Control device for internal combustion engine

Abstract

To enable air-fuel ratio feedback control to be started early after starting of an internal combustion engine.SOLUTION: When the element temperature of an air-fuel ratio sensor 20 becomes equal to or higher than an activation temperature, an electronic control unit 21 starts detection of an air-fuel ratio based on the sensor output of the air-fuel ratio sensor 20 and air-fuel ratio feedback control based on the detection result. In a period from the start of the air-fuel ratio until the element temperature becomes equal to or higher than a predetermined desorption convergence temperature, the electronic control unit 21 calculates an air-fuel ratio detection value by setting the gain of the air-fuel ratio detection value with respect to the sensor output when the sensor output has a value indicating an air-fuel ratio richer than a stoichiometric air-fuel ratio to a value smaller than that after the elapse of the period.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a control device for an internal combustion engine that performs feedback control of the air-fuel ratio based on the detection result of an air-fuel ratio sensor. [Background technology]

[0002] The air-fuel ratio sensor used in the above-mentioned internal combustion engine control device has a detection element that generates an electromotive force in response to the difference in oxygen partial pressure between the ambient exhaust gas and the reference gas. The detection element does not become active until it reaches a certain temperature. Therefore, after starting the internal combustion engine, the control device begins feedback control of the air-fuel ratio only after the temperature of the detection element reaches or exceeds the activation temperature.

[0003] On the other hand, while the internal combustion engine is stopped, HC components in the exhaust gas may be adsorbed onto the detection element of the air-fuel ratio sensor. When the temperature of the detection element rises to a certain level, the adsorbed HC components begin to desorb from the detection element. The detection element also generates an electromotive force due to the desorption of these HC components. As a result, a phenomenon known as cold shoot occurs, in which the output of the air-fuel ratio sensor shifts to the rich side.

[0004] In contrast, the control device for an internal combustion engine described in Patent Document 1 determines whether the desorption of HC components has converged based on the time elapsed since the start of the internal combustion engine. That is, this control device determines that the desorption of HC components has converged when the time elapsed since the start reaches a predetermined time. Then, this control device starts feedback control of the air-fuel ratio on the condition that it is determined that the desorption of HC components has converged and the temperature of the detection element is equal to or higher than the activation temperature. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2010 / 041585 Summary of the Invention [Problem to be solved by the invention]

[0006] There are cases where the temperature of the detection element reaches the activation temperature before it is determined that the desorption of HC components has converged. In such cases, because it is not determined that the desorption has converged, there is a period during which feedback control of the air-fuel ratio cannot be performed even though the detection element is activated.

[0007] The time required for HC components to desorb from the detection element varies significantly depending on the amount of HC components adsorbed by the detection element at the start of the internal combustion engine and the temperature of the detection element after the start of the internal combustion engine. Therefore, the elapsed time after start at which it is determined that desorption has converged must be set to a relatively long time, taking into account the variation in the time required for desorption. Therefore, in the conventional control device described above, the start of air-fuel ratio feedback control may be delayed. [Means for solving the problem]

[0008] A control device for an internal combustion engine that solves the above problem detects an air-fuel ratio by calculating an air-fuel ratio detection value based on the sensor output of an air-fuel ratio sensor installed in an exhaust passage of the internal combustion engine. The control device then performs air-fuel ratio feedback control based on the air-fuel ratio detection result. The control device also starts air-fuel ratio detection when the element temperature of the air-fuel ratio sensor reaches or exceeds a predetermined activation temperature. During the period from the start of air-fuel ratio detection until a predetermined condition is met, the control device for an internal combustion engine calculates the air-fuel ratio detection value by setting a gain of the air-fuel ratio detection value for the sensor output when the sensor output indicates an air-fuel ratio richer than the stoichiometric air-fuel ratio to a smaller value than after the same period has elapsed.

[0009] The above-described internal combustion engine control device detects the air-fuel ratio by calculating an air-fuel ratio detection value based on the sensor output of the air-fuel ratio sensor. Here, a sensor output indicating an air-fuel ratio richer than the stoichiometric air-fuel ratio is defined as a rich output. When the air-fuel ratio detection value is calculated by reducing the gain for the rich output, the air-fuel ratio detection value indicates a rich air-fuel ratio that is less rich than the air-fuel ratio originally indicated by the sensor output. Therefore, even if the sensor output deviates rich due to the cold shoot phenomenon, the air-fuel ratio feedback control is less likely to control the air-fuel ratio to a value significantly leaner than the stoichiometric air-fuel ratio. This makes it possible to perform air-fuel ratio feedback control even during a period when the cold shoot phenomenon may occur. Therefore, the above-described internal combustion engine control device has the advantage of being able to start air-fuel ratio feedback control early after the internal combustion engine is started. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a diagram schematically illustrating a configuration of an embodiment of a control device for an internal combustion engine. [Figure 2] 2 is a diagram showing a schematic configuration of an air-fuel ratio sensor installed in an internal combustion engine controlled by the control device. FIG. [Figure 3] 3 is a graph showing the relationship between the element temperature and the sensor output in the air-fuel ratio sensor of FIG. 2 and the excess air ratio. [Figure 4] 4 is a graph showing the relationship between the element temperature and the sensor output and the excess air ratio in a calculation map used by the control device of FIG. 1 to calculate the excess air ratio. DETAILED DESCRIPTION OF THE INVENTION

[0011] An embodiment of a control device for an internal combustion engine will be described in detail below with reference to FIGS. <Configuration of the control device for the internal combustion engine> First, the configuration of this embodiment will be described with reference to FIG. 1. The control device of this embodiment controls an internal combustion engine 10 shown in FIG. 1. The internal combustion engine 10 includes a combustion chamber 11 in which an air-fuel mixture is combusted, and an intake passage 12 and an exhaust passage 13 connected to the combustion chamber 11. The intake passage 12 is an introduction passage for intake air used for combustion in the combustion chamber 11. The exhaust passage 13 is an exhaust passage for exhaust gas generated by the combustion of the air-fuel mixture in the combustion chamber 11. The internal combustion engine 10 also includes an air flow meter 14 that detects the intake air flow rate in the intake passage 12, and a throttle valve 15 that adjusts the intake air flow rate in the intake passage 12. The internal combustion engine 10 also includes an injector 16 that injects fuel into the intake air introduced into the combustion chamber 11, and an ignition device 17 that ignites the air-fuel mixture in the combustion chamber 11 by spark discharge. The internal combustion engine 10 also includes a catalytic converter 18 that purifies exhaust gas flowing through the exhaust passage 13. An air-fuel ratio sensor 20 is installed in the exhaust passage 13 at a position upstream of the catalytic converter 18.

[0012] The internal combustion engine 10 is controlled by an electronic control unit 21 serving as a control device. The electronic control unit 21 includes a processing device 22 and a storage device 23. The storage device 23 stores in advance programs and data used to control the internal combustion engine 10. The processing device 22 executes the programs read from the storage device 23 to perform various processes for controlling the internal combustion engine 10.

[0013] Various sensors for detecting the operating state of the internal combustion engine 10 are connected to the electronic control unit 21. The sensors connected to the electronic control unit 21 include the air flow meter 14 and the air-fuel ratio sensor 20. The electronic control unit 21 controls the operating state of the internal combustion engine 10 by operating the throttle valve 15, the injector 16, the ignition device 17, etc. based on the detection results of these sensors.

[0014] The electronic control unit 21 detects the air-fuel ratio of the mixture burned in the combustion chamber 11 based on the output of the air-fuel ratio sensor 20. The electronic control unit 21 controls the internal combustion engine 10 based on the detection result of the air-fuel ratio.

[0015] <Air-fuel ratio feedback control> The electronic control unit 21 performs feedback control of the air-fuel ratio of the mixture burned in the combustion chamber 11 based on the air-fuel ratio detection result by the air-fuel ratio sensor 20. During the air-fuel ratio feedback control, the electronic control unit 21 calculates the amount of air in the combustion chamber 11 based on the detection result of the air flow meter 14. The electronic control unit 21 then calculates an amount of air whose ratio to the amount of air in the combustion chamber 11 is a stoichiometric air-fuel ratio as the value of a base injection amount. The base injection amount is a feedforward value of the fuel injection amount of the injector 16 that is the subject of feedback in the air-fuel ratio feedback control. The electronic control unit 21 also calculates a feedback correction value for the fuel injection amount based on the air-fuel ratio detection result by the air-fuel ratio sensor 20. Specifically, when the air-fuel ratio detection value indicates an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the electronic control unit 21 calculates a positive value as the feedback correction value, i.e., a value that corrects the fuel injection amount to an increased side. On the other hand, when the detected air-fuel ratio value indicates an air-fuel ratio richer than the stoichiometric air-fuel ratio, the electronic control unit 21 calculates a negative feedback correction value, i.e., a value for correcting the fuel injection amount to a smaller value. The electronic control unit 21 then operates the injector 16 to inject an amount of fuel equal to the sum of the base injection amount and the feedback correction value. The electronic control unit 21 maintains the air-fuel ratio of the mixture burned in the combustion chamber 11 at the stoichiometric air-fuel ratio through this air-fuel ratio feedback control. In the following description, an air-fuel ratio leaner than the stoichiometric air-fuel ratio will be referred to as a lean air-fuel ratio. Also, an air-fuel ratio richer than the stoichiometric air-fuel ratio will be referred to as a rich air-fuel ratio. This air-fuel ratio feedback control is performed, for example, through PID control, with the detected air-fuel ratio value as the controlled variable, the stoichiometric air-fuel ratio as the target value, and the fuel injection amount as the manipulated variable.

[0016] <Configuration of the air-fuel ratio sensor> Next, the configuration of the air-fuel ratio sensor 20 will be described with reference to Figure 2. The air-fuel ratio sensor 20 includes a sensor element 30. The sensor element 30 has a solid electrolyte layer 31 in the form of a flat plate made of a solid electrolyte whose main component is zirconia. A diffusion-controlling layer 34 is laminated on the surface of the solid electrolyte layer 31. The diffusion-controlling layer 34 is a layer made of porous ceramics that limits the diffusion of gas molecules. A ceramic substrate 35 made of an insulating ceramic material is laminated on the surface of the solid electrolyte layer 31 opposite to the side on which the diffusion-controlling layer 34 is laminated. An electric heater 38 for heating the sensor element 30 is attached to the ceramic substrate 35.

[0017] An exhaust chamber 36 and an atmospheric chamber 37 are provided inside the sensor element 30. The exhaust chamber 36 is a closed space surrounded by the solid electrolyte layer 31 and the diffusion-controlling layer 34. Exhaust air around the sensor element 30 is introduced into the exhaust chamber 36 after passing through the diffusion-controlling layer 34. On the other hand, the atmospheric chamber 37 is a space surrounded by the solid electrolyte layer 31 and the ceramic base material 35, and is open to the atmosphere.

[0018] The sensor element 30 is further provided with an exhaust gas-side electrode 32 and an atmosphere-side electrode 33. The exhaust gas-side electrode 32 is provided on the surface of the solid electrolyte layer 31 on the side where the diffusion-controlling layer 34 is laminated, so as to be exposed to the exhaust gas chamber 36. On the other hand, the atmosphere-side electrode 33 is provided on the surface of the solid electrolyte layer 31 on the side where the ceramic substrate 35 is laminated, so as to be exposed to the atmosphere chamber 37.

[0019] When a voltage is applied between the exhaust-side electrode 32 and the atmosphere-side electrode 33, oxygen ions migrate within the solid electrolyte layer 31 in response to the oxygen partial pressure difference between the exhaust chamber 36 and the atmosphere chamber 37. As a result, a current flows between the exhaust-side electrode 32 and the atmosphere-side electrode 33. In the following description, this current will be referred to as the sensor current. The air-fuel ratio sensor 20 is provided with an ammeter 39 that detects the sensor current. The sensor current increases as the oxygen partial pressure difference between the exhaust chamber 36 and the atmosphere chamber 37 increases and as the voltage applied between the electrodes increases. However, the migration of oxygen ions through the solid electrolyte layer 31 reduces the oxygen partial pressure difference between the exhaust chamber 36 and the atmosphere chamber 37. In addition, the diffusion-controlling layer 34 limits the migration of exhaust gas from the exhaust passage 13 to the exhaust chamber 36. Therefore, even if the voltage applied between the electrodes is increased beyond a certain level, the sensor current saturates and no longer increases. In the following description, the value of the sensor current at this point will be referred to as the limiting current value IL. The limiting current value IL is proportional to the difference in oxygen partial pressure between the exhaust gas around the sensor element 30 and the atmosphere.

[0020] The air-fuel ratio sensor 20 measures the limiting current value IL based on the change in the sensor current when an AC voltage is applied between the exhaust-side electrode 32 and the atmosphere-side electrode 33. The air-fuel ratio sensor 20 uses the measured limiting current value IL as the sensor output. In the case of the internal combustion engine 10 of FIG. 1, exhaust gas with the same properties as when it is discharged from the combustion chamber 11, before passing through the catalytic device 18, reaches the air-fuel ratio sensor 20. In this case, the oxygen partial pressure difference between the atmosphere and the exhaust correlates with the air-fuel ratio of the mixture combusted in the combustion chamber 11. Therefore, the sensor output is a value that reflects the air-fuel ratio of the mixture combusted in the combustion chamber 11.

[0021] In the case of this air-fuel ratio sensor 20, the limit current value IL is "0" when the air-fuel ratio is stoichiometric, a positive value when the air-fuel ratio is lean, and a negative value when the air-fuel ratio is rich. In the following description, the sensor output (IL=0) corresponding to the stoichiometric air-fuel ratio will be referred to as a stoichiometric output. Furthermore, the sensor output (IL<0) corresponding to a rich air-fuel ratio will be referred to as a rich output. Furthermore, the sensor output (IL>0) corresponding to a lean air-fuel ratio will be referred to as a lean output.

[0022] The electronic control unit 21 calculates the excess air ratio λ based on the sensor output. The electronic control unit 21 then uses the calculated excess air ratio λ as the detected air-fuel ratio value in air-fuel ratio feedback control. The excess air ratio λ is the ratio of the amount of air in the mixture to the stoichiometric air amount. The stoichiometric air amount is the minimum amount of air required to completely combust all the fuel in the mixture. The excess air ratio λ is equal to the value obtained by dividing the actual air-fuel ratio by the stoichiometric air-fuel ratio.

[0023] <Air-fuel ratio sensor output characteristics> Next, the output characteristics of the air-fuel ratio sensor 20 will be described with reference to FIG. 3. In the following description, the temperature of the sensor element 30 will be referred to as the element temperature TS. FIG. 3 shows the relationship between the limit current value IL and the excess air factor λ when the element temperature TS is T1 to T3. The temperatures T1 to T3 are higher in the order of T1, T2, and T3. In FIG. 3, curve L1 shows the relationship between the limit current value IL and the excess air factor λ when the element temperature TS is T1. Curve L2 shows the relationship between the limit current value IL and the excess air factor λ when the element temperature TS is T2. Curve L3 shows the relationship between the limit current value IL and the excess air factor λ when the element temperature TS is T3.

[0024] The lower the element temperature TS, the higher the electrical resistance of the solid electrolyte layer 31, making it more difficult for the sensor current to flow. Therefore, the lower the element temperature TS, the larger the absolute value of the limiting current value IL at the same excess air factor λ.

[0025] The relationship between the element temperature TS, the excess air factor λ, and the limiting current value IL can be determined in advance by experiments, etc. Therefore, the excess air factor λ can be determined based on the element temperature TS and the limiting current value IL, which is the sensor output. Furthermore, the element temperature TS can be determined based on, for example, the impedance of the solid electrolyte layer 31.

[0026] <Cold shoot phenomenon of air-fuel ratio sensor> During the warm-up process of the air-fuel ratio sensor 20, a phenomenon occurs in which the output of the air-fuel ratio sensor 20 deviates to the rich side from the value corresponding to the actual air-fuel ratio. This phenomenon is called the cold shoot phenomenon. Next, the cold shoot phenomenon of the air-fuel ratio sensor 20 will be explained.

[0027] After the internal combustion engine 10 is stopped, exhaust gas containing HC components remains in the exhaust passage 13. While the internal combustion engine 10 is stopped, the HC components in the exhaust gas may be adsorbed onto the sensor element 30 of the air-fuel ratio sensor 20. After the internal combustion engine 10 is started, the temperature of the sensor element 30 gradually increases due to heat received from the exhaust gas and heating by the electric heater 38. The HC components adsorbed onto the sensor element 30 are desorbed from the sensor element 30 when the element temperature TS exceeds a certain level. At this time, the concentration of unburned fuel components in the exhaust gas in the exhaust chamber 36 is higher than that in the exhaust gas in the exhaust passage 13 due to the desorbed HC components. Therefore, when the HC components are desorbed, the output of the air-fuel ratio sensor 20 shifts toward the rich side. This shift in the output of the air-fuel ratio sensor 20 toward the rich side continues until the desorption of the HC components from the sensor element 30 is completed.

[0028] The HC components adsorbed to the sensor element 30 include HC with a small number of carbon atoms and HC with a large number of carbon atoms, such as aromatic hydrocarbons. The desorption of HC with a large number of carbon atoms occurs at a higher element temperature TS than the desorption of HC with a small number of carbon atoms. Therefore, the cold shoot phenomenon can occur over a wide range of element temperatures TS, from a low element temperature TS at which HC with a small number of carbon atoms desorbs to a high element temperature TS at which HC with a large number of carbon atoms desorbs. In the following description, the element temperature TS at which the desorption of HC with a large number of carbon atoms converges is referred to as the desorption convergence temperature TCO.

[0029] <Air-fuel ratio detection> Next, the detection of the air-fuel ratio performed by the electronic control unit 21 based on the sensor output will be described with reference to Fig. 4. The electronic control unit 21 starts detecting the air-fuel ratio based on the sensor output when the element temperature TS becomes equal to or higher than the activation temperature TAC of the sensor element 30. In addition, the electronic control unit 21 starts air-fuel ratio feedback control together with the detection of the air-fuel ratio.

[0030] In the air-fuel ratio detection process, the electronic control unit 21 acquires the limit current value IL and the element temperature TS from the air-fuel ratio sensor 20 at every predetermined control cycle. Then, the electronic control unit 21 detects the air-fuel ratio by calculating the excess air ratio λ based on the acquired limit current value IL and element temperature TS. The electronic control unit 21 calculates the excess air ratio λ by referring to a calculation map stored in advance in the storage device 23. The calculation map stores the value of the excess air ratio λ for each combination of the limit current value IL and the element temperature TS.

[0031] Fig. 4 shows the relationship between the limit current value IL and the excess air factor λ when the element temperature TS is T1 to T3 in the calculation map. In Fig. 4, curve L1A shows the relationship between the limit current value IL and the excess air factor λ when the element temperature TS is T1. Curve L2A shows the relationship between the limit current value IL and the excess air factor λ when the element temperature TS is T2. Curve L3A shows the relationship between the limit current value IL and the excess air factor λ when the element temperature TS is T3.

[0032] The calculation map is set to reflect the output characteristics of the air-fuel ratio sensor 20 when the sensor output is a stoichiometric output or a lean output (IL≧0). Therefore, when the limit current value IL is in the range of 0 or more, the curves L1A-L3A in FIG. 4 overlap with the curves L1-L3 in FIG. 3. As described above, the air-fuel ratio sensor 20 has an output characteristic in which the sensor output decreases as the element temperature TS decreases at the same air-fuel ratio. Reflecting this, when the limit current value IL is a positive value, the electronic control unit 21 calculates the air excess ratio λ by setting the gain of the air-fuel ratio detection value relative to the sensor output to a larger value when the element temperature TS is low than when the element temperature TS is high. The gain here is a conversion coefficient of the sensor output to the air-fuel ratio. In this embodiment, the value "G" that satisfies the relationship of Equation (1) for the limit current value IL and the air excess ratio λ corresponds to the gain of the air-fuel ratio detection value relative to the sensor output.

[0033]

number

[0034] Even when the sensor output is a rich output (IL<0), if the element temperature TS is equal to or higher than the desorption convergence temperature TCO, the electronic control unit 21 sets the gain of the air-fuel ratio detection value relative to the sensor output so as to reflect the output characteristics of the air-fuel ratio sensor 20. For example, in FIG. 4, curve L3A corresponding to a temperature "T3" higher than the desorption convergence temperature TCO overlaps with curve L3 in FIG. 3 corresponding to the same temperature "T3." On the other hand, when the sensor output is a rich output (IL<0), if the element temperature TS is below the desorption convergence temperature TCO, the electronic control unit 21 sets a smaller value as the gain of the air-fuel ratio detection value relative to the sensor output than when the element temperature TS is equal to or higher than the desorption convergence temperature TCO. In other words, when the element temperature TS is below the desorption convergence temperature TCO and the sensor output is a rich output (IL<0), the electronic control unit 21 calculates an air-fuel ratio that is less rich than the value actually indicated by the sensor output as the detected air-fuel ratio.

[0035] <Effects of the embodiment> After the internal combustion engine 10 starts, the electronic control unit 21 starts detecting the air-fuel ratio based on the sensor output when the element temperature TS of the air-fuel ratio sensor 20 becomes equal to or higher than the activation temperature TAC. In addition, the electronic control unit 21 starts air-fuel ratio feedback control together with the detection of the air-fuel ratio.

[0036] The electronic control unit 21 detects the air-fuel ratio by calculating the value of the excess air factor λ used as the air-fuel ratio detection value based on the limit current value IL and the element temperature TS of the air-fuel ratio sensor 20. When the element temperature TS is below the desorption convergence temperature TCO, the electronic control unit 21 calculates the excess air factor λ by setting the gain of the air-fuel ratio detection value for the rich output to a value smaller than when the element temperature TS is equal to or higher than the desorption convergence temperature TCO. After the internal combustion engine 10 starts, the element temperature TS rises through the activation temperature TAC until it reaches a temperature equal to or higher than the desorption convergence temperature TCO. Therefore, during the period from the start of air-fuel ratio detection until the element temperature TS reaches or exceeds the desorption convergence temperature TCO, the electronic control unit 21 detects the air-fuel ratio by setting the gain of the air-fuel ratio detection value for the rich output to a value smaller than that after the same period has elapsed. That is, during the period from when the detection of the air-fuel ratio starts until the element temperature TS becomes equal to or higher than the desorption convergence temperature TCO, the electronic control unit 21 sets the gain of the air-fuel ratio detection value relative to the rich output to a smaller value than after the same period has elapsed, and calculates the air-fuel ratio detection value based on the sensor output.

[0037] As described above, the air-fuel ratio sensor 20 has an output characteristic in which the lower the element temperature TS, the smaller the sensor output. Therefore, if the cold shoot phenomenon does not occur, it is desirable to detect the air-fuel ratio by setting a larger value as the element temperature TS decreases, regardless of the sensor output, to the gain of the air-fuel ratio detection value relative to the sensor output. However, in this case, if the cold shoot phenomenon occurs, the air-fuel ratio detection result will deviate toward the rich side. In this case, the deviation in the air-fuel ratio detection result will occur in one of the following ways (1) to (3).

[0038] (1) The actual air-fuel ratio is lean, and the detected air-fuel ratio is also lean, but the detected air-fuel ratio underestimates the degree to which the air-fuel ratio is lean. (2) The actual air-fuel ratio is lean, but the detected air-fuel ratio is rich.

[0039] (3) The actual air-fuel ratio is a rich air-fuel ratio, and the detected air-fuel ratio is also a rich air-fuel ratio, but the detected result overestimates the degree to which the air-fuel ratio is rich. If air-fuel ratio feedback control is performed according to the detection result in case (2), the air-fuel ratio will be controlled in a direction that deviates from the stoichiometric air-fuel ratio. Furthermore, if air-fuel ratio feedback control is performed according to the detection result in case (3), the air-fuel ratio will be overcorrected to an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio. Therefore, in cases (2) and (3), the air-fuel ratio may significantly deviate from the stoichiometric air-fuel ratio to the lean side.

[0040] On the other hand, when air-fuel ratio feedback control is performed according to the detection result in case (1), the convergence speed is slow, but the air-fuel ratio can be controlled to the stoichiometric air-fuel ratio. Therefore, even if the detection result of the air-fuel ratio deviates to the rich side due to the cold shoot phenomenon, if the sensor output is lean, the effect on the air-fuel ratio feedback control is limited.

[0041] In contrast, during the period from the start of detection until the element temperature TS becomes equal to or higher than the desorption convergence temperature TCO, the electronic control unit 21 detects the air-fuel ratio by setting the gain of the air-fuel ratio detection value for the rich output to a value smaller than that after the same period has elapsed. This reduces the impact on the air-fuel ratio feedback control of the deviation in the air-fuel ratio detection result in cases (2) and (3). This allows the implementation of air-fuel ratio feedback control even during the period when the cold shoot phenomenon occurs. In other words, the air-fuel ratio feedback control can be started earlier after the start of the internal combustion engine 10.

[0042] Even in such a case, air-fuel ratio feedback control can be performed to correct at least the deviation of the air-fuel ratio toward the lean side during the period until the element temperature TS reaches the desorption convergence temperature TCO. Furthermore, even if the cold shoot phenomenon occurs, it is possible to prevent the air-fuel ratio from significantly deviating from the stoichiometric air-fuel ratio toward the lean side. Therefore, the control device of this embodiment can converge the air-fuel ratio to the stoichiometric air-fuel ratio earlier than when the start of control is delayed until the desorption of HC components has converged, or when control is performed directly in accordance with the detection result of the air-fuel ratio that has deviated toward the rich side due to the cold shoot phenomenon.

[0043] <Other embodiments> This embodiment can be modified as follows: This embodiment and the following modifications can be combined and implemented within the scope of technical compatibility.

[0044] During the period in which the gain of the air-fuel ratio detection value for rich output is set to a small value, the gain of the air-fuel ratio detection value for lean output may also be set to a smaller value than after the same period has elapsed.

[0045] The air-fuel ratio itself may be obtained as the detected air-fuel ratio value instead of the excess air ratio λ. In the above embodiment, the period during which the gain of the air-fuel ratio detection value for the rich output is set to a value smaller than the value after the period has elapsed ended when the element temperature TS became equal to or higher than the desorption convergence temperature TCO. In other words, the period ended when the condition that the element temperature TS was equal to or higher than the desorption convergence temperature TCO was met. The condition for ending this period may be changed. For example, the time when the desorption of HC components from the sensor element 30 will converge can be predicted to some extent based on the elapsed time from the start of air-fuel ratio detection or the accumulated air volume since the start of air-fuel ratio detection. Therefore, the condition for ending the period may be that a predetermined time has elapsed since the start of air-fuel ratio detection, or that the accumulated air volume of the internal combustion engine 10 since the start of air-fuel ratio detection has reached a predetermined value or greater.

[0046] A sensor including a sensor element having a configuration different from that of the sensor element 30 shown in Fig. 2 may be used as the air-fuel ratio sensor 20. In that case, the air-fuel ratio sensor 20 may become a sensor that generates a sensor output other than the limit current value IL.

[0047] A sensor having the same function as the air-fuel ratio sensor 20 may be installed in the exhaust passage 13 downstream of the catalytic device 18, and sub-feedback control of the air-fuel ratio may be performed based on the detection result of the sensor. Exhaust gas reformed by the catalytic device 18 reaches such a sensor. Therefore, the sensor output of such a sensor does not necessarily reflect the air-fuel ratio of the mixture burned in the combustion chamber 11. In sub-feedback control of the air-fuel ratio, the air-fuel ratio of the exhaust gas after passing through the catalyst, which is an index value of the properties of the exhaust gas that has passed through the catalytic device 18, is detected based on the sensor output of such a sensor. Therefore, such a sensor is also included in the air-fuel ratio sensor. The air-fuel ratio detection process in the above embodiment may also be applied to detection of the air-fuel ratio of the exhaust gas after passing through the catalyst based on the sensor output of such a sensor. [Explanation of symbols]

[0048] 10...internal combustion engine, 11...combustion chamber, 12...intake passage, 13...exhaust passage, 14...air flow meter, 15...throttle valve, 16...injector, 17...ignition device, 18...catalytic device, 20...air-fuel ratio sensor, 21...electronic control unit, 22...processing device, 23...memory device, 30...sensor element, 31...solid electrolyte layer, 32...exhaust side electrode, 33...atmosphere side electrode, 34...diffusion-controlling layer, 35...ceramic substrate, 36...exhaust chamber, 37...atmosphere chamber, 38...electric heater, 39...ammeter

Claims

1. A control device for an internal combustion engine, which detects an air-fuel ratio by calculating an air-fuel ratio detection value based on a sensor output of an air-fuel ratio sensor installed in an exhaust passage of the internal combustion engine, and performs air-fuel ratio feedback control based on the detection result of the air-fuel ratio, When the element temperature of the air-fuel ratio sensor reaches a predetermined activation temperature or higher, detection of the air-fuel ratio is started, and during a period from the start of detection of the air-fuel ratio until a predetermined condition is established, a gain of the air-fuel ratio detection value with respect to the sensor output when the sensor output is a value indicating an air-fuel ratio on the rich side relative to a stoichiometric air-fuel ratio is set to a value smaller than that after the same period has elapsed, and the air-fuel ratio detection value is calculated; Furthermore, during the period from the start of detection of the air-fuel ratio until a predetermined condition is established, when the sensor output is a value indicating an air-fuel ratio that is richer than the stoichiometric air-fuel ratio, the gain of the air-fuel ratio detection value when the element temperature is low is set to a smaller value than when the element temperature is high, and the air-fuel ratio detection value is calculated. Control device for internal combustion engines.

2. 2. The control device for an internal combustion engine according to claim 1, wherein the predetermined condition is that the element temperature is equal to or higher than a predetermined temperature that is higher than the activation temperature.

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