Engine control device

Abstract

To provide a control device of an engine capable of preventing damage of a catalyst device while preventing excessive temperature rise of the catalyst device.SOLUTION: When the accelerator opening degree is equal to or less than a predetermined accelerator determination opening degree, fuel cut for stopping fuel injection by the fuel supply device 11 is performed, and when the fuel cut is performed in a state where the catalyst temperature is equal to or higher than a predetermined determination temperature, intake air amount increase control for controlling the intake air amount adjustment device 12 so that the intake air amount becomes larger than when the catalyst temperature is lower than the determination temperature is performed.SELECTED DRAWING: Figure 6

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] As in Patent Document 1, deterioration of the catalytic converter can be suppressed by appropriately cooling the catalytic converter. However, if the catalytic converter is cooled suddenly while it is still hot, cracks may form in the catalytic converter, causing damage to the catalytic converter. The configuration of Patent Document 1 leaves room for improvement in this regard, i.e., in terms of preventing damage to the catalytic converter.

[0006] The present invention has been made in consideration of the above circumstances, and aims to provide an engine control device that can prevent the temperature of a catalytic converter from becoming excessively high while preventing damage to the catalytic converter. [Means for solving the problem]

[0007] The engine control device according to the present invention is an engine control unit 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 connected to the engine body, a fuel supply device for supplying fuel to the combustion chamber, an intake amount adjusting device for adjusting the intake amount which is the amount of air taken into the combustion chamber, a catalytic device provided in the exhaust passage for purifying exhaust gas, a catalyst temperature specifying device for specifying the catalyst temperature which is the temperature of the catalytic device, an accelerator opening detection device for detecting an accelerator opening which is the opening of the accelerator pedal, and a control unit for controlling the fuel supply device and the intake amount adjusting device. and a control unit for controlling the intake air amount adjustment device so that the intake air amount increases when the catalyst temperature is lower than the predetermined judgment temperature, when the fuel cut is performed in a state where the catalyst temperature specified by the catalyst temperature specifying device is equal to or higher than a predetermined judgment temperature, and when the control unit performs the fuel cut when the catalyst temperature specified by the catalyst temperature specifying device is higher than the predetermined judgment temperature, the control unit controls the intake air amount adjustment device so that the intake air amount increases at a slower rate when the catalyst temperature is high than when the catalyst temperature is low (claim 1).

[0008] According to the present invention, when the catalyst temperature is equal to or higher than the threshold temperature, intake air amount increase control is implemented to increase the amount of air in the combustion chamber and therefore the amount of air introduced into the exhaust passage during fuel cut. Therefore, the catalyst device in the exhaust passage can be cooled by a large amount of air by utilizing the timing of fuel cut, and the temperature of the catalyst device can be prevented from becoming excessively high.

[0009] Furthermore, when the intake air amount is increased as a result of the intake air amount increase control being performed when the catalyst temperature is equal to or higher than the threshold temperature, the rate of increase in the intake air amount is slower when the catalyst temperature is high than when it is low. Therefore, when the temperature of the catalytic converter is high and the temperature difference between the catalytic converter temperature and the air temperature is large, the catalytic converter is easily cooled by the air, preventing a large amount of air from being introduced into the catalytic converter in a short period of time. This prevents a sudden drop in the temperature of the catalytic converter, and ultimately damage to the catalytic converter, such as cracks, that would otherwise occur due to this temperature drop. On the other hand, when the temperature of the catalytic converter is relatively low and the temperature drop of the catalytic converter is kept relatively small, a large amount of air is introduced into the catalytic converter, appropriately cooling the catalytic converter and preventing the temperature of the catalytic converter from becoming excessively high.

[0010] The configuration for increasing the intake amount when the intake amount increase control is implemented includes a configuration in which, when the fuel cut is implemented, the control unit prohibits the intake amount increase control if a predetermined prohibition condition is met, and starts the intake amount increase control to increase the intake amount if the prohibition condition is not met (Claim 2).

[0011] This configuration can prevent damage to the catalytic converter when the intake air amount is increased as the prohibition condition is no longer satisfied.

[0012] In the above configuration, preferably, when the fuel cut is performed, the control unit controls the intake air amount adjustment device so that when the intake air amount is reduced, the rate of reduction of the intake air amount is faster when the catalyst temperature is high than when it is low (claim 3).

[0013] When a fuel cut is performed because the catalyst temperature is equal to or higher than the threshold temperature, if the rate of decrease in the intake air volume is slow, the amount of air flowing into the catalytic converter increases, which may cause a sudden drop in the temperature of the catalytic converter. In contrast, with the above-described configuration, when a fuel cut is performed because the catalyst temperature is equal to or higher than the threshold temperature, the rate of decrease in the intake air volume is faster when the catalyst temperature is high than when it is low, so the amount of air flowing into the catalytic converter is reduced earlier. This prevents a sudden drop in the temperature of the catalytic converter and damage to the catalytic converter. Furthermore, when the catalyst temperature is relatively low and the catalytic converter is unlikely to be damaged, the temperature of the catalytic converter can be reduced earlier using a large amount of air.

[0014] In the above configuration, preferably, when the fuel cut is not being performed and the catalyst temperature is equal to or higher than the judgment temperature, the control unit limits the intake air amount to a predetermined upper limit intake air amount or less using the intake air amount adjustment device (claim 4).

[0015] According to this configuration, when fuel cut is not being performed and the catalyst temperature is equal to or higher than the threshold temperature, 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.

[0016] In the above configuration, the intake amount adjusting device may be a throttle valve that is provided in the intake passage and opens and closes the intake passage (claim 5). [Effects of the Invention]

[0017] As described above, the engine control device of the present invention can prevent the temperature of the catalyst device from becoming excessively high and also prevent damage to the catalyst device. [Brief explanation of the drawings]

[0018] [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. [Figure 4] 4 is a time chart for explaining an outline of deterioration diagnosis of the rear O2 sensor. [Figure 5] 10 is a flowchart showing a procedure for setting a catalyst protection flag. [Figure 6] 4 is a flowchart showing the contents of control performed by the PCM. [Figure 7] 10 is a flowchart showing the details of throttle control during non-FC. [Figure 8] 4 is a flowchart showing the content of non-FC fuel injection control. [Figure 9] 4 is a time chart showing the time changes of each parameter during fuel cut. [Figure 10] 4 is a time chart showing the time changes of each parameter during fuel cut. DETAILED DESCRIPTION OF THE INVENTION

[0019] (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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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 lean 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 rich voltage. When the exhaust gas air-fuel ratio is stoichiometric, the RO2 output voltage is a stoichiometric voltage that is higher than the lean voltage and lower than the rich voltage. When the exhaust gas air-fuel ratio shifts from a leaner state to the stoichiometric air-fuel ratio, the RO2 output voltage increases from a voltage lower than the lean voltage through the lean voltage to the stoichiometric voltage. When the exhaust gas air-fuel ratio shifts from a richer state to the stoichiometric air-fuel ratio, the RO2 output voltage decreases from a voltage higher than the rich voltage through the rich voltage to the stoichiometric voltage.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] (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.

[0039] 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.

[0040] 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.

[0041] The vehicle is equipped with a braking device 81, a brake pedal 82, and a brake sensor SN8. The braking 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 device that switches between driving and stopping the braking device 81 and increases or decreases the braking force that the braking 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, which is the opening degree of the brake pedal 82, and the operating state of the braking device 81.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] The PCM 100 functionally includes a main control unit 101 and a diagnosis unit 102. The main control unit 101 controls the amount of intake air and the amount of fuel introduced into each combustion chamber 5. The diagnosis unit 102 performs abnormality diagnosis of the engine system E. In this embodiment, the diagnosis unit 102 diagnoses whether the rear O2 sensor SN6 has deteriorated.

[0046] (Fuel cut and basic control during fuel cut) When a fuel cut condition is met, that is, the engine speed is higher than a predetermined idle speed and the accelerator opening is equal to or smaller than a predetermined accelerator judgment opening, the PCM 100 (main control unit 101) performs a fuel cut by stopping the drive of the injector 11 of each cylinder 2A and stopping fuel injection into each combustion chamber 5. The accelerator judgment opening is set to 0 (zero), i.e., an opening close to fully closed, and the fuel cut is essentially performed when the accelerator pedal 91 is not depressed (accelerator-off). During the 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., an opening close to fully closed, and is preset to an opening smaller (closed) than the throttle opening achieved when the fuel cut is not being performed. Furthermore, during the fuel cut except when the EGR valve failure diagnosis described below is being performed, the PCM 100 fully closes the EGR valve 43.

[0047] (Rear O2 sensor deterioration diagnosis) The deterioration diagnosis of the rear O2 sensor SN6 performed by the PCM 100 (diagnosis unit 102) will be described with reference to Figure 4. Figure 4 is a time chart showing changes over time in the fuel cut flag, rear air-fuel ratio, RO2 output voltage, and diagnosis execution flag. The fuel cut flag is a flag that is set to 1 when a fuel cut, which will be described later, is performed, and is set to 0 when a fuel cut is not performed. The diagnosis execution flag is a flag that is set to 1 when a deterioration diagnosis of the rear O2 sensor SN6 is performed, and is set to 0 at other times.

[0048] When fuel cut is not being performed, the air-fuel ratio of the gas in the combustion chamber 5 and the exhaust gas is controlled to be close to the stoichiometric air-fuel ratio. On the other hand, when fuel cut is performed, the exhaust gas becomes almost entirely air. Therefore, when fuel cut is started, the air-fuel ratio of the exhaust gas becomes significantly leaner (larger) than close to the stoichiometric air-fuel ratio, and the RO2 output voltage drops. In the example of FIG. 4, fuel cut is started at time t1, and the RO2 output voltage drops accordingly.

[0049] 4, the solid line indicates the RO2 output voltage when the rear O2 sensor SN6 is not degraded, and the dashed line indicates the RO2 output voltage when the rear O2 sensor SN6 is degraded. As shown in this graph, the response speed of the rear O2 sensor SN6 decreases as it degrades, and the time dt2 required for the RO2 output voltage to decrease from the predetermined first voltage V1 to the predetermined second voltage V2 when the rear O2 sensor SN6 is degraded is longer than the time dt1 required when the rear O2 sensor SN6 is not degraded.

[0050] Using the above characteristics, in this embodiment, the rear O2 sensor SN6 is determined to be degraded based on the time it takes for the RO2 output voltage to drop from a predetermined first voltage V1 to a second voltage V2 after the start of fuel cut. Specifically, the PCM 100 performs a degradation diagnosis of the rear O2 sensor SN6 from the start of fuel cut until the RO2 output voltage drops to the predetermined second voltage V2. The PCM 100 also determines the time it takes for the RO2 output voltage to drop from the first voltage V1 to the second voltage V2 after the start of fuel cut, and determines that the rear O2 sensor SN6 is degraded if the determined time is longer than a predetermined diagnostic time. The first voltage V1 is preset to a value smaller than the stoichiometric voltage and larger than the lean voltage, and is stored in the PCM 100. The second voltage V2 is preset to a value smaller than the first voltage V1, and is stored in the PCM 100.

[0051] As described above, in this embodiment, a deterioration diagnosis of the rear O2 sensor SN6 is performed from the time when fuel cut starts until the RO2 output voltage drops to the second voltage V2, and the diagnosis execution flag becomes 1 when fuel cut starts and becomes 0 when the RO2 voltage drops to the second voltage V2.

[0052] In this embodiment, the number of times the deterioration diagnosis of the rear O2 sensor SN6 is performed in one driving cycle is limited. Specifically, when the number of times the deterioration diagnosis of the rear O2 sensor SN6 is performed in one driving cycle reaches a predetermined number of determinations, the PCM 100 prohibits the performance of subsequent deterioration diagnosis of the rear O2 sensor SN6 until the end of that driving cycle. The number of determinations is preset and stored in the PCM 100. The number of determinations is, for example, one time. Note that a driving cycle is the period from when the IG_ON signal is turned on, enabling the engine 1 to start, to when the IG_OFF signal is turned on, disabling the engine 1 from being driven.

[0053] (Catalyst protection control) Next, the catalyst protection control performed by the PCM 100 to prevent the catalyst device 31 from overheating will be described.

[0054] (Catalyst temperature conditions) 5 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.

[0055] Steps S51 to S56 shown in FIG. 5 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.

[0056] 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).

[0057] 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).

[0058] 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.

[0059] Here, the above-mentioned first judgment temperature and the above-mentioned second judgment temperature under the condition that the catalyst temperature has become equal to or higher than the first judgment temperature correspond to the "judgment temperature" of the present invention.

[0060] (Fuel cut control) Fig. 6 is a flowchart showing control mainly performed during fuel cut by the PCM 100 (main control unit 101). Steps S61 to S100 shown in Fig. 7 are repeatedly performed at predetermined intervals while IG_ON.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] If the determination in step S63 is NO and the catalyst protection flag is 0, the PCM 100 proceeds to step S67. In step S67, the PCM 100 sets the normal FC opening to the target throttle opening, which is the target value for the throttle opening. As described above, the normal FC opening is preset to an opening close to fully closed. After step S67, the PCM 100 proceeds to step S68.

[0066] If the determination in step S63 is YES and the catalyst protection flag is 1, the PCM 100 determines whether the diagnosis execution flag is 0 (step S64). That is, the PCM 100 determines whether a deterioration diagnosis of the rear O2 sensor SN6 is being performed.

[0067] If the determination in step S64 is NO and the diagnosis execution flag is 1, that is, if a deterioration diagnosis of the rear O2 sensor is being performed, the PCM 100 proceeds to step S67, sets the normal FC opening to the target throttle opening, and then proceeds to step S68. On the other hand, if the determination in step S64 is YES and a deterioration diagnosis of the rear O2 sensor is not being performed, the PCM 100 next determines whether the inflow of exhaust gas containing correction fuel into the catalytic device 31 has ended (step S65). As will be described in detail later, in this embodiment, the fuel injection amount is increased for a predetermined period after a fuel cut. In step S65, it is determined whether the inflow of exhaust gas containing this increased and corrected fuel into the catalytic device 31 has ended.

[0068] If the determination in step S65 is NO, meaning that the inflow of exhaust gas containing the increased fuel into the catalytic converter 31 has not yet ended, the PCM 100 proceeds to step S67, sets the normal FC opening to the target throttle opening, and then proceeds to step S68. On the other hand, if the determination in step S65 is YES, meaning that the inflow of exhaust gas containing the increased fuel into the catalytic converter 31 has ended, the PCM 100 proceeds to step S66. In step S66, the PCM 100 performs intake amount increase control, which is one type of catalyst protection control.

[0069] 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 S66, the PCM 100 sets the target throttle opening to an opening larger (more open) than the normal FC opening. In steps S70 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 S67 is performed). After step S66, the PCM 100 proceeds to step S68.

[0070] In step S68, similarly to step S63, the PCM 100 determines whether the catalyst protection flag is 1. If the determination in step S68 is YES and the catalyst protection flag is 1, the PCM 100 determines whether the target throttle opening set in step S66 or S67 is smaller than the current throttle opening (step S69). The current throttle opening is determined 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.

[0071] If the determination in step S69 is YES, that is, the target throttle opening is smaller than the current throttle opening, the PCM 100 controls the throttle valve 22 to close so that the throttle opening becomes the target throttle opening (step S70). By controlling the throttle valve 22 to close, the intake amount is reduced.

[0072] When step S70 is performed, the PCM 100 drives the throttle valve 22 so that the closing speed of the throttle valve 22 increases as the catalyst temperature increases. For example, the closing speed of the throttle valve 22 increases in proportion to the catalyst temperature. After step S70, the PCM 100 ends the processing (returns to step S61). Note that when the closing speed of the throttle valve 22 is high, the rate at which the intake air amount decreases also increases. That is, in step S70, the PCM 100 controls the throttle valve 22 so that the rate at which the intake air amount decreases increases as the catalyst temperature increases.

[0073] On the other hand, if the determination in step S69 is NO and the target throttle opening is equal to or greater than the current throttle opening, the PCM 100 determines whether the target throttle opening set in step S66 or S67 is greater than the current throttle opening (step S71).

[0074] If the determination in step S71 is YES and the target throttle opening is greater than the current throttle opening, the PCM 100 controls the throttle valve 22 to open so that the throttle opening becomes the target throttle opening (step S72). By controlling the throttle valve 22 to open, the intake amount increases.

[0075] When step S72 is performed, the PCM 100 drives the throttle valve 22 so that the opening speed of the throttle valve 22 slows as the catalyst temperature increases. For example, the opening speed of the throttle valve 22 is slowed in proportion to the catalyst temperature. After step S72, the PCM 100 ends the processing (returns to step S61). Note that when the opening speed of the throttle valve 22 is slow, the rate of increase in the intake air amount also increases. That is, in step S72, the PCM 100 controls the throttle valve 22 so that the rate of increase in the intake air amount increases as the catalyst temperature increases.

[0076] If the determination in step S71 is NO and the throttle opening degree matches the target throttle opening degree, the PCM 100 controls the throttle valve 22 so as to maintain the current throttle opening degree, and ends the process (return to step S61).

[0077] Returning to step S68, if the determination is NO and the catalyst protection flag is 0, the PCM 100 opens and closes the throttle valve 22 so that the throttle opening becomes the target throttle opening set in step S66 or step S68 (step S73). Here, in step S73, unlike steps S70 and S71, the opening and closing speed of the throttle valve 22 is not changed depending on the catalyst temperature, but is opened and closed at a predetermined speed. Note that even when step S73 is performed, if the target throttle opening and the current throttle opening match, the PCM 100 controls the throttle valve 22 to maintain that opening, and then ends the process (returning to step S61).

[0078] As described above, when a fuel cut is in progress (the determination in step S62 is YES) and the catalyst protection flag is 1 (the determination in step S63 is YES), if both of the following conditions are met: the deterioration diagnosis of the rear O2 sensor SN6 is not being performed and the diagnosis execution flag is 0 (the determination in step S64 is YES), and the inflow of exhaust gas containing correction fuel into the catalytic device 31 has ended (the determination in step S65 is YES), intake air amount increase control is implemented, and the throttle opening is set to an opening greater than the normal FC opening. On the other hand, even if a fuel cut is in progress (the determination in step S62 is NO) and the catalyst protection flag is 1 (the determination in step S63 is NO), if either of the above two conditions is not met (the determination in step S64 or step S65 is NO), intake air amount increase control is prohibited, and the throttle opening is controlled to the normal FC opening. Furthermore, when the catalyst protection flag is 1, intake air amount increase control is not implemented, and the throttle opening is controlled to the normal FC opening. As a result, when the catalyst protection flag is 1 and the above two conditions are met during fuel cut, the intake air amount is increased compared to when the catalyst flag is 0 or when at least one of the above two conditions is not met. Here, the condition that the deterioration diagnosis of the rear O2 sensor SN6 is not being performed and the condition that the inflow of exhaust gas containing correction fuel into the catalytic device 31 has ended correspond to the "prohibition conditions" in the present invention.

[0079] Furthermore, when fuel is being cut (the determination in step S62 is YES) and the catalyst protection flag is 1 (the determination in step S63 is YES), if the throttle valve 22 is controlled to close (the determination in step S69 is YES), the higher the catalyst temperature, the faster the closing speed of the throttle valve 22 is, and the faster the rate at which the intake air amount decreases. Furthermore, when fuel is being cut and the catalyst protection flag is 1, if the throttle valve 22 is controlled to open (the determination in step S71 is YES), the higher the catalyst temperature, the slower the opening speed of the throttle valve 22 is, and the slower the rate at which the intake air amount increases.

[0080] (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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] (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.

[0088] 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. 8.

[0089] 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 a value close to the stoichiometric air-fuel ratio, based on the target intake air amount set in step S102 or step S105 of non-FC fuel throttle control, and the engine speed detected by the crank angle sensor SN1, etc.

[0090] Next, the PCM 100 determines whether the time elapsed since the fuel supply was restored is equal to or shorter than a predetermined time (S202). Step S202 is a step for determining whether the air-fuel ratio in the catalytic converter 31, which became leaner than the stoichiometric air-fuel ratio due to the fuel cut, is maintained at a lean state. The predetermined time is the time from the fuel supply restoration until the air-fuel ratio in the catalytic converter 31 becomes stoichiometric or richer than the stoichiometric air-fuel ratio. Here, when the air-fuel ratio in the catalytic converter 31 changes from a state leaner than the stoichiometric air-fuel ratio to the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio, the RO2 output voltage rises above a predetermined threshold voltage that is lower than the stoichiometric voltage. Therefore, in this embodiment, the period from the fuel supply restoration until the RO2 output voltage exceeds the threshold voltage is set as the predetermined time. That is, in step S202, the PCM 100 determines whether the RO2 output voltage exceeds the threshold voltage after the fuel supply restoration. The threshold voltage is preset and stored in the PCM 100.

[0091] If the determination in step S202 is NO, the time elapsed since fuel recovery exceeds a predetermined period, and the air-fuel ratio in the catalytic device 31 has become the stoichiometric air-fuel ratio or richer than that, PCM100 proceeds to step S204 while maintaining the fuel injection amount at the value set in step S201 without performing the fuel injection amount increase correction described below.

[0092] On the other hand, if the determination in step S202 is YES, the elapsed time since fuel recovery is equal to or less than the predetermined time, and the air-fuel ratio in the catalytic converter 31 is leaner than the stoichiometric air-fuel ratio, the PCM 100 corrects and increases the fuel injection amount (step S203). Specifically, in step S203, 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 corrected and increased, the air-fuel ratio of the mixture in the combustion chamber 5 becomes richer than the stoichiometric air-fuel ratio. After step S203, the PCM 100 proceeds to step S204.

[0093] In step S204, the PCM 100 drives the injector 11 to achieve the fuel injection amount set in step S201 or the fuel injection amount reset in step S203. When step S205 is performed, the non-FC fuel injection control ends.

[0094] As described above, by implementing fuel increase control after fuel recovery, the oxygen stored in the catalytic device 31 due to the fuel cut is consumed, and the air-fuel ratio of the catalytic device 31 is returned to the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio.

[0095] Returning to the flowchart of Figure 6, step S65 will be described in detail. Step S65 is a step for determining whether or not there is any influence of the fuel increase correction described above. Specifically, it takes time for the burned gas in the combustion chamber 5 to reach the catalytic device 31. Therefore, even after the fuel increase control ends, the exhaust gas containing the increased fuel continues to flow into the catalytic device 31 for a while. In step S65, it is determined whether or not the flow of the exhaust gas containing the increased fuel into the catalytic device 31 has ended.

[0096] The air-fuel ratio of the mixture and exhaust gas in the combustion chamber 5 containing the increased fuel is 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 is 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 and exhaust gas in the combustion chamber 5 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 and exhaust gas in the combustion chamber 5 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. The front O2 sensor SN5 is provided immediately upstream of the catalytic converter 31, and the front air-fuel ratio detected by the front O2 sensor SN5 is substantially the same as the air-fuel ratio of the exhaust gas flowing into the catalytic converter 31. Thus, in this embodiment, in step S65, the PCM 100 determines NO from the time the fuel amount increase control ends until the front air-fuel ratio detected by the front O2 sensor SN5 becomes the stoichiometric air-fuel ratio or leaner than the stoichiometric air-fuel ratio; otherwise, it determines YES. As described above, when the determination in step S65 is NO, meaning that the inflow of exhaust gas containing the increased fuel into the catalytic converter 31 has not ended, the PCM 100 proceeds to step S66 and sets the normal FC opening to the target throttle opening. On the other hand, when the determination in step S65 is YES, meaning that the inflow 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.

[0097] (effect, etc.) As described above, in the above embodiment, when the catalyst protection flag is 1 and the catalyst temperature is high (when the deterioration diagnosis of the rear O2 sensor SN6 is not being performed and the inflow of exhaust gas containing correction fuel into the catalytic converter 31 has ended), the intake air amount increase control is basically performed, 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, the catalyst temperature is equal to or higher than the first threshold temperature, or when the catalyst temperature has once exceeded the first threshold temperature but has not yet fallen below the second threshold temperature. When the catalyst flag is 0 and the catalyst temperature is low, the catalyst temperature is equal to or lower than the second threshold temperature, or when the catalyst temperature has exceeded the second threshold temperature but has not yet reached the first threshold temperature.

[0098] Furthermore, when the throttle valve 22 is opened to increase the intake air amount while the catalyst protection flag is set to 1, the higher the catalyst temperature, the slower the opening speed of the throttle valve 22, thereby slowing down the rate at which the intake air amount increases. Also, when the throttle valve 22 is closed to reduce the intake air amount at the start of a fuel cut, the higher the catalyst temperature, the faster the closing speed of the throttle valve 22, thereby accelerating the rate at which the intake air amount decreases. This makes it possible to prevent damage such as cracks in the catalyst device 31, and also to properly cool the catalyst device 31 to prevent it from overheating.

[0099] This will be explained in detail using Figure 9. Figure 9 is a time chart that schematically shows the time changes of each parameter when a fuel cut is initiated in a state where the catalyst protection flag is 1 and the number of diagnoses of the rear O2 sensor SN6 has not yet reached the number of determinations. Figure 9 shows, from top to bottom, the charts for the catalyst protection flag, fuel cut flag, diagnosis execution flag, throttle opening, intake air amount, RO2 output voltage, and catalyst temperature.

[0100] In the example of FIG. 9, at time t11, fuel cut starts, deterioration diagnosis of the rear O2 sensor SN6 starts, and the diagnosis execution flag changes from 0 to 1. As described above, when the diagnosis execution flag is 1 and deterioration diagnosis of the rear O2 sensor SN6 is being performed, the throttle opening is set to the normal FC opening. As described above, the throttle opening when not in a fuel cut state is larger than the normal FC opening. Therefore, after time t11, the throttle valve 22 is controlled to close toward the normal FC opening, and the intake amount decreases.

[0101] In the example of Figure 9, the deterioration diagnosis of the rear O2 sensor SN6 ends at time t12 as the RO2 output voltage drops to the second voltage V2. When the deterioration diagnosis of the rear O2 sensor SN6 ends, the intake air amount increase control is started. Accordingly, after time t12, the throttle opening is increased (opened) from the normal FC opening, and the intake air amount increases.

[0102] In the charts of throttle opening, intake air amount, and catalyst temperature in Fig. 9, the dashed dotted line indicates the change over time of each parameter when the closing speed of the throttle valve 22 is slower than that indicated by the solid line. In the charts of throttle opening, intake air amount, and catalyst temperature in Fig. 9, the dashed two-dotted line indicates the change over time of each parameter when the opening speed of the throttle valve 22 is faster than that indicated by the solid line.

[0103] As shown by the dashed line in FIG. 9 , if the throttle valve 22 is closed slowly, the intake air volume remains high even after time t11. Therefore, in this case, a large amount of air flows into the catalytic converter 31 immediately after fuel cut begins at time t11. Here, when the temperature of the catalytic converter 31 is high, the difference between the temperature and the air temperature is large. Therefore, if the throttle valve 22 is closed slowly, a large amount of low-temperature air flows into the catalytic converter 31 while the temperature of the catalytic converter 31 is high, resulting in a significant drop in catalyst temperature. In contrast, as shown by the solid line in FIG. 9 , if the throttle valve 22 is closed quickly, the intake air volume decreases quickly after time t11. Therefore, if the throttle valve 22 is closed quickly, the amount of air flowing into the catalytic converter 31 immediately after fuel cut begins at time t11 is kept small, so the catalyst temperature decreases gradually.

[0104] In this way, if the closing speed of the throttle valve 22 is slowed when the temperature of the catalytic converter 31 is high, the catalyst temperature drops significantly, and if the closing speed of the throttle valve 22 is increased when the temperature of the catalytic converter 31 is high, the catalyst temperature drops gradually. If the catalyst temperature drops too much, there is a high possibility that the catalytic converter 31 will be damaged, such as cracked.

[0105] In contrast, in this embodiment, when fuel cut is performed while the catalyst flag is 1, i.e., the catalyst temperature is high, and the throttle valve 22 is controlled to close to reduce the intake air amount, the higher the catalyst temperature, the faster the throttle valve 22 closes, and the faster the intake air amount decreases. This makes it possible to prevent a large amount of low-temperature air from flowing into the catalytic converter 31 when the catalyst temperature is high. This prevents the catalyst temperature from decreasing excessively, and ultimately prevents damage such as cracking of the catalytic converter 31. On the other hand, when the catalyst temperature is low and the catalytic converter 31 is unlikely to be damaged, the catalytic converter 31 can be cooled quickly by flowing a large amount of low-temperature air into it.

[0106] Furthermore, as shown by the two-dot chain line in Figure 9, if the opening speed of the throttle valve 22 is fast, the intake air amount increases early after time t12, and a large amount of low-temperature air flows into the catalytic converter 31 all at once. As described above, if a large amount of low-temperature air flows into the catalytic converter 31 when the temperature of the catalytic converter 31 is high, the catalyst temperature drops significantly. Therefore, in this case, as shown by the two-dot chain line, the catalyst temperature drops sharply after time t12. In contrast, as shown by the solid line in Figure 9, if the opening speed of the throttle valve 22 is slow, the intake air amount increases gradually after time t12, and the catalyst temperature drops relatively gradually.

[0107] In this way, if the opening speed of the throttle valve 22 is increased when the temperature of the catalytic converter 31 is high, the catalyst temperature drops sharply, and if the opening speed of the throttle valve 22 is decreased when the temperature of the catalytic converter 31 is high, the catalyst temperature drops gradually. If the catalyst temperature drops too much, there is a high possibility that the catalytic converter 31 will be damaged, such as cracked.

[0108] In contrast, in this embodiment, when fuel cut is performed while the catalyst flag is 1, i.e., the catalyst temperature is high, and the throttle valve 22 is controlled to open to increase the intake air amount, the higher the catalyst temperature, the slower the opening speed of the throttle valve 22 is, and the slower the rate of increase in the intake air amount is. This makes it possible to prevent a large amount of low-temperature air from flowing into the catalytic converter 31 when the catalyst temperature is high. This prevents the catalyst temperature from decreasing excessively, and ultimately prevents damage such as cracking of the catalytic converter 31. On the other hand, when the catalyst temperature is low and the catalytic converter 31 is unlikely to be damaged, the catalytic converter 31 can be cooled quickly by flowing a large amount of low-temperature air into it.

[0109] Here, when the catalyst flag is 1 and fuel cut is being performed, the timing for opening and closing the throttle valve 22 is not limited to the start and end of the deterioration diagnosis of the rear O2 sensor SN6, as described above. For example, the throttle valve 22 is also closed and opened when the inflow of exhaust gas containing correction fuel into the catalytic converter 31 ends during fuel cut. FIG. 10 is a time chart that schematically shows the time changes of each parameter when the inflow of exhaust gas containing correction fuel into the catalytic converter 31 ends during fuel cut. Specifically, FIG. 10 is a time chart when fuel cut is initiated when the catalyst protection flag is 1 and the inflow of exhaust gas containing correction fuel into the catalytic converter 31 has not yet ended. FIG. 10 shows, from top to bottom, charts of the catalyst protection flag, fuel cut flag, fuel increase flag, front air-fuel ratio, throttle opening, and intake air volume. The fuel increase flag is a flag that is 1 when the fuel injection amount is being increased and corrected, and is 0 otherwise.

[0110] In the example of FIG. 10, the increase correction of the fuel injection amount ends at time t21. At time t22 after time t21, a fuel cut starts. Furthermore, at time t23 after time t22, the front air-fuel ratio reaches the stoichiometric air-fuel ratio (St), and the inflow of exhaust gas containing the increase-corrected fuel into the catalytic converter 31 ends. In this way, if a fuel cut starts before the inflow of exhaust gas containing the increase-corrected fuel into the catalytic converter 31 ends, intake air amount increase control is not performed even when the fuel cut starts, and the throttle valve 22 is closed toward the normal FC opening at the start of the fuel cut (time t21). Then, when the inflow of exhaust gas containing the increase-corrected fuel into the catalytic converter 31 ends (time t23), intake air amount increase control is started, and the throttle valve 22 is controlled to open from the normal FC opening. Thus, in the example of FIG. 10, at time t21, the higher the catalyst temperature, the faster the closing speed of the throttle valve 22 becomes, and the faster the rate at which the intake air amount decreases. Furthermore, at time t23, the higher the catalyst temperature, the slower the opening speed of the throttle valve 22, thereby slowing the rate of increase in the intake air amount. By controlling in this manner, according to this embodiment, even if the inflow of exhaust gas containing correction fuel into the catalytic device 31 ends while fuel cut is being performed, the throttle valve 22 and therefore the intake air amount can be appropriately controlled in accordance with the catalyst temperature, and the catalytic device 31 can be prevented from becoming excessively hot while preventing damage to the catalytic device 31. Note that in the example of Figure 10, the fuel cut is started after the air-fuel mixture in the combustion chamber 5 is controlled to the stoichiometric air-fuel ratio for a while after the fuel injection amount increase correction is completed, so 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.

[0111] In addition, 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.

[0112] 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.

[0113] (Variation) In the above embodiment, when the intake air amount increasing 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 capable of changing 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.

[0114] In the above embodiment, a case has been described in which a deterioration diagnosis of the rear O2 sensor SN6 is performed as an abnormality diagnosis of the engine system E. However, instead of or in addition to the deterioration diagnosis of the rear O2 sensor SN6, a diagnosis of another device may be performed as an abnormality diagnosis. Furthermore, even when an abnormality diagnosis other than the deterioration diagnosis of the rear O2 sensor SN6 is performed, the abnormality diagnosis may be configured to take priority over the intake air amount increase control during fuel cut. For example, a configuration may be adopted in which a diagnosis of whether the EGR device 40 has a malfunction is performed during fuel cut, and when fuel cut begins, the intake air amount increase control is prohibited and the diagnosis is performed, and the intake air amount increase control is started after the diagnosis is completed.

[0115] 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.

[0116] In addition, in the above embodiment, a case has been described in which, during a single fuel cut, once the deterioration diagnosis of the rear O2 sensor SN6 is completed, the intake amount increase control is then started. However, during the fuel cut in which the deterioration diagnosis of the rear O2 sensor SN6 is performed, the intake amount increase control may be prohibited until the fuel cut is completed.

[0117] In the above embodiment, the catalytic device 31 includes a three-way catalyst as a catalyst, but the catalyst included in the catalytic device 31 is not limited to this. Also, in the above embodiment, the injector 11 is a side injection type, but the injection type of the injector 11 is not limited to this. Also, in the above embodiment, the fuel is injected directly into the combustion chamber 5, but the fuel injection form is not limited to this. Also, the specific structure of the engine body 1, such as the number of cylinders, is not limited to the above. [Explanation of symbols]

[0118] 1 Engine body 5 Combustion chamber 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) 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 provided in the exhaust passage for purifying exhaust gas; 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 for controlling the fuel supply device and the intake air amount adjustment device, The control unit a fuel cut to stop fuel injection by the fuel supply device when the accelerator opening detected by the accelerator opening detection device is equal to or smaller than a predetermined accelerator judgment opening; When the fuel cut is performed when the catalyst temperature identified by the catalyst temperature identifying device is equal to or higher than a predetermined judgment temperature, 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 below the judgment temperature, and the intake amount adjustment device is controlled so that the rate of increase of the intake amount is slower when the catalyst temperature is high than when the catalyst temperature is low.

2. 2. The engine control device according to claim 1, When the fuel cut is performed, the control unit prohibits the intake air amount increase control if a predetermined prohibition condition is met, and starts the intake air amount increase control to increase the intake air amount if the prohibition condition is not met.

3. 2. The engine control device according to claim 1, The control unit controls the intake air amount adjustment device so that when the fuel cut is performed and the intake air amount is reduced, the rate of reduction of the intake air amount is faster when the catalyst temperature is high than when the catalyst temperature is low.

4. 2. The engine control device according to claim 1, The control unit is characterized in that, when the fuel cut is not being performed and the catalyst temperature is equal to or higher than the judgment temperature, the control unit limits the intake amount to a predetermined upper limit intake amount or less using the intake amount adjustment device.

5. The engine control device according to any one of claims 1 to 4, 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