JP4060427B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine, and more particularly to a method for determining the start timing of air-fuel ratio feedback control according to the active state of an air-fuel ratio sensor.
[0002]
[Prior art]
Conventionally, in an air-fuel ratio control apparatus for an internal combustion engine that controls the air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine, when the internal combustion engine is in a normal operating state, the target air-fuel ratio is set to the stoichiometric air-fuel ratio and the exhaust gas is set. In order to improve the characteristics, the target air-fuel ratio is set to a value on the lean side of the stoichiometric air-fuel ratio between the start of the engine and the activation of the three-way catalyst disposed in the exhaust system of the engine. Executes lean burn control that sets the target air-fuel ratio to a value on the lean side of the stoichiometric air-fuel ratio in order to reduce the HC component in the engine and to improve fuel efficiency in the high-speed cruising operation state of the engine, or to prevent knocking In addition, the target air-fuel ratio is set in accordance with the operating state of the internal combustion engine, such as setting the target air-fuel ratio to a richer value than the stoichiometric air-fuel ratio in order to protect the engine.
[0003]
Conventionally, a wide range air-fuel ratio sensor (limit current type oxygen concentration sensor) having an output characteristic substantially proportional to the oxygen concentration in the exhaust gas is provided in the exhaust system, and activation for determining the activation of the air-fuel ratio sensor is performed. Until the air-fuel ratio sensor is activated by providing the determination means, only the rich / lean of the air-fuel ratio is determined based on the presence / absence of a signal from the air-fuel ratio sensor, and proportional / integral control (PI control) is executed. An air-fuel ratio control apparatus for an internal combustion engine that starts precise air-fuel ratio feedback control such as modern control based on the output of the air-fuel ratio sensor after activation is known, for example, from Japanese Patent Laid-Open No. 7-127502.
[0004]
In the known air-fuel ratio control device, the activation of the air-fuel ratio sensor is determined based on the element temperature of the wide-range air-fuel ratio sensor (limit current type oxygen concentration sensor).
[0005]
[Problems to be solved by the invention]
However, the wide-range air-fuel ratio sensor as in the known air-fuel ratio control apparatus usually has a different element temperature at which the limit current of the air-fuel ratio sensor is stabilized depending on the control air-fuel ratio. In other words, in the air-fuel ratio region near the stoichiometric air-fuel ratio, the limit current stabilizes at a relatively low element temperature, whereas the limit current increases until the element temperature becomes higher as the control air-fuel ratio departs from the stoichiometric air-fuel ratio. It is usually not stable.
[0006]
For this reason, in the known air-fuel ratio control apparatus, when the element temperature at which the limit current is stabilized over the entire air-fuel ratio is adopted as the reference value for air-fuel ratio sensor activation determination, the determination of air-fuel ratio sensor activation is made. Since the start timing of the air-fuel ratio feedback control is delayed, the exhaust gas characteristics are deteriorated. On the other hand, if the reference value is set to a lower element temperature in order to advance the start timing of the air-fuel ratio feedback control, The feedback control is started before the air-fuel ratio sensor is sufficiently activated in the air-fuel ratio region other than the vicinity of the air-fuel ratio, such as the air-fuel ratio feedback control in the rich or lean air-fuel ratio region other than the vicinity of the theoretical air-fuel ratio. There is a problem that reliability is lowered and exhaust gas characteristics are deteriorated.
[0007]
The present invention has been made to solve the above-described problems, and appropriately determines the active state of the air-fuel ratio sensor so that the air-fuel ratio sensor is empty as soon as possible without impairing the reliability of the output of the air-fuel ratio sensor. It is an object of the present invention to provide an air-fuel ratio control apparatus for an internal combustion engine that can start fuel ratio feedback control and thereby improve exhaust gas characteristics.
[0008]
[Means for Solving the Problems]
  The invention according to claim 1 is provided in an exhaust system of an internal combustion engine.,An air-fuel ratio sensor composed of a limiting current oxygen concentration sensor that outputs a signal having a value substantially proportional to the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine, and an operating state detection that detects the operating state of the internal combustion engine Means, air-fuel ratio feedback control means for feedback-controlling the air-fuel ratio so that the air-fuel ratio matches the target air-fuel ratio according to the output signal of the air-fuel ratio sensor, and the target according to the detected operating state The target air-fuel ratio setting means for setting the air-fuel ratio and the degree of activity of the air-fuel ratio sensor are determined.As a parameter to represent, a parameter representing the element temperature of the air-fuel ratio sensorThe target air-fuel ratio set by the target air-fuel ratio setting means is a degree-of-activity detection means for detecting and an activity determination reference value for determining whether or not the air-fuel ratio sensor is activated.The further away from the stoichiometric air-fuel ratio, the higher the element temperature side.An activation determination reference value setting means to be set and execution of air-fuel ratio feedback control to the target air-fuel ratio is permitted when the detected degree of activity exceeds the activation determination reference value to the side representing a higher active state The permission means to perform is provided.
[0009]
  According to this configuration, the air-fuel ratio sensor isOutputs a signal with a value approximately proportional to the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engineIt is composed of a limiting current type oxygen concentration sensor, and an activation determination reference value for determining whether or not the air-fuel ratio sensor is activated isAs the target air-fuel ratio set according to the operating state of the internal combustion engine is further away from the stoichiometric air-fuel ratio, the element temperature of the higher-temperature air-fuel ratio sensor is increased.The degree of activation of the air-fuel ratio sensorA parameter that represents the element temperature of the air-fuel ratio sensorIs detected, and feedback control based on the output of the air-fuel ratio sensor is started when the detected degree of activity of the air-fuel ratio sensor exceeds the set activity determination reference value to the side representing a higher active state. Therefore, it is possible to appropriately determine the active state of the air-fuel ratio sensor and start the air-fuel ratio feedback control as early as possible without impairing the reliability of the output of the air-fuel ratio sensor. Gas characteristics can be improved.
  The invention according to claim 2 is provided in the exhaust system of the internal combustion engine, and the element temperature to be activated is low when the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is the stoichiometric air-fuel ratio, and the air-fuel ratio is theoretically An air-fuel ratio sensor that has a characteristic of increasing as it goes away from the air-fuel ratio, and that outputs a signal having a value substantially proportional to the air-fuel ratio; an operating state detecting means that detects an operating state of the internal combustion engine; and the air-fuel ratio sensor Air-fuel ratio feedback control means for feedback-controlling the air-fuel ratio so that the air-fuel ratio matches the target air-fuel ratio according to the output signal, and the target air-fuel ratio setting the target air-fuel ratio according to the detected operating state An air-fuel ratio setting means; an activity detection means for detecting a parameter representing the element temperature of the air-fuel ratio sensor as a parameter representing the degree of activity of the air-fuel ratio sensor; and the air-fuel ratio sensor An activation determination reference value for determining whether or not the element is activated is set on the side representing the element temperature that is higher as the target air-fuel ratio set by the target air-fuel ratio setting means is farther from the stoichiometric air-fuel ratio. An activity determination reference value setting means that permits execution of air-fuel ratio feedback control to the target air-fuel ratio when the detected degree of activity exceeds the activity determination reference value to a side representing a higher active state And a permission unit.
  According to a third aspect of the present invention, there is provided a limiting current type oxygen concentration sensor which is provided in an exhaust system of an internal combustion engine and outputs a signal having a value substantially proportional to an air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine. An air-fuel ratio sensor, an operating condition detecting means for detecting an operating condition of the internal combustion engine, and an air-fuel ratio feedback-controlling the air-fuel ratio in accordance with an output signal of the air-fuel ratio sensor so that the air-fuel ratio matches a target air-fuel ratio. An internal resistance of the air-fuel ratio sensor is detected as a parameter representing the degree of activity of the air-fuel ratio sensor, a target air-fuel ratio setting means for setting the target air-fuel ratio according to the detected operating state, and a parameter representing the degree of activity of the air-fuel ratio sensor And an activity determination reference value for determining whether or not the air-fuel ratio sensor is activated, and the target air-fuel ratio set by the target air-fuel ratio setting means An activation determination reference value setting unit that sets the activation determination reference value with respect to the internal resistance of the air-fuel ratio sensor, the maximum value when the target air-fuel ratio is the stoichiometric air-fuel ratio. When the target air-fuel ratio is set to a value that gradually decreases from the maximum value as the target air-fuel ratio goes away from the stoichiometric air-fuel ratio, and is set for the reciprocal of the internal resistance, the target air-fuel ratio is the stoichiometric air-fuel ratio. An activity determination reference value setting means for setting to a minimum value when the fuel is an air-fuel ratio and setting the target air-fuel ratio to a value that gradually increases from the minimum value as the target air-fuel ratio departs from the stoichiometric air-fuel ratio; and the detected activity level is the activity determination Permission means for permitting execution of the air-fuel ratio feedback control to the target air-fuel ratio when a reference value exceeds a side representing a higher active state.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
[0011]
FIG. 1 is a diagram showing an overall configuration of an internal combustion engine (hereinafter referred to as “engine”) and a control device according to a first embodiment of the present invention. In the figure, reference numeral 1 denotes a DOHC in-line four-cylinder engine in which each cylinder is provided with a pair of an intake valve and an exhaust valve (not shown).
[0012]
A throttle body 3 is provided in the middle of the intake pipe 2 of the engine 1, and a throttle valve 3 'is disposed therein. A throttle valve opening (θTH) sensor 4 is connected to the throttle valve 3 ′ and outputs an electric signal corresponding to the opening of the throttle valve 3 ′ to an electronic control unit (hereinafter referred to as “ECU”) 5. Supply.
[0013]
The fuel injection valve 6 is provided for each cylinder between the engine 1 and the throttle valve 3 and slightly upstream of the intake valve (not shown) of the intake pipe 2, and each injection valve is connected to a fuel pump (not shown). At the same time, it is electrically connected to the ECU 5 and the valve opening time of the fuel injection valve 6 is controlled by a signal from the ECU 5.
[0014]
An intake pipe absolute pressure (PBA) sensor 8 is provided immediately downstream of the throttle valve 3 via a pipe 7. The absolute pressure signal converted into an electric signal by the absolute pressure sensor 8 is supplied to the ECU 5. Is done. Further, an intake air temperature (TA) sensor 9 is attached downstream thereof, detects the intake air temperature TA, outputs a corresponding electrical signal, and supplies it to the ECU 5.
[0015]
An engine water temperature (TW) sensor 10 mounted on the main body of the engine 1 is composed of a thermistor or the like, detects the engine water temperature (cooling water temperature) TW, outputs a corresponding temperature signal, and supplies it to the ECU 5. The engine speed (NE) sensor 11 and the cylinder discrimination (CYL) sensor 12 are attached around the cam shaft or crank shaft (not shown) of the engine 1. The engine speed sensor 11 outputs a signal pulse (hereinafter referred to as “TDC signal pulse”) at a predetermined crank angle position every 180 ° rotation of the crankshaft of the engine 1, and the cylinder discrimination sensor 12 is a predetermined crank of a specific cylinder. A signal pulse (hereinafter referred to as “CYL signal pulse”) is output at an angular position, and each of these signal pulses is supplied to the ECU 5.
[0016]
The three-way catalyst 14 is disposed in the exhaust pipe 13 of the engine 1 and purifies components such as HC, CO, NOx in the exhaust gas. A limit current type oxygen concentration sensor (hereinafter referred to as “LAF sensor”) 15 as an air-fuel ratio sensor is mounted upstream of the three-way catalyst 14 in the exhaust pipe 13.
[0017]
As will be described later, the LAF sensor 15 constitutes an oxygen concentration detection device 16 together with an oxygen concentration detection activation control device (hereinafter referred to as “control device”) 25 as internal resistance detection means. The LAF sensor 15 is connected to the ECU 5 via the control device 25, outputs an electrical signal that is substantially proportional to the oxygen concentration (air-fuel ratio) in the exhaust gas, and outputs the electrical signal to the control device 25. The detected oxygen concentration value stored in the control device 25 is read out by the ECU 5.
[0018]
The ECU 5 shapes input signal waveforms from various sensors, corrects the voltage level to a predetermined level, converts an analog signal value into a digital signal value, and the like, and a central processing circuit (hereinafter referred to as “CPU”). 5b, a storage device 5c that stores various calculation programs executed by the CPU 5b, calculation results, and the like, an output circuit 5d that supplies a drive signal to the fuel injection valve 6, and the like.
[0019]
The CPU 5b discriminates various engine operation states such as an air-fuel ratio feedback control operation region and an open loop control operation region according to the oxygen concentration in the exhaust gas based on the various engine parameter signals described above, and according to the engine operation state. Based on the following Equation 1, the fuel injection time Tout of the fuel injection valve 6 synchronized with the TDC signal pulse is calculated.
[0020]
[Expression 1]
Tout = Ti × KCMDM × KLAF × K1 + K2
Here, Ti is a basic fuel amount, specifically, a basic fuel injection time determined according to the engine speed NE and the intake pipe absolute pressure PBA, and a Ti map for determining this Ti value is stored in the storage means. 5c.
[0021]
KCMDM is a corrected target air-fuel ratio coefficient, which is set according to the engine operating state, and is calculated by multiplying the target air-fuel ratio coefficient KCMD representing the target air-fuel ratio by the fuel cooling correction coefficient KETV. The correction coefficient KETV is a coefficient for correcting the fuel injection amount in consideration of the change in the air-fuel ratio of the air-fuel mixture supplied to the engine 1 due to the cooling effect by actually injecting the fuel, and the target air-fuel ratio. It is set according to the value of the coefficient KCMD. As is clear from Equation 1, the fuel injection time (fuel injection amount) Tout increases as the corrected target air-fuel ratio coefficient KCMDM increases, so that the KCMD value and the KCMDM value are the reciprocals of the so-called air-fuel ratio A / F. Proportional value. The target air-fuel ratio coefficient KCMD is proportional to the reciprocal of the air-fuel ratio A / F, that is, the fuel-air ratio F / A, and takes a value of 1.0 when the stoichiometric air-fuel ratio is used.
[0022]
KLAF is an air-fuel ratio correction coefficient, which is set so that the air-fuel ratio detected by the LAF sensor 15 coincides with the target air-fuel ratio during air-fuel ratio feedback control, and is set to a predetermined value according to the engine operating state during open loop control. Is set.
[0023]
K1 and K2 are other correction coefficients and correction variables that are calculated according to various engine parameter signals, and are values that optimize various characteristics such as fuel efficiency characteristics and engine acceleration characteristics according to engine operating conditions. Set to
[0024]
The CPU 5b outputs a signal for driving the fuel injection valve 6 via the output circuit 5d based on the result calculated and determined as described above.
[0025]
FIG. 2 is a block diagram showing a detailed configuration of the oxygen concentration detection device 16 in FIG. 1, and the same components in FIG. 1 as those in FIG.
[0026]
The oxygen concentration detection device 16 includes a LAF sensor 15 and a control device 25. The LAF sensor 15 is provided in the exhaust pipe 13 as described above, and the signal line of the LAF sensor 15 is detachably connected to the control device 25 by a connector (not shown). The LAF sensor 15 is composed of a solid electrolyte element or the like, and incorporates a heater 54 inside thereof. The heater 54 has a sufficient heat generation capacity to activate the LAF sensor 15. In addition, a cover 59 having a small hole 60 is provided around the LAF sensor 15, and the cover 59 prevents the LAF sensor 15 from directly contacting the exhaust gas by flowing the exhaust gas through the small hole 60, It serves to protect and keep warm the LAF sensor 15.
[0027]
The control device 25 includes a bias control unit 63, a current detection unit 67, and a control unit 69. One of the lead wires 61 connected to the LAF sensor 15 is connected to the bias control unit 63, and the other lead wire 61 is connected to the current detection unit 67. Further, two lead wires 62 connected to the heater 54 are connected to the heating control unit 71 of the control unit 69.
[0028]
The bias control unit 63 includes a positive bias source 64, a negative bias source 65, and a switching unit 66. The current detection unit 67 is connected to the switching unit 66 and the control unit 69, and the switching unit 66 is also connected to the control unit 69. The switching unit 66 switches the polarity of the voltage applied to the LAF sensor 15 according to the signal from the control unit 69, and the current detection unit 67 outputs the detected current to the control unit 69.
[0029]
The control unit 69 includes an amplifier 72 that amplifies and shapes a signal, an A / D conversion unit 68 that converts an analog signal value into a digital signal value, a storage unit 70, and a heating control unit 71 that controls the heat generation state of the heater 54. Composed. The storage unit 70 stores various calculation programs executed by the control unit 69, ROMs and RAMs for storing maps and calculation results described later, and a ring for storing oxygen concentration (air-fuel ratio A / F) detection values of the LAF sensor 15. It consists of a buffer.
[0030]
The control unit 69 receives the CYL signal pulse, the TDC signal pulse, the engine speed NE signal, and the intake pipe absolute pressure PBA signal from the ECU 5, while the oxygen concentration detection value and the internal resistance value of the LAF sensor 15 selected by the processing described later. Is supplied to the ECU 5.
[0031]
The LAF sensor 15 can detect the oxygen concentration in the exhaust gas linearly because the limit current value when a predetermined positive voltage V1 is applied is proportional to the oxygen partial pressure, but the LAF sensor 15 Since the activation requires a high temperature and the activation temperature range is narrow, the activation state of the LAF sensor 15 cannot be appropriately controlled only by the exhaust gas temperature of the engine 1. Therefore, a process for maintaining the active state of the LAF sensor 15 by detecting the internal resistance (hereinafter referred to as “LAF sensor activation process”) is required. The oxygen concentration detection device 16 switches between LAF sensor activation processing and oxygen concentration detection processing at a constant switching cycle T. The switching period T is set in consideration of the elements of the LAF sensor 15 and the heat capacity of the heater 54, the cooling characteristics of the LAF sensor 15, the activation temperature range of the LAF sensor 15, and the like.
[0032]
In the oxygen concentration detection device 25, a predetermined positive voltage V <b> 1 is applied to the LAF sensor 15 by connecting the switching unit 66 to the positive bias source 64. At this time, the current value I1 output from the LAF sensor 15 is detected by the current detection unit 67, the current value I1 is amplified and shaped by the amplifier 72, and then converted into a digital value by the A / D conversion unit 68. Based on this digital value, the oxygen concentration (air-fuel ratio) in the exhaust gas is detected.
[0033]
On the other hand, by connecting the switching unit 66 to the negative bypass source 65, a predetermined negative voltage V2 is applied to the LAF sensor 15, and the current value I2 output from the LAF sensor 15 at this time is applied by the current detection unit 67. The current value I2 is amplified and shaped by the amplifier 72 and then converted into a digital value by the A / D converter 68. Based on this digital value, the internal resistance LAFRI of the LAF sensor 15 is detected.
[0034]
When the detected internal resistance LAFRI is equal to or greater than a predetermined reference value, the heater 54 is controlled to be heated by the heating control unit 71. When the internal resistance LAFRI is lower than the predetermined reference value, the heating control unit 71 Heating of the heater 54 is stopped. Thus, the heat generation state of the heater 54 is controlled so that the detected internal resistance LAFRI is always constant, and the temperature of the LAF sensor 15 is always maintained within the activation temperature range.
[0035]
The operation of the air-fuel ratio control apparatus according to the present embodiment will be described below with reference to the drawings.
[0036]
FIG. 3 is a flowchart showing a target air-fuel ratio setting process for setting the target air-fuel ratio in accordance with the operating state of the internal combustion engine. This target air-fuel ratio setting process is executed every time each TDC signal pulse is generated.
[0037]
First, in step S101, it is determined whether an engine protection enrichment condition is satisfied. This engine protection rich condition is a condition for enriching the air-fuel ratio in order to prevent excessive engine temperature rise to avoid knocking or protect the engine. This is established in a predetermined high-load operation state of the engine 1 such as the throttle valve opening degree. When the engine protection rich condition is satisfied, the target air-fuel ratio coefficient KCMD is set to a predetermined rich correction value KCMDPR (for example, a value corresponding to A / F = 12) (step S102), and the process is terminated. When the engine protection rich condition is not satisfied, the process proceeds to step S103, and it is determined whether or not the three-way catalyst 14 is activated. This determination of activation is performed, for example, by determining that the three-way catalyst 14 is activated when a known temperature sensor is provided in the three-way catalyst 14 and the catalyst temperature reaches a predetermined temperature.
[0038]
When it is determined in step S103 that the three-way catalyst 14 is activated, the process proceeds to step S104, and it is further determined whether the lean burn condition is satisfied. This lean burn condition is a condition that is satisfied in the operating state of the internal combustion engine capable of combustion under a lean air-fuel ratio, such as when the vehicle is traveling at high speed cruising, and if the lean burn condition is satisfied, the step Proceeding to S105, the KCMD value is set to a predetermined value KCMDLB for lean burn control (for example, a value corresponding to A / F = 22), and the process ends. If the lean burn condition is not satisfied, the process proceeds to step S106, the KCMD value is set to a predetermined value KCMDS corresponding to the theoretical air-fuel ratio (step S106), and the process is terminated.
[0039]
In step S103, if the three-way catalyst 14 is not activated, the process proceeds to step S107, and it is further determined whether the after-start lean condition is satisfied. The after-start lean condition is a condition that is established in a state in which the three-way catalyst 14 cannot sufficiently purify the HC component in the exhaust gas immediately after the internal combustion engine is started. In order to reduce the amount of HC generated due to the combustion of 1, the KCMD value is set to a predetermined value KCMDSL for lean after starting (for example, a value corresponding to A / F = 15) (step S108), and the process is terminated. If the after-start lean condition is not satisfied, the process proceeds to step S106 and the process is terminated.
[0040]
Next, the air-fuel ratio sensor activity determination process will be described with reference to FIGS. This air-fuel ratio sensor activity determination process detects the degree of activity of the LAF sensor 15 and sets an activity determination reference value for determining the activity of the LAF sensor 15 in accordance with the target air-fuel ratio. The detected air-fuel ratio sensor This is a process for determining whether or not to start the air-fuel ratio feedback control based on the output of the LAF sensor 15 by comparing the degree of activation of the current and the set activity determination reference value.
[0041]
First, in step S201, it is determined whether or not the heater 54 is energized. If power is being supplied, the activation determination reference value RILMT corresponding to the target air-fuel ratio coefficient KCMD value is retrieved from the RILMT table stored in the storage device 5C. This RILMT value is a reference value set for the internal resistance LAFRI of the LAF sensor 15, and is set to a relatively large value when the target air-fuel ratio is the stoichiometric air-fuel ratio as shown in FIG. The air-fuel ratio is set to a smaller value as the theoretical air-fuel ratio increases.
[0042]
This is because the element temperature at which the LAF sensor 15 is activated, that is, the element temperature when the above-described limit current value is stabilized is low in the stoichiometric air-fuel ratio region and becomes higher as the distance from the stoichiometric air-fuel ratio is increased. Accordingly, when the target air-fuel ratio is set to the stoichiometric air-fuel ratio, the RILMT value is set to a large value so that the LAF sensor 15 is determined to be activated at a relatively low element temperature (for example, 530 ° C.), For example, when the target air-fuel ratio is a predetermined air-fuel ratio for lean burn control (for example, A / F = 22), the LAF sensor 15 is activated when the element temperature becomes relatively high (for example, 700 ° C.). As determined, the RILMT value is set to a small value.
[0043]
Returning to FIG. 4, when the search for the activation determination reference value RILMT according to the KCMD value is completed in step S <b> 202, the process proceeds to step S <b> 203 to determine whether or not the internal resistance LAFRI of the LAF sensor 15 is smaller than the reference value RILMT. . Here, if LAFRI <RILMT, the process proceeds to step S204, the LAF sensor activation flag FLAFACT indicating “1” that the LAF sensor 15 is activated is set to “1”, and the process ends. If LAFRI ≧ RILMT, the process proceeds to step S205, the LAF sensor activation flag FLAFACT is set to “0”, and the process ends. Therefore, in the present embodiment, the reciprocal of the internal resistance LAFRI of the LAF sensor 15 is the degree of activity of the air-fuel ratio sensor described in the claims, and the reciprocal of the RILMT value corresponds to the activity determination reference value.
[0044]
If the heater 54 is not energized in step S201, the process proceeds to step S205, the LAF sensor activation flag FLAFACT is set to “0”, and the process ends.
[0045]
Next, the air-fuel ratio feedback control execution determination process will be described with reference to FIG. FIG. 6 is a flowchart showing the air-fuel ratio feedback control execution determination process. This process is a process of determining whether or not to execute the air-fuel ratio feedback control based on the LAF sensor output in accordance with the determination result of the air-fuel ratio sensor activation determination process described above.
[0046]
First, in step S301, it is determined whether or not the engine water temperature TW is higher than a predetermined temperature TWLAF (for example, −20 ° C.). If TW> TWLAF, the process proceeds to step S302, and it is further determined whether or not the engine speed NE is within a predetermined intermediate range (NHOP> NE> NLOP, for example, NHOP = 5000 rpm, NLOP = 500 rpm). If NHOP> NE> NLOP, the process proceeds to step S303, and it is further determined whether or not the LAF sensor activation flag FLAFACT is “1”.
[0047]
Here, if FLAFACT = 1, it is assumed that the air-fuel ratio feedback control based on the output signal of the LAF sensor 15 is to be executed, the process proceeds to step S304, the air-fuel ratio feedback control execution flag FLAFFB is set to “1”, and the process is terminated. To do. This air-fuel ratio feedback control execution flag FLAFFB is a flag used in the calculation process of the air-fuel ratio correction coefficient KLAF, and when this flag is “1”, a known PID control based on the output of the LAF sensor 15 is executed. To do.
[0048]
In any of the processes in steps S301 to S303, if the determination result is negative, that is, whether TW ≦ TWLAF, NE ≧ NHOP or NE ≦ NLOP, or FLAFFB = 0. In such a case, the process proceeds to step S305, the air-fuel ratio feedback control execution flag is set to “0”, and the process ends.
[0049]
Next, the operation of the air-fuel ratio control apparatus of the present embodiment will be specifically described with reference to FIG. FIG. 7 is a graph showing changes in the internal resistance LAFRI, the LAF sensor activation flag FLAFACT, and the supply air-fuel ratio (actual air-fuel ratio) of the LAF sensor 15 immediately after the engine 1 is started.
[0050]
First, immediately after the engine 1 is started (time t0 to time t1), the internal resistance LAFRI is high (that is, the element temperature is low), and the LAF sensor 15 is not activated, so the LAF sensor activation flag FLAFACT is set to “0”. Is done. At this time, since the air-fuel ratio feedback control based on the output of the LAF sensor 15 is not executed, a deviation occurs between the air-fuel ratio (actual air-fuel ratio) of the air-fuel mixture supplied to the engine 1 and the target air-fuel ratio.
[0051]
When the internal resistance LAFRI of the LAF sensor 15 falls below the air-fuel ratio sensor activation determination reference value RILMT set according to the target air-fuel ratio coefficient KCMD at time t1, the value of the LAF sensor activation flag FLAFACT is switched to “1”. As a result, air-fuel ratio feedback control based on the output of the LAF sensor 15 is started, and thereafter, the actual air-fuel ratio is controlled to become the target air-fuel ratio.
[0052]
As described above, according to the air-fuel ratio control apparatus for the internal combustion engine of the present embodiment, the target air-fuel ratio coefficient KCMD is set according to the operating state of the internal combustion engine, and the activation of the LAF sensor 15 according to the KCMD value. Is determined as the activation determination reference value RILMT, and the LAF sensor 15 is activated when the internal resistance LAFRI of the LAF sensor 15 falls below the set activation determination reference value RILMT. Based on the air-fuel ratio feedback control is started. As a result, the activation state of the LAF sensor 15 is appropriately determined, and the air-fuel ratio feedback control is started as early as possible without impairing the reliability of the output of the LAF sensor. Characteristics can be improved.
[0053]
In the above embodiment, the internal resistance LAFRI (or a parameter inversely proportional thereto) of the LAF sensor 15 is used as a parameter indicating the degree of activity of the air-fuel ratio sensor. However, the present invention is not limited to this. The LAF sensor element temperature itself such as current or all parameters related to the element temperature can be used as the parameters indicating the degree of activity.
[0054]
【The invention's effect】
  According to the air-fuel ratio control apparatus for an internal combustion engine according to claim 1, the activity determination for determining whether or not the air-fuel ratio sensor is activated according to the target air-fuel ratio set according to the operating state of the internal combustion engine. While the reference value is set, the degree of activity of the air-fuel ratio sensor that outputs a signal having a value substantially proportional to the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is detected, and the degree of activity of the detected air-fuel ratio sensor is determined. Since the feedback control based on the output of the air / fuel ratio sensor is started when the set activity determination reference value is exceeded, the reliability of the output of the air / fuel ratio sensor is determined by appropriately determining the active state of the air / fuel ratio sensor. The air-fuel ratio feedback control can be started at the earliest possible time without impairing the exhaust gas, thereby improving the exhaust gas characteristics.
  According to the air-fuel ratio control apparatus for an internal combustion engine according to claim 2, the activation determination reference value is set to the maximum value or the minimum value when the target air-fuel ratio is the stoichiometric air-fuel ratio, and the target air-fuel ratio is the theoretical air-fuel ratio. Since it is set to a value that gradually decreases from the maximum value or a value that gradually increases from the minimum value as the distance from the air-fuel ratio increases, the active state of the air-fuel ratio sensor can be determined more appropriately without impairing the reliability of the output of the air-fuel ratio sensor. The air-fuel ratio feedback control can be started as early as possible.
[Brief description of the drawings]
FIG. 1 is an overall configuration diagram of an air-fuel ratio control apparatus for an internal combustion engine according to an embodiment of the present invention.
2 is a block diagram showing a detailed configuration of the oxygen concentration detection device 16 in FIG. 1. FIG.
FIG. 3 is a flowchart of a program for setting a target air-fuel ratio.
FIG. 4 is a flowchart of a program for determining the activity of the LAF sensor 15;
FIG. 5 is a diagram showing a table for determining an activity determination reference value for determining the activity of the air-fuel ratio sensor.
FIG. 6 is a flowchart of a program for determining whether or not to execute air-fuel ratio feedback control based on the output of the LAF sensor 15;
FIG. 7 is a graph for explaining the operation of the air-fuel ratio control apparatus of the present embodiment.
[Explanation of symbols]
1 engine
5 ECU
14 Three-way catalyst
15 LAF sensor
16 Oxygen concentration detector
25 Control device

Claims (3)

An air-fuel ratio sensor, which is provided in an exhaust system of an internal combustion engine and outputs a signal having a value substantially proportional to the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine;
An operating state detecting means for detecting an operating state of the internal combustion engine;
Air-fuel ratio feedback control means for feedback-controlling the air-fuel ratio so that the air-fuel ratio matches a target air-fuel ratio according to an output signal of the air-fuel ratio sensor;
Target air-fuel ratio setting means for setting the target air-fuel ratio according to the detected operating state;
An activity level detecting means for detecting a parameter indicating an element temperature of the air / fuel ratio sensor as a parameter indicating the activity level of the air / fuel ratio sensor ;
The element temperature that is higher as the target air-fuel ratio set by the target air-fuel ratio setting means becomes farther from the stoichiometric air-fuel ratio is used as an activation determination reference value for determining whether or not the air-fuel ratio sensor is activated. Activity determination reference value setting means for setting on the side representing
Permission means for permitting execution of air-fuel ratio feedback control to the target air-fuel ratio when the detected degree of activity exceeds the activity determination reference value to a side representing a higher active state;
An air-fuel ratio control apparatus for an internal combustion engine, comprising: The element temperature provided and activated in the exhaust system of the internal combustion engine is low when the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is the stoichiometric air-fuel ratio, and increases as the air-fuel ratio departs from the stoichiometric air-fuel ratio. An air-fuel ratio sensor having a characteristic and outputting a signal having a value substantially proportional to the air-fuel ratio;
An operating state detecting means for detecting an operating state of the internal combustion engine;
Air-fuel ratio feedback control means for feedback-controlling the air-fuel ratio so that the air-fuel ratio matches a target air-fuel ratio according to an output signal of the air-fuel ratio sensor;
Target air-fuel ratio setting means for setting the target air-fuel ratio according to the detected operating state;
An activity detection means for detecting a parameter representing the element temperature of the air-fuel ratio sensor as a parameter representing the activity level of the air-fuel ratio sensor;
The element temperature that is higher as the target air-fuel ratio set by the target air-fuel ratio setting means becomes farther from the stoichiometric air-fuel ratio is used as an activation determination reference value for determining whether or not the air-fuel ratio sensor is activated. Activity determination reference value setting means for setting on the side representing
Permission means for permitting execution of air-fuel ratio feedback control to the target air-fuel ratio when the detected degree of activity exceeds the activity determination reference value to a side representing a higher active state;
An air-fuel ratio control apparatus for an internal combustion engine , comprising: An air-fuel ratio sensor, which is provided in an exhaust system of an internal combustion engine and outputs a signal having a value substantially proportional to the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine;
An operating state detecting means for detecting an operating state of the internal combustion engine;
Air-fuel ratio feedback control means for feedback-controlling the air-fuel ratio so that the air-fuel ratio matches a target air-fuel ratio according to an output signal of the air-fuel ratio sensor;
Target air-fuel ratio setting means for setting the target air-fuel ratio according to the detected operating state;
As a parameter representing the activity level of the air-fuel ratio sensor, an activity level detection means for detecting an internal resistance of the air-fuel ratio sensor;
Activity determination reference value setting means for setting an activity determination reference value for determining whether or not the air-fuel ratio sensor is activated according to the target air-fuel ratio set by the target air-fuel ratio setting means; When the reference value for determining the activity is set with respect to the internal resistance of the air-fuel ratio sensor, it is set to a maximum value when the target air-fuel ratio is the stoichiometric air-fuel ratio, and the target air-fuel ratio is set to the stoichiometric air-fuel ratio. When set to a value that gradually decreases from the maximum value as the distance from the fuel ratio increases, and to set the reciprocal of the internal resistance, when the target air-fuel ratio is the stoichiometric air-fuel ratio, the minimum value is set, and the An activity determination reference value setting means for setting the target air-fuel ratio to a value that gradually increases from the minimum value as the target air-fuel ratio departs from the theoretical air-fuel ratio;
Permission means for permitting execution of air-fuel ratio feedback control to the target air-fuel ratio when the detected degree of activity exceeds the activity determination reference value to a side representing a higher active state;
An air-fuel ratio control apparatus for an internal combustion engine , comprising:

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