Air-fuel ratio calculation device

Abstract

To provide an air-fuel ratio calculation device that suppresses decrease in calculation accuracy of the air-fuel ratio.SOLUTION: An air-fuel ratio calculation device applied to an air-fuel ratio sensor including a sensor element and a heater heating the sensor element comprises: an acquisition unit that acquires an output value of the sensor element; an impedance calculation unit that calculates an impedance of the sensor element; a power calculation unit that calculates a power integration value input to the heater; and an air-fuel ratio calculation unit that calculates an air-fuel ratio on the basis of the output value. The air-fuel ratio calculation unit calculates the air-fuel ratio so that the air-fuel ratio when the air-fuel ratio corresponding to the output value indicates a rich air-fuel ratio that is less than a theoretical air-fuel ratio becomes a lean side as the impedance is higher and the power integration 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] BACKGROUND ART There is known an apparatus for calculating an air-fuel ratio based on an output value of an air-fuel ratio sensor that includes a sensor element and a heater that heats the sensor element (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-055279 Summary of the Invention [Problem to be solved by the invention]

[0004] While the heater is heating the sensor element to its activation temperature, the output value of the air-fuel ratio sensor may shift to the rich side relative to the actual air-fuel ratio of the gas, a phenomenon known as cold shoot. This phenomenon may reduce the accuracy of the air-fuel ratio calculation.

[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 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; 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 an air-fuel ratio based on the output value, wherein the air-fuel ratio calculation unit calculates the air-fuel ratio 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 such that the air-fuel ratio becomes leaner as the impedance increases and the integrated value of power decreases.

[0007] The air-fuel ratio calculation unit may calculate the air-fuel ratio without using the integrated power value when the integrated power value is equal to or greater than a predetermined value at which it can be considered that desorption of HC components from the sensor element has been completed.

[0008] The power calculation unit may reset the integrated power value to zero when the impedance is equal to or greater than a predetermined value and the heater has not been energized for a predetermined period of time or more.

[0009] 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]

[0010] 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]

[0011] [Figure 1] FIG. 1 is a schematic diagram of an exhaust system. [Figure 2] FIG. 2 is a schematic diagram of the air-fuel ratio sensor. [Figure 3] FIG. 3 is a flowchart illustrating an example of the air-fuel ratio calculation control. [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

[0012] [Outline of exhaust system] 1 is a schematic diagram of an exhaust system 1. This exhaust system 1 is equipped with an engine 10 and 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.

[0013] An exhaust passage 20 is connected to the engine 10. A catalytic converter 30 is provided midway through the exhaust passage 20. The catalytic converter 30 is configured, for example, in a honeycomb shape, and has multiple internal passages formed in the direction of exhaust flow. A three-way catalyst is supported on the partition walls that separate these internal passages. The three-way catalyst 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.

[0014] An air-fuel ratio sensor 40 is provided upstream of the catalytic device 30. The air-fuel ratio sensor 40 outputs a signal corresponding to the air-fuel ratio of the exhaust gas flowing into the catalytic device 30. The air-fuel ratio sensor 40 may also be provided downstream of the catalytic device 30.

[0015] The exhaust 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.

[0016] 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 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. The ECU 50 receives power from a battery 60.

[0017] [Outline of the air-fuel ratio sensor] FIG. 2 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 the battery 60 via the ECU 50. The ECU 50 also controls the duty ratio of the current supplied to the heater 42.

[0018] 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 20 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.

[0019] [Air-fuel ratio calculation control] FIG. 3 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). Step S1 is an example of processing executed by an acquisition unit. Next, the ECU 50 calculates the impedance of the sensor element 41 (step S2). Step S2 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 S3). Step S3 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 S3 does not matter.

[0020] Next, the ECU 50 calculates the air-fuel ratio based on the output current value, impedance, and integrated power value (step S4). Step S4 is an example of processing executed by the air-fuel ratio calculation unit. FIGS. 4A and 4B are exemplary diagrams of maps for calculating the air-fuel ratio. In FIGS. 4A and 4B, the vertical axis indicates the air-fuel ratio, and the horizontal axis indicates the output current value. FIG. 4A shows a case where the impedance is high, and FIG. 4B shows a case where the impedance is low. In addition to the maps shown in FIGS. 4A and 4B, a plurality of maps corresponding to impedances are stored in advance in the ROM of the ECU 50.

[0021] Each map specifies an air-fuel ratio according to the power integrated value W. In the examples of FIGS. 4A and 4B, the dotted lines indicate the case where the power integrated value W is a1 and the case where the power integrated value W is a2 greater than a1, and the solid lines indicate the case where the power integrated value W is equal to or greater than a3 greater than a2. When the output current value is zero, the air-fuel ratio is calculated as stoichiometric (theoretical air-fuel ratio) ST. 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 above map, the higher the impedance of the sensor element 41 is when the output current value is 0 or less, the leaner the calculated air-fuel ratio is. As described above, the higher the impedance of the sensor element 41, the lower the temperature of the sensor element 41 is, and the lower the impedance of the sensor element 41, the higher the temperature of the sensor element 41 is. When the impedance is high, the temperature of the sensor element 41 is low, so HC components attached to the sensor element 41 are less likely to desorb. When the impedance is low, the temperature of the sensor element 41 is high, so 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 calculated based not only on the impedance of the sensor element 41 but also on the integrated value W of power to the heater 42, as explained below.

[0023] 4A and 4B, the air-fuel ratio is calculated to be leaner as the output current value is 0 or less and the integrated power value W is smaller. This is because the rate at which the desorption of HC components progresses is slow while the integrated power value W is small, and as the integrated power value W increases, the rate at which the desorption of HC components progresses increases, eliminating any deviation from the actual air-fuel ratio on the rich side. In this way, the air-fuel ratio is calculated with high accuracy based on the output current value, impedance, and integrated power value.

[0024] 4A and 4B, when the integrated power value W is equal to or greater than a3, the air-fuel ratio is calculated without using the integrated power value W. This is because it is believed that the HC components have already been sufficiently desorbed when the integrated power value W is equal to or greater than a3. Therefore, the value a3 is set to an integrated power value at which the HC components adsorbed on the sensor element 41 are sufficiently desorbed and the cold shoot phenomenon does not occur. Note that, although the above example illustrates a case in which the air-fuel ratio is calculated based on a map, the air-fuel ratio may also be calculated using an arithmetic expression that uses the output current value, impedance, and integrated power value as arguments.

[0025] [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).

[0026] 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 low enough that HC components are adsorbed by the sensor element 41. 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 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 power integration value to zero (step S13).

[0027] 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.

[0028] 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]

[0029] 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 and a heater that heats the sensor element, an acquisition unit that acquires an output value of the sensor element; 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, the air-fuel ratio calculation unit calculates the air-fuel ratio 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 such that the air-fuel ratio becomes leaner as the impedance increases and the integrated power value decreases.

2. 2. The air-fuel ratio calculation device according to claim 1, wherein the air-fuel ratio calculation unit calculates the air-fuel ratio without relying on the integrated power value when the integrated power value is equal to or greater than a predetermined value at which desorption of HC components from the sensor element can be considered to have been completed.

3. 3. The air-fuel ratio calculation device according to claim 2, wherein 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.

4. 4. 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.

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