Exhaust emission control device for engine

Abstract

To provide an exhaust emission control device for an engine capable of earlier completing regeneration of an exhaust filter.SOLUTION: An exhaust emission control device for an engine includes: an exhaust emission control catalyst 331 installed in an exhaust passage 32 of the engine E; an exhaust filter 332 installed downstream of the exhaust emission control catalyst 331; operating state sensors 201-211; and a controller 101 that controls an operating state of the engine E. When detecting that the exhaust filter 332 reaches its regeneration timing, the controller 101 performs air-fuel ratio vibration control for vibrating an air-fuel ratio of exhaust gas flowing into the exhaust emission control catalyst 331 to a rich side and a lean side of a stoichiometric equivalent value, and forcedly sets gas fuel as fuel to be used when performing the air-fuel ratio vibration control.SELECTED DRAWING: Figure 7

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 switch between liquid fuel and gas fuel for operation. This engine is called a bi-fuel engine, and is equipped with a liquid fuel tank that stores liquid fuel and a gas fuel tank that stores gas fuel, and the fuel supplied to the combustion chamber can be switched depending on the remaining amount in the fuel tank or the driver's selection. A known bi-fuel engine uses gasoline as the liquid fuel and compressed natural gas (CNG) as the gas fuel.

[0003] On the other hand, in an engine equipped with an exhaust filter in which an oxidation catalyst is carried on a filter carrier, when particulate matter accumulates on the filter carrier in an amount exceeding a standard value, it is known to operate the engine by switching the air-fuel ratio of the exhaust gas that comes into contact with the oxidation catalyst between rich and lean in order to regenerate the exhaust filter (Patent Document 1).

[0004] 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]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-299521 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when exhaust filter regeneration is performed by switching the exhaust air-fuel ratio between rich and lean, the following problem arises.

[0007] When gasoline is used as fuel, switching between rich and lean exhaust air-fuel ratios increases the amount of particulate matter contained in the exhaust due to the relative increase in fuel when the exhaust air-fuel ratio is made rich. This particulate matter flows into the exhaust filter undergoing regeneration and accumulates on the filter substrate, causing the exhaust filter to take longer to complete regeneration.

[0008] As a result, pressure loss in the exhaust filter may not be resolved for a long period of time, which may result in a deterioration in engine performance.

[0009] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an exhaust gas purification device for an engine that can complete the regeneration of an exhaust filter more quickly. [Means for solving the problem]

[0010] In order to solve the above problems, an exhaust purification device for an engine according to one aspect of the present invention is an engine that can be operated by switching between liquid fuel and gaseous fuel as a used fuel supplied to a combustion chamber, and includes: an exhaust purification catalyst installed in an exhaust passage of the engine; an exhaust filter installed in the exhaust passage downstream of the exhaust purification catalyst, supporting a catalyst component and configured to be able to capture particulate matter in the exhaust; an operating condition sensor configured to be able to output a signal corresponding to an 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 filter regeneration time detection means that detects that the exhaust filter is due to be regenerated, filter regeneration means that, when the filter regeneration time detection means detects that the exhaust filter is due to be regenerated, performs air-fuel ratio oscillation control to oscillate the air-fuel ratio of the exhaust flowing into the exhaust purification catalyst between rich and lean sides with respect to a value equivalent to stoichiometry, and used fuel setting means that forcibly sets the used fuel to gaseous fuel when the air-fuel ratio oscillation control is performed by the filter regeneration means. [Effects of the Invention]

[0011] According to the present invention, by performing air-fuel ratio oscillation control when it is detected that the exhaust filter is due for regeneration, it is possible to burn particulate matter accumulated on the exhaust filter by reacting with oxygen, thereby promoting exhaust filter regeneration. Here, by setting the fuel used when performing air-fuel ratio oscillation control to gaseous fuel, it is possible to suppress the generation of soot due to combustion and reduce the amount of particulate matter that flows into the exhaust filter during regeneration. This shortens the time required to regenerate the exhaust filter, making it possible to complete regeneration more quickly. [Brief explanation of the drawings]

[0012] [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] 10 is a flowchart showing a basic flow of air-fuel ratio oscillation control according to another embodiment of the present invention. [Figure 16] 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 17] 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

[0013] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] The exhaust system 3 includes an exhaust manifold 31 and an exhaust pipe 32, as well as a catalytic converter 33. After combustion, 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 an exhaust purification catalyst 331 and an exhaust filter 332 housed in the catalytic converter 33, in that order. Harmful exhaust components including total hydrocarbons (THC) and nitrogen oxides (NOx) are purified from the exhaust gas by the exhaust purification catalyst 331, and further, particulate matter in the exhaust is collected by the exhaust filter 332 before being released into the atmosphere.

[0025] The exhaust purification catalyst 331 has a structure in which a catalytic component is supported on a ceramic honeycomb carrier, and the exhaust filter 332 has a structure in which a catalytic component is supported on a filter element. The exhaust filter 332 is a so-called wall-flow type exhaust filter, also known as a gasoline particulate filter (GPF), and has the same honeycomb structure as the carrier of the exhaust purification catalyst 331, with partition walls that define the internal space being 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. A three-way catalyst can be used as the catalytic component supported on both the exhaust purification catalyst 331 and the exhaust filter 332.

[0026] The exhaust filter 332 can be accommodated not only in a common housing together with the exhaust purification catalyst 331 (in this embodiment, the housing of the catalytic converter 33), but also in a housing separate from that of the exhaust purification catalyst 331. Specifically, the housing of the exhaust purification catalyst 331 and the housing of the exhaust filter 332 are formed separately, and the exhaust purification catalyst 331 and the exhaust filter 332 are accommodated in their own housings, and both housings are arranged in series and connected to each other via an exhaust pipe.

[0027] In addition to the above, the engine E includes an engine controller 101 and various sensors 201-211.

[0028] 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.

[0029] 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, a filter inlet pressure sensor 208, a filter 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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").

[0034] The catalyst temperature sensor 205 detects the temperature Tcat (hereinafter referred to as "catalyst temperature") of the exhaust purification catalyst 331 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 exhaust purification catalyst 331 (hereinafter referred to as "catalyst inlet gas temperature") is detected as the catalyst temperature Tcat.

[0035] In this way, in this embodiment, the temperature of the exhaust gas close to the temperature of the 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 through the exhaust pipe between the exhaust purification catalyst 331 and the exhaust filter 332, or the temperature of the carrier of the exhaust purification catalyst 331 (catalyst bed temperature).

[0036] 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 exhaust purification catalyst 331 (hereinafter referred to as the "upstream air-fuel ratio").

[0037] The downstream exhaust sensor 207 is installed in the exhaust pipe 32 downstream of the catalytic converter 33, and detects the air-fuel ratio λr of the exhaust that has passed through the exhaust purification catalyst 331 (hereinafter referred to as the "downstream air-fuel ratio").

[0038] The filter inlet side pressure sensor 208 is installed in the exhaust pipe 32 upstream of the exhaust filter 332, and detects the pressure Pexh1 of the exhaust gas before it flows into the exhaust filter 332 (hereinafter referred to as "filter inlet gas pressure"). The filter inlet side pressure sensor 208 constitutes the "first pressure sensor" according to this embodiment.

[0039] The filter outlet side pressure sensor 209 is installed in the exhaust pipe 32 downstream of the exhaust filter 332, and detects the pressure Pexh2 of the exhaust gas that has passed through the exhaust filter 332 (hereinafter referred to as the "filter outlet gas pressure"). The filter outlet side pressure sensor 209 constitutes the "second pressure sensor" according to this embodiment.

[0040] 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.

[0041] 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.

[0042] 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 211 described above, while controlling the operating state of the engine E. The engine controller 101 constitutes the "controller" according to this embodiment.

[0043] The engine controller 101 switches between air-fuel ratio oscillation control and air-fuel ratio feedback control in the air-fuel ratio control during normal operation of the engine E. The air-fuel ratio oscillation control 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 air-fuel ratio feedback control 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 the value equivalent to stoichiometry.

[0044] 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.

[0045] Here, catalyst temperature Tcat is an indicator of the activity state of exhaust purification catalyst 331, and catalyst temperature Tcat being equal to or higher than a predetermined temperature Tcat1 indicates that activation of exhaust purification catalyst 331 has progressed and that exhaust purification catalyst 331 is 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 catalyst 331, and exhaust flow rate Qexh being equal to or lower than a predetermined flow rate Qexh1 indicates that the amount of harmful exhaust substances that need to be purified is relatively small and that the conditions are such that processing by the highly activated exhaust purification catalyst 331 is easy. In other words, performing air-fuel ratio feedback control in operating range B means that air-fuel ratio feedback control is performed when activation of exhaust purification catalyst 331 has progressed and it is determined that processing of harmful exhaust components by exhaust purification catalyst 331 is easy.

[0046] FIG. 3 is an operating region map showing the implementation regions of air-fuel ratio oscillation control and air-fuel ratio feedback control. 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 catalyst 331 is advanced, and the conditions are such that processing by the exhaust purification catalyst 331 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 catalyst 331 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 catalyst 331 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.

[0047] Furthermore, in this embodiment, the fuel used to operate the engine E is switched depending on the amount of fuel remaining in the fuel tank or a selection made by the driver.

[0048] 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 or a driver's selection. The fuel can also be switched depending on the operating state of the engine E, for example, the required engine load. For example, the fuel used is switched from liquid fuel to gas fuel when the remaining amount in the liquid fuel tank decreases and reaches a predetermined remaining amount while the engine is running on liquid fuel, or when the required engine load is less than the predetermined load. As an example of switching depending on the engine load, gas fuel is selected when the engine E is in an operating range other than a predetermined high-load range, specifically, when the engine E is not in a high-speed driving range (for example, an operating range in which the load ratio of the engine E relative to the full load is 80% or more), and liquid fuel is selected when the engine is in a high-load range. On the other hand, when the remaining amount in the gas fuel tank decreases and reaches a predetermined remaining amount while the engine is running on gas fuel, the fuel used is switched from gas fuel to liquid fuel. Furthermore, if the driver selects gas fuel while driving on liquid fuel, the fuel used is switched from liquid fuel to gas fuel, provided that the remaining amount in the gas fuel tank is equal to or greater than a predetermined remaining amount, and if the driver selects liquid fuel while driving on gas 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 a predetermined remaining amount.

[0049] During operation of the engine E, particulate matter contained in the exhaust accumulates on the exhaust filter 332, specifically, on the filter element of the exhaust filter 332. As the accumulation of particulate matter progresses, the exhaust filter 332 becomes clogged, increasing pressure loss in the exhaust filter 332 and raising concerns about impeding engine performance. The accumulation of particulate matter tends to progress significantly when the engine is operating using liquid fuel.

[0050] In this embodiment, the amount of particulate matter deposited on the exhaust filter 332 (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 exhaust filter 332 and regenerate the exhaust filter 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 and Pexh2 upstream and downstream of the exhaust filter 332, that is, the differential pressure across the exhaust filter 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 has exceeded the upper limit of deposition amount and that the exhaust filter 332 is due for regeneration.

[0051] FIG. 2 is a flowchart showing the overall flow of air-fuel ratio control according to this embodiment.

[0052] 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.

[0053] 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.

[0054] 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 catalyst 331 are present, and the conditions are such that it is not easy to treat them with the exhaust purification catalyst 331, so the process of S104 is executed without going through the process of S103.

[0055] 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.

[0056] In step S104, air-fuel ratio oscillation control is executed in accordance with the procedure shown in the flowchart of FIG.

[0057] In step S105, air-fuel ratio feedback control is executed in accordance with the procedure shown in the flowchart of FIG.

[0058] FIG. 4 is a flowchart showing the basic flow of the air-fuel ratio oscillation control according to this embodiment.

[0059] 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 catalyst 331 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] In S205, the value of the flag FRG is set to 1.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] In S211, the fuel injector 41 (41a, 41b) is driven with the fuel injection amount Qf.

[0071] FIG. 5 is a flowchart showing the basic flow of air-fuel ratio feedback control according to this embodiment.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] In S304, the fuel injectors 41 (41a, 41b) are driven with the fuel injection amount Qf.

[0076] 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.

[0077] Now, the processes from S206 to S209 in FIG. 4 will be described in more detail with reference to FIGS.

[0078] 9 to 12 are graphs of experimental data measured under a plurality of conditions by changing the catalyst temperature Tcat and the exhaust flow rate Qexh, showing the change in the purification rate η of the exhaust purification catalyst 331 and the downstream minimum air-fuel ratio λr_min relative to the frequency Frq of air-fuel ratio oscillation in air-fuel ratio oscillation control. Fig. 9 shows data when the catalyst is highly active and the exhaust flow rate is low, Fig. 10 shows data when the catalyst is highly active and the exhaust flow rate is high, Fig. 11 shows data when the catalyst is low activity and the exhaust flow rate is low, and Fig. 12 shows data when the catalyst is low activity and the exhaust flow rate is high.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] Based on the above findings, the procedure for extracting the optimum frequency, that is, for specifying the control frequency Fcn, will be described below.

[0085] 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 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, the air-fuel ratio change amount Δλr_min is divided by ΔF (in this embodiment, the amount of change in the logarithm of the frequency log(Frq)) to calculate the gradient glmba of the change in the downstream minimum air-fuel ratio λr_min with respect to the logarithm of the frequency log(Frq), and the absolute value (=|glmba|) of this gradient is compared with a predetermined value g01. The frequency Frq when the absolute value of the gradient glmba (=|glmba|) reaches the predetermined value g01 is identified, and this is set as the control frequency Fcn.

[0086] 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.

[0087] 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.

[0088] FIG. 6 is a flowchart showing the basic flow of the filter regeneration control (regeneration time detection process) according to this embodiment.

[0089] In S401, the filter inlet gas pressure Pexh1 and the filter outlet gas pressure Pexh2 are read.

[0090] 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 filter inlet side pressure sensor 208 and the exhaust pressure Pexh2 detected by the filter outlet side pressure sensor 209 using the following equation (1). ΔP=Pexh1-Pexh2 …(1)

[0091] In S403, it is determined whether the filter differential pressure ΔP is greater than a first predetermined pressure ΔP1, that is, whether the accumulation of particulate matter in the exhaust filter 332 has progressed and the filter accumulation 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 exhaust filter 332 when the filter accumulation amount Qgpf has reached its upper limit (i.e., the upper limit accumulation 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 "filter regeneration timing detection means" according to this embodiment.

[0092] In S404, filter regeneration is performed according to the procedure shown in the flowchart of FIG.

[0093] FIG. 7 is a flowchart showing the flow of filter regeneration control (regeneration execution process).

[0094] 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.

[0095] In S502, the fuel used is switched from liquid fuel to gaseous fuel. As a result, when filter regeneration is started, if the fuel being used is gaseous fuel, the gaseous fuel continues to be used, and if the fuel being used is liquid fuel, the fuel being used is switched to gaseous fuel. In other words, in S501 and S502, the fuel being used during filter regeneration is forcibly set to gaseous fuel. The processing of S501 and S502 corresponds to the processing executed by the engine controller 101 as the "fuel used setting means" according to this embodiment.

[0096] In S503, the control frequency Fcn is read. For the air-fuel ratio oscillation control during filter regeneration, it is possible to use the control frequency Fcn that was specified 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. 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 then the control frequency Fcn may be specified 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.

[0097] In S504, air-fuel ratio oscillation control is performed using the control frequency Fcn. By performing air-fuel ratio oscillation control, particulate matter accumulated on the exhaust filter 332 reacts with oxygen and burns, causing a decrease in the filter differential pressure ΔP. Here, while air-fuel ratio oscillation control is being performed, the exhaust purification catalyst 331 upstream of the exhaust filter 332 repeatedly stores excess oxygen in the exhaust and releases the stored oxygen from the catalyst 331 due to its oxygen storage capacity, thereby adjusting the air-fuel ratio of the exhaust passing through the exhaust purification catalyst 331 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 exhaust filter 332 rises, which has the effect of heating the exhaust filter 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 "filter regeneration means" according to this embodiment.

[0098] In S505, it is determined whether the filter differential pressure ΔP is equal to or less than a second predetermined pressure ΔP2, that is, whether the regeneration of the exhaust filter 332 has progressed due to the combustion of particulate matter, and the filter differential pressure ΔP has decreased to 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 exhaust filter 332 has been completed, and has a value smaller than the first predetermined pressure ΔP1. If the filter differential pressure 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 is continued. The process of S505 corresponds to the process executed by the engine controller 101 as the "filter regeneration completion determination means" according to this embodiment.

[0099] In S506, the air-fuel ratio oscillation control is stopped and filter regeneration is terminated. Accordingly, the forced setting of the fuel to be used is cancelled. By canceling the forced setting, the fuel to be used can be set according to the remaining amount in the fuel tank or a fuel selected by the driver. For example, if the driver selects liquid fuel and the fuel to be used is switched to gaseous fuel during filter regeneration, canceling the forced setting upon completion of regeneration will return the fuel to liquid fuel.

[0100] 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.

[0101] While the engine E is operating, particulate matter contained in the exhaust gas after combustion passes through the exhaust purification catalyst 331 and accumulates on the exhaust filter 332. As the accumulation of particulate matter progresses and the filter accumulation amount Qgpf increases, the filter differential pressure ΔP increases. When the filter differential pressure ΔP exceeds a first predetermined pressure (regeneration start pressure) ΔP1, it is determined that it is time to regenerate the exhaust filter 332, and filter regeneration using air-fuel ratio oscillation control begins. The particulate matter accumulated on the exhaust filter 332 decreases due to combustion, and the filter differential pressure ΔP decreases. When the filter differential pressure ΔP reaches a second predetermined pressure (regeneration completion pressure) ΔP2, it is determined that regeneration of the exhaust filter 332 is completed, and the air-fuel ratio oscillation control is stopped, and filter regeneration ends.

[0102] 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.

[0103] First, when it is detected that it is time to regenerate the exhaust filter 332, air-fuel ratio oscillation control is performed to oscillate the air-fuel ratio of the exhaust gas flowing into the catalytic converter 33, specifically the exhaust purification catalyst 331, between the rich side and the lean side relative to a value equivalent to stoichiometry. This makes it possible to burn the particulate matter deposited on the exhaust filter 332 by reacting with oxygen, thereby promoting the regeneration of the exhaust filter 332.

[0104] Here, by forcibly setting the fuel used when performing air-fuel ratio oscillation control to gaseous fuel, it is possible to suppress the generation of particulate matter during regeneration of the exhaust filter 332 and reduce the amount of particulate matter that flows into the exhaust filter 332 during the regeneration process. This shortens the time required to regenerate the exhaust filter 332 and enables regeneration to be completed more quickly.

[0105] Secondly, by detecting that it is time to regenerate the exhaust filter 332 when the difference between the exhaust pressure Pexh1 detected by the filter inlet side pressure sensor 208 and the exhaust pressure Pexh2 detected by the filter outlet side pressure sensor 209, i.e., the differential pressure ΔP before and after the filter, exceeds a first predetermined pressure ΔP1, it is possible to accurately detect that it is time to regenerate the exhaust filter 332 with a relatively simple configuration.

[0106] Then, after regeneration of the exhaust filter 332 has begun, if the differential pressure ΔP across the filter decreases and reaches a second predetermined pressure ΔP2, it is determined that regeneration of the exhaust filter 332 has been completed. This makes it possible to accurately detect that it is time to regenerate the exhaust filter 332 and determine that regeneration has been completed without requiring the addition of any special configuration.

[0107] Thirdly, by using methane-based fuel as the gaseous fuel, it is possible to suppress the generation of soot due to combustion, thereby suppressing the accumulation of particulate matter on the exhaust filter 332 and extending the regeneration interval of the exhaust filter 332. Furthermore, suppressing the accumulation of particulate matter when using gaseous fuel increases the degree of freedom in using or selecting liquid fuel, making it possible to further promote both the improvement of the output of the engine E and the improvement of the exhaust properties.

[0108] 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.

[0109] 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.

[0110] Fourth, 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 (including the temperature of the exhaust purification catalyst 331, i.e., the activation state of the catalyst 331), making it possible to further improve the purification rate of the exhaust purification catalyst 331.

[0111] Fifth, 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.

[0112] 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.

[0113] 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.

[0114] FIG. 15 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.

[0115] 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.

[0116] Figures 16 and 17 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 331, and the air-fuel ratio range Rlmb, for different catalyst temperatures Tcat. Figure 16(a) shows the case of a relatively low catalyst temperature Tcat1, and Figure 16(b) shows the case of a catalyst temperature Tcat2 that is higher than Tcat1. Figure 17(a) shows the case of a catalyst temperature Tcat3 that is higher than Tcat2, and Figure 17(b) shows the case of a catalyst temperature Tcat4 that is higher than Tcat3.

[0117] 16 and 17, 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.

[0118] 16 and 17, 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.

[0119] 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.

[0120] 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 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.

[0121] 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.

[0122] 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.

[0123] Furthermore, in the above explanation, the catalyst inlet gas temperature Tcat_in is used as the catalyst temperature Tcat of the exhaust purification catalyst 331, but the catalyst temperature Tcat is not limited to this, and it is also possible to obtain it directly by installing a temperature sensor such as a thermocouple in the bed portion of the exhaust purification catalyst 331. [Explanation of symbols]

[0124] 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...exhaust filter, 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 switching between liquid fuel and gaseous fuel as the fuel supplied to a combustion chamber, an exhaust purification catalyst installed in an exhaust passage of the engine; an exhaust filter that is installed downstream of the exhaust purification catalyst in the exhaust passage, supports a catalyst component, and is configured to be able to capture particulate matter in the exhaust; 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 a filter regeneration time detection means for detecting when the exhaust filter is due for regeneration; filter regeneration means for performing air-fuel ratio oscillation control, when the filter regeneration time detection means detects that it is time to regenerate the exhaust filter, to oscillate the air-fuel ratio of the exhaust flowing into the exhaust purification catalyst between a rich side and a lean side with respect to a value equivalent to stoichiometry; and a fuel use setting means for forcibly setting the fuel use when the air-fuel ratio oscillation control is performed by the filter regeneration means to gaseous fuel.

2. the used fuel setting means switches the used fuel from liquid fuel to gas fuel when the used fuel is liquid fuel when the filter regeneration time detection means detects that it is time to regenerate the exhaust filter, 2. The exhaust gas purification device for an engine according to claim 1, wherein the filter regeneration means performs the air-fuel ratio oscillation control after the used fuel is switched by the used fuel setting means.

3. a first pressure sensor disposed in the exhaust passage so as to be able to detect the pressure of the exhaust gas upstream of the exhaust filter; a second pressure sensor disposed in the exhaust passage so as to be able to detect the pressure of the exhaust gas downstream of the exhaust filter, 2. The engine exhaust purification device according to claim 1, wherein the filter regeneration time detection means detects that it is time to regenerate the exhaust filter when a difference between the exhaust pressure detected by the first pressure sensor and the exhaust pressure detected by the second pressure sensor exceeds a first predetermined value.

4. The controller further includes a filter regeneration completion determination means for determining whether or not regeneration of the exhaust filter has been completed, the filter regeneration completion determination means determines that regeneration of the exhaust filter is completed when the pressure difference reaches a second predetermined value after the air-fuel ratio oscillation control is started by the filter regeneration means, 4. The exhaust gas purification device for an engine according to claim 3, wherein when the filter regeneration completion determination means determines that regeneration of the exhaust filter has been completed, the filter regeneration means stops the air-fuel ratio oscillation control and cancels the forced setting of the used fuel by the used fuel setting means.

5. the operating condition sensor includes a downstream exhaust sensor that is installed in the exhaust passage downstream of the exhaust purification catalyst and detects an air-fuel ratio of the exhaust gas that has passed through the 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; 2. The exhaust gas purification device for an engine according to claim 1, wherein the filter regeneration means oscillates the air-fuel ratio at the control frequency specified by the control frequency specifying means.

6. the controller further includes air-fuel ratio oscillation control means for performing the air-fuel ratio oscillation control during normal operation of the engine before the filter regeneration time detection means detects that it is time to regenerate the exhaust filter, 6. The engine exhaust gas purification device according to claim 5, wherein the control frequency specifying means specifies the control frequency based on the air-fuel ratio detected by the downstream exhaust sensor while the air-fuel ratio oscillation control means is performing the air-fuel ratio oscillation control.

7. 7. The engine exhaust gas purification device according to claim 6, 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.

8. 8. The engine exhaust gas purification device according to claim 7, 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.

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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