Engine catalytic converter diagnostic device

Abstract

In an engine catalytic converter diagnostic device that diagnoses the catalytic converter that purifies engine exhaust gases, the diagnostic accuracy of the catalytic converter is significantly improved. [Solution] The engine catalyst diagnostic device 100 includes an exhaust passage 50 for discharging exhaust gas from the combustion chamber 17 of the engine 1, a catalyst device 51 provided on the exhaust passage 50 for purifying the exhaust gas, a linear A / F sensor SW7 provided on the exhaust passage 50 downstream of the catalyst device 51 and capable of detecting the air-fuel ratio of the exhaust gas, and a processing device 60 configured to diagnose the catalyst device 51 based on the air-fuel ratio detected by the linear A / F sensor SW7. The processing device 60 acquires the temperature of the catalyst device 51 when the engine 1 is started after soaking, calculates the fluctuation range of the air-fuel ratio detected by the linear A / F sensor SW7 within a predetermined time, and diagnoses the catalyst device 51 based on the relationship between this fluctuation range and the catalyst temperature.

Description

【Technical Field】 【0001】 The present invention relates to an engine catalyst diagnosis device that diagnoses a catalyst device for purifying exhaust gas of an engine. 【Background Art】 【0002】 Conventionally, a catalyst device for purifying exhaust gas has been provided in the exhaust passage of an engine, and this catalyst device has been diagnosed (deterioration diagnosis) based on the air-fuel ratio of the exhaust gas or the like. For example, in Patent Document 1, oxygen sensors are provided on the upstream side and the downstream side of the catalyst device, and the deterioration of the catalyst device is determined by comparing the standard dispersion state of the oxygen concentration fluctuation values in the exhaust gas on the upstream side and the downstream side of the catalyst device. 【Prior Art Documents】 【Patent Documents】 【0003】 【Patent Document 1】 Japanese Patent Application Laid-Open No. 2013-83195 【Summary of the Invention】 【Problems to be Solved by the Invention】 【0004】 In recent years, vehicle exhaust gas regulations have become very strict, and therefore, it is required to diagnose the catalyst device with higher accuracy. In order to meet such demands, the inventors of the present application have conducted intensive research. As a result, the inventors of the present application have found that the catalyst device can be diagnosed with higher accuracy when starting the engine after soaking, that is, during cold start (cold start). 【0005】 When the engine is cold, the catalytic converter gradually becomes active as the temperature rises, meaning its exhaust gas purification performance (purification rate) gradually increases (this is called the catalytic converter's "light-off" performance). In this case, if the catalytic converter is functioning normally, the activation is quick, but if it is degraded, the activation tends to be slow. Therefore, when the engine is cold, the difference depending on the degree of catalytic converter degradation becomes particularly apparent. Accordingly, the inventors of this application considered diagnosing the catalytic converter when the engine is cold. 【0006】 Furthermore, the inventors of this invention considered using the air-fuel ratio of the exhaust gas downstream of the catalytic converter to determine the activation state of the catalytic converter. This is because, when the catalytic converter is inactive, the air-fuel ratio downstream of the catalytic converter fluctuates relatively large, but as the catalytic converter becomes active, the oxygen consumption in the catalytic converter increases (because the oxidation-reduction efficiency of the catalytic converter with respect to the exhaust gas increases), causing the fluctuation in the air-fuel ratio downstream of the catalytic converter to decrease. Therefore, it can be said that the activation state of the catalytic converter can be accurately determined based on the fluctuation in the air-fuel ratio of the exhaust gas downstream of the catalytic converter. 【0007】 Furthermore, the inventors of this invention considered detecting the air-fuel ratio of the exhaust gas downstream of such a catalytic converter using a linear A / F sensor. This is because a linear A / F sensor can detect the air-fuel ratio with high accuracy. Specifically, a linear A / F sensor outputs a signal (voltage or current signal) corresponding to the magnitude of the air-fuel ratio, so it has a wide detection range for the air-fuel ratio. In contrast, a lambda O2 sensor can basically only detect air-fuel ratios near the stoichiometric air-fuel ratio; in other words, it can only detect whether or not the air-fuel ratio of the exhaust gas deviates from the stoichiometric air-fuel ratio. Therefore, the detection range of the air-fuel ratio of a lambda O2 sensor is much narrower than that of a linear A / F sensor. 【0008】 This invention is based on the above findings and aims to significantly improve the diagnostic accuracy of a catalytic converter in an engine catalytic converter diagnostic device that diagnoses the catalytic converter used to purify engine exhaust gases. [Means for solving the problem] 【0009】 To achieve the above objective, the present invention provides an engine catalyst diagnostic device comprising: an exhaust passage for discharging exhaust gas from the combustion chamber of an engine; a catalyst device provided on the exhaust passage for purifying the exhaust gas; a linear A / F sensor provided on the exhaust passage downstream of the catalyst device and capable of detecting the air-fuel ratio of the exhaust gas; and a processing device configured to diagnose the catalyst device based on the air-fuel ratio detected by at least the linear A / F sensor, wherein the processing device is configured to acquire the temperature of the catalyst device during engine startup after soaking, calculate the fluctuation range of the air-fuel ratio detected by the linear A / F sensor within a predetermined time, and diagnose the catalyst device based on the relationship between the fluctuation range and the temperature of the catalyst device. 【0010】 In the present invention configured as described above, the treatment device diagnoses the catalytic converter during startup after soaking (i.e., when cold), when differences in the degree of catalytic converter degradation are particularly evident. In particular, since the temperature at which the catalytic converter becomes active (catalyst temperature) changes depending on the degree of catalytic converter degradation, the treatment device uses the catalyst temperature when diagnosing the catalytic converter. Furthermore, the treatment device uses fluctuations in the air-fuel ratio downstream of the catalytic converter to determine the activity state of the catalytic converter. This is because when the catalytic converter is inactive, the air-fuel ratio downstream of the catalytic converter fluctuates relatively large, but as the catalytic converter becomes active, oxygen consumption in the catalytic converter increases, and fluctuations in the air-fuel ratio downstream of the catalytic converter become smaller. Therefore, the activity state of the catalytic converter can be accurately determined based on such fluctuations in the air-fuel ratio (especially the range of fluctuation). Thus, in the present invention, the treatment device diagnoses the catalytic converter when cold, based on the relationship between the range of fluctuations in the air-fuel ratio downstream of the catalytic converter and the catalyst temperature. This significantly improves the diagnostic accuracy of the catalytic converter. 【0011】 In the present invention, preferably, the processing apparatus is configured to determine the activity level of the catalyst based on the fluctuation range, obtain the temperature of the catalyst when it reaches a predetermined activity level based on the determination result of the activity level, and diagnose the catalyst based on that temperature. According to the present invention configured in this way, the catalyst device can be accurately diagnosed by using the catalyst temperature at which the catalyst device reaches a predetermined level of activity. 【0012】 In the present invention, preferably, the processing apparatus diagnoses that the catalyst is degraded if the temperature of the catalyst is above a predetermined temperature when the catalyst reaches a predetermined level of activity. In this case, the treatment device diagnoses that the catalytic converter is deteriorating because its exhaust gas purification performance is slow to start up. This allows for an accurate diagnosis of the catalytic converter. 【0013】 In the present invention, preferably, the linear A / F sensor is a second linear A / F sensor, the air-fuel ratio detected by the second linear A / F sensor is defined as the second air-fuel ratio, and the fluctuation range of the second air-fuel ratio detected by the second linear A / F sensor within a predetermined time is defined as the second fluctuation range. The engine catalyst diagnostic device further includes a first linear A / F sensor provided in the exhaust passage upstream of the catalyst device and capable of detecting the first air-fuel ratio of the exhaust gas. The processing device is configured to calculate the first fluctuation range of the first air-fuel ratio detected by the first linear A / F sensor within a predetermined time, along with the second fluctuation range, at the time of starting the engine after soaking, calculate the fluctuation range ratio, which is the ratio of the first fluctuation range to the second fluctuation range, and determine the activity level of the catalyst device based on the fluctuation range ratio. Since the first fluctuation range upstream of the catalytic converter is stable because it is not affected by the exhaust gas purification performance of the catalytic converter, this invention uses this first fluctuation range as a reference to evaluate the magnitude of the second fluctuation range downstream of the catalytic converter. This ensures the diagnostic accuracy of the catalytic converter. 【0014】 In the present invention, preferably, the processing apparatus normalizes the fluctuation range ratio, and when the normalized fluctuation range ratio reaches a predetermined value, it determines that the catalyst apparatus has reached a predetermined level of activity, and diagnoses the catalyst apparatus based on the temperature of the catalyst apparatus at that time. The fluctuation range ratio is unitless and changes depending on various factors such as the amount of precious metal in the catalyst device, its durability, and its degradation state. Therefore, in the present invention described above, in order to eliminate the influence of these factors and ensure versatility, the fluctuation range ratio is processed using a normalized value. This effectively ensures the diagnostic accuracy of the catalyst device. 【0015】 In the present invention, preferably, the linear A / F sensor is a second linear A / F sensor, and the air-fuel ratio detected by the second linear A / F sensor is defined as the second air-fuel ratio. The engine catalyst diagnostic device further includes a first linear A / F sensor located upstream of the catalyst and capable of detecting the first air-fuel ratio of the exhaust gas. The processing device is configured to calculate the oxygen storage capacity (OSC) of the catalyst based on the first air-fuel ratio detected by the first linear A / F sensor and the second air-fuel ratio detected by the second linear A / F sensor when the engine is fuel-cut off after engine startup, and to further diagnose the catalyst based on the oxygen storage capacity. During engine fuel cut-off, essentially only air is supplied to the catalytic converter, so the catalytic converter tends to become oxygen-saturated. According to the present invention described above, by diagnosing the catalytic converter based on the amount of oxygen stored in this oxygen-saturated state, the catalytic converter can be diagnosed with high accuracy. 【0016】 In the present invention, preferably, the engine catalyst diagnostic device further includes a fuel injector for supplying fuel to the combustion chamber, and the processing device is configured to control the fuel injector to increase the amount of fuel supplied to the combustion chamber when the engine recovers from fuel cut-off, and to reduce the amount of fuel increased compared to when it is diagnosed that the catalytic converter is degraded, if it is diagnosed that the catalytic converter is degraded. According to the present invention configured in this way, it is possible to suppress unnecessary increases in fuel quantity and improve fuel efficiency. 【0017】 In the present invention, preferably, the catalyst diagnostic device for an engine further includes a warning lamp for notifying an abnormality to the vehicle occupants, and the processing device is configured to perform control to turn on the warning lamp when it is diagnosed that the catalyst device is deteriorated. According to the present invention configured as described above, it is possible to accurately convey the deterioration of the catalyst device to the occupants and prompt the replacement of the catalyst device or the like. 【Advantages of the Invention】 【0018】 According to the present invention, in an engine catalyst diagnostic device that diagnoses a catalyst device for purifying the exhaust gas of an engine, the diagnostic accuracy of the catalyst device can be significantly improved. 【Brief Description of the Drawings】 【0019】 [Figure 1] It is a schematic configuration diagram of an engine catalyst diagnostic device according to an embodiment of the present invention. [Figure 2] It is a block diagram showing the electrical configuration of an engine catalyst diagnostic device according to an embodiment of the present invention. [Figure 3] In an embodiment of the present invention, it is an explanatory diagram of the catalyst temperature obtained according to the normalized fluctuation ratio. [Figure 4] In an embodiment of the present invention, it is an explanatory diagram of deterioration determination based on the catalyst temperature obtained from FIG. 3. [Figure 5] In an embodiment of the present invention, it is a flowchart showing the catalyst diagnostic process performed during cold operation. [Figure 6] In an embodiment of the present invention, it is a time chart showing the catalyst diagnostic process performed during cold operation. [Figure 7] In an embodiment of the present invention, it is a flowchart showing the catalyst diagnostic process performed during warm operation. 【Modes for Carrying Out the Invention】 【0020】 Hereinafter, an engine catalyst diagnostic device according to an embodiment of the present invention will be described with reference to the accompanying drawings. 【0021】 [Device configuration] First, with reference to Figure 1, the overall configuration of the engine catalyst diagnostic device according to this embodiment will be described. Figure 1 is a schematic diagram of the engine catalyst diagnostic device according to this embodiment. 【0022】 The engine catalytic converter diagnostic device 100 is mounted on a vehicle (not shown) and, as shown in Figure 1, mainly comprises an engine 1 as an internal combustion engine that generates power (propulsion) for the vehicle, an intake passage 40 that supplies air (intake) to the engine 1, and an exhaust passage 50 that discharges exhaust gas from the engine 1. 【0023】 Engine 1 is a four-stroke engine that performs intake, compression, expansion, and exhaust strokes. Engine 1 is a gasoline engine that uses gasoline as fuel. This fuel may be any liquid fuel containing at least gasoline, and may be gasoline containing, for example, bioethanol. 【0024】 Specifically, engine 1 mainly comprises a cylinder block 11, a cylinder head 12 mounted on the cylinder block 11 and forming a cylinder 13 together with the cylinder block 11, a piston 14 that reciprocates within the cylinder 13, a connecting rod 15 connected to the piston 14, and a crankshaft 16 connected to the connecting rod 15. Engine 1 is, for example, a multi-cylinder engine containing multiple cylinders 13 (only one cylinder 13 is shown in Figure 1). The cylinder block 11, cylinder head 12, and piston 14 form the combustion chamber 17 of engine 1. 【0025】 Furthermore, the engine 1 has fuel injectors 18 and spark plugs 19 provided in the cylinder head 12. The fuel injectors 18 inject fuel into the cylinder 13 (combustion chamber 17), and the spark plugs 19 ignite the fuel-air mixture in the cylinder 13. A fuel supply system (not shown) is connected to the fuel injectors 18, and fuel is supplied from this system. In the engine 1 shown in Figure 1, the fuel injectors 18 are shown positioned to inject fuel from above the combustion chamber 17, but the fuel injectors 18 may also be positioned to inject fuel from the side of the combustion chamber 17. In the latter case, the fuel injectors 18 can be provided in the cylinder block 11. 【0026】 On the other hand, the intake passage 40 is provided with an air cleaner 41 and a throttle valve 43. The throttle valve 43 adjusts the amount of air introduced into the cylinder 13 according to its opening. An intake valve 21 is also provided between the intake passage 40 and the cylinder 13. The intake valve 21 is opened and closed at predetermined timings by a valve train. Typically, the valve train is an electrically or hydraulically operated variable valve train that varies the valve timing and / or valve lift. For example, the valve train is an intake S-VT (Sequential-Valve Timing) that can continuously change the rotational phase of the intake camshaft relative to the crankshaft 16 within a predetermined angular range. 【0027】 Next, an exhaust valve 22 is provided in the exhaust passage 50. Specifically, the exhaust valve 22 is located between the cylinder 13 and the exhaust passage 50. The exhaust valve 22 is opened and closed at predetermined timings by a valve train. Typically, the valve train is an electrically or hydraulically operated variable valve train that varies the valve timing and / or valve lift. For example, the valve train is an exhaust S-VT that continuously changes the rotational phase of the exhaust camshaft relative to the crankshaft 16 within a predetermined angular range. 【0028】 Furthermore, the exhaust passage 50 is provided with two catalytic converters 51 and 52, each containing a three-way catalytic converter. Catalytic converter 51 is located upstream of catalytic converter 52, and catalytic converter 52 is located downstream of catalytic converter 51. The three-way catalytic converter contains platinum group elements (PGMs) such as platinum (Pt), palladium (Pd), and rhodium (Rh), and purifies HC, CO, NOx, etc., in the exhaust gas. Basically, the three-way catalytic converter purifies (oxidizes) HC and CO when the air-fuel ratio of the exhaust gas is near the stoichiometric air-fuel ratio or higher than the stoichiometric air-fuel ratio (lean), and purifies (reduces) NOx when the air-fuel ratio of the exhaust gas is near the stoichiometric air-fuel ratio or lower than the stoichiometric air-fuel ratio (rich). Note that it is not limited to using two catalytic converters 51 and 52; at least catalytic converter 51 is sufficient, and catalytic converter 52 does not need to be located downstream of catalytic converter 51. 【0029】 Furthermore, as shown in Figure 1, the engine catalytic converter diagnostic device 100 includes an airflow sensor SW1, an intake air temperature sensor SW2, a water temperature sensor SW3, a crank angle sensor SW4, an accelerator opening sensor SW5, and linear A / F sensors SW6 and SW7. 【0030】 The airflow sensor SW1 is located on the intake passage 40 downstream of the air cleaner 41 and detects the flow rate of air flowing through the intake passage 40. The intake air temperature sensor SW2 is located on the intake passage 40 downstream of the air cleaner 41 and detects the temperature of the air flowing through the intake passage 40. The water temperature sensor SW3 is located on the engine 1 and detects the temperature of the coolant in the engine 1. The crank angle sensor SW4 is located on the engine 1 and detects the rotation angle of the crankshaft 16. The accelerator opening sensor SW5 is located on the accelerator pedal mechanism 30 and detects the accelerator opening corresponding to the amount of accelerator pedal operation. The linear A / F sensor SW6 is located on the exhaust passage 50 upstream of the catalytic converter 51 and detects the air-fuel ratio of the exhaust gas flowing into the catalytic converter 51 (hereinafter referred to as the "first detected air-fuel ratio" as appropriate). The linear A / F sensor SW7 is installed in the exhaust passage 50 downstream of the catalytic converter 51 and detects the air-fuel ratio of the exhaust gas flowing out of the catalytic converter 51 (hereinafter referred to as the "second detected air-fuel ratio" as appropriate). The linear A / F sensors SW6 and SW7 output a signal (voltage or current signal) corresponding to the magnitude of the air-fuel ratio. 【0031】 Linear A / F sensors SW6 and SW7 correspond to the "first linear A / F sensor" and "second linear A / F sensor" in the present invention, respectively, and the first and second detected air-fuel ratios correspond to the "first air-fuel ratio" and "second air-fuel ratio" in the present invention, respectively. 【0032】 Next, with reference to Figure 2, the electrical configuration of the engine catalyst diagnostic device 100 according to this embodiment will be described. Figure 2 is a block diagram showing the electrical configuration of the engine catalyst diagnostic device 100 according to this embodiment. 【0033】 As shown in Figure 2, the engine catalyst diagnostic device 100 has a processing unit 60 configured to perform various controls and processes in the device 100. The processing unit 60 is composed of circuits and is a controller based on a well-known microcomputer. The processing unit 60 includes one or more processors 60a as a central processing unit (CPU) that executes programs, a memory 60b that stores programs and data and is composed of, for example, RAM (Random Access Memory) or ROM (Read Only Memory), and an input / output bus for inputting and outputting electrical signals. For example, the processing unit 60 is an ECU (Electronic Control Unit). 【0034】 In addition to the sensors SW1 to SW7 described above, the processing unit 60 is supplied with detection signals (output signals) from the ambient temperature sensor SW8, which detects the ambient temperature. Based on these detection signals, the processing unit 60 controls the fuel injectors 18, spark plugs 19, throttle valve 43, etc., of the engine 1. The processing unit 60 also controls the warning lights 70 to notify the vehicle occupants of any abnormalities. In particular, in this embodiment, the processing unit 60 diagnoses the catalytic converter 51 based on the air-fuel ratio detected by the linear A / F sensors SW6 and SW7, and performs control according to the diagnosis result (hereinafter, this control process will be referred to as the "catalytic converter diagnostic process"). 【0035】 [Catalyst diagnostic processing] Next, the catalyst diagnostic process performed by the processing apparatus 60 in this embodiment will be described. 【0036】 (Cold working treatment) First, in this embodiment, the catalyst diagnostic process performed during engine 1 startup after soaking, i.e., during cold operation (assuming the catalyst device 51 is not in an active state), will be described. 【0037】 As described above, the inventors of this application have found that the catalytic converter 51 can be diagnosed more accurately when the engine 1 is cold (during cold start). The reason for this is as follows: When the engine is cold, the catalytic converter 51 gradually becomes active as the temperature rises, that is, the exhaust gas purification performance (purification rate) gradually increases (light-off performance). At this time, if the catalytic converter 51 is in good condition, the increase in activity is fast, but if the catalytic converter 51 is deteriorated, the increase in activity tends to be slow. This is because when the catalytic converter 51 deteriorates, noble metals condense on the catalyst surface due to thermal stress, the active surface shrinks, and the exhaust gas purification performance decreases. Therefore, when the engine is cold, the difference depending on the degree of deterioration of the catalytic converter 51 is clearly evident. 【0038】 In particular, the temperature at which the catalyst 51 is activated varies depending on the degree of deterioration of the catalyst 51. Specifically, when the catalyst 51 is functioning normally, it is activated at a relatively low temperature (thus resulting in a faster rise in exhaust gas purification performance), while when the catalyst 51 is deteriorated, it is not activated until a relatively high temperature is reached (thus resulting in a slower rise in exhaust gas purification performance). Therefore, the inventors of this application considered diagnosing the catalyst 51 based on the temperature at which the catalyst 51 reaches a predetermined level of activity (catalyst temperature) when it is cold. 【0039】 Furthermore, the inventors of the present invention considered using the air-fuel ratio of the exhaust gas downstream of the catalytic converter 51 to determine the degree of activity of the catalytic converter 51. This is because, when the catalytic converter 51 is inactive, the air-fuel ratio downstream of the catalytic converter 51 fluctuates relatively large, but as the catalytic converter 51 becomes active, the oxygen consumption in the catalytic converter 51 increases (because the oxidation-reduction efficiency of the catalytic converter 51 with respect to the exhaust gas increases), and the fluctuation of the air-fuel ratio downstream of the catalytic converter 51 becomes smaller. Therefore, it can be said that the degree of activity of the catalytic converter 51 can be accurately determined based on the fluctuation of the air-fuel ratio of the exhaust gas downstream of the catalytic converter 51. 【0040】 Based on the above, in this embodiment, the processing unit 60 acquires the temperature of the catalytic converter 51 (catalyst temperature) when the engine 1 is cold, calculates the fluctuation range of the second detected air-fuel ratio detected within a predetermined time by the linear A / F sensor SW7, and diagnoses the catalytic converter 51 based on the relationship between this fluctuation range and the catalyst temperature. Specifically, the processing unit 60 determines the degree of activity of the catalytic converter 51 based on the fluctuation range of the second detected air-fuel ratio, and diagnoses the catalytic converter 51 based on the catalyst temperature when the catalytic converter 51 reaches a predetermined degree of activity. If the acquired catalyst temperature is above a predetermined temperature, the processing unit 60 diagnoses that the catalytic converter 51 is degraded. This is because the rise of the exhaust gas purification performance of the catalytic converter 51 is slow. 【0041】 Furthermore, in this embodiment, the processing unit 60 not only calculates the fluctuation range of the second detected air-fuel ratio (second fluctuation range) detected within a predetermined time by the linear A / F sensor SW7 provided downstream of the catalyst device 51, but also calculates the fluctuation range of the first detected air-fuel ratio (first fluctuation range) detected within a predetermined time by the linear A / F sensor SW6 provided upstream of the catalyst device 51, and calculates the fluctuation range ratio (first fluctuation range / second fluctuation range), which is the ratio of the first fluctuation range to the second fluctuation range, and determines the degree of activity of the catalyst device 51 based on this fluctuation range ratio. Since the first fluctuation range upstream of the catalyst device 51 is stable because it is not affected by the exhaust gas purification performance of the catalyst device 51, in this embodiment, such a first fluctuation range is used as a reference to evaluate the magnitude of the second fluctuation range downstream of the catalyst device 51. 【0042】 In particular, in this embodiment, the processing apparatus 60 normalizes the above-mentioned fluctuation range ratio, and when the normalized fluctuation range ratio (hereinafter referred to as the "normalized fluctuation range ratio") reaches a predetermined value, it determines that the catalyst device 51 has reached a predetermined level of activity, and diagnoses the catalyst device 51 based on the catalyst temperature at that time. The fluctuation range ratio has no units and changes depending on various factors such as the amount of precious metal in the catalyst device 51, its durability, and its deterioration state, so in order to eliminate the influence of these factors and provide versatility, the processing is performed using a normalized value of the fluctuation range ratio (normalized fluctuation range ratio). 【0043】 Here, with reference to Figures 3 and 4, the catalyst diagnostic process performed during cold conditions in this embodiment will be specifically described. Figure 3 is an explanatory diagram of the catalyst temperature obtained according to the normalized fluctuation range ratio in this embodiment, and Figure 4 is an explanatory diagram of the degradation determination based on the catalyst temperature obtained from Figure 3. 【0044】 In Figure 3, the horizontal axis represents time, and the vertical axis represents the normalized fluctuation range ratio and catalyst temperature. Specifically, graph G11 shows an example of the time change of the normalized fluctuation range ratio when the catalyst device 51 is functioning normally, graph G12 shows an example of the time change of the normalized fluctuation range ratio when the catalyst device 51 is at a degradation level that does not require replacement (hereinafter referred to as the "first degradation level"), and graph G13 shows an example of the time change of the normalized fluctuation range ratio when the catalyst device 51 is at a degradation level that requires replacement (corresponding to a malfunction, hereinafter referred to as the "second degradation level"). The normalized fluctuation range ratio is expressed in the range of 0 to 1 by performing a normalization process in which the maximum value of the fluctuation range ratio obtained in a series of processes (corresponding to the value when the time change of the fluctuation range ratio becomes very small) is defined as 1. Also, graph G14 shows an example of the time change of catalyst temperature. For example, this catalyst temperature is the temperature estimated by the processing device 60. 【0045】 Graphs G11, G12, and G13 show that when the catalyst 51 is degraded, the rise in the normalized fluctuation range ratio is slower than when the catalyst 51 is functioning normally. This means that the rise in the exhaust gas purification performance of the catalyst 51 is slower, or in other words, the catalyst 51 is activated more slowly. The treatment device 60 uses the time change of catalyst temperature shown in graph G14 to obtain the catalyst temperature when the normalized fluctuation range ratio reaches a predetermined value. For example, in graphs G11, G12, and G13, the treatment device 60 obtains temperatures T11, T12, and T13 as the catalyst temperatures when the normalized fluctuation range ratio reaches a predetermined value (0.5). When the catalyst 51 is degraded, the obtained catalyst temperatures are higher than when the catalyst 51 is functioning normally (T13 > T12 > T11). 【0046】 Next, in Figure 4, the horizontal axis shows the exhaust gas flow rate and the vertical axis shows the catalyst temperature. Specifically, graph G15 is a determination line defined by the exhaust gas flow rate and catalyst temperature that distinguishes between the normal state and the first degradation level of the catalyst device 51, and graph G16 is a determination line defined by the exhaust gas flow rate and catalyst temperature that distinguishes between the first degradation level and the second degradation level of the catalyst device 51. For example, when the treatment device 60 obtains a catalyst temperature T11, it determines that the catalyst device 51 is normal because T11 is below the determination line G15. When the catalyst temperature T12 is obtained, it determines that the catalyst device 51 is at the first degradation level because T12 is above the determination line G15 and below the determination line G16. When the catalyst temperature T13 is obtained, it determines that the catalyst device 51 is at the second degradation level because T13 is above the determination line G16. 【0047】 Next, with reference to Figure 5, a flowchart showing the catalyst diagnostic process performed during cold operation in this embodiment will be described. This flow is repeatedly executed by the processing unit 60 at predetermined intervals. Specifically, the processor 60a within the processing unit 60 reads a program stored in memory 60b and executes the program, thereby realizing the control related to this flow. Note that the flow shown in Figure 5 is assumed to be performed when the engine 1 is started. 【0048】 First, in step S20, the processing unit 60 acquires various information, including detected values ​​from sensors SW1 to SW8 (Figure 2) as described above. Typically, the processing unit 60 acquires the first detected air-fuel ratio detected by the linear A / F sensor SW6, the second detected air-fuel ratio detected by the linear A / F sensor SW7, and the ambient temperature detected by the ambient temperature sensor SW8. The processing unit 60 also acquires the temperature of the catalytic converter 51 (catalyst temperature). For example, the processing unit 60 estimates the catalyst temperature based on the heat balance, taking into account the heat generated by the engine 1 (determined from the engine speed and load of the engine 1), the heat consumed in the exhaust passage 50 up to the catalytic converter 51, and the reaction heat in the catalytic converter 51. By comparing this catalyst temperature with the ambient temperature, it is possible to determine whether it is the time of starting the engine after a soak. In other words, if the catalyst temperature and the ambient temperature are roughly the same at the start of the catalyst diagnostic process, it can be said that it is the time of starting the engine after a soak. Alternatively, instead of estimating the catalyst temperature, a temperature sensor may be installed in the catalyst device 51 to directly detect the catalyst temperature. 【0049】 Next, in step S21, the processing unit 60 determines whether the linear A / F sensor SW6 has been activated. For example, the processing unit 60 determines the activation of the linear A / F sensor SW6 based on the magnitude of its internal resistance (which indicates the temperature of the linear A / F sensor SW6). If, as a result of step S21, the processing unit 60 determines that the linear A / F sensor SW6 has been activated (step S21: Yes), it proceeds to step S22 and controls the fuel injector 18 to adjust the air-fuel ratio upstream of the catalytic converter 51 to the target air-fuel ratio. Conversely, if the processing unit 60 does not determine that the linear A / F sensor SW6 has been activated (step S21: No), it returns to step S21. 【0050】 Next, in step S23, the processing unit 60 determines whether the linear A / F sensor SW7 has been activated. For example, the processing unit 60 determines the activation of the linear A / F sensor SW7 based on the magnitude of its internal resistance (which indicates the temperature of the linear A / F sensor SW7). If, as a result of step S23, the processing unit 60 determines that the linear A / F sensor SW7 has been activated (step S23: Yes), it proceeds to step S24 and controls the fuel injector 18 to adjust the air-fuel ratio downstream of the catalytic converter 51 to the target air-fuel ratio. Conversely, if the processing unit 60 does not determine that the linear A / F sensor SW7 has been activated (step S23: No), it returns to step S23. 【0051】 Next, in step S25, the processing unit 60 calculates the first fluctuation range for the first detected air-fuel ratio from the maximum and minimum values ​​of the first detected air-fuel ratio detected within a predetermined time by the linear A / F sensor SW6 located upstream of the catalytic converter 51. At the same time, the processing unit 60 calculates the second fluctuation range for the second detected air-fuel ratio from the maximum and minimum values ​​of the second detected air-fuel ratio detected within a predetermined time by the linear A / F sensor SW7 located downstream of the catalytic converter 51. Then, the processing unit 60 proceeds to step S26 and calculates the fluctuation range ratio (first fluctuation range / second fluctuation range), which is the ratio between the first and second fluctuation ranges calculated in step S25. Then, the processing unit 60 proceeds to step S27 and performs a moving average processing on the fluctuation range ratio calculated in step S26 (the fluctuation range ratio used in subsequent processing will refer to the value after the moving average processing). 【0052】 Next, in step S28, the processing unit 60 determines whether the fluctuation range ratio is stable or not. Here, the processing unit 60 determines whether the catalyst device 51 is active or not by checking whether the fluctuation range ratio is stable or not. For example, the processing unit 60 determines whether the fluctuation value of the fluctuation range ratio is less than a predetermined threshold or not. As a result of step S28, if the processing unit 60 determines that the fluctuation range ratio is stable (step S28: Yes), it proceeds to step S29, and if it does not determine that the fluctuation range ratio is stable (step S28: No), it returns to step S25. 【0053】 Next, in step S29, the processing unit 60 determines whether the current situation is one in which the engine 1 (vehicle) has been started after being soaked. In this case, the processing unit 60 reads the soak time and determines whether the engine 1 has been soaked for a predetermined time or longer. This ensures that the catalyst diagnostic process is performed when the engine 1 and the catalyst device 51 have cooled sufficiently before starting. If the processing unit 60 determines in step S29 that it is a start after soaking (step S29: Yes), it proceeds to step S30. If it does not determine that it is a start after soaking (step S29: No), it terminates the catalyst diagnostic process. 【0054】 Next, in step S30, the processing unit 60 determines whether the deviation between the second detected air-fuel ratio detected by the linear A / F sensor SW7 and the target value is less than a threshold. Here again, the processing unit 60 determines whether the catalyst device 51 is active by checking whether the second detected air-fuel ratio is stable. As a result of step S30, if the processing unit 60 determines that the deviation of the second detected air-fuel ratio is less than a threshold (step S30: Yes), it proceeds to step S31. If it does not determine that the deviation of the second detected air-fuel ratio is less than a threshold (step S30: No), it terminates the catalyst diagnostic process. 【0055】 Next, in step S31, the processing unit 60 normalizes the fluctuation range ratios calculated so far. Specifically, the processing unit 60 defines the maximum value of the fluctuation range ratios calculated continuously from the start of this catalyst diagnostic process (corresponding to the value when the time change of the fluctuation range ratio becomes very small) as 1, and thus expresses all the fluctuation range ratios calculated continuously in the range of 0 to 1. 【0056】 Next, in step S32, the processing apparatus 60 obtains the catalyst temperature when the normalized fluctuation range ratio reaches a predetermined value (for example, 0.5), based on the normalized fluctuation range ratio calculated in step S31 and the catalyst temperature estimated so far. 【0057】 Next, in step S33, the processing unit 60 determines the degradation of the catalyst device 51 based on the catalyst temperature obtained in step S32. Specifically, the processing unit 60 determines that the catalyst device 51 is normal if the catalyst temperature is below the temperature defined by the determination line G15, determines that the catalyst device 51 is at the first degradation level if the catalyst temperature is above the temperature defined by the determination line G15 and below the temperature defined by the determination line G16, and determines that the catalyst device 51 is at the second degradation level if the catalyst temperature is above the temperature defined by the determination line G16. 【0058】 Next, in step S34, the processing unit 60 performs control according to the determination result of step S34. Specifically, if the processing unit 60 diagnoses that the catalytic converter 51 is at the second deterioration level, it illuminates the warning light 70. Also, if the processing unit 60 diagnoses that the catalytic converter 51 is at the first deterioration level, it controls the fuel injector 18 so that when the engine 1 is later cut off from fuel cut-off, the amount of fuel increased when the fuel cut-off is restored is less than when the catalytic converter 51 is functioning normally. The reason for performing this control is as follows. 【0059】 During fuel cut-off, only air is supplied to the catalytic converter 51, increasing its oxygen storage capacity (OSC). This increase in the catalytic converter 51 reduces the NOx purification performance after the fuel cut-off is restored. Therefore, the treatment device 60 temporarily increases the fuel amount to reduce the oxygen storage capacity of the catalytic converter 51 when the fuel cut-off is restored. When the catalytic converter 51 is functioning normally, it has a high oxygen storage capacity, so a larger fuel amount is needed. However, when the catalytic converter 51 is degraded, it has a low oxygen storage capacity, so a larger fuel amount is not needed. For this reason, the amount of fuel increased when the engine 1 restores from fuel cut-off is less when the catalytic converter 51 is degraded than when it is functioning normally. 【0060】 Next, with reference to Figure 6, the time chart for the catalyst diagnostic process performed during cold operation in this embodiment will be described. From top to bottom, Figure 6 shows the maximum and minimum values ​​of the first detected air-fuel ratio, the maximum and minimum values ​​of the second detected air-fuel ratio, the fluctuation range ratio, the table value between the fluctuation range ratio and the catalyst temperature, the fluctuation value of the fluctuation range ratio, the deviation of the second detected air-fuel ratio, the normalized fluctuation range ratio, and the catalyst temperature at which the normalized fluctuation range ratio reaches a predetermined value. 【0061】 Before time t1, the following conditions are met: the vehicle ignition is turned on, engine 1 is started, the catalytic converter 51 is soaked (in this case, the catalytic converter temperature is approximately equal to the ambient temperature), and the linear A / F sensors SW6 and SW7 are activated. Therefore, the processing unit 60 starts the specific processing for the catalytic converter diagnostic process in cold conditions from time t1. 【0062】 Specifically, from time t1, the processing unit 60 acquires the maximum and minimum values ​​of the first detected air-fuel ratio detected by the linear A / F sensor SW6 located upstream of the catalytic converter 51, and acquires the maximum and minimum values ​​of the second detected air-fuel ratio detected by the linear A / F sensor SW7 located downstream of the catalytic converter 51. The processing unit 60 then calculates the first fluctuation range from the maximum and minimum values ​​of the first detected air-fuel ratio, and the second fluctuation range from the maximum and minimum values ​​of the second detected air-fuel ratio, and calculates the fluctuation range ratio (first fluctuation range / second fluctuation range) from these first and second fluctuation ranges. The processing unit 60 also performs a moving average processing on the fluctuation range ratio thus calculated. 【0063】 In Figure 6, graph G21 shows an example of the fluctuation range ratio before moving average processing when the catalyst device 51 is functioning normally, and graph G22 shows an example of the fluctuation range ratio after moving average processing of graph G21. Furthermore, graph G23 shows an example of the fluctuation range ratio after moving average processing when the catalyst device 51 is at the first degradation level, and graph G24 shows an example of the fluctuation range ratio after moving average processing when the catalyst device 51 is at the second degradation level. 【0064】 Furthermore, the processing unit 60 estimates the temperature of the catalyst device 51 (catalyst temperature) and uses this catalyst temperature to calculate a table value between the fluctuation range ratio and the catalyst temperature, that is, it establishes a correspondence between the fluctuation range ratio and the catalyst temperature. In addition, the processing unit 60 calculates the fluctuation value of the fluctuation range ratio and the deviation between the second detected air-fuel ratio and the target value in order to determine whether the catalyst device 51 is active or not. At time t2, the deviation of the second detected air-fuel ratio falls below a threshold, and at time t3, the fluctuation value of the fluctuation range ratio also falls below a threshold. After time t3, the fluctuation range ratio reaches its maximum value, so the processing unit 60 defines the maximum value of the fluctuation range ratio as 1 and calculates the normalized fluctuation range ratio by representing the fluctuation range ratio, which has been continuously calculated since the start of the catalyst diagnostic process, in the range of 0 to 1. Then, the processing unit 60 obtains the catalyst temperature when the normalized fluctuation range ratio reaches a predetermined value (for example, 0.5). In Figure 6, the table values ​​of the fluctuation range ratio and catalyst temperature, the fluctuation value of the fluctuation range ratio, the deviation of the second detected air-fuel ratio, and the normalized fluctuation range ratio are illustrated as examples for the case where the catalytic converter 51 is functioning normally (Graph G22). 【0065】 In the example shown in Figure 6, if the catalyst device 51 is functioning normally (Graph G22), the processing unit 60 obtains the catalyst temperature at time t3 when the normalized fluctuation range ratio reaches a predetermined value. In this case, the processing unit 60 determines that the catalyst device 51 is functioning normally because the obtained catalyst temperature is below the temperature defined by the determination line G15. On the other hand, if the catalyst device 51 is at the first degradation level (Graph G23), the processing unit 60 obtains the catalyst temperature at a later time t4 when the normalized fluctuation range ratio reaches a predetermined value. In this case, the processing unit 60 determines that the catalyst device 51 is at the first degradation level because the obtained catalyst temperature is above the temperature defined by the determination line G15 and below the temperature defined by the determination line G16. Furthermore, if the catalyst device 51 is at the second degradation level (Graph G24), the processing unit 60 obtains the catalyst temperature at a later time t5 when the normalized fluctuation range ratio reaches a predetermined value. In this case, the processing unit 60 determines that the catalyst device 51 is at the second degradation level because the acquired catalyst temperature is above the temperature defined by the determination line G16. 【0066】 (Treatment while warm) Next, in this embodiment, the catalyst diagnostic process performed after the engine 1 is started, that is, when it is warm (assuming the catalyst 51 is in an active state) will be described. In this embodiment, when the engine 1 is warm, the processing unit 60 calculates the oxygen storage amount of the catalyst 51 based on the first detected air-fuel ratio detected by the first linear A / F sensor SW6 and the second detected air-fuel ratio detected by the second linear A / F sensor SW7, and diagnoses the catalyst 51 based on this oxygen storage amount. 【0067】 When diagnosing the catalytic converter 51 based on its oxygen storage capacity, it is desirable for the catalytic converter 51 to be in an oxygen-saturated state in order to perform the diagnosis accurately. The oxygen storage capacity changes depending on the degree of deterioration of the catalytic converter 51, but in an oxygen-saturated state, the oxygen storage capacity of the catalytic converter 51 is almost at its maximum, so the degree of deterioration of the catalytic converter 51 is clearly reflected in the size of the oxygen storage capacity, and the diagnosis can be performed accurately. On the other hand, while the engine 1 is in a fuel-cut state, basically only air is supplied to the catalytic converter 51, so the catalytic converter 51 can be quickly brought into an oxygen-saturated state. For these reasons, in this embodiment, the catalytic converter 51 is diagnosed when the fuel is cut off. 【0068】 Referring to Figure 7, a flowchart illustrating the catalyst diagnostic process performed at warm temperatures in this embodiment will be described. This flow is repeatedly executed by the processing unit 60 at predetermined intervals. Specifically, the processor 60a within the processing unit 60 reads a program stored in memory 60b and executes the program, thereby realizing the control related to this flow. Note that the flow shown in Figure 7 is performed when control (air-fuel ratio control) is being performed to set the air-fuel ratio downstream of the catalyst device 51 to the target air-fuel ratio. 【0069】 First, in step S40, the processing unit 60 acquires various information, including detected values ​​from sensors SW1 to SW8 (Figure 2) as described above. Typically, the processing unit 60 acquires the first detected air-fuel ratio detected by the linear A / F sensor SW6, the second detected air-fuel ratio detected by the linear A / F sensor SW7, and the intake air flow rate detected by the airflow sensor SW1. The processing unit 60 also acquires the temperature of the catalyst device 51 (catalyst temperature). For example, the processing unit 60 estimates the catalyst temperature. Alternatively, instead of estimating the catalyst temperature, a temperature sensor may be provided on the catalyst device 51 to directly detect the catalyst temperature. 【0070】 Next, in step S41, the processing unit 60 determines whether the fuel cut execution conditions have been met. For example, the processing unit 60 determines that the fuel cut execution conditions have been met if the accelerator opening detected by the accelerator opening sensor SW5 is 0 and the engine speed detected by the crank angle sensor SW4 is greater than or equal to a predetermined value (step S41: Yes). In this case, the processing unit 60 proceeds to step S42. On the other hand, if the processing unit 60 does not determine that the fuel cut execution conditions have been met (step S41: No), it terminates the catalyst diagnostic process. 【0071】 Next, in step S42, the processing unit 60 calculates the oxygen mass upstream of the catalyst device 51 (hereinafter referred to as the "first oxygen mass") based on the first detected air-fuel ratio detected by the linear A / F sensor SW6. Then, in step S43, the processing unit 60 calculates the oxygen mass downstream of the catalyst device 51 (hereinafter referred to as the "second oxygen mass") based on the second detected air-fuel ratio detected by the linear A / F sensor SW7. Then, in step S44, the processing unit 60 calculates the oxygen storage capacity of the catalyst device 51 by subtracting the second oxygen mass calculated in step S43 from the first oxygen mass calculated in step S42. 【0072】 Next, in step S45, the processing unit 60 determines whether the oxygen storage amount calculated in step S44 is less than a predetermined deviation threshold. Here, the processing unit 60 determines whether the oxygen storage amount is stable, that is, whether there is little fluctuation in the oxygen storage amount. If, as a result of step S45, the processing unit 60 determines that the oxygen storage amount is less than the deviation threshold (step S45: Yes), it proceeds to step S46. On the other hand, if the processing unit 60 does not determine that the oxygen storage amount is less than the deviation threshold (step S45: No), it terminates the catalyst diagnostic process. 【0073】 Next, in step S46, the processing device 60 determines the deterioration of the catalyst device 51 based on the calculated oxygen storage amount. Specifically, the processing device 60 determines the deterioration of the catalyst device 51 by comparing the oxygen storage amount with predetermined thresholds. Specifically, the processing device 60 determines that the catalyst device 51 is normal if the oxygen storage amount is equal to or greater than the first threshold, determines that the catalyst device 51 is at the first deterioration level if the oxygen storage amount is less than the first threshold but equal to or greater than the second threshold (< first threshold), and determines that the catalyst device 51 is at the second deterioration level if the oxygen storage amount is less than the second threshold. The processing device 60 sets such thresholds based on the catalyst temperature. 【0074】 Next, in step S47, the processing unit 60 performs control according to the determination result in step S46. Specifically, if the processing unit 60 diagnoses that the catalytic converter 51 is at the second deterioration level, it illuminates the warning light 70. If the processing unit 60 diagnoses that the catalytic converter 51 is at the first deterioration level, it controls the fuel injector 18 so that the amount of fuel increased when the engine 1 recovers from fuel cut is less than when the catalytic converter 51 is functioning normally. 【0075】 [Mechanism of Action and Effects] Next, the operation and effects of the engine catalyst diagnostic device 100 according to this embodiment will be described. 【0076】 In this embodiment, the engine catalyst diagnostic device 100 includes an exhaust passage 50 for discharging exhaust gas from the combustion chamber 17 of the engine 1, a catalyst device 51 provided on the exhaust passage 50 for purifying the exhaust gas, a linear A / F sensor SW7 provided on the exhaust passage 50 downstream of the catalyst device 51 and capable of detecting the air-fuel ratio of the exhaust gas, and a processing device 60 configured to diagnose the catalyst device 51 based on the air-fuel ratio detected by at least the linear A / F sensor SW7. The processing device 60 is configured to acquire the temperature of the catalyst device 51 (catalyst temperature) when the engine 1 is started after soaking, calculate the fluctuation range of the air-fuel ratio detected by the linear A / F sensor SW7 within a predetermined time, and diagnose the catalyst device 51 based on the relationship between this fluctuation range and the catalyst temperature. 【0077】 In this embodiment, the treatment device 60 diagnoses the catalyst device 51 when it is cold, when differences in the degree of deterioration of the catalyst device 51 are clearly evident. In particular, since the temperature at which the catalyst device 51 becomes active (catalyst temperature) changes depending on the degree of deterioration of the catalyst device 51, the treatment device 60 uses the catalyst temperature when diagnosing the catalyst device 51. Furthermore, the treatment device 60 uses fluctuations in the air-fuel ratio downstream of the catalyst device 51 to determine the activity state of the catalyst device 51. This is because when the catalyst device 51 is not active, the air-fuel ratio downstream of the catalyst device 51 fluctuates relatively large, but as the catalyst device 51 becomes active, the oxygen consumption in the catalyst device 51 increases, and the fluctuations in the air-fuel ratio downstream of the catalyst device 51 become smaller. Therefore, the activity state of the catalyst device 51 can be accurately determined based on these fluctuations in the air-fuel ratio. Based on the above, in this embodiment, the processing unit 60 diagnoses the catalytic converter 51 when cold, based on the relationship between the fluctuation range of the air-fuel ratio downstream of the catalytic converter 51 and the catalytic converter temperature. This significantly improves the diagnostic accuracy of the catalytic converter 51. 【0078】 Furthermore, in this embodiment, the processing apparatus 60 determines the activity level of the catalyst device 51 based on the fluctuation range and diagnoses the catalyst device 51 based on the catalyst temperature when the catalyst device 51 reaches a predetermined activity level. This allows for accurate diagnosis of the catalyst device 51 by using the catalyst temperature when the catalyst device 51 reaches a predetermined activity level. 【0079】 Furthermore, in this embodiment, the processing unit 60 diagnoses that the catalyst device 51 is degraded if the acquired catalyst temperature is above a predetermined temperature. In this case, the processing unit 60 diagnoses that the catalyst device 51 is degraded because the start-up of the exhaust gas purification performance of the catalyst device 51 is slow. This allows for accurate diagnosis of the catalyst device 51. 【0080】 Furthermore, in this embodiment, the engine catalyst diagnostic device 100 is provided on the exhaust passage 50 upstream of the catalyst device 51 and has a linear A / F sensor SW6 capable of detecting the air-fuel ratio of the exhaust gas (first detected air-fuel ratio). The processing device 60 calculates the first fluctuation range of the first detected air-fuel ratio detected within a predetermined time by the linear A / F sensor SW6, along with the second fluctuation range of the second detected air-fuel ratio detected within a predetermined time by the linear A / F sensor SW7. It then calculates the fluctuation range ratio, which is the ratio of the first fluctuation range to the second fluctuation range, and determines the activity level of the catalyst device 51 based on this fluctuation range ratio. Since the first fluctuation range upstream of the catalyst device 51 is stable because it is not affected by the exhaust gas purification performance of the catalyst device 51, in this embodiment, such a first fluctuation range is used as a reference to evaluate the magnitude of the second fluctuation range downstream of the catalyst device 51. This ensures the diagnostic accuracy of the catalyst device 51. 【0081】 Furthermore, in this embodiment, the processing device 60 normalizes the fluctuation range ratio, and when the normalized fluctuation range ratio reaches a predetermined value, it determines that the catalyst device 51 has reached a predetermined level of activity, and diagnoses the catalyst device 51 based on the catalyst temperature at that time. The fluctuation range ratio has no units and changes depending on various factors such as the amount of precious metal in the catalyst device 51, its durability, and its deterioration state. Therefore, in order to eliminate the influence of these factors and ensure versatility, the processing is performed using a normalized value of the fluctuation range ratio (normalized fluctuation range ratio). This effectively ensures the diagnostic accuracy of the catalyst device 51. 【0082】 Furthermore, in this embodiment, when the engine 1 is started and the engine 1 is fuel-cut off, the processing unit 60 calculates the oxygen storage amount of the catalytic converter 51 based on the first detected air-fuel ratio detected by the first linear A / F sensor SW6 and the second detected air-fuel ratio detected by the second linear A / F sensor SW7, and further diagnoses the catalytic converter 51 based on this oxygen storage amount. Since only air is supplied to the catalytic converter 51 while the engine 1 is fuel-cut off, the catalytic converter 51 tends to become oxygen-saturated. By diagnosing the catalytic converter 51 based on the oxygen storage amount in such an oxygen-saturated state, the catalytic converter 51 can be diagnosed with high accuracy. 【0083】 Furthermore, in this embodiment, when the engine 1 recovers from fuel cut-off, the processing unit 60 controls the fuel injector 18 to increase the amount of fuel supplied to the combustion chamber 17, and if it diagnoses that the catalytic converter 51 is degraded, it reduces the amount of fuel increased compared to when it does not diagnose that the catalytic converter 51 is degraded. This suppresses unnecessary increases in fuel supply and improves fuel efficiency. 【0084】 Furthermore, in this embodiment, the processing unit 60 controls the system to illuminate the warning light 70 when it diagnoses that the catalytic converter 51 is deteriorating. This allows the occupants to be accurately informed of the deterioration of the catalytic converter 51 and to be prompted to replace the catalytic converter 51. [Explanation of Symbols] 【0085】 1 Engine 13 cylinders 17 Combustion chamber 18 Fuel Injector 19 Spark plugs 40 Intake passage 50 Exhaust passage 51, 52 Catalyst device 60 Processing Unit 70 warning light 100 Engine catalytic converter diagnostic device SW6, SW7 Linear A / F Sensors

Claims

[Claim 1] An engine catalytic converter diagnostic device, An exhaust passage for discharging exhaust gas from the combustion chamber of the engine, A catalytic converter is provided on the exhaust passage for purifying the exhaust gas, A linear A / F sensor is provided in the exhaust passage downstream of the catalytic converter and capable of detecting the air-fuel ratio of the exhaust gas, A processing device configured to diagnose the catalytic converter based on the air-fuel ratio detected by at least the linear A / F sensor, It has, The aforementioned processing device, when starting the engine after soaking, The temperature of the catalyst device is obtained, The fluctuation range of the air-fuel ratio detected within a predetermined time by the linear A / F sensor is calculated. Based on the relationship between the fluctuation range and the temperature of the catalyst device, the catalyst device is diagnosed. An engine catalyst diagnostic device characterized by being configured in such a way. [Claim 2] The aforementioned processing apparatus is Based on the aforementioned fluctuation range, the activity level of the catalyst device is determined. Based on the determination result of the activity level, the temperature of the catalyst device when it reaches a predetermined activity level is obtained, and the catalyst device is diagnosed based on that temperature. The catalyst diagnostic device for an engine according to claim 1, configured as described above. [Claim 3] The catalyst diagnostic device for an engine according to claim 2, wherein the processing device diagnoses that the catalyst device is degraded when the temperature of the catalyst device reaches the predetermined level of activity and the temperature of the catalyst device is above a predetermined temperature. [Claim 4] If the linear A / F sensor is designated as a second linear A / F sensor, the air-fuel ratio detected by the second linear A / F sensor is designated as the second air-fuel ratio, and the fluctuation range of the second air-fuel ratio detected by the second linear A / F sensor within the predetermined time is designated as the second fluctuation range, then the catalytic converter diagnostic device of the engine further includes a first linear A / F sensor provided on the exhaust passage upstream of the catalytic converter, which is capable of detecting the first air-fuel ratio of the exhaust gas. The aforementioned processing device, when starting the engine after soaking, Along with the second fluctuation range, the first fluctuation range of the first air-fuel ratio detected by the first linear A / F sensor within the predetermined time is calculated. The variation range ratio, which is the ratio of the first variation range to the second variation range, is calculated. The activity level of the catalyst device is determined based on the aforementioned fluctuation range ratio. The catalyst diagnostic device for an engine according to claim 2 or 3, configured as described above. [Claim 5] The catalyst diagnostic device for an engine according to claim 4, wherein the processing device normalizes the fluctuation range ratio, determines that the catalyst device has reached a predetermined level of activity when the normalized fluctuation range ratio reaches a predetermined value, and diagnoses the catalyst device based on the temperature of the catalyst device at that time. [Claim 6] If the linear A / F sensor is designated as a second linear A / F sensor, and the air-fuel ratio detected by the second linear A / F sensor is designated as the second air-fuel ratio, then the catalytic converter diagnostic device of the engine further includes a first linear A / F sensor provided on the exhaust passage upstream of the catalytic converter, which is capable of detecting the first air-fuel ratio of the exhaust gas. The aforementioned processing device, when the engine has been started and the engine's fuel supply has been cut off, Based on the first air-fuel ratio detected by the first linear A / F sensor and the second air-fuel ratio detected by the second linear A / F sensor, the oxygen storage capacity of the catalyst is calculated. Based on the oxygen storage capacity, the catalyst device is further diagnosed. The catalyst diagnostic device for an engine according to claim 1, configured as described above. [Claim 7] The catalytic converter diagnostic device for the engine further includes a fuel injector for supplying fuel to the combustion chamber. The aforementioned processing apparatus is When the engine recovers from a fuel cut-off, the fuel injector is controlled to increase the amount of fuel supplied to the combustion chamber. If it is diagnosed that the catalytic converter is degraded, the amount of fuel added will be reduced compared to when it is not diagnosed that the catalytic converter is degraded. The catalyst diagnostic device for an engine according to claim 1, configured as described above. [Claim 8] The aforementioned engine catalytic converter diagnostic device further includes a warning light for notifying the vehicle occupants of an abnormality. The catalytic converter diagnostic device for an engine according to claim 1, wherein the processing device is configured to control the illumination of the warning light when it is determined that the catalytic converter is deteriorating.

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