Patent Publication Number: US-2005139491-A1

Title: Oxygen concentration detecting apparatus and method

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an oxygen concentration detecting apparatus and method used to detect an oxygen concentration in, for example, exhaust gas in an internal combustion engine.  
      2. Description of the Related Art  
      Japanese Unexamined Patent Publication No. 59-148857 discloses an oxygen concentration detecting apparatus for detecting an oxygen concentration in to-be-measured gas. The oxygen concentration detecting apparatus is arranged such that a substrate, a standard electrode, an oxygen ion transmissive solid electrolyte, and a measuring electrode are laminated, the measuring electrode is divided into an energizing electrode and a reference electrode, an oxygen partial pressure in the standard electrode is controlled by applying current between the standard electrode and the energizing electrode, and the oxygen concentration in the to-be-measured gas is detected based on an electromotive force produced between the standard electrode and the reference electrode.  
      Incidentally, since internal combustion engines with a small engine displacement mounted on motor cycles uses an exhaust pipe having a small diameter, an oxygen concentration detecting element mounted on the exhaust pipe must be reduced in size.  
      However, the thickness of laminated members must be reduced to reduce the size of the detecting element, thereby the mechanical strength of the detecting element is reduced.  
      In contrast, in the above oxygen concentration detecting apparatus, the pressure in the detecting element may be increased by the oxygen excessively accumulated to the standard electrode. Accordingly, when the strength of the detecting element is reduced, there is a possibility that detecting element is broken by an increase in the internal pressure thereof.  
     SUMMARY OF THE INVENTION  
      Accordingly, an object of the present invention is to prevent the breakage of a detecting element due to oxygen excessively accumulated to a standard electrode.  
      To achieve the above object, in the present invention, the amount of oxygen accumulated to a standard electrode is estimated, and when it is estimated that the accumulated amount of oxygen reaches a threshold value, the amount of manipulation of a detecting element is changed in a direction where the amount of oxygen flowing to the standard electrode is suppressed.  
      The other objects and features of this invention will become understood from the following description with reference to the accompanying drawing. 
    
    
     BRIEF EXPLANATION OF THE DRAWINGS  
       FIG. 1  is a block diagram showing an arrangement of an oxygen concentration detecting apparatus according to an embodiment;  
       FIG. 2  is a sectional view showing an arrangement of the oxygen concentration detecting element according to the embodiment; and  
       FIG. 3  is a flowchart showing a sequence for setting a bias voltage and a heater voltage according to the embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       FIG. 1  is a block diagram showing an air fuel ratio control system of an internal combustion engine including an oxygen concentration detecting apparatus according to an embodiment of the present invention.  
      The oxygen concentration detecting apparatus according to the embodiment detects the oxygen concentration in an exhaust gas which has a close relation to the air fuel ratio in the internal combustion engine by mounting a detecting element  12  on an exhaust pipe of the internal combustion engine.  
      In the air fuel ratio control system, the amount of fuel injected into the internal combustion engine is feed-back controlled based on the air fuel ratio determined from the oxygen concentration in the exhaust gas.  
      The internal combustion engine is mounted on, for example, a motor cycle.  
      In  FIG. 1 , an engine control unit (ECU)  11 , which controls a fuel injection amount as well as controls the detecting element  12 , includes a microcomputer  111 .  
      The detecting element  12  is controlled by the microcomputer  111 , a bias voltage output unit  112 , and a heater voltage output unit  113 .  
      The microcomputer  111  includes an air fuel ratio detecting/correction value calculating unit  1111 , a fuel injection amount calculating unit  1112 , a drive condition determining unit  1113 , an element state determining unit  1114 , a voltage correction determining unit  1115 , a bias voltage calculating unit  1116 , and a heater voltage calculating unit  1117 .  
      The air fuel ratio detecting/correction value calculating unit  1111  detects the air fuel ratio in response to the bias voltage output from the bias voltage output unit  112  and to the signal of the oxygen concentration detected by the detecting element  12 . Further, the air fuel ratio detection/correction value calculation unit  1111  calculates the correction value of the fuel injection amount based on a detection result of the air fuel ratio and outputs the correction value to the fuel injection amount calculating unit  1112 .  
      The fuel injection amount calculating unit  1112  corrects the fuel injection amount based on the correction value supplied from the air fuel ratio detecting/correction value calculating unit  1111  and controls a fuel injection device  13  based on the corrected fuel injection amount.  
      The drive condition determining unit  1113  is supplied with, for example, an engine rotational speed of the internal combustion engine, a fuel injection amount, an intake pipe pressure, a vehicle velocity, an air fuel ratio, an exhaust gas temperature, and the like as the drive state of a vehicle, and determines the drive state of a vehicle based on the information supplied thereto.  
      Further, the element state determining unit  1114  is supplied with actually measured values of, for example, an element temperature, an element impedance, an element internal stress, and the like as a state of detecting element  12  and determines the state of detecting element  12  based on the information supplied thereto.  
      Estimated values of the element temperature, the element impedance and the element internal stress may be used in place of the actually measured values thereof. The element temperature can be estimated based on an exhaust gas temperature, and the element impedance can be estimated based on the impedance of a heater for heating the detecting element.  
      Results of determination of the drive condition determining unit  1113  and the element state determining unit  1114  are output to the voltage correction determining unit  1115 .  
      The voltage correction determining unit  1115  estimates the amount of oxygen accumulated to the standard electrode of the detecting element  12  from the drive state and the element state and determines whether a bias voltage and a heater voltage that applied to the detecting element  12  are to be changed. The voltage correction determining unit  1115  outputs results of determination as to whether the voltages are to be changed to the bias voltage calculating unit  1116  and the heater voltage calculating unit  1117 .  
      When the state that the temperature of the detecting element  12 , which is estimated from the engine rotational speed, the fuel injection amount, the intake pipe pressure, the vehicle velocity, the air fuel ratio, the exhaust gas temperature, and the like, exceeds, for example, 650° C. or the state that the temperature of the detecting element  12  detected by a sensor exceeds, for example, 650° C. continues for a predetermined time, the voltage correction determining unit  1115  determines that the amount of oxygen accumulated to the standard electrode of the detecting element  12  reaches a threshold value and instructs to reduce the bias voltage and the heater voltage.  
      Further, when the state that the air fuel ratio is leaner than, for example, a theoretical air fuel ratio continues for a predetermined time, the voltage correction determining unit  1115  determines that the amount of oxygen accumulated to the standard electrode of the detecting element  12  reaches the threshold value and instructs to reduce the bias voltage and the heater voltage.  
      On receiving the instruction for reducing the bias voltage, the bias voltage calculating unit  1116  reduces the bias voltage to about 1.0 V when it is an ordinary voltage of about 1.2 V.  
      Further, on receiving the instruction for reducing the heater voltage, the heater voltage calculating unit  1117  reduces the heater voltage to about 10 V when it is an ordinary voltage of about 13 V.  
      The bias voltage output unit  112  applies the bias voltage calculated by the bias voltage calculating unit  1116  to the detecting element  12 .  
      The heater voltage output unit  113  controls the turning on/off of a switching unit  15  so that a target voltage calculated by the heater voltage calculating unit  1117  is applied to a heater unit  122 .  
      The switching unit  15  has a function for turning off a heater drive current upstream of the heater unit  122 .  
      When the healer drive current supplied to the heater unit  122  is shut off by a switching means disposed downstream of the heater unit  122 , that is, interposed between the heater unit  122  and a ground potential, a potential is produced to the heater unit  122  before the heater drive current is shut off. When the heater drive current is shut off, a large amount of oxygen flows from the heater unit  122  to the standard electrode of the detecting element  12 . As a result, there is a possibility that the detecting element  12  is broken by an increase in the internal pressure of the detecting element  12 .  
      In contrast, the switching unit  15  disposed upstream of the heater unit  122  can prevent the oxygen from flowing to the standard electrode when the drive current to the heater unit  122  is shut off, thereby the breakage of the detecting element  12  can be prevented.  
      The detecting element  12  includes a signal unit  121  and the heater unit  122 , the signal unit  121  detecting the oxygen concentration in a to-be-measured gas (exhaust gas) based on the bias voltage applied from the bias voltage output unit  112 , and the heater unit  122  heating the detecting element  12  based on the heater voltage applied from the heater voltage output unit  113 .  
       FIG. 2  is a sectional view showing an arrangement of the detecting element  12 .  
      In  FIG. 2 , the detecting element  12  includes a base member  22 , an oxygen ion transmissive solid electrolyte layer  23 , a porous layer  24 , an inside electrode  25  (standard electrode), an inside dense layer  26 , an outside electrode  27  (measuring electrode), an outside dense layer  28 , and a protection layer  29 . The solid electrolyte layer  23  is formed on the outside surface side of the base member  22 . The porous layer  24  is interposed between the inside surface of the solid electrolyte layer  23  and the outside surface of the base member  22  and composed of a porous material. The inside electrode  25  (standard electrode) is formed on the inside surface of the solid electrolyte layer  23 . The inside dense layer  26  is formed on the outside surface of the solid electrolyte layer  23  and has an electrode window  26   a . The outside electrode  27  (measuring electrode) is formed on the outside surface of the inside dense layer  26  and on the outside surface of the solid electrolyte layer  23  exposed by the electrode window  26   a . The outside dense layer  28  is formed on the outside surface of the outside electrode  27  and has an oxygen introducing window  28   a  at the same position as the electrode window  26   a . The protection layer  29  is formed on the outside surface of the outside dense layer  28  and the outside surface of the outside electrode  27  exposed by the oxygen introducing window  28   a.    
      The outside dense layer  28  and the protection layer  29  are exposed to the to-be-measured gas (exhaust gas in the exhaust pipe) on the outsides thereof.  
      The base member  22  is composed of a rod  210 , a heater pattern  211 , which is formed around the outer periphery of the rod  210 , and a heater covering layer  212  as an insulation material formed around the outer periphery of the rod  210  so as to cover the heater pattern  211 .  
      The rod  210  is formed of a ceramic material, for example, alumina, and the like.  
      The heater pattern  211  is formed of a heat generating conductive material such as tungsten, platinum, and the like, and the temperature of the solid electrolyte layer  23  and the like are increased to an activation temperature by the heat generated by the heater pattern  211 .  
      The solid electrolyte layer  23  is formed of, for example, a paste-like material composed of, for example, zirconia powder mixed with yttria powder at a predetermined mixing ratio by weight.  
      The solid electrolyte layer  23  can generate an electromotive force between the inside electrode  25  (standard electrode) and the outside electrode  27  (measuring electrode) according to a difference between oxygen densities, and transport oxygen ions.  
      The porous layer  24  is formed of a ceramic material such as alumina, and the like and constitutes a path for escaping the oxygen transported to the inside electrode  25  through the solid electrolyte layer  23 .  
      The inside electrode  25  and the outside electrode  27  are formed of platinum and the like which have conductivity as well as is a material through which the oxygen passes.  
      Lead wires  25   a  and  27   a  are disposed to the inside electrodes  25  and outside electrodes  27  integrally therewith, respectively so that a potential difference between the inside electrode  25  and the outside electrode  27  can be detected using the lead wires  25   a  and  27   a.    
      The inside dense layer  26  is formed of a material, for example, a ceramic material such as alumina and the like through which the oxygen in the to-be-measured gas cannot pass to the inside surface thereof.  
      The inside dense layer  26  covers the entire outside surface of the solid electrolyte layer  23 , and the electrode window  26   a  is formed by cutting off a part of the inside dense layer  26 .  
      The electrode window  26   a  has a dimension smaller than that of the inside electrode  25  in both an axial direction and a circumferential direction.  
      The outside dense layer  28  is formed of a material, for example, the ceramic material such as alumina and the like through which the to-be-measured gas cannot pass to the inside surface thereof likewise the inside dense layer  26 , and the oxygen introducing window  28   a  is formed by cutting a part of the outside dense layer  28  at the same position as the electrode window  26   a.    
      The protection layer  29  covers the outside electrode  27 , which is exposed to the outside through the oxygen introducing window  28   a  of the outside dense layer  28 , from the outside and is formed of a porous structural member composed of a material, for example, a mixture of alumina and magnesium oxide through which the harmful gases, dusts, and the like in the to-be-measured gas cannot pass to the inside surface side but the oxygen in the to-be-measured gas can pass to the inside surface side.  
      The detecting element  12  arranged as described above controls the oxygen partial pressure in the inside electrode  25  (standard electrode) by causing the oxygen ions in the solid electrolyte layer  23  to migrate by connecting an external power supply between the inside electrode  25  and the outside electrode  27 . Further, the detection device  12  measures an electromotive force, which corresponds to a difference between the oxygen partial pressure in the inside electrode  25  (standard electrode) and the oxygen partial pressure in the outside electrode  27  (measuring electrode) exposed to the to-be-measured gas as a value corresponding to the oxygen concentration in the to-be-measured gas.  
      Next, a sequence for setting the bias voltage and the heater voltage which is applied to the detecting element  12  will be explained with reference to a flowchart shown in  FIG. 3 .  
      The various drive conditions such as an engine rotational speed, an engine load, an air fuel ratio, and the like are input at step S 1 , and it is determined at step S 2  whether the present air fuel ratio in the internal combustion engine is leaner than that of the theoretical air fuel ratio.  
      The determination of lean is executed based on the air fuel ratio detected by the detecting element  12  or by a target air fuel ratio at the time.  
      When the air fuel ratio is lean, the process goes to step S 3  at which a lean duration time is measured by incrementing a lean counter CL by 1.  
      At step S 4 , whether the lean duration time reaches a predetermined time (for example, 10 seconds) is determined by comparing the value of the lean counter CL with a predetermined value CL1.  
      When the value of the lean counter CL is equal to or more than the predetermined value CL1, the process goes to step S 5 , at which a voltage change flag FL is set to 1.  
      In contrast, when the value of the lean counter CL is less than the predetermined value CL1, the process go to step S 8  by bypassing step S 5 , thereby the voltage change flag FL up to the last time is maintained.  
      When it is determined at step S 2  that the air fuel ratio is not lean, the process goes to step S 6  at which the lean counter CL is reset to zero, and further the voltage change flag FL is reset to zero at next step S 7 .  
      When the air fuel ratio is lean, oxygen continuously flows to the inside electrode  25  as the standard electrode and is excessively accumulated to the inside electrode  25 , thereby the internal pressure of the inside electrode  25  increases.  
      Thus, whether the amount of oxygen accumulated to the inside electrode  25  reaches the threshold value is determined from the lean duration time, and when it is estimated that the amount of oxygen accumulated to the inside electrode  25  reaches the threshold value, the voltage change flag FL is set to 1.  
      When the air fuel ratio is leaner, the lean counter CL may be incremented by a larger value, and when a larger amount of oxygen flows to the inside electrode  25 , the lean counter CL may be incremented at a higher speed.  
      Further, as a simplified method, when the air fuel ratio is leaner, the predetermined value CL1 may be changed to a smaller value.  
      At step S 8 , it is determined whether the temperature of the detecting element  12  is equal to or more than a predetermined temperature (for example, 650° C.).  
      The temperature of the detecting element  12  can be detected by the sensor, in addition to that it can be estimated by the drive conditions and an environmental temperature.  
      When the temperature of the detecting element  12  is equal to or more than the predetermined temperature, the process goes to step  89  at which a temperature counter CT is incremented by 1, thereby a time during which the detecting element  12  is kept at a high temperature is measured.  
      When the detecting element  12  has a higher temperature, the temperature counter CT may be incremented by a larger value, and when a larger amount of oxygen flows to the inside electrode  25 , the temperature counter CT may be incremented at a higher speed.  
      Further, as a simplified method, when the detecting element  12  has a higher temperature, a predetermined value CT 1  may be changed to a smaller value.  
      At step  10 , whether the high temperature continuing time reaches a predetermined time is determined by comparing the value of the temperature counter CT with the predetermined value CT 1 .  
      When the value of the temperature counter CT is equal to or more than the predetermined value CT 1 , the process goes to step S 11 , at which a voltage change flag FT is set to 1.  
      In contrast, when the value of the temperature counter CT is less than the predetermined value CT 1 , the process go to step S 14  by bypassing step S 11 , thereby the voltage change flag FT up to the last time is maintained.  
      When it is determined at step S 8  that the temperature of the detecting element  12  is less than the predetermined temperature, the process goes to step S 12  at which the temperature counter CT is reset to zero, and further the voltage change flag FT is reset to zero at next step S 13 .  
      When the detecting element  12  has a high temperature, the internal resistance thereof decreases and an excessive current flows between the electrodes  25  and  27 , thereby a large amount of oxygen flows to the inside electrode  25  as the standard electrode. With the above operation, oxygen is excessively accumulated to the inside electrode  25  and the internal pressure thereof is increased.  
      Whether the amount of oxygen accumulated to the inside electrode  25  reaches the threshold value is determined from the high temperature continuing time, and when it is estimated that the amount of oxygen accumulated to the inside electrode  25  reaches the threshold value, the voltage change flag FT is set to 1.  
      At step S 14 , it is determined whether the voltage change flag FL is set to 1.  
      When the voltage change flag FL is set to 1, it is estimated that the lean air fuel ratio continues and the amount of oxygen accumulated to the inside electrode  25  reaches the threshold value. Accordingly, the process goes to step S 16  at which processing for reducing the bias voltage and/or the heater voltage is executed to suppress the accumulation of oxygen.  
      In contrast, when the voltage change flag FL is set to 0, the process goes to step S 15  at which whether the voltage change flag FT is set to 1 is determined.  
      When the voltage change flag FT is set to 1, it is estimated that the high temperature of the detecting element  12  continues and the amount of oxygen accumulated to the inside electrode  25  reaches the threshold value. Accordingly, the process goes to step S 16  at which the processing for reducing the bias voltage and/or the heater voltage is executed to suppress the accumulation of oxygen.  
      When both the voltage change flags FL and FT are set to zero, it is not estimated that an excessive amount of oxygen is accumulated to the inside electrode  25 . Accordingly, the process goes to step S 17  at which the bias voltage and the heater voltage are set to ordinary values.  
      When the ordinary value of the bias voltage is, for example, 1.2 V, and the accumulation of oxygen is to be suppressed, the bias voltage is reduced to, for example, about 1.0 V.  
      When the ordinary value of the heater voltage is, for example, 13 V and the accumulation of oxygen is to be suppressed, the heater voltage is reduced to, for example, about 10 V.  
      The amounts of reduction of the bias voltage and the heater voltage are set within the range by which the detection of the air fuel ratio is not affected. Further, the amounts of reduction of the bias voltage and the heater voltage may be changed according to the air fuel ratio and the atmospheric temperature of the detecting element  12  at the time.  
      Since a decrease in the bias voltage decreases the current flowing between the electrodes  25  and  27 , the amount of oxygen flowing to the inside electrode  25  can be suppressed. In contrast, a decrease in the heater voltage can increase the internal resistance of the detection device  12  by decreasing the temperature thereof, thereby the amount of oxygen flowing to the inside electrode  25  can be suppressed.  
      When the oxygen flowing to the inside electrode  25  can be suppressed, an increase in the internal pressure of the detection device  12  due to the accumulation of oxygen can be suppressed, thereby the detecting element  12  can be prevented from being broken by an excessive internal pressure.  
      The same operation/working effect can be obtained by applying the above processing for setting the bias voltage and the heater voltage also in a detecting element in which the outside electrode as the measuring electrode is divided into an energizing electrode and the reference electrode.  
      The entire contents of Japanese Patent Application No. 2003-435777, filed Dec. 26, 2003 and Japan Patent Application No. 2004-331453 filed Nov. 16, 2004 are incorporated herein by reference.  
      While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims.  
      Furthermore, the foregoing description of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.