Device for estimating air-fuel ratio sensor

Abstract

To grasp an abnormal output value when an element crack occurs, without prototyping an air-fuel ratio sensor with the element crack caused.SOLUTION: A control device 100 estimates an abnormal output value of an air-fuel ratio sensor 85 when an element crack occurs, which causes exhaust gas to flow into an atmospheric duct of the air-fuel ratio sensor 85 provided in an exhaust system of an internal combustion engine 10. The control device 100 performs a process to calculate a value indicating an amount of oxygen deficiency in the atmospheric duct when it is assumed that the element crack has occurred, and a process to calculate the abnormal output value based on the value indicating the amount of oxygen deficiency.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an estimation device for an air-fuel ratio sensor. [Background technology]

[0002] For example, Patent Document 1 describes a device that performs abnormality diagnosis of an exhaust gas sensor for cracked elements based on the output value of the exhaust gas sensor. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-14670 Summary of the Invention [Problem to be solved by the invention]

[0004] When an element crack occurs in an air-fuel ratio sensor, which is an exhaust gas sensor, due to exhaust gas flowing into the air duct, an output abnormality occurs in which the output value of the air-fuel ratio sensor becomes leaner than the stoichiometric air-fuel ratio, even though the actual air-fuel ratio is richer than the stoichiometric air-fuel ratio. To detect such an abnormality, it is necessary to prototype an air-fuel ratio sensor with a cracked element and measure the abnormal output value to adjust the abnormality judgment value. However, in recent years, due to the miniaturization of elements and the requirement to detect even minute element cracks due to stricter regulations, it is becoming more difficult to prototype an air-fuel ratio sensor with a cracked element. [Means for solving the problem]

[0005] The air-fuel ratio sensor estimating device for solving the above problem is a device that estimates an abnormal output value of an air-fuel ratio sensor provided in an exhaust system of an internal combustion engine when an element crack occurs, causing exhaust gas to flow into an air duct of the air-fuel ratio sensor. This estimation device executes a process of calculating a value that indicates the amount of oxygen deficiency in the air duct when an element crack is assumed to have occurred, and a process of calculating the abnormal output value based on the value that indicates the amount of oxygen deficiency. [Effects of the Invention]

[0006] This air-fuel ratio sensor estimation device can grasp the abnormal output value when an element crack occurs, without having to prototype an air-fuel ratio sensor with a cracked element. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a schematic diagram showing the configuration of an internal combustion engine and a control device according to an embodiment; [Figure 2] FIG. 2 is a cross-sectional view of an element provided in the air-fuel ratio sensor. [Figure 3] 4 is a graph showing the relationship between the oxygen concentration in the atmospheric duct and the air-fuel ratio output by the air-fuel ratio sensor. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, an embodiment of an estimation device for an air-fuel ratio sensor will be described with reference to FIGS. <Configuration of internal combustion engine and control device> 1, an internal combustion engine 10 has a plurality of cylinders 11 that combust an air-fuel mixture. The internal combustion engine 10 is provided with an intake passage 15 that serves as a passage for introducing air to each cylinder 11.

[0009] A throttle valve 16 that adjusts the amount of intake air is provided in the intake passage 15. The portion of the intake passage 15 downstream of the throttle valve 16 branches off for each cylinder. The branches of the intake passage 15 for each cylinder are connected to intake ports provided for each cylinder. Each intake port is provided with a fuel injection valve 17, for example, for port injection.

[0010] Each cylinder 11 is provided with an ignition plug 19 that ignites the air-fuel mixture introduced into the cylinder 11 by spark discharge. The internal combustion engine 10 is provided with an exhaust passage 21 that serves as a discharge path for exhaust gas generated by the combustion of an air-fuel mixture in each cylinder 11. The upstream portion of the exhaust passage 21 is branched off for each cylinder. The branched portions of the exhaust passage 21 are connected to exhaust ports provided for each cylinder.

[0011] An air-fuel ratio sensor 85 that outputs a signal according to the oxygen concentration in the exhaust gas, and a catalyst 22 that purifies the exhaust gas are installed in the middle of an exhaust passage 21 that is an exhaust system. An air-fuel mixture containing fuel injected by a fuel injection valve 17 is introduced into each cylinder 11 of the internal combustion engine 10. When an ignition plug 19 ignites the air-fuel mixture, combustion occurs in the cylinder 11. Exhaust gas generated by the combustion is discharged from the cylinder 11 into an exhaust passage 21 and purified by a catalyst 22.

[0012] The control device 100 includes a CPU 110 and a memory 120 that stores control programs and data. The CPU 110 executes the programs stored in the memory 120, thereby carrying out various engine controls.

[0013] An air flow meter 81, which is an intake air amount sensor that detects an intake air amount GA, and a water temperature sensor 82 that detects a coolant temperature THW, which is the temperature of the coolant for the internal combustion engine 10, are connected to the control device 100, and output signals from these sensors are input to the control device 100. A crank angle sensor 83 that detects a rotation angle of a crankshaft and an accelerator sensor 84 that detects an accelerator operation amount ACCP, which is the amount of operation of an accelerator pedal, are also connected to the control device 100, and output signals from these sensors are input to the control device 100. The air-fuel ratio sensor 85 is also connected to the control device 100, and an output signal from this sensor is input to the control device 100.

[0014] Based on output signals from the various sensors described above, the control device 100 performs various engine controls, such as fuel injection control by the fuel injection valve 17, intake air amount control by the throttle valve 16, and ignition control by the spark plug 19.

[0015] <Configuration of the air-fuel ratio sensor> The configuration of the air-fuel ratio sensor 85 will be described with reference to FIG. 2. The air-fuel ratio sensor 85 includes an element 30. The 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 element 30 is provided on the ceramic substrate 35.

[0016] An exhaust duct 36 and an atmosphere duct 37 are provided inside the element 30. The exhaust duct 36 is a closed space surrounded by the solid electrolyte layer 31 and the diffusion-controlling layer 34. Exhaust air around the element 30 is introduced into the exhaust duct 36 after passing through the diffusion-controlling layer 34. On the other hand, the atmosphere duct 37 is a space surrounded by the solid electrolyte layer 31 and the ceramic base material 35, and is open to the atmosphere.

[0017] The 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 an exhaust duct 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 an atmosphere duct 37.

[0018] When a voltage is applied between the exhaust-side electrode 32 and the atmosphere-side electrode 33, oxygen ions move within the solid electrolyte layer 31 in accordance with the oxygen partial pressure difference between the exhaust duct 36 and the atmosphere duct 37. As a result, an output current corresponding to the oxygen concentration in the exhaust flows between the exhaust-side electrode 32 and the atmosphere-side electrode 33. When the air-fuel ratio of the mixture, which is closely related to the oxygen concentration in the exhaust, is the stoichiometric air-fuel ratio, the output current of the air-fuel ratio sensor 85 is "0." Furthermore, as the air-fuel ratio becomes richer, the output current increases in the negative direction, and as the air-fuel ratio becomes leaner, the output current increases in the positive direction. Therefore, the degree to which the air-fuel ratio of the mixture is lean or rich can be detected based on the output current of the air-fuel ratio sensor 85.

[0019] <Air-fuel ratio sensor output abnormality> As shown in FIG. 2, when an element crack CR occurs, which causes exhaust gas to flow into the atmospheric duct 37, an output abnormality occurs in which the output value of the air-fuel ratio sensor 85 becomes leaner than the stoichiometric air-fuel ratio, even though the actual air-fuel ratio is richer than the stoichiometric air-fuel ratio.

[0020] Here, the inventors have found that when an element crack occurs in the air-fuel ratio sensor 85, causing exhaust gas to flow into the air duct 37 of the air-fuel ratio sensor 85, the abnormal output value AFe output from the air-fuel ratio sensor 85 correlates with the amount of oxygen deficiency in the air duct 37.

[0021] The oxygen deficiency is the amount of oxygen that is insufficient relative to the minimum amount of oxygen required for complete combustion of all fuel in the mixture. This oxygen deficiency is a value that correlates with the absolute value of the atmospheric duct oxygen concentration Ct (described later) when it is a negative value.

[0022] Furthermore, the oxygen concentration Ct in the atmospheric duct decreases as the inflow exhaust gas amount QE flowing into the atmospheric duct 37 increases, and therefore the oxygen concentration Ct in the atmospheric duct is a value that correlates with the inflow exhaust gas amount QE.

[0023] FIG. 3 shows a schematic diagram of the change in output of the air-fuel ratio sensor 85 in which the element crack occurs in an environment where the air-fuel ratio is rich. As shown in FIG. 3 , in the oxygen-presence region where oxygen is present in the atmospheric duct 37, the air-fuel ratio sensor 85 outputs a normal air-fuel ratio AF. Meanwhile, as exhaust gas flows into the atmospheric duct 37, the oxygen in the atmospheric duct 37 decreases. When this decrease in oxygen causes the atmospheric duct oxygen concentration Ct, which indicates the oxygen concentration in the atmospheric duct 37, to reach an abnormal output start concentration Tc, which is approximately 0%, the output of the air-fuel ratio sensor 85 begins to lean, and the abnormal output value AFe begins to be output. Then, in the transition region until the atmospheric duct oxygen concentration Ct falls to a predetermined upper concentration limit Cn, the abnormal output value AFe increases as the atmospheric duct oxygen concentration Ct falls. Then, in the oxygen-deficient region after the atmospheric duct oxygen concentration Ct reaches the upper concentration limit Cn, the increase in the abnormal output value AFe stops, saturates, and becomes approximately constant. The abnormal output start concentration Tc and the upper concentration limit Cn can be determined in advance through experiments, etc.

[0024] Therefore, the control device 100 of this embodiment executes a process to calculate the atmospheric duct oxygen concentration Ct, which is a value indicating the amount of oxygen deficiency when it is assumed that an element crack has occurred, based on the inflow exhaust gas amount QE, etc. Then, the control device 100 executes a process to calculate the abnormal output value AFe based on the calculated atmospheric duct oxygen concentration Ct. In this way, the control device 100 functions as an estimation device that estimates the abnormal output value AFe of the air-fuel ratio sensor 85 when an element crack CR has occurred, causing exhaust gas to flow into the atmospheric duct 37.

[0025] The procedure for calculating the abnormal output value AFe will be described below. <Calculation of the amount of air entering> First, the control device 100 calculates the inflow air amount Qa (unit: ml / min), which is the amount of air flowing into the air duct 37. This inflow air amount Qa is affected by the air-fuel ratio AF of the exhaust gas and the oxygen concentration in the air duct 37.

[0026] The maximum inflow air amount Qa(AF), which is a value required to derive the inflow air amount Qa and is the maximum value of the inflow air amount Qa when the exhaust air-fuel ratio AF is a predetermined rich value, can be estimated, for example, as follows. That is, in an experimental environment where exhaust gas (model gas) can be caused to flow steadily / stably under predetermined constant exhaust pressure conditions, a relational expression (simple correlation expression) between the exhaust air-fuel ratio AF and the maximum inflow air amount Qa(AF) is created. The following expression (1) is the relational expression obtained from such an estimation.

[0027] Qa(AF) = α × AF + β…(1) The values α and β in the formula (1) are constants obtained from experimental results, and will vary depending on the structure of the air-fuel ratio sensor 85.

[0028] The value of Qa(AF) calculated by this formula (1) is the inflow air amount Qa at the concentration upper limit value Cn at which the abnormal output value AFe shown in Fig. 3 saturates at a predetermined air-fuel ratio AF. Formula (1) is an approximation formula obtained from the results of an experiment in which the inflow air amount Qa at the concentration upper limit value Cn was measured at a plurality of rich air-fuel ratios.

[0029] Then, the control device 100 calculates the amount of inflowing air Qa based on the above formula (1) and the following formula (2). Qa = Qa(AF) × (Ca-Ct) / Ca…(2) Ca: Atmospheric oxygen concentration = 20.95% Ct: Currently calculated oxygen concentration in the atmospheric duct (%) <Calculating the inflow volume> The control device 100 calculates the inflow exhaust gas amount QE (unit: ml / min), which is the amount of exhaust gas flowing into the atmospheric duct 37. The inflow exhaust gas amount QE is a value correlated to the hole diameter when drilling a hole simulating an element crack, and the larger the hole diameter, the greater the inflow exhaust gas amount QE. Furthermore, the higher the exhaust pressure P around the air-fuel ratio sensor 85, the greater the inflow exhaust gas amount QE. The control device 100 calculates the inflow exhaust gas amount QE at the exhaust pressure P based on the following equation (3).

[0030] QE=QPa×(P / Pa)^(1 / 2)…(3) QPa: Inflow exhaust volume at a given exhaust pressure (e.g., 400 kPa) for a given hole diameter P: Exhaust pressure around the air-fuel ratio sensor 85 (kPa) Pa: The specified exhaust pressure (e.g., 400 kPa) The value of QPa is determined in advance by experimentation or the like based on the inflow exhaust gas volume corresponding to various hole diameters. The inflow exhaust gas volume corresponding to a hole diameter under a given exhaust pressure can be determined from a theoretical formula, map data, or the like. The abnormal output value AFe for a different hole diameter is calculated by changing the value of QPa to a value corresponding to the hole diameter.

[0031] The exhaust pressure P is calculated based on the intake air volume GA. <Calculation of oxygen concentration in atmospheric duct> The control device 100 calculates the oxygen concentration Ct in the atmospheric duct based on the following equation (4).

[0032] Ct=Ctp+{(CE-Ctp)×QE+(Ca-Ctp)×Qa}×t / V…(4) Ctp: Oxygen concentration in the atmospheric duct calculated in the previous calculation cycle Ct (%) CE: Exhaust oxygen concentration (%) QE: Inflow displacement (ml / min) Ca: Atmospheric oxygen concentration = 20.95% Qa: Inflow air volume (ml / min) t: Calculation period of oxygen concentration Ct in the atmospheric duct (ms) V: Effective detection volume (ml) The exhaust oxygen concentration CE in equation (4) is calculated from the following equation (5).

[0033] CE=20.95×(1-AFs / AF)…(5) AFs: Stoic AF AF: Air-fuel ratio detected by the air-fuel ratio sensor The effective detection volume V in equation (4) is the volume of the region facing the atmosphere-side electrode 33 in the atmosphere duct 37.

[0034] And <Calculation of Abnormal Output Value> When the oxygen concentration Ct in the air duct is a value within the above-described transition region (when Cn < Ct < Tc), the control device 100 calculates the abnormal output value AFe based on the following formula (6).

[0035] AFe = AF + Sp × (Ct - Tc)…(6) AF: Air-fuel ratio detected by the air-fuel ratio sensor Sp: Slope of the abnormal output value AFe Ct: Oxygen concentration in the air duct (%) Tc: Abnormal output start concentration Tc (%) The slope Sp in formula (6) is the change amount of the abnormal output value AFe with respect to the unit change amount of the oxygen concentration Ct in the air duct, and is a negative value obtained through preliminary experiments.

[0036] Further, when the oxygen concentration Ct in the air duct is equal to or higher than the above-described concentration upper limit value Cn, the control device 100 calculates the abnormal output value AFe by substituting the value of "Cn" for "Ct" in the above formula (6).

[0037] Thus, in this embodiment, the value of QPa is set according to the pore diameter of the hole simulating element cracking. And the abnormal output value AFe is calculated based on the intake air amount GA and the air-fuel ratio AF generally measured in the internal combustion engine 10.

[0038] <Operations and Effects of this Embodiment> (1) The inventor of the present invention has found that when an element crack occurs in the air duct 37 of the air-fuel ratio sensor 85 where exhaust gas flows in, the abnormal output value AFe output from the air-fuel ratio sensor 85 correlates with the oxygen deficiency amount in the air duct 37. Therefore, the oxygen concentration Ct in the air duct indicating such an oxygen deficiency amount is calculated, and the abnormal output value AFe is calculated based on the calculated oxygen concentration Ct in the air duct. Therefore, even without prototyping the air-fuel ratio sensor 85 that has caused an element crack, it becomes possible to grasp the abnormal output value AFe when an element crack occurs.

[0039] (2) The abnormal output value AFe when an element crack occurs can be determined without having to prototype an air-fuel ratio sensor 85 with an element crack. Therefore, it is possible to inexpensively adapt the abnormality determination value for detecting an abnormality in the air-fuel ratio sensor 85.

[0040] <Example of change> The above embodiment can be modified as follows: The above embodiment and the following modifications can be combined with each other within the scope of technical compatibility.

[0041] The oxygen concentration Ct in the atmospheric duct was calculated based on the inflow exhaust volume QE, but it may be calculated using other methods. Although exhaust pressure P is calculated from the intake air amount GA, it can also be detected by a pressure sensor. [Explanation of symbols]

[0042] 10...internal combustion engine, 11...cylinder, 15...intake passage, 16...throttle valve, 17...fuel injector, 19...spark plug, 21...exhaust passage, 22...catalyst, 30...element, 31...solid electrolyte layer, 32...exhaust side electrode, 33...atmosphere side electrode, 34...diffusion-controlling layer, 35...ceramic substrate, 36...exhaust duct, 37...atmosphere duct, 38...electric heater, 81...air flow meter, 82...water temperature sensor, 83...crank angle sensor, 84...accelerator sensor, 85...air-fuel ratio sensor, 100...control device, 110...CPU, 120...memory

Claims

[Claim 1] 1. A device for estimating an abnormal output value of an air-fuel ratio sensor provided in an exhaust system of an internal combustion engine when an element crack occurs due to exhaust gas flowing into an atmospheric duct of the air-fuel ratio sensor, A process of calculating a value indicating the amount of oxygen deficiency in the atmospheric duct when it is assumed that an element crack has occurred; and calculating the abnormal output value based on the value indicating the amount of oxygen deficiency. Air-fuel ratio sensor estimation device.

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