Internal combustion engine control device and internal combustion engine control method

Abstract

This internal combustion engine control device integrates an oxygen storage amount stored by a three-dimensional catalyst in a period from an integration start position at which combustion gas of an internal combustion engine calculated from an air-fuel ratio of exhaust gas starts changing from lean to rich or rich to lean to an integration stop position at which the concentration of oxygen increasing or decreasing before the integration start position inverts.

Description

[Technical Field]

[0001] The present invention relates to an internal combustion engine control device and an internal combustion engine control method. [Background technology]

[0002] A conventional control technology is known in which a three-way catalyst is provided in the exhaust pipe of an internal combustion engine, and exhaust gas sensors installed before and after the three-way catalyst are used to detect the oxygen storage state within the catalyst and correct the air-fuel ratio based on the detected oxygen storage state. In this control technology, a rich correction of the air-fuel ratio is determined based on the oxygen storage state detected by time integration of the product of the amount of air taken into the internal combustion engine and the difference between the air-fuel ratio of the exhaust gas and the stoichiometric air-fuel ratio. Furthermore, an exhaust gas sensor installed downstream of the three-way catalyst detects the presence or absence of oxygen released downstream of the three-way catalyst, thereby performing feedback correction of the air-fuel ratio control. In the following description, the three-way catalyst is also abbreviated as "catalyst." [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2017-31833 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-57545 Summary of the Invention [Problem to be solved by the invention]

[0004] The oxygen storage capacity within the catalyst changes as the catalyst deteriorates. Changes in oxygen storage capacity affect the process performed by the electronic control unit to determine whether or not to execute air-fuel ratio correction control. Therefore, the electronic control unit must accurately diagnose the degree of catalyst deterioration. If the electronic control unit erroneously diagnoses the catalyst's deterioration state, it will be unable to execute optimal air-fuel ratio correction control and will be unable to maintain the oxygen storage capacity within an appropriate range.

[0005] The technology described in Patent Document 1 determines catalyst deterioration based on the number of times that the lean air-fuel ratio sensor provided upstream of the catalyst and the oxygen sensor provided downstream of the catalyst reverse (fluctuation) between the lean and rich air-fuel ratios. However, the technology described in Patent Document 1 does not take into consideration the effect of delay in the sensor signal due to oxygen sensor deterioration. As a result, even if the catalyst is in a deteriorated state, if the sensor signal is delayed, it will have a waveform similar to that of a normal sensor signal, which can lead to the electronic control unit incorrectly diagnosing catalyst deterioration.

[0006] To address this issue, the technology described in Patent Document 2 improves diagnostic accuracy by eliminating the effect of response delays from an oxygen sensor located in the exhaust passage downstream of the catalyst. The technology described in Patent Document 2 eliminates the effect of oxygen sensor degradation by correcting the catalyst's oxygen storage capacity using the sensor response time measurement from the previous trip to determine catalyst degradation. However, even if the oxygen sensor is degraded, it is necessary to receive a signal from the degraded oxygen sensor. In this case, the electronic control unit cannot perform correction control in response to catalyst degradation. During the period when correction control cannot be performed, there is a risk that the electronic control unit will not be able to respond to any deterioration in emissions performance (performance in reducing emissions of NOx, CO, and HC emitted from the tailpipe of the internal combustion engine). Therefore, to prevent erroneous diagnosis of catalyst degradation, it is necessary to accurately determine the amount of oxygen stored in the catalyst.

[0007] The present invention has been made in view of the above circumstances, and has as its object to accurately determine the amount of oxygen stored in a three-way catalyst. [Means for solving the problem]

[0008] The internal combustion engine control device according to the present invention controls an internal combustion engine that is equipped with an air-fuel ratio sensor that is disposed upstream of a three-way catalyst provided in an exhaust pipe and detects the air-fuel ratio of exhaust gas, and an oxygen concentration sensor that is disposed downstream of the three-way catalyst and detects the oxygen concentration of exhaust gas. Exclusion gas Air-fuel ratioan oxygen storage amount integrating unit that integrates the oxygen storage amount stored in the three-way catalyst during a period from an integration start position where the oxygen concentration starts to change from lean to rich or from rich to lean to an integration stop position where the oxygen concentration that has been increasing or decreasing since before the integration start position is reversed; an integration stop determination unit that determines an integration stop position; an oxygen flow rate estimation unit that estimates an oxygen flow rate of the exhaust gas based on the flow rate and air-fuel ratio of the exhaust gas; and an integration start determination unit that determines an integration start position based on the air-fuel ratio, wherein the oxygen storage amount integration unit integrates the oxygen storage amount based on the oxygen flow rate, the integration start position, and the integration stop position, and the oxygen storage amount integration unit detects when a voltage value output by the oxygen concentration sensor with a delay from the integration start position for a rich condition in which the air-fuel ratio of the exhaust gas changes from lean to rich and then starts to change from rich to lean becomes a voltage value that exceeds a voltage upper limit threshold due to a maximum electromotive force caused by the rich air-fuel ratio, The timing at which the voltage then decreases and enters the range between the upper and lower voltage thresholds, and the differential value of the voltage value at the time of the decrease exceeds the differential value threshold, is set as the integration stop position for the rich condition. The voltage value output by the oxygen concentration sensor, which lags behind the integration start position for the lean condition at which the air-fuel ratio of the exhaust gas changes from rich to lean and then begins to change from lean to rich, becomes a voltage value below the lower voltage threshold due to the minimum electromotive force caused by the lean air-fuel ratio, and then increases and enters the range between the upper and lower voltage thresholds, and the differential value of the voltage value at the time of the increase exceeds the differential value threshold, is set as the integration stop position for the lean condition. do. [Effects of the Invention]

[0009] According to the present invention, by accurately determining the amount of oxygen stored in the three-way catalyst, it is possible to prevent erroneous diagnosis of catalyst deterioration. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a schematic diagram illustrating the overall configuration of an internal combustion engine control system according to a first embodiment of the present invention. [Figure 2] 1 is a block diagram showing an example of the hardware configuration of an ECU according to a first embodiment of the present invention. [Figure 3] 1 is a block diagram showing an example of the internal configuration of an ECU according to a first embodiment of the present invention. [Figure 4A] 1 is a schematic diagram showing an example of the configuration of a post-processing system according to a first embodiment of the present invention. [Figure 4B] 3 is a diagram illustrating the relationship between the equivalence ratio of exhaust gas and the output of the air-fuel ratio sensor according to the first embodiment of the present invention. FIG. [Figure 4C] FIG. 3 is a diagram illustrating the relationship between the equivalence ratio of exhaust gas and the output of the rear oxygen sensor according to the first embodiment of the present invention. [Figure 5A] 1 is a diagram illustrating the tendency of the equivalence ratio of H2O (water), CO (carbon monoxide), CO2 (carbon dioxide), H2 (hydrogen), and O2 (oxygen) according to the first embodiment of the present invention. FIG. [Figure 5B] FIG. 2 is a diagram illustrating the tendency of HC (hydrocarbons) and NOx (nitrogen oxides) with respect to the equivalence ratio according to the first embodiment of the present invention. [Figure 6] FIG. 2 is a diagram illustrating the main reaction process of a three-way catalyst (ceria-based) used in the aftertreatment system according to the first embodiment of the present invention. [Figure 7]FIG. 3 is a diagram illustrating the tendency of the purification efficiency of a three-way catalyst with respect to the exhaust gas equivalence ratio at temperatures equal to or higher than the catalyst activation temperature according to the first embodiment of the present invention. [Figure 8] 3 is a diagram illustrating the catalyst upstream equivalence ratio, the catalyst downstream equivalence ratio, and the output behavior of the rear oxygen sensor 22 according to the first embodiment of the present invention. FIG. [Figure 9] FIG. 3 is a diagram illustrating the hysteresis of the output characteristics of the rear oxygen sensor according to the first embodiment of the present invention. [Figure 10] 3 is a diagram illustrating the output of the rear oxygen sensor and the changes over time in the NOx concentration and HC concentration downstream of the catalyst according to the first embodiment of the present invention. FIG. [Figure 11] FIG. 2 is a diagram illustrating the relationship between the degree of catalyst deterioration and the oxygen storage capacity of a three-way catalyst according to the first embodiment of the present invention. [Figure 12] FIG. 2 is a diagram illustrating the relationship between the oxygen storage ratio of the three-way catalyst according to the first embodiment of the present invention and the NOx purification efficiency of the three-way catalyst. [Figure 13] FIG. 10 is a diagram illustrating the results of a comparison of the catalyst upstream equivalence ratio, catalyst downstream equivalence ratio, oxygen storage ratio, and output behavior of the rear oxygen sensor when a new catalyst and a degraded catalyst according to the first embodiment of the present invention are used. [Figure 14A] FIG. 4 is a diagram showing a change in sensor voltage, which is the output of the rear oxygen sensor, when the rear oxygen sensor according to the first embodiment of the present invention is in a brand new state. [Figure 14B] FIG. 4 is a diagram showing a change in sensor voltage, which is the output of the rear oxygen sensor, when the rear oxygen sensor according to the first embodiment of the present invention is in a deteriorated state. [Figure 15] FIG. 4 is a diagram illustrating a method for eliminating the delay in the sensor voltage of the rear oxygen sensor in a deteriorated state according to the first embodiment of the present invention. [Figure 16] FIG. 4 is an explanatory diagram showing an example of timings for starting and stopping integration of the oxygen storage amount when the fuel injection amount decreases (from rich to lean) according to the first embodiment of the present invention. [Figure 17] 5 is an explanatory diagram showing an example of timings for starting and stopping accumulation of the oxygen storage amount when the fuel injection amount decreases (from lean to rich) according to the first embodiment of the present invention. FIG. [Figure 18] 4 is a flowchart showing how to stop integrating the oxygen storage amount according to the first embodiment of the present invention. [Figure 19A] FIG. 10 is a diagram showing an example of a diagnosis result of a catalyst state using a conventional technique. [Figure 19B] FIG. 3 is a diagram showing an example of a diagnosis result of a catalyst state using the technology according to the first embodiment of the present invention. [Figure 20] FIG. 10 is a block diagram showing an example of the configuration of an oxygen storage amount integrating unit according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functions or configurations are designated by the same reference numerals, and redundant description will be omitted.

[0012] [First embodiment] FIG. 1 is a schematic diagram of the overall configuration of an internal combustion engine control system 100. As shown in FIG.

[0013] The internal combustion engine control system 100 includes an internal combustion engine 1, a flow sensor 2, a turbocharger 3, an air bypass valve 4, an intercooler 5, a boost temperature sensor 6, a throttle valve 7, an intake manifold 8, a boost pressure sensor 9, a flow enhancement valve 10, an intake valve 11, an exhaust valve 13, a fuel injection valve 15, a spark plug 16, a knock sensor 17, a crank angle sensor 18, a wastegate valve 19, a front air-fuel ratio sensor 20, an exhaust purification catalyst (three-way catalyst) 21, a rear oxygen sensor 22, an EGR (Exhausted Gas Recirculation) pipe 23, an EGR cooler 24, an EGR valve 25, a temperature sensor 26, a differential pressure sensor 27, and an ECU (Engine Control Unit) 28. The rear oxygen sensor 22 can be replaced with an air-fuel ratio sensor.

[0014] An intake passage and an exhaust passage are connected to an internal combustion engine 1. The intake passage is fitted with a flow rate sensor 2 that detects the flow rate of intake air, and an intake temperature sensor built into the flow rate sensor 2. The intake temperature sensor measures the intake temperature. The turbocharger 3 is composed of a compressor 3a and a turbine 3b, with the compressor 3a connected to the intake passage and the turbine 3b connected to the exhaust passage. The turbine 3b of the turbocharger 3 converts the energy of the exhaust gas from the internal combustion engine 1 into the rotational energy of the turbine blades. The compressor 3a of the turbocharger 3 compresses the intake air that flows in from the intake passage by the rotation of the compressor blades connected to the turbine blades.

[0015] The intercooler 5 is provided downstream of the compressor 3a of the turbocharger 3, and cools the intake air temperature that has been increased by adiabatic compression by the compressor 3a. The supercharger temperature sensor 6 is mounted downstream of the intercooler 5, and measures the temperature of the intake air cooled by the intercooler 5 (supercharger temperature).

[0016] The throttle valve 7 is provided downstream of the intercooler 5 and narrows the intake passage to control the amount of intake air flowing into the cylinders of the internal combustion engine 1. The throttle valve 7 is configured as an electronically controlled butterfly valve whose valve opening can be controlled independently of the accelerator pedal depression amount by the driver. Downstream of the throttle valve 7 is connected to an intake manifold 8 equipped with a boost pressure sensor 9.

[0017] The intake manifold 8 provided downstream of the throttle valve 7 may be integrated with the intercooler 5. In this case, the volume from downstream of the compressor 3a to the cylinder can be reduced, which improves acceleration / deceleration response and controllability.

[0018] The flow enhancement valve 10 is disposed downstream of the intake manifold 8 and enhances the turbulence of the flow inside the cylinder by causing a bias in the intake air drawn into the cylinder. When exhaust gas recirculation combustion, which will be described later, is performed, the flow enhancement valve is closed to promote and stabilize turbulent combustion. The internal combustion engine 1 is equipped with an intake valve 11 and an exhaust valve 13.

[0019] The intake valve 11 and the exhaust valve 13 each have a variable valve mechanism for continuously varying the valve opening / closing phase. Sensors 12 and 14 for detecting the valve opening / closing phase are attached to the variable valve mechanisms of the intake valve 11 and the exhaust valve 13, respectively. Each cylinder of the internal combustion engine 1 is equipped with a direct injection fuel injection valve 15 that injects fuel directly into the cylinder. Note that the fuel injection valve 15 may also be a port injection type that injects fuel into the intake port.

[0020] Spark plugs 16, which have electrodes exposed inside the cylinders and generate sparks to ignite a combustible mixture, are installed in the cylinders of the internal combustion engine 1. Knock sensors 17 are installed in the cylinder block and detect the presence or absence of knock by detecting cylinder block vibrations caused by combustion pressure vibrations generated in the combustion chamber. Crank angle sensor 18 is installed on the crankshaft and outputs a signal corresponding to the rotation angle of the crankshaft to ECU 28 as a signal indicating the rotation speed.

[0021] The front air-fuel ratio sensor 20 is disposed downstream of the turbine 3b of the turbocharger 3 and upstream of a three-way catalyst (exhaust purification catalyst 21) provided in the exhaust pipe. The air-fuel ratio sensor (front air-fuel ratio sensor 20) outputs a signal indicating the exhaust gas composition detected from the exhaust gas, i.e., the air-fuel ratio, to the ECU 28. The exhaust purification catalyst 21 is a three-way catalyst, and is provided downstream of the front air-fuel ratio sensor 20. The exhaust purification catalyst 21 purifies harmful exhaust gas components such as carbon monoxide, nitrogen compounds, and unburned hydrocarbons in the exhaust gas through catalytic reactions. A rear oxygen sensor 22 is disposed downstream of the exhaust purification catalyst 21. The rear oxygen sensor 22 detects the presence or absence of oxygen in the exhaust gas after purification by the exhaust purification catalyst 21, i.e., the oxygen concentration of the exhaust gas. Hereinafter, the exhaust purification catalyst 21 will also be referred to as a three-way catalyst or a catalyst.

[0022] The turbocharger 3 is equipped with an air bypass valve 4 and a wastegate valve 19. The air bypass valve 4 is arranged in a bypass flow path connecting the upstream and downstream of the compressor 3a to prevent an excessive increase in pressure from the downstream of the compressor 3a to the upstream of the throttle valve 7. If the throttle valve 7 is suddenly closed in a supercharging state, the air bypass valve 4 is opened under the control of the ECU 28, causing the compressed intake air downstream of the compressor 3a to flow back through the bypass flow path to the upstream of the compressor 3a. As a result, the supercharging pressure is immediately reduced, preventing a phenomenon known as surging and appropriately preventing damage to the compressor 3a.

[0023] The wastegate valve 19 is disposed in a bypass flow path connecting the upstream and downstream of the turbine 3b. The wastegate valve 19 is an electrically operated valve whose valve opening can be freely controlled according to the boost pressure under the control of the ECU 28. When the ECU 28 adjusts the opening of the wastegate valve 19 based on the boost pressure detected by the boost pressure sensor 9, part of the exhaust gas passes through the bypass flow path, thereby reducing the work that the exhaust gas imparts to the turbine 3b. As a result, the wastegate valve 19 can maintain the boost pressure at the target pressure.

[0024] The EGR pipe 23 connects the exhaust flow path downstream of the exhaust purification catalyst 21 with the intake flow path upstream of the compressor 3a, and diverts exhaust gas from downstream of the exhaust purification catalyst 21 and recirculates it upstream of the compressor 3a. An EGR cooler 24 provided in the EGR pipe 23 cools the exhaust gas. An EGR valve 25 is provided downstream of the EGR cooler 24 and controls the flow rate of the exhaust gas. The EGR pipe 23 is provided with a temperature sensor 26 that detects the temperature of the exhaust gas upstream of the EGR valve 25, and a differential pressure sensor 27 that detects the differential pressure between the upstream and downstream of the EGR valve 25.

[0025] The ECU 28 is an arithmetic circuit that includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), an A / D (Analog-to-Digital) converter, a driver circuit, etc., and controls the various components of the internal combustion engine control system 100 and executes various data processing. The various sensors and actuators described above are connected to the ECU 28. The ECU 28 controls the operation of actuators such as the throttle valve 7, the intake valve 11, the exhaust valve 13, the fuel injection valve 15, and the EGR valve 25. The ECU 28 also detects the operating state of the internal combustion engine 1 based on signals input from the various sensors, and ignites the spark plug 16 at a timing determined according to the operating state.

[0026] [Example of hardware configuration for an internal combustion engine control device] FIG. 2 is a block diagram showing an example of the hardware configuration of the ECU 28.

[0027] The ECU 28 includes an input circuit 201 , an input / output port 202 , a RAM 203 , a ROM 204 , a CPU 205 , a throttle valve drive circuit 206 , a fuel injection valve drive circuit 207 , and an ignition output circuit 208 .

[0028] Output signals from various sensors are input to the input circuit 201. In FIG. 2, an example is shown in which output signals such as a throttle sensor of the throttle valve 7, a flow rate sensor 2, a supercharging temperature sensor 6, a supercharging pressure sensor 9, valve opening / closing phase sensors 12 and 14, a knock sensor 17, a crank angle sensor 18, a front air-fuel ratio sensor 20, and a rear oxygen sensor 22 are input to the input circuit 201 of the ECU 28. The signals input to the input circuit 201 are transmitted to the input / output port 202.

[0029] The signals transmitted to the input / output port 202 are stored in the RAM 203 and processed by the CPU 205. A control program describing the content of the arithmetic processing is pre-written in the ROM 204 and executed by the CPU 205. The ROM 204 records programs, data, etc. necessary for the operation of the CPU 205, and is used as an example of a computer-readable non-transitory storage medium storing programs executed by the ECU 28.

[0030] The control signals calculated by the CPU 205 according to the control program are output to each device such as the throttle valve drive circuit 206, the combustion injection valve drive circuit 207, and the ignition output circuit 208. The throttle valve drive circuit 206 outputs a drive signal for controlling the opening and closing drive of the throttle valve 7 to the throttle valve 7. The combustion injection valve drive circuit 207 outputs a drive signal for controlling the opening and closing drive of the fuel injection valve 15 at the fuel injection timing to the fuel injection valve 15. The ignition output circuit 208 outputs a drive signal for controlling the ignition of the spark plug 6 at the ignition timing to the ignition output circuit 208.

[0031] <Example of the internal configuration of the ECU> FIG. 3 is a block diagram showing an example of the internal configuration of the ECU 28. Although the ECU 28 has various functions, here, the function of the ECU 28 to obtain a target air-fuel ratio based on the air-fuel ratio sensor signal and the rear oxygen sensor signal will be described.

[0032] The ECU 28 includes an integration stop determination unit 120 , an oxygen flow rate estimation unit 131 , an integration start determination unit 132 , an oxygen storage amount integration unit 133 , a catalyst deterioration diagnosis unit 134 , and an air-fuel ratio correction unit 135 .

[0033] First, an example of the internal configuration of the accumulation stop determination unit 120 will be described. The integration stop determination unit (integration stop determination unit 120) determines the integration stop position in the integration process of the oxygen storage amount. This integration stop determination unit 120 includes a differential value determination unit 121, a voltage value determination unit 122, and a condition determination unit 123.

[0034] The differential value determination unit 121 determines the differential value of the rear oxygen sensor voltage and includes a differentiator 121a and a determination unit 121b. The differentiator 121a calculates a differential value of the rear oxygen sensor voltage based on the rear oxygen sensor voltage signal input from the rear oxygen sensor 22. The differential value has a positive or negative sign depending on the direction of change in the voltage value output by the oxygen concentration sensor (rear oxygen sensor 22). The positive or negative sign of the differential value threshold differs depending on whether the air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio.

[0035] The determination unit 121b compares the differential value of the rear oxygen sensor voltage with a differential value threshold, and detects the timing when the differential value of the rear oxygen sensor voltage becomes larger than the differential value threshold as the rising timing (ON) of the differential value. Therefore, the determination unit 121b converts the differential value of the rear oxygen sensor voltage and the differential value threshold into absolute values. The rising timing of the differential value is output to the condition determination unit 123.

[0036] The voltage value determination unit 122 detects whether the rear oxygen sensor voltage is within a predetermined range, and includes a voltage lower limit determination unit 122a, a voltage upper limit determination unit 122b, and a voltage range determination unit 122c.

[0037] The voltage lower limit determination unit 122a compares the rear oxygen sensor voltage with a voltage lower limit threshold based on the rear oxygen sensor voltage signal input from the rear oxygen sensor 22. If the rear oxygen sensor voltage is equal to or greater than the voltage lower limit threshold, the voltage lower limit determination unit 122a outputs an ON signal to the voltage range determination unit 122c.

[0038] The voltage upper limit determination unit 122b compares the rear oxygen sensor voltage with the voltage upper limit threshold based on the rear oxygen sensor voltage signal input from the rear oxygen sensor 22. If the rear oxygen sensor voltage is less than the voltage upper limit threshold, the voltage upper limit determination unit 122b outputs an ON signal to the voltage range determination unit 122c.

[0039] When the voltage range determining unit 122c receives an ON signal from both the lower voltage limit determining unit 122a and the upper voltage limit determining unit 122b (AND condition), the voltage range determining unit 122c outputs an in-voltage range signal (ON signal) to the condition determining unit 123.

[0040] When an ON signal is input from both the differential value determination unit 121 and the voltage value determination unit 122 (AND condition), the condition determination unit 123 outputs an integration stop signal to the oxygen storage amount integration unit 133. The integration stop signal is a signal that instructs the oxygen storage amount integration unit 133 to stop integrating the oxygen storage amount, and indicates the timing to stop integrating the oxygen storage amount.

[0041] Next, an example of the configuration of functional units other than the integration stop determination unit 120 will be described. The oxygen flow rate estimation unit (oxygen flow rate estimation unit 131) estimates the oxygen flow rate of the exhaust gas based on the flow rate of the exhaust gas and the air-fuel ratio calculated from the air-fuel ratio sensor signal. The estimated oxygen flow rate is output to the oxygen storage amount integration unit 133. An accumulation start determination unit (accumulation start determination unit 132) determines the start of accumulation of the oxygen storage amount based on the air-fuel ratio calculated from the air-fuel ratio sensor signal. Then, the accumulation start determination unit 132 outputs an accumulation start signal to instruct the oxygen storage amount accumulation unit 133 to start accumulating the oxygen storage amount.

[0042] The oxygen storage amount integrating section (oxygen storage amount integrating section 133) integrates the amount of oxygen stored in the three-way catalyst (exhaust purification catalyst 21) during the period from the integration start position to the integration stop position. The integration start position is indicated by an integration start signal. The integration stop position is indicated by an integration stop signal. The integration start position is determined based on the amount of oxygen detected by the front air-fuel ratio sensor 20. Exhaust gas air fuel The ratio is The integration stop position is the timing when the oxygen concentration starts to change from lean to rich or from rich to lean. The integration stop position is the timing when the oxygen concentration, which has been increasing or decreasing since before the integration start position, reverses. Therefore, when the integration start signal is input, the oxygen storage amount integration unit 133 starts integrating the oxygen storage amount based on the oxygen flow rate, and when the integration stop signal is input, it stops integrating the oxygen storage amount. In this way, the oxygen storage amount integration unit (oxygen storage amount integration unit 133) integrates the oxygen storage amount based on the oxygen flow rate, the integration start position, and the integration stop position. This allows the oxygen storage amount integration unit 133 to accurately determine the oxygen storage amount. Then, the oxygen storage amount integration unit 133 outputs the integrated oxygen storage amount to the catalyst deterioration diagnosis unit 134 as an estimated oxygen storage amount.

[0043] The catalyst diagnosis unit (catalyst deterioration diagnosis unit 134) diagnoses the state of the three-way catalyst (exhaust purification catalyst 21) based on the oxygen storage amount (estimated oxygen storage amount) and outputs the diagnosis result. The diagnosis result of the deterioration diagnosis of the three-way catalyst is output to the air-fuel ratio correction unit 135. Here, the diagnosis result is expressed as a state level value assigned to each state of the three-way catalyst (exhaust purification catalyst 21). This state level value indicates the deterioration state of the three-way catalyst. For example, if the three-way catalyst is new, it is not deteriorated and a state level value of "1" is assigned, and if it is deteriorated, a state level value of "2" is assigned. Alternatively, the deterioration state may be further subdivided so that if the three-way catalyst is not deteriorated, a state level value of "21" is assigned, and if it is deteriorated, a state level value of "22" is assigned. The state level value may also be expressed using alphabets or a combination of alphanumeric characters.

[0044] The air-fuel ratio correction unit (air-fuel ratio correction unit 135) corrects the target air-fuel ratio of the internal combustion engine (internal combustion engine 1) according to the air-fuel ratio target value and the diagnosis result, and outputs the corrected target air-fuel ratio. Then, the air-fuel ratio correction unit 135 outputs the calculated target value of the air-fuel ratio as the corrected target air-fuel ratio. Because the diagnosis result is expressed as a state level value, the air-fuel ratio target value correction unit 135 can speed up the correction process of the target air-fuel ratio by previously creating a table of correction amounts of the target air-fuel ratio according to the state level value. Thereafter, the ECU 28 controls the output of the internal combustion engine 1 in accordance with the corrected target air-fuel ratio.

[0045] <Example of aftertreatment system configuration> Next, a configuration example of an aftertreatment system 110 that purifies exhaust gas from an internal combustion engine will be described with reference to FIGS. 4A to 4C.

[0046] FIG. 4A is a schematic diagram showing an example of the configuration of the post-processing system 110. As shown in FIG. As described above, the aftertreatment system 110 uses a three-way catalyst as the exhaust purification catalyst 21. In order to maintain the purification efficiency of the three-way catalyst at an optimum point, sensors for detecting the exhaust gas composition are provided upstream and downstream of the three-way catalyst. For example, the aftertreatment system 110 is provided with a front air-fuel ratio sensor 20 at its upstream portion (front) and a rear oxygen sensor 22 at its downstream portion (rear). With this configuration, the front air-fuel ratio sensor 20 can measure the air-fuel ratio of the exhaust gas flowing into the three-way catalyst. Furthermore, the rear oxygen sensor 22 can detect the presence or absence of oxygen in the exhaust gas after purification by the catalyst.

[0047] 4B is a diagram illustrating the relationship between the equivalence ratio of exhaust gas (=stoichiometric air-fuel ratio / air-fuel ratio) and the output of the front air-fuel ratio sensor 20. The horizontal axis of FIG.

[0048] The output (air-fuel ratio sensor output) of the front air-fuel ratio sensor 20 tends to decrease as the equivalence ratio increases (in other words, as the exhaust gas becomes richer). By previously acquiring the relationship between the equivalence ratio and the air-fuel ratio sensor output shown in Fig. 4B, the ECU 28 can accurately detect the equivalence ratio over a wide range from a lean state to a rich state of the exhaust gas.

[0049] 4C is a diagram illustrating the relationship between the equivalence ratio of exhaust gas and the output of the rear oxygen sensor 22. The horizontal axis of FIG. 4C represents the equivalence ratio, and the vertical axis represents the rear oxygen sensor output.

[0050] The rear oxygen sensor output, also known as the rear oxygen sensor voltage, varies depending on the electromotive force caused by the difference between the oxygen concentration in the exhaust gas and the oxygen concentration in the air. The rear oxygen sensor output exhibits a minimum electromotive force under lean conditions and a maximum electromotive force under rich conditions. Therefore, under catalytic control, the rear oxygen sensor output exhibits a sudden change at the stoichiometric air-fuel ratio (equivalence ratio of 1.0). By detecting the timing of changes in the rear oxygen sensor output and feeding this timing back to the air-fuel ratio control, the ECU 28 can maintain the exhaust gas equivalence ratio near the stoichiometric air-fuel ratio.

[0051] <Concentration of chemical species in exhaust gas> 5A and 5B are diagrams illustrating the trends of the concentrations of chemical species in the exhaust gas with respect to the equivalence ratio. FIG. 5A is a diagram illustrating the trend with respect to the equivalence ratio of H2O (water), CO (carbon monoxide), CO2 (carbon dioxide), H2 (hydrogen), and O2 (oxygen). 5A and 5B are graphs illustrating the trends of HC (hydrocarbons) and NOx (nitrogen oxides) relative to the equivalence ratio. The horizontal axis of each graph represents the equivalence ratio, and the vertical axis represents the concentration of the chemical species in the exhaust gas.

[0052] As shown in Figure 5A, the combustion gas composition of hydrocarbon fuels tends to increase CO (carbon monoxide) and H2 (hydrogen) on the rich side of the stoichiometric air-fuel ratio, and increase O2 (oxygen) on the lean side. On the other hand, as shown in Figure 5B, NOx (nitrogen oxides) reaches a maximum value slightly leaner than the stoichiometric air-fuel ratio, and tends to decrease on both the leaner and richer sides.

[0053] 5A and 5B, even under theoretical air-fuel ratio conditions where fuel and air (oxygen) are supplied in the right amounts, a certain amount of CO (carbon monoxide) and NOx (nitrogen oxides) is emitted in the high-temperature combustion gas without converting to HO (water) or CO2 (carbon dioxide). For this reason, it is necessary to properly purify the exhaust gas using the aftertreatment system 110.

[0054] <Three-way catalyst reaction process> 6 is a diagram illustrating the main reaction process of the three-way catalyst (ceria-based) used in the aftertreatment system 110. In the reaction process shown in FIG. 6, coefficients are omitted.

[0055] The three-way catalytic reaction process mainly consists of an oxidation reaction, a NOx reduction reaction, and an oxygen storage and release reaction. In the oxidation reaction, CO, H2, and HC, which are produced under rich or high-temperature conditions, react with oxygen to produce harmless CO2 and H2O. Unburned hydrocarbons (HC) include components such as methane, propane, ethylene, and butane, and each reaction proceeds at a different rate.

[0056] The NOx reduction reaction is mainly represented by the reaction of CO and NO, producing harmless CO2 and N2. In the oxygen storage and release reaction, the storage and release of oxygen (O2) and the oxidation and reduction reactions of HC, CO, and NO proceed via the catalytic material Ce (cerium). That is, the reaction between cerium dioxide (CeO2) and CO and HC produces harmless CO2 and H2O, while the reaction between cerium trioxide (Ce2O3) and NO produces harmless N2. The oxygen storage ratio Ψ of the three-way catalyst is determined by the balance between the CeO2 and Ce2O3 that are produced at the same time. In other words, if all the Ce2O3 in the catalyst becomes CeO2, it cannot react with NO, and NO cannot be purified.

[0057] Thus, in order to maintain the purification efficiency of the three-way catalyst appropriately, it is necessary to maintain the balance between CeO2 and Ce2O3, i.e., the oxygen storage ratio Ψ, at a predetermined value. Since all of the above-mentioned reaction processes are strongly dependent on the catalyst temperature, the ECU 28 needs to properly manage the catalyst temperature so that the temperature reaches or exceeds the activation temperature as soon as possible after the internal combustion engine 1 is started.

[0058] Although the system shown in this embodiment is configured to use a ceria-based three-way catalyst, the present invention is not limited to this. Even with catalysts using other materials that exhibit similar effects, the same effects can be achieved without changing the configuration of the present invention by adjusting the constants of the control model. In addition to the reaction process shown in FIG. 6, the catalytic reaction may involve a water-gas shift reaction or the like. The ECU 28 can also accommodate these reaction processes by adjusting the control model constants.

[0059] Figure 7 is a diagram illustrating the tendency of the purification efficiency of a three-way catalyst versus the exhaust gas equivalence ratio at temperatures above the catalyst activation temperature. The horizontal axis of Figure 7 represents the equivalence ratio, and the vertical axis represents the catalyst purification rate. Figure 7 also shows the catalyst purification rates for NOx, HC, and CO when the equivalence ratio changes from lean to rich. The closer the catalyst purification efficiency is to 100%, the more the components in the exhaust gas are purified and not emitted.

[0060] This graph shows that the purification efficiency characteristics of a three-way catalyst change around the stoichiometric air-fuel ratio. Under lean conditions, the purification efficiency of CO and HC is maintained at approximately 90% or higher. On the other hand, as the equivalence ratio decreases, the purification efficiency of NOx decreases. Also, on the rich side, the purification efficiency of HC and CO tends to decrease as the equivalence ratio increases. On the other hand, as the equivalence ratio increases, the purification efficiency of NOx decreases. Near the stoichiometric air-fuel ratio, purification efficiencies of 90% or higher for NOx, HC, and CO can all be achieved. Therefore, the point near the stoichiometric air-fuel ratio is called the "three-way point." The ECU 28 controls the purification efficiency of the three-way catalyst to maintain a high level by maintaining the equivalence ratio at the stoichiometric air-fuel ratio, which is the three-way point.

[0061] Figure 8 is a diagram illustrating the behavior of the catalyst upstream equivalence ratio, catalyst downstream equivalence ratio, and output of the rear oxygen sensor 22. The horizontal axis of Figure 8 represents time, and the vertical axis represents the catalyst upstream equivalence ratio, catalyst downstream equivalence ratio, and rear oxygen sensor output. This diagram shows the behavior of the catalyst upstream and downstream equivalence ratios and the output of the rear oxygen sensor 22 installed downstream of the catalyst when the air-fuel ratio is changed stepwise over time from the lean side to the rich side around an equivalence ratio of 1.0.

[0062] 8, the equivalence ratios upstream and downstream of the catalyst are 1.0, and the output of the rear oxygen sensor 22 is a constant value. Even when the equivalence ratio is set to the stoichiometric air-fuel ratio, a very small amount of oxygen is discharged downstream of the catalyst, thereby maintaining the intermediate state.

[0063] In step (2) of Figure 8, the catalyst upstream and downstream equivalence ratios fall below 1.0 due to the lean fuel mixture. While the catalyst upstream equivalence ratio immediately falls, the catalyst downstream equivalence ratio falls gradually with a delay. Furthermore, the output of the rear oxygen sensor 22 suddenly falls to its minimum value after a delay.

[0064] In step (3) of Figure 8, the catalyst upstream and downstream equivalence ratios increase from 1.0 due to the rich side shift. Here, the catalyst upstream equivalence ratio increases immediately, while the catalyst downstream equivalence ratio increases gradually with a delay. Furthermore, the output of the rear oxygen sensor 22 exhibits hysteresis 31, changing suddenly after a delay greater than that of the catalyst downstream equivalence ratio.

[0065] In step (4) of Figure 8, as in step (2), the catalyst upstream and downstream equivalence ratios fall below 1.0 due to a lean shift. Here, the catalyst upstream equivalence ratio falls immediately, while the catalyst downstream equivalence ratio falls gradually with a delay. The output of the rear oxygen sensor 22 also exhibits hysteresis 32, changing suddenly after a delay greater than the catalyst downstream equivalence ratio.

[0066] As described above, the delay time of the catalyst downstream equivalence ratio and the delay time of the output of the rear oxygen sensor 22 tend to differ when the mixture changes from lean to rich and when it changes from rich to lean. This tendency is due to the different reaction rates of CeO2 and Ce2O3 in the three-way catalyst, as described in Figure 6. Furthermore, since the reaction rate depends on the catalyst temperature and exhaust gas flow rate, the hysteresis characteristics 31 and 32 described above also change depending on the catalyst temperature and exhaust gas flow rate.

[0067] 9 is a diagram illustrating the hysteresis of the output characteristic of the rear oxygen sensor 22. In this graph, the horizontal axis represents the equivalence ratio, and the vertical axis represents the output of the rear oxygen sensor 22.

[0068] The static characteristics of the rear oxygen sensor 22 have been described with reference to Figure 4C. However, because the rear oxygen sensor 22 itself uses a catalytic material, it has hysteresis due to a delay in oxygen detection. That is, when the air condition changes suddenly from lean to rich, the equivalence ratio of the timing of the change in sensor output shifts to the rich side. On the other hand, when the air condition changes suddenly from rich to lean, the equivalence ratio of the timing of the change in sensor output shifts to the lean side.

[0069] Furthermore, the behavior of the rear oxygen sensor 22 described above is affected by temperature and deterioration of the characteristics of the materials that make up the rear oxygen sensor 22. For example, a delay occurs in the output of the rear oxygen sensor due to aging deterioration of the rear oxygen sensor 22. The control model must take into account not only catalyst deterioration but also changes in dynamic characteristics including the effects of temperature and deterioration of the rear oxygen sensor 22.

[0070] Figure 10 is a diagram illustrating the change over time in the output of the rear oxygen sensor 22 and the NOx and HC concentrations downstream of the catalyst. The horizontal axis of Figure 10 represents time, and the vertical axis represents the catalyst upstream equivalence ratio, the rear oxygen sensor output, the NOx concentration downstream of the catalyst, and the HC concentration downstream of the catalyst. Figure 10 shows the change over time in the output of the rear oxygen sensor 22 and the NOx and HC concentrations downstream of the catalyst when the engine goes from a state controlled at the stoichiometric air-fuel ratio to a period of motoring operation (fuel cut), and then returns to firing operation (internal combustion engine operation) at the stoichiometric air-fuel ratio.

[0071] 10, the equivalence ratio upstream of the catalyst is 1.0, the output of the rear oxygen sensor 22 is a constant value, and the NOx concentration and HC concentration downstream of the catalyst are low.

[0072] In step (2) of Figure 10, fuel cut is performed, so the output of the rear oxygen sensor 22 decreases, and the equivalence ratios upstream and downstream of the catalyst fall below 1.0. As shown in Figure 5B, in a lean condition, the NOx concentration is low. Also, as shown in Figure 7, in a lean condition, the catalyst purification rate of HC is high, so the HC concentration decreases.

[0073] In step (3) of Figure 10, firing operation at the stoichiometric air-fuel ratio is started again. Here, we will explain the temporal changes in the output of the rear oxygen sensor 22 and the NOx and HC concentrations downstream of the catalyst in the order of stoichiometry control, appropriate rich correction, and excessive rich correction.

[0074] (Stoichiometry control) The solid lines show the output of the rear oxygen sensor 22 and the changes over time in the NOx and HC concentrations downstream of the catalyst when stoichiometry control is performed. As shown in Figure 8, the output of the rear oxygen sensor 22 increases with a delay when the engine changes from a lean to a rich condition. The NOx concentration downstream of the catalyst exhibits a behavior in which a momentary increase in NOx occurs during the delay period until the output of the rear oxygen sensor 22 returns to normal, resulting in the emission of NOx. On the other hand, the HC concentration downstream of the catalyst remains almost unchanged, preventing the emission of HC.

[0075] (appropriate rich correction) The dashed lines show the output of the rear oxygen sensor 22 and the changes over time in the NOx and HC concentrations downstream of the catalyst when appropriate rich correction is performed. When firing operation is restarted after a fuel cut, once rich correction is performed, the equivalence ratio upstream of the catalyst becomes rich. The output of the rear oxygen sensor 22 reaches the equivalence ratio more quickly than under stoichiometry control. Furthermore, because the NOx and HC concentrations downstream of the catalyst remain almost unchanged, NOx and HC emissions are prevented.

[0076] (Excessive rich correction) The rough dashed lines show the output of the rear oxygen sensor 22 and the changes over time in the NOx and HC concentrations downstream of the catalyst when excessive rich correction is made. The rear oxygen sensor 22 detects the oxygen content of the gas downstream of the catalyst. Therefore, when excessive rich correction is made, the internal state of the catalyst has already changed to the maximum or minimum oxygen storage state by the time the output of the rear oxygen sensor 22 reacts. Furthermore, when excessive rich correction is made, the NOx concentration decreases as shown in Figure 5B, and the catalyst purification rate of NOx increases as shown in Figure 7, preventing NOx emissions.

[0077] On the other hand, the HC concentration increases as shown in Figure 5B, and the catalyst purification rate of HC decreases as shown in Figure 7, so HC is emitted. In other words, if a control method is used in which the rich correction is stopped after the output of the rear oxygen sensor 22 reacts, the timing to stop the rich correction is too late for the catalyst, and HC cannot be properly prevented. For this reason, in air-fuel ratio control for an internal combustion engine, it is necessary to perform rich correction control for an appropriate period of time, taking into account the state inside the catalyst, which cannot be directly observed from the outside.

[0078] 11 is a diagram illustrating the relationship between the degree of catalyst deterioration and the oxygen storage capacity of a three-way catalyst, where the horizontal axis represents the degree of catalyst deterioration and the vertical axis represents the oxygen storage capacity.

[0079] Catalyst degradation refers to a state in which the catalytic activity of a three-way catalyst is reduced due to thermal effects or poisoning from sulfur contained in the fuel. When a three-way catalyst is new, it has a high oxygen storage capacity. However, as catalyst degradation progresses, the oxygen storage capacity of the three-way catalyst tends to decrease.

[0080] The following describes the effect that changes in oxygen storage capacity have on the purification function of a three-way catalyst. Figure 12 is a diagram illustrating the relationship between the oxygen storage ratio of a three-way catalyst and its NOx conversion efficiency. The horizontal axis of Figure 12 represents the oxygen storage ratio, and the vertical axis represents the NOx conversion efficiency. In Figure 12, the solid line represents the change in the NOx conversion efficiency of a new three-way catalyst, and the dashed line represents the change in the NOx conversion efficiency of a deteriorated three-way catalyst.

[0081] When the oxygen storage ratio is low, both new and deteriorated three-way catalysts have high oxygen storage ratios. Also, new three-way catalysts have a higher NOx reduction efficiency than deteriorated three-way catalysts.

[0082] When the oxygen storage ratio exceeds a predetermined value, the NOx purification efficiency of both a new and a deteriorated three-way catalyst drops significantly. This is because, as explained with reference to FIG. 6, Ce2O3 in the catalyst is important for NOx purification. If all Ce2O3 reacts and converts to CeO2, Ce2O3 cannot react with NO, and NO is not purified. Therefore, as explained with reference to FIG. 7, in order to maintain high catalytic conversion efficiency of a three-way catalyst, it is necessary not only to maintain the exhaust gas air-fuel ratio at the catalyst inlet at the three-way point, but also to appropriately correct and control the exhaust gas air-fuel ratio at the catalyst inlet so that the oxygen storage ratio falls within a predetermined range. As shown in FIG. 12, the predetermined range of the oxygen storage ratio is represented by a deteriorated catalyst control range 36 and a new catalyst control range 35, with the new catalyst control range 35 being wider than the deteriorated catalyst control range 36.

[0083] Figure 13 is a diagram illustrating the results of a comparison of the catalyst upstream equivalence ratio, catalyst downstream equivalence ratio, oxygen storage fraction, and output behavior of the rear oxygen sensor 22 when a new catalyst and a degraded catalyst are used. In Figure 13, the horizontal axis represents time, and the vertical axis represents the catalyst upstream equivalence ratio, catalyst downstream equivalence ratio, oxygen storage fraction, and rear oxygen sensor output. Figure 13 shows the output behavior of the rear oxygen sensor 22 installed downstream of the catalyst when the air-fuel ratio is varied over time in steps toward the leaner and richer sides, with an equivalence ratio of 1.0 as the center, when using a new catalyst and a degraded catalyst. The dashed line in the diagram represents the degraded catalyst, and the solid line represents the new catalyst.

[0084] 13, the equivalence ratios upstream and downstream of the catalyst are 1.0, and the output of the rear oxygen sensor 22 is a constant value. Even when the equivalence ratio is set to the stoichiometric air-fuel ratio, a very small amount of oxygen is discharged downstream of the catalyst, thereby maintaining the intermediate state.

[0085] In step (2) of FIG. 13, the catalyst upstream and downstream equivalence ratios fall below 1.0 due to a shift to the lean side. Here, the catalyst upstream equivalence ratio falls immediately, while the catalyst downstream equivalence ratio falls gradually and with a delay. Furthermore, because a new catalyst has a large oxygen storage capacity, the change in the catalyst downstream equivalence ratio of the new catalyst lags behind the change in the catalyst downstream equivalence ratio of a degraded catalyst. Furthermore, because a degraded catalyst has a small oxygen storage capacity, the oxygen storage ratio of the degraded catalyst saturates quickly, while it takes time for the oxygen storage ratio of the new catalyst to saturate. For this reason, the output of the rear oxygen sensor 22 changes more quickly for a degraded catalyst than for a new catalyst.

[0086] In step (3) of Figure 13, the catalyst upstream and downstream equivalence ratios increase from 1.0 due to the shift to the rich side. Here, the catalyst upstream equivalence ratio increases immediately, while the catalyst downstream equivalence ratio increases gradually with a delay. Furthermore, the change in the equivalence ratio of the new catalyst lags behind the change in the equivalence ratio of the deteriorated catalyst.

[0087] In step (4) of Figure 13, as in step (2), the catalyst upstream and downstream equivalence ratios fall below 1.0 due to a shift to the lean side. Here, the catalyst upstream equivalence ratio falls immediately, while the catalyst downstream equivalence ratio falls gradually with a delay. Furthermore, the oxygen storage fraction and the output of the rear oxygen sensor 22 change more slowly with a new catalyst than with a deteriorated catalyst.

[0088] As shown in Figure 13, a deteriorated catalyst reduces the delay in the output behavior of the rear oxygen sensor 22 in response to lean and rich air-fuel ratio changes compared to a new catalyst. This can be explained by the change in the oxygen storage ratio over time in the catalyst. That is, as the oxygen storage capacity of the catalyst decreases due to deterioration, the oxygen storage ratio reaches its maximum or minimum value more quickly, which speeds up the release of oxygen downstream of the catalyst and reduces the delay in the output behavior of the rear oxygen sensor 22. Therefore, the rich correction period after returning from a fuel cut, as described with reference to Figure 10, needs to be set taking into account the deteriorated state of the catalyst.

[0089] 14A and 14B are explanatory diagrams showing the influence of deterioration of the rear oxygen sensor 22 on catalyst diagnosis.

[0090] The estimated oxygen storage amount of a deteriorated catalyst starts to be accumulated when the air-fuel ratio switches from lean to rich (the Start position in the figure), and stops to be accumulated when the air-fuel ratio switches from lean to rich (the Reset position in the figure).

[0091] 14A is a diagram showing changes in the rear oxygen sensor voltage when the rear oxygen sensor 22 is in a brand new condition. The rear oxygen sensor 22 outputs a rear oxygen sensor voltage that corresponds to the catalyst deterioration state, and because the rear oxygen sensor voltage returns to its original value at the Reset position, catalyst diagnosis is performed correctly.

[0092] FIG. 14B shows changes in rear oxygen sensor voltage when the rear oxygen sensor 22 is in a deteriorated state. The rear oxygen sensor 22 has a delayed rise in rear oxygen sensor voltage. Therefore, although the Reset position shown in FIG. 14A should be the timing for stopping the accumulation of oxygen storage capacity, the rear oxygen sensor voltage returns to its original value at the Reset position, which is later than the Stop position, as shown by the delay 41 due to sensor deterioration in FIG. 14B. This delays the timing for stopping the accumulation of oxygen storage capacity, resulting in an excessive oxygen storage capacity being calculated. As a result, even though the catalyst is in a deteriorated state, the catalyst diagnosis may determine that it can store a large amount of oxygen, resulting in an erroneous diagnosis that the catalyst is in new condition.

[0093] Fig. 15 is a diagram illustrating a method for eliminating the delay in the rear oxygen sensor voltage when the rear oxygen sensor 22 is in a deteriorated state. The horizontal axis of Fig. 15 represents time, and the vertical axis represents the catalyst upstream equivalence ratio and the rear oxygen sensor output. Fig. 15 shows a method for eliminating the delay in the rear oxygen sensor voltage output by a deteriorated rear oxygen sensor 22, utilizing the hysteresis characteristics of the rear oxygen sensor 22.

[0094] The upper part of Fig. 15 shows the catalyst upstream equivalence ratio, and the lower part of Fig. 15 shows the change in rear oxygen sensor voltage as the rear oxygen sensor output. The rear oxygen sensor 22 has a switch-like nonlinear characteristic in which the output of the rear oxygen sensor voltage is accompanied by a delay (hysteresis). For example, as shown by the catalyst upstream equivalence ratio in the upper part of Fig. 15, when the air-fuel ratio changes from lean to rich or from rich to lean, the rear oxygen sensor output changes with a delay.

[0095] When the rear oxygen sensor output enters band 51, which is represented by the range indicated by the double-headed arrow in Fig. 15, the value of the rear oxygen sensor output suddenly reverses. The integration stop determination unit 120 (see Fig. 3) of the ECU 28 according to this embodiment has the function of determining the integration stop position, and by utilizing the characteristic that the rear oxygen sensor output reverses when it enters band 51, it captures the rising timing of the rear oxygen sensor voltage.

[0096] <Timing for starting and stopping oxygen storage accumulation> The oxygen storage amount integration unit (oxygen storage amount integration unit 133) of the ECU 28 determines the timing at which a signal is input from the integration stop determination unit (integration stop determination unit 120) when the voltage value output by the oxygen concentration sensor (rear oxygen sensor 22) falls within the range between the upper voltage threshold and the lower voltage threshold and the derivative value of the voltage value exceeds the derivative value threshold as the integration stop position. For example, the oxygen storage amount integration unit 133 performs processing to stop integrating the oxygen storage amount when the following conditions are met: the rear oxygen sensor voltage falls within the threshold of band 51 and the time derivative value of the sensor voltage value exceeds a certain threshold. The timing to start and stop integrating the oxygen storage amount will now be described with reference to FIGS. 16 and 17.

[0097] FIG. 16 is an explanatory diagram showing an example of the timings for starting and stopping the accumulation of the oxygen storage amount when the fuel injection amount decreases (from rich to lean).

[0098] 14A and 14B, the voltage rise timing of the rear oxygen sensor 22 is not affected by deterioration of the rear oxygen sensor 22. Therefore, the integration stop determination unit 120 of the ECU 28 can accurately determine the timing to stop oxygen storage amount integration by accurately determining the voltage rise timing of the rear oxygen sensor 22. Therefore, the timing to stop oxygen storage amount integration is determined to be the moment when the AND condition is met, which satisfies both that the rear oxygen sensor voltage has entered a predetermined value range (band 51) and that the differential value of the rear oxygen sensor voltage is equal to or greater than a certain threshold.

[0099] Furthermore, the voltage value determination unit 122 of the integration stop determination unit 120 detects the rising timing of the rear oxygen sensor voltage by detecting that the rear oxygen sensor 22 has entered a predetermined value range (band 51). Furthermore, the differential value determination unit 121 of the integration stop determination unit 120 detects the rising timing of the differential value by detecting that the differential value of the sensor voltage is equal to or greater than a threshold value.

[0100] In Figure 16, the point in time (Start) when the fuel injection amount decreases represents the timing when integration of the oxygen storage amount begins. In Figure 16, the point in time (Stop) when the rear oxygen sensor voltage increases, goes out of a predetermined value range (Band 51), and then inverts, decreases, and re-enters the predetermined value range (Band 51) represents the timing when integration of the oxygen storage amount stops.

[0101] At the point (Stop) when the rear oxygen sensor voltage enters the predetermined range (band 51), the differential value of the rear oxygen sensor voltage is negative, but this has been converted to an absolute value by the determination unit 121b. Because the differential value of the rear oxygen sensor voltage and the differential value threshold have been converted to absolute values, the determination unit 121b can easily compare the differential value of the rear oxygen sensor voltage with the differential value threshold. Therefore, the differential value determination unit 121 can determine the timing when the differential value exceeds the differential value threshold as the timing when the rear oxygen sensor voltage enters the predetermined range (band 51).

[0102] The oxygen storage amount integrating unit 133 starts integrating the oxygen storage amount from the Start position where the mixture changes from rich to lean, and stops integrating at the Stop position. The catalyst deterioration diagnosis unit 134 performs a deterioration diagnosis of the catalyst based on the estimated oxygen storage amount calculated by the oxygen storage amount integrating unit 133.

[0103] FIG. 17 is an explanatory diagram showing an example of the timings for starting and stopping the accumulation of the oxygen storage amount when the fuel injection amount decreases (from lean to rich).

[0104] In Figure 17, the point in time (Start) when the fuel injection amount increases represents the timing when integration of the oxygen storage amount begins. In Figure 17, the rear oxygen sensor voltage decreases, leaves a predetermined value range (Band 51), and then the inverted rear oxygen sensor voltage increases and enters the predetermined value range (Band 51) represents the timing when integration of the oxygen storage amount stops. At the point in time (Stop) when the rear oxygen sensor voltage enters the predetermined value range (Band 51), the differential value of the rear oxygen sensor voltage takes a positive value. Therefore, the integration stop determination unit 120 can determine the point in time when the differential value exceeds the differential value threshold as the point in time when the rear oxygen sensor voltage enters the predetermined value range (Band 51).

[0105] The oxygen storage amount integrating unit 133 starts integrating the oxygen storage amount from the Start position where the mixture changes from lean to rich, and stops integrating at the Stop position. The catalyst deterioration diagnosis unit 134 performs a deterioration diagnosis of the catalyst based on the estimated oxygen storage amount calculated by the oxygen storage amount integrating unit 133.

[0106] 18 is a flowchart showing how to stop the accumulation of oxygen storage capacity. The accumulation of oxygen storage capacity is stopped when the rear oxygen sensor voltage falls within the threshold of band 51 and the time derivative of the rear oxygen sensor voltage exceeds a certain derivative threshold. Here, the processing of each functional unit will be described with reference to FIG. 3.

[0107] First, the integration start determination unit 132 detects that the air-fuel ratio has reversed based on the air-fuel ratio sensor signal (S1). Exclusion gas Air-fuel ratio The oxygen flow rate estimation unit 131 estimates the oxygen flow rate based on the air-fuel ratio sensor signal and the exhaust gas flow rate. Then, the oxygen storage amount integration unit 133 starts integrating the oxygen storage amount (S2).

[0108] Next, the differentiator 121a of the differential value determination unit 121 calculates a differential value of the rear oxygen sensor voltage based on the rear oxygen sensor signal input from the rear oxygen sensor 22 (S3). Next, the voltage value determination unit 122 checks whether the rear oxygen sensor voltage is within a specified range (between a lower voltage threshold and an upper voltage threshold) based on the rear oxygen sensor signal input from the rear oxygen sensor 22 (S4). In parallel with step S4, the differential value determination unit 121 checks whether the differential value (absolute value) of the rear oxygen sensor voltage is equal to or greater than a certain differential value threshold (S5).

[0109] Next, the integration stop determination unit 120 determines whether steps S4 and S5 are simultaneously satisfied (S6). If steps S4 and S5 are not simultaneously satisfied (NO in S6), the process returns to step S3 and is repeated. If steps S4 and S5 are simultaneously satisfied (YES in S6), the integration stop determination unit 120 outputs an integration stop signal to the oxygen storage amount integration unit 133.

[0110] When the accumulation stop signal is input, oxygen storage amount accumulation unit 133 stops accumulating the oxygen storage amount (S7). Thereafter, oxygen storage amount accumulation unit 133 outputs the accumulated oxygen storage amount as an estimated oxygen storage amount to catalyst deterioration diagnosis unit 134. Catalyst deterioration diagnosis unit 134 performs catalyst deterioration diagnosis based on the estimated oxygen storage amount (S8), and outputs the diagnosis result to air-fuel ratio correction unit 135.

[0111] Then, the air-fuel ratio correction unit 135 calculates an air-fuel ratio correction value corresponding to the diagnosis result for the input air-fuel ratio target value (S9), and ends this process. Note that the air-fuel ratio correction unit 135 outputs the corrected target air-fuel ratio based on the calculated air-fuel ratio correction value.

[0112] 19A and 19B are diagrams illustrating the effects of the conventional technology and the effects of the ECU 28 according to this embodiment. Here, the effects on catalyst diagnosis due to the deterioration of the rear oxygen sensor 22 are shown. It is also assumed that both the catalyst and the rear oxygen sensor 22 are deteriorated.

[0113] 19A is a diagram showing an example of a diagnosis result of a catalyst state using a conventional technique, and is the same as FIG. 14B. As described above, in the conventional technology, the rear oxygen sensor voltage is output with a delay due to the influence of the delay 41 caused by deterioration of the rear oxygen sensor 22. For this reason, the conventional ECU integrated the oxygen storage amount from the point when the fuel injection amount increased (Start) to the point when the fuel injection amount decreased (Reset), and calculated an excessive oxygen storage amount. As a result, the catalyst diagnosis by the conventional ECU would erroneously determine that the catalyst was in a deteriorated state, thinking that it could store a large amount of oxygen, even though it was actually in a deteriorated state, and would therefore diagnose the catalyst as being in a new condition.

[0114] FIG. 19B is a diagram showing an example of the diagnosis result of the catalyst state using the technology according to this embodiment. The ECU 28 according to this embodiment detects that the rear oxygen sensor voltage is within a predetermined range and that the differential value of the rear oxygen sensor voltage is equal to or greater than a certain threshold. This allows the ECU 28 to detect the rising timing of the rear oxygen sensor voltage without being affected by deterioration of the rear oxygen sensor 22. The ECU 28 can then accurately detect the timing to stop integrating the oxygen storage amount and calculate the oxygen storage amount according to the state of the catalyst.

[0115] Furthermore, the ECU 28 can minimize delays due to deterioration of the rear oxygen sensor 22 by detecting the rising timing of the differential value of the rear oxygen sensor voltage. However, this method is susceptible to noise associated with the differentiation operation, which may result in an erroneous diagnosis of the catalyst condition. Therefore, the ECU 28 uses an AND condition between the rising timing of the differential value of the rear oxygen sensor voltage and whether the rear oxygen sensor 22 is within a predetermined value range, thereby eliminating the influence of noise associated with the differentiation operation and preventing an erroneous diagnosis of the catalyst condition.

[0116] Furthermore, due to the characteristics of the rear oxygen sensor 22, the rear oxygen sensor voltage may fluctuate around a certain voltage value (for example, around 0.15V or 0.65V). For this reason, the ECU 28 cannot determine whether there is a transient movement in the rear oxygen sensor voltage by using only the threshold value, but also the differential value. By using both as an AND condition, it is possible to determine an appropriate timing to stop the oxygen storage amount integration.

[0117] The ECU 28 according to the first embodiment described above can accurately determine when to stop integrating the oxygen storage amount by detecting when the rear oxygen sensor voltage enters a predetermined threshold range (band 51) and when the time derivative of the rear oxygen sensor voltage exceeds a certain derivative threshold. This allows the ECU 28 to calculate the oxygen storage amount appropriate to the catalyst condition. Detecting the rise timing of the derivative can minimize delays due to deterioration of the rear oxygen sensor. Furthermore, by ANDing the rise timing of the derivative and the rear oxygen sensor being within a predetermined range, the influence of noise associated with differentiation can be eliminated, preventing misdiagnosis. This allows the ECU 28 to maintain a high purification efficiency of the three-way catalyst and prevent a deterioration in emissions performance.

[0118] [Second embodiment] 3, the catalyst deterioration diagnosis unit 134 diagnoses the deterioration of the three-way catalyst based on the accumulated estimated oxygen storage amount, and the air-fuel ratio correction unit 135 calculates the corrected target air-fuel ratio. Here, another embodiment for calculating the estimated oxygen storage amount will be described.

[0119] 20 is a block diagram showing an example of the internal configuration of an oxygen storage amount integrating unit 133A according to the second embodiment. The oxygen storage amount integrating unit 133A has a configuration including a plurality of different oxygen storage amount models for estimating the oxygen storage amount of the three-way catalyst.

[0120] The oxygen storage amount integrating unit 133A includes a new catalyst oxygen storage amount model 136, a deteriorated catalyst oxygen storage amount model 137, and a model selecting unit 138. The plurality of oxygen storage amount models (new catalyst oxygen storage amount model 136, deteriorated catalyst oxygen storage amount model 137) are selected by the model selecting unit 138 according to the state of the three-way catalyst (exhaust purification catalyst 21). The new catalyst oxygen storage amount model 136 is a model that estimates the amount of oxygen that can be stored by a new catalyst based on the oxygen flow rate input from the oxygen flow rate estimation unit 131. The deteriorated catalyst oxygen storage amount model 137 is a model that estimates the amount of oxygen that can be stored by the deteriorated catalyst based on the oxygen flow rate input from the oxygen flow rate estimation unit 131 .

[0121] A model selection unit (model selection unit 138) integrates the oxygen storage amount using one oxygen storage amount model selected from a plurality of oxygen storage amount models (new catalyst oxygen storage amount model 136, degraded catalyst oxygen storage amount model 137) according to the diagnosis result input from catalyst degradation diagnosis unit 134. If the result of the degradation diagnosis is new, this model selection unit 138 selects new catalyst oxygen storage amount model 136 as a model for estimating the oxygen storage amount, and integrates the oxygen storage amount using new catalyst oxygen storage amount model 136 from the time an integration start signal is input until an integration stop signal is input. On the other hand, if the result of the degradation diagnosis is deteriorated, model selection unit 138 selects degraded catalyst oxygen storage amount model 137 as a model for estimating the oxygen storage amount, and integrates the oxygen storage amount using degraded catalyst oxygen storage amount model 137 from the time an integration start signal is input until an integration stop signal is input.

[0122] The oxygen storage amount integrated by the model selection unit 138 is output as an estimated oxygen storage amount to the catalyst deterioration diagnosis unit 134. Then, the catalyst deterioration diagnosis unit 134 performs catalyst deterioration diagnosis again. In addition, as shown in FIG. 3, the diagnosis result by the catalyst deterioration diagnosis unit 134 is output to the air-fuel ratio correction unit 135.

[0123] The oxygen storage capacity integrating unit 133A according to the second embodiment described above can determine an estimated oxygen storage capacity by switching between oxygen storage capacity models for a new catalyst and a deteriorated catalyst based on the catalyst deterioration diagnosis results. By providing a plurality of oxygen storage capacity models for minor deterioration and major deterioration in accordance with the usage time of the three-way catalyst in addition to new and deteriorated, it becomes possible to determine an estimated oxygen storage capacity that matches the deterioration state of the three-way catalyst.

[0124] The present invention is not limited to the above-described embodiment, and it goes without saying that various other applications and modifications are possible without departing from the gist of the present invention as set forth in the claims. For example, the above-described embodiment has described the system configuration in detail and specifically to clearly explain the present invention, and is not necessarily limited to a system including all of the described configurations. Furthermore, it is also possible to add, delete, or replace part of the configuration of this embodiment with other configurations. In addition, the control lines and information lines shown are those that are considered necessary for the explanation, and do not necessarily show all the control lines and information lines in the product. In reality, it can be assumed that almost all components are interconnected. [Explanation of symbols]

[0125] 1...internal combustion engine, 20...front air-fuel ratio sensor, 21...exhaust gas purification catalyst, 22...rear oxygen sensor, 28...ECU, 100...internal combustion engine control system, 110...aftertreatment system, 120...integration stop determination unit, 121...differential value determination unit, 122...voltage value determination unit, 123...condition determination unit, 131...oxygen flow rate estimation unit, 132...integration start determination unit, 133...oxygen storage amount integration unit, 134...catalyst deterioration diagnosis unit, 135...air-fuel ratio correction unit

Claims

1. 1. An internal combustion engine control device for controlling an internal combustion engine including an air-fuel ratio sensor disposed upstream of a three-way catalyst provided in an exhaust pipe and detecting an air-fuel ratio of exhaust gas, and an oxygen concentration sensor disposed downstream of the three-way catalyst and detecting an oxygen concentration of the exhaust gas, an oxygen storage amount integrating unit that integrates an oxygen storage amount stored in the three-way catalyst during a period from an integration start position where the air-fuel ratio of the exhaust gas starts to change from lean to rich or from rich to lean to an integration stop position where the oxygen concentration, which has been increasing or decreasing since before the integration start position, is reversed; an integration stop determination unit that determines the integration stop position; an oxygen flow rate estimation unit that estimates an oxygen flow rate of the exhaust gas based on the flow rate of the exhaust gas and the air-fuel ratio; an integration start determination unit that determines the integration start position based on the air-fuel ratio, the oxygen storage amount integrating unit integrates the oxygen storage amount based on the oxygen flow rate, the integration start position, and the integration stop position; the oxygen storage amount integrating unit determines the integration stop position for the rich condition as a timing at which the voltage value output by the oxygen concentration sensor, delayed from the integration start position under a rich condition in which the air-fuel ratio of the exhaust gas changes from lean to rich and then starts to change from rich to lean, exceeds an upper voltage threshold due to a maximum electromotive force caused by the rich air-fuel ratio, and then decreases to fall within a range between the upper voltage threshold and the lower voltage threshold, and a differential value of the voltage value at the time of the decrease exceeds a differential value threshold, The integration stop position for the lean condition is determined to be the timing at which the voltage value output by the oxygen concentration sensor, delayed from the integration start position under a lean condition in which the air-fuel ratio of the exhaust gas changes from rich to lean and then starts to change from lean to rich, becomes a voltage value below the lower voltage threshold due to a minimum electromotive force caused by the lean air-fuel ratio, and then increases to fall within the range between the upper voltage threshold and the lower voltage threshold, and the differential value of the voltage value during the increase exceeds the differential value threshold. Internal combustion engine control device.

2. The differential value has a positive or negative sign depending on the direction of change in the voltage value output by the oxygen concentration sensor, and the differential value threshold has a different positive or negative sign when determining the integration stop position under the rich condition or when determining the integration stop position under the lean condition. The internal combustion engine control device according to claim 1.

3. The differential value and the differential value threshold are converted into absolute values.

3. The internal combustion engine control device according to claim 2.

4. a catalyst diagnosis unit that diagnoses the state of the three-way catalyst based on the oxygen storage amount and outputs a diagnosis result; an air-fuel ratio correction unit that corrects the target air-fuel ratio of the internal combustion engine in accordance with the air-fuel ratio target value and the diagnosis result, and outputs the corrected target air-fuel ratio.

3. The internal combustion engine control device according to claim 2.

5. The diagnostic results are expressed as a state level value assigned to each state of the three-way catalyst.

5. The internal combustion engine control device according to claim 4.

6. The oxygen storage amount integrating unit a plurality of oxygen storage amount models selected according to the state of the three-way catalyst; a model selection unit that calculates the oxygen storage amount using one oxygen storage amount model selected from a plurality of oxygen storage amount models according to the diagnosis result.

5. The internal combustion engine control device according to claim 4.

7. 1. An internal combustion engine control method performed by an internal combustion engine control device that controls an internal combustion engine including an air-fuel ratio sensor that is disposed upstream of a three-way catalyst provided in an exhaust pipe and detects an air-fuel ratio of exhaust gas, and an oxygen concentration sensor that is disposed downstream of the three-way catalyst and detects an oxygen concentration of the exhaust gas, comprising: a voltage value output by the oxygen concentration sensor with a delay from the integration start position under a rich condition, where the air-fuel ratio of the exhaust gas changes from lean to rich and then starts to change from rich to lean, exceeds an upper voltage threshold due to a maximum electromotive force caused by the rich air-fuel ratio, and then decreases to fall within a range between the upper voltage threshold and the lower voltage threshold, and a differential value of the voltage value during the decrease exceeds a differential value threshold, is determined as the integration stop position under the rich condition; determining, as the integration stop position for the lean condition, a timing at which a voltage value output by the oxygen concentration sensor with a delay from the integration start position under a lean condition in which the air-fuel ratio of the exhaust gas changes from rich to lean and then starts to change from lean to rich, increases from a voltage value that is below the voltage lower limit threshold due to a minimum electromotive force under the lean condition, enters a range between the voltage upper limit threshold and the voltage lower limit threshold, and a derivative value of the voltage value during the increase exceeds the derivative value threshold; During a period from the integration start position under the rich condition to the integration stop position under the rich condition, or during a period from the integration start position under the lean condition to the integration stop position under the lean condition, a process of integrating the oxygen storage amount stored in the three-way catalyst is performed based on the flow rate of the exhaust gas and the oxygen flow rate estimated based on the air-fuel ratio, the integration start position, and the integration stop position. Internal combustion engine control method.

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