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United States Patent Application 20060248881
Kind Code A1
YOKOTA; Haruyuki November 9, 2006

System for Injecting Fuel into Diesel Engine Exhaust Pipe

Abstract

Disclosed is a system for injecting fuel into a diesel engine exhaust pipe that prevents injected fuel from adhering to an exhaust pipe inner wall, adequately reduces and regenerates an exhaust gas purification catalyst, and remarkably increases fuel efficiency. This system includes a plurality of fuel injection nozzles, which are positioned in an exhaust pipe 10 of a diesel engine 1, injects fuel from the fuel injection nozzles, and reduces and regenerates the exhaust gas purification catalyst 11 that is installed in the exhaust pipe. The fuel injection nozzles are positioned so that injection axis lines 20a intersect with each other. It is preferred that the fuel injection nozzles be arranged in a circumferential direction of the exhaust pipe and positioned at substantially equal spacing intervals, and that the injection axis lines intersect at substantially one point. It is also preferred that the fuel injection nozzles be holed or slit fuel injection nozzles having one or two injection holes.


Inventors: YOKOTA; Haruyuki; (Tokyo, JP)
Correspondence Address:
    REED SMITH, LLP;ATTN: PATENT RECORDS DEPARTMENT
    599 LEXINGTON AVENUE, 29TH FLOOR
    NEW YORK
    NY
    10022-7650
    US
Serial No.: 459520
Series Code: 11
Filed: July 24, 2006

Current U.S. Class: 60/286; 60/303
Class at Publication: 060/286; 060/303
International Class: F01N 3/10 20060101 F01N003/10


Foreign Application Data

DateCodeApplication Number
Jan 22, 2004JP2004-013747

Claims



1-7. (canceled)

8. A system for injecting fuel into a diesel engine exhaust pipe comprising a plurality of fuel injection nozzles (20,36,38) positioned in an exhaust pipe (10) of a diesel engine (1) for injecting fuel into the exhaust pipe to reduce and regenerate an exhaust gas purification catalyst (11) installed in the exhaust pipe, wherein the fuel injection nozzles are positioned so that injection axis lines (20a) of the injected fuel intersect with each other.

9. The system for injecting fuel into a diesel engine exhaust pipe according to claim 8, wherein the plurality of fuel injection nozzles (20,36,38) are arranged in a circumferential direction of the exhaust pipe (10) and positioned at substantially equal spaced intervals from one another.

10. The system for injecting fuel into a diesel engine exhaust pipe according to claim 8 wherein the plurality of fuel injection nozzles (20,36,38) are positioned so that the injection axis lines (20a) intersect at substantially one point.

11. The system for injecting fuel into a diesel engine exhaust pipe according to claim 9 wherein the plurality of fuel injection nozzles (20,36,38) are positioned so that the injection axis lines (20a) intersect at substantially one point.

12. The system for injecting fuel into a diesel engine exhaust pipe according to claim 8, wherein the fuel injection nozzles are holed fuel injection nozzles (20,36) or slit fuel injection nozzles (38) having one or two injection holes (25,37,39).

13. The system for injecting fuel into a diesel engine exhaust pipe according to claim 9, wherein the fuel injection nozzles are holed fuel injection nozzles (20,36) or slit fuel injection nozzles (38) having one or two injection holes (25,37,39).

14. The system for injecting fuel into a diesel engine exhaust pipe according to claim 10, wherein the fuel injection nozzles are holed fuel injection nozzles (20,36) or slit fuel injection nozzles (38) having one or two injection holes (25,37,39).

15. The system for injecting fuel into a diesel engine exhaust pipe according to claim 11, wherein the fuel injection nozzles (20,36,38) are formed so that the injection axis lines (20a) are inclined rearward from a nozzle main body center line (21a).

16. The system for injecting fuel into a diesel engine exhaust pipe according to claim 12, wherein the fuel injection nozzles (20,36,38) are formed so that the injection axis lines (20a) are inclined rearward from a nozzle main body center line (21a).

17. The system for injecting fuel into a diesel engine exhaust pipe according to claim 13, wherein the fuel injection nozzles (20,36,38) are formed so that the injection axis lines (20a) are inclined rearward from a nozzle main body center line (21a).

18. The system for injecting fuel into a diesel engine exhaust pipe according to claim 14, wherein the fuel injection nozzles (20,36,38) are formed so that the injection axis lines (20a) are inclined rearward from a nozzle main body center line (21a).

19. The system for injecting fuel into a diesel engine exhaust pipe according to claim 8 wherein the diesel engine (1) has a feed pump (4) for supplying fuel to the fuel injection nozzles (20,36,38).

20. The system for injecting fuel into a diesel engine exhaust pipe according to claim 9 wherein the diesel engine (1) has a feed pump (4) for supplying fuel to the fuel injection nozzles (20,36,38).

21. The system for injecting fuel into a diesel engine exhaust pipe according to claim 10 wherein the diesel engine (1) has a feed pump (4) for supplying fuel to the fuel injection nozzles (20,36,38).

22. The system for injecting fuel into a diesel engine exhaust pipe according to claim 11 wherein the diesel engine (1) has a feed pump (4) for supplying fuel to the fuel injection nozzles (20,36,38).

23. The system for injecting fuel into a diesel engine exhaust pipe according to claim 12 wherein the diesel engine (1) has a feed pump (4) for supplying fuel to the fuel injection nozzles (20,36,38).

24. The system for injecting fuel imnto a diesel engine exhaust pipe according to claim 13 wherein the diesel engine (1) has a feed pump (4) for supplying fuel to the fuel injection nozzles (20,36,38).

25. The system for injecting fuel into a diesel engine exhaust pipe according to claim 14 wherein the diesel engine (1) has a feed pump (4) for supplying fuel to the fuel injection nozzles (20,36,38).

26. The system for injecting fuel into a diesel engine exhaust pipe according to claim 15 wherein the diesel engine (1) has a feed pump (4) for supplying fuel to the fuel injection nozzles (20,36,38).

27. The system for injecting fuel into a diesel engine exhaust pipe according to claim 16 wherein the diesel engine (1) has a feed pump (4) for supplying fuel to the fuel injection nozzles (20,36,38).

28. The system for injecting fuel into a diesel engine exhaust pipe according to claim 17 wherein the diesel engine (1) has a feed pump (4) for supplying fuel to the fuel injection nozzles (20,36,38).

29. The system for injecting fuel into a diesel engine exhaust pipe according to claim 18 wherein the diesel engine (1) has a feed pump (4) for supplying fuel to the fuel injection nozzles (20,36,38).
Description



TECHNICAL FIELD

[0001] The present invention relates to a system for injecting fuel into a diesel engine exhaust pipe, suitable for use in a diesel engine that includes a NOx occlusion reduction type catalyst or DPNR or other reduction regeneration type catalyst as an exhaust gas posttreatment device.

BACKGROUND ART

[0002] In recent years, various systems have been developed for reducing particulate matter (hereinafter referred to as the PM) and NOx in an exhaust gas. The PM is composed of an SOF, soot, and sulphate+absorbed water. The SOF, which is composed of unburned fuel and oil, and HC and CO in the exhaust gas are oxidized and purified by an oxidation catalyst. A diesel particulate filter (hereinafter referred to as the DPF) is capable of considerably reducing the PM and used to make diesel engine automobiles environmentally friendly. The DPF comprises a heat-resistant filter, which is capable of capturing the PM with high efficiency, and a filter regeneration device, which is used to remove the PM captured by the filter. Some filter regeneration devices regenerate the filter by burning the captured PM away with a light oil burner or the like.

[0003] Meanwhile, the exhaust gas composition for use in a diesel engine is in an extremely unfavorable environment for catalysts because it is always in an oxygen excess region, scarce of an element that serves as a reducer, and distributed over a wide temperature range. In this oxygen excess environment, a NOx occlusion reduction type catalyst is available as a catalyst for NOx purification. The NOx occlusion reduction type catalyst exhibits an extremely high NOx reduction rate. Therefore, when very stringent NOx control requirements are to be complied with, the use of a NOx occlusion reduction type catalyst is a workable choice. A diesel particulate-NOx reduction system (hereinafter referred to as the DPNR), which is a combination of the NOx occlusion reduction type catalyst and the aforementioned DPF, is also available.

[0004] When the aforementioned NOx occlusion reduction type catalyst or DPNR is employed, it is necessary to remove oxygen from NO.sub.3, which is formed upon NOx occlusion, and use the removed oxygen to perform a filter regeneration process. A system for injecting fuel into an exhaust pipe (disclosed, for instance, by Patent Document 1), which injects fuel into an exhaust pipe for the purpose of increasing the amount of HC, CO, CO.sub.2, or H.sub.2, is used as a system for oxygen removal and filter regeneration. For example, this system introduces pressurized fuel from an engine feed pump, and injects the fuel from a holed fuel injection nozzle 50 (see FIGS. 8 and 9) or a slit fuel injection nozzle 51 (see FIGS. 10 and 11), which is installed in the exhaust pipe and positioned upstream of the catalyst.

[0005] The holed fuel injection nozzle 50 injects fuel, for instance, from eight circular injection openings 50a in the leading end. The slit fuel injection nozzle 51 injects fuel from one or a plurality of injection slits 51a in the leading end. To provide adequate catalyst reduction and regeneration, it is necessary to provide a rich air-fuel ratio by quickly raising the fuel vapor concentration in the exhaust pipe.

Patent Document 1: Japanese Patent Laid-open No.2000-356137

DISCLOSURE OF THE INVENTION

Problem to Be Solved By the Invention

[0006] As described above, the conventional system for injecting fuel into a diesel engine exhaust pipe injects fuel into the exhaust pipe via a holed fuel injection nozzle 50 or a slit fuel injection nozzle 51. The holed fuel injection nozzle 50 injects the fuel, for instance, from eight circular injection openings 50a in the leading end. The slit fuel injection nozzle 51 injects the fuel from one or a plurality of injection slits 51a in the leading end.

[0007] Therefore, the injected fuel streaks in the exhaust pipe 52 as shown in FIG. 12. The greater part of the injected fuel then adheres to an inner wall 53 of the exhaust pipe 52. Consequently, as shown in FIG. 13, the fuel vapor directly reaching the catalyst is substantially reduced to half. Meanwhile, the fuel adhering to the inner wall of the exhaust pipe vaporizes, and the resulting fuel vapor reaches the catalyst with a small time delay.

[0008] As such being the case, the conventional system for injecting fuel into a diesel engine exhaust pipe cannot provide adequate catalyst reduction and regeneration because it cannot quickly raise the fuel vapor concentration in the exhaust pipe. To raise the fuel vapor concentration as needed for catalyst reduction and regeneration, it is necessary to inject a large amount of fuel. However, injecting a large amount of fuel decreases fuel efficiency.

[0009] The present invention has been made to solve the above problem. It is an object of the present invention to provide a system for injecting fuel into diesel engine exhaust pipe for adequately reducing and regenerating an exhaust gas purification catalyst and remarkably increasing fuel efficiency by preventing injected fuel from adhering to the inner wall of the exhaust pipe and quickly raising the fuel vapor concentration in the exhaust pipe to a level close to a theoretical fuel vapor concentration.

Means for Solving the Problem

[0010] To solve the above problem, the present invention provides a system for injecting fuel into a diesel engine exhaust pipe for use in a diesel engine that includes a fuel injection nozzle in its exhaust pipe. This system injects fuel from the fuel injection nozzle and reduces and regenerates an exhaust gas purification catalyst that is installed in the exhaust pipe. The fuel injection nozzle comprises a plurality of injection nozzles whose injection axis lines intersect with each other. The injection axis line means an injection center line that indicates the injection direction of each nozzle.

[0011] The employed fuel injection nozzle comprises the plurality of fuel injection nozzles, which are installed so that their injection axis lines intersect with each other. Therefore, the fuels injected from the fuel injection nozzles collide with each other and mix to provide optimum atomization, thereby preventing the injected fuels from adhering to the inner wall of the exhaust pipe.

[0012] It is preferred that the plurality of fuel injection nozzles be arranged in the circumferential direction of the exhaust pipe and positioned at substantially equal spacing intervals. When the plurality of fuel injection nozzles are arranged in the circumferential direction of the exhaust pipe and positioned at substantially equal spacing intervals, the fuels injected from the fuel injection nozzles uniformly collide with each other and mix within the exhaust pipe. As a result, the fuel vapor is evenly supplied to the catalyst.

[0013] It is preferred that the plurality of fuel injection nozzles be positioned so as to intersect their injection axis lines at substantially one point. When the plurality of fuel injection nozzles are positioned so as to intersect their injection axis lines at substantially one point, the fuels injected from the nozzles collide with each other and mix with increased certainty, thereby further preventing the injected fuels from adhering to the inner wall of the exhaust pipe.

[0014] It is preferred that the fuel injection nozzles be holed or slit fuel injection nozzles having one or two injection openings. The system according to the present invention causes the fuels injected from the fuel injection nozzles to collide with each other and mix, thereby preventing the injected fuels from adhering to the inner wall of the exhaust pipe. Therefore, the optimum nozzles are holed or slit fuel injection nozzles that inject fuel linearly. Further, the use of one or two injection openings is more appropriate than the conventional use of eight or more injection openings because the use of one or two injection openings provides more effective fuel collision and mixture.

[0015] It is preferred that the injection axis lines of the fuel injection nozzles be inclined rearward from the nozzle main body center lines. The system according to the present invention assumes that the injection axis lines of the fuel injection nozzles are inclined rearward, that is, in an exhaust gas flow direction. When the injection axis lines are inclined rearward from the nozzle main body center line, the injection nozzle mounting angle relative to the exhaust pipe is allowed to be great. Consequently, the injection nozzle structure is such that the injection nozzles can be installed with ease.

[0016] For example, a diesel engine feed pump supplies fuel to the fuel injection nozzles. The system for injecting fuel into diesel engine exhaust pipe fuel according to the present invention is particularly suitable for a situation where relatively low pressure fuel supplied from the feed pump is to be injected.

Effects of the Invention

[0017] As described in detail above, the present invention provides a system for injecting fuel into a diesel engine exhaust pipe for use in a diesel engine that includes a fuel injection nozzle in its exhaust pipe. This system injects fuel from the fuel injection nozzle and reduces and regenerates an exhaust gas purification catalyst that is installed in the exhaust pipe. The fuel injection nozzle comprises a plurality of injection nozzles whose injection axis lines intersect with each other. Therefore, the present invention prevents injected fuel from adhering to the inner wall of the exhaust pipe, and ensures that the fuel vapor concentration in the exhaust pipe is close to a theoretical fuel vapor concentration. This makes it possible to adequately reduce and regenerate the exhaust gas purification catalyst and remarkably increase fuel efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 is a system diagram illustrating a system for injecting fuel into a diesel engine exhaust pipe according to the present invention.

[0019] FIG. 2 is a cross-sectional side view illustrating a fuel injection nozzle that is shown in FIG. 1.

[0020] FIG. 3 is a bottom view illustrating the fuel injection nozzle that is shown in FIG. 2.

[0021] FIG. 4 is a bottom view illustrating another fuel injection nozzle.

[0022] FIG. 5 is a bottom view illustrating still another fuel injection nozzle.

[0023] FIG. 6 is a cross-sectional side view illustrating how the fuel injection nozzle shown in FIG. 1 is mounted.

[0024] FIG. 7 is a graph illustrating an operation that is performed by the fuel injection nozzle shown in FIG. 1.

[0025] FIG. 8 is a cross-sectional side view illustrating a conventional holed fuel injection nozzle.

[0026] FIG. 9 is a bottom view illustrating the holed fuel injection nozzle that is shown in FIG. 8.

[0027] FIG. 10 is a cross-sectional side view illustrating a conventional slit fuel injection nozzle.

[0028] FIG. 11 is a bottom view illustrating the slit fuel injection nozzle that is shown in FIG. 10.

[0029] FIG. 12 is a cross-sectional side view illustrating how a conventional fuel injection nozzle is mounted.

[0030] FIG. 13 is a graph illustrating an operation that is performed by a conventional fuel injection nozzle.

DESCRIPTION OF REFERENCE NUMERALS

[0031] 1: Diesel engine [0032] 2: Common rail [0033] 3: Supply pump [0034] 4: Feed pump [0035] 5: Fuel tank [0036] 6: Fuel filter [0037] 7: ECU [0038] 10: Exhaust pipe [0039] 11: Catalyst [0040] 12: Muffler [0041] 20: Fuel injection nozzle [0042] 20a: Injection axis line [0043] 21: Nozzle main body [0044] 21a: Nozzle main body center line [0045] 22: Fuel supply hole [0046] 23: Valve seat [0047] 24: Guidance hole [0048] 25: Injection hole [0049] 30: Valve stem [0050] 31: Seat section [0051] 36: Fuel injection nozzle [0052] 37: Injection hole [0053] 38: Fuel injection nozzle [0054] 39: Injection hole [0055] 50: Holed fuel injection nozzle [0056] 50a: Injection hole [0057] 51: Slit fuel injection nozzle [0058] 51a: Injection hole [0059] 52: Exhaust pipe [0060] 53: Inner wall

BEST MODE FOR CARRYING OUT THE INVENTION

[0061] A system for injecting fuel into a diesel engine exhaust pipe according to an embodiment of the present invention will now be described in detail with reference to FIGS. 1 to 7.

[0062] As shown in FIG. 1, a diesel engine 1 includes a common rail 2, which supplies fuel to a fuel injection nozzle in the diesel engine 1; a supply pump 3, which supplies high-pressure fuel to the common rail 2; a feed pump 4, which supplies fuel from a fuel tank 5 to the supply pump 3; a fuel filter 6, which is positioned between the supply pump 3 and feed pump 4; and an ECU 7, which exercises fuel control over the diesel engine 1.

[0063] An exhaust pipe 10 contains, for instance, two fuel injection nozzles 20, a NOx occlusion reduction type catalyst 11, which is an example of a reduction regeneration type catalyst, and a muffler 12. The ECU 7 is electrically connected to the fuel injection nozzle in the engine 1, the supply pump 3, the feed pump 4, the fuel injection nozzles 20 in the exhaust pipe 10, and the like.

[0064] The NOx occlusion reduction type catalyst 11 is a so-called NOx occlusion reduction type catalyst that occludes NOx in an exhaust gas flow in the exhaust pipe 10, and performs a regeneration process on the occluded NOx by increasing the amount of HC, CO, CO.sub.2, or H.sub.2 in the exhaust gas. The catalyst 11 includes a monolith carrier for which a lattice-shaped path is formed in the direction of an exhaust gas flow, and a coat layer that is formed on the monolith carrier and used to support a precious metal and a NOx occlusion agent. Pt is used, for instance, as the precious metal. Li, Na, K, Cs, or other alkaline metal, Mg, Ca, Ba, or other alkaline earth metal, or Y, La, Ce, Pr, Nd, Eu, Gd, Dy, or other rare earth metal is used, for instance, as the NOx occlusion agent. Alumina or the like is used as the coat layer.

[0065] As shown in FIG. 2, a holed fuel injection nozzle is used, for instance, as the fuel injection nozzle 20. The fuel injection nozzle 20 includes a cylindrical nozzle main body 21, and a column-shaped valve stem 30, which is inserted into the nozzle main body 21 and allowed to move in axial direction. The nozzle main body 21 includes a fuel supply hole 22, a valve seat 23 on which a conical seat section 31 at the leading end of the valve stem 30 is seated, a guidance hole 24, which is extended from the valve seat 23, and an injection hole 25, which is extended in a circumferential direction from the guidance hole 24. As shown in FIG. 3, the fuel injection nozzle 20 is provided with only one injection hole 25, which is a small circular hole. As indicated in FIG. 2, the fuel injection nozzle 20 is formed so that its injection axis line 20a is inclined rearward from a nozzle main body center line 21a. The injection axis line 20a means an injection center line that indicates the injection direction of each nozzle 20.

[0066] The valve stem 30 is opened/closed by a solenoid, not shown, that is controlled by the ECU 7. The fuel injection nozzle may be a holed fuel injection nozzle 36 having two injection holes 37 as shown in FIG. 4 or a slit fuel injection nozzle 38 having an injection hole 39 as shown in FIG. 5. The injection holes 37 in the holed fuel injection nozzle 36 and the injection hole 39 in the slit fuel injection nozzle 38 are formed so that their injection axis lines are inclined rearward from the nozzle main body center line as is the case with the aforementioned fuel injection nozzle 20.

[0067] The fuel injection nozzle described above is not limited to a holed fuel injection nozzle or slit fuel injection nozzle. Further, the number of injection holes is not limited to one or two.

[0068] As shown in FIG. 6, two fuel injection nozzles 20 are arranged in a circumferential direction of the exhaust pipe 10 and positioned at equal spacing intervals, that is, placed at diametrical positions of the exhaust pipe 10. Further, the two fuel injection nozzles 20 are installed so that their injection axis lines 20a incline rearward, that is, in the direction of an exhaust gas flow, and intersect at substantially one point. As described above, the fuel injection nozzles 20 are formed so that their injection axis lines 20a are inclined rearward from the nozzle main body center line 21a. Therefore, the fuel injection nozzle mounting angle relative to the exhaust pipe 10 is allowed to be great. Consequently, the fuel injection nozzle structure is such that the fuel injection nozzles can be installed with ease. However, the installation conditions such as the angle formed between the two fuel injection nozzles 20 and the angle formed between the fuel injection nozzles 20 and the axis line of the exhaust pipe 10 are set as appropriate depending, for instance, on the diameter of the exhaust pipe 10, the flow velocity of an exhaust gas, and the distance to the catalyst 11.

[0069] The operation performed by the system for injecting fuel into a diesel engine exhaust pipe will now be described.

[0070] The amount of NOx that is discharged from the engine 1 varies, for instance, with the engine speed and accelerator opening. The ECU 7, which is shown in FIG. 1, stores a NOx discharge amount map that relates to the amount of NOx discharge from the engine 1, which varies with the operating state as described above. In accordance with the NOx discharge amount map, the ECU 7 calculates the amount of NOx discharge from the engine 1 and determines the cumulative amount of NOx that is occluded by the catalyst 11.

[0071] When, for instance, Ba is used as the NOx occlusion agent, which is supported by the coat layer of the catalyst 11, the NOx discharged from the engine 1 reacts with O.sub.2 in the exhaust gas within the catalyst 11. The NOx further reacts with BaO and BaCO.sub.3 in the catalyst 11 to generate Ba(NO.sub.3).sub.2. In the resulting state, the NOx is occluded by the catalyst 11. With the NOx concentration extremely lowered, the exhaust gas is then discharged into the atmosphere via the muffler 12 and the like.

[0072] The ECU 7 activates the solenoid for the fuel injection nozzle 20 in the exhaust pipe 10 when it judges that a predetermined amount is exceeded by the amount of NOx occluded by the catalyst 11 and that the catalyst temperature is not lower than a catalyst reduction temperature (e.g., 200 to 450.degree. C.). When the solenoid is activated, the valve stem 30 shown in FIG. 2 is lifted in the fuel injection nozzle 20 so that the seat section 31 of the valve stem 30 leaves the valve seat 23 of the nozzle main body 21. The pressurized fuel supplied from the feed pump 4 is then injected into the exhaust pipe 10 from the injection hole 25 via the fuel supply hole 22 and guidance hole 24. Since the feed pump 4 supplies fuel from the fuel tank 5 to the supply pump 3, the fuel pressure is relatively low.

[0073] When the fuel is injected into the exhaust pipe 10 as described above, the oxygen concentration in the exhaust gas decreases and HC, CO, CO.sub.2, or H.sub.2 in the exhaust gas, which serves as a reducer, increases in amount. As a result, Ba(NO.sub.3).sub.2, which is occluded by the catalyst 11, reacts with the above reducer and reduces to N.sub.2. Further, the catalyst 11 acts as a highly selective reduction catalyst so that the above NO.sub.3 reacts with HC and CO in the exhaust gas to generate harmless N.sub.2, CO.sub.2, and H.sub.2O, which are eventually discharged into the atmosphere. As described above, the ECU 7 activates the solenoid for the fuel injection nozzle 20 to properly control the amount of fuel injection in the exhaust pipe 10 and the fuel injection timing. When it is estimated that the reduction and regeneration of the catalyst 11 are virtually completed, the ECU 7 stops the fuel injection from the fuel injection nozzle 20.

[0074] In the system for injecting fuel into a diesel engine exhaust pipe, the two fuel injection nozzles 20 are installed so that their injection axis lines 20a intersect with each other as shown in FIG. 6. Therefore, the fuels injected from the injection nozzles 20 collide with each other and mix to provide optimum atomization, thereby preventing the injected fuels from adhering to the inner wall of the exhaust pipe. Consequently, the greater part of the fuel injected from the fuel injection nozzles 20 is instantaneously vaporized within the exhaust gas.

[0075] Therefore, even when the amount of fuel is small, the fuel vapor concentration in the exhaust pipe 10 suddenly increases to a theoretical fuel vapor concentration as indicated in FIG. 7. This causes the exhaust gas air-fuel ratio to Instantaneously become rich, thereby making it possible to adequately reduce and regenerate the exhaust gas purification catalyst. Since adequate reduction and regeneration can be achieved with a small amount of fuel as described above, fuel efficiency remarkably increases.

[0076] Further, since the two fuel injection nozzles 20 are arranged in a circumferential direction of the exhaust pipe 10 and positioned at equal spacing intervals, the injected fuels uniformly collide with each other and mix. As a result, the fuel vapor is evenly supplied to the catalyst 11. The fuel injection nozzles 20 need not always be arranged in a circumferential direction and positioned substantially at equal spacing intervals. Furthermore, the two fuel injection nozzles 20 are positioned so that their injection axis lines 20a intersect at substantially one point. Therefore, the injected fuels collide with each other and mix with increased certainty, thereby further preventing the injected fuels from adhering to the inner wall of the exhaust pipe. The fuel injection nozzles 20 need not always be positioned so as to intersect their injection axis lines 20a at substantially one point.

[0077] In the system for injecting fuel into a diesel engine exhaust pipe described above, the exhaust pipe 10 is provided with two fuel injection nozzles 20. These two fuel injection nozzles 20 also prevent the injected fuels from adhering to the inner wall of the exhaust pipe with an extremely high probability. However, the number of fuel injection nozzles 20 is not always limited to two. Alternatively, three or more fuel injection nozzles may be used.

[0078] The feed pump 4 of the diesel engine 1 supplies fuel to the fuel injection nozzles 20. However, the source of fuel supply is not always limited to the feed pump. For example, the holed fuel injection nozzles may be modified to provide high-pressure injection and permit the use of high-pressure fuel supplied from the common rail 2.

[0079] The system for injecting fuel into a diesel engine exhaust pipe described above is applied to the NOx occlusion reduction type catalyst 11. However, the present invention is not limited to the use of the NOx occlusion reduction type catalyst 11. The present invention can also be applied to a DPNR or other reduction regeneration type catalyst. The foregoing description of the system for injecting fuel into a diesel engine exhaust pipe is to be considered in all respects only as illustrative and not restrictive. It should be understood by those skilled in the art that various modifications may be made without departing from the spirit and scope of the present invention.

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