JP2005009478A - Lpg fuel engine and method for purifying emission thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To easily and adequately carry out a NOx reduction process for a NOx storage-reduction catalyst using LPG fuel. <P>SOLUTION: An LPG fuel engine 1 which generates drive power by burning LPG fuel in a combustion chamber 2 comprises a fuel tank 10 for storing the LPG fuel; an exhaust pipe 7 for introducing emissions from the combustion chamber 2; and a rear catalyst device 9 provided on the exhaust pipe 7 and including the NOx storage-reduction catalyst. To carry out the NOx reduction process for the rear catalyst device 9 of the LPG fuel engine 1, the LPG fuel in gaseous phase existing within the fuel tank 10 is introduced into the area of the exhaust pipe 2 closer to the combustion chamber 2 than to the rear catalyst device 9. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an LPG fuel engine that generates power by burning LPG fuel in a combustion chamber and an exhaust gas purification method for the LPG fuel engine.

  Conventionally, as an LPG fuel engine that generates power by burning LPG fuel in a combustion chamber, an injector for liquid phase fuel injection that injects LPG fuel in a liquid phase state into the combustion chamber, and an LPG fuel in a gas phase state. There is known an LPG fuel engine including a gas-phase fuel injection injector that injects into a combustion chamber (see, for example, Patent Document 1). In addition, an injector for liquid phase fuel injection that injects LPG fuel in a liquid phase state into an intake port, and an injector for gas phase fuel injection that injects LPG fuel in a gas phase state into an intake pipe upstream of the intake port Also known is an LPG fuel engine equipped with the above (for example, see Patent Document 2).

  On the other hand, in recent years, an engine capable of so-called lean combustion operation has been popularized from the viewpoint of reducing fuel consumption and high output of the engine, and performing the lean combustion operation on the LPG fuel engine as described above. Has come to be required. Here, in the case where the lean combustion operation is executed by the LPG fuel engine, the catalyst capable of storing NOx in the exhaust gas in order to remove NOx (nitrogen oxide) in the exhaust gas, as in other engines. Must be provided in the exhaust system. At a predetermined timing, NOx stored in the catalyst must be reduced.

Japanese Patent Publication No. 7-26596 JP 2001-107805 A

  However, LPG fuel has a lower so-called rich limit concentration than gasoline. For this reason, even if it is attempted to perform the NOx reduction process for the NOx storage reduction catalyst by the rich spike operation in which the LPG fuel is temporarily and excessively supplied into the combustion chamber, it is difficult to perform the reduction process at a level that can sufficiently satisfy NOx. Become.

  Accordingly, an object of the present invention is to provide an LPG fuel engine and an exhaust gas purification method for an LPG fuel engine that can easily and satisfactorily perform NOx reduction treatment on a NOx storage reduction catalyst using LPG fuel.

  An LPG fuel engine according to the present invention is an LPG fuel engine that generates power by burning LPG fuel in a combustion chamber, a fuel tank that stores LPG fuel, an exhaust passage through which exhaust gas from the combustion chamber is introduced, and an exhaust gas A NOx occlusion reduction catalyst provided in the passage, and in order to perform NOx reduction treatment on the NOx occlusion reduction catalyst, the LPG fuel in the gas phase existing in the fuel tank is removed from the NOx occlusion reduction catalyst in the exhaust passage. Can also be introduced into the region on the combustion chamber side.

  This LPG fuel engine includes a NOx occlusion reduction catalyst in an exhaust passage through which exhaust gas from the combustion chamber is introduced so that a lean combustion operation using LPG fuel can be performed. In this LPG fuel engine, in order to perform the NOx reduction process for the NOx storage reduction catalyst, the LPG fuel in the gas phase existing in the fuel tank is placed closer to the combustion chamber than the NOx storage reduction catalyst in the exhaust passage. It can be introduced into the area.

  As a result, in this LPG fuel engine, the amount required for the NOx reduction process while eliminating the rich misfire of the LPG fuel that tends to occur when the supply amount of the LPG fuel to the combustion chamber is increased for the NOx reduction process. The LPG fuel can be accurately supplied to the NOx storage reduction catalyst. Further, by introducing the LPG fuel in the gas phase state into the exhaust passage, it becomes possible to uniformly mix the LPG fuel into the exhaust gas flowing into the NOx storage reduction catalyst. Therefore, according to the LPG fuel engine, it is possible to easily and satisfactorily execute the NOx reduction process for the NOx storage reduction catalyst using the LPG fuel. In addition, in this LPG fuel engine, LPG fuel vaporized inside the fuel tank can be used effectively.

  Further, it is preferable that the LPG fuel in a gas phase state is introduced into the exhaust passage using the vapor pressure in the fuel tank.

  Accordingly, it is possible to reliably introduce the LPG fuel in the gas phase state into the exhaust passage, and to simplify and reduce the configuration for introducing the LPG fuel into the exhaust passage.

  The LPG fuel engine according to the present invention further includes an addition valve for introducing gas phase LPG fuel into the exhaust passage, and a surge tank provided between the fuel tank and the addition valve. When performing the NOx reduction process on the catalyst, it is preferable to introduce LPG fuel in a gas phase state from the addition valve to the exhaust path when the internal pressure of the surge tank is higher than the exhaust pressure in the exhaust pipe.

  If such a configuration is adopted, even if the internal pressure of the surge tank (steam pressure in the fuel tank) fluctuates, the addition valve (the valve opening time) should be controlled according to the fluctuation of the internal pressure of the surge tank. As a result, it is possible to accurately supply the LPG fuel in the gas phase state to the NOx occlusion reduction catalyst in an amount necessary for the NOx reduction treatment.

  The LPG fuel engine of the present invention preferably further includes means for adding a cetane improver to the LPG fuel prior to introduction into the exhaust passage.

  Thus, by adding the cetane number improver to the LPG fuel in the gas phase state sent from the fuel tank to the exhaust passage before introduction into the exhaust passage, the LPG fuel as the reducing agent is added. Ignition and flammability can be improved. Therefore, if such a configuration is adopted, the NOx reduction process for the NOx storage reduction catalyst can be performed very well.

  Another LPG fuel engine according to the present invention is an LPG fuel engine that generates power by burning LPG fuel in a combustion chamber, a fuel tank that stores LPG fuel, and an exhaust passage through which exhaust gas from the combustion chamber is introduced. In order to execute the NOx storage reduction catalyst provided in the exhaust passage and the NOx reduction treatment for the NOx storage reduction catalyst, the LPG fuel is taken out from the fuel tank, and the taken out LPG fuel is burned more than the NOx storage reduction catalyst in the exhaust passage. It is characterized by comprising a reducing agent adding means to be introduced into the chamber side region and a means for adding a cetane number improver to the LPG fuel taken out from the fuel tank by the reducing agent adding means.

  This LPG fuel engine is also provided with a NOx occlusion reduction catalyst in an exhaust passage through which exhaust gas from the combustion chamber is introduced so that a lean combustion operation using LPG fuel can be performed. In this LPG fuel engine, the LPG fuel is taken out from the fuel tank by the reducing agent adding means in order to execute the NOx reduction treatment on the NOx storage reduction catalyst, and the taken out LPG fuel is used as the NOx storage reduction catalyst in the exhaust passage. It is introduced into the region closer to the combustion chamber. At this time, the cetane number improver is added to the LPG fuel taken out from the fuel tank by the reducing agent adding means prior to introduction into the exhaust passage.

  As a result, also in this LPG fuel engine, it is necessary for the NOx reduction process while eliminating the rich misfire of the LPG fuel that tends to occur when the supply amount of the LPG fuel to the combustion chamber is increased for the NOx reduction process. An amount of LPG fuel can be accurately supplied to the NOx storage reduction catalyst. In addition, since the cetane number improver is added to the LPG fuel sent from the fuel tank to the exhaust passage prior to introduction into the exhaust passage, the ignitability and combustibility of the LPG fuel as a reducing agent is improved. In addition, the NOx reduction process for the NOx occlusion reduction catalyst can be performed extremely well. Therefore, also with this LPG fuel engine, it is possible to easily and satisfactorily execute the NOx reduction process for the NOx storage reduction catalyst using the LPG fuel.

  An exhaust gas purification method for an LPG fuel engine according to the present invention is an exhaust gas purification method for an LPG fuel engine in which LPG fuel from a fuel tank is burned in a combustion chamber to generate power, and an exhaust passage through which exhaust gas from the combustion chamber is introduced In order to perform NOx reduction treatment on the NOx storage reduction catalyst, the LPG fuel in the gas phase existing in the fuel tank is placed closer to the combustion chamber than the NOx storage reduction catalyst in the exhaust passage. It is characterized by being introduced into a region.

  Another exhaust gas purification method for an LPG fuel engine according to the present invention is an exhaust gas purification method for an LPG fuel engine that generates power by burning LPG fuel from a fuel tank in a combustion chamber, and exhaust gas from the combustion chamber is introduced. A NOx occlusion reduction catalyst is provided in the exhaust passage, and in order to perform NOx reduction treatment on the NOx occlusion reduction catalyst, LPG fuel is taken out from the fuel tank, and after adding a cetane number improver to the taken out LPG fuel, NOx in the exhaust passage It is characterized in that it is introduced into a region closer to the combustion chamber than the storage reduction catalyst.

  According to the present invention, it is possible to realize an LPG fuel engine and an exhaust gas purification method for an LPG fuel engine that can easily and satisfactorily perform NOx reduction treatment on a NOx storage reduction catalyst using LPG fuel.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings.

  Hereinafter, preferred embodiments of an LPG fuel engine and an operation method thereof according to the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a schematic configuration diagram showing a first embodiment of an LPG fuel engine according to the present invention. The LPG fuel engine 1 shown in the figure is configured as an in-line four-cylinder engine using LPG fuel (liquefied petroleum gas fuel) such as liquefied propane gas fuel or liquefied butane gas fuel, and is preferably used as a travel drive source of a vehicle. It is a thing. The LPG fuel engine 1 has a plurality of combustion chambers 2 formed in an engine block. The LPG fuel (in this embodiment, liquefied propane gas fuel) is burned in each combustion chamber 2 to reciprocate the piston 3. Power is generated by moving it.

  As shown in FIG. 1, the LPG fuel engine 1 has a plurality of injectors 4 and a plurality of spark plugs (not shown). Each of the injectors 4 and the spark plugs is provided inside each combustion chamber 2. It is arranged to face. Each piston 3 of the LPG fuel engine 1 is configured as a so-called deep dish top surface type, and a recess 3a is formed on the top surface thereof. In the LPG fuel engine 1, LPG fuel is directly injected from each injector 4 toward the recess 3 a of the piston 3 in each combustion chamber 2 in a state where air is sucked into each combustion chamber 2.

  Thereby, in the LPG fuel engine 1, a layer of a mixture of LPG fuel and air is formed (stratified) in the vicinity of a spark plug (not shown) in a state separated from the surrounding air layer. As a result, the LPG fuel engine 1 can perform stable stratified combustion using an extremely lean air-fuel mixture. Although the LPG fuel engine 1 of the present embodiment is described as a so-called direct injection engine, the present invention is not limited to this, and the present invention can be applied to an intake pipe (intake port) injection type engine. Needless to say.

  In addition, an intake valve that opens and closes the intake port and an exhaust valve that opens and closes the exhaust port are disposed in the cylinder head of the LPG fuel engine 1 for each combustion chamber 2. These intake valves and exhaust valves are opened and closed by a valve operating mechanism (not shown) having a variable valve timing function, for example. Each intake port of each combustion chamber 2 is connected to an intake pipe (intake manifold) 5, and a drive-by-wire throttle valve 6 is installed in the intake pipe 5, for example. On the other hand, the exhaust port of each combustion chamber 2 is connected to an exhaust pipe (exhaust manifold) 7.

  Furthermore, when the stratified lean combustion operation as described above is executed, a three-way catalyst is installed in the exhaust pipe 7 in view of the increase in NOx (nitrogen oxide) in the exhaust gas from the LPG fuel engine 1. In addition to the pre-catalyst device 8 including, a post-catalyst device 9 including a NOx storage reduction catalyst is connected. Thereby, NOx in the exhaust gas discharged from each combustion chamber 2 via each exhaust valve is stored by the NOx storage reduction catalyst of the post-catalyst device 9. In the LPG fuel engine 1, a NOx reduction process is performed on the rear catalyst device 9 at a predetermined timing as will be described later, whereby the NOx stored in the NOx storage reduction catalyst is reduced. The exhaust pipe 7 is provided with an air-fuel ratio sensor AFS that detects the air-fuel ratio of the exhaust gas that has passed through the rear-stage catalyst device 9.

  In the LPG fuel engine 1, the LPG fuel is stored in a liquid phase inside the fuel tank 10, and the LPG fuel is supplied from each injector 4 from the viewpoint of ensuring a sufficient amount of air necessary for combustion. It is injected into the combustion chamber 2 in a liquid phase state. In order to inject LPG fuel into each combustion chamber 2 in a liquid phase state, the LPG fuel engine 1 includes a low pressure pump 11 connected to a fuel tank 10, and a low pressure pump 11 and each injector 4. And a high-pressure pump 12 arranged.

  In the present embodiment, as the low-pressure pump 11 and the high-pressure pump 12, an electric pump capable of duty control is employed. The discharge port of the low pressure pump 11 is connected to the suction port of the high pressure pump 12 via the low pressure pipe 14. Further, the discharge port of the high-pressure pump 12 is connected to a delivery pipe (distribution pipe) 15, and the fuel inlet of each injector 4 is connected to the delivery pipe 15. Thereby, the LPG fuel in the liquid phase is sucked out of the fuel tank 10 by the low-pressure pump 11 and sent to the high-pressure pump 12, and further pressurized by the high-pressure pump 12 to each injector 4 through the delivery pipe 15. Will be supplied.

  As shown in FIG. 1, the fuel tank 10 is provided with a pressure sensor Pt that detects its internal pressure (vapor pressure) and a temperature sensor Tt that detects its internal temperature. Further, in the vicinity of the high pressure pump 12 of the low pressure pipe 14 connecting the low pressure pump 11 and the high pressure pump 12, there is a pressure sensor Pp for detecting the pressure of the LPG fuel flowing through the inside, and the temperature of the LPG fuel flowing through the inside. And a temperature sensor Tp for detecting. Similarly, in the delivery pipe 15 to which the fuel inlet of each injector 4 is connected, a pressure sensor Pd that detects the pressure of the LPG fuel existing therein, and a temperature that detects the temperature of the LPG fuel existing therein. And a sensor Td.

  Furthermore, in the LPG fuel engine 1 of the present embodiment, the suction port of the high-pressure pump 12 and the fuel tank 10 are communicated with each other by a return pipe 16. A branching portion 16 a is branched from the end of the return pipe 16 on the high-pressure pump 12 side, and the end of the branching portion 16 a is connected to the delivery pipe 15. As a result, in the LPG fuel engine 1, the gas phase region inside the fuel tank 10, the vicinity of the suction port of the high-pressure pump 12 and the inside of the delivery pipe 15, that is, the vicinity of the fuel inlet of each injector 4 are connected via the return pipe 16. Will be communicated.

  By providing such a return pipe 16 with respect to the LPG fuel engine 1, the LPG fuel should be supplied around the suction port of the high-pressure pump 12, inside the delivery pipe 15, that is, around the fuel inlet of each injector 4. Even if vaporization occurs, the vapor of LPG fuel can be returned to the fuel tank 10 via the return pipe 16. Therefore, in the LPG fuel engine 1, it is possible to extremely reliably prevent the LPG fuel in the gas-liquid mixed state from flowing into the boosting portion of the high-pressure pump 12 and each injector 4.

  The LPG fuel engine 1 configured as described above includes an electronic control unit (ECU) 20 that functions as control means. The ECU 20 includes a CPU, a ROM, a RAM, an input / output port, a storage device, etc., all not shown. As shown in FIG. 1, the air-fuel ratio sensor AFS, the pressure sensor Pt and temperature sensor Tt of the fuel tank 10, the pressure sensor Pp and temperature sensor Tp of the low-pressure pipe 14, and the pressure sensor Pd and temperature sensor Td of the delivery pipe 15 are Each is connected to an input / output port of the ECU 20. Each pressure sensor Pt, Pp, and Pd and each temperature sensor Tt, Tp, and Td give the ECU 20 a signal that indicates a detected value.

  The input / output ports of the ECU 20 include various sensors such as the low pressure pump 11 and the high pressure pump 12, the injectors 4, the throttle valve 6, the ignition plug (igniter), the valve mechanism, the accelerator position sensor, and the rotation speed sensor. Is connected. The ECU 20 uses the various maps stored in the storage device and operates the low-pressure pump 11 and the high-pressure pump 12 so that the LPG fuel engine 1 generates a desired output based on the detection values of various sensors. The valve opening timing of the injector 4, the opening degree of the throttle valve 6, and the opening / closing operation of each intake valve and each exhaust valve are controlled.

  Here, when the stratified lean combustion operation of the LPG fuel engine 1 is executed under the control of the ECU 20, the NOx in the exhaust gas discharged from each combustion chamber 2 through each exhaust valve is converted into the post-catalyst device 9. Is stored by the NOx storage reduction catalyst. However, since the NOx occlusion amount in the post-catalyst device 9 has a certain limit, it is necessary to appropriately reduce the NOx occluded in the NOx occlusion reduction catalyst.

  For this reason, the LPG fuel engine 1 includes a reducing agent addition system 30 for performing a NOx reduction process on the NOx storage reduction catalyst included in the post-catalyst device 9. This reducing agent addition system 30 can introduce LPG fuel in a gas phase state as a reducing agent into the exhaust pipe (exhaust passage) 7 using the vapor pressure in the fuel tank 10, and is simple and low cost. In addition, the LPG fuel vaporized inside the fuel tank 10 can be effectively used.

  As shown in FIG. 1, the reducing agent addition system 30 includes a communication pipe 31, a surge tank 32, and an LPG addition valve 33. One end of the communication pipe 31 is connected to the upper part of the fuel tank 10, and the other end of the communication pipe 31 is connected to the surge tank 32. The LPG addition valve 33 is arranged so that its fuel injection port faces the inside of the exhaust pipe 7 between the front-stage catalyst device 8 and the rear-stage catalyst device 9, and its fuel inlet is connected to the surge tank 32. . Thereby, the gas phase region inside the fuel tank 10 and the inside of the surge tank 32 are communicated with each other by the communication pipe 31, and the gas phase state existing inside the fuel tank 10 through the communication pipe 31 and the surge tank 32. The LPG fuel can be supplied to the LPG addition valve 33. Further, the LPG addition valve 33 is controlled by the ECU 20 described above, and when the LPG addition valve 33 is opened, LPG fuel in a gas phase can be introduced into the exhaust pipe 7.

  The surge tank 32 constituting the reducing agent addition system 30 is provided with a pressure sensor Ps for detecting the internal pressure (pressure of LPG fuel in a gas phase state). Further, the exhaust pipe 7 is provided with a pressure sensor Pe for detecting the pressure of the exhaust gas flowing from the front catalyst device 8 to the rear catalyst device 9. These pressure sensors Ps and Pe are connected to an input / output port of the ECU 20, and give signals indicating the detected values to the ECU 20. The ECU 20 uses a predetermined map and the like, and controls the LPG addition valve 33 based on the signals from the pressure sensors Ps and Pe, thereby executing the NOx reduction process for the rear-stage catalyst device 9 (NOx storage reduction catalyst). To do.

  Next, the procedure of the NOx reduction process for the rear catalyst device 9 of the LPG fuel engine 1 configured as described above will be described with reference to FIG.

  The NOx reduction routine of FIG. 2 is executed by the ECU 20 every predetermined time. When the execution timing of the NOx reduction routine is reached, the ECU 20 first determines the LPG fuel engine 1 at that time based on a predetermined operation state detection signal or the like. It is then determined whether or not the stratified lean combustion operation is being executed (S10). As shown in FIG. 2, when it is determined in S10 that the stratified lean combustion operation is not executed (for example, when the homogeneous combustion operation is executed in the LPG fuel engine 1), at this stage, The NOx reduction process for the catalyst device 9 is not executed.

  When it is determined in S10 that the stratified lean combustion operation is being executed, the ECU 20 determines whether or not a NOx reduction process is required for the post-catalyst device 9 (S12). In the present embodiment, the ECU 20 integrates the injection amount of LPG fuel and the operation time of the engine 1 during operation of the LPG fuel engine 1, 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 20 determines in S12 whether or not the NOx occlusion amount in the post-catalyst device 9 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 9 is not executed.

  If it is determined in S12 that the NOx reduction process is required for the rear catalyst device 9, the ECU 20 is further provided in the pressure sensor Ps and the exhaust pipe 7 provided in the surge tank 32 of the reducing agent addition system 30. Based on the signal from the pressure sensor Pe, the internal pressure of the surge tank 32 and the exhaust pressure in the exhaust pipe 7 are acquired and compared (S14).

  When it is determined in S14 that the internal pressure of the surge tank 32 exceeds the exhaust pressure in the exhaust pipe 7, that is, the pressure of the exhaust gas flowing from the front catalyst device 8 to the rear catalyst device 9 (NOx storage reduction catalyst). The ECU 20 uses, for example, a predetermined map or the like, and uses the LPG addition valve 33 for a time corresponding to the deviation between the internal pressure of the surge tank 32 and the exhaust pressure in the exhaust pipe 7 and the NOx occlusion amount in the post-catalyst device 9. Is opened (S16).

  As a result, due to the pressure difference between the internal pressure of the surge tank 32 (steam pressure in the fuel tank 10) and the exhaust pressure in the exhaust pipe 7, the LPG fuel in the gas phase existing inside the fuel tank 10 is It is led to the LPG addition valve 33 through the communication pipe 31 and the surge tank 32, and is introduced from the LPG addition valve 33 into the region closer to the combustion chamber 2 than the rear catalyst device 9 of the exhaust pipe 7 (near the rear catalyst device 9). Is done. The gas phase LPG fuel introduced from the LPG addition valve 33 into the exhaust pipe 7 mixes with the exhaust gas quickly and uniformly, and flows into the post-catalyst device 9 (NOx storage reduction catalyst). Then, in the post-catalyst device 9, the NOx stored in the NOx storage reduction catalyst is reduced using the introduced LPG fuel as a reducing agent.

  Thus, in the LPG fuel engine 1, when the internal pressure of the surge tank 32 is higher than the exhaust pressure in the exhaust pipe 7, the LPG fuel in the gas phase state is introduced from the LPG addition valve 33 to the exhaust pipe 7. In the LPG fuel engine 1, even if the internal pressure of the surge tank 32, that is, the vapor pressure in the fuel tank 10 fluctuates, the valve opening time of the LPG addition valve 33 is changed by the ECU 20 in the fluctuation of the internal pressure of the surge tank 32. Therefore, the LPG fuel in the gas phase is supplied to the NOx occlusion reduction catalyst in the exhaust pipe 7, that is, the NOx occlusion reduction catalyst of the post-catalyst device 9 accurately by the amount necessary for the NOx reduction process.

  As a result, the LPG fuel engine 1 is necessary for the NOx reduction process while eliminating the rich misfire of the LPG fuel that tends to occur when the supply amount of the LPG fuel to each combustion chamber 2 is increased for the NOx reduction process. It is possible to easily and satisfactorily execute the NOx reduction process by accurately supplying an appropriate amount of LPG fuel to the NOx storage reduction catalyst of the post-catalyst device 9.

  Further, when the stratified lean combustion operation is executed in the LPG fuel engine 1 including the plurality of combustion chambers 2 as described above, the components and temperatures of the exhaust gas from the respective combustion chambers 2 are not necessarily the same. Even if the amount of LPG fuel into each combustion chamber 2 is increased and the NOx reduction process is performed on the post-catalyst device 9, the NOx occlusion reduction catalyst may not be subjected to a uniform and good NOx reduction process. On the other hand, in the present embodiment, the LPG fuel in the gas phase is mixed with the exhaust gas discharged from each combustion chamber 2 and joined immediately before the rear catalyst device 9 (NOx storage reduction catalyst). Therefore, in the LPG fuel engine 1, it is possible to uniformly mix the LPG fuel in the gas phase state with the exhaust gas having relatively little concentration and temperature unevenness, and the NOx reduction process can be executed satisfactorily.

  In general, it is considered that the internal pressure of the surge tank 32 is hardly lower than the exhaust pressure in the exhaust pipe 7. However, in S14, the internal pressure of the surge tank 32 is exhausted in the exhaust pipe 7. It may be determined that the pressure is below the atmospheric pressure. In view of this point, in the LPG fuel engine 1, when it is determined in S14 that the internal pressure of the surge tank 32 is equal to or lower than the exhaust pressure in the exhaust pipe 7, the ECU 20 causes each injector 4 to move into each combustion chamber 2. The amount of LPG fuel injected into the engine is increased for a predetermined time so as not to misfire (S18), whereby NOx reduction processing is performed on the rear catalyst device 9. In the present embodiment, when the NOx reduction process is performed on the post-catalyst device 9 in S18, the air-fuel ratio between the air and the LPG fuel in each combustion chamber 2 is set to an ideal air-fuel ratio or slightly richer than the ideal air-fuel ratio. The time for increasing the amount of LPG fuel is also set slightly longer than a general rich spike.

  In this way, when the processing of S16 or S18 is completed, NOx stored in the NOx storage reduction catalyst of the post-catalyst device 9 is reduced. Then, at the next execution timing, the above-described NOx reduction routine is executed again by the ECU 20.

  FIG. 3 is a schematic configuration diagram showing a second embodiment of the LPG fuel engine according to the present invention. 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.

  An LPG fuel engine 1A shown in FIG. 3 is further provided with a cetane number improver addition system 35 in addition to the LPG fuel engine 1 described above. As shown in FIG. 3, the cetane number improver addition system 35 includes a storage tank 36, a pump 37 connected to the storage tank 36, and a supply pipe 38 connected to the discharge port of the pump 37. The storage tank 36 stores a cetane number improver (liquid) that can improve the cetane number of LPG fuel such as amine nitrate, alkylene nitrate, or 2,2-dinitropropane. The pump 37 can suck out the cetane number improver from the storage tank 36 and send it out to the supply pipe 38. As the pump 37, an electric pump capable of duty control is adopted, and the pump 37 is controlled by the ECU 20A. Further, the supply pipe 38 is connected to an addition valve (injection valve) 39 arranged so that the fluid jet port faces the inside of the surge tank 32. The addition valve 39 is also connected to the input / output port of the ECU 20A and is controlled to be opened and closed by the ECU 20A.

  In the LPG fuel engine 1A configured as described above, the ECU 20A executes the stratified lean combustion operation and the internal pressure of the surge tank 32 is reduced to the exhaust pipe 7 when the NOx reduction process is required for the post-catalyst device 9. When it is determined that the internal exhaust pressure is exceeded, the pump 37 is operated and the addition valve 39 is opened at an appropriate timing according to the opening timing of the LPG addition valve 33. Thus, prior to introduction into the exhaust passage 7, the cetane number improver atomized from the addition valve 39 in the surge tank 32 is added to the LPG fuel sent from the fuel tank 10 to the exhaust pipe 7. Therefore, the ignitability and combustibility of the LPG fuel as the reducing agent can be improved. Therefore, in the LPG fuel engine 1A, it is possible to execute the NOx reduction process for the NOx storage reduction catalyst very well.

  FIG. 4 is a schematic configuration diagram showing a third embodiment of the LPG fuel engine according to the present invention. Note that the same elements as those described in relation to the above-described embodiments are denoted by the same reference numerals, and redundant descriptions are omitted.

  The LPG fuel engine 1B shown in FIG. 4 includes a reducing agent addition system 40 and a cetane number improver addition system 50 in order to perform NOx reduction processing on the NOx storage reduction catalyst of the post-catalyst device 9.

  The reducing agent addition system 40 includes an LPG addition pump 41 connected to the fuel tank 10 and a supply pipe 42 connected to the discharge port of the LPG addition pump 41. The LPG addition pump 41 sucks (takes out) the liquid phase LPG fuel from the fuel tank 10 and sends it out to the supply pipe 42. The supply pipe 42 has a flow meter 43 in the middle, and an LPG addition valve (LPG injection valve) disposed so that the fluid outlet faces the inside of the exhaust pipe 7 between the front-stage catalyst device 8 and the rear-stage catalyst device 9. ) 44. The flow meter 43 is connected to the input / output port of the ECU 20B, detects the flow rate of the LPG fuel flowing through the supply pipe 42, and gives a signal indicating the detected value to the ECU 20B. The LPG addition valve 44 is connected to an input / output port of the ECU 20B and is controlled to be opened and closed by the ECU 20B. Thus, by operating the LPG addition pump 41 and opening the LPG addition valve 44, the LPG fuel in the liquid phase is taken out from the fuel tank 10, and the taken-out LPG fuel is removed from the rear stage catalytic device 9 (NOx occlusion reduction) of the exhaust passage 7. It becomes possible to introduce into the region closer to the combustion chamber 2 than the catalyst).

  On the other hand, as shown in FIG. 4, the cetane number improver addition system 50 includes a storage tank 51, a pump 52 connected to the storage tank 51, and a supply pipe 53 connected to the discharge port of the pump 52. . The storage tank 51 stores a cetane number improver (liquid) that can improve the cetane number of LPG fuel, such as amine nitrate, alkylene nitrate, or 2,2-dinitropropane. The pump 52 sucks out the cetane number improver from the storage tank 51 and sends it out to the supply pipe 53. As the pump 52, an electric pump capable of duty control is adopted, and the pump 52 is controlled by the ECU 20B. The supply pipe 53 is connected to an addition valve 54 that is controlled to be opened and closed by the ECU 20B. The addition valve 54 is arranged so that the fluid jet port faces the inside of the supply pipe 42 on the upstream side of the LPG addition valve 44 of the reducing agent addition system 40.

  In the LPG fuel engine 1B configured as described above, the NOx reduction routine shown in FIG. 5 is repeatedly executed at predetermined time intervals by the ECU 20B in order to execute the NOx reduction process for the post-catalyst device 9. When it becomes the execution timing of the NOx reduction routine, the ECU 20B first determines whether or not the stratified lean combustion operation is being executed in the LPG fuel engine 1 at that time based on a predetermined operation state detection signal or the like (S20). ). When it is determined in S20 that the stratified lean combustion operation has not been executed, the NOx reduction process for the rear catalyst device 9 is not executed at this stage.

  When it is determined in S20 that the stratified lean combustion operation is being executed, the ECU 20B further determines whether or not the NOx reduction process is required for the rear catalyst device 9 (S22). Also in this embodiment, the ECU 20B integrates the injection amount of LPG fuel and the operation time of the engine 1B during operation of the LPG fuel engine 1B, and based on these values, the ECU 20B uses the predetermined map or the like as the NOx storage reduction catalyst. Obtain the amount of NOx occluded. Then, in S22, the ECU 20B determines whether or not the NOx occlusion amount in the rear catalyst device 9 exceeds a predetermined threshold value. Even when it is determined in S22 that the NOx reduction process is not necessary, the NOx reduction process is not executed for the rear catalyst device 9.

  If it is determined in S22 that the NOx reduction process is required for the rear catalytic device 9, the ECU 20B determines the air-fuel ratio (exhaust air / fuel ratio) of the exhaust gas flowing out from the rear catalytic device 9 based on the signal from the air / fuel ratio sensor AFS. After that, the amount of LPG fuel added as a reducing agent corresponding to the exhaust air / fuel ratio is obtained using a predetermined map or the like (S24). Further, the ECU 20B uses a predetermined map or the like to reduce the NOx based on the LPG fuel addition amount obtained in S24 and the properties (fuel properties) of the LPG fuel in the fuel tank 10 that are known in advance. The amount of cetane improver to be added to the LPG fuel supplied from the fuel tank 10 to the exhaust pipe 7 for processing is determined (S26).

  When the addition amounts of the LPG fuel and the cetane number improver are obtained, the ECU 20B operates the LPG addition pump 41 (S28). As a result, the LPG fuel is sucked out of the fuel tank 10 by the LPG addition pump 41 and is pumped toward the exhaust pipe 7. In S28, the ECU 20B operates the pump 52 of the cetane number improver addition system 50 at an appropriate timing, and the addition amount of the cetane number improver obtained in S26 based on the indicated value of the flow meter 43. The addition valve 54 is opened for a time corresponding to the above. Thus, since the cetane number improver is added to the LPG fuel sucked from the fuel tank 10 by the LPG addition pump 41 prior to introduction into the exhaust passage 7, the LPG fuel as the reducing agent is added. Ignition and flammability can be improved.

  The ECU 20B then adds the LPG fuel to which the amount of cetane number improver obtained in S26 has been added to the region closer to the combustion chamber 2 than the rear catalyst device 9 (NOx storage reduction catalyst) of the exhaust passage 7 in S24. In order to introduce (inject) a predetermined amount, the LPG addition pump 41 is controlled and the LPG addition valve 44 is opened (S30). As a result, the exhaust passage 7 on the upstream side of the post-catalyst device 9 temporarily and suddenly has an excessive fuel state (rich spike state), and the LPG fuel introduced (injected) into the exhaust pipe 7 is discharged into the exhaust pipe 7. It self-ignites and vaporizes almost simultaneously with its introduction. The LPG fuel thus vaporized mixes with the exhaust gas quickly and uniformly, and flows into the post-catalyst device 9 (NOx storage reduction catalyst). As a result, in the post-catalyst device 9, NOx occluded in the NOx occlusion reduction catalyst is reduced extremely well by using the LPG fuel vaporized in the exhaust passage 7 as a reducing agent.

  As described above, the LPG fuel engine 1B also performs the NOx reduction process while eliminating the rich misfire of the LPG fuel that tends to occur when the supply amount of the LPG fuel to the combustion chamber 2 is increased for the NOx reduction process. A required amount of LPG fuel can be accurately supplied to the NOx storage reduction catalyst. In addition, since the cetane number improver is added to the LPG fuel sent from the fuel tank 10 to the exhaust passage 7 prior to introduction into the exhaust passage 7, the ignitability and combustibility of the LPG fuel as a reducing agent are reduced. As a result, the NOx reduction treatment for the NOx occlusion reduction catalyst can be performed very well. In the present embodiment, LPG fuel that is self-ignited and vaporized immediately before the post-catalyst device 9 (NOx storage reduction catalyst) is mixed with the exhaust gas discharged from each combustion chamber 2 and joined. Become. Therefore, the LPG fuel engine 1B can easily and satisfactorily execute the NOx reduction process for the NOx storage reduction catalyst using the LPG fuel.

  Further, in the LPG fuel engine 1B, in order to execute the NOx reduction process for the NOx storage reduction catalyst, the operation mode is switched from the stratified lean combustion to the homogeneous combustion, and then the LPG fuel is temporarily and excessively supplied into each combustion chamber 2. Therefore, it is not necessary to execute the rich spike operation to be supplied to the engine, so that the deterioration of fuel consumption associated with the NOx reduction process can be suppressed. Note that after the processing of S30, the ECU 20B once terminates the NOx reduction routine of FIG. 5 and executes the above-described NOx reduction routine again at the next execution timing.

1 is a schematic configuration diagram showing a first embodiment of an LPG fuel engine according to the present invention. 3 is a flowchart for explaining a NOx reduction routine executed in the LPG fuel engine of FIG. 1. It is a schematic block diagram which shows 2nd Embodiment of the LPG fuel engine by this invention. It is a schematic block diagram which shows 3rd Embodiment of the LPG fuel engine by this invention. 5 is a flowchart for illustrating a NOx reduction routine executed in the LPG fuel engine of FIG.

Explanation of symbols

1, 1A, 1B LPG fuel engine 2 Combustion chamber 3 Piston 3a Recess 4 Injector 5 Intake pipe 6 Throttle valve 7 Exhaust pipe (exhaust passage)
8 Pre-catalyst device 9 Subsequent catalyst device (NOx storage reduction catalyst)
10 Fuel tank 11 Low pressure pump 12 High pressure pump 14 Low pressure pipe 15 Delivery pipe 16 Return pipe 20, 20A, 20B ECU
30 Reducing agent addition system 31 Communication pipe 32 Surge tank 33 LPG addition valve 35 Cetane improver addition system 36 Storage tank 37 Pump 38 Supply pipe 39 Addition valve 40 Reductant addition system 41 LPG addition pump 42 Supply pipe 43 Flow meter 44 LPG Addition valve 50 Cetane improver addition system 51 Storage tank 52 Pump 53 Supply pipe 54 Addition valve AFS Air-fuel ratio sensor Pd, Pe, Pp, Ps, Pt Pressure sensor Td, Tp, Tt Temperature sensor

Claims (7)

In an LPG fuel engine that generates power by burning LPG fuel in a combustion chamber,
A fuel tank for storing LPG fuel;
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 NOx reduction treatment on the NOx occlusion reduction catalyst, gas phase LPG fuel existing in the fuel tank is supplied to the exhaust passage. An LPG fuel engine, which can be introduced into a region closer to the combustion chamber than the NOx storage reduction catalyst.   2. The LPG fuel engine according to claim 1, wherein the LPG fuel in a gas phase is introduced into the exhaust passage by utilizing a vapor pressure in the fuel tank. 3.   An NOx reduction treatment for the NOx occlusion reduction catalyst, further comprising: an addition valve for introducing gas phase LPG fuel into the exhaust passage; and a surge tank provided between the fuel tank and the addition valve. 2. The gas phase LPG fuel is introduced into the exhaust passage from the addition valve when the internal pressure of the surge tank is higher than the exhaust pressure in the exhaust pipe. 2. The LPG fuel engine according to 2.   The LPG fuel engine according to any one of claims 1 to 3, further comprising means for adding a cetane number improver to the LPG fuel prior to introduction into the exhaust passage. In an LPG fuel engine that generates power by burning LPG fuel in a combustion chamber,
A fuel tank for storing LPG fuel;
An exhaust passage through which exhaust gas from the combustion chamber is introduced;
A NOx storage reduction catalyst provided in the exhaust passage;
In order to perform the NOx reduction process for the NOx storage reduction catalyst, the LPG fuel is taken out from the fuel tank, and the taken out LPG fuel is introduced into the region closer to the combustion chamber than the NOx storage reduction catalyst in the exhaust passage. Agent addition means,
Means for adding a cetane number improver to the LPG fuel taken out from the fuel tank by the reducing agent addition means. In an LPG fuel engine exhaust gas purification method for generating power by burning LPG fuel from a fuel tank in a combustion chamber,
A NOx storage reduction catalyst is provided in an exhaust passage through which the exhaust gas from the combustion chamber is introduced, and in order to perform a NOx reduction process on the NOx storage reduction catalyst, a gas phase LPG present in the fuel tank is present. An exhaust gas purification method for an LPG fuel engine, characterized in that fuel is introduced into a region closer to the combustion chamber than the NOx storage reduction catalyst in the exhaust passage. In an LPG fuel engine exhaust gas purification method for generating power by burning LPG fuel from a fuel tank 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 in order to perform NOx reduction processing on the NOx storage reduction catalyst, LPG fuel is taken out from the fuel tank, and the taken out LPG fuel is used as the extracted LPG fuel. An exhaust gas purification method for an LPG fuel engine, comprising adding a cetane number improver and introducing the cetane number improver into a region closer to the combustion chamber than the NOx storage reduction catalyst in the exhaust passage.

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