JP7613191B2 - Supercharged engine control device - Google Patents

Description

The present invention relates to a control device for a supercharged engine.

As an exhaust purification device for vehicle engines, there is a three-way catalytic converter with an oxygen storage function. Some engines equipped with such a three-way catalytic converter are equipped with two air-fuel ratio sensors, one at the front and one at the rear. The front air-fuel ratio sensor is installed in the exhaust passage upstream of the three-way catalytic converter. The rear air-fuel ratio sensor is installed in the exhaust passage downstream of the three-way catalytic converter. In engines equipped with such two air-fuel ratio sensors, air-fuel ratio control is performed through the following two feedbacks: a main feedback based on the output of the front air-fuel ratio sensor, and a sub-feedback based on the output of the rear air-fuel ratio sensor.

The output characteristics of the air-fuel ratio sensor vary due to individual differences and aging. Patent Document 1 describes a technique for calibrating the output deviation of the rear air-fuel ratio sensor based on the difference between the output of the rear air-fuel ratio sensor during a predetermined period and the standard value of the same output corresponding to the theoretical air-fuel ratio. The above-mentioned predetermined period is the period during which the output of the rear air-fuel ratio sensor becomes stable after returning from a fuel cut.

JP 2016-31041 A

After fuel cut is resumed, there is a period during which the composition of the exhaust gas that has passed through the three-way catalytic converter temporarily becomes equivalent to the theoretical air-fuel ratio. During this period, the output of the rear air-fuel ratio sensor becomes equivalent to the theoretical air-fuel ratio of the sensor. Therefore, the output of the rear air-fuel ratio sensor can be calibrated based on the above deviation.

However, with the above conventional technology, the output of the rear air-fuel ratio sensor can only be calibrated when the engine returns to normal after a fuel cut. Therefore, depending on the engine operating conditions, it may be difficult to obtain an opportunity to calibrate the output.

The control device for solving the above problem controls a supercharged engine having a combustion chamber in which the mixture is burned, an exhaust passage through which exhaust gas generated by the combustion of the mixture in the combustion chamber flows, a turbine installed in the exhaust passage , a three-way catalytic device having oxygen storage capacity installed in a portion downstream of the turbine in the exhaust passage, and an air-fuel ratio sensor installed in a portion downstream of the three-way catalytic device in the exhaust passage, the turbine being provided with a turbine wheel that rotates by receiving the exhaust flow, a bypass passage that is a passage for exhaust gas that bypasses the turbine wheel, and a wastegate valve that is a valve that changes the flow passage area of the exhaust gas in the bypass passage. Here, the combustion of the mixture in the combustion chamber at an air-fuel ratio richer than the theoretical air-fuel ratio is referred to as rich combustion, and the combustion of the mixture in the combustion chamber at an air-fuel ratio leaner than the theoretical air-fuel ratio is referred to as lean combustion. Also, the output of the air-fuel ratio sensor indicating that the exhaust gas that has passed through the three-way catalytic device is in a rich state is referred to as a rich output, and the output of the air-fuel ratio sensor indicating that the exhaust gas that has passed through the three-way catalytic device is in a lean state is referred to as a lean output. At this time, the control device for the supercharged engine performs a sub air-fuel ratio feedback process during operation of the supercharged engine, which switches from rich combustion to lean combustion in response to a change in the output of the air-fuel ratio sensor from lean output to rich output, and switches from lean combustion to rich combustion in response to a change in the output of the air-fuel ratio sensor from rich output to lean output. The control device also performs a sensor output calibration process to calibrate the output of the air-fuel ratio sensor based on the output of the air-fuel ratio sensor during a period in which rich combustion is being performed by the sub air-fuel ratio feedback process and a state in which a change in the output of the air-fuel ratio sensor is maintained at or below a preset value. The control device performs the sensor output calibration process under the condition that the opening degree of the wastegate valve is equal to or less than a preset opening degree.

In the above engine control device, the target air-fuel ratio is alternately switched between the lean side target air-fuel ratio and the rich side target air-fuel ratio in the sub air-fuel ratio feedback process. During lean combustion with the lean side target air-fuel ratio as the target air-fuel ratio, exhaust gas containing excess oxygen flows into the catalytic device. At this time, the catalytic device stores the excess oxygen in the exhaust gas and creates a stoichiometric atmosphere inside, thereby facilitating the purification of the exhaust gas by the three-way catalyst. If lean combustion continues, the catalytic device will eventually be unable to store any more excess oxygen, causing the exhaust gas containing excess oxygen to flow out, i.e., a so-called lean failure. At this time, if the target air-fuel ratio is switched to the rich side target air-fuel ratio and rich combustion is started, exhaust gas containing unburned fuel components will flow into the catalytic device. At this time, the catalytic device releases the oxygen stored during lean combustion and creates a stoichiometric atmosphere inside, thereby facilitating the purification of the exhaust gas by the three-way catalyst. If rich burn continues, the catalytic converter will eventually release all of the stored oxygen, causing exhaust containing unburned fuel components to flow out, a phenomenon known as rich failure. At this time, the target air-fuel ratio can be switched to a lean target air-fuel ratio to start lean burn, alternating between lean and rich burn to reduce the steady-state deviation of the air-fuel ratio in the main air-fuel ratio feedback process. The occurrence of lean failure or rich failure can be confirmed from the output of the rear air-fuel ratio sensor. Therefore, in the sub air-fuel ratio feedback process, the timing of switching the target air-fuel ratio is determined based on the output of the rear air-fuel ratio sensor.

The turbine in an exhaust turbine type supercharged engine is equipped with a turbine wheel that rotates due to the force of the exhaust flow. The exhaust flow that passes through the turbine wheel and heads towards the three-way catalyst device becomes a swirling flow. On the other hand, when the wastegate valve is opened, part of the exhaust flows around the turbine wheel. The exhaust flow that passes through the wastegate valve and heads towards the three-way catalyst device becomes a blowdown flow. Therefore, the smaller the opening of the wastegate valve, the greater the bias in the exhaust flow within the three-way catalyst device, and the shorter the period in which lean and rich breakdowns occur.

During rich combustion after a lean breakdown, there is a period when the output of the air-fuel ratio sensor corresponds to the stoichiometric air-fuel ratio. Therefore, the amount of deviation in the output of the air-fuel ratio sensor can be confirmed based on the output of the air-fuel ratio sensor at this time. However, if the cycle in which lean breakdown and rich breakdown occur is short, the period in which the output of the air-fuel ratio sensor corresponds to the stoichiometric air-fuel ratio will also be short. Therefore, when the wastegate valve opening is large and the cycle in which lean breakdown and rich breakdown occur is short, it will not be possible to accurately confirm the output of the air-fuel ratio sensor corresponding to the stoichiometric air-fuel ratio from the output of the air-fuel ratio sensor during rich combustion.

In response to this, the control device calibrates the output of the air-fuel ratio sensor based on the sensor output when output fluctuations during rich combustion are small, when the opening of the wastegate valve is equal to or smaller than a default opening. This allows the output deviation of the air-fuel ratio sensor to be appropriately calibrated. As a result, deterioration in the control accuracy of the air-fuel ratio due to deviations in the output of the air-fuel ratio sensor is suppressed.

Prior to starting the sensor output calibration process in the control device, it is advisable to perform a drive process that drives the wastegate valve so that the opening is equal to or less than a default opening. In this case, if the opening of the wastegate valve is controlled as the situation progresses, the sensor output calibration process can be performed even if it is not possible to perform the process because the opening exceeds the default opening. This makes it easier to obtain an opportunity to perform the sensor output calibration process.

The sensor output calibration process may be performed by calculating the difference between a standard value of the output of the air-fuel ratio sensor corresponding to the stoichiometric air-fuel ratio and an average value of the output of the air-fuel ratio sensor during the above period as an output calibration amount of the air-fuel ratio sensor. In this case, the sub air-fuel ratio feedback process may be performed in the following manner (a) or (b) to suppress deterioration of the control accuracy of the air-fuel ratio due to deviation of the air-fuel ratio sensor. (a) The output of the air-fuel ratio sensor is corrected by the output calibration amount to determine the output change from lean output to rich output and the output change from rich output to lean output in the sub air-fuel ratio feedback process. (b) The judgment value of the output of the air-fuel ratio sensor used for determining the output change from lean output to rich output and the output change from rich output to lean output in the sub air-fuel ratio feedback process is corrected by the output calibration amount.

1 is a diagram showing a schematic configuration of an embodiment of a control device for a supercharged engine; FIG. 2 is a control block diagram of air-fuel ratio control executed by the engine control device. 4A is a time chart showing the transitions of the front air-fuel ratio and the target air-fuel ratio, FIG. 4B is a time chart showing the transitions of the oxygen storage amount, and FIG. 4C is a time chart showing the transitions of the rear air-fuel ratio during the air-fuel ratio control by the control device. 4 is a flowchart of a sensor output calibration routine executed by the control device. 6 is a graph showing a calculation manner of an output calibration amount in the sensor output calibration routine; FIG. 13 is a diagram showing the exhaust flow and oxygen storage state of the front three-way catalytic converter immediately after the start of rich combustion when the WGV is fully closed. FIG. 13 is a diagram showing the exhaust flow and oxygen storage state of a front three-way catalytic converter when rich collapse occurs with the WGV fully closed. FIG. 13 is a diagram showing the exhaust flow and oxygen storage state of the front three-way catalytic converter immediately after the start of rich combustion when the WGV is fully open. FIG. 13 is a diagram showing the exhaust flow and oxygen storage state of a front three-way catalytic converter when rich collapse occurs with the WGV fully open. FIG. 13 is a diagram showing the output waveforms of the rear air-fuel ratio sensor when the WGV is fully open and when it is fully closed.

Below, one embodiment of a control device for a supercharged engine will be described in detail with reference to Figures 1 to 10. The control device 50 of this embodiment is applied to a supercharged engine 10 mounted on a vehicle.

<Configuration of supercharged engine 10>
First, the configuration of a supercharged engine 10 to which a control device 50 of this embodiment is applied will be described with reference to FIG. 1. The supercharged engine 10 includes a combustion chamber 11 in which a mixture is burned. The supercharged engine 10 also includes an intake passage 12 which is an introduction passage for intake air to the combustion chamber 11, and an exhaust passage 13 which is an exhaust passage for exhaust gas from the combustion chamber 11. The supercharged engine 10 includes a plurality of cylinders, each of which has an individual combustion chamber 11. FIG. 1 shows only one of the plurality of combustion chambers 11. The supercharged engine 10 includes an injector 14 for injecting fuel into the intake air to be burned in the combustion chamber 11, which is provided for each cylinder. Furthermore, each cylinder of the supercharged engine 10 includes an ignition device 15 for igniting the mixture in the combustion chamber 11 by spark discharge. The supercharged engine 10 generates a driving force for a vehicle by rotating a crankshaft 16 by the combustion of the mixture in the combustion chamber 11.

The turbocharged engine 10 is equipped with a turbocharger 20. The turbocharger 20 has a compressor 21 installed in the intake passage 12 and a turbine 22 installed in the exhaust passage 13. The compressor 21 is provided with a compressor wheel 23 that compresses the intake air according to its rotation. The turbine 22 is provided with a turbine wheel 24 that rotates by receiving the exhaust flow. The turbine wheel 24 is connected to the compressor wheel 23 via a turbine shaft 25. As a result, when the turbine wheel 24 rotates, the compressor wheel 23 also rotates in conjunction with it. The turbine 22 is also provided with a bypass passage 26 and a wastegate valve 27. The bypass passage 26 is a flow path for exhaust that bypasses the turbine wheel 24. The wastegate valve 27 is a valve that changes the flow path area of the exhaust in the bypass passage 26. In this embodiment, a diaphragm-type valve that uses the differential pressure between the boost pressure PB and atmospheric pressure as power for changing the opening degree is adopted as the wastegate valve 27.

An intercooler 30 is provided in the intake passage 12 downstream of the compressor 21. A throttle valve 31 is provided in the intake passage 12 downstream of the intercooler 30. The intercooler 30 is a cooler that cools the intake air, which has been heated by adiabatic compression in the compressor 21, by heat exchange with the cooling water of the supercharged engine 10. The throttle valve 31 is a valve that changes the flow area of the intake air in the intake passage 12. An airflow meter 32, a supercharging pressure sensor 33, and an intake manifold pressure sensor 34 are also provided in the intake passage 12. The airflow meter 32 is provided in the intake passage 12 upstream of the compressor 21. The airflow meter 32 detects the intake flow rate GA, which is the flow rate of the intake air flowing through the intake passage 12. The boost pressure sensor 33 detects the pressure of the intake air in the portion of the intake passage 12 downstream of the intercooler 30 and upstream of the throttle valve 31 as the boost pressure PB. The intake manifold pressure sensor 34 detects the pressure of the intake air in the portion of the intake passage 12 downstream of the throttle valve 31 as the intake manifold pressure PM.

A first three-way catalytic device 40 is installed in a portion downstream of the turbine 22 in the exhaust passage 13. Furthermore, a second three-way catalytic device 41 is installed in a portion downstream of the first three-way catalytic device 40 in the exhaust passage 13. A three-way catalyst made of a precious metal such as platinum is supported in the first three-way catalytic device 40. The three-way catalyst is a catalytic material that simultaneously promotes the oxidation of unburned fuel components in the exhaust gas, such as hydrocarbons (HC) and carbon monoxide (CO), and the reduction of nitrogen oxides (NOx) in the exhaust gas. In addition, the first three-way catalytic device 40 also supports an oxygen storage agent made of a metal oxide such as ceria. The oxygen storage agent stores the surrounding oxygen in an oxidizing atmosphere with an excess of oxygen, and releases a rapidly increased amount of oxygen in a reducing atmosphere with a shortage of oxygen. The first three-way catalytic device 40, which supports such an oxygen storage agent, is a three-way catalytic device with oxygen storage ability. Like the first three-way catalytic converter 40, the second three-way catalytic converter 41 also supports a three-way catalyst and is a three-way catalytic converter with oxygen storage capacity.

Furthermore, a front air-fuel ratio sensor 42 and a rear air-fuel ratio sensor 43 are installed in the exhaust passage 13. The front air-fuel ratio sensor 42 is installed in a portion of the exhaust passage 13 downstream of the turbine 22 and upstream of the first three-way catalytic device 40. The rear air-fuel ratio sensor 43 is installed in a portion of the exhaust passage 13 downstream of the first three-way catalytic device 40 and upstream of the second three-way catalytic device 41. The front air-fuel ratio sensor 42 has a detection portion exposed to the exhaust and outputs an electric signal according to the composition of the exhaust that touches the detection portion. The output of the front air-fuel ratio sensor 42 corresponds to the air-fuel ratio of the mixture burned in the combustion chamber 11. In the following description, the air-fuel ratio indicated by the output of the front air-fuel ratio sensor 42 is referred to as the front air-fuel ratio FrAF.

On the other hand, the rear air-fuel ratio sensor 43 is also a sensor with a structure similar to that of the front air-fuel ratio sensor 42. However, the detection section of the rear air-fuel ratio sensor 43 is exposed to the exhaust gas after passing through the first three-way catalytic device 40. Therefore, the air-fuel ratio indicated by the output of the rear air-fuel ratio sensor 43 is a value corresponding to the components of the exhaust gas reformed in the first three-way catalytic device 40. In the following description, the air-fuel ratio indicated by the output of the rear air-fuel ratio sensor 43 is referred to as the rear air-fuel ratio RrAF. In this embodiment, the first three-way catalytic device 40 corresponds to a three-way catalytic device for exhaust gas purification installed downstream of the turbine 22 and upstream of the rear air-fuel ratio sensor 43 in the exhaust passage 13.

A vehicle equipped with a supercharged engine 10 is equipped with a control device 50. The control device 50 is an electronic control device that includes a processing circuit that executes various processes related to engine control, and a storage device that stores programs and data for engine control.

The outputs of the air flow meter 32, the boost pressure sensor 33, the intake manifold pressure sensor 34, the front air-fuel ratio sensor 42, and the rear air-fuel ratio sensor 43 are input to the control device 50. The outputs of the crank angle sensor 51 and the accelerator pedal sensor 52 are also input to the control device 50. The crank angle sensor 51 is a sensor that detects the crank angle θc, which is the rotation angle of the crankshaft 16 of the supercharged engine 10. The accelerator pedal sensor 52 is a sensor that detects the accelerator pedal opening ACC, which is the amount of depression of the accelerator pedal by the driver. The control device 50 obtains the engine speed NE from the output of the crank angle sensor 51. The control device 50 also obtains the engine load factor KL from the intake flow rate GA, the boost pressure PB, the intake manifold pressure PM, the throttle opening TA, and the like. The engine load factor KL indicates the intake filling efficiency ηc of the combustion chamber 11.

The control device 50 controls the throttle opening TA, fuel injection amount QINJ, ignition timing AOP, etc. based on the detection results of these sensors. The control device 50 also calculates a target value for the boost pressure PB according to the operating conditions of the supercharged engine 10. The control device 50 performs feedback control of the WGV opening so that the target value of the boost pressure PB is obtained.

<Air-fuel ratio control>
Furthermore, the control device 50 controls the air-fuel ratio of the mixture burned in the combustion chamber 11. Next, the air-fuel ratio control will be described with reference to FIG.

Figure 2 shows the flow of processing by the control device 50 related to air-fuel ratio control. As shown in Figure 2, the control device 50 performs feedback control of the air-fuel ratio by setting the fuel injection amount QINJ of the injector 14 through a base injection amount calculation process P1, a main air-fuel ratio feedback process P2, and a sub air-fuel ratio feedback process P3. The control device 50 repeatedly executes the series of processes shown in Figure 2 at predetermined control intervals while the supercharged engine 10 is in operation. In the following explanation and drawings, feedback is abbreviated as "F/B."

In the base injection amount calculation process P1, the value of the base injection amount QBSE is calculated based on the target air-fuel ratio AFT, which is the target value of the air-fuel ratio, and the engine load factor KL. Specifically, in the base injection amount calculation process P1, the quotient (= KL x ρ/AFT) obtained by dividing the product of the intake density ρ and the engine load factor KL by the target air-fuel ratio AFT is calculated as the value of the base injection amount QBSE.

In the main air-fuel ratio F/B process P2, an air-fuel ratio feedback correction value FAF, which is a correction value for the fuel injection amount QINJ, is calculated based on the front air-fuel ratio FrAF and the target air-fuel ratio AFT. Specifically, in the main air-fuel ratio F/B process P2, the value of the air-fuel ratio feedback correction value FAF is gradually updated so that the deviation between the front air-fuel ratio FrAF and the target air-fuel ratio AFT approaches "0". In the air-fuel ratio control, the product (=QBSE x FAF) of the base injection amount QBSE multiplied by the air-fuel ratio feedback correction value FAF is set as the value of the fuel injection amount QINJ of the injector 14.

In the sub air-fuel ratio F/B process P3, the target air-fuel ratio AFT is alternately switched between the rich side target air-fuel ratio AFTR and the lean side target air-fuel ratio AFTL. In the sub air-fuel ratio F/B process P3, the timing of switching the target air-fuel ratio AFT is determined based on the rear air-fuel ratio RrAF, which is the output of the rear air-fuel ratio sensor 43. The value of the rich side target air-fuel ratio AFTR is preset to an air-fuel ratio slightly richer than the stoichiometric air-fuel ratio. The value of the lean side target air-fuel ratio AFTL is preset to an air-fuel ratio slightly leaner than the stoichiometric air-fuel ratio. The switching of the target air-fuel ratio AFT from the rich side target air-fuel ratio AFTR to the lean side target air-fuel ratio AFTL is performed when the rear air-fuel ratio RrAF changes from an air-fuel ratio richer than the lean failure determination value LE to an air-fuel ratio leaner than the lean failure determination value LE. The target air-fuel ratio AFT is switched from the lean target air-fuel ratio AFTL to the rich target air-fuel ratio AFTR when the rear air-fuel ratio RrAF changes from an air-fuel ratio leaner than the rich collapse determination value RI to an air-fuel ratio richer than the rich collapse determination value RI. The lean collapse determination value LE is set to an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio and richer than the lean target air-fuel ratio AFTL. The rich collapse determination value RI is set to an air-fuel ratio that is richer than the stoichiometric air-fuel ratio and leaner than the rich target air-fuel ratio AFTR.

Figure 3 shows an example of an embodiment of air-fuel ratio control by the control device 50. Note that Figure 3 shows an embodiment of air-fuel ratio control when the supercharged engine 10 is operating steadily with the wastegate valve 27 fully closed. Note that Figure 3(a) shows the trends in the front air-fuel ratio FrAF and the target air-fuel ratio AFT, Figure 3(b) shows the trends in the oxygen storage amount OSA of the first three-way catalyst device 40, and Figure 3(c) shows the trends in the rear air-fuel ratio RrAF.

When the lean target air-fuel ratio AFTL is set as the value of the target air-fuel ratio AFT, the mixture in the combustion chamber 11 is burned at an air-fuel ratio leaner than the theoretical air-fuel ratio. In the following description, the combustion of the mixture at the theoretical air-fuel ratio is described as stoichiometric combustion, the combustion of the mixture at an air-fuel ratio leaner than the theoretical air-fuel ratio is described as lean combustion, and the combustion of the mixture at an air-fuel ratio richer than the theoretical air-fuel ratio is described as rich combustion. In addition, in the following description, exhaust gas containing an amount of oxygen equal to the amount required for complete combustion of the unburned fuel components in the exhaust gas is described as stoichiometric exhaust gas. Furthermore, in the following description, exhaust gas containing an amount of oxygen greater than the amount required for complete combustion of the unburned fuel components in the exhaust gas is described as lean exhaust gas. In addition, in the following description, exhaust gas containing an amount of oxygen less than the amount required for complete combustion of the unburned fuel components in the exhaust gas is described as rich exhaust gas. Exhaust produced by stoichiometric combustion is called stoichiometric exhaust, exhaust produced by lean combustion is called lean exhaust, and exhaust produced by rich combustion is called rich exhaust.

During lean combustion, lean exhaust gas flows into the first three-way catalytic device 40. At this time, the first three-way catalytic device 40 absorbs excess oxygen in the exhaust gas, thereby reducing and purifying the NOx in the exhaust gas. As a result, the exhaust gas flowing out of the first three-way catalytic device 40 has a composition close to that of stoichiometric exhaust gas. Therefore, the rear air-fuel ratio RrAF at this time is close to the theoretical air-fuel ratio.

However, there is a limit to the amount of oxygen that the first three-way catalytic device 40 can store. Therefore, if lean combustion continues, the oxygen storage amount OSA of the first three-way catalytic device 40 will eventually reach its upper limit MAX, and it will no longer be able to store any more oxygen. Then, lean exhaust gas will flow out of the first three-way catalytic device 40. In the following explanation, this state in which lean exhaust gas flows out of the first three-way catalytic device 40 will be referred to as a lean failure state. When lean failure occurs, the exhaust gas flowing around the detection portion of the rear air-fuel ratio sensor 43 becomes lean exhaust gas. Therefore, the rear air-fuel ratio RrAF at this time will be an air-fuel ratio that is leaner than the theoretical air-fuel ratio.

The control device 50 confirms the occurrence of lean failure when the rear air-fuel ratio RrAF becomes leaner than the lean failure judgment value LE during lean combustion. When the control device 50 confirms the occurrence of lean failure, it switches the target air-fuel ratio AFT from the lean side target air-fuel ratio AFTL to the rich side target air-fuel ratio AFTR. This switches the combustion in the combustion chamber 11 from lean combustion to rich combustion. In FIG. 3, switching from lean combustion to rich combustion in response to lean failure is performed at times t2, t4, and t6.

When rich combustion begins, rich exhaust gas flows into the first three-way catalytic device 40. At this time, the first three-way catalytic device 40 releases the oxygen stored during lean combustion to oxidize and purify the unburned fuel components in the exhaust gas. As a result, the exhaust gas flowing out of the first three-way catalytic device 40 has a composition close to that of stoichiometric exhaust gas. Therefore, the rear air-fuel ratio RrAF returns to a value close to the theoretical air-fuel ratio.

If rich combustion continues, eventually the oxygen storage amount OSA of the first three-way catalyst device 40 will reach "0" and the unburned fuel components will no longer be able to be sufficiently oxidized. As a result, rich exhaust gas will begin to flow out of the first three-way catalyst device 40, and the rear air-fuel ratio RrAF will change from a value close to the theoretical air-fuel ratio to the rich side. In the following explanation, this state in which rich exhaust gas begins to flow out of the first three-way catalyst device 40 will be referred to as a rich failure state.

The control device 50 confirms the occurrence of rich-side collapse when the rear air-fuel ratio RrAF becomes richer than the rich-side collapse determination value RI during rich combustion. When the control device 50 confirms the occurrence of rich-side collapse, it switches the target air-fuel ratio AFT from the rich-side target air-fuel ratio AFTR to the lean-side target air-fuel ratio AFTL and starts lean combustion. After that, lean combustion and rich combustion are alternately switched each time lean collapse and the occurrence of rich collapse are confirmed. Note that in FIG. 3, switching from rich combustion to lean combustion in response to rich collapse is performed at times t1, t3, t5, and t7.

When lean breakdown occurs, the amount of NOx emissions from the first three-way catalytic device 40 temporarily increases. When rich breakdown occurs, the amount of THC emissions from the first three-way catalytic device 40 temporarily increases. THC emissions are the sum of methane emissions and non-methane hydrocarbon emissions. Incidentally, the second three-way catalytic device 41 plays a role in purifying CO, HC, and NOx that could not be completely purified by the first three-way catalytic device 40.

As described above, the control device 50 performs air-fuel ratio control through two feedback processes: the main air-fuel ratio feedback process P2 based on the output of the front air-fuel ratio sensor 42, and the sub air-fuel ratio feedback process P3 based on the output of the rear air-fuel ratio sensor 43. This air-fuel ratio control ensures the exhaust performance of the supercharged engine 10.

<Sensor output calibration process>
The output characteristics of the rear air-fuel ratio sensor 43 used in the sub air-fuel ratio feedback process P3 have variations due to individual differences and aging. The control device 50 performs a sensor output calibration process to calibrate the deviation of the output of the rear air-fuel ratio sensor 43 due to such variations.

The rear air-fuel ratio sensor 43 is manufactured so that the output for each air-fuel ratio falls within a predetermined tolerance range. In the following description, the rear air-fuel ratio sensor 43 having output characteristics in which the output is the median value of the tolerance range is referred to as the standard sensor for the rear air-fuel ratio sensor 43. In the sensor output calibration process, the deviation amount of the output of the rear air-fuel ratio sensor 43 relative to the standard sensor is obtained as the value of the output calibration amount ΔS.

Figure 4 shows a flow chart of the sensor output calibration routine executed by the control device 50 for the sensor output calibration process. The control device 50 repeatedly executes the processing of this routine at each predetermined control period while the supercharged engine 10 is operating.

When this routine starts, the control device 50 first determines in step S100 whether the conditions for executing output calibration are met. The conditions for executing output calibration include that the sub air-fuel ratio feedback process P3 is being executed and that output calibration for the current trip is incomplete. If the conditions for execution are not met (NO), the control device 50 ends the processing of this routine for the current control cycle. On the other hand, if the conditions for execution are met (YES), the control device 50 proceeds to step S110.

When the process proceeds to step S110, the control device 50 drives the wastegate valve 27 so that the WGV opening is equal to or less than the default opening in step S110. After completing this drive, the control device 50 proceeds to step S120.

When the process proceeds to step S120, the control device 50 acquires the average output AVE of the rear air-fuel ratio sensor 43 in the next period in step S120. That is, this is the period during which rich combustion is performed by the sub air-fuel ratio feedback process P3 and the change in the output of the rear air-fuel ratio sensor 43 is small. In the following description, this period is referred to as the stable period after lean failure. In this embodiment, the period from when lean failure is confirmed to when rich failure is confirmed, during which the change in the output of the rear air-fuel ratio sensor 43 is maintained at or below a preset value, is defined as the stable period of the sensor output after lean failure. Then, the control device 50 determines the average value of the output of the rear air-fuel ratio sensor 43 during the stable period of the sensor output after lean failure as the value of the average output AVE.

Then, in step S130, the control device 50 calculates the difference between the average output AVE and the predetermined standard stoichiometric output S0 as the value of the output calibration amount ΔS. The standard stoichiometric output S0 is set to the output of the standard sensor corresponding to the stoichiometric air-fuel ratio. In other words, the standard stoichiometric output S0 is the standard value of the output of the rear air-fuel ratio sensor 43 corresponding to the stoichiometric air-fuel ratio. After calculating the output calibration amount ΔS, the control device 50 ends the processing of this routine for the current control cycle.

The control device 50 reflects the output calibration amount ΔS calculated in this manner in the sub air-fuel ratio feedback process P3 in the following manner. That is, the control device 50 sets the value obtained by adding the output calibration amount ΔS to the default standard rich failure judgment value RI0 as the value of the rich failure judgment value RI. The standard rich failure judgment value RI0 is set to the value of the rich failure judgment value RI set on the premise of a standard sensor. The control device 50 also sets the value obtained by adding the output calibration amount ΔS to the default standard lean failure judgment value LE0 as the value of the lean failure judgment value LE. The standard lean failure judgment value LE0 is set to the value of the lean failure judgment value LE set on the premise of a standard sensor.

In this embodiment, the processing of steps S120 and S130 in the sensor output calibration routine corresponds to a sensor output calibration process that calibrates the output of the rear air-fuel ratio sensor 43. In addition, in this embodiment, the processing of step S110 in the sensor output calibration routine corresponds to a drive process that drives the wastegate valve 27 to an opening degree equal to or less than a predetermined opening degree prior to the start of the sensor output calibration process. Furthermore, in this embodiment, the output of the rear air-fuel ratio sensor 43 when the rear air-fuel ratio RrAF is an air-fuel ratio richer than the stoichiometric air-fuel ratio corresponds to a rich output. In this embodiment, the output of the rear air-fuel ratio sensor 43 when the rear air-fuel ratio RrAF is an air-fuel ratio leaner than the stoichiometric air-fuel ratio corresponds to a lean output.

<Actions and Effects of the Present Embodiment>
The operation and effects of this embodiment will be described.
5, the output waveform of the standard sensor during air-fuel ratio control is shown by a dashed line, and the output waveform of the rear air-fuel ratio sensor 43, the output of which is shifted to the rich side compared to the standard sensor, is shown by a solid line.

As described above, during the rich combustion from lean breakdown to rich breakdown, there is a period in which the exhaust gas that has passed through the first three-way catalytic device 40 becomes stoichiometric exhaust gas. The output of the rear air-fuel ratio sensor 43 at this time corresponds to the theoretical air-fuel ratio of the sensor. In the following description, the output of the rear air-fuel ratio sensor 43 corresponding to the theoretical air-fuel ratio is referred to as stoichiometric output. If the period in which the exhaust gas that has passed through the first three-way catalytic device 40 becomes stoichiometric exhaust gas continues for a certain period of time, the stoichiometric output of the sensor can be confirmed from the output of the rear air-fuel ratio sensor 43 after rich breakdown. Therefore, in the sensor output calibration process, the average output AVE of the rear air-fuel ratio sensor 43 during the stable period of the sensor output after lean breakdown is obtained as the stoichiometric output of the sensor. Then, the difference between the average output AVE and the standard stoichiometric output S0 is obtained as the value of the output calibration amount ΔS indicating the deviation amount of the output of the rear air-fuel ratio sensor 43 relative to the standard sensor.

In this embodiment, the lean failure determination value LE and the rich failure determination value RI are corrected by the value of the output calibration amount ΔS. Therefore, lean failure and rich failure can be determined by compensating for the deviation in the sensor output.

In this embodiment, prior to the sensor output calibration process, a drive process is performed to drive the wastegate valve 27 to an opening degree equal to or less than a default opening degree. Therefore, the sensor output calibration process in this embodiment is performed when the opening degree of the wastegate valve 27 is equal to or less than the default opening degree. The reason for performing the sensor output calibration process under these conditions will be explained below.

Figure 6 shows the flow of exhaust gas flowing into the first three-way catalytic device 40 when the wastegate valve 27 is fully closed. In the following description, the upstream end of the first three-way catalytic device 40 in the exhaust gas flow direction is referred to as the front end of the first three-way catalytic device 40. The downstream end of the first three-way catalytic device 40 in the exhaust gas flow direction is referred to as the rear end of the first three-way catalytic device 40.

At this time, because the bypass passage 26 is blocked, all of the exhaust gas discharged from the combustion chamber 11 reaches the first three-way catalytic device 40 via the turbine wheel 24. By passing through the turbine wheel 24, the exhaust gas flow becomes a swirling flow as shown in FIG. 6. Therefore, the bias of the exhaust gas hitting the front end face of the first three-way catalytic device 40 at this time is reduced. In other words, the exhaust gas hits the entire front end face of the first three-way catalytic device 40 evenly.

In Fig. 6 and Figs. 7 to 9 described later, the oxygen storage state of each part of the first three-way catalytic device 40 is also shown. The parts indicated by white circles in the figures indicate parts where the oxygen storage agent has stored oxygen. The parts indicated by hatched circles in the figures indicate parts where the oxygen storage agent has released oxygen. Fig. 6 shows the oxygen storage state of the first three-way catalytic device 40 immediately after switching from lean combustion to rich combustion in response to the occurrence of lean breakdown. Fig. 7 shows the oxygen storage state of the first three-way catalytic device 40 when rich breakdown occurs thereafter.

As described above, when the wastegate valve 27 is fully closed, the exhaust gas hits the entire end face on the front end side of the first three-way catalytic device 40 uniformly, so that the release of oxygen from the oxygen storage agent after the start of rich combustion proceeds evenly from the front end to the rear end of the first three-way catalytic device 40. Therefore, as shown in FIG. 7 , when rich breakdown occurs, the first three-way catalytic device 40 is in a state in which almost all of the oxygen stored during lean combustion has been released, that is, the oxygen storage amount OSA is almost zero. Similarly, the oxygen storage during lean combustion also proceeds evenly from the front end to the rear end of the first three-way catalytic device 40. Therefore, when lean breakdown occurs, the first three-way catalytic device 40 is in a state in which oxygen has been stored to the limit, that is, the oxygen storage amount OSA has reached the upper limit MAX.

Figure 8 shows the flow of exhaust gas flowing into the first three-way catalytic converter 40 when the wastegate valve 27 is fully open. Because the pressure loss of the exhaust gas in the route via the turbine wheel 24 is greater than that in the bypass passage 26, most of the exhaust gas discharged from the combustion chamber 11 reaches the first three-way catalytic converter 40 through the bypass passage 26. At this time, as shown in Figure 8, the strong blowdown flow ejected from the bypass passage 26 hits a limited area of the front end face of the first three-way catalytic converter 40. Therefore, the first three-way catalytic converter 40 at this time has a portion through which a large amount of exhaust gas flows and a portion through which only a small amount of exhaust gas flows.

Note that FIG. 8 shows the oxygen storage state of the first three-way catalytic device 40 immediately after switching from lean combustion to rich combustion in response to the occurrence of lean breakdown. FIG. 9 shows the oxygen storage state of the first three-way catalytic device 40 when rich breakdown occurs thereafter. At this time, the release of oxygen from the first three-way catalytic device 40 is concentrated in the area where a large amount of exhaust gas flows. Therefore, as shown in FIG. 9, rich breakdown occurs in a state where the area where oxygen is stored remains inside the first three-way catalytic device 40. Therefore, when the wastegate valve 27 is fully opened, the time from the occurrence of lean breakdown to the occurrence of rich breakdown is shorter than when it is fully closed.

In Figure 10, the output waveform of the rear air-fuel ratio sensor 43 when the wastegate valve 27 is fully open is shown by a solid line. Also in Figure 10, the output waveform of the rear air-fuel ratio sensor 43 when the wastegate valve 27 is fully closed is shown by a dashed line. When the wastegate valve 27 is fully closed, there is a period after lean failure during which the amount of change in the rear air-fuel ratio sensor 43 hardly changes, i.e., a stable period of sensor output. Therefore, the stoichiometric output of the sensor can be confirmed from the output of the rear air-fuel ratio sensor 43 during such a stable period. On the other hand, when the wastegate valve 27 is fully closed, there is almost no stable period of sensor output after lean failure.

The flow rate of the exhaust gas blown from the bypass passage 26 to the front end of the first three-way catalytic converter 40 increases as the opening of the wastegate valve 27 increases. Therefore, the time from the occurrence of lean breakdown to the occurrence of rich breakdown increases as the opening of the wastegate valve 27 decreases. The stable period of the sensor output after lean breakdown also increases as the opening of the wastegate valve 27 decreases. Therefore, in this embodiment, the sensor output calibration process is performed with the opening of the wastegate valve 27 set to a default opening or less. Incidentally, the default opening is set to the upper limit of the opening range of the wastegate valve 27 that provides a stable period of the sensor output that allows the stoichiometric output to be accurately confirmed.

In addition, since a naturally aspirated engine does not have a turbine wheel 24 that generates a swirling flow in the exhaust gas blown into the first three-way catalytic converter 40, the period during which the sensor output remains stable after a lean-state breakdown is short. Therefore, it is difficult to calibrate the sensor output based on the output of the rear air-fuel ratio sensor 43 during the period during which the sensor output remains stable after a lean-state breakdown.

According to the control device 50 for the supercharged engine 10 of the present embodiment described above, the following effects can be achieved.
(1) In this embodiment, a sensor output calibration process is performed to calibrate the output of the rear air-fuel ratio sensor 43 based on the output of the rear air-fuel ratio sensor 43 during rich burn and during a period in which the change in the output of the rear air-fuel ratio sensor 43 is small. Furthermore, this sensor output calibration process is performed when the opening of the wastegate valve 27 is equal to or smaller than a default opening. This makes it possible to appropriately calibrate the output of the rear air-fuel ratio sensor 43, and prevents deterioration of the control accuracy of the air-fuel ratio due to the output deviation.

(2) Prior to the sensor output calibration process, a drive process is performed to drive the wastegate valve 27 so that the opening is equal to or less than the default opening. In such a case, if the opening of the wastegate valve 27 is controlled as the situation progresses, the sensor output calibration process can be performed even in a situation in which the sensor output calibration process cannot be performed because the opening is not equal to or less than the default opening. This makes it easier to obtain an opportunity to perform the sensor output calibration process.

(3) Even during a stable period of the sensor output, the output of the rear air-fuel ratio sensor 43 may temporarily fluctuate slightly. If the output of the rear air-fuel ratio sensor 43 is based on such a temporary fluctuating output, accurate output calibration cannot be performed. In contrast, in this embodiment, the average output AVE, which is the average value of the output of the rear air-fuel ratio sensor 43 during a stable period of the sensor output, is obtained as the stoichiometric output of the sensor. The difference between the average output AVE and the standard stoichiometric output S0 is then obtained as the value of the output calibration amount ΔS. This improves the accuracy of the output calibration of the rear air-fuel ratio sensor 43.

(4) The sub air-fuel ratio feedback process P3 is performed after correcting the lean failure determination value LE and the rich failure determination value RI using the output calibration amount ΔS. Therefore, even if a deviation occurs in the output of the rear air-fuel ratio sensor 43, the control accuracy of the air-fuel ratio can be ensured.

Other Embodiments
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, prior to the sensor output calibration process, the wastegate valve 27 was driven to set the opening to a default opening or less. This drive process may be omitted. Even in this case, if the sensor output calibration process is performed on the condition that the wastegate valve 27 opening is set to a default opening or less, the deviation in the output of the rear air-fuel ratio sensor 43 can be appropriately calibrated.

- In the above embodiment, the lean failure judgment value LE and the rich failure judgment value RI were corrected by the output calibration amount ΔS. Instead of the lean failure judgment value LE and the rich failure judgment value RI, the output of the rear air-fuel ratio sensor 43 may be corrected by the output calibration amount ΔS. Even in such a case, by determining lean failure or rich failure using the corrected output of the rear air-fuel ratio sensor 43, deterioration of the air-fuel ratio control accuracy due to deviations in the output of the rear air-fuel ratio sensor 43 can be suppressed, as in the above embodiment.

In the above embodiment, the sensor output was calibrated based on the average output AVE, which is the average value of the output of the rear air-fuel ratio sensor 43 during the stable period of the sensor output after lean failure. If the upper limit of the output change amount of the rear air-fuel ratio sensor 43, which is determined as the stable period, is sufficiently small, the output of the rear air-fuel ratio sensor 43 may be calibrated based on the instantaneous output of the rear air-fuel ratio sensor 43 during the stable period.

LIST OF SYMBOLS 10...Supercharged engine 11...Combustion chamber 12...Intake passage 13...Exhaust passage 14...Injector 15...Ignition device 16...Crankshaft 20...Turbocharger 21...Compressor 22...Turbine 23...Compressor wheel 24...Turbine wheel 25...Turbine shaft 26...Bypass passage 27...Wastegate valve 30...Intercooler 31...Throttle valve 32...Air flow meter 33...Supercharge pressure sensor 34...Intake manifold pressure sensor 40...First three-way catalytic device 41...Second three-way catalytic device 42...Front air-fuel ratio sensor 43...Rear air-fuel ratio sensor 50...Control device 51...Crank angle sensor 52...Accelerator pedal sensor

Claims (5)

the compressor is disposed in an intake passage which is a passage through which intake air is introduced into a combustion chamber; a turbine is disposed in an exhaust passage which is a passage through which exhaust gas is discharged from the combustion chamber ; a three-way catalytic device which is disposed in a portion of the exhaust passage downstream of the turbine and has an oxygen storage capacity; and an air-fuel ratio sensor which is disposed in a portion of the exhaust passage downstream of the three-way catalytic device ,
A control device for controlling a supercharged engine, the control device comprising: a turbine wheel that rotates by receiving an exhaust flow; a bypass passage that is a flow path of exhaust that bypasses the turbine wheel; and a wastegate valve that is a valve that changes a flow path area of the exhaust of the bypass passage ,
When combustion of an air-fuel mixture at an air-fuel ratio richer than the stoichiometric air-fuel ratio is defined as rich combustion, combustion of an air-fuel mixture in the combustion chamber at an air-fuel ratio leaner than the stoichiometric air-fuel ratio is defined as lean combustion, an output of the air-fuel ratio sensor indicating that the exhaust gas that has passed through the three-way catalytic device is in a rich state is defined as a rich output, and an output of the air-fuel ratio sensor indicating that the exhaust gas that has passed through the three-way catalytic device is in a lean state is defined as a lean output,
a sub air-fuel ratio feedback process for switching from the rich combustion to the lean combustion in response to a change in an output of the air-fuel ratio sensor from the lean output to the rich output during operation of the supercharged engine , and for switching from the lean combustion to the rich combustion in response to a change in an output of the air-fuel ratio sensor from the rich output to the lean output;
a sensor output calibration process for calibrating the output of the air-fuel ratio sensor based on the output of the air-fuel ratio sensor during a period in which the rich combustion is being performed by the sub air-fuel ratio feedback process and a state in which a change in the output of the air-fuel ratio sensor is maintained at or below a predetermined value ;
and performing the sensor output calibration process when the opening degree of the wastegate valve is equal to or smaller than a predetermined opening degree. The control device for a supercharged engine according to claim 1, further comprising a drive process for driving the wastegate valve to an opening degree equal to or less than the default opening degree prior to the start of the sensor output calibration process. 3. The control device for a supercharged engine according to claim 1, wherein the sensor output calibration process is performed by calculating a difference between a standard value of the output of the air-fuel ratio sensor corresponding to a stoichiometric air-fuel ratio and an average value of the output of the air-fuel ratio sensor during the period as an output calibration amount of the air-fuel ratio sensor. The control device for a supercharged engine according to claim 3, wherein the determination of the output change from the lean output to the rich output in the sub air-fuel ratio feedback process and the determination of the output change from the rich output to the lean output are performed using a value obtained by correcting the output of the air-fuel ratio sensor with the output calibration amount. The control device for a supercharged engine according to claim 3, wherein the output calibration amount is used to correct the output judgment value of the air-fuel ratio sensor used to judge the output change from the lean output to the rich output and the output change from the rich output to the lean output in the sub air-fuel ratio feedback process.