JP7600893B2 - Engine Control Unit - Google Patents

Description

The present invention relates to an engine control device that diagnoses whether or not there is an abnormality in an exhaust system component.

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 engines equipped with a three-way catalyst device, the exhaust purification efficiency of the three-way catalyst device is increased by basically performing stoichiometric combustion, where the air-fuel ratio of the mixture is the theoretical air-fuel ratio.

Meanwhile, the device described in Patent Document 1 is known as a device that diagnoses whether or not there is an abnormality in a catalytic converter installed in the exhaust passage of an engine. The device in this document diagnoses whether or not there is an abnormality in the catalytic converter while performing the active air-fuel ratio control described below. Active air-fuel ratio control is a control that alternates 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.

JP 2010-255490 A

However, if the above abnormality diagnosis is performed when the temperature of the catalytic converter is lower than a certain level, it may result in a deterioration of emissions or an erroneous diagnosis. For this reason, the conditions for performing the diagnosis may include that the catalyst temperature be equal to or higher than a predetermined diagnostic lower limit temperature.

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 into the exhaust passage drops. As the temperature of the exhaust gas drops, the temperature of the catalytic device also drops. 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, causing the diagnosis to 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. For this reason, there is a risk that the diagnosis will be interrupted and restarted repeatedly.

The same problem can occur when diagnosing exhaust system components 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 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. The engine control device includes, as a condition for executing the above diagnosis, that the estimated value of the catalyst temperature by the second estimation process is equal to or higher than a predetermined diagnostic lower limit temperature. Note that the rich air-fuel ratio here 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.

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. In other words, a decrease in catalyst temperature due to a change in the air-fuel ratio during diagnosis of exhaust system components 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 the conditions for performing the diagnosis of exhaust system components are met. Therefore, even if the actual catalyst temperature falls below the diagnostic lower limit temperature after the start of the diagnosis of exhaust system components, the conditions for performing the diagnosis are unlikely to be met. Therefore, according to the engine control device, the abnormality diagnosis of exhaust system components is unlikely to be interrupted.

1 is a diagram illustrating a schematic configuration of an embodiment of an engine control device; 10A is a time chart showing the progress of an actual air-fuel ratio, the progress of an output of a front air-fuel ratio sensor, the progress of an output of a rear air-fuel ratio sensor, and the progress of an oxygen storage amount OSA of the catalytic converter during an abnormality diagnosis of the catalytic converter by the engine control device. 4 is a control block diagram showing a flow of processing related to estimation of a first catalyst temperature THC1 in a first estimation process performed by the engine control device. FIG. 4 is a control block diagram showing a flow of processing related to estimation of a second catalyst temperature THC2 in a second estimation process performed by the engine control device. FIG.

Hereinafter, an embodiment of an engine control device will be described in detail with reference to FIGS.
<Configuration of engine 11>
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 an intake flow rate control valve. An injector 16 which injects fuel into the intake air is installed 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 downstream of the catalyst device 18 in the exhaust passage 14. The catalytic device 18 and the filter device 19 support a three-way catalyst that simultaneously oxidizes HC and CO in the exhaust gas and reduces NOx. The catalytic device 18 and the filter device 19 also support an oxygen storage agent as a promoter to enhance the catalytic action of the three-way catalyst.

<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 including 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.

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 downstream of the throttle valve 15 in the intake passage 13. The crank angle sensor 25 is a sensor that detects the 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 device 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 coolant. 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.

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 amount of the engine 11 determined by the engine control device 10 includes 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 amount. 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.

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 by the catalytic device 18 during a predetermined calculation period is described 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, etc. 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, etc., 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, for example, from the detection result of the front air-fuel ratio sensor 26. 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.

<Diagnosis of Abnormality in Catalyst Device 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.

When diagnosing an abnormality of 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 during abnormality diagnosis of 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. A rich air-fuel ratio is an air-fuel ratio richer than the stoichiometric air-fuel ratio, and a lean air-fuel ratio is an air-fuel ratio leaner than the stoichiometric air-fuel ratio. In the following description, combustion of a mixture whose air-fuel ratio is the stoichiometric air-fuel ratio is described as stoichiometric combustion. In addition, combustion of a mixture whose air-fuel ratio is a rich air-fuel ratio is described as rich combustion, and combustion of a mixture whose air-fuel ratio is a lean air-fuel ratio is described as lean combustion. Exhaust 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 is 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 is referred to as lean exhaust gas.

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

In FIG. 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 due to 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 theoretical air-fuel ratio.

When the abnormality diagnosis is started at time t0, the engine control device 10 starts active air-fuel ratio control at that time t0. In this embodiment, during active air-fuel ratio control, the air-fuel ratio feedback control is performed with the feedback gain set smaller than normal. In FIG. 2, at time t0, the engine control device 10 changes the actual air-fuel ratio AF to a rich air-fuel ratio. When the combustion of a rich air-fuel ratio mixture, i.e., rich combustion, begins 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 will release oxygen even if the rich exhaust gas flows in, and oxidize the unburned fuel components in the exhaust gas. Therefore, immediately after the start of rich combustion, the exhaust gas flowing out of the catalytic converter 18 is maintained at stoichiometric exhaust gas. The output λr of the rear air-fuel ratio sensor 27 is also maintained at a value close to the stoichiometric output value ST.

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 rich exhaust gas flowing out of the catalytic converter 18 at this time is referred to as rich failure.

When rich-flag failure 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-flag failure when the output λr of the rear air-fuel ratio sensor 27 becomes a value richer than a predefined rich-flag failure determination value XR. The rich-flag failure determination value XR is preset to 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-flag failure, 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-flag failure is performed at times t1 and t3.

When lean combustion begins, lean exhaust gas containing a large amount of excess oxygen is discharged from the combustion chamber 12. 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, lean exhaust gas also flows into the catalytic device 18. At this time, the catalytic device 18 stores the excess oxygen in the lean exhaust gas that flows in. Therefore, for a while after lean combustion begins, the output λr of the rear air-fuel ratio sensor 27 is maintained at a value close to the stoichiometric output value ST.

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

When lean failure 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 failure when the output λr of the rear air-fuel ratio sensor 27 becomes a value leaner than a preset lean failure determination value XL. The lean failure determination value XL is preset to 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 failure, 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 failure is performed at times t2 and t4.

In this way, the engine control device 10 performs active air-fuel ratio control to alternate between rich combustion and lean combustion depending on rich breakdown and lean breakdown. 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 depending on lean breakdown until rich breakdown occurs is referred to as the rich combustion period. Also, the period during which lean combustion continues from the start of lean combustion depending on rich breakdown until lean breakdown occurs is referred to as the 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 measurement periods. 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 the measurement value of the maximum oxygen storage amount Cmax.

When diagnosing an abnormality, the engine control device 10 averages the measured values of the maximum oxygen storage amount Cmax over multiple 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 the lower limit value that is acceptable for ensuring the exhaust purification performance of the engine 11.

The engine control device 10 performs 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 pre-set requirements are met. 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, the changes in the engine speed NE and the engine load factor KL are small. Requirement (d) is that the catalyst temperature is equal to or higher than a preset diagnosis lower limit temperature. The diagnosis lower limit temperature is set to a temperature higher than the lower limit of the catalyst temperature range at which the catalyst and oxygen storage agent supported in the catalytic device 18 are fully activated.

<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.

<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 process related to estimation of the first catalyst temperature THC1 shown in FIG. 3 corresponds to the first estimation process.

In the engine outlet gas temperature estimation process P1, the engine control device 10 first calculates a steady-state engine outlet gas temperature T1 based on the engine speed NE and the engine load factor KL. The steady-state 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 operating steadily under the current engine speed NE and engine load factor KL. Note that 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 (ro) is a state in which the outside air temperature THA is at the standard temperature. State (ha) is a state in which the ignition timing SA is not retarded to promote the warm-up of the catalytic device 18. 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-state engine output 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 engine load factor KL, which have been determined in advance through experiments, and the steady-state engine output gas temperature T1. Furthermore, in the engine output gas temperature estimation process P1, the engine control device 10 calculates a value obtained by correcting the steady-state engine output 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 output gas temperature T2.

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. Also, 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 compared to stoichiometric combustion due to the excess fuel or excess air. And, 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. And 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 to reflect the change in the combustion gas temperature due to the actual air-fuel ratio AF.

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.

Furthermore, in the catalyst inlet gas temperature estimation process P4, the engine control device 10 calculates the catalyst inlet gas temperature T4 based on the engine outlet gas temperature T3, the exhaust flow rate GE, the outside air temperature THA, the driving speed V, etc. The catalyst inlet gas temperature T4 represents the temperature of the exhaust gas flowing into the catalyst 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 therein, and the heat exchange between the exhaust passage 14 and the outside air flowing around it.

Then, in the 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 driving 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 catalyst device 18 and the exhaust gas flowing therethrough, the heat exchange between the catalyst device 18 and the outside air flowing around it, and the heat generated by the combustion of unburned fuel components within the catalyst device 18.

<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.

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 into the catalyst inlet gas temperature estimation process P4 without going through the air-fuel ratio temperature correction process P2. In this case, in the catalyst inlet gas temperature estimation process P4, the catalyst inlet gas temperature T4 is calculated using the stoichiometric engine outlet gas temperature T2 instead of the engine outlet gas temperature T3.

The engine control device 10 uses the second catalyst temperature THC2 calculated in this manner to determine whether the conditions for executing the abnormality diagnosis of the catalytic device 18 are met. In other words, the requirement (iv) for the execution condition to be met is actually determined based on whether the second catalyst temperature THC2 is equal to or higher than the diagnostic lower limit temperature.

In contrast, the engine control device 10 uses the first catalyst temperature THC1 as the catalyst temperature 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. Catalyst warm-up promotion control is a control that promotes the warm-up of the catalyst device 18 by increasing the exhaust temperature by, for example, retarding the ignition timing SA when the catalyst of the catalyst device 18 is in an inactive state, such as immediately after a cold start of the engine 11. 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 oxygen storage amount OSA described above.

<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 implementation of 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.

In contrast, 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 an abnormality diagnosis of the catalytic device 18 is performed. In this embodiment, the determination of whether the conditions for executing such an abnormality diagnosis are met 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 conditions for executing the abnormality diagnosis are met.

When 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 due to the excess fuel or excess 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, due to the larger heat capacity. 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 abnormality diagnosis is satisfied, the following situation may occur. That is, after the start of abnormality diagnosis, the first catalyst temperature THC1 may fall below the diagnosis lower limit temperature due to the decrease in exhaust temperature caused by active air-fuel ratio control, and the abnormality diagnosis may be interrupted. When the abnormality diagnosis is interrupted, stoichiometric combustion is resumed and the exhaust temperature rises. As a result, when the first catalyst temperature THC1 rises above the diagnostic 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 drops, and the first catalyst temperature THC1 drops. Therefore, if the first catalyst temperature THC1 is used to determine whether the conditions for executing the abnormality diagnosis are met, the abnormality diagnosis may be repeatedly interrupted and resumed.

In contrast, in this embodiment, the second catalyst temperature THC2, which is estimated assuming that the air-fuel ratio is fixed at the theoretical air-fuel ratio, is used to determine whether or not to perform an abnormality diagnosis. This second catalyst temperature THC2 does not reflect the effects of the decrease in exhaust temperature due to active air-fuel ratio control. Therefore, the condition for performing an abnormality diagnosis is not satisfied after the start of the abnormality diagnosis due to the decrease in exhaust temperature caused by active air-fuel ratio control alone.

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, which reflects the change in exhaust temperature due to the air-fuel ratio, and the second catalyst temperature THC2, which is 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 conditions for executing the abnormality diagnosis of the catalytic device 18 are satisfied. In this case, after the abnormality diagnosis is started, the conditions for executing the abnormality diagnosis are 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.

(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, the deterioration of the exhaust performance of the engine 11 due to the execution of active air-fuel ratio control during an abnormality diagnosis is suppressed.

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 be smaller than normal even during the active air-fuel ratio control. During the active air-fuel ratio control, the air-fuel ratio feedback control may be stopped.

- 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 theoretical air-fuel ratio, the details of the estimation logic may be changed as appropriate.

-Some abnormality diagnoses of exhaust system parts other than the catalytic device 18 are performed by changing the air-fuel ratio from the theoretical air-fuel ratio to another air-fuel ratio. For example, abnormality diagnosis of the front air-fuel ratio sensor 26 may be performed 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 abnormality diagnosis of such exhaust system parts other than the catalytic device 18 on the condition that the catalyst temperature is equal to or higher than the diagnostic lower limit temperature. In this case, it is desirable 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.

LIST OF SYMBOLS 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: COMPUTING DEVICE 21: MEMORY 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 (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 purifying exhaust gas 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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