JP3656453B2 - Heater control device for air-fuel ratio sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a heater control device for an air-fuel ratio sensor, and more particularly to a heater control device for an air-fuel ratio sensor for preventing element cracking of the air-fuel ratio sensor during cold start.
[0002]
[Prior art]
In recent air-fuel ratio control of an engine, an air-fuel ratio sensor and a catalyst are disposed in the exhaust system of the engine, and the exhaust gas is exhausted to the maximum extent by the catalyst. Feedback control is performed so that the exhaust air-fuel ratio of the engine detected by the fuel ratio sensor becomes a target air-fuel ratio, for example, the stoichiometric air-fuel ratio. As this air-fuel ratio sensor, a limiting current type oxygen concentration detecting element (oxygen sensor) that outputs a limiting current in proportion to the oxygen concentration contained in exhaust gas discharged from the engine is used. The limiting current type oxygen concentration detection element detects the exhaust air / fuel ratio of the engine in a wide area and linearly from the oxygen concentration, and improves the air / fuel ratio control accuracy, or the rich / theoretical air / fuel ratio (stoichi) / lean wide area sky. This is useful for controlling the exhaust air-fuel ratio of the engine to the target air-fuel ratio between the fuel ratios.
[0003]
It is essential that the oxygen concentration detection element be kept in an active state in order to maintain air-fuel ratio detection accuracy. Normally, the element is heated by energizing a heater attached to the element from the start of the engine. Then, energization control of the heater is performed so as to be activated early and maintain the activated state.
The heater control device for an oxygen sensor disclosed in JP-A-8-278279 supplies all power to the heater until the heater temperature reaches a predetermined temperature for early activation of the sensor element at the initial stage of energization of the heater. When the temperature reaches a predetermined temperature, electric power corresponding to the heater temperature is supplied to the heater, and when the temperature of the sensor element reaches the predetermined temperature temperature, electric power corresponding to the element temperature of the oxygen sensor is supplied to the heater.
[0004]
[Problems to be solved by the invention]
However, the oxygen sensor heater control device disclosed in the above-mentioned JP-A-8-278279 supplies power to the heater in order to activate the oxygen sensor element early in the initial stage of energization of the heater, particularly at the cold start of the engine. Since power is supplied to the heater at full power, that is, 100% duty ratio without considering the voltage of the battery, a large current flows through the heater to rapidly heat the heater, and the heater temperature rapidly rising and the element temperature of the oxygen sensor There is a problem that the temperature difference increases rapidly, and so-called thermal shock causes an element crack of an oxygen sensor (referred to as an air-fuel ratio sensor).
[0005]
SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a heater control device for an air-fuel ratio sensor that solves the above problems and prevents cracking of the air-fuel ratio sensor due to a thermal shock when the engine is cold started.
[0006]
[Means for Solving the Problems]
A heater control device for an air-fuel ratio sensor according to a first aspect of the present invention that solves the above problems includes an air-fuel ratio sensor provided in an exhaust passage of an internal combustion engine, a heater that heats the air-fuel ratio sensor, and the air-fuel ratio sensor. A heater control unit for controlling an electric power supplied to the heater so as to reach an activation temperature, and a battery voltage detecting unit for detecting a voltage of a battery for supplying electric power to the heater. Heater temperature detecting means for detecting the temperature of the heater, and when the internal combustion engine is started , the heater temperature detected by the heater temperature detecting means after the start of power supply to the heater is the heat resistant limit temperature of the heater until it reaches a temperature corresponding to the heater resistor that does not exceed the battery voltage detected by the battery voltage detecting means is lower than a predetermined voltage If the battery voltage is higher than or equal to the predetermined voltage, a predetermined maximum power that does not cause a sensor element crack according to the battery voltage level is set. And a power setting unit configured to supply power at a supply cycle .
[0007]
With the above configuration, when power is supplied to the heater at the time of cold start, power is supplied to the heater according to the voltage of the battery, so the heater is not heated suddenly, and element cracking of the air-fuel ratio sensor due to thermal shock is prevented. .
A heater control apparatus for an air-fuel ratio sensor according to a second embodiment of the present invention that solves the above problems includes an air-fuel ratio sensor provided in an exhaust passage of an internal combustion engine, a heater that heats the air-fuel ratio sensor, and the air-fuel ratio sensor. A heater control unit for controlling an electric power supplied to the heater so as to reach an activation temperature, and a battery voltage detecting unit for detecting a voltage of a battery for supplying electric power to the heater. When the heater temperature detecting means for detecting a temperature of said heater, and the heater current detection means for detecting a current flowing through the heater, and the element temperature detection means for detecting the element temperature of the air-fuel ratio sensor, the starting of the internal combustion engine upon, such as a heater temperature detected by the heater temperature detection means from the start of power supply to the heater does not exceed the heat resistance limit temperature of the heater heating Until it reaches a temperature corresponding to the resistance, the supply power to the battery voltage the heater calculated from the detected heater current by battery voltage and detected by the detection means and the heater current detection means, the sensor element crack Heater power setting means for setting the power supplied to the heater so that the heater maximum supply power does not cause
It is provided with.
[0008]
With the above configuration, the power supplied to the heater is set so that the power supplied to the heater calculated from the battery voltage and the heater current is the maximum heater power that does not cause sensor element cracking. The element cracking is prevented.
A heater control device for an air-fuel ratio sensor according to another embodiment of the second embodiment according to the present invention that solves the above-described problem includes an air-fuel ratio sensor provided in an exhaust passage of an internal combustion engine, a heater that heats the air-fuel ratio sensor, And a heater control unit for controlling the electric power supplied to the heater so that the air-fuel ratio sensor reaches an activation temperature. Battery voltage detection means for detecting, heater temperature detection means for detecting the temperature of the heater, element temperature detection means for detecting the element temperature of the air-fuel ratio sensor, and power supply to the heater when the internal combustion engine is started From the start until the heater temperature detected by the heater temperature detecting means reaches a temperature corresponding to the heater resistance that does not exceed the heat resistance limit temperature of the heater When the temperature difference between the heater temperature and the element temperature detected by the element temperature detecting means is larger than a predetermined value, the supply power to the heater is set to a predetermined power that does not cause a sensor element crack, and the temperature When the difference is less than or equal to a predetermined value, the heater is set to supply power at a cycle for supplying a predetermined maximum power that does not cause the sensor element cracking according to the battery voltage height, or does not cause the sensor element cracking. Heater power setting means for setting the power supplied to the heater so as to obtain the maximum power supply.
With the above configuration, since the element crack of the air-fuel ratio sensor is predicted from the temperature difference between the heater temperature at the cold start and the element temperature of the air-fuel ratio sensor, the power supplied to the heater is set based on the temperature difference. Element breakage of the air-fuel ratio sensor due to thermal shock is prevented.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a schematic configuration diagram of an embodiment of a heater control apparatus for an air-fuel ratio sensor according to the present invention. In FIG. 1 and subsequent figures, the same components are denoted by the same reference numerals. An air-fuel ratio sensor 1 that is disposed in an exhaust passage of an internal combustion engine (not shown) and detects an exhaust air-fuel ratio of the engine includes an air-fuel ratio sensor element (hereinafter referred to as a sensor element) 2 and a heater 4. A voltage is applied from an air-fuel ratio sensor circuit (hereinafter referred to as a sensor circuit) 3, and electric power is supplied to the heater 2 from the battery 5 via the heater control circuit 6. The sensor circuit 3 receives an analog applied voltage from an air-fuel ratio control unit (A / FCU) 10 formed of a microcomputer via a low-pass filter (LPF) 7 and applies it to the sensor element 2.
[0010]
The A / FCU 10 forms part of the electronic control unit (ECU) 100 together with the sensor circuit 3, the heater control circuit 6 and the LPF 7, and converts digital data into a rectangular analog voltage by a D / A converter provided therein. After that, it outputs to the sensor circuit 3 through the LPF 7. The LPF 7 outputs a smoothed signal from which a high-frequency component of the rectangular analog voltage signal is removed, and prevents an error in detecting the output current of the sensor element 2 due to high-frequency noise. As the voltage of the annealing signal is applied to the sensor element 2, the A / FCU 10 supplies the current flowing through the sensor element 2 that changes in proportion to the oxygen concentration in the gas to be detected, that is, to the sensor element 2 at that time. The applied voltage is detected. The A / FCU 10 is provided with an A / D converter in order to detect these currents and voltages, and these A / D converters are analog voltages and sensors corresponding to the current flowing from the sensor circuit 3 to the sensor element 2. The voltage applied to the element 2 is received and converted into digital data.
[0011]
The air-fuel ratio sensor 1 cannot use its output for air-fuel ratio control unless the sensor element 2 is activated. For this reason, the A / FCU 10 supplies power from the battery 5 to the heater 4 built in the sensor element 2 when the engine is started to energize the heater 4, activates the sensor element 2 early, and activates the sensor element 2. Supplies power to the heater 4 to maintain its active state. The voltage of the battery 5 is converted into digital data by an A / D converter provided in the A / FCU 10.
[0012]
However, focusing on the fact that the resistance of the sensor element 2 depends on the temperature of the sensor element 2, that is, the resistance of the sensor element 2 attenuates as the sensor element temperature increases, the temperature at which the resistance of the sensor element 2 maintains the active state of the sensor element 2 Control is performed to maintain the temperature of the sensor element 2 at a target temperature, for example, 700 ° C., by supplying electric power to the heater 4 so that the resistance value corresponds to, for example, 30Ω. The air-fuel ratio control unit (A / FCU) 10 includes an A / D converter that receives analog voltage corresponding to the voltage and current of the heater 4 from the heater control circuit 6 that heats the sensor element 2 and converts the analog voltage into digital data. Provided. Using these digital data, for example, the resistance value of the heater 4 is calculated, power is supplied to the heater 4 according to the operating state of the engine based on the calculated resistance value, and overheating (OT) of the heater 4 is prevented. Thus, the temperature of the heater 4 is controlled. In the embodiment of the present invention, a limiting current type oxygen concentration detection element (oxygen sensor) is used as the air-fuel ratio sensor 1. However, the present invention is not limited to this, and is also applicable to the case where a λ-type oxygen sensor (referred to as an O ₂ sensor) having a Z characteristic for determining whether the air-fuel ratio is rich or lean is used as the air-fuel ratio sensor 1. it can.
[0013]
The air-fuel ratio control unit (A / FCU) 10 includes, for example, a CPU, ROM, RAM, B (battery backup). A RAM, an input port, an output port, an A / D converter, and a D / A converter are provided to perform heater control of an air-fuel ratio sensor 1 of the present invention described later.
Here, the reason why element cracking of the air-fuel ratio sensor occurs during cold start will be described.
[0014]
2 is a cross-sectional view of the air-fuel ratio sensor shown in FIG. The sensor body 20 of the air-fuel ratio sensor has a diffusion resistance layer 21 having a cup-shaped cross section, and this diffusion resistance layer 21 is fitted into a mounting hole portion of an engine exhaust pipe 27 at its opening end 21a and fixed. ing. The diffusion resistance layer 21 is formed by a plasma spraying method such as ZrO2. The sensor body 20 has a solid electrolyte layer 22, and the solid electrolyte layer 22 is formed of an oxygen ion conductive oxide sintered body and an inner peripheral wall of the resistance diffusion layer 21 through an exhaust gas side electrode layer 23 having a cup-shaped cross section. It is fitted and fixed uniformly. On the inner surface of the solid electrolyte layer 22, an atmosphere-side electrode layer 24 is uniformly fixed in a cross-sectional cup shape. In this case, both the exhaust-side electrode layer 23 and the atmosphere-side electrode layer 24 are formed so as to be sufficiently porous by chemical plating or the like with a noble metal having high catalytic activity such as platinum (Pt). The area and thickness of the exhaust gas side electrode layer 23 are about 10 to 100 mm ² and about 0.5 to 2.0 μm. On the other hand, the area and thickness of the atmosphere-side electrode layer 24 are 10 mm ² or more and about 0.5 to 2.0 μm. The sensor body 20 is surrounded by a protective cover 28. The protective cover 28 is provided in order to ensure the heat insulation of the sensor body 20 while preventing direct contact with the exhaust gas of the sensor body 20. The protective cover 28 is provided with a large number of small holes for communicating the inside and outside of the cover.
[0015]
Since it is necessary to supply a large amount of power to the heater 26 in order to heat the sensor body 20 early when the engine is cold-started, according to the prior art, power is supplied from the battery 5 to the heater 26 at a duty ratio of 100%. . Then, a large current flows through the heater 26 to heat the heater 26 abruptly, the temperature of the heater 26 rises rapidly, the temperature difference between the temperature of the heater 26 and the temperature of the sensor body 20 increases abruptly, and the air-fuel ratio sensor. Element cracking occurs. This element crack includes a microcrack of the diffusion layer. In the present invention, in order to prevent element cracking of the air-fuel ratio sensor during cold start of the engine, heater control is performed so as not to supply excessive power to the heater 26 during cold start as will be described below.
[0016]
FIG. 3 is a flowchart of the heater control routine. The processing of this routine and the flowcharts shown in FIGS. 4, 6, 7 and 8 are executed at predetermined processing cycles, for example, every 64 ms. First, at step 301, it is determined whether an ignition switch IGSW (not shown) is on or off. When the IGSW is on, the routine proceeds to step 302, and when the IGSW is on, this routine is terminated.
[0017]
Briefly explaining the processing in steps 302 to 312, for the early activation of the air-fuel ratio sensor 1, the power supply from the battery 5 to the heater 4 is started, and until the heater temperature reaches a predetermined temperature, duty control at the time of start-up is performed. Is supplied to the heater 4 (duty control at start-up), and when the heater temperature reaches a predetermined temperature, the electric power corresponding to the heater temperature is supplied to the heater 4 (heater upper limit resistance F / B control). When the temperature of the fuel ratio sensor 1 reaches a predetermined temperature, electric power for maintaining the sensor element 2 in the active state is supplied to the heater 4 according to the element temperature of the air fuel ratio sensor 1 (element temperature F / B control).
[0018]
In step 302, the element DC impedance Zdc of the air-fuel ratio sensor 1 is calculated. The impedance Zdc is obtained by applying a negative voltage Vneg to the sensor element 2, detecting the current Ineg, and calculating Zdc = Vneg / Ineg. In general, there is a correlation that the element DC impedance decreases as the element temperature rises. For example, when the sensor element 2 has an activation temperature of 700 ° C., the element DC impedance is 30Ω.
[0019]
In step 303, it is determined whether or not the activation flag F1 of the air-fuel ratio sensor 1 is set. If F1 = 1, the process proceeds to step 304. In step 304, element temperature F / B control is executed, and F1 = 0. If so, go to Step 305.
In step 305, the activation of the sensor element 2 is determined based on the element DC impedance. That is, when Zdc> 30, it is determined that the sensor element 2 has been activated, the activation flag F1 of the air-fuel ratio sensor 1 is set to 1 at step 306, and then element temperature F / B control is executed at step 304, Zdc ≦ If it is 30, it is determined that the sensor element 2 is in an inactive state, and the process proceeds to step 307 to perform heater control for activating the sensor element 2. The flag F1 is reset by a one-shot pulse signal when the ignition switch IGSW is switched from OFF to ON.
[0020]
In step 307, the applied voltage Vn and current In to the heater 4 are detected. In step 308, the resistance Rh of the heater 4 is calculated from Rh = Vn / In.
In step 309, it is determined whether or not the heater upper limit temperature 1020 ° C., which is lower than the heat-resistant limit temperature 1200 ° C. of the heater 4 by a predetermined temperature, has been reached. DUTY control for supplying as much power as possible to 4 is executed. If the determination result is NO, the process proceeds to step 311 to perform control to maintain the heater 4 at the heater upper limit temperature of 1020 ° C. Steps 310 and 311 will be described in detail later with reference to FIGS. Here, the reason why the upper limit temperature of the heater is not set to the heat resistant limit temperature of the heater 4 is that the resistance temperature characteristic of the heater 4 varies. When the median value of the variation is used, the heater resistance Rh corresponding to the heater upper limit temperature of 1020 ° C. is 2.1Ω, and when the heater control is performed so that the heater resistance Rh is 2.1Ω, the heater temperature variation is 870 to 1200. It falls within the range of ° C and does not exceed the heat resistance limit temperature of the heater 4.
[0021]
In step 312, the voltage of the battery 5 is applied to the heater according to the DUTY ratio set in steps 310 and 311. Here, DUTY control means that when the cycle of turning on and off the voltage of the battery 5 to the heater 4 is 100 ms, for example, when the DUTY ratio is 20%, the on time is 20 ms, the off time is 80 ms, and the DUTY ratio is 50%. When the ON time is 50 ms, the OFF time is 50 ms, and the DUTY ratio is 100%, the control means that the voltage of the battery 5 is applied to the heater 4 in each cycle of the ON time of 100 ms. Next, step 311 in FIG. 3 will be described in detail with reference to FIG.
[0022]
FIG. 4 shows heater control based on the heater upper limit resistance. First, in step 401, it is determined whether or not the heater power control flag F2 indicating that the heater power control is being executed is set. If F2 = 1, the process proceeds to step 402. If F1 = 0, the process proceeds to step 403. In step 403, 20% is set as the initial duty ratio of the heater power control. This 20% is a value selected so that a sudden change in the heater temperature is suppressed when the heater voltage control is shifted to the power control. Next, at step 404, F2 is set. The flag F2 is reset by a one-shot pulse signal when the ignition switch IGSW is switched from OFF to ON.
[0023]
In step 402, it is determined whether or not the heater resistance Rh is greater than 2.5Ω in order to perform control for protecting the heater 4 from being abnormally heated due to a rise in exhaust temperature caused by a sudden change in engine operating conditions. When Rh> 2.5Ω, the process proceeds to step 405, and when Rh ≦ 2.5Ω, the process proceeds to step 406. In step 406, DUTY = DUTY-10 is calculated, and the calculated value is set to a new DUTY ratio. When DUTY becomes a negative value, DUTY = 0 is set.
[0024]
In step 405, the heater power Wh is calculated from the following equation.
Wh = Vn.times.In.times.DUTY / 100
Here, Vn and In indicate the voltage value and current value detected in step 307 in FIG. 3, and DUTY indicates the DUTY ratio set in step 403, 406, 408 or 409 in the previous processing cycle.
[0025]
In step 407, the heater power Wh in the current processing cycle is compared with the heater power supply 21W corresponding to the heat-resistant limit temperature 1200 ° C of the heater 4, and when Wh ≦ 21, the power supplied to the heater 4 is lower than the target power. In step 408, the duty ratio is increased by 3% (DUTY = DUTY + 3 is calculated) to increase the power supplied to the heater 4. If Wh> 21, the power supplied to the heater 4 is the target. It is determined that the electric power is higher than the electric power, and the process proceeds to step 409. In step 409, the duty ratio is subtracted by 3% (DUTY = DUTY-3 is calculated), and the electric power supplied to the heater 4 is decreased.
[0026]
By controlling the heater based on the DUTY set as described above, the actual power supplied to the heater 4 can be controlled to the target power 21 (W).
Next, the element temperature F / B control in step 304 will be described. Based on the element DC impedance Zdc detected in step 303, the duty ratio of the voltage applied to the heater 4 is set based on the following equation so that the element DC impedance Zdc is 30 (Ω) corresponding to an element temperature of 700 ° C. Calculate.
[0027]
DUTY = GP + GI + c
GP = a (Zdc-30)... Proportional term GI = GI + b (Zdc-30)... Integral term where a, b and c are constants such as a = 4.2, b = 0.2 and c = 20, for example. It is. By controlling the heater 4 with the calculated duty ratio, the element DC impedance Zdc can be controlled in the vicinity of 30 (Ω), the sensor element can always be maintained in a good active state, and damage to the sensor element due to abnormal heating can be prevented. it can. Next, step 310 in FIG. 3 will be described below with reference to FIG.
[0028]
FIG. 5 is a flowchart showing the heater control of the first embodiment when the engine is started. When it is determined in step 309 in FIG. 3 that the heater temperature has not reached the heater upper limit temperature of 1020 ° C., steps 501 and 502 are executed, and DUTY control for supplying as much power as possible to the heater 4 is performed. In step 501, the voltage of the battery 5 is read. In step 502, the DUTY ratio of the heater supply power is calculated from the battery voltage read in step 501 using the map shown in FIG.
[0029]
According to the heater control of the first embodiment, even when the battery voltage VB is higher than 12V when the engine is started, the duty ratio is set lower than 100%. Can be prevented.
In the map shown in FIG. 6, when the battery voltage V _B is 12V and DUTY = 100, a voltage of 12V is continuously applied to the heater resistance Rh = 2.1 (Ω), and the average supply power to the heater 4 is , (V _B ) ² / R, which is about 69 (W). By setting the average supply power to the heater 4 at this time to an upper limit value, it is possible to supply power to the heater 4 without causing a sensor element crack. When the battery voltage V _B exceeds 12 V, the DUTY ratio is calculated so that the average power supply to the heater 4 does not exceed 69 (W). For example, when the battery voltage V _B is 14 V and the power is supplied with 100% duty as it is, the average supply power becomes (V _B ) ² / R = 93 (W), and this is reduced to 69 (W). In order to achieve this, the DUTY ratio may be 69/93 = 74 (%). The DUTY ratio for the battery voltage V _B exceeding 12V in the map is thus calculated. Note that the DUTY ratio for the battery voltage V _B of 12 V or less is set to 100% for early activation of the sensor element 2 because the average supply power is 69 (W) or less even when power is supplied at 100% duty. Next, step 310 of FIG. 3 will be described below with reference to FIG.
[0030]
FIG. 7 is a flowchart showing the heater control of the second embodiment when the engine is started. When it is determined in step 309 in FIG. 3 that the heater temperature has not reached the heater upper limit temperature of 1020 ° C., Rh <2.1 (Ω), steps 701 and 702 are executed to supply as much power as possible to the heater 4. DUTY control is performed. In step 701, the heater supply power Wa to the heater 4 at the current processing cycle DUTY = 100 (%) is calculated from the heater voltage Vn and heater current In detected in step 307 in FIG. 3 (Wa = Vn × In). . Here, it should be noted that the heater voltage Vn corresponds to the battery voltage VB, and Wa is calculated according to VB.
[0031]
In step 702, the DUTY ratio is calculated from the following equation using the heater maximum supply power Whm and the heater supply power Wa calculated in step 701 for the current processing cycle.
DUTY = (Whm / Wa) × 100 = (144 / Wa) × 100
Here, the heater maximum supply power Whm is the average supply power 144 that can be supplied to the heater 4 without cracking the sensor element when the heater resistance Rh is 1.0 (Ω) and the battery voltage VB is 12 (V). Say (W). When DUTY> 100 as a result of the above calculation, DUTY = 100 is set.
[0032]
According to the heater control of the second embodiment, the heater supply power Wa is the maximum heater supply depending on the battery voltage VB when the heater resistance Rh is a small resistance value before reaching 2.1 (Ω) when the engine is started. Since there is a possibility that the heater temperature rapidly rises beyond the electric power Whm (= 144 (W)) and the sensor element breaks due to thermal shock, the heater is set so that the upper limit of the heater supply electric power Wa is set to the heater maximum supply electric power Whm. The DUTY ratio of the voltage applied to 4 is set.
[0033]
FIG. 8 is a flowchart showing the heater control of the third embodiment when the engine is started. When it is determined in step 309 in FIG. 3 that the heater temperature has not reached the heater upper limit temperature of 1020 ° C., steps 801 to 806 are executed, and DUTY control for supplying as much power as possible to the heater 4 is performed. In step 801, the heater temperature is calculated from the heater resistance Rh calculated in step 308 of FIG. In step 802, the temperature of the sensor element 2 is calculated from the element DC impedance Zdc calculated in step 302 of FIG. In step 803, the temperature difference between the heater temperature calculated in step 801 and the sensor element temperature calculated in step 802 is calculated. In step 804, it is determined whether or not the temperature difference exceeds a predetermined value, for example, 500 ° C., and if the determination result is YES, the process proceeds to step 805, and in step 805, the heater temperature is rapidly increased and a sensor element caused by thermal shock is detected. DUTY ratio that does not cause element cracking of, for example, 20% is set. When the determination result in step 804 is NO, a DUTY ratio corresponding to the voltage of the battery 5 is calculated. Specifically, the processing of the above-described flowchart of FIG. 5 (first embodiment) or the flowchart of FIG. 7 (second embodiment) is executed. The predetermined value 500 ° C. is a value obtained experimentally. Since the heater upper limit temperature is 1020 ° C. and the sensor activation temperature is 700 ° C., a thermal shock does not occur at a difference of about 320 ° C. Until the sensor element 2 is activated, the normal temperature difference rarely exceeds 500 ° C., and this setting has little effect on the early activation of the sensor element 2.
[0034]
In step 802, the temperature of the sensor element 2 is calculated from the element DC impedance Zdc. Alternatively, the temperature of the sensor element 2 may be calculated from the element AC impedance Zac. Normally, 0.3 (V), for example, is applied to the sensor element 2, and the exhaust current is calculated by detecting the limit current every predetermined period. The AC impedance Zac applies a pulse voltage of 0.3 ± 0.2 (V) to the sensor element 2 every predetermined period, for example, every 64 ms, and detects the voltage Vac and current Iac of the sensor element 2 at that time, Calculate Zac = Vac / Iac. In general, there is a correlation that the element AC impedance is attenuated as the element temperature rises, similarly to the element DC impedance. In the case of detecting the element AC impedance, there is no need to apply a negative voltage to the sensor element 2 as in the case of detecting the element DC impedance, so that there is an advantage that the control circuit can be simplified.
[0035]
【The invention's effect】
As described above, according to the present invention, when power is supplied to the heater at the time of cold start, power is supplied to the heater in accordance with the voltage of the battery. Element breakage of the sensor is prevented.
As described above, according to the present invention, the element crack of the air-fuel ratio sensor is predicted from the temperature difference between the heater temperature and the sensor element temperature during cold start, and the heater is supplied based on the temperature difference. Since the electric power to be set is set, the element breakage of the air-fuel ratio sensor due to the thermal shock is prevented.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an embodiment of a heater control device for an air-fuel ratio sensor according to the present invention.
FIG. 2 is a cross-sectional view of the air-fuel ratio sensor shown in FIG.
FIG. 3 is a flowchart of a heater control routine.
FIG. 4 is a flowchart showing heater control based on a heater upper limit resistance.
FIG. 5 is a flowchart showing heater control of the first embodiment at the time of engine start.
FIG. 6 is a map for calculating a DUTY ratio of heater supply power from a battery voltage.
FIG. 7 is a flowchart showing heater control of the second embodiment at the time of engine start.
FIG. 8 is a flowchart showing heater control of a third embodiment at the time of engine start.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... A / F sensor 2 ... Sensor element 3 ... A / F sensor circuit 4 ... Heater 5 ... Battery 6 ... Heater control circuit 7 ... LPF
10 ... A / F control unit 100 ... Electronic control unit (ECU)

Claims (3)

An air-fuel ratio sensor provided in an exhaust passage of the internal combustion engine, a heater for heating the air-fuel ratio sensor, heater control means for controlling electric power supplied to the heater so that the air-fuel ratio sensor becomes an activation temperature, In the air-fuel ratio sensor heater control device comprising:
Battery voltage detecting means for detecting a voltage of a battery for supplying power to the heater;
Heater temperature detecting means for detecting the temperature of the heater;
When starting the internal combustion engine, from the start of power supply to the heater until the heater temperature detected by the heater temperature detecting means reaches a temperature corresponding to the heater resistance that does not exceed the heat resistance limit temperature of the heater. In the meantime, when the battery voltage detected by the battery voltage detecting means is lower than a predetermined voltage, it is set to continuously supply power to the heater, and when the battery voltage is higher than or equal to the predetermined voltage, Power setting means for setting power to be supplied at a cycle for supplying a predetermined maximum power that does not cause the sensor element cracking according to the height of the battery voltage ;
A heater control device for an air-fuel ratio sensor. An air-fuel ratio sensor provided in an exhaust passage of the internal combustion engine, a heater for heating the air-fuel ratio sensor, heater control means for controlling electric power supplied to the heater so that the air-fuel ratio sensor becomes an activation temperature, In the air-fuel ratio sensor heater control device comprising:
Battery voltage detecting means for detecting a voltage of a battery for supplying power to the heater;
Heater temperature detecting means for detecting the temperature of the heater;
Heater current detecting means for detecting a current flowing through the heater;
An element temperature detecting means for detecting an element temperature of the air-fuel ratio sensor;
When starting the internal combustion engine, from the start of power supply to the heater until the heater temperature detected by the heater temperature detecting means reaches a temperature corresponding to the heater resistance that does not exceed the heat resistance limit temperature of the heater. During this time, the power supplied to the heater calculated from the battery voltage detected by the battery voltage detection means and the heater current detected by the heater current detection means becomes the heater maximum supply power that does not cause sensor element cracking. A heater power setting means for setting the power supplied to the heater,
A heater control device for an air-fuel ratio sensor. An air-fuel ratio sensor provided in an exhaust passage of the internal combustion engine, a heater for heating the air-fuel ratio sensor, heater control means for controlling electric power supplied to the heater so that the air-fuel ratio sensor becomes an activation temperature, In the air-fuel ratio sensor heater control device comprising:
Battery voltage detecting means for detecting a voltage of a battery for supplying power to the heater;
Heater temperature detecting means for detecting the temperature of the heater;
An element temperature detecting means for detecting an element temperature of the air-fuel ratio sensor;
When starting the internal combustion engine, from the start of power supply to the heater until the heater temperature detected by the heater temperature detecting means reaches a temperature corresponding to the heater resistance that does not exceed the heat resistance limit temperature of the heater. Meanwhile, when the temperature difference between the heater temperature and the element temperature detected by the element temperature detecting means is larger than a predetermined value, the power supplied to the heater is set to be a predetermined power that does not cause a sensor element crack, When the temperature difference is smaller than or equal to a predetermined value, the sensor element is set to be supplied at a cycle for supplying a predetermined maximum power that does not cause the sensor element cracking according to the battery voltage level, or the sensor element cracking occurs. Heater power setting means for setting the supply power to the heater so as not to have a heater maximum supply power;
A heater control device for an air-fuel ratio sensor.

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