Engine control device

Abstract

To provide an engine control device capable of improving NOx purification performance while preventing a catalyst device from becoming excessively high in temperature.SOLUTION: When the accelerator opening degree is less than a predetermined accelerator determination opening degree, execute a fuel cut for stopping fuel supply by the fuel supply device 11, and when the fuel cut is executed, execute intake air amount increase control for controlling the intake air amount adjusting device 22 such that the intake air amount is larger when the catalyst temperature specified by the catalyst temperature specifying device is high than when the catalyst temperature is low; When fuel cut is finished and fuel supply by a fuel supply device 11 is restarted, fuel amount increase control for controlling the fuel supply device 11 so that a fuel amount supplied to a combustion chamber 5 becomes larger than a basic fuel amount is performed, and intake air amount increase control is prohibited for a predetermined period after the fuel amount increase control is finished.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to an engine control device. [Background technology]

[0002] A catalytic converter installed in the engine's exhaust passage to purify exhaust gas deteriorates when exposed to high temperatures. Therefore, in order to maintain good engine exhaust gas performance, it is necessary to prevent the catalytic converter from becoming excessively hot.

[0003] Patent document 1 discloses an engine that, in order to prevent the catalytic converter from overheating, increases the amount of air drawn into the combustion chamber during fuel cutoff (fuel cut), which stops the fuel supply to the combustion chamber, and introduces a large amount of air into the exhaust passage and ultimately the catalytic converter, thereby cooling the catalytic converter with that air. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-132185 Summary of the Invention [Problem to be solved by the invention]

[0005] When a catalytic converter includes a three-way catalyst, oxygen is stored in the catalyst during a fuel cut. Therefore, when the fuel cut ends and fuel supply resumes, the air-fuel ratio in the catalytic converter is leaner than the stoichiometric air-fuel ratio, making it difficult to reduce NOx. Therefore, simply applying the configuration of Patent Document 1 to a vehicle with a catalytic converter that includes a three-way catalyst could result in a significant decrease in NOx purification performance due to the large amount of oxygen stored in the catalytic converter during a fuel cut. In other words, the configuration of Patent Document 1 leaves room for improvement in terms of NOx purification performance.

[0006] The present invention has been made in consideration of the above circumstances, and aims to provide an engine control device that can improve NOx purification performance while preventing the catalyst device from becoming excessively hot. [Means for solving the problem]

[0007] The engine control device according to the present invention is an engine control device provided in a vehicle equipped with an accelerator pedal, and includes an engine body having a combustion chamber formed therein, an exhaust passage and an intake passage respectively connected to the engine body, a fuel supply device that supplies fuel to the combustion chamber, an intake amount adjustment device that adjusts the intake amount which is the amount of air taken into the combustion chamber, a catalytic device including a three-way catalyst, a catalyst temperature specifying device that specifies the catalyst temperature which is the temperature of the catalytic device, an accelerator opening detection device that detects an accelerator opening which is the opening of the accelerator pedal, and a control unit that calculates a basic fuel amount based on the accelerator opening detected by the accelerator opening detection device, and controls the fuel supply device so that the amount of fuel supplied to the combustion chamber becomes the basic fuel amount. The control unit performs a fuel cut to stop fuel supply by the fuel supply device when the accelerator opening detected by the accelerator opening detection device is less than a predetermined accelerator judgment opening, and when the catalyst temperature identified by the catalyst temperature identification device is high during the fuel cut, performs intake amount increase control to control the intake amount adjustment device so that the intake amount is larger than when the catalyst temperature is low, and when the fuel cut is ended and fuel supply by the fuel supply device is resumed, performs fuel amount increase control to control the fuel supply device so that the amount of fuel supplied to the combustion chamber is larger than the basic fuel amount, and prohibits the intake amount increase control for a predetermined period after the fuel amount increase control is ended (claim 1).

[0008] In this invention, when the catalyst temperature is high, intake air amount increase control is implemented during fuel cut, which increases the intake air amount (the amount of air in the combustion chamber) and therefore the amount of air introduced into the exhaust passage. Therefore, by utilizing the timing of fuel cut, a large amount of air can be used to cool the catalyst device in the exhaust passage, preventing the temperature of the catalyst device from becoming excessively high.

[0009] Furthermore, when the fuel cut ends and fuel supply is resumed, fuel amount increase control is implemented to increase the amount of fuel supplied to the combustion chamber above the base fuel amount, which allows the oxygen stored in the three-way catalyst during the fuel cut to be consumed quickly through fuel oxidation, improving the NOx purification performance of the three-way catalyst.

[0010] However, if intake air amount increase control is performed when the exhaust gas contains a large amount of fuel due to the implementation of fuel increase control, the fuel resulting from the fuel increase control and the air resulting from the intake air amount increase control may react in the exhaust passage and the catalytic converter, causing the temperature of the catalytic converter to rise. In contrast, the present invention prohibits intake air amount increase control for a predetermined period after the fuel increase control ends. This prevents the above-mentioned reaction from occurring in the exhaust passage and the catalytic converter, which would otherwise cause the temperature of the catalytic converter to rise. Therefore, the present invention can reliably prevent the catalytic converter from overheating while improving the NOx purification performance of the catalytic converter.

[0011] In the above configuration, it is preferable to further include an upstream air-fuel ratio detection device that detects the air-fuel ratio of the exhaust gas flowing into the catalytic device, and the control unit prohibits the intake amount increase control from the time the fuel amount increase control is completed until the air-fuel ratio of the exhaust gas detected by the upstream air-fuel ratio detection device becomes equal to or greater than the stoichiometric air-fuel ratio (claim 2).

[0012] With this configuration, the timing at which the inflow of exhaust gas containing increased fuel due to the implementation of fuel augmentation control into the catalytic device ends is estimated based on the air-fuel ratio of the exhaust gas detected by the upstream air-fuel ratio detection device, and intake air amount augmentation control is prohibited until this timing. This prevents the prohibition period of intake air amount augmentation control from becoming excessively long. This ensures the implementation period of intake air amount augmentation control, and more reliably cools the catalytic device.

[0013] In the above configuration, preferably, the control unit prohibits the intake amount increase control from the time the fuel amount increase control ends until the volume of exhaust gas discharged into the exhaust passage after the fuel amount increase control ends becomes equal to or greater than the volume of the exhaust passage between the engine body and the catalytic device (claim 3).

[0014] With this configuration, the timing at which the inflow of exhaust gas containing increased fuel due to the execution of the fuel amount increase control into the catalytic device ends is estimated based on the volume of the exhaust gas, and the intake air amount increase control is prohibited until this timing. This prevents the prohibition period of the intake air amount increase control from becoming excessively long. Therefore, the execution period of the intake air amount increase control can be secured, and the catalytic device can be cooled more reliably.

[0015] In the above configuration, it is preferable to further include a downstream air-fuel ratio detection device that detects the air-fuel ratio of the exhaust gas flowing out from the catalytic device, and the control unit performs the fuel amount increase control for a period from when fuel supply by the fuel supply device is resumed until the air-fuel ratio of the exhaust gas detected by the downstream air-fuel ratio detection device becomes equal to or lower than the stoichiometric air-fuel ratio (claim 4).

[0016] In this configuration, the timing at which the air-fuel ratio in the catalytic converter is no longer leaner than the stoichiometric air-fuel ratio is estimated based on the air-fuel ratio of the exhaust gas detected by the downstream air-fuel ratio detection device, and fuel amount increase control is performed until this timing. This prevents the implementation period of fuel amount increase control from becoming excessively long, which in turn prevents the prohibition period of intake air amount increase control from becoming long. This ensures the implementation period of intake air amount increase control, and more reliably cools the catalytic converter.

[0017] In the above configuration, when the fuel cut is not being performed and the catalyst temperature is high, the control unit limits the intake air amount to a predetermined upper limit intake air amount or less by the intake air amount adjusting device (claim 5).

[0018] According to this configuration, when fuel cut is not being performed and the catalyst temperature is high, the intake air volume is prevented from exceeding the upper intake air volume limit, thereby minimizing the combustion energy generated in the combustion chamber. This prevents the catalyst temperature from rising excessively. However, increasing the number of times the intake air volume is restricted increases the number of times engine output is restricted, which may result in a deterioration in vehicle drivability. In contrast, the present invention cools the catalyst device when fuel cut is performed as described above, thereby reducing the chances of the catalyst temperature rising excessively when fuel cut is not being performed. This prevents the catalyst temperature from rising excessively while suppressing a deterioration in drivability.

[0019] In the above-described configuration, the intake amount adjusting device may be a throttle valve provided in the intake passage for opening and closing the intake passage (claim 6). [Effects of the Invention]

[0020] As described above, the engine control device of the present invention can prevent the catalyst from becoming excessively hot while improving the NOx purification performance. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a schematic configuration diagram of an engine system according to an embodiment of the present invention. [Figure 2] 10 is a graph showing the relationship between the air-fuel ratio and the RO2 output voltage. [Figure 3] FIG. 2 is a diagram showing a control block of the engine system. [Figure 4] 10 is a flowchart showing a procedure for setting a catalyst protection flag. [Figure 5] 4 is a flowchart showing the contents of control performed by the PCM. [Figure 6] 10 is a flowchart showing the details of throttle control during non-FC. [Figure 7] 4 is a flowchart showing the content of non-FC fuel injection control. [Figure 8] 4 is a time chart showing the time changes of each parameter when the engine is running. DETAILED DESCRIPTION OF THE INVENTION

[0022] (Overall configuration of the engine system) FIG. 1 is a schematic diagram showing a preferred embodiment of an engine system E to which an engine control device according to the present invention is applied. The engine system E includes an engine body 1 that is driven by a supply of fuel, and an intake passage 20 and an exhaust passage 30 connected to the engine body 1. The intake passage 20 is a passage through which intake air, which is air introduced into the engine body 1, flows. The exhaust passage 30 is a passage through which exhaust gas discharged from the engine body 1 flows. The engine system E is installed in a vehicle such as an automobile as a power source for driving the vehicle.

[0023] The engine body 1 is a multi-cylinder engine having a plurality of cylinders 2A (only one of which is shown in FIG. 1). In this embodiment, the engine body 1 is a four-cylinder in-line engine, with the four cylinders 2A aligned in a direction perpendicular to the plane of the paper in FIG. 1. The engine body 1 includes a cylinder block 2 having the plurality of cylinders 2A formed therein, a cylinder head 3 attached to the upper surface of the cylinder block 2 so as to close the upper end openings of each cylinder 2A, and a plurality of pistons 4 housed in each cylinder 2A so as to be able to slide back and forth.

[0024] A combustion chamber 5 is defined above the piston 4 of each cylinder 2A. As will be described later, fuel is supplied to the combustion chamber 5. The mixture of the supplied fuel and air is burned in the combustion chamber 5, and the expansion force caused by the combustion causes the piston 4 to reciprocate up and down.

[0025] A crankshaft 13, which is the output shaft of the engine body 1, is provided at the bottom of the cylinder block 2 (below the pistons 4). The crankshaft 13 is connected to the pistons 4 of each cylinder 2A via connecting rods. The crankshaft 13 rotates around its central axis in response to the reciprocating motion (up and down movement) of the pistons 4.

[0026] A vehicle equipped with the engine system E is equipped with a multi-speed transmission as the transmission 60. The transmission 60 is also an automatic transmission, and the transmission 60 and the crankshaft 13 are connected via a torque converter and the like. The output of the engine body 1 is transmitted to the wheels 70 via the crankshaft 13, the torque converter, the transmission 60 and the like. In this embodiment, the transmission 60 is a six-speed multi-speed transmission, and realizes six forward gears with different gear ratios.

[0027] A crank angle sensor SN1 is attached to the cylinder block 2. The crank angle sensor SN1 detects the crank angle, which is the rotation angle of the crankshaft 13, and the engine speed, which is the rotation speed of the crankshaft 13.

[0028] An intake port 6 and an exhaust port 7 that communicate with the combustion chamber 5 are formed in the cylinder head 3 for each cylinder 2A. The cylinder head 3 is also equipped with an intake valve 8 that opens and closes the opening of the intake port 6 on the combustion chamber 5 side, and an exhaust valve 9 that opens and closes the opening of the exhaust port 7 on the combustion chamber 5 side, for each cylinder 2A.

[0029] The cylinder head 3 is fitted with one injector 11 for each cylinder 2A that supplies fuel into the combustion chamber 5. The engine body 1 is a gasoline engine, and the injector 11 injects fuel, including gasoline, into the combustion chamber 5. The injector 11 injects fuel into the combustion chamber 5. The injector 11 is a side-injection fuel injection valve, and its tip faces the combustion chamber 5 from the inner circumferential surface of the combustion chamber 5. The cylinder head 3 is fitted with one spark plug 10 for each cylinder 2A that ignites the fuel-air mixture in the combustion chamber 5. The spark plug 10 is arranged so that its tip, including the spark plug, faces the inside of the combustion chamber 5 from near the center of the ceiling surface of the combustion chamber 5. The injector 11 corresponds to the "fuel supply device" of the present invention.

[0030] The intake passage 20 is connected to the cylinder head 3 so as to communicate with the intake port 6 of each cylinder 2A. In the intake passage 20, an air cleaner 21, a throttle valve 22, and a surge tank 23 are arranged in this order from the upstream side in the flow direction of intake air.

[0031] The air cleaner 21 is a filter that removes foreign matter from the intake air. The throttle valve 22 is a valve that opens and closes the intake passage 20. The amount of intake air flowing through the intake passage 20, and therefore the amount of air drawn into the combustion chamber 5, is changed depending on the opening of the throttle valve 22. The surge tank 23 is a tank that provides space for evenly distributing the intake air to each cylinder 2A. The throttle valve 22 corresponds to the "intake air amount adjustment device" of the present invention.

[0032] An air flow sensor SN2, an intake air temperature sensor SN3, and an intake air pressure sensor SN4 are arranged in the intake passage 20. The air flow sensor SN2 detects the intake air volume, which is the amount of air taken into each combustion chamber 5 through the intake passage 20. The intake air temperature sensor SN3 detects the intake air temperature, which is the temperature of the air flowing through the intake passage 20. The intake air pressure sensor SN4 detects the intake air pressure, which is the pressure inside the intake passage 20. The air flow sensor SN2 and the intake air temperature sensor SN3 are arranged near the air cleaner 21 and detect the flow rate and temperature of air passing through the intake passage 20 near the air cleaner 21, respectively. The intake air pressure sensor SN4 is arranged in the surge tank 23 and detects the pressure inside the surge tank 23.

[0033] The exhaust passage 30 is connected to the cylinder head 3 so as to communicate with the exhaust ports 7 of each cylinder 2A. A catalytic device 31 is disposed in the exhaust passage 30. The catalytic device 31 includes a catalyst and is a device that purifies exhaust gas by utilizing the action of the catalyst.

[0034] The catalytic device 31 incorporates a three-way catalyst. As a result, when the air-fuel ratio of the exhaust gas is at or near the stoichiometric air-fuel ratio, the catalytic device 31 oxidizes HC (hydrocarbons) and CO (carbon monoxide) while reducing NOx (nitrogen oxides). Here, the three-way catalyst has the property of storing oxygen. Therefore, when the exhaust gas contains a large amount of oxygen, the catalytic device 31 stores the oxygen. When the amount of stored oxygen is large, the catalytic device 31 is unable to sufficiently reduce NOx.

[0035] A front O2 sensor SN5 and a rear O2 sensor SN6 are disposed in the exhaust passage 30. The front O2 sensor SN5 detects the oxygen concentration and air-fuel ratio contained in the exhaust gas. The rear O2 sensor SN6 detects the air-fuel ratio of the exhaust gas. The front O2 sensor SN5 is attached to a portion of the exhaust passage 30 upstream of the catalytic converter 31 (in the direction of exhaust gas flow) and detects the oxygen concentration and air-fuel ratio of the exhaust gas flowing into the catalytic converter. In this embodiment, the front O2 sensor SN5 is disposed near the upstream end of the catalytic converter 31. The rear O2 sensor SN6 is attached to a portion of the exhaust passage 30 downstream of the catalytic converter 31 (in the direction of exhaust gas flow) and detects the air-fuel ratio of the exhaust gas flowing out of the catalytic converter 31. Here, no component capable of changing the air-fuel ratio, such as a catalyst, is disposed between the rear O2 sensor SN6 and the catalytic converter 31, and the rear air-fuel ratio detected by the rear O2 sensor SN6 is approximately equal to the air-fuel ratio in the catalytic converter 31. The front O2 sensor SN5 corresponds to the "upstream air-fuel ratio detecting device" of the present invention, and the rear O2 sensor SN6 corresponds to the "downstream air-fuel ratio detecting device".

[0036] In the following, the air-fuel ratio of the exhaust gas passing through the location where the front O2 sensor SN5 is installed, and that is upstream of the catalytic device 31, will be referred to as the front air-fuel ratio. Also, the air-fuel ratio of the exhaust gas passing through the location where the rear O2 sensor SN6 is installed, and that is downstream of the catalytic device 31, will be referred to as the rear air-fuel ratio. Also, in the following, a large / small air-fuel ratio will be referred to as lean / rich, and specifically, when the proportion of oxygen in the exhaust gas is high and the air-fuel ratio of the exhaust gas is large, it will be referred to as lean, and the opposite will be referred to as rich.

[0037] The front O2 sensor SN5 is a so-called linear O2 sensor that outputs a voltage proportional to the oxygen concentration and air-fuel ratio of the exhaust gas. Meanwhile, the rear O2 sensor SN6 is a so-called λ sensor that detects whether the exhaust gas is near the stoichiometric air-fuel ratio, leaner than the stoichiometric air-fuel ratio, or richer than the stoichiometric air-fuel ratio. Specifically, the output voltage of the rear O2 sensor SN6 is as shown in FIG. 2. Hereinafter, the output voltage of the rear O2 sensor SN6 will be referred to as the RO2 output voltage where appropriate. When the exhaust gas air-fuel ratio is leaner than the stoichiometric air-fuel ratio, the RO2 output voltage is lower than a predetermined first voltage. When the exhaust gas air-fuel ratio is richer than the stoichiometric air-fuel ratio, the RO2 output voltage is higher than a predetermined second voltage. When the exhaust gas air-fuel ratio is stoichiometric, the RO2 output voltage is a stoichiometric voltage higher than the first voltage and lower than the second voltage. When the air-fuel ratio of the exhaust gas shifts from a state leaner than the stoichiometric air-fuel ratio to the stoichiometric air-fuel ratio, the RO2 output voltage increases from a voltage lower than the first voltage, via the first voltage, to the stoichiometric voltage. When the air-fuel ratio of the exhaust gas shifts from a state richer than the stoichiometric air-fuel ratio to the stoichiometric air-fuel ratio, the RO2 output voltage decreases from a voltage higher than the second voltage, via the second voltage, to the stoichiometric voltage.

[0038] In this embodiment, the exhaust passage 30 is a so-called 4-2-1 type exhaust passage. That is, the exhaust passage 30 is configured such that four exhaust passages extending from the engine body 1 converge into two passages, which then converge into one downstream (in the direction of exhaust gas flow). The catalytic device 31, front O2 sensor SN5, and rear O2 sensor SN6 are all located downstream of the point where the exhaust passages converge into one passage.

[0039] The engine system E is provided with an EGR device 40. The EGR device 40 includes an EGR passage 41. The EGR passage 41 is a passage that connects the exhaust passage 30 and the intake passage 20 and recirculates EGR gas, which is a part of the exhaust gas, to the intake passage 20. The EGR passage 41 connects a portion of the exhaust passage 30 downstream of the catalytic device 31 (in the flow direction of the exhaust gas) with a portion of the intake passage 20 between the throttle valve 22 and the surge tank 23.

[0040] An EGR cooler 42 and an EGR valve 43 are provided in the EGR passage 41. The EGR cooler 42 cools the EGR gas flowing through the EGR passage 41 by heat exchange. The EGR valve 43 is a valve that opens and closes the EGR passage 41. The amount of EGR gas recirculated to the intake passage 20 is changed depending on the opening degree of the EGR valve 43. The EGR valve 43 is provided in the EGR passage 41 closer to the intake passage 20 than the EGR cooler 42.

[0041] (Control system) FIG. 3 is a functional block diagram showing the control system of the engine system E. The PCM 100 shown in this diagram is a device that is mounted on a vehicle and performs overall control of the engine system E. The PCM 100 is composed of a microcomputer that includes a processor (CPU) that performs various arithmetic processing, memories such as ROM and RAM, and various input / output buses. The PCM 100 corresponds to the "control unit" in this invention.

[0042] The PCM 100 is electrically connected to a crank angle sensor SN1, an air flow sensor SN2, an intake air temperature sensor SN3, an intake air pressure sensor SN4, a front O2 sensor SN5, and a rear O2 sensor SN6. Information detected by each of the sensors SN1 to SN6 is sequentially input to the PCM 100.

[0043] A vehicle equipped with the engine system E is equipped with an accelerator pedal 91 that is depressed by the driver, and an accelerator sensor SN7. The accelerator pedal 91 is an operating device for changing and adjusting the output of the engine main body 1 and, ultimately, the vehicle speed. The accelerator sensor SN7 detects the amount of depression of the accelerator pedal 91, i.e., the accelerator opening, which is the opening degree of the accelerator pedal 91. The accelerator opening is a parameter that is 0 (%) when the accelerator pedal 91 is not depressed and 100 (%) when the accelerator pedal 91 is at its maximum depression amount. The accelerator sensor SN7 corresponds to the "accelerator opening detection device" of the present invention.

[0044] The vehicle is equipped with a brake device 81, a brake pedal 82, and a brake sensor SN8. The brake device 81 is a device that applies braking force to the wheels 70 to brake the wheels 70. The brake pedal 82 is an operating means for switching between driving and stopping the brake device 81 and increasing or decreasing the braking force that the brake device 81 applies to the wheels 70. The brake pedal 82 is operated by the driver by depressing it. The brake sensor SN8 detects the amount of depression of the brake pedal 82, i.e., the brake opening degree, which is the opening degree of the brake pedal 82.

[0045] The vehicle is equipped with a vehicle speed sensor SN9 for detecting vehicle speed. The vehicle is provided with a start switch SW1 that is operated by the driver to start the engine main body 1. When a predetermined operation is performed on the start switch SW1, the switch turns to IG_ON, power is supplied to each part of the engine system E, and the engine main body 1 becomes startable. Furthermore, when a predetermined operation is performed on the start switch SW1 while the switch is in the IG_ON state, the switch turns to IG_OFF, power supply to each part of the engine system E is stopped, and the engine main body 1 becomes unable to start.

[0046] The PCM 100 sequentially receives information detected by the accelerator sensor SN7 and the brake sensor SN8, and a signal from the start switch SW1.

[0047] The PCM 100 controls each part of the engine system E while making various determinations and calculations based on input information from the sensors SN1 to SN9 and the start switch SW1. The PCM 100 is electrically connected to the spark plug 10, the injector 11, the throttle valve 22, the EGR valve 43, etc., and outputs control signals to these devices based on the results of the calculations.

[0048] (Fuel cut and basic control during fuel cut) When a fuel cut condition is met—engine speed is higher than a predetermined idle speed and accelerator opening is equal to or smaller than a predetermined accelerator determination opening—the PCM 100 stops driving the injectors 11 of each cylinder 2A to stop fuel injection into each combustion chamber 5. The accelerator determination opening is set to 0 (zero), i.e., a nearly fully closed position, and fuel cut is essentially performed when the accelerator pedal 91 is not depressed (accelerator-off). During fuel cut, except when the intake amount increase control described below is being performed, the PCM 100 controls the throttle opening to a predetermined normal FC opening. The normal FC opening is 0 (zero), i.e., a nearly fully closed position, and is preset to a smaller (closer) position than the throttle opening achieved when fuel cut is not being performed. Furthermore, during fuel cut, except when the EGR valve malfunction diagnosis described below is being performed, the PCM 100 fully closes the EGR valve 43.

[0049] (Catalyst protection control) Next, the catalyst protection control performed by the PCM 100 to prevent the catalyst device 31 from overheating will be described.

[0050] (Catalyst temperature conditions) 4 is a flowchart showing the procedure for calculating the catalyst protection flag. Catalyst protection control is performed when the catalyst temperature, which is the temperature of the catalytic converter 31, is high. Specifically, catalyst protection control is performed when the catalyst temperature is equal to or higher than a first judgment temperature, and when the catalyst temperature has reached or exceeded the first judgment temperature but has not yet fallen below a second judgment temperature. The catalyst protection flag is a flag that indicates whether the conditions for performing this catalyst protection control are met.

[0051] Steps S51 to S56 shown in FIG. 4 are repeatedly performed at predetermined intervals while IG_ON is on. First, the PCM 100 reads various information including the detected values ​​of sensors SN1 to SN9 (step S51). In step S51, the PCM 100 reads at least the intake air amount detected by the air flow sensor SN2, the engine speed detected by the crank angle sensor SN1, and the intake air temperature detected by the intake air temperature sensor SN3. Next, the PCM 100 estimates the catalyst temperature, which is the temperature of the catalytic converter 31 (step S52). Specifically, the PCM 100 estimates the temperature of the exhaust gas based on the intake air amount, the engine speed, the intake air temperature, the amount of fuel injected from the injector 11, and the like, and estimates the catalyst temperature based on the estimated exhaust gas temperature. In this embodiment, the PCM 100 determines the catalyst temperature in this way, and the PCM 100 corresponds to a "catalyst temperature determining device" in addition to the "control device" of the present invention.

[0052] Next, the PCM 100 determines whether the catalyst temperature estimated in step S52 is equal to or higher than a first judgment temperature (step S53). The first judgment temperature is set in advance and stored in the PCM 100. The first judgment temperature is set to, for example, about 900°C. If the determination in step S53 is YES, meaning that the catalyst temperature is equal to or higher than the first judgment temperature, the PCM 100 sets a catalyst protection flag to 1 (step S54).

[0053] On the other hand, if the determination in step S53 is NO and the catalyst temperature is less than the first judgment temperature, the PCM 100 determines whether the conditions that the catalyst protection flag is 1 and the catalyst temperature is less than the second judgment temperature are met (step S55). If the determination in step S55 is NO and the catalyst protection flag is 0 or the catalyst temperature is equal to or greater than the second judgment temperature, the PCM 100 ends the process (returning to step S51). That is, the PCM 100 maintains the value of the catalyst protection flag at its current value. On the other hand, if the determination in step S55 is YES, the catalyst protection flag is 1, and the catalyst temperature is less than the second judgment temperature, that is, if the catalyst temperature becomes equal to or greater than the first judgment temperature and the catalyst protection flag is set to 1, and then the catalyst temperature drops to less than the second judgment temperature, the PCM 100 sets the catalyst protection flag to 0 (step S56) and ends the process (returning to step S51).

[0054] In this way, the catalyst protection flag is set to 1 when the catalyst temperature is equal to or higher than the first judgment temperature, or when the catalyst temperature has reached or exceeded the first judgment temperature but has not yet dropped below the second judgment temperature, and the conditions for implementing catalyst protection control are met. In other cases, when the conditions for implementing catalyst protection control are not met, the flag is set to 0. The catalyst protection flag is set to 0 when IG_OFF is selected.

[0055] (Fuel cut control) Fig. 5 is a flowchart showing the control mainly performed during fuel cut by the PCM 100. Steps S61 to S200 shown in Fig. 5 are repeatedly performed at predetermined intervals while the IG_ON state is maintained.

[0056] First, the PCM 100 reads various information including the detected values ​​of the sensors SN1 to SN9 (step S61). In step S61, the PCM 100 reads at least the engine speed detected by the crank angle sensor SN1 and the accelerator opening detected by the accelerator sensor SN7.

[0057] Next, the PCM 100 determines whether or not a fuel cut is in progress (step S62). Specifically, the PCM 100 determines whether or not the above-described fuel cut condition is met and the fuel injection of the injectors 11 of all the cylinders 2A has stopped.

[0058] If the determination in step S62 is NO and fuel cut is not in progress, the PCM 100 performs non-FC throttle control (step S100). The PCM 100 also performs non-FC fuel injection control (step S200) and then ends the process (returns to step S61). Non-FC throttle control is control of the throttle valve 22 that is performed during normal operation without fuel cut. Non-FC fuel injection control is control of the injector 11 that is performed during normal operation without fuel cut. Non-FC throttle control and non-FC fuel injection control will be described later.

[0059] If the determination in step S62 is YES and fuel cut is in progress, the PCM 100 determines whether the catalyst protection flag is 1 (step S63). In step S63, the determination is made using the catalyst protection flag that is calculated separately based on the catalyst temperature as described above.

[0060] If the determination in step S63 is NO and the catalyst protection flag is 0, the PCM 100 proceeds to step S66. In step S66, the PCM 100 sets the normal FC opening to the target throttle opening, which is the target value of the throttle opening. As described above, the normal FC opening is preset to an opening close to fully closed. After step S66, the PCM 100 proceeds to step S67.

[0061] If the determination in step S63 is YES and the catalyst protection flag is 1, the PCM 100 determines whether or not the flow of exhaust gas containing the correction fuel into the catalytic device 31 has ended (step S64). The details of step S64 will be described later.

[0062] If the determination in step S64 is NO and the flow of exhaust gas containing correction fuel into the catalytic device 31 has not yet ended, the PCM 100 proceeds to step S66, sets the normal FC opening to the target throttle opening, and then proceeds to step S67.

[0063] On the other hand, if the determination in step S64 is YES and the flow of the exhaust gas containing the correction fuel into the catalytic converter 31 has ended, the PCM 100 proceeds to step S65 and performs intake amount increase control, which is one type of catalyst protection control.

[0064] The intake air amount increase control is a control for increasing the intake air amount, which is the amount of air taken into the combustion chamber 5. In step S65, the PCM 100 sets the target throttle opening to an opening that is larger (more open) than the normal FC opening. In steps S69 and S71, which will be described later, the throttle valve 22 is opened and closed so as to achieve the target throttle opening. Therefore, when the intake air amount increase control is performed, the intake air amount is increased compared to when the intake air amount increase control is not performed (when step S66 is performed). After step S65, the process proceeds to step S67.

[0065] In step S67, the PCM 100 opens or closes the throttle valve 22 so that the opening of the throttle valve 22 becomes the target throttle opening set in step S65 or step S66. For example, the PCM 100 identifies the current opening of the throttle valve 22 based on the drive current of the throttle valve 22, the output of a throttle valve opening sensor capable of detecting the opening of the throttle valve 22, etc., and drives the throttle valve 22 based on the current opening of the throttle valve 22 and the target throttle opening. After step S67, the PCM 100 ends the process (returns to step S61).

[0066] As described above, during fuel cut (determination in step S62 is YES), when the catalyst protection flag is 1 (determination in step S63 is YES) and the inflow of exhaust gas containing the correction fuel into the catalytic device 31 has ended (determination in step S64 is YES), intake amount increase control is implemented and the throttle opening is set to the normal FC opening, that is, an opening greater than the opening when the catalyst flag is 0. This increases the intake amount compared to when the catalyst protection flag is 0 (determination in step S63 is NO). Also, during fuel cut, even if the catalyst protection flag is 1 (determination in step S63 is YES) but the inflow of exhaust gas containing the correction fuel into the catalytic device 31 has not ended (determination in step S64 is NO), intake amount increase control is prohibited and the throttle opening is controlled to the normal FC opening.

[0067] (Throttle control when not in FC mode) Next, the non-FC throttle control in step S100 will be described with reference to the flowchart in FIG.

[0068] When the non-FC throttle control is started, the PCM 100 first calculates a target torque, which is a target value of the engine torque (step S101). The PCM 100 calculates the target torque based on the accelerator opening detected by the accelerator sensor SN7, the vehicle speed detected by the vehicle speed sensor SN9, etc.

[0069] Next, the PCM 100 sets a target intake air amount, which is a target value of the intake air amount (step S102). The PCM 100 sets the target intake air amount based on the target torque, the engine speed detected by the crank angle sensor SN1, and the like.

[0070] Next, the PCM 100 determines whether the catalyst protection flag is 1 (step S103). If the determination in step S103 is YES and the catalyst protection flag is 1, the PCM 100 determines whether the target intake air amount set in step S102 is greater than the upper limit intake air amount (step S104). The upper limit intake air amount is set in advance and stored in the PCM 100. If the determination in step S104 is YES and the target intake air amount is greater than the upper limit intake air amount, the PCM 100 resets the target intake air amount to the upper limit intake air amount (step S105). That is, the target intake air amount is changed from the value set in step S102 to the upper limit intake air amount. After step S105, the PCM 100 proceeds to step S106.

[0071] On the other hand, if the judgment in step S103 is NO and the catalyst protection flag is 0, or if the target intake air volume set in step S102 is equal to or less than the upper limit intake air volume, PCM100 proceeds to step S106 without performing step S105, i.e., while maintaining the target intake air volume at the value set in step S102.

[0072] In step S106, the PCM 100 opens and closes the throttle valve 22 so as to achieve the target intake air amount set in step S102 or the target intake air amount reset to the upper limit intake air amount in step S105. When step S106 is performed, the non-FC throttle control ends.

[0073] As described above, when fuel cut is not being performed and the catalyst protection flag is 1, the PCM 100 performs control to limit the target intake air amount, and therefore the intake air amount, to an upper limit intake air amount or less, as one of catalyst protection controls.

[0074] (Fuel injection control when not in FC mode) Next, the non-FC fuel injection control in step S200 will be described. In the following, the end of fuel cut and the resumption of fuel supply to the combustion chamber 5 will be referred to as fuel restoration, where appropriate.

[0075] During fuel cut, fuel supply to the combustion chamber 5 is stopped, and the exhaust gas becomes almost entirely air. Accordingly, oxygen is stored in the catalytic device 31 during fuel cut. If the amount of oxygen stored in the catalytic device 31 becomes large, the air-fuel ratio in the catalytic device 31 after fuel recovery becomes leaner than the stoichiometric air-fuel ratio, resulting in a decrease in NOx purification performance. Therefore, in this embodiment, fuel amount increase control is implemented to increase the amount of fuel supplied to the combustion chamber 5 after fuel recovery, and unburned fuel is supplied to the exhaust passage 30 and the catalytic device 31, causing the oxygen stored in the catalytic device 31 to be consumed by reaction with the unburned fuel. Non-FC fuel injection control, including this fuel amount increase control, will be described using the flowchart in FIG. 7.

[0076] When non-FC fuel injection control is started, the PCM 100 first calculates a basic fuel amount, which is the basic amount of fuel to be supplied to the combustion chamber 5, and sets this as the fuel injection amount (step S201). The PCM 100 calculates, as the basic fuel amount, the amount of fuel that will make the air-fuel ratio of the air-fuel mixture in the combustion chamber 5 close to the stoichiometric air-fuel ratio, based on the target intake air amount set in step S102 or step S105 of the non-FC fuel throttle control and the engine speed detected by the crank angle sensor SN1, etc. Here, as described above, the target intake air amount is set based on the target torque and therefore the accelerator opening, and the basic fuel amount is set based on the accelerator opening.

[0077] Next, the PCM 100 determines whether the following conditions are met: it is time to restore fuel (immediately after the fuel cut ends and fuel supply is resumed), or a fuel increase flag, which will be described later, is set to 1 (S202). If the determination in step S202 is YES, meaning it is time to restore fuel, or if the fuel increase flag is set to 1, the PCM 100 determines whether the RO2 output voltage, which is the output voltage of the rear O2 sensor SN6, is equal to or less than a predetermined determination voltage (step S203). The determination voltage is preset and stored in the PCM 100. The determination voltage is set to a value higher than the first voltage and lower than the stoichiometric voltage, and is stored in the PCM 100. The RO2 output voltage is equal to or less than the determination voltage when the rear air-fuel ratio is leaner than the stoichiometric air-fuel ratio, or when the rear air-fuel ratio is in the process of transitioning from a state leaner than the stoichiometric air-fuel ratio to the stoichiometric air-fuel ratio. That is, the RO2 output voltage becomes equal to or lower than the determination voltage when the rear air-fuel ratio is substantially leaner than the stoichiometric air-fuel ratio, and in step S203, it is determined whether the rear air-fuel ratio is leaner than the stoichiometric air-fuel ratio. Furthermore, as described above, the rear air-fuel ratio detected by the rear O2 sensor SN6 is equivalent to the air-fuel ratio in the catalytic device 31. Therefore, in step S203, it is determined whether the air-fuel ratio in the catalytic device 31 is substantially leaner than the stoichiometric air-fuel ratio (larger than the stoichiometric air-fuel ratio).

[0078] If the determination in step S203 is YES and the RO2 output voltage is equal to or lower than the determination voltage, i.e., if the rear air-fuel ratio and the air-fuel ratio in the catalytic converter 31 are leaner than the stoichiometric air-fuel ratio (greater than the stoichiometric air-fuel ratio), the PCM 100 performs fuel amount increase control to increase the fuel injection amount (step S204). Specifically, in step S204, the PCM 100 resets the fuel injection amount to a value greater than the basic injection amount calculated in step S201. For example, the PCM 100 resets the fuel injection amount to a value obtained by adding a predetermined amount to the fuel injection amount set in step S201. As described above, the fuel injection amount set in step S201 is an amount that brings the air-fuel ratio of the mixture in the combustion chamber 5 close to the stoichiometric air-fuel ratio. Therefore, when the fuel injection amount is increased, the air-fuel ratio of the mixture in the combustion chamber 5 becomes richer than the stoichiometric air-fuel ratio. After step S204, the PCM 100 sets the fuel increase flag to 1 (step S205).

[0079] On the other hand, if the determination in step S202 is NO, meaning it is not time to restore fuel or the fuel increase flag is 0, the PCM 100 proceeds to step S207 without performing steps S204 and S205. In other words, if the determination in step S202 is NO, the PCM 100 does not increase the fuel injection amount, but instead proceeds to step S207 while maintaining the fuel injection amount at the value set in step S201.

[0080] Furthermore, if the determination in step S202 is YES while the determination in step S203 is NO and the RO2 output voltage is higher than the determination voltage, that is, if the rear air-fuel ratio and the air-fuel ratio in the catalytic device 31 are the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio (the rear air-fuel ratio and the air-fuel ratio in the catalytic device 31 are equal to or lower than the stoichiometric air-fuel ratio), the PCM 100 sets the fuel increase flag to 0 (step S206) and proceeds to step S207. In this case as well, the PCM 100 proceeds to step S207 without performing step S204, that is, without increasing the fuel injection amount.

[0081] In step S207, the PCM 100 drives the injector 11 so as to achieve the fuel injection amount set in step S201 or the fuel injection amount reset in step S204. When step S207 is performed, the non-FC fuel injection control ends.

[0082] As described above, the PCM 100 performs fuel augmentation control, which increases the fuel injection amount beyond the basic injection amount to make the mixture in the combustion chamber 5 richer than the stoichiometric air-fuel ratio, from when fuel is restored until the RO2 output voltage becomes equal to or lower than the determination voltage and the rear air-fuel ratio and the air-fuel ratio in the catalytic device 31 become the stoichiometric air-fuel ratio or richer than this (below the stoichiometric air-fuel ratio), and sets the fuel increase flag to 1 while the fuel augmentation control is being performed. In this way, the fuel increase flag is set to 1 while the fuel augmentation control is being performed and is set to 0 at other times. Note that the fuel increase flag is set to 0 when a fuel cut starts while the flag is set to 1.

[0083] Returning to the flowchart of Figure 5, step S64 will be described in detail. In step S64, it is determined whether all of the exhaust gas containing the fuel-increased fuel, which is the air-fuel mixture formed in the combustion chamber 5 when the fuel increase control is being performed (burned gas after combustion of the air-fuel mixture), has reached the catalytic device 31. Specifically, it takes time for the burned gas to reach the catalytic device 31 after being discharged from the combustion chamber 5. Therefore, even after the fuel increase control has ended, the exhaust gas containing the fuel that has been increased continues to flow into the catalytic device 31 for a while. In step S64, it is determined whether the exhaust gas containing the fuel that has been increased has finished flowing into the catalytic device 31.

[0084] As described above, when the fuel injection amount is increased, the air-fuel ratio of the mixture in the combustion chamber 5 becomes richer than the stoichiometric air-fuel ratio. Therefore, while the exhaust gas containing the increased fuel is flowing into the catalytic device 31, the air-fuel ratio of the exhaust gas flowing into the catalytic device 31 becomes richer than the stoichiometric air-fuel ratio. On the other hand, after the fuel increase control ends, the fuel injection amount is set to the basic fuel amount, and the air-fuel ratio of the mixture in the combustion chamber 5 and the exhaust gas becomes the stoichiometric air-fuel ratio. Therefore, when the flow of the exhaust gas containing the increased fuel into the catalytic device 31 ends, the air-fuel ratio of the exhaust gas flowing into the catalytic device 31 becomes the stoichiometric air-fuel ratio. Furthermore, if fuel is cut during the fuel increase control, the air-fuel ratio of the mixture in the combustion chamber 5 and the exhaust gas becomes leaner than the stoichiometric air-fuel ratio as soon as the fuel increase control ends. Therefore, in this case, when the flow of the exhaust gas containing the increased fuel into the catalytic device 31 ends, the air-fuel ratio of the exhaust gas flowing into the catalytic device 31 becomes leaner than the stoichiometric air-fuel ratio. Therefore, in this embodiment, when the air-fuel ratio of the exhaust gas flowing into the catalytic device 31 changes from a state richer than the stoichiometric air-fuel ratio to the stoichiometric air-fuel ratio or leaner than that, it is determined that the flow of exhaust gas containing the increased corrected fuel into the catalytic device 31 has ended.

[0085] As described above, the front air-fuel ratio detected by the front O2 sensor SN5 is equal to the air-fuel ratio of the exhaust gas flowing into the catalytic device 31. Thus, in step S64, the PCM 100 determines NO from the time when the fuel amount increase control ends and the fuel increase flag changes from 1 to 0 until the front air-fuel ratio detected by the front O2 sensor SN5 becomes the stoichiometric air-fuel ratio or leaner than this (until it becomes the stoichiometric air-fuel ratio or greater), and otherwise determines YES in step S64. Then, as described above with respect to the flowchart of FIG. 5, if the determination in step S64 is NO and the inflow of exhaust gas containing the fuel that has been increased and corrected into the catalytic device 31 has not ended after the fuel amount increase control ends, the PCM 100 prohibits the intake amount increase control and sets the normal FC opening to the target throttle opening (step S66). On the other hand, if the determination in step S64 is YES, that is, the flow of exhaust gas containing the increased fuel into the catalytic converter 31 has ended, the PCM 100 proceeds to step S65 and performs intake amount increase control.

[0086] (action, etc.) As described above, in the above embodiment, basically (when the inflow of exhaust gas containing the increased fuel into the catalytic converter 31 has ended), when the catalyst protection flag is 1 and the catalyst temperature is high, the intake air amount increase control is implemented, and the throttle opening and intake air amount during fuel cut are increased compared to when the catalyst protection flag is 0 and the catalyst temperature is low. Therefore, by utilizing the timing of fuel cut, the high-temperature catalytic converter 31 can be cooled with a large amount of air, and its temperature can be reduced quickly. Here, as described above, when the catalyst flag is 1 and the catalyst temperature is high, it means that the catalyst temperature is equal to or higher than the first judgment temperature, or when the catalyst temperature has once exceeded the first judgment temperature but has not yet fallen below the second judgment temperature. When the catalyst flag is 0 and the catalyst temperature is low, it means that the catalyst temperature is lower than the second judgment temperature, or when the catalyst temperature has exceeded the second judgment temperature but has not yet reached the first judgment temperature.

[0087] Furthermore, when the fuel cut ends and fuel supply is resumed, fuel amount increase control is performed to increase the amount of fuel supplied to the combustion chamber 5 from the basic fuel amount. Therefore, the oxygen stored in the catalytic device 31 during the fuel cut can be used to oxidize the fuel and consumed early. Therefore, the NOx purification performance of the catalytic device 31 after the fuel cut ends can be improved.

[0088] However, if the intake amount increase control is performed to increase the throttle opening while the exhaust gas containing the increased fuel has not yet finished flowing into the catalytic converter 31, there is a risk that the unburned fuel will react with air in the exhaust passage 30 and the catalytic converter 31, causing the temperature of the catalytic converter 31 to rise. In contrast, in the above embodiment, the intake amount increase control is prohibited when the exhaust gas containing the increased fuel has not yet finished flowing into the catalytic converter 31. This prevents the above reaction from occurring in the exhaust passage 30 and the catalytic converter 31, and suppresses a rise in the temperature of the catalytic converter 31. Therefore, according to the above embodiment, the NOx purification performance of the catalytic converter 31 can be improved while reliably preventing the catalytic converter 31 from overheating.

[0089] This will be explained in detail using FIG. 8. FIG. 8 is a time chart that schematically shows the time changes of each parameter while the engine is running. From top to bottom, FIG. 8 shows charts for the fuel cut flag, fuel increase flag, front air-fuel ratio detected by the front O2 sensor SN5, RO2 output voltage, throttle opening, catalyst protection flag, and catalyst temperature. The fuel cut flag is a flag that is set to 1 when fuel cut is being performed and set to 0 otherwise. In the charts for throttle opening and catalyst temperature, the dotted lines represent the throttle opening and catalyst temperature according to a comparative example. When the catalyst protection flag is set to 1 during fuel cut, intake amount increase control is performed regardless of whether the inflow of exhaust gas containing the increased fuel into the catalytic converter 31 has ended. Note that in FIG. 8, "FC in progress" in the chart for the fuel cut flag indicates that fuel cut is being performed, and "fuel increase in progress" in the chart for the fuel increase flag indicates that fuel increase control is being performed.

[0090] In the example of Figure 8, with the catalyst protection flag set to 0, fuel cut is performed until time t1, at which point fuel is restored. Because fuel cut is performed with the catalyst protection flag set to 0, the throttle opening is controlled to the normal FC opening until time t1. Also, in the example of Figure 8, because fuel cut is performed until time t1, the front air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio (St), and the RO2 output voltage becomes smaller (leaner) than the stoichiometric voltage (Vst).

[0091] When fuel is restored at time t1, fuel augmentation control is implemented and the fuel increase flag is set to 1. By restoring fuel, the throttle opening is controlled to an opening corresponding to the target intake air amount. In the example of FIG. 8, after time t1, the throttle opening is set to an opening larger than the normal FC opening. Furthermore, after time t1, fuel augmentation control is implemented, causing the air-fuel ratio of the mixture in the combustion chamber 5 to become richer than the stoichiometric air-fuel ratio. However, as described above, there is a delay time before the burned gas in the combustion chamber 5 reaches the catalytic device 31 and the front O2 sensor SN5. As a result, the front air-fuel ratio is maintained leaner than the stoichiometric air-fuel ratio for a while after time t1. Similarly, the rear air-fuel ratio also becomes leaner than the stoichiometric air-fuel ratio for a while after time t1, and the RO2 output voltage is maintained at a value smaller than the stoichiometric voltage (VSt).

[0092] In the example of Figure 8, exhaust gas containing fuel that has been increased and corrected reaches the catalytic converter 31 and the front O2 sensor SN5 at time t2, and the front air-fuel ratio becomes richer than the stoichiometric air-fuel ratio at time t2. However, because fuel cut was performed until time t1, oxygen is stored in the catalytic converter 31 at time t2. Therefore, although exhaust gas that is richer than the stoichiometric air-fuel ratio flows into the catalytic converter 31 at time t2, the air-fuel ratio in the catalytic converter 31 is maintained leaner than the stoichiometric air-fuel ratio for a while after time t2, and the RO2 output voltage is maintained at a value smaller than the stoichiometric voltage (Vst).

[0093] In the example of FIG. 8 , after time t2, almost all of the excess oxygen stored in the catalytic converter 31 is consumed, causing the air-fuel ratio in the catalytic converter 31 and the rear air-fuel ratio to become stoichiometric, and at time t4, the RO2 output voltage becomes equal to or greater than the threshold voltage. When the RO2 output voltage becomes equal to or greater than the threshold voltage, the fuel amount increase control is terminated and the fuel increase flag is set to 0. When the fuel amount increase control is terminated, the fuel injection amount is set to the basic injection amount, and the air-fuel ratio of the mixture in the combustion chamber 5 becomes stoichiometric. However, due to the exhaust gas delay time dt, for a while after time t4, exhaust gas containing the increased fuel flows into the front O2 sensor SN5 and the catalytic converter 31, maintaining the front air-fuel ratio richer than the stoichiometric air-fuel ratio. In the example of FIG. 8 , at time t6, the flow of exhaust gas containing the increased fuel into the catalytic converter 31 ends, and at time t6, the front air-fuel ratio increases to the stoichiometric air-fuel ratio (St).

[0094] 8, at time t3 between times t2 and t4, the catalyst temperature becomes equal to or higher than the first judgment temperature (Xtc1), and the catalyst protection flag switches from 0 to 1. Also in the example of FIG. 8, with the catalyst protection flag at 1, fuel cut is started again at time t5 between times t4 and t6, and the fuel cut flag becomes 1.

[0095] In the above embodiment, during fuel cut, if the catalyst protection flag is 1 and the inflow of exhaust gas containing increased corrected fuel into the catalytic device 31 has finished, and intake amount increase control is implemented, but in other cases intake amount increase control is prohibited. At time t5 in the example of Fig. 8, the conditions that fuel cut is being implemented and the catalyst protection flag is 1 are met, but the inflow of exhaust gas containing increased corrected fuel into the catalytic device 31 has not finished. Therefore, in the above embodiment, the implementation of intake amount increase control at time t5 is prohibited, and when fuel cut starts at time t5, the throttle opening is controlled to the normal FC opening.

[0096] 8, the flow of exhaust gas containing the increased fuel into the catalytic device 31 ends at time t6 after time t5. Thus, in the above embodiment, the throttle opening is maintained at the normal FC opening between time t5 and time t6. Then, at time t6, intake amount increase control is started, and the throttle valve 22 is controlled to open to an opening greater than the normal FC opening.

[0097] On the other hand, in the comparative example, the intake amount increase control is performed regardless of whether the inflow of exhaust gas containing the increased-corrected fuel into the catalytic converter 31 has ended. Therefore, the intake amount increase control is started at time t5, and the throttle valve 22 is controlled to open. As described above, even though fuel cut starts at time t5, the inflow of exhaust gas containing the increased-corrected fuel into the catalytic converter 31 does not end until time t6. Therefore, unburned fuel is present in the exhaust passage 30 and the catalytic converter 31 upstream of the catalytic converter 31 between time t5 and time t6. Therefore, in the comparative example, the throttle valve 22 is controlled to open between time t5 and time t6, and a large amount of air is introduced into the combustion chamber 5 and the exhaust passage 30, causing a reaction between the unburned fuel and the air in the exhaust passage 30 and the catalytic converter 31. As a result, as shown by the dashed line in the catalyst temperature chart of FIG. 8 , in the comparative example, the temperature of the catalytic converter rises after time t5 due to the heat generated by the reaction.

[0098] In contrast to this, in the above embodiment, from time t5 to time 6, the intake amount increase control is prohibited and the throttle opening is controlled to the normal FC opening, thereby reducing the amount of air introduced into the combustion chamber 5 and ultimately the exhaust passage 30. Therefore, according to the above embodiment, the reaction between unburned fuel and air in the exhaust passage 30 is suppressed, and the increase in catalyst temperature after time t5 is suppressed, as shown by the solid line in the catalyst temperature chart in Figure 8.

[0099] 8, since the fuel injection amount increase correction is completed at time t4, and the air-fuel mixture in the combustion chamber 5 is controlled to the stoichiometric air-fuel ratio for a while, and then fuel cut is started (at time t5), the front air-fuel ratio becomes close to the stoichiometric air-fuel ratio for a while after time t23, and then becomes leaner than the stoichiometric air-fuel ratio. Also, after time t6, in both the above embodiment and the comparative example, the intake amount increase control is performed in a state where the inflow of exhaust gas containing the increase correction fuel into the catalytic device 31 has ended, and the catalyst temperature begins to decrease.

[0100] Furthermore, in the above embodiment, the front air-fuel ratio detected by the front O2 sensor SN5 is used to determine that the inflow of exhaust gas containing the increased fuel into the catalytic device 31 has ended. Therefore, according to the above embodiment, it is possible to accurately estimate the timing at which the inflow of exhaust gas containing the increased fuel into the catalytic device 31 will end, and it is possible to avoid an excessively long period during which the intake air amount increase control is prohibited. Therefore, according to the above embodiment, it is possible to ensure the period during which the intake air amount increase control is performed, and it is possible to more reliably cool the catalytic device.

[0101] In addition, in the above embodiment, the fuel amount increase control is terminated when the RO2 output voltage exceeds the threshold voltage. That is, the detection value of the rear O2 sensor SN6 is used to estimate that the air-fuel ratio in the catalytic converter 31 has become stoichiometric or richer. In other words, the state in which the air-fuel ratio in the catalytic converter 31 is leaner than the stoichiometric air-fuel ratio has been resolved. Therefore, the above embodiment accurately identifies the timing when the lean state of the catalytic converter 31 is resolved, thereby optimizing the implementation period of the fuel amount increase control. Specifically, the implementation period of the fuel amount increase control can be prevented from being excessively longer or shorter than the period required to consume the oxygen stored in the catalytic converter 31. By preventing the implementation period of the fuel amount increase control from being excessively long, the timing at which the inflow of exhaust gas containing the increased fuel into the catalytic converter 31 ends can be delayed, which in turn prevents the prohibition period of the intake air amount increase control from being extended. This ensures opportunities for the intake air amount increase control, thereby more reliably cooling the catalytic converter 31. Furthermore, it is possible to avoid an increase in the amount of unburned fuel discharged. Furthermore, by preventing the implementation period of the fuel amount increase control from becoming excessively short, the oxygen stored in the catalytic converter 31 can be appropriately consumed, and the NOx purification performance of the catalytic converter 31 can be ensured.

[0102] Furthermore, in the above embodiment, when the catalyst protection flag is 1 and the catalyst temperature is high, the intake air amount is prevented from exceeding the upper intake air amount while fuel cut is not being performed, and the combustion energy generated in the combustion chamber is kept low. This makes it possible to more reliably prevent the catalyst temperature from becoming excessively high.

[0103] However, if the number of opportunities to restrict the intake air amount increases, the number of opportunities to restrict engine output also increases, which may result in a deterioration in vehicle drivability. In contrast, in the above embodiment, the catalytic converter 31 is cooled when a fuel cut is performed to prevent the temperature from rising excessively. This reduces the opportunities for the catalyst temperature to become high when a fuel cut is not performed, thereby suppressing a deterioration in vehicle drivability.

[0104] (Variation) In the above embodiment, the timing when the front air-fuel ratio becomes the stoichiometric air-fuel ratio or leaner after the fuel amount increase control is ended is identified as the timing when the inflow of exhaust gas containing the increased corrected fuel into the catalytic device 31 ends (hereinafter referred to as the inflow end timing as appropriate), but the configuration for identifying the inflow end timing is not limited to this.

[0105] For example, instead of the above configuration, the timing when the volume of exhaust gas led into the exhaust passage 30 after the fuel amount increase control ends becomes equal to or greater than the volume of the exhaust passage 30 between the engine body 1 and the catalytic device 31 may be specified as the inflow end timing. In other words, when a fuel cut is in progress and the catalyst flag is 1, the intake amount increase control may be prohibited from the time when the fuel amount increase control ends until the volume of exhaust gas led into the exhaust passage 30 after the fuel amount increase control ends becomes equal to or greater than the volume of the exhaust passage 30 between the engine body 1 and the catalytic device 31.

[0106] The volume of exhaust gas led into the exhaust passage 30 after the fuel amount increase control is completed can be obtained by estimating the volumetric flow rate of the exhaust gas led from the combustion chamber 5 to the exhaust passage 30 based on the engine speed and the intake air amount, and integrating the estimated volumetric flow rate after the fuel amount increase control is completed. The volume of the exhaust passage 30 between the engine body 1 and the catalytic device 31 may be set in advance by measurement or the like.

[0107] The inflow end timing may be set to a timing when a preset period has elapsed since the end of the fuel increase control. In this case, the preset period may be determined based on the engine speed or the like.

[0108] Furthermore, in the above embodiment, when the intake air amount increase control is performed and when the control for limiting the intake air amount to equal to or less than the upper limit intake air amount is performed, the intake air amount is increased or decreased by increasing or decreasing the opening degree of the throttle valve 22. However, the device for increasing or decreasing the intake air amount is not limited to the throttle valve 22. For example, a variable valve mechanism that can change the opening and closing timing of the intake valve 8 may be provided in the device that drives the intake valve 8, and the intake air amount may be increased or decreased by changing the opening and closing timing of the intake valve 8.

[0109] In the above embodiment, the catalyst protection control (intake air amount increase control and control to limit the intake air amount to below the upper limit intake air amount) is described as being performed when the catalyst temperature is above the first judgment temperature, and when the catalyst temperature has reached above the first judgment temperature but has not yet dropped to below the second judgment temperature. However, the condition of when the catalyst temperature has reached above the first judgment temperature but has not yet dropped to below the second judgment temperature may be excluded from the conditions for performing catalyst protection control.

[0110] Furthermore, the specific structure of the engine body 1, such as the number of cylinders, is not limited to that described above. [Explanation of symbols]

[0111] 1 Engine body 11 Injector (fuel supply device) 20 Intake passage 22 Throttle valve (intake volume control device) 30 Exhaust passage 31 Catalytic converter 91 Accelerator pedal 100 PCM (control unit, catalyst temperature identification device) SN5 Front O2 sensor (upstream air-fuel ratio detection device) SN6 Rear O2 sensor (downstream air-fuel ratio detection device) SN7 Accelerator sensor (accelerator opening detection device)

Claims

1. In a control device for an engine provided in a vehicle equipped with an accelerator pedal, an engine body in which a combustion chamber is formed; an exhaust passage and an intake passage, each connected to the engine body; a fuel supply device for supplying fuel to the combustion chamber; an intake air amount adjusting device that adjusts an intake air amount, which is the amount of air taken into the combustion chamber; a catalytic converter including a three-way catalyst; a catalyst temperature specifying device for specifying a catalyst temperature, which is the temperature of the catalyst device; an accelerator opening detection device that detects an accelerator opening, which is the opening of the accelerator pedal; a control unit that calculates a basic fuel amount based on the accelerator opening detected by the accelerator opening detection device and controls the fuel supply device so that the amount of fuel supplied to the combustion chamber becomes the basic fuel amount, The control unit a fuel cut is performed to stop fuel supply by the fuel supply device when the accelerator opening detected by the accelerator opening detection device is less than a predetermined accelerator judgment opening; When the catalyst temperature specified by the catalyst temperature specifying device is high during the fuel cut, an intake amount increase control is performed to control the intake amount adjustment device so that the intake amount is larger than when the catalyst temperature is low, When the fuel cut is ended and fuel supply by the fuel supply device is resumed, a fuel amount increase control is performed to control the fuel supply device so that the amount of fuel supplied to the combustion chamber becomes larger than the basic fuel amount, 10. An engine control device comprising: a control unit for inhibiting the intake air amount increase control for a predetermined period of time after the fuel amount increase control has ended;

2. 2. The engine control device according to claim 1, an upstream air-fuel ratio detection device for detecting the air-fuel ratio of exhaust gas flowing into the catalytic device; the control unit prohibits the intake air amount increase control from the time the fuel amount increase control is completed until the air-fuel ratio of the exhaust gas detected by the upstream air-fuel ratio detection device becomes equal to or higher than the stoichiometric air-fuel ratio.

3. 2. The engine control device according to claim 1, The control unit prohibits the intake amount increase control from the time when the fuel amount increase control is completed until the volume of exhaust gas discharged into the exhaust passage after the fuel amount increase control is completed becomes equal to or greater than the volume of the exhaust passage between the engine body and the catalytic device.

4. 2. The engine control device according to claim 1, a downstream air-fuel ratio detection device for detecting the air-fuel ratio of exhaust gas flowing out from the catalytic device; the control unit performs the fuel amount increase control for a period from when fuel supply by the fuel supply device is resumed until the air-fuel ratio of the exhaust gas detected by the downstream air-fuel ratio detection device becomes equal to or lower than a stoichiometric air-fuel ratio.

5. 2. The engine control device according to claim 1, The control unit is configured to limit the intake air amount to a predetermined upper limit intake air amount or less by the intake air amount adjusting device when the catalyst temperature is high and the fuel cut is not being performed.

6. The engine control device according to any one of claims 1 to 5, 10. An engine control device, wherein the intake amount adjusting device is a throttle valve provided in the intake passage for opening and closing the intake passage.

Images