JP2012036814A - Air fuel ratio detecting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an air fuel ratio detecting device that can surely prevent an element from being damaged by being moistened with a condensate water, and can start an air fuel ratio feed back control at an early stage.SOLUTION: The air fuel ratio detecting device includes: an air fuel ratio sensor 7 provided in an exhaust passage 4 of an internal combustion engine 1; a heater built in the air fuel ratio sensor 7 to increase a temperature of the element of the air fuel ratio sensor 7; and a control means 8 that starts to increase of the element when the internal combustion engine 1 is started, and decreases the speed of the temperature increase when the element reaches an active temperature.

Description

  The present invention relates to an air-fuel ratio detection device that detects an exhaust air-fuel ratio of an internal combustion engine.

  In order to reduce harmful components discharged from the vehicle, an exhaust purification catalyst is interposed in the exhaust passage. In order to exhibit the purification performance of the exhaust purification catalyst, an air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust discharged from the internal combustion engine is provided, and air-fuel ratio feedback control is performed based on the detected value of the air-fuel ratio sensor. It has been broken.

  By the way, generally, at the time of normal operation after the completion of warm-up, the element temperature is controlled to 700 ° C. or higher where the air-fuel ratio sensor exhibits high accuracy and high response. For this reason, after the engine is started, heating by a heater or the like is performed in order to quickly raise the element temperature to this temperature. However, if there is condensed water in the exhaust passage, the element whose temperature is rising may be exposed to the condensed water, and element cracking may occur due to the thermal stress caused by the temperature difference between the wetted part and other parts. There is.

  Therefore, in Patent Document 1, when the cooling water temperature immediately after starting is below a predetermined value, the temperature is rapidly raised to about 300 ° C., which is lower than the activation temperature of the air-fuel ratio sensor. A technique is disclosed in which control is started and the temperature is rapidly raised to a temperature of 700 ° C. or higher when the cooling water temperature exceeds a predetermined value. The characteristic of the air-fuel ratio sensor is that if the element temperature is about 300 ° C., it is possible to detect whether lean or rich based on the stoichiometric air-fuel ratio, even if linear output with respect to the air-fuel ratio cannot be performed. Is used. This is a control for starting air-fuel ratio feedback at an earlier stage while preventing element cracking due to moisture by immediately starting air-fuel ratio food back control when it becomes possible to detect lean or rich.

JP 2003-155953 A

  By the way, the thermal stress that causes element cracking increases as the element temperature increases, but it also has a characteristic that it increases as the temperature rise rate of the element increases. Therefore, when the cooling water temperature exceeds a predetermined value in a state where there is a temperature difference between the part that has been flooded but other parts, although the condensed water has disappeared, the temperature is rapidly raised to a high temperature as in Patent Document 1. Then, there exists a possibility of producing an element crack by a thermal stress.

  Therefore, an object of the present invention is to provide an air-fuel ratio detection device capable of starting air-fuel ratio feedback control at an earlier stage while reliably preventing element cracking due to water when the engine is started.

  An air-fuel ratio detection device of the present invention includes an air-fuel ratio sensor provided in an exhaust passage of an internal combustion engine, a heater that raises the temperature of an element of the air-fuel ratio sensor, and starts the temperature of the element when the internal combustion engine is started. And a control means for controlling the heater so as to reduce the heating rate of the element when the activation temperature is reached.

  According to the present invention, the temperature can be quickly raised to the activation temperature, and the generation of thermal stress can be suppressed by reducing the rate of temperature rise when the activation temperature is reached. That is, air-fuel ratio feedback control can be started early while preventing element cracking.

1 is a diagram showing an embodiment of an internal combustion engine system to which the present invention is applied. It is a flowchart which shows the control routine for the element temperature rise of the air fuel ratio sensor which a controller performs. It is a figure which shows the relationship between the intensity | strength with respect to the moisture of an element, and element temperature. It is a figure which shows the relationship between the element temperature of an air fuel ratio sensor, and detection accuracy. It is a figure which shows the relationship between the element temperature when the air fuel ratio of an air-fuel mixture changes, and the responsiveness of an air fuel ratio sensor. It is a time chart at the time of performing the control routine of FIG.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic configuration diagram of an internal combustion engine system to which the present embodiment is applied.

  A throttle valve 3 disposed at the inlet of the intake manifold 2 adjusts the intake air amount of the internal combustion engine 1. A fuel injection valve 5 arranged at a branch portion branched from the intake manifold 2 and connected to each cylinder injects an amount of fuel corresponding to the intake air amount. The opening degree of the throttle valve 3 and the fuel injection amount are determined by a control unit (ECU) 8 described later.

  Exhaust gas from the internal combustion engine 1 is discharged to the exhaust passage 4 and purified by an exhaust purification catalyst 6 interposed in the exhaust passage 4. An air-fuel ratio sensor 7 that detects the air-fuel ratio of the exhaust is disposed on the upstream side of the catalyst 6 in the exhaust passage 4. The air-fuel ratio sensor 7 uses ceramics such as zirconia and titania as elements as well as the known air-fuel ratio sensor, and adjusts the air-fuel ratio of the exhaust gas over a wide range from the stoichiometric air-fuel ratio to the lean side and the rich side. On the other hand, linear output characteristics are shown.

  The air-fuel ratio sensor 7 has a built-in heater. The air-fuel ratio sensor 7 cannot detect the air-fuel ratio unless the element temperature reaches the active temperature, that is, about 550 ° C., and the air-fuel ratio detection accuracy and the responsiveness to changes in the air-fuel ratio are sufficiently high. Since the temperature is higher than 700 ° C., the temperature of the element for detection of the air-fuel ratio sensor 7 is raised by the heater for heating at the time of cold start.

  A detection signal from the air-fuel ratio sensor 7 is read into the ECU 8. In addition to this, the ECU 8 reads signals from various sensors relating to the operation state such as the crank angle sensor 10, the accelerator opening sensor 11, the cooling water temperature sensor 12, and the like. Based on the read signal, the ECU 8 sets the fuel injection amount and ignition timing according to the operating state, the throttle opening, the heater on / off of the air-fuel ratio sensor 7, and the exhaust air-fuel ratio as the stoichiometric air-fuel ratio. Air-fuel ratio feedback control, etc. for maintaining the temperature is performed.

  The ECU 8 is composed of a microcomputer including a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), and an input / output interface (I / O interface). The ECU 8 can also be composed of a plurality of microcomputers.

  By the way, when the internal combustion engine 1 is started, condensed water may exist in the exhaust passage 4. When the wall surface temperature of the exhaust passage 4 is low, such as when starting the cold machine, the condensed water is condensed by the moisture in the exhaust discharged from the internal combustion engine 1 being cooled by the wall surface of the exhaust passage 4 or the like. After the operation is stopped, the air in the exhaust passage 4 is cooled and condensed as the temperature of the exhaust passage 4 decreases.

  When the element for detection of the air-fuel ratio sensor 7 is flooded with condensed water, a thermal stress is generated in the element due to a temperature difference between the wetted part and the non-watered part, which may cause element cracking.

  The thermal stress that causes element cracking increases as the temperature difference between the wetted part and the unwatered part increases. In other words, the higher the element temperature, the greater the thermal stress due to water exposure. Therefore, if the element is exposed to water at a high temperature, the possibility of element cracking increases. In addition, even when the temperature rising rate of the element is high, the temperature difference between the wetted part and the non-watered part becomes large, and the possibility of element cracking increases. For this reason, it is not a good idea to raise the temperature of the element during a period of condensed water.

  However, if the element temperature is lower than the activation temperature, the air-fuel ratio sensor 7 has a marked decrease in air-fuel ratio detection accuracy. Therefore, from the viewpoint of improving the exhaust performance, it is desirable that the air-fuel ratio sensor 7 be quickly raised after the engine is started.

  As control for satisfying these conflicting requirements, Japanese Patent Application Laid-Open No. 2003-155953 can detect whether lean or rich based on the theoretical air-fuel ratio when it is determined that there is condensed water. A control is disclosed in which air-fuel ratio feedback control is started when the temperature of the element is raised to a certain temperature, for example, about 300 ° C. On the other hand, when it is determined that there is no condensed water, the temperature is controlled to rise rapidly.

  However, the ECU 8 cannot perform highly accurate air-fuel ratio feedback control only by knowing whether the air-fuel ratio is lean or rich with respect to the theoretical air-fuel ratio. As will be described later, at an element temperature of about 300 ° C., the responsiveness of the air-fuel ratio sensor 7 to changes in the air-fuel ratio is significantly lower than that in the active state. Therefore, with the control disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 2003-155953, it is impossible to reliably avoid element cracking and to improve exhaust performance. Furthermore, when it is determined that there is no condensed water, the temperature is rapidly increased. However, as described above, there is a possibility that element cracking may occur even when the rate of temperature increase is too high.

  On the other hand, in the present embodiment, element cracking of the air-fuel ratio sensor 7 is reliably prevented and exhaust performance is improved by the control described below.

  FIG. 2 is a flowchart showing the contents of a temperature increase control routine of the air-fuel ratio sensor 7 executed by the ECU 8 when the engine is started. This control routine is repeatedly executed at a constant time interval, for example, 10 milliseconds from the start of the internal combustion engine. Hereinafter, it demonstrates according to the step of a flowchart.

  In step S100, the ECU 8 determines whether or not the engine is being started based on the state of the ignition switch or the like. If it is at the time of engine starting, the process of step S110 will be performed, and if that is not right, a process will be complete | finished.

  In step S110, the ECU 8 sets the heater duty to 100%. That is, it heats with the upper limit of the capability of a heater.

  In step S120, it is determined whether or not the internal resistance of the element of the air-fuel ratio sensor 7 is A (Ω) or less. If the determination result is A (Ω) or less, the process of step S130 is executed, and A ( If it is greater than Ω), the process is terminated. This process determines whether or not the air-fuel ratio sensor 7 has a temperature at which air-fuel ratio feedback control is possible. Since the internal resistance has a characteristic that it decreases as the element temperature increases, if the lower limit temperature at which the air-fuel ratio sensor 7 can perform feedback control, that is, the internal resistance of the element at the active temperature is set as A (Ω), the element It is possible to determine whether or not the temperature is a lower limit temperature at which air-fuel ratio feedback control is possible without measuring the temperature with a sensor or the like. The lower limit element temperature at which air-fuel ratio feedback control is possible, that is, the activation temperature is about 550 ° C.

  Here, the reason why the start temperature of the air-fuel ratio feedback control is set to the activation temperature, that is, 550 ° C. will be described.

  FIG. 3 is a diagram showing the relationship between the strength of the element against moisture and the element temperature, where the vertical axis represents the strength against moisture of the element, the horizontal axis represents the element temperature, and the broken line X is required when used for vehicles. The lower limit of the strength to be applied (hereinafter referred to as required strength). The required strength is a value that can be arbitrarily set.

  The strength against water is evaluated by the amount of dripping when element is cracked by dropping water while gradually increasing the amount of the element. That is, the vertical axis in FIG. 3 corresponds to the amount of dripping, and the greater the amount of dripping when an element crack occurs, the higher the strength against moisture. In addition, the upper limit of the vertical axis | shaft of FIG. 3 is the upper limit of the amount of condensed water which can generally be assumed.

  FIG. 3 shows that when the element temperature is 500 ° C. or less, there is no fear of element cracking with the amount of condensed water that can be generally assumed, and sufficient strength is obtained even at about 550 ° C.

  FIG. 4 is a diagram showing the relationship between the element temperature of the air-fuel ratio sensor 7 and the detection accuracy. The horizontal axis represents the element temperature, and the vertical axis represents the detected air-fuel ratio. Solid lines a-d in the figure show the results of measuring the air-fuel ratios of the air-fuel ratios and the air-fuel ratios set to A / F = 13, 14.5 and 18, respectively, and the element temperatures.

  As shown in FIG. 4, when the element temperature exceeds 600 ° C., the solid line a-d almost coincides with the set air-fuel ratio and the measurement result. At 600 ° C. or lower, A / F = 13, 14.5, that is, the set value and the measurement result almost coincide with each other for the air-fuel ratio richer than the stoichiometric air-fuel ratio, but A / F = 18 and the atmosphere, that is, the stoichiometric air-fuel ratio. For the leaner air-fuel ratio, the difference between the set value and the measurement result increases. However, since the element is activated at 550 ° C. or higher, the difference between the air-fuel ratios can be determined.

  FIG. 5 is a diagram showing the relationship between the element temperature and the responsiveness of the air-fuel ratio sensor 7 when the air-fuel ratio of the air-fuel mixture changes, here, when A / F = 13 changes to A / F = 18. Yes, the vertical axis represents the response, and the horizontal axis represents the element temperature. The responsiveness is the time (milliseconds) from when the setting of the air-fuel ratio is changed until the detection value of the air-fuel ratio sensor 7 is changed. From FIG. 5, it can be seen that when the element temperature is 600 ° C. or lower, the responsiveness starts to deteriorate significantly.

  As described above, if the element temperature is 550 ° C., the element has sufficient strength against water, and if it is up to this temperature, there is no risk of element cracking even if the temperature is rapidly increased. In addition, when the engine is started, the amount of increase at the time of starting is performed, so the detection accuracy of the air-fuel ratio richer than the stoichiometric air-fuel ratio is important. However, at 550 ° C., the air-fuel ratio sensor 7 is on the rich side of the stoichiometric air-fuel ratio. Has sufficient detection accuracy. Further, since control is not performed to greatly change the air-fuel ratio when the engine is started, even responsiveness at 550 ° C. can be sufficiently used for feedback control. Therefore, the element temperature for starting the air-fuel ratio feedback control is set to 550 ° C., which is the activation temperature.

  Return to the description of the flowchart. In step S130, the ECU 8 starts air-fuel ratio feedback control and reduces the duty of the heater to B%. For B%, for example, a value of about 70% is set. As a result, the increasing speed of the element temperature decreases.

  In step S140, the ECU 8 determines whether or not the internal resistance of the element has become equal to or less than C (Ω). If it is equal to or less than C (Ω), the process of step S150 is executed. Exit. In step S150, the ECU 8 executes heater duty feedback control with the target internal resistance set to C (Ω).

  Here, C (Ω) is the element temperature when the element temperature is 700 ° C. However, the internal resistance value may be a predetermined temperature within a range of 600 ° C. to 700 ° C. For example, if the above-described required strength is set higher than that shown in FIG. You may set to the temperature lower than 700 degreeC.

  As shown in FIG. 3, if it is 700 degrees C or less, the intensity | strength with respect to water will become higher than the minimum of a required intensity | strength. As shown in FIGS. 4 and 5, when the element temperature is in the range of 600 ° C. to 700 ° C., the detection accuracy of the air / fuel ratio sensor 7 can be ensured regardless of whether the air / fuel ratio is leaner or richer than the theoretical air / fuel ratio. Responsiveness to changes can be secured. Therefore, the target internal resistance for feedback control is set to an internal resistance C (Ω) corresponding to an element temperature of 700 ° C.

  In step S150, the ECU 8 executes heater duty feedback control with the target internal resistance set to C (Ω).

  In step S160, the ECU 8 determines whether or not the engine operating time has exceeded a preset threshold value D (minutes). If it has elapsed, the process of step S170 is executed, and if not, the process is performed. finish. The threshold value D is the time until the condensed water disappears from the exhaust passage 4, and is determined and set by experiment or the like for each specification of the internal combustion engine 1. Here, it is set to about 4 to 5 (minutes).

  In step S170, the ECU 8 performs heater duty feedback control with the target internal resistance set to E (Ω). The target internal resistance E (Ω) is an internal resistance value corresponding to an element temperature during normal operation (hereinafter referred to as normal control temperature), for example, 730 (° C.).

  Note that steps S160 and 170 are not essential processes. As shown in FIGS. 4 and 5, when the element temperature reaches 730 ° C., the detection accuracy of the air-fuel ratio sensor 7 is improved particularly on the lean side of the theoretical air-fuel ratio. Further, the responsiveness of the air-fuel ratio sensor 7 is better at 730 ° C. than at 700 ° C. However, if the element temperature is 700 ° C., sufficient detection accuracy and responsiveness can be ensured. Therefore, when the target air-fuel ratio does not change from the theoretical air-fuel ratio, it is not necessary to raise the temperature to 730 ° C. On the other hand, when the target air-fuel ratio is switched to the stoichiometric air-fuel ratio or an air-fuel ratio leaner than the stoichiometric air-fuel ratio according to the operating state, etc., the response and the detection accuracy of the air-fuel ratio leaner than the stoichiometric air-fuel ratio are higher 730. By raising the temperature to ° C., more accurate air-fuel ratio feedback control becomes possible. Therefore, the necessity of steps S160 and S170 is determined according to the required detection accuracy and responsiveness.

  Next, the effect obtained when the ECU 8 executes the control routine will be described with reference to the time chart of FIG. The solid line in FIG. 6 shows the case where the control routine of the present embodiment is executed, and the broken line shows the case where the temperature is increased at a heater duty of 100% until the element temperature reaches 730 ° C.

  After starting the engine at t0, the temperature of the element is raised at a heater duty of 100%. Thereby, the element temperature starts to rise and the thermal stress also starts to increase. If the element temperature is 550 ° C. or less, the strength against element cracking is ensured, so there is no risk of element cracking even if the temperature is rapidly increased at a heater duty of 100%. In addition, by rapidly raising the temperature of the element, it is possible to shorten the time until the start of air-fuel ratio feedback control.

  When the internal resistance of the element reaches A (Ω) at t1, that is, when the element temperature reaches the activation temperature of 550 ° C., the heater duty is reduced to B%, that is, 70% in this embodiment, and air-fuel ratio feedback is performed. Start control. In addition, a heater having a temperature raising capability such that t1 is about 4 seconds is used.

  The heater duty is maintained at 70% until the internal resistance of the element is C (Ω), that is, until t3 when the element temperature becomes 700 ° C. By reducing the heater duty, the increasing speed of the element temperature is decreased, and the increasing speed of the thermal stress is also decreased accordingly. Thereby, the temperature of the element can be raised while avoiding the element cracking.

  On the other hand, when the temperature of the element is raised to 730 ° C. at a heater duty of 100%, the element temperature reaches 730 ° C. at t2 before t3, but the thermal stress is larger than when the heater duty is reduced. It increases rapidly to a value and peaks before t2. And the amount of condensed water is increasing during this period. Therefore, there is a high possibility that element cracking occurs due to water exposure.

  The heater duty B% may be an intermediate value between the heater duty 100% and the target heater duty 40% at the time of heater duty feedback control described later, and is not limited to 70%. It may be set to 80%.

  When the internal resistance of the element is C (Ω) at t3, that is, when the element temperature reaches 700 ° C., the heater duty is feedback controlled so as to maintain it until t4 when condensed water runs out. Note that the target value of the heater duty at this time is about 40%.

  As a result, it is possible to prevent element cracking due to water exposure while performing air-fuel ratio feedback control with high accuracy. Note that t4 is D (seconds) used as the threshold for determination in step S160 of the control routine described above.

  When the condensed water disappears at t4, the heater duty is increased and the temperature of the element is raised to 730 ° C. This enables more accurate air-fuel ratio feedback control.

  Although the heater duty is gradually reduced to 70% and 40%, it may be reduced to 40% at once when the element temperature reaches 550 ° C. This is because the element temperature will eventually reach 700 ° C. if the heater duty is 40%. However, the time from starting the air-fuel ratio feedback control to reaching 700 ° C. becomes longer.

  The effects of the present embodiment are summarized as follows.

  (1) The ECU 8 starts the temperature increase of the element when the internal combustion engine 1 is started, and reduces the temperature increase rate of the element when the element reaches the activation temperature. The temperature can be raised at an early stage, and the thermal stress generated when the temperature rises above the activation temperature can be reduced. That is, the air-fuel ratio feedback control can be started early while preventing element cracking.

  (2) The ECU 8 controls the heater so that the element temperature is higher than the activation temperature and lower than the normal control temperature after the element reaches the activation temperature during the period when the condensed water remains in the exhaust passage 4. The air-fuel ratio feedback control can be performed with sufficient accuracy and responsiveness while preventing the element from being cracked even during the period when the condensed water remains.

  The present invention is not limited to the above-described embodiments, and it goes without saying that various modifications can be made within the scope of the technical idea described in the claims.

DESCRIPTION OF SYMBOLS 1 Internal combustion engine 2 Intake passage 3 Throttle valve 4 Exhaust passage 5 Fuel injection valve 6 Catalyst 7 Air fuel ratio sensor 8 Control unit 10 Crank angle sensor 11 Accelerator opening sensor 12 Cooling water temperature sensor

Claims (2)

An air-fuel ratio sensor provided in the exhaust passage of the internal combustion engine;
A heater for raising the temperature of the air-fuel ratio sensor element;
Control means for starting the temperature rise of the element when the internal combustion engine is started, and controlling the heater so as to reduce the temperature rise rate of the element when the element reaches an activation temperature;
An air-fuel ratio detection apparatus comprising:   The control means is configured so that, during a period in which condensed water remains in the exhaust passage, after the element reaches the activation temperature, the element temperature is higher than the activation temperature and the target temperature during operation after the condensed water disappears is exceeded. The air-fuel ratio detection apparatus according to claim 1, wherein the heater is controlled to be low.

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