EXHAUST GAS PURIFYING SYSTEM

Abstract

In an exhaust gas purifying system having a catalyst provided in an exhaust pipe for carrying out chemical reduction reaction so as to remove NOx contained in exhaust gas from an engine, an additive injection valve is provided in the exhaust pipe at an upstream side of the catalyst for injecting additive agent into the exhaust pipe, so that the chemical reduction reaction at the catalyst is facilitated. An injection port of the additive injection valve is formed into a slit-shape, so that a sheet-like spray is formed when the additive is injected from the additive injection valve.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2007-253630, which is filed on Sep. 28, 2007, the disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an exhaust gas purifying system, such as a urea SCR system (selective catalytic reduction), for purifying exhaust gas of an internal combustion engine by specific purifying reaction of the exhaust gas. The present invention further relates to an additive injection valve for injecting addition agent in liquid form which will be used for the specific purifying reaction of the exhaust gas, and relates to an additive injection device.

BACKGROUND OF THE INVENTION

Recently, a urea SCR system (selective catalytic reduction) is under development and partly in practical use, as disclosed in Japanese Patent Publication No. 2003-293739, which will be applied to an electric power plant, various kinds of factories, and vehicles (in particular, vehicles having diesel engines), as an exhaust gas purifying system for purifying NOx (nitrogen oxides) contained in exhaust gas at a high purifying ratio.

The urea SCR system is composed of a catalyst for facilitating purifying reaction of exhaust gas, an exhaust pipe for supplying exhaust gas from an exhaust gas generating device (for example, an internal combustion engine) to the catalyst, and an additive injection valve provided in the exhaust pipe for injecting urea aqueous solution (ammonia) into the exhaust gas flowing through the exhaust pipe. The catalyst facilitates reduction reaction (the purifying reaction of exhaust gas) for NOx based on the urea aqueous solution. According to the above structure, the urea aqueous solution is supplied to the catalyst (provided at a downstream side) together with the exhaust gas by use of the exhaust gas flow. As a result, the reduction reaction of NOx based on the urea aqueous solution (ammonia) is generated at the catalyst to purify the exhaust gas. More exactly, the urea aqueous solution injected from the additive injection valve is hydrolyzed by heat of the exhaust gas to generate ammonia (NH3), and the NOx contained in the exhaust gas is reduced by the ammonia on the catalyst, so that the exhaust gas is purified.

An exhaust gas purifying system is also known in the art, for example, as disclosed in Japanese Patent Publication No. 2001-3737, according to which a center axis for injection of an additive injection valve 100 is arranged in parallel to a center axis 10a of an exhaust pipe 10, as shown in FIGS. 14A and 14B. In this injection valve, multiple injection ports are circularly (or concentrically) provided at a nozzle portion 101 thereof.

It is desired in the above exhaust gas purifying system that ammonia generated by the hydrolysis of the urea aqueous solution is supplied to whole area of the catalyst, so that NOx is reduced at the whole area of the catalyst to increase the purifying ratio of the catalyst.

According to the exhaust gas purifying system shown in FIGS. 14A and 14B, a spray form of the urea aqueous solution injected by the additive injection valve 100 becomes a mid-air conical shape, because the multiple injection ports are circularly arranged. As a result, the sprayed ammonia (the urea aqueous solution) reaches at an annular area 102 of an upstream side of SCR catalyst 20 (an annular hatched area in FIGS. 14A and 14B), whereas the sprayed ammonia (the urea aqueous solution) may not reach at any other areas than the annular area 102 In such a case, reduction reaction for NOx differs from area to area on the SCR catalyst 20, and thereby the purifying ratio will be decreased.

SUMMARY OF THE INVENTION

The present invention is made in view of the above problems. It is an object of the present invention to provide an exhaust gas purifying system, an additive injection valve, and an additive injection device, according to which addition agent will be supplied to a larger area of a catalyst for facilitating purifying reaction of exhaust gas.

According to one of features of the present invention, an exhaust gas purifying system for a vehicle has a catalyst provided in an exhaust pipe of an internal combustion engine for facilitating chemical reduction reaction to purify NOx contained in exhaust gas from the engine. An additive injection valve is provided in the exhaust pipe at an upstream side of the catalyst for injecting additive agent into the exhaust pipe, so that the chemical reduction reaction at the catalyst is facilitated. And an injection port of the additive injection valve is formed into a slit-shape, so that a sheet-like spray is formed when the additive is injected from the additive injection valve.

According to another feature of the invention, the slit-shape injection port is formed as a straightly extending slit.

According to a further feature of the invention, a width of the slit-shape injection port in a short side direction is not uniform.

According to a still further feature of the invention, the width of the slit-shape injection port at a position away from a center in a longitudinal direction is made smaller than the width of the slit-shape injection port at the center.

According to a still further feature of the invention, a center line of the injection by the additive injection valve is arranged to be in parallel to a center line of the exhaust pipe.

According to a still further feature of the invention, the slit-shape injection port crosses a center of the exhaust pipe and straightly extends in a radial direction of the exhaust pipe.

According to a still further feature of the invention, DPF is provided in the exhaust pipe at an upstream side of the additive injection valve.

According to a still further feature of the invention, the exhaust pipe has a straight pipe portion, the catalyst is provided at a downstream side of the straight pipe portion, and the additive injection valve is provided at an upstream side of the straight pipe portion. And the additive injection valve is provided in the straight pipe portion in such a manner that a center line of a nozzle portion of the additive injection valve comes into line with a center line of the straight pipe portion.

According to a still further feature of the invention, a center line of the catalyst comes into line with the center line of the straight pipe portion.

According to a still further feature of the invention, the exhaust pipe has a first curved pipe at the upstream end of the straight pipe portion, and the additive injection valve is provided at the first curved pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:

FIG. 1 is a schematic view showing an SCR system according to an embodiment of the present invention;

FIG. 2 is a cross sectional view showing an additive injection valve according to the present invention;

FIG. 3 is a top plan view showing a plate member of the present invention;

FIG. 4 is a schematic perspective illustration for a spray form of urea aqueous solution injected from the additive injection valve according to the present invention;

FIG. 5 is a schematic view showing condition in which the urea aqueous solution is supplied to a catalyst;

FIG. 6 is a top plan view showing a plate member of a comparison example;

FIG. 7 is a top plan view showing a plate member according to a modification of the present invention;

FIG. 8 is also a top plan view showing a plate member according to another modification of the present invention;

FIG. 9 is a view showing velocity distribution of exhaust gas;

FIGS. 10 to 13 are schematic views respectively showing further modifications of the present invention; and

FIGS. 14A and 14B are schematic views showing a conventional urea SCR system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be hereinafter explained with reference to the drawings. In the embodiment of the present invention, the invention is applied to a diesel engine (exhaust gas generating device) mounted on a vehicle as an SCR system (an exhaust gas purifying system).

A general outline of the SCR system will be explained with reference to FIG. 1. In FIG. 1, the SCR system purifies exhaust gas emitted from a diesel engine of a vehicle. A diesel particulate filter (DPF) 40, an exhaust pipe 10 and an SCR catalyst 20 are arranged in this order from an upstream of exhaust gas flow, so that the exhaust gas flows through the exhaust pipe 10, being swirled as indicated by an arrow A.

The DPF 40 is a filter of a continuous re-generative type for continuously trapping and removing diesel particulates (PM=Particulate Matter) contained in the exhaust gas. For example, the trapped PM is combusted (as a re-generation treatment) by a post fuel injection, which is carried out after a main fuel injection. As a result, the DPF 40 is continuously used as the filter. The DPF 40 carries an oxidation catalyst made of platinum or platinated metal for removing HC and CO together with soluble organic constituent (SOF), which is one of PM compounds.

An additive injection valve 30 is provided at the exhaust pipe 10, for injecting (supplying) urea aqueous solution (addition agent) to exhaust gas flowing through the exhaust pipe 10. The urea aqueous solution injected from the additive injection valve 30 is converted to ammonia (NH3) by heat of the exhaust gas, and such converted ammonia is supplied to the SCR catalyst 20 on a downstream side together with the exhaust gas so that they are used for exhaust gas purifying reaction.

The SCR catalyst 20 facilitates well-known reduction reaction (the purifying reaction of the exhaust gas), and carries out chemical reactions as indicated by the following reaction formulas (Formula 1 to Formula 3), so as to reduce NOx contained in the exhaust gas:

4NO+4NH₃+O₂→4N₂+6H₂O   (Formula 1)

6NO₂+8NH₃→7N₂+12H₂O   (Formula 2)

NO+NO₂+2NH₃+O₂→2N₂+3H₂O   (Formula 3)

The urea aqueous solution is drawn up by a pump (not shown) from a urea aqueous solution tank (not shown), and supplied to the additive injection valve 30 through a supply pipe.

As shown in FIG. 1, the exhaust pipe 10 of an upstream side of the SCR catalyst 20 is composed of a cylindrical straight pipe 11 connected to the SCR catalyst 20, a first curved pipe 12 connected to the straight pipe 11 and bent into an arc shape, and a second curved pipe 13 connected to the DPF 40 and bent into an arc shape, but bent in a direction opposite to that of the first curved pipe 12. A projected portion 14 is provided at the first curved pipe 12 and the additive injection valve 30 is provided in the projected portion 14.

The projected portion 14 is formed into a cylindrical shape, and is opened to the first curved pipe 12 in such a manner that a center line of the projected portion 14 comes into line with a center line 11a of the straight pipe 11. In other words, the projected portion 14 is fixed to a curved outer peripheral portion of the first curved pipe 12 in such a manner that the projected portion 14 is projected in a direction opposite to the straight pipe 11. The center line 11a of the straight pipe 11 is also in line with a center line of the SCR catalyst 20. The additive injection valve 30 is attached to an end portion 14a of the projected portion 14 (at an opposite side of the straight pipe 11), so that an injection port of the additive injection valve 30 is opened to the inside of the projected portion 14. A center line of a nozzle portion of the additive injection valve 30 is also arranged to come into line with the center line 11a of the straight pipe 11. As a result of the above structure, injection direction of the urea aqueous solution by the additive injection valve 30 is made to be in parallel to the center line 11a of the straight pipe 11.

A structure of the additive injection valve 30 will be explained with reference to FIG. 2. The additive injection valve 30 is an electromagnetic valve basically having the same structure to a fuel injection valve for a gasoline engine. A nozzle portion 31 formed at a forward end of the additive injection valve 30 has a needle 33, which is accommodated in a valve body 32 in a sliding manner in an axial direction. The needle 33 is seated on (or separated from) a valve seat 34 formed in the valve body 32. An injection port 35a is formed in a plate member 35, which is provided at a downstream side of the valve seat 34.

An electromagnetic solenoid 36 is arranged at an upper side of the nozzle portion 31. A terminal 37 is connected to the electromagnetic solenoid 36. An inlet port 39, communicated with a passage 38, which is formed between the valve body 32 and the needle 33, is connected to a tank for storing the urea aqueous solution. According to such structure, the urea aqueous solution will be supplied to a valve seating portion of the valve seat 34 through the inlet port 39 and the passage 38.

When electric power is supplied to the above electromagnetic solenoid 36 via the terminal 37 under control of ECU 41 (an electronic control unit mounted on the vehicle, see FIG. 1), the needle 33 is moved in a valve opening direction. As a result, the urea aqueous solution reaching at the valve seating portion flows through an opened space between the needle 33 and the valve seat 34 in a downstream direction. And the urea aqueous solution having passed between the needle 33 and the valve seat 34 is injected through the injection port formed in the plate member 35.

An effect of the SCR system of the present invention will be explained with reference to FIGS. 3 to 5. In the following explanation, it is assumed that the exhaust gas flows in the exhaust pipe 10 in a swirled form.

As shown in FIG. 3, a slit-shape injection port 35a is formed in the disc-shaped plate member 35. The slit-shape injection port 35a is straightly extending in a radial direction of the disc-shaped plate member 35 (the radial direction of the exhaust pipe 10), wherein the straightly extending line for the injection port 35a crosses a center of the plate member 35 (a center of the nozzle portion 31). As shown in FIG. 4, the urea aqueous solution injected from the injection port 35a forms a sheet-like spray. More exactly, the spray of the injected urea aqueous solution flows towards the SCR catalyst 20, wherein, at a beginning of the injection, the injected spray is not rotated by the swirled flow of the exhaust gas due to penetration of injection. However, as the injected spray flows in the downstream direction thereafter, the injected spray starts with rotation by the swirled flow of the exhaust gas. As above, the injected spray of the urea aqueous solution has a plate form at an area adjacent to the injection port 35a, but the shape of the injected spray is formed into a spiral shape at a position, which is separated by more than a predetermined distance from the injection port 35a toward the SCR catalyst 20, wherein the spiral shape is formed depending on flow speed of the swirled flow of the exhaust gas. The spray of ammonia, which is generated by hydrolysis of the urea aqueous solution, (including the spray of the urea aqueous solution) arrives at an upstream side surface 20a of the SCR catalyst 20, as indicated by a hatched portion in FIG. 5.

An area 21 of the upstream side surface 20a of the SCR catalyst 20, at which the spray of ammonia (the urea aqueous solution) arrives, has a shape corresponding to the slit-like shape of the injection port 35a. Namely, the area 21 (also referred to as the additive agent arriving area) is formed as a rectangular area extending in the radial direction of the exhaust pipe 10. This is because the center line of the injection by the additive injection valve 30 is arranged to be in parallel to the center line of the exhaust pipe 10. In other words, the center lines of the nozzle portion 31, the SCR catalyst 20 and the straight pipe 11 are so made as to come into line with each other, and furthermore the injection direction of the urea aqueous solution by the additive injection valve 30 is arranged to be in parallel to the center line 11a of the straight pipe 11. According to such arrangement, the cross sectional shape of the spiral spray of the urea aqueous solution (the shape in the cross section extending in the radial direction of the exhaust pipe 10) is expanded, as a distance from the injection port 35a becomes larger as a result of spreading the injected spray. However, the cross sectional shape still corresponds to the slit-like shape of the injection port 35a. As above, the additive agent arriving area 21 has the shape corresponding to the slit-like shape of the injection port 35a.

The area 21 (the additive agent arriving area 21) of the upstream side surface 20a of the SCR catalyst 20, at which the spray of ammonia (the urea aqueous solution) arrives, is changed from time to time as the flow speed (flow speed in an axial flow direction and flow speed in a rotational direction) of the swirl flow of the exhaust gas varies. For example, the additive agent arriving area 21 is rotated in a circumferential direction around the center line of the exhaust pipe 10, as indicated by dotted lines in FIG. 5.

The above embodiment has the following advantages.

According to the above embodiment, the injection port 35a of the nozzle portion 31 is formed into the slit-shape, the center lines of the nozzle portion 31 and the SCR catalyst 20 and the center line 11a of the straight pipe 11 are so made as to come into line with each other, and the injection direction of the urea aqueous solution by the additive injection valve 30 is arranged to be in parallel to the center line 11a of the straight pipe 11. As a result, the area 21 (the additive agent arriving area 21) of the upstream side surface 20a of the SCR catalyst 20, at which the spray of ammonia (the urea aqueous solution) arrives, is formed as the rectangular area continuously extending in the radial direction of the exhaust pipe 10, as indicated by the hatched area in FIG. 5. In addition, the additive agent arriving area 21 is changed from time to time as the flow speed of the swirl flow of the exhaust gas varies. Therefore, as explained above, the additive agent arriving area 21 is rotated in a circumferential direction around the center line of the exhaust pipe 10, as indicated by the dotted lines in FIG. 5. As a result, ammonia can be supplied to a wide area of the SCR catalyst 20.

Furthermore, since the injection port 35a is formed into the slit-shape extending in the radial direction of the exhaust pipe 10, density of the spray for ammonia (the urea aqueous solution) arriving at the additive agent arriving area 21 can be uniformized. More exactly, the density of the spray for ammonia becomes more uniform, when compared with a comparison example shown in FIG. 6, in which multiple small injection ports 103 are provided in the plate member 35.

(Modifications)

The present invention is not limited to the above explained embodiment, but may be modified in the following manners.

(a) In the above embodiment, the injection port 35a is formed into the slit-shape, which straightly extends in the radial direction of the plate member 35 passing through the center thereof. The slit-shape injection port 35a may be, however, offset from the center of the plate member 35. The slit-shape injection port 35a may be formed into a bent-shape. Furthermore, the injection port 35a may be formed into a cross-slit-shape, which extends in the radial direction from a certain point (e.g. the center) of the plate member, as shown in FIG. 7.

(b) In the above embodiment, the slit-shape injection port 35a has a rectangular shape, so that a width in a short-side direction (a slit width) has a predetermined constant value. The slit width of the injection port 35a may be, however, changed along a longitudinal direction. In case of the slit-shape injection port, spray condition (for example, particle diameter, penetration, etc.) of the urea aqueous solution injected from the injection port 35a varies at positions in the longitudinal direction of the slit. Accordingly, the spray condition may be adjusted by setting the width in the short-side direction (the slit-width) of the injection port 35a for respective longitudinal positions.

For example, in the case that particulate diameter of the urea aqueous solution tends to become larger at a longitudinal position, a distance of which is longer away from the center of the plate member, the particulate diameter of the spray for the urea aqueous solution can be made uniform by adjusting the slit-width in such a manner that the slit-width is made smaller as the longitudinal position is more away from the center, as shown in FIG. 8.

The velocity distribution of the exhaust gas (a result of measurement, in which flow speed of the exhaust gas in the axial direction of the exhaust pipe 10 is measured at respective points in the radial direction and at the same time) shows a certain tendency, as shown in FIG. 9, that the flow speed varies depending on a distance in the radial direction from the center line 10a of the exhaust pipe 10. The flow speed becomes at its maximum at an intermediate point between the center line 10a of the exhaust pipe 10 and an inner surface 10b. The flow speed is decreased at a point coming closer to the inner surface 10b of the exhaust pipe 10. In FIG. 9, arrows designate vectors for the flow speed of the exhaust gas flowing in the exhaust pipe 10. Accordingly, the penetration of the spray for the urea aqueous solution may be adjusted by setting the width in the short-side direction (the slit-width) of the injection port 35a in accordance with the above velocity distribution.

(c) In the above embodiment, the additive injection valve 30 is provided at the projected portion 14 of the first curved pipe 12. However, as shown in FIG. 10, the additive injection valve 30 may be provided at a corner of an L-shaped pipe.

(d) In the above embodiment, the urea aqueous solution is injected in parallel to the center line of the exhaust pipe 10. More exactly, the additive injection valve 30 is provided in such a manner that the injection direction (the center line of injection) of the urea aqueous solution by the additive injection valve 30 becomes in parallel to the center line 11a of the straight pipe 11. The present invention is not limited to such embodiment. For example, as shown in FIGS. 11 and 12, the center line of the injection by the additive injection valve 30 may be inclined by a predetermined angle with respect to the center line 10a of the exhaust pipe 10.

(e) In the above embodiment, the additive injection valve 30 is disclosed as an example of a spray forming means, according to which the additive is supplied to the passage 38 (the passage from the inlet port 39 to the injection port 35a, as shown in FIG. 2) and the additive is injected by opening and closing the passage 38. The spray forming means may be arranged such that gas, in which the additive is compressed (such as, compressed air), maybe injected. For example, as shown in FIG. 13, the exhaust gas purifying system may have an additive supply device 52, a compressed air supply device 53, a mixing device 54, and an injection nozzle 55, wherein the additive and the compressed air respectively supplied from the additive supply device 52 and the compressed air supply device 53 are mixed in the mixing device 54, and the mixed gas is injected from the injection nozzle 55. An injection port of the injection nozzle 55 is made as a slit-shape port, so that a sheet-like spray of the additive is formed.

(f) In the above embodiment, the present invention is applied to the urea SCR system for the engine mounted on the vehicle. The present invention is not limited to such application, but may be applied to any other system for purifying the exhaust gas by use of the additive and the catalyst.

Claims

1. An exhaust gas purifying system for a vehicle comprising:

a catalyst provided in an exhaust pipe of an internal combustion engine for facilitating chemical reduction reaction to purifying NOx contained in exhaust gas from the engine; and

an additive injection valve provided in the exhaust pipe at an upstream side of the catalyst for injecting additive agent into the exhaust pipe, so that the chemical reduction reaction at the catalyst is facilitated,

wherein an injection port of the additive injection valve is formed into a slit-shape.

2. The exhaust gas purifying system according to the claim 1, wherein

the slit-shape injection port is formed as a straightly extending slit.

3. The exhaust gas purifying system according to the claim 1, wherein

a width of the slit-shape injection port in a short side direction is not uniform.

4. The exhaust gas purifying system according to the claim 3, wherein

the width of the slit-shape injection port at a position away from a center in a longitudinal direction is made smaller than the width of the slit-shape injection port at the center.

5. The exhaust gas purifying system according to the claim 1, wherein

a center line of the injection by the additive injection valve is arranged to be in parallel to a center line of the exhaust pipe.

6 The exhaust gas purifying system according to the claim 1, wherein

the slit-shape injection port crosses a center of the exhaust pipe and straightly extends in a radial direction of the exhaust pipe.

7. An exhaust gas purifying system for a vehicle comprising:

a catalyst provided in an exhaust pipe of an internal combustion engine for facilitating chemical reduction reaction to purify NOx contained in exhaust gas from the engine; and

a spray forming means provided in the exhaust pipe at an upstream side of the catalyst for injecting additive agent into the exhaust pipe, so that the chemical reduction reaction at the catalyst is facilitated,

wherein a sheet-like spray is formed when the additive is injected from the spray forming means.

8. The exhaust gas purifying system according to the claim 1, further comprising:

DPF provided in the exhaust pipe at an upstream side of the additive injection valve or the spray forming means.

9. The exhaust gas purifying system according to the claim 1, wherein

the exhaust pipe has a straight pipe portion,

the catalyst is provided at a downstream side of the straight pipe portion,

the additive injection valve is provided at an upstream side of the straight pipe portion, and

the additive injection valve is provided in the straight pipe portion in such a manner that a center line of a nozzle portion of the additive injection valve comes into line with a center line of the straight pipe portion.

10. The exhaust gas purifying system according to the claim 9, wherein

a center line of the catalyst comes into line with the center line of the straight pipe portion.

11. The exhaust gas purifying system according to the claim 9, wherein

the exhaust pipe has a first curved pipe at the upstream end of the straight pipe portion, and

the additive injection valve is provided at the first curved pipe.