Exhaust emission control device for engine

Abstract

To provide an exhaust emission control device for an engine capable of attaining a higher exhaust emission control ratio.SOLUTION: An exhaust emission control device for an engine E includes exhaust emission control catalysts 331, 332, operating state sensors 201-211 and a controller 101. During an operation of the engine E by using gas fuel, the controller 101 performs air-fuel ratio vibration control for vibrating an air-fuel ratio of exhaust gas to a rich side and a lean side of a stoichiometric equivalent value. The exhaust emission control catalyst includes: a first catalyst support region (first exhaust emission control catalyst 331) on which only a CeZr-based catalyst is supported as a catalyst component that contributes to elimination of exhaust gas harmful components out of the CeZr-based catalyst and an alumina-based catalyst; and a second catalyst support region (second exhaust emission control catalyst 332) that is provided downstream of the first catalyst support region in a flow of exhaust gas and on which only the alumina-based catalyst is supported as a catalyst component out of the CeZr-based catalyst and the alumina-based catalyst.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an exhaust gas purification device for an engine. [Background technology]

[0002] There are engines that can be run using compressed natural gas (CNG) as fuel.

[0003] There are also engines that can use liquid fuels in addition to gaseous fuels such as compressed natural gas, and are configured to be able to switch between liquid and gaseous fuels during operation. These engines are called bi-fuel engines, and have separate fuel tanks for storing liquid and gaseous fuels, and the fuel supplied to the combustion chamber can be switched depending on the remaining amount in the fuel tank or the driver's choice.

[0004] On the other hand, in engines equipped with a three-way catalyst in the exhaust passage, it is known that the exhaust purification rate of the catalyst can be improved by forcibly oscillating the air-fuel ratio of the mixture between the rich side and the lean side relative to the stoichiometric air-fuel ratio.

[0005] Here, a rich air-fuel ratio of the exhaust gas means that the amount of fuel contained in the exhaust gas is greater than the equivalent amount, and a lean air-fuel ratio of the exhaust gas means that the amount of fuel contained in the exhaust gas is less than the equivalent amount. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2023-076990 Summary of the Invention [Problem to be solved by the invention]

[0007] There is a demand for further improvements in exhaust purification efficiency in engines that can be operated on gaseous fuels.

[0008] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an engine exhaust purification device that can achieve a higher exhaust purification rate. [Means for solving the problem]

[0009] In order to solve the above problems, one aspect of the present invention provides an exhaust purification device for an engine that can be operated by supplying gaseous fuel to a combustion chamber, and includes an exhaust purification catalyst installed in an exhaust passage of the engine, an operating condition sensor configured to output a signal corresponding to the operating condition of the engine, and a controller that controls the operating condition of the engine based on the signal output by the operating condition sensor. The controller includes air-fuel ratio oscillation control means that performs air-fuel ratio oscillation control to oscillate the air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst between rich and lean sides of a value equivalent to stoichiometry when the engine is operated with the gaseous fuel. The exhaust purification catalyst has a first catalyst supporting region that supports only the CeZr-based catalyst out of a CeZr-based catalyst and an alumina-based catalyst as catalytic components that contribute to purifying exhaust harmful components, and a second catalyst supporting region that is located downstream of the first catalyst supporting region in the exhaust flow and that supports only the alumina-based catalyst out of the CeZr-based catalyst and the alumina-based catalyst as catalytic components. [Effects of the Invention]

[0010] According to the present invention, in the exhaust purification catalyst, a first catalyst supporting region and a second catalyst supporting region are formed separately in the direction of exhaust flow, and the upstream first catalyst supporting region supports a CeZr-based catalyst as a catalytic component that contributes to purifying harmful exhaust components, while the downstream second catalyst supporting region supports an alumina-based catalyst as a catalytic component.By performing air-fuel ratio oscillation control when the engine is operating on gaseous fuel, it is possible to achieve both exhaust purification performance in both low temperature ranges and high temperature ranges, thereby achieving a higher exhaust purification rate. [Brief explanation of the drawings]

[0011] [Figure 1]1 is a schematic diagram showing the overall configuration of an engine according to an embodiment of the present invention. [Figure 2] 4 is a flowchart showing the overall flow of air-fuel ratio control during normal engine operation. [Figure 3] 1 is an operating region map used to switch control methods in air-fuel ratio control. [Figure 4] 1 is a flowchart showing a basic flow of air-fuel ratio oscillation control. [Figure 5] 1 is a flowchart showing a basic flow of air-fuel ratio feedback control. [Figure 6] 4 is a flowchart showing the flow of filter regeneration control (regeneration time detection process). [Figure 7] 4 is a flowchart showing a flow of filter regeneration control (regeneration execution process). [Figure 8] 1 is a graph showing the change in the amount of particulate matter deposited (differential pressure before and after filter regeneration). [Figure 9] 10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation in air-fuel ratio oscillation control, the purification rate η of the exhaust purification catalyst, and the minimum value λr_min of the downstream air-fuel ratio when the catalyst is highly active and the exhaust flow rate is low. [Figure 10] 10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation in air-fuel ratio oscillation control, the purification rate η of the exhaust purification catalyst, and the minimum value λr_min of the downstream air-fuel ratio when the catalyst is highly active and the exhaust flow rate is high. [Figure 11] 10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation in air-fuel ratio oscillation control, the purification rate η of the exhaust purification catalyst, and the minimum value λr_min of the downstream air-fuel ratio when the catalyst is low in activity and the exhaust flow rate is low. [Figure 12] 10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation in air-fuel ratio oscillation control, the purification rate η of the exhaust purification catalyst, and the minimum value λr_min of the downstream air-fuel ratio when the catalyst is low in activity and the exhaust flow rate is high. [Figure 13]10 is a graph showing output waveforms of an upstream exhaust sensor and a downstream exhaust sensor in the high frequency range during air-fuel ratio oscillation control. [Figure 14] 10 is a graph showing output waveforms of an upstream exhaust sensor and a downstream exhaust sensor in a low frequency range during air-fuel ratio oscillation control. [Figure 15] 1 is a graph of experimental data showing methane conversion as a function of different catalyst configurations. [Figure 16] 1 is a graph of experimental data showing methane conversion as a function of different catalyst configurations. [Figure 17] 1 is a graph of experimental data showing the methane conversion efficiency of a CeZr-based catalyst and an alumina-based catalyst under different exhaust conditions. [Figure 18] 10 is a flowchart showing a basic flow of air-fuel ratio oscillation control according to another embodiment of the present invention. [Figure 19] 10 is a graph of experimental data showing the relationship between the frequency Frq of air-fuel ratio oscillation in air-fuel ratio oscillation control, the purification rate η of the exhaust purification catalyst, and the difference between the maximum value λr_max and the minimum value λr_min of the downstream air-fuel ratio (air-fuel ratio range Rlmb) for different catalyst temperatures (a) Tcat1 and (b) Tcat2 (>Tcat1). [Figure 20] 10 is a graph of experimental data showing the relationship between the frequency Frq of the air-fuel ratio oscillation in the air-fuel ratio oscillation control, the purification rate η of the exhaust purification catalyst, and the air-fuel ratio range Rlmb for different catalyst temperatures (a) Tcat3 (>Tcat2) and (b) Tcat4 (>Tcat3). DETAILED DESCRIPTION OF THE INVENTION

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0013] FIG. 1 is a schematic diagram showing the overall configuration of an internal combustion engine (hereinafter simply referred to as "engine") E according to one embodiment of the present invention.

[0014] In the following description, the terms "upstream" and "downstream" are used in relation to the direction of flow of normal exhaust gas discharged from the engine E. For example, the upstream side of the exhaust purification catalyst refers to the upstream side of the exhaust purification catalyst in the direction of exhaust gas flow, and the downstream side of the exhaust purification catalyst refers to the downstream side of the exhaust purification catalyst in the direction of exhaust gas flow.

[0015] In this embodiment, engine E is mounted on a vehicle and serves as its drive source. Engine E is a bi-fuel engine, capable of switching between liquid and gaseous fuels. As described below, engine E includes a first fuel system for supplying liquid fuel and a second fuel system for supplying gaseous fuel. Liquid fuel refers to fuel that is in a liquid state at room temperature and normal pressure or when supplied to engine E. In this embodiment, gasoline is used. In contrast, gaseous fuel refers to fuel that is in a gaseous state at room temperature and normal pressure. In this embodiment, compressed natural gas (CNG) is used. Using compressed natural gas as fuel can suppress the generation of soot due to combustion and reduce carbon dioxide emissions compared to using gasoline as fuel. Liquid fuels include gasoline, a mixture of gasoline and ethanol, alcohol fuel, or synthetic fuel. Gas fuels include methane-based fuels other than compressed natural gas (CNG) (including biogas fuels derived from livestock manure such as cow dung) or a mixture of compressed natural gas and other methane-based fuels.

[0016] The engine E includes an engine body 1 having a combustion chamber, an intake system 2, and an exhaust system 3. In this embodiment, the engine E is an in-line four-cylinder engine, but the type of the engine E, that is, the number and arrangement of cylinders in the engine E, is not limited to this. Various types of engines can be used, such as single-cylinder, two-cylinder, six-cylinder, V-type, and horizontally opposed type.

[0017] The engine body 1 includes a cylinder block, a cylinder head, and a crankcase. A piston is inserted into the cylinder block, and the space formed between the piston crown surface and the inner surface of the cylinder head serves as a combustion chamber.

[0018] The intake system 2 includes an intake pipe 21 and an intake manifold 22, as well as an air cleaner 23 attached to the inlet of the intake pipe 21. Air from which foreign matter such as dust has been removed via the air cleaner 23 is introduced into the intake pipe 21. The intake pipe 21 is connected to a collecting section of the intake manifold 22, and the intake manifold 22 branches off from the collecting section and is connected to a side section of the cylinder head. The air that flows from the intake pipe 21 into the intake manifold 22 is distributed to each cylinder via the branching sections of the intake manifold 22.

[0019] In this embodiment, a port injection type fuel supply system is adopted. The engine E is equipped with a plurality of fuel injectors 41 (41 a, 41 b) embedded in the cylinder head, and fuel is injected from each of the plurality of fuel injectors 41 toward the corresponding cylinder.

[0020] Engine E is a bi-fuel engine and includes a second fuel system for supplying gaseous fuel in addition to a first fuel system for supplying liquid fuel. Specifically, engine E includes a liquid fuel tank (not shown) for storing liquid fuel, a first fuel injector 41a for injecting liquid fuel, a gaseous fuel tank (not shown) for storing gaseous fuel, and a second fuel injector 41b for injecting gaseous fuel. The first fuel injector 41a is connected to the liquid fuel tank via a first fuel pipe and receives a supply of liquid fuel from the liquid fuel tank. The second fuel injector 41b is connected to the gaseous fuel tank via a second fuel pipe and receives a supply of gaseous fuel from the gaseous fuel tank.

[0021] First fuel injector 41a and second fuel injector 41b are both installed at branching portions of intake manifold 22 and inject fuel toward the intake ports of the corresponding cylinders. The fuel supply method is not limited to this, and a supply method other than port injection, such as direct injection, can also be adopted.

[0022] The liquid fuel injected by the first fuel injector 41a and the gaseous fuel injected by the second fuel injector 41b are mixed with air that has passed through a branching portion of the intake manifold 22 and are introduced into the corresponding cylinder. In each cylinder, the fuel and air continue to mix together to form an air-fuel mixture. This mixture is then ignited by a spark plug 51, causing the mixture to burn.

[0023] The exhaust system 3 includes an exhaust manifold 31 and an exhaust pipe 32, as well as a catalytic converter 33. After combustion, the exhaust gas remaining in the cylinders is discharged to a branching section of the exhaust manifold 31. The exhaust gas is collected from the branching section to a collecting section in the exhaust manifold 31 and introduced into the exhaust pipe 32. The catalytic converter 33 is installed in the exhaust pipe 32, and the exhaust gas flowing through the exhaust pipe 32 is introduced into the catalytic converter 33 and passes through exhaust purification catalysts 331 and 332 housed in the catalytic converter 33. When the exhaust gas passes through the exhaust purification catalysts 331 and 332, harmful exhaust components are purified, and further, particulate matter in the exhaust is collected before being released into the atmosphere.

[0024] In this embodiment, a first exhaust purification catalyst 331 and a second exhaust purification catalyst 332 are provided as exhaust purification catalysts housed in the catalytic converter 33. The first exhaust purification catalyst 331 and the second exhaust purification catalyst 332 can be housed not only in a common or the same casing (that is, the casing of the catalytic converter 33), but also in separate casings and installed in the exhaust pipe 32. Specifically, the casing of the first exhaust purification catalyst 331 and the casing of the second exhaust purification catalyst 332 are formed as separate bodies, and the first exhaust purification catalyst 331 and the second exhaust purification catalyst 332 are housed in their own casings, and both casings are arranged in series and connected to each other via the exhaust pipe.

[0025] The first exhaust purification catalyst 331 is configured with a ceramic honeycomb carrier carrying predetermined catalytic components, and cordierite ceramic can be used as the ceramic. The first exhaust purification catalyst 331 is composed of a CeZr-based catalyst primarily composed of cerium-zirconium (CeZr) composite oxide and an alumina-based catalyst primarily composed of alumina, with only the CeZr-based catalyst being the catalytic component that substantially contributes to purifying harmful exhaust components. The CeZr-based catalyst has a structure in which a noble metal is supported on a CeZr composite oxide, and may contain rare earth elements such as lanthanum (La) and neodymium (Nd) as additives in addition to the CeZr composite oxide as the primary component. The CeZr-based catalyst may further contain a binder (e.g., alumina) to the extent necessary to coat and fix the catalytic components on the carrier. The amount of binder added is, for example, approximately 10% by weight or less. Here, the carrier provided in the first exhaust purification catalyst 331 corresponds to the "first carrier", and the region on this carrier where the coating of catalyst components is formed corresponds to the "first catalyst supporting region".

[0026] The second exhaust purification catalyst 332 has a configuration in which a predetermined catalytic component is supported on a filter carrier. The filter carrier has a honeycomb structure similar to the carrier of the first exhaust purification catalyst 331, and the partition walls that divide the internal space are porous enough to allow exhaust gas to pass through. As a result, particulate matter in the exhaust gas is captured by the partition walls as the exhaust gas passes through them. Of the CeZr-based catalyst and the alumina-based catalyst, the second exhaust purification catalyst 332 contains only the alumina-based catalyst as its catalytic component, and the alumina-based catalyst has a structure in which a precious metal is supported on alumina. The alumina-based catalyst may contain a rare earth such as lanthanum (La) as an additive in addition to aluminum oxide, which is the main component. Here, the filter carrier provided in the second exhaust purification catalyst 332 corresponds to the "second carrier," and the area on this carrier where the catalytic component coating is formed corresponds to the "second catalyst supported area."

[0027] The volume of the CeZr-based catalyst in the first exhaust purification catalyst 331 is set to a ratio of approximately 0.3 to 0.7 times the volume of the catalyst components in the entire exhaust purification catalysts 331, 332, that is, the total volume of the CeZr-based catalyst and the alumina-based catalyst. The volume of the CeZr-based catalyst can be changed as appropriate depending on the application of the engine, etc. For example, in cases where the range of air-fuel ratio fluctuation during operation is narrow or fluctuations occur infrequently, such as in stationary engines or engines dedicated to power generation in series hybrid systems, it is also possible to further reduce the volume of the CeZr-based catalyst from the above ratio.

[0028] In addition to the above, the engine E includes an engine controller 101 and various sensors 201-211.

[0029] The engine controller 101 is an electronic control unit that is configured by a microcomputer equipped with a central processing unit (CPU), storage devices such as ROM and RAM, an input / output interface, and the like.

[0030] The engine E is equipped with an accelerator sensor 201 and an engine rotation speed sensor 202, as well as an air flow meter 203, a coolant temperature sensor 204, a catalyst temperature sensor 205, an upstream exhaust sensor 206, a downstream exhaust sensor 207, an inlet pressure sensor 208, an outlet pressure sensor 209, and a remaining fuel sensor 210. Detection signals output from these sensors 201 to 210 are transmitted to the engine controller 101 and received by the engine controller 101 as its inputs. In addition to the above, the engine E is equipped with a fuel selector switch 211. An output signal from the fuel selector switch 211 is also transmitted to the engine controller 101.

[0031] The accelerator sensor 201 detects the amount of depression of the accelerator pedal by the driver as the accelerator opening APO, which is an index of the target load required for the engine E.

[0032] The engine rotation speed sensor 202 detects the rotation speed Ne of the engine E. A crank angle sensor can be used as the engine rotation speed sensor 202, and converts the elapsed time per unit crank angle or reference crank angle detected by the crank angle sensor into the rotation speed Ne.

[0033] The air flow meter 203 detects, as the intake air amount Qa, the flow rate of air introduced into the engine E. In this embodiment, the intake air amount Qa is referred to as an index of the flow rate of exhaust gas discharged from the combustion chamber.

[0034] The coolant temperature sensor 204 detects the temperature Tw of the coolant flowing through a coolant passage formed in the cylinder block of the engine body 1 (hereinafter referred to as the "coolant temperature").

[0035] The catalyst temperature sensor 205 detects the temperature Tcat (hereinafter referred to as "catalyst temperature") of the exhaust purification catalysts 331, 332 provided in the catalytic converter 33. In this embodiment, the temperature of the exhaust gas at the inlet of the catalytic converter 33, in other words, the temperature Tcat_in of the exhaust gas flowing into the first exhaust purification catalyst 331 (hereinafter referred to as "catalyst inlet gas temperature") is detected as the catalyst temperature Tcat.

[0036] In this way, in this embodiment, the temperature of the exhaust gas close to the temperature of the first exhaust purification catalyst 331 (catalyst inlet gas temperature Tcat_in) is used as the catalyst temperature Tcat, but what can be used as the catalyst temperature Tcat is not limited to this, and it may be the temperature of the exhaust gas flowing between the first exhaust purification catalyst 331 and the second exhaust purification catalyst 332, or the temperature of the carrier of the first exhaust purification catalyst 331 (catalyst bed temperature).

[0037] The upstream exhaust sensor 206 is installed in the exhaust pipe 32 upstream of the catalytic converter 33, and detects the air-fuel ratio λf of the exhaust before it flows into the first exhaust purification catalyst 331 (hereinafter referred to as the “upstream air-fuel ratio”).

[0038] The downstream side exhaust sensor 207 is installed between the first exhaust purification catalyst 331 and the second exhaust purification catalyst 332, penetrating the housing of the catalytic converter 33 from the inside to the outside, and detects the air-fuel ratio of the exhaust gas flowing between the first exhaust purification catalyst 331 and the second exhaust purification catalyst 332, in other words, the air-fuel ratio λr of the exhaust gas that has passed through the first exhaust purification catalyst 331 (hereinafter referred to as the "downstream side air-fuel ratio"). The downstream side exhaust sensor 207 may be installed in the exhaust pipe 32 downstream of the catalytic converter 33, and may detect the air-fuel ratio of the exhaust gas that has passed through the second exhaust purification catalyst 332.

[0039] The inlet side pressure sensor 208 is installed upstream of the second exhaust purification catalyst 332, in this embodiment between the first exhaust purification catalyst 331 and the second exhaust purification catalyst 332, penetrating the housing of the catalytic converter 33 from the inside to the outside, and detects the pressure of the exhaust gas before it flows into the second exhaust purification catalyst 332 (hereinafter referred to as the "inlet gas pressure") Pexh1.

[0040] The outlet side pressure sensor 209 is installed in the exhaust pipe 32 downstream of the second exhaust purification catalyst 332, and detects the pressure of the exhaust that has passed through the second exhaust purification catalyst 332 (hereinafter referred to as "outlet gas pressure") Pexh2.

[0041] The remaining fuel quantity sensors 210 (210a, 210b) detect the amount of fuel remaining in the fuel tank, i.e., the remaining fuel quantity Lf (Lfa, Lfb) in the fuel tank. In this embodiment, a liquid fuel tank for storing liquid fuel and a gas fuel tank for storing gas fuel are provided as fuel tanks, and the first remaining fuel quantity sensor 210a detects the remaining quantity in the liquid fuel tank, and the second remaining fuel quantity sensor 210b detects the remaining quantity in the gas fuel tank.

[0042] The fuel selector switch 211 is installed, for example, on the dashboard inside the vehicle so as to be operable by the driver of the vehicle, and outputs an instruction signal to select between liquid fuel and gaseous fuel to be used to operate the engine E. By operating the fuel selector switch 211, the driver can select the fuel to be used under specific circumstances, such as when starting the engine E, and can also switch the fuel to gaseous fuel while operating on liquid fuel, or switch the fuel to liquid fuel while operating on gaseous fuel.

[0043] The engine controller 101 controls the air-fuel ratio of the mixture used for combustion in the cylinders based on the detection signals output from the various sensors 201 to 210 described above, while controlling the operating state of the engine E. The engine controller 101 constitutes the "controller" according to this embodiment.

[0044] The engine controller 101 switches between air-fuel ratio oscillation control and air-fuel ratio feedback control in air-fuel ratio control during normal operation of the engine E. The air-fuel ratio oscillation control is a control that forcibly oscillates the air-fuel ratio of the exhaust gas flowing into the catalytic converter 33 between the rich side and the lean side of a value equivalent to stoichiometry, and in this embodiment, this is achieved by oscillating the air-fuel ratio of the mixture formed in the combustion chamber. Specifically, the fuel injection amount of the fuel injector 41 (41a, 41b) is forcibly increased or decreased. The amplitude and frequency of the air-fuel ratio oscillation in the air-fuel ratio oscillation control, in other words, the width and period of the air-fuel ratio fluctuation, are set to a degree that allows the oscillation of the air-fuel ratio in the exhaust gas to be mitigated by the oxygen storage capacity (OSC) of the first exhaust purification catalyst 331, preferably to a degree that allows the air-fuel ratio of the exhaust gas passing through the first exhaust purification catalyst 331 to be kept at or near the stoichiometric equivalent value. Air-fuel ratio feedback control is a control that adjusts the air-fuel ratio of the mixture to the stoichiometric air-fuel ratio based on a signal from an exhaust sensor, in this embodiment, the upstream exhaust sensor 206. As a result of the air-fuel ratio feedback control, the air-fuel ratio of the exhaust gas flowing into the catalytic converter 33 approaches a value equivalent to stoichiometry.

[0045] Switching between air-fuel ratio oscillation control and air-fuel ratio feedback control depends on the operating range to which the operating state of engine E belongs. In this embodiment, the entire operating range of engine E is divided into multiple regions A and B determined according to the catalyst temperature Tcat and the exhaust flow rate Qexh, and the operating state of engine E is determined to be either in a first predetermined region A where the catalyst temperature Tcat is lower than a predetermined temperature Tcat1 or the exhaust flow rate Qexh is higher than the predetermined flow rate Qexh1, or in a region other than the first predetermined region A, that is, in a second predetermined region B where the catalyst temperature Tcat is equal to or higher than the predetermined temperature Tcat1 and the exhaust flow rate Qexh is equal to or lower than the predetermined flow rate Qexh1. If the operating state of engine E is in the first predetermined region A, air-fuel ratio oscillation control is executed, and if it is in the second predetermined region B, air-fuel ratio feedback control is executed.

[0046] Here, catalyst temperature Tcat is an indicator of the activity state of exhaust purification catalysts 331, 332, and catalyst temperature Tcat being equal to or higher than a predetermined temperature Tcat1 indicates that activation of exhaust purification catalysts 331, 332 has progressed and that exhaust purification catalysts 331, 332 are in a highly activated state. On the other hand, exhaust flow rate Qexh is an indicator of the amount of harmful exhaust substances that need to be purified by exhaust purification catalysts 331, 332, and exhaust flow rate Qexh being equal to or lower than a predetermined flow rate Qexh1 indicates that the amount of harmful exhaust substances to be purified is relatively small and that conditions are such that processing by the highly activated exhaust purification catalysts 331, 332 is easy. In other words, performing air-fuel ratio feedback control in operating range B means performing air-fuel ratio feedback control when activation of exhaust purification catalysts 331, 332 has progressed and it is determined that processing of harmful exhaust components by exhaust purification catalysts 331, 332 is easy.

[0047] 3 is an operating region map showing regions in which air-fuel ratio oscillation control and air-fuel ratio feedback control are performed. As shown in FIG. 3, when the catalyst inlet gas temperature Tcat_in is equal to or higher than a predetermined temperature Tcat1 and the exhaust flow rate Qexh is equal to or lower than a predetermined flow rate Qexh1, the operating state of the engine E falls within the second predetermined region B, the activation of the exhaust purification catalysts 331, 332 is advanced, and the conditions are such that processing by the exhaust purification catalysts 331, 332 is easy, and air-fuel ratio feedback control is selected and executed. In other cases, the operating state of the engine E falls within the first predetermined region A, and the activation of the exhaust purification catalysts 331, 332 is insufficient or there is a large amount of exhaust harmful substances to be purified, so the conditions are such that processing by the exhaust purification catalysts 331, 332 is relatively difficult, and air-fuel ratio oscillation control is selected and executed. In this embodiment, the catalyst inlet gas temperature Tcat_in is used as an index of the catalyst temperature Tcat, and the intake air amount Qa is used as an index of the exhaust flow rate Qexh. The intake air amount Qa is an example of a state variable that has a high correlation with the exhaust flow rate Qexh.

[0048] Furthermore, in this embodiment, the fuel used to operate the engine E is switched depending on the remaining amount in the fuel tank and the driver's selection.

[0049] Specifically, the fuel is switched from liquid fuel to gas fuel or from gas fuel to liquid fuel depending on the remaining amount in the fuel tank and the driver's selection. In this embodiment, operation using gas fuel is the basis from the viewpoint of exhaust properties. During operation using gas fuel, if the remaining amount in the gas fuel tank decreases and reaches a predetermined remaining amount, or if the driver operates the fuel selector switch 211 and selects liquid fuel, the fuel used is switched from gas fuel to liquid fuel, provided that the remaining amount in the liquid fuel tank is equal to or greater than the predetermined remaining amount. After switching to liquid fuel, if the gas fuel tank is refilled or the driver operates the fuel selector switch 211 again and selects gas fuel, the fuel used is switched from liquid fuel to gas fuel.

[0050] While the engine E is operating, particulate matter contained in the exhaust gas accumulates on the second exhaust purification catalyst 332, specifically, on the filter carrier provided in the second exhaust purification catalyst 332. As the accumulation of particulate matter progresses, the filter carrier becomes clogged, increasing the pressure loss in the second exhaust purification catalyst 332 and raising concerns that this may impair engine performance. The accumulation of particulate matter tends to progress significantly when the engine is operating using liquid fuel.

[0051] In this embodiment, the second exhaust purification catalyst 332, i.e., the amount of particulate matter deposited on the filter carrier (hereinafter referred to as the "filter deposition amount") Qgpf is detected, and when the filter deposition amount Qgpf exceeds a predetermined upper limit of deposition amount, control is executed to reduce the particulate matter deposited on the second exhaust purification catalyst 332 and regenerate the second exhaust purification catalyst 332 (hereinafter referred to as "filter regeneration control"). The filter deposition amount Qgpf correlates with the difference ΔP (=Pexh1-Pexh2) between the exhaust pressures Pexh1, Pexh2 on the upstream and downstream sides of the second exhaust purification catalyst 332, i.e., the differential pressure before and after the filter carrier of the second exhaust purification catalyst 332 (hereinafter referred to as the "filter differential pressure"). The filter differential pressure ΔP is compared with a predetermined pressure, and when the filter differential pressure ΔP exceeds the predetermined pressure, it is detected that the filter deposition amount Qgpf exceeds the upper limit of deposition amount and that the second exhaust purification catalyst 332 is at the time of regeneration.

[0052] FIG. 2 is a flowchart showing the overall flow of air-fuel ratio control according to this embodiment.

[0053] The engine controller 101 executes the air-fuel ratio control according to the routine shown in FIG. 2 at predetermined time intervals after the engine E is started.

[0054] In S101, various state parameters that serve as indicators of the operating state of the engine E, such as the engine rotation speed Ne, the intake air amount Qa, the upstream air-fuel ratio λf, the downstream air-fuel ratio λr, and the catalyst inlet gas temperature Tcat_in, are read.

[0055] In S102, it is determined whether the intake air amount Qa is greater than a predetermined flow rate Qa1. If it is greater than the predetermined flow rate Qa1, the process proceeds to S104, and if it is equal to or less than the predetermined flow rate Qa1, the process proceeds to S103. As mentioned above, the intake air amount Qa is an example of a state variable that replaces the exhaust flow rate Qexh, and if the intake air amount Qa is greater than the predetermined flow rate Qa1, it is determined that a large amount of harmful exhaust substances that need to be purified by the exhaust purification catalysts 331, 332 are present, and the conditions are such that it is not easy to treat them with the exhaust purification catalysts 331, 332, so the process of S104 is executed without going through the process of S103.

[0056] In S103, it is determined whether the catalyst inlet gas temperature Tcat_in is higher than a predetermined temperature Tcat1. If the catalyst inlet gas temperature Tcat_in is higher than the predetermined temperature Tcat1, the operating state of the engine E is determined to be in the second predetermined region B shown in Figure 3, and the process proceeds to S105. Otherwise, the operating state of the engine E is determined to be in the first predetermined region A, and the process proceeds to S104.

[0057] In step S104, air-fuel ratio oscillation control is executed in accordance with the procedure shown in the flowchart of FIG.

[0058] In step S105, air-fuel ratio feedback control is executed in accordance with the procedure shown in the flowchart of FIG.

[0059] FIG. 4 is a flowchart showing the basic flow of the air-fuel ratio oscillation control according to this embodiment.

[0060] In S201, it is determined whether or not setting of the control frequency Fcn has already been completed. The control frequency Fcn is an optimum frequency at which the highest purification rate (hereinafter referred to as "maximum purification rate") η can be obtained by the exhaust purification catalysts 331, 332 when performing air-fuel ratio oscillation control, and in this embodiment, the control frequency Fcn is set for each operating range of the engine E determined according to the rotation speed and load of the engine E. If setting of the control frequency Fcn has already been completed, proceed to S202, and if not yet completed, proceed to S203. As the load of the engine E, the intake air amount Qa, which is a state variable correlated with the load, can be used.

[0061] In S202, the control frequency Fcn is read. In this embodiment, an operating range map is provided in which the control frequency Fcn can be assigned in accordance with the rotation speed and load (e.g., intake air amount Qa) of the engine E, and the corresponding control frequency Fcn is read from an operating range for which setting has already been completed. After reading the control frequency Fcn, the process proceeds to S210.

[0062] In S203, it is determined whether the value of the flag FRG is 0. If the value of the flag FRG is 0, the process proceeds to S204, and if not, the process proceeds to S206.

[0063] In S204, a reference frequency F0 is set for the frequency Frq of the air-fuel ratio oscillation. The reference frequency F0 is a value that is temporarily set when starting to specify the control frequency Fcn, and is set in advance for each operating range in the operating range map as an initial value for the frequency Frq. This is not limitative, and it is also possible to simply set the reference frequency F0 to 1 [Hz] across the entire operating range of the engine E.

[0064] In S205, the value of the flag FRG is set to 1.

[0065] In S206, the frequency Frq of the air-fuel ratio oscillation is decreased by a predetermined frequency ΔF. Specifically, the frequency Frq is updated to a frequency that is decreased from the current frequency Frq by the predetermined frequency ΔF (Frq=Frq-ΔF), and air-fuel ratio oscillation control is executed using the new updated frequency Frq.

[0066] In S207, based on the downstream air-fuel ratio λr, the minimum value of the downstream air-fuel ratio λr within a predetermined time (hereinafter referred to as the "downstream minimum air-fuel ratio") λr_min is calculated, and the amount of change in the downstream minimum air-fuel ratio λr_min before and after decreasing the frequency Frq by ΔF (hereinafter referred to as the "air-fuel ratio change amount") Δλr_min is calculated. Here, the predetermined time may be a time equivalent to one cycle of the air-fuel ratio oscillation, or may be a time longer than this. In this embodiment, the predetermined time is a time longer than one cycle of the air-fuel ratio oscillation.

[0067] In S208, the gradient glmba of the change in the downstream minimum air-fuel ratio λr_min with respect to the frequency Frq is calculated by dividing the air-fuel ratio change amount Δλr_min by ΔF. In this embodiment, the gradient glmba is the gradient of the change in the downstream minimum air-fuel ratio λr_min with respect to the logarithm log(Frq) of the frequency Frq, which is obtained when the frequency Frq is decreased from the reference frequency F0 by a predetermined frequency ΔF. Then, it is determined whether the absolute value of the gradient glmba (=|glmba|) is equal to or greater than a predetermined value g01. If the gradient glmba is equal to or greater than the predetermined value g01, in other words, if the gradient glma of the change in the downstream minimum air-fuel ratio λr_min before and after the decrease in the frequency Frq decreases and its absolute value (=|glmba|) reaches the predetermined value g01, the process proceeds to S209. If the gradient glmba is less than the predetermined value g01, the process bypasses S209 and proceeds to S210.

[0068] In S209, the control frequency Fcn is set. In this embodiment, the frequency Frq when the absolute value of the gradient glmba (=|glmba|) reaches a predetermined value g01 is set as the control frequency Fcn.

[0069] The processing from S206 to S209 corresponds to the processing executed by the engine controller 101 as the "control frequency specifying means" according to this embodiment.

[0070] In S210, a fuel injection amount Qf is calculated. In the air-fuel ratio oscillation control, a basic injection amount Qfb of fuel equivalent to the intake air amount Qa is calculated, and the basic injection amount Qfb is multiplied by a correction coefficient α for air-fuel ratio oscillation. Furthermore, various increase correction amounts H according to the coolant temperature Tw and the like are added to calculate the fuel injection amount Qf. Here, the correction coefficient α is set to switch between values ​​greater than 1 and less than 1 every time equivalent to half a cycle of the control frequency Fcn (=1 / (2Fcn)). By multiplying by the correction coefficient α, the air-fuel ratio of the air-fuel mixture periodically fluctuates or oscillates, for example, between 0.95 and 1.05 in terms of an excess air ratio. The processing of S210 corresponds to processing executed by the engine controller 101 as the "air-fuel ratio oscillation control means" according to this embodiment.

[0071] In S211, the fuel injector 41 (41a, 41b) is driven with the fuel injection amount Qf.

[0072] FIG. 5 is a flowchart showing the basic flow of air-fuel ratio feedback control according to this embodiment.

[0073] In S301, a basic injection amount Qfb is calculated. The basic injection amount Qfb is an injection amount corresponding to the amount of fuel equivalent to the intake air amount Qa.

[0074] In S302, an air-fuel ratio feedback correction amount Hqf is calculated. In this embodiment, the air-fuel ratio feedback correction amount Hqf is calculated as a function of the difference between the upstream air-fuel ratio λf detected by the upstream exhaust sensor 206 and the stoichiometric equivalent value λst (i.e., 1). When the difference between the upstream air-fuel ratio λf and the stoichiometric equivalent value λst is greater than 0 and the exhaust is in an air-excess state, the air-fuel ratio feedback correction amount Hqf is calculated as an increase correction amount that increases the amount of fuel, and when the difference is less than 0 and the exhaust is in a fuel-excess state, the air-fuel ratio feedback correction amount Hqf is calculated as a decrease correction amount that decreases the amount of fuel.

[0075] In S303, the fuel injection amount Qf is set. In the air-fuel ratio feedback control, the fuel injection amount Qf is calculated by adding the air-fuel ratio feedback correction amount Hqf to the basic injection amount Qfb, and also adding various increase correction amounts H according to the coolant temperature Tw, etc.

[0076] In S304, the fuel injectors 41 (41a, 41b) are driven with the fuel injection amount Qf.

[0077] In S211 and S304, the engine controller 101 outputs a drive pulse signal for liquid fuel injection corresponding to the fuel injection amount Qf to the drive circuit of the first fuel injector 41a during operation using liquid fuel, and outputs a drive pulse signal for gas fuel injection corresponding to the fuel injection amount Qf to the drive circuit of the second fuel injector 41b during operation using gas fuel.

[0078] Now, the processes from S206 to S209 in FIG. 4 will be described in more detail with reference to FIGS.

[0079] 9 to 12 are graphs of experimental data measured under a number of conditions by changing the catalyst temperature Tcat and exhaust flow rate Qexh, showing the change in the purification rate η of the exhaust purification catalyst and the downstream minimum air-fuel ratio λr_min relative to the air-fuel ratio oscillation frequency Frq in air-fuel ratio oscillation control. The purification rate η indicates the purification rate measured downstream of the second exhaust purification catalyst 332, that is, the purification rate obtained by the entire first and second exhaust purification catalysts 331, 332.

[0080] FIG. 9 shows data when the catalyst activity is high and the exhaust flow rate is low, FIG. 10 shows data when the catalyst activity is high and the exhaust flow rate is high, FIG. 11 shows data when the catalyst activity is low and the exhaust flow rate is low, and FIG. 12 shows data when the catalyst activity is low and the exhaust flow rate is high.

[0081] 9 to 12, the horizontal axis represents the logarithm log(Frq) of the frequency Frq, and the vertical axis represents the purification efficiency η and the downstream minimum air-fuel ratio λr_min. log(Frq)=0 corresponds to 1 [Hz]. The open squares represent the purification efficiency ηthc of total hydrocarbons THC, and the open circles represent the purification efficiency ηnox of nitrogen oxides NOx. The triangles represent the downstream minimum air-fuel ratio λr_min.

[0082] Here, the downstream minimum air-fuel ratio λr_min measured in the region on the lower frequency side than the frequency at which the purification rate η is maximized is indicated by an open triangle, and the downstream minimum air-fuel ratio λr_min measured in the region on the higher frequency side is indicated by a filled triangle. The thick dotted line is an approximation line of the downstream minimum air-fuel ratio λr_min in the region on the lower frequency side, and the thick solid line is an approximation line of the downstream minimum air-fuel ratio λr_min in the region on the higher frequency side.

[0083] 9 to 12, it can be seen that the downstream minimum air-fuel ratio λr_min changes discontinuously near the frequency at which the purification rate η is maximized, and that the slope of the change in the downstream minimum air-fuel ratio λr_min with respect to the frequency Frq (specifically, the logarithm of the frequency log(Frq)) also changes. By setting the frequency Frq on the horizontal axis to the logarithm log(Frq), it is possible to more clearly see the change in slope. In this embodiment, attention is focused on the downstream minimum air-fuel ratio λr_min and the frequency Frq at which its slope shows a discontinuous change, and this is called the "optimum frequency" and set as the control frequency Fcn. When taking the logarithm of the frequency Frq, it is preferable to set the predetermined value g01 to a value in the range from 0.015 to 0.025, which makes it possible to set an appropriate control frequency Fcn.

[0084] 13 and 14 are graphs showing the output waveforms of the upstream exhaust sensor 206 and the downstream exhaust sensor 207 in air-fuel ratio oscillation control, with Fig. 13 showing these output waveforms in the high-frequency (short-cycle) region and Fig. 14 showing them in the low-frequency (long-cycle) region. The "high-frequency region" in Fig. 13 is part of the "region on the higher frequency side than the frequency at which the purification rate η becomes maximum" in Figs. 9 to 12, and the "low-frequency region" in Fig. 14 is part of the "region on the lower frequency side than the frequency at which the purification rate η becomes maximum." In Figs. 13 and 14, thin solid lines indicate the upstream air-fuel ratio λf, and thick solid lines indicate the downstream air-fuel ratio λr. Dotted lines indicate the catalyst temperature Tcat.

[0085] 13, it can be seen that in the high-frequency range, regardless of an increase in the catalyst temperature Tcat, the oscillation of the downstream-side air-fuel ratio λr is greatly attenuated relative to the upstream-side air-fuel ratio λf. In contrast, in the low-frequency range, it can be seen that no attenuation of the downstream-side air-fuel ratio λr is observed, and in fact the oscillation is amplified. This is presumably due to the oxygen storage capacity (OSC) of the exhaust purification catalyst 331. That is, in the high-frequency range, the imbalance between fuel and air caused by the air-fuel ratio oscillation is compensated for by the oxygen storage capacity, and the air-fuel ratio of the exhaust gas after passing through the catalyst 331 (downstream air-fuel ratio λr) is maintained near a value equivalent to stoichiometry. In contrast, in the low-frequency range, the imbalance between fuel and air cannot be compensated for by the oxygen storage capacity, and the fluctuation in the air-fuel ratio in the mixture is directly reflected in the change in the downstream air-fuel ratio λr. As a result, at the boundary between the region where the oxygen storage capacity works effectively and the region where the oxygen storage capacity fails, the downstream minimum air-fuel ratio λr_min changes discontinuously, and the slope of the change in the downstream minimum air-fuel ratio λr_min with respect to the frequency Frq changes to an extent that can be determined by comparison with a threshold value.

[0086] Based on the above findings, the procedure for extracting the optimum frequency, that is, the procedure for specifying the control frequency Fcn according to this embodiment will be described below.

[0087] Air-fuel ratio oscillation control is started, and the frequency Frq of the air-fuel ratio oscillation is decreased from the reference frequency F0 in increments of a predetermined frequency ΔF. The minimum value of the air-fuel ratio λr downstream of the first exhaust purification catalyst 331, that is, the downstream minimum air-fuel ratio λr_min, is detected, and the amount of change in the downstream minimum air-fuel ratio λr_min detected before and after decreasing the frequency Frq by ΔF is calculated as the air-fuel ratio change amount Δλr_min. Then, by dividing the air-fuel ratio change amount Δλr_min by ΔF (in this embodiment, the amount of change in the logarithm log(Frq) of the frequency), the gradient glmba of the change in the downstream minimum air-fuel ratio λr_min with respect to the logarithm log(Frq) of the frequency is calculated, and the absolute value (=|glmba|) of this gradient is compared with a predetermined value g01. The frequency Frq when the absolute value (=|glmba|) of the gradient glmba reaches the predetermined value g01 is identified, and this is set as the control frequency Fcn. In this embodiment, the control frequency Fcn is set to a frequency at which the air-fuel ratio oscillations imparted to the exhaust by the air-fuel ratio oscillation control can be mitigated by the oxygen storage capacity of the first exhaust purification catalyst 331, in other words, a frequency that is low enough not to cause a decrease in the purification rate η.

[0088] As described above, instead of the method of comparing the gradient glmba of the change in the downstream minimum air-fuel ratio λr_min with a predetermined value, the control frequency Fcn can also be set by identifying the frequency Frq when a large change (i.e., a difference) that exceeds a predetermined value occurs in the downstream minimum air-fuel ratio λr_min before and after decreasing the frequency Frq by ΔF as the optimal frequency and setting the frequency Frq as the control frequency Fcn.

[0089] The reference frequency F0 is a value that is initially set when performing air-fuel ratio oscillation, and may be set uniformly for the entire operating range of the engine E, or may be set to a different value for each operating range. When the reference frequency F0 is set uniformly, 1 [Hz] (i.e., logF0=0) can be simply adopted as the reference frequency F0. Furthermore, an operating range map may be set so that the control frequency Fcn can be stored for each operating range, and the identified control frequency Fcn may be stored or updated for each operating range.

[0090] FIG. 6 is a flowchart showing the basic flow of the filter regeneration control (regeneration time detection process) according to this embodiment.

[0091] In S401, the inlet gas pressure Pexh1 and the outlet gas pressure Pexh2 are read.

[0092] In S402, the differential pressure across the filter ΔP is calculated. The differential pressure across the filter ΔP is calculated as the difference between the exhaust pressure Pexh1 detected by the inlet pressure sensor 208 and the exhaust pressure Pexh2 detected by the outlet pressure sensor 209 using the following equation (1). ΔP=Pexh1-Pexh2 …(1)

[0093] In S403, it is determined whether the filter differential pressure ΔP is greater than a first predetermined pressure ΔP1, that is, whether the deposition of particulate matter in the second exhaust purification catalyst 332 has progressed and the filter deposition amount Qgpf has increased, causing the filter differential pressure ΔP to exceed the first predetermined pressure ΔP1. The first predetermined pressure ΔP1 corresponds to the differential pressure formed before and after the second exhaust purification catalyst 332 when the filter deposition amount Qgpf has reached its upper limit (i.e., the upper limit deposition amount). If the filter differential pressure ΔP is greater than the first predetermined pressure ΔP1, the process proceeds to S404, and if it is equal to or less than the first predetermined pressure ΔP1, the filter regeneration control is terminated. The process of S403 corresponds to the process executed by the engine controller 101 as the "regeneration timing detection means" according to this embodiment.

[0094] In S404, filter regeneration is performed according to the procedure shown in the flowchart of FIG.

[0095] FIG. 7 is a flowchart showing the flow of filter regeneration control (regeneration execution process).

[0096] In S501, it is determined whether the fuel being used is gas fuel. If the fuel being used is gas fuel, the process proceeds to S503, and if the fuel being used is liquid fuel, the process proceeds to S502.

[0097] In S502, the fuel used is switched from liquid fuel to gas fuel. As a result, if the fuel being used is gas fuel when filter regeneration begins, the use of gas fuel continues, and if the fuel being used is liquid fuel, the fuel being used is switched to gas fuel. If the use of liquid fuel is due to a lack of fuel in the gas fuel tank, the switch to gas fuel is prohibited, and liquid fuel can continue to be used when filter regeneration is performed.

[0098] In S503, the control frequency Fcn is read. For the air-fuel ratio oscillation control during filter regeneration, the control frequency Fcn that was identified and set when performing the air-fuel ratio oscillation control during normal operation of the engine E, specifically during normal operation before the current filter regeneration, that is, the control frequency Fcn that was set by the processing shown in S206 to S209 of the flowchart in Fig. 4 is used. If the control frequency Fcn is in an operating region where setting has not yet been completed, it may be set to the reference frequency F0 at the start of regeneration, and the control frequency Fcn may be identified and used during filter regeneration, or a control frequency Fcn that has already been set for an adjacent operating region may be used approximately as the control frequency for that operating region.

[0099] In S504, air-fuel ratio oscillation control is performed using the control frequency Fcn. By performing air-fuel ratio oscillation control, particulate matter accumulated in the second exhaust purification catalyst 332 is burned by reacting with oxygen, and the filter differential pressure ΔP decreases. Here, while air-fuel ratio oscillation control is being performed, in the first exhaust purification catalyst 331, the oxygen storage capacity of the CeZr-based catalyst, which is a catalyst component, is used to repeatedly store excess oxygen in the exhaust and release the stored oxygen from the catalyst, and the air-fuel ratio of the exhaust passing through the first exhaust purification catalyst 331 is adjusted to a value equivalent to stoichiometry. Due to the quantitative balance between unburned components and oxygen contained in the exhaust, oxidation of the unburned components progresses and the temperature of the exhaust flowing into the second exhaust purification catalyst 332 rises, which has the effect of heating the second exhaust purification catalyst 332 and promotes regeneration processing by burning the particulate matter. The processing of S503 and S504 corresponds to processing performed by the engine controller 101 as a "regeneration implementation means" according to this embodiment.

[0100] In S505, it is determined whether the pressure difference across the filter ΔP is equal to or less than a second predetermined pressure ΔP2, that is, whether the regeneration of the second exhaust purification catalyst 332 has progressed due to the combustion of particulate matter, causing the pressure difference across the filter ΔP to decrease and reach the second predetermined pressure ΔP2. The second predetermined pressure ΔP2 is a pressure used as a reference for determining whether the regeneration of the second exhaust purification catalyst 332 has been completed, and has a value smaller than the first predetermined pressure ΔP1. If the pressure difference across the filter ΔP is equal to or less than the second predetermined pressure ΔP2, the process proceeds to S506, and if it is greater than the second predetermined pressure ΔP2, the process returns to S504, and filter regeneration by air-fuel ratio oscillation control continues.

[0101] In S506, the air-fuel ratio oscillation control is stopped and filter regeneration is terminated. Accordingly, the switch to gaseous fuel is canceled and the fuel used is restored to the fuel used before the switch. In other words, if liquid fuel was selected as the fuel used and the switch to gaseous fuel was made when filter regeneration was performed, the fuel used is restored to liquid fuel upon completion of filter regeneration.

[0102] FIG. 8 is a graph showing the change in the amount of particulate matter deposited (the pressure difference ΔP across the filter) before and after filter regeneration.

[0103] While the engine E is operating, particulate matter contained in the exhaust gas after combustion passes through the first exhaust purification catalyst 331 and accumulates on the second exhaust purification catalyst 332 (specifically, the filter substrate). As the accumulation of particulate matter progresses and the filter accumulation amount Qgpf increases, the differential pressure across the filter ΔP increases. When the differential pressure across the filter ΔP exceeds a first predetermined pressure (regeneration start pressure) ΔP1, it is determined that it is time to regenerate the second exhaust purification catalyst 332, and filter regeneration by air-fuel ratio oscillation control begins. The particulate matter accumulated on the second exhaust purification catalyst 332 decreases due to combustion, and the differential pressure across the filter ΔP decreases. When the differential pressure across the filter ΔP reaches a second predetermined pressure (regeneration completion pressure) ΔP2, it is determined that regeneration of the second exhaust purification catalyst 332 is completed, and the air-fuel ratio oscillation control is stopped, and filter regeneration ends.

[0104] The exhaust gas purification device for the engine E according to this embodiment has the above-described configuration. The effects obtained by this embodiment will be described below.

[0105] First, in the exhaust purification catalysts 331, 332, a first catalyst supporting region and a second catalyst supporting region are formed separately in the direction of exhaust flow, and the upstream first catalyst supporting region supports a CeZr-based catalyst as a catalytic component that substantially contributes to purifying harmful exhaust components, while the downstream second catalyst supporting region supports an alumina-based catalyst as a catalytic component.

[0106] As a result, by carrying out air-fuel ratio oscillation control during operation using gaseous fuel, it is possible to achieve both exhaust purification performance in low temperature ranges and exhaust purification performance in high temperature ranges.

[0107] In this embodiment, a first exhaust purification catalyst 331 and a second exhaust purification catalyst 332 are installed, and a first catalyst supporting region is formed on the carrier of the first exhaust purification catalyst 331, and a second catalyst supporting region is formed on the carrier of the second exhaust purification catalyst 332, respectively, thereby making it possible to satisfactorily achieve the function expected of a CeZr-based catalyst (the function of mitigating air-fuel ratio oscillations through oxygen storage capacity) and the function expected of an alumina-based catalyst (the function of efficiently purifying exhaust gas under stoichiometric conditions).

[0108] 15 and 16 are graphs of experimental data showing the methane conversion efficiency ηch4 for different catalyst configurations. In each of Fig. 15 and Fig. 16, the horizontal axis represents the catalyst temperature Tcat, and the vertical axis represents the methane conversion efficiency ηch4.

[0109] 15 shows the methane conversion efficiency ηch4 for each catalyst configuration when air-fuel ratio oscillation is applied. In the figure, solid circles represent the catalyst configuration according to this embodiment (i.e., a configuration in which a CeZr-based catalyst and an alumina-based catalyst are supported on different supports), open triangles represent the configuration in which only a CeZr-based catalyst is supported, and solid triangles represent the configuration in which only an alumina-based catalyst is supported. Furthermore, open squares represent the configuration in which a CeZr-based catalyst and an alumina-based catalyst are mixed and supported on a common support, as in a typical three-way catalyst, and open circles represent the configuration in which the alumina-based catalyst is supported on the upstream side and the CeZr-based catalyst is supported on the downstream side.

[0110] 16 shows the methane purification efficiency (ηch4) under conditions in which air-fuel ratio oscillation is applied to the exhaust gas, and under conditions in which the air-fuel ratio of the exhaust gas is adjusted to a steady stoichiometric ratio, for different catalyst configurations. In the figure, the solid circles represent the catalyst configuration according to this embodiment, the open triangles represent the configuration in which only a CeZr-based catalyst is supported, and the open squares represent the configuration in which a CeZr-based catalyst and an alumina-based catalyst are mixed and supported on a common support, as in a conventional three-way catalyst, under conditions in which air-fuel ratio oscillation is applied to the exhaust gas. The solid triangles represent the configuration in which only an alumina-based catalyst is supported under conditions in which the air-fuel ratio of the exhaust gas is adjusted to a steady stoichiometric ratio.

[0111] 15 and 16, the volume of the catalyst component, the amount of precious metal to be supported, and the conditions related to the exhaust are all approximately the same. Specifically, the length and cross-sectional area of ​​the carrier supporting the catalyst component, the mass of precious metal per unit volume of the carrier, and the exhaust composition, flow rate, and temperature are all the same. In the catalyst configuration according to this embodiment, the carrier supporting the CeZr-based catalyst and the carrier supporting the alumina-based catalyst each have the same cross-sectional area, and are half the length of the catalyst supporting only the CeZr-based catalyst and half the length of the catalyst supporting only the alumina-based catalyst.

[0112] In the case of the catalyst configuration according to this embodiment, as shown in FIG. 15, low-temperature performance equivalent to or better than that obtained when only a CeZr-based catalyst is supported and air-fuel ratio oscillation is applied to the exhaust gas, and high-temperature performance equivalent to or better than that obtained when only an alumina-based catalyst is supported and the air-fuel ratio of the exhaust gas is adjusted to steady stoichiometry, is also obtained, as shown in FIG. 16.

[0113] Specifically, comparing the case of the catalyst configuration according to this embodiment (filled circles) with the case of a configuration supporting only a CeZr-based catalyst (open triangles) in Fig. 15, it can be seen that the curve for the methane purification rate ηch4 for the catalyst configuration according to this embodiment tends to rise from a lower catalyst temperature Tcat. Furthermore, comparing the case of the catalyst configuration according to this embodiment (filled circles) with the case of a configuration supporting only an alumina-based catalyst (filled triangles) in Fig. 16, it can be seen that the catalyst configuration according to this embodiment achieves a final purification rate (methane purification rate ηch4 in the high temperature range) that is nearly 100% as compared to a case in which only an alumina-based catalyst is supported and the exhaust air-fuel ratio is adjusted to steady stoichiometry.

[0114] In contrast, when a CeZr-based catalyst and an alumina-based catalyst are mixed and supported on a common carrier (open square), or when an arrangement opposite to that of this embodiment (open circle) is used, the low-temperature performance and high-temperature performance are inferior to those of the catalyst configuration of this embodiment.

[0115] FIG. 17 is a graph of experimental data showing the methane conversion efficiency ηch4 by the CeZr-based catalyst and the alumina-based catalyst under different exhaust conditions.

[0116] Figure 17(a) shows the methane purification efficiency ηch4 by the CeZr-based catalyst when air-fuel ratio oscillation is applied to the exhaust gas (solid circles) and when the air-fuel ratio of the exhaust gas is adjusted to a steady stoichiometric ratio (open circles).

[0117] Figure 17(b) shows the methane purification rate ηch4 by the alumina-based catalyst when air-fuel ratio oscillation is applied to the exhaust gas (solid circles) and when the air-fuel ratio of the exhaust gas is adjusted to a steady stoichiometric ratio (open circles).

[0118] 17(a) and 17(b), the air-fuel ratio oscillation applied to the exhaust gas is set to a frequency of 0.2 Hz, with the rich and lean air-fuel ratios converted to an excess air ratio of 0.95 and 1.03, respectively. The air-fuel ratio of the exhaust gas when adjusted to a steady stoichiometric ratio is set to 0.99, converted to an excess air ratio.

[0119] By imparting air-fuel ratio oscillation to the exhaust, CeZr-based catalysts exhibit a higher methane purification rate ηch4 across the entire temperature range, including low and high temperatures, than when the exhaust air-fuel ratio is adjusted to a steady stoichiometric range. CeZr-based catalysts exhibit particularly excellent low-temperature performance when air-fuel ratio oscillation is imparted to the exhaust, but at high temperatures, they tend not to achieve a 100% methane purification rate ηch4, even when air-fuel ratio oscillation is imparted to the exhaust.

[0120] In contrast, alumina-based catalysts exhibit a higher methane purification rate ηch4 when the exhaust air-fuel ratio is adjusted to a steady stoichiometric ratio than when air-fuel ratio oscillation is applied to the exhaust. Alumina-based catalysts excel in high-temperature performance when the exhaust air-fuel ratio is adjusted to a steady stoichiometric ratio, achieving a methane purification rate ch4 of nearly 100%, but tend to exhibit slightly inferior low-temperature performance under all exhaust conditions.

[0121] Therefore, in the case of the catalyst configuration according to this embodiment, by performing air-fuel ratio oscillation control, it is possible to achieve a high methane purification rate ηch4 over the entire temperature range from low to high temperatures.

[0122] In the low temperature range, the CeZr-based catalyst placed upstream causes the curve for the methane conversion rate ηch4 to rise from a lower catalyst temperature Tcat, making it possible to achieve high low-temperature performance.

[0123] Furthermore, even though the methane purification rate ηch4 of the CeZr-based catalyst is not sufficient in the high temperature range, the oxygen storage capacity of the CeZr-based catalyst alleviates air-fuel ratio fluctuations in the exhaust gas, and the exhaust gas flowing into the alumina-based catalyst located downstream is adjusted to a state close to the steady stoichiometry, making it possible to achieve high high-temperature performance for the entire catalyst system, including the CeZr-based catalyst and the alumina-based catalyst.

[0124] It is presumed that with a CeZr-based catalyst, a purification reaction proceeds in which methane in the exhaust gas is oxidized by oxygen released from the catalyst in the early stages of the transition after the exhaust gas air-fuel ratio shifts from the lean side to the rich side, while with an alumina-based catalyst, a purification reaction proceeds in which the remaining methane is oxidized by oxygen in the exhaust gas in a state close to the steady stoichiometric ratio.

[0125] Secondly, in addition to the above, by using methane-based fuel as gaseous fuel, it is possible to suppress the generation of soot itself due to combustion, thereby suppressing the accumulation of particulate matter in the second exhaust purification catalyst 332 and extending the regeneration interval of the second exhaust purification catalyst 332. And, by suppressing the accumulation of particulate matter when using gaseous fuel, the degree of freedom in using or selecting liquid fuel increases accordingly, so it is possible to further promote the compatibility of improving the output of the engine E and improving the properties of the exhaust.

[0126] While the compressed natural gas used as fuel in this embodiment can reduce carbon dioxide emissions compared to gasoline, methane, a major component of exhaust gas, is known to have a greenhouse effect approximately 25 times greater than that of carbon dioxide. Therefore, reducing methane emissions is important in order to ensure the benefits over gasoline. However, methane is chemically more stable than other HC species, and it is difficult to achieve a sufficient purification rate by oxidizing it in the same way as other HC species.

[0127] In contrast, air-fuel ratio oscillation control in which the control frequency Fcn is appropriately adjusted can specifically improve the methane purification rate, making it possible to more effectively use compressed natural gas as fuel.

[0128] Thirdly, based on the air-fuel ratio λr detected by the downstream exhaust sensor 207, an appropriate frequency is identified as the frequency of the air-fuel ratio oscillation (control frequency Fcn), and air-fuel ratio oscillation control can be performed at a more appropriate control frequency Fcn regardless of the operating state of the engine E or changes in the properties of the exhaust purification catalysts 331, 332 (for example, deterioration of the catalyst components), making it possible to further improve the purification rate of the exhaust purification catalysts 331, 332.

[0129] Here, according to a configuration in which the downstream exhaust sensor 207 is disposed between the first exhaust purification catalyst 331 and the second exhaust purification catalyst 332, it becomes possible to avoid a situation in which the second exhaust purification catalyst 332 becomes a disturbance factor for the detection of the downstream air-fuel ratio λr, causing the control frequency Fcn to deviate from the appropriate value.

[0130] Fourth, by calculating the gradient glmba of the change in the minimum value of the air-fuel ratio λr detected by the downstream exhaust sensor 207 (downstream minimum air-fuel ratio λr_min) relative to the logarithm of the frequency of the air-fuel ratio oscillation, and specifying the frequency at which the absolute value of this gradient (=|glmba|) reaches a predetermined value g01 as the control frequency Fcn, it becomes possible to specify an appropriate control frequency Fcn with a relatively simple configuration, and to execute air-fuel ratio oscillation control based on a more appropriate control frequency Fcn.

[0131] By setting the predetermined value g01 in the range of 0.015 to 0.025, it becomes possible to specify a more appropriate frequency as the control frequency Fcn.

[0132] Fifth, by using a filter carrier as the carrier of the second exhaust purification catalyst 332, it is possible to remove particulate matter in the exhaust in addition to purifying harmful exhaust components (gas components), making it possible to configure the exhaust purification device for engine E more compactly as a whole.

[0133] In the above explanation, the setting of the control frequency Fcn focuses on the gradient of change glmba of the downstream minimum air-fuel ratio λr_min with respect to the frequency Frq or the difference between the downstream minimum air-fuel ratio λr_min before and after the decrease in frequency Frq. The setting of the control frequency Fcn is not limited to this, and can also be performed by focusing on the gradient of change Rlmb between the maximum value (hereinafter referred to as the "downstream maximum air-fuel ratio") λr_max and the minimum value λr_min of the air-fuel ratio λr detected by the downstream exhaust sensor 207 with respect to the frequency Frq.

[0134] FIG. 18 is a flowchart showing the basic flow of air-fuel ratio oscillation control according to another embodiment of the present invention, as an application example in this case.

[0135] 4, the air-fuel ratio range Rlmb (=λr_max-λr_min) is calculated (S601), and instead of the process of S208, the gradient glmbb of the change in the air-fuel ratio range Rlmb with respect to the frequency Frq or its logarithm log(Frq) is calculated, and the frequency at which the absolute value of this gradient (=|glmbb|) reaches a predetermined value g02 is identified as the optimal frequency and set as the control frequency Fcn (S602). When taking the logarithm of the frequency Frq, it is preferable to set the predetermined value g02 in the range from 0.015 to 0.025, which makes it possible to set an appropriate control frequency Fcn.

[0136] 19 and 20 are graphs of experimental data showing the relationship between the frequency Frq of air-fuel ratio oscillation in the air-fuel ratio oscillation control according to this embodiment, the purification rate η of the exhaust purification catalyst, and the air-fuel ratio range Rlmb, for different catalyst temperatures Tcat. Fig. 19(a) shows the case of a relatively low catalyst temperature Tcat1, and Fig. 19(b) shows the case of a catalyst temperature Tcat2 that is higher than Tcat1. Fig. 20(a) shows the case of a catalyst temperature Tcat3 that is higher than Tcat2, and Fig. 20(b) shows the case of a catalyst temperature Tcat4 that is higher than Tcat3.

[0137] 19 and 20, similarly to FIGS. 9 to 12, the purification rate η indicates the purification rate obtained by the first and second exhaust purification catalysts 331, 332 as a whole.

[0138] 19 and 20, the horizontal axis represents the logarithm of the frequency Frq, and the vertical axis represents the purification efficiency η and the air-fuel ratio range Rlmb. The open squares represent the total hydrocarbon (THC) purification efficiency ηthc, and the open circles represent the nitrogen oxide (NOx) purification efficiency ηnox. The triangles represent the air-fuel ratio range Rlmb, with open triangles representing measurements in the lower frequency region than the frequency at which the purification efficiency η is maximized, and filled triangles representing measurements in the higher frequency region. The thick dotted line is an approximation straight line of the air-fuel ratio range Rlmb in the lower frequency region, and the thick solid line is an approximation straight line of the air-fuel ratio range Rlmb in the higher frequency region.

[0139] 19 and 20, it can be seen that the purification efficiency η increases for both total hydrocarbons THC and nitrogen oxides NOx as the catalyst temperature Tcat rises, and that a high purification efficiency η is maintained up to a higher frequency Frq. Here, when the frequency Frq is decreased in increments of a predetermined frequency ΔF, it can be seen that in the vicinity of the frequency at which the purification efficiency η is maximized, the approximation curve for the air-fuel ratio range Rlmb shifts from the solid line to the dotted line, and the gradient glmbb of the change with respect to the frequency Frq and the absolute value of that gradient (=|glmbb|) increase.

[0140] In this embodiment, the frequency at which this phenomenon occurs is extracted as a singular point by comparing it with a predetermined value g02, and set as the control frequency Fcn.

[0141] In this way, by calculating the gradient glmbb of the change that the difference between the maximum and minimum values ​​of the air-fuel ratio detected by the downstream exhaust sensor 207 (air-fuel ratio range Rlmb) makes with respect to the logarithm of the frequency Frq of the air-fuel ratio oscillation, and specifying the frequency at which the absolute value of this gradient (=|glmbb|) reaches a predetermined value g02 as the control frequency Fcn, it is possible to specify an appropriate control frequency Fcn with a relatively simple configuration, and it becomes possible to execute air-fuel ratio oscillation control based on a more appropriate control frequency Fcn.

[0142] By setting the predetermined value g02 in the range of 0.015 to 0.025, it becomes possible to specify a more appropriate frequency as the control frequency Fcn.

[0143] Setting the control frequency Fcn based on the air-fuel ratio range Rlmb is not limited to engine E in which multiple cylinders are connected in parallel as shown in Figure 1, but can also be applied to engines in which multiple cylinders are divided into multiple cylinder groups and different cylinder groups are connected in parallel. For example, an engine in which four cylinders are divided into two cylinder groups, different cylinder groups are connected in parallel, and each cylinder constituting a cylinder group is connected in parallel. Setting the control frequency Fcn based on the air-fuel ratio range Rlmb is preferably applied to such engine E when the phases of the air-fuel ratio oscillations between different cylinder groups are set to be opposite to each other, in other words, when the phases of the air-fuel ratio oscillations are set to be inverted.

[0144] In the above description, the frequency when air-fuel ratio oscillation control is performed, i.e., the control frequency Fcn, is determined as the frequency at which the downstream minimum air-fuel ratio λr_min or the air-fuel ratio range Rlmb reaches a predetermined value based on the slope of change relative to the logarithm of the air-fuel ratio oscillation frequency Frq. The control frequency Fcn can also be determined by detecting the amplitude of the downstream air-fuel ratio λr and determining a frequency that can keep this amplitude within a predetermined range. For example, the frequency of the air-fuel ratio oscillation is gradually reduced from 1 Hz while detecting the amplitudes of the upstream air-fuel ratio λf and the downstream air-fuel ratio λr. The frequency at which the amplitude of the downstream air-fuel ratio λr falls within 5% of the amplitude of the upstream air-fuel ratio λf is determined and set as the control frequency Fcn.

[0145] Furthermore, the carrier used for the second exhaust purification catalyst 332 is not limited to a filter carrier, but may be the same carrier as that used for the first exhaust purification catalyst 331, for example, a ceramic honeycomb carrier, and the second exhaust purification catalyst 332 may be configured such that predetermined catalytic components are carried on this carrier. The engine E is not limited to a bi-fuel engine, but may be one that uses only gaseous fuel as fuel.

[0146] A mixing promotion means may be installed between the region where the CeZr-based catalyst is supported and the region where the alumina-based catalyst is supported, for example, between the first exhaust gas purification catalyst 331 and the second exhaust gas purification catalyst 332. This promotes turbulence or diffusion of the exhaust gas that has passed through the first exhaust gas purification catalyst 331, making it possible to bring the exhaust gas flowing into the second exhaust gas purification catalyst 332 closer to a steady stoichiometric state. Various means, including baffles, vortex generators, and heaters, can be used as the mixing promotion means. It is also possible to promote exhaust gas turbulence by installing a third support separate from the first and second supports between the first and second exhaust gas purification catalysts 331 and 332 and flowing the exhaust gas through this support. For example, multiple holes may be formed through the partition wall of the third support in the thickness direction, and exhaust gas turbulence may be promoted by flowing the exhaust gas through these multiple holes. The third support may be made of metal, or may have a zeolite coating on the surface that contacts the exhaust gas. [Explanation of symbols]

[0147] E...engine, 1...engine body, 2...intake system, 21...intake pipe, 22...intake manifold, 23...air cleaner, 3...exhaust system, 31...exhaust manifold, 32...exhaust pipe, 33...catalytic converter, 331...exhaust purification catalyst, 332...second exhaust purification catalyst, 41...fuel injector, 41a...first fuel injector (for liquid fuel), 41b...second fuel injector (for gaseous fuel), 51...spark plug, 101...engine controller, 201...accelerator sensor, 202...engine rotation speed sensor, 203...air flow meter, 204...coolant temperature sensor, 205...catalyst temperature sensor, 206...upstream exhaust sensor, 207...downstream exhaust sensor, 208...filter inlet pressure sensor, 209...filter outlet pressure sensor, 210a, 210b...fuel level sensor, 211...fuel selector switch.

Claims

1. An exhaust gas purification device for an engine configured to be operable by supplying gaseous fuel to a combustion chamber, an exhaust purification catalyst installed in an exhaust passage of the engine; an operating condition sensor configured to be able to output a signal corresponding to the operating condition of the engine; a controller that controls the operating state of the engine based on the signal output by the operating state sensor, the controller comprises air-fuel ratio oscillation control means for performing air-fuel ratio oscillation control that oscillates the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst between a rich side and a lean side with respect to a stoichiometric equivalent value during operation using the gaseous fuel, The exhaust purification catalyst is a first catalyst supporting region in which only a CeZr-based catalyst is supported among a CeZr-based catalyst and an alumina-based catalyst as a catalytic component that contributes to purifying harmful exhaust components; an exhaust gas purification device for an engine, the device comprising: a second catalyst supporting region that is provided downstream of the first catalyst supporting region in terms of the flow of exhaust gas, the second catalyst supporting region supporting only the alumina-based catalyst out of the CeZr-based catalyst and the alumina-based catalyst as the catalytic component;

2. The exhaust purification catalyst is a first exhaust purification catalyst having a first support as a support for supporting the catalytic component, the first support having the first catalyst supporting region formed thereon; an exhaust purification catalyst disposed downstream of the first exhaust purification catalyst with respect to the flow of the exhaust gas, the second exhaust purification catalyst having a second carrier provided separately from the first carrier as a carrier for supporting the catalytic component, the second catalyst supporting region being formed on the second carrier;

3. the operating condition sensor includes a downstream exhaust sensor that is installed in the exhaust passage downstream of the first exhaust purification catalyst and detects an air-fuel ratio of the exhaust gas that has passed through the first exhaust purification catalyst, the controller further includes control frequency specifying means for specifying, as a control frequency, a frequency at which the air-fuel ratio is oscillated by the air-fuel ratio oscillation control, based on the air-fuel ratio detected by the downstream exhaust sensor; 3. The engine exhaust purification device according to claim 2, wherein the air-fuel ratio oscillation control means oscillates the air-fuel ratio of the exhaust gas at the control frequency after the control frequency is specified by the control frequency specifying means.

4. 4. The exhaust gas purification device for an engine according to claim 3, wherein the downstream side exhaust sensor is installed in the exhaust passage between the first exhaust gas purification catalyst and the second exhaust gas purification catalyst.

5. 4. The engine exhaust gas purification device according to claim 3, wherein the control frequency specifying means specifies, as the control frequency, the frequency at which a slope of a change in a minimum value of the air-fuel ratio detected by the downstream exhaust sensor or a difference between the maximum and minimum values ​​of the air-fuel ratio with respect to a logarithm of a frequency of air-fuel ratio oscillation reaches a predetermined value.

6. 6. The engine exhaust gas purification device according to claim 5, wherein the control frequency specifying means specifies, as the control frequency, the frequency when the absolute value of the gradient reaches a preset value in a range from 0.015 to 0.

025.

7. 3. The engine exhaust purification device according to claim 2, wherein the second exhaust purification catalyst has, as the second support, a filter support configured to be able to capture particulate matter in the exhaust gas.

8. The controller a regeneration time detection means for detecting that the second exhaust purification catalyst is at a time to regenerate from the accumulation of the particulate matter when the amount of particulate matter accumulated on the second support reaches an upper limit of the accumulation amount; 8. The exhaust gas purification device for an engine according to claim 7, further comprising: a regeneration implementation means that implements the air-fuel ratio oscillation control when the regeneration timing detection means detects that the second exhaust gas purification catalyst is in the regeneration timing.

9. The exhaust gas purification device for an engine according to any one of claims 1 to 8, wherein the gaseous fuel is a methane-based fuel.

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