Air-fuel ratio calculation device

Abstract

To provide an air-fuel ratio calculation device in which deterioration in air-fuel ratio calculation accuracy is suppressed.SOLUTION: An air-fuel ratio calculation device applied to an air-fuel ratio sensor including a sensor element installed in an exhaust passage of an engine and a heater for heating the sensor element includes: an acquisition unit for acquiring an output value of the sensor element and a target air-fuel ratio of the engine; an impedance calculation unit for calculating impedance of the sensor element; an electric power calculation unit for calculating an electric power integrated value fed to the heater; and an air-fuel ratio calculation unit for calculating an air-fuel ratio on the basis of the output value. When the air-fuel ratio corresponding to the output value indicates a rich air-fuel ratio smaller than a theoretical air-fuel ratio, the air-fuel ratio calculation unit calculates the air-fuel ratio more on a lean side as the target air-fuel ratio is on a leaner side, as the impedance is higher, and as the electric power integrated value is smaller.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] A device is known that calculates an air-fuel ratio based on an output value of an air-fuel ratio sensor that includes a sensor element disposed in an exhaust passage of an engine and a heater that heats the sensor element. While the heater is heating the sensor element to an activation temperature, a so-called cold shoot phenomenon may occur, in which the output value of the air-fuel ratio sensor shifts to the rich side relative to the actual air-fuel ratio of the gas (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-114992 Summary of the Invention [Problem to be solved by the invention]

[0004] Such a phenomenon may result in a decrease in the accuracy of the calculation of the air-fuel ratio.

[0005] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an air-fuel ratio calculation device that suppresses a decrease in the accuracy of air-fuel ratio calculation. [Means for solving the problem]

[0006] The above object can be achieved by an air-fuel ratio calculation device that is applied to an air-fuel ratio sensor that includes a sensor element installed in an exhaust passage of an engine and a heater that heats the sensor element, the air-fuel ratio calculation device comprising: an acquisition unit that acquires an output value of the sensor element and a target air-fuel ratio of the engine; an impedance calculation unit that calculates the impedance of the sensor element; a power calculation unit that calculates an integrated value of power input to the heater; and an air-fuel ratio calculation unit that calculates the air-fuel ratio based on the output value, wherein when the air-fuel ratio corresponding to the output value indicates a rich air-fuel ratio that is smaller than the stoichiometric air-fuel ratio, the air-fuel ratio calculation unit calculates the air-fuel ratio to be leaner as the target air-fuel ratio becomes leaner, the impedance becomes higher, and the integrated value of power becomes smaller.

[0007] The power calculation unit resets the integrated power value to zero when the impedance is equal to or greater than a predetermined value and the heater has been deenergized for a predetermined time or more, the predetermined value being the impedance indicating the temperature of the sensor element at which adsorption of HC components into the sensor element begins, and the predetermined time may be the minimum time at which a cold shoot phenomenon occurs the next time the engine is started.

[0008] The power calculation unit may calculate the integrated power value based on a voltage applied to the heater and a duty ratio of power supply to the heater. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide an air-fuel ratio calculation device in which a decrease in the accuracy of calculating the air-fuel ratio is suppressed. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1A is a schematic diagram of an engine system, and FIG. 1B is a schematic diagram of an air-fuel ratio sensor. [Figure 2] FIG. 2 is a flowchart illustrating an example of the air-fuel ratio calculation control. [Figure 3]3A and 3B are diagrams showing examples of maps for calculating the air-fuel ratio. [Figure 4] 4A and 4B are diagrams showing examples of maps for calculating the air-fuel ratio. [Figure 5] FIG. 5 is a flowchart illustrating an example of the power integration value calculation control. DETAILED DESCRIPTION OF THE INVENTION

[0011] [Engine system overview] 1A is a schematic diagram of an engine system 1. The engine system 1 has an engine 10, an intake passage 20, and an exhaust passage 30. The engine system 1 is mounted on a vehicle. The vehicle may be, for example, an engine vehicle equipped with only the engine 10 as a power source for running, or may be a hybrid vehicle equipped with both the engine 10 and a motor as a power source for running.

[0012] The engine 10 is a multi-cylinder engine having multiple cylinders. The engine 10 is provided with an in-cylinder injection valve 12 and a spark plug 14. The in-cylinder injection valve 12 directly injects fuel into the combustion chamber of the engine 10. Note that a port injection valve may be provided instead of or in addition to the in-cylinder injection valve 12. The spark plug 14 ignites the mixture of fuel and air. A throttle valve 22 is provided in the intake passage 20. The throttle valve 22 is driven, for example, by an actuator (not shown) to adjust the amount of intake air.

[0013] A catalytic device 32 is provided in the exhaust passage 30. The catalytic device 32 purifies harmful components in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into it is within a narrow range near stoichiometry. An air-fuel ratio sensor 40 is provided upstream of the catalytic device 32. The air-fuel ratio sensor 40 outputs a signal corresponding to the air-fuel ratio of the exhaust gas flowing into the catalytic device 32. The air-fuel ratio sensor 40 may also be provided downstream of the catalytic device 32.

[0014] The engine system 1 includes an ECU (Electronic Control Unit) 50. The ECU 50 is mainly composed of a computer including a CPU (Central Processing Unit) and volatile and non-volatile memories such as RAM (Random Access Memory) and ROM (Read Only Memory). The ECU 50 executes various control processes related to the engine 10 by running programs installed in the memory on the CPU. The ECU 50 is an example of an air-fuel ratio calculation device, and functionally implements an acquisition unit, an impedance calculation unit, a power calculation unit, and an air-fuel ratio calculation unit, which will be described in detail later.

[0015] The ECU 50 calculates the air-fuel ratio of the exhaust gas based on the output value of the air-fuel ratio sensor 40. Based on the calculated air-fuel ratio, the ECU 50 feedback-controls the intake air amount and fuel injection amount of the engine 10 so that the air-fuel ratio of the exhaust gas from the engine 10 becomes a target air-fuel ratio. The target air-fuel ratio is set by the ECU 50 according to the operating state of the engine 10. The ECU 50 also executes control to maintain the temperature of the sensor element of the air-fuel ratio sensor 40 within an active temperature range.

[0016] [Outline of the air-fuel ratio sensor] FIG. 1B is a schematic diagram of an air-fuel ratio sensor. The air-fuel ratio sensor 40 includes a sensor element 41 and a heater 42. The sensor element 41 outputs an output current value corresponding to the air-fuel ratio of the exhaust gas to the ECU 50. As will be described in detail later, the ECU 50 acquires the output current value of the sensor element 41 and calculates the air-fuel ratio based on this output current value. The ECU 50 also detects the voltage applied between the electrodes of the sensor element 41 and the current flowing between the electrodes, and calculates the impedance based on these. A higher impedance indicates a lower temperature of the sensor element 41. The heater 42 heats the sensor element 41 to its activation temperature and maintains it at the activation temperature. A voltage is applied to the heater 42 from a battery 60 via the ECU 50. The ECU 50 also controls the duty ratio of the current supplied to the heater 42.

[0017] For example, while the heater 42 is heating the sensor element 41 to its activation temperature during engine start-up, a so-called cold shoot phenomenon may occur, in which the output current value of the air-fuel ratio sensor deviates toward the rich side relative to the actual air-fuel ratio of the gas. The cold shoot phenomenon occurs when HC (hydrocarbon) components adsorbed to the sensor element 41 in the exhaust passage 30 during engine stop are desorbed by the heating, causing the atmosphere around the sensor element 41 to become rich. The desorption of HC components from the sensor element 41 progresses with the temperature rise of the sensor element 41 and the passage of time, eliminating the cold shoot phenomenon. However, while the cold shoot phenomenon is occurring, the accuracy of the air-fuel ratio calculation decreases. As a result, it becomes difficult to control the actual air-fuel ratio to the target air-fuel ratio, which may result in a deterioration in emissions and drivability. Therefore, the ECU 50 of this embodiment executes the following air-fuel ratio calculation control.

[0018] [Air-fuel ratio calculation control] FIG. 2 is a flowchart illustrating an example of air-fuel ratio calculation control. The ECU 50 acquires an output current value of the sensor element 41 (step S1). Next, the ECU 50 acquires a target air-fuel ratio of the engine 10 (step S2). Steps S1 and S2 are an example of processing executed by an acquisition unit. Next, the ECU 50 calculates the impedance of the sensor element 41 (step S3). Step S3 is an example of processing executed by an impedance calculation unit. Next, the ECU 50 calculates an integrated value of the power input to the heater 42 (step S4). Step S4 is an example of processing executed by a power calculation unit. The method of calculating the integrated power value will be described in detail later. The order of steps S1 to S4 does not matter.

[0019] Next, the ECU 50 refers to the map and calculates the air-fuel ratio based on the output current value, the target air-fuel ratio, the impedance, and the integrated power value (step S5). Step S5 is an example of a process executed by the air-fuel ratio calculation unit.

[0020] 3A to 4B are exemplary diagrams of maps for calculating the air-fuel ratio. In FIGS. 3A to 4B, the vertical axis represents the air-fuel ratio and the horizontal axis represents the output current value of the sensor element 41. FIG. 3A shows the air-fuel ratio as a function of impedance when the integrated power value is small. FIG. 3B shows the air-fuel ratio as a function of impedance when the integrated power value is large. In the maps of FIGS. 3A and 3B, the target air-fuel ratio is set to the same value. FIG. 4A shows the air-fuel ratio as a function of integrated power value when the target air-fuel ratio is the stoichiometric air-fuel ratio (14.6). FIG. 4B shows the air-fuel ratio as a function of integrated power value when the target air-fuel ratio is a rich air-fuel ratio (13.0). In the maps of FIGS. 4A and 4B, the impedance has the same value. In addition to the maps shown in FIGS. 3A to 4B, a plurality of maps with different integrated power values ​​and target air-fuel ratios are pre-stored in the ROM of the ECU 50.

[0021] In the examples of Figures 3A and 3B, the case where impedance IP is value i1 is shown by a solid line. The case where impedance IP is value i2 higher than value i1 and the case where impedance IP is value i3 higher than value i2 are shown by dotted lines, respectively. When the output current value is zero, the air-fuel ratio is calculated as the stoichiometric air-fuel ratio (14.6). When the output current value is greater than zero, the air-fuel ratio is calculated as a lean air-fuel ratio. When the output current value is less than zero, the air-fuel ratio is calculated as a rich air-fuel ratio.

[0022] According to the maps of Figures 3A and 3B, when the output current value is 0 or less and the impedance of the sensor element 41 is higher, the calculated air-fuel ratio is leaner. As described above, the higher the impedance of the sensor element 41, the lower the temperature of the sensor element 41. The lower the impedance of the sensor element 41, the higher the temperature of the sensor element 41. When the sensor element 41 is at a low temperature, HC components attached to the sensor element 41 are less likely to desorb. When the sensor element 41 is at a high temperature, desorption of HC components from the sensor element 41 progresses. However, the actual desorption of HC components progresses with the elapsed time after the sensor element 41 reaches a predetermined temperature. For this reason, the air-fuel ratio is also calculated based on the integrated value of power to the heater 42.

[0023] 3A and 3B, when the output current value is 0 or less, the smaller the integrated power value, the leaner the calculated air-fuel ratio. This is because the rate at which the desorption of HC components progresses is slow while the integrated power value is small, and as the integrated power value increases, the rate at which the desorption of HC components progresses increases, eliminating any deviation on the rich side from the actual air-fuel ratio.

[0024] 4A and 4B, the case where the integrated power value W is a3 is shown by a solid line. The cases where the integrated power value W is a2, which is lower than a3, and a1, which is lower than a2, are shown by dotted lines. As described above, the smaller the integrated power value is when the output current value is 0 or less, the leaner the calculated air-fuel ratio is.

[0025] As shown in FIGS. 4A and 4B, the leaner the target air-fuel ratio, the leaner the calculated air-fuel ratio. Here, the leaner the target air-fuel ratio, the leaner the air-fuel ratio of the actual gas. As described above, the cold shoot phenomenon occurs when HC components adhering to the sensor element 41 are desorbed, causing the atmosphere around the sensor element 41 to become rich. Therefore, it is considered that the output current value of the sensor element 41 when the cold shoot phenomenon occurs does not depend on the air-fuel ratio of the actual gas. As a result, it is considered that the leaner the target air-fuel ratio, the larger the deviation of the output current value of the sensor element 41 from the air-fuel ratio of the actual gas on the rich side, and the richer the target air-fuel ratio, the smaller the deviation of the output current value of the sensor element 41 from the air-fuel ratio of the actual gas on the rich side.

[0026] As described above, the air-fuel ratio is calculated with high accuracy based on the output current value, the target air-fuel ratio, the impedance, and the integrated power value. In the above example, the air-fuel ratio is calculated based on a map, but the air-fuel ratio may also be calculated using an arithmetic expression that uses the output current value, the target air-fuel ratio, the impedance, and the integrated power value as arguments.

[0027] [Integrated power calculation control] Next, the control for calculating the integrated power value will be described. Fig. 5 is a flowchart illustrating the control for calculating the integrated power value. The ECU 50 calculates the integrated power value using the following equation (1) (step S11). Integrated power value = previous value + (heater voltage) 2 × Duty ratio…(1) The previous value is the integrated power value calculated last time. The heater voltage is the voltage applied to the heater 42, which corresponds to the voltage of the battery 60 in this embodiment. The duty ratio is the power duty ratio of the heater 42. The ECU 50 calculates the integrated power value for each unit time based on the above formula (1).

[0028] Next, the ECU 50 determines whether a predetermined time has elapsed during which the impedance of the sensor element 41 is equal to or greater than a predetermined value and the duty ratio is 0 (step S12). An impedance equal to or greater than the predetermined value indicates that the temperature of the sensor element 41 is equal to or lower than the temperature at which the sensor element 41 begins to adsorb HC components. A duty ratio of 0 indicates that the engine 10 is in a stopped state and power supply to the heater 42 is stopped. The predetermined time indicates the minimum time during which the cold shoot phenomenon may occur the next time the engine 10 is started. Therefore, if the determination in step S12 is Yes, it is determined that the cold shoot phenomenon may occur the next time the engine 10 is started, and the ECU 50 resets the integrated power value to zero (step S13).

[0029] If the answer is No in step S12, this control ends. Note that if the answer is No in step S12, for example, the engine 10 is restarted immediately after the engine 10 is stopped and before the HC components are adsorbed. In this case, the HC components are not adsorbed to the sensor element 41, so the integrated power value is not reset and calculation of the integrated power value continues.

[0030] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and variations are possible within the scope of the gist of the present invention as defined in the claims. [Explanation of symbols]

[0031] 10 Engine 40 Air-fuel ratio sensor 41 Sensor element 42 Heater 50 ECU (air-fuel ratio calculation device, acquisition unit, impedance calculation unit, power calculation unit, air-fuel ratio calculation unit)

Claims

1. An air-fuel ratio calculation device applied to an air-fuel ratio sensor including a sensor element installed in an exhaust passage of an engine and a heater for heating the sensor element, an acquisition unit that acquires an output value of the sensor element and a target air-fuel ratio of the engine; an impedance calculation unit that calculates the impedance of the sensor element; a power calculation unit that calculates an integrated value of power input to the heater; an air-fuel ratio calculation unit that calculates an air-fuel ratio based on the output value, an air-fuel ratio calculation device, wherein, when the air-fuel ratio corresponding to the output value indicates a rich air-fuel ratio that is smaller than a stoichiometric air-fuel ratio, the air-fuel ratio calculation unit calculates the air-fuel ratio to be leaner as the target air-fuel ratio becomes leaner, as the impedance becomes higher, and as the power integrated value becomes smaller.

2. the power calculation unit resets the integrated power value to zero when the impedance is equal to or greater than a predetermined value and the heater has been deenergized for a predetermined period of time or more; the predetermined value is the impedance indicating a temperature of the sensor element at which adsorption of HC components into the sensor element begins, 2. The air-fuel ratio calculation device according to claim 1, wherein the predetermined time is the minimum time within which the cold shoot phenomenon occurs the next time the engine is started.

3. 3. The air-fuel ratio calculation device according to claim 1, wherein the power calculation unit calculates the integrated power value based on a voltage applied to the heater and a duty ratio of power supply to the heater.

Images