JP4196334B2 - Multi-fuel engine and exhaust gas purification method for multi-fuel engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a multi-fuel engine that generates power by burning at least one of gaseous fuel and liquid fuel in a combustion chamber, and an exhaust gas purification method for the multi-fuel engine.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, there is known a multi-fuel engine capable of switching and using a plurality of types of fuel including gas fuel such as compressed natural gas (CNG) and liquid fuel such as gasoline (for example, see Patent Document 1). .) In this type of multi-fuel engine, the supply of gasoline is usually stopped and only CNG is often used as fuel. When the remaining amount of CNG falls below a predetermined value, the supply of CNG is stopped and gasoline is used.
[0003]
[Patent Document 1]
Japanese Patent Laid-Open No. 11-294212
[0004]
[Problems to be solved by the invention]
By the way, in recent years, from the viewpoint of low fuel consumption and high output of an engine, an engine capable of so-called lean combustion operation has been popularized, and a multi-fuel engine using a plurality of types of fuel is similarly lean. It has been demanded that combustion operation can be performed. Here, when the lean combustion operation is executed in the multi-fuel engine, it is difficult to sufficiently remove NOx in the exhaust gas by the three-way catalyst. For this reason, even in a multi-fuel engine capable of lean combustion operation, it is necessary to store NOx in the exhaust gas in the NOx storage reduction catalyst and to reduce NOx stored in the catalyst at a predetermined timing.
[0005]
However, when a NOx reduction process is performed using a gaseous fuel such as CNG in a multi-fuel engine operated in a lean burn mode, sufficient NOx reduction performance may not be obtained due to the occurrence of so-called rich misfire. In addition, since liquid fuel such as gasoline contains a large amount of sulfur components, if a NOx reduction process is performed using liquid fuel in a multifuel engine that is operated with lean combustion, the NOx storage reduction catalyst may be deteriorated. There is.
[0006]
Accordingly, an object of the present invention is to provide a multi-fuel engine and an exhaust gas purification method for a multi-fuel engine that can reliably perform a NOx reduction process while suppressing deterioration of the NOx storage reduction catalyst.
[0007]
[Means for Solving the Problems]
A multi-fuel engine according to the present invention is a multi-fuel engine that generates power by burning at least one of gaseous fuel and liquid fuel in a combustion chamber, and is an injector for gaseous fuel capable of directly injecting gaseous fuel into the combustion chamber A liquid fuel injector capable of introducing liquid fuel into the combustion chamber, an exhaust passage through which exhaust gas from the combustion chamber is introduced, and a NOx storage reduction catalyst provided in the exhaust passage, In order to perform the NOx reduction process, an amount of gaseous fuel necessary for the NOx reduction process is injected into the combustion chamber by the gaseous fuel injector during the exhaust stroke.
[0008]
This multi-fuel engine has a NOx occlusion reduction catalyst in the exhaust passage in order to remove NOx generated by the execution of the lean combustion operation. When the NOx occluded in the NOx occlusion reduction catalyst is reduced, in this multi-fuel engine, an amount of gaseous fuel necessary for the NOx reduction treatment is injected into the combustion chamber by the gaseous fuel injector during the exhaust stroke. . That is, in the present invention, it is not intended that the gaseous fuel injected into the fuel chamber during the exhaust stroke is combusted in the combustion chamber, and the gaseous fuel is transferred from the combustion chamber to the NOx storage reduction catalyst via the exhaust passage. Supplied as a reducing agent.
[0009]
As a result, in this multi-fuel engine, it is possible to reliably supply the amount of gaseous fuel necessary for the NOx reduction treatment to the NOx storage reduction catalyst without the problem of so-called rich misfire. As a result, it is possible to reliably perform the NOx reduction process while preventing thermal damage to the engine components and the catalyst due to rich misfire. Furthermore, in this multi-fuel engine, NOx reduction treatment is performed using gaseous fuel having a lower sulfur content compared to liquid fuel such as gasoline, so that the sulfur poisoning of the NOx storage reduction catalyst is greatly suppressed. It becomes possible to do.
[0010]
Further, it is preferable that the rich spike operation using the liquid fuel is performed when the NOx reduction process is required and the remaining amount of the gaseous fuel is equal to or less than a predetermined value.
[0011]
As a result, even when the amount of gaseous fuel necessary for the NOx reduction process cannot be secured, the NOx stored in the NOx storage reduction catalyst can be reliably reduced.
[0012]
An exhaust gas purification method for a multi-fuel engine according to the present invention is an exhaust gas purification method for a multi-fuel engine that generates power by burning at least one of gaseous fuel and liquid fuel in a combustion chamber. NOx occlusion reduction catalyst is provided in the exhaust passage through which NOx is introduced, and in order to perform NOx reduction treatment for the NOx occlusion reduction catalyst, an amount of gaseous fuel necessary for NOx reduction treatment is directly injected into the combustion chamber during the exhaust stroke It is.
[0013]
Another multi-fuel engine according to the present invention is an multi-fuel engine that generates power by burning at least one of a gaseous fuel and a liquid fuel in a combustion chamber, and an exhaust passage through which exhaust gas from the combustion chamber is introduced. And a NOx occlusion reduction catalyst provided in the exhaust passage, and in order to perform NOx reduction treatment for the NOx occlusion reduction catalyst, gaseous fuel can be introduced into the region closer to the combustion chamber than the NOx occlusion reduction catalyst in the exhaust passage It is characterized by that.
[0014]
This multi-fuel engine also has a NOx occlusion reduction catalyst in the exhaust passage in order to remove NOx generated by the execution of the lean combustion operation. When the NOx occluded in the NOx occlusion reduction catalyst is reduced, gaseous fuel is introduced into a region closer to the combustion chamber than the NOx occlusion reduction catalyst in the exhaust passage in this multi-fuel engine. In this way, even if gaseous fuel as a reducing agent is directly supplied to the NOx occlusion reduction catalyst via the exhaust passage, the amount of gaseous fuel necessary for the NOx reduction treatment can be stored without causing the so-called rich misfire problem. It is possible to reliably supply to the reduction catalyst.
[0015]
Therefore, even with this multi-fuel engine, it is possible to reliably execute the NOx reduction process while preventing thermal damage to engine components and the catalyst due to rich misfire. Also in this multi-fuel engine, NOx reduction treatment is performed using gaseous fuel having a low content of sulfur components compared to liquid fuel such as gasoline, which significantly reduces sulfur poisoning of the NOx storage reduction catalyst. It becomes possible to suppress.
[0016]
Furthermore, it is preferable that the rich spike operation using the liquid fuel is performed when the NOx reduction process is required and the remaining amount of the gaseous fuel is equal to or less than a predetermined value.
[0017]
Further, an injector for gaseous fuel capable of directly injecting gaseous fuel into the combustion chamber, a gaseous fuel container for storing gaseous fuel, and an addition valve connected to the gaseous fuel container for introducing the gaseous fuel into the exhaust passage When the NOx reduction treatment is required and the pressure of the gaseous fuel in the addition valve is lower than the exhaust pressure in the exhaust passage, the amount of gaseous fuel required for the NOx reduction treatment is gas during the exhaust stroke Preferably, the fuel is injected directly into the combustion chamber by a fuel injector or a rich spike operation using liquid fuel is performed.
[0018]
By adopting such a configuration, it is possible to reliably introduce gaseous fuel into the exhaust passage with a simple configuration. Under such a configuration, even when the pressure of the gaseous fuel in the addition valve is lower than the exhaust pressure in the exhaust passage, it is possible to reliably reduce the NOx stored in the NOx storage reduction catalyst. Become.
[0019]
Further, the gaseous fuel introduced into the exhaust passage is preferably CNG or LPG fuel in a gas phase.
[0020]
According to another aspect of the present invention, there is provided an exhaust gas purification method for a multi-fuel engine, wherein at least one of a gaseous fuel and a liquid fuel is burned in a combustion chamber to generate power. A NOx occlusion reduction catalyst is provided in the exhaust passage through which the exhaust gas is introduced, and gaseous fuel is introduced into a region closer to the combustion chamber than the NOx occlusion reduction catalyst in the exhaust passage in order to perform NOx reduction treatment on the NOx occlusion reduction catalyst. Is.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, preferred embodiments of a multi-fuel engine and an exhaust gas purification method for a multi-fuel engine according to the present invention will be described in detail with reference to the drawings.
[0022]
[First Embodiment]
FIG. 1 is a schematic configuration diagram showing a first embodiment of a multi-fuel engine according to the present invention. A multi-fuel engine 1 shown in FIG. 1 is a so-called bi-fuel engine that can be used by switching a plurality of types of fuel including gaseous fuel such as compressed natural gas (CNG) and LPG, and liquid fuel such as gasoline. In the multi-fuel engine 1, CNG (gas fuel) or gasoline (liquid fuel) is directly injected into each of the plurality of combustion chambers 4 defined by the cylinder block 2 and the cylinder head 3, and the air-fuel mixture in each combustion chamber 4. As a result of burning and reciprocating the piston 5, power is obtained.
[0023]
Thus, in order to inject CNG directly into each combustion chamber 4, the multifuel engine 1 of the present embodiment includes a plurality of gas injectors (injectors for gaseous fuel) 6 disposed in the cylinder head 3, and And a gas cylinder (gaseous fuel container) 7 for storing CNG. The gas injector 6 is arranged for each combustion chamber 4 so that the fuel jet port faces the inside of the cylinder. As shown in FIG. 1, a gas delivery pipe 9 is connected to the gas cylinder 7 via a gas supply pipe (gaseous fuel supply path) 8, and the fuel of each gas injector 6 is connected to the gas delivery pipe 9. The entrance is connected.
[0024]
Further, a pressure sensor (CNG residual pressure sensor) Pt for detecting an internal pressure (CNG residual pressure) of the gas cylinder 7 is disposed at the fuel outlet of the gas cylinder 7. Similarly, the gas delivery pipe 9 is provided with a pressure sensor Pd for detecting the pressure of the CNG inside the gas delivery pipe 9 and a temperature sensor Td for detecting the temperature of the CNG inside the gas delivery pipe 9. Further, the gas supply pipe 8 is provided with a regulator 10 together with a fuel cutoff valve (not shown). In the present embodiment, the CNG is filled in the gas cylinder 7 at a filling pressure of, for example, 20 MPa, and the CNG from the gas cylinder 7 is reduced to a constant adjustment pressure (for example, 5 MPa in the present embodiment) by the regulator 10. Then, the gas is introduced into each gas injector 6. In the multi-fuel engine 1, CNG is normally injected directly into the corresponding combustion chamber 4 from each gas injector 6 at this adjusted pressure (injection pressure).
[0025]
In addition to the gas injector 6 described above, the multi-fuel engine 1 includes a plurality of gasoline injectors (liquid fuel injectors) 11 disposed in the cylinder head 3 in order to directly inject gasoline into each combustion chamber 4. And a gasoline tank (liquid fuel container) 12 for storing gasoline. The gasoline injector 11 is also arranged for each combustion chamber 4 so that its fuel outlet faces the inside of the cylinder. As shown in FIG. 1, a gasoline delivery pipe 14 is connected to the gasoline tank 12 via a gasoline supply pipe 13, and a fuel inlet of each gasoline injector 11 is connected to the gasoline delivery pipe 14. Yes.
[0026]
The gasoline supply pipe 13 is provided with a fuel pump (not shown), and the gasoline tank 12 is provided with a gasoline remaining amount sensor 15 for detecting the amount of gasoline remaining in the gasoline tank 12. In the present embodiment, each gasoline injector 11 has been described as directly injecting gasoline into the corresponding combustion chamber 4. However, the present invention is not limited to this. That is, each gasoline injector 11 may be arranged to inject gasoline (liquid fuel) into the intake pipe (intake port).
[0027]
The intake port 16 connected to each combustion chamber 4 is connected to a surge tank 21 via an intake manifold 20, and the surge tank 21 is connected to an air cleaner 23 via an intake duct 22. The surge tank 21 is provided with a pressure sensor Ps for detecting the internal pressure thereof, and a throttle valve 25 driven by a stepping motor 24 is installed in the intake duct 22. On the other hand, the exhaust port 17 connected to each combustion chamber 4 is connected to an exhaust manifold (exhaust passage) 26.
[0028]
By the way, as shown in FIG. 1, the piston 5 of the multifuel engine 1 is configured as a so-called deep dish top surface type, and a concave portion 5a is formed on the upper surface thereof. In the multifuel engine 1 of the present embodiment, CNG or gasoline is directly injected from the gas injector 6 or the gasoline injector 11 toward the recess 5a of each piston 5 in a state where a large amount of air is sucked into each combustion chamber 4. The As a result, a layer of a mixture of fuel and air is formed (stratified) in the vicinity of the spark plug 19 while being separated from the surrounding air layer. As a result, the multifuel engine 1 can perform stable combustion using an extremely lean air-fuel mixture, and can achieve low fuel consumption and low pollution.
[0029]
In contrast, when the stratified lean combustion operation is performed as described above, the NOx component in the exhaust gas of the multi-fuel engine 1 increases. For this reason, an exhaust pipe (exhaust passage) 27 connected to the exhaust manifold 26 connected to each combustion chamber 4 has a post-stage catalyst device 29 including a NOx storage reduction catalyst in addition to a pre-stage catalyst device 28 including a three-way catalyst. It has been incorporated.
[0030]
Furthermore, the multifuel engine 1 includes an electronic control unit (ECU) 30 that functions as control means. The ECU 30 includes a CPU 31, a ROM 32, a RAM 33, a backup RAM 34, an input port 35, an output port 36, and the like. Each gas injector 6 and each gasoline injector 11 described above are connected to an output port 36 of the ECU 30 via a drive circuit (not shown) and are controlled by the ECU 30. Similarly, a spark plug 19, a stepping motor 24 for the throttle valve 25, a valve operating mechanism for the intake valve 18i and the exhaust valve 18e, and the like are connected to the output port 36 of the ECU 30 through a drive circuit or the like (not shown). Has been.
[0031]
Further, the CNG residual pressure sensor Pt for detecting the internal pressure of the gas cylinder 7, the pressure sensor Pd of the gas delivery pipe 9, and the temperature sensor Td are connected to the input port 35 of the ECU 30 via an A / D converter (not shown). Yes. Similarly, a gasoline remaining amount sensor 15 of the gasoline tank 12, a pressure sensor Ps of the surge tank 21, and the like are connected to an input port 35 of the ECU 30 via an A / D converter (not shown). Further, an accelerator position sensor 40 that detects an operation amount (depression amount) of an accelerator pedal, a rotation speed sensor 41 that detects an engine speed, and the like are connected to the input port 35 of the ECU 30.
[0032]
Then, the ECU 30 uses various maps or the like prepared in advance, and each gas injector 6 or each gasoline injector 11 so that the multi-fuel engine 1 generates a desired output based on detection values of various sensors. And the opening timing of the throttle valve 25, and the opening / closing operation of each intake valve 18i and each exhaust valve 18e. Here, when the stratified lean combustion operation of the multi-fuel engine 1 is executed under the control of the ECU 30, the NOx in the exhaust gas discharged from each combustion chamber 4 through each exhaust valve 18 e is converted into the post-catalyst device 29. The NOx occlusion / reduction catalyst occludes the NOx occlusion amount in the post-catalyst device 29, but has a certain limit. For this reason, the ECU 30 executes a NOx reduction process for the post-catalyst device 29 (NOx storage reduction catalyst) according to the procedure shown in FIG.
[0033]
Next, the procedure of the NOx reduction process for the rear catalyst device 29 of the multi-fuel engine 1 configured as described above will be described with reference to FIG.
[0034]
The NOx reduction process routine of FIG. 2 is executed by the ECU 30 every predetermined time. When the execution timing of the NOx reduction process routine is reached, the ECU 30 determines that the multi-fuel engine 1 at that time is based on a predetermined operation state detection signal or the like. Whether or not (stratified) lean fuel operation is being executed is determined (S10). As shown in FIG. 2, when it is determined in S10 that the lean combustion operation is not executed (for example, when the homogeneous combustion operation is executed in the multifuel engine 1), at this stage, the post-stage catalyst is The NOx reduction process for the device 29 is not executed.
[0035]
When it is determined in S10 that the lean combustion operation is being performed, the ECU 30 determines whether or not the NOx reduction process is required for the post-catalyst device 29 (S12). In the present embodiment, the ECU 30 integrates the injection amount of each fuel and the operation time of the engine 1 while the multifuel engine 1 is operating, and stores the NOx storage reduction catalyst from a predetermined map or the like based on these values. The amount of NOx that is being used is determined. Then, the ECU 30 determines whether or not the obtained storage amount of NOx exceeds a predetermined threshold value. As shown in FIG. 2, even when it is determined in S12 that the NOx reduction process is not necessary, the NOx reduction process for the subsequent catalyst device 29 is not executed.
[0036]
When it is determined in S12 that the NOx reduction process is required for the rear catalyst device 29, the ECU 30 further determines whether CNG is used as fuel (S14). When the ECU 30 determines that CNG is used as fuel in S14, the ECU 30 executes a rich spike operation during the exhaust stroke using the CNG using a predetermined map or the like (S16). That is, in S16, the ECU 30 controls each gas injector 6 so that an amount of CNG required for the NOx reduction process is directly injected into each combustion chamber 4 within a relatively short time during the exhaust stroke. (S16).
[0037]
Here, the CNG injected into the fuel chambers 4 during the exhaust stroke for the NOx reduction process in S16 is only from the respective combustion chambers 4 through the exhaust passages such as the exhaust manifold 26, and so on. NOx occlusion reduction catalyst) is supplied as a reducing agent. That is, the CNG injected into each combustion chamber 4 in S16 basically does not burn in each combustion chamber 4 and does not contribute to the generation of power.
[0038]
Therefore, the multifuel engine 1 can reliably supply the amount of CNG necessary for the NOx reduction process to the post-catalyst device 29 (NOx storage reduction catalyst) without the problem of so-called rich misfire. As a result, it is possible to reliably perform the NOx reduction process on the rear-stage catalyst device 29 while preventing thermal damage to the engine components, the three-way catalyst, the NOx storage reduction catalyst, and the like due to rich misfire. In the multi-fuel engine 1, the NOx reduction treatment is performed using CNG that has a lower sulfur component content than gasoline, so that it is possible to significantly suppress sulfur poisoning of the NOx storage reduction catalyst. .
[0039]
On the other hand, when the ECU 30 determines in S14 that CNG is not used as fuel and gasoline is used, the remaining amount of CNG in the gas cylinder 7 is further determined based on a signal from the CNG residual pressure sensor Pt. Ask for. Then, the ECU 30 determines whether or not the remaining amount of CNG is equal to or less than a predetermined value, that is, whether or not the remaining amount of CNG is insufficient (S18).
[0040]
If the ECU 30 determines in S18 that the remaining amount of CNG exceeds a predetermined value, the ECU 30 executes the process in S16 described above. Further, when the ECU 30 determines that the remaining amount of CNG is equal to or less than a predetermined value in S18, each gasoline injector 11 so that gasoline is temporarily and excessively supplied into each combustion chamber 4 at a predetermined timing. Is controlled (S20).
[0041]
Thus, in the multi-fuel engine 1, when the NOx reduction process is required for the post-catalyst device 29 and the remaining amount of CNG is equal to or less than a predetermined value, the rich spike operation using gasoline is executed ( S20). As a result, the multifuel engine 1 can reliably reduce the NOx occluded in the NOx occlusion reduction catalyst even when the amount of CNG necessary for the NOx reduction treatment cannot be secured.
[0042]
When the process of S16 or S20 is completed, the NOx stored in the NOx storage reduction catalyst of the post-catalyst device 29 is reduced. Then, at the next execution timing, the above-described NOx reduction routine is executed again by the ECU 30.
[0043]
[Second Embodiment]
Hereinafter, a second embodiment of the multi-fuel engine according to the present invention will be described with reference to FIGS. 3 and 4. The same elements as those described in relation to the first embodiment described above are denoted by the same reference numerals, and redundant description is omitted.
[0044]
The multi-fuel engine 1A shown in FIG. 3 includes a reducing agent addition system 50 for performing a NOx reduction process on the NOx storage reduction catalyst of the post-catalyst device 29. This reducing agent addition system 50 can introduce CNG reliably in the region between the front catalyst device 28 and the rear catalyst device 29 (NOx occlusion reduction catalyst) of the exhaust pipe 27, and is simple and low in cost. It is possible.
[0045]
As shown in FIG. 3, the reducing agent addition system 50 includes a branch pipe 51, a surge tank 52, and a CNG addition valve 53. The branch pipe 51 is branched from the gas supply pipe 8 between the regulator 10 for setting CNG from the gas cylinder (gaseous fuel container) 7 to a predetermined adjustment pressure and the gas delivery pipe 9 and connected to the surge tank 52. . The CNG addition valve 53 is disposed such that its fuel injection port faces the inside of the exhaust pipe 27 between the front-stage catalyst device 28 and the rear-stage catalyst device 29, and its fuel inlet is connected to the surge tank 52. . As a result, CNG regulated by the regulator 10 can be supplied to the addition valve 53 via the branch pipe 51 and the surge tank 52. The CNG addition valve 53 is controlled by the ECU 30, and the CNG can be introduced into the exhaust pipe 27 by opening the CNG addition valve 53.
[0046]
The surge tank 52 constituting the reducing agent addition system 50 is provided with a pressure sensor Pm for detecting the internal pressure (CNG pressure in the CNG addition valve 53). Further, the exhaust pipe 27 is provided with a pressure sensor Pe for detecting the pressure of the exhaust gas flowing from the front stage catalyst device 28 to the rear stage catalyst device 29. These pressure sensors Pm and Pe are connected to an input / output port of the ECU 30, and give signals indicating the detected values to the ECU 30, respectively. Then, the ECU 30 uses a predetermined map or the like, and controls the CNG addition valve 53 based on signals from the pressure sensors Pm and Pe and the like, thereby performing NOx reduction processing on the rear-stage catalyst device 29 (NOx storage reduction catalyst). Execute.
[0047]
Next, the procedure of the NOx reduction process for the post-catalyst device 29 of the multifuel engine 1A according to the second embodiment will be described with reference to FIG.
[0048]
The NOx reduction process routine of FIG. 4 is also executed by the ECU 30 of the multi-fuel engine 1A every predetermined time. When the ECU 30 comes to the execution timing of the NOx reduction process routine, based on a predetermined operating state detection signal and the like, It is determined whether or not the (stratified) lean fuel operation is being executed in the multifuel engine 1A (S30). When it is determined in S30 that the lean combustion operation is not being executed (for example, when the homogeneous combustion operation is being executed in the multi-fuel engine 1A), at this stage, the NOx reduction process for the post-catalyst device 29 is executed. Not.
[0049]
When it is determined in S30 that the lean combustion operation is being executed, the ECU 30 determines whether or not the NOx reduction process is required for the rear catalyst device 29 (S32). As shown in FIG. 4, even when it is determined in S32 that the NOx reduction process is not necessary, the NOx reduction process for the subsequent catalyst device 29 is not executed. On the other hand, if it is determined in S32 that the NOx reduction process is required for the subsequent catalyst device 29, the ECU 30 further determines whether or not CNG is used as fuel (S34).
[0050]
If it is determined in S34 that CNG is used as the fuel, the ECU 30 further includes a pressure sensor Pm provided in the surge tank 52 of the reducing agent addition system 50 and a pressure sensor Pe provided in the exhaust pipe 27. The internal pressure of the surge tank 52 (CNG addition pressure, that is, the pressure of CNG in the addition valve 53) and the exhaust pressure in the exhaust pipe 27 are acquired and compared with each other (S36).
[0051]
When it is determined in S36 that the internal pressure of the surge tank 52 exceeds the exhaust pressure in the exhaust pipe 27, that is, the pressure of the exhaust gas flowing from the front catalyst device 28 to the rear catalyst device 29 (NOx storage reduction catalyst). The ECU 30 uses a predetermined map or the like, for example, for the time corresponding to the deviation between the internal pressure of the surge tank 52 and the exhaust pressure in the exhaust pipe 27 and the NOx occlusion amount in the post-catalyst device 29, for the CNG addition valve 53. Is opened (S38).
[0052]
As a result, the pressure difference between the internal pressure of the surge tank 52 (the pressure of CNG in the addition valve 53) and the exhaust pressure in the exhaust pipe 27 causes the combustion valve 4 side from the addition valve 53 to the rear catalyst device 29 of the exhaust pipe 27. CNG is introduced into the region (near the post-catalyst device 29). CNG introduced into the exhaust pipe 27 from the CNG addition valve 53 is mixed with the exhaust gas quickly and uniformly, and flows into the post-catalyst device 29 (NOx storage reduction catalyst). Then, in the post-catalyst device 29, NOx occluded in the NOx occlusion reduction catalyst is reduced using the introduced CNG as a reducing agent.
[0053]
Thus, even if CNG as a reducing agent is directly supplied to the post-catalyst device 29 (NOx storage reduction catalyst) via the exhaust pipe 27, the amount necessary for the NOx reduction treatment without the problem of so-called rich misfire. The CNG can be reliably supplied to the post-catalyst device 29. Therefore, even with the multifuel engine 1A of the second embodiment, it is possible to reliably perform the NOx reduction process while preventing thermal damage to the engine components and the catalyst due to rich misfire. Also, in the multifuel engine 1A, the NOx reduction treatment is performed using a gaseous fuel having a lower sulfur component content than liquid fuel such as gasoline, so that the sulfur poisoning of the NOx occlusion reduction catalyst is greatly reduced. It becomes possible to suppress.
[0054]
Furthermore, in the present embodiment, the CNG fuel in the gas phase is mixed with the exhaust gas discharged from each combustion chamber 4 and joined immediately before the post-stage catalyst device 29 (NOx storage reduction catalyst). Therefore, in the multi-fuel engine 1A, it becomes possible to uniformly mix CNG with the exhaust gas having relatively little concentration and temperature unevenness, and the NOx reduction process can be performed satisfactorily.
[0055]
On the other hand, when the ECU 30 determines in S34 that CNG is not used as fuel and gasoline is used, the remaining amount of CNG in the gas cylinder 7 is further determined based on a signal from the CNG residual pressure sensor Pt. Ask for. Then, the ECU 30 determines whether or not the remaining amount of CNG is equal to or less than a predetermined value, that is, whether or not the remaining amount of CNG is insufficient (S40).
[0056]
When ECU 30 determines in S40 that the remaining amount of CNG exceeds a predetermined value, it executes the process in S36 described above. Further, when the ECU 30 determines that the remaining amount of CNG is equal to or less than a predetermined value in S40, each gasoline injector 11 is configured so that gasoline is temporarily and excessively supplied into each combustion chamber 4 at a predetermined timing. Is controlled (S42).
[0057]
As described above, also in the multifuel engine 1A, the rich spike operation using gasoline is executed when the NOx reduction process for the rear catalyst device 29 is required and the remaining amount of CNG is equal to or less than the predetermined value. (S42). As a result, in the multifuel engine 1A, even when the amount of CNG necessary for the NOx reduction process cannot be ensured, the NOx stored in the NOx storage reduction catalyst can be reliably reduced.
[0058]
In the present embodiment, since CNG is directly injected into the combustion chamber 4, a comparison is made in the surge tank 52 so that the pressure can be injected into the combustion chamber 4 by the regulator 10. High pressure CNG fuel is stored. Therefore, it is generally considered that the internal pressure of the surge tank 52 is hardly lower than the exhaust pressure in the exhaust pipe 27. However, in S36, it may be determined that the internal pressure of the surge tank 52 is equal to or lower than the exhaust pressure in the exhaust pipe 27.
[0059]
In view of this point, in the multifuel engine 1A of the present embodiment, when it is determined in S36 that the internal pressure of the surge tank 52 is equal to or lower than the exhaust pressure in the exhaust pipe 27, gasoline is used in S42. Rich spike operation is executed. If it is determined in S36 that the internal pressure of the surge tank 52 is equal to or lower than the exhaust pressure in the exhaust pipe 27, instead of performing the rich spike operation using gasoline in S42, NOx reduction processing is performed during the exhaust stroke. A necessary amount of CNG may be introduced into each combustion chamber 4.
[0060]
When the process of S36 or S42 is completed, the NOx stored in the NOx storage reduction catalyst of the post-catalyst device 29 is reduced. Then, at the next execution timing, the NOx reduction routine of FIG.
[0061]
As described above, the multifuel engine 1A of the second embodiment has been described as using CNG as the gaseous fuel, but LPG fuel that instantly vaporizes when injected into the combustion chamber 4 is used as the gaseous fuel. May be. In this case, it is preferable that the surge tank 52 communicates with the upper part of the fuel tank that stores the LPG fuel. This makes it possible to easily and reliably introduce the LPG fuel in the gas phase state into the exhaust pipe 27 using the vapor pressure in the fuel tank.
[0062]
【The invention's effect】
As described above, according to the present invention, a multi-fuel engine and an exhaust gas purification method for a multi-fuel engine that can reliably perform a NOx reduction process while preventing thermal damage associated with the NOx reduction process for the NOx storage reduction catalyst. Can be realized.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing a first embodiment of a multi-fuel engine according to the present invention.
FIG. 2 is a flowchart for explaining the operation of the multi-fuel engine of FIG. 1;
FIG. 3 is a schematic configuration diagram showing a second embodiment of a multi-fuel engine according to the present invention.
4 is a flowchart for explaining the operation of the multi-fuel engine of FIG. 3;
[Explanation of symbols]
1,1A Multi-fuel engine
2 Cylinder block
3 Cylinder head
4 Combustion chamber
5 piston
5a recess
6 Gas injector (Injector for gaseous fuel)
7 Gas cylinder
8 Gas supply pipe
9 Gas delivery pipe
10 Regulator
11 Gasoline injector (Liquid fuel injector)
12 Gasoline tank
13 Gasoline supply pipe
14 Gasoline delivery pipe
15 Fuel level sensor
16 Intake port
17 Exhaust port
18e Exhaust valve
18i Intake valve
19 Spark plug
20 Intake manifold
21 Surge tank
22 Intake duct
23 Air cleaner
24 Stepping motor
25 Throttle valve
26 Exhaust manifold
27 Exhaust pipe (exhaust passage)
28 Pre-catalyst device
29 Second-stage catalyst device (NOx storage reduction catalyst)
30 ECU
40 Accelerator position sensor
41 Rotational speed sensor
50 Reducing agent addition system
51 branch pipe
52 Surge Tank
53 CNG addition valve
Pd, Pe, Pm, Ps, Pt Pressure sensor
Td temperature sensor

Claims (8)

In a multi-fuel engine that generates power by burning at least one of gaseous fuel and liquid fuel in a combustion chamber,
A gaseous fuel injector capable of directly injecting gaseous fuel into the combustion chamber;
A liquid fuel injector capable of introducing liquid fuel into the combustion chamber;
An exhaust passage through which exhaust gas from the combustion chamber is introduced;
A NOx occlusion reduction catalyst provided in the exhaust passage, and an amount of gaseous fuel necessary for the NOx reduction treatment during the exhaust stroke for performing the NOx reduction treatment on the NOx occlusion reduction catalyst. A multi-fuel engine which is injected into the combustion chamber by an injector. 2. The multifuel according to claim 1, wherein a rich spike operation using liquid fuel is performed when the NOx reduction treatment is required and the remaining amount of gaseous fuel is a predetermined value or less. engine. In an exhaust gas purification method for a multi-fuel engine that generates power by burning at least one of gaseous fuel and liquid fuel in a combustion chamber,
An NOx storage reduction catalyst is provided in an exhaust passage through which exhaust gas from the combustion chamber is introduced, and an amount of gas necessary for the NOx reduction process is performed during the exhaust stroke in order to perform the NOx reduction process on the NOx storage reduction catalyst. An exhaust gas purification method for a multi-fuel engine, characterized in that fuel is directly injected into the combustion chamber. In a multi-fuel engine that generates power by burning at least one of gaseous fuel and liquid fuel in a combustion chamber,
An exhaust passage through which exhaust gas from the combustion chamber is introduced;
A NOx occlusion reduction catalyst provided in the exhaust passage, and in order to perform a NOx reduction treatment on the NOx occlusion reduction catalyst, a gaseous fuel is provided in a region closer to the combustion chamber than the NOx occlusion reduction catalyst in the exhaust passage. A multi-fuel engine characterized in that it can be introduced. 5. The multifuel according to claim 4, wherein rich spike operation using liquid fuel is performed when the NOx reduction treatment is required and the remaining amount of gaseous fuel is equal to or less than a predetermined value. engine. Injector for gaseous fuel capable of directly injecting gaseous fuel into the combustion chamber, a gaseous fuel container for storing gaseous fuel, and an addition valve connected to the gaseous fuel container for introducing gaseous fuel into the exhaust passage When the NOx reduction treatment is required and the pressure of the gaseous fuel in the addition valve is lower than the exhaust pressure in the exhaust passage, the amount required for the NOx reduction treatment during the exhaust stroke is 6. The multi-fuel engine according to claim 5, wherein gaseous fuel is injected into the combustion chamber by the gaseous fuel injector, or rich spike operation using liquid fuel is executed. The multifuel engine according to any one of claims 4 to 6, wherein the gaseous fuel introduced into the exhaust passage is CNG or LPG fuel in a gas phase. In an exhaust gas purification method for a multi-fuel engine that generates power by burning at least one of gaseous fuel and liquid fuel in a combustion chamber,
An NOx storage reduction catalyst is provided in an exhaust passage through which exhaust gas from the combustion chamber is introduced, and the combustion chamber is more disposed than the NOx storage reduction catalyst in the exhaust passage in order to perform NOx reduction processing on the NOx storage reduction catalyst. An exhaust gas purification method for a multi-fuel engine, characterized in that gaseous fuel is introduced into a side region.

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