Engine air-fuel ratio control device

Abstract

To avoid adverse effects caused by execution of switching of a fuel simultaneously with vibration of an air-fuel ratio, in an engine operable while switching the fuel.SOLUTION: An air-fuel ratio control device for an engine includes: air-fuel ratio vibration control means for executing air-fuel ratio vibration control to vibrate an air-fuel ratio of the exhaust gas flowing to an exhaust purification catalyst, to a rich side and a lean side with respect to a stoichiometric equivalent value; fuel switching request detection means for detecting a fuel switching request for switching from a liquid fuel to a gas fuel or from the gas fuel to the liquid fuel; and fuel switching execution means for switching the fuel when the fuel switching request detection means detects the fuel switching request. The fuel switching execution means executes the switching of the fuel from the liquid fuel to the gas fuel at a timing excluding shift of the air-fuel ratio from the rich side to the lean side by the air-fuel ratio vibration control, and executes the switching of the fuel from the gas fuel to the liquid fuel at a timing excluding the shift of the air-fuel ratio from the lean side to the rich side by the air-fuel ratio vibration control.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to an air-fuel ratio control apparatus for an engine. [Background Art]

[0002] There exists an engine configured to be capable of switching the fuel used for operation between liquid fuel and gaseous fuel. This engine is called a bi-fuel engine, and is equipped with a liquid fuel tank that stores liquid fuel and a gaseous fuel tank that stores gaseous fuel. The fuel supplied to the combustion chamber is switched according to the remaining amount of fuel and the selection made by the driver. As a bi-fuel engine, one that uses gasoline as the liquid fuel and compressed natural gas (CNG) as the gaseous fuel is known.

[0003] On the other hand, in an engine provided with a three-way catalyst in an exhaust passage, it is known that forcibly oscillating the air-fuel ratio of the air-fuel mixture between the rich side and the lean side with respect to the theoretical air-fuel ratio improves the exhaust purification rate by the catalyst. The air-fuel mixture being on the rich side means that the amount of fuel contained in the air-fuel mixture is larger than the stoichiometric amount, and being on the lean side means that the amount of fuel contained in the air-fuel mixture is smaller than the stoichiometric amount. [Prior Art Literature] [Patent Literature]

[0004] [Patent Literature 1] Japanese Examined Patent Publication No. 56-017533 [Summary of the Invention] [Problem to be Solved by the Invention]

[0005] When forced oscillation of the air-fuel ratio is performed in the bi-fuel engine described above, the following issues may arise.

[0006] In forced air-fuel ratio oscillations, a decrease in engine torque occurs when the air-fuel ratio shifts from rich to lean, and an increase in engine torque occurs when it shifts from lean to rich.

[0007] Engine torque fluctuations tend to occur due to fuel switching; a decrease occurs when switching from liquid fuel to gaseous fuel, and an increase occurs when switching from gaseous fuel to liquid fuel.

[0008] Here, there are concerns that the fuel switching, being synchronized with fluctuations in the air-fuel ratio, will result in significant fluctuations in engine torque, potentially causing substantial problems with drivability and ride comfort. Specifically, the switching from liquid fuel to gaseous fuel, when synchronized with the shift in the air-fuel ratio from rich to lean, will exacerbate fluctuations in the direction of decreasing engine torque, while the switching from gaseous fuel to liquid fuel, when synchronized with the shift in the air-fuel ratio from lean to rich, will exacerbate fluctuations in the direction of increasing engine torque.

[0009] Furthermore, there are concerns that this exacerbation of engine torque fluctuations could impair drivability and ride comfort.

[0010] Therefore, the present invention aims to provide an air-fuel ratio control device for an engine capable of switching between liquid fuel and gaseous fuel, which can improve the purification rate of the exhaust gas purification catalyst while avoiding a situation in which the fuel switching is performed in synchronization with the fluctuation of the air-fuel ratio, thereby causing substantial adverse effects on drivability and ride comfort. [Means for solving the problem]

[0011] To solve the aforementioned problems, an air-fuel ratio control device for an engine according to one embodiment of the present invention is an air-fuel ratio control device for an engine equipped with an exhaust gas purification catalyst in the exhaust passage and configured to operate by switching the fuel supplied to the combustion chamber between liquid fuel and gaseous fuel, comprising: an operating state sensor that outputs a signal according to the operating state of the engine; and a controller that controls the operating state of the engine based on the signal output by the operating state sensor, wherein the controller comprises: air-fuel ratio vibration control means that performs air-fuel ratio vibration control to vibrate the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification catalyst between the rich side and the lean side with respect to the stoichiometric equivalent value; fuel switching request detection means that detects a fuel switching request to switch the fuel from liquid fuel to gaseous fuel or from gaseous fuel to liquid fuel; and fuel switching execution means that switches the fuel when a fuel switching request is detected by the fuel switching request detection means. The fuel switching execution means performs the switching of fuel from liquid fuel to gaseous fuel at a timing other than the transition of the air-fuel ratio from rich to lean due to air-fuel ratio oscillation control, while also performing the switching of fuel from gaseous fuel to liquid fuel at a timing other than the transition of the air-fuel ratio from lean to rich due to air-fuel ratio oscillation control. [Effects of the Invention]

[0012] According to the present invention, in an engine capable of switching between liquid fuel and gaseous fuel, the purification rate by the exhaust gas purification catalyst is improved by controlling the air-fuel ratio vibration, and it is possible to avoid a situation in which fluctuations in engine torque associated with fuel switching are exacerbated by synchronization with the vibration of the air-fuel ratio, thereby causing substantial adverse effects on drivability and ride comfort. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic diagram showing the overall configuration of an engine according to one embodiment of the present invention. [Figure 2] This flowchart shows the overall flow of air-fuel ratio control according to the same embodiment as described above. [Figure 3] This is an operating domain map showing the implementation domains of air-fuel ratio oscillation control and air-fuel ratio feedback control in the air-fuel ratio control according to the above embodiment. [Figure 4] This flowchart shows the basic flow of air-fuel ratio vibration control according to the same embodiment. [Figure 5] This flowchart shows the basic flow of air-fuel ratio feedback control according to the same embodiment. [Figure 6] This flowchart shows the basic flow of fuel switching control according to the same embodiment. [Figure 7] This graph shows experimental data illustrating the relationship between the frequency Frq of air-fuel ratio oscillation in air-fuel ratio oscillation control, the purification rate η of the exhaust gas purification catalyst, and the minimum value λr_min of the downstream air-fuel ratio, under conditions of high catalyst activity and low exhaust gas flow rate. [Figure 8] This graph shows experimental data illustrating the relationship between the frequency Frq of air-fuel ratio oscillation in air-fuel ratio oscillation control, the purification rate η of the exhaust gas purification catalyst, and the minimum value λr_min of the downstream air-fuel ratio, under conditions of high catalyst activity and high exhaust gas flow rate. [Figure 9] This graph shows experimental data illustrating the relationship between the frequency Frq of air-fuel ratio oscillation in air-fuel ratio oscillation control, the purification rate η of the exhaust gas purification catalyst, and the minimum value λr_min of the downstream air-fuel ratio, under conditions of low catalyst activity and low exhaust gas flow rate. [Figure 10] This graph shows experimental data illustrating the relationship between the frequency Frq of air-fuel ratio oscillation in air-fuel ratio oscillation control, the purification rate η of the exhaust gas purification catalyst, and the minimum value λr_min of the downstream air-fuel ratio, under conditions of low catalyst activity and high exhaust gas flow rate. [Figure 11] This graph shows the output waveforms of the upstream and downstream exhaust sensors in the air-fuel ratio oscillation control system, in the high-frequency range. [Figure 12] This graph shows the output waveforms of the upstream and downstream exhaust sensors in the air-fuel ratio oscillation control system, in the low-frequency range. [Figure 13] This diagram illustrates the effect of synchronizing air-fuel ratio oscillations with fuel switching on fluctuations in engine torque. [Figure 14] This flowchart shows the basic flow of fuel switching control according to another embodiment of the present invention. [Figure 15]It is a flowchart showing the basic flow of air-fuel ratio vibration control according to still another embodiment of the present invention. [Figure 16] It is a graph of experimental data showing the relationships among the frequency Frq of air-fuel ratio vibration, the purification rate η of an 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) in the air-fuel ratio vibration control according to the above embodiment, for different catalyst temperatures (a) Tcat1 and (b) Tcat2 (>Tcat1). [Figure 17] It is a graph of experimental data showing the relationships among the frequency Frq of air-fuel ratio vibration, the purification rate η of an exhaust purification catalyst, and the air-fuel ratio range Rlmb in the air-fuel ratio vibration control according to the above embodiment, for different catalyst temperatures (a) Tcat3 (>Tcat2) and (b) Tcat4 (>Tcat3). [Figure 18] It is a schematic diagram showing the overall configuration of an engine according to still another embodiment of the present invention. [Figure 19] It is a flowchart showing the overall flow of air-fuel ratio control according to still another embodiment of the present invention. [Figure 20] It is a graph showing the relationship between catalyst temperature Tcat and the purification rate ηthc of total hydrocarbons (THC) for a plurality of frequencies of air-fuel ratio vibration. [Figure 21] It is a graph showing the relationship between catalyst temperature Tcat and the purification rate ηnox of nitrogen oxides (NOx) for a plurality of frequencies of air-fuel ratio vibration. [Figure 22] It is a graph showing the relationship between catalyst temperature Tcat and the minimum output value (downstream minimum air-fuel ratio λr_min) of a downstream exhaust sensor for a plurality of frequencies of air-fuel ratio vibration. [Figure 23] It is a graph showing the relationship between catalyst temperature Tcat and the maximum output value (downstream maximum air-fuel ratio λr_max) of a downstream exhaust sensor for a plurality of frequencies of air-fuel ratio vibration. [Figure 24] It is a distribution diagram showing the relationship among the minimum output value λr_min, the maximum output value λr_max of the downstream exhaust sensor, and the purification rate ηthc of total hydrocarbons. [Figure 25]This is a distribution diagram showing the relationship between the minimum output value λr_min, the maximum output value λr_max of the downstream exhaust sensor, and the nitrogen oxide purification rate ηnox. [Figure 26] This is a distribution diagram showing the relationship between the minimum output value λr_min and the maximum output value λr_max of the downstream exhaust sensor and the average purification rate ηave for total hydrocarbons and nitrogen oxides. [Modes for carrying out the invention]

[0014] Embodiments of the present invention will be described below with reference to the drawings.

[0015] Figure 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.

[0016] In the following explanation, the terms "upstream" and "downstream" are used in relation to the direction of the normal exhaust flow emitted from engine E. For example, the upstream side of the exhaust purification device refers to the upstream side of the exhaust purification device in the direction of the exhaust flow, and the downstream side of the exhaust purification catalyst refers to the downstream side of the exhaust purification catalyst in the direction of the exhaust flow.

[0017] In this embodiment, engine E is mounted on a vehicle and constitutes its power source. Engine E is a bi-fuel engine and is configured to operate by switching between liquid fuel and gaseous fuel. As will be described later, 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 normal temperature and pressure or when supplied to engine E, and in this embodiment, gasoline is used. In contrast, gaseous fuel refers to fuel that is in a gaseous state at normal temperature and pressure, and in this embodiment, compressed natural gas (CNG) is used.

[0018] Engine E comprises an engine body 1 having a combustion chamber, as well as an intake system 2 and an exhaust system 3. In this embodiment, engine E is an inline four-cylinder engine, but the type of engine E, that is, the number and arrangement of cylinders in engine E, is not limited to this. Various types of engines, such as single-cylinder, two-cylinder, six-cylinder, V-type, and horizontally opposed types, can be used.

[0019] The engine body 1 comprises a cylinder block, a cylinder head, and a crankcase. A piston is inserted into the cylinder block, and the space formed between the crown surface of the piston and the inner surface of the cylinder head becomes the combustion chamber.

[0020] 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 dust and other foreign matter has been removed is introduced into the intake pipe 21 via the air cleaner 23. The intake pipe 21 is connected to the collector of the intake manifold 22, and the intake manifold 22 branches off from the collector and is connected to the side of the cylinder head. The air flowing from the intake pipe 21 into the intake manifold 22 is distributed to each cylinder via the branching section of the intake manifold 22.

[0021] In this embodiment, a port injection type fuel supply system is employed. The engine E is equipped with a plurality of fuel injectors 41 (41a, 41b) embedded in the cylinder head, and fuel is injected from each of these fuel injectors 41 to the corresponding cylinder. The engine E is a bi-fuel engine and is equipped with a second fuel system for supplying gaseous fuel in addition to a first fuel system for supplying liquid fuel. Specifically, the engine E is equipped with 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 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 gaseous fuel from the gaseous fuel tank. The first fuel injector 41a and the second fuel injector 41b are both installed at the branching point of the intake manifold 22 and inject fuel toward the intake port of the corresponding cylinder. The fuel supply method is not limited to this, and other supply methods besides port injection can be used, for example, direct injection for liquid fuel.

[0022] In this embodiment, 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 the branch section of the intake manifold 22 and introduced into the corresponding cylinders. Inside each cylinder, the fuel and air are mixed to form a fuel-air mixture. This mixture is then ignited by the spark plug 51, causing it 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 remaining in the cylinder is discharged to the branch section of the exhaust manifold 31. The exhaust is collected from the branch section to the collection section in the exhaust manifold 31 and introduced into the exhaust pipe 32. A catalytic converter 33 is installed in the exhaust pipe 32, and the exhaust flowing through the exhaust pipe 32 is introduced into the catalytic converter 33. After the exhaust harmful components, including total hydrocarbons (THC) and nitrogen oxides (NOx), are purified by the exhaust purification catalyst 331 housed in the catalytic converter 33, the exhaust is released into the atmosphere. In this embodiment, the catalytic converter 33 includes a three-way catalyst as the exhaust purification catalyst 331.

[0024] In addition to the above, engine E is equipped with an engine controller 101 and various sensors 201 to 209.

[0025] The engine controller 101, as an electronic control unit, consists of a microcomputer equipped with a central processing unit (CPU), memory devices such as ROM and RAM, and input / output interfaces.

[0026] Engine E is equipped with an accelerator sensor 201 and an engine speed sensor 202, as well as an airflow meter 203, a coolant temperature sensor 204, a catalyst temperature sensor 205, an upstream exhaust sensor 206, a downstream exhaust sensor 207, a fuel level sensor 208 (208a, 208b), and a fuel selector switch 209. Detection signals output from these sensors 201 to 208 and instruction signals output from the fuel selector switch 209 are input to the engine controller 101. The accelerator sensor 201, engine speed sensor 202, airflow meter 203, coolant temperature sensor 204, catalyst temperature sensor 205, upstream exhaust sensor 206, and downstream exhaust sensor 207 constitute the "operating state sensor" according to this embodiment, and the detection signals output from these sensors 201 to 207 indicate the operating state of engine E.

[0027] The accelerator sensor 201 detects the amount the driver depresses the accelerator pedal, which is expressed as the accelerator opening degree APO. The accelerator opening degree APO is an indicator of the target load required for engine E.

[0028] The engine speed sensor 202 detects the rotational speed Ne of the engine E. A crank angle sensor can be used as the engine speed sensor 202, and the elapsed time per unit crank angle or reference crank angle detected by the crank angle sensor is converted into the rotational speed Ne.

[0029] The airflow meter 203 detects the flow rate of air introduced into the engine E as the intake air volume Qa. In this embodiment, the intake air volume Qa is used as an indicator of the flow rate of exhaust gas discharged from the combustion chamber.

[0030] The coolant temperature sensor 204 detects the temperature Tw of the coolant flowing through the coolant passages formed in the cylinder block of the engine body 1.

[0031] The catalyst temperature sensor 205 detects the temperature (hereinafter referred to as "catalyst temperature") Tcat of the exhaust gas 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 (hereinafter referred to as "catalyst inlet gas temperature") Tcat_in is detected as the catalyst temperature Tcat.

[0032] The upstream exhaust sensor 206 detects the air-fuel ratio λf of the exhaust gas flowing through the exhaust pipe 32 upstream of the catalytic converter 33, that is, the exhaust gas before it flows into the catalytic converter 33 (hereinafter referred to as the "upstream air-fuel ratio"). The upstream exhaust sensor 206 constitutes the "exhaust sensor" according to this embodiment.

[0033] The downstream exhaust sensor 207 detects the air-fuel ratio λr of the exhaust gas flowing through the exhaust pipe 32 downstream of the catalytic converter 33, that is, the exhaust gas after it has passed through the catalytic converter 33 (hereinafter referred to as the "downstream air-fuel ratio"). The downstream exhaust sensor 207 constitutes the "downstream exhaust sensor" according to this embodiment.

[0034] The fuel level sensors 208 (208a, 208b) detect the amount of fuel remaining in the fuel tank, i.e., the remaining fuel amount Lf (Lfa, Lfb). In this embodiment, the fuel tank is provided as a liquid fuel tank for storing liquid fuel and a gaseous fuel tank for storing gaseous fuel. The first fuel level sensor 208a detects the remaining amount in the liquid fuel tank, and the second fuel level sensor 208b detects the remaining amount in the gaseous fuel tank.

[0035] The fuel selector switch 209 is operated by the vehicle driver, for example, by being installed inside the vehicle cabin, and outputs an instruction signal to select the fuel to be used to operate engine E, either liquid fuel or gaseous fuel. The driver can operate the fuel selector switch 209 to select the fuel to be used when starting engine E, and can switch the fuel to gaseous fuel while operating with liquid fuel, or switch the fuel to liquid fuel while operating with gaseous fuel.

[0036] The engine controller 101 controls the operating state of the engine E while controlling the air-fuel ratio of the mixture used for combustion, based on detection signals output from the various sensors 201 to 207 described above. The engine controller 101 constitutes the "controller" according to this embodiment.

[0037] The engine controller 101 switches between air-fuel ratio oscillation control and air-fuel ratio feedback control when controlling the air-fuel ratio. 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 relative to the stoichiometric value, and in this embodiment, this is done by oscillating the air-fuel ratio of the mixture formed in the combustion chamber. Specifically, it forcibly increases or decreases the fuel injection amount of the fuel injectors 41 (41a, 41b). 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 signals from the exhaust sensor, in this embodiment, the upstream exhaust sensor 206, and as a result of the air-fuel ratio feedback control, the air-fuel ratio of the exhaust gas flowing into the catalytic converter 33 is brought closer to the stoichiometric value.

[0038] The switching between air-fuel ratio oscillation control and air-fuel ratio feedback control depends on the operating region to which the engine E is operating. In this embodiment, the operating region of the engine E is divided into multiple regions A and B determined according to the catalyst temperature Tcat and the exhaust flow rate Qexh (Figure 3). It is determined whether the engine E is in the first region A, where the catalyst temperature Tcat is lower than a predetermined temperature Tcat1 or the exhaust flow rate Qexh is greater than a predetermined flow rate Qexh1, or in a region other than the first region A, i.e., in the second region B, where the catalyst temperature Tcat is at or above the predetermined temperature Tcat1 and the exhaust flow rate Qexh is at or below the predetermined flow rate Qexh1. If the engine E is operating in the first region A, air-fuel ratio oscillation control is performed; if it is in the second region B, air-fuel ratio feedback control is performed.

[0039] Here, the catalyst temperature Tcat is an indicator of the activity state of the exhaust gas purification catalyst 331. When the catalyst temperature Tcat is above a predetermined temperature Tcat1, it indicates that the exhaust gas purification catalyst 331 is highly activated and in a highly active state. On the other hand, the exhaust gas flow rate Qexh is an indicator of the amount of exhaust gas harmful substances that need to be purified by the exhaust gas purification catalyst 331. When the exhaust gas flow rate Qexh is below a predetermined flow rate Qexh1, it indicates that the amount of exhaust gas harmful substances to be purified is relatively small, and that the activated exhaust gas purification catalyst 331 can easily process them. In other words, executing air-fuel ratio feedback control in operating region B means executing air-fuel ratio feedback control when it is determined that the exhaust gas purification catalyst 331 is highly activated and can easily process the exhaust gas harmful components.

[0040] Figure 3 is an operating region map showing the implementation regions for air-fuel ratio oscillation control and air-fuel ratio feedback control. As shown in Figure 3, when the catalyst temperature Tcat is above a predetermined temperature Tcat1 and the exhaust flow rate Qexh is below a predetermined flow rate Qexh1, the operating state of the engine E belongs to the second region B, where the activation of the exhaust purification catalyst 331 is progressing and conditions for easy treatment by the exhaust purification catalyst 331 are assumed, and air-fuel ratio feedback control is selected and executed. Otherwise, the operating state of the engine E belongs to the first region A, where the activation of the exhaust purification catalyst 331 is insufficient, or there is a large amount of exhaust harmful substances to be purified, making treatment by the exhaust purification catalyst 331 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 the catalyst temperature Tcat, and the intake air amount Qa is used as 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.

[0041] In addition to the above, this embodiment performs control (hereinafter referred to as "fuel switching control") to switch the fuel used to operate engine E according to the remaining fuel level or selection by the driver.

[0042] Specifically, the fuel switching between liquid fuel and gaseous fuel will be performed according to the remaining fuel level or the driver's selection. For example, if the liquid fuel tank level decreases during operation with liquid fuel and reaches a predetermined level, or if engine E is not generating torque or is generating only very small torque (for example, during coasting or idling), the fuel used will be switched from liquid fuel to gaseous fuel. On the other hand, if the gaseous fuel tank level decreases during operation with gaseous fuel and reaches a predetermined level, the fuel used will be switched from gaseous fuel to liquid fuel. Furthermore, if the driver selects gaseous fuel during operation with liquid fuel, the fuel used will be forcibly switched from liquid fuel to gaseous fuel, provided that the gaseous fuel tank level is above a predetermined level. Similarly, if the driver selects liquid fuel during operation with gaseous fuel, the fuel used will be forcibly switched from gaseous fuel to liquid fuel, provided that the liquid fuel tank level is above a predetermined level.

[0043] In this embodiment, when switching fuels, the engine torque fluctuations associated with the fuel switching are amplified by synchronization with the air-fuel ratio vibrations caused by air-fuel ratio vibration control. This control is performed as part of the fuel switching control to avoid situations where large torque fluctuations occur.

[0044] Specifically, in air-fuel ratio oscillation control, when the air-fuel ratio shifts from rich to lean, a decrease in engine torque occurs, and when the air-fuel ratio shifts from lean to rich, an increase in engine torque occurs. Engine torque fluctuations tend to occur not only when the air-fuel ratio crosses the stoichiometric value, but also when the fuel is switched. A decrease in fluctuation occurs when switching from liquid fuel to gaseous fuel, and an increase in fluctuation occurs when switching from gaseous fuel to liquid fuel. Here, it is conceivable that if the fuel switching is performed in synchronization with the air-fuel ratio oscillation, the fluctuation in engine torque associated with the fuel switching will be amplified by the synchronization with the air-fuel ratio oscillation, resulting in a large torque fluctuation. For example, if the switch from liquid fuel to gaseous fuel is performed in sync with the transition of the air-fuel ratio from rich to lean, it promotes a decrease in engine torque (decreasing torque fluctuation), and if the switch from gaseous fuel to liquid fuel is performed in sync with the transition of the air-fuel ratio from lean to rich, it promotes an increase in engine torque (increasing torque fluctuation).

[0045] To avoid such a situation, in this embodiment, the fuel switching is performed at a timing that avoids the fluctuation in engine torque associated with the fuel switching, which is exacerbated by synchronization with the fluctuation of the air-fuel ratio.

[0046] Specifically, the switching from liquid fuel to gaseous fuel is performed while avoiding the timing when the air-fuel ratio shifts from rich to lean (i.e., the timing when fluctuations in engine torque are amplified in the decreasing direction), and the switching from gaseous fuel to liquid fuel is performed while avoiding the timing when the air-fuel ratio shifts from lean to rich (i.e., the timing when fluctuations in engine torque are amplified in the increasing direction). Furthermore, in this embodiment, the switching from liquid fuel to gaseous fuel is performed in synchronization with the air-fuel ratio shift, particularly from lean to rich, and the switching from gaseous fuel to liquid fuel is performed in synchronization with the air-fuel ratio shift, particularly from rich to lean. In other words, the fuel switching is performed at a timing when the resulting fluctuations in engine torque are offset by the torque fluctuations in the opposite direction associated with the air-fuel ratio shift.

[0047] Figure 2 is a flowchart showing the overall flow of air-fuel ratio control according to this embodiment.

[0048] In this embodiment, the air-fuel ratio control routine shown in Figure 2 is performed by the engine controller 101 at predetermined intervals after the engine E is started. When starting the air-fuel ratio control, the engine controller 101 selects air-fuel ratio oscillation control as the air-fuel ratio control method.

[0049] In S101, indicators of the engine E's operating state are read, such as engine speed Ne, intake air volume Qa, upstream air-fuel ratio λf, downstream air-fuel ratio λr, and catalyst temperature Tcat (catalyst inlet gas temperature Tcat_in).

[0050] In S102, it is determined whether the intake air volume Qa is greater than a predetermined flow rate Qa1. If it is greater than the predetermined flow rate Qa1, the process proceeds to S104; if it is less than or equal to the predetermined flow rate Qa1, the process proceeds to S103. As mentioned earlier, the intake air volume Qa is an example of a state variable that replaces the exhaust flow rate Qexh. If the intake air volume Qa is greater than the predetermined flow rate Qa1, it is assumed that there is a large amount of exhaust hazardous substances that require purification by the exhaust purification catalyst 331, and that the treatment by the exhaust purification catalyst 331 is not easy. Therefore, the process in S104 is executed without going through the process in S103.

[0051] In S103, it is determined whether the catalyst temperature Tcat is above a predetermined temperature Tcat1. If the catalyst temperature Tcat is above the predetermined temperature Tcat1, the operating state of engine E is assumed to be in the second region B shown in Figure 3, and the process proceeds to S105. Otherwise, the operating state of engine E is assumed to be in the first region A, and the process proceeds to S104.

[0052] In S104, air-fuel ratio vibration control is performed. The air-fuel ratio vibration control follows the flowchart shown in Figure 4. The processes shown in the flowchart in Figure 4 correspond to the processes performed by the engine controller 101 as the "air-fuel ratio vibration control means" according to this embodiment.

[0053] In S105, air-fuel ratio feedback control is performed. The air-fuel ratio feedback control follows the flowchart shown in Figure 5. The processes shown in the flowchart in Figure 5 correspond to the processes performed by the engine controller 101 as the "air-fuel ratio feedback control means" according to this embodiment.

[0054] Figure 4 is a flowchart showing the basic flow of air-fuel ratio oscillation control according to this embodiment.

[0055] In S201, it is determined whether the setting of the control frequency Fcn has already been completed. The control frequency Fcn is the optimal frequency at which the exhaust gas purification catalyst 331 can obtain the highest purification rate (hereinafter referred to as the "maximum purification rate") η when performing air-fuel ratio oscillation control. In this embodiment, the control frequency Fcn is set for each operating region of the engine E, which is determined according to the rotational speed and load of the engine E. If the setting of the control frequency Fcn has already been completed, the process proceeds to S202; otherwise, the process proceeds 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.

[0056] In S202, the control frequency Fcn is read. In this embodiment, an operating region map is provided that is set to allow the assignment of the control frequency Fcn in accordance with the rotational speed and load (e.g., intake air volume Qa) of the engine E, and the corresponding control frequency Fcn is read from the operating region that has already been set. After reading the control frequency Fcn, the process proceeds to S210.

[0057] In S203, it is determined whether the value of flag FRG is 0 or not. If the value of flag FRG is 0, proceed to S204; otherwise, proceed to S206.

[0058] In the S204, a reference frequency F0 is set for the air-fuel ratio oscillation frequency Frq. The reference frequency F0 is a value that is temporarily set when the determination of the control frequency Fcn begins, and is pre-set for each operating region as the initial value of frequency Frq in the operating region map. However, it is not limited to this, and for simplicity, it is also possible to set it uniformly to 1 [Hz] for the entire operating region of engine E.

[0059] In S205, the value of flag FRG is set to 1.

[0060] In S206, the frequency Frq of the air-fuel ratio oscillation is reduced by a predetermined frequency ΔF. Specifically, the frequency Frq is updated to a frequency that is reduced by a predetermined frequency ΔF from the current frequency Frq (Frq = Frq - ΔF), and air-fuel ratio oscillation control is performed using the new updated frequency Frq.

[0061] In S207, the minimum value of the downstream air-fuel ratio λr within a predetermined time (hereinafter referred to as the "minimum downstream air-fuel ratio") λr_min is calculated based on the downstream air-fuel ratio λr, and the change in the minimum downstream air-fuel ratio λr_min before and after reducing the frequency Frq by ΔF (hereinafter referred to as the "air-fuel ratio change") Δλr_min is calculated. Here, the predetermined time may be the time of one period of air-fuel ratio oscillation, or it may be a longer time. In this embodiment, it is a time longer than one period of air-fuel ratio oscillation.

[0062] In S208, the slope glmba of the change in the downstream minimum air-fuel ratio λr_min with respect to frequency Frq is calculated by dividing the air-fuel ratio change amount Δλr_min by ΔF. In this embodiment, the slope glmba is the slope of the change in the downstream minimum air-fuel ratio λr_min with respect to the logarithm log(Frq) of frequency Frq, obtained when the frequency Frq is decreased by a predetermined frequency ΔF from the reference frequency F0. Then, it is determined whether the absolute value of the slope glmba (=|glmba|) is greater than or equal to a predetermined value g01. If it is greater than or equal to the predetermined value g01, in other words, if the slope glma of the change in the downstream minimum air-fuel ratio λr_min before and after the decrease in frequency Frq decreases and its absolute value (=|glmba|) reaches the predetermined value g01, the process proceeds to S209. If it is less than the predetermined value g01, the process in S209 is bypassed and the process proceeds to S210.

[0063] In S209, the control frequency Fcn is set. In this embodiment, the frequency Frq when the absolute value of the slope glmba (=|glmba|) reaches a predetermined value g01 is set as the control frequency Fcn.

[0064] The processing from S206 to S209 corresponds to the processing performed by the engine controller 101 as the "control frequency identification means" according to this embodiment.

[0065] In the S210, the fuel injection amount Qf is calculated. In air-fuel ratio oscillation control, the basic fuel injection amount Qfb 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 enrichment correction amounts H corresponding to the coolant temperature Tw, etc., are added to calculate the fuel injection amount Qf. Here, the correction coefficient α is set to switch between a value greater than 1 and a value less than 1 every half period of the control frequency Fcn (=1 / (2Fcn)), and by multiplying by the correction coefficient α, the air-fuel ratio of the mixture periodically fluctuates or oscillates between, for example, 0.95 and 1.05 in terms of excess air ratio.

[0066] In the S211, the fuel injectors 41 (41a, 41b) are driven by the fuel injection amount Qf.

[0067] Figure 5 is a flowchart showing the basic flow of the air-fuel ratio feedback control according to this embodiment.

[0068] In S301, the basic injection amount Qfb is calculated. The basic injection amount Qfb is the injection amount equivalent to the amount of fuel that is equivalent to the intake air amount Qa.

[0069] In S302, the 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). If the difference between the upstream air-fuel ratio λf and the stoichiometric equivalent value λst is greater than 0 and the exhaust is in a state of excess air, the air-fuel ratio feedback correction amount Hqf is calculated as an increase correction amount to increase the fuel amount. If the difference is less than 0 and the exhaust is in a state of excess fuel, the amount of reduction correction amount to decrease the fuel amount is calculated.

[0070] In S303, the fuel injection amount Qf is set. In air-fuel ratio feedback control, it is calculated by adding the air-fuel ratio feedback correction amount Hqf to the basic injection amount Qfb, as well as various enrichment correction amounts H according to the coolant temperature Tw, etc.

[0071] In S304, the fuel injectors 41 (41a, 41b) are driven by the fuel injection amount Qf.

[0072] In S211 and S304, the 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 when operating with liquid fuel, and outputs a drive pulse signal for gaseous fuel injection corresponding to the fuel injection amount Qf to the drive circuit of the second fuel injector 41b when operating with gaseous fuel.

[0073] Now, referring to Figures 7 to 12, we will explain in more detail the processes from S206 to S209 in Figure 4.

[0074] Figures 7 to 10 are graphs of experimental data measured under multiple conditions by varying the catalyst temperature Tcat and exhaust flow rate Qexh, showing the change in the purification rate η of the exhaust gas purification catalyst 331 and the downstream minimum air-fuel ratio λr_min with respect to the air-fuel ratio oscillation frequency Frq in air-fuel ratio oscillation control. Figure 7 shows the data when the catalyst is highly active and the exhaust flow rate is low, Figure 8 shows the data when the catalyst is highly active and the exhaust flow rate is high, Figure 9 shows the data when the catalyst is less active and the exhaust flow rate is low, and Figure 10 shows the data when the catalyst is less active and the exhaust flow rate is high.

[0075] In Figures 7 through 10, the horizontal axis represents the logarithm of the frequency Frq (log(Frq)), and the vertical axis represents the purification rate η and the downstream minimum air-fuel ratio λr_min. log(Frq)=0 corresponds to 1 [Hz]. The open squares represent the purification rate ηthc for total hydrocarbons THC, and the open circles represent the purification rate ηnox for nitrogen oxides NOx. The triangles represent the downstream minimum air-fuel ratio λr_min.

[0076] Here, the downstream minimum air-fuel ratio λr_min measured in the region below the frequency at which the purification rate η is maximum is shown by open triangles, and the downstream minimum air-fuel ratio λr_min measured in the region below the frequency is shown by filled triangles. The thick dotted line is an approximate straight line of the downstream minimum air-fuel ratio λr_min in the low-frequency region, and the thick solid line is an approximate straight line of the downstream minimum air-fuel ratio λr_min in the high-frequency region.

[0077] In each of Figures 7 to 10, it can be observed that near the frequency at which the purification rate η is maximum, 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 (specifically, the logarithm of the frequency log(Frq)) also changes. By using the logarithm of the frequency Frq on the horizontal axis, the change in the slope can be observed more clearly. In this embodiment, we focus on the frequency Frq at which the downstream minimum air-fuel ratio λr_min and its slope show a discontinuous change, and call this the "optimal frequency," which is 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 of 0.015 to 0.025, thereby making it possible to set an appropriate control frequency Fcn.

[0078] Figures 11 and 12 are graphs showing the output waveforms of the upstream exhaust sensor 206 and the downstream exhaust sensor 207 in air-fuel ratio oscillation control. Figure 11 shows these output waveforms in the high-frequency (short-period) region, and Figure 12 shows them in the low-frequency (long-period) region. The "high-frequency region" in Figure 11 refers to a part of the "region higher than the frequency at which the purification rate η is maximum" as shown in Figures 7 to 10, and the "low-frequency region" in Figure 12 refers to a part of the "region lower than the frequency at which the purification rate η is maximum." In Figures 11 and 12, the thin solid line represents the upstream air-fuel ratio λf, and the thick solid line represents the downstream air-fuel ratio λr. The dotted line represents the catalyst temperature Tcat.

[0079] Referring to Figure 11, it can be seen that in the high-frequency region, the oscillation of the downstream air-fuel ratio λr is significantly attenuated relative to the upstream air-fuel ratio λf, regardless of the increase in catalyst temperature Tcat. In contrast, referring to Figure 12, it can be seen that in the low-frequency region, no attenuation of the downstream air-fuel ratio λr is observed; rather, the oscillation is amplified. This is presumed to be due to the oxygen storage capacity (OSC) of the exhaust gas purification catalyst 331. In other words, in the high-frequency region, the imbalance between fuel and air associated with air-fuel ratio oscillations 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 the stoichiometric value. In contrast, in the low-frequency region, the imbalance between fuel and air cannot be fully compensated for by the oxygen storage capacity, and fluctuations in the air-fuel ratio in the mixture are directly reflected in the change of the downstream air-fuel ratio λr. As a result, at the boundary between the region where oxygen storage capacity is effective and the region where oxygen storage capacity fails, the downstream minimum air-fuel ratio λr_min changes discontinuously, and the slope of the change with respect to frequency Frq changes to a degree that can be determined by comparing it with a threshold.

[0080] Based on the above findings, the procedure for extracting the optimal frequency, that is, for determining the control frequency Fcn, is described below.

[0081] Air-fuel ratio oscillation control is initiated, and the frequency Frq of the air-fuel ratio oscillation is decreased from the reference frequency F0 by a predetermined frequency ΔF. The downstream minimum air-fuel ratio λr_min is detected, and the change in the downstream minimum air-fuel ratio λr_min detected before and after the frequency Frq is decreased by ΔF is calculated as the air-fuel ratio change Δλr_min. Then, by dividing the air-fuel ratio change Δλr_min by ΔF (in this embodiment, the change in the logarithm of the frequency log(Frq)), the slope glmba of the change in the downstream minimum air-fuel ratio λr_min with respect to the logarithm of the frequency log(Frq) is calculated, and its absolute value (=|glmba|) is compared with a predetermined value g01. The frequency Frq at which the absolute value of the slope glmba (=|glmba|) reaches the predetermined value g01 is identified, and this is set as the control frequency Fcn.

[0082] As mentioned earlier, setting the control frequency Fcn can be done not by comparing the slope glmba of the change in the downstream minimum air-fuel ratio λr_min with a predetermined value, but by identifying the frequency Frq at which a large change (i.e., a difference) exceeding a predetermined value occurs in the downstream minimum air-fuel ratio λr_min before and after decreasing the frequency Frq by ΔF, and setting that frequency Fcn as the optimal frequency.

[0083] The reference frequency F0 is the value initially set when performing air-fuel ratio oscillation. It may be set uniformly across the entire operating range of the engine E, or it may be set to a different value for each operating range. For a simple example, when setting it uniformly, 1 Hz (i.e., logF0=0) can be used as the reference frequency F0. Furthermore, an operating range map may be set up so that the control frequency Fcn can be stored for each operating range, and the specified control frequency Fcn may be stored or updated for each operating range.

[0084] Figure 6 is a flowchart showing the basic flow of fuel switching control according to this embodiment.

[0085] In S401, it is determined whether or not there is a fuel switching request. A fuel switching request is generated when the remaining fuel level decreases to a predetermined level or when the driver instructs a fuel switching. For example, it is generated when the remaining amount Lfa in the liquid fuel tank decreases to a predetermined level Lfa1 while operating with liquid fuel, when the remaining amount Lfb in the gaseous fuel tank decreases to a predetermined level Lfb1 while operating with gaseous fuel, when the driver operates the fuel switching switch 209 to instruct a switch to gaseous fuel while operating with liquid fuel, or when the driver operates the fuel switching switch 209 to instruct a switch to liquid fuel while operating with gaseous fuel. If there is a fuel switching request, the process proceeds to S402; if there is no fuel switching request, the current control is terminated. The process in S401 corresponds to the process executed by the engine controller 101 as the "fuel switching request detection means" according to this embodiment.

[0086] In S402, it is determined whether or not air-fuel ratio vibration control is currently being performed. If air-fuel ratio vibration control is being performed, the process proceeds to S403; otherwise, the current control is terminated.

[0087] In S403, the currently used fuel is determined. If the currently used fuel is a liquid fuel (gasoline in this embodiment), the process proceeds to S404; if it is a gaseous fuel (compressed natural gas), the process proceeds to S407.

[0088] In S404, it is determined whether the current air-fuel ratio is lean or not. If it is lean, the process proceeds to S405. If it is not lean, i.e., if the current air-fuel ratio is rich, the process in S404 is repeated until the air-fuel ratio shifts to lean.

[0089] In S405, it is determined whether or not it is time to shift the air-fuel ratio from lean to rich. If it is time to shift to rich, the process proceeds to S406. If it is not yet time to shift to rich, the process returns to S404 and waits until it is time to shift to rich.

[0090] In the S406, the fuel switch from liquid fuel to gaseous fuel, that is, the switch from gasoline to compressed natural gas, is synchronized with the transition of the air-fuel ratio from lean to rich.

[0091] The timing of the air-fuel ratio transition from lean to rich can be estimated by determining it as the point when half a cycle of the air-fuel ratio oscillation has elapsed since the previous transition to the lean side, i.e., half a period corresponding to the control frequency Fcn. In simpler terms, the system can be instructed to switch to gaseous fuel when the air-fuel ratio is lean, resulting in the fuel switching occurring at or near the timing of the air-fuel ratio transition.

[0092] In S407, it is determined whether the current air-fuel ratio is on the rich side. If it is on the rich side, the process proceeds to S408. If it is not on the rich side, i.e., if the current air-fuel ratio is on the lean side, the process in S407 is repeated until the air-fuel ratio shifts to the rich side.

[0093] In S408, it is determined whether or not it is time to shift the air-fuel ratio from rich to lean. If it is time to shift to lean, the process proceeds to S409. If it is not yet time to shift to lean, the process returns to S407 and waits until it is time to shift to lean.

[0094] In the S409, the fuel switch from gaseous fuel to liquid fuel, specifically from compressed natural gas to gasoline, is synchronized with the transition of the air-fuel ratio from rich to lean.

[0095] As mentioned earlier, the timing of the transition of the air-fuel ratio from rich to lean can be determined as the time elapsed equivalent to half a cycle of the air-fuel ratio oscillation since the previous transition to the rich side. Alternatively, instead of determining the timing of the transition to lean, the fuel may be switched at or near the timing of the air-fuel ratio transition as a result of instructing the switch to liquid fuel when the air-fuel ratio is rich.

[0096] The processes from S403 to S409 correspond to the processes executed by the engine controller 101 as the "fuel switching execution means" according to this embodiment.

[0097] The air-fuel ratio control device for engine E according to this embodiment has the above configuration. The effects obtained by this embodiment will be described below.

[0098] In this embodiment, when the controller 101 detects a fuel switching request to switch between liquid fuel and gaseous fuel (fuel switching request detection means), it switches the fuel according to the request (fuel switching execution means). Here, when switching from liquid fuel to gaseous fuel, a decrease in engine torque occurs (decreasing torque fluctuation), while when switching from gaseous fuel to liquid fuel, an increase in engine torque occurs (increasing torque fluctuation). This torque fluctuation is amplified because the fuel switching is performed in synchronization with the air-fuel ratio oscillation due to air-fuel ratio oscillation control. Specifically, the decrease in torque fluctuation is amplified when the switching from liquid fuel to gaseous fuel is performed in synchronization with the transition of the air-fuel ratio from the rich side to the lean side, and the increase in torque fluctuation is amplified when the switching from gaseous fuel to liquid fuel is performed in synchronization with the transition of the air-fuel ratio from the lean side to the rich side. Figure 3 is an explanatory diagram illustrating the effect of synchronization of air-fuel ratio oscillations in response to fuel switching on engine torque fluctuations. In this embodiment, synchronization patterns that result in large torque fluctuations are targeted for avoidance.

[0099] In contrast, in this embodiment, firstly, the fuel switching from liquid fuel to gaseous fuel is performed in synchronization with the transition of the air-fuel ratio from lean to rich, while the fuel switching from gaseous fuel to liquid fuel is performed in synchronization with the transition of the air-fuel ratio from rich to lean. This makes it possible to avoid a situation where fluctuations in engine torque associated with fuel switching are amplified by synchronization with the vibration of the air-fuel ratio. In particular, according to this embodiment, fluctuations in engine torque associated with fuel switching can be canceled out by torque fluctuations in the opposite direction associated with the transition of the air-fuel ratio, thus making it possible to suppress fluctuations in engine torque when switching fuel (Figure 3). Furthermore, this control can be performed while continuing air-fuel ratio vibration control.

[0100] Thus, according to this embodiment, in an engine E capable of switching between liquid fuel and gaseous fuel, the purification rate by the exhaust gas purification catalyst 331 is improved by air-fuel ratio vibration control, and fluctuations in engine torque associated with fuel switching are exacerbated by synchronization with the vibration of the air-fuel ratio, thereby avoiding situations that would have substantial adverse effects on drivability and ride comfort.

[0101] In this embodiment, compressed natural gas, used as fuel, can reduce carbon dioxide emissions compared to gasoline. However, methane, which is the main component of the exhaust, is known to have about 25 times the greenhouse effect of carbon dioxide. Therefore, it is important to suppress methane emissions in order to maintain the advantages over using gasoline. However, methane is chemically stable compared to other HC species, and it is difficult to achieve a sufficient purification rate by applying the same oxidation purification methods as for other HC species.

[0102] By appropriately adjusting the control frequency Fcn, air-fuel ratio oscillation control makes it possible to specifically improve the methane purification rate.

[0103] Secondly, based on the air-fuel ratio λr detected by the downstream exhaust sensor 207, an appropriate frequency is identified as the air-fuel ratio oscillation frequency (control frequency Fn) when performing air-fuel ratio oscillation control. This makes it possible to perform air-fuel ratio oscillation control at an 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), thereby further improving the purification rate of the exhaust purification catalyst 331.

[0104] Thirdly, by calculating the slope glmba of the change in the minimum air-fuel ratio (downstream minimum air-fuel ratio λr_min) detected by the downstream exhaust sensor 207 with respect to the logarithm of the air-fuel ratio oscillation frequency, and identifying the frequency at which the absolute value of this slope (=|glmba|) reaches a predetermined value g01 as the control frequency Fcn, it is possible to identify an appropriate control frequency Fcn with a relatively simple configuration, and to perform air-fuel ratio oscillation control under a more appropriate control frequency Fcn.

[0105] Furthermore, by setting a predetermined value g01 within the range of 0.015 to 0.025, it becomes possible to identify a more appropriate frequency as the control frequency Fcn.

[0106] Control to avoid the exacerbation of engine torque fluctuations associated with fuel switching by synchronization with air-fuel ratio oscillations is not limited to canceling out engine torque fluctuations associated with fuel switching with torque fluctuations in the opposite direction associated with the air-fuel ratio transition, but can also be achieved by temporarily stopping air-fuel ratio oscillation control and stopping air-fuel ratio oscillations. In this embodiment, this is achieved by switching the air-fuel ratio control from air-fuel ratio oscillation control to air-fuel ratio feedback control.

[0107] Figure 14 is a flowchart showing the basic flow of fuel switching control according to another embodiment of the present invention, as an example of its application in that case.

[0108] The processing in S501 and S502 is the same as the processing in S401 and S402 in the flowchart shown in Figure 6. Specifically, in S501, it is determined whether or not there is a fuel switching request. If there is a fuel switching request, the process proceeds to S502. If there is no fuel switching request, the current control is terminated. In S502, it is determined whether or not air-fuel ratio vibration control is being executed. If air-fuel ratio vibration control is being executed, the process proceeds to S503. If it is not being executed, the current control is terminated. The processing in S501 corresponds to the processing executed by the engine controller 101 as the "fuel switching request detection means" according to this embodiment.

[0109] In S503, the air-fuel ratio vibration control is forcibly disabled.

[0110] In S504, air-fuel ratio feedback control is initiated. In other words, the processes in S503 and S504 switch the air-fuel ratio control from air-fuel ratio vibration control to air-fuel ratio feedback control. The processes in S503 and S504 correspond to the processes executed by the engine controller 101 as the "air-fuel ratio vibration stopping means" according to this embodiment.

[0111] In S505, the currently used fuel is determined. If the currently used fuel is a liquid fuel (gasoline in this embodiment), the process proceeds to S506; if it is a gaseous fuel (compressed natural gas), the process proceeds to S507.

[0112] In the S506, the fuel is switched from liquid fuel to gaseous fuel, that is, from gasoline to compressed natural gas.

[0113] In S507, the fuel is switched from a gaseous fuel to a liquid fuel, that is, from compressed natural gas to gasoline. The processes from S505 to S507 correspond to the processes executed by the engine controller 101 as the "fuel switching execution means" according to this embodiment.

[0114] In this embodiment, when the controller 101 detects a fuel switching request to switch between liquid fuel and gaseous fuel (fuel switching request detection means), it stops the vibration of the air-fuel ratio by air-fuel ratio vibration control (air-fuel ratio vibration stopping means), and while the vibration of the air-fuel ratio is stopped, it performs the fuel switching (fuel switching execution means). Here, when switching from liquid fuel to gaseous fuel, a torque fluctuation in the decreasing direction occurs, and when switching from gaseous fuel to liquid fuel, a torque fluctuation in the increasing direction occurs. This fluctuation is amplified because the fuel switching is performed in synchronization with the vibration of the air-fuel ratio by air-fuel ratio vibration control (Figure 3).

[0115] In contrast, in this embodiment, when switching fuels, the vibration of the air-fuel ratio is stopped by air-fuel ratio vibration control, thereby suppressing the fluctuation in engine torque caused by the vibration of the air-fuel ratio itself, and avoiding a situation where the fluctuation in engine torque associated with fuel switching is exacerbated by synchronization with the vibration of the air-fuel ratio.

[0116] Thus, according to this embodiment, in an engine E capable of switching between liquid fuel and gaseous fuel, the purification rate by the exhaust gas purification catalyst 331 is improved by air-fuel ratio vibration control, and fluctuations in engine torque associated with fuel switching are exacerbated by synchronization with the vibration of the air-fuel ratio, thereby avoiding situations that would have substantial adverse effects on drivability and ride comfort.

[0117] Furthermore, in this embodiment, when a fuel switching request is detected, the air-fuel ratio control is switched from air-fuel ratio oscillation control to air-fuel ratio feedback control, and the fuel switching is performed while the air-fuel ratio feedback control is being executed.

[0118] This ensures that the air-fuel ratio is maintained at a stoichiometric value when switching fuels, thus avoiding fluctuations in engine torque caused by air-fuel ratio fluctuations. In other words, it prevents the situation where fluctuations in engine torque associated with fuel switching are exacerbated by synchronization with air-fuel ratio fluctuations.

[0119] In the above explanation, when setting the control frequency Fcn, we focused on the slope glmba of the change in the downstream minimum air-fuel ratio λr_min with respect to the frequency Frq, or the difference in 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 it is also possible to set it by focusing on the slope of the change in the difference (hereinafter referred to as the "air-fuel ratio range") Rlmb with respect to frequency Frq between the maximum value (hereinafter referred to as the "downstream maximum air-fuel ratio") λr_max and the minimum value λr_min detected by the downstream exhaust sensor 207.

[0120] Figure 15 is a flowchart showing the basic flow of air-fuel ratio oscillation control according to yet another embodiment of the present invention, as an example of its application in that case.

[0121] In this embodiment, instead of the process in S207 of the flowchart shown in Figure 4, the air-fuel ratio range Rlmb (=λr_max-λr_min) is calculated (S601), and instead of the process in S208, the slope glmbb of the change of 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 slope (=|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 to a value in the range of 0.015 to 0.025, thereby making it possible to set an appropriate control frequency Fcn.

[0122] Figures 16 and 17 are graphs of experimental data showing the relationship between the air-fuel ratio oscillation frequency Frq, the purification rate η of the exhaust gas purification catalyst 331, and the air-fuel ratio range Rlmb in the air-fuel ratio oscillation control according to this embodiment, for different catalyst temperatures Tcat. Figure 16(a) shows the case for a relatively low catalyst temperature Tcat1, and Figure 16(b) shows the case for a catalyst temperature Tcat2, which is higher than Tcat1. Figure 17(a) shows the case for a catalyst temperature Tcat3, which is higher than Tcat2, and Figure 17(b) shows the case for a catalyst temperature Tcat4, which is higher than Tcat3.

[0123] In Figures 16 and 17, the horizontal axis represents the logarithm of the frequency Frq, and the vertical axis represents the purification rate η and the air-fuel ratio range Rlmb. The open squares represent the purification rate ηthc for total hydrocarbons THC, and the open circles represent the purification rate ηnox for nitrogen oxides NOx. The triangles represent the air-fuel ratio range Rlmb, with open triangles indicating measurements in the low-frequency range below the frequency at which the purification rate η is maximum, and black triangles indicating measurements in the high-frequency range. The thick dotted line is an approximate straight line for the air-fuel ratio range Rlmb in the low-frequency range, and the thick solid line is an approximate straight line for the air-fuel ratio range Rlmb in the high-frequency range.

[0124] As shown in Figures 16 and 17, the purification rate η increases with increasing catalyst temperature Tcat for both total hydrocarbons and nitrogen oxides, and it can be seen that a high purification rate η tends to be maintained up to higher frequencies Frq. Here, when the frequency Frq is decreased by a predetermined frequency ΔF, it can be observed that near the frequency at which the purification rate η is maximum, the approximation curve of the air-fuel ratio range Rlmb shifts from a solid line to a dotted line, and the slope of the change with respect to frequency Frq, glmbb, and the absolute value of that slope (=|glmbb|) increase.

[0125] In this embodiment, the frequency at which this phenomenon occurs is extracted as a singularity by comparing it with a predetermined value g02, and set as the control frequency Fcn.

[0126] In this way, the slope glmbb of the change in 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) with respect to the logarithm of the air-fuel ratio oscillation frequency is calculated, and the frequency at which the absolute value of this slope (=|glmbb|) reaches a predetermined value g02 is identified as the control frequency Fcn. This makes it possible to identify an appropriate control frequency Fcn with a relatively simple configuration, and to perform air-fuel ratio oscillation control under a more appropriate control frequency Fcn.

[0127] Furthermore, by setting a predetermined value g02 within the range of 0.015 to 0.025, it becomes possible to identify a more appropriate frequency as the control frequency Fcn.

[0128] The setting of the control frequency Fcn using the air-fuel ratio range Rlmb can be applied not only to engine E, which has multiple cylinders connected in parallel to each other, as shown in Figure 1, but also to engines in which multiple cylinders are divided into multiple cylinder groups and different cylinder groups are connected in parallel.

[0129] Figure 18 is a schematic diagram showing the overall configuration of an engine E according to yet another embodiment of the present invention, as an example of its application in this case. In this embodiment, the engine E divides four cylinders into two cylinder groups, connects the different cylinder groups in parallel, and also connects each cylinder constituting a cylinder group in parallel. The setting of the control frequency Fcn by the air-fuel ratio range Rlmb is suitably applicable in such an engine E when the phases of the air-fuel ratio oscillations between different cylinder groups are set in opposite directions, in other words, the phases of the air-fuel ratio oscillations are inverted.

[0130] The determination of which operating region A or B the engine E is in can be performed not only directly using the catalyst temperature Tcat and exhaust flow rate Qexh, but also indirectly using the air-fuel ratio λr detected by the downstream exhaust sensor 207.

[0131] Specifically, based on the downstream air-fuel ratio λr, the maximum and minimum values ​​of the exhaust air-fuel ratio in air-fuel ratio oscillation control are detected. If the detected minimum air-fuel ratio (i.e., the downstream minimum air-fuel ratio λr_min) is less than or equal to a predetermined judgment value and the detected maximum air-fuel ratio (i.e., the downstream maximum air-fuel ratio λr_max) is less than or equal to a predetermined judgment value, it is determined that the operating state of engine E is in the second region B, and air-fuel ratio feedback control is executed.

[0132] Figure 19 is a flowchart showing the overall flow of air-fuel ratio control according to yet another embodiment of the present invention, as an example of its application in that case.

[0133] In this embodiment, instead of the process at S103 in the flowchart shown in Figure 2, the process at S1031 is executed.

[0134] After reading predetermined state parameters indicating the operating state of engine E, such as engine rotation speed Ne (S101), S102 determines whether the intake air volume Qa is greater than a predetermined flow rate Qa1. If it is greater than the predetermined flow rate Qa1, the process proceeds to S104; if it is less than or equal to the predetermined flow rate Qa1, the process proceeds to S1031. The flow rate Qa1 is the intake air volume corresponding to the first exhaust flow rate Qexh1 that defines the boundary between the first region A and the second region B shown in Figure 3.

[0135] In S1031, the maximum value λr_max and minimum value λr_min of the downstream air-fuel ratio λr are detected, and it is determined whether the downstream maximum air-fuel ratio λr_max is less than or equal to a predetermined determination value SL12 and the downstream minimum air-fuel ratio λr_min is less than or equal to a predetermined determination value SL11. If the downstream maximum air-fuel ratio λr_max is less than or equal to a predetermined determination value SL12 and the downstream minimum air-fuel ratio λr_min is less than or equal to a predetermined determination value SL11, the process proceeds to S105; otherwise, the process proceeds to S104.

[0136] In S104, air-fuel ratio vibration control is implemented.

[0137] The S105 implements air-fuel ratio feedback control.

[0138] The process of S1031 shown in Figure 19 will be explained in more detail below with reference to Figures 20 to 26.

[0139] Figures 20 to 23 are graphs of experimental data obtained when the frequency Frq of the air-fuel ratio oscillation was switched between several different frequencies, showing the changes in the purification rate η, the downstream minimum air-fuel ratio λr_min, and the downstream maximum air-fuel ratio λr_max with respect to the catalyst temperature Tcat. In this embodiment, the catalyst inlet gas temperature Tcat_in is used as the catalyst temperature Tcat. Figure 20 shows the change in the total hydrocarbon purification rate ηthc by the exhaust gas purification catalyst 331, and Figure 21 shows the change in the nitrogen oxide purification rate ηnox by the exhaust gas purification catalyst 331. Figure 22 shows the change in the downstream minimum air-fuel ratio λr_min, and Figure 23 shows the change in the downstream maximum air-fuel ratio λr_max.

[0140] In Figures 20 to 23, the white triangles connected by a dashed line represent a frequency Frq of 0.05 [Hz], and the white squares connected by a long dotted line represent a frequency Frq of 0.1 [Hz]. Furthermore, the white circles connected by a dashed line represent a frequency Frq of 0.2 [Hz], and the white triangles connected by a short dotted line represent a frequency Frq of 0.5 [Hz]. The black circles connected by a thick solid line represent a frequency Frq of 1 [Hz], a frequency of 1 [Hz] that corresponds to the frequency obtained by air-fuel ratio feedback control.

[0141] Referring to Figures 20 and 21, the overall trend is that as the catalyst temperature Tcat increases, that is, as the exhaust gas purification catalyst 331 becomes more active, the purification rates ηthc and ηnox for total hydrocarbons and nitrogen oxides also increase. Furthermore, when air-fuel ratio oscillation control is performed at frequencies below 0.5 Hz, compared to when oscillation is applied at a frequency of 1 Hz, which corresponds to air-fuel ratio feedback control, higher purification rates ηthc and ηnox are obtained, especially in the low-temperature range. It can be seen that at 0.5 Hz and 0.2 Hz, higher purification rates ηthc and ηnox are obtained across the entire temperature range than at 1 Hz.

[0142] Referring to Figure 22, it can be seen that the downstream minimum air-fuel ratio λr_min decreases as the catalyst temperature Tcat increases. This corresponds to an increase in the purification rate η of the exhaust gas purification catalyst 331, that is, the degree of activation of the catalyst 331. This is because when the air-fuel ratio shifts to the rich side, the exhaust gas purification catalyst 331 generates hydrogen, and the downstream exhaust sensor 207 reacts to this hydrogen. Thus, there is a correlation between the catalyst temperature Tcat and the downstream minimum air-fuel ratio λr_min, and it is possible to indirectly grasp the catalyst temperature Tcat, that is, the activity state of the exhaust gas purification catalyst 331, from the downstream minimum air-fuel ratio λr_min. In this embodiment, the downstream minimum air-fuel ratio λr_min is used for region determination in air-fuel ratio control (S1031 in Figure 19).

[0143] Here, when the frequency is 0.05 Hz, as with other frequencies, the downstream minimum air-fuel ratio λr_min decreases as the catalyst temperature Tcat increases (Figure 22), but the nitrogen oxide purification rate ηnox is particularly low compared to other frequencies. This is presumed to be because the frequency of 0.05 Hz is too low relative to the oxygen storage capacity of the exhaust gas purification catalyst 331, and the oxygen storage capacity can no longer absorb the fluctuations in the air-fuel ratio due to air-fuel ratio oscillations, resulting in a state where the air-fuel ratio in the reaction field of catalyst 331 is close to repeatedly fluctuating between a steady rich state and a steady lean state.

[0144] Thus, simply having a decrease in the downstream minimum air-fuel ratio λr_min is not sufficient to determine if the oxygen storage capacity has failed. In fact, it could lead to an incorrect judgment that the exhaust gas purification catalyst 331 has become more active, even though a sufficient purification rate η has not been achieved.

[0145] Such a situation can be avoided by referring to the downstream maximum air-fuel ratio λr_max. As shown in Figure 23, the downstream maximum air-fuel ratio λr_max maintains a relatively high value, for example, a value higher than 1, even when the catalyst temperature Tcat rises at a frequency of 0.05 [Hz]. In this embodiment, in the region determination (S1031) of the flowchart shown in Figure 19, the downstream maximum air-fuel ratio λr_max is referred to in addition to the downstream minimum air-fuel ratio λr_min.

[0146] Figures 24 to 26 are distribution diagrams showing the exhaust gas purification catalyst purification rate η assigned to the downstream minimum air-fuel ratio λr_min and the downstream maximum air-fuel ratio λr_max. In each of Figures 24 to 26, the purification rate η obtained under different exhaust gas flow rates Qexh is shown superimposed. For example, in addition to the purification rate η obtained when the SV value is 26k, the purification rates η obtained when the SV values ​​are 49k and 98k are also shown superimposed. The purification rate η when controlled by air-fuel ratio oscillation is shown by a circle plot, and the purification rate η when controlled by air-fuel ratio feedback is shown by an X plot. For each of the circle and X plots, a darker color indicates a higher purification rate η. Figure 24 shows the total hydrocarbon purification rate ηthc, Figure 25 shows the nitrogen oxide purification rate ηnox, and Figure 26 shows the average purification rate ηave for total hydrocarbons and nitrogen oxides.

[0147] Referring to Figures 24 to 26, it can be seen that the lower left of the figure, that is, the lower both the downstream minimum air-fuel ratio λr_min and the downstream maximum air-fuel ratio λr_max, the higher the purification rate η tends to be. Due to this trend, in Figures 24 to 26, within the thick dotted box where the downstream minimum air-fuel ratio λr_min is below the threshold SL11 and the downstream maximum air-fuel ratio λr_max is below the threshold SL12, the catalyst 331 is activated to such an extent that a high purification rate η can be obtained by either air-fuel ratio feedback control or air-fuel ratio oscillation control. For example, when the threshold SL11 is set to 0.965 and the threshold SL12 is set to 1.000, it has been confirmed that a purification rate η of 80% or more can be obtained for total hydrocarbons, nitrogen oxides, and their averages within the boxed region. In other words, in step S1031 of the flowchart shown in Figure 19, by setting the judgment value SL11 to, for example, 0.965 and the judgment value SL12 to, for example, 1.000, it is possible to appropriately determine that the operating state of engine E is in the second region B, where exhaust gas purification is easy. The judgment values ​​SL11 and SL12 can be set as appropriate within the ranges of 0.960 ± 0.01 and 1.000 ± 0.01, respectively.

[0148] Thus, by configuring the system to estimate whether the operating state of engine E is in the second region B, or in other words, whether the exhaust gas purification is easy or difficult, based on the air-fuel ratio (downstream air-fuel ratio λr) detected by the downstream exhaust sensor 207 during the execution of air-fuel ratio vibration control, it becomes possible to appropriately switch from air-fuel ratio vibration control to air-fuel ratio feedback control, and to implement a simpler configuration by suppressing an increase in the number of parts.

[0149] Thus, in this embodiment, by comparing the downstream minimum air-fuel ratio λr_min and the downstream maximum air-fuel ratio λr_max with their respective thresholds, it is possible to determine whether the exhaust gas is being purified under conditions that are easy or difficult, without relying on the exhaust gas flow rate Qexh. Therefore, the determination based on the exhaust gas flow rate Qexh, or in this embodiment, the intake air volume Qa (S102 in Figure 19), may be omitted. However, the intake air volume Qa is a state variable that is generally acquired for engine control, not limited to this embodiment, and does not require the addition of any parts to acquire it. Furthermore, it is possible to determine when air-fuel ratio oscillation control should be performed using a simple method that utilizes the intake air volume Qa. Therefore, the determination based on the intake air volume Qa is effective in ensuring the stability of the control.

[0150] Furthermore, in the above explanation, the catalyst inlet gas temperature Tcat_in was used as the catalyst temperature Tcat of the exhaust gas purification catalyst 331. However, the catalyst temperature Tcat is not limited to this, and can also be directly obtained by installing a temperature sensor such as a thermocouple in the bed portion of the exhaust gas purification catalyst 331. [Explanation of Symbols]

[0151] E...Engine, 1...Engine block, 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 catalytic converter, 41...Fuel injector, 41a...First (liquid fuel) fuel injector, 41b...Second (gaseous fuel) fuel injector, 51...Spark plug, 101...Engine controller, 201...Accelerator sensor, 202...Engine rotation speed sensor, 203...Airflow meter, 204...Coolant temperature sensor, 205...Catalytic converter temperature sensor, 206...Upstream exhaust sensor, 207...Downstream exhaust sensor, 208...Fuel level sensor, 208a...First (liquid fuel tank) fuel level sensor, 208b...Second (gaseous fuel tank) fuel level sensor, 209...Fuel selection switch.

Claims

1. An air-fuel ratio control device for an engine equipped with an exhaust gas purification catalyst in the exhaust passage and configured to operate by switching between liquid fuel and gaseous fuel supplied to the combustion chamber, An operating status sensor that outputs a signal according to the engine's operating status, The system includes a controller that controls the engine's operating state based on a signal output by the aforementioned operating state sensor, The aforementioned controller, An air-fuel ratio vibration control means that performs air-fuel ratio vibration control to cause the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification catalyst to fluctuate between the rich side and the lean side with respect to the stoichiometric value, A fuel switching request detection means for detecting a fuel switching request to switch the fuel from the liquid fuel to the gaseous fuel or from the gaseous fuel to the liquid fuel, The system includes a fuel switching execution means that switches the fuel when the fuel switching request is detected by the fuel switching request detection means, The fuel switching execution means is, The switching of the fuel from the liquid fuel to the gaseous fuel is performed at a timing other than the transition of the air-fuel ratio from the rich side to the lean side due to the air-fuel ratio oscillation control, An engine air-fuel ratio control device that performs the switching of the fuel from the gaseous fuel to the liquid fuel at a timing other than the transition of the air-fuel ratio from lean to rich due to the air-fuel ratio oscillation control.

2. The fuel switching execution means is, The switching of the fuel from the liquid fuel to the gaseous fuel is performed in synchronization with the transition of the air-fuel ratio from lean to rich due to the air-fuel ratio oscillation control, The air-fuel ratio control device for an engine according to claim 1, wherein the switching of the fuel from the gaseous fuel to the liquid fuel is performed in synchronization with the transition of the air-fuel ratio from the rich side to the lean side by the air-fuel ratio oscillation control.

3. An air-fuel ratio control device for an engine equipped with an exhaust gas purification catalyst in the exhaust passage and configured to operate by switching between liquid fuel and gaseous fuel supplied to the combustion chamber, An operating status sensor that outputs a signal according to the engine's operating status, The system includes a controller that controls the engine's operating state based on a signal output by the aforementioned operating state sensor, The aforementioned controller, An air-fuel ratio vibration control means that performs air-fuel ratio vibration control to cause the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification catalyst to fluctuate between the rich side and the lean side with respect to the stoichiometric value, A fuel switching request detection means for detecting a fuel switching request to switch the fuel from the liquid fuel to the gaseous fuel or from the gaseous fuel to the liquid fuel, When the fuel switching request is detected by the fuel switching request detection means, the fuel switching execution means switches the fuel, When the fuel switching request is detected by the fuel switching request detection means, the system includes an air-fuel ratio vibration stopping means for stopping the vibration of the air-fuel ratio, The fuel switching execution means is an engine air-fuel ratio control device that performs the fuel switching while the air-fuel ratio vibration stopping means is stopping the vibration of the air-fuel ratio.

4. The aforementioned operating state sensor includes an exhaust sensor installed in the exhaust passage and configured to output a signal corresponding to the air-fuel ratio of the exhaust flowing through the exhaust passage, The controller further comprises an air-fuel ratio feedback control means that performs air-fuel ratio feedback control to control the air-fuel ratio of the exhaust flowing into the exhaust purification catalyst to a stoichiometric value based on the signal output by the exhaust sensor. The air-fuel ratio control device for an engine according to claim 3, wherein the air-fuel ratio vibration stopping means stops the vibration of the air-fuel ratio when the fuel switching request is detected by the fuel switching request detection means by switching the control of the air-fuel ratio from the air-fuel ratio vibration control means to the air-fuel ratio feedback control means.

5. The aforementioned operating state sensor includes a downstream exhaust sensor installed downstream of the exhaust purification catalyst in the exhaust passage and configured to output a signal corresponding to the air-fuel ratio of the exhaust flowing through the exhaust passage. The controller further includes a control frequency identification means that identifies the frequency at which the air-fuel ratio is vibrated by the air-fuel ratio vibration control, based on the air-fuel ratio detected by the downstream exhaust sensor, as the control frequency. The air-fuel ratio vibration control means, after the control frequency is identified by the control frequency identification means, causes the air-fuel ratio to vibrate at the control frequency, as described in any one of claims 1 to 4.

6. The air-fuel ratio control device for an engine according to claim 5, wherein the control frequency determination means determines the frequency at which the slope of the change in the minimum value of the air-fuel ratio detected by the downstream exhaust sensor or the difference between the maximum and minimum values ​​of the air-fuel ratio reaches a predetermined value with respect to the logarithm of the frequency of the air-fuel ratio oscillation, the control frequency.

7. The air-fuel ratio control device for an engine according to claim 6, wherein the control frequency determining means determines the frequency at which the absolute value of the slope reaches a preset value in the range of 0.015 to 0.025 as the control frequency.

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