JP4609407B2 - Air-fuel ratio control device - Google Patents

Description

  The present invention relates to an air-fuel ratio control device. More preferably, the present invention relates to an air-fuel ratio control apparatus for an internal combustion engine that can use an alternative fuel other than gasoline.

  Japanese Patent Laid-Open No. 4-362242 discloses a control system for an internal combustion engine that can use alcohol fuel in addition to gasoline as fuel for the internal combustion engine. This prior art system includes an alcohol concentration sensor for detecting the alcohol concentration in the fuel in the fuel supply path between the fuel tank and the injector. When starting the internal combustion engine in this system, the temperature and alcohol concentration of the internal combustion engine are detected, and it is determined whether or not the internal combustion engine can be started according to this.

  By the way, when gasoline contains alcohol as fuel, gasoline and alcohol may be separated in the fuel tank or in the fuel supply path when the temperature is low or when the alcohol is replenished. When the internal combustion engine is started in this state, the alcohol concentration in the fuel supply path changes after the internal combustion engine is started. For this reason, the alcohol concentration detected when the internal combustion engine is started may be different from the alcohol concentration in the actual fuel after the internal combustion engine is started. Accordingly, it is conceivable that an erroneous determination occurs in the start determination based on the alcohol concentration at the start of the internal combustion engine.

  In order to prevent such a misjudgment, the system of the prior art detects the alcohol concentration at the start of the internal combustion engine. First, the energization of the starter motor is prohibited and the pump disposed in the fuel tank is operated to operate the fuel. While the fuel is circulated through the supply path, the alcohol concentration sensor repeatedly detects the alcohol concentration. Each time the alcohol concentration is detected, the difference between the alcohol concentration detected last time and the alcohol concentration detected this time is obtained, and when this difference becomes smaller than the judgment value, the alcohol and gasoline are mixed uniformly. Judge that The alcohol concentration at this point is detected as the actual alcohol concentration of the fuel, and start of energization of the starter motor and execution of fuel injection are permitted. Further, control such as start determination of the internal combustion engine is performed by the alcohol concentration and the temperature of the internal combustion engine.

JP-A-4-362242 Japanese Patent Laid-Open No. 5-209549 JP-A-7-197837

  According to the above prior art system, the alcohol concentration can be appropriately detected after the gasoline and alcohol are sufficiently mixed when the internal combustion engine is started. However, in order to detect the alcohol concentration, it is necessary to install a dedicated alcohol concentration sensor in the fuel supply path. Further, it is considered that the starter motor is not energized while the alcohol concentration at the time of starting is detected, so that the starting of the internal combustion engine cannot be started and the startability is lowered.

  Further, when a fuel containing alcohol is used as in the above prior art system, the theoretical air-fuel ratio of the fuel is different from that of gasoline. Therefore, it is difficult to detect the air-fuel ratio of the exhaust gas accurately according to the fuel and perform the air-fuel ratio control according to the fuel properties only by using the exhaust gas sensor used when gasoline is the fuel. In this regard, the above-described prior art system only detects the alcohol concentration, and does not contribute to the air-fuel ratio control by detecting the air-fuel ratio according to the detected alcohol concentration.

  The present invention has been made to solve the above-described problems, and detects a fuel property without separately providing a dedicated sensor for determining the fuel property, and performs air-fuel ratio control in accordance with this. It is an object of the present invention to provide an air-fuel ratio control apparatus improved so as to be able to achieve the above.

In order to achieve the above object, a first invention is an air-fuel ratio control apparatus for controlling an air-fuel ratio of an internal combustion engine that can use gasoline and fuel other than gasoline as fuel.
A positive voltage applying means for applying a positive voltage for detecting the air-fuel ratio of the exhaust gas between the atmosphere-side electrode and the exhaust-side electrode of the air-fuel ratio sensor disposed in the exhaust passage of the internal combustion engine;
Sensor output detecting means for detecting a sensor output of the air-fuel ratio sensor;
Fuel property determining means for determining the fuel property of the fuel used in the internal combustion engine according to the sensor output of the air-fuel ratio sensor;
A property determination time air-fuel ratio control means for controlling the air-fuel ratio of the internal combustion engine to a target air-fuel ratio while determining the fuel property;
Completion determination means for determining whether or not the fuel property determination is completed;
An air-fuel ratio control means for performing air-fuel ratio control according to the determined fuel property based on a sensor output detected during application of the positive voltage when completion of the fuel property determination is recognized;
It is characterized by providing.

In a second aspect based on the first aspect, the fuel is gasoline, alcohol, or a mixed fuel of gasoline and alcohol,
The property determination time air-fuel ratio control means sets the target air-fuel ratio to a theoretical air-fuel ratio,
The fuel property determining means includes
A variation detecting means for detecting a variation of the sensor output detected during application of the positive voltage with respect to a calculated value of the sensor output with respect to the theoretical air-fuel ratio;
Alcohol content rate detecting means for detecting the alcohol content rate in the fuel according to the variation;
It is characterized by providing.

According to a third invention, in the first invention, when the completion of the fuel property determination is not recognized, a large voltage applying means for applying a large voltage larger than the positive voltage between both electrodes of the air-fuel ratio sensor. Further comprising
The fuel is gasoline or alcohol or a mixed fuel of gasoline and alcohol,
The property determination time air-fuel ratio control means sets the target air-fuel ratio to a theoretical air-fuel ratio,
The fuel property determining means includes alcohol content rate detecting means for detecting an alcohol content rate in the fuel in accordance with a sensor output detected during application of the large voltage.

According to a fourth invention, in any one of the first to third inventions,
Map selecting means for selecting a map that defines the relationship between the sensor output detected in the positive voltage application state and the air-fuel ratio in accordance with the determined fuel property when the fuel property is determined;
Air-fuel ratio detecting means for detecting an air-fuel ratio according to the selected map in accordance with a sensor output detected during application of the positive voltage,
The air-fuel ratio control means performs air-fuel ratio control according to the air-fuel ratio when completion of the fuel property determination is recognized.

According to a fifth invention, in any one of the first to fourth inventions, an electromotive force generated between both electrodes of the air-fuel ratio sensor is detected without applying a voltage between both electrodes of the air-fuel ratio sensor. With electromotive force detection means,
The property determination time air-fuel ratio control means performs air-fuel ratio control according to the electromotive force.

According to a sixth invention, in the fifth invention,
Impedance detection voltage application means for applying an impedance detection voltage for detecting an element impedance of a sensor element of the air / fuel ratio sensor to the air / fuel ratio sensor;
An element impedance detection means for detecting the element impedance in accordance with a sensor output detected during application of the impedance detection voltage;
First element temperature determining means for determining whether or not the temperature of the sensor element of the air-fuel ratio sensor has reached a first activation temperature based on the element impedance when completion of the fuel property determination is not permitted; ,
Second element temperature determining means for determining whether or not the temperature of the sensor element has reached a second activation temperature based on the element impedance when completion of the fuel property determination is recognized;
Further comprising
The electromotive force detection means detects the electromotive force when it is recognized that the temperature of the sensor element has reached the first activation temperature,
The air-fuel ratio control means performs air-fuel ratio control according to the sensor output during application of the positive voltage when it is recognized that the temperature of the sensor element has reached the second activation temperature.

  According to the first invention, the output of the air-fuel ratio sensor arranged in the exhaust passage of the internal combustion engine while controlling the air-fuel ratio of the internal combustion engine that can use gasoline and fuel other than gasoline as the fuel becomes the target air-fuel ratio. Accordingly, the fuel properties of the fuel used in the internal combustion engine can be determined. In addition, after the fuel property determination is completed, air-fuel ratio control is performed according to the determined fuel property based on the sensor output detected during application of the positive voltage of the air-fuel ratio sensor. Thereby, it is not necessary to install a dedicated sensor for the fuel property, and the fuel property can be determined with a simpler system while preventing the startability from being deteriorated. In addition, air-fuel ratio control during fuel property determination is ensured, and after the fuel property determination is completed, air-fuel ratio control using an air-fuel ratio sensor is performed according to the fuel property. Thereby, it is possible to improve the exhaust emission characteristics even in an internal combustion engine using fuel other than gasoline.

  According to the second invention, in a state where the air-fuel ratio is controlled to the stoichiometric air-fuel ratio, the sensor output detected during application of the positive voltage is detected from the sensor output corresponding to the stoichiometric air-fuel ratio. The alcohol content can be determined by analyzing the above. Therefore, the fuel property can be determined using the air-fuel ratio sensor without installing a dedicated sensor.

  According to the third aspect of the invention, the sensor output is detected by applying a large voltage larger than the positive voltage while the air-fuel ratio is controlled to the stoichiometric air-fuel ratio. From this sensor output, the alcohol content in the fuel can be detected. Therefore, the fuel property can be determined by reliably detecting the alcohol concentration in the mixed fuel of gasoline and alcohol using an air-fuel ratio sensor without installing a dedicated sensor.

  According to the fourth aspect of the invention, it is possible to select a map that defines the relationship between the sensor output during application of positive voltage and the air-fuel ratio according to the determined fuel property, and to detect the air-fuel ratio according to this map. Therefore, even if the fuel property changes, the air-fuel ratio corresponding to the fuel can be detected, and the air-fuel ratio control can be realized with higher accuracy.

  According to the fifth aspect, when the completion of the fuel property determination is not recognized, the air-fuel ratio control is performed according to the electromotive force detected in a state where no voltage is applied to the air-fuel ratio sensor. Here, the electromotive force of the air-fuel ratio sensor changes abruptly in the vicinity of the theoretical air-fuel ratio, regardless of the fuel properties, and shows a substantially constant low value in a state where oxygen is excessive with respect to the theoretical air-fuel ratio, In the state of oxygen deficiency, the value is almost constant and high. Therefore, according to the electromotive force, it is possible to detect whether at least the current air-fuel ratio is excessive or insufficient. Therefore, even in the state where the fuel property until the fuel property determination is completed is not clarified, the air-fuel ratio control at least in the vicinity of the theoretical air-fuel ratio can be realized using the air-fuel ratio sensor. Thereby, even when the fuel property to be used changes, it is possible to improve the exhaust emission characteristics during the fuel property determination.

  According to the sixth aspect of the present invention, the element impedance of the air-fuel ratio sensor can be calculated according to the sensor output detected while applying the impedance detection voltage. Therefore, it is possible to easily detect the element impedance by switching to the impedance detection voltage application while detecting the electromotive force without applying the voltage to the air-fuel ratio sensor. The electromotive force is detected when the sensor element temperature is found to have reached the first activation temperature according to the element impedance, and is positive when the sensor element temperature is found to have reached the second activation temperature. Apply voltage. Thereby, the element temperature of the air-fuel ratio sensor is surely controlled by the activation temperature corresponding to the use state, and a more accurate output of the air-fuel ratio sensor can be obtained.

  Embodiments of the present invention will be described below with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals, and the description thereof is simplified or omitted.

Embodiment 1 FIG.
[Configuration of Air-Fuel Ratio Sensor of Embodiment 1]
FIG. 1 is a schematic diagram for explaining the structure of a system in which the air-fuel ratio control apparatus according to Embodiment 1 of the present invention is mounted. The system shown in FIG. 1 includes an internal combustion engine 10. As the fuel for the internal combustion engine 10, gasoline, ethanol, or a mixed fuel in which gasoline and ethanol are mixed is used. The internal combustion engine 10 includes a plurality of cylinders, and an injector (not shown) that supplies fuel to each cylinder. The gasoline, ethanol or a mixed fuel thereof is supplied from the fuel tank 12 to the injector of each cylinder.

  An exhaust passage 16 is provided in the exhaust system of the internal combustion engine 10. A catalyst 18 is installed in the exhaust passage 16. An air-fuel ratio sensor 20 is attached upstream of the catalyst 18 in the exhaust passage 16. An oxygen sensor 22 is attached downstream of the catalyst 18 in the exhaust passage 16.

  This system includes an ECU (Electronic Control Unit) 24 as an air-fuel ratio control device. An air-fuel ratio sensor 20, an oxygen sensor 22, and the like are connected to the input side of the ECU 24, and outputs from these sensors are taken into the ECU 24 to detect information on fuel properties, air-fuel ratio, and the like. Further, a necessary control signal is issued from the output side of the ECU 24, thereby performing control necessary for the operation of the internal combustion engine 10.

[Air-fuel ratio sensor drive circuit]
FIG. 2 is a circuit diagram for explaining a drive circuit that is connected to the air-fuel ratio sensor 20 and drives the air-fuel ratio sensor 20 in the first embodiment of the present invention. The drive circuit 30 shown in FIG. 2 includes an applied voltage operation unit 32 for supplying an appropriate voltage to the air-fuel ratio sensor 20. Each of the switching circuits 34 and 36 is connected to the applied voltage operation unit 32 via a feedback circuit that controls the supplied voltage to be constant. An anode terminal 38 is connected to the switching circuit 34. A negative terminal 40 is connected to the switching circuit 36. An air electrode (not shown) of the air-fuel ratio sensor 20 is connected to the anode terminal 38 and an exhaust electrode (not shown) is connected to the negative terminal 40.

  The switching circuits 34 and 36 are connected to an output port (not shown) of the ECU 24. The ECU 24 issues a control signal to switch the connection between the switches SW1 and SW2 of the switching circuits 34 and 36. When the switches of the switching circuits 34 and 36 are switched to the SW1 side and SW1 is turned on, the potential of the anode terminal 38 and the potential of the negative terminal 40 are maintained at the voltages supplied from the applied voltage operation unit 32, respectively. The

  Specifically, the applied voltage operation unit 32 can supply a voltage at which the anode terminal 38 has an anode reference potential (eg, 3.3 V) and the negative terminal 40 has a negative electrode reference potential (eg, 2.9 V). In a state where the anode terminal 38 and the negative electrode terminal 40 are maintained at these reference potentials, a positive voltage for air-fuel ratio detection is applied between the anode terminal 38 and the negative electrode terminal 40. That is, in a state where the electrodes of the air-fuel ratio sensor 20 are connected to the terminals 38 and 40, a positive voltage is applied to the air-fuel ratio sensor 20. The potential of the anode terminal 38 at this time is guided to AD1. The ECU 24 takes in the electric potential guided to AD1 to apply the sensor output voltage [V] generated between both electrodes of the air-fuel ratio sensor 20 when a positive voltage is applied, and when the positive voltage is applied to the air-fuel ratio sensor 20. The flowing sensor current can be detected.

  In addition, the applied voltage operation unit 32 can supply a voltage obtained by superimposing an AC voltage on a positive voltage for air-fuel ratio detection between the anode terminal 38 and the negative electrode terminal 40. Thereby, an impedance detection voltage is applied between the electrodes of the air-fuel ratio sensor 20. The ECU 24 can detect the sensor current based on the output from the AD1, and further detect the element impedance based on the sensor current. The element impedance has a correlation with the temperature (element temperature) of the sensor element of the air-fuel ratio sensor 20. Therefore, the element temperature can be detected based on the element impedance, or the element temperature can be controlled according to the element impedance.

  On the other hand, when the switches of the switching circuits 34 and 36 are switched to the SW2 side and SW2 is turned ON, the voltage applied between the anode terminal 38 and the negative electrode terminal 40, that is, the applied voltage to the air-fuel ratio sensor 20 is It becomes zero. In this voltage application OFF state, the negative terminal 40 is grounded, and the positive terminal is connected to AD2 of the ECU 24. As a result, an electromotive force generated in the air-fuel ratio sensor 20 is guided to AD2. The ECU 24 can detect the electromotive force when the voltage application of the air-fuel ratio sensor 20 is turned off by taking in the output of AD2.

[Detecting air-fuel ratio according to fuel properties]
FIG. 3 is a diagram showing the relationship between the output of the air-fuel ratio sensor 20 and the air-fuel ratio of the exhaust gas when a positive voltage is applied to the air-fuel ratio sensor 20 in the first embodiment of the present invention. In FIG. 3, the horizontal axis represents the air-fuel ratio, and the vertical axis represents the sensor current. Also, the solid line (a) in FIG. 3 represents the sensor current when gasoline is used as the fuel for the internal combustion engine 10, and the dotted line (b) is the sensor current when 100% ethanol fuel is used as the fuel for the internal combustion engine 10. Represents.

  When a positive voltage is applied between both electrodes of the air-fuel ratio sensor 20, a sensor current corresponding to the oxygen concentration in the exhaust gas flows between the electrodes. The normal ECU 24 stores a map that defines the relationship between the sensor current and the air-fuel ratio when gasoline is used as fuel, as shown by the solid line (a). When gasoline is used as the fuel, the air-fuel ratio is detected according to this map according to the output (sensor current) of the air-fuel ratio sensor 20 taken into the ECU 24, and the fuel injection amount is calculated based on the air-fuel ratio. Will be performed.

  Here, the sensor current detected when a positive voltage is applied corresponds to the oxygen concentration in the exhaust gas. However, the air-fuel ratio with respect to the oxygen concentration in the exhaust gas differs for each fuel property. Therefore, the relationship between the sensor current and the air-fuel ratio also differs for each fuel property.

  Specifically, for example, as shown by the dotted line (b) in FIG. 3, the relationship between the sensor current and the air-fuel ratio in the case of 100% ethanol is different from that in the case of gasoline (solid line (a)). Here, in the case of 100% ethanol, the theoretical air-fuel ratio A / F = 9. The air-fuel ratio (dotted line (b)) with respect to the sensor current at 100% ethanol shown in FIG. 3 is calculated based on the air-fuel ratio (solid line (a)) in the case of gasoline and when the sensor current is 0 [mA]. It becomes a value approximated to a value shifted parallel to the horizontal axis to a position where the fuel ratio becomes 9. However, especially in the region richer than the stoichiometric air-fuel ratio, the slope of the output characteristics tends to differ between gasoline and ethanol, and the ethanol output characteristics graph (dotted line (b)) shows the gasoline output. The characteristic graph (solid line (a)) is not exactly the same as the parallel shift.

  The characteristics of the air-fuel ratio with respect to such a sensor current are different for each fuel property. For example, in a fuel in which gasoline and ethanol are mixed, the air-fuel ratio corresponding to the sensor current decreases as the mixing ratio of ethanol increases.

  As described above, in this system, gasoline, ethanol, or a mixed fuel thereof is used as the fuel, and the ethanol content in the fuel is appropriately selected and changed between 0% and 100%. For this reason, the ECU 24 stores a map that defines the relationship between the sensor current and the air-fuel ratio for each fuel property (ethanol content). When the fuel is filled or replenished at the time of starting the internal combustion engine 10 and the like, and the fuel properties in the fuel become different, the fuel properties are detected, and the sensor current and empty capacity corresponding to the ethanol content of the fuel are detected. It is possible to switch to a map that defines the relationship with the fuel ratio. By switching to the map corresponding to the fuel property in this way, the air-fuel ratio that matches the fuel property can be calculated based on the sensor current of the air-fuel ratio sensor 20.

[Air-fuel ratio control until map switching]
As described above, even when fuel other than gasoline is used, if the fuel property is determined, the air-fuel ratio sensor 10 switches the map according to the fuel property, so that the air-fuel ratio is based on the sensor current. Can be detected. However, in order to switch maps in this way, a process for determining the fuel property of the internal combustion engine 10 is required every time the fuel property changes.

  The fuel property can be detected based on the output of the air-fuel ratio sensor 20 by a method described later. However, it is difficult to accurately detect the fuel property instantaneously, and a certain amount of time is required. Until this fuel property determination, the relationship between the sensor current of the air-fuel ratio sensor 20 and the air-fuel ratio cannot be specified, so the air-fuel ratio based on the output of the air-fuel ratio sensor 20 cannot be detected. However, in order to improve the exhaust emission characteristics, it is preferable that air-fuel ratio control can be performed even during the fuel property determination.

  Therefore, in the system of the first embodiment, after the fuel property is changed, the electromotive voltage of the air-fuel ratio sensor 20 is detected as voltage application OFF until the fuel property is correctly determined. FIG. 4 is a diagram showing the relationship between the electromotive force of the air-fuel ratio sensor 20 and the air-fuel ratio. In FIG. 4, the horizontal axis represents the air-fuel ratio, and the vertical axis represents the sensor output (electromotive force) [V]. In FIG. 4, the solid line (a) represents the electromotive force when the fuel is 100% gasoline, and the dotted line (b) represents the electromotive force when the fuel is 100% ethanol.

  As shown in FIG. 4, even when the fuel is 100% gasoline (solid line (a)) or when the fuel is 100% ethanol (dotted line (b)), the sensor output is detected with the voltage applied OFF. There is no significant difference in the characteristics of (electromotive force). In other words, the electromotive force shows an almost constant output of about 1 [V] when the exhaust gas is leaner than the stoichiometric air-fuel ratio (excess oxygen), regardless of the fuel properties, and is rich (oxygen deficient) Shows an almost constant output of about 0 [V], and changes suddenly in the vicinity of the theoretical air-fuel ratio.

  Therefore, even in a state where the fuel properties are not known, it is possible to detect whether or not at least the exhaust gas is richer or leaner than the stoichiometric air-fuel ratio by the detected electromotive force. The system of the first embodiment uses this to perform air-fuel ratio control with the stoichiometric air-fuel ratio as the target air-fuel ratio based on the detected electromotive force until the fuel property is determined. Thereafter, when the fuel property is detected, the map is switched to a map corresponding to the fuel property, and application of a positive voltage to the air-fuel ratio sensor 20 is started. Thereby, the air-fuel ratio can be detected correctly according to the fuel property.

[Control of element temperature]
In the above use, when the electromotive force is correctly detected in the voltage application OFF state, the sensor element is preferably used in a state of reaching an activation temperature (first activation temperature) of about 550 ° C. On the other hand, when detecting a sensor current by applying a positive voltage, it is preferable to use the sensor in a state where the activation temperature (second activation temperature) of a normal air-fuel ratio sensor of about 650 ° C. has been reached. Therefore, in the first embodiment, the element temperature is controlled so that the activation temperature is appropriate for the case where the electromotive force is detected and the case where the sensor current is detected by applying a positive voltage. Specifically, since the element temperature has a correlation with the element impedance, the impedance detection voltage is periodically applied to detect the element impedance, and accordingly, the element temperature is installed in the vicinity of the sensor element of the air-fuel ratio sensor 20. The element temperature can be controlled by controlling energization to a heater (not shown).

[Fuel property determination using air-fuel ratio sensor in embodiment 1]
FIG. 5 is a diagram for explaining the variation between the air-fuel ratio and the output of the air-fuel ratio sensor in the first embodiment of the present invention. In FIG. 5, the horizontal axis represents the excess air ratio λ, and the vertical axis represents the output voltage [V] of the air-fuel ratio sensor detected in the AD1 sky when a positive voltage is applied. In FIG. 5, the solid line (a) represents the case of 100% gasoline, and the dotted line (b) represents the case of 100% ethanol.

  From FIG. 5, when the excess air ratio λ is small, that is, when the air-fuel ratio is rich, ethanol is 100% (dotted line (a)), and the slope of the output characteristic of the air-fuel ratio sensor 20 is gasoline 100%. It can be seen that it is larger. That is, in the region where the air-fuel ratio is rich, the sensor output voltage with respect to the excess air ratio λ in the case of 100% ethanol tends to be smaller than that in the case of 100% gasoline.

  FIG. 6 is a diagram showing an output distribution of an air-fuel ratio sensor detected when a positive voltage is applied in a state where the air-fuel ratio is controlled to the stoichiometric air-fuel ratio in the first embodiment of the present invention. FIG. 6A shows a case where the fuel is 100% gasoline, and FIG. 6B shows a case where the fuel is 100% ethanol. As shown in FIG. 6A, if 100% gasoline fuel is used, the output of the air-fuel ratio sensor 20 while the air-fuel ratio is controlled to the stoichiometric air-fuel ratio is the sensor output at the stoichiometric air-fuel ratio. And has a peak at the center.

  On the other hand, in the case of 100% ethanol fuel, the sensor output tends to decrease in the rich region as described above. Accordingly, the distribution of the output of the air-fuel ratio sensor 20 during the control of the air-fuel ratio to the stoichiometric air-fuel ratio is shifted to a richer side than the sensor output voltage at the stoichiometric air-fuel ratio, as shown in FIG. When compared with the output distribution in the case of 100% gasoline, it is shifted to the rich side (the side where the output is small).

  As described above, the deviation of the output distribution of the air-fuel ratio sensor 20 tends to increase (shift toward the rich side) as the content of ethanol fuel increases. This deviation in power distribution has a correlation with the ethanol content in the fuel. Therefore, in Embodiment 1, the sensor output is repeatedly detected by switching to positive voltage application while detecting the electromotive force and controlling the air-fuel ratio to the theoretical air-fuel ratio. Thereafter, a variation in the output (sensor current = 0) corresponding to the theoretical air-fuel ratio of the output is detected, and the fuel property is determined according to the variation.

  More specifically, the difference between the sensor output and the output corresponding to the stoichiometric air-fuel ratio, the sensor output distribution, the average value of the sensor output, or the like is calculated as a parameter indicating variation. On the other hand, a map of the ethanol content as such a parameter is obtained in the ECU 24 through experiments or the like and stored in advance. In the determination of the fuel property, the parameter can be calculated from the sensor output, and the fuel property can be determined according to the map stored in advance.

  FIG. 7 is a timing chart of the control executed by the system in the first embodiment of the present invention. As shown in FIG. 7, SW2 of the switching circuits 34 and 36 is turned on and SW1 is turned off at regular intervals. When SW2 is ON, the voltage application to the air-fuel ratio sensor 10 is turned off, and the electromotive force generated between both electrodes of the air-fuel ratio sensor 20 is guided to AD2. The ECU 24 detects whether the exhaust gas is rich or lean based on the electromotive force. Based on this result, the ECU 24 executes air-fuel ratio control such as correcting the fuel injection amount so that the air-fuel ratio becomes the stoichiometric air-fuel ratio.

  After a predetermined time has elapsed since the switch was set to SW2, the switches of the switching circuits 36 and 38 are switched, and SW1 is turned on and SW2 is turned off. As a result, a positive voltage is applied between both electrodes of the air-fuel ratio sensor 20. At this time, the sensor output of the air-fuel ratio sensor 20 is detected from AD1. The sensor output is detected each time the switching circuits 34 and 36 are turned off and a positive voltage is applied. After the sensor output is detected as many times as necessary, the sensor output is analyzed, and the above parameters (for example, average value) indicating the variation are calculated. Based on this, the fuel property is determined.

  After detecting the sensor output during the application of the positive voltage, the impedance detection voltage is supplied from the applied voltage operation unit 32 with the SW1 being ON each time. That is, an alternating voltage is applied so as to be superimposed on the positive voltage. The sensor output at this time is detected from AD1, the sensor current is obtained, and the element impedance is calculated based on the applied voltage and the fluctuation of the sensor current. Based on this element impedance, the element temperature of the air-fuel ratio sensor 20 is controlled.

  The element impedance of the sensor element is detected by applying an AC voltage to the positive voltage and supplying it after applying the positive voltage for property determination. Therefore, even when the electromotive force is detected with the air-fuel ratio sensor 20 in the voltage application OFF state, it is not necessary to install a voltage application circuit for impedance detection, and the element temperature control can be executed with a normal circuit.

  After the fuel property determination is completed, SW1 of the switching circuits 34 and 36 remains ON. Therefore, the air-fuel ratio sensor 20 is maintained in a state where a positive voltage is applied. Moreover, an alternating voltage is superimposed on a positive voltage at a fixed timing. Thereby, the element impedance can be detected, the air-fuel ratio can be detected as the normal air-fuel ratio sensor 20 even after the fuel property determination, and the element temperature can be controlled to an appropriate temperature by detecting the element impedance.

[Specific Control Routine of First Embodiment]
FIG. 8 is a flowchart for illustrating a control routine executed by ECU 24 in the first embodiment of the present invention. FIG. 8 is a routine that is executed every time the internal combustion engine 10 is started. In the routine of FIG. 8, it is first determined whether or not the internal combustion engine 10 is currently being started (S100). Since it is considered that the addition of fuel is normally performed in a state in which the internal combustion engine 10 is stopped, there is a low possibility that a change in the fuel property will occur at a time other than when the internal combustion engine 10 is started. Therefore, if it is not recognized that the internal combustion engine 10 is being started, the current process ends.

  On the other hand, if it is determined in step S100 that the internal combustion engine 10 is being started, then an impedance detection voltage is applied to the air-fuel ratio sensor 20 (S110). Specifically, a predetermined AC voltage is superimposed on the positive voltage from the applied voltage operation unit 32 in a state in which SW1 of the switching circuits 34 and 36 is controlled to be ON and SW2 is OFF by a control signal from the ECU 24. The impedance detection voltage is supplied and applied between both electrodes of the air-fuel ratio sensor 20 connected between both terminals 38 and 40.

  Next, the element impedance is detected (S112). Specifically, in a state where the impedance detection voltage is applied to the air-fuel ratio sensor 20, the output guided to AD1 is taken into the ECU 24, and the sensor current is calculated based on this. Thereafter, the element impedance is calculated from the impedance detection voltage and the sensor current.

  Next, it is determined whether or not the current element temperature is equal to or higher than an activation temperature for detecting electromotive force (here, 550 ° C.) (S114). The element temperature is determined based on whether the element impedance obtained in step S112 is smaller than the element impedance indicating 550 ° C. In step S114, when it is not recognized that the element temperature ≧ 550 ° C., the sensor temperature is calculated while the sensor element is warmed up until the element temperature ≧ 550 ° C. is established. The determination of the element temperature is repeatedly executed.

  On the other hand, if the formation of element temperature ≧ 550 ° C. is recognized in step S114, the voltage application to the air-fuel ratio sensor 20 is turned off (S120). Specifically, SW1 of the switching circuits 34 and 36 is turned off and SW2 is turned on. Next, the electromotive force of the air-fuel ratio sensor 20 is detected (S122). The electromotive force is obtained in the ECU 24 when the output of AD2 is taken into the ECU 24. Based on the magnitude of the electromotive force, it is detected whether the current air-fuel ratio is deviated from the stoichiometric air-fuel ratio to the rich side or the lean side, and air-fuel ratio control is performed with the stoichiometric air-fuel ratio as the target air-fuel ratio (S124). ).

  Next, a positive voltage for air-fuel ratio detection is applied to the air-fuel ratio sensor 20 (S130). Here, specifically, SW1 of switching circuits 34 and 36 is switched ON and SW2 is switched OFF. In this state, a positive voltage is supplied from the applied voltage operation unit 32. As a result, a positive voltage is applied between both electrodes of the air-fuel ratio sensor 20 connected to both terminals 38 and 40. Next, a sensor output is detected (S132). The sensor output is detected by capturing the output of AD1 with a positive voltage applied.

  Next, it is determined whether the monitoring of the sensor output is completed (S134). Specifically, it is determined whether or not sensor output data has been detected a predetermined number of times necessary for detecting a parameter indicating variation in sensor output for determining fuel properties.

  In step S134, when the sensor output monitoring completion is not recognized, the process returns to S110 again, and the positive voltage is applied, that is, SW1 of the switching circuits 34 and 36 is turned on, SW2 is turned off, and the positive voltage is supplied. In this state, a predetermined AC voltage is further superimposed, impedance detection is performed, and the sensor element temperature is controlled to 550 ° C. or higher (S110 to S114). Thereafter, in a state where the control to the theoretical air-fuel ratio by the detection of electromotive force (S120 to S124) is performed, the switching circuits 34 and 36 are switched to detect the sensor output during the application of the positive voltage (S130 to S132). These processes in steps S110 to S132 are repeatedly executed until the completion of the air-fuel ratio monitoring is recognized in step S134.

  If it is determined in step S134 that the air-fuel ratio monitoring is completed, the detected sensor output is analyzed, a necessary parameter is obtained, and the fuel property is determined according to this parameter (S140). When the fuel property is determined, a map that defines the relationship between the sensor current and the air-fuel ratio is switched according to the fuel property (S142). Specifically, a map corresponding to the fuel property determined in S140 is read, and this map is set as a map that defines the relationship between the sensor current and the air-fuel ratio according to the currently used fuel property. . A map for each fuel property is stored in the ECU 24 in advance.

  Next, an impedance detection voltage is applied to the air-fuel ratio sensor 20 (S150), and the element impedance is detected (S152). The element impedance detection is calculated based on the sensor current detected in a state where SW1 of the switching circuit is turned on and the impedance detection voltage is applied, as in step S110.

  Next, it is determined whether or not the element temperature of the air-fuel ratio sensor 20 has reached the activation temperature (here, 650 ° C.) in the air-fuel ratio detection mode (S154). Specifically, the determination is made based on whether or not the detected element impedance is smaller than the impedance corresponding to 650 ° C. If it is not recognized in step S154 that the element temperature ≧ 650 ° C., the element temperature determination in steps S150 to S154 is repeatedly performed while the sensor element is warmed up until this condition is confirmed. Is done.

  On the other hand, if it is recognized in step S154 that the element temperature ≧ 650 ° C. is established, then a positive voltage is applied to the air-fuel ratio sensor 20 (S160). Specifically, the superposition of the AC voltage for impedance detection is turned off while SW1 of the switching circuits 34 and 36 is turned on, and only the positive voltage is supplied. As a result, a sensor current corresponding to the oxygen concentration of the exhaust gas flows through the air-fuel ratio sensor 20. The sensor current is calculated based on the anode potential taken into the ECU 24 from AD1 (S162). Thereafter, the air-fuel ratio is obtained according to the map switched in step S142 (S164). Thereafter, air-fuel ratio control is executed so as to reach the target air-fuel ratio according to the air-fuel ratio (S166). The control after the map is switched in step S142 is a normal air-fuel ratio control method. Even after step S166, the air-fuel ratio is calculated based on the sensor current of the air-fuel ratio sensor 20 according to the normal air-fuel ratio control technique, and the air-fuel ratio control and the element temperature control are executed.

  As described above, according to the first embodiment, when the fuel property at the time of starting is unknown, the stoichiometric air-fuel ratio of the air-fuel ratio sensor 20 is set to the target air-fuel ratio by switching the switching circuits 34 and 36. While performing the air-fuel ratio control, it is possible to determine the fuel property by obtaining the variation in the sensor output during application of the positive voltage. Accordingly, even when the fuel property is unknown, the fuel property is determined using the air-fuel ratio sensor 20 without providing a dedicated sensor for air-fuel ratio control and a dedicated sensor for determining the fuel property. On the other hand, it is possible to realize control of the air-fuel ratio in a state where the fuel property at the time of starting is unknown. Further, after the determination of the fuel property is completed, the air-fuel ratio sensor 20 can detect the air-fuel ratio corresponding to the fuel property by switching the map. Therefore, air-fuel ratio control can be performed with higher accuracy.

  In the first embodiment, the case where the fuel property is determined every time the internal combustion engine 10 is started has been described. However, the present invention is not limited to this. For example, it is possible to detect that the fuel has been actually replenished and to perform the control of the first embodiment only when the fuel is replenished. The same applies to the second embodiment below.

  Further, in the first embodiment, the case has been described in which the internal combustion engine 10 can use a mixed fuel of gasoline and ethanol containing ethanol at different contents of 0 to 100%. However, the fuel to be used is not limited to gasoline, ethanol and a mixed fuel thereof, but may be another fuel or a mixture of other fuel and gasoline. Even in such a case, the relationship between the output variation of the air-fuel ratio sensor and the fuel property when the air-fuel ratio is controlled to be close to the theoretical air-fuel ratio as shown in FIG. Thus, the fuel property can be determined. Further, by storing a map of the sensor current and the air-fuel ratio in advance for each fuel property in the ECU 24, air-fuel ratio detection using the air-fuel ratio sensor 20 can be performed. Even when the fuel properties are different in this way, the electromotive force characteristics are almost the same. Therefore, until the fuel property determination is completed, control close to the theoretical air-fuel ratio can be performed based on the electromotive force regardless of the fuel property.

  In the first embodiment, the sensor output voltage is detected in the fuel property determination, and the fuel property is determined based on this variation. On the other hand, the sensor current is obtained in the air-fuel ratio detection and the element impedance detection. Although the case where calculation is performed using this has been described, the present invention is not limited to this. For example, the fuel property can be obtained from variations in sensor current, the air-fuel ratio can be detected corresponding to the sensor output voltage, and the element impedance can be obtained from the sensor output voltage.

  Moreover, in Embodiment 1, the case where the map according to the fuel property was memorize | stored in ECU24 was demonstrated. However, for example, in the case of a fuel containing gasoline and ethanol, the value of the air-fuel ratio with respect to the sensor current corresponding to the fuel properties is the air-fuel ratio in the case of gasoline, and the air-fuel ratio when the sensor current is zero is the theory of the fuel. The air-fuel ratio is approximated to a parallel shift. Therefore, instead of a map for each fuel property, for example, based on the relationship as described above, a correction value corresponding to the fuel content of ethanol or the like is obtained by a map or calculation. By correcting the air-fuel ratio calculated from the above, the air-fuel ratio corresponding to the fuel property can be obtained. The same applies to the second embodiment below.

  In the first embodiment, the case where the control to the vicinity of the theoretical air-fuel ratio is performed based on the electromotive force of the air-fuel ratio sensor 20 while the fuel property determination is performed has been described. As a result, in the system of the first embodiment, the control to the vicinity of the theoretical air-fuel ratio and the fuel property determination can be realized simultaneously by one air-fuel ratio sensor 20. However, the present invention is not limited to this. For example, an oxygen sensor or an air-fuel ratio sensor may be provided in the exhaust passage 16 separately from the air-fuel ratio sensor 20. However, the fuel property determination is performed based on variations in the output distribution of the air-fuel ratio sensor 20 when the air-fuel ratio is controlled in the vicinity of the theoretical air-fuel ratio. Therefore, in order to perform such fuel property determination, an air-fuel ratio detecting means capable of performing air-fuel ratio control in which the target air-fuel ratio becomes clear is required.

  The present invention may control the target air-fuel ratio as a value different from the theoretical air-fuel ratio as long as air-fuel ratio control to the target air-fuel ratio in the fuel property is ensured. In this case, the relationship between variations in the output distribution of the air-fuel ratio sensor for each fuel property when the target air-fuel ratio is controlled is detected in advance by experiments or the like, and the relationship between the parameter indicating this variation and the fuel property is stored. Thus, as described above, the fuel property can be detected from the sensor output of the air-fuel ratio sensor.

  In the first embodiment, the case where the element temperature at the time of detecting the electromotive force is set to about 550 ° C. or higher and the element temperature at the time of detecting the air-fuel ratio is set to about 650 ° C. or higher has been described. However, the element temperature is not limited to this in the present invention. For example, when it is necessary to start air-fuel ratio control sooner, detection of electromotive force may be started even at an element temperature of 550 ° C. or lower. However, in this case as well, the element temperature is preferably about 350 ° C. or higher in order to ensure a certain degree of accuracy of the air-fuel ratio control based on the electromotive force. The same applies to the second embodiment below.

  In the first embodiment, the case where the element temperature is controlled based on the detected element impedance has been described. However, the control of the element temperature is not limited to such a method, and may be performed by another method. The same applies to the second embodiment below.

  In the first embodiment, the case where the drive circuit 30 controls the application of a positive voltage or impedance detection voltage, the applied voltage OFF state, or the like, and the detection of sensor output or electromotive force has been described. However, in the present invention, the circuit for applying the necessary voltage to the air-fuel ratio sensor 20 or detecting the output thereof is not limited to the drive circuit shown in FIG. 2, and may be performed by another circuit. Good. The same applies to the second embodiment below.

  In the above embodiment, when referring to the number of each element, quantity, quantity, range, etc., unless otherwise specified or clearly specified in principle, the number referred to It is not limited. Further, the structures described in the embodiments, steps in the method, and the like are not necessarily essential to the present invention unless otherwise specified or clearly specified in principle. The same applies to the second embodiment.

  For example, the “positive voltage applying unit” of the present invention is realized by executing step S130 or 160 of FIG. 8, and the “sensor output detecting unit” is realized by executing step S132 or S162. By executing step S140, the “fuel property determination means” is realized, and by executing step S124, the “characteristic determination air-fuel ratio control means” is realized, and by executing step S134, “completion determination” is realized. "Means" is realized, and step S166 is executed to realize "air-fuel ratio control means".

  Further, for example, by executing step S140, the “variation detecting means” and the “alcohol content rate detecting means” of the present invention are realized. Further, for example, “map selection means” is realized by executing step S142, and “air-fuel ratio detection means” is realized by executing step S164. Further, for example, “electromotive force detection means” is realized by executing step S122, and “impedance detection voltage applying means” is realized by executing step S110 or S150, and steps S112 or S152 are executed. As a result, “element impedance calculation means” is realized, step S114 is executed to realize “first element temperature determination means”, and step S154 is executed to execute “second element temperature determination means”. Is realized.

Embodiment 2. FIG.
The system of the second embodiment has the same configuration as the system of the first embodiment shown in FIGS. However, in the system of the second embodiment, the applied voltage operation unit 32 of the drive circuit 30 can supply the positive voltage, the AC voltage for impedance detection, and a large voltage of about 1.0 [V] larger than the positive voltage. it can.

  FIG. 9 is a diagram for explaining changes in the sensor current flowing through the air-fuel ratio sensor 20 when the voltage applied between both electrodes of the air-fuel ratio sensor 20 in the second embodiment of the present invention is changed. In FIG. 9, the horizontal axis represents the applied voltage [V], and the vertical axis represents the sensor current [mA]. As shown in FIG. 9, when a positive voltage (in the embodiment, about 0.4 V) is applied to the air-fuel ratio sensor 20, a limit current corresponding to the oxygen concentration of the exhaust gas flows through the air-fuel ratio sensor 20. By detecting this limit current as a sensor current, the air-fuel ratio can be obtained.

  If the voltage applied to the air-fuel ratio sensor 20 has a magnitude near the positive voltage, the sensor current is stable at the limit current. However, when the applied voltage is increased to about 1.0 [V], the sensor current increases beyond the limit current. This is considered to be because electrolysis of water near the exhaust side electrode starts when the applied voltage increases. That is, it can be considered that the increase of the sensor current flowing through the air-fuel ratio sensor 20 with respect to the limit current when a large voltage is applied corresponds to the decomposition current due to water splitting.

  By the way, when the fuel is a mixed fuel of gasoline and ethanol, the amount of water molecules present in the exhaust gas has a correlation with the ethanol content contained in the fuel, and the amount of water molecules generated increases as the ethanol content increases. Become more. Therefore, as the ethanol content increases, the decomposition current due to water splitting when a large voltage is applied increases, and the amount of increase in sensor current with respect to the limit current increases as shown in FIG.

  The ethanol content, the generation amount of water molecules, and the increase amount of the sensor current with respect to the amount of water molecules have a certain correlation. Therefore, the ethanol content in the fuel can be calculated based on this correlation. Here, in order to calculate the ethanol content, it is necessary to clarify the amount of increase in sensor current due to water splitting with respect to the limit current. Therefore, it is necessary that the sensor current (limit current) when a positive voltage is applied is clear and stable.

  As described above, even when the fuel property is not determined, the air-fuel ratio can be controlled to the stoichiometric air-fuel ratio by detecting the electromotive force of the air-fuel ratio sensor 20. Therefore, the system according to the second embodiment uses this to control the air-fuel ratio to the stoichiometric air-fuel ratio during fuel property determination. Thereby, the sensor current (limit current) when a positive voltage is applied can be stably set to a state where the sensor current = 0. Accordingly, it is possible to accurately grasp the current increase due to water splitting with respect to the limit current (= 0), and it is possible to accurately determine the fuel property.

  Specifically, the process of detecting the electromotive force by turning on SW2 and controlling it to the theoretical air-fuel ratio based on this, and the process of detecting sensor current by switching SW1 to ON and applying a large voltage in this state Repeat several times. As a result, it is estimated that the state of limiting current = 0 is maintained, so the average value of the detected sensor current is the amount of increase in the sensor current due to water splitting, and based on this, the ethanol content in the fuel Can be calculated. In addition, the relationship between the increase amount of the sensor current and the ethanol content is obtained in advance through experiments or the like and stored in the ECU 24 as a map.

  The specific control routine executed by the system of the second embodiment is the same as the routine shown in FIG. However, in the state where SW1 is turned on and SW2 is turned off in step S130 of FIG. 8, a large voltage of about 1.0 V is applied to the air-fuel ratio sensor 20 instead of applying a positive voltage. In step S132, which continues in this state, a sensor current is obtained. When S110 to S134 are repeatedly executed and the sensor current is detected a predetermined number of times, the completion of the monitor is recognized in step S134. Thereafter, in step S140, the average value of the sensor current when a large voltage is applied is obtained, and the ethanol content in the fuel is calculated.

  Note that the impedance detection voltage applied in step S110 of the second embodiment is obtained by superimposing an AC voltage on a large voltage. That is, in the second embodiment, the voltage applied for detecting the sensor current is a large voltage of 1.0 V in the routine that is repeatedly executed in steps S110 to S134. Therefore, when the process of step S110 is performed after steps S130 to S134, the AC voltage may be superimposed by the applied voltage operation unit 32 without changing the large voltage to the positive voltage. Since the element impedance can be calculated by a change in the sensor current with respect to the impedance detection voltage, the element impedance can be detected even when the impedance detection voltage increases.

  As described above, in the second embodiment, the property determination is performed by detecting the ethanol content in the fuel according to the increase amount of the sensor current when a large voltage is applied to the air-fuel ratio sensor 20. it can. Here, the air-fuel ratio control during fuel property detection can be performed based on the electromotive force of the air-fuel ratio sensor 20, and the air-fuel ratio during property determination can be maintained near the theoretical air-fuel ratio. While realizing the control, the fuel property can be determined at the same time.

  In the second embodiment, the case where the target air-fuel ratio is set as the theoretical air-fuel ratio and the fuel property determination is performed in a state where the air-fuel ratio is controlled in the vicinity of the theoretical air-fuel ratio based on the electromotive force of the air-fuel ratio sensor has been described. However, the present invention is not limited to this. For example, a dedicated sensor for detecting the air-fuel ratio during fuel property determination may be provided and controlled to a predetermined target air-fuel ratio based on this sensor. Even in such a case, the property determination can be performed by detecting the sensor current and obtaining an increase of the sensor current with respect to the sensor current (limit current) when a positive voltage is applied according to the target air-fuel ratio.

  In the second embodiment, the case where the internal combustion engine 10 can use a mixed fuel of gasoline and ethanol containing ethanol at different contents of 0 to 100% has been described. However, the fuel used is not limited to gasoline, ethanol, and a mixed fuel thereof, but may be another alcohol fuel, or a mixture of other alcohol fuel and gasoline. Not only ethanol but also other alcohol fuels are included, water corresponding to the alcohol content is generated in the exhaust gas. Therefore, the relationship between the alcohol content and the amount of water generated can be calculated, and based on this, the relationship between the magnitude of the applied large voltage, the increase in sensor current, and the alcohol content can be specified. By storing this in the ECU 24 in advance, the above method can also be applied to fuels containing other alcohols.

  In the second embodiment, in the fuel property determination, the fuel property is determined based on the increase amount of the sensor current, and the air-fuel ratio detection and the element impedance detection are performed based on the sensor current. However, the present invention is not limited to this. Since the sensor output voltage and the sensor current, which are sensor outputs when a predetermined voltage is applied, have a correlation, for example, the fuel property can be obtained by the increase amount of the sensor output voltage. Detection can be performed in correspondence with the output voltage, or the element impedance can be obtained from the sensor output voltage.

  In the second embodiment, the “large voltage applying unit” of the present invention is realized by executing step S130, and the “alcohol content rate detecting unit” is realized by executing step S140.

It is a schematic diagram for demonstrating the structure of the system in Embodiment 1 of this invention. It is a schematic diagram for demonstrating the drive circuit of the air fuel ratio sensor in Embodiment 1 of this invention. It is a figure for demonstrating the relationship between the sensor electric current and an air fuel ratio of the air fuel ratio sensor in Embodiment 1 of this invention. It is a figure for demonstrating the relationship between an electromotive force and an air fuel ratio of the air fuel ratio sensor in Embodiment 1 of this invention. It is a figure for demonstrating the relationship between the sensor output voltage and fuel property of the air fuel ratio sensor in Embodiment 1 of this invention. It is a figure showing distribution of the sensor output for every fuel property of the air fuel ratio sensor in Embodiment 1 of this invention. It is a timing chart of control in Embodiment 1 of this invention. It is a flowchart for demonstrating the control routine which ECU performs in Embodiment 1 of this invention. It is a figure for demonstrating the relationship between the sensor electric current of the air fuel ratio sensor in Embodiment 2 of this invention, and a fuel property.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Internal combustion engine 12 Fuel tank 16 Exhaust passage 18 Catalyst 20 Air-fuel ratio sensor 22 Oxygen sensor 24 ECU
Reference Signs List 30 drive circuit 32 applied voltage operation section 34, 36 switching circuit 38 anode terminal 40 negative terminal

Claims (5)

An air-fuel ratio control apparatus for controlling an air-fuel ratio of an internal combustion engine that can use gasoline and fuel other than gasoline as fuel,
A positive voltage applying means for applying a positive voltage for detecting the air-fuel ratio of the exhaust gas between the atmosphere-side electrode and the exhaust-side electrode of the air-fuel ratio sensor disposed in the exhaust passage of the internal combustion engine;
Sensor output detecting means for detecting a sensor output of the air-fuel ratio sensor;
Fuel property determining means for determining the fuel property of the fuel used in the internal combustion engine according to the sensor output of the air-fuel ratio sensor;
An electromotive force detecting means for detecting an electromotive force generated between the two electrodes in a state where no voltage is applied between the electrodes of the air-fuel ratio sensor;
While determining the fuel property , based on the electromotive force, the property determination time air-fuel ratio control means for controlling the air-fuel ratio of the internal combustion engine to a target air-fuel ratio;
Completion determination means for determining whether or not the fuel property determination is completed;
Air-fuel ratio control means for performing air-fuel ratio control according to the determined fuel property based on the sensor output detected during application of the positive voltage after completion of the fuel property determination is recognized;
An air-fuel ratio control apparatus comprising: The fuel is mixed fuel of gasoline or alcohol or gasoline and alcohol,
The target air-fuel ratio is a theoretical air-fuel ratio ,
Before Symbol fuel property determining means,
For calculated value of the sensor output corresponding sense Ronsora ratio, and variation detection means for detecting the variation of the sensor output detected during application of the positive voltage,
Alcohol content rate detecting means for detecting the alcohol content rate in the fuel according to the variation;
The air-fuel ratio control apparatus according to claim 1, comprising: A high voltage applying means for applying a larger voltage than the positive voltage between both electrodes of the air-fuel ratio sensor when the fuel property determination is not completed;
The fuel, Ri mixed fuel der of gasoline or alcohol or gasoline and alcohol,
Before Symbol target air-fuel ratio is a stoichiometric air-fuel ratio,
The said fuel property determination means is equipped with the alcohol content rate detection means which detects the alcohol content rate in the said fuel according to the sensor output detected during the application of the said high voltage. Air-fuel ratio control device. Map selecting means for selecting a map that defines the relationship between the sensor output detected in the positive voltage application state and the air-fuel ratio in accordance with the determined fuel property when the fuel property is determined;
Air-fuel ratio detecting means for detecting an air-fuel ratio according to the selected map in accordance with a sensor output detected during application of the positive voltage,
4. The air-fuel ratio control according to claim 1, wherein the air-fuel ratio control means performs air-fuel ratio control according to the air-fuel ratio when completion of the fuel property determination is recognized. apparatus. Impedance detection voltage applying means for applying an impedance detection voltage for detecting an element impedance of a sensor element of the air / fuel ratio sensor to the air / fuel ratio sensor;
An element impedance detection means for detecting the element impedance in accordance with a sensor output detected during application of the impedance detection voltage;
First element temperature determining means for determining whether or not the temperature of the sensor element of the air-fuel ratio sensor has reached a first activation temperature based on the element impedance when completion of the fuel property determination is not permitted; ,
Second element temperature determining means for determining whether or not the temperature of the sensor element has reached a second activation temperature based on the element impedance when completion of the fuel property determination is recognized;
Further comprising
The electromotive force detection means detects the electromotive force when it is recognized that the temperature of the sensor element has reached the first activation temperature,
The air-fuel ratio control means performs air-fuel ratio control according to the sensor output during application of the positive voltage when it is recognized that the temperature of the sensor element has reached the second activation temperature. Item 5. The air-fuel ratio control device according to any one of Items 1 to 4 .

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