Engine Control Unit

Abstract

To prevent interruption of abnormality diagnosis of an exhaust system component.SOLUTION: An engine control device 10 finds two temperatures as estimated values for catalyst temperatures, including a first catalyst temperature to which an exhaust temperature at an air-fuel ratio is reflected and a second catalyst temperature which is found assuming that the air-fuel ratio is fixed to a theoretical air-fuel ratio. Besides, the engine control device 10 performs abnormality diagnosis of a catalyst device 18 while alternately switching the air-fuel ratio between a rich air-fuel ratio and a lean air-fuel ratio. Then, the engine control device 10 determines whether the conditions for executing the abnormality diagnosis are established or not, using the second catalyst temperature out of the estimated values for the two catalyst temperatures.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an engine control device that diagnoses the presence or absence of abnormalities in exhaust system components. [Background technology]

[0002] As an exhaust purification device for engines, there is a three-way catalyst device that purifies exhaust gas using a three-way catalyst that simultaneously oxidizes hydrocarbons (HC) and carbon monoxide (CO) and reduces nitrogen oxides (NOx). In an engine equipped with a three-way catalyst device, the efficiency of purifying exhaust gas by the three-way catalyst device is increased by basically performing stoichiometric combustion in which the air-fuel ratio of the mixture is the theoretical air-fuel ratio.

[0003] Meanwhile, a device described in Patent Document 1 is known as a device for diagnosing the presence or absence of an abnormality in a catalytic converter installed in an exhaust passage of an engine. The device in this document diagnoses the presence or absence of an abnormality in the catalytic converter while performing the following active air-fuel ratio control. The active air-fuel ratio control alternates the air-fuel ratio of the mixture burned in the combustion chamber between a rich air-fuel ratio, which is an air-fuel ratio richer than the stoichiometric air-fuel ratio, and a lean air-fuel ratio, which is an air-fuel ratio leaner than the stoichiometric air-fuel ratio. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2010-255490 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, when the above abnormality diagnosis is performed when the temperature of the catalytic converter is lower than a certain level, it may cause the emission to deteriorate or may result in an erroneous diagnosis. For this reason, the condition for executing the diagnosis may include that the catalyst temperature is equal to or higher than a predetermined diagnostic lower limit temperature.

[0006] On the other hand, when the air-fuel ratio of the mixture burning in the combustion chamber is changed from the stoichiometric air-fuel ratio to a rich air-fuel ratio or a lean air-fuel ratio, the temperature of the exhaust gas discharged from the combustion chamber to the exhaust passage drops. The temperature of the catalytic device also drops together with the drop in the temperature of the exhaust gas. Therefore, when the above-mentioned catalytic device is diagnosed while performing active air-fuel ratio control, the catalyst temperature drops after the diagnosis starts. Then, the catalyst temperature may fall below the diagnostic lower limit temperature, and the diagnosis may be interrupted. Note that when the diagnosis is interrupted and the air-fuel ratio of the mixture burning in the combustion chamber is returned to the stoichiometric air-fuel ratio, the temperature of the exhaust gas rises and the temperature of the catalytic device also rises. Therefore, there is a risk that the diagnosis may be interrupted and resumed repeatedly.

[0007] The same problem may occur when diagnosing exhaust system components by changing the air-fuel ratio of the mixture burned in the combustion chamber from the stoichiometric air-fuel ratio to a rich air-fuel ratio or a lean air-fuel ratio. [Means for solving the problem]

[0008] The engine control device that solves the above problem diagnoses whether there is an abnormality in an exhaust system part of an engine in which a catalytic device for exhaust purification is installed in an exhaust passage by changing the air-fuel ratio of the mixture burning in the combustion chamber from the stoichiometric air-fuel ratio to a rich air-fuel ratio or a lean air-fuel ratio. The engine control device also includes, as a process for estimating the catalyst temperature of the catalytic device, a first estimation process that estimates the catalyst temperature by reflecting the change in exhaust temperature due to the air-fuel ratio of the mixture burning in the combustion chamber, and a second estimation process that estimates the catalyst temperature assuming that the air-fuel ratio of the mixture burning in the combustion chamber is fixed to the stoichiometric air-fuel ratio. In the engine control device, the execution condition of the above diagnosis includes that the estimated value of the catalyst temperature by the second estimation process is equal to or higher than a predetermined diagnosis lower limit temperature. Here, the rich air-fuel ratio indicates an air-fuel ratio richer than the stoichiometric air-fuel ratio, and the lean air-fuel ratio indicates an air-fuel ratio leaner than the stoichiometric air-fuel ratio.

[0009] The catalyst temperature estimated by the engine control device in the first estimation process is a value that reflects the effect of the change in the actual air-fuel ratio on the temperature of the catalyst device. In contrast, the second catalyst temperature estimated by the engine control device in the second estimation process is a value that does not reflect the effect of the change in the actual air-fuel ratio on the temperature of the catalyst device. That is, a decrease in the catalyst temperature due to a change in the air-fuel ratio during the diagnosis of the exhaust system parts is reflected in the first catalyst temperature but is not reflected in the second catalyst temperature. The engine control device uses the second catalyst temperature to determine whether or not the execution condition for the diagnosis of the exhaust system parts is satisfied. Therefore, even if the actual catalyst temperature falls below the diagnosis lower limit temperature after the start of the diagnosis of the exhaust system parts, the execution condition for the diagnosis is unlikely to be unsatisfied. Therefore, according to the engine control device, the abnormality diagnosis of the exhaust system parts is unlikely to be interrupted. [Brief description of the drawings]

[0010] [Figure 1] 1 is a diagram illustrating a schematic configuration of an embodiment of an engine control device; [Diagram 2] FIG. 11 is a time chart showing, during an abnormality diagnosis of a catalytic converter by the engine control device, (a) the progress of an actual air-fuel ratio, (b) the progress of an output of a front air-fuel ratio sensor, (c) the progress of an output of a rear air-fuel ratio sensor, and (d) the progress of an oxygen storage amount OSA of the catalytic converter. [Diagram 3] 4 is a control block diagram showing a process flow relating to estimation of a first catalyst temperature THC1 in a first estimation process performed by the engine control device. FIG. [Figure 4] 4 is a control block diagram showing a process flow relating to estimation of a second catalyst temperature THC2 in a second estimation process performed by the engine control device. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of an engine control device will be described in detail with reference to FIGS. <Engine 11 Configuration> First, referring to FIG. 1, the configuration of an engine 11 to which an engine control device 10 of the present embodiment is applied will be described. The engine 11 is mounted on a vehicle. The engine 11 includes a combustion chamber 12 in which a mixture is burned. The engine 11 also includes an intake passage 13 which is an intake passage to the combustion chamber 12, and an exhaust passage 14 which is an exhaust passage from the combustion chamber 12. The intake passage 13 includes a throttle valve 15 which is a flow rate control valve for the intake air. An injector 16 which injects fuel into the intake air is installed in a portion downstream of the throttle valve 15 in the intake passage 13. An ignition device 17 which ignites the mixture by spark discharge is installed in the combustion chamber 12 to which the intake air mixed with fuel, i.e., the mixture, is introduced through the intake passage 13. On the other hand, a catalyst device 18 for purifying exhaust gas is installed in the exhaust passage 14. A filter device 19 for collecting particulate matter in the exhaust gas is installed in a portion downstream of the catalyst device 18 in the exhaust passage 14. A three-way catalyst that simultaneously oxidizes HC and CO in the exhaust gas and reduces NOx is supported in the catalytic device 18 and the filter device 19. In addition, an oxygen storage agent is supported in the catalytic device 18 and the filter device 19 as a promoter to enhance the catalytic action of the three-way catalyst.

[0012] <Configuration of engine control device 10> Next, a configuration of the engine control device 10 that controls the engine 11 will be described. The engine control device 10 is configured as an electronic control unit that includes an arithmetic processing device 20 and a storage device 21. The arithmetic processing device 20 is a device that executes arithmetic processing for engine control. The storage device 21 is a device that stores programs and data for engine control.

[0013] The engine control device 10 is connected to various sensors that detect state quantities that indicate the operating state of the engine 11. These sensors include an air flow meter 22, a throttle opening sensor 23, an intake pressure sensor 24, a crank angle sensor 25, a front air-fuel ratio sensor 26, and a rear air-fuel ratio sensor 27. In addition, the above sensors also include an accelerator pedal sensor 28, a vehicle speed sensor 29, an outside air temperature sensor 30, and a water temperature sensor 31. The air flow meter 22 is a sensor that detects the intake flow rate GA of the intake passage 13. The throttle opening sensor 23 is a sensor that detects the throttle opening TA, which is the opening of the throttle valve 15. The intake pressure sensor 24 is a sensor that detects the intake pressure PM, which is the pressure of the intake air in the portion of the intake passage 13 downstream of the throttle valve 15. The crank angle sensor 25 is a sensor that detects a crank angle θ, which is the rotation angle of the crankshaft, which is the output shaft of the engine 11. The front air-fuel ratio sensor 26 is a sensor that detects the air-fuel ratio of the exhaust gas flowing into the catalytic device 18. The rear air-fuel ratio sensor 27 is a sensor that detects the air-fuel ratio of the exhaust gas flowing out from the catalytic converter 18. The accelerator pedal sensor 28 is a sensor that detects the accelerator pedal opening degree ACC, which is the amount of accelerator pedal operation by the driver. The vehicle speed sensor 29 is a sensor that detects the vehicle's traveling speed V. The outside air temperature sensor 30 is a sensor that detects the outside air temperature THA, which is the temperature of the air outside the vehicle. The water temperature sensor 31 is a sensor that detects the engine water temperature THW, which is the temperature of the engine cooling water. The engine control device 10 obtains the engine speed NE from the detection result of the crank angle θ by the crank angle sensor 25. The engine control device 10 also obtains the engine load factor KL from the intake flow rate GA, the throttle opening degree TA, the engine speed NE, etc. The engine load factor KL represents the intake filling rate of the combustion chamber 12.

[0014] The engine control device 10 determines each operation amount of the engine 11 according to the operating state of the engine 11 grasped from the detection results of these sensors. The operation amounts of the engine 11 determined by the engine control device 10 include the throttle opening TA, the fuel injection amount Q of the injector 16, and the ignition timing SA of the mixture by the ignition device 17. The engine control device 10 controls the engine by driving the throttle valve 15, the injector 16, the ignition device 17, etc. according to the determined operation amounts. Note that the engine control device 10 performs air-fuel ratio feedback control as part of the engine control. In the air-fuel ratio feedback control, the engine control device 10 feedback-adjusts the fuel injection amount Q of the injector 16 so that the output λf of the front air-fuel ratio sensor 26 approaches a value indicating the theoretical air-fuel ratio.

[0015] In addition, the engine control device 10 calculates the oxygen storage amount OSA, which is the amount of oxygen stored in the catalytic device 18, during the operation of the engine 11. The calculation of the oxygen storage amount OSA is performed in the following manner. In the following description, the amount of oxygen stored in the catalytic device 18 during a predetermined calculation period is referred to as the oxygen storage speed VO of the catalytic device 18. Note that the oxygen storage speed VO when the catalytic device 18 is releasing oxygen is a negative value. The oxygen storage speed VO of the catalytic device 18 is determined by the flow rate of the exhaust gas flowing into the catalytic device 18, the concentration of unburned fuel components / excess oxygen in the exhaust gas, the catalyst temperature, the oxygen storage amount OSA, and the like. The flow rate of the exhaust gas flowing into the catalytic device 18 can be calculated from the engine speed NE and the engine load factor KL. The concentration of unburned fuel components and excess oxygen in the exhaust gas flowing into the catalytic device 18 can be calculated from the air-fuel ratio of the mixture being burned in the combustion chamber 12. The engine control device 10 also estimates the catalyst temperature, which is the temperature of the catalyst supported by the catalytic device 18. Furthermore, the engine control device 10 calculates the oxygen storage speed VO of the catalytic device 18 from the engine speed NE, engine load factor KL, actual air-fuel ratio AF, catalyst temperature, oxygen storage amount OSA, and the like for each predetermined calculation period. The engine control device 10 then calculates the oxygen storage amount OSA of the catalytic device 18 as the integrated value of the oxygen storage speed VO for each calculation period. The actual air-fuel ratio AF represents the air-fuel ratio of the mixture being burned in the combustion chamber 12. The actual air-fuel ratio AF is calculated from the detection result of the front air-fuel ratio sensor 26, for example. The actual air-fuel ratio AF can also be calculated from the result of calculation based on the engine load factor KL and the fuel injection amount Q.

[0016] <Abnormality diagnosis of catalytic converter 18> The catalytic converter 18 installed in the exhaust passage 14 of the engine 11 configured as described above may lose its oxygen storage capacity due to deterioration, and may become unable to perform sufficient exhaust purification performance. As part of engine control, the engine control device 10 performs an abnormality diagnosis of the catalytic converter 18 to determine whether the catalytic converter 18 has deteriorated to the point where it is unable to perform sufficient exhaust purification performance.

[0017] When diagnosing the catalytic device 18, the engine control device 10 performs active air-fuel ratio control to set the air-fuel ratio of the mixture burned in the combustion chamber 12 to a value other than the stoichiometric air-fuel ratio. In the active air-fuel ratio control when diagnosing the catalytic device 18, the air-fuel ratio of the mixture burned in the combustion chamber 12 is alternately switched between a rich air-fuel ratio and a lean air-fuel ratio. The rich air-fuel ratio is an air-fuel ratio richer than the stoichiometric air-fuel ratio, and the lean air-fuel ratio is an air-fuel ratio leaner than the stoichiometric air-fuel ratio. In the following description, the combustion of the mixture whose air-fuel ratio is the stoichiometric air-fuel ratio is referred to as stoichiometric combustion. Also, the combustion of the mixture whose air-fuel ratio is the rich air-fuel ratio is referred to as rich combustion, and the combustion of the mixture whose air-fuel ratio is the lean air-fuel ratio is referred to as lean combustion. Exhaust gas containing a large amount of unburned fuel components such as HC and CO is discharged from the combustion chamber 12 into the exhaust passage 14 during rich combustion. In the following description, exhaust gas containing a large amount of unburned fuel components discharged from the combustion chamber 12 during rich combustion will be referred to as rich exhaust gas. On the other hand, exhaust gas containing a large amount of surplus oxygen not used in combustion is discharged from the combustion chamber 12 into the exhaust passage 14 during lean combustion. In the following description, exhaust gas containing a large amount of surplus oxygen discharged from the combustion chamber 12 during lean combustion will be referred to as lean exhaust gas.

[0018] Fig. 2 shows an example of an embodiment of abnormality diagnosis of the catalytic device 18. Fig. 2(a) to (d) respectively show the transitions of the following parameters during abnormality diagnosis of the catalytic device 18. That is, Fig. 2(a) shows the transitions of the actual air-fuel ratio AF, Fig. 2(b) shows the transitions of the output λf of the front air-fuel ratio sensor 26, Fig. 2(c) shows the transitions of the output λr of the rear air-fuel ratio sensor 27, and Fig. 2(d) shows the transitions of the oxygen storage amount OSA of the catalytic device 18.

[0019] 2, at time t0, abnormality diagnosis of the catalytic converter 18 is started. Before time t0, stoichiometric combustion is performed in the combustion chamber 12 by air-fuel ratio feedback control. At this time, the output λf of the front air-fuel ratio sensor 26 and the output λr of the rear air-fuel ratio sensor 27 are both stoichiometric output values ​​ST, which are values ​​corresponding to the stoichiometric air-fuel ratio.

[0020] When the abnormality diagnosis is started at time t0, the engine control device 10 starts the active air-fuel ratio control at the time t0. In this embodiment, during the active air-fuel ratio control, the air-fuel ratio feedback control is performed with the feedback gain set smaller than normal. In FIG. 2, the engine control device 10 changes the actual air-fuel ratio AF to a rich air-fuel ratio at time t0. When the combustion of a rich air-fuel ratio mixture, i.e., rich combustion, is started in the combustion chamber 12, rich exhaust gas containing a large amount of unburned fuel components is discharged from the combustion chamber 12. Then, when the rich exhaust gas reaches the front air-fuel ratio sensor 26, the output λf of the front air-fuel ratio sensor 26 changes from the stoichiometric output value ST to a rich output value RI, which is a value corresponding to the rich air-fuel ratio. After that, the rich exhaust gas also flows into the catalytic device 18. If oxygen is stored in the catalytic device 18 at this time, the catalytic device 18 releases oxygen even when the rich exhaust gas flows in, and oxidizes the unburned fuel components in the exhaust gas. Therefore, immediately after the start of rich combustion, the exhaust gas flowing out of the catalytic device 18 is maintained at stoichiometric exhaust gas. And the output λr of the rear air-fuel ratio sensor 27 is also maintained at a value close to the stoichiometric output value ST.

[0021] After rich combustion begins, the oxygen storage amount OSA of the catalytic converter 18 gradually decreases over time. Eventually, the catalytic converter 18 releases all of its oxygen and its oxygen storage amount OSA becomes "0." As a result, the catalytic converter 18 is no longer able to sufficiently oxidize the unburned fuel components in the exhaust gas, and emits rich exhaust gas. In the following explanation, the start of the outflow of rich exhaust gas from the catalytic converter 18 at this time is referred to as rich failure.

[0022] When rich bankruptcy occurs, the output λr of the rear air-fuel ratio sensor 27 changes from a value close to the stoichiometric output value ST to the rich side. The engine control device 10 confirms the occurrence of rich bankruptcy when the output λr of the rear air-fuel ratio sensor 27 becomes a value richer than a predefined rich bankruptcy determination value XR. The rich bankruptcy determination value XR is preset with a value of the output λr corresponding to an air-fuel ratio richer than the stoichiometric air-fuel ratio. When the engine control device 10 confirms rich bankruptcy, it switches the combustion in the combustion chamber 12 from rich combustion to lean combustion. In FIG. 2, switching from rich combustion to lean combustion in response to rich bankruptcy is performed at time t1 and time t3.

[0023] When lean combustion starts, lean exhaust gas containing a large amount of excess oxygen is discharged from the combustion chamber 12. Then, when the lean exhaust gas reaches the front air-fuel ratio sensor 26, the output λf of the front air-fuel ratio sensor 26 changes from the rich output value RI to the lean output value LE, which is a value corresponding to the lean air-fuel ratio. After that, the lean exhaust gas also flows into the catalytic device 18. At this time, the catalytic device 18 stores the excess oxygen in the inflowing lean exhaust gas. Therefore, for a while after the start of lean combustion, the output λr of the rear air-fuel ratio sensor 27 is maintained at a value close to the stoichiometric output value ST.

[0024] After lean combustion starts, the oxygen storage amount OSA of the catalytic converter 18 gradually increases over time. There is a limit to the amount of oxygen that the catalytic converter 18 can store. Therefore, if lean combustion continues, the catalytic converter 18 will eventually reach a state where it cannot store any more oxygen. As a result, the catalytic converter 18 will discharge lean exhaust gas containing the oxygen that was not stored. In the following description, the start of the outflow of lean exhaust gas from the catalytic converter 18 at this time will be referred to as lean failure.

[0025] When lean breakdown occurs, the output λr of the rear air-fuel ratio sensor 27 changes from a value close to the value "λst" corresponding to the stoichiometric air-fuel ratio to a value corresponding to the lean air-fuel ratio. The engine control device 10 confirms the occurrence of lean breakdown when the output λr of the rear air-fuel ratio sensor 27 becomes a value leaner than a preset lean breakdown determination value XL. The lean breakdown determination value XL is preset with a value of the output λr corresponding to an air-fuel ratio leaner than the stoichiometric air-fuel ratio. When the engine control device 10 confirms lean breakdown, it switches the combustion in the combustion chamber 12 from lean combustion to rich combustion. In FIG. 2, switching from lean combustion to rich combustion in response to lean breakdown is performed at time t2 and time t4.

[0026] In this way, the engine control device 10 performs active air-fuel ratio control so as to alternately switch between rich combustion and lean combustion in response to rich bankruptcy and lean bankruptcy. The engine control device 10 measures the maximum oxygen storage amount Cmax of the catalytic device 18 in the following manner while performing active air-fuel ratio control. In the following description, the period during which rich combustion continues from the start of rich combustion in response to lean bankruptcy until the occurrence of rich bankruptcy is referred to as a rich combustion period. Also, the period during which lean combustion continues from the start of lean combustion in response to rich bankruptcy until the occurrence of lean bankruptcy is referred to as a lean combustion period. The engine control device 10 measures the maximum oxygen storage amount Cmax of the catalytic device 18 using each of the rich combustion period and the lean combustion period as a measurement period. Specifically, the engine control device 10 obtains the integrated value of the oxygen storage speed VO for each calculation cycle in each measurement period as a measurement value of the maximum oxygen storage amount Cmax.

[0027] When diagnosing an abnormality, the engine control device 10 averages the measured values ​​of the maximum oxygen storage amount Cmax during a plurality of measurement periods to determine the current maximum oxygen storage amount Cmax of the catalytic device 18. The engine control device 10 then diagnoses whether or not there is an abnormality in the catalytic device 18 based on whether or not the maximum oxygen storage amount Cmax is below a lower limit value that is allowable for ensuring the exhaust purification performance of the engine 11.

[0028] The engine control device 10 executes such abnormality diagnosis of the catalytic device 18 in response to the establishment of a preset execution condition. The execution condition is established when all of a plurality of predefined requirements are satisfied. The requirements for the establishment of such execution conditions include the following requirements (a) to (d). Requirement (a) is that the abnormality diagnosis of the catalytic device 18 is incomplete in the current trip. Requirement (b) is that the warm-up of the engine 11 is completed. Requirement (c) is that the operating conditions of the engine 11 are stable, that is, that there is little change in the engine speed NE and the engine load factor KL. Requirement (d) is that the catalyst temperature is equal to or higher than a preset diagnosis lower limit temperature. Note that the diagnosis lower limit temperature is set to a value higher than the lower limit of the catalyst temperature range at which the catalyst and the oxygen storage agent supported on the catalytic device 18 are sufficiently activated.

[0029] <Catalyst temperature estimation> Next, the details of the process for estimating the catalyst temperature will be described. The engine control device 10 estimates two temperatures, a first catalyst temperature THC1 and a second catalyst temperature THC2, as the catalyst temperatures. The first catalyst temperature THC1 is an estimated value of the catalyst temperature that is estimated in a form that reflects the change in the exhaust temperature due to the actual air-fuel ratio AF. On the other hand, the second catalyst temperature THC2 is an estimated value of the catalyst temperature that is estimated assuming that the actual air-fuel ratio AF is fixed to the theoretical air-fuel ratio.

[0030] <Estimation of first catalyst temperature THC1> 3 shows a flow of processing by the engine control device 10 related to estimation of the first catalyst temperature THC1. The engine control device 10 estimates the first catalyst temperature THC1 through an engine outlet gas temperature estimation process P1, an air-fuel ratio temperature correction process P2, an exhaust flow rate calculation process P3, a catalyst inlet gas temperature estimation process P4, and a catalyst temperature estimation process P5. In this embodiment, the processing related to estimation of the first catalyst temperature THC1 shown in FIG. 3 corresponds to the first estimation process.

[0031] In the engine outlet gas temperature estimation process P1, the engine control device 10 first calculates a steady engine outlet gas temperature T1 based on the engine speed NE and the engine load factor KL. The steady engine outlet gas temperature T1 represents the temperature of the exhaust gas discharged from the combustion chamber 12 to the exhaust passage 14 when the engine 11 is in steady operation under the current engine speed NE and engine load factor KL. The steady operation here refers to an operating state that matches all of the following states (i) to (ii). State (i) is a state in which the warm-up of the engine 11 is completed. State (ii) is a state in which the outside air temperature THA is at a standard temperature. State (i) is a state in which the retard control of the ignition timing SA for promoting the warm-up of the catalytic device 18 is not performed. State (ii) is a state in which the air-fuel ratio of the mixture burned in the combustion chamber 12 is the theoretical air-fuel ratio. The engine control device 10 calculates the steady engine outlet gas temperature T1 using a calculation map M1 stored in advance in the storage device 21. The calculation map M1 stores the relationship between the engine speed NE and the engine load factor KL, which have been obtained in advance by experiments or the like, and a steady-state engine outlet gas temperature T1. Furthermore, in the engine outlet gas temperature estimation process P1, the engine control device 10 calculates a value obtained by correcting the steady-state engine outlet gas temperature T1 based on the engine water temperature THW, the outside air temperature THA, the ignition timing SA, etc., as the value of the stoichiometric engine outlet gas temperature T2.

[0032] In the subsequent air-fuel ratio temperature correction process P2, the engine control device 10 calculates a value obtained by correcting the stoichiometric engine outlet gas temperature T2 based on the actual air-fuel ratio AF as the engine outlet gas temperature T3. The mixture during rich combustion contains excess fuel that does not contribute to combustion. The mixture during lean combustion contains excess air that does not contribute to combustion. Therefore, even if the amount of fuel burned in the combustion chamber 12 is the same, the heat capacity of the combustion gas is larger during rich combustion or lean combustion than during stoichiometric combustion due to the excess fuel or excess air. During rich combustion or lean combustion, the temperature of the combustion gas is lower than that during stoichiometric combustion due to the larger heat capacity. In this way, the temperature of the combustion gas is highest when the actual air-fuel ratio AF is the stoichiometric air-fuel ratio. The temperature of the combustion gas decreases according to the change in the actual air-fuel ratio AF from the stoichiometric air-fuel ratio to the rich side or the lean side. The correction of the stoichiometric engine outlet gas temperature T2 based on the actual air-fuel ratio AF in the air-fuel ratio temperature correction process P2 is performed so as to reflect the change in the combustion gas temperature due to the actual air-fuel ratio AF.

[0033] Meanwhile, in the exhaust flow rate calculation process P3, the engine control device 10 calculates the exhaust flow rate GE, which is the flow rate of the exhaust discharged from the combustion chamber 12 to the exhaust passage 14. The exhaust flow rate GE is calculated as the sum of the intake flow rate GA and the fuel flow rate GF. The fuel flow rate GF is the total amount of fuel injected per unit time. The value of the fuel flow rate GF is calculated from the fuel injection amount Q and the engine speed NE.

[0034] Furthermore, in a catalyst inlet gas temperature estimation process P4, the engine control device 10 calculates a catalyst inlet gas temperature T4 based on the engine outlet gas temperature T3, the exhaust flow rate GE, the outside air temperature THA, the traveling speed V, etc. The catalyst inlet gas temperature T4 represents the temperature of the exhaust gas flowing into the catalytic device 18. The engine control device 10 calculates the catalyst inlet gas temperature T4 using a heat balance model for the heat exchange between the exhaust passage 14 and the exhaust gas flowing therethrough, and the heat exchange between the exhaust passage 14 and the outside air flowing around it.

[0035] Then, in catalyst temperature estimation process P5, the engine control device 10 calculates the first catalyst temperature THC1 based on the catalyst inlet gas temperature T4, the exhaust flow rate GE, the outside air temperature THA, the traveling speed V, etc. The engine control device 10 calculates the first catalyst temperature THC1 using a heat balance model for the heat exchange between the catalytic device 18 and the exhaust gas flowing therethrough, the heat exchange between the catalytic device 18 and the outside air flowing around it, and the heat generated by the combustion of unburned fuel components inside the catalytic device 18.

[0036] <Estimation of second catalyst temperature THC2> 4 shows a flow of processing by the engine control device 10 related to estimation of the second catalyst temperature THC2. In this embodiment, the processing related to estimation of the second catalyst temperature THC2 shown in FIG 4 corresponds to the second estimation processing.

[0037] As shown in Fig. 4, the engine control device 10 calculates the second catalyst temperature THC2 through four processes, excluding the air-fuel ratio temperature correction process P2, out of the five processes used to calculate the first catalyst temperature THC1. Specifically, when calculating the second catalyst temperature THC2, the engine control device 10 inputs the stoichiometric engine outlet gas temperature T2 calculated in the engine outlet gas temperature estimation process P1 directly to the catalyst inlet gas temperature estimation process P4 without passing through the air-fuel ratio temperature correction process P2. In this case, the catalyst inlet gas temperature estimation process P4 calculates the catalyst inlet gas temperature T4 using the stoichiometric engine outlet gas temperature T2 instead of the engine outlet gas temperature T3.

[0038] The engine control device 10 uses the second catalyst temperature THC2 calculated in this manner to determine whether the execution condition for abnormality diagnosis of the catalytic device 18 is satisfied. That is, the requirement (iv) for the execution condition to be satisfied is actually determined based on whether the second catalyst temperature THC2 is equal to or higher than the diagnosis lower limit temperature.

[0039] In contrast, the engine control device 10 uses the first catalyst temperature THC1 as the catalyst temperature to be referenced in other engine controls. For example, the first catalyst temperature THC1 is used to determine whether or not to perform catalyst warm-up promotion control and catalyst OT prevention control. The catalyst warm-up promotion control is a control that promotes the warm-up of the catalyst device 18 by increasing the exhaust temperature by retarding the ignition timing SA or the like when the catalyst of the catalyst device 18 is in an inactive state, such as immediately after a cold start of the engine 11. The catalyst OT prevention control is a control that prevents the catalyst device 18 from overheating by limiting the output of the engine 11 when the temperature of the catalyst device 18 becomes too high. The engine control device 10 also uses the first catalyst temperature THC1 as the catalyst temperature for calculating the above-mentioned oxygen storage amount OSA.

[0040] <Actions and Effects of the Embodiment> As described above, in this embodiment, two temperatures, the first catalyst temperature THC1 and the second catalyst temperature THC2, are obtained as estimated values ​​of the catalyst temperature. The first catalyst temperature THC1 is estimated as a value reflecting the change in exhaust temperature due to the air-fuel ratio. On the other hand, the second catalyst temperature THC2 is estimated assuming that the air-fuel ratio is fixed to the theoretical air-fuel ratio. While stoichiometric combustion continues, both the first catalyst temperature THC1 and the second catalyst temperature THC2 have values ​​close to the actual catalyst temperature. On the other hand, even during the active air-fuel ratio control, the first catalyst temperature THC1 is maintained at a value close to the actual catalyst temperature. On the other hand, the second catalyst temperature THC2 during the active air-fuel ratio control may have a value deviated from the actual catalyst temperature.

[0041] On the other hand, in this embodiment, the air-fuel ratio of the mixture burned in the combustion chamber 12 is alternately switched between a rich air-fuel ratio and a lean air-fuel ratio by active air-fuel ratio control, while the catalytic converter 18 is diagnosed for an abnormality. In this embodiment, the determination as to whether the execution condition for such an abnormality diagnosis is satisfied is performed using the second catalyst temperature THC2, not the first catalyst temperature THC1. This is because the following problem occurs when the first catalyst temperature THC1 is used to determine whether the execution condition for the abnormality diagnosis is satisfied.

[0042] When the active air-fuel ratio control is started, rich combustion and lean combustion are alternately repeated. As described above, during rich combustion or lean combustion, the heat capacity of the combustion gas is larger by the amount of surplus fuel or surplus air compared to stoichiometric combustion, even if the amount of fuel burned in the combustion chamber 12 is the same. Therefore, during rich combustion or lean combustion, the exhaust temperature is lower than during stoichiometric combustion, even if the amount of heat generated by combustion is the same, because the heat capacity is larger. If the temperature of the exhaust gas flowing into the catalytic device 18 decreases, the catalyst temperature also decreases. The first catalyst temperature THC1 is a value that reflects the actual decrease in the catalyst temperature at this time. Therefore, if the first catalyst temperature THC1 is used to determine whether the execution condition for the abnormality diagnosis is satisfied, the following situation may occur. That is, after the abnormality diagnosis is started, the first catalyst temperature THC1 may fall below the diagnosis lower limit temperature due to the decrease in the exhaust temperature caused by the active air-fuel ratio control, and the abnormality diagnosis may be interrupted. When the abnormality diagnosis is interrupted, the stoichiometric combustion is resumed and the exhaust temperature rises. As a result, when the first catalyst temperature THC1 rises to or above the diagnosis lower limit temperature, the abnormality diagnosis is resumed. When the abnormality diagnosis is resumed and active air-fuel ratio control is started, the exhaust temperature falls, and the first catalyst temperature THC1 falls. Therefore, if the first catalyst temperature THC1 is used to determine whether the execution condition for the abnormality diagnosis is satisfied, the abnormality diagnosis may be repeatedly interrupted and resumed.

[0043] In contrast, in this embodiment, the second catalyst temperature THC2 estimated assuming that the air-fuel ratio is fixed to the theoretical air-fuel ratio is used to determine whether or not abnormality diagnosis should be performed. Such second catalyst temperature THC2 does not reflect the effect of the decrease in exhaust temperature due to active air-fuel ratio control. Therefore, the condition for performing abnormality diagnosis is not satisfied after the start of abnormality diagnosis due to the decrease in exhaust temperature caused by active air-fuel ratio control alone.

[0044] According to the engine control device 10 of the present embodiment described above, the following effects can be achieved. (1) In this embodiment, the first catalyst temperature THC1 reflecting the change in exhaust temperature due to the air-fuel ratio and the second catalyst temperature THC2 calculated assuming that the air-fuel ratio is fixed to the theoretical air-fuel ratio are calculated as the estimated values ​​of the catalyst temperature. Then, the second catalyst temperature THC2 is used to determine whether or not the execution condition for abnormality diagnosis of the catalytic device 18 is satisfied. In this case, after the start of the abnormality diagnosis, the execution condition for abnormality diagnosis is not unsatisfied due to a decrease in exhaust temperature caused by active air-fuel ratio control. Therefore, the abnormality diagnosis of the catalytic device 18 is less likely to be interrupted.

[0045] (2) Since the abnormality diagnosis is unlikely to be interrupted, the time required from the start of the engine 11 to the completion of the abnormality diagnosis is likely to be short. (3) When active air-fuel ratio control is being performed, the properties of the exhaust gas released into the atmosphere are worse than those during stoichiometric combustion. Meanwhile, the execution time of active air-fuel ratio control until the abnormality diagnosis is completed becomes longer each time the abnormality diagnosis is interrupted. Therefore, according to this embodiment, since the abnormality diagnosis is less likely to be interrupted, deterioration of the exhaust performance of the engine 11 due to the execution of active air-fuel ratio control during abnormality diagnosis is suppressed.

[0046] This embodiment can be modified as follows: This embodiment and the following modifications can be combined with each other to the extent that there is no technical contradiction. In the above embodiment, the air-fuel ratio feedback control is continued with the feedback gain set to a value smaller than normal even during active air-fuel ratio control. The air-fuel ratio feedback control may be stopped during active air-fuel ratio control.

[0047] As long as the first catalyst temperature THC1 is calculated as a value that reflects the change in exhaust temperature due to the air-fuel ratio, and the second catalyst temperature THC2 is calculated assuming that the air-fuel ratio is fixed to the stoichiometric air-fuel ratio, the details of the estimation logic for these may be changed as appropriate.

[0048] Some abnormality diagnoses of exhaust system components other than the catalytic converter 18 are performed by changing the air-fuel ratio from the stoichiometric air-fuel ratio to another air-fuel ratio. For example, the front air-fuel ratio sensor 26 may be diagnosed for abnormality based on a change in the output λf of the front air-fuel ratio sensor 26 in response to the change in the air-fuel ratio. It is also possible to perform such abnormality diagnoses of exhaust system components other than the catalytic converter 18 on the condition that the catalyst temperature is equal to or higher than the diagnostic lower limit temperature. In this case, it is also preferable to use the second catalyst temperature THC2 to determine whether the conditions for executing the abnormality diagnosis are met. In such a case, the abnormality diagnosis is less likely to be interrupted. [Explanation of symbols]

[0049] 10...Engine control device 11…Engine 12…Combustion chamber 13…Intake passage 14...Exhaust passage 15...Throttle valve 16...Injector 17...Ignition device 18...Catalyst device 19...Filter device 20...Processing device 21…Storage device 22...Air flow meter 23...Throttle opening sensor 24...Intake pressure sensor 25...Crank angle sensor 26...Front air-fuel ratio sensor 27...Rear air-fuel ratio sensor 28...Accelerator pedal sensor 29...Vehicle speed sensor 30...Water temperature sensor

Claims

[Claim 1] An engine control device that diagnoses the presence or absence of an abnormality in an exhaust system component of an engine in which a catalytic device for exhaust purification is installed in an exhaust passage by changing an air-fuel ratio of a mixture combusted in a combustion chamber from a stoichiometric air-fuel ratio to a rich air-fuel ratio or a lean air-fuel ratio, The engine control device estimates the catalyst temperature of the catalyst device by: a first estimation process for estimating the catalyst temperature by reflecting a change in an exhaust temperature caused by an air-fuel ratio of the mixture being burned in the combustion chamber; a second estimation process for estimating the catalyst temperature assuming that the air-fuel ratio of the mixture burning in the combustion chamber is fixed to a stoichiometric air-fuel ratio; and the execution condition of the diagnosis includes that the estimated value of the catalyst temperature by the second estimation process is equal to or higher than a predetermined diagnosis lower limit temperature, The first estimation process is a process for estimating the catalyst temperature to be a temperature lower than the catalyst temperature estimated in the second estimation process when the air-fuel ratio of the mixture is the rich air-fuel ratio or the lean air-fuel ratio. Engine control device.

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