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] There is known a device 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 sensor element is being heated to an activation temperature by the heater, the output value of the air-fuel ratio sensor may shift to the rich side relative to the air-fuel ratio of the actual gas, which is called a cold shoot phenomenon (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2009-114992 A 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 PRESENT EMBODIMENTS Accordingly, an object of the present invention is to provide an air-fuel ratio calculation device in which a decrease in the accuracy of calculation of the air-fuel ratio is suppressed. [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 including: 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 an 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 an 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 a stoichiometric air-fuel ratio, the air-fuel ratio calculation unit calculates the air-fuel ratio to be on the leaner side as the target air-fuel ratio is on the leaner side, the impedance is higher, and the integrated power value is 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 the 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. Effect 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 calculation accuracy of the air-fuel ratio is suppressed. [Brief description 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. [Diagram 2] FIG. 2 is a flowchart illustrating an example of the air-fuel ratio calculation control. [Diagram 3]3A and 3B are diagrams illustrating maps for calculating the air-fuel ratio. [Figure 4] 4A and 4B are diagrams illustrating maps for calculating the air-fuel ratio. [Diagram 5] FIG. 5 is a flowchart illustrating an example of the power integration value calculation control. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] [Outline of engine system configuration] 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 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 a 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 a mixture of fuel and air. The intake passage 20 is provided with a throttle valve 22. The throttle valve 22 is driven by, for example, 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 the catalytic device 32 is in a narrow range near stoichiometric. 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 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 a RAM (Random Access Memory) and a ROM (Read Only Memory). The ECU 50 executes various control processes related to the engine 10 by executing programs installed in the memory on the CPU. The ECU 50 is an example of an air-fuel ratio calculation device, and functionally realizes 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 of 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] 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 according to the air-fuel ratio of the exhaust gas to the ECU 50. The ECU 50, which will be described in detail later, 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. The higher the impedance, the lower the temperature of the sensor element 41. The heater 42 raises the sensor element 41 to an activation temperature and maintains the sensor element 41 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, when the heater 42 is heating the sensor element 41 to an activation temperature at the start of the engine 10, the output current value of the air-fuel ratio sensor may deviate toward the rich side with respect to the air-fuel ratio of the actual gas, which is called a cold shoot phenomenon. The cold shoot phenomenon occurs when the HC (hydrocarbon) components in the exhaust passage 30 adsorbed to the sensor element 41 at the time of stopping the engine 10 are desorbed by the above heating, and the atmosphere in the vicinity of the sensor element 41 becomes rich. The desorption of the HC components from the sensor element 41 progresses with the temperature rise of the sensor element 41 and the passage of time, and the cold shoot phenomenon is eliminated. However, when such a cold shoot phenomenon occurs, the calculation accuracy of the air-fuel ratio decreases. As a result, it becomes difficult to control the actual air-fuel ratio to the target air-fuel ratio, which may lead to 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 the 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 the 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 the 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 the 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] Figs. 3A to 4B are exemplary diagrams of maps for calculating the air-fuel ratio. In Figs. 3A to 4B, the vertical axis indicates the air-fuel ratio and the horizontal axis indicates the output current value of the sensor element 41. Fig. 3A shows the air-fuel ratio according to the impedance when the power integration value is small. Fig. 3B shows the air-fuel ratio according to the impedance when the power integration value is large. In the maps of Fig. 3A and Fig. 3B, the target air-fuel ratio is set to the same value. Fig. 4A shows the air-fuel ratio according to the integrated power value when the target air-fuel ratio is the stoichiometric air-fuel ratio (14.6). Fig. 4B shows the air-fuel ratio according to the integrated power value when the target air-fuel ratio is a rich air-fuel ratio (13.0). In the maps of Fig. 4A and Fig. 4B, the impedance is the same value. In the ROM of the ECU 50, in addition to the maps illustrated in Figs. 3A to 4B, a plurality of maps with different power integration values and target air-fuel ratios are stored in advance.

[0021] In the examples of Fig. 3A and Fig. 3B, the case where the impedance IP is the value i1 is shown by a solid line. The cases where the impedance IP is a value i2 higher than the value i1 and a value i3 higher than the 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 Fig. 3A and Fig. 3B, when the output current value is 0 or less and the impedance of the sensor element 41 is higher, it is calculated as a lean-side air-fuel ratio. 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, the HC components attached to the sensor element 41 are difficult to desorb. When the sensor element 41 is at a high temperature, the desorption of HC components from the sensor element 41 progresses. However, the actual desorption of HC components progresses with the passage of time after the sensor element 41 reaches a predetermined temperature. For this reason, the air-fuel ratio is also calculated based on the power integration value to the heater 42.

[0023] 3A and 3B, when the output current value is equal to or less than 0, the smaller the integrated power value, the leaner the air-fuel ratio calculated is. This is because while the integrated power value is small, the rate at which the desorption of HC components progresses is slow, 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 a value a3 is shown by a solid line. The cases where the integrated power value W is a value a2 lower than the value a3, and a value a1 lower than the value 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 air-fuel ratio is calculated.

[0025] As shown in FIG. 4A and FIG. 4B, the leaner the target air-fuel ratio is, the leaner the air-fuel ratio is calculated. Here, the leaner the target air-fuel ratio is, the leaner the air-fuel ratio of the actual gas is. As described above, the cold shoot phenomenon occurs when the HC components attached to the sensor element 41 are desorbed and the atmosphere in the vicinity of the sensor element 41 becomes rich. For this reason, it is considered that the output current value of the sensor element 41 does not depend on the air-fuel ratio of the actual gas when the cold shoot phenomenon occurs. As a result, it is considered that the leaner the target air-fuel ratio is, 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 becomes, and the richer the target air-fuel ratio is, 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 becomes.

[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 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] [Power integration value calculation control] Next, the power integration value calculation control will be described. Fig. 5 is a flowchart illustrating the power integration value calculation control. The ECU 50 calculates the power integration value by the following formula (1) (step S11). Accumulated power value = previous value + (heater voltage) 2 × Duty ratio…(1) The previous value is the power integrated 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 power integrated 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). The impedance being 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 starts to adsorb HC components. The duty ratio being 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 answer is Yes in step S12, it is deemed that the cold shoot phenomenon may occur the next time the engine 10 is started, and the ECU 50 resets the power integrated 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, since the HC components are not adsorbed by the sensor element 41, the power integrated value is not reset and the calculation of the power integrated 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 described 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 that is applied to an air-fuel ratio sensor including a sensor element installed in an exhaust passage of an engine and a heater that heats 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 for calculating an 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 unit that calculates the air-fuel ratio to be on the leaner side as the target air-fuel ratio becomes leaner, as the impedance becomes higher, and as the power integrated value becomes smaller, when the air-fuel ratio corresponding to the output value indicates a rich air-fuel ratio which is smaller than a theoretical air-fuel ratio.

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 a time period during which power supply to the heater has been stopped has elapsed for a predetermined 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 a 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 current supply to the heater.

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