carrier, the filter is formed by the first portion including the inlet-side end edge of the inner wall of the inlet-side passage which is a cell passage opened on the inlet side, and the filter material. The urea hydrolysis catalyst is supported on the inlet-side end surface of the carrier, and the first NOx reduction catalyst is supported on the second portion of the inner wall of the inlet-side passage on the downstream side of the first portion. . Then, the filter carrier, of the inner wall of the outlet-side passage is a cell passage opening at the outlet side, have coupling overlap in the direction intersecting the flow of the exhaust gas flowing into the filter carrier with respect to the first portion, the first The third NOx reduction catalyst is supported on the third part extending to the downstream side of the part, and the second wall of the outlet side passage is located on the downstream side of the third part. wherein the oxidation catalyst is supported on overlapping the fourth portion intends case in a direction crossing the flow of exhaust to the portion.

In the present invention, urea is hydrolyzed by the hydrolysis catalyst supported on the first portion of the inner wall of the inlet side passage and the end surface on the inlet side of the filter carrier, and the ammonia generated thereby is converted into the first and second ammonia. The NOx can be efficiently reduced by supplying the NOx reduction catalyst.
Specifically, the first and second NOx reduction catalysts include an inner wall portion (second portion) of the inlet side passage downstream of the first portion and an inner wall portion (third portion) of the outlet side passage. ) and it is supported in the. Here, the third portion, extending Mashimasu from Rukoto to the downstream side of the agreement, the first portion overlapping in a direction crossing the flow of the exhaust gas flowing into the filter carrier with respect to the first portion, Ammonia passing through the partition wall of the filter carrier from the first portion is supplied to the second NOx reduction catalyst, and other ammonia is supplied to the first NOx reduction catalyst to contribute to the reduction of NOx. it can. In the present invention, the slip ammonia is oxidized by the oxidation catalyst carried on the inner wall portion (fourth portion) of the outlet side passage downstream of the third portion, and this is released into the atmosphere without being purified. Can be avoided.

  Thus, according to the present invention, a series of chemical reactions relating to selective catalytic reduction with urea, including urea hydrolysis, NOx reduction and slip ammonia oxidation, can be performed in the filter carrier. The release of NOx and particulates into the atmosphere can be suppressed with a compact configuration as a whole system while ensuring a required NOx conversion rate.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 shows a configuration of an engine (hereinafter referred to as “engine”) 1 according to an embodiment of the present invention. In the present embodiment, a direct injection type diesel engine is employed as the engine 1.
An air cleaner (not shown) for removing dust in the intake air is attached to the introduction portion of the intake passage 11. Further, a compressor 12a of a variable nozzle type turbocharger 12 is installed in the intake passage 11, and the intake air compressed by the compressor 12a flows into the manifold portion via the surge tank 13 and enters each cylinder. Distributed.

  In the cylinder head of the engine body, fuel supply injectors 21, 21,... Are installed for each cylinder. The injector 21 is operated by a fuel injection control signal from a control unit (hereinafter referred to as “ECU”) 101 of the engine 1. Fuel delivered by a fuel pump (not shown) is supplied to the injector 21 via the common rail 22 and injected into the combustion chamber of each cylinder.

A turbine 12b of the turbocharger 12 is installed in the exhaust passage 31 downstream of the manifold portion, and the compressor 12a rotates when the turbine 12b is driven by exhaust.
A diesel particulate filter (corresponding to “particulate filter”, hereinafter referred to as “DPF”) 32 as an “exhaust purification device” according to the present embodiment is installed downstream of the turbine 12b. The DPF 32 incorporates a cylindrical filter carrier 321, and the filter carrier 321 carries a catalyst (NOx reduction catalyst) for promoting a NOx reduction reaction by ammonia. When exhaust from the engine 1 passes through the DPF 32, particulates in the exhaust are collected by the filter carrier 321 and NOx in the exhaust is reduced by the NOx reduction catalyst.

The exhaust passage 31 is connected to the intake passage 11 (in this embodiment, the surge tank 13) by an EGR pipe 33. Exhaust gas is recirculated to the intake passage 11 via the EGR pipe 33. An EGR valve 34 is interposed in the EGR pipe 33, whereby the flow rate of the exhaust gas recirculated is controlled.
Hereinafter, the configuration of the exhaust purification system and the exhaust purification process according to this embodiment will be described.

  The exhaust purification system according to the present embodiment collects particulates in the exhaust by the filter carrier 321 of the DPF 32, and reduces and purifies NOx in the exhaust by the NOx reduction catalyst carried on the filter carrier 321. is there. A urea water tank 41 is provided, and ammonia as a reducing agent is stored in the urea water tank 41 in the form of a urea aqueous solution. The urea water stored in the urea water tank 41 is supplied to the urea addition unit 42 via the urea water supply pipe. In the present embodiment, the urea addition unit 42 and the next urea water injection nozzle 44 constitute an air assist type injector, and the urea addition unit 42 incorporates an electric feed pump. The urea water supplied to the urea addition unit 42 is mixed with compressed air supplied from an air tank (not shown) through the air supply pipe 43 and is injected into the exhaust gas through the urea water injection nozzle 44. Urea in the injected urea water is hydrolyzed by exhaust heat and generates ammonia. The urea water injection nozzle 44 is installed upstream of the DPF 32 so as to penetrate the tube wall of the exhaust passage 31 inward. The injection direction is not particularly limited, but is parallel to the exhaust flow. The filter carrier 321 is set toward the upstream end surface. In the present embodiment, the urea supply unit 42 and the urea water injection nozzle 44 constitute a “supply device” for urea water.

  The urea addition mode is not limited to the air assist type described here, but a pressure increase pump for forming an injection pressure is provided instead of the urea addition unit 42 (and the air supply pipe 43), and a predetermined pump is provided by this pump. The urea water whose pressure has been increased to the pressure may be jetted by the urea water jet nozzle. In such a form, the “supply device” of urea water is constituted by a booster pump and a urea water injection nozzle.

FIG. 2 shows the configuration of the DPF 32 by a longitudinal section including the central axis of the filter carrier 321.
In the present embodiment, the DPF 32 has both a particulate collection function and a NOx reduction function. The filter carrier 321 is a wall flow type honeycomb carrier, and the inside thereof is defined by a plurality of cell passages p1 and p2 along the flow of exhaust gas by partition walls having pores of a size capable of collecting particulates. Yes. The pores form a passage when the exhaust gas passes through the partition wall of the filter carrier 321. Adjacent cell passages p1 and p2 are alternately closed on the inlet side or outlet side by the sealing materials 322 and 322, so that the cell passage (hereinafter referred to as “inlet side passage”) p1 opened on the inlet side, and the outlet A cell passage (hereinafter referred to as “exit-side passage”) p <b> 2 that opens on the side is formed. The inlet side passage p1 communicates with the exhaust passage upstream of the DPF 32, and the outlet side passage p2 communicates with the exhaust passage downstream of the DPF 32.

  The filter carrier 321 carries a plurality of catalysts c1 to c4 having different functions along the flow of exhaust gas inside the carrier. Specifically, a urea hydrolysis catalyst c1 is formed on a portion (corresponding to a “first portion”) forming an upstream portion of the inlet-side passage p1 in the inner wall of the filter carrier 321 forming the inlet-side passage p1. And a NOx reduction catalyst c2 is supported on a portion downstream of the inner wall portion on the inlet side (corresponding to a “second portion”). In addition, the NOx reduction catalyst c3 is supported on a portion (corresponding to a “third portion”) forming the upstream portion of the outlet side passage p2 in the inner wall of the carrier forming the outlet side passage p2, and the outlet side An oxidation catalyst c4 is supported on a portion downstream of the inner wall portion (corresponding to a “fourth portion”). The oxidation catalyst c4 is for purifying slip ammonia that has passed through the NOx reduction catalysts c2 and c3 without contributing to the reduction of NOx.

  Further, in the present embodiment, the urea hydrolysis catalyst c1 is added to the inner wall portion on the inlet side and is also carried on the end surface 321a on the inlet side of the filter carrier 321. FIG. 3 shows the filter carrier 321 as viewed from the upstream side with respect to the exhaust flow. Thus, in this embodiment, the urea hydrolysis catalyst c1 is supported over the entire end surface 321a of the filter carrier 321 formed by the plugging material 322 and the partition walls.

In this embodiment, the NOx reduction catalyst c1 supported on the inner wall portion on the inlet side is the “first NOx reduction catalyst”, and the NOx reduction catalyst c2 supported on the inner wall portion on the outlet side is the “second NOx reduction catalyst”. Respectively. Further, the oxidation catalyst c4 supported on the inner wall portion on the outlet side corresponds to the “first oxidation catalyst”.
Returning to FIG. 1, the operation of the urea addition unit 42 is controlled by a command signal (urea addition control signal) from the ECU 101 for exhaust purification. The ECU 101 is configured as an electronic control unit with a built-in microcomputer. The ECU 101 receives signals relating to the operating conditions of the engine 1 (in accordance with the engine speed NE and the fuel injection amount QF) from an engine control ECU (not shown). Signal) and a signal from the exhaust pipe temperature sensor 151 for detecting the exhaust temperature Texh upstream of the DPF 32 are input. The ECU 101 calculates an optimum urea water injection amount according to the operating condition of the engine 1 based on the input signal, and outputs a urea addition control signal corresponding to the calculated urea water injection amount to the urea addition unit 42. To do.

Next, an exhaust purification process by the system according to the present embodiment will be described with reference to FIG.
During the operation of the engine 1, urea water in an amount corresponding to the operating conditions of the engine 1 is supplied to the exhaust gas by the urea addition unit 42 and the urea water injection nozzle 44. The urea in the urea water undergoes hydrolysis on the hydrolysis catalyst c1 supported on the inner wall portion (first portion) on the inlet side, and ammonia as a reducing agent is generated. In the present embodiment, the urea hydrolysis catalyst c1 is added to the inner wall portion on the inlet side, and is also supported on the end surface 321a on the inlet side of the filter carrier 321 that is easily in contact with exhaust gas. This also occurs on the end face 321a of the filter carrier 321. Part of the generated ammonia passes through the partition wall of the filter carrier 321 from the inner wall portion on which the hydrolysis catalyst c1 is supported, and is supplied to the NOx reduction catalyst c3 on the outlet side, as indicated by an arrow Fa in the figure. On the other hand, as shown by the arrow Fb in the figure, other ammonia is supplied to the NOx reduction catalyst c2 on the inlet side, and passes through the partition from the inner wall portion (second portion) on which the NOx reduction catalyst c2 is supported. . NOx reduction reaction is promoted by the NOx reduction catalysts c2 and c3 on the inlet side and the outlet side, and NOx in the exhaust gas is reduced. The slip ammonia that has not contributed to the reduction of NOx by the NOx reduction catalysts c2 and c3 is purified by the oxidation catalyst c4 supported on the inner wall of the carrier that forms the most downstream part of the outlet side passage p2.

A series of chemical reactions relating to selective catalytic reduction of this embodiment including urea hydrolysis, NOx reduction, and slip ammonia oxidation are expressed by the following equations (1) to (3).
(NH ₂ ) ₂ CO + H ₂ O → 2NH ₃ + CO ₂ (1)
NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O (2)
4NH ₃ + 3O ₂ → 2N ₂ + 6H ₂ O (3)
According to this embodiment, the following effects can be obtained.

In the present embodiment, urea is hydrolyzed by the hydrolysis catalyst c1 supported on the inner wall portion on the inlet side of the filter carrier 321 and the end surface 321a on the inlet side, and the ammonia generated thereby is converted into the inlet side and the outlet side. The NOx can be reduced by being supplied to the NOx reduction catalysts c1 and c2 carried on the inner wall portions.
Specifically, in this embodiment, the inner wall portion (second portion) on the downstream side of the inner wall portion of the filter carrier 321 carrying the hydrolysis catalyst c1 and the inner wall portion (third portion) on the outlet side. And NOx reduction catalysts c2 and c3 are carried on each. Here, the NOx reduction catalyst c3 on the outlet side overlaps with the inner wall portion supporting the hydrolysis catalyst c1 in the direction intersecting the flow of the exhaust gas flowing into the filter carrier 321 and is supported on the portion shifted to the downstream side. I am letting. Therefore, the ammonia passing through the partition wall of the filter carrier 321 from the inner wall portion supporting the hydrolysis catalyst c1 is supplied to the NOx reduction catalyst c3 on the outlet side, and the other ammonia is supplied to the NOx reduction catalyst c2 on the inlet side. Thus, it can contribute to the reduction of NOx. In this embodiment, slip ammonia is oxidized by the oxidation catalyst c4 supported on the inner wall of the carrier that forms the most downstream portion of the outlet side passage p2, and this is prevented from being released into the atmosphere without being purified. Can do.

  Thus, according to this embodiment, the urea hydrolysis catalyst c1, the inlet-side and outlet-side NOx reduction catalysts c2 and c3, and the oxidation catalyst c4 for slip ammonia purification are subjected to a series of selective catalytic reduction with urea. Since they are arranged in the order of chemical reaction, NOx in the exhaust can be efficiently purified in the filter carrier 321 including the treatment of slip ammonia. Further, according to the present embodiment, in addition to the reduction of NOx, particulates in the exhaust gas can be collected and removed by the filter carrier 321, so that the release of NOx and particulates into the atmosphere is reduced throughout the system. Can be suppressed by a compact configuration.

In this embodiment, the urea hydrolysis catalyst c1 is added to the inner wall portion on the inlet side, and is also carried on the end surface 321a on the inlet side of the filter carrier 321. The end surface 321a on the entrance side is
Since it faces the flow of exhaust, it is easy to contact the exhaust. Accordingly, the amount of ammonia generated can be supplemented by the hydrolysis catalyst c1 supported on the end surface 321a, so that the hydrolysis catalyst c1 supported on the inner wall of the carrier is reduced and the inner wall portion on the inlet side supporting the NOx reduction catalyst c2. Can be secured.

  In addition to the above configuration, an oxidation catalyst as a “second oxidation catalyst” may be installed upstream of the DPF 32. This oxidation catalyst oxidizes hydrocarbons and carbon monoxide in the exhaust gas, and converts nitrogen monoxide in the exhaust gas into NOx mainly containing nitrogen dioxide (hereinafter referred to as “NO 2”) and is contained in the exhaust gas. This is for adjusting the ratio of NO and NO2 to be optimal for the reduction reaction of NOx.

  The present invention can be applied not only to a direct injection type diesel engine but also to other types of diesel engines and gasoline engines such as a sub-chamber type.

The schematic block diagram of the diesel engine which concerns on one Embodiment of this invention. Longitudinal sectional view of a diesel particulate filter as an exhaust purification device according to the same embodiment Front enlarged view of the filter carrier according to the same embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Diesel engine, 11 ... Intake passage, 12 ... Turbocharger, 12a ... Compressor, 12b ... Turbine, 13 ... Surge tank, 21 ... Injector, 22 ... Common rail, 31 ... Exhaust passage, 32 ... Exhaust purification device Diesel particulate filter, 321... Filter carrier, 321a ... end face of filter carrier on the inlet side, 322 ... sealant, 33 ... EGR pipe, 34 ... EGR valve, 41 ... urea water tank, 42 ... urea addition unit, 44 ... Urea water injection nozzle, 101 ... Control unit of exhaust purification system, 151 ... Exhaust pipe temperature sensor, c1 ... Urea hydrolysis catalyst, c2 ... NOx reduction catalyst as "first NOx reduction catalyst", c3 ... "second" NOx reduction catalyst as "NOx reduction catalyst", c4 ... as "first oxidation catalyst" Catalyst, p1 ... inlet channel, p2 ... outlet passage.

Claims (3)

It has a plurality of cell passages along the flow of exhaust gas, which are defined by partition walls that can collect particulates in the exhaust gas by filtration, and the adjacent cell passages are alternately closed by the plugging material on the inlet side or outlet side. A filter carrier configured to
In the filter carrier, a first portion including an inlet side edge of an inner wall of the inlet side passage which is a cell passage opened on the inlet side, and an inlet side of the filter carrier formed by the plugging material. An end surface, and a urea hydrolysis catalyst supported on the end surface;
In the filter carrier, a first NOx reduction catalyst supported on a second portion downstream of the first portion of the inner wall of the inlet side passage;
In the filter carrier, the inner wall of the outlet side passage which is a cell passage opened on the outlet side overlaps the first portion in a direction intersecting the flow of the exhaust gas flowing into the filter carrier, A second NOx reduction catalyst supported on a third portion extending to the downstream side of the portion;
In the filter carrier, a fourth portion of the inner wall of the outlet-side passage that is downstream of the third portion and overlaps the second portion in a direction intersecting the exhaust flow. A supported first oxidation catalyst;
An exhaust emission control device for an engine comprising: An exhaust emission control device according to claim 1 ,
A urea water supply device arranged to be able to supply urea water upstream of the exhaust purification device with respect to engine exhaust,
An exhaust purification system composed of. Further comprising configured claim the second oxidation catalyst provided upstream of the exhaust gas purifier
The exhaust purification system according to 2.

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